A ligand-specific bimetallic electrocatalyst for efficient oxygen evolution reaction at higher current density

Abstract

The oxygen evolution reaction (OER) is a critical and bottleneck process in electrochemical energy applications. This study presents a straightforward hydrothermal method for preparing a NiCo bimetallic organic framework (NiCo-MOF) with three unique ligands. The NiCo-trimesic acid-based MOF on carbon cloth (NiCo-t-MOF/CC) can sustain the industrially relevant current density of 100 mA cm⁻² for over 62 hours despite the observed gradual increase in potential in 1 M KOH without replacing the electrolyte. The NiCo-t-MOF/CC electrocatalyst achieved a significantly lower overpotential of 440 mV to reach a current density of 100 mA cm⁻², outperforming the benchmark RuO₂ catalyst, which required 581 mV. A Tafel slope of 83 mV dec⁻¹ at NiCo-t-MOF/CC indicates faster oxygen evolution kinetics than at RuO₂/CC (97 mV dec⁻¹). Interestingly, NiCo-t-MOF/CC||Pt–C/CC exhibited a relatively diminished cell voltage of 1.54 V to deliver a current density of 10 mA cm⁻², which is close to the thermodynamic water splitting energy of 1.23 V. The performance of NiCo-t-MOF/CC is promising at higher current densities for industrial applications.

---

Hydrogen fuel, as a clean, renewable, and efficient secondary energy source, offers various advantages such as abundant resources, high calorific value, less pollution, and safety.^1–3 It has emerged as an important resource for driving energy transformation and achieving carbon neutrality. Hydrogen production from water electrolysis is poised to become the mainstream approach, leading to a decline in the cost of electricity if hydrogen is produced from renewable energy sources.^4–6 However, the oxygen evolution reaction (OER), a vital half-cell reaction in water splitting, poses a significant bottleneck because of the involvement of multi-proton and electron transfers.^7–9 Noble metal-based catalysts, particularly IrO₂ and RuO_2, are currently well-known benchmark OER catalysts. However, the high cost, limited availability, and poor stability of these catalysts have hindered their widespread applications despite their effective and efficient OER activity.^10,11 Therefore, designing an earth-abundant transition metal-based highly efficient electrocatalyst is a promising approach and viable solution.

Metal–organic frameworks (MOFs) are crystalline, highly porous materials constructed from metal ions or clusters in coordination with organic linkers.^12–14 Due to their tunable porosity and readily accessible active sites, MOFs have received tremendous attention in several electrochemical applications. In recent years, first-row transition metals such as Ni, Co, and Fe are considered as promising electrocatalysts owing to their superior electrochemical activity, low cost and stability towards the OER.^12,15–17 Sun et al.¹⁸ reported that NiFe-based MOF nanosheets directly grown on a Ni foam substrate act as a robust electrocatalyst for the electrochemical OER. Duan et al.¹⁹ demonstrated that a bimetallic MOF nanosheet-based electrocatalyst with highly exposed molecular active sites enhances the catalytic activity for overall water-splitting reactions. Here, this study brings three unique ligands, i.e., 2-methyl imidazole, trimesic acid, and terephthalic acid, to prepare NiCo bimetallic MOFs (named as NiCo-im-MOF, NiCo-t-MOF and NiCo-tp-MOF respectively) with different morphologies and electronic structures via one-pot hydrothermal synthesis. We investigated the influence of ligands on the OER activity of NiCo bimetallic MOF electrocatalysts for three different MOFs under industrially relevant conditions (i.e., higher current density and longer stability), Fig. S1.† Surprisingly, the NiCo-t-MOF electrocatalyst showed extraordinary catalytic activity. Without changing the electrolyte, the NiCo-t-MOF catalyst maintained a current density of 100 mA cm⁻² for more than 62 hours in 1 M KOH. The superior OER activity of the NiCo-t-MOF likely results from the robust coordination environment, higher surface area, and better hydrophilicity, which improves the electron transfer, active site availability, and interaction with electrolyte leading to enhanced electrocatalytic performance over NiCo-im-MOF and NiCo-tp-MOF. The nitrogen atoms in the imidazole ring can coordinate to metal centers in catalytic materials, forming strong metal–ligand interactions. This can enhance the material's ability to stabilize reaction intermediates or facilitate electron transfer. The symmetrical and rigid structure of terephthalic acid can lead to the formation of highly ordered catalytic sites in metal–organic frameworks, enhancing the material's structural integrity and stability under reaction conditions, whereas the carboxyl groups in trimesic acid can coordinate with metal ions, creating a stable porous structure that enhances the material surface area and porosity, facilitating access to active sites and improving the overall catalytic efficiency. As shown in Fig. 1, trimesic acid has a tricarboxylate structure, whereas terephthalic acid is a dicarboxylate ligand, and 2-methyl imidazole is a nitrogen-based ligand. The additional carboxyl groups in trimesic acid facilitate better interaction with metal sites, improving the electrocatalytic activity by offering more active coordination environments for OER activity. A more interconnected and denser coordination network with Ni and Co metal centres creates stronger metal–ligand interactions and, increases the overall stability of the MOF and expand the electron transfer process.^20–22

Fig. 1 Chemical structures of the ligands.

0.2 M each solution of 2-methyl imidazole, trimesic acid, and terephthalic acid was prepared in DMF, followed by ultrasonication for 10 minutes. Then, 0.05 M nickel nitrate hexahydrate and 0.1 M cobalt nitrate hexahydrate were added to the same solution and ultrasonicated for an additional 30 minutes. The thoroughly mixed solutions were transferred to Teflon-lined autoclaves and subjected to heat treatment at 180 °C for 12 hours. After the heat treatment, the autoclaves were allowed to cool naturally, and the resulting products were collected by centrifugation. The solids obtained were washed with deionized water and ethanol and then dried at 70 °C for 10 hours. The final solid materials were categorised as NiCo-im-MOF, NiCo-t-MOF, and NiCo-tp-MOF based on their coordinated ligands. The synthetic scheme of the catalysts is illustrated in Fig. 2a.

Fig. 2 (a) Schematic illustration of preparation, (b) XRD pattern, (c) SEM image, and (d) EDS spectrum of NiCo-t-MOF.

XRD measurement was performed to explore the crystalline structure of the synthesized NiCo-t-MOF, and the pattern is displayed in Fig. 2b. The XRD pattern showed distinct, well-defined peaks revealing excellent crystallinity. Three characteristic peaks at 17.55°, 18.74°, and 26.52° can be attributed to the (220), (111), and (311) crystallographic planes, respectively²³ of NiCo-t-MOF. The crystal structure of NiCo-t-MOF matches well with the JCPDS file no. 96-7.2-6689, which shows a monoclinic crystal system with lattice constant values of a, b, c, α, β, and γ as 17.41 Å, 12.95 Å, 6.50 Å, 90.00°, 111.90°, and 90.00° respectively. Along with the mentioned crystalline peaks, there are other significant peaks at 7.29°, 10.9°, and 21.96° which are ascribed to (100), (010), and (101) lattice planes. Comparison of the simulated PXRD pattern from the Cambridge Crystallographic Data Centre (CCDC) with the experimental pattern of the NiCo t-MOF showed comparable peaks with the simulated pattern of Ni-MOF with CCDC no. 802889 as shown in Fig. 2b.^24,25 The SEM micrographs of NiCo-t-MOF in Fig. 2c reveal an elongated or rod-like structure with sizes of approximately 1.9 μm to 2.6 μm, as marked by the measurement labels. The images indicate the formation of crystallites, which potentially influence the material surface area and catalytic or electronic properties. The elemental composition by Energy Dispersive X-ray spectroscopy (EDS) shown in Fig. 2d infers the presence of all elements of the NiCo-t-MOF material.

The wide-scan XPS spectrum in Fig. 3 represents distinctive Ni 2p, Co 2p, O 1s, and C 1s peaks. The Ni 2p spectrum showed two significant peaks at 873.28 and 855.48 eV, corresponding to Ni 2p_1/2 and Ni 2p_3/2 states, respectively, with a spin-energy difference of 17.8 eV. In addition, satellite peaks were noticed at 878.68 and 860.18 eV due to the existence of high-spin Ni²⁺. Similarly, the Co 2p spectrum displayed peaks at 796.39 and 780.63 eV, inferring the +2 oxidation state of Co with two satellite peaks at 801.37 and 783.27 eV. The XPS data depict a +2 oxidation state for both Co and Ni in NiCo-t-MOF.^26,27 The deconvoluted C 1s HR-XPS spectrum showed binding energy maxima at 284.08, 285.58, and 287.87 eV, which are attributed to C–C, C–O, and O–C–O environments, respectively.²⁸ Further, the O 1s deconvoluted spectrum displayed peaks at 530.9 and 532.5 eV, responsible for oxygen bound to metal and –OH species.²⁹

Fig. 3 Wide range XPS spectrum of NiCo-t-MOF and high resolution deconvoluted spectra of Ni 2p, Co 2p, C 1s and O 1s.

The N₂ adsorption–desorption isotherms were recorded at 77 K to study the porous structure of the NiCo-t-MOF. The adsorption–desorption hysteresis loop presented in Fig. 4 illustrates mesoporous properties of the NiCo-t-MOF material. The synthesized NiCo-t-MOF material had a surface area of 9.6 m² g⁻¹ and an average pore diameter of about 1.33 nm, which suggests that the micro–mesoporous structure is interlaced. The micro–mesoporous structure of NiCo-t-MOF would facilitate the quick transport of ions in aqueous electrolytes and is advantageous for enhancing the OER activity.

Fig. 4 N₂ adsorption–desorption isotherms for NiCo-t-MOF.

A three-electrode assembly was designed to examine the electrocatalytic OER activity of the NiCo-t-MOF electrocatalyst in 1 M KOH solution.

The cyclic voltammogram (CV), linear sweep voltammetry (LSV) curves and the electrochemical active surface area (ECSA) were recorded with NiCo-t-MOF coated on glassy carbon (GC)/carbon cloth (CC) as the working electrode, the Hg/HgO reference electrode and a graphite rod as the counter electrode. The details of the experimental section are provided in the ESI.† The electrochemical properties of the prepared samples were studied by CV in 1 M KOH by selecting GC and CC as the substrate, Fig. S2.† The CV experiments were performed in the potential window of 1.2 to −1.2 V vs. Hg/HgO at a scan rate of 50 mV s⁻¹. Fig. S2a and b† show clear redox peaks for corresponding metal ions in the NiCo-t-MOF sample.³⁰Fig. 5a compares the OER polarization curves of bare CC, NiCo-t-MOF/CC, and RuO₂/CC in 1 M KOH solution. LSV curves were collected at a scan rate of 5 mV s⁻¹ after activating the electrode surface with 20 cycles of cyclic voltammograms (CVs) to minimize the capacitive current and to reach a relatively stable surface. The LSV curve of NiCo-t-MOF/CC showed a clear oxidation peak, caused by the oxidation of low valence metal ions to Ni³⁺ and Co³⁺ before the water oxidation. The NiCo-t-MOF/CC exhibited an overpotential of 440 mV to attain a current density of 100 mV cm⁻², which is much lower than that of the benchmark OER catalyst RuO₂ (581 mV), demonstrating that the fabricated electrocatalyst outperformed the noble metal catalyst even under industrially relevant conditions of higher current density. The low overpotential of NiCo-t-MOF/CC for the OER may be due to the higher catalytic activity sites and larger specific surface area of NiCo-t-MOF, indicating a greater area available for interaction between the active sites and the electrolyte. The formation of oxidised species of Ni³⁺ (NiOOH) and Co³⁺ (CoOOH) before the onset of the OER may act as the real active species for the OER activity.³¹ Also, additional species are involved during the OER as the electrochemical reaction occurs at a greater negative potential than Co²⁺/Co³⁺ oxidation and as evidenced by XPS, the smaller overpotential is attained because of the oxidation of both Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺, which suggests that both Co and Ni ions are involved in facilitating the OER.³² However, the probable detailed OER mechanism is discussed in the ESI.† The overpotentials of commercial RuO₂/CC and bare CC at any current density are much higher than that of the NiCo-t-MOF/CC, indicating superior electrocatalytic activity of the fabricated electrode for the OER process.

Fig. 5 Electrochemical activity of OER studies (a) polarization curves, (b) Tafel slopes, (c) C_dl values and (d) stability test of the NiCo-t-MOF catalyst.

Furthermore, Fig. 5b represents the Tafel slopes calculated from the linear fits of the potential (V) as a function of the logarithm of current density (mA cm⁻²) extracted from the LSV measurements, Fig. 5a. The Tafel slope value represents the charge transfer rate at the catalyst surface during the electrochemical OER process. The drop in the Tafel slope for NiCo-t-MOF/CC (81 mV dec⁻¹) indicates faster oxygen evolution kinetics than at RuO₂/CC (96 mV dec⁻¹) and bare CC (287 mV dec⁻¹).³³ In addition, the faradaic efficiency measured by the drainage method was found to be 92.7% for the OER. The detailed calculation for finding the faradaic efficiency is presented in ESI.† The electrochemical double-layer capacitance (C_dl) is directly proportional to the electrochemical active area (ECSA) of an electrocatalyst. CV was used to study the ECSA in the non-faradaic region, Fig. S3 and S4 (ESI).† The CVs were recorded at various scan rates of 10 to 100 mV s⁻¹. The current density curves were plotted, (j_a − j_c)/2 versus scan rate, where j_a and j_c are the anode and cathode current densities, and the linear fitting is used to obtain the C_dl value from CVs.³⁴ As shown in Fig. 5c, NiCo-t-MOF/CC exhibited a C_dl value of 8.64 mFcm⁻², indicating that the fabricated electrocatalyst possesses more active sites than RuO₂/CC (3.31 mF cm⁻²). To assess the charge transfer resistance during the electrocatalytic reaction, electrochemical impedance spectroscopy (EIS) was performed. The Nyquist plots of bare CC, NiCo-t-MOF/CC, and RuO₂/CC at respective OER overpotentials are shown in Fig. S5 (ESI).† The plot of NiCo-t-MOF/CC suggests a smaller charge transfer resistance (∼1.32 Ω) than those of bare CC (17.45 Ω) and RuO₂/CC (1.71 Ω), inferring favourable OER charge transfer kinetics at NiCo-t-MOF/CC. The NiCo-t-MOF/CC catalyst showed excellent stability for more than 62 hours at a current density of 100 mA cm⁻² without changing the electrolyte in the electrochemical cell. In comparison with the recently reported transition metal-based alkaline OER electrocatalysts (Table S2†), the NiCo-t-MOF/CC exhibited surprisingly lower overpotential and outstanding stability at a current density of 100 mA cm⁻², which is of high relevance for industrial applications.

To understand the decay in the electrocatalytic performance after 62 h of stability studies, the catalyst was withdrawn after 62 h and then characterized by powder XRD, XPS and N₂ adsorption–desorption isotherms. As shown in Fig. S6a,† no significant change in the sample morphology was noticed, but the careful observation of the images depicted that the crystalline structure, i.e. rod-type structure (Fig. 2c) of NiCo-t-MOF is slightly reformed as shown in the magnified portion of the SEM image. The magnified SEM image displayed both crystalline and non-crystalline portions. The powder XRD pattern of NiCo-t-MOF after 62 h stability is shown in Fig. S6b,† which revealed that the catalyst has retained some crystallinity along with a broad amorphous peak around 25°. In addition, peaks at 18° and 25.8° were noticed corresponding to nickel oxalate (JCPDS no. 98-019-0774). Furthermore, there were crystalline peaks at 43.7° and 50.9° due to the CC substrate (JCPDS card no. 04-0850)³⁵ which was used to deposit the electrocatalyst for OER studies.

To assess the changes in the composition of the surface elements, XPS analysis of the NiCo-t-MOF sample after 62 hours of stability was also performed, Fig. S6c.† The sample showed peaks corresponding to Ni 2p, Co 2p, O 1s, and C 1s, along with a distinctive peak at 689 eV, which corresponds to F 1s. This can be attributed to the fluoride content of the Nafion binder used to produce the electrode for electrochemical investigations.³⁵ The BET surface area analysis of the NiCo t-MOF sample after 62 hours of stability (Fig. S6d† showed a surface area of 1.24 m² g⁻¹ and an average pore diameter of about 1.49 nm, which suggested a slight change in the surface area with no significant changes in the average pore diameter.

To investigate the practical applicability of the designed NiCo-t-MOF, an alkaline two-electrode water electrolyser was constructed using commercial 20 wt% Pt–C/CC as the cathode and the as-synthesized catalyst as the anode, shown in Fig. 6a. Interestingly, NiCo-t-MOF/CC||Pt–C/CC exhibited a relatively small cell voltage of 1.54 V to deliver a current density of 10 mA cm⁻², which is close to the thermodynamic water splitting energy of 1.23 V and lower than that of the various reported catalyst materials in the literature, shown in Table S3, ESI.† Table S3† presents a few OER electrocatalysts in the full-cell water electrolyser configuration where commercial Pt-based catalysts are used as the cathodic HER electrocatalysts and the designed catalysts are used as the anodic OER electrocatalysts in the two-electrode configuration. Additionally, the NiCo-t-MOF/CC||Pt/C/CC net-constructed water electrolyzer achieved a working voltage of 1.93 V at a higher current density of 100 mA cm⁻² and demonstrated reliable usability without any degradation behaviour in the chronopotentiometric curve even after water electrolysis for 32 h (Fig. 6b). Both sides of the electrolytic cell generated visible bubbles vigorously (Fig. 6b) suggesting the viability of the fabricated electrolyser for hydrogen generation in practical applications.

Fig. 6 (a) Polarization curve for the full cell water electrolyser set-up (commercial 20 wt% Pt/C as the cathode and NiCo-t-MOF as the anode), (b) chronopotentiometric stability test for NiCo-t-MOF/CC||Pt–C/CC. Inset, image shows vigorous bubble evolution during the continuous operation of the water-splitting reaction for about 30 hours.

It should be noted that the onset potential observed at less than 1.23 V in Fig. 5a and 6a is not the actual onset potential for the OER. Instead, it reflects the influence of the oxidation peaks associated with Ni and Co species. In alkaline media, it is well documented that transition metal-based electrocatalysts, particularly those involving Ni and Co, undergo surface redox transitions before the OER begins. These redox processes, such as the oxidation of Ni²⁺ to Ni³⁺ and Co²⁺ to Co³⁺, typically occur in the potential range of 1.2 to 1.4 V vs. RHE and may overlap with the region where the OER starts. In our study, the onset potential observed below 1.23 V is primarily attributed to the Ni/Co oxidation processes and does not represent the actual onset of the OER. These pre-OER redox activities are inherent of the electrocatalyst surface chemistry and significantly influence the voltammetric curves. Because of this interference, accurate reporting of the onset potential or calculating the overpotential at a standard specific current density (e.g., 10 mA cm⁻²) has become challenging. Therefore, given this overlap, it is not possible to report overpotential values at a current density of 10 mA cm⁻² with high accuracy for this system. Instead, we focused on benchmarking the catalyst performance at higher and more distinguishable current density (e.g., 100 mA cm⁻²) to avoid the ambiguities.

All the results, especially the lower overpotential and long-term stability at higher current density, suggest that the NiCo-t-MOF/CC demonstrates promising potential for practical applications.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We sincerely acknowledge DST, India (TDP/BDTD/32/2019), DST, India (DST/TDT/DDP-31/2021) and VGST, India (VGST/K-FISTL1/GRD No. 1053/2021-22) along with Jain University, Minor Project (JU/MRP/Univ/3/2022) for the financial support.

Notes and references

1. R. M. Navarro, et al., Hydrogen production from renewable sources: biomass and photocatalytic opportunities, Energy Environ. Sci., 2009, 2(1), 35–54.
2. S. Singh, et al., Hydrogen: a sustainable fuel for future of the transport sector, Renewable Sustainable Energy Rev., 2015, 51, 623–633.
3. P. J. Megía, et al., Hydrogen Production Technologies: From Fossil Fuels toward Renewable Sources. A Mini Review, Energy Fuels, 2021, 35(20), 16403–16415.
4. S. Aralekallu, K. Sannegowda Lokesh and V. Singh, Advanced bifunctional catalysts for energy production by electrolysis of earth-abundant water, Fuel, 2024, 357, 129753.
5. S. Aralekallu, L. K. Sannegowda and V. Singh, Developments in electrocatalysts for electrocatalytic hydrogen evolution reaction with reference to bio-inspired phthalocyanines, Int. J. Hydrogen Energy, 2023, 48(44), 16569–16592.
6. X. Li, et al., Nanostructured catalysts for electrochemical water splitting: current state and prospects, J. Mater. Chem. A, 2016, 4(31), 11973–12000.
7. S. Aralekallu, et al., Ni foam-supported azo linkage cobalt phthalocyanine as an efficient electrocatalyst for oxygen evolution reaction, J. Power Sources, 2020, 449, 227516.
8. N.-T. Suen, et al., Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives, Chem. Soc. Rev., 2017, 46(2), 337–365.
9. K. Yang, et al., Reconstruction of bimetal CoFe0.13-MOF to enhance the catalytic performance in the oxygen evolution reaction, Chem. Commun., 2022, 58(8), 1115–1118.
10. J. Gao, H. Tao and B. Liu, Progress of Nonprecious-Metal-Based Electrocatalysts for Oxygen Evolution in Acidic Media, Adv. Mater., 2021, 33(31), 2003786.
11. R. Solanki, et al., Investigation of recent progress in metal-based materials as catalysts toward electrochemical water splitting, J. Environ. Chem. Eng., 2022, 10(4), 108207.
12. S. Mukhopadhyay, et al., Evolution of metal organic frameworks as electrocatalysts for water oxidation, Chem. Commun., 2020, 56(79), 11735–11748.
13. M. Ko, L. Mendecki and K. A. Mirica, Conductive two-dimensional metal–organic frameworks as multifunctional materials, Chem. Commun., 2018, 54(57), 7873–7891.
14. W. Liu and X.-B. Yin, Metal–organic frameworks for electrochemical applications, TrAC, Trends Anal. Chem., 2016, 75, 86–96.
15. Q. Liang, et al., Multiscale structural regulation of metal–organic framework nanofilm arrays for efficient oxygen evolution reaction, Chem. Commun., 2022, 58(49), 6966–6969.
16. B. Singh and H. Gupta, Metal–organic frameworks (MOFs) for hybrid water electrolysis: structure–property–performance correlation, Chem. Commun., 2024, 60(62), 8020–8038.
17. A. Morozan and F. Jaouen, Metal organic frameworks for electrochemical applications, Energy Environ. Sci., 2012, 5(11), 9269–9290.
18. F. Sun, et al., NiFe-Based Metal–Organic Framework Nanosheets Directly Supported on Nickel Foam Acting as Robust Electrodes for Electrochemical Oxygen Evolution Reaction, Adv. Energy Mater., 2018, 8(21), 1800584.
19. J. Duan, S. Chen and C. Zhao, Ultrathin metal-organic framework array for efficient electrocatalytic water splitting, Nat. Commun., 2017, 8(1), 15341.
20. I. Soni, P. Kumar and G. Kudur Jayaprakash, Recent advancements in the synthesis and electrocatalytic activity of two-dimensional metal–organic framework with bimetallic nodes for energy-related applications, Coord. Chem. Rev., 2022, 472, 214782.
21. R. Jaryal, R. Kumar and S. Khullar, Mixed metal-metal organic frameworks (MM-MOFs) and their use as efficient photocatalysts for hydrogen evolution from water splitting reactions, Coord. Chem. Rev., 2022, 464, 214542.
22. J. Liu, et al., Recent advances of functional heterometallic-organic framework (HMOF) materials: Design strategies and applications, Coord. Chem. Rev., 2022, 463, 214521.
23. M. Ezzati, S. Shahrokhian and H. Hosseini, In Situ Two-Step Preparation of 3D NiCo-BTC MOFs on a Glassy Carbon Electrode and a Graphitic Screen Printed Electrode as Nonenzymatic Glucose-Sensing Platforms, ACS Sustain. Chem. Eng., 2020, 8(38), 14340–14352.
24. P. Maniam and N. Stock, Investigation of Porous Ni-Based Metal–Organic Frameworks Containing Paddle-Wheel Type Inorganic Building Units via High-Throughput Methods, Inorg. Chem., 2011, 50(11), 5085–5097.
25. K. Sel, et al., Benign Preparation of Metal–Organic Frameworks of Trimesic Acid and Cu, Co or Ni for Potential Sensor Applications, J. Electron. Mater., 2015, 44(1), 136–143.
26. H. Guo, et al., Trimesic acid-modified 2D NiCo-MOF for high-capacity supercapacitors, J. Alloys Compd., 2023, 934, 167779.
27. T. V. M. Sreekanth, et al., NiCo bimetallic metal-organic framework (NiCo-MOFs) with distinct morphologies for efficient HER activity, Inorg. Chem. Commun., 2024, 161, 112128.
28. T. Zhang, et al., Hybrids of Cobalt/Iron Phosphides Derived from Bimetal–Organic Frameworks as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction, ACS Appl. Mater. Interfaces, 2017, 9(1), 362–370.
29. X. Y. Liu, et al., Self-assembled porous NiCo₂O₄ hetero-structure array for electrochemical capacitor, J. Power Sources, 2013, 239, 157–163.
30. H. Zhang, et al., NiCo-MOF directed NiCoP and coconut shell derived porous carbon as high-performance supercapacitor electrodes, J. Energy Storage, 2022, 54, 105356.
31. Z.-W. Gao, et al., Engineering NiO/NiFe LDH Intersection to Bypass Scaling Relationship for Oxygen Evolution Reaction via Dynamic Tridimensional Adsorption of Intermediates, Adv. Mater., 2019, 31(11), 1804769.
32. S. Hadimane, et al., Bioinspired Precious-Metal-Free N4 Macrocycle as an Electrocatalyst for the Hydrogen Evolution Reaction, ACS Appl. Energy Mater., 2021, 4(10), 10826–10834.
33. K. P. Cp, et al., Non-precious cobalt phthalocyanine-embedded iron ore electrocatalysts for hydrogen evolution reactions, Sustainable Energy Fuels, 2021, 5(5), 1448–1457.
34. S. Hadimane, S. Aralekallu and L. K. Sannegowda, Tuning a palladium(II) phthalocyanine embedded hybrid electrocatalyst for the hydrogen evolution reaction, Sustainable Energy Fuels, 2024, 8(8), 1775–1787.
35. S. Sun, et al., Nickel-foam-supported β-Ni(OH)2 as a green anodic catalyst for energy efficient electrooxidative degradation of azo-dye wastewater, RSC Adv., 2018, 8(35), 19776–19785.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4se01656d

---