A decade of breakthroughs: MOF-graphene oxide catalysts for water splitting efficiency

2.1 Mechanism of electrocatalysis for HER and OER water splitting

Electrocatalysis is a unique field of electrochemistry that mainly focuses on the type of catalyst used to enhance the speed of electrochemical reactions. In recent decades, progress in electrochemistry has been crucial for the development of a sustainable environment and energy conversion, especially in fuel cells and electrocatalytic cells. ³⁶ Electrochemistry technologies utilize electrical energy to facilitate energy-demanding reactions such as conversion of carbon dioxide to hydrocarbons, nitrate conversion reactions and water electrolysis in hydrogen fuel cells. The electrolysis of water results in oxidation for the production of oxygen gas at the anode, and hydrogen gas evolves at the cathode due to the reduction of protons, as exhibited in Figure 1a. The overall reaction results in chemical oxidation-reduction of electrical energy and water. ^{37 , 38} Water splitting involves two crucial processes which result in hydroxide ions (OH‾) plus hydrogen gas (H₂) production at the cathodic end during the HER process when water is reduced. In the OER mechanism, the oxidation of hydroxide ions takes place at the anode to produce water (H₂O) and oxygen gas (O₂). In addition, the half-reaction of water electrolysis specifically in an acidic medium exhibits a polarization curve for HER and OER as shown in Figure 1b.

Figure 1:

Overview of general experimental setup for water splitting. (a) The graphical representation of H₂ and O₂ generation from water dissociation; (b) polarization curve and half-reaction of water dissociation in acid electrolyte. Reproduced with permission from Ref. 39]. Copyright 2020. The Royal Society of Chemistry.

The oxygen evolution reaction in neutral and acidic media follows a sequence of proton-coupled electron transfer (PCET) steps, although the mechanisms differ slightly due to the environment. In a neutral medium, water molecules adsorb onto the catalyst surface, forming a hydroxyl intermediate (M–OH), which is deprotonated and oxidized to an oxo species (M = O). This oxo intermediate reacts with water to form a hydroperoxo species (M–OOH), which eventually releases oxygen (O₂) while regenerating the catalytic site. The presence of buffering species in neutral media plays a crucial role in stabilizing intermediates and facilitating proton removal. In acidic media, the reaction also progresses through similar intermediates, starting with the adsorption of a water molecule to form M–OH₂⁺. Successive PCET steps lead to the formation of M = O and M–OOH intermediates, which release oxygen and regenerate the active site. The acidic environment enhances proton transfer kinetics but requires robust catalysts, such as IrO₂ or RuO₂, to resist corrosion and maintain stability under harsh conditions. Both mechanisms underscore the importance of intermediate stabilization and efficient catalyst design for improved OER performance. ⁴⁰ In an alkaline medium, HER occurs through three primary steps: the Volmer step, which involves the electrochemical reduction of water to form adsorbed hydrogen atoms (H), and hydroxide ions (OH⁻); the Heyrovsky step, combining H with another water molecule to release molecular hydrogen (H₂); or the Tafel step, where two H atoms combine to form H₂ directly. The alkaline environment introduces challenges due to slower kinetics compared to acidic media, requiring optimized catalysts with strong water-splitting and hydrogen adsorption/desorption capabilities. These mechanisms highlight the critical role of intermediate species and the need for advanced electrocatalysts for efficient water-splitting reactions. ⁴¹

2.2 Factors influencing electrocatalytic HER and OER water splitting

It is essential to understand and optimize the factors that influence electrocatalytic performance to achieve high efficiency in electrochemical water splitting for hydrogen and oxygen production. Electrocatalysts must possess specific qualities to ensure fast reaction kinetics, high catalytic activity, and stable operation under various conditions. Several key parameters, such as surface area, overpotential, pH, and Tafel slope, have a direct impact on the effectiveness and stability of electrocatalysts in HER and OER. By analyzing and optimizing these factors, researchers can enhance the reaction rate, reduce energy input requirements, and improve overall system efficiency. Below is a detailed discussion of each factor and its role in electrocatalysis:

Surface area: The surface area of the electrocatalyst highly affects the available active sites, catalytic activity and thermal stability of the material being used in an electrochemical reaction. The interaction among the active sites of the catalyst is automatically improved by the expansion of surface area. However, the larger surface areas of the supporting material increase intrinsic activity, maintain electrical conductivity, avoid agglomeration, and uniformly distribute nanomaterials.

Overpotential is a challenging yet crucial factor in electrocatalytic HER and OER water-splitting hydrogen production. The lower value of overpotential plays a significant role. Although it is challenging to maintain, it can be reduced by stabilizing electrocatalysts’ intrinsic properties and concentration polarization.

pH of the electrolyte: It is a critical factor affecting the choice of electrocatalyst, operational conditions, and the available electrolyte source. Maintaining the pH in both acidic and alkaline mediums is crucial, as it minimizes operational costs and ensures smooth electron transfer dynamics at fluctuating current densities.

Tafel slope: One of the most critical factors in electrocatalysis is the Tafel slope. It offers essential details regarding the electrocatalysts kinetics, efficiency, and reaction mechanism. A smaller value of the Tafel slope indicates better reaction kinetics and efficiency of the electrocatalyst. A few additional parameters also influence the efficiency and structural characteristics of electrocatalysts for the production of oxygen and hydrogen gas, as shown in Figure 2.

Figure 2:

The flow chart displays key factors and characterizations for the electrocatalytic performance of electrocatalysts.

3.1 Electrocatalytic water splitting by MOFs and MOF-GO composites

In recent years, MOF based electrocatalysts have gained significant attention to enhance the efficiency of water-splitting reactions. Many types of research have been done on electrocatalytic water splitting processes, such as the alteration of the core metallic framework of MOF, introducing variable functionalizable linkers and integration of additional metal atom/substrate to the MOF structure. ^{53 , 54 , 55 , 56 , 57} As a result, desirable materials electronic structures altered. This trend reveals the emergence of novel MOF materials for electrocatalytic water splitting, such as pure, bimetallic composites, MOF-based materials, and MOF-derived materials. ^{58 , 59 , 60 , 61} Incorporating graphene oxide into a MOF results in a hybrid (MOF-GO) composite, exhibiting nanosheets-like morphology for improving electron and mass transmission and enabling prominent exposure to catalytic active sites. Ultimately, this leads to an electrochemical reaction that improves the production of hydrogen (H₂) as well oxygen (O₂) via water splitting.

3.2 Plausible mechanism of electrocatalytic water splitting by MOF and MOF-GO composites

The MOF-based electrocatalytic water splitting plays a pivotal role due to the remarkable structural properties of MOFs, such as high porosity, specific surface area, functionalizable linkers and tunable pore structures. ^{62 , 63} This mechanism promotes the most directed means of hydrogen production at a given voltage of 1.23 V at standard conditions (25 °C and 1 atm) by absorbing water molecules onto the surface of MOF electrocatalyst. ⁶⁴ Additionally, transition metal nitrides, carbides, phosphides and sulfides exhibit cost-effective and efficient electrocatalytic HER and OER water splitting across the suitable electrolytic system. ^{65 , 66 , 67} Moreover, the intrinsic electrocatalytic properties of electrocatalysts can be enhanced through the introduction of additional metal atoms or substrates. This addition modulates the electronic structures, facilitating enhanced charge transfer and active sites of electrocatalysts for hydrogen and oxygen generation, as shown in Figure 3a. The synthesis of clean energy materials addresses global energy needs for sustainability. ^{68 , 69 , 70 , 71} Nowadays, metal-organic frameworks excel in catalytic activity with the combined effect of graphene in the form of graphene oxides (GO) or reduced graphene (rGO), as depicted in Figure 3b, with enhanced electrochemical performance through efficient electron transfer, including active site exposure, catalyst dispersion, charge mobility and stability. ⁷² The introduction of GO increases both HER and OER exchange current density and reduces electrochemical resistance. ^{73 , 74}

Figure 3:

The graphical illustration of a plausible mechanism of electrocatalytic water splitting (a) metal-organic framework nanocomposites; (b) metal-organic framework clusters on graphene oxides for MOF-GO nanocomposites.

3.3 MOFs and MOF-GO electrocatalysts for HER

The creation of electrocatalysts with improved efficiency plays a pivotal role in a green environment for HER water splitting. Some recent electrocatalysts have been mentioned, which explore the catalytic activities of highly efficient MOF and MOF-GO-dependent HER electrocatalysts listed in Table 1. Jiang et al. presented electrodeposited nickel-phosphorus (Ni–P) film, which exhibits remarkable overpotential values −93 mV (at 10 mA/cm²). ⁷⁵ The remarkable activity of Ni–P film is due to the amazing performance of Pt and IrO₂. Zheng et al. reported the material’s significant current density HER activity is demonstrated by the nanofoam catalyst, which employs its trifunctional layer and contains selenium on the surface and cobalt (Co/Se–MoS₂–NF) on the inner layer. ⁷⁶ The lower overpotential value of 382 mV reveals that cobalt atoms are confined in inner layers to stimulate neighboring sulphur atoms. Dai et al. proved the chemical interaction of a one-pot solvothermal technique for the hydrogen evolution process to create molybdenum polysulfide (MoSx) embedded over a porous Zr-based MOF (Zr-MOF/UiO-66-NH₂). ⁷⁷ Under acidic conditions, the stabilized electrolytic HER performance displays an extraordinary Tafel slope with 59 mV dec⁻¹ and an overpotential of 200 mV. These results show the versatility of Mo-based HER electrocatalysts, which offer faster and less expensive proton-transfer electrocatalysts. An alternative method to create cobalt diselenide (MOF-CoSe₂) nanoparticles formed from metal-organic frameworks is to use the in-situ selenium of Co-based MOFs to attach the particles on nitrogen-doped graphitic carbon. ⁷⁸ MOF-CoSe₂ electrocatalytic performance is less significant than that of the MoS₂/CoSe₂ hybrid catalyst, created with identical 0.5 M H₂SO₄ conditions, as shown in Figure 4a. ⁷⁹ The significant part of the MoS₂/CoSe₂ hybrid catalyst was confirmed by the high-resolution STEM-EDX elemental mapping (Figure 4b). Although molybdenum disulfide and cobalt diselenide materials work synergistically, MoS₂/CoSe₂ has high HER performance, as evidenced by binding energy XPS spectra (Figure 4c) overpotential of 68 mV. This suggests that MoS₂/CoSe₂ exhibits a superior activity surface area and many active sites as shown in the Volmer-Tafel pathway for hydrogen evolution reaction (Figure 4d).

Figure 4:

Effect of structural characteristics and synthesis methods on the HER performance of the MoS2/CoSe2 hybrid catalyst. (a) The synthesis route of MoS₂/CoSe₂ hybrid catalyst; (b) STEM-EDX elemental mapping; (c) XPS spectra; (d) reaction route of HER according to Volmer-Tafel pathway. Reproduced with permission from Ref. 79]. Copyright 2015. Nature Communications.

Li et al. revealed that halogen atoms enhance the catalyst activity via the desorption of hydrogen from cobalt sites via synthesizing 2D polyhalogenated Co(II)-based MOF (Co–Cl₄-MOF) to explore the effect of substituted halogen for electrochemical HER with the resulting yield of 81 % based on Cl₄–H₂pta and lower overpotential (283 mV). ⁸⁰ Khrizanforova et al. prepared a 3D Ni redox active MOF as an efficient electrocatalyst, which is based on 4,4′-Bipyridine and Ferrocenyl Diphosphinate ligands for HER. ⁸¹ The addition of a 4,4′-bipyridine linker enhances Ni-MOF HER kinetic properties in both an organic and aqueous environment. The electrochemical studies demonstrate that Ni-MOF has superior HER efficiency due to a small overpotential of 350 mV and excellent durability (∼10,000 cycles). Shekurov et al. synthesized a novel 1D helical Zn redox active coordinated ZnfcdHp polymer as an efficient electrocatalyst for HER based on ferrocenylenbis (H-phosphinic) acid (H₂fcdHp) ligand and the Zn nitrate salt, as depicted in Figure 5a. Figure 5b depicts the Helical chain’s chirality (R-enantiomer, S-enantiomer). The ZnfcdHp catalyst’s hydrogen evolution reaction shows an overpotential of 340 mV and Tafel slope of 110 mV dec⁻¹ with linear sweep voltammetry (LSV) polarization curve for extended stability (Figure 5c and d). Furthermore, these catalysts were considered superior substitutes for Pt-catalysts in hydrogen evolution reactions. ⁸²

Figure 5:

Synthesis and structural features of the 1D helical ZnfcdHp polymer catalyst and its HER performance. (a) The graphical illustration for synthesis route of 1D helical Zn redox active coordinated ZnfcdHp polymer; (b) helical chains chirality (R-enantiomer, S-enantiomer) and, (c) LSV polarization curves with corresponding (d) Tafel slopes. Reproduced with permission from Ref. 82]. Copyright 2019. Royal Society of Chemistry.

Roy et al. introduced the metal-organic frameworks composed of cobaloximes-based catalyst UU-100(Co), which act as metallo-linkers in the clusters of hexanuclear zirconium for efficient hydrogen evolution catalytic activity. ⁸³ The obtained Tafel slope value (250 mV dec⁻¹) claims better HER activity with 309 µmol H₂ cm⁻² generation with 18 h of electrolysis. The study reveals that cobaloximes have enhanced structural integrity with MOFs, leading to at least 50–200 times increase in turnover numbers with high porosity and redox-active metallo-linkers than traditional cobaloximes for hydrogen generation. B. Patel et al. presented the development of an effective self-sacrificial Nickel-based metal-organic framework (Ni@NCS) bifunctional electrocatalyst by facile pyrolysis method. ⁸⁴ The remarkable catalytic qualities of the Ni@NCS-800 nanomaterial, Ni@NCS-800 displays an overpotential of 366 mV for HER while reaching a current density of 10 mA cm⁻² and a Tafel slope value of 93 mV dec⁻¹ supported by the exceptional catalytic properties of Ni@NCS-800 nanomaterial. The stability of the catalyst assures that it is a simple, non-precious and stable catalytic material for HER activity. Lin et al. synthesized Ru-doped Nickel thiophene dicarboxylic acid (Ni_XRu-TDA/NF) composites using a one-pot solvothermal method, as shown in Figure 6a. ⁸⁵ The morphological structure and elemental analysis of Ni_XRu-TDA/NF were present in TEM images and EDX mapping (Figure 6b). However, the LSV curves with histograms overpotential of the catalyst’s HER performance reported, showing a Tafel slope around 49 mV dec⁻¹ (Figure 6c and d) and a charge transfer resistance (R_ct) of 5.9 Ω for hydrogen generation by water splitting. Additionally, the catalyst performs more effectively when the cathode and anode are submerged in a water electrolysis system, achieving a cell voltage of 1.53 V with a current density of 10 mA cm⁻².

Figure 6:

Synthesis, structural analysis, and HER performance of NiXRu-TDA/NF catalyst. (a) The schematic diagram for synthesis of Ru-doped Nickel thiophene dicarboxylic acid (Ni _X Ru-TDA/NF); (b) TEM images; (c) LSV curves, histograms overpotentials and (d) Tafel slopes of material. Reproduced with permission from Ref. 85]. Copyright 2023. Elsevier.

Emerging research has been performed for the development of innovative MOF-GO-based composites and promising applications of MOF-GO materials as electrocatalysts owing to their specialized topologies and diverse functional groups and central metal atoms, as listed in Table 1. Jiao et al. prepared a layered metal-organic framework CoP/reduced graphene oxide composite (COP/rGO-400) followed by pyrolysis and subsequent phosphating process for overall water splitting (Figure 7a) in an alkaline environment of 1 M KOH demonstrated the HER performance at an overpotential of 150 mV having Tafel slope value 38 mV dec⁻¹ and LSV polarization curves with 100 % faradaic efficiency (Figure 7e–g). The morphological structure of COP/rGO-400 was present in SEM and TEM images (Figure 7b–d). This work describes how graphene oxide and metal-organic frameworks can be integrated with MOF to create effective electrocatalysts for various applications. ⁸⁶

Figure 7:

Synthesis, structural analysis, and HER performance of CoP/rGO-400 composite catalyst. (a) Graphical illustration for synthesis route of CoP/reduced graphene oxide composite (COP/rGO-400); (b) scanning electron microscope (SEM) image; (c) TEM, and (d) high resolution TEM image of COP/rGO-400; (e) LSV curve, (f) Tafel slope, and (g) time-dependent current density curves at 1 M KOH solution. Reproduced with permission from Ref. 86]. Copyright 2016. Royal Society of Chemistry.

Khiarak et al. prepared a CoNi MOFon incredibly porous conductive graphene (PCG) for HER by using the high-quality template-assisted chemical vapour deposition (CVD) graphene and solvothermal production deposition of bimetallic CoNi metal-organic framework (CoNi-MOF-PCG) particles over a graphene substrate. ⁸⁷ The electrocatalyst shows excellent results in a highly alkaline solution (1 M KOH), demonstrating a 12-fold increase in Electrochemical Surface Area (ECSA) and an active catalytic site for HER exhibiting an overpotential of 265 mV and improved rate of diffusion and charge transfer efficiency. Yan and colleagues created a hybrid composite that splits electrocatalytically by introducing a nickel organic framework onto reduced graphene oxide (Ni₂P/rGO), allowing it to undergo a low-temperature phosphorization procedure, as shown in Figure 8a. ⁸⁸ The electrocatalyst shows excellent results in a highly alkaline solution (1 M KOH) with the current-time curve and Tafel slope (Figure 8c) of 58 mV dec⁻¹ with LSV polarization curves (Figure 8c). The composite provides improved HER catalytic capabilities with an overpotential of 142 mV, a greater surface area and an average particle size of roughly 2.6 nm.

Figure 8:

Synthesis, structural analysis, and HER performance of Ni₂P/rGO composite catalyst. (a) Graphical illustration for the synthesis of Ni₂P/rGO; (b) Tafel slopes of HER; (c) polarization curves of Ni₂P/rGO in 1.0 M KOH. Reproduced with permission from Ref. 88]. Copyright 2018. The Royal Society of Chemistry.

Additionally, Gopi et al. developed the bimetallic VeNi_xFe_yMOF/GO composite using solvothermal synthesis to create a carbon-supported architecture. ⁸⁹ The reduced R_ct value of 12 Ω and the tafel slope of 150 mV/dec indicated a high-speed transfer of electrons for hydrogen generation with an overpotential of 90 mV in a very acidic solution. Makhafola et al. prepared an effective electrocatalyst Pd@GO/MOF nanocomposite by loading the MOF and Pd@GO through an impregnation process. The stability was enhanced because the Pd interacted with GO and MOF, limiting the loss of those functional groups. The turn over frequency (TOF) value of 4.71 mol·H₂s⁻¹ and a Tafel slope of 169 mV dec⁻¹ exhibits that the Pd@GO/MOF application performance was discovered as an H₂-evolving catalyst, demonstrating the composite’s outstanding conductivity that results from the presence of Pd@GO. ⁹⁰ Another unique electrocatalytic composite can be found in Makhafola’s study. Specifically, the authors prepared GO/Cu-MOF composite for hydrogen production via the impregnation method, as shown in Figure 9a. ⁹¹ The morphological structure of GO/Cu-MOF composites was present in SEM and TEM images (Figure 9b–d). The TOF value (4.57 mol·H₂ s⁻¹) with the concentration of 0.3 M H₂SO₄ resulted in higher Hydrogen generation. The synergistic interaction between GO with the MOF crystals is responsible for the enhanced performance in HER studies and better log current pH with the Pourbaix diagram reported after GO was added, as depicted in Figure 9e and f.

Figure 9:

Synthesis, structural analysis, and HER performance of GO/Cu-MOF composite catalyst. (a) Graphical illustration for GO/Cu-MOF synthesis route; (b–d) SEM and TEM images of catalyst; (e) log current of pH of solution, and (f) Pourbaix diagram of HER. Reproduced with permission from Ref. 91]. Copyright 2020. Electrochemical Science Group.

3.4 MOFs and MOF-GO electrocatalysts for OER

Depending on unique approaches, several metal-dependent electrocatalysts were reported for electrochemical water splitting for oxygen evolution reactions, as listed in Table 2. Jiang et al. presented electrodeposited nickel-phosphorus (Ni–P) film, which exhibits remarkable progress for OER with an overpotential value of 344 mV. ⁷⁵ The increased OER performance was attributed to the high catalytic activity of Ni–P film. Maity et al. introduced the nanoporous bimetallic phosphide (Ni_0.2Co_0.8P) catalyst with a 3D network structure and an enhanced surface area. ⁹² The resulting catalyst Ni_0.2Co_0.8P displays exceptional performance under alkaline OER conditions having an overpotential of 230 mV and a Tafel slope of 44 mV dec⁻¹. These results revealed that there is a synergetic effect between d and p bands with rapid mass transfer properties by providing a three-dimensional network morphology of Ni_0.2Co_0.8P catalyst. Ni exhibits highly efficient, durable and cost-effective electrocatalysts that take part for the large-scale water splitting. Wang et al. presented tricomponent metal phosphide with a hollow structure synthesized from cobalt-containing MOFs (ZIF-67) followed by high-temperature reduction at a lower overpotential value of 329 mV. ⁹³ The greatly enhanced water oxidation performance presents it as a versatile approach to utilize MOFs as a precursor to synthesize highly effective water-splitting electrocatalysts. Xu et al. used a simple hydrothermal surfactant-assisted method to synthesize an ultrathin two-dimensional cobalt metal-organic framework nanosheets [Co₂(OH)₂BDC] (Figure 10a). The resultant 2D Co-MOFs were applied as working electrode. The LSV curves of electrodes observed at a slow scan rate of 5 mV s⁻¹ are shown in Figure 10b. Moreover, the 2D Co-MOFs material shows a low overpotential of 263 mV at 10 mA cm⁻² and a Tafel slope of 74 mV dec⁻¹ in 1.0 M KOH (Figure 10c and d). ⁹⁴ The superior catalytic kinetics, reflected by the LSV polarization curves and the Tafel slope of 74 mV dec⁻¹, highlight the material’s ability to target more active sites, resulting in enhanced durability and improved OER performance.

Figure 10:

Synthesis, structural characterization, and OER performance of ultrathin 2D Co-MOF nanosheets. (a) Graphical illustration of ultrathin 2D Co-MOF synthesis; (b) LSV curve; (c) Tafel slopes; (d) durability test for 1,000 cycles, and (e) Chronoamperometric testing of ultrathin 2D Co-MOF at 1.0 M KOH electrolyte. Reproduced with permission from Ref. 94]. Copyright 2018. Royal Society of Chemistry.

Besides, mixing MOF with heterometallic atoms resulted in exceptional catalysts through phosphidation, carbonation and oxidation. Guo et al. developed a well-controlled Co₃O₄@Ni₂P–CoP/NF bimetallic electrocatalyst through phosphating and carbonation of Co₃O₄ nanowires by template-directed construction of MOF arrays. ⁹⁵ The resulting hierarchal Co₃O₄@Ni₂P–CoP/NF nanomaterials exhibit exceptional OER electrocatalytic properties at an overpotential of 298 mV and the long-term durability of 40 h in 1 M KOH solution, suggesting high catalytic bimetallic electrocatalyst for water oxidation reaction, high porosity, and N, P-codoping. Wei et al. introduced ultrathin NiFeZn-MOF nanosheet fabrication on nickel foams for electrocatalytic production of oxygen with a durability of 120 h. ⁹⁶ The recorded BET surface area of 227.1 m² g⁻¹ indicates the relative gap between the pores on the surface of the catalyst material, exhibiting the Tafel slope value of 51 mV dec⁻¹ for better OER electrochemical performance and water decomposition. Sondermann et al. synthesized the combination of NiCo-BTC (1,3,5-benzene dicarboxylate) MOF with highly conductive carbon material ketjenblack (KB) through a solvothermal reaction. ⁹⁷ The overpotential value shifts from 366 mV to 347 mV when KB is linked with NiCo-BTC (MOF), exhibiting higher rate kinetics of OER catalyst. Ge et al. demonstrated the facile 2D bimetallic MOF-Fe/Co synthesis (Figure 11a) via stirring with a significant overpotential value of 238 mV, which exhibits that the addition of Fe in 2D MOF-Co modifies Co electronic states and reduces the free energy of oxygen evolution reaction for using on industrial level. ⁹⁸ The energy dispersive spectroscopy (EDS) elemental mapping (Figure 11b) and the LSV polarization curves are shown in Figure 11c. The lower charge transfer resistance (Rct) value of 3.3 Ω of the catalyst between the electrode and electrolyte exhibited charge transfer on the catalyst’s surface.

Figure 11:

Synthesis, structural characterization, and OER performance of ultrathin 2D MOF-Fe/Co ultrathin nanosheets. (a) The graphical representation of ultrathin 2D MOF-Fe/Co synthesis route; (b) EDS elemental mapping and, (c) LSV polarization curve of 2D MOF-Fe/Co nanosheets. Reproduced with permission from Ref. 98]. Copyright 2021. Wiley.

Ma et al. reported the in-situ synthesis of bimetallic nanoparticles encapsulated within metal-organic frameworks (MOFs) by fabricating nanoparticles of Ni–Cu alloy. ⁹⁹ The Ni–Cu@Cu–Ni-MOF-74 exhibits 71.18 times higher OER activity at overpotential 624 mV and 98 mV dec⁻¹, which results in an enhancement ofECSA and more catalytic active sites. Ai et al. presented an imperative approach for the synthesis of cobalt/cerium-based metal-organic frameworks (Co/Ce-based MOFs) via a simple solvothermal method which promotes OER electrocatalysis due to the presence of catalytically inert Ce-BTC as an OER promoter, as visualized in Figure 12a. ¹⁰⁰ The LSV polarization curves and comparisons of OER current densities are presented in Figure 12b and c. The synthesized Co/Ce-based MOFs exhibit remarkable OER activity with a low overpotential value of 308 mV and prolonged electrocatalyst stability over 25 h compared to commercial catalysts.

Figure 12:

Synthesis, structural characterization, and OER performance of Co/Ce-based MOFs. (a) The schematic representation for crystal growth of Co/Ce-based MOFs; (b) polarization curves and (c) comparisons of OER current densities and enhancement factor for Co/Ce-based MOFs. Reproduced with permission from Ref. 100]. Copyright 2022. Elsevier.

Numerous investigations have highlighted (Table 2) that the efficiency of these electrocatalysts has remarkable intrinsic properties when graphene or reduced graphene oxides are templated to emphasize their potential. The excellent OER performance of a layered metal-organic framework CoP/reduced graphene oxide composite (COP/rGO-400) at an overpotential of 340 mV exhibits a stable electron transfer process displayed superior activity to that of the integrated Pt/C and IrO₂ catalyst couple. Additionally, COP/rGO-400 was reported for the very first time as a bifunctional catalyst due to the sandwiched layer of reduced graphene oxide. ⁸⁶ Yaqoob et al. investigated the OER electrocatalytic maintenance of cobalt benzene tricarboxylic acid-based metal-organic frameworks with reduced graphene oxide (Co BTC-rGO) in alkaline conditions by solvothermal methods. ¹⁰¹ The overpotential of 290 mV exhibits a Tafel slope value of 71.4 mV dec⁻¹ due to the π–π interaction between the MOF and reduced graphene oxide. This results in long-term stability and excellent OER catalytic activity. Yan et al. synthesized an electrocatalytic hybrid composite through the implantation of a nickel organic framework on graphene oxide (Ni₂P/rGO) with an enhanced oxygen evolution reaction. ⁸⁸ This resulted in an overpotential of 260 mV for OER and dispersion of ultra-small Ni₂P active sites adjacent to graphene oxide. Gopi et al. synthesized bifunctional water-splitting electrocatalysts from a bimetallic V-doped Ni_xFe_yMOF@Graphene oxide composite, which was created by solvothermal synthesis on a carbon-supported architecture, as presented in Figure 13a. ⁸⁹ Furthermore, our results showed that vanadium doping might work well with Ni/Fe to produce a modest overpotential for OER of 210 mV at 10 mA cm⁻². The morphological structures in TEM images (Figure 13b–d) along with the LSV curves (Figure 13e) and Tafel slopes comparisons with overpotential are shown in Figure 13g. The positive shift from Ni²⁺ to Ni³⁺ in the catalyst V-Ni_xFe_y-MOF/GO resulted in the smallest Tafel slope of 97 mV/dec, indicating the faster flow of electrons for the oxygen evolution reaction.

Figure 13:

Synthesis, structural characterization, and OER performance of V-Ni_xFe_y-MOF/GO catalyst. (a) Synergistic route illustration to synthesize V-Ni_xFe_y-MOF/GO catalyst; (b–d) TEM images; (e) LSV curve and (f) Tafel slopes; (g) Tafel slope and overpotential comparison for the synthesized catalyst with different ratio of Ni and Fe metal. Reproduced with permission from Ref. 89]. Copyright 2022. Elsevier.

Abazari et al. reported a cost-effective Fe–Ni–P/rGO composite through the solvothermal method in an alkaline medium, revealing a lower overpotential of 264 mV with a Tafel slope of 62 mV dec⁻¹. ¹⁰² The development of a bimetallic FeNi catalyst facilitates (–OH) transfer through its non-porous structure and exhibits amazing OER performance. Fang et al. used a novel rational strategy to synthesize a bimetallic iron-nickel phosphide reduced graphene oxide (R@FeNi) composite by pyrolysis and subsequent phosphidation process, as shown in Figure 14a. ¹⁰³ The catalyst presents superior OER activity by achieving a low overpotential value of 240 mV at a current density of 10 mA cm⁻². The morphological structures in the form of TEM images and water contact angle of the R@FeNi composite is shown in Figure 14b and c. Additionally, the R@FeNi composite exhibits exceptional OER kinetics with a LSV curves and Tafel slope (63 mV dec⁻¹) compared to other non-noble MOFs due to the high conductance of the reduced graphene oxide (Figure 14d and e). Xie et al. reported a novel three-dimensional graphene oxide nickel-based metal-organic framework (3D Gr/Ni-MOF) composite synthesized by combining freeze drying and ultrasonic technique. ¹⁰⁴ The OER electrochemical performance of 3D Gr/Ni-MOF catalyst exhibits a lower overpotential of 370 mV due to exposed Ni active sites with enhanced charge and mass transfer, resulting in improved OER performance.

Figure 14:

Synthesis, structural characterization, and OER performance of R@FeNi composite. (a) Graphical illustration for preparation of R@FeNi composite; (b) TEM images; (c) water contact angle of R@FeNi and, (d) LSV curves and (e) Tafel slopes for OER performance of R@FeNi composite Reproduced with permission from Ref. 103]. Copyright 2022. American Chemical Society.