Incremental Co Enhances Charge Transfer Efficiency and Accelerates Hydrogen Evolution Kinetics in NiCoP

Incremental Co Enhances Charge Transfer Efficiency and Accelerates Hydrogen Evolution Kinetics in NiCoP | 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 Research Article Incremental Co Enhances Charge Transfer Efficiency and Accelerates Hydrogen Evolution Kinetics in NiCoP Gang Wang, Chuwei Han, Ailing Song, Wenqing Geng, Dongkai Sun, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7545454/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 13 You are reading this latest preprint version Abstract Developing high-performance electrocatalysts for the hydrogen evolution reaction (HER) is crucial for advancing hydrogen production in clean energy systems. In this study, we investigated the role of incremental Co incorporation in nickel-cobalt phosphide (NiCoP) and its influence on the charge transfer efficiency and reaction kinetics of HER. Electrochemical analyses, including Tafel slope measurements and electrochemical impedance spectroscopy (EIS), reveal that incremental Co-based NiCoP with an optimal Co content exhibits a reduced overpotential and faster charge transfer, providing the most pronounced improvements. The XPS results corroborated the experimental findings, indicating that increased Co strengthened the electronic structure interaction among Ni, Co, and P in NiCoP. The availability of Co atoms and the low electronegativity promoted the electron flow to Ni and/or P, accelerated electron migration, and improved HER dynamics. This research demonstrates that adjusting the composition of NiCoP by increasing the incorporation of Co is a practical approach to enhance the catalytic activity and stability in HER applications. Metal-organic framework Transition metal phosphide Lamellar structure Electronic regulation Hydrogen evolution reaction Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction With the deep exploitation and depletion of traditional fossil fuels, various global problems have been caused, including but not limited to the greenhouse effect, environmental pollution, and energy shortage [ 1 , 2 ]. The search for cleaner and more renewable alternative energy sources is becoming an urgent need to realize the mission of a carbon-neutral society [ 3 , 4 ]. Electrocatalytic water splitting to produce hydrogen is considered as a clean and efficient alternative energy development method, helping to solve the energy crisis and environmental problems brought about by traditional fossil fuels [ 5 , 6 ]. However, due to the slow dynamics of the cathode hydrogen evolution reaction (HER), the large overpotential hinders the practical application of water splitting [ 7 , 8 ]. Although the precious metal Pt-based electrocatalysts have been found to have low overpotential and excellent HER kinetics, their high cost and limited supply limit their wide industrial applications [ 9 , 10 ]. Therefore, to improve the overall efficiency of the hydroelectrolysis system, it is necessary to focus on the development of a cost-effective and highly active HER electrocatalyst [ 11 , 12 ]. Transition metals such as phosphides, sulfides, selenides, nitrides, and boride [ 13 – 18 ] have been explored to replace Pt-based catalysts. Among them, transition metal phosphates (TMPs) have attracted much attention due to their advantages in abundant reserves, high mechanical strength, strong chemical tolerance, and low cost [ 19 ], such as Ni x P y [ 20 ], Co x P y [ 21 ], and Fe x P y [ 22 ]. The metal cation and negatively charged P have higher electronegativity for HER activity, pushing H 2 O dissociation and H 2 adsorption. However, problems such as low electrical conductivity, small specific surface area, and slow ion diffusion lead to poor electrocatalytic performance [ 23 , 24 ]. Studies have found that relative to the monometal phosphoride, the catalytic activity of bimetal phosphide is better [ 25 , 26 ]. Introducing a second metal to form double metal phosphides can effectively improve the catalytic performance, and due to the synergy between the two metal components, the electronic structure of TMPs can be optimized to regulate the kinetic energy barrier [ 27 – 29 ]. Li et al. prepared a Mo-CoP catalyst material by one-step hydrothermal and subsequent phosphating, with an over-potential as low as 112 mV at 100 mA cm − 2 , and this excellent activity comes from the synergistic action of two different metal phosphates [ 30 ]. Gao and coworkers demonstrated the successful modification of the electronic structure of the CoP electrocatalyst by chemically doping Ce, allowing for its excellent HER catalytic performance [ 31 ]. To this end, cooperative modulation of two atoms may be an effective way to improve the catalytic behavior of monometal phosphates. Compared with a single metal electrode, binary phospho-based compounds have higher electrical conductivity, enhanced structural and chemical stability, and good electrochemical properties [ 32 ]. These findings have inspired more researchers to explore the synergy between multiple components to regulate catalytic activity. Proper association between polymetallic components is essential to achieve high catalytic activity in water decomposition. NiCoP with good electronic conductivity has good catalytic activity on HER, Ni can promote the adsorption and desorption of hydrogen, and Co plays a stabilizing role in the HER process, thus reducing the energy barrier of hydrogen generation [ 33 ]. For example, Ma et al. prepared Mn-doped NiCoP with a nanoparticle array on a nickel foam (NF) substrate and greatly reduced the overpotential of HER to 148 mV at a current density of 100 mA cm − 2 [ 34 ]. The excellent catalytic performance can be attributed to the bimetallic synergy, d-band center regulation, and a secondary catalyst on the NiCoP surface to generate [ 35 ]. In addition, a metal-organic framework (MOF) has a high specific surface area and far-sequence structure through a strong connection of metal ions to organic ligands [ 36 , 37 ]. Therefore, functional TMP materials derived from MOFs have a high specific surface area and uniform element distribution [ 38 , 39 ], greatly enhancing the exposure of the active site, accelerating the penetration of the electrolyte, and shortening the diffusion path of ions/electrons in electrochemical processes. A series of electrocatalytic NiCo-TMP materials have been prepared using this method [ 40 – 42 ]. Rajpure M M et al [ 43 ]. prepared NiCo-LDH by electrodeposition, and then used LDH sheet structure as the sacrificial template for the formation of MOF, and finally prepared NiCoP from MOF by low temperature phosphating. Zhang et al [ 44 ]. synthesized Co MOFs on NF. Ni was introduced into the synthesis of NiCo MOFs by ion exchange, and NiCoP was obtained by one-step phosphating of MOF at low temperature. In this work, we choose Ni and Co transition metals, terephthalic acid as organic ligand to synthesize bimetallic MOF in one step, and then phosphating MOF to synthesize NiCoP at low temperature. This method is relatively simple and convenient compared to others. Additionally, the synthesized NiCoP can retain the original flake morphology of the MOF, enhance electrolyte diffusion and bubble desorption, which is advantageous for the catalytic reaction. However, balancing the Ni: Co ratio to maximize both the number of active sites and electronic conductivity based on MOF-derived NiCoP is still challenging. In this study, we selected MOF as a self-sacrificing template to make bimetal phosphates and obtained the NiCoP/NF catalytic electrode by hydrothermal method and low-temperature phosphorization. As shown in Scheme 1 , NiCo MOF/NF precursors with different proportions of metals were prepared by changing the metal Co content and the low-temperature phosphating in a subsequent step to obtain the catalytic electrode material. We explored the role of increasing Co incorporation in NiCoP and its effect on the charge transfer efficiency and reaction kinetics of HER. Incremental Co brings enhanced electron structure interactions, promotes rapid electron migration, and improves HER dynamics, as the synergistic interaction between Ni and incremental Co atoms creates favorable binding energy for hydrogen intermediates. The overpotential of the NiCoP-2 electrode is 200 mV at 100 mA cm − 2 , with excellent electrocatalytic performance and no significant decay of stability in 1.0 M KOH electrolyte for up to 120 h. The results show that a moderate amount of Co can significantly improve the HER performance of NiCoP, while excess Co can reduce its HER performance. Adjusting the composition of NiCoP by increasing Co incorporation is an effective way to improve the catalytic activity and stability of HER applications. Moreover, the electronic structure of NiCo bimetal was further adjusted to obtain better HER performance. Scheme 1. The schematic diagram of the synthesis process of NiCoP/NF. Experimental Preparation of NiCo-MOF on Nickel Foam All the reagents used in this experiment are of analytical grade and can be used without further purification. First, the NF (Ni-foam) was pretreated before the preparation of the NiCo-P. The nickel foam (1 cm×3 cm) was sonicated in 3 M hydrochloric acid and deionized water for 15 min to remove the surface oxide layer. Weigh 0.5 mmol NiCl 2 ·6H 2 O, 0.5 mmol CoCl 2 ·6H 2 O, 0.21 g of 1,4-benzendicarboxylic acid (1,4-BDC) dissolved in a mixed solution containing 4 mL of deionized water, 4 mL of ethanol and 42 mL N,N-Dimethylformamide (DMF), and fully dissolved by magnetic stirring for 10 min at room temperature, the samples were transferred to a stainless steel autoclave and subjected to a hydrothermal reaction at 140 ℃ for 6 h. After the reaction, the samples were removed, washed alternately several times with deionized water and ethanol, and dried under vacuum at 60℃ for 12 h. Finally, the NiCo MOF nanosheets were grown uniformly on the NF. The ratio of different metals was changed by changing CoCl 2 ·6H 2 O to 0.25 mmol and 0.75 mmol, and the resulting MOF precursors were named NiCo MOF-1, NiCo MOF-2, and NiCo MOF-3. At the same time, we prepared MOF powder under the same method, collected it after the reaction, and washed and dried it for the XRD test Preparation of NiCo-P on nickel foam and Pt/C electrodes In the phosphorization reaction, the prepared NiCo MOF / NF was placed downstream of the tubular furnace and weighed 0.5 g NaH 2 PO 2 ·H 2 O upstream. Subsequently, we heated the tube furnace to 300 ℃ at a heating rate of 2 ℃ min − 1 under an argon atmosphere and kept for 2 h. After natural cooling, the best sample obtained was named NiCoP-2. For comparison, the ratio of different metals was changed by changing CoCl 2 ·6H 2 O to 0.25 mmol and 0.75 mmol. Phosphosamples obtained at different ratios were labeled as NiCoP-1, NiCoP-2, and NiCoP-3. For comparison of electrocatalytic activity, 20 wt% Pt/C was loaded on nickel foam as follows. An amount of 5 mg of catalyst was dispersed in 200 mL of ethanol, along with 0.6 mg of acetylene black and 0.6 mg of polyvinylidene fluoride (PVDF), and sonicated for 1 h to form an ink-like mixture. Then, the ink-like mixture was drop-cast onto nickel foam with a geometric area of 1 cm×1 cm and dried at 60 ℃ to obtain the benchmark electrode. Material characterization XRD patterns of as-prepared materials were performed on a Rigaku Smart Lab X-ray diffractometer operated at 40 kV using Cu ka radiation source at a scan rate of 5°. All XRD data were analyzed using MDI Jade software and compared with the PDF standard database, ensuring accurate identification of the crystal phase. The morphology of as-prepared samples was characterized by Field emission scanning electron microscopy (FESEM, ZeissSupra 55) with an accelerating voltage of 15 kV and transmission electron microscopy (TEM, JEM 2100F) operated at 100 kV. Cut and paste the sample of appropriate size on the sample tray, and put the sample into the sample chamber of Thermo Scientific K-Alpha XPS instrument. When the pressure of the sample chamber is less than 4.0×10 − 7 mbar, the sample is sent to the analysis chamber, the spot size is 400 µm, and the working voltage is 12 kV. Filament current 6 mA; Full spectrum scanning energy 150 eV, step size 1 eV. The narrow-spectrum scanning energy is 50 eV and the step size is 0.1 eV. In order to ensure the accuracy of the data, the spectrum uses 284.8 eV of the C1s spectral line as the calibration standard. XPS test was performed on the ESCALab MKl X-ray photoelectron spectrometer equipped with Al Ka X-ray radiation as the excitation source. The data processing software (Avantage) of XPS test equipment is used to fit the XPS high resolution spectrum. As for the background removal method, we mainly used Shirley or Tougaard background correction. Among them, Shirley is selected for metal Ni and Co fitting, and Tougaard is selected for P fitting, and then the entire layer is fitted by the "Gauss-Lorentz mixing" method. In addition, high-resolution transmission electron microscopy (HR-TEM, FEI Talos F200X) was used to study the high-resolution crystal structure of the material. Electrochemical measurements All the electrochemical measurements were performed on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai, China) at room temperature. All measurements were performed in 1 M KOH aqueous electrolyte. In a three-electrode system, the final catalyst NiCo-P/NF is used as the working electrode directly, with the Hg/HgO electrode and Pt foil as the reference electrode and counter electrode, respectively. Linear sweep voltammetry (LSV) curves were measured at a scanning speed of 5 mV s − 1 and iR correction was performed on the obtained results. Electrochemical impedance spectroscopy (EIS) data collection was performed in the frequency range of 100 kHz to 0.01 Hz with an AC voltage of 5 mV. The double-layer capacitance of the electrode was characterized by cyclic voltammetry (CV) and tested at different scan rates from 20 to 100 mV s − 1 in the potential region of − 0.725 ~ − 0.825 V. The chronopotentiometry was used for long-term stability testing at a constant current density of 100 mA cm − 2 . All potentials reported in our work were displayed vs. the reversible hydrogen electrode (RHE) according to the equation: E (RHE) = E(Hg/HgO/OH − ) + 0.925 V (1) ECSA = C dl /C s (2) C s is the specific capacitance for a flat surface of electrodes in an alkaline electrolyte. Results and discussion Synthesis, morphology, and microstructure analysis Scheme 1 presents a comprehensive schematic diagram that effectively illustrates the structure evolution during the synthesis processes. We combined the metal ion and organic ligand through a simple one-step hydrothermal reaction to obtain bimetallic MOF, which yielded a low-temperature phosphoride catalytic electrode. The appropriate addition of metal Co instead of Ni can regulate the electron transfer between different quantities of bimetallic atoms by adjusting the content of metal Co to improve the catalytic activity of hydrogen evolution. The morphology of the obtained catalysts was characterized by a field emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM). Figures 1a, b show the morphology of NiCo MOF-2 precursor after hydrothermal reaction. It can be seen that the precursor is evenly distributed and loaded on the nickel foam substrate, the MOF through hydrothermal synthesis is a nanosheet morphology, and the sheet size is about 2 μm. Meanwhile, the uniformly overlapping nanosheets gave rise to a pore channel structure. The combination of the nanosheet layer and the pore structure augmented the surface area of the catalyst, exposed more active sites, and facilitated the transfer of electrolytes and ions, providing an excellent channel for the precipitation of catalytic products. Figure S1 shows the SEM images of MOF precursor obtained under different metal ratios. Figure S2 shows the elemental mapping of the NiCo MOF-2 precursor, showing the associated elemental composition and its uniform distribution. The uniform morphology of NiCo MOF-2 allows for optimal catalytic activity after phosphating. Figure 1c shows the morphology of NiCoP-2 obtained after the phosphorization of the NiCo MOF-2 precursor, which shows that the phosphorization samples maintain the nanosheet morphology of the precursor with a thickened surface. Through the TEM test in Figure 1d , we can see the nanosheet structure of the bimetallic phosphide, which is beneficial to the exposure of the active sites. Figure 1f shows the distinct color lattice stripes observed in the high-resolution TEM (HRTEM) of Figure 1e and the Fourier transform of the selected box areas. It can be seen that the (201) crystal surface with a lattice spacing of 0.201 nm corresponds to NiCoP, indicating the main component of the phosphorized electrode structure. Figure 1g shows the total spacing of five lattice stripes, which defines the crystalline phase. The total distance is 1.005 nm, which is confirmed again that the (201) crystal surface with a lattice spacing of 0.201 nm corresponds to NiCoP. Figure 1h shows the corresponding elemental mapping, and it can be seen that the nanosheets are composed of Ni, Co, and P elements, and these elements are evenly distributed. Then, the crystal structure of the material was characterized by X-ray diffraction (XRD). As shown in Figure 1i , the synthesized NiCo MOF precursor has a good crystal form[45,46] . Furthermore, we found that NiCoP prepared at different metal ratios in the second step had different crystallinity. With the content change in incremental Co, NiCoP-2 showed a better crystallinity than the control sample of NiCoP-1 and NiCoP-3. X-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition, chemical states and bonding, charge redistribution and electron transfer of elements within NiCoP-1, NiCoP-2, and NiCoP-3 samples. The spectra confirm the presence of Ni, Co, P, C, and O elements on the surface of NiCoP-2, consistent with the detection results of the control samples of NiCoP-1 and NiCoP-3 ( Figure S3 ). Then, we focus on the high-resolution XPS spectra of Ni 2p, Co 2p, and P 2p in NiCoP-2 and comparative analysis with NiCoP-1 and NiCoP-3 to elucidate electronic structure modifications induced by Co incorporation. In the high-resolution Ni 2p spectrum of NiCoP-2 ( Figure 2a ), the peaks at 856.67 eV (Ni 2p 3/2 ) and 874.58 eV (Ni 2p 1/2 ) are attributed to Ni 2+ species [47]. Additional peaks at 860.26 eV and 879.16 eV are shakeup satellites, indicative of electronic excitations [48,49]. The Ni-P bond is represented by binding energies at 853.10 eV and 870.24 eV, signifying the phosphide phase [48]. With the change of Co content, NiCoP-1 and NiCoP-3 related Ni-P bonds were located at 852.68 eV and 852.53 eV, respectively. The binding energy of NiCoP-2 is shifted by 0.42 eV in the direction of higher binding energy than that of NiCoP-1. This shift to a higher binding energy indicates that Ni loses electrons around which electron density decreases. In contrast, the binding energy NiCoP-3 associated with the Ni-P bond shifted 0.57 eV in the direction of lower binding energy than NiCoP-2, where Ni gains more incremental cobalt donated electrons. The change of Co content effectively regulated the electronic environment of NiCoP, accelerated the electron transfer, and thus improved the inherent electrocatalytic activity of the catalyst. For Ni 2p 1/2 and Ni 2p 3/2 , due to the effect of metal oxidation, other samples have a certain deviation from NiCoP-2. The Co 2p spectrum of NiCoP-2 ( Figure 2b ) reveals three pairs of peaks corresponding to Co-P, Co 2+ , and satellite features [50]. The peaks at 786.20 eV and 803.43 eV are identified as shakeup satellites, while the peaks at 781.93 eV (Co 2p 3/2 ) and 797.92 eV (Co 2p 1/2 ) correspond to Co 2+ species [51,52]. The 777.47 eV is the Co-P bond related characteristic peak. Relative to NiCoP-2, the Co-P bond peak binding energy of NiCoP-1 is in 777.66 eV. The positive shift of NiCoP-1 indicates that Co gains available Ni-donated electrons. In contrast, the Co-P bond peak binding energy of NiCoP-3 is in 777.95 eV, which significant positive shift relative to NiCoP-1, mainly due to the availability of incremental Co and its low electronegativity, Co donates electrons in the system. In addition, a pair of Co 2+ and satellite feature peaks also exist in NiCoP-1 and NiCoP-3. The P 2p spectrum of NiCoP-2 ( Figure 2c ) exhibits a peak at 134.22 eV, attributed to P-O bonds arising from inevitable surface oxidation [53]. Peaks at 129.50 eV and 130.31 eV indicate the M-P bonds, demonstrating the phosphide structure [54]. The 2p 3/2 and 2p 1/2 binding energies associated with the metal-P bond in samples NiCoP-1 and NiCoP-3 are 129.27 eV, 130.10 eV and 129.82 eV, 130.74 eV, respectively. A positive P 2p binding energy shift with increased Co content suggests electron transfer from P to Ni and Co atoms. This situation is unexpected, especially when we examine the shifts in Ni and Co. In this context, P appears to acquire additional electron density from the presence of Ni and Co. However, at the same time, P is subjected to oxidation due to various environmental factors. This dual behavior indicates a complex interplay between the electron donation from the transition metals and the oxidative processes influenced by the surrounding conditions, resulting in a more nuanced effect on chemical kinetics. This electron redistribution, especially the electron donation characteristic of Ni in NiCoP-2 and rapid electron transfer by incremental Co, results in enhanced catalytic performance for HER ( Figure 2d ) [55,56]. Characterization of Electrochemical Performance Simultaneously, the prepared NiCoP catalysts were tested to evaluate the catalytic HER activity in a 1 M KOH (pH=14). Now it is generally accepted that pH is 14 for alkaline hydrogen evolution test. Furthermore, at pH 14, the OH concentration was very high and more conducive to hydrogen evolution reaction[57,58]. Based on the three-electrode system, the platinum foil was used as the opposite electrode, and Hg/HgO as the reference electrode. NiCoP-1, NiCoP-2, NiCoP-3, bare NF electrodes, and Pt/C (20%) electrodes were characterized and compared. The polarization curves recorded by linear scanning voltammetry (LSV) are shown in Figure 3a . All polarization curves were subjected with iR compensation. Figure 3b reveals that at a current density of 200 mA cm -2 , NiCoP-2 possesses the smallest overpotential (218 mV), which is lower than that of Pt/C (224 mV), NiCoP-1 holds the second position (262 mV), and NiCoP-3 has the largest overpotential (268 mV). The slope of the Tafel curve, derived from the strong polarization region, provides crucial insights into the kinetic processes involved in hydrogen evolution at the electrode. A smaller Tafel slope, as determined through linear fitting, signifies a reduced change in overpotential with increasing current density, indicative of enhanced electrocatalytic performance. Moreover, variations in Tafel slope values can be associated with different steps involved in the HER. Specifically, a lower Tafel slope suggests that the rate-determining step occurs at the double electron transfer reaction within the HER mechanism. This relationship implies that a lower slope correlates with a faster Tafel step, further validating the efficiency of the electrocatalyst under investigation. As shown in Figure 3c , the Tafel slope of NiCoP-2 (124 mV dec -1 ) is lower than that of NiCoP-1 (133mV dec -1 ), NiCoP-3 (142 mV dec -1 ) and NF (217 mV dec -1 ), indicating that the HER mechanism of NiCoP-2 is the Volmer-Heyrovsky mechanism[59,60]. The charge transfer rate of the electrode was evaluated using electrochemical impedance spectroscopy, which provides insights into the resistive components affecting electrochemical processes. In this context, the charge transfer resistance (R ct ) is particularly significant, reflecting the resistance at the interface between the electrode and the electrolyte. A lower R ct indicates enhanced charge transfer capability, essential for optimizing catalytic activity. As illustrated in Figure 3d , the NiCoP-2 electrode demonstrates the smallest charge transfer resistance, with an R ct value of 3.95 Ω. This superior performance positions NiCoP-2 as the most effective among the electrodes tested in terms of charge transfer ability. Through EIS, we observed that the conductivity of NiCoP varies with increasing cobalt content, highlighting modifications in the electronic structure. Concurrently, XPS results indicated that variations in cobalt content also influence the electronic structure. Although charge transfer processes may influence binding energy, they do not fully account for the observed variations in binding energy, other factors, such as surface electron density and local potential, play a significant role. By comparing Tafel and EIS, NiCoP-2 has the lowest Tafel slope and the lowest R ct , indicating that with the introduction of incremental Co, NiCoP-2 has the most favorable electronic structure and the fastest electron conduction rate, which accelerates the hydrogen evolution kinetics and greatly reduces the hydrogen evolution overpotential. Then, previously reported NiCo-based materials for HER are listed to evaluate the electrocatalytic properties ( Table 1 ). To relate structural properties to intrinsic activity, double layer capacitance values (C dl ) are used to reflect the electrochemical active surface area (ECSA) level of the material [61]. C dl values for different samples were calculated using cyclic voltammetry (CV) curves. Figure S4 shows the CV curves of NiCoP-1, NiCoP-2, NiCoP-3, and bare NF. Due to the proportional relationship between C dl and ECSA, this can be used to assess the active surface area of the electrodes. From Figure 3e , the C dl value of NiCoP-2 (67 mF dec -2 ) is relatively higher than that of NiCoP-1 (62 mF dec -2 ), NiCoP-3 (43 mF dec -2 ) and bare NF (3.5 mF dec -2 ). In comparison, the C dl values of NiCoP-1 and NiCoP-2 are not much different, and that of NiCoP-3 is slightly smaller, indicating that the main reason affecting the performance at this time is the influence of electronic interaction. In addition to evaluating the HER activity of the catalyst, the stability of the electrode is also one of the most effective means of determining the application value of the catalyst. Figure 3f shows the trend of the electrode potential overtime at a current density of 100 mA cm -2 . After conducting the HER test for 120 h, we observed minimal decay in the overpotential of the electrode, measuring only a 10 mV increase at a current density of 100 mA cm -2 . This limited change suggests that the active sites of the catalyst remain stable and well-maintained throughout the duration of the test, highlighting the durability and effectiveness of the catalyst in sustaining performance under continuous operation. Table 1 －Comparison of the HER activity of NiCoP-2 with other electrocatalysts Number Catalysis Electrolyte η@j ( mA cm -2 ) Ref 1 NiCoP-2 1 M KOH KOH 108 mV@10 This work 2 Co-P11/Cu 1 M KOH 98.8 mV@10 [62] 3 MoS 2 /rGO(2) 1 M KOH 176mV@10 [63] 4 NiCo-MOF-P 1 M KOH 191 mV@10 [64] 5 Ni 3 S 2 @NiS/Ni-Net-2 1 M KOH 207 mV@100 [65] 6 NiCo-MOF-rod 1 M KOH 125mV@10 [66] 7 NiCoP/CoP 2 -SCBC 1 M KOH 159.5 mV@10 [67] 8 NiCoP@NC 1 M KOH 77 mV@10 [68] 9 Fe 2 P/NiCoP/NF 1 M KOH 125 mV@10 [69] 10 Ni@CoP 1 M KOH 181 mV@10 [70] 11 NiCo-NiCoP@PCT/CC 1 M KOH 135 mV@10 [71] After 120 h of stability test, we recorded the changes in the morphology, physical phase, and catalytic properties of the catalyst. The results of SEM and XRD after the stability test showed in Figures 4a-c , the surface-growing nanosheets became larger and thicker after the long-term stability test. However, on the whole, the original morphology is still maintained, and the phase of the material has not changed significantly, which once again confirms its good durability. From the XPS spectra after stability, it can also be seen that there is no significant change in the chemical state of the material before and after testing ( Figures 4d-f ). After a 120 h stability test, the XPS spectra show that the surfaces of NiCoP-2 also contain Ni, Co, P, C, and O elements ( Figure S5 ). In addition, the Si 2p (~103 eV) and Si 2s (~153 eV) peaks observed in the spectra suggest the presence of silicon, which may arise from environmental contamination or the silicon substrate used for catalyst deposition. It can be observed that the M-P peak in the P 2p XPS spectrum is strengthened and the P-O bond is attenuated compared to that before the stability test, suggesting that the oxidation of metal phosphates is mitigated due to the continuous hydrogen reduction test. The enhancement of HER activity is closely associated with the intrinsic activity determined by electronic structure, charge and mass transfer efficiency, and the stability of the active substance. The NiCoP-2 catalyst possesses a more appropriate electronic structure, stable structural properties, and higher conductivity, and exhibits good HER activity with the proper addition of Co. In addition, The contents of various elements in the NiCoP-2 samples were analyzed and compared before and after stability testing. The specific content comparison is presented in Table S1 . Notably, the Ni content significantly increases after stability testing, indicating a rise in the surface concentration of Ni during the constant current test. This phenomenon may be attributed to the enrichment of Ni on the catalyst surface, as it is likely to form active sites with other substances during the hydrogen evolution reaction (HER), thus enhancing its concentration in the surface active layer. The increased presence of Ni aids in catalyzing the reaction, suggesting that Ni plays a more critical role in the catalytic process[72]. In contrast, the change in Co content is minimal, which may imply that Co's contribution to the catalytic reaction is less stable than that of Ni. Alternatively, Co might be more active during the reaction and more susceptible to dissolution or surface migration[73]. Another key change observed is a significant reduction in phosphorus, which may indicate a change or partial removal of phosphide on the catalyst surface. The loss of phosphorus could impact the stability of the catalyst, particularly during long-term electrocatalysis[74,75]. This removal may alter the active site on the catalyst surface, consequently affecting its hydrogen evolution activity. Theoretical calculation into HER A stable NiCoP (201) surface model was applied based on the observed lattice planes from XRD patterns and TEM images, as shown in Figure 5a . The electron density of Ni and Co atoms was further verified by constructing the corresponding structural model of NiCoP. Bader charge analysis in Figure 5a shows that the average electron density of Ni (Ni1 and Ni2) atoms at the NiCoP (201) interface decreases by 0.22e, indicating that significant electron redistribution can provide rapid electron transfer, and then adjust the adsorption capacity of intermediates during the electro catalytic reaction. The Gibbs free energy (ΔGH*) of surface hydrogen adsorption at different sites of NiCoP was determined using density functional theory (DFT). The ideal hydrogen evolution reaction electrocatalyst surface should have the best affinity, and the hydrogen atom adsorption free energy can be used to evaluate the HER activity of the electrocatalytic material. When the hydrogen adsorption free energy approaches zero, that is, ΔGH*=0, it is conducive to hydrogen adsorption and desorption, and the catalytic activity is optimal, which is used to explain the bimetallic synergistic sites enhancing the electrocatalytic performance of NiCoP on HER. The atomic hydrogen adsorption free energy at three sites on the NiCoP (201) surface is shown in Figure 5b . The ΔG H* on the surfaces of Ni, Co, and Ni-Co(201) were calculated to be -0.66, -0.60, and 0.42 eV, respectively. Compared with the other two sites, the hydrogen adsorption free energy of bimetallic synergistic Ni Co is closer to the thermal neutral point, indicating that the bimetallic site Ni Co has the highest catalytic activity for HER, which is consistent with the experimental results. The negative free energy of Ni and Co indicates that the adsorption process is prone to occur, and the desorption process is a rate determining step. Conversely, the free energy of Ni-Co shows a positive value, indicating that the adsorption process (Volmer step) is a rate determining step. Therefore, the bimetallic site Ni-Co leads to changes in electronic structure and a significant decrease in ΔG H* value, thereby enhancing HER activity. Conclusions This study successfully synthesized a high-performance HER electrocatalyst, NiCoP-2, using a solvothermal method to grow a bimetallic MOF precursor in situ on nickel foam, followed by low-temperature phosphorization. By regulating the ratio of Ni and Co during synthesis, the material exhibited abundant catalytic sites, enhanced diffusion channels, and reduced charge transfer resistance, resulting in excellent electrocatalytic performance. Incorporating incremental Co into NiCoP improved electrical conductivity and optimized the electronic structure, significantly enhancing electrocatalytic activity. The electrochemical analysis demonstrated that NiCoP with the optimal Co content exhibited a reduced overpotential, faster charge transfer, and superior performance. The incorporation of incremental Co modulated the electronic structure of NiCoP, as evidenced by significant shifts in the binding energies of Ni 2p and Co 2p, confirmed through XPS analysis. These shifts, driven by synergistic interactions between Ni and Co, enabled charge redistribution and enhanced catalytic activity. Electrons primarily transferred from incremental Co to Ni and/or P. This electronic state modulation created an optimal electron-rich/electron-deficient balance for hydrogen evolution reactions. Overall, this study highlights that tuning the composition of NiCoP through Co incorporation effectively enhances its catalytic activity and stability, paving the way for advanced electrocatalysts for hydrogen production. Declarations Supplementary Materials: The online version contains supplemen tary material available at: https://doi.org/10.1007/xxx. Author Contributions: Conceptualization, A.S. and G.S.; methodology, G.W. and C.H.; validation, W.G. and W.Z.; formal analysis, W.G. and D.S.; investigation, J.Y.; data curation, Z.F. and J.Z.; writing—original draft preparation, G.W., C.H. and W.Z.; writing—review and editing, A.S., W.Z., H.D. and Z.M.; resources, A.S. Z.M. and G.S.;supervision, A.S. and G.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by the Hebei Provincial Department of Human Resources and Social Security (C20230328), the Natural Science Foundation of Hebei Province (E2023203209), the Natural Science Foundation of Hebei Province (B2021203016), State Key Laboratory of High Performance Complex Manufacturing (22567616H). Data availability No datasets were generated or analysed during the current study. 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SEM images of (a-b) NiCo MOF-2, (c) NiCoP-2, and (d-e) TEM image of NiCoP-2, (f) HRTEM image of NiCoP-2. (g) The selected area in Figure 1f corresponds to the contour intensity. (h) SEM image and EDS elemental mapping of NiCoP-2. (i) XRD patterns of NiCo-precursor, NiCoP-1, NiCoP-2, and NiCoP-3.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/ba8a8164ffb944d0bd03b3ff.png"},{"id":91624524,"identity":"957bd72e-246e-42f0-9c1c-0deca4ee6068","added_by":"auto","created_at":"2025-09-18 12:04:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":277161,"visible":true,"origin":"","legend":"\u003cp\u003eElectron transfer\u003cstrong\u003e \u003c/strong\u003eanalysis. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, and (c) P 2p of NiCoP-1, NiCoP-2, and NiCoP-3. (d) Schematic illustration of electron transfer on a NiCoP-2 crystal model.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/13b07ff66025caa34ea9a1a3.png"},{"id":91624522,"identity":"54a51ad1-da1b-4ee4-b13a-02321ed84ccd","added_by":"auto","created_at":"2025-09-18 12:04:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":63745,"visible":true,"origin":"","legend":"\u003cp\u003eHER performances and kinetics for as-prepared and benchmark electrodes in 1.0 M KOH. (a) iR-corrected LSV polarization curves and (b) Overpotential of catalysts at 100 mA cm\u003csup\u003e-2\u003c/sup\u003e. (c) The corresponding Tafel plots, (d) EIS spectra, and (f) C\u003csub\u003edl\u003c/sub\u003e of different electrodes. (f) Chronopotentiometry curve of NiCoP-2 performed at geometric current densities of −100 mA cm\u003csup\u003e-2 \u003c/sup\u003eand LSV curves of NiCoP-2 before and after the stability test.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/8d98762355bcb3caa9826e06.png"},{"id":91626404,"identity":"35608375-16df-4a72-aeac-3c120ba32bcb","added_by":"auto","created_at":"2025-09-18 12:12:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":344288,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterization after long-term stability test. (a) SEM, (b) TEM, (c) XRD images, and (d-f) XPS spectra of NiCoP-2 after stability test for 120 h.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/9e4b1df549225d333e9ef306.png"},{"id":91624527,"identity":"da36b6ba-98e0-40ee-b6e3-d6eadbaab91e","added_by":"auto","created_at":"2025-09-18 12:04:31","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":151796,"visible":true,"origin":"","legend":"\u003cp\u003eTheoretical predictions of the properties of NiCoP. (a) Atomic model of NiCoP.(b) Gibbs free energy of hydrogen adsorpion diagrams for the Tafel step.\u003c/p\u003e","description":"","filename":"image6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/e782e66295e072f39e6ef2f9.jpg"},{"id":91628143,"identity":"4aa455b7-3af5-4cb6-bc34-c9c79876e0ea","added_by":"auto","created_at":"2025-09-18 12:28:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3340645,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/4fe912ca-5bec-468a-ad34-6a78628aa798.pdf"},{"id":91626992,"identity":"8b7752c8-989c-490e-a2e3-a4dee9aab2be","added_by":"auto","created_at":"2025-09-18 12:20:31","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":45413,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1. \u003c/strong\u003eThe schematic diagram of the synthesis process of NiCoP/NF.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7545454/v1/f997709719e69d595600c62b.jpeg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Incremental Co Enhances Charge Transfer Efficiency and Accelerates Hydrogen Evolution Kinetics in NiCoP","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWith the deep exploitation and depletion of traditional fossil fuels, various global problems have been caused, including but not limited to the greenhouse effect, environmental pollution, and energy shortage [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The search for cleaner and more renewable alternative energy sources is becoming an urgent need to realize the mission of a carbon-neutral society [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Electrocatalytic water splitting to produce hydrogen is considered as a clean and efficient alternative energy development method, helping to solve the energy crisis and environmental problems brought about by traditional fossil fuels [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. However, due to the slow dynamics of the cathode hydrogen evolution reaction (HER), the large overpotential hinders the practical application of water splitting [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Although the precious metal Pt-based electrocatalysts have been found to have low overpotential and excellent HER kinetics, their high cost and limited supply limit their wide industrial applications [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Therefore, to improve the overall efficiency of the hydroelectrolysis system, it is necessary to focus on the development of a cost-effective and highly active HER electrocatalyst [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTransition metals such as phosphides, sulfides, selenides, nitrides, and boride [\u003cspan additionalcitationids=\"CR14 CR15 CR16 CR17\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] have been explored to replace Pt-based catalysts. Among them, transition metal phosphates (TMPs) have attracted much attention due to their advantages in abundant reserves, high mechanical strength, strong chemical tolerance, and low cost [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], such as Ni\u003csub\u003ex\u003c/sub\u003eP\u003csub\u003ey\u003c/sub\u003e [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], Co\u003csub\u003ex\u003c/sub\u003eP\u003csub\u003ey\u003c/sub\u003e [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], and Fe\u003csub\u003ex\u003c/sub\u003eP\u003csub\u003ey\u003c/sub\u003e [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The metal cation and negatively charged P have higher electronegativity for HER activity, pushing H\u003csub\u003e2\u003c/sub\u003eO dissociation and H\u003csub\u003e2\u003c/sub\u003e adsorption. However, problems such as low electrical conductivity, small specific surface area, and slow ion diffusion lead to poor electrocatalytic performance [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Studies have found that relative to the monometal phosphoride, the catalytic activity of bimetal phosphide is better [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Introducing a second metal to form double metal phosphides can effectively improve the catalytic performance, and due to the synergy between the two metal components, the electronic structure of TMPs can be optimized to regulate the kinetic energy barrier [\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Li et al. prepared a Mo-CoP catalyst material by one-step hydrothermal and subsequent phosphating, with an over-potential as low as 112 mV at 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, and this excellent activity comes from the synergistic action of two different metal phosphates [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Gao and coworkers demonstrated the successful modification of the electronic structure of the CoP electrocatalyst by chemically doping Ce, allowing for its excellent HER catalytic performance [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. To this end, cooperative modulation of two atoms may be an effective way to improve the catalytic behavior of monometal phosphates. Compared with a single metal electrode, binary phospho-based compounds have higher electrical conductivity, enhanced structural and chemical stability, and good electrochemical properties [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. These findings have inspired more researchers to explore the synergy between multiple components to regulate catalytic activity. Proper association between polymetallic components is essential to achieve high catalytic activity in water decomposition. NiCoP with good electronic conductivity has good catalytic activity on HER, Ni can promote the adsorption and desorption of hydrogen, and Co plays a stabilizing role in the HER process, thus reducing the energy barrier of hydrogen generation [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. For example, Ma et al. prepared Mn-doped NiCoP with a nanoparticle array on a nickel foam (NF) substrate and greatly reduced the overpotential of HER to 148 mV at a current density of 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The excellent catalytic performance can be attributed to the bimetallic synergy, d-band center regulation, and a secondary catalyst on the NiCoP surface to generate [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. In addition, a metal-organic framework (MOF) has a high specific surface area and far-sequence structure through a strong connection of metal ions to organic ligands [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Therefore, functional TMP materials derived from MOFs have a high specific surface area and uniform element distribution [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], greatly enhancing the exposure of the active site, accelerating the penetration of the electrolyte, and shortening the diffusion path of ions/electrons in electrochemical processes. A series of electrocatalytic NiCo-TMP materials have been prepared using this method [\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Rajpure M M et al [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. prepared NiCo-LDH by electrodeposition, and then used LDH sheet structure as the sacrificial template for the formation of MOF, and finally prepared NiCoP from MOF by low temperature phosphating. Zhang et al [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. synthesized Co MOFs on NF. Ni was introduced into the synthesis of NiCo MOFs by ion exchange, and NiCoP was obtained by one-step phosphating of MOF at low temperature. In this work, we choose Ni and Co transition metals, terephthalic acid as organic ligand to synthesize bimetallic MOF in one step, and then phosphating MOF to synthesize NiCoP at low temperature. This method is relatively simple and convenient compared to others. Additionally, the synthesized NiCoP can retain the original flake morphology of the MOF, enhance electrolyte diffusion and bubble desorption, which is advantageous for the catalytic reaction. However, balancing the Ni: Co ratio to maximize both the number of active sites and electronic conductivity based on MOF-derived NiCoP is still challenging.\u003c/p\u003e\u003cp\u003eIn this study, we selected MOF as a self-sacrificing template to make bimetal phosphates and obtained the NiCoP/NF catalytic electrode by hydrothermal method and low-temperature phosphorization. As shown in Scheme \u003cspan refid=\"Sch2\" class=\"InternalRef\"\u003e1\u003c/span\u003e, NiCo MOF/NF precursors with different proportions of metals were prepared by changing the metal Co content and the low-temperature phosphating in a subsequent step to obtain the catalytic electrode material. We explored the role of increasing Co incorporation in NiCoP and its effect on the charge transfer efficiency and reaction kinetics of HER. Incremental Co brings enhanced electron structure interactions, promotes rapid electron migration, and improves HER dynamics, as the synergistic interaction between Ni and incremental Co atoms creates favorable binding energy for hydrogen intermediates. The overpotential of the NiCoP-2 electrode is 200 mV at 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, with excellent electrocatalytic performance and no significant decay of stability in 1.0 M KOH electrolyte for up to 120 h. The results show that a moderate amount of Co can significantly improve the HER performance of NiCoP, while excess Co can reduce its HER performance. Adjusting the composition of NiCoP by increasing Co incorporation is an effective way to improve the catalytic activity and stability of HER applications. Moreover, the electronic structure of NiCo bimetal was further adjusted to obtain better HER performance.\u003c/p\u003e\u003cp\u003eScheme 1. The schematic diagram of the synthesis process of NiCoP/NF.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003ePreparation of NiCo-MOF on Nickel Foam\u003c/h2\u003e\u003cp\u003eAll the reagents used in this experiment are of analytical grade and can be used without further purification. First, the NF (Ni-foam) was pretreated before the preparation of the NiCo-P. The nickel foam (1 cm\u0026times;3 cm) was sonicated in 3 M hydrochloric acid and deionized water for 15 min to remove the surface oxide layer. Weigh 0.5 mmol NiCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, 0.5 mmol CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, 0.21 g of 1,4-benzendicarboxylic acid (1,4-BDC) dissolved in a mixed solution containing 4 mL of deionized water, 4 mL of ethanol and 42 mL N,N-Dimethylformamide (DMF), and fully dissolved by magnetic stirring for 10 min at room temperature, the samples were transferred to a stainless steel autoclave and subjected to a hydrothermal reaction at 140 ℃ for 6 h. After the reaction, the samples were removed, washed alternately several times with deionized water and ethanol, and dried under vacuum at 60℃ for 12 h. Finally, the NiCo MOF nanosheets were grown uniformly on the NF. The ratio of different metals was changed by changing CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO to 0.25 mmol and 0.75 mmol, and the resulting MOF precursors were named NiCo MOF-1, NiCo MOF-2, and NiCo MOF-3. At the same time, we prepared MOF powder under the same method, collected it after the reaction, and washed and dried it for the XRD test\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003ePreparation of NiCo-P on nickel foam and Pt/C electrodes\u003c/h3\u003e\n\u003cp\u003eIn the phosphorization reaction, the prepared NiCo MOF / NF was placed downstream of the tubular furnace and weighed 0.5 g NaH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO upstream. Subsequently, we heated the tube furnace to 300 ℃ at a heating rate of 2 ℃ min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under an argon atmosphere and kept for 2 h. After natural cooling, the best sample obtained was named NiCoP-2. For comparison, the ratio of different metals was changed by changing CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO to 0.25 mmol and 0.75 mmol. Phosphosamples obtained at different ratios were labeled as NiCoP-1, NiCoP-2, and NiCoP-3. For comparison of electrocatalytic activity, 20 wt% Pt/C was loaded on nickel foam as follows. An amount of 5 mg of catalyst was dispersed in 200 mL of ethanol, along with 0.6 mg of acetylene black and 0.6 mg of polyvinylidene fluoride (PVDF), and sonicated for 1 h to form an ink-like mixture. Then, the ink-like mixture was drop-cast onto nickel foam with a geometric area of 1 cm\u0026times;1 cm and dried at 60 ℃ to obtain the benchmark electrode.\u003c/p\u003e\n\u003ch3\u003eMaterial characterization\u003c/h3\u003e\n\u003cp\u003eXRD patterns of as-prepared materials were performed on a Rigaku Smart Lab X-ray diffractometer operated at 40 kV using Cu ka radiation source at a scan rate of 5\u0026deg;. All XRD data were analyzed using MDI Jade software and compared with the PDF standard database, ensuring accurate identification of the crystal phase. The morphology of as-prepared samples was characterized by Field emission scanning electron microscopy (FESEM, ZeissSupra 55) with an accelerating voltage of 15 kV and transmission electron microscopy (TEM, JEM 2100F) operated at 100 kV. Cut and paste the sample of appropriate size on the sample tray, and put the sample into the sample chamber of Thermo Scientific K-Alpha XPS instrument. When the pressure of the sample chamber is less than 4.0\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003embar, the sample is sent to the analysis chamber, the spot size is 400 \u0026micro;m, and the working voltage is 12 kV. Filament current 6 mA; Full spectrum scanning energy 150 eV, step size 1 eV. The narrow-spectrum scanning energy is 50 eV and the step size is 0.1 eV. In order to ensure the accuracy of the data, the spectrum uses 284.8 eV of the C1s spectral line as the calibration standard. XPS test was performed on the ESCALab MKl X-ray photoelectron spectrometer equipped with Al Ka X-ray radiation as the excitation source. The data processing software (Avantage) of XPS test equipment is used to fit the XPS high resolution spectrum. As for the background removal method, we mainly used Shirley or Tougaard background correction. Among them, Shirley is selected for metal Ni and Co fitting, and Tougaard is selected for P fitting, and then the entire layer is fitted by the \"Gauss-Lorentz mixing\" method. In addition, high-resolution transmission electron microscopy (HR-TEM, FEI Talos F200X) was used to study the high-resolution crystal structure of the material.\u003c/p\u003e\n\u003ch3\u003eElectrochemical measurements\u003c/h3\u003e\n\u003cp\u003eAll the electrochemical measurements were performed on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai, China) at room temperature. All measurements were performed in 1 M KOH aqueous electrolyte. In a three-electrode system, the final catalyst NiCo-P/NF is used as the working electrode directly, with the Hg/HgO electrode and Pt foil as the reference electrode and counter electrode, respectively. Linear sweep voltammetry (LSV) curves were measured at a scanning speed of 5 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and iR correction was performed on the obtained results. Electrochemical impedance spectroscopy (EIS) data collection was performed in the frequency range of 100 kHz to 0.01 Hz with an AC voltage of 5 mV. The double-layer capacitance of the electrode was characterized by cyclic voltammetry (CV) and tested at different scan rates from 20 to 100 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the potential region of \u0026minus;\u0026thinsp;0.725\u0026thinsp;~\u0026thinsp;\u0026minus;\u0026thinsp;0.825 V. The chronopotentiometry was used for long-term stability testing at a constant current density of 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. All potentials reported in our work were displayed vs. the reversible hydrogen electrode (RHE) according to the equation:\u003c/p\u003e\u003cp\u003eE\u003csub\u003e(RHE)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;E(Hg/HgO/OH\u003csup\u003e\u0026minus;\u003c/sup\u003e)\u0026thinsp;+\u0026thinsp;0.925 V (1)\u003c/p\u003e\u003cp\u003eECSA\u0026thinsp;=\u0026thinsp;C\u003csub\u003edl\u003c/sub\u003e/C\u003csub\u003es\u003c/sub\u003e (2)\u003c/p\u003e\u003cp\u003eC\u003csub\u003es\u003c/sub\u003e is the specific capacitance for a flat surface of electrodes in an alkaline electrolyte.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003e\u003cstrong\u003eSynthesis, morphology, and microstructure analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eScheme 1\u003c/strong\u003e presents a comprehensive schematic diagram that effectively illustrates the structure evolution during the synthesis processes. We combined the metal ion and organic ligand through a simple one-step hydrothermal reaction to obtain bimetallic MOF, which yielded a low-temperature phosphoride catalytic electrode. The appropriate addition of metal Co instead of Ni can regulate the electron transfer between different quantities of bimetallic atoms by adjusting the content of metal Co to improve the catalytic activity of hydrogen evolution. The morphology of the obtained catalysts was characterized by a field emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM). \u003cstrong\u003eFigures 1a, b\u003c/strong\u003e show the morphology of NiCo MOF-2 precursor after hydrothermal reaction. It can be seen that the precursor is evenly distributed and loaded on the nickel foam substrate, the MOF through hydrothermal synthesis is a nanosheet morphology, and the sheet size is about 2 \u0026mu;m. Meanwhile, the uniformly overlapping nanosheets gave rise to a pore channel structure. The combination of the nanosheet layer and the pore structure augmented the surface area of the catalyst, exposed more active sites, and facilitated the transfer of electrolytes and ions, providing an excellent channel for the precipitation of catalytic products.\u003cstrong\u003e\u0026nbsp;Figure S1\u003c/strong\u003e shows the SEM images of MOF precursor obtained under different metal ratios. \u003cstrong\u003eFigure S2\u003c/strong\u003e shows the elemental mapping of the NiCo MOF-2 precursor, showing the associated elemental composition and its uniform distribution. The uniform morphology of NiCo MOF-2 allows for optimal catalytic activity after phosphating. \u003cstrong\u003eFigure 1c\u003c/strong\u003e shows the morphology of NiCoP-2 obtained after the phosphorization of the NiCo MOF-2 precursor, which shows that the phosphorization samples maintain the nanosheet morphology of the precursor with a thickened surface. Through the TEM test in \u003cstrong\u003eFigure 1d\u003c/strong\u003e, we can see the nanosheet structure of the bimetallic phosphide, which is beneficial to the exposure of the active sites. \u003cstrong\u003eFigure 1f\u003c/strong\u003e shows the distinct color lattice stripes observed in the high-resolution TEM (HRTEM) of \u003cstrong\u003eFigure 1e\u003c/strong\u003e and the Fourier transform of the selected box areas. It can be seen that the (201) crystal surface with a lattice spacing of 0.201 nm corresponds to NiCoP, indicating the main component of the phosphorized electrode structure.\u003cstrong\u003e\u0026nbsp;Figure 1g\u003c/strong\u003e shows the total spacing of five lattice stripes, which defines the crystalline phase. The total distance is 1.005 nm, which is confirmed again that the (201) crystal surface with a lattice spacing of 0.201 nm corresponds to NiCoP. \u003cstrong\u003eFigure 1h\u003c/strong\u003e shows the corresponding elemental mapping, and it can be seen that the nanosheets are composed of Ni, Co, and P elements, and these elements are evenly distributed. Then, the crystal structure of the material was characterized by X-ray diffraction (XRD). As shown in \u003cstrong\u003eFigure 1i\u003c/strong\u003e, the synthesized NiCo MOF precursor has a good crystal form[45,46] . Furthermore, we found that NiCoP prepared at different metal ratios in the second step had different crystallinity. With the content change in incremental Co, NiCoP-2 showed a better crystallinity than the control sample of NiCoP-1 and NiCoP-3.\u003c/p\u003e\n\u003cp\u003eX-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition, chemical states and bonding, charge redistribution and electron transfer of elements within NiCoP-1, NiCoP-2, and NiCoP-3 samples. The spectra confirm the presence of Ni, Co, P, C, and O elements on the surface of NiCoP-2, consistent with the detection results of the control samples of NiCoP-1 and NiCoP-3 (\u003cstrong\u003eFigure S3\u003c/strong\u003e). Then, we focus on the high-resolution XPS spectra of Ni 2p, Co 2p, and P 2p in NiCoP-2 and comparative analysis with NiCoP-1 and NiCoP-3 to elucidate electronic structure modifications induced by Co incorporation. In the high-resolution Ni 2p spectrum of NiCoP-2 (\u003cstrong\u003eFigure 2a\u003c/strong\u003e), the peaks at 856.67 eV (Ni 2p\u003csub\u003e3/2\u003c/sub\u003e) and 874.58 eV (Ni 2p\u003csub\u003e1/2\u003c/sub\u003e) are attributed to Ni\u003csup\u003e2+\u003c/sup\u003e species [47]. Additional peaks at 860.26 eV and 879.16 eV are shakeup satellites, indicative of electronic excitations [48,49]. The Ni-P bond is represented by binding energies at 853.10 eV and 870.24 eV, signifying the phosphide phase [48]. With the change of Co content, NiCoP-1 and NiCoP-3 related Ni-P bonds were located at 852.68 eV and 852.53 eV, respectively. The binding energy of NiCoP-2 is shifted by 0.42 eV in the direction of higher binding energy than that of NiCoP-1. This shift to a higher binding energy indicates that Ni loses electrons around which electron density decreases. In contrast, the binding energy NiCoP-3 associated with the Ni-P bond shifted 0.57 eV in the direction of lower binding energy than NiCoP-2, where Ni gains more incremental cobalt donated electrons. The change of Co content effectively regulated the electronic environment of NiCoP, accelerated the electron transfer, and thus improved the inherent electrocatalytic activity of the catalyst. For Ni 2p\u003csub\u003e1/2\u0026nbsp;\u003c/sub\u003eand Ni 2p\u003csub\u003e3/2\u003c/sub\u003e, due to the effect of metal oxidation, other samples have a certain deviation from NiCoP-2. The Co 2p spectrum of NiCoP-2 (\u003cstrong\u003eFigure 2b\u003c/strong\u003e) reveals three pairs of peaks corresponding to Co-P, Co\u003csup\u003e2+\u003c/sup\u003e, and satellite features [50]. The peaks at 786.20 eV and 803.43 eV are identified as shakeup satellites, while the peaks at 781.93 eV (Co 2p\u003csub\u003e3/2\u003c/sub\u003e) and 797.92 eV (Co 2p\u003csub\u003e1/2\u003c/sub\u003e) correspond to Co\u003csup\u003e2+\u003c/sup\u003e species [51,52]. The 777.47 eV is the Co-P bond related characteristic peak. Relative to NiCoP-2, the Co-P bond peak binding energy of NiCoP-1 is in 777.66 eV. The positive shift of NiCoP-1 indicates that Co gains available Ni-donated electrons. In contrast, the Co-P bond peak binding energy of NiCoP-3 is in 777.95 eV, which significant positive shift relative to NiCoP-1, mainly due to the availability of incremental Co and its low electronegativity, Co donates electrons in the system. In addition, a pair of Co\u003csup\u003e2+\u003c/sup\u003e and satellite feature peaks also exist in NiCoP-1 and NiCoP-3. The P 2p spectrum of NiCoP-2 (\u003cstrong\u003eFigure 2c\u003c/strong\u003e) exhibits a peak at 134.22 eV, attributed to P-O bonds arising from inevitable surface oxidation [53]. Peaks at 129.50 eV and 130.31 eV indicate the M-P bonds, demonstrating the phosphide structure [54]. The 2p\u003csub\u003e3/2\u0026nbsp;\u003c/sub\u003eand 2p\u003csub\u003e1/2\u0026nbsp;\u003c/sub\u003ebinding energies associated with the metal-P bond in samples NiCoP-1 and NiCoP-3 are 129.27 eV, 130.10 eV and 129.82 eV, 130.74 eV, respectively. A positive P 2p binding energy shift with increased Co content suggests electron transfer from P to Ni and Co atoms. This situation is unexpected, especially when we examine the shifts in Ni and Co. In this context, P appears to acquire additional electron density from the presence of Ni and Co. However, at the same time, P is subjected to oxidation due to various environmental factors. This dual behavior indicates a complex interplay between the electron donation from the transition metals and the oxidative processes influenced by the surrounding conditions, resulting in a more nuanced effect on chemical kinetics. This electron redistribution, especially the electron donation characteristic of Ni in NiCoP-2 and rapid electron transfer by incremental Co, results in enhanced catalytic performance for HER (\u003cstrong\u003eFigure 2d\u003c/strong\u003e) [55,56].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization of Electrochemical Performance\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSimultaneously, the prepared NiCoP catalysts were tested to evaluate the catalytic HER activity in a 1 M KOH (pH=14). Now it is generally accepted that pH is 14 for alkaline hydrogen evolution test. Furthermore, at pH 14, the OH concentration was very high and more conducive to hydrogen evolution reaction[57,58]. Based on the three-electrode system, the platinum foil was used as the opposite electrode, and Hg/HgO as the reference electrode. NiCoP-1, NiCoP-2, NiCoP-3, bare NF electrodes, and Pt/C (20%) electrodes were characterized and compared. The polarization curves recorded by linear scanning voltammetry (LSV) are shown in \u003cstrong\u003eFigure 3a\u003c/strong\u003e. All polarization curves were subjected with iR compensation. \u003cstrong\u003eFigure 3b\u003c/strong\u003e reveals that at a current density of 200 mA cm\u003csup\u003e-2\u003c/sup\u003e, NiCoP-2 possesses the smallest overpotential (218 mV), which is lower than that of Pt/C (224 mV), NiCoP-1 holds the second position (262 mV), and NiCoP-3 has the largest overpotential (268 mV). The slope of the Tafel curve, derived from the strong polarization region, provides crucial insights into the kinetic processes involved in hydrogen evolution at the electrode. A smaller Tafel slope, as determined through linear fitting, signifies a reduced change in overpotential with increasing current density, indicative of enhanced electrocatalytic performance. Moreover, variations in Tafel slope values can be associated with different steps involved in the HER. Specifically, a lower Tafel slope suggests that the rate-determining step occurs at the double electron transfer reaction within the HER mechanism. This relationship implies that a lower slope correlates with a faster Tafel step, further validating the efficiency of the electrocatalyst under investigation. As shown in \u003cstrong\u003eFigure 3c\u003c/strong\u003e, the Tafel slope of NiCoP-2 (124 mV dec\u003csup\u003e-1\u003c/sup\u003e) is lower than that of NiCoP-1 (133mV dec\u003csup\u003e-1\u003c/sup\u003e), NiCoP-3 (142 mV dec\u003csup\u003e-1\u003c/sup\u003e) and NF (217 mV dec\u003csup\u003e-1\u003c/sup\u003e), indicating that the HER mechanism of NiCoP-2 is the Volmer-Heyrovsky mechanism[59,60]. The charge transfer rate of the electrode was evaluated using electrochemical impedance spectroscopy, which provides insights into the resistive components affecting electrochemical processes. In this context, the charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e) is particularly significant, reflecting the resistance at the interface between the electrode and the electrolyte. A lower R\u003csub\u003ect\u003c/sub\u003e indicates enhanced charge transfer capability, essential for optimizing catalytic activity. As illustrated in \u003cstrong\u003eFigure 3d\u003c/strong\u003e, the NiCoP-2 electrode demonstrates the smallest charge transfer resistance, with an R\u003csub\u003ect\u003c/sub\u003e value of 3.95 \u0026Omega;. This superior performance positions NiCoP-2 as the most effective among the electrodes tested in terms of charge transfer ability. Through EIS, we observed that the conductivity of NiCoP varies with increasing cobalt content, highlighting modifications in the electronic structure. Concurrently, XPS results indicated that variations in cobalt content also influence the electronic structure. Although charge transfer processes may influence binding energy, they do not fully account for the observed variations in binding energy, other factors, such as surface electron density and local potential, play a significant role. By comparing Tafel and EIS, NiCoP-2 has the lowest Tafel slope and the lowest R\u003csub\u003ect\u003c/sub\u003e, indicating that with the introduction of incremental Co, NiCoP-2 has the most favorable electronic structure and the fastest electron conduction rate, which accelerates the hydrogen evolution kinetics and greatly reduces the hydrogen evolution overpotential. Then, previously reported NiCo-based materials for HER are listed to evaluate the electrocatalytic properties (\u003cstrong\u003eTable 1\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eTo relate structural properties to intrinsic activity, double layer capacitance values (C\u003csub\u003edl\u003c/sub\u003e) are used to reflect the electrochemical active surface area (ECSA) level of the material [61]. C\u003csub\u003edl\u003c/sub\u003e values for different samples were calculated using cyclic voltammetry (CV) curves. \u003cstrong\u003eFigure S4\u003c/strong\u003e shows the CV curves of NiCoP-1, NiCoP-2, NiCoP-3, and bare NF. Due to the proportional relationship between C\u003csub\u003edl\u003c/sub\u003e and ECSA, this can be used to assess the active surface area of the electrodes. From \u003cstrong\u003eFigure 3e\u003c/strong\u003e, the C\u003csub\u003edl\u003c/sub\u003e value of NiCoP-2 (67 mF dec\u003csup\u003e-2\u003c/sup\u003e) is relatively higher than that of NiCoP-1 (62 mF dec\u003csup\u003e-2\u003c/sup\u003e), NiCoP-3 (43 mF dec\u003csup\u003e-2\u003c/sup\u003e) and bare NF (3.5 mF dec\u003csup\u003e-2\u003c/sup\u003e). In comparison, the C\u003csub\u003edl\u003c/sub\u003e values of NiCoP-1 and NiCoP-2 are not much different, and that of NiCoP-3 is slightly smaller, indicating that the main reason affecting the performance at this time is the influence of electronic interaction. In addition to evaluating the HER activity of the catalyst, the stability of the electrode is also one of the most effective means of determining the application value of the catalyst. \u003cstrong\u003eFigure 3f\u003c/strong\u003e shows the trend of the electrode potential overtime at a current density of 100 mA cm\u003csup\u003e-2\u003c/sup\u003e. After conducting the HER test for 120 h, we observed minimal decay in the overpotential of the electrode, measuring only a 10 mV increase at a current density of 100 mA cm\u003csup\u003e-2\u003c/sup\u003e. This limited change suggests that the active sites of the catalyst remain stable and well-maintained throughout the duration of the test, highlighting the durability and effectiveness of the catalyst in sustaining performance under continuous operation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e－Comparison of the HER activity of NiCoP-2 with other electrocatalysts\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eNumber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eCatalysis\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003eElectrolyte\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e\u0026eta;@j ( mA cm\u003csup\u003e-2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003eRef\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNiCoP-2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e108 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003eThis work\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eCo-P11/Cu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e98.8 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[62]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eMoS\u003csub\u003e2\u003c/sub\u003e/rGO(2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e176mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[63]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNiCo-MOF-P\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e191 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[64]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNi\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e@NiS/Ni-Net-2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e207 mV@100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[65]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNiCo-MOF-rod\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e125mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[66]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNiCoP/CoP\u003csub\u003e2\u003c/sub\u003e-SCBC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e159.5 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[67]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNiCoP@NC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e77 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[68]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eP/NiCoP/NF\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e125 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[69]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNi@CoP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e181 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[70]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 167px;\"\u003e\n \u003cp\u003eNiCo-NiCoP@PCT/CC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 89px;\"\u003e\n \u003cp\u003e1 M KOH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 147px;\"\u003e\n \u003cp\u003e135 mV@10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e[71]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eAfter 120 h of stability test, we recorded the changes in the morphology, physical phase, and catalytic properties of the catalyst. The results of SEM and XRD after the stability test showed in \u003cstrong\u003eFigures 4a-c\u003c/strong\u003e, the surface-growing nanosheets became larger and thicker after the long-term stability test. However, on the whole, the original morphology is still maintained, and the phase of the material has not changed significantly, which once again confirms its good durability. From the XPS spectra after stability, it can also be seen that there is no significant change in the chemical state of the material before and after testing (\u003cstrong\u003eFigures 4d-f\u003c/strong\u003e). After a 120 h stability test, the XPS spectra show that the surfaces of NiCoP-2 also contain Ni, Co, P, C, and O elements (\u003cstrong\u003eFigure S5\u003c/strong\u003e). In addition, the Si 2p (~103 eV) and Si 2s (~153 eV) peaks observed in the spectra suggest the presence of silicon, which may arise from environmental contamination or the silicon substrate used for catalyst deposition. It can be observed that the M-P peak in the P 2p XPS spectrum is strengthened and the P-O bond is attenuated compared to that before the stability test, suggesting that the oxidation of metal phosphates is mitigated due to the continuous hydrogen reduction test. The enhancement of HER activity is closely associated with the intrinsic activity determined by electronic structure, charge and mass transfer efficiency, and the stability of the active substance. The NiCoP-2 catalyst possesses a more appropriate electronic structure, stable structural properties, and higher conductivity, and exhibits good HER activity with the proper addition of Co.\u003c/p\u003e\n\u003cp\u003eIn addition, The contents of various elements in the NiCoP-2 samples were analyzed and compared before and after stability testing. The specific content comparison is presented in \u003cstrong\u003eTable S1\u003c/strong\u003e. Notably, the Ni content significantly increases after stability testing, indicating a rise in the surface concentration of Ni during the constant current test. This phenomenon may be attributed to the enrichment of Ni on the catalyst surface, as it is likely to form active sites with other substances during the hydrogen evolution reaction (HER), thus enhancing its concentration in the surface active layer. The increased presence of Ni aids in catalyzing the reaction, suggesting that Ni plays a more critical role in the catalytic process[72]. In contrast, the change in Co content is minimal, which may imply that Co\u0026apos;s contribution to the catalytic reaction is less stable than that of Ni. Alternatively, Co might be more active during the reaction and more susceptible to dissolution or surface migration[73]. Another key change observed is a significant reduction in phosphorus, which may indicate a change or partial removal of phosphide on the catalyst surface. The loss of phosphorus could impact the stability of the catalyst, particularly during long-term electrocatalysis[74,75]. This removal may alter the active site on the catalyst surface, consequently affecting its hydrogen evolution activity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTheoretical calculation into HER\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA stable NiCoP (201) surface model was applied based on the observed lattice planes from XRD patterns and TEM images, as shown in \u003cstrong\u003eFigure 5a\u003c/strong\u003e. The electron density of Ni and Co atoms was further verified by constructing the corresponding structural model of NiCoP. Bader charge analysis in Figure 5a shows that the average electron density of Ni (Ni1 and Ni2) atoms at the NiCoP (201) interface decreases by 0.22e, indicating that significant electron redistribution can provide rapid electron transfer, and then adjust the adsorption capacity of intermediates during the electro catalytic reaction. The Gibbs free energy (\u0026Delta;GH*) of surface hydrogen adsorption at different sites of NiCoP was determined using density functional theory (DFT). The ideal hydrogen evolution reaction electrocatalyst surface should have the best affinity, and the hydrogen atom adsorption free energy can be used to evaluate the HER activity of the electrocatalytic material. When the hydrogen adsorption free energy approaches zero, that is, \u0026Delta;GH*=0, it is conducive to hydrogen adsorption and desorption, and the catalytic activity is optimal, which is used to explain the bimetallic synergistic sites enhancing the electrocatalytic performance of NiCoP on HER. The atomic hydrogen adsorption free energy at three sites on the NiCoP (201) surface is shown in \u003cstrong\u003eFigure 5b\u003c/strong\u003e. The \u0026Delta;G\u003csub\u003eH*\u003c/sub\u003e on the surfaces of Ni, Co, and Ni-Co(201) were calculated to be -0.66, -0.60, and 0.42 eV, respectively. Compared with the other two sites, the hydrogen adsorption free energy of bimetallic synergistic Ni Co is closer to the thermal neutral point, indicating that the bimetallic site Ni Co has the highest catalytic activity for HER, which is consistent with the experimental results. The negative free energy of Ni and Co indicates that the adsorption process is prone to occur, and the desorption process is a rate determining step. Conversely, the free energy of Ni-Co shows a positive value, indicating that the adsorption process (Volmer step) is a rate determining step. Therefore, the bimetallic site Ni-Co leads to changes in electronic structure and a significant decrease in \u0026Delta;G\u003csub\u003eH*\u003c/sub\u003e value, thereby enhancing HER activity.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study successfully synthesized a high-performance HER electrocatalyst, NiCoP-2, using a solvothermal method to grow a bimetallic MOF precursor in situ on nickel foam, followed by low-temperature phosphorization. By regulating the ratio of Ni and Co during synthesis, the material exhibited abundant catalytic sites, enhanced diffusion channels, and reduced charge transfer resistance, resulting in excellent electrocatalytic performance. Incorporating incremental Co into NiCoP improved electrical conductivity and optimized the electronic structure, significantly enhancing electrocatalytic activity. The electrochemical analysis demonstrated that NiCoP with the optimal Co content exhibited a reduced overpotential, faster charge transfer, and superior performance. The incorporation of incremental Co modulated the electronic structure of NiCoP, as evidenced by significant shifts in the binding energies of Ni 2p and Co 2p, confirmed through XPS analysis. These shifts, driven by synergistic interactions between Ni and Co, enabled charge redistribution and enhanced catalytic activity. Electrons primarily transferred from incremental Co to Ni and/or P. This electronic state modulation created an optimal electron-rich/electron-deficient balance for hydrogen evolution reactions. Overall, this study highlights that tuning the composition of NiCoP through Co incorporation effectively enhances its catalytic activity and stability, paving the way for advanced electrocatalysts for hydrogen production.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupplementary Materials:\u003c/strong\u003e The online version contains supplemen tary material available at: https://doi.org/10.1007/xxx.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e Conceptualization, A.S. and G.S.; methodology, G.W. and C.H.; validation, W.G. and W.Z.; formal analysis, W.G. and D.S.; investigation, J.Y.; data curation, Z.F. and J.Z.; writing\u0026mdash;original draft preparation, G.W., C.H. and W.Z.; writing\u0026mdash;review and editing, A.S., W.Z., H.D. and Z.M.; resources, A.S. Z.M. and G.S.;supervision, A.S. and G.S. All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eThis work was financially supported by the Hebei Provincial Department of Human Resources and Social Security (C20230328), the Natural Science Foundation of Hebei Province (E2023203209), the Natural Science Foundation of Hebei Province (B2021203016), State Key Laboratory of High Performance Complex Manufacturing (22567616H).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e No datasets were generated or analysed during the current study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest:\u0026nbsp;\u003c/strong\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFan J, Fu C, Liang R, Lv H, Fang C, Guo Y, Hao W. 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Journal of Power Sources 2021;516:230657\u0026ndash;67.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang H, Aierke A, Zhou Y, Ni, Z, Feng L, Chen A, Wagberg T, Hu G. A high-performance transition-metal phosphide electrocatalyst for converting solar energy into hydrogen at 19.6% STH efficiency. Carbon Energy 2023;51:217\u0026ndash;29.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Scheme","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\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":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Metal-organic framework, Transition metal phosphide, Lamellar structure, Electronic regulation, Hydrogen evolution reaction","lastPublishedDoi":"10.21203/rs.3.rs-7545454/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7545454/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDeveloping high-performance electrocatalysts for the hydrogen evolution reaction (HER) is crucial for advancing hydrogen production in clean energy systems. In this study, we investigated the role of incremental Co incorporation in nickel-cobalt phosphide (NiCoP) and its influence on the charge transfer efficiency and reaction kinetics of HER. Electrochemical analyses, including Tafel slope measurements and electrochemical impedance spectroscopy (EIS), reveal that incremental Co-based NiCoP with an optimal Co content exhibits a reduced overpotential and faster charge transfer, providing the most pronounced improvements. The XPS results corroborated the experimental findings, indicating that increased Co strengthened the electronic structure interaction among Ni, Co, and P in NiCoP. The availability of Co atoms and the low electronegativity promoted the electron flow to Ni and/or P, accelerated electron migration, and improved HER dynamics. This research demonstrates that adjusting the composition of NiCoP by increasing the incorporation of Co is a practical approach to enhance the catalytic activity and stability in HER applications.\u003c/p\u003e","manuscriptTitle":"Incremental Co Enhances Charge Transfer Efficiency and Accelerates Hydrogen Evolution Kinetics in NiCoP","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-18 12:04:26","doi":"10.21203/rs.3.rs-7545454/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-05T10:40:23+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-20T06:43:58+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-17T03:09:23+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-16T06:05:54+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"167624716162794091887122131658083746421","date":"2025-09-15T01:16:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"291549738299011082522684417362001364839","date":"2025-09-14T13:07:47+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"153303748480404781792313317121337849867","date":"2025-09-14T04:52:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"159971305670231162130271057263891906317","date":"2025-09-13T12:16:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"185703428913124190463606805458680145472","date":"2025-09-11T16:47:56+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-11T13:29:10+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-11T07:42:30+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-09-11T07:42:09+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2025-09-05T14:56:51+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"de1cf935-9883-4c6e-a6d0-c403d87febe1","owner":[],"postedDate":"September 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-11-09T21:53:22+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-18 12:04:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7545454","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7545454","identity":"rs-7545454","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}