Minireview: Ni–Fe and Ni–Co Metal–Organic Frameworks for Electrocatalytic Water‐Splitting Reactions

Electrolysis is one of the clean, environmentally friendly, and sustainable pathways to produce hydrogen for renewable energy storage. However, to make electrolysis a competitive technology for hydrogen production, developing nonprecious metal‐based catalysts for oxygen evolution reaction (OER) is mandatory. Several new classes of electrocatalysts are developed with outstanding OER catalytic activity, stability, and commercial viability. Owing to the structural diversity, porosity, and accessibility of catalytically active metal centers, nickel‐based metal–organic frameworks (MOFs) are intensively explored as OER catalysts. In particular, bi‐ and trimetallic Ni MOFs with Fe and Co as additional metal nodes show excellent OER activity which can be tailored through the fine tuning of the metal compositions. Herein, the current state of research in Ni‐based MOFs as OER catalyst materials for alkaline electrolysis is presented. Strategies to improve the catalytic performance like compositional variations, choice of synthetic routes, and support materials are presented. Furthermore, OER activities are compared and presented based on the performance metrics (current density, overpotential, and Tafel slopes). Finally, concluding remarks featuring the key findings in Ni‐based MOFs and the possible rooms for future developments are summarized.


Introduction
Sustainable renewable energy sources such as solar and wind energy are the two principal routes to meet the global green energy demand with no or minimal carbon footprint. [1,2] Yet, the energy production from both sources is characterized by fluctuations and are intermittent in nature. Therefore, finding a suitable and efficient storage solution is researched intensively at fundamental and technological levels. Several storage solutions are proposed and applied; among them, the electrolysis of water is very promising because hydrogen, as one of the reaction products, is a facile energy carrier. [3][4][5] The water electrolysis reaction is composed of two half reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). To drive both reactions at appreciable rate and with low overvoltage, active and stable catalysts are required. [6] Though both reactions are sluggish, the OER is kinetically very demanding and much of the potential losses are associated with it. [7,8] As such it is considered as the performance limiting factor for the overall water-splitting reactions. [9] Today's benchmark OER catalysts are based on rare and costly elements such as Ru and Ir. [10] The best-performing OER catalysts, RuO 2 and IrO 2 , require an overpotential from 200 mV (in acid) to 300 mV (in base) to achieve a current density of 10 mA cm À2 (the bottom threshold current density for commercially intended OER catalysts). [11,12] To deploy water electrolysis in larger scales and make it a competitive energy storage technology on the market, developing stable, efficient, and cost-effective OER/ HER catalysts based on Earth-abundant elements is essential. [2] Transition metal oxides and hydroxides are in the forefront of this research and show tremendous potential as OER/HER catalysts with outstanding performances. [13][14][15] Particularly the Ni/Fe and Co/Ni oxyhydroxides demonstrated the best OER activities, mostly comparable or even surpass the performance of the benchmark catalysts. [11,16,17] Motivated by these results, a wide class of transition metal compounds are investigated for OER/ HER catalysis and among them are metal-organic frameworks (MOFs). [18,19] MOFs are inorganic-organic hybrid crystalline materials with very high porosity (>60% free volume) displaying huge internal surface areas. [20] The modulation or variation of the two components, the metal ions and the organic linkers, enables the fine tuning of the chemical and morphological structures, leading to intriguing properties of MOFs with potential applications in areas of gas storage, separation, and catalysis. [21][22][23] The interest on MOFs as electrocatalysts for OER/HER fundamentally relies on 1) tunable porosity and morphology for accelerated diffusion of reactive components and 2) well-coordinated but spatially separated single (multi)-active metal centers which function synergistically for the targeted catalytic reaction. [18] The present minireview focuses on Ni-based bi-and trimetallic MOFs containing Fe and Co mainly for electrocatalytic oxygen evolution and to a less extent hydrogen evolution reactions. Revisiting the literatures published in the past 10 years clearly demonstrate that MOF electrocatalysts for OER and HER are quite dominated by Ni-based MOFs and their derivatives. [24] We have strictly limited the scope of the review to Ni-MOFs retaining the inherent 3D network structure during and after the electrocatalytic reactions. A recent review by Mukherjee et al. convincingly presented MOFs' inherent electrocatalytic activities. [25] This minireview is also aimed at presenting recent findings that pristine Ni-based MOFs containing Fe and Co are highly active catalysts for OER/HER reactions and highlight that postsynthesis treatments are not necessarily required and if so, do not guarantee to boost their catalytic activities. Other types of electrocatalysts such as pyrolysis products of Ni-MOFs or Ni-MOF-templated nanocomposite materials will not be covered, and readers can refer other excellent reviews on the topic. [18,26] Furthermore, we have presented the activities of pure Ni-MOFs for comparison reasons without presenting details as the topic extensively reviewed in recently published works. [27,28] Thus the article gives brief accounts on recent developments of Ni-Co and Ni-Fe bimetallic MOFs with particular emphasis on 1) the synthesis strategies of Ni-based MOFs and the commonly used organic linkers, 2) application as OER and HER catalysts, and 3) prospects and future challenges.

Linkers
In MOFs, the metal ions are connected by organic linkers. These linkers provide the superordinate structure of the MOF through their structure and the arrangement and number of functional groups involved. These organic linkers contain functional groups that could form coordination bonds, such as carboxylate, amine, nitrile, sulfonates, etc. For Ni-based transition metal MOFs for electrochemical water splitting, terephthalic acid and trimesic acid are the two commonly used organic linkers which are shown in Scheme 1. In the case of terephthalic acid, the structure of the MOFs formed can be modified by addition functional groups, such as amino or hydroxy groups, as steric hindrances and repulsions can occur. These linkers are used because they are relatively inexpensive for the number of functional groups, they have good solubilities in different solvents, which allow the MOFs to be synthesized in different ways, and most importantly, they form stable coordination to the metal centers. However, there are also synthesis routes which involve significantly more complex linkers, for example, biphenyl-3,3',5,5'-tetra-(phenyl-4-carboxylic acid). [29]

Synthesis Methods
In the MOF synthesis, the metal ions and the linkers are connected to each other in such a way that a crystalline and porous structure is created. In recent years, several synthetic procedures were developed to synthesize transition metal MOFs for electrochemical water splitting (Scheme 2). These are mainly categorized into three synthesis methods, which in turn can be divided into a few subgroups.

Solvothermal/Hydrothermal
Solvothermal or hydrothermal synthesis are the most widely used routes for the synthesis of transition metal MOFs. [30][31][32][33] The metal salts, for example, nitrates, sulfates, or chlorides, and the organic linkers are dissolved in a suitable solvent, in most cases N,N-dimethylformamide (DMF), water, or ethanol. The mixture is put and sealed in a reaction vessel (an autoclave or a glass tube), which withstands the high synthesis temperature and pressure. Various process parameters can be varied, such as pressure, time, temperature, solvent, reagent concentrations, etc. The influence of temperature is vital, and it is greatly dependent on the heating method. Thus, conventional heating creates the significant concentration gradient which increases the local ion metal concentration and size of crystals. As a result, the morphology and size of the MOFs are strongly affected by the gradient and method of heating. In many cases, the substrate to be functionalized is placed directly (e.g., metal foams, foils, and carbon cloth [CC]) into the reaction vessel so that a later electrode preparation step can be avoided and a good contact between the substrate and the MOF is ensured. In the cases of MOFs that are not formed directly on the substrate, a washing and drying process is necessary to remove the residues of the metal salts and linkers. Subsequently, a catalyst ink is prepared from the MOF powder using appropriate solvent and binder (Nafion solution) and is applied to the substrate with different coating methods. Nafion ensures high adhesion to the electrode and it has high electrical conductivity, which does not greatly reduce the activity.

Precipitation/Sonochemical
Some MOFs for electrochemical water splitting can also be synthesized at ambient pressure and temperature. [34,35] The most common and easy route is dissolving the linker and metal salt in a solvent together and letting the MOF to precipitate out from the mixture. However, in some syntheses, the solution must be stirred or ultrasonically mixed for a long time to precipitate the MOF. This approach is more attractive from both industrial and environmental aspects, due to its simplicity, and no energy input is necessary in the MOF formation processes. However, in some cases, the crystallinity of the MOF materials prepared at room temperature is relatively poor. Thus, this synthesis route can only be used for certain MOFs.

Electrosynthesis/Electrodeposition
The third main way to synthesize MOFs for electrochemical water splitting is electrosynthesis/electrodeposition, where the MOF is directly formed on the electrode. [36,37] Electrochemical methods for the synthesis of MOFs offer several advantages including short reaction times and mild synthesis conditions, fine control over the deposition parameters, potential for scale-up, and more importantly direct growth of MOFs on various types of conducting substrates, a prerequisite for many electrocatalytic applications including OER/HER. In the synthesis, the metal salts and the linkers are dissolved in a solvent together with organic or inorganic electrolyte salts (e.g., Tributyl methylammonium methyl sulfate (MTBS)). A two-or three-electrode setup is used, with a voltage applied between the cathode and anode or between the working and reference electrodes. This forms a homogeneous MOF layer on flat and highly porous conductive surfaces.

Microwave
The microwave synthesis of MOFs is like the solvo-/ hydrothermal synthesis; however, the reaction vessel is heated with electromagnetic microwaves (λ ¼ 10 À3 mÀ1 m) instead of a conventional furnace. [35,38,39] This microwave radiation can cause the (polar) molecules in the solution to oscillate, causing them to heat up. Due to this direct irradiation on the molecules, the microwave synthesis is significantly faster and more energy efficient than the conventional solvo-/hydrothermal synthesis. In addition, most microwave ovens offer higher heating rates and stirring in the reaction vessel, which leads to a more homogeneous heat distribution in the solution. As a result, the size and size distribution of the MOF crystals are highly controlled. Microwave ovens use usually significantly smaller reaction vessels than the solvo-/hydrothermal reactors, which lead to a smaller yield per batch, but this can be compensated by the much faster reaction. In addition, the direct high-energy input results in high pressures, which limit the choice of reaction vessels.

Mechanochemical
In mechanochemical synthesis, the reactions between the metal ions and the linker are initiated using mechanical energy produced during a milling or grinding process. [40,41] The energy can be generated by mechanical force produced manually by hand, with a mortar and pestle, or with automated ball-mill and grinding devices. The heat created through friction induces the mixtures to react, allowing the formation of MOF. No solvents or catalytic amounts of solvents are used in most mechanochemical synthesis methods, which is highly attractive for reducing the cost of production and environmental pollution. However, if milling or grinding is manually performed, the reaction conditions are difficult to control, and products are less reproducible. However, modern and automated ball mills avoid these problems since reaction conditions can be easily adjusted.
Few representative examples of Ni-based MOFs with the respective synthesis routes are summarized in Table 1.

Co-Ni MOFs
Ni-MOFs can act directly as OER electrocatalysts from the point of view of their unique structures and compounds but show poor conductivity. This is due to the blocking of active sites by organic linkers and poor interactions between metal centers and ligands. To improve the electrocatalytic efficiency of MOFs, various strategies have been developed such as annealing, second metal addition, heterometal doping, defect engineering, morphology tuning, heterostructure construction, and hybridization.
To improve the properties (e.g., porosity, surface area, crystallinity, conductivity, mechanical strength, structural complexity, and stability) of the single-metal framework materials, the construction of bimetallic organic frameworks (bimetallic MOF) with controlled compositions, morphologies, and structures has received significant attention. According to the distribution of metal ions, bimetallic MOFs can adopt "solid solution" or "hybrid" structures. In the solid-solution bimetallic MOFs, the metals show delocalized or homogeneous distributions through the whole crystal. In hybrid bimetallic MOFs, the chemically different MOFs form next to each other. [42,43] In this section, we focus only on Co/Ni solid-solution bimetallic MOFs.
Recently, the performance of Ni-MIL-77 was improved by forming bimetallic MOFs by Xiao et al. [44] Figure 1a shows the schematic preparation of Ni/Co MOF. The electrocatalytic OER activity of different mole ratios of Ni:Co was examined (Ni-MOF as R1, Ni/Co(20:1)-MOF as R2, Ni/Co(10:1)-MOF as R3, Ni/Co(5:1)-MOF as R4, Co-MOF as R5) at a scan rate of 5 mV s À1 . It can be seen in Figure 1b that the addition of Co increases the electron transfer rates by enhancing the electroactive surface area of the material compared to the other MOF materials. R3 reveals the highest OER performance among all samples, which shows an overpotential of 249 mV with a small Tafel slope of 40.92 mV dec À1 for OER (at a current density of 10 mA cm À2 ). The authors claimed that the high performance of ultrathin nanobelts (R3) is related to several factors. 1) The elongated structure facilitates electron transfer; in addition, the ultrathin belt-like structures have good dispersion in alkaline solution. 2) the higher surface area arising from the more porous structure plays an important role in higher efficiency. 3) the incorporation of Co leads to better conductivity in MOF. 4) The presence of different metal centers in MOF makes them potential electrocatalytic active centers. With the second metal doping, the electronic environment of the metal centers could be modulated, which enhances the performance. 5) comparing the valence electron configuration of Ni (3d 8 4s 2 ), Co (3d 7 4s 2 ) is more easily oxidized to high-valence states, known to be highly active for the OER.  Figure 1. a) Schematics of preparation and catalytic processes. b) The OER performance of the five kinds of MOFs. Reproduced with permission. [44] Copyright 2018, Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com In a recent report, Zhou et al. synthesized a series of bimetallic Co/Ni-MOFs (CTGU-10c2) for the electrocatalytic OER through the solvothermal method. [45] The as-prepared Co/Ni-MOF (CTGU-10c2) with a hierarchical nanobelt structure demonstrated low onset potential of 140 mV at 10 mA cm À2 in 0.1 M KOH ( Figure 2a). The authors explained that the high OER performance of Co/Ni-MOF (CTGU-10c2) can be attributed to the coupling effect between Ni and Co and the existence of unsaturated metal sites. Furthermore, the CTGU-10c2 displays low charge transfer resistance, a smallest Tafel slope (58 mV dec À1 ), and high electrochemical active area (Figure 2b-d). To explain the high performance, especially the synergic effect of Co and Ni, the authors used density functional theory (DFT) calculations with various metal ratios. Figure 2e exhibits the simplified model catalyst, focusing on the effect of various ratios of the metals with the same linker. The calculated OER energy profile and overpotential are shown in Figure 2f. It can be concluded that the bimetallic MOFs act better as OER catalysts than single-metal MOFs; among the four ratios the best activity is observed for Co:Ni ratios of 1:2, with a calculated overpotential of 420 mV. The improvement associated with the new metal originates from the shift of the d-band center to a higher-energy level. The change in the electronic level occurs because of the slight difference between the Co─O and Ni─O bonds in the bimetallic MOFs. This is crucial to catalyst design, which helps to select specific metals or new ligands that lead to heavier distortion and higher performance. Based on calculations, Co is the active center in the CoNi structure that proves that the role of the introduced Ni center is to cause distortion and thus improve the activity of the Co center.
Liu et al. presented a 2D Co-Ni MOF deposited on a Cu foil that acts as a high-performance electrocatalyst for OER with a low overpotential of 265 mV at 10 mA cm À2 in an alkaline solution. [33] The Co-Ni MOF with Co:Ni ratio of 1:1 displays outstanding activity with Tafel slope of 56 mV dec À1 , the smallest among the series (Figure 3a). The Co-Ni MOF was prepared under solvothermal conditions (80°C, 2 h) on Cu foil using 2,4-naphtalenedicarboxlyic acid (NDCA) as the linker to form vertically oriented nanosheets (Figure 3d). In situ conducting atomic force microscopy and two-point conductivity measurements proved that the electron transfer along the Z-axis is better than other orientational axes. This could be one of the attributing factors for the improved electrocatalytic activities. After heat treatment, the 2D Co-Ni MOF transforms to a hybrid nanoplate array of metallic nitrides on an amorphous carbon network that catalyzes the HER with a modest overpotential of 120 mV at 10 mA cm À2 and shows great stability ( Figure 3b). The overall water splitting using 2D Co-Ni MOF catalysts demonstrated a 99% Faradaic efficiency at 1.64 V, suggesting that the 2D Co-Ni MOF is a bifunctional electrocatalyst with promising performance for the total water-splitting systems.
Li et al. reported their findings on the expansion of amorphous nickel-cobalt bimetal MOF nanosheets with crystalline motifs through an easy "ligands hybridization engineering" method. [46] The ligands include inorganic ligands (NO 3 À and H 2 O) and organic ones, hexamethylenetetramine (HMT). Further, they www.advancedsciencenews.com www.small-structures.com analyzed a sequence of composite metals with multiligand substances as OER catalysts to examine their probable benefits and characteristics. It is found that Ni doping is an efficient approach for optimizing the electronic structure, varying lattice ordering degrees, and thus improving the activities of HMT-based electrocatalysts. Furthermore, the crystalline-amorphous boundaries of different HMT-based electrocatalysts can be easily controlled by simply changing the amount of the Ni precursor added. As a result, the optimized ultrathin MOF of (Co, 0.3Ni)-HMT nanosheets can achieve a current density of 10 mA cm À2 at a low overpotential of 330 mV with a small Tafel slope of 66 mV dec À1 . The electronic structure change generated by Ni doping and 2D nature of the MOF nanosheet structure with multiple ligands plays an essential role in facilitating the kinetically slow OER process. The report emphasizes the significance of active metals and the use of different ligands in a single MOF as a novel approach to prepare effective electrocatalysts. In another work by Zheng et al., bimetallic CoNi-MOF nanosheets/reduced graphene oxide (rGO) hybrid electrocatalysts are reported. The CoNi-MOF nanosheets were in situ grown onto rGO using the surfactant modulation method using tetrakis(4-carboxyphenyl) porphyrin (TCPP) as linker. The CoNi-MOF/rGO hybrids, nanosheets which are homogeneously encapsulated in rGO, display improved electrocatalytic activities toward OER and oxygen reduction reaction (ORR). The rGO acts as the catalyst support for the composite due to its large specific surface area and exhibits high electrical conductivity. In addition, rGO can also be a pillar connector in MOFs because of the hydroxyl and epoxy functional groups on the surface. The improved catalytic performances of CoNi-MOF/rGO can be related to the improved electron-conductive property of MOF/ rGO hybrids and the large surface area of rGO which allow the growth of dense CoNi-MOF nanosheets. Thus, the highly dense catalytic sites with facilitated electron transport pathway are crucial for the enhanced performance during the electrochemical reaction. [47] Recently, the bimetallic alloys (e.g., FeCo, FeNi, and CoNi) show higher performance in electrocatalytic activities than their single metals for OER, because the combination of two metals can show inherent polarity to cause synergetic effects. Among them, CoNi alloy is more interesting because of their features of low cost and good environmental friendliness. Previous studies showed, in an alkaline solution, that nickel tends to desorb OH À more than cobalt, and cobalt is more effective in accelerating the rate-determining step. The recent report of Liu et al. demonstrated a strategy to form the Co-Ni alloy nanoparticles encapsulated in nitrogen-doped porous carbon frameworks by annealing a Co-Ni MOF. The optimized catalyst (Co2Ni@NC) exhibits excellent electrocatalytic activities and durability in alkaline solution. In summary, the Ni-Co MOFs are not only highly active OER catalysts by their own, they are also flexible platforms to synthesize other OER catalysts. [48] The recent Ni/Co-based MOFs for water splitting are summarized in Table 2.

Ni-Fe MOFs
Ni-Fe bimetal organic frameworks are investigated for OER and HER considering many benefiting factors: 1) having similar  radii, the bimetallic organic framework is facilely formed by replacing the Ni atom by Fe atom (or vice versa), keeping the structural integrity and stability of the MOF structure, 2) the bimetal organic framework is expected to display superior electrocatalytic properties due to the synergetic catalytic properties of both Fe and Ni, as shown for Ni/Fe oxy-hydroxide systems, and [10,49] 3) performance comparison of the bimetallic electrocatalysts with single-component Ni or Fe MOFs is feasible, as their structures and morphologies largely remain unchanged, thus guaranteeing to systematically study composition-dependent electrocatalytic activities.
The first systematic study of the OER activity of Ni-Fe MOFs based on 1,3,5-benzenetricarboxylic acid (BTC) was reported by Wang et al. The authors prepared a series of Fe/Ni MOFs with varying Ni/Fe (1:1, 3:1 and 12:1) ratios using a solvothermal synthetic approach and the OER catalytic performances were investigated. [36] The mixed-metal Fe/Ni-BTC with Ni/Fe molar ratio of 12:1 shows high OER catalytic activity with low onset potential of 170 mV on a glassy carbon electrode (GCE) (Figure 4a). However, due to the poor conductivity and adhesion of the film, the current density was much lower than the benchmark value (10 mA cm À2 ). To overcome this problem, the Electrode material was immersed inside the reaction mixture; GCE: glassy carbon electrode; CC: carbon cloth; DMI: dimethylimidazole; H 6 BHB: 5 0 (3,5-dicarboxylphenyl)-[1,1 0 :3 0 ,1 00 -terphenyl]-3,3 00 ,5,5 00 -tetracarboxylicacid.  authors used a facile electrochemical deposition (ED) approach to directly grow Fe/Ni-BTC MOF thin film from a solution containing the metal precursors and BTC. A highly active and stable binder-free film with improved electrical conductivity was obtained on a nickel foam (NF) substrate. The ED approach is fast, reduces electrode preparation steps, and avoids the use of binders. This electrode shows high activity with a low overpotential of 270 mV at 10 mA cm À2 and a small Tafel slope of 47 mV dec À1 (Figure 4b). The MOF film maintains its OER catalytic activity for 15 h without detectable activity loss. On the other hand, thin films prepared with similar Ni/Fe ratios but without the organic linker BTC lead to bulk Ni/Fe-mixed hydroxides with inferior OER activity. This suggests that the intrinsic porosity achieved by the BTC and the uniform distribution of the metal ions in the MOF structure are key for enhanced OER activities.
Performance comparison at higher current densities (Figure 4c) clearly shows that the Fe/Ni-BTC MOF has the best OER activity. Furthermore, the ED synthetic strategy can be scaled up for large-area electrode fabrication with potential application for real-world commercial electrolysis. Duan and his co-workers prepared ultrathin 2D nanosheet array NiFe-MOF on various supports via a facile one-step chemical bath deposition method. [31] The MOFs are formed directly on the substrate by adding the organic linker NDCA to an aqueous solution of nickel and iron metal salts. The MOF crystal structure consists of alternating units NDCA and octahedrally coordinated metal-oxygen layers (MO 6 units; M (Ni, Fe, or Cu) as shown in Figure 5a. The formation of the 2D nanosheet arrays is only realized in the presence of the substrates; without the substrate a bulk material of aggregates of nanosheets with microsized secondary nanoparticles was obtained. Varying the reaction time led to NiFe-MOF displaying different morphologies, microrods at 3 h, or nanosheets of varying sizes (10 or 20 h), suggesting that the nanocrystal growth follows the dissolution-crystallization mechanism.
The OER electrocatalytic activity test shown in Figure 5b-d indicates that the NiFe-MOF prepared on nickel foam can deliver a current density of 10 mA cm À2 at an overpotential of 240 mV, which is smaller than the pure Ni-MOF (296 mV) and Fe-MOF (324 mV). In addition, NiFe-MOF/NF outperforms those grown on glassy carbon (NiFe-MOF/GC, 406 mV), suggesting that meso-and macroporosity that stem from NF is crucial for the increased activity. On the other hand, the NiFe-MOF nanocomposite after calcination under N 2 requires higher overpotential (336 mV) to reach the same current density, indicating that the inherent structures of the MOF with molecular Ni/Fe centers are responsible for the higher catalytic activities. Interestingly, the 2D NiFe-MOF exhibits higher conductivity (3 orders of magnitude) compared to the bulk NiFe-MOF, as evidenced from a four-point conductivity measurement, suggesting that the intrinsically low conductivity of MOFs, which limits their application as electrocatalysts, can be significantly improved by modulating the morphologies. Furthermore, the authors demonstrate an electrolytic cell for the complete water splitting using NiFe-MOF as both a cathode and an anode. The cell can deliver 10 mA cm À2 at a voltage of 1.55 V without showing noticeable  to deliver 10 mA cm À2 . The work demonstrates a useful strategy to overcome some limitations of MOFs as electrocatalysts, namely, 1) the ultrathin nature of the nanosheet arrays aids in the exposure of more Ni/Fe metal centers for the catalytic activity and improves electrical conductivity, 2) the mesopores between the nanosheets decrease the diffusion path length and facilitate the mass transport of reaction products, which would otherwise be highly hindered by the very narrow pores of the MOF, and 3) the catalyst support, NF, featuring large macropores allows easy access of electrolytes and gaseous products to and away from the reaction sites. However, as the authors only presented NiFe-MOF with one composition (23% Fe according to the X-ray photoelectron spectroscopy (XPS)), further studies focusing on the compositional variations will help to understand the synergy of the two metals for the OER/HER activities. Sun et al. reported a novel self-templating one-step solvothermal route to synthesize a binder-free 3D electrode by developing MOF nanosheet materials on Ni foam (NF) surface. [32] MIL-53(FeNi)/NF was prepared from a slightly acidic solution (pH ¼ 5) containing FeCl 2 , DMF, ethanol, and water with NF immersed in it under hydrothermal conditions. The added Fe atoms and the Ni atoms released from NF substrate (due to the acidic etching) would link with terephthalic acid linker molecules to form a stacked nanosheet morphology on the surface of the 3D NF, as shown in Figure 6a,b. The formation of MIL-53 crystal structure was confirmed from the selected-area electron diffraction (SAED) patterns ( Figure 6d) and X-ray diffraction (XRD) measurements. The quantitative energy-dispersive Xray spectroscopy (EDS) compositional analysis reveals that Fe and Ni atoms are uniformly distributed and exist in 1:1 ratio (Figure 6e). The XPS results showed that both Fe and Ni atoms exhibit 2þ oxidation states. However, XPS quantitative compositional analysis was not reported, which would help to compare the surface composition to the bulk. As most electrocatalytic reactions, including OER/HER, are surface initiated, surface compositions are more relevant and give useful insights on catalytically active sites.
The OER activity in 1 M KOH solution showed that the as-prepared MIL-53(FeNi)/NF electrode has superior OER performance, with a current density of 50 mA cm À2 at an overpotential of 233 mV and Tafel slope of 31.3 mV dec À1 (Figure 6f,g).  However, at the same overpotential, the control electrodes prepared under similar conditions, MIL-53(Ni)/NF, TPA/NF, and NF, deliver only 3.25, 1.69, and 0.22 mA cm À2 , respectively. The authors also reported the technically relevant mass activities which should be encouraged in the community for fair activity comparison in newly reported catalyst systems. Consequently, at the overpotential of 233 mV, mass activities of 19.02, 6.05, and 1.76 A g À1 were achieved for the MIL-53(FeNi)/NF, MIL-53(Ni)/ NF, and TPA/NF electrodes, respectively. The authors suggested that the higher OER activity is related to: 1) catalytically active octahedral MO 6 structures and exposed hydrophilic carboxyl groups, 2) increased ECSA as evidenced from capacitance measurement (Figure 6h), 3) high electrical conductivity, and 4) DFT calculation that shows increased 3d orbital electron density for Ni ions, which enhance the OER. Furthermore, calculations show that MIL-53(FeNi) structure has penta-coordinated Ni atoms which facilitate foreign atoms adsorption. The current synthesis route cuts the electrode preparation steps for OER significantly and provides a single-step process that ensures the intimate contact of the catalyst layer with the underlying current collector without the use of binder materials. This has manifold advantages: 1) guarantees good adherence which withstands the harsh OER conditions with massive gas bubble formation, 2) avoids the screening of effective electrochemical area by the binders, and 3) reduces contact resistance (improved conductivity). All these parameters are critical for MOF-based OER catalysts which intrinsically have relatively low electrical conductivity, low mass permeability, and blockage of active metal centers by organic ligands.
In a related work, Xing et al. reported the hydrothermal synthesis of NiFe-MOF-74 using the organic linker 2,5-dihydroxyterephthalic acid. [50] The advantages of MOF-74 compared to other MOF structures is that it contains coordinately unsaturated metal sites, which provide abundant metal sites. Furthermore, the fully coordinated oxygen in H 4 DOBDC makes MOF-74 very stable. [51] The MOF is directly grown on NF under hydrothermal reaction conditions (120°C, 24 h) from a mixed solution containing the H 4 DOBDC ligand in the absence or presence of FeCl 2 to form Ni-MOF-74/NF and NiFe-MOF-74/NF, respectively. The reaction temperature and amount of Fe were optimized to obtain high-performing OER catalysts. Note that the Ni-ion source is the NF which undergoes the slow-etching process under hydrothermal conditions. The authors suggested that the presence of Fe led to the uniform distribution of wellordered rhombic MOF crystals throughout the NF substrate (Figure 7c,d). However, without the Fe ions sparsely populated, MOF particles are obtained as evidenced from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements (Figure 7a,b). EDS mapping shows uniform distribution of Fe and the ratio of Ni to Fe was obtained from inductively coupled plasma atomic emission spectroscopy (ICP-AES) and was found to be 22:1 for the most active sample. The selfsupported NiFe-MOF-74/NF exhibits superior electrocatalytic performance toward OER in alkaline solutions (1.0 M KOH), requiring only 223 mV overpotential to deliver 10 mA cm À2 with a Tafel slope of 71.6 mV dec À1 (Figure 7e   require 310 and 360 mV of overpotential to deliver the same current, respectively. The high OER activity NiFe-MOF-74/NF sample is attributed to large electrochemical surface areas, rapid charge transfer capability, and mechanical stability. The authors also demonstrated an electrolytic cell consist of NiFe-MOF-74/ NF anode & Pt/C/NF cathode operating at a cell voltage of 1.54 V and 10 mA cm À2 and with good long-term stability. Zheng et al. also reported the preparation and OER activities of bimetallic FeNi-MOF samples with the Ni/Fe molar ratios of 9:1, 5:1, 3:1, and 1:1 using the same organic ligand of H 4 DOBDC following a reported synthesis procedure. [52] The as-prepared samples exhibit MOF-74-type crystal structure except that of the pure Fe-MOF, as confirmed from XRD measurements. The synthesis route fails to produce pure Fe-MOF; instead, it forms the Fe/H 4 DOBDC complex. The addition of Fe induces the nanosheet morphologies, and the thickness and regularity of the nanosheets are highly dependent on the Fe concentrations. Furthermore, the presence of Fe significantly decreases the BET surface area (6-7-fold) compared to the pure Ni-MOF. The higher OER activities were obtained for samples prepared from Ni/Fe molar ratios of 3:1 (FeNi-DOBDC-3). However, analysis from ICP-AES shows higher Fe content (Fe:Ni, 1:1.171) compared to the molar synthesis ratios, suggesting that Fe ions easily coordinate with H 4 DOBDC. Current densities of 50 and 100 mA cm À2 can be achieved at overpotentials of only 270 and 287 mV, respectively. Furthermore, a low Tafel slope of 49 mV dec À1 indicates that the OER is kinetically favored. The authors suggested that the outstanding OER catalytic activity could originate from the highly porous structure, large surface area, high conductivity, and synergy interaction between Fe and Ni in the MOFs. As mentioned above, the high conductivity may play a significant role compared to the other properties.
Boettcher et al. reported that the conductivity of Ni 1Àx Fe x OOH increased from 0.2 to 6.5 mS cm À1 , when the Fe content reached 25%, and the overpotential for OER significantly dropped by a value of 200 mV. [53] A similar trend is also expected for Ni-Fe MOF.
Yang et al. utilized a hydrothermally grown NiFe-layered double-hydroxide nanoarray on a nickel foam (NiFe-LDH/NF) as both the template and precursor for the fabrication of a highly oriented layered MOF nanoarray (Fe 0.1 -Ni-MOF/NF) (Figure 8a). [54] The Ni 2þ and Fe 3þ ions in the solution replaced the H atoms of the NiFe-LDH layer, which later coordinated with the organic linker terephthalic acid. As the reaction progresses, the metal sites are fully reacted to form the nanosheet MOF self-assembled Fe 0.1 -Ni-MOF (Figure 8b). The transformation of NiFe-LDH to the NiFe-MOF is tracked and confirmed by XRD. The energy-dispersive X-ray spectroscopy (EDX) mapping shows uniform distribution of Ni and Fe, with Fe accounting only %1% of the total composition. Such low amounts of Fe may result from the fact that there is high amount of Ni coming from the nickel foam substrate.
The as-prepared Fe 0.1 -Ni-MOF/NF was tested as the electrocatalyst for alkaline water oxidation. It requires low overpotentials of 243 and 263 mV to generate 50 and 100 mA cm À2 in 1.0 M KOH, respectively (Figure 8a). The Tafel slope analysis shows (Figure 8b) that the OER process is not kinetically hindered on Fe 0.1 -Ni-MOF/NF with a small slope of 69.8 mV dec À1 . Furthermore, its catalytic activity could be maintained for at least 20 h at a high current density of 150 mA cm À2 (Figure 8e). Notably, it also achieved high turnover frequency values of 0.018 and 0.086 O 2 s À1 at low overpotentials of 250 and 300 mV, respectively. The authors also investigated the influence of Fe concentration on the OER activities and found out that both  lower and higher concentrations compared to Fe 0.1 -Ni-MOF/NF resulted in higher overpotentials delivering the same current densities, suggesting that 1% Fe atomic concentration is the optimal. In addition, Fe 0.1 -Ni-MOF/NF possesses a higher electrochemical surface area as evidenced from the double-layer capacitance measurement, which possibly contributes to its higher OER activity. Interestingly, the electrocatalyst also shows promising activities under high KOH concentration (30 wt%, mostly used by commercial electroyzers) and requires overpotentials of 233 and 295 mV to drive 100 and 400 mA cm À2 , respectively, promising its practical applications.
Recently, Ji et al. prepared NiFe bimetallic MOF nanosheets by room-temperature hydrothermal condition and investigated the effect of lattice strain using XPS and X-ray absorption spectroscopy (XAS). [55] The strain was controlled via the addition of monocarboxylic acid linkers resulting in the formation of NiFe-MOF with a missing linker. The XPS investigation of the lattice-strained NiFe-MOF (LS-NiFe-MOF) presented in Figure 9a,b shows the slight shift of Ni 2p XPS peaks to higher binding energy while the Fe 2p XPS peaks shifted to lower energy. This suggests a partial Ni-to-Fe electron transfer, leading to the formation of empty Ni 3d orbitals and filled Fe 3d orbitals induced by the lattice strain. Interestingly, the Ni and Fe L 2 , 3 edge XAS spectra, which indicate the metal 3d occupancy and the changes in the local ligand environment, support the XPS findings. In Figure 9c,d for LS-NiFe-MOF sample, the peak intensity of Ni 3d e.g., is increased, inferring the decreased 3d electron occupation of the Ni atoms. In contrast, the Fe 3d t 2g peak intensity decreased due to the electron transfer. The changes in the Ni and Fe electronic structure could effectively affect the interaction of the two atoms with the reaction intermediates, leading to improved catalytic activity for OER. The polarization curves of NiFe-MOFs and LS-NiFe-MOFs show overpotentials of 320 and 230 mV at current density of 10 mA cm À2 and the Tafel slopes of 164.9 and 86.6 mV dec À1 for NiFe-MOFs and LS-NiFe-MOFs, respectively. To understand the origin of the enhanced OER performance of the LS-NiFe-MOFs, operando X-ray absorption fine structure (XAFS) studies were conducted as a function of the applied potential. As shown in Figure 9e,f, the Ni K-edge operando X-ray absorption near edge structure (XANES) spectra significantly shifted toward the higher-energy side with increasing potential. This suggests the presence of Ni 3þ/4þ corroborating the gradual oxidation of Ni during the OER process as previously reported. [56,57] In contrast, the Fe K-edge position hardly shifts under different applied potentials, signifying that the Fe sites are not active during the OER. The studies show the power of XAS in unraveling OER catalytic centers in multimetal Ni-based MOFs.
A series of MIL-88B-type Fe/Ni MOFs were prepared by Ling et al. in a one-pot synthesis method on nickel foam (NF) using terephthalic acid as linker. [58] The authors prepared three types of MOFs, namely, Ni-MOF, Fe 2 Ni-MOF, and Fe-MOF under hydrothermal conditions. SEM measurements show that MOFs exhibited different microstructures (Ni-MOF nanosheet, Fe 2 Ni-MOF spindle, and Fe-MOF shell-like morphologies). Such morphological variations may have important implications regarding the electrocatalytic activities. XPS investigation reveals, in comparison with Fe-MOF and Ni-MOF, for Fe 2 Ni-MOF that the Fe 2p spectrum shifted to higher binding energy and Ni 2p spectrum shifted to lower binding energy, suggesting partial electron transfer from Fe 3þ to Ni 2þ through the oxygen atoms of the linker. OER tests demonstrated that, compared with the monometallic  [55] Copyright 2020, American Chemical Society. MOFs (Fe-MOF/NF and Ni-MOF/NF), Fe 2 Ni-MOF/NF shows superior performance. It exhibited a low overpotential of 222 mV at a current density of 10 mA cm À2 and a small Tafel slope of 42.39 mV dec À1 . Additionally, lower Fe/Ni ratios (5:5 and 3:7) were tested and showed low activities. The authors attributed the high performance of the Fe 2 Ni-MOF/NF due to 1) increased 3d orbital electron density of Ni which facilitates the OER, 2) improved electrical conductivity, and 3) hybrid morphology with increased active sites. Metal foam substrates possess many appealing properties; however, there is always a danger that MOFs can easily shed from the support surface due to lose contact, especially with the smooth surface. To ensure close contact, some organic polymers were used as binders. However, this may lead to other problems such as hindered diffusion of electrolytes or high contact resistance. Hence Chen et al. developed a facile two-step strategy to fabricate MOF/GA/NF nanocomposite. [59] The graphene aerogel (GA) layer is grafted in nickel foam by dispersing graphene oxide and reductant (ascorbic acid (AA)) under ultrasonication followed by freeze drying to produce GA/NF templates (Figure 10a). Addition of Fe 2þ and terephthalic acid under solvothermal conditions led to the formation of a lamellar bimetallic MFN (MIL-53(FeNi)) encapsulated in the NF (denoted as MFN@GA/NF, Figure 10b-d). The graphene aerogel with a highly porous structure would facilitate the in situ growth of the MOFs and serve as a conductive bridge to connect with metal foam.
The hierarchical MFN@GA/NF was tested in 1 M O 2 -saturated KOH solution to study the electrocatalytic performance. It demonstrated competitive and stable electrocatalytic activities for OER. As shown in Figure 10e, the MFN @GA/NF only requires an overpotential of 250 mV to reach current density of 20 mA cm À2 . However, GA/NF and pure NF require 349 and 377 mV to deliver 20 mA cm À2 , respectively. Interestingly, MFN@GA/NF can deliver 100 and 250 mA cm À2 at overpotentials of 299 and 335 mV, respectively, showing its potential for commercial application. The catalyst exhibited excellent OER activity and possessed remarkable stability over 100 h of continuous electrolysis, which are superior to most pristine MOF active catalysts in alkaline media. Furthermore, the Tafel analysis (Figure 10f ) indicates that the smallest Tafel slope (66 mV dec À1 ) belongs to MFN@GA/NF, suggesting that the oxidation of water is not kinetically hindered. The work provides a new approach to develop a hierarchical structure of 2D MOFs into NF-supported porous graphene aerogel materials. The GA facilitates the OER catalytic activities by reducing the contact resistance and increasing the electrochemical active surface areas (ECSA) (Figure 10g).
Very recently, Öztürk et al. proposed a strategy to overcome some limitations of MOF electrocatalysts using conductive carbon support. [60] They employed a commercially available high-surface-area ketjenblack (KB) porous carbon as a support to synthesize iron containing Ni-MOF-74. Through a simple one-step solvothermal reaction, Ni(Fe)-MOF-74 was directly formed on the KB in the presence of metal ions and 2,5dihydroxyterephtalic acid (Figure 11a). The KB colloidal particles can interact with MOFs via the condensation between surface hydroxyl groups or via the Van der Waals forces, thereby serving as a very good conductive support. Furthermore, KB also provides mechanical stability to the MOF micropores and suppresses the formation of larger MOF particles; as such it facilitates the transport of electrolytes and reaction products. The MOF is uniformly distributed on the porous carbon support, as shown in TEM image (Figure 11b). Higher amounts of Fe led to deactivation or low activities due the delayed transition of Ni 2þ/3þ . [61] Figure 11c shows that to achieve a current density of 10 mA cm À2 overpotentials of 274, 318, and 410 mV are required for Ni(Fe)-MOF-74/KB, Ni(Fe)-MOF-74, and KB, respectively. Furthermore, the Tafel slopes for KB, Ni(Fe)-MOF74, and Ni(Fe)-MOF/KB are found to be 76.6, 58.3, and 40.4 mV dec À1 , respectively. Inevitably, the Ni(Fe)-MOF/KB showed the lowest Tafel slope, and the introduction of KB was quite effective to reduce the kinetic barrier for OER. In addition, the electrochemical impedance spectroscopy (EIS) results show that the KB considerably decreases the charge transfer resistance for the OER (Figure 11f ). The work demonstrates simple but effective use of the highly conductive and porous carbon material ketjenblack with high potential of overcoming the intrinsic drawbacks of MOF for electrocatalysis, namely, low electrical conductivity, unstable microporosity, and poor wettability. However, the postmortem analysis after the stability test under constant potential conditions for 12 h reveals that the MOF structure is altered, and the distribution of Fe and Ni in the catalyst system was not uniform. These suggest that the MOF is converted to another compound, most likely to metal oxyhydroxides, which are also electrochemically active. Such postmortem analysis should be encouraged in the community to strengthen the notion that MOFs are stable under the alkaline electrolysis conditions or to identify the "true" catalyst after their conversion during electrolysis.
Morphology and microstructure of materials significantly affect their electroctalytic properties. Cheng et al. studied the morphology-dependent electrocatalytic performance of a 2D Ni-Fe MOF derived from Fe(py) 2 Ni(CN) 4 (py ¼ pyridine) precursor. Fe-Ni MOFs exhibiting different nanostructures like nanoboxes, nanocubes, nanoplates, and nanosheets were prepared by adjusting the reaction time and temperature. [62] Variations in morphologies expose different active crystal planes with varied electrocatalytic activities. Notably, the nanoboxes with a hollow structure exhibit excellent electrocatalytic activity and stability for OER through the higher active surface area and intrinsic activity of the exposed crystal planes. The nanocubes show a regular hexahedral shape with an average size of %170 nm and a smooth surface (Figure 12a). When the reaction time was extended from 10 to 24 h, the cube-like solid particles were changed into hollow nanoboxes (Figure 12b). The transformation of nanocubes into nanoboxes is due to the dissolutionrecrystallization process during the prolonged reaction time. As reaction time proceeds, a well-defined square nanoplate morphology with a lateral dimension of %180 nm is obtained (Figure 12c). When the reaction was prolonged to 24 h, the nanoplates were changed into thinner nanosheets with more lateral dimension of %300 nm (Figure 12d). The morphological evolution from nanocubes to nanoplates and nanosheets essentially resulted from the declining growth rate along the [a] direction. Based on the linear sweep voltammogram (LSV) curves (Figure 12e), nanoboxes showed the best OER activity with an overpotential of 285 mV at the current density of 10 mA cm À2 (Figure 12f ). The OER electrocatalytic kinetics was studied according to the Tafel plots in Figure 12g. The smaller Tafel  Table 3).

Trimetallic Ni-Fe-Co MOFs
Very recently, the Ni-based trimetallic MOFs containing Co and Fe were studied as potential multifunctional catalysts for OER/HER. [37,63,64] It has been theoretically and experimental demonstrated that multimetal oxide systems show exceptionally high OER/HER catalytic activities as they exhibit favorable surface adsorption energies and electronic structures. [65] Similarly, in multimetal MOF systems, charge transfer occurs between the metal centers via the linkers due to their differences in electron accepting/donating abilities causing electronic structure changes. Accordingly, the lattice parameters and the degree of overlap between the atomic orbitals will change leading to lattice strain.  Figure 13a. The FCN-BTC MOF electrode showed excellent electrocatalytic performance for the OER, only requiring 218, 238, and 250 mV of overpotentials to deliver current densities of 10, 50, and 100 mA cm À2 , respectively. The Tafel analysis in Figure 13b shows that the smallest Tafel slope of 29.3 mV dec À1 was calculated for FCN-BTC MOF, suggesting that the OER reaction mechanism follows the four-electron transfer pathway. Based on this the authors proposed that the MO 6 metal centers in the MOF will be oxidized to form MO 6 /MOOH active centers and the OH À will oxidize to oxygen under the alkaline environment. The high activity of FCN-BTC MOF is attributed to the low charge transfer resistance and high ECSA evidenced from the electrochemical impedance and double-layer capacitance measurements. The XPS measurements presented in Figure 13c  www.advancedsciencenews.com www.small-structures.com  Reproduced with permission. [64] Copyright 2022, Royal Society of Chemistry.
www.advancedsciencenews.com www.small-structures.com The recent report of Farahani et al. demonstrates a novel and controllable electrodeposition approach for the in situ growth of a trimetallic Fe-Co-Ni MOF on Ni foam. [37] The authors follow a layer-by-layer (LbL) reductive electrodeposition approach (Figure 14a,b) to fabricate a unique Fe-Co-Ni trilayer architecture from a DMF/H 2 O solution containing metal ions, cetyltrimethylammonium bromide (CTAB), and 2-amino-1,4-benzene dicarboxylic acid. The successful stepwise formation of each MOF layer is presented in the cross-sectional SEM image of Figure 14c. The as-prepared Fe-Co-Ni MOF is a multifunctional catalyst which proved to catalyze different electrochemical reactions including ORR, HER, and OER, as schematically presented in Figure 14d.
The authors perform XPS investigations to get insights on the surface chemical states. The core-level XPS spectrum of Fe 2p clearly shows that the binding energies generally shift positively to higher values, moving from the Fe MOF to Fe-Co MOF and then to Fe-Co-Ni MOF, respectively. A similar trend is also observed for the core-level Co 2p XPS spectra. This suggested the local electronic structure and coordination changes of the metal nodes in the LbL MOF layers. The positive shift of the binding energies entails higher oxidation states of the metal centers, which likely improve the electrocatalytic activities. The OER activity tests in 1.0 M KOH displayed in Figure 15a follow the trend as Fe-Co-Ni MOF > Fe-Co MOF > Fe MOF > NF. Overpotentials of 440, 290, 280, and 254 mV are required to achieve a current density of 10 mA cm À2 for the NF, Fe MOF, bilayer Fe-Co MOF, and trilayer Fe-Co-Ni MOF, respectively. The Tafel slope of the trilayer Fe-Co-Ni MOF is calculated to be 51.3 mV dec À1 (Figure 15b), which is lower compared to the bilayer Fe-Co MOF (63.0 mV dec À1 ) and the Fe MOF (67.7 mV dec À1 ) catalysts. In addition, Fe-Co-Ni MOF shows nickel substrate. d) As-synthesized material can be directly used as electrocatalysts active for following electrochemical reactions: the OER, the HER, and the ORR. Furthermore, the practical application of the Fe-Co-Ni MOF as an active material for pseudocapacitive energy storage devices is demonstrated. Reproduced with permission. [37] Copyright 2022, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com no detectable activity loss for 48 h at current densities of 10 and 100 mA cm À2 (Figure 15c), demonstrating its long-term stability. The authors attributed the superior OER activity of the trilayer Fe-Co-Ni MOF to its well-defined ordered porous structure, synergistic effects between optimally distributed metal catalysts, and a large accessible surface area. Figure 15d,e reveals the potential of Fe-Co-Ni MOF as HER catalyst. Interestingly, the HER catalytic activity also follows similar activity trends like the OER, Fe-Co-Ni MOF > Fe-Co MOF > Fe MOF > NF. An overpotential of 116 mV is required at 10 mA cm À2 current density, a smallest value among the MOF series, but larger than the benchmark Pt/C (74 mV) catalyst. The dual-catalytic functionality of the Fe-Co-Ni MOF can be exploited for constructing total water-splitting cells (Figure 15f ). The authors assembled two-electrode setup and measured the OER and HER potential difference and found out 1.60 V is required to achieve a current density of 10 mA cm À2 for the Fe-Co-Ni MOF-based cell. The value is slightly better than that of the benchmark Pt/C(À)||RuO 2 (þ) electrolyzer (1.62 V). The long-term stability and durability of the cell recorded under chronopotentiometry response of the FeÀCoÀNi MOF(À)|| FeÀCoÀNi MOF(þ), two-electrode electrolyzer, at a current density of 10 mA cm À2 show that the cell maintains a voltage of 1.6 V over the course of 150 h. To assess the stability of the catalysts the authors perform post-operation analysis. After 2 and 150 h operation time, the morphology of the catalyst remains unaltered. However, the XPS investigation after 150 h continuous operation indicates a slight increase of trivalent species, hinting the likely generation of the (oxy)hydroxides during catalysis. Furthermore, DFT calculations show that FeÀCoÀNi MOF displays more negative H 2 O adsorption energy, smaller bandgap energy, and higher density of states around the Fermi level compared to the other mono-/bimetallic MOFs. These results suggest strong interaction of H 2 O with metal nodes and increased electron conductivity, both of which are essential for catalyzing the water-splitting reactions. Notably, the FeÀCoÀNi MOF is also an efficient catalyst for ORR and successfully employed in Zn-air battery and as charge storage material in supercapacitors.
In summary, the authors demonstrated a synthetic approach which is highly flexible with MOF catalyst with multifunctional catalytic activities.

Conclusion and Perspective
Propelled by the development of highly active transition metal oxides, [66,67] Ni based Co and Fe containing mono-, bi-or tri-metallic MOFs are promising classes of materials for alkaline OER catalysis with huge potential to replace noble metal based OER catalysts. The high specific surface area, accessible porosity, spatially distributed and varied metal sites, tunable and rationally engineered metal-linker combination with controllable morphologies and structures, are highlighted properties which stand out compared to their metal oxides analogues. These properties Figure 15. OER and HER activities and overall water-splitting studies on Fe-Co-Ni MOF, Co-Ni MO, Fe MOF, NF, and benchmark RuO 2 and Pt/C electrodes. a) OER polarization curves at a sweep rate of 5 mV s À1 and b) Tafel curves in a 1.0 M KOH solution. c) Long-term (48 h) chronopotentiometry curves of the Fe-Co-Ni MOF electrode at current densities of 10 and 100 mA cm À2 (inset: CV and LSV curves of the Fe-Co-Ni MOF electrode before and after 1000 CV cycles). d) HER polarization curves at a sweep rate of 5 mV s À1 and e) corresponding Tafel plots in a 1.0 M KOH solution. f ) Overall OER and HER polarization curves in three-electrode cell setups. Reproduced with permission. [37] Copyright 2022, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com make Ni based MOFs the most active research domains in the field of electrocatalysis. Promisingly, the pristine Ni based MOFs display comparable or even superior catalytic performances compared to other materials as presented in this brief overview. The reports prove that pristine Ni based MOFs are robust and catalytically active materials without the need of further post-synthetic treatments. The recent progress in Ni based MOFs showed: 1) hydrothermal and electrodeposition synthetic routes are highly suited for the preparation of Ni based MOFs, 2) the direct growth of the MOFs on macroporous substrates (e.g., Ni foams) is key to address the issue of the inherent low conductivity of MOFs and enhance the mass transport properties, and 3) the Ni-Fe, Co-Ni and Ni-Fe-Co MOFs display higher OER catalytic activities than the single component MOFs, signifying the synergetic catalytic effects of the metals in a single MOF structure. However, there are still questions surrounding the catalytic role of each metal, optimal ratios in the MOF structure, and stability under the harsh OER conditions. Few computational attempts show that in the bi and tri metallic Ni MOFs, the electronic properties of the metal nodes is highly modulated resulting electron deficient (high valent metal centers) and electron rich (oxygen centers). The theoretical predictions are supported by XPS measurements and partly unravel the specific role of the constituent metals during the OER. But such studies are scarce, and more studies should be directed to establish composition activity trends. It has been shown that Ni based MOFs undergo a partial conversion to the respective metal oxyhydroxides which are catalytically active for alkaline OER. The degree of conversion and the structural integrity of the MOF should be checked using post-mortem analysis and commonly practiced to ensure the long term stability and to pave the way to real world applications. In this regard in-situ spectroscopic techniques such as Raman and XAS are providing conclusive atomistic information and identify the 'true' catalyst phases in Ni based MOFs under the OER conditions. On the other hand, most reports use only a handful of organic linkers based on benzyl carboxylic acid derivatives. Understanding the role of the organic linkers in the context of the OER catalysis is rare but also crucial for developing new linkers or to use the existing ones from the vast library of MOF linkers. Noteworthy mentioning, very recent attempts to introduce lattice strain and defects in the MOF structure via missing linkers and controlled cleavage, [55,68] have showed promising results and will further shape the future research directions in the quest of developing highly active Ni based OER catalysts. Lastly, hydrothermal methods will still take the center stage for Ni based MOF synthesis and the microwave local heating bring new prospects to finely adjust the sizes and morphologies.