Highly Effective Bifunctional Electrocatalysts: Synthesizing NiCo 2 O 4 Nanostructures via Chemical Precipitation for Enhanced Oxygen Evolution and Reduction Reaction

Highly Effective Bifunctional Electrocatalysts: Synthesizing NiCo 2 O 4 Nanostructures via Chemical Precipitation for Enhanced Oxygen Evolution and Reduction Reaction | 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 Short Report Highly Effective Bifunctional Electrocatalysts: Synthesizing NiCo 2 O 4 Nanostructures via Chemical Precipitation for Enhanced Oxygen Evolution and Reduction Reaction Ananta Sasmal, Dipankar Gogoi, T D Das This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4092883/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract In this investigation, we successfully produced NiCo 2 O 4 nanostructures using a simple chemical precipitation method, wherein we adjusted molarity concentration of sodium bicarbonate (NaHCO 3 ) and precursor ratios of Ni and Co. Analysis of surface features revealed a diverse range of shapes, including particles, flowers, rods, and flakes. Notably, the NiCo 2 O 4 nanorods (NCO3) demonstrated a significant threefold increase in BET surface area compared to NCO5. The alterations observed in the physical and chemical characteristics significantly influenced the electrocatalytic efficacy in alkaline environments for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In the context of the oxygen reduction reaction, NCO5 displayed a commencement potential of 0.72 V compared to the reversible hydrogen electrode (RHE), surpassing NCO4 by 110 mV, albeit falling short by 90 mV when compared to Pt/C, the standard benchmark material with a potential of 0.82 V. In terms of OER, NCO3 displayed a potential difference of 152 mV@10mA/cm 2 compared to other NiCo 2 O 4 materials and Pt/C. The increased level of activity observed can be attributed not only to the increased surface area but also to enhancements in electrical properties. This is supported by the lower charge transfer resistance measured in NCO3 (215.2 Ω.cm 2 ) compared to NCO5 (350.2 Ω.cm 2 ) as revealed by electrochemical impedance spectroscopy (EIS). Inverse spinel bi-functional oxygen evolution and oxygen reduction Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Lately, there has been a noticeable escalation in the imperative concerning energy consumption and environmental degradation. [ 1 – 2 ]. To mitigate our reliance on fossil fuels, innovative approaches to green energy storage and conversion are imperative [ 3 ]. However, a significant obstacle emerges from the slow reaction rates experienced during the process of oxygen reduction at the cathode and oxygen evolution at the anode during the charging of batteries. [ 4 – 6 ]. Fuel cells and air batteries present promising solutions to this challenge [ 7 ]. Historically, noble metals such as platinum have been esteemed for their efficacy in catalyzing the oxygen reduction reaction (ORR), while iridium oxide and ruthenium oxide have been favored for the oxygen evolution reaction (OER). However, their substantial cost, restricted availability, and insufficient durability in alkaline conditions have hindered their widespread commercial application [ 8 – 12 ]. In contemporary research endeavors, considerable focus is directed towards minimizing platinum (Pt) loading in catalysts [ 13 ]. One prominent approach involves the development of alloys utilizing metals such as Cobalt (Co), Nickel (Ni), and Iron (Fe), alongside the synthesis of core-shell structures [ 14 – 16 ]. These strategies aim to employ the non-noble metal as the active catalyst core, thereby reducing the necessity for Pt content [ 17 – 20 ]. However, the continued dependence on Pt as the principal electroactive substance presents an enduring obstacle, predominantly attributed to its exorbitant expense and constrained accessibility. Consequently, there is a pressing need for alternative methodologies to engineer highly efficient materials [ 21 – 24 ]. Transition metal oxides (TMOs) emerge as a promising substitute in this context. TMOs, exhibiting diverse oxidation states, demonstrate heightened activity in oxygen reactions attributed to enhanced electron transport capabilities [ 25 ]. Moreover, their inherent ability to seamlessly integrate within a single material matrix is a crucial attribute for electrocatalysts [ 26 – 28 ]. The continuous oxidation and reduction processes within electrochemical cells necessitate materials capable of swift interconversion between oxidation states, a requirement fulfilled by TMOs [ 29 – 30 ]. Additionally, TMOs offer versatility, allowing for their utilization either in combination with other TMOs to create mixed transition metal oxides or as standalone electrocatalysts (MTMOs). Notably, compounds like spinels and perovskites exhibit well-ordered crystalline structures. Spinels, characterized by the general chemical formula AB 2 X 4 , offer particularly enticing prospects due to their potential for amalgamating the catalytic properties of various oxides within a single crystal lattice [ 31 ]. Co 3 O 4 , a widely studied spinel electrocatalyst renowned for its efficacy and durability in oxygenevolution reaction (OER) and oxygen reduction reaction (ORR) in basic mediums, has undergone enhancement with the incorporation of Ni atoms to form NiCo 2 O 4 [ 32 – 34 ]. This modification aims to bolster electrical conductivity and augment the electroactive sites, thereby improving the overall electrochemical performance [ 35 ]. In contrast, nickel oxide, a transition metal oxide (TMO), though less explored, exhibits commendable conductivity, barrier properties, and superior stability compared to precious metals [ 36 ]. While demonstrating promising OER capability, it falls short in ORR performance, rendering it unsuitable for standalone ORR applications. However, when combined with other oxides, it holds potential as a bifunctional material. The incorporation of nickel oxides into the spinel structure gives rise to the NiCo2O4 spinel, presenting a notable potential as an electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in various applications [ 37 ]. Notably, the electrical conductivity of Co, Ni, and their compositions follows the sequence: Co 3 O 4 < NiO < NiCo 2 O 4 [ 38 ]. Despite its inherent advantages such as high impact strength, corrosion resistance, and abundance, NiCo 2 O 4 faces limitations due to its poor conductivity and large structure [ 39 – 40 ]. Various strategies including compositional modification, valence control, defect generation, and morphology optimization have been explored to enhance its electrochemical performance. Diverse morphologies such as nanoparticles, nanowires, nanoneedles, and urchin-like structures have been synthesized via hydrothermal methods, albeit with scalability challenges for commercial applications [ 41 ]. In this investigation, we introduce a facile chemical precipitation approach to fabricate NiCo 2 O 4 nanostructures by varying molarity of concentration of NaHCO 3 and precursor ratios of Ni and Co chloride source. The effectiveness of electrocatalysis for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was evaluated using experiments performed utilizing a rotating disk electrode. Our contribution lies in the facile synthesis of this ternary compound as a bifunctional electrocatalyst, crucial for "air electrode" applications in various energy systems, and exploring the influence of Ni incorporation on cobalt oxides in forming NiCo 2 O 4 spinel, elucidating structural and electrocatalytic modifications. This study utilizes synthesized NiCo2O4 nanostructures to investigate their electrocatalytic activity for the oxygen reduction and oxygen evolution processes, which were notably influenced by changes in morphology and electrical properties in a systematics approach. 2. Experimental Section 2.1. Chemicals required Nickel chloride hexahydrate (NiCl 2 .6H 2 O), cobalt chloride hexahydrate (CoCl 2 .6H 2 O) and sodium bicarbonate (NaHCO 3 ) were of analytical grade and employed without additional purification. 2.2. Synthesis of NiCo 2 O 4 nanomaterials The chemicals were employed immediately upon acquisition without the need for further refinement or purification processes. For a conventional preparation procedure, solutions of 0.05 M NiCl 2 ·6H2O, 0.1 M CoCl 2 ·6H 2 O, and 1.5 M NaHCO 3 were prepared in advance. Subsequently, 80 mL of the NiCl 2 ·6H 2 O solution was mixed with the NaHCO 3 solution under continuous stirring, leading to a 20-minute reaction to form a homogenous suspension. Subsequently, 80 milliliters of the CoCl 2 ·6H 2 O solution were added to the previously mentioned suspension and agitated for a duration of 8 hours. The resulting precipitate was collected via centrifugation, subjected to multiple washes with acetone and water to eliminate residual impurities, and finally dried by maintaining at 70°C in hot-oven. To synthesize the nanostructured NiCo 2 O 4 material, the carbonate precursor underwent annealing in an air environment at 300°C for a duration of 3 hours, employing a gradual heating rate of 5°C/min. Furthermore, the identical procedure was employed to generate samples with varied molarities of NaHCO 3 precursors, specifically 0.5 M, 1 M, and 2 M, while maintaining consistency in other parameters. The samples synthesized using NaHCO 3 at concentrations of 0.5 M, 1 M, and 2 M were denoted as NCO1, NCO2, NCO3, and NCO4 respectively. Furthermore, two additional materials were created by adjusting the ratio of Ni and Co precursors while maintaining consistent synthesis conditions. These materials, designated as NCO5 and NCO6, were prepared using precursor molar ratios of Ni to Co of 1:0.5 and 1:1 respectively, in accordance with the established protocol. 2.3. Materials Characterizations The investigation of morphology and surface structure was carried out utilizing scanning electron microscopy (SEM) (LEO FESEM 1530) and transmission electron microscopy (TEM) (Philips CM300). Surface structure and morphology were further analyzed using Fast Fourier Transformation (FFT) and high-resolution TEM (HR-TEM). The crystalline nature of the spinel nanostructure within NiCo 2 O 4 was explored employing HR-TEM, FFT, and selected area electron diffraction (SAED) patterns. The surface area and porosity characteristics were determined using a Micromeritics ASAP 2020 surface area and porosity analyzer, with the nitrogen adsorption-desorption isotherm analyzed through the Brunauer-Emmett-Teller (BET) method, and pore size distributions assessed via the Barrett-Joyner-Halenda (BJH) model. The spinel crystal structure of the nanoplatelets in NiCo 2 O 4 was confirmed through X-ray diffraction (XRD) (BrukerAXS D8 Advance). Additionally, X-ray photoelectron spectroscopy (XPS) was conducted to ascertain the composition available within the NiCo 2 O 4 nanostructure. 2.4. Electrochemical characterization and electrocatalytic evaluation Cyclic voltammetry (CV) for OER and rotating disc electrode (RDE) voltammogram for ORR were used to examine the electrochemical performances of the nanomaterials used for the current study in 1 M aqueous KOH electrolyte. A potentiostat and a rotation speed controller make up the setup for RDE. All electrochemical tests were performed at room temperature, with Ag/AgCl acting as a reference electrode. The counter electrode was made of platinum wire. The working electrode was a glassy carbon electrode (5 mm OD) coated with 20 mL of 4 mg mL − 1 suspension produced by combining NiCo 2 O 4 and a 0.5 wt% Nafion solution in mixed solution (375 µL H2O + 125 µL IPA). All additional comparative materials were prepared in the same way as the working electrode. ORR curves were recorded from − 0.7 to 1V using an O 2 -saturated electrolyte at a scan rate of 5 mV/s at varied electrode rotation speeds (1000, 1200,1400,1600,1800,2000 rpm). In order to spin off the oxygen evolved during voltammetry testing, the OER curves were recorded from 0 to 0.8 V at a scan rate of 5 mV/s with a N 2 -saturated electrolyte rotating at 1000 rpm. Linear sweep voltammetry was taken from 0.3 to 0.93 V vs RHE (Reversible Hydrogen Electrode) at a sweep rate of 5 mV/s for ORR in a O 2 -rich solution. The experiments of OER were taken by taking potential range from 1 to 1.9 V vs RHE at a sweep rate of 5 mV/s in a N 2 -rich solution. The RDE measurement for ORR was carried out by taking different rotation (1000–2000 rpm) from potential range 0 to 0.93 V vs RHE. Moreover, electrochemical impedance spectroscopy (EIS) was taken to check the electrical properties of the NiCo 2 O 4 materials, particularly the resistance of charge-discharge (Rct). The measurements were performed at 0.64 V vs. RHE with the frequency ranging from 0.01 HZ to 100 kHZ. 3. Results and Discussions 3.1. Structural analysis The X-ray diffraction (XRD) analysis was conducted to assess the crystallinity, structural characteristics, and phase purity of the synthesized nanostructures. Figure 1 illustrates the powder XRD patterns obtained from samples prepared using various concentrations of NaHCO 3 and different precursor ratios of Ni and Co. The synthesis involved chemical co-precipitation followed by calcination at 300°C for 3 hours, employing NaHCO 3 concentrations of 0.5 M, 1 M, and 2 M, and precursor molar ratios of Ni to Co at 1:0.5 and 1:1, respectively. Distinct samples were synthesized with different molarities, including 1.5 M (NCO1), 0.5 M (NCO2), 1 M (NCO3), and 2 M (NCO4), as well as precursor molar ratios of Ni to Co at 1:0.5 (NCO5) and 1:1 (NCO6), respectively. The diffraction peaks observed at 18.6°, 30.9°, 35.5°, 38.3°, 44.4°, 54.9°, 58.8°, and 75.0° correspond to the crystallographic planes of (111), (220), (311), (222), (400), (422), (511), and (440), respectively. These peaks are consistent with the cubic spinel NiCo 2 O 4 phase, as confirmed by comparison with the JCPDS file (JCPDS card No.00-20-0781) [ 42 ]. The XRD patterns (Fig. 1 A) exhibit prominent peaks indicative of well-crystalline samples, while low-intensity peaks remain unindexed. Notably, no characteristic peaks associated with impurities, such as NiO or CoO, were detected, underscoring the high purity of the synthesized NiCo 2 O 4 samples [ 43 ]. Furthermore, variation in the stoichiometric ratio of precursor concentrations during the synthesis of NCO1, from 1:0.5 (NCO5) to 1:1 (NCO6), revealed consistent formation of the cubic spinel NiCo 2 O 4 phase across all XRD peaks. Importantly, no significant alterations in peak position or additional peaks were observed in the XRD patterns of NiCo 2 O 4 samples synthesized under different parameters, including molarity and precursor molar ratios. The FTIR analysis of NiCo 2 O 4 revealed significant peaks near 2104 cm − 1 , suggesting potential bending of water molecules. Additionally, the presence of the H–O–H bending vibration mode was indicated by peaks observed at 1602 cm − 1 and 1078 cm − 1 . Weak signals at 640 cm − 1 and 487 cm − 1 , consistent with all peaks detected in the NiCo 2 O 4 nanostructured materials illustrated in Fig. 2 , were attributed to metal oxide vibrations [ 44 ]. 3.2. Morphological analysis By varying the concentration of sodium bicarbonate and adjusting the precursor molar ratio, the surface characteristics of the produced NiCo 2 O 4 nanostructures (as depicted in Fig. 3 ) were investigated. Figure 3 (NCO1) presents a scanning electron microscope (FESEM) image of nickel cobaltite nanoparticles synthesized using water as the solvent and a concentration of 1.5 M NaHCO 3 .Additionally, nanoflowers were produced when the molarity concentration of NaHCO 3 was switched from 1.5 M to 0.5 M (NCO2) while keeping the remaining synthesis parameters constant (Fig. 3 (NCO2)). In addition, compared to using little high molarity concentration, the morphology of the sample is significantly changed when using 1 M concentration of sodium bicarbonate (NCO3). Figure 3 (NCO3) shows how nanorods are shaped which have also good electrochemical performance for making efficient electrocatalyst. Using the highest molar concentration of 2 M (NCO4) and keeping the other synthesis variables constant, the shape of nanoparticles similar to those shown in Fig. 3 (NCO4) was found. It is worth emphasised that the solvent is essential for the creation of different NiCo 2 O 4 nanostructures with morphologies resembling nanoparticles, nanoflowers, and nanorods [ 45 ]. Further, we achieved 1:0.5 and 1:1 NiCl 2 .6H 2 O and CoCl 2 .6H 2 O precursor ratios throughout the synthesis of NCO1, while maintaining the other reaction conditions constant. This allowed us to better understand the impact of precursor molar ratio. NiCl 2 .6H 2 O and CoCl 2 .6H 2 O were mixed in a 1:0.5 molar ratio to produce nanosheets (Fig. 3 , NCO5). CoCl 2 .6H 2 O and NiCl 2 .6H 2 O were combined in a 1:1 molar ratio to create encapsulated nanoparticles with the same shape (Fig. 3 , NCO6). The comprehensive microstructural analysis of NCO3 was completed utilising TEM in addition to support FESEM. Using a 0.05 M of NiCl 2 .6H 2 O and 0.1 M of CoCl 2 .6H 2 O and 1 M of NaHCO 3 in 80 mL of aqueous solution and calcinated at 300°C for 3 hours, NCO3 was synthesised. Figure 4 illustrates the TEM image, selected area electron diffraction (SAED) pattern, and high-resolution transmission electron microscopy (HRTEM) images of NCO3.The TEM picture (Fig. 4 a) displays a morphology resembling nanorods made of nanoplates. Moreover, the location of the selected area electron diffraction (SAED) pattern of NCO3 (Fig. 4 b) indicates the poly-crystalline structure of the samples. Furthermore, the edge of a nanosheet was used to catch the high-resolution TEM (HRTEM) (Fig. 4 c). The (311) crystal plane of the synthesized nanocomposite, identified as the most prominent peak in XRD, corresponds to a fringe spacing of 0.22 nm, as observed in Fig. 1 a (NCO3) [ 46 ]. 3.3. Compositional analysis X-ray photoelectron spectroscopy (XPS) is employed to gain insights into the elemental composition and chemical states of the prepared NiCo 2 O 4 sample.. Figure 5 a shows the Ni, Co, and O components present in the survey region from the XPS survey pattern of NiCo 2 O 4 nanoflowers (1200-0 eV). Using high resolution Ni, Co, and O spectra, the sample's spinel structure's oxidation condition was determined. In Fig. 5 b, the high-resolution Ni 2p spectrum is depicted, delineated into the Ni 2p 3/2 peak at 850.2 eV and the Ni 2p 1/2 peak at 856.6 eV, forming a spin-orbit doublet. The Ni 3+ 2p 3/2 and Ni 3+ 2p 1/2 states exhibit dual peaks with spin-orbit binding energies of 852.5 and 860.7 eV, correspondingly, while Ni 2+ 2p 3/2 and Ni 2+ 2p 1/2 display two peaks with binding energies of 848.5 and 856.8 eV, respectively. Additionally, the Co 2p spectrum demonstrates a well-fitted profile with two prominent peaks corresponding to Co 2p 3/2 and Co 2p 1/2 , positioned at 772.5 eV and 792.4 eV, as illustrated in Fig. 5 c. In the O 1s spectra (depicted in Fig. 5 d), the binding energies of 526.1 eV, 528.2 eV, and 529.5 eV are attributed to the metal-oxygen bond in NiCo 2 O 4 , as well as to defect sites with reduced oxygen coordination and oxygen absorption from the surrounding atmosphere[ 47 ]. 3.4. BET analysis The BET technique, founded on the adsorption of particular molar species in their gaseous state on the surface, stands as one of the pivotal methods for accurately determining the total specific area of porous samples [ 48 ]. The crucial factor in any electrochemical reaction, the active surface area of the synthesized NiCo 2 O 4 -based samples, was assessed utilizing the Quantachrome Nova station 100 apparatus through gas adsorption analysis conducted at 77 K. The synthesised NCO3 (Fig. 6 (a)) has high surface area of 91.2 m 2 /g, compared to 30.76 m 2 /g for the NCO5 (Fig. 6 (b)). When compared to NCO3, the nanorod morphology of the synthesized NCO3 substantially enhances the specific surface area. The total pore volume was calculated by assessing the quantity of gas absorbed relative to the applied pressure. The investigation of the pore size distribution of the synthesized NiCo 2 O 4 active materials was conducted using non-local density functional theory. The synthesised NCO3 has a pore volume of 2.184 cm 3 /g, while the synthesised NCO5 has a pore volume of 0.135 cm 3 /g. Due to high pore volume of as synthesized NCO3, it indicates the superior electrochemical performance. 3.5. Electrocatalytic study on OER/ORR The electrocatalytic characteristics of NiCo 2 O 4 nanostructures were investigated using the polarisation plot of the oxygen evolution reaction (OER) (Fig. 7a). The onset potentials of the NiCo 2 O 4 nanostructures (NCO1, NCO2, NCO3, NCO4, NCO5, NCO6, and IrO 2 ) synthesised using the various parameters stated were 1.52, 1.5, 1.49, 1.52, 1.54, 1.56, 1.57, and 1.6, respectively, vs. RHE. In compared to other nickel cobaltite samples and IrO 2 , the NiCo 2 O 4 material (NCO3) used 260 mV less energy to conduct the oxygen evolution when a reference current density of 10 mA.cm − 2 was used. The highest electrocatalytic characteristics for oxygen evolution are defined by the high double layer region in NiCo 2 O 4 samples with IrO 2 , while NCO3 revealed the low double layer region in Cdl plots (Fig. 7b). The decreased Tafel slope, which denotes a successful charge-transfer process, can be linked to the increased OER activity NCO3 exhibits IrO 2 's Tafel slope indicated that the dissociative water adsorption mechanism was its limiting factor [ 49 ]. All of the NiCO 2 O 4 samples' tafel slopes were further displayed (SI). In addition to NCO5's outstanding ORR performance in compared to Pt/C, NCO3 also demonstrated strong OER activity. Figure 7(c) shows the Nyquist plot of the synthesized materials using a frequency range of 0–10 kHz and amplitude of 5 kV. In comparison to other NiCo 2 O 4 samples, the synthesised materials NCO3 and NCO5 have a lower Rct value, which indicates that they have good electrocatalytic capabilities for oxygen reduction and evolution reactions. NCO5 was found to achieve a greater current density than other NiCo 2 O 4 nanomaterials, it was also discovered. Thus, linear sweep voltammetry curve for the oxygen reduction reaction of NiCo 2 O 4 nanoparticles with Pt/C at different rotatory speeds are given in Fig. 7e and the LSV curve of other synthesized NiCo 2 O 4 nanomaterials were shown in supporting information (SI, Fig. S1 ). As can be seen, at all rotational speeds, all materials reached clearly defined diffusion limiting currents. An activity comparison of Pt/C and NiCo 2 O 4 nanomaterials at 1600 rpm is shown in Fig. 7(d). While Pt/C displayed 15.8 mA.cm − 2 at the same potential, NCO5 displayed a highest current density of -12.3 mA.cm − 2 among all NiCo 2 O 4 catalysts. So Changes in surface area cause a shift in the onset potential. With a nearly 2-fold greater surface area than Co 3 O 4 , NiCo 2 O 4 exhibits better activity in terms of current density. As a result, there are more active sites available for oxygen molecule reactions in NiCo 2 O 4 . The introduction of Ni species into the NiCo 2 O 4 catalyst enhanced its activity by boosting surface area and shifting the onset potential. The performance exhibited by the NiCo 2 O 4 material surpassed that of comparable reported materials, with the added advantage of being synthesized through a simpler process [ 50 – 51 ]. The presence of nickel (Ni) molecules within the NiCo 2 O 4 catalyst facilitated an improvement in the onset potential. Ni played a crucial role in augmenting electrical conductivity, thereby enhancing electronic properties and facilitating electron transfer [ 52 ]. The enhancement in onset potential resulting from nickel substitution is attributed to the reduction in activation energy [ 53 ]. Because of the enhancement of the electronic characteristics, the NiCo 2 O 4 material demonstrated an enhancement in the electrocatalytic performance for ORR. The following equations were used to conduct a Koutecky-Levich correlation and further analyse the ORR mechanism. $$\frac{1}{J}=\frac{1}{{J}_{L}}+\frac{1}{{J}_{K}}=\frac{1}{B{\omega }^{\frac{1}{2}}}+\frac{1}{{J}_{K}}$$ 1 B = 0.62nFC 0 (D 0 ) 2/3 \({\vartheta }^{-1/6}\) (2) J K =nFkC 0 (3) Where J d , J k , and J l are the disc current density, kinetic current density, and diffusion current density, respectively. Furthermore, n represents the electron transfer number, F represents the Faraday constant (96485 C/mol), CO 2 represents the oxygen concentration in KOH solution (1.4x10 − 6 mol/cm 3 ), DO 2 represents the oxygen diffusion coefficient in KOH (1.73x10 − 5 cm 2 /s), is the kinematic viscosity of the KOH solution (0.01 cm 2 /s), and is the electrode rotation rate (rad/s). Figure 7(f) shows the KL plots obtained at two different potentials of NCO5 samples and other plots were shown in Fig S2. This phenomenon can be visually illustrated by contrasting the outcomes with theoretical K–L plots for 2 and 4 electrons, demonstrating a consistent adherence to a 4-electron trend across all materials. The number of transferred electrons was determined using Eq. 2, yielding values of 3.12, 1.2, 1.9, 1.4, 3.3, 1.9, and 4e- for NCO1, NCO2, NCO3, NCO4, NCO5, NCO6, and Pt/C, respectively. Otherwise, KL plots showed linear behaviour, indicating that these materials had a first-order influence on O 2 kinetics [ 54 ]. 4. Conclusions NiCo 2 O 4 nanostructures were made using a simple chemical precipitation method followed by callcination, which is a quick and efficient way to produce spinel type oxides. The morphology of the samples transforms when nickel is added, transitioning from particles to a flake-like structure for NiCo 2 O 4 . X-ray diffraction and X-ray photoelectron spectroscopy, which revealed NiCo 2 O 4 in the form of an inverse spinel, corroborated the production of these spinel type structures. These modifications led to a shift in the surface area determined by BET, which was altered to discover areas of 121.26 and 32.27 m 2 /g for NCO3 and NCO5 materials, respectively. EIS measurements supported the claim that the Ni inclusion also changed the NiCo 2 O 4 material's electrical characteristics. When compared to other NiCo 2 O 4 material, NCO5's charge transfer resistance dropped from 350.2 cm 2 to 215.2 cm 2 . The electrocatalytic activity for the oxygen reduction and oxygen evolution processes was significantly impacted by these morphological and electrical alterations. During the ORR, the NCO5 had an onset potential of 0.72 V vs. RHE, which was 110 mV higher than the NCO4 (0.61 V) and 90 mV lower than the Pt/C as reference material (0.82 V). And for the OER, the NCO3 material exhibiting a potential difference of 152 mV@10 mA.cm − 2 when compared to other NiCo 2 O 4 materials and Pt/C. Declarations Author Contributions Statement Ananta Sasmal: Experimentation, Investigation, Writing – review & editing; Dipankar Gogoi: Conceptualization, Experimentation, Investigation, Methodology, & Writing – original draft; and T.D. Das: Visualization, & Supervision. Funding No financial support was provided by any funding agency or organizations for this work. 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In International Conference on Advanced Nanomaterials & Emerging Engineering Technologies ,468-470 IEEE (2013). Wang, Wenjun, et al. "NiCo 2 O 4 nanowire arrays for electrochemical energy storage." ACS nano 6.5 , 4532-4540 (2012). Y. Wang, Q. Zhang, J. Zhang, Challenges and opportunities for NiCo2O4 as high-performance electrochemical energy storage materials. Adv. Energy Mater., 9 (28), 1901218 (2019). H. Liu, X. Xu, J. Qiao, Recent progress on nickel cobaltite-based materials for energy storage and conversion. Small Methods, 4 (1), 1900622 (2020). Y. Wang, H. Wang, H. Zhang, J. Wang, Hydrothermal synthesis of nanomaterials with diverse morphologies and their applications in energy storage and conversion. Nano Energy, 61 , 246-265 (2019). X. Ren, C. Cai, C. Yang, P. Yang, Z. Guo, Facile Synthesis of Hierarchical NiCo 2 O 4 Microspheres with Enhanced Electrochemical Performance for Supercapacitors. Mater. Lett. 259 , 126872 (2020). H. Zhang, L. Li, G. Zhang, J. Wang, Fabrication of impurity-free NiCo 2 O 4 nanoflowers for efficient electrocatalytic water oxidation. J. Power Sources., 429 , 34-41 (2019). M. Haripriya, R. Sivasubramanian, A. M. Ashok, S. Hussain, G. Amarendra, Hydrothermal synthesis of NiCo 2 O 4 –NiO nanorods for high performance supercapacitors. J Mater Sci Mater Electron, 30 , 7497-7506 (2019). L. Feng, Y. Jiang, X. Wang, Y. Xia, Shape-Controlled Synthesis of Spinel Cobaltite (Co 3 O 4 , NiCo 2 O 4 , and CoNi 2 O 4 ) Nanostructures and Their Application in Oxygen Reduction Reaction and Lithium Ion Batteries. Adv. Funct. Mater., 26 (23), 4165–4174 (2016). N. Naseri, B. Alinejad, Synthesis and Characterization of Nanostructured NiCo 2 O 4 and Its Catalytic Activity for Oxidation of CO in the Presence of Hydrogen. Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 48 (5), 645-650 (2018). A. Ghadage, P. Kodam, D. Nadargi, K. P. Shinde, I. Mulla, J. S. Park, S. Suryavanshi, Sponge microflowers of NiCo 2 O 4 : a versatile material for high performance supercapacitor. J. Porous Mater., 29 (4), 1239-1252 (2022). Y. Wang, M. Cui, L. Wang, G. Lu, Recent advances in Brunauer–Emmett–Teller (BET) method for surface area measurement: Instrumentation, methodologies and applications. Advances in Colloid and Interface Science, 292 , 102427 (2021). L. Zeng, T.S. Zhao, R.H. Zhang, J.B. Xu, NiCo 2 O 4 nanowires@ MnOx nanoflakes supported on stainless steel mesh with superior electrocatalytic performance for anion exchange membrane water splitting. Electrochem. commun., 87 , 66-70 (2018). M. Li, H. Zhang, T. Xiao, B. Zhang, J. Yan, D. Chen, Y. Chen, Rose flower-like nitrogen-doped NiCo2O4/carbon used as cathode electrocatalyst for oxygen reduction in air cathode microbial fuel cell. Electrochim. Acta., 258 , 1219-1227 (2017). J. Wang, Y. Fu, Y. Xu, J. Wu, J.H. Tian, R. Yang, Hierarchical NiCo2O4 hollow nanospheres as high efficient bi-functional catalysts for oxygen reduction and evolution reactions. Int. J. Hydrog. Energy, 41 (21), 8847-8854 (2016). Z.F. Huang, J. Wang, Y. Peng, C.Y. Jung, A. Fisher, X. Wang, Design of efficient bifunctional oxygen reduction/evolution electrocatalyst: recent advances and perspectives. Adv. Energy Mater., 7 (23), 1700544 (2017). N. Heller-Ling, M. Prestat, J.L. Gautier, J.F. Koenig, G. Poillerat, P. Chartier, Oxygen electroreduction mechanism at thin Ni x Co 3 − XO 4 spinel films in a double channel electrode flow cell (DCEFC). Electrochim. Acta., 42 (2), 197-202 (1997). V.P. Zhdanov, B. Kasemo, Kinetics of electrochemical O 2 reduction on Pt. Electrochem. commun., 8 (7), 1132-1136 (2006). Top of Form Additional Declarations No competing interests reported. Supplementary Files Supplementaryinformation.docx GraphicalAbstract.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4092883","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Short Report","associatedPublications":[],"authors":[{"id":283613935,"identity":"c273be29-8f19-442d-b2a6-680098c729dd","order_by":0,"name":"Ananta Sasmal","email":"","orcid":"","institution":"National Institute of Technology Arunachal Pradesh, Jote, Itanagar-791113","correspondingAuthor":false,"prefix":"","firstName":"Ananta","middleName":"","lastName":"Sasmal","suffix":""},{"id":283613936,"identity":"924b829a-fd1f-41b4-bb05-4fd3ba32d127","order_by":1,"name":"Dipankar Gogoi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4UlEQVRIiWNgGAWjYFACHhgjgfEBiMtHihZmAxCXjRQtbBIgiqAWc/beYxKMO+zy+NnTr1V+zbGTYWNgfvjoBh4tlj3n0iQYzyQXS/a8Kbstuy0Z6DA2Y+McPFoMbuSYSTC2MSduuJGTdltyGzNQCw+bNF4t99+AtNSDtRRLbqsnQssNHpCWw0At6ccYP247TISWMznGFoltxxNn9rxhlmbcdpyHjZmQX46fMbzxsa06sZ89/eHHn9uq7fnZmx8+xqcFDBLAJI8BMziOmAkpRwD2B4w/iFc9CkbBKBgFIwgAAFvJRW9HNVjNAAAAAElFTkSuQmCC","orcid":"","institution":"National Institute of Technology Arunachal Pradesh, Jote, Itanagar-791113","correspondingAuthor":true,"prefix":"","firstName":"Dipankar","middleName":"","lastName":"Gogoi","suffix":""},{"id":283613938,"identity":"5a5ff0e2-0a19-46f1-a7d9-1b66c8d597b6","order_by":2,"name":"T D Das","email":"","orcid":"","institution":"National Institute of Technology Arunachal Pradesh, Jote, Itanagar-791113","correspondingAuthor":false,"prefix":"","firstName":"T","middleName":"D","lastName":"Das","suffix":""}],"badges":[],"createdAt":"2024-03-13 13:00:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4092883/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4092883/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":53473951,"identity":"773e6e08-0ef8-4442-baca-35fe39a5c54e","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":20557,"visible":true,"origin":"","legend":"\u003cp\u003eillustrates the XRD patterns of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures synthesized at different molarities: 1.5 M (NCO1), 0.5 M (NCO2), 1 M (NCO3), and 2 M (NCO4), along with precursor molar ratios of Ni to Co at 1:0.5 (NCO5) and 1:1 (NCO6), respectively.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/074fda78e773281e6d456fa8.png"},{"id":53474513,"identity":"eacfc922-77d2-4599-b49b-254065cc0acc","added_by":"auto","created_at":"2024-03-26 12:18:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":19203,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR plots of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003enanostructures produced by altering the sodium bicarbonate concentration and the molar ratios of Ni and Co precursors.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/b511d46c687af5b99f775630.png"},{"id":53474514,"identity":"4498aaa5-b46a-4921-896b-710ef2877a03","added_by":"auto","created_at":"2024-03-26 12:18:56","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":481262,"visible":true,"origin":"","legend":"\u003cp\u003eFESEM images of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructured materials with a variation of molarity concentration of sodium bicarbonates (NCO1-NCO4) and the variation of precursor ratio (NCO5, NCO6) respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/7a95e7640d5b9da91a3cafd8.png"},{"id":53473954,"identity":"4799a4d1-15b2-4249-8bd6-6f865b06c9f4","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":408815,"visible":true,"origin":"","legend":"\u003cp\u003epresents the microstructural analysis of NCO3, comprising (a) a TEM image, (b) an SAED pattern, and (c) an HRTEM image.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/c055d62d96e8fb09b3bcddaf.png"},{"id":53473960,"identity":"fd89ef99-0af4-409f-802d-84572cb41ccf","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":54367,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XPS survey pattern of NCO3, (b) high-resolution spectra of Ni, (c) high-resolution spectra of Co and (d) high-resolution spectra of oxygen (O\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/7244e0d7a413a191ad9b7d8d.png"},{"id":53473957,"identity":"120f1397-6891-4203-82b9-ff7f8387431e","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":41876,"visible":true,"origin":"","legend":"\u003cp\u003eDisplays the BET analysis of N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms and pore size distribution curves (inserted images) for (a) NCO3 and (b) NCO5.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/68f542fe3795c60f892c674c.png"},{"id":53473956,"identity":"ae8a7394-fa38-43bf-85ef-6a0d1eecde8c","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":80750,"visible":true,"origin":"","legend":"\u003cp\u003eshows (a) polarisation plots of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples with IrO\u003csub\u003e2\u003c/sub\u003e for OER, (b) capacitance of double layer plots, (c) EIS plots NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples, (d) polarisation plot of different NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples with Pt/c for ORR, (e) LSV plot of NCO5 for ORR and (f) KL-plot of NCO5 respectively.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/b21bacb0b082b537fa964221.png"},{"id":54761542,"identity":"538ad217-e900-4101-8d91-3ce70c36cfd7","added_by":"auto","created_at":"2024-04-16 11:30:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1336271,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/2272676f-6af4-47cc-aefd-b08f317544d4.pdf"},{"id":53473953,"identity":"289626fe-1b66-4f6c-9c64-dce13118f52b","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1183107,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/2dc18aa46e837b397662e54a.docx"},{"id":53473958,"identity":"acce95ed-50c4-49e4-8dde-118c2cdf29b5","added_by":"auto","created_at":"2024-03-26 12:10:56","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":478093,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-4092883/v1/03c81aae14960507abcf3239.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Highly Effective Bifunctional Electrocatalysts: Synthesizing NiCo 2 O 4 Nanostructures via Chemical Precipitation for Enhanced Oxygen Evolution and Reduction Reaction","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eLately, there has been a noticeable escalation in the imperative concerning energy consumption and environmental degradation. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. To mitigate our reliance on fossil fuels, innovative approaches to green energy storage and conversion are imperative [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, a significant obstacle emerges from the slow reaction rates experienced during the process of oxygen reduction at the cathode and oxygen evolution at the anode during the charging of batteries. [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Fuel cells and air batteries present promising solutions to this challenge [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Historically, noble metals such as platinum have been esteemed for their efficacy in catalyzing the oxygen reduction reaction (ORR), while iridium oxide and ruthenium oxide have been favored for the oxygen evolution reaction (OER). However, their substantial cost, restricted availability, and insufficient durability in alkaline conditions have hindered their widespread commercial application [\u003cspan additionalcitationids=\"CR9 CR10 CR11\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn contemporary research endeavors, considerable focus is directed towards minimizing platinum (Pt) loading in catalysts [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. One prominent approach involves the development of alloys utilizing metals such as Cobalt (Co), Nickel (Ni), and Iron (Fe), alongside the synthesis of core-shell structures [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. These strategies aim to employ the non-noble metal as the active catalyst core, thereby reducing the necessity for Pt content [\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. However, the continued dependence on Pt as the principal electroactive substance presents an enduring obstacle, predominantly attributed to its exorbitant expense and constrained accessibility. Consequently, there is a pressing need for alternative methodologies to engineer highly efficient materials [\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Transition metal oxides (TMOs) emerge as a promising substitute in this context. TMOs, exhibiting diverse oxidation states, demonstrate heightened activity in oxygen reactions attributed to enhanced electron transport capabilities [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Moreover, their inherent ability to seamlessly integrate within a single material matrix is a crucial attribute for electrocatalysts [\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The continuous oxidation and reduction processes within electrochemical cells necessitate materials capable of swift interconversion between oxidation states, a requirement fulfilled by TMOs [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Additionally, TMOs offer versatility, allowing for their utilization either in combination with other TMOs to create mixed transition metal oxides or as standalone electrocatalysts (MTMOs). Notably, compounds like spinels and perovskites exhibit well-ordered crystalline structures. Spinels, characterized by the general chemical formula AB\u003csub\u003e2\u003c/sub\u003eX\u003csub\u003e4\u003c/sub\u003e, offer particularly enticing prospects due to their potential for amalgamating the catalytic properties of various oxides within a single crystal lattice [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCo\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, a widely studied spinel electrocatalyst renowned for its efficacy and durability in oxygenevolution reaction (OER) and oxygen reduction reaction (ORR) in basic mediums, has undergone enhancement with the incorporation of Ni atoms to form NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e [\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. This modification aims to bolster electrical conductivity and augment the electroactive sites, thereby improving the overall electrochemical performance [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. In contrast, nickel oxide, a transition metal oxide (TMO), though less explored, exhibits commendable conductivity, barrier properties, and superior stability compared to precious metals [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. While demonstrating promising OER capability, it falls short in ORR performance, rendering it unsuitable for standalone ORR applications. However, when combined with other oxides, it holds potential as a bifunctional material. The incorporation of nickel oxides into the spinel structure gives rise to the NiCo2O4 spinel, presenting a notable potential as an electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in various applications [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Notably, the electrical conductivity of Co, Ni, and their compositions follows the sequence: Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;NiO\u0026thinsp;\u0026lt;\u0026thinsp;NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Despite its inherent advantages such as high impact strength, corrosion resistance, and abundance, NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e faces limitations due to its poor conductivity and large structure [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Various strategies including compositional modification, valence control, defect generation, and morphology optimization have been explored to enhance its electrochemical performance. Diverse morphologies such as nanoparticles, nanowires, nanoneedles, and urchin-like structures have been synthesized via hydrothermal methods, albeit with scalability challenges for commercial applications [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. In this investigation, we introduce a facile chemical precipitation approach to fabricate NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures by varying molarity of concentration of NaHCO\u003csub\u003e3\u003c/sub\u003e and precursor ratios of Ni and Co chloride source. The effectiveness of electrocatalysis for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was evaluated using experiments performed utilizing a rotating disk electrode. Our contribution lies in the facile synthesis of this ternary compound as a bifunctional electrocatalyst, crucial for \"air electrode\" applications in various energy systems, and exploring the influence of Ni incorporation on cobalt oxides in forming NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e spinel, elucidating structural and electrocatalytic modifications. This study utilizes synthesized NiCo2O4 nanostructures to investigate their electrocatalytic activity for the oxygen reduction and oxygen evolution processes, which were notably influenced by changes in morphology and electrical properties in a systematics approach.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Chemicals required\u003c/h2\u003e \u003cp\u003eNickel chloride hexahydrate (NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO), cobalt chloride hexahydrate (CoCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO) and sodium bicarbonate (NaHCO\u003csub\u003e3\u003c/sub\u003e) were of analytical grade and employed without additional purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Synthesis of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanomaterials\u003c/h2\u003e \u003cp\u003eThe chemicals were employed immediately upon acquisition without the need for further refinement or purification processes. For a conventional preparation procedure, solutions of 0.05 M NiCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H2O, 0.1 M CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, and 1.5 M NaHCO\u003csub\u003e3\u003c/sub\u003e were prepared in advance. Subsequently, 80 mL of the NiCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO solution was mixed with the NaHCO\u003csub\u003e3\u003c/sub\u003e solution under continuous stirring, leading to a 20-minute reaction to form a homogenous suspension. Subsequently, 80 milliliters of the CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO solution were added to the previously mentioned suspension and agitated for a duration of 8 hours. The resulting precipitate was collected via centrifugation, subjected to multiple washes with acetone and water to eliminate residual impurities, and finally dried by maintaining at 70\u0026deg;C in hot-oven. To synthesize the nanostructured NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e material, the carbonate precursor underwent annealing in an air environment at 300\u0026deg;C for a duration of 3 hours, employing a gradual heating rate of 5\u0026deg;C/min.\u003c/p\u003e \u003cp\u003eFurthermore, the identical procedure was employed to generate samples with varied molarities of NaHCO\u003csub\u003e3\u003c/sub\u003e precursors, specifically 0.5 M, 1 M, and 2 M, while maintaining consistency in other parameters. The samples synthesized using NaHCO\u003csub\u003e3\u003c/sub\u003e at concentrations of 0.5 M, 1 M, and 2 M were denoted as NCO1, NCO2, NCO3, and NCO4 respectively. Furthermore, two additional materials were created by adjusting the ratio of Ni and Co precursors while maintaining consistent synthesis conditions. These materials, designated as NCO5 and NCO6, were prepared using precursor molar ratios of Ni to Co of 1:0.5 and 1:1 respectively, in accordance with the established protocol.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Materials Characterizations\u003c/h2\u003e \u003cp\u003eThe investigation of morphology and surface structure was carried out utilizing scanning electron microscopy (SEM) (LEO FESEM 1530) and transmission electron microscopy (TEM) (Philips CM300). Surface structure and morphology were further analyzed using Fast Fourier Transformation (FFT) and high-resolution TEM (HR-TEM). The crystalline nature of the spinel nanostructure within NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was explored employing HR-TEM, FFT, and selected area electron diffraction (SAED) patterns. The surface area and porosity characteristics were determined using a Micromeritics ASAP 2020 surface area and porosity analyzer, with the nitrogen adsorption-desorption isotherm analyzed through the Brunauer-Emmett-Teller (BET) method, and pore size distributions assessed via the Barrett-Joyner-Halenda (BJH) model. The spinel crystal structure of the nanoplatelets in NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was confirmed through X-ray diffraction (XRD) (BrukerAXS D8 Advance). Additionally, X-ray photoelectron spectroscopy (XPS) was conducted to ascertain the composition available within the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Electrochemical characterization and electrocatalytic evaluation\u003c/h2\u003e \u003cp\u003eCyclic voltammetry (CV) for OER and rotating disc electrode (RDE) voltammogram for ORR were used to examine the electrochemical performances of the nanomaterials used for the current study in 1 M aqueous KOH electrolyte. A potentiostat and a rotation speed controller make up the setup for RDE. All electrochemical tests were performed at room temperature, with Ag/AgCl acting as a reference electrode. The counter electrode was made of platinum wire. The working electrode was a glassy carbon electrode (5 mm OD) coated with 20 mL of 4 mg mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e suspension produced by combining NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and a 0.5 wt% Nafion solution in mixed solution (375 \u0026micro;L H2O\u0026thinsp;+\u0026thinsp;125 \u0026micro;L IPA). All additional comparative materials were prepared in the same way as the working electrode. ORR curves were recorded from \u0026minus;\u0026thinsp;0.7 to 1V using an O\u003csub\u003e2\u003c/sub\u003e-saturated electrolyte at a scan rate of 5 mV/s at varied electrode rotation speeds (1000, 1200,1400,1600,1800,2000 rpm). In order to spin off the oxygen evolved during voltammetry testing, the OER curves were recorded from 0 to 0.8 V at a scan rate of 5 mV/s with a N\u003csub\u003e2\u003c/sub\u003e-saturated electrolyte rotating at 1000 rpm. Linear sweep voltammetry was taken from 0.3 to 0.93 V vs RHE (Reversible Hydrogen Electrode) at a sweep rate of 5 mV/s for ORR in a O\u003csub\u003e2\u003c/sub\u003e-rich solution. The experiments of OER were taken by taking potential range from 1 to 1.9 V vs RHE at a sweep rate of 5 mV/s in a N\u003csub\u003e2\u003c/sub\u003e-rich solution. The RDE measurement for ORR was carried out by taking different rotation (1000\u0026ndash;2000 rpm) from potential range 0 to 0.93 V vs RHE. Moreover, electrochemical impedance spectroscopy (EIS) was taken to check the electrical properties of the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e materials, particularly the resistance of charge-discharge (Rct). The measurements were performed at 0.64 V vs. RHE with the frequency ranging from 0.01 HZ to 100 kHZ.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussions","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Structural analysis\u003c/h2\u003e\n \u003cp\u003eThe X-ray diffraction (XRD) analysis was conducted to assess the crystallinity, structural characteristics, and phase purity of the synthesized nanostructures. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the powder XRD patterns obtained from samples prepared using various concentrations of NaHCO\u003csub\u003e3\u003c/sub\u003e and different precursor ratios of Ni and Co. The synthesis involved chemical co-precipitation followed by calcination at 300\u0026deg;C for 3 hours, employing NaHCO\u003csub\u003e3\u003c/sub\u003e concentrations of 0.5 M, 1 M, and 2 M, and precursor molar ratios of Ni to Co at 1:0.5 and 1:1, respectively. Distinct samples were synthesized with different molarities, including 1.5 M (NCO1), 0.5 M (NCO2), 1 M (NCO3), and 2 M (NCO4), as well as precursor molar ratios of Ni to Co at 1:0.5 (NCO5) and 1:1 (NCO6), respectively. The diffraction peaks observed at 18.6\u0026deg;, 30.9\u0026deg;, 35.5\u0026deg;, 38.3\u0026deg;, 44.4\u0026deg;, 54.9\u0026deg;, 58.8\u0026deg;, and 75.0\u0026deg; correspond to the crystallographic planes of (111), (220), (311), (222), (400), (422), (511), and (440), respectively. These peaks are consistent with the cubic spinel NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase, as confirmed by comparison with the JCPDS file (JCPDS card No.00-20-0781) [\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e]. The XRD patterns (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA) exhibit prominent peaks indicative of well-crystalline samples, while low-intensity peaks remain unindexed. Notably, no characteristic peaks associated with impurities, such as NiO or CoO, were detected, underscoring the high purity of the synthesized NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples [\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e]. Furthermore, variation in the stoichiometric ratio of precursor concentrations during the synthesis of NCO1, from 1:0.5 (NCO5) to 1:1 (NCO6), revealed consistent formation of the cubic spinel NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase across all XRD peaks. Importantly, no significant alterations in peak position or additional peaks were observed in the XRD patterns of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples synthesized under different parameters, including molarity and precursor molar ratios.\u003c/p\u003e\n \u003cp\u003eThe FTIR analysis of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e revealed significant peaks near 2104 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, suggesting potential bending of water molecules. Additionally, the presence of the H\u0026ndash;O\u0026ndash;H bending vibration mode was indicated by peaks observed at 1602 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003eand 1078 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Weak signals at 640 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 487 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, consistent with all peaks detected in the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructured materials illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, were attributed to metal oxide vibrations [\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Morphological analysis\u003c/h2\u003e\n \u003cp\u003eBy varying the concentration of sodium bicarbonate and adjusting the precursor molar ratio, the surface characteristics of the produced NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures (as depicted in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) were investigated. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (NCO1) presents a scanning electron microscope (FESEM) image of nickel cobaltite nanoparticles synthesized using water as the solvent and a concentration of 1.5 M NaHCO\u003csub\u003e3\u003c/sub\u003e.Additionally, nanoflowers were produced when the molarity concentration of NaHCO\u003csub\u003e3\u003c/sub\u003e was switched from 1.5 M to 0.5 M (NCO2) while keeping the remaining synthesis parameters constant (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(NCO2)). In addition, compared to using little high molarity concentration, the morphology of the sample is significantly changed when using 1 M concentration of sodium bicarbonate (NCO3). Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (NCO3) shows how nanorods are shaped which have also good electrochemical performance for making efficient electrocatalyst. Using the highest molar concentration of 2 M (NCO4) and keeping the other synthesis variables constant, the shape of nanoparticles similar to those shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e (NCO4) was found. It is worth emphasised that the solvent is essential for the creation of different NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures with morphologies resembling nanoparticles, nanoflowers, and nanorods [\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e]. Further, we achieved 1:0.5 and 1:1 NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO and CoCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO precursor ratios throughout the synthesis of NCO1, while maintaining the other reaction conditions constant. This allowed us to better understand the impact of precursor molar ratio. NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO and CoCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO were mixed in a 1:0.5 molar ratio to produce nanosheets (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, NCO5). CoCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO and NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO were combined in a 1:1 molar ratio to create encapsulated nanoparticles with the same shape (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, NCO6).\u003c/p\u003e\n \u003cp\u003eThe comprehensive microstructural analysis of NCO3 was completed utilising TEM in addition to support FESEM. Using a 0.05 M of NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO and 0.1 M of CoCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO and 1 M of NaHCO\u003csub\u003e3\u003c/sub\u003e in 80 mL of aqueous solution and calcinated at 300\u0026deg;C for 3 hours, NCO3 was synthesised. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the TEM image, selected area electron diffraction (SAED) pattern, and high-resolution transmission electron microscopy (HRTEM) images of NCO3.The TEM picture (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea) displays a morphology resembling nanorods made of nanoplates. Moreover, the location of the selected area electron diffraction (SAED) pattern of NCO3 (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb) indicates the poly-crystalline structure of the samples. Furthermore, the edge of a nanosheet was used to catch the high-resolution TEM (HRTEM) (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec). The (311) crystal plane of the synthesized nanocomposite, identified as the most prominent peak in XRD, corresponds to a fringe spacing of 0.22 nm, as observed in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea (NCO3) [\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Compositional analysis\u003c/h2\u003e\n \u003cp\u003eX-ray photoelectron spectroscopy (XPS) is employed to gain insights into the elemental composition and chemical states of the prepared NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e sample.. Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the Ni, Co, and O components present in the survey region from the XPS survey pattern of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoflowers (1200-0 eV). Using high resolution Ni, Co, and O spectra, the sample\u0026apos;s spinel structure\u0026apos;s oxidation condition was determined. In Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb, the high-resolution Ni 2p spectrum is depicted, delineated into the Ni 2p\u003csub\u003e3/2\u003c/sub\u003e peak at 850.2 eV and the Ni 2p\u003csub\u003e1/2\u003c/sub\u003e peak at 856.6 eV, forming a spin-orbit doublet. The Ni\u003csup\u003e3+\u003c/sup\u003e2p\u003csub\u003e3/2\u003c/sub\u003e and Ni\u003csup\u003e3+\u003c/sup\u003e2p\u003csub\u003e1/2\u003c/sub\u003e states exhibit dual peaks with spin-orbit binding energies of 852.5 and 860.7 eV, correspondingly, while Ni\u003csup\u003e2+\u003c/sup\u003e2p\u003csub\u003e3/2\u003c/sub\u003e and Ni\u003csup\u003e2+\u003c/sup\u003e2p\u003csub\u003e1/2\u003c/sub\u003e display two peaks with binding energies of 848.5 and 856.8 eV, respectively. Additionally, the Co 2p spectrum demonstrates a well-fitted profile with two prominent peaks corresponding to Co 2p\u003csub\u003e3/2\u003c/sub\u003e and Co 2p\u003csub\u003e1/2\u003c/sub\u003e, positioned at 772.5 eV and 792.4 eV, as illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec. In the O 1s spectra (depicted in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed), the binding energies of 526.1 eV, 528.2 eV, and 529.5 eV are attributed to the metal-oxygen bond in NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, as well as to defect sites with reduced oxygen coordination and oxygen absorption from the surrounding atmosphere[\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4. BET analysis\u003c/h2\u003e\n \u003cp\u003eThe BET technique, founded on the adsorption of particular molar species in their gaseous state on the surface, stands as one of the pivotal methods for accurately determining the total specific area of porous samples [\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e]. The crucial factor in any electrochemical reaction, the active surface area of the synthesized NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-based samples, was assessed utilizing the Quantachrome Nova station 100 apparatus through gas adsorption analysis conducted at 77 K. The synthesised NCO3 (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(a)) has high surface area of 91.2 m\u003csup\u003e2\u003c/sup\u003e/g, compared to 30.76 m\u003csup\u003e2\u003c/sup\u003e/g for the NCO5 (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(b)). When compared to NCO3, the nanorod morphology of the synthesized NCO3 substantially enhances the specific surface area. The total pore volume was calculated by assessing the quantity of gas absorbed relative to the applied pressure. The investigation of the pore size distribution of the synthesized NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e active materials was conducted using non-local density functional theory. The synthesised NCO3 has a pore volume of 2.184 cm\u003csup\u003e3\u003c/sup\u003e/g, while the synthesised NCO5 has a pore volume of 0.135 cm\u003csup\u003e3\u003c/sup\u003e/g. Due to high pore volume of as synthesized NCO3, it indicates the superior electrochemical performance.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5. Electrocatalytic study on OER/ORR\u003c/h2\u003e\n \u003cp\u003eThe electrocatalytic characteristics of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures were investigated using the polarisation plot of the oxygen evolution reaction (OER) (Fig. 7a). The onset potentials of the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures (NCO1, NCO2, NCO3, NCO4, NCO5, NCO6, and IrO\u003csub\u003e2\u003c/sub\u003e) synthesised using the various parameters stated were 1.52, 1.5, 1.49, 1.52, 1.54, 1.56, 1.57, and 1.6, respectively, vs. RHE. In compared to other nickel cobaltite samples and IrO\u003csub\u003e2\u003c/sub\u003e, the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e material (NCO3) used 260 mV less energy to conduct the oxygen evolution when a reference current density of 10 mA.cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e was used. The highest electrocatalytic characteristics for oxygen evolution are defined by the high double layer region in NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples with IrO\u003csub\u003e2\u003c/sub\u003e, while NCO3 revealed the low double layer region in Cdl plots (Fig.\u0026nbsp;7b). The decreased Tafel slope, which denotes a successful charge-transfer process, can be linked to the increased OER activity NCO3 exhibits IrO\u003csub\u003e2\u003c/sub\u003e\u0026apos;s Tafel slope indicated that the dissociative water adsorption mechanism was its limiting factor [\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e]. All of the NiCO\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples\u0026apos; tafel slopes were further displayed (SI). In addition to NCO5\u0026apos;s outstanding ORR performance in compared to Pt/C, NCO3 also demonstrated strong OER activity. Figure 7(c) shows the Nyquist plot of the synthesized materials using a frequency range of 0\u0026ndash;10 kHz and amplitude of 5 kV. In comparison to other NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples, the synthesised materials NCO3 and NCO5 have a lower Rct value, which indicates that they have good electrocatalytic capabilities for oxygen reduction and evolution reactions.\u003c/p\u003e\n \u003cp\u003eNCO5 was found to achieve a greater current density than other NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanomaterials, it was also discovered. Thus, linear sweep voltammetry curve for the oxygen reduction reaction of NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles with Pt/C at different rotatory speeds are given in Fig. 7e and the LSV curve of other synthesized NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanomaterials were shown in supporting information (SI, Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e). As can be seen, at all rotational speeds, all materials reached clearly defined diffusion limiting currents. An activity comparison of Pt/C and NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanomaterials at 1600 rpm is shown in Fig. 7(d). While Pt/C displayed 15.8 mA.cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at the same potential, NCO5 displayed a highest current density of -12.3 mA.cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e among all NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e catalysts. So Changes in surface area cause a shift in the onset potential. With a nearly 2-fold greater surface area than Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e exhibits better activity in terms of current density. As a result, there are more active sites available for oxygen molecule reactions in NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. The introduction of Ni species into the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e catalyst enhanced its activity by boosting surface area and shifting the onset potential. The performance exhibited by the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e material surpassed that of comparable reported materials, with the added advantage of being synthesized through a simpler process [\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e]. The presence of nickel (Ni) molecules within the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e catalyst facilitated an improvement in the onset potential. Ni played a crucial role in augmenting electrical conductivity, thereby enhancing electronic properties and facilitating electron transfer [\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e]. The enhancement in onset potential resulting from nickel substitution is attributed to the reduction in activation energy [\u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eBecause of the enhancement of the electronic characteristics, the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e material demonstrated an enhancement in the electrocatalytic performance for ORR. The following equations were used to conduct a Koutecky-Levich correlation and further analyse the ORR mechanism.\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\frac{1}{J}=\\frac{1}{{J}_{L}}+\\frac{1}{{J}_{K}}=\\frac{1}{B{\\omega }^{\\frac{1}{2}}}+\\frac{1}{{J}_{K}}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eB\u0026thinsp;=\u0026thinsp;0.62nFC\u003csub\u003e0\u003c/sub\u003e (D\u003csub\u003e0\u003c/sub\u003e)\u003csup\u003e2/3\u003c/sup\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\vartheta }^{-1/6}\\)\u003c/span\u003e\u003c/span\u003e (2)\u003c/p\u003e\n \u003cp\u003eJ\u003csub\u003eK\u003c/sub\u003e =nFkC\u003csub\u003e0\u003c/sub\u003e (3)\u003c/p\u003e\n \u003cp\u003eWhere J\u003csub\u003ed\u003c/sub\u003e, J\u003csub\u003ek\u003c/sub\u003e, and J\u003csub\u003el\u003c/sub\u003e are the disc current density, kinetic current density, and diffusion current density, respectively. Furthermore, n represents the electron transfer number, F represents the Faraday constant (96485 C/mol), CO\u003csub\u003e2\u003c/sub\u003e represents the oxygen concentration in KOH solution (1.4x10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mol/cm\u003csup\u003e3\u003c/sup\u003e), DO\u003csub\u003e2\u003c/sub\u003e represents the oxygen diffusion coefficient in KOH (1.73x10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e/s), is the kinematic viscosity of the KOH solution (0.01 cm\u003csup\u003e2\u003c/sup\u003e/s), and is the electrode rotation rate (rad/s). Figure\u0026nbsp;7(f) shows the KL plots obtained at two different potentials of NCO5 samples and other plots were shown in Fig S2. This phenomenon can be visually illustrated by contrasting the outcomes with theoretical K\u0026ndash;L plots for 2 and 4 electrons, demonstrating a consistent adherence to a 4-electron trend across all materials. The number of transferred electrons was determined using Eq.\u0026nbsp;2, yielding values of 3.12, 1.2, 1.9, 1.4, 3.3, 1.9, and 4e- for NCO1, NCO2, NCO3, NCO4, NCO5, NCO6, and Pt/C, respectively. Otherwise, KL plots showed linear behaviour, indicating that these materials had a first-order influence on O\u003csub\u003e2\u003c/sub\u003e kinetics [\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eNiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures were made using a simple chemical precipitation method followed by callcination, which is a quick and efficient way to produce spinel type oxides. The morphology of the samples transforms when nickel is added, transitioning from particles to a flake-like structure for NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. X-ray diffraction and X-ray photoelectron spectroscopy, which revealed NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in the form of an inverse spinel, corroborated the production of these spinel type structures. These modifications led to a shift in the surface area determined by BET, which was altered to discover areas of 121.26 and 32.27 m\u003csup\u003e2\u003c/sup\u003e/g for NCO3 and NCO5 materials, respectively. EIS measurements supported the claim that the Ni inclusion also changed the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e material's electrical characteristics. When compared to other NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e material, NCO5's charge transfer resistance dropped from 350.2 cm\u003csup\u003e2\u003c/sup\u003e to 215.2 cm\u003csup\u003e2\u003c/sup\u003e. The electrocatalytic activity for the oxygen reduction and oxygen evolution processes was significantly impacted by these morphological and electrical alterations. During the ORR, the NCO5 had an onset potential of 0.72 V vs. RHE, which was 110 mV higher than the NCO4 (0.61 V) and 90 mV lower than the Pt/C as reference material (0.82 V). And for the OER, the NCO3 material exhibiting a potential difference of 152 mV@10 mA.cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e when compared to other NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e materials and Pt/C.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contributions Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAnanta Sasmal:\u003c/strong\u003e Experimentation, Investigation, Writing \u0026ndash; review \u0026amp; editing; \u003cstrong\u003eDipankar Gogoi:\u003c/strong\u003e Conceptualization, Experimentation, Investigation, Methodology, \u0026amp; Writing \u0026ndash; original draft; and \u003cstrong\u003eT.D. Das:\u003c/strong\u003e Visualization, \u0026amp; Supervision.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo financial support was provided by any funding agency or organizations for this work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no direct or indirect competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS. Chen, S. Saud, S. Bano, A. Haseeb, The nexus between financial development, globalization, and environmental degradation: Fresh evidence from Central and Eastern European Countries. Environ. Sci. Pollut. Res, \u003cstrong\u003e26\u003c/strong\u003e, 24733-24747 (2019).\u003c/li\u003e\n\u003cli\u003eA. Bilal, X. Li, N. Zhu, R. Sharma, A. 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Chartier, Oxygen electroreduction mechanism at thin Ni\u003csub\u003ex\u003c/sub\u003eCo\u003csub\u003e3\u003c/sub\u003e\u0026minus; XO\u003csub\u003e4\u003c/sub\u003e spinel films in a double channel electrode flow cell (DCEFC). Electrochim. Acta., \u003cstrong\u003e42\u003c/strong\u003e(2), 197-202 (1997).\u003c/li\u003e\n\u003cli\u003eV.P. Zhdanov, B. Kasemo, Kinetics of electrochemical O\u003csub\u003e2\u003c/sub\u003e reduction on Pt. Electrochem. commun., \u003cstrong\u003e8\u003c/strong\u003e(7), 1132-1136 (2006). Top of Form\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Inverse spinel, bi-functional, oxygen evolution and oxygen reduction","lastPublishedDoi":"10.21203/rs.3.rs-4092883/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4092883/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this investigation, we successfully produced NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanostructures using a simple chemical precipitation method, wherein we adjusted molarity concentration of sodium bicarbonate (NaHCO\u003csub\u003e3\u003c/sub\u003e) and precursor ratios of Ni and Co. Analysis of surface features revealed a diverse range of shapes, including particles, flowers, rods, and flakes. Notably, the NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanorods (NCO3) demonstrated a significant threefold increase in BET surface area compared to NCO5. The alterations observed in the physical and chemical characteristics significantly influenced the electrocatalytic efficacy in alkaline environments for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In the context of the oxygen reduction reaction, NCO5 displayed a commencement potential of 0.72 V compared to the reversible hydrogen electrode (RHE), surpassing NCO4 by 110 mV, albeit falling short by 90 mV when compared to Pt/C, the standard benchmark material with a potential of 0.82 V. In terms of OER, NCO3 displayed a potential difference of 152 mV@10mA/cm\u003csup\u003e2\u003c/sup\u003e compared to other NiCo\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e materials and Pt/C. The increased level of activity observed can be attributed not only to the increased surface area but also to enhancements in electrical properties. This is supported by the lower charge transfer resistance measured in NCO3 (215.2 Ω.cm\u003csup\u003e2\u003c/sup\u003e) compared to NCO5 (350.2 Ω.cm\u003csup\u003e2\u003c/sup\u003e) as revealed by electrochemical impedance spectroscopy (EIS).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"Highly Effective Bifunctional Electrocatalysts: Synthesizing NiCo 2 O 4 Nanostructures via Chemical Precipitation for Enhanced Oxygen Evolution and Reduction Reaction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-26 12:10:51","doi":"10.21203/rs.3.rs-4092883/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"65f06735-a509-461a-8f9f-e5c9afd602ff","owner":[],"postedDate":"March 26th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-05-09T06:10:17+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-26 12:10:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4092883","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4092883","identity":"rs-4092883","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}