**5. MOFs‐based catalysts for OER**

MOFs possess high specific surface area and tunable pore structures, and they are easily functionalized by different metal centers and organic linkers, highlighting their great poten‐ tials in electrocatalysis. Similar to those utilized for HER, MOFs have already been devel‐ oped for OER catalysis basically in two ways: direct and indirect ones. In the first case, MOFs will be received directly as electrocatalysts, and therefore element selection plays a key role to construct coordination modes which should facilitate oxygen‐species adsorption/desorption and water dissociation. While in the second case, MOFs will be transformed into other composite materials in which metal, in the form of metallic–, oxide– or nitrite–phase, is associated with carbon. Despite having potential, MOFs‐derived materials are at the expense of MOFs. For instance, the specific surface area decreased and the ordered porosities were destroyed. Therefore, structure modification is crucial to make the materials maintain higher specific surface area and hieratical pore structures through pyrolysis in order to maximize the catalytic site density and provide accessible channels for mass transfer.

Regarding element selection, metal‐O, metal‐N, or mixed N‐metal‐O coordination modes have already been developed to construct OER catalysts. In MOFs, the metal ion redox typically involves in the electrochemical process, and the electron‐accepting ability of the nitrogen and oxygen atoms in organic linkers may polarize the adjacent metal atoms to afford better catalytic performance.

Babu et al. [55] investigated the electrocatalytic activity of commercially available Fe(BTC) MOF (BasoliteTM F300), with BTC=bezene‐1,3,5‐tricarboxylate. When studying the hydroxide on the voltammetric responses for Fe(BTC), a well‐defined hydroxide oxidation response was clearly observed if NaOH was added to a 0.1 M KCl background electrolyte. Such an oxida‐ tion current was ascribed to OER current. However, such a current could be seen only by pre‐ scanning the Fe(BTC)‐modified electrode lower than 0.5 V vs. saturated calomel electrode (SCE), a reduction potential zone where the hydrous iron oxide intermediates could be formed. Fe(III/II) played a key role in determining the electrochemical behavior of Fe(BTC).

In addition to MoS2, MoCx has also been widely studied for HER. The porous MoCx nano‐ octahedrons were prepared through an MOFs‐derived strategy [54]. Cu‐based MOF [HKUST‐ 1; Cu3 (BTC)2(H2O)3] was first used to hold the Mo‐based Keggin‐type POMs (H3PMo12O40), forming an MOF, i.e., NENU‐5 nano‐octahedrons. The MoCx‐Cu intermediate was prepared by carbonizing NENU‐5 in inert atmosphere, and the final MoCx porous nanoparticles were obtained by removing Cu by Fe3+ etching. The obtained porous MoCx catalysts were evaluat‐ ed for HER in both acidic and basic aqueous solutions. The polarization curves showed that the overpotentials of this catalyst were only 142 and 151 mV to deliver a current density of 10

In summary, the highly porous MOFs containing redox sites have been developed as HER catalysts. However, their low conductivity limits their electrocatalytic performance for HER. Their combination with highly conductive substrates is an efficient way to enhance their HER activities. Furthermore, many researches have demonstrated that the MOFs‐derived cata‐ lysts may exhibit greatly improved performance in HER. These MOFs‐derived catalysts were obtained mainly through carbonization and a following phosphidation or sulfuration of MOFs containing active metal species (usually Co, Ni and Mo) at appropriate temperatures in inert or air atmosphere. Due to the active species, the novel porous structures, the enhanced conductivity and the protection from the heteroatom doped carbons, these MOF‐derived catalysts usually exhibit higher activity and stability as compared to the pristine MOFs catalysts. Hence, because oftheseunique structures andproperties, theMOF‐derived materials

benefit the development of HER catalysts with low costs and high performance.

catalytic site density and provide accessible channels for mass transfer.

MOFs possess high specific surface area and tunable pore structures, and they are easily functionalized by different metal centers and organic linkers, highlighting their great poten‐ tials in electrocatalysis. Similar to those utilized for HER, MOFs have already been devel‐ oped for OER catalysis basically in two ways: direct and indirect ones. In the first case, MOFs will be received directly as electrocatalysts, and therefore element selection plays a key role to construct coordination modes which should facilitate oxygen‐species adsorption/desorption and water dissociation. While in the second case, MOFs will be transformed into other composite materials in which metal, in the form of metallic–, oxide– or nitrite–phase, is associated with carbon. Despite having potential, MOFs‐derived materials are at the expense of MOFs. For instance, the specific surface area decreased and the ordered porosities were destroyed. Therefore, structure modification is crucial to make the materials maintain higher specific surface area and hieratical pore structures through pyrolysis in order to maximize the

Regarding element selection, metal‐O, metal‐N, or mixed N‐metal‐O coordination modes have already been developed to construct OER catalysts. In MOFs, the metal ion redox typically involves in the electrochemical process, and the electron‐accepting ability of the nitrogen and oxygen atoms in organic linkers may polarize the adjacent metal atoms to afford better catalytic

mA cm−2 in acidic and alkaline solutions, respectively.

122 124Metal-Organic Frameworks

**5. MOFs‐based catalysts for OER**

performance.

Gong et al. [56] prepared two CoII metal‐organic frameworks, complex **1** (using 4,5‐di(4′‐ carboxylphenyl) phthalic acid and 4,4′‐bipyridine as co‐ligands) and complex **2** (using 4,5‐ di(4′‐carboxylphenyl)phthalic acid and azene as co‐ligands) as electrocatalysts. The OER catalytic activity of these two complexes in an aqueous buffer solution (pH = 6.8) was investigated. The results showed that complex **1** possessed an oxidation peak of water at a more negative potential of 1.02 V vs. SCE accompanied by a much higher OER current than bare glassy‐carbon (GC) electrode and ligand‐modified glassy‐carbon electrode. As con‐ firmed by electrochemical impedance spectra, complex **1**‐modified GC showed lower charge‐ transfer resistance than the bare GC, indicating that the framework promoted the charge‐ transfer, thus the OER catalytic activity. When applied complex **2**, the **2**‐modified GC showed lower OER overpotential and higher OER current than complex **1**‐modified GC. Such an enhancement could be ascribed to the better charge build‐up of complex **2** than that of complex **1**, further due to the different co‐linkers and frameworks in the two MOFs.

To verify the Co contributions for OER in MOFs, Yin's group [57] first synthesized an MOF(Fe) catalystby hydrothermal processusing FeIII as metal precursor and 1,3,5‐BTC as organic ligand. The synthesized MOF(Fe) afforded a specific surface area up to 1600 m2 g−1. MOF(Fe) afford‐ ed excellent OER activity with a delivered current density of 2.30 mA·cm−2 at 0.90 V vs. Ag/ AgCl in 0.1 M KOH, higher than the activated carbon (Super P). Further, they prepared an MOF(Fe/Co) as ORR/OER catalyst by a hydrothermal process using Fe and Co as mixed metal precursors, and the same organic ligand [58]. Surprisingly, MOF(Fe/Co) exhibited an im‐ proved OER activity with a delivered current density of 2.97 mA·cm−2 at 0.90 V vs. Ag/AgCl than the aforementioned MOF(Fe). Although MOF(Fe/Co) possessed lower specific surface area (∼1070.1 m2 ·g−1) than MOF(Fe), MOF(Fe/Co) exhibited better OER activity than MOF(Fe) in 0.1 M KOH electrolyte, primarily due to the aid of Co species.

Wang et al. [59] synthesized a cobalt‐based ZIF (Co‐ZIF‐9) by assembling Co ions with benzimidazolate ligands for water oxidation. Co‐ZIF‐9 had open‐framework structure, in which Co ions were coordinated by N atoms in benzimidazolate linkers. It had been found that this catalyst was thermodynamically feasible for catalytically oxidizing water molecule by density functional theory (DFT) calculation because Co ions had redox function and could bond ‐OH resulting in low activation barriers. In addition, the nearby benzimidazolate also promoted the OER reaction by accepting the eliminated ‐H atoms. Thus, Co‐ZIF‐9 showed obvious OER activity in wide pH basic solutions. Besides, the activity stability of Co‐ZIF‐9 was also investigated and there was no obvious deactivation in current density after 25 h test in potassium phosphate buffer.

Tan et al. [60] synthesized an MOF‐74 to study the water dissociation mechanism on open metal sites in MOFs through *in situ* IR spectroscopy and first‐principles calculations (**Figure 2**). The *in situ* IR spectroscopy provided a direct evidence of water reaction occurred on the metal centers. The water dissociation mechanism in MOF‐74 primarily depended on two aspects: (1) the covalent bond between water and metal center and (2) the hydrogen bonding between the O atoms of the linker and the H atoms of the water molecule. This work was of significance to demonstrate the coordinatively unsaturated metal centers as active sites for water dissociation.

**Figure 2.** Schematic illustration of the OER catalytic mechanism on MOF‐74 [60].

Based on the above results, it has been theoretically or experimentally demonstrated that bare MOFs can be designed by element selection to show excellent OER catalytic activity. However, compared with the noble metal‐based and transition metal oxide‐based electrocatalysts, MOFs are still insufficient to catalyze OER with higher current probably due to their intrinsic poor electronic conductivity. Therefore, Loh et al. [27] prepared a Cu‐centered MOF from copper nitrate trihydrate, 1,4‐benzenedicarboxylic acid and triethylene‐diamine in the presence of GO. An optimal composition (GO 8 wt%) Cu‐MOF afforded an OER onset potential of 1.19 V vs. RHE in acid electrolytes, a ∼200 mV positive shift than that of pure Cu‐MOF. The 8 wt% GO‐ incorporated Cu‐MOF also showed fast OER kinetics with a smaller Tafel slope of 65 mV·dec −1 than pure Cu‐MOF (89 mV·dec−1). The results demonstrated that the enhanced electrocata‐ lytic properties and stability in acid of the GO‐MOF composite was due to the unique porous scaffold structure, improved charge transport, and synergistic interactions between the GO and MOF.

Because of the high specific surface area and the well‐defined porosities, MOFs are excellent catalyst support materials. Therefore, doping MOFs with other active species in the forms of oxides, ions, and complexes can open up a new route for MOFs in OER electrocatalysis.

Yin et al. [61] prepared α‐MnO2/MIL‐101(Cr) catalyst through a hydrothermal process. In this catalyst, α‐MnO2 particles were embedded in MIL‐101(Cr) matrix, resulting in strong interac‐ tions between α‐MnO2 and MIL‐101(Cr). The OERcatalytic activity ofthe composite was tested using a carbon paper containing this catalyst as working electrode. The results revealed that α‐MnO2/MIL‐101(Cr) composite afforded an excellent OER catalytic activity with a deliv‐ ered current density of 23.67 mA cm−2 at 0.9 V in 0.1 M KOH solution which was about two times higher than that of pure α‐MnO2 under the same conditions. The high specific surface area of MIL‐101(Cr) and abundant micropores of MIL‐101(Cr) were advantageous for the diffusion of electrolyte and the high dispersion of α‐MnO2 particles made it easy to contact the electrolyte, resulting in enhanced OER catalytic activity.

Tan et al. [60] synthesized an MOF‐74 to study the water dissociation mechanism on open metal sites in MOFs through *in situ* IR spectroscopy and first‐principles calculations (**Figure 2**). The *in situ* IR spectroscopy provided a direct evidence of water reaction occurred on the metal centers. The water dissociation mechanism in MOF‐74 primarily depended on two aspects: (1) the covalent bond between water and metal center and (2) the hydrogen bonding between the O atoms of the linker and the H atoms of the water molecule. This work was of significance to demonstrate the coordinatively unsaturated metal centers as active sites for water dissociation.

Based on the above results, it has been theoretically or experimentally demonstrated that bare MOFs can be designed by element selection to show excellent OER catalytic activity. However, compared with the noble metal‐based and transition metal oxide‐based electrocatalysts, MOFs are still insufficient to catalyze OER with higher current probably due to their intrinsic poor electronic conductivity. Therefore, Loh et al. [27] prepared a Cu‐centered MOF from copper nitrate trihydrate, 1,4‐benzenedicarboxylic acid and triethylene‐diamine in the presence of GO. An optimal composition (GO 8 wt%) Cu‐MOF afforded an OER onset potential of 1.19 V vs. RHE in acid electrolytes, a ∼200 mV positive shift than that of pure Cu‐MOF. The 8 wt% GO‐ incorporated Cu‐MOF also showed fast OER kinetics with a smaller Tafel slope of 65 mV·dec −1 than pure Cu‐MOF (89 mV·dec−1). The results demonstrated that the enhanced electrocata‐ lytic properties and stability in acid of the GO‐MOF composite was due to the unique porous scaffold structure, improved charge transport, and synergistic interactions between the GO

Because of the high specific surface area and the well‐defined porosities, MOFs are excellent catalyst support materials. Therefore, doping MOFs with other active species in the forms of oxides, ions, and complexes can open up a new route for MOFs in OER electrocatalysis.

Yin et al. [61] prepared α‐MnO2/MIL‐101(Cr) catalyst through a hydrothermal process. In this catalyst, α‐MnO2 particles were embedded in MIL‐101(Cr) matrix, resulting in strong interac‐ tions between α‐MnO2 and MIL‐101(Cr). The OERcatalytic activity ofthe composite was tested using a carbon paper containing this catalyst as working electrode. The results revealed that α‐MnO2/MIL‐101(Cr) composite afforded an excellent OER catalytic activity with a deliv‐ ered current density of 23.67 mA cm−2 at 0.9 V in 0.1 M KOH solution which was about two times higher than that of pure α‐MnO2 under the same conditions. The high specific surface

**Figure 2.** Schematic illustration of the OER catalytic mechanism on MOF‐74 [60].

and MOF.

124 126Metal-Organic Frameworks

Subsequently, Yin's group [62] decorated MIL‐101(Cr) with Co ions with various oxidiza‐ tion states through impregnation followed by post‐treatment under oxidant or reducing reagents. The Co species were highly dispersed on the MOF surface and showed various CoIII/ CoII ratios. Primary results demonstrated that the OER activity is related to the CoIII contents since the catalyst showed better OER catalytic activity as the surface CoIII content increased. The reason might be the CoIII species could promote the OH- adsorbed onto the electrocata‐ lyst surface. In addition, the porous and open structures of MIL‐101(Cr) support were in favor of the contact between oxygen species and the active Co sites.

Wang et al. [19] decorated a Zr‐MOF, namely UiO‐67, with **1‐3** Ir‐containing complexes to form MOF **1‐3**, respectively. Many MOFs lack stability in water, but UiO‐67 is one of the excep‐ tions. The OER performance of the samples was investigated in a pH = 1 solution with Ce4+ as an oxidant. The Ce4+ was reduced to Ce3+, while water was oxidized to form oxygen. The results showed that after the incorporation of Ir‐containing complexes in UiO‐67, MOF **1‐3** were the effective water oxidation catalysts with turnover frequencies of up to 4.8 h−1. The parent UiO‐ 67 showed negligible OER catalytic activity, demonstrating the significance of the Ir‐contain‐ ing dopants as active centers.

The OER current density at 10 mA·cm−2 is a criterion used to judge an OER catalyst since such a current density is an important metric for practical solarfuel production. Unfortunately, most of the aforementioned MOF‐based OER catalysts fail to reach such a current density even at high overpotentials. Apart from directly being the OER catalysts, MOFs can be converted into other forms of materials through pyrolysis. Through pyrolysis, the organic linkers can be transformed into carbon materials, and the well‐dispersed metal centers in the MOFs' frameworks can be transformed into phosphate‐ or oxide‐phase but still maintain excellent dispersion in carbons. Such a strategy is helpful to obtain materials with excellent charge‐ transfer properties.

You et al. [50] prepared a CoPx/NC catalyst derived from ZIF‐67 and its polyhedron‐like morphology survived from the pyrolysis. The specific surface area of CoPx/NC was up to 183 m2 ·g−1, and the pore volume is 0.276 m<sup>3</sup> ·g−1. In OER activity test, the overpotential of CoPx/NC was about 354 mV at a current density of 10 mA cm−2, while IrO2 is about 368 mV at the same current density. Moreover, after 1000th continuous CV cycles, the overpotential of CoPx/NC at 10 mA cm−2 showed only slight difference, demonstrating the excellent OER activity durability of the catalyst.

Apart from CoPx, Co3O4 also has been reported to be active for OER, and can significantly improve the conductivity and stability properties of the catalysts when combined with carbon materials. The porous Co3O4‐based hybrids were typically obtained through a one‐step carbonization of Co‐MOFs. Li et al. [63] introduced MWCNT to MOFs and obtained the Co3O4@MWCNTs by carbonization and subsequent oxidation process. The thermal oxida‐ tion led to Co3O4‐N‐C active sites uniformly dispersed on MWCNTs (20–50 nm). And the

introduction of MWCNT and the *in‐situ* N‐doped carbon carbonized from ligands could significantly improve the electronic conductivity of the catalysts. Thus, better OERactivity was observed with an onset potential of only 1.5 V (vs. RHE). Ma et al. [64] synthesized MOF‐ derived Co3O4‐carbon porous nanowire arrays. The Co3O4‐carbon were directly prepared on Cu foil as a working electrode. Since this electrode is binder‐free and carbon is formed *in situ*, the charge conductivity performance is greatly improved, resulting in excellent OER catalyt‐ ic activity in 0.1 M KOH solution. It shows a sharp onset potential of 1.47 V (vs. RHE), very close to that of IrO2/C (1.45 V vs. RHE). The durability is also an important criterion for OER catalysts. The chronopotentiometric response at a current density of 10 mA·cm−2 was also recorded, and only 6.5% attenuation was observed within 30 h on Co3O4‐carbon, while that of IrO2/C is 4.7 times lager under the same condition.

The above results demonstrate that MOFs‐derived catalysts show much enhanced OER catalytic performance than pristine MOFs. There are likely several reasons for that. On one hand, carbon materials formed *in situ* or *ex situ* in catalysts can promote electronic conductiv‐ ity and accelerate charge transfer. On the other hand, the OER active species can be well‐ dispersed on carbon materials, resulting in improved OER activity. In addition, the strong interactions between OER active species and carbon materials stabilize the OER active site structures, thus leading to enhanced OER activity durability.

## **6. Conclusions**

MOFs‐based electrocatalysts for HER and/or OER are rapidly developed in recent years due to unique structures of MOFs. These catalysts mainly include MOFs catalysts, MOFs sup‐ ports for catalysts, and MOFs‐derived catalysts. Due to the fact that the pore structures and functions are tunable and devisable, it is convenient to directly design and construct the active sites for HER and/or OER in MOFs during the synthesis process. However, the vast majority of the synthesized MOFs suffer from poor electronic conductivity, leading to low electron transfer efficiency, which restricts catalytic performance. MOFs are highly porous materials and have ultrahigh specific surface area, thus they are regarded as the most promising support materials for catalysts. The active species for HER or OER can be well dispersed at the surfaces of MOFs or embedded in MOFs matrix, resulting in improved catalytic performance for HER or OER. However, MOFs are microporous materials with small aperture size (<2 nm).On one hand, the active species are difficult to be introduced in their pore channels. Hence, one cannot obtain the catalysts with high activity and stability for HER or OER. On the other hand, the accommodation of electrolyte in the pore channels is very limited. Thus, the active centers in MOFs cannot access the electrolytes with high efficiency. In addition, the poor electronic conductivity of MOFs is another drawback of these catalysts. Some post‐treatment methods can greatly improve the electrical conductivity of MOFs‐derived catalysts. However, the collapse of the pore structures of MOFs usually occurs during the preparation of the MOFs‐ derived catalysts leading to a decreased specific surface area of the catalysts, which is adverse to the development of MOFs‐derived catalysts with high catalytic performance for HER or OER.

Although many MOFs‐based catalysts with high catalytic performance for HER and/or OER have been developed in recent years, there are still a lot of scientific and technical problems to solve before the developed MOFs‐based electrocatalysts can meet the requirements for commercialization. The problems mainly involve how to improve the electronic conductivi‐ ty of MOFs, enlarge pore channels of MOFs to accommodate more electrolytes, limit the collapse of pore structure of MOFs, and maintain high specific surface area during pyrolysis. In addition, the reaction mechanisms of HER and OER, the transfer and diffusion properties of reactants and products, and the effects of electrolyte on the catalytic performance of MOFs‐ based catalysts need to be clarified.
