Electrochemical performance of Molybdenum Carbide MXene-few layer graphene hybrid electrodes for aqueous supercapacitors

Electrochemical performance of Molybdenum Carbide MXene-few layer graphene hybrid electrodes for aqueous supercapacitors | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Electrochemical performance of Molybdenum Carbide MXene-few layer graphene hybrid electrodes for aqueous supercapacitors Samaneh Vaez, Ahmad Bagheri, Hossein Beydaghi, Sebastiano Bellani, and 10 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8910897/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract This work investigates the influence of ionic radius on the charge storage behavior of molybdenum carbide MXene electrodes (Mo 2 CCl 2 ) produced through the molten salt etching method for application in aqueous-based supercapacitors (SCs). Various electrolytes, i.e ., 3 M H 2 SO 4 , 1 M Li 2 SO 4 , 1 M Na 2 SO 4 , and 0.6 M K 2 SO 4 were investigated, revealing that the small cation (H + ) enhances the SCs capacitance through fast redox kinetics and high ionic mobility. To elucidate the role of anions, neutral electrolytes (8 m NaNO 3 , 2 M NaCl, and 1 M Na 2 SO 4 ) were also explored, enabling a wide voltage window and stable operation of SCs. An asymmetric supercapacitor was assembled using Mo 2 CCl 2 /FLG (few-layer graphene) as the pseudocapacitive electrode and FLG/CG/ (curved graphene) as the EDLC counterpart. In this configuration, FLG prevents MXene restacking while CG provides abundant electroactive sites, resulting in enhanced energy density and cycling durability. These results highlight the combined effect of electrolyte ion selection and hybrid electrode engineering toward high-performance, durable aqueous energy-storage devices. Physical sciences/Chemistry Physical sciences/Energy science and technology Physical sciences/Materials science Physical sciences/Nanoscience and technology MXenes Molten Salt Etching Supercapacitor Few-layer Graphene Curved Graphene Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The demand for energy is increasing worldwide due to population growth and industrial development. 1 , 2 At the same time, renewable energy sources are increasingly being used to reduce dependence on fossil fuels. 3 Supercapacitors (SCs) are considered promising energy storage devices due to their unique properties, including fast charge-discharge, high specific power, and excellent cyclic stability. 4 , 5 Supercapacitors are mainly divided into two types: electrochemical double-layer capacitors (EDLCs) 6 and pseudocapacitors (PCs). 6 – 8 In EDLCs, energy is stored electrostatically through the adsorption and desorption of ions at the electrode–electrolyte interface. 9 However, EDLCs are limited by low energy density, which restricts their use in large-scale energy storage applications. 10 – 13 In contrast, PCs utilize rapid and reversible faradaic processes, including redox reactions, ion intercalation, and electrosorption, to achieve significantly higher charge storage capacity compared to EDLCs. 1 , 14 , 15 This faradaic contribution enhances the energy density of PCs while maintaining the fast charge–discharge kinetics characteristic of SCs. 16 , 17 As a result, PCs are considered as strong candidates for next-generation energy storage systems, although further advances in electrode materials and device design are still needed to overcome unsolved challenges. The latter include sluggish ion transport within dense electrode architectures, 18,19 structural degradation during repeated redox cycling, 20,21 and mismatched potential windows between electrodes and electrolytes, 22,23 all of which adversely affect the long-term cycling stability of PCs. Several two-dimensional (2D) materials, including, manganese dioxide (MnO 2 ), molybdenum disulfide (MoS 2 ), tungsten disulfide (WS 2 ), and MXenes, have emerged as promising candidates for pseudocapacitive energy storage devices. 24 – 27 These materials offer high surface area ( i.e ., ~ 18–67 m 2 /g for Ti 3 C 2 MXenes), although the reported values vary substantially with processing and architecture. 28 , 29 This variability primarily reflects differences in the degree of MXene delamination (which controls interlayer separation and restacking) 30 and the formation of composites ( e.g ., with graphene or other carbonaceous nanomaterials) that inhibit restacking and create more accessible, ion-reachable surface area. 31 , 32 Other relevant properties include tunable redox activity 33 , and rapid ion-access pathways 33 , 34 . However, when used in symmetric PCs, their practical application is often hindered by slow faradaic kinetics ( i.e. , with charge-transfer resistances \(\:{R}_{ct}\) ranging from 0.5 to 4 Ω and estimated heterogeneous rate constants \(\:{k}^{0}\) on the order of 10 − 9 –10 − 5 cm/s) 35–38 and structural instability 39 , leading to limited rate capability ( i.e. , capacitance retention drops to 30–60% when current density increases from 1–2 A/g to 10 A/g) 37,40 and capacity fading over extended cycling ( i.e. , capacity retention decreases to 70–85% after 3000–5000 cycles). 16 , 17 , 37 , 40 , 41 . To overcome these issues in symmetric PCs, MXenes have recently emerged as promising electrode materials in asymmetric supercapacitors (ASCs) due to their tunable surface chemistry, high electrical conductivity, and intrinsic pseudocapacitive behavior. 42 – 45 Although other 2D materials can also be used in ASCs, MXenes are highlighted here due to their electrochemical properties. In ASCs, MXenes are often combined with EDLC-type electrodes, enabling simultaneous faradaic and non-faradaic charge storage, which improves energy density while retaining the excellent cycling stability characteristic of EDLCs. 45 , 46 In particular, molybdenum carbide MXene (Mo 2 CT x ) stands out for its metallic-level electrical conductivity, surface redox chemistry, and layered structure. 47 – 50 These properties enable efficient charge transfer and fast redox kinetics, making Mo 2 CT x particularly suitable for SC applications. 51 , 52 Nevertheless, the practical use of Mo 2 C is hindered by structural restacking, which occurs during electrode fabrication or repeated cycling. This restacking reduces the ion-accessible surface area ( i.e ., ~ 8.9–4.9 m 2 /g) 53 and consequently lowers capacitance and rate performance in Mo 2 C MXene-based supercapacitors. 47 , 54 , 55 In this scenario, the integrating chloride-terminated Mo 2 C MXene (Mo 2 CCl 2 ) with conductive carbon materials enhances electron transport and mitigates restacking of Mo 2 CCl 2 layers, thereby increasing the ion-accessible surface area and improving both rate capability and cycling stability.. 56 In this work, we propose the design and realization of a hybrid electrode composed of Mo 2 CCl 2 produced via molten salt etching method 57 and few-layer graphene (FLG). Although Mo 2 CCl 2 exhibits metallic-level conductivity, the actual conductivity strongly depends on surface terminations, 58 structural defects, flake restacking, and interlayer resistance. 59 , 61 In our case, the incorporation of FLG improves the conductive network by reducing interlayer resistance and preventing restacking, thereby enhancing effective charge transport compared to pristine Mo 2 CCl 2 .. 55,62–65 The synergistic interaction between Mo 2 CCl 2 and FLG enables efficient charge transport, rapid redox kinetics, and enhanced structural stability, leading to improved energy storage performance compared to the pristine counterpart ( i.e ., the Mo 2 CCl 2 /FLG electrode delivers 136.3 F/g at 50 mV/s in 3 M H 2 SO 4 , compared to ~ 79 F/g reported for Mo 2 C-based MXenes). 54 This hybrid electrode (Mo 2 CCl 2 /FLG) design represents a novel approach for the development of next-generation high-performance SCs. Based on this rationale, the electrochemical behavior of Mo 2 CCl 2 /FLG electrodes was evaluated in different aqueous electrolytes, beginning with a series of sulfates: 3 M H 2 SO 4 , 1 M Li 2 SO 4 , 1 M Na 2 SO 4 , and 0.6 M K 2 SO 4 , to investigate the effect of cation size on electrode performance. Beyond material selection, electrolyte ion characteristics significantly affect charge storage behavior, ionic transport, and overall SC coulombic efficiency (CE). 66 Aqueous electrolytes, with their inherently high ionic conductivity compared to organic electrolytes and ionic liquids, enable fast charge–discharge operation, offering advantages for high-power applications. 67 , 68 Notably, smaller hydrated cations such as H + exhibit greater mobility and conductivity than their larger alkali-metal counterparts (Li + , Na + , K + ), resulting in enhanced capacitive performance in 3 M H 2 SO 4 electrolyte. 66 , 69 To elucidate the role of anionic radius, 2 M NaCl,8 m NaNO 3 and 1M Na 2 SO 4 electrolytes were used to investigate the influence of anions on charge transport resistance. The electrodes studied are referred to as X–Y, in which X represents the active material (Mo 2 CCl 2 /FLG) and Y denotes the type of electrolyte used ( e.g. , H 2 SO 4 , NaNO 3 , etc.). Electrochemical characterization was carried out in a three-electrode Swagelok configuration, enabling evaluation of the Mo 2 CCl 2 /FLG working electrode. A combination of cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) was used to comprehensively investigate the charge-storage mechanism, rate capability, and interfacial resistance of the electrodes in various electrolyte systems. The results demonstrated a strong dependence of electrochemical performance on ionic radius. Among the electrolytes investigated, the half-cell based on 3 M H 2 SO 4 delivered the highest effective gravimetric capacitance (C g ) of 136.3 F/g at 50 mV/s. This superior performance arises from faster interfacial charge transfer in acidic electrolyte, shown by a lower series resistance (Rₛ ≈ 0.04 Ω in 3 M H 2 SO 4 ) versus 0.08 Ω (2 M NaCl) and 0.13 Ω (8 m NaNO 3 ). Stronger ion adsorption is enabled by the smaller hydrated proton radius (H⁺: 2.76 Å) compared with K⁺ (3.31 Å), Na⁺ (3.58 Å), and Li⁺ (3.82 Å). Enhanced surface redox activity is evidenced by persistent reversible peaks in 3 M H 2 SO 4 half-cells across increasing scan rates. The capacitance at 50 mV/s is 136.3 F/g, higher than 42.85 F/g (1 M Li 2 SO 4 ), 38.75 F/g (1 M Na 2 SO 4 ), and 32.12 F/g (0.6 M K 2 SO 4 ), ~ 3.2–4.2× higher. However, to enhance the device-level performance, the pseudocapacitive Mo 2 CCl 2 /FLG hybrid electrode was assembled with an EDLC electrode offering fast and reversible ion transport capability, forming an asymmetric device. Concerning the EDLC electrode, the composition of curved graphene (CG) with FLG (CG/FLG) was used to prevent nanosheets restacking and enhance the exposure of electroactive surface sites of the CG/FLG electrode, providing excellent structural and electrochemical compatibility with the Mo 2 CCl 2 /FLG electrode. 70 The ASCs were assembled and evaluated in three aqueous electrolytes: 3 M H 2 SO 4 , 2 M NaCl, and 8 m NaNO 3 , each selected to balance ionic mobility (3 M H 2 SO 4 ), environmental safety, and electrochemical stability (2 M NaCl and 8 m NaNO₃). The as-prepared ASCs are denoted as negative electrode // positive electrode –electrolyte , i.e ., Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 indicates a device with Mo 2 CCl 2 /FLG as the negative, CG/FLG as the positive electrode and 3 M H 2 SO 4 as the electrolyte. Among the assembled ASCs, the device using 8 m NaNO 3 as the electrolyte delivered the highest electrochemical performance, achieving an energy density of approximately 8 Wh/kg and remarkable cyclic stability (94% gravimetric capacitance -C g - retention after 12000 GCD cycles). The 2 M NaCl-based ASCs also demonstrated long-term stability (81% C g retention after 12000 GCD cycles), reaching an energy density of 7.39 Wh/kg, higher than that of the 3 M H 2 SO 4 -based ASCs (4 Wh/kg). The 3 M H 2 SO 4 -based ASCs delivered the highest C g but operating at lower cell voltage (1 V), compared with 2 M NaCl- and 8 M NaNO 3 -based ASCs (1.6 V), due to water decomposition under acidic conditions. This study offers significant insights into the electrochemical behavior of the hybrid Mo 2 CCl 2 /FLG electrode, revealing correlations between its structural properties and potential application in electrochemical energy storage systems. Results and Discussion Figure 1 a presents the schematic representation of the Mo 2 CCl 2 synthesis. The morphological and structural characterization of the synthesized Mo 2 CCl 2 is shown in Fig. 1 b-f. Figure 1 b shows the X-ray diffraction (XRD) patterns of both the synthesized MXene after copper removal by washing and its parent phase, Mo 2 Ga 2 C. After etching and washing, the XRD pattern changes significantly. Compared with Mo 2 Ga 2 C, the peaks at ~ 34˚, 37˚, and 43˚ 2θ reduce in intensity by approximately 50–70% and slightly shifted by 0.2°–0.4°, while the (002) peak at around 9° strongly reduced in intensity. These structural changes are consistent with previously reported molten-salt-synthesized Mo 2 CT x MXenes in literature. 57 The Scanning Electron Microscopy (SEM) image reported in Fig. 1 c reveals several multilayer MXene flakes exhibiting visible slit-like pores. The flakes reach sizes of up to a few micrometers in length and display textured surfaces. Figure 1 d shows a Selected Area Electron Diffraction (SAED) image of Mo 2 CCl 2 along the [001] axis, confirming its hexagonal symmetry and high crystallinity. The High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF STEM) image in Fig. 1 e, together with the Transmission Electron Microscopy (TEM) images in Fig. 1 f and Figure j , further confirm high-quality multilayer MXene flakes. While molten salt etching of MXene 71 , 72 differs significantly from conventional aqueous acid etching methods 73 – 75 , the resulting materials retain the characteristic features of MXenes and exhibit excellent structural quality. The Mo 2 CCl 2 /FLG electrode was evaluated in a three-electrode configuration in various sulfate-based electrolytes (3 M H 2 SO 4 , 1 M Li 2 SO 4 , 1 M Na 2 SO 4 , and 0.6 M K 2 SO 4 ) to investigate the effect of cation radius and mobility on charge storage. Figure 2 a illustrates the schematic of the three-electrode configuration for the Mo 2 CCl 2 /FLG electrode tested in various aqueous electrolytes. Figure 2 b presents the CV curves recorded at a scan rate of 20 mV/s for all electrolytes. The results reveal a predominantly capacitive and irreversible redox behaviour (small oxidation peak) for Mo 2 CCl 2 /FLG–1 M Li 2 SO 2 , Mo 2 CCl 2 /FLG–1 M Na 2 SO 4 , and Mo 2 CCl 2 /FLG–0.6 M K 2 SO 4 . In contrast, Mo 2 CCl 2 /FLG–3 M H 2 SO 4 exhibits a reversible redox process, indicating a distinct electrochemical mechanism under acidic conditions. Among the tested systems, the Mo 2 CCl 2 /FLG–3 M H 2 SO 4 electrode delivers a markedly higher specific current in the CV profiles, indicating a higher C g compared to the other assembled systems with different electrolytes. 69 Figure S1 a–d displays the CV curves of Mo 2 CCl 2 /FLG–3 M H 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Li 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Na 2 SO 4 , and Mo 2 CCl 2 /FLG–0.6 M K 2 SO 4 recorded at various potential scan rates, ranging from 5 to 50 mV/s. These curves exhibit distinct reversible redox peaks, further confirming the pseudocapacitive properties of the Mo 2 CCl 2 /FLG–3 M H 2 SO 4 devices. On the contrary, Mo 2 CCl 2 /FLG–1 M Li 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Na 2 SO 4 , and Mo 2 CCl 2 /FLG–0.6 M K 2 SO 4 ASCs display a predominantly capacitive behavior. Notably, the redox peaks of the Mo 2 CCl 2 /FLG–3 M H 2 SO 4 remain visible with increasing the potential scan rates, indicating efficient charge transfer ( i.e ., persistence of oxidation and redox peaks in the CV profiles across different scan rates) and electrochemical reversibility. 76 Figure S2a–d shows the GCD profiles of Mo 2 CCl 2 /FLG–3 M H 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Li 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Na 2 SO 4 and Mo 2 CCl 2 /FLG–0.6 M K 2 SO 4 electrodes, respectively, measured at specific currents ranging from 1 to 20 A/g. All electrodes tested in different electrolytes exhibit non-linear voltage–time profiles at various specific currents. The non-linear GCD profile of the Mo 2 CCl 2 /FLG– 3 M H 2 SO 4 displays redox features indicative of substantial pseudocapacitive contributions, whereas the GCD curves obtained in the devices based on the other electrolytes exhibit mainly capacitive behavior. Figure 2 c shows C g of Mo 2 CCl 2 /FLG electrode in four different electrolytes as a function of the potential scan rate, calculated from the CV profiles (see Supporting Information, Equation (S1)). Among the investigated systems, the Mo 2 CCl 2 /FLG–3 M H 2 SO 4 demonstrates the highest C g ( e. g. , 136.3 F/g at 50 mV/s), outperforming those measured in 1 M Li 2 SO 4 , 1 M Na 2 SO 4 , and 0.6 M K 2 SO 4 electrolyte systems (42.85, 38.75, and 32.12 F/g for 1 M Li 2 SO 4, 1 M Na 2 SO 4 and 0.6 M K 2 SO 4 , respectively, at the same potential scan rate). This enhancement in 3 M H 2 SO 4 arises from the high ionic mobility and small hydration sphere radius of H + ions (2.76 Å), which enable a proton hopping between water molecules via hydrogen bonding. Compared with K + (3.31 Å), Na + (3.58 Å), and Li + (3.82 Å) ions, H + exhibits the highest molar ionic conductivity. 66 , 69 The enhanced conductivity and ionic mobility facilitate rapid charge transfer, while the smaller hydration radius increases ion adsorption at the electrolyte/electrode interface, further promoting faradaic reactions. 69,77 Consequently, the superior proton dynamics in 3 M H 2 SO 4 result in the highest C g observed (136.3 F/g at 50 mV/s). Table S1 compares these results with Mo 2 C-based electrodes previously reported in literature, highlighting the material strong potential for supercapacitor applications. The electrochemical stability of Mo 2 CCl 2 /FLG electrodes was assessed over 12000 GCD cycles at a specific current of 5 A/g. As demonstrated in Figure S3a–d , Mo 2 CCl 2 /FLG–3 M H 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Li 2 SO 4 , Mo 2 CCl 2 /FLG–1 M Na 2 SO 4 , and Mo 2 CCl 2 /FLG–0.6 M K 2 SO 4 samples retained 89%, 90%, 93%, and 90% of their initial C g , respectively. Additionally, all electrodes maintained nearly 100% CE throughout cycling (up to 12000 cycles). These results highlight the long-term stability and electrochemical reversibility of the Mo 2 CCl 2 /FLG electrodes across these electrolytes. Considering both stability and cost factors, i.e. , Na salts are much more abundant and cost-effective than Li salts, Na-based electrolytes are attractive for their use in large-scale and environmentally friendly energy storage devices. 78 , 79 To evaluate the effect of anion size on the electrochemical properties, we compared Mo 2 CCl 2 /FLG electrode in 2 M NaCl, 8 m NaNO 3 , and 1 M Na 2 SO 4 using a three-electrode cell configuration. The electrolytes have a near-neutral pH (~ 6–7), contain the same cation (Na + ) but different anions: Cl⁻, NO 3 ⁻, and SO 4 2 ⁻ ion. Figure 2 d displays the non-rectangular shape of CV curves of Mo 2 CCl 2 /FLG–2 M NaCl, Mo 2 CCl 2 /FLG–8 m NaNO 3 , and Mo 2 CCl 2 /FLG-1 M Na 2 SO 4 systems at 20 mV/s, indicating the pseudocapacitive behavior. 80 Figure S4a–b presents the CV curves of Mo 2 CCl 2 /FLG in 2 M NaCl and 8 m NaNO 3 electrolytes measured at scan rates from 5 to 50 mV/s. When assembled and tested in three-electrode cell configuration with both 8 m NaNO 3 and 2 M NaCl electrolytes, the electrodes predominantly exhibit capacitive behavior. Whereas a weak redox reaction is observed in the cell assembled with 2 M NaCl, indicating a minor pseudocapacitive contribution. 76 Figure S4c–d illustrate the GCD profiles of the Mo 2 CCl 2 /FLG–2 M NaCl and Mo 2 CCl 2 /FLG–8 m NaNO 3 system recorded at specific current, ranging from 1 to 20 A/g. All electrodes exhibit approximately linear voltage–time characteristics across the tested specific current range. These results are consistent with the primarily capacitive behavior previously shown by the CV analyses. The C g of the Mo 2 CCl 2 /FLG electrode, measured in 2 M NaCl, 8 m NaNO 3 , and 1 M Na 2 SO 4 electrolytes at various potential scan rates, was calculated from the CV curves shown in Fig. 2 e. C g increases with decreasing the potential scan rate, reaching 57.5, 51.25, and 42.5 F/g at 5 mV/s for Mo 2 CCl 2 /FLG–2 M NaCl, Mo 2 CCl 2 /FLG–8 m NaNO 3 and Mo 2 CCl 2 /FLG–1 M Na 2 SO 4 systems, respectively. This trend is attributed to diffusion limitations of electrolyte ions into the interlayer spaces of the electrode. 81 , 82 The SO 4 2 ⁻ ions possess a larger ionic radius compared to NO 3 ⁻ and Cl⁻ ions. 83 Moreover, SO 4 2 ⁻ exhibits lower ionic conductivity and mobility compared to Cl⁻ and NO 3 ⁻. 84,85 As a result, 1 M Na 2 SO 4 aqueous electrolyte delivered poor capacitive performance. As shown in Figure S5a-b , the Mo 2 CCl 2 /FLG electrode demonstrated excellent cycling stability in 2 M NaCl and 8 m NaNO 3 , retaining 80% and 82% of its initial C g , respectively, after 12000 GCD cycles at 5A/g. Additionally, the CE remained close to 100%, confirming the superior electrochemical reversibility of the electrode. The cyclic stability of representative Mo 2 CCl 2 /FLG–3 M H 2 SO 4 and Mo 2 CCl 2 /FLG–8 m NaNO 3 systems after 12000 GCD cycles was investigated by post-measurement XRD and Raman spectroscopy. Figure S6a displays the XRD patterns of pristine Mo 2 CCl 2 and the Mo 2 CCl 2 /FLG electrodes after long-term cycling stability in 3 M H 2 SO 4 and 8 m NaNO 3 electrolytes, enabling a comparison of the structural changes resulting from electrochemical cycling. The electrodes used Mo 2 CCl 2 /FLG–3 M H 2 SO 4 and Mo 2 CCl 2 /FLG–8 m NaNO 3 systems after 12000 GCD cycles show main diffraction peaks at 34.5°, 38.0°, 39.6°, 52.3°, 61.9°, 69.8°, 75.0°, and 76.0°, corresponding to the (100), (002), (101), (102), (110), (103), (112), and (201) planes of Mo 2 C (JCPDS 65-8766). 86 – 89 More in detail, the XRD pattern of the as-prepared pristine Mo 2 CCl 2 show sharp, well-defined peaks, indicating a highly crystalline and ordered structure. After the cyclic stability test, some structural changes were observed in the Mo 2 CCl 2 /FLG. These changes are likely due to repeated ion intercalation and deintercalation, which can cause mechanical stress such as expansion and contraction of Mo 2 CCl 2 /FLG during the cycling process. 67 , 90 – 92 Additionally, new diffraction peaks appear at 43.5°, 44.9°, and 60° after cycling, indicating the formation of new crystalline phases. These phases may occur from side reactions, the generation of byproducts, or partial decomposition of Mo 2 CCl 2 . 93 , 94 A sharp diffraction peak observed at 54.9° corresponds to the (004) reflection, which is characteristic of graphite (substrate) and FLG, confirming the presence of well-ordered graphitic structures within the electrode. 63 , 95 To further investigate structural changes in Mo 2 CCl 2 /FLG electrodes after GCD cycling, ex-situ Raman spectroscopy was performed on the electrode of Mo 2 CCl 2 /FLG–3 M H 2 SO 4 and Mo 2 CCl 2 /FLG–8 m NaNO 3 systems (Figure S6b). Raman spectra reveal characteristic peaks at 1358 cm⁻ 1 (D band) and 1583 cm⁻ 1 (G band), which are typically observed in graphite, FLG and Mo 2 CCl 2 . 86 , 89 , 96 , 97 Additionally, a distinct peak at 886cm⁻ 1 is attributed to Mo 2 CCl 2 . 96–98 Notably, these characteristic peaks remain present in the samples after the GCD cycling, indicating that Mo 2 CCl 2 /FLG electrodes preserved their structural integrity. The Raman I(D)/I(G) ratios for pristine Mo 2 CCl 2 and the Mo 2 CCl 2 /FLG electrode are ~ 0.77 and ~ 0.52, respectively, indicating that the incorporation of FLG reduces the overall defect density and improves structural ordering within the composite. 99 Controlled defect sites are known to enhance the electrochemical performance of carbon-based and MXene electrodes by providing additional electroactive sites and improving ion accessibility, whereas excessive defects can hinder electrical conductivity and limit power performance. 97 , 100 , 101 After cycling in different electrolytes, the I(D)/I(G) ratios for Mo 2 CCl 2 /FLG–8 m NaNO 3 and Mo 2 CCl 2 /FLG–3 M H 2 SO 4 increase to ~ 0.57 and ~ 0.71, respectively. The increased defects (higher ID/IG) observed in Mo 2 CCl 2 /FLG–3 M H 2 SO 4 sample suggests that the stronger acidic environment induces a higher density of defect sites, which can partially modify the Mo–C bonding environment. 102 This findings correlates with the electrochemical results, in which the higher defect density in Mo 2 CCl 2 /FLG–3 M H 2 SO 4 contributes to greater pseudocapacitive activity and higher capacitance. 103 To further assess the charge storage performance of the MXnene-based electrodes in practical applications, aqueous ASCs were assembled using 8 m NaNO 3 , 2 M NaCl, and 3 M H 2 SO 4 as electrolytes. These were selected based on the high C g obtained in 3 M H 2 SO 4 and the excellent cycling stability demonstrated in the environmentally friendly 8 m NaNO 3 and 2 M NaCl electrolytes. Based on the electrochemical evaluation of Mo 2 CCl 2 /FLG in a three-electrode setup, Mo 2 CCl 2 /FLG–3 M H 2 SO 4 was selected as the PC electrode, whereas Mo 2 CCl 2 /FLG–8 m NaNO 3 and Mo 2 CCl 2 /FLG–2 M NaCl exhibited predominantly capacitive behavior ( Figure S9a-f ). The CG/FLG functioned as an EDLCs electrode, as shown by its electrochemical characterization in 8 m NaNO 3 ( Figure S7a-f ), 2 M NaCl ( Figure S8a-f ). Before ASCs assembly, the active material mass loading of the electrodes was optimized to balance the C g of the positive and negative electrodes. The working voltage windows (WVW) of the ASCs were determined by combining the stable potential ranges of the separate positive and negative electrodes obtained from separate three-electrode CV measurements carried out at 20 mV/s (Fig. 3 a-c). Figure 3 d-f shows the electrode potentials vs. Ag/AgCl recorded over time during GCD measurements at a specific current of 1 A/g in the investigated electrolytes. The Mo 2 CCl 2 /FLG electrode in 8 m NaNO 3 and 2 M NaCl operates in a potential range of − 0.7 to + 0.2 V, confirming mainly capacitive characteristics, whereas the CG/FLG electrode operated between 0.2 and 0.9 V. Both electrodes remained in the electrochemical stability window during GCD process, thereby effectively avoiding parasitic hydrogen and oxygen evolution reactions (HER and OER). The potential profile of CG/FLG is reproduced over subsequent cycles, confirming the excellent electrochemical stability of this EDLC-type electrodes. Mo 2 CCl 2 /FLG provides a minor pseudocapacitive contribution at potentials below 0.0V vs. Ag/AgCl. Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 ASCs stably operate within a WVW of 1.0 V. Figure 4 a reports the CV curves measured for Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , − 2 M NaCl, and − 3 M H 2 SO 4 ASCs, measured at scan rate of 20mV/s, with WVW of 1.6 V, 1.6 V, and 1 V, respectively. The CV profiles indicate that the charge storage process is primarily ruled by EDLC rather than pseudocapacitive contributions. Figure 4 b presents the GCD profiles measured for Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , − 2 M NaCl, and − 3 M H 2 SO 4 , at a specific current of 5 A/g. The nearly linear GCD profiles confirm the predominantly capacitive behavior of the ASCs, consistent with the CV analysis. Figure 4 c shows the C g calculated from the GCD curves (Supporting Information, Equation (S2)). The Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 ASCs achieved the highest C g of 29.57 F/g at 1 A/g. The Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl and Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 ASCs displayed C g values of 24.67 F/g and 23.13 F/g at 1 A/g, respectively. The superior performance of the Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 compared to Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl and Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , is consistent with the results obtained from the three-electrode cell configuration for the negative electrodes. 69 , 104 For all ASCs, the C g decreased with increasing the specific current, although the devices demonstrated remarkable rate capabilities ( e.g. , at 20 A/g, the C g retention was 86%, 65%, and 63% for Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 , Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl, and Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , respectively). As demonstrated in Fig. 4 c, CE of the ASCs exceeds 90% at high specific currents for both Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl and Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , indicating excellent reversibility of the redox processes. In contrast, the Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 ASCs exhibit a slight reduction in CE at lower specific currents (< 5 A/g), which is probably attributed to parasitic reactions commonly encountered in ASCs based on acidic electrolyte, which easily promote HER. 23 The impedance of the electrode–electrolyte interfaces was analyzed through EIS measurements. The complex impedance (Z) of the systems was characterized by analysing the Nyquist plots (− Im[Z] vs. Re[Z]), offering valuable information on charge-transfer and diffusion processes. 105 A Nyquist plot of an SC typically exhibits three distinct regions corresponding to different electrochemical processes: redox reactions appearing in the high-to-mid frequency range, ion diffusion processes dominating the mid-frequency region, and capacitive behavior prevailing at low frequencies. 6 , 106 In the high-frequency region, the plot reflects the electrical conductivity of the electrode and the charge-transfer reactions occurring at solid/solid interface. 107 – 110 Thus, the diameter of the semicircle observed in this region is related to the interfacial resistance between the current collector and the electrode. 111 – 113 Instead, the x-axis (Z re ) intercept at the highest frequency corresponds to the equivalent series resistance (R s ), which arises from the ionic resistance of the electrolyte and the intrinsic electronic resistance of the electrodes, distinct from the ion-diffusion processes observed in the mid-frequency region. 78 , 105 , 114 Figure S10 shows the Nyquist plots for Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl, and Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 . The measurements were recorded over a frequency range of 100 kHz to 10 mHz at open-circuit potential with an AC amplitude of 10 mV. From the analysis of the Nyquist plots, calculated R s were 0.04, 0.13 and 0.08 Ω for Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 , Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 and Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl, respectively. These values indicate significantly lower resistance and higher ionic conductivity in the acidic electrolytes compared to the near-neutral ones. The pseudocapacitive energy storage of the Mo 2 C active material is based on redox reactions involving charge transfer. This behavior occurs along with surface-controlled processes that contribute to EDLC. 115 The main type of charge storage can be identified by analyzing CV curves at different scan rates. As previously shown, the CV curves of Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl, and Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 were measured at different voltage scan rates from 5 to 1000 mV/s. These data were evaluated using the power law given in Eq. 1 . $$\:i=a{v}^{b}$$ 1 In which i represents the measured current (A), v is the voltage scan rate (mV/s), and a and b are constants that reflect the contributions of capacitive and faradaic processes. Specifically, for diffusion-controlled behavior, the current is proportional to the square root of the scan rate ( b = 0.5). 80 In contrast, for capacitive processes, the current is directly proportional to the scan rate ( b = 1). 80 Figures S11a,12a,13a display the b -values for the Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl, and Mo 2 CCl 2 /FLG // CG/FLG − 3 M H 2 SO 4 as a function of their voltage. Notably, b -values for Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , range from 0.72 to 0.93 ( Figure S11a ), indicating that the charge storage involves a combination of both capacitive and diffusion-controlled processes. Similarly, the Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl and Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 show b values ranging from 0.5 to 1, indicating a mixed charge storage mechanism involving both capacitive and diffusion-controlled contributions, as expressed by the following equation: 6 $$\:i\left(V\right)={{k}_{1}v+k}_{2}{v}^{1/2}$$ 2 in which \(\:v\) represents the voltage scan rate (mV/s), \(\:{k}_{1}v\) corresponds to the current from surface capacitive contributions, and \(\:{k}_{2}{v}^{1/2}\) accounts for the current arising from diffusion-controlled faradaic processes. Eq. ( 3 ) can also be written in a different form as: $$\:\frac{i\left(V\right)}{{v}^{1/2}}={{k}_{1}v+k}_{2}$$ 3 Accordingly, the values of k₁ and k₂ were obtained from the linear fitting of i(V)/v¹ᐟ² vs. v¹ᐟ² (Figures S11b,12b,13b). As shown in Figures S11c-d,12c-d,13c-d , the diffusion-controlled charge contributions at a voltage scan rate of 5 mV/s were calculated 64% for Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 , 40% for Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl, and 49% for Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 . Figures S11c,12c,13c confirm that capacitive charge storage increases with broadening WVW, while diffusion-controlled faradaic processes are progressively decreased with increasing WVW due to the limited kinetics of faradaic reactions. Figure 4 d displays the Ragone plots (energy density, Eₛ, vs. power density, Pₛ) for the investigated ASCs. The Ragone plots were derived from the GCD analysis, as detailed in the Supporting Information (Equations S3 and S4). Despite its highest C g Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 has shown the lowest energy density among the investigated devices, i.e ., 4.07 Wh/kg at 338.54 W/kg. The poor energy density of the ASCs based on H 2 SO 4 as the electrolyte is primarily due to the limited WVW. 66 The cyclic stability of Mo 2 CCl 2 /FLG // CG/FLG ASCs was assessed over 12000 GCD cycles at a specific current of 5 A/g. As illustrated in Fig. 4 e, the Mo 2 CCl 2 /FLG // CG/FLG–8 m NaNO 3 and Mo 2 CCl 2 /FLG // CG/FLG–2 M NaCl retained 94% and 81% of their initial C g , respectively. In contrast, Mo 2 CCl 2 /FLG // CG/FLG–3 M H 2 SO 4 has shown a significant performance reduction, retaining only 20% of its initial C g after 12000 GCD cycles. The CE remained nearly 100% for the 8 m NaNO 3 -based and 2 M NaCl ASCs, while it dropped to 83.74% when using 3 M H 2 SO 4 electrolyte. Methods Materials Synthesis: Mo 2 Ga 2 C was supplied by Carbon Ukraine. Sodium chloride (NaCl, 99.5%), potassium chloride (KCl, 99.5%), and hydrochloric acid (HCl, 37%) were obtained from Sigma Aldrich. Copper (II) chloride anhydrous (CuCl 2 , 98%), was purchased from Alfa Aesar. To perform the molten salt etching of gallium from Mo 2 Ga 2 C, a procedure similar to the one in Ref 116 was followed. The ratio was adjusted to account for the extra stoichiometry of gallium. Powders of Mo 2 Ga 2 C, NaCl, KCl, and CuCl 2 were combined in a 1:6:6:6 ratio and ground with a mortar and pestle for 10 min to ensure uniform particle size and to mix the MAX phase and salts together. The mixed powders were then added to an alumina crucible, covered with a lid, and then the crucible was placed in a glove box transfer chamber and evacuated three times, refilling with Argon, to displace extra oxygen trapped within the powders. The crucible was then promptly placed in a tube furnace with argon flowing at 450 sccm. The tube was allowed to purge for 1 h, and then heated at a rate of 5 ˚C/min up to 700 ˚C. It was held at 700 ˚C for 4 h and then allowed to cool to room temperature, all under flowing argon. After cooling, deionized (DI) water was used to dissolve leftover chloride salts and transfer the materials to a 50 mL centrifuge tube. Extra DI water was used to fill the tube as needed. This was centrifuged for 10 min at 5000 rpm, after which the supernatant was discarded. Three additional cycles of adding 50 mL DI water, resuspending the solid material with vortex mixing, centrifuging, and discarding the supernatant were performed to ensure all leftover chloride salts were dissolved and removed from the product. To remove copper deposited on the multilayer MXene particles during the molten salt synthesis, the wet powders were added to a solution of 1M CuCl 2 in 2M HCl, with 4x the mass of the CuCl 2 used in the molten salt mixture. This mixture was stirred for 4 h, after which the mixture was decanted into 50 mL centrifuge tubes and washed with excess DI water by centrifuging for 5 min at 5000 rpm, discarding the supernatant, and refilling with DI water and resuspending using vortex mixing until the pH of the supernatant reached ~ 6. The resulting powders were dried using vacuum filtration followed by vacuum annealing at 200 ˚C for 2 h to remove bulk water. Samples were stored in a vacuum desiccator until needed. Characterization XRD patterns were produced using a Bruker D8 ADVANCE powder diffractometer using Cu K α1 (λ = 0.154 nm) radiation. Patterns were collected between 3˚and 80 2θ with a step size of 0.02˚ and a dwell time of 1.5 s. SEM was performed on a Zeiss Ultra Plus field-emission SEM electron microscope with an accelerating voltage of 3 keV. Both TEM and STEM images were produced using an uncorrected FEI Titan with Schottky field emission S-FEG source operated at 300 kV Electrode Preparation: The hybrid Mo 2 CCl 2 /FLG electrode was prepared by mixing Mo 2 CCl 2 powder and FLG, exfoliated from graphite by BeDimensional S.p.A. using its proprietary wet-jet milling method, 95,117–120 with a carboxymethyl cellulose:Styrene-Butadiene Rubber (CMC:SBR) binder (weight ratio 90:5:5). A homogenized slurry was obtained by dispersing Mo 2 CCl 2 powder in deionized water by ultrasonic bath, and then FLG, CMC:SBR were added in to the composition and using a planetary centrifugal mixer. 121 , 122 The as prepared slurry was deposited onto a graphite sheet by doctor blade method, which allowed for controlled thickness and even distribution. 7 , 10 The electrodes were dried at 60°C in an oven to completely remove residual moisture and solvents. After drying, the electrodes were precisely punched into discs with a diameter of 8 m. The prepared electrodes exhibited an average active material mass loading of approximately 3 mg/cm 2 . 123 Three-Electrode Cell Configuration: Electrochemical characterization was carried out in three-electrode Swagelok cells based on 316L stainless steel or titanium pistons, an insulating PTFE-coated 316L stainless steel body, and PTFE sealing rings. Each cell was assembled with a Mo 2 CCl 2 /FLG working electrode, an Ag/AgCl reference electrode, high-mass loading activated carbon as counter electrode, and a glass fiber (Whatman GF/A) as the separator. The cell components were tightened in the Swagelok cell to maintain good contact and prevent leakage. All measurements were performed at room temperature (~ 25°C). Various aqueous electrolytes were used to investigate the effect of ionic radius of different cations and anions on SC performance. These included 3 M H 2 SO 4, 1 M Li 2 SO 4 , 1 M Na 2 SO 4 , 0.6 M K 2 SO 4 (sulfate-based electrolytes for cation comparison), as well as 2 M NaCl and 8 m NaNO 3 (for anion comparison). ASC Assembly: Two-electrode ASCs were assembled using Mo 2 CCl 2 /FLG as the negative electrode and CG/FLG as the positive electrode. The CG/FLG positive electrode was prepared by mixing CG, FLG, and CMC:SBR in water by a planetary centrifugal mixer (weight ratio 90:5:5), and casting the resulting slurry onto a graphite sheet. Before assembling the ASCs, the electrochemical properties of both Mo 2 CCl 2 /FLG and CG/FLG electrodes were individually evaluated through three-electrode cell measurements. Charge balance between the two electrodes was achieved by adjusting their mass loadings according to their respective charge storage capacities within the selected potential window. 107 , 124 – 126 The optimal mass ratio between the positive and negative electrodes (m₊/m₋) is determined using the following Eq. 4 : $$\:\frac{{m}_{+}}{{m}_{\_}}=\:\frac{{C}_{\_\:}\times\:\varDelta\:V\_}{{C}_{+}\times\:\varDelta\:{V}_{+}}$$ 4 The Mo 2 CCl 2 /FLG // CG/FLG ASCs were tested in three different aqueous electrolytes: 3 M H 2 SO 4 , 2 M NaCl, and 8 m NaNO 3 . For the 3 M H 2 SO 4 electrolyte, the cell voltage was limited to 1.0 V to avoid parasitic water splitting reaction, in particular the acid-promoted HER. 127 , 128 In near-neutral 2 M NaCl and 8 m NaNO 3 electrolytes, an upper cell voltage of 1.6 V was reached, taking advantage of the larger overpotential for gas evolution in near-neutral electrolyte. 23 The assembled ASCs were evaluated through CV and GCD analyses. Long-term cyclic stability was assessed by repeating GCD cycles up to 12000 times at a fixed specific current (5 A/g). Conclusions This study highlights the successful integration of Mo 2 CCl 2 /FLG as a high-performance negative electrode and CG/FLG as a complementary positive electrode in aqueous ASCs. The Mo 2 CCl 2 /FLG hybrid leverages the synergistic combination of pseudocapacitive Mo 2 CCl 2 and highly conductive FLG, leading to enhanced charge-transfer kinetics, superior reversibility, and excellent electrochemical performance. By systematically investigating different aqueous electrolytes, 3 M H 2 SO 4 , 2 M NaCl, and 8 m NaNO 3 , the study elucidates the role of ionic species in modulating charge storage behavior and interfacial resistance. The ASCs with 3 M H 2 SO 4 delivered the highest specific capacitance (29.57 F/g at 1 A/g), attributed to the fast proton mobility and efficient surface redox reactions of Mo 2 CCl 2 . However, in 3 M H 2 SO 4 the device voltage had to be limited to 1.0 V due to acid-promoted water splitting/HER, whereas near-neutral Na-based electrolytes (2 M NaCl, 8 m NaNO 3 ) enabled a broader 1.6 V window. Moreover, Na-based ASCs have shown superior cycling stability, retaining 94% (8 m NaNO₃) and 81% (2 M NaCl) of the initial capacitance after 12000 cycles, compared with only 20% retention in 3 M H 2 SO 4 -based ASCs. Remarkably, the 8 m NaNO 3 ASC achieved an energy density of ~ 8 Wh/kg with 94% capacitance retention after 12000 GCD cycles, demonstrating outstanding long-term durability. Similarly, the 2 M NaCl ASCs exhibited excellent energy retention and cycling stability. The EIS further revealed a strong electrolyte dependence of the internal resistance, with the lowest R s (~ 0.04 Ω) observed in 3 M H 2 SO 4 -based ASC, while 8 m NaNO 3 -one displayed a higher Rₛ due to reduced ion mobility. Overall, the Mo 2 CCl 2 /FLG // CG/FLG architecture, when paired with optimized electrolytes, is demonstrating promising for high-performance, durable, and scalable aqueous supercapacitors. These findings establish valuable design guidelines to tailor electrode–electrolyte interfaces and pave the way toward next-generation electrochemical energy storage systems that balance high energy density, power capability, and long-term stability. Declarations Acknowledgments This project received funding from the European Union’s 2D Printable Horizon Europe research and innovation program under Grant Agreement No. 101135196 and the European Union’s GREENCAP Horizon Europe research and innovation program under Grant Agreement No. 101091572. S.V. acknowledges financial support from the European Union Next Generation EU program (D.M. 117 del 02/03/2023 Ministero dell′ Università e della Ricerca). T.G. thanks the European Research Council for the project JANUS BI (grant agreement no. [101041229]) and the European Innovation Council for the project LEAF (grant agreement n. [101186701]). V.N. and X.G. wish to thank the support of the Research Ireland-funded AMBER Research Centre and the SFI Frontiers for the Future award (Grant Nos. 12/RC/2278_P2 and 20/FFP-A/8950 respectively). Furthermore, V.N. and X.G. wish to thank the Advanced Microscopy Laboratory (AML) in CRANN for the provision of their facilities and thank Clive Downing for optimizing the microscope. Author contributions S.V.: Collected data, performed the electrochemical analysis, and original draft preparation. A.B. and H.B.: Conceptualization, performed the electrochemical analysis, Supervision, Reviewing & Editing. K.P., X.G., A.S.N., and V.N.: synthesized and characterized the MXenes material. A. M. and T. M. synthesized and characterized the graphene-based material. J-K.P.: performed and analyzed the Raman data and M.W. performed the XRD data. S.B., T.G., and F.B.: Supervision, Reviewing & Editing. Competing interests Samaneh Vaez, Ahmad Bagheri, Hossein Beydaghi, Jaya Kumar Panda, Alberto Morenghi and Francesco Bonaccorso are employees of BeDimensional S.p.A., a company producing 2D materials. All the others authors declare no conflict of interest. Additional information Supplementary information The online version contains supplementary material available at Correspondence and requests for materials should be addressed to Francesco Bonaccorso. [email protected] Reprints and permissions information is available at http://www.nature.com/reprints. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Data availability All data supporting the findings of this study are available from the corresponding author upon request. References Qiu, L. et al. 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Ahn, H., Kim, D., Lee, M. & Nam, K. W. Challenges and possibilities for aqueous battery systems. Communications Materials 4, 37 (2023). Additional Declarations Competing interest reported. Samaneh Vaez, Ahmad Bagheri, Hossein Beydaghi, Jaya Kumar Panda, Alberto Morenghi and Francesco Bonaccorso are employees of BeDimensional S.p.A., a company producing 2D materials. All the others authors declare no conflict of interest. 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16:26:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8910897/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8910897/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103852955,"identity":"dd301bf7-32f9-48d8-a9cc-c05c4b415d31","added_by":"auto","created_at":"2026-03-03 17:13:24","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":497148,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCharacterization of Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e.\u003c/em\u003e \u003cem\u003ea) schematic illustration of the synthesized Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. b) XRD patterns of Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and parent Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eGa\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eC. c) SEM image of Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e particles. d) SAED along the [001] axis. e) HAADF STEM image of Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl2 particle. f) and j) are TEM images at low and high magnification.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8910897/v1/27ef3689d48f8ec41d4eb82e.png"},{"id":103852952,"identity":"19c6ddbf-f769-4a62-b3ad-101cc3ba09a7","added_by":"auto","created_at":"2026-03-03 17:13:24","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":302714,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003ea\u003c/em\u003e) schematic of the three-electrode configuration used for the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode b, d)\u003cem\u003e\u003cstrong\u003e \u003c/strong\u003e\u003c/em\u003e\u003cem\u003eCV curves measured for Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG\u003c/em\u003e electrodes in different electrolytes\u003cem\u003e at a potential scan rate of 20 mV/s. c, e) \u003c/em\u003eC\u003csub\u003eg\u003c/sub\u003e\u003cem\u003e of the electrodes as a function of the potential scan rate.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8910897/v1/7e9ead1475cd6ebbab9f6b32.png"},{"id":103852951,"identity":"762d39bd-2713-430e-a053-959303a468ae","added_by":"auto","created_at":"2026-03-03 17:13:24","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":91606,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCV curves of a) Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG–8 m NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, b) Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG\u003c/em\u003e–\u003cem\u003e2 M NaCl and c) Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG\u003c/em\u003e–\u003cem\u003e3 M H\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, measured at 20 mV/s in a three-electrode cell configuration. GCD curves with real-time electrode potentials and cell voltage of ASCs d) Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG–8 m NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, e) Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG\u003c/em\u003e–\u003cem\u003e2 M NaCl and f) Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG\u003c/em\u003e–\u003cem\u003e3 M H\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e .\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8910897/v1/79f166e570ccf445991d1b00.png"},{"id":104401044,"identity":"ad10ac3f-47db-4d0e-a724-2cca565f1f87","added_by":"auto","created_at":"2026-03-11 12:11:44","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":83753,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eElectrochemical characterization of the investigated Mo\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eCCl\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/FLG // CG/FLG ASCs. a) CV curves of the ASC recorded in 8 m NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, 2 M NaCl, and 3 M H\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. b) GCD profiles of the SCs recorded in 8 m NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, 2 M NaCl, and 3 M H\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. c) C\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e and d) CE of the investigated ASCs as a function of the specific current (data extrapolated from the GCD profiles), e) Ragone plots and f) C\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e retention of the investigated ASCs over 12000 GCD cycles at 5 A/g.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8910897/v1/82c7dd8890c653498d7633f7.png"},{"id":104408356,"identity":"6ee2329a-eec7-4071-8d16-1a244d0c2491","added_by":"auto","created_at":"2026-03-11 12:42:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2194299,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8910897/v1/4dbf5dca-b7b9-4108-90bb-baed241f2743.pdf"},{"id":103852954,"identity":"22e91649-d4cd-402c-bab8-b50e80431954","added_by":"auto","created_at":"2026-03-03 17:13:24","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":559354,"visible":true,"origin":"","legend":"","description":"","filename":"SVaezSIfinalrevised.docx","url":"https://assets-eu.researchsquare.com/files/rs-8910897/v1/bfeea96541d96e10d8b01744.docx"}],"financialInterests":"Competing interest reported. Samaneh Vaez, Ahmad Bagheri, Hossein Beydaghi, Jaya Kumar Panda, Alberto Morenghi and Francesco Bonaccorso are employees of BeDimensional S.p.A., a company producing 2D materials. All the others authors declare no conflict of interest.","formattedTitle":"Electrochemical performance of Molybdenum Carbide MXene-few layer graphene hybrid electrodes for aqueous supercapacitors","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe demand for energy is increasing worldwide due to population growth and industrial development.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e At the same time, renewable energy sources are increasingly being used to reduce dependence on fossil fuels.\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Supercapacitors (SCs) are considered promising energy storage devices due to their unique properties, including fast charge-discharge, high specific power, and excellent cyclic stability.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e Supercapacitors are mainly divided into two types: electrochemical double-layer capacitors (EDLCs)\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e and pseudocapacitors (PCs).\u003csup\u003e\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e In EDLCs, energy is stored electrostatically through the adsorption and desorption of ions at the electrode\u0026ndash;electrolyte interface.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e However, EDLCs are limited by low energy density, which restricts their use in large-scale energy storage applications.\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn contrast, PCs utilize rapid and reversible faradaic processes, including redox reactions, ion intercalation, and electrosorption, to achieve significantly higher charge storage capacity compared to EDLCs.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e This faradaic contribution enhances the energy density of PCs while maintaining the fast charge\u0026ndash;discharge kinetics characteristic of SCs.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e As a result, PCs are considered as strong candidates for next-generation energy storage systems, although further advances in electrode materials and device design are still needed to overcome unsolved challenges. The latter include sluggish ion transport within dense electrode architectures,\u003csup\u003e18,19\u003c/sup\u003e structural degradation during repeated redox cycling,\u003csup\u003e20,21\u003c/sup\u003e and mismatched potential windows between electrodes and electrolytes,\u003csup\u003e22,23\u003c/sup\u003e all of which adversely affect the long-term cycling stability of PCs. Several two-dimensional (2D) materials, including, manganese dioxide (MnO\u003csub\u003e2\u003c/sub\u003e), molybdenum disulfide (MoS\u003csub\u003e2\u003c/sub\u003e), tungsten disulfide (WS\u003csub\u003e2\u003c/sub\u003e), and MXenes, have emerged as promising candidates for pseudocapacitive energy storage devices.\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e These materials offer high surface area (\u003cem\u003ei.e\u003c/em\u003e., ~\u0026thinsp;18\u0026ndash;67 m\u003csup\u003e2\u003c/sup\u003e/g for Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXenes), although the reported values vary substantially with processing and architecture.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e This variability primarily reflects differences in the degree of MXene delamination (which controls interlayer separation and restacking)\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e and the formation of composites (\u003cem\u003ee.g\u003c/em\u003e., with graphene or other carbonaceous nanomaterials) that inhibit restacking and create more accessible, ion-reachable surface area.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e Other relevant properties include tunable redox activity\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e, and rapid ion-access pathways\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. However, when used in symmetric PCs, their practical application is often hindered by slow faradaic kinetics (\u003cem\u003ei.e.\u003c/em\u003e, with charge-transfer resistances \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{ct}\\)\u003c/span\u003e\u003c/span\u003eranging from 0.5 to 4 Ω and estimated heterogeneous rate constants \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}^{0}\\)\u003c/span\u003e\u003c/span\u003eon the order of 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e\u0026ndash;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e cm/s)\u003csup\u003e35\u0026ndash;38\u003c/sup\u003e and structural instability\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e, leading to limited rate capability (\u003cem\u003ei.e.\u003c/em\u003e, capacitance retention drops to 30\u0026ndash;60% when current density increases from 1\u0026ndash;2 A/g to 10 A/g)\u003csup\u003e37,40\u003c/sup\u003e and capacity fading over extended cycling (\u003cem\u003ei.e.\u003c/em\u003e, capacity retention decreases to 70\u0026ndash;85% after 3000\u0026ndash;5000 cycles).\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo overcome these issues in symmetric PCs, MXenes have recently emerged as promising electrode materials in asymmetric supercapacitors (ASCs) due to their tunable surface chemistry, high electrical conductivity, and intrinsic pseudocapacitive behavior.\u003csup\u003e\u003cspan additionalcitationids=\"CR43 CR44\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e Although other 2D materials can also be used in ASCs, MXenes are highlighted here due to their electrochemical properties. In ASCs, MXenes are often combined with EDLC-type electrodes, enabling simultaneous faradaic and non-faradaic charge storage, which improves energy density while retaining the excellent cycling stability characteristic of EDLCs.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e In particular, molybdenum carbide MXene (Mo\u003csub\u003e2\u003c/sub\u003eCT\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e) stands out for its metallic-level electrical conductivity, surface redox chemistry, and layered structure.\u003csup\u003e\u003cspan additionalcitationids=\"CR48 CR49\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e These properties enable efficient charge transfer and fast redox kinetics, making Mo\u003csub\u003e2\u003c/sub\u003eCT\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e particularly suitable for SC applications.\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e,\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e Nevertheless, the practical use of Mo\u003csub\u003e2\u003c/sub\u003eC is hindered by structural restacking, which occurs during electrode fabrication or repeated cycling. This restacking reduces the ion-accessible surface area (\u003cem\u003ei.e\u003c/em\u003e., ~ 8.9\u0026ndash;4.9 m\u003csup\u003e2\u003c/sup\u003e/g)\u003csup\u003e53\u003c/sup\u003e and consequently lowers capacitance and rate performance in Mo\u003csub\u003e2\u003c/sub\u003eC MXene-based supercapacitors.\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e,\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e,\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn this scenario, the integrating chloride-terminated Mo\u003csub\u003e2\u003c/sub\u003eC MXene (Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e) with conductive carbon materials enhances electron transport and mitigates restacking of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e layers, thereby increasing the ion-accessible surface area and improving both rate capability and cycling stability..\u003csup\u003e56\u003c/sup\u003e In this work, we propose the design and realization of a hybrid electrode composed of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e produced via molten salt etching method\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e and few-layer graphene (FLG). Although Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e exhibits metallic-level conductivity, the actual conductivity strongly depends on surface terminations,\u003csup\u003e58\u003c/sup\u003e structural defects, flake restacking, and interlayer resistance.\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e,\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e In our case, the incorporation of FLG improves the conductive network by reducing interlayer resistance and preventing restacking, thereby enhancing effective charge transport compared to pristine Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e..\u003csup\u003e55,62\u0026ndash;65\u003c/sup\u003e The synergistic interaction between Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e and FLG enables efficient charge transport, rapid redox kinetics, and enhanced structural stability, leading to improved energy storage performance compared to the pristine counterpart (\u003cem\u003ei.e\u003c/em\u003e., the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode delivers 136.3 F/g at 50 mV/s in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, compared to ~\u0026thinsp;79 F/g reported for Mo\u003csub\u003e2\u003c/sub\u003eC-based MXenes).\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e This hybrid electrode (Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG) design represents a novel approach for the development of next-generation high-performance SCs. Based on this rationale, the electrochemical behavior of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrodes was evaluated in different aqueous electrolytes, beginning with a series of sulfates: 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and 0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, to investigate the effect of cation size on electrode performance. Beyond material selection, electrolyte ion characteristics significantly affect charge storage behavior, ionic transport, and overall SC coulombic efficiency (CE).\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e Aqueous electrolytes, with their inherently high ionic conductivity compared to organic electrolytes and ionic liquids, enable fast charge\u0026ndash;discharge operation, offering advantages for high-power applications.\u003csup\u003e\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e,\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e Notably, smaller hydrated cations such as H\u003csup\u003e+\u003c/sup\u003e exhibit greater mobility and conductivity than their larger alkali-metal counterparts (Li\u003csup\u003e+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e), resulting in enhanced capacitive performance in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte.\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e,\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e To elucidate the role of anionic radius, 2 M NaCl,8 m NaNO\u003csub\u003e3\u003c/sub\u003e and 1M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolytes were used to investigate the influence of anions on charge transport resistance. The electrodes studied are referred to as X\u0026ndash;Y, in which X represents the active material (Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG) and Y denotes the type of electrolyte used (\u003cem\u003ee.g.\u003c/em\u003e, H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, NaNO\u003csub\u003e3\u003c/sub\u003e, etc.). Electrochemical characterization was carried out in a three-electrode Swagelok configuration, enabling evaluation of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG working electrode. A combination of cyclic voltammetry (CV), galvanostatic charge\u0026ndash;discharge (GCD), and electrochemical impedance spectroscopy (EIS) was used to comprehensively investigate the charge-storage mechanism, rate capability, and interfacial resistance of the electrodes in various electrolyte systems. The results demonstrated a strong dependence of electrochemical performance on ionic radius. Among the electrolytes investigated, the half-cell based on 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e delivered the highest effective gravimetric capacitance (C\u003csub\u003eg\u003c/sub\u003e) of 136.3 F/g at 50 mV/s. This superior performance arises from faster interfacial charge transfer in acidic electrolyte, shown by a lower series resistance (Rₛ \u0026asymp; 0.04 Ω in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) versus 0.08 Ω (2 M NaCl) and 0.13 Ω (8 m NaNO\u003csub\u003e3\u003c/sub\u003e). Stronger ion adsorption is enabled by the smaller hydrated proton radius (H⁺: 2.76 \u0026Aring;) compared with K⁺ (3.31 \u0026Aring;), Na⁺ (3.58 \u0026Aring;), and Li⁺ (3.82 \u0026Aring;). Enhanced surface redox activity is evidenced by persistent reversible peaks in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e half-cells across increasing scan rates. The capacitance at 50 mV/s is 136.3 F/g, higher than 42.85 F/g (1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), 38.75 F/g (1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), and 32.12 F/g (0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), ~\u0026thinsp;3.2\u0026ndash;4.2\u0026times; higher.\u003c/p\u003e \u003cp\u003eHowever, to enhance the device-level performance, the pseudocapacitive Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG hybrid electrode was assembled with an EDLC electrode offering fast and reversible ion transport capability, forming an asymmetric device. Concerning the EDLC electrode, the composition of curved graphene (CG) with FLG (CG/FLG) was used to prevent nanosheets restacking and enhance the exposure of electroactive surface sites of the CG/FLG electrode, providing excellent structural and electrochemical compatibility with the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode.\u003csup\u003e\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e The ASCs were assembled and evaluated in three aqueous electrolytes: 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 2 M NaCl, and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e, each selected to balance ionic mobility (3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), environmental safety, and electrochemical stability (2 M NaCl and 8 m NaNO₃). The as-prepared ASCs are denoted as \u003cem\u003enegative electrode // positive electrode \u0026ndash;electrolyte\u003c/em\u003e, \u003cem\u003ei.e\u003c/em\u003e., Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG\u0026ndash;3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e indicates a device with Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG as the negative, CG/FLG as the positive electrode and 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as the electrolyte. Among the assembled ASCs, the device using 8 m NaNO\u003csub\u003e3\u003c/sub\u003e as the electrolyte delivered the highest electrochemical performance, achieving an energy density of approximately 8 Wh/kg and remarkable cyclic stability (94% gravimetric capacitance -C\u003csub\u003eg\u003c/sub\u003e- retention after 12000 GCD cycles). The 2 M NaCl-based ASCs also demonstrated long-term stability (81% C\u003csub\u003eg\u003c/sub\u003e retention after 12000 GCD cycles), reaching an energy density of 7.39 Wh/kg, higher than that of the 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-based ASCs (4 Wh/kg). The 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-based ASCs delivered the highest C\u003csub\u003eg\u003c/sub\u003e but operating at lower cell voltage (1 V), compared with 2 M NaCl- and 8 M NaNO\u003csub\u003e3\u003c/sub\u003e-based ASCs (1.6 V), due to water decomposition under acidic conditions. This study offers significant insights into the electrochemical behavior of the hybrid Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode, revealing correlations between its structural properties and potential application in electrochemical energy storage systems.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea presents the schematic representation of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e synthesis. The morphological and structural characterization of the synthesized Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb-f. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb shows the X-ray diffraction (XRD) patterns of both the synthesized MXene after copper removal by washing and its parent phase, Mo\u003csub\u003e2\u003c/sub\u003eGa\u003csub\u003e2\u003c/sub\u003eC. After etching and washing, the XRD pattern changes significantly. Compared with Mo\u003csub\u003e2\u003c/sub\u003eGa\u003csub\u003e2\u003c/sub\u003eC, the peaks at ~ 34˚, 37˚, and 43˚ 2θ reduce in intensity by approximately 50–70% and slightly shifted by 0.2°–0.4°, while the (002) peak at around 9° strongly reduced in intensity. These structural changes are consistent with previously reported molten-salt-synthesized Mo\u003csub\u003e2\u003c/sub\u003eCT\u003csub\u003ex\u003c/sub\u003e MXenes in literature.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e The Scanning Electron Microscopy (SEM) image reported in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec reveals several multilayer MXene flakes exhibiting visible slit-like pores. The flakes reach sizes of up to a few micrometers in length and display textured surfaces. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed shows a Selected Area Electron Diffraction (SAED) image of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e along the [001] axis, confirming its hexagonal symmetry and high crystallinity. The High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF STEM) image in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee, together with the Transmission Electron Microscopy (TEM) images in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ef and \u003cb\u003eFigure j\u003c/b\u003e, further confirm high-quality multilayer MXene flakes. While molten salt etching of MXene\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e71\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e72\u003c/span\u003e\u003c/sup\u003e differs significantly from conventional aqueous acid etching methods\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e73\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e75\u003c/span\u003e\u003c/sup\u003e, the resulting materials retain the characteristic features of MXenes and exhibit excellent structural quality. The Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode was evaluated in a three-electrode configuration in various sulfate-based electrolytes (3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and 0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) to investigate the effect of cation radius and mobility on charge storage.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea illustrates the schematic of the three-electrode configuration for the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode tested in various aqueous electrolytes. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb presents the CV curves recorded at a scan rate of 20 mV/s for all electrolytes. The results reveal a predominantly capacitive and irreversible redox behaviour (small oxidation peak) for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e2\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. In contrast, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e exhibits a reversible redox process, indicating a distinct electrochemical mechanism under acidic conditions. Among the tested systems, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrode delivers a markedly higher specific current in the CV profiles, indicating a higher C\u003csub\u003eg\u003c/sub\u003e compared to the other assembled systems with different electrolytes.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e \u003cb\u003eFigure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003ea–d\u003c/b\u003e displays the CV curves of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e recorded at various potential scan rates, ranging from 5 to 50 mV/s. These curves exhibit distinct reversible redox peaks, further confirming the pseudocapacitive properties of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e devices. On the contrary, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e ASCs display a predominantly capacitive behavior. Notably, the redox peaks of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e remain visible with increasing the potential scan rates, indicating efficient charge transfer (\u003cem\u003ei.e\u003c/em\u003e., persistence of oxidation and redox peaks in the CV profiles across different scan rates) and electrochemical reversibility.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e76\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure S2a–d\u003c/b\u003e shows the GCD profiles of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrodes, respectively, measured at specific currents ranging from 1 to 20 A/g. All electrodes tested in different electrolytes exhibit non-linear voltage–time profiles at various specific currents. The non-linear GCD profile of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG– 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e displays redox features indicative of substantial pseudocapacitive contributions, whereas the GCD curves obtained in the devices based on the other electrolytes exhibit mainly capacitive behavior. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec shows C\u003csub\u003eg\u003c/sub\u003e of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode in four different electrolytes as a function of the potential scan rate, calculated from the CV profiles (see Supporting Information, Equation (S1)). Among the investigated systems, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e demonstrates the highest C\u003csub\u003eg\u003c/sub\u003e (\u003cem\u003ee. g.\u003c/em\u003e, 136.3 F/g at 50 mV/s), outperforming those measured in 1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and 0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte systems (42.85, 38.75, and 32.12 F/g for 1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4,\u003c/sub\u003e1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and 0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, respectively, at the same potential scan rate). This enhancement in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e arises from the high ionic mobility and small hydration sphere radius of H\u003csup\u003e+\u003c/sup\u003e ions (2.76 Å), which enable a proton hopping between water molecules \u003cem\u003evia\u003c/em\u003e hydrogen bonding. Compared with K\u003csup\u003e+\u003c/sup\u003e (3.31 Å), Na\u003csup\u003e+\u003c/sup\u003e (3.58 Å), and Li\u003csup\u003e+\u003c/sup\u003e (3.82 Å) ions, H\u003csup\u003e+\u003c/sup\u003e exhibits the highest molar ionic conductivity.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e66\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e The enhanced conductivity and ionic mobility facilitate rapid charge transfer, while the smaller hydration radius increases ion adsorption at the electrolyte/electrode interface, further promoting faradaic reactions. \u003csup\u003e69,77\u003c/sup\u003e Consequently, the superior proton dynamics in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e result in the highest C\u003csub\u003eg\u003c/sub\u003e observed (136.3 F/g at 50 mV/s). \u003cb\u003eTable \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e compares these results with Mo\u003csub\u003e2\u003c/sub\u003eC-based electrodes previously reported in literature, highlighting the material strong potential for supercapacitor applications.\u003c/p\u003e \u003cp\u003eThe electrochemical stability of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrodes was assessed over 12000 GCD cycles at a specific current of 5 A/g. As demonstrated in \u003cb\u003eFigure S3a–d\u003c/b\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e samples retained 89%, 90%, 93%, and 90% of their initial C\u003csub\u003eg\u003c/sub\u003e, respectively. Additionally, all electrodes maintained nearly 100% CE throughout cycling (up to 12000 cycles). These results highlight the long-term stability and electrochemical reversibility of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrodes across these electrolytes. Considering both stability and cost factors, \u003cem\u003ei.e.\u003c/em\u003e, Na salts are much more abundant and cost-effective than Li salts, Na-based electrolytes are attractive for their use in large-scale and environmentally friendly energy storage devices.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e78\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e79\u003c/span\u003e\u003c/sup\u003e To evaluate the effect of anion size on the electrochemical properties, we compared Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode in 2 M NaCl, 8 m NaNO\u003csub\u003e3\u003c/sub\u003e, and 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e using a three-electrode cell configuration. The electrolytes have a near-neutral pH (~ 6–7), contain the same cation (Na\u003csup\u003e+\u003c/sup\u003e) but different anions: Cl⁻, NO\u003csub\u003e3\u003c/sub\u003e⁻, and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e⁻ ion. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed displays the non-rectangular shape of CV curves of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–2 M NaCl, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG-1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e systems at 20 mV/s, indicating the pseudocapacitive behavior.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e80\u003c/span\u003e\u003c/sup\u003e \u003cb\u003eFigure S4a–b\u003c/b\u003e presents the CV curves of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG in 2 M NaCl and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e electrolytes measured at scan rates from 5 to 50 mV/s. When assembled and tested in three-electrode cell configuration with both 8 m NaNO\u003csub\u003e3\u003c/sub\u003e and 2 M NaCl electrolytes, the electrodes predominantly exhibit capacitive behavior. Whereas a weak redox reaction is observed in the cell assembled with 2 M NaCl, indicating a minor pseudocapacitive contribution.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e76\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure S4c–d\u003c/b\u003e illustrate the GCD profiles of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–2 M NaCl and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e system recorded at specific current, ranging from 1 to 20 A/g. All electrodes exhibit approximately linear voltage–time characteristics across the tested specific current range. These results are consistent with the primarily capacitive behavior previously shown by the CV analyses.\u003c/p\u003e \u003cp\u003eThe C\u003csub\u003eg\u003c/sub\u003e of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode, measured in 2 M NaCl, 8 m NaNO\u003csub\u003e3\u003c/sub\u003e, and 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolytes at various potential scan rates, was calculated from the CV curves shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee. C\u003csub\u003eg\u003c/sub\u003e increases with decreasing the potential scan rate, reaching 57.5, 51.25, and 42.5 F/g at 5 mV/s for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–2 M NaCl, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e systems, respectively. This trend is attributed to diffusion limitations of electrolyte ions into the interlayer spaces of the electrode.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e81\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e82\u003c/span\u003e\u003c/sup\u003e The SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e⁻ ions possess a larger ionic radius compared to NO\u003csub\u003e3\u003c/sub\u003e⁻ and Cl⁻ ions.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e83\u003c/span\u003e\u003c/sup\u003e Moreover, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e⁻ exhibits lower ionic conductivity and mobility compared to Cl⁻ and NO\u003csub\u003e3\u003c/sub\u003e⁻.\u003csup\u003e84,85\u003c/sup\u003e As a result, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e aqueous electrolyte delivered poor capacitive performance. As shown in \u003cb\u003eFigure S5a-b\u003c/b\u003e, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode demonstrated excellent cycling stability in 2 M NaCl and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e, retaining 80% and 82% of its initial C\u003csub\u003eg\u003c/sub\u003e, respectively, after 12000 GCD cycles at 5A/g. Additionally, the CE remained close to 100%, confirming the superior electrochemical reversibility of the electrode.\u003c/p\u003e \u003cp\u003eThe cyclic stability of representative Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e systems after 12000 GCD cycles was investigated by post-measurement XRD and Raman spectroscopy. \u003cb\u003eFigure S6a\u003c/b\u003e displays the XRD patterns of pristine Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e and the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrodes after long-term cycling stability in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e electrolytes, enabling a comparison of the structural changes resulting from electrochemical cycling. The electrodes used Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e systems after 12000 GCD cycles show main diffraction peaks at 34.5°, 38.0°, 39.6°, 52.3°, 61.9°, 69.8°, 75.0°, and 76.0°, corresponding to the (100), (002), (101), (102), (110), (103), (112), and (201) planes of Mo\u003csub\u003e2\u003c/sub\u003eC (JCPDS 65-8766).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e86\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e89\u003c/span\u003e\u003c/sup\u003e More in detail, the XRD pattern of the as-prepared pristine Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003eshow sharp, well-defined peaks, indicating a highly crystalline and ordered structure. After the cyclic stability test, some structural changes were observed in the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG. These changes are likely due to repeated ion intercalation and deintercalation, which can cause mechanical stress such as expansion and contraction of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG during the cycling process.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e67\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e90\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e92\u003c/span\u003e\u003c/sup\u003e Additionally, new diffraction peaks appear at 43.5°, 44.9°, and 60° after cycling, indicating the formation of new crystalline phases. These phases may occur from side reactions, the generation of byproducts, or partial decomposition of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e93\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e94\u003c/span\u003e\u003c/sup\u003e A sharp diffraction peak observed at 54.9° corresponds to the (004) reflection, which is characteristic of graphite (substrate) and FLG, confirming the presence of well-ordered graphitic structures within the electrode.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e63\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e95\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eTo further investigate structural changes in Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrodes after GCD cycling, ex-situ Raman spectroscopy was performed on the electrode of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e systems (Figure S6b). Raman spectra reveal characteristic peaks at 1358 cm⁻\u003csup\u003e1\u003c/sup\u003e (D band) and 1583 cm⁻\u003csup\u003e1\u003c/sup\u003e (G band), which are typically observed in graphite, FLG and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e86\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e89\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e96\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e97\u003c/span\u003e\u003c/sup\u003e Additionally, a distinct peak at 886cm⁻\u003csup\u003e1\u003c/sup\u003e is attributed to Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e. \u003csup\u003e96–98\u003c/sup\u003e Notably, these characteristic peaks remain present in the samples after the GCD cycling, indicating that Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrodes preserved their structural integrity. The Raman I(D)/I(G) ratios for pristine Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e and the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode are ~ 0.77 and ~ 0.52, respectively, indicating that the incorporation of FLG reduces the overall defect density and improves structural ordering within the composite.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e99\u003c/span\u003e\u003c/sup\u003e Controlled defect sites are known to enhance the electrochemical performance of carbon-based and MXene electrodes by providing additional electroactive sites and improving ion accessibility, whereas excessive defects can hinder electrical conductivity and limit power performance.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e97\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e100\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e101\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAfter cycling in different electrolytes, the I(D)/I(G) ratios for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e increase to ~ 0.57 and ~ 0.71, respectively. The increased defects (higher ID/IG) observed in Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e sample suggests that the stronger acidic environment induces a higher density of defect sites, which can partially modify the Mo–C bonding environment.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e102\u003c/span\u003e\u003c/sup\u003e This findings correlates with the electrochemical results, in which the higher defect density in Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e contributes to greater pseudocapacitive activity and higher capacitance.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e103\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eTo further assess the charge storage performance of the MXnene-based electrodes in practical applications, aqueous ASCs were assembled using 8 m NaNO\u003csub\u003e3\u003c/sub\u003e, 2 M NaCl, and 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as electrolytes. These were selected based on the high C\u003csub\u003eg\u003c/sub\u003e obtained in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and the excellent cycling stability demonstrated in the environmentally friendly 8 m NaNO\u003csub\u003e3\u003c/sub\u003e and 2 M NaCl electrolytes. Based on the electrochemical evaluation of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG in a three-electrode setup, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was selected as the PC electrode, whereas Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG–2 M NaCl exhibited predominantly capacitive behavior (\u003cb\u003eFigure S9a-f\u003c/b\u003e). The CG/FLG functioned as an EDLCs electrode, as shown by its electrochemical characterization in 8 m NaNO\u003csub\u003e3\u003c/sub\u003e (\u003cb\u003eFigure S7a-f\u003c/b\u003e), 2 M NaCl (\u003cb\u003eFigure S8a-f\u003c/b\u003e). Before ASCs assembly, the active material mass loading of the electrodes was optimized to balance the C\u003csub\u003eg\u003c/sub\u003e of the positive and negative electrodes. The working voltage windows (WVW) of the ASCs were determined by combining the stable potential ranges of the separate positive and negative electrodes obtained from separate three-electrode CV measurements carried out at 20 mV/s (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea-c).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed-f shows the electrode potentials vs. Ag/AgCl recorded over time during GCD measurements at a specific current of 1 A/g in the investigated electrolytes. The Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode in 8 m NaNO\u003csub\u003e3\u003c/sub\u003e and 2 M NaCl operates in a potential range of − 0.7 to + 0.2 V, confirming mainly capacitive characteristics, whereas the CG/FLG electrode operated between 0.2 and 0.9 V. Both electrodes remained in the electrochemical stability window during GCD process, thereby effectively avoiding parasitic hydrogen and oxygen evolution reactions (HER and OER). The potential profile of CG/FLG is reproduced over subsequent cycles, confirming the excellent electrochemical stability of this EDLC-type electrodes. Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG provides a minor pseudocapacitive contribution at potentials below 0.0V vs. Ag/AgCl. Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e ASCs stably operate within a WVW of 1.0 V. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea reports the CV curves measured for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, − 2 M NaCl, and − 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e ASCs, measured at scan rate of 20mV/s, with WVW of 1.6 V, 1.6 V, and 1 V, respectively. The CV profiles indicate that the charge storage process is primarily ruled by EDLC rather than pseudocapacitive contributions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb presents the GCD profiles measured for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, − 2 M NaCl, and − 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, at a specific current of 5 A/g. The nearly linear GCD profiles confirm the predominantly capacitive behavior of the ASCs, consistent with the CV analysis. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec shows the C\u003csub\u003eg\u003c/sub\u003e calculated from the GCD curves (Supporting Information, Equation (S2)). The Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e ASCs achieved the highest C\u003csub\u003eg\u003c/sub\u003e of 29.57 F/g at 1 A/g. The Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e ASCs displayed C\u003csub\u003eg\u003c/sub\u003e values of 24.67 F/g and 23.13 F/g at 1 A/g, respectively. The superior performance of the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e compared to Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e /FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, is consistent with the results obtained from the three-electrode cell configuration for the negative electrodes.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e69\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e104\u003c/span\u003e\u003c/sup\u003e For all ASCs, the C\u003csub\u003eg\u003c/sub\u003e decreased with increasing the specific current, although the devices demonstrated remarkable rate capabilities (\u003cem\u003ee.g.\u003c/em\u003e, at 20 A/g, the C\u003csub\u003eg\u003c/sub\u003e retention was 86%, 65%, and 63% for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, respectively). As demonstrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec, CE of the ASCs exceeds 90% at high specific currents for both Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, indicating excellent reversibility of the redox processes. In contrast, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e ASCs exhibit a slight reduction in CE at lower specific currents (\u0026lt; 5 A/g), which is probably attributed to parasitic reactions commonly encountered in ASCs based on acidic electrolyte, which easily promote HER.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe impedance of the electrode–electrolyte interfaces was analyzed through EIS measurements. The complex impedance (Z) of the systems was characterized by analysing the Nyquist plots (− Im[Z] vs. Re[Z]), offering valuable information on charge-transfer and diffusion processes.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e105\u003c/span\u003e\u003c/sup\u003e A Nyquist plot of an SC typically exhibits three distinct regions corresponding to different electrochemical processes: redox reactions appearing in the high-to-mid frequency range, ion diffusion processes dominating the mid-frequency region, and capacitive behavior prevailing at low frequencies.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e106\u003c/span\u003e\u003c/sup\u003e In the high-frequency region, the plot reflects the electrical conductivity of the electrode and the charge-transfer reactions occurring at solid/solid interface.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e107\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e110\u003c/span\u003e\u003c/sup\u003e Thus, the diameter of the semicircle observed in this region is related to the interfacial resistance between the current collector and the electrode.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e111\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e113\u003c/span\u003e\u003c/sup\u003e Instead, the x-axis (Z\u003csub\u003ere\u003c/sub\u003e) intercept at the highest frequency corresponds to the equivalent series resistance (R\u003csub\u003es\u003c/sub\u003e), which arises from the ionic resistance of the electrolyte and the intrinsic electronic resistance of the electrodes, distinct from the ion-diffusion processes observed in the mid-frequency region.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e78\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e105\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e114\u003c/span\u003e\u003c/sup\u003e \u003cb\u003eFigure S10\u003c/b\u003e shows the Nyquist plots for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e /FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The measurements were recorded over a frequency range of 100 kHz to 10 mHz at open-circuit potential with an AC amplitude of 10 mV. From the analysis of the Nyquist plots, calculated R\u003csub\u003es\u003c/sub\u003e were 0.04, 0.13 and 0.08 Ω for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl, respectively. These values indicate significantly lower resistance and higher ionic conductivity in the acidic electrolytes compared to the near-neutral ones.\u003c/p\u003e \u003cp\u003eThe pseudocapacitive energy storage of the Mo\u003csub\u003e2\u003c/sub\u003eC active material is based on redox reactions involving charge transfer. This behavior occurs along with surface-controlled processes that contribute to EDLC.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e115\u003c/span\u003e\u003c/sup\u003e The main type of charge storage can be identified by analyzing CV curves at different scan rates. As previously shown, the CV curves of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e were measured at different voltage scan rates from 5 to 1000 mV/s. These data were evaluated using the power law given in Eq.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:i=a{v}^{b}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003eIn which \u003cem\u003ei\u003c/em\u003e represents the measured current (A), \u003cem\u003ev\u003c/em\u003e is the voltage scan rate (mV/s), and \u003cem\u003ea\u003c/em\u003e and \u003cem\u003eb\u003c/em\u003e are constants that reflect the contributions of capacitive and faradaic processes. Specifically, for diffusion-controlled behavior, the current is proportional to the square root of the scan rate (\u003cem\u003eb\u003c/em\u003e = 0.5).\u003csup\u003e80\u003c/sup\u003e In contrast, for capacitive processes, the current is directly proportional to the scan rate (\u003cem\u003eb\u003c/em\u003e = 1).\u003csup\u003e80\u003c/sup\u003e \u003cb\u003eFigures S11a,12a,13a\u003c/b\u003e display the \u003cem\u003eb\u003c/em\u003e-values for the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl, and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG − 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as a function of their voltage. Notably, \u003cem\u003eb\u003c/em\u003e-values for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, range from 0.72 to 0.93 (\u003cb\u003eFigure S11a\u003c/b\u003e), indicating that the charge storage involves a combination of both capacitive and diffusion-controlled processes. Similarly, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e show b values ranging from 0.5 to 1, indicating a mixed charge storage mechanism involving both capacitive and diffusion-controlled contributions, as expressed by the following equation:\u003csup\u003e6\u003c/sup\u003e\u003c/p\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:i\\left(V\\right)={{k}_{1}v+k}_{2}{v}^{1/2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003ein which \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:v\\)\u003c/span\u003e\u003c/span\u003e represents the voltage scan rate (mV/s), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}_{1}v\\)\u003c/span\u003e\u003c/span\u003e corresponds to the current from surface capacitive contributions, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}_{2}{v}^{1/2}\\)\u003c/span\u003e\u003c/span\u003eaccounts for the current arising from diffusion-controlled faradaic processes. Eq.\u0026nbsp;(\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) can also be written in a different form as:\u003c/p\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\frac{i\\left(V\\right)}{{v}^{1/2}}={{k}_{1}v+k}_{2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e \u003cp\u003eAccordingly, the values of k₁ and k₂ were obtained from the linear fitting of i(V)/v¹ᐟ² vs. v¹ᐟ² (Figures S11b,12b,13b). As shown in \u003cb\u003eFigures S11c-d,12c-d,13c-d\u003c/b\u003e, the diffusion-controlled charge contributions at a voltage scan rate of 5 mV/s were calculated 64% for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e /FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e, 40% for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–2 M NaCl, and 49% for Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. \u003cb\u003eFigures S11c,12c,13c\u003c/b\u003e confirm that capacitive charge storage increases with broadening WVW, while diffusion-controlled faradaic processes are progressively decreased with increasing WVW due to the limited kinetics of faradaic reactions.\u003c/p\u003e \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed displays the Ragone plots (energy density, Eₛ, vs. power density, Pₛ) for the investigated ASCs. The Ragone plots were derived from the GCD analysis, as detailed in the Supporting Information (Equations S3 and S4). Despite its highest C\u003csub\u003eg\u003c/sub\u003e Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e has shown the lowest energy density among the investigated devices, \u003cem\u003ei.e\u003c/em\u003e., 4.07 Wh/kg at 338.54 W/kg. The poor energy density of the ASCs based on H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as the electrolyte is primarily due to the limited WVW.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e The cyclic stability of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG ASCs was assessed over 12000 GCD cycles at a specific current of 5 A/g. As illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ee, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e /FLG // CG/FLG–8 m NaNO\u003csub\u003e3\u003c/sub\u003e and Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e /FLG // CG/FLG–2 M NaCl retained 94% and 81% of their initial C\u003csub\u003eg\u003c/sub\u003e, respectively. In contrast, Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e /FLG // CG/FLG–3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e has shown a significant performance reduction, retaining only 20% of its initial C\u003csub\u003eg\u003c/sub\u003e after 12000 GCD cycles. The CE remained nearly 100% for the 8 m NaNO\u003csub\u003e3\u003c/sub\u003e-based and 2 M NaCl ASCs, while it dropped to 83.74% when using 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003cdiv id=\"Sec4\" class=\"Section3\"\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Methods","content":"\u003ch2\u003eMaterials Synthesis:\u003c/h2\u003e\u003cp\u003eMo\u003csub\u003e2\u003c/sub\u003eGa\u003csub\u003e2\u003c/sub\u003eC was supplied by Carbon Ukraine. Sodium chloride (NaCl, 99.5%), potassium chloride (KCl, 99.5%), and hydrochloric acid (HCl, 37%) were obtained from Sigma Aldrich. Copper (II) chloride anhydrous (CuCl\u003csub\u003e2\u003c/sub\u003e, 98%), was purchased from Alfa Aesar. To perform the molten salt etching of gallium from Mo\u003csub\u003e2\u003c/sub\u003eGa\u003csub\u003e2\u003c/sub\u003eC, a procedure similar to the one in Ref \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e116\u003c/span\u003e\u003c/sup\u003e was followed. The ratio was adjusted to account for the extra stoichiometry of gallium. Powders of Mo\u003csub\u003e2\u003c/sub\u003eGa\u003csub\u003e2\u003c/sub\u003eC, NaCl, KCl, and CuCl\u003csub\u003e2\u003c/sub\u003e were combined in a 1:6:6:6 ratio and ground with a mortar and pestle for 10 min to ensure uniform particle size and to mix the MAX phase and salts together. The mixed powders were then added to an alumina crucible, covered with a lid, and then the crucible was placed in a glove box transfer chamber and evacuated three times, refilling with Argon, to displace extra oxygen trapped within the powders. The crucible was then promptly placed in a tube furnace with argon flowing at 450 sccm. The tube was allowed to purge for 1 h, and then heated at a rate of 5 ˚C/min up to 700 ˚C. It was held at 700 ˚C for 4 h and then allowed to cool to room temperature, all under flowing argon. After cooling, deionized (DI) water was used to dissolve leftover chloride salts and transfer the materials to a 50 mL centrifuge tube. Extra DI water was used to fill the tube as needed. This was centrifuged for 10 min at 5000 rpm, after which the supernatant was discarded. Three additional cycles of adding 50 mL DI water, resuspending the solid material with vortex mixing, centrifuging, and discarding the supernatant were performed to ensure all leftover chloride salts were dissolved and removed from the product.\u003c/p\u003e\u003cp\u003eTo remove copper deposited on the multilayer MXene particles during the molten salt synthesis, the wet powders were added to a solution of 1M CuCl\u003csub\u003e2\u003c/sub\u003e in 2M HCl, with 4x the mass of the CuCl\u003csub\u003e2\u003c/sub\u003e used in the molten salt mixture. This mixture was stirred for 4 h, after which the mixture was decanted into 50 mL centrifuge tubes and washed with excess DI water by centrifuging for 5 min at 5000 rpm, discarding the supernatant, and refilling with DI water and resuspending using vortex mixing until the pH of the supernatant reached ~ 6. The resulting powders were dried using vacuum filtration followed by vacuum annealing at 200 ˚C for 2 h to remove bulk water. Samples were stored in a vacuum desiccator until needed.\u003c/p\u003e\n\u003ch3\u003eCharacterization\u003c/h3\u003e\n\u003cp\u003eXRD patterns were produced using a Bruker D8 ADVANCE powder diffractometer using Cu K\u003csub\u003eα1\u003c/sub\u003e (λ\u0026thinsp;=\u0026thinsp;0.154 nm) radiation. Patterns were collected between 3˚and 80 2θ with a step size of 0.02˚ and a dwell time of 1.5 s. SEM was performed on a Zeiss Ultra Plus field-emission SEM electron microscope with an accelerating voltage of 3 keV. Both TEM and STEM images were produced using an uncorrected FEI Titan with Schottky field emission S-FEG source operated at 300 kV\u003c/p\u003e\n\u003ch3\u003eElectrode Preparation:\u003c/h3\u003e\n\u003cp\u003eThe hybrid Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG electrode was prepared by mixing Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e powder and FLG, exfoliated from graphite by BeDimensional S.p.A. using its proprietary wet-jet milling method,\u003csup\u003e95,117\u0026ndash;120\u003c/sup\u003e with a carboxymethyl cellulose:Styrene-Butadiene Rubber (CMC:SBR) binder (weight ratio 90:5:5). A homogenized slurry was obtained by dispersing Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e powder in deionized water by ultrasonic bath, and then FLG, CMC:SBR were added in to the composition and using a planetary centrifugal mixer.\u003csup\u003e\u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e121\u003c/span\u003e,\u003cspan citationid=\"CR122\" class=\"CitationRef\"\u003e122\u003c/span\u003e\u003c/sup\u003e The as prepared slurry was deposited onto a graphite sheet by doctor blade method, which allowed for controlled thickness and even distribution.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e The electrodes were dried at 60\u0026deg;C in an oven to completely remove residual moisture and solvents. After drying, the electrodes were precisely punched into discs with a diameter of 8 m. The prepared electrodes exhibited an average active material mass loading of approximately 3 mg/cm\u003csup\u003e2\u003c/sup\u003e.\u003csup\u003e123\u003c/sup\u003e\u003c/p\u003e\n\u003ch3\u003eThree-Electrode Cell Configuration:\u003c/h3\u003e\n\u003cp\u003eElectrochemical characterization was carried out in three-electrode Swagelok cells based on 316L stainless steel or titanium pistons, an insulating PTFE-coated 316L stainless steel body, and PTFE sealing rings. Each cell was assembled with a Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG working electrode, an Ag/AgCl reference electrode, high-mass loading activated carbon as counter electrode, and a glass fiber (Whatman GF/A) as the separator. The cell components were tightened in the Swagelok cell to maintain good contact and prevent leakage. All measurements were performed at room temperature (~\u0026thinsp;25\u0026deg;C). Various aqueous electrolytes were used to investigate the effect of ionic radius of different cations and anions on SC performance. These included 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4,\u003c/sub\u003e1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (sulfate-based electrolytes for cation comparison), as well as 2 M NaCl and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e (for anion comparison).\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eASC Assembly:\u003c/h2\u003e \u003cp\u003eTwo-electrode ASCs were assembled using Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG as the negative electrode and CG/FLG as the positive electrode. The CG/FLG positive electrode was prepared by mixing CG, FLG, and CMC:SBR in water by a planetary centrifugal mixer (weight ratio 90:5:5), and casting the resulting slurry onto a graphite sheet. Before assembling the ASCs, the electrochemical properties of both Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG and CG/FLG electrodes were individually evaluated through three-electrode cell measurements. Charge balance between the two electrodes was achieved by adjusting their mass loadings according to their respective charge storage capacities within the selected potential window.\u003csup\u003e\u003cspan citationid=\"CR107\" class=\"CitationRef\"\u003e107\u003c/span\u003e,\u003cspan additionalcitationids=\"CR125\" citationid=\"CR124\" class=\"CitationRef\"\u003e124\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR126\" class=\"CitationRef\"\u003e126\u003c/span\u003e\u003c/sup\u003eThe optimal mass ratio between the positive and negative electrodes (m₊/m₋) is determined using the following Eq.\u0026nbsp;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\frac{{m}_{+}}{{m}_{\\_}}=\\:\\frac{{C}_{\\_\\:}\\times\\:\\varDelta\\:V\\_}{{C}_{+}\\times\\:\\varDelta\\:{V}_{+}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG ASCs were tested in three different aqueous electrolytes: 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 2 M NaCl, and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e. For the 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte, the cell voltage was limited to 1.0 V to avoid parasitic water splitting reaction, in particular the acid-promoted HER.\u003csup\u003e\u003cspan citationid=\"CR127\" class=\"CitationRef\"\u003e127\u003c/span\u003e,\u003cspan citationid=\"CR128\" class=\"CitationRef\"\u003e128\u003c/span\u003e\u003c/sup\u003e In near-neutral 2 M NaCl and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e electrolytes, an upper cell voltage of 1.6 V was reached, taking advantage of the larger overpotential for gas evolution in near-neutral electrolyte.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e The assembled ASCs were evaluated through CV and GCD analyses. Long-term cyclic stability was assessed by repeating GCD cycles up to 12000 times at a fixed specific current (5 A/g).\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study highlights the successful integration of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG as a high-performance negative electrode and CG/FLG as a complementary positive electrode in aqueous ASCs. The Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG hybrid leverages the synergistic combination of pseudocapacitive Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e and highly conductive FLG, leading to enhanced charge-transfer kinetics, superior reversibility, and excellent electrochemical performance.\u003c/p\u003e \u003cp\u003eBy systematically investigating different aqueous electrolytes, 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 2 M NaCl, and 8 m NaNO\u003csub\u003e3\u003c/sub\u003e, the study elucidates the role of ionic species in modulating charge storage behavior and interfacial resistance. The ASCs with 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e delivered the highest specific capacitance (29.57 F/g at 1 A/g), attributed to the fast proton mobility and efficient surface redox reactions of Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e. However, in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e the device voltage had to be limited to 1.0 V due to acid-promoted water splitting/HER, whereas near-neutral Na-based electrolytes (2 M NaCl, 8 m NaNO\u003csub\u003e3\u003c/sub\u003e) enabled a broader 1.6 V window. Moreover, Na-based ASCs have shown superior cycling stability, retaining 94% (8 m NaNO₃) and 81% (2 M NaCl) of the initial capacitance after 12000 cycles, compared with only 20% retention in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-based ASCs.\u003c/p\u003e \u003cp\u003eRemarkably, the 8 m NaNO\u003csub\u003e3\u003c/sub\u003e ASC achieved an energy density of ~\u0026thinsp;8 Wh/kg with 94% capacitance retention after 12000 GCD cycles, demonstrating outstanding long-term durability. Similarly, the 2 M NaCl ASCs exhibited excellent energy retention and cycling stability. The EIS further revealed a strong electrolyte dependence of the internal resistance, with the lowest R\u003csub\u003es\u003c/sub\u003e (~\u0026thinsp;0.04 Ω) observed in 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-based ASC, while 8 m NaNO\u003csub\u003e3\u003c/sub\u003e-one displayed a higher Rₛ due to reduced ion mobility.\u003c/p\u003e \u003cp\u003eOverall, the Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG // CG/FLG architecture, when paired with optimized electrolytes, is demonstrating promising for high-performance, durable, and scalable aqueous supercapacitors. These findings establish valuable design guidelines to tailor electrode\u0026ndash;electrolyte interfaces and pave the way toward next-generation electrochemical energy storage systems that balance high energy density, power capability, and long-term stability.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project received funding from the European Union’s 2D Printable Horizon Europe research and innovation program under Grant Agreement No. 101135196 and the European Union’s GREENCAP Horizon Europe research and innovation program under Grant Agreement No. 101091572. S.V. acknowledges financial support from the European Union Next Generation EU program (D.M. 117 del 02/03/2023 Ministero dell′ Università e della Ricerca). T.G. thanks the European Research Council for the project JANUS BI (grant agreement no. [101041229]) and the European Innovation Council for the project LEAF (grant agreement n. [101186701]). V.N. and X.G. wish to thank the support of the Research Ireland-funded AMBER Research Centre and the SFI Frontiers for the Future award (Grant Nos. 12/RC/2278_P2 and 20/FFP-A/8950 respectively). Furthermore, V.N. and X.G. wish to thank the Advanced Microscopy Laboratory (AML) in CRANN for the provision of their facilities and thank Clive Downing for optimizing the microscope.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eS.V.: Collected data, performed the electrochemical analysis, and original draft preparation. A.B. and H.B.: Conceptualization, performed the electrochemical analysis, Supervision, Reviewing \u0026amp; Editing. K.P., X.G., A.S.N., and V.N.: synthesized and characterized the MXenes material.\u0026nbsp;A. M. and T. M. synthesized and characterized the graphene-based material. J-K.P.: performed and analyzed the Raman data and M.W. performed the XRD data. S.B., T.G., and F.B.: Supervision, Reviewing \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSamaneh Vaez, Ahmad Bagheri, Hossein Beydaghi, Jaya Kumar Panda, Alberto Morenghi and Francesco Bonaccorso are employees of BeDimensional S.p.A., a company producing 2D materials. All the others authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e The online version contains supplementary material available at\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence\u003c/strong\u003e and requests for materials should be addressed to \u0026nbsp;Francesco Bonaccorso. [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReprints and permissions information\u003c/strong\u003e is available at http://www.nature.com/reprints.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePublisher’s note\u003c/strong\u003e Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eOpen Access\u003c/strong\u003e This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. 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W. Challenges and possibilities for aqueous battery systems. \u003cem\u003eCommunications Materials\u003c/em\u003e 4, 37 (2023).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-2d-materials-and-applications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npj2dmaterials","sideBox":"Learn more about [npj 2D Materials and Applications](http://www.nature.com/npj2dmaterials/)","snPcode":"41699","submissionUrl":"https://submission.springernature.com/new-submission/41699/3","title":"npj 2D Materials and Applications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"MXenes, Molten Salt Etching, Supercapacitor, Few-layer Graphene, Curved Graphene","lastPublishedDoi":"10.21203/rs.3.rs-8910897/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8910897/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis work investigates the influence of ionic radius on the charge storage behavior of molybdenum carbide MXene electrodes (Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e) produced through the molten salt etching method for application in aqueous-based supercapacitors (SCs). Various electrolytes, \u003cem\u003ei.e\u003c/em\u003e., 3 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and 0.6 M K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e were investigated, revealing that the small cation (H\u003csup\u003e+\u003c/sup\u003e) enhances the SCs capacitance through fast redox kinetics and high ionic mobility. To elucidate the role of anions, neutral electrolytes (8 m NaNO\u003csub\u003e3\u003c/sub\u003e, 2 M NaCl, and 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) were also explored, enabling a wide voltage window and stable operation of SCs. An asymmetric supercapacitor was assembled using Mo\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e/FLG (few-layer graphene) as the pseudocapacitive electrode and FLG/CG/ (curved graphene) as the EDLC counterpart. In this configuration, FLG prevents MXene restacking while CG provides abundant electroactive sites, resulting in enhanced energy density and cycling durability. These results highlight the combined effect of electrolyte ion selection and hybrid electrode engineering toward high-performance, durable aqueous energy-storage devices.\u003c/p\u003e","manuscriptTitle":"Electrochemical performance of Molybdenum Carbide MXene-few layer graphene hybrid electrodes for aqueous supercapacitors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-03 17:13:16","doi":"10.21203/rs.3.rs-8910897/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-02T12:49:19+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-01T14:29:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"272894820390814375123733970136927929533","date":"2026-03-16T09:18:43+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-10T16:04:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"322275723194498985327519430986378924647","date":"2026-02-26T12:59:53+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"206581760019001562972548468627302691055","date":"2026-02-26T11:33:15+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-26T10:13:30+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-25T18:47:09+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-22T16:38:14+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj 2D Materials and Applications","date":"2026-02-18T15:40:45+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-2d-materials-and-applications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npj2dmaterials","sideBox":"Learn more about [npj 2D Materials and Applications](http://www.nature.com/npj2dmaterials/)","snPcode":"41699","submissionUrl":"https://submission.springernature.com/new-submission/41699/3","title":"npj 2D Materials and Applications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"977aa4ac-41cd-4656-97f6-a425ca9d929e","owner":[],"postedDate":"March 3rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":63591852,"name":"Physical sciences/Chemistry"},{"id":63591853,"name":"Physical sciences/Energy science and technology"},{"id":63591854,"name":"Physical sciences/Materials science"},{"id":63591855,"name":"Physical sciences/Nanoscience and technology"}],"tags":[],"updatedAt":"2026-05-12T13:11:21+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-03 17:13:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8910897","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8910897","identity":"rs-8910897","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}