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. 2026 Feb 17;11(35):52399–52410. doi: 10.1021/acsomega.5c10265

Mono- and Bimetallic Nickel–Cobalt Metal–Organic Framework Electrocatalysts for Overall Water Splitting

Çağdaş Yavuz †, Henrik Haspel †,‡, Ákos Szamosvölgyi †, Robert Vajtai §,*, Zoltán Kónya †,‡
PMCID: PMC13564021  PMID: 42730194

Abstract

The use of carbon-neutral energy carriers could reduce the ecological footprint of some of the most polluting industries (e.g., ammonia, cement, and steel production). A promising energy carrier is “green” hydrogen, i.e., H2 produced by electrochemical water splitting driven by electricity from renewable sources. Metal–organic frameworks (MOFs) are constructed by the assorted coordination of metal clusters and polytopic organic linkers. MOFs containing 3d transition metals, like Fe, Co, and Ni, can be active catalysts in the hydrogen evolution reaction (HER) and/or the oxygen evolution reaction (OER). In this study, we fabricated nickel-, cobalt-, and bimetallic nickel- and cobalt-metal–organic frameworks (Ni/Co-MOFs) by a simple wet-chemical method and deposited them onto porous nickel foam (NF) and carbon paper substrates via drop-casting and electrochemical deposition. The alkaline water splitting activity of the electrodes was studied in a standard 3-electrode electrochemical cell by cyclic voltammogram, linear sweep voltammetry, electrochemical impedance spectroscopy, etc. The bimetallic Ni/Co-MOF drop-cast on NF showed the highest activity and the lowest overpotential among the investigated samples. Onset potentials of 1.52 and 0.24 V (vs reversible hydrogen electrode) at current densities of 20 and 10 mA cm–2 were obtained in OER and HER, respectively, maintaining stable oxygen and hydrogen evolution for 50 h without any deterioration in the catalytic performance. In two-electrode overall water splitting, the NF-supported Ni/Co-MOF exhibits high HER and OER activity and enables efficient overall water splitting, requiring only 1.41 V to reach 10 mA cm–2, demonstrating its practical electrocatalytic applicability.

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1. Introduction

The advancement of clean and sustainable energy processes, such as metal-air batteries, solar cells, and water electrolysis, has become exceedingly critical during the past decades as a result of pressing environmental issues. Although renewable energy sources, such as solar, wind, wave, and geothermal, have the capacity to provide solutions to existing energy and environmental problems, their continuous and efficient applicability is limited due to climatic and geographical conditions. In recent decades, hydrogen has been recognized as a potential energy carrier that might replace conventional fossil fuels due to its zero-carbon emission and three times higher energy density (120 MJ/kg) than that of gasoline (44 MJ/kg).

At present, there are several well-established carbon-intensive methods for generating hydrogen, like coal gasification, partial oxidation of methane, or steam reforming. Nevertheless, electrochemical water splitting driven by renewable energy has gained significant attention as the hydrogen production of the future. The two half-reactions in electrochemical water splitting are hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the cathode and anode, respectively. The complicated four-electron transfer of OER, its slow kinetics, and large overpotential, brought on by a complex mechanism including a multistep proton-coupled electron transfer, makes OER the bottleneck process in water splitting. Noble-metals and their compounds are known electrocatalysts in OER (e.g., RuO2 and IrO2) and HER (Pt group metals, PGM), but their high cost, scarcity, and limited availability prevent them from being commercialized on an industrial scale. Nonprecious transition metal-based electrocatalysts have been actively developed as potential replacements for costly PGM catalysts.

Metal–organic frameworks (MOFs) have controlled crystalline architectures with a porous structure by the coordination of metal clusters or nodes with polytopic organic linkers. They have drawn interest due to the numerous potential applications in gas storage and separation, energy storage, catalysis, electrocatalysis, and drug delivery. For electrochemical energy storage and conversion, such as in batteries, supercapacitors, and fuel cells, MOFs are potential electrode materials owing to their large surface area and high micropore volume, along with numerous active sites for redox reactions. MOF porosity, however, not only provides high specific surface area (SSA), but the homogeneously distributed metal nodes in the structure serve as potential catalytically active sites.

The main limiting factors of MOF applications are their poor ion transport characteristics, low stability, and comparatively low conductivity. Several methods have been developed to increase the catalytic performance of MOFs. A few of the practices are photoinduced lattice strain, low-dimensional nanocrystallization, multimetal doping, MOF hybridization, phase transition, and controlling the active sites. The 3d transition metal (TM = Fe, Co, Ni) nodes in MOFs improved HER and OER activity in Ni-, Fe-, NiCo-, and NiFe-MOFs. In particular, NiCo-MOFs perform better than other monometallic MOFs due to the electrical collaborative impact of bimetallic metal centers. Zeolitic imidazolate frameworks (ZIFs) are an MOF subclass of more than 150 members, where tetrahedral metal cations are connected by imidazolate (IM) linkers. Their structure resembles those of aluminosilicate zeolites, as the M-IM-M angle is close to that of the Si–O–Si angle. ZIFs combine the advantages of MOFs and zeolites, like high SSA and porosity along with thermal and chemical stability.

In this study, electrocatalyst electrodes of mono- and bimetallic nickel/cobalt-metal–organic frameworks (Ni-, Co-, Ni/Co-MOFs) were constructed and tested in alkaline HER and the OER. Electrodes were fabricated by (1) drop-casting (DC) and (2) electrochemical deposition (ECD) onto Ni foam (NF) and carbon paper (CP) substrates. Porous carbon gas-diffusion layers are traditionally used in electrocatalytic systems, like electrolyzers and fuel cells, whereas three-dimensional (3D) metal foams offer a large surface area with a highly conducting porous network, and are an emerging class of gas-diffusion substrates. The optimized drop-casted Ni/Co-MOF/NF exhibited an overpotential of 296 and 245 mV at 20 and 10 mA cm–2 in a 1 M KOH electrolyte for the OER and HER, respectively.

2. Experimental Section

2.1. Materials

Nickel nitrate hexahydrate (Ni­(NO3)2·6H2O, 99%), cobalt­(II) nitrate hexahydrate (Co­(NO3)2·6H2O, ≥98%), 2-methyl imidazole (C4H6N2, 99%), and methanol were used for MOF synthesis. All chemicals were purchased from Sigma-Aldrich and used without any further purifications. Electrodes were fabricated on nickel foam (NF, Recemat BV) and CP gas-diffusion layers (CP, Quintech Freudenberg H23C2).

2.2. Synthesis of Mono- and Bimetallic MOFs

Numerous ZIF-type materials can be solvothermally synthesized by using various transition metals and a 2-methylimidazole (2-mIM) linker at high temperature and pressure. Here, we synthesized a Ni-MOF by a one-step method at room temperature, where 2-methyl imidazole (2-mIM) was used as the organic ligand and nickel­(II) nitrate hexahydrate as the Ni precursor. First, solutions of 1.32 g of 2-mIM and 1.16 g of Ni-salt were prepared separately in 30 mL of MeOH under vigorous stirring, mixed until complete dissolution, and stirred at room temperature for 24 h. The Ni-ZIF product was filtered and washed by methanol and DI water several times and then dried at 60 °C for 12 h. The same method was followed for Co-MOF (ZIF-67) synthesis using 1.16 g of cobalt­(II) nitrate hexahydrate as Co-source and 1.32 g of 2-mIM as the organic ligand. Ni/Co-MOF was prepared with a Ni/Co atomic ratio of 1:1 using the same precursors.

2.3. Preparation of MOF Electrodes on NF and CP Substrates

Two deposition methods were used to fabricate the electrodes on NF and CP. For DC, an ink was prepared by suspending 5 mg of MOF in a solution of 600 μL of DI water, 350 μL of IPA, and 50 μL of Nafion (20%) in an ultrasound bath for 30 min. Afterward, 100 μL of ink was drop-casted onto precleaned NFs and CPs, and the substrates were dried at 50 °C. In ECD, the filtrate of the final separation was directly used as an electrolyte in a two-electrode cell using NF or CP as the working and Pt counter electrode at 5 V and 2 A for 5 min. The schematics of the coating methods is given in Figure .

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Schematics of MOF synthesis and the subsequent coating processes.

2.4. Materials Characterization

X-ray diffraction (XRD) patterns were recorded using a Rigaku Miniflex-II X-ray diffractometer (Co Kα radiation (6.9257 keV), λ = 1.7902 Å) operated at 30 kV and 30 mA with a scanning rate of 5° min–1 over a 2θ range of 5–80°. Raman spectra were measured with a Bruker Senterra II Raman microscope by using a 532 nm laser at 12.5 mW power. FT-IR spectroscopy was carried out with a Bruker Vertex 70 instrument (16 scan s–1, 4 cm–1 resolution) in the wavenumber range of 4500–440 cm–1 using KBr pastille. The morphology and chemical composition of the samples were investigated in a Thermo Fisher Apreo C scanning electron microscope (SEM) using an accelerating voltage of 10 kV and with a Bruker XFlash 6-30 energy-dispersive X-ray spectroscopy (EDS) detector. Additional elemental analysis was carried out via X-ray fluorescence (XRF) using an Olympus Vanta VMR-CCX spectrometer with a 40 keV excitation beam. The diffuse reflectance spectra of the catalysts were recorded in the 200–800 nm wavelength range by using a JASCO-V650 spectrophotometer equipped with an integration sphere (ILV-724). The SSA and the pore size distribution of the materials were determined via N2 adsorption by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda methods with a Quantachrome NOVA 3000e surface area analyzer. The thermal behavior of the samples was examined by thermogravimetric analysis (TA Instruments Q500 TGA) under air and nitrogen atmospheres at a heating rate of 5 °C min–1. The elemental composition and the oxidation state of the elements in the MOFs before and after the OER were investigated by X-ray photoelectron spectroscopy (XPS). Powdered materials were pressed onto double-sided carbon tapes attached to stainless steel sample holders. The samples were analyzed in a Specs XPS instrument equipped with an XR50 dual anode X-ray source and a Phoibos 150 hemispherical electron analyzer. The samples were irradiated with Mg Kα X-ray, operated with 150 W power (12 kV). An electron flood gun was operated during data acquisition for all measurements to negate charging of the sample surface. Survey spectra were collected with a pass energy of 80 and 1 eV step size, while high-resolution spectra were collected with a pass energy of 20 and 0.1 eV step size. The Casa XPS software package version 2.3.26rev1.2Y was used for data analysis. For all high-resolution spectra, Shirley background correction was used. All peaks were fitted with a Gauss–Lorentzian product function, where the Lorentzian contribution is 0.3, unless otherwise stated. The aliphatic component of the C 1s spectra at 284.8 eV was used as an inner binding energy reference. All of the carbon detected is adventitious carbon.

2.5. Electrochemical Analysis

The electrochemical measurements were performed in a three-electrode system with 1.0 M KOH (pH 13.8) as the electrolyte using an Autolab PGSTAT302N electrochemical workstation at room temperature. A 1 × 1 cm2 MOF-coated NF or CP electrode was the working electrode, whose electrode potential was measured against a Ag/AgCl (saturated KCl) reference electrode, and a platinum spiral served as the counter electrode. Nernst-equation was used to transform the measured potentials to the reversible hydrogen electrode (RHE) scale (E RHE = E Ag/AgCl + 0.195 V + 0.059 × pH), where E Ag/AgCl is the electrode potential of the reference electrode, and corresponding overpotentials (η) were obtained by using the formula η­(OER) = E RHE – 1.23 V and η­(HER) = E RHE. The catalytic performance was evaluated by linear sweep voltammetry (LSV) at a sweep rate of 10 mV s–1 in the 0.1 to −0.6 V (vs RHE) and 1 to 1.9 V (vs RHE) potential ranges for the HER and OER, respectively. The MOF/substrate interface in the electrode was characterized by electrochemical impedance spectroscopy (EIS) recorded from 1 MHz to 0.1 Hz with an excitation potential of 10 mV (RMS). The high-frequency resistance from the EIS spectra was used to correct the raw potential values for the uncompensated solution resistance (iR-correction). Long-term electrolyses were carried out at the constant current density (chronopotentiometry (CP)) for 50 h. OER and HER kinetics were assessed by the corresponding Tafel slope (TS) computed via fitting the linear part of the LSV curves by the equation: η = b log j + a, where η is the overpotential, b is the Tafel slope, j denotes the current density, and a is a constant. In general, a smaller Tafel slope implies a steeper increase in current density with increasing driving force, i.e., potential. Electrochemically active surface area (ECSA) was determined from the electrochemical double-layer capacitance (C dl) of the electrode/electrolyte interface. Recording scan-rate-dependent cyclic voltammograms (CVs) between 10 and 500 mV s–1, the non-Faradaic capacitive current associated with double-layer charging was measured in the potential range from 0.1 to 0.40 V (vs RHE), where no discernible Faradaic processes take place. ECSA was calculated from the electrochemical capacitance using the following equation:

ECSA=double−layercapacitance(Cdl)specificcapacitance(Cs)cm2 1

where C s is the specific capacitance of the ideal surface in 1 M KOH (0.04 mF cm–2), and C dl is the double-layer capacitance of the electrode.

3. Results and Discussion

3.1. Structure and Morphology

The crystal structure of the MOFs was studied by powder XRD. Fe, Co, and Mo-rich samples have strong absorption around the Cu Kα radiation, but the resulting intense fluorescence and high background-to-signal ratio can be eliminated by using a Co X-ray source. Thus, XRD measurements were carried out with a Co Kα source, and raw diffractograms were converted to Cu Kα wavelength for the ease of comparison. XRD patterns for Ni-, Co-, and Ni/Co-MOFs, along those of the MOF-coated NF and CP electrodes, are shown in Figure .

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XRD patterns of MOFs (a) and the MOF-coated NF (b) and CP (c) electrodes.

All samples were crystalline, and the reflections of Ni-MOF at 2θ = 11.2, 22.5, 33.4, 33.9, and 38.3° agree well with those reported earlier on Ni-ZIF structures, while peaks at 2θ = 7.2, 10.3, 12.6, 14.6, 16.3, 17.9, 22.0, 24.4, 25.5, 26.6, 29.5, 31.4, and 32.3° of Co-MOF correspond to the (011), (002), (112), (022), (013), (222), (114), (233), (002), (134), (044), (244), and (235) planes of the MOF-67 structure (Figure a). According to the reference card JCPDS No. 004-0850, the peaks at 2θ = 43.9, 51.2, and 75.8° in the NF substrate correspond to cubic Ni(111), (200), and (220) reflections. The sharp peak at 2θ = 18.1° originates from the (100) planes of the PTFE used to treat the GDL, while the diffuse reflection at 2θ = 25.6° is assigned to the (002) reflection of the graphitic part of the microporous layer on the CP. The patterns appearing between 2θ = 8–20° in MOF-coated CP electrodes correspond to those found for the pure materials and prove the successful coating of the substrate. The drop in the PTFE (100) relative intensity in the MOF/CP electrodes in Figure c is due to the MOF layer deposited on top of the carbon powder. The weak XRD peaks of the MOF-coated NF and CP electrodes primarily arise from the combination of low MOF loading, nanoscale crystal effects, and substrate signal interference. Both NF and CP exhibit strong background reflections or broad peaks due to their metallic and graphitic nature. These intense substrate reflections may overlap or suppress the diffraction peaks of the MOF layer. XRD measurements of the MOF-coated NF substrates were performed at a low scanning rate (0.2° min–1) over a 2θ range of 5–20° to enhance signal resolution in the low-angle region. The corresponding diffraction patterns are presented in the inset of Figure b. The average crystallite size in the perpendicular direction to the lattice planes (d) in Ni- and Co-MOF was estimated by using the Scherrer-equation:

d=Kλβcos⁡Θ 2

where K is a shape factor (K = 0.94), λ is the X-ray wavelength, b is the FWHM, and θ is the Bragg angle of the peak center. Crystallite sizes of (15.1 ± 1.3) and (26.5 ± 1.5) nm were calculated for Ni- and Co-MOF, respectively.

The morphologies of Ni-MOF, Co-MOF, and Ni/Co-MOF can be seen in the SEM images in Figure a–d. Ni-MOF has a polyhedral shape with a diameter of 3–5 μm (Figure a), while the Co-MOF shows a rounded morphology of 1 μm diameter with some cuboid crystals in the sample (Figure b). Ni/Co-MOF particles possessed a thin, flakelike morphology as shown in Figure c,d. SEM images of the catalysts coated on NF are displayed in Figure S2. These images clearly demonstrate the uniform coverage and good adhesion of the MOF layers on the NF substrate, confirming the deposition of the active materials.

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SEM images of (a) Ni-MOF, (b) Co-MOF, (c,d) Ni/Co-MOF, (e) elemental mapping image of Ni/Co-MOF, (f) EDS spectrum of Ni/Co-MOF with elemental composition in at %.

The elemental composition of Ni/Co-MOF was determined by EDS in Figure e,f (spectra for the single metallic MOFs are presented in Figure S1d,e), where elemental mapping in Figure e shows the presence of Ni and Co and the uniform distribution of Ni and Co throughout the bimetallic sample; in addition, Figure S1f represents the elemental mapping of O, N, and C. The relative at % of the constituents are summarized in the inset table of Figure f. Additional XRF analysis was employed to determine the elemental composition and Ni/Co ratio by an independent method in Ni/Co-MOF. Results are shown in Figure S1a–c and compared to those calculated from the EDS measurements in Table S1. Both EDS and XRF analyses show the composite formation with a ratio of Ni/Co ≈ 1.2 (1.24 from EDS and 1.22 from XRF).

Diffuse reflectance UV–vis spectroscopy was employed to investigate electronic transitions in the MOFs (Figure S3). These processes are well-studied in ZIF-67 (Co-MOF), where three absorption peaks were found in the visible range at around 580, 560, and 540 nm along with a rising absorption toward the IR region, like in earlier reports on Co-ZIF67 structures. In tetrahedral Co­(II)-complexes, one absorption band is seen in the visible range [4A2(F)–4T1(4P) transition] and two more can be found in the near-IR range [4A2(F)–4T1(4F) and 4A2(F)–4T2(4F) transitions, out-of-range here] attributed to ligand-to-metal charge transfer (LMCT), both split by spin–orbit coupling. The absorption characteristics of Ni/Co-MOF are similar to that of the Co-MOF with a minor red shift and decreased absorbance due to the dilution of Co2+ in the bimetallic structure like it was found in a Zn–Co-ZIF. The Ni-MOF showed three complex peaks in the region of 200–350 nm, 350–500 nm, and 550–800 nm as it is known for Ni-2MiM-MOFs. The rising absorbance below 400 nm is characteristic to ZIF structures due to strong LMCT, while imidazole compounds also possess strong absorption in the 200–350 nm region.

The N2 adsorption–desorption analyses in Figure S4 were performed to evaluate the SSA, pore volume, and average pore size (Table S2) of the as-synthesized Ni-, Co-, and Ni/Co-MOFs, which are critical parameters influencing the electrocatalytic activity. A higher surface area and well-developed porosity enhance the accessibility of active sites, facilitate electrolyte penetration, and improve charge and mass transport during the OER and HER processes. The isotherms can be identified as type IV with H3-type hysteresis loops, confirming the presence of mesopores. The Co-MOF sample shows a significantly higher SSA of 1185 m2 g–1 compared to those of the Ni-MOF (18 m2 g–1) and the Ni/Co-MOF (26 m2 g–1), in conjunction with the highest microporosity and total pore volume of 0.635 cm3 g–1 among the studied materials. Care must be taken in the determination of the BET surface area of MOFs as a round-robin study found enormous spread in SSA values calculated from the same raw isotherms, whereas the BET method can only be utilized for microporous materials with restrictions. Furthermore, although the desorption branch of a N2 adsorption isotherm should fall back onto the adsorption branch at p/p 0 < 0.42 at 77 K, the hysteresis persists in the low relative pressure region for all studied materials. The occurrence of “open isotherm” or “low-pressure hysteresis” (LPH) is known in a wide range of materials, like in MOFs or nanoporous glasses, which can be caused by the structural flexibility or adsorbent rearrangement in genuine LPH or insufficient equilibration or degassing time in nongenuine LPH.

The possible instability of MOFs could have led to the collapse or irreversible restructuring of the internal porosity during solvent removal (activation) or drying, likely due to the loss of coordinated solvent molecules. Such degradation is frequently encountered in MOFs that are highly sensitive to thermal or vacuum treatments. This hypothesis is supported by thermogravimetric analyses (Figure S5), which confirm the higher thermal durability of the Co-MOF. Furthermore, SEM images reveal large particles for the Ni-MOF and an amorphous morphology for the Ni/Co-MOF, consistent with structural collapse or the formation of a nonporous phase. In contrast, the high surface area of Co-MOF is supported by SEM images that show small, distinct, and well-defined particles.

Thermal stability of the samples was investigated by thermogravimetry analysis (TGA) under air (Figure S5a) and N2 (Figure S5b). The low-temperature weight losses are the removal of physisorbed water and methanol used as solvents in the synthesis and trapped in the structure and pores of the MOFs. Pure 2-mIM linker undergoes a single-step degradation between 100 and 200 °C under both atmospheres in Figure S5, which causes the major weight loss in MOFs above 300 °C, resulting in the collapse of the MOF structures. Similar increase in the degradation temperatures compared to that of 2-mIM was found in Cu- and Ni-imidazole complexes. The stability increases in the Ni/Co-MOF < Ni-MOF < Co-MOF order under both inert and oxidizing atmospheres. However, the stability of Co-MOF is notably higher under nitrogen than in air, whereas the thermal stability of the Ni-MOF and Ni/Co-MOF is not influenced by the atmosphere to that extent.

FT-IR spectra in Figure S6a confirmed the incorporation of organic ligands and functional groups in the synthesized MOFs. All samples exhibited characteristic bands in the 650–1700 cm–1 region, corresponding to stretching and bending vibrations of the imidazole ring. For 2-mIM, absorption peaks at 1675, 1594, and 1302 cm–1 were assigned to the N–H, CN, and C–N bonds, respectively. These features were also present in Ni-MOF, Co-MOF, and Ni/Co-MOF, although with slight shifts and intensity variations. The reduced intensity of the CN stretching (1594 cm–1) and out-of-plane bending mode (755 cm–1) further supported the coordination of the imidazolate group with Ni or Co, leading to the formation of metal–nitrogen bonds. Additional peaks at 680 and 740 cm–1 were assigned to aromatic C–H stretching, while the absorption at 1383 cm–1 was attributed to NO stretching vibration. Moreover, the Co-MOF spectrum displayed a distinct band at 425 cm–1, characteristic of Co–N bonding, confirming successful framework formation.

Figure S6b shows the Raman spectra of the MOFs compared to those of the 2-mIM linker. For the pure organic linker, characteristic bands appear at 680 and 746 cm–1 (aromatic C–H stretching), 1126 and 1168 cm–1 (C–C stretching/C–H bending of the imidazole ring), 1305 cm–1 (C–N stretching with N–H bending), 1370 cm–1 (CH3 symmetric bending), and 1482 cm–1 (methyl bending). , Additional peaks in the low Raman shift region at 460 and 525 cm–1 correspond to symmetric vibrations of M–O (M = Ni and Co). The appearance of these features in the MOF spectra confirms successful framework formation. The reduced intensity of the C–N stretching band indicates coordination of Ni/Co centers to the nitrogen atom of the imidazole ring. The band near 1460 cm–1 is further assigned to symmetric NO3 – stretching within the MOFs.

3.2. Electrocatalytic Performance

MOFs can act as OER and HER electrocatalysts, as redox-active metal centers (frequently Fe, Co, Ni, etc.) and functionalized organic ligands are present in their structure often in a porous architecture. Unfortunately, due to the limited accessibility of active centers obscured by surrounding organic ligands and the poor electronic interaction between organic ligands and metal centers, the majority of pristine MOFs exhibit intrinsically poor conductivities, which in turn limits their electrocatalytic performances. Therefore, using pure MOFs directly as OER and HER electrocatalysts is still in its early stage, while there is an ongoing debate whether MOFs are the catalytically active materials or only the precatalysts. Recently, several approaches have been put forth, mostly centered on changing the compositions (metal nodes and organic ligands) and structures of pristine MOFs to increase their conductivity and electrocatalytic activity. Tables S5 and S6 provide a summary of the current Ni and Co-based MOFs including the electrolyte and substrate type for the OER and HER, respectively.

3.2.1. OER Performance

The electrocatalytic OER activity of the as-synthesized MOFs deposited onto CP and NF substrates by DC and ECD was assessed under alkaline conditions (1.0 M KOH, pH = 13.8). The corresponding IR-compensated polarization curves (LSV) are displayed in Figures a and S7a–c, where the OER activity was compared to that of an IrO2/NF benchmark electrocatalyst. Neither the nickel nor the carbon substrate showed any OER activity in contrast to the MOF electrodes.

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OER performance of the catalysts. (a) LSV curves of MOF electrodes and IrO2 fabricated by DC on NF, (b) the Tafel-plots derived from the OER polarization curves, (c) the corresponding overpotential values of the electrodes, and (d) the chronopotentiometric curve of Ni/Co-MOF by DC on NF at a current density of 20 mA cm–2.

TSs were derived from the linear region of the LSV curves and used to assess the OER reaction kinetics of the catalysts in Figures b and S7d–f. The TSs of Ni-MOF, Co-MOF, and Ni/Co-MOF were found to be 112 mV dec–1, 124 mV dec–1, and 108 mV dec–1, respectively. Activities were characterized by the overpotential recorded at 20 mA cm–2 (η20) for the Ni-MOF, Co-MOF, and Ni/Co-MOF catalysts on NF and CP substrates deposited by ECD and DC (Figure c). In general, NF-supported and drop-casted MOFs showed the highest activity among the samples, in particular, DC-coated Ni/Co-MOF on NF and CP showed the lowest overpotentials of 296 and 306 mV (at 20 mA cm–2). Overpotentials of 270 and 300 mV were obtained on DC-deposited benchmark IrO2 on NF and CP, respectively. Figure d depicts the stability test of the electrode of the highest activity, i.e., the CP curve of the Ni/Co-MOF drop-casted on NF at 20 mA cm–2 for 50 h. After a small initial drop, the electrode potential remained stable throughout the measurement, proving the stability of the Ni/Co-MOF electrode.

3.2.2. HER Performance

The alkaline HER activity of the catalysts was investigated in 1.0 M KOH, and the LSV curves recorded on NF-supported MOF electrodes are plotted in Figures a and S8a–c, where HER activity was compared to that of a Pt/C/NF benchmark electrocatalyst.

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HER performance of the catalysts. (a) LSV curves of MOF and Pt/C fabricated by DC on NF, (b) the Tafel-plots derived from the HER polarization curves, (c) the corresponding overpotential values of the electrodes, and (d) the chronopotentiometric curve of the drop-casted Ni/Co-MOF on NF at a current density of −10 mA cm–2.

Bare NF and CP substrates showed the highest onset potentials as anticipated (Figures S7a–c and S8a–c), while NF-supported MOFs exhibited the highest HER activity with an overpotential of 245 mV at 10 mA cm–2 on the Ni/Co-MOF/NF electrode fabricated by DC (Figure c). DC-deposited benchmark Pt/C electrodes showed the lowest overpotentials of 72 (Figure a) and 91 mV (Figure S8c) on NF and CP, respectively. TSs of 201 mV dec–1, 99 mV dec–1, and 96 mV dec–1 were found for Ni-MOF, Co-MOF, and Ni/Co-MOF, respectively (Figure b). The low overpotential along with the low Tafel slope make the NF-supported Ni/Co-MOF a promising HER catalyst candidate with a stable performance at −10 mA cm–2 for 50 h (Figure d). The combination of high conductivity, large surface area, mechanical robustness, chemical stability, and favorable support–catalyst interface makes NF the preferred substrate over CP for the OER and HER.

The active species are in situ-formed Ni–Co (oxy)­hydroxides, in which Ni centers serve as the dominant intrinsic OER active sites, while Co plays a synergistic role. In the HER, mixed Ni–Co phases facilitate both water dissociation and hydrogen adsorption, so both metals contribute to the enhanced performance. The incorporation of Co into the Ni lattice modulates the electronic structure of Ni and lowers the energy barrier for the Ni2+/Ni3+ oxidation. This improves the conductivity and structural stability of the reconstructed oxyhydroxide layer, thereby accelerating charge transfer during the OER. Operando investigations have confirmed that Ni–Co catalysts reconstruct under OER conditions into mixed (oxy)­hydroxides, with oxidized Ni centers acting as the primary active sites and Co facilitating Ni oxidation and charge transfer. , DFT studies show that Co incorporation into Ni-based catalysts tunes the adsorption free energies of H, OH, and OOH, lowering the calculated OER overpotential. ,

EIS was employed to characterize the electrode/electrolyte interface. Impedance spectra in Figure S9 indicated the lowest charge-transfer resistance and the highest conductivity in Ni/Co-MOF among the studied catalysts, advantageous in OER and HER. This is especially true, when the Ni/Co-MOF electrode was fabricated by DC on the NF substrate (Figure S9b), which aligns well with the electrocatalytic results in Figures S7 and S8.

Moreover, having shown high activity in both the HER and OER, the NF-supported Ni/Co-MOF electrode can be employed as an overall water splitting electrocatalyst, as it is seen in Figure a. The tests were conducted using a two-electrode configuration, where Ni/Co-MOF served as both the anode and the cathode. The cell required a potential of 1.41 V to achieve a current of 10 mA cm–2. The result confirms the practical applicability of Ni/Co-MOF as an overall water splitting electrocatalyst.

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(a) The combined OER/HER activity of the drop-casted Ni/Co-MOF electrode on NF, (b) the schematics of the electronic interaction between Co2+ and Ni2+ via O-bridges.

ECSA was calculated from the electrochemical double-layer capacitance (C dl) at the electrode/electrolyte interface via eq . Figures S10 and S11 show CV curves recorded at different scan rates between 10 and 500 mV s–1 and the corresponding ECSA determination for the two electrodes with the highest activity (Ni/Co-MOF drop-casted on NF and CP) in Figure S10 and for the drop-casted NF- and CP-supported Ni-MOF and Co-MOF electrodes in Figure S11. The calculated C dl and ECSA values of the drop-casted electrodes were tabulated in Table S3. The C dl values for the Ni/Co-MOF samples were 0.82 and 0.11 mF cm–2, which equal to ECSAs of 20.4 and 2.8 cm2 for Ni/Co-MOF on NF and CP, respectively.

To investigate the charge-transfer kinetics at the electrode/electrolyte interface under reaction conditions, EIS was performed at 1.52 and 1.54 V (vs RHE) offset potentials in the OER and at −0.24 and −0.29 V (vs RHE) offset potentials in the HER on drop-casted Ni/Co-MOF on NF. Impedance spectra are semicircles in the Nyquist plots in all cases (Figure S12) fitted by the equivalent circuit (R s + CPE/R ct) taking solution (R s) and charge-transfer resistance (R ct) and a nonideal capacitive behavior (constant phase element (CPE)) into account. Fitting parameters are presented in Table S4, where reduced interfacial/internal resistances and enhanced charge transfer are indicative of an active catalyst. The lowest R ct in Ni/Co-MOF DC-NF implies fast electron transfer and mass transport during the OER and HER, whereas the Ni/Co-MOF DC-CP electrode has a more capacitive behavior represented by higher CPE values in both reactions.

Structural and spectroscopic characterizations of the Ni/Co-MOF catalyst were done before and after the OER by performing 5 CV cycles between −0.5 and 1.0 V (vs Ag/AgCl) at 10 mV s–1 scan rate, followed by a chronopotentiometric (CP) electrolysis at 20 mA cm–2 for 1 h (Figure S13). As shown in the XRD patterns in Figure S13a, part of the Ni/Co-MOF transforms into metal hydroxides owing to alkaline hydrolysis after the cycling tests, which agrees well with earlier studies. SEM images in Figure S13b show no major changes; Ni/Co-MOF retained its lamellar, flake-like morphology. FT-IR and Raman analyses in Figure S13d,e revealed an increase in the intensity of the M–O/MOOH stretching vibration bands after the reaction, indicating the formation of additional M–O bonds. The transformation of MOFs into metal (oxy)­hydroxide phases under OER conditions is a well-known phenomenon. During electrochemical oxidation, the initially crystalline MOF structure undergoes partial decomposition as the organic linkers are oxidized and leached into the electrolyte, while the metal nodes (Ni and Co) reorganize into amorphous or poorly crystalline (oxy)­hydroxide. This reconstruction process is typically accompanied by the decrease or disappearance of the characteristic vibrational bands associated with organic linkers and the emergence of new M–O and O–H stretching and bending vibrations corresponding to metal (oxy)­hydroxides.

XPS was employed to determine the elemental composition and chemical states of the as-synthesized and postreaction MOFs (Figures S14 and S15). In the survey spectra of the Ni/Co-MOF, the peaks located at 285.74 and 530.53 eV belong to C 1s and O 1s, respectively. The high-resolution N 1s XPS spectrum of the Ni/Co-MOF exhibits a peak at 407.26 eV, which is characteristic to the nitrogen (N−) in the 2-mIM linker molecules, confirming their successful incorporation into the framework. The Ni 2p spectrum in Figure S14 shows two major peaks at binding energies of 872.62 and 854.91 eV, corresponding to Ni 2p1/2 and Ni 2p3/2, respectively. The peaks at binding energies of 780.83 and 796.42 eV in Figure S14c are assigned to Co 2p3/2 and Co 2p1/2, respectively, corresponding to Co2+. In addition, the peaks observed in the O 1s spectra in Figure S14 represent Co–O/Ni–O, CO, and C–O bands.

The postelectrocatalysis XPS spectra in Figure S15 provide evidence for the structural evolution of the MOF samples into active (oxy)­hydroxide phases. A comparative analysis shows a slight positive binding energy shift for the Ni 2p peaks, with Ni 2p3/2 and Ni 2p1/2 located at 855.32 and 873.04 eV, respectively. This shift is characteristic of the oxidation from Ni2+ to Ni3+, signifying the formation of nickel oxyhydroxide (NiOOH). Meanwhile, the spectral features of Co 2p confirm conversion to cobalt oxyhydroxide (CoOOH). The presence of the 2-methylimidazole linker is critical, as its coordination environment likely enhances the adsorption of hydroxyl ions (OH–), thereby facilitating the rapid and efficient transformation of metal centers into their respective conductive (oxy)­hydroxides. The slight shifting binding energy of Ni2+ and the Co2+ peak suggested the generation of nickel and cobalt hydroxides/oxyhydroxides during electrocatalysis. ,

There are various descriptors for predicting electrocatalytic activity, such as the d-band center position, coordination number, and adsorption energy of intermediates. Metal-ion electronegativity has been proposed as a novel descriptor entirely based on intrinsic properties, i.e., Pauling EN, ionic radius, electron affinity, valence electron number, coordination number, and spin state. EN is an atom’s tendency to attract electrons in chemical bonds, and by the introduction of metals (metal ions) of different EN into a parent structure, it can redistribute its electron density. This provides a means of fine-tuning intermediate adsorption energy and thus catalytic activity. It was shown that exchanging Fe3+ in Fe­(III)-MIL-88A MOF with metal ions of lower EN (Ni2+) shifts electron density from the Ni2+ to Fe3+ via bridging oxygen and decreases the overall EN of the structure, which in turn enhances *O adsorption, and eventually OER performance. Exceptionally high OER performance was reported in ultrathin NiCo-UMOFNs, where the four unpaired electrons of Co2+ in its π-symmetry (t2g) d-orbitals interact with bridging O2– by π-donation. Ni2+ ions, on the other hand, filled the entire t2g orbital, and by connecting Ni2+ and Co2+, the e––e– repulsion between the bridging O2– and Ni2+ nodes might amplify π-donation through Co–O. This leads to a partial charge transfer from Ni2+ to Co2+ nodes. Metal-ion doping in MOFs also influences their electronic structure and the local coordination, allowing to fine-tune the environment of the active sites and adsorption properties of reaction intermediates. It has been shown that doping modifies the electronic structure of Ni2+-doped ferrocene (Fc)-based Co-MOFs (CoNiFc-MOFs) and increases their conductivity. The improved OER and HER activity of the Ni/Co-MOF is mainly caused by the enhanced charge transfer compared to that in the monometallic Ni- and Co-MOF counterparts. Based on the mechanistic consideration above, we propose an EN-driven enhancement via electronic coupling between Co2+ and Ni2+ in the catalytic activity of the bimetallic Ni/Co-MOF (Figure b). Furthermore, postreaction analyses from this study and previous reports indicate that MOFs containing organic ligands undergo structural changes by prolonged or varying electrocatalytic conditions (OER, HER, CP, CA). In particular, the active metal nodes (Ni2+ → Ni3+, Co2+ → Co3+) become oxidized, leading to the formation of electroactive metal (oxy)­hydroxide species (NiOOH, CoOOH). ,,,

4. Conclusions

In summary, Ni- and Co-containing mono- and bimetallic metal–organic frameworks (MOFs) were synthesized by a wet chemical method with the reaction of Ni- and Co-salts and 2-methyl imidazole (2-mIM) organic linker. The obtained MOFs were deposited onto porous CP and NF substrates by DC and ECD. The constructed electrodes underwent electrochemical characterization in a standard three-electrode setup, and their electrocatalytic performance was also tested in HER and OER under alkaline conditions. The NF-supported bimetallic Ni/Co-MOF electrode fabricated by DC showed the highest activity among all electrodes in this study. It demonstrated an overpotential of 296 mV with a 108 mV dec–1 Tafel slope at 20 mA cm–2 in the OER and an overpotential of 245 mV with a 96 mV dec–1 Tafel slope at 10 mA cm–2 in the HER. The higher catalytic activity of the bimetallic MOF compared to that of the monometallic counterparts was discussed based on the electronic interaction between Co2+ and Ni2+ via O-bridges, as the introduction of a metal with different EN into the parent structure results in the redistribution of electron density, which in turn affects the adsorption energy of intermediates, a key factor in electrocatalytic activity. The reported NF-supported Ni–Co-MOF system is a promising candidate for applications in overall water splitting.

Supplementary Material

ao5c10265_si_001.pdf (1.5MB, pdf)

Acknowledgments

Ç.Y. acknowledges financial support of the MTA Sustainable Development and Technologies National Programme of MTA, Sustainable Technologies Sub-Programme (NP2022-II-7/2022). H.H. acknowledges financial support of the János Bolyai Research Grant of the Hungarian Academy of Sciences (BO/682/22). Project no. RRF-2.3.1-21-2022-00009 (National Laboratory for Renewable Energy) has been implemented with the support provided by the Recovery and Resilience Facility of the European Union within the framework of Programme Széchenyi Plan Plus. The project received funding from the HUN-REN Hungarian Research Network. Z.K. is grateful for K 21 138714 and SNN_135918 projects of the National Research, Development and Innovation Fund. TKP2021-NVA-19 under the TKP2021-NVA funding scheme of the Ministry for Innovation and Technology is acknowledged. R.V. acknowledges the MTA Distinguished Guest Scientist Fellowship Programme 2024 of the Hungarian Academy of Sciences (MTA).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c10265.

  • Additional morphological and structural analysis (XRF spectra, EDS mapping, and elemental composition, SEM images, UV–vis DRS spectra, N2 adsorption–desorption isotherms with the corresponding BET parameters, TGA curves, and FT-IR and Raman spectra), electrochemical results (LSV curves with the corresponding Tafel plots for HER and OER, EIS spectra, ECSA determination and EIS fitting parameters), and catalyst stability assessment (XRD patterns, SEM images, FT-IR, Raman, and XPS spectra of the as-synthesized and spent catalysts along with comparative summaries of OER and HER performance of previously reported Ni- and Co-MOFs and their derivatives) (PDF)

Çağdaş Yavuz: Writingoriginal draft, Investigation, Visualization, Formal analysis, Data curation, Conceptualization. Henrik Haspel: Writingreview and editing, Conceptualization, Methodology, Project administration, Supervision. Ákos Szamosvölgyi: Investigation, Visualization, Formal analysis, Data curation, Writingreview and editing. Robert Vajtai: Writingreview and editing, Supervision, Funding acquisition. Zoltán Kónya: Writingreview and editing, Supervision, Funding acquisition.

The authors declare no competing financial interest.

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