**1. Introduction**

Electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the key reactions for many energy storage and conversion systems, including water split‐ ting, rechargeable metal‐air batteries, and the unitized regenerative fuel cells [1–3]. Howev‐ er, the two reactions are kinetically sluggish, which leads to large overpotentials in the electrodes, decreases the performance of these systems, and makes them difficult to meet the requirements for commercialization at present. It has been reported that suitable electrocata‐ lysts can effectively accelerate the HER/OER and reduce their overpotentials in the electro‐ des. At present, noble metal catalysts show the best catalytic performance for HER and OER. For example, carbon‐supported Pt catalysts exhibit the best catalytic performance for HER [4], while IrO<sup>2</sup> and RuO2 are very effective for OER [5]. However, the reserves of these noble metals are extremely limited in nature resulting in a high price of these noble metal catalysts, which restricts their practical applications. Therefore, the development of highly efficient electroca‐ talysts with low cost for HER/OER is indispensable. In recent years, a large amount of non‐ noble metal catalysts have been synthesized for HER/OER. The non‐noble metal catalysts for HER mainly include transition metal phosphides [6–8], such as CoP, NiP, transition metal sulfides [9–11], such as MoS2, CoS, Ni‐S, and their alloys [12, 13] such as Ni‐Mo, Ni‐Cu. As for OER, the non‐noble metal catalysts mainly focus on metal oxides [14–16] such as Co3O4, MnOx, NixCo3-xO4, and metal sulfides [17, 18] such as Ni3S2, NiCo2S4. Although these non‐noble metal catalysts have shown excellent catalytic performance for HER and/or OER, to date, there is no one catalyst that can fully meet the demands of the practical applications. It is necessary to develop more economically viable electrocatalysts with higher catalytic performance for HER and/or OER, and further accelerate the commercial applications of these energy storage and conversion systems.

Metal‐organic frameworks (MOFs) are porous crystalline materials with topological struc‐ tures. MOFs are formed by assembling metal nodes and organic ligands. The metal node precursors mainly come from metal nitrates or chlorides, while the organic ligands mainly include benzimidazolate, dicarboxylic acid, and others. MOFs can be synthesized by some simple methods under mild conditions such as solvothermal, diffusion, microwave, and ionothermal methods [19–22]. MOFs possess unique structural characterizations such as high porosity, ultrahigh specific surface area (∼6240 m2 g−1) and tunable pore structure and easy functionalization [23], and they have been widely investigated and applied in gas adsorp‐ tion/separation [24], magnetism [25], optoelectronics [26], and catalysis [27]. In recent years, MOFs have attracted great attention for use in energy conversion and storage systems such as water splitting, fuel cells, metal‐air batteries, and supercapacitors. More recently, the applica‐ tions of MOFs have been expanded to the HER and OER catalysis. To date, some novel MOFs‐ based catalysts for HER and OER have been synthesized. On the one hand, the active species can be introduced into the structures of MOFs, directly acting as effective catalysts for HER and/orOER; on the other hand, MOFs also can be usedas supports to disperse the active species for HER and/or OER due to their porosity. In addition, the highly porous MOFs can be transformed to the derived catalysts by self‐sacrifice methods. These MOFs‐derived cata‐ lysts usually possess improved electronic conductivity and remarkable HER and/or OER catalytic performance. This chapter reviews recent progress of MOFs and MOFs‐derived electrocatalysts for HER and/or OER. The MOFs are first introduced briefly. The applica‐ tions of MOFs‐based catalysts for HER and/or OER are the main contents and involve details. This work provides a helpful reference for researchers who are studying the MOFs‐based electrocatalysts for HER and/or OER.

## **2. Overview of metal‐organic frameworks**

**1. Introduction**

114 116Metal-Organic Frameworks

conversion systems.

Electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the key reactions for many energy storage and conversion systems, including water split‐ ting, rechargeable metal‐air batteries, and the unitized regenerative fuel cells [1–3]. Howev‐ er, the two reactions are kinetically sluggish, which leads to large overpotentials in the electrodes, decreases the performance of these systems, and makes them difficult to meet the requirements for commercialization at present. It has been reported that suitable electrocata‐ lysts can effectively accelerate the HER/OER and reduce their overpotentials in the electro‐ des. At present, noble metal catalysts show the best catalytic performance for HER and OER. For example, carbon‐supported Pt catalysts exhibit the best catalytic performance for HER [4], while IrO<sup>2</sup> and RuO2 are very effective for OER [5]. However, the reserves of these noble metals are extremely limited in nature resulting in a high price of these noble metal catalysts, which restricts their practical applications. Therefore, the development of highly efficient electroca‐ talysts with low cost for HER/OER is indispensable. In recent years, a large amount of non‐ noble metal catalysts have been synthesized for HER/OER. The non‐noble metal catalysts for HER mainly include transition metal phosphides [6–8], such as CoP, NiP, transition metal sulfides [9–11], such as MoS2, CoS, Ni‐S, and their alloys [12, 13] such as Ni‐Mo, Ni‐Cu. As for OER, the non‐noble metal catalysts mainly focus on metal oxides [14–16] such as Co3O4, MnOx, NixCo3-xO4, and metal sulfides [17, 18] such as Ni3S2, NiCo2S4. Although these non‐noble metal catalysts have shown excellent catalytic performance for HER and/or OER, to date, there is no one catalyst that can fully meet the demands of the practical applications. It is necessary to develop more economically viable electrocatalysts with higher catalytic performance for HER and/or OER, and further accelerate the commercial applications of these energy storage and

Metal‐organic frameworks (MOFs) are porous crystalline materials with topological struc‐ tures. MOFs are formed by assembling metal nodes and organic ligands. The metal node precursors mainly come from metal nitrates or chlorides, while the organic ligands mainly include benzimidazolate, dicarboxylic acid, and others. MOFs can be synthesized by some simple methods under mild conditions such as solvothermal, diffusion, microwave, and ionothermal methods [19–22]. MOFs possess unique structural characterizations such as high porosity, ultrahigh specific surface area (∼6240 m2 g−1) and tunable pore structure and easy functionalization [23], and they have been widely investigated and applied in gas adsorp‐ tion/separation [24], magnetism [25], optoelectronics [26], and catalysis [27]. In recent years, MOFs have attracted great attention for use in energy conversion and storage systems such as water splitting, fuel cells, metal‐air batteries, and supercapacitors. More recently, the applica‐ tions of MOFs have been expanded to the HER and OER catalysis. To date, some novel MOFs‐ based catalysts for HER and OER have been synthesized. On the one hand, the active species can be introduced into the structures of MOFs, directly acting as effective catalysts for HER and/orOER; on the other hand, MOFs also can be usedas supports to disperse the active species for HER and/or OER due to their porosity. In addition, the highly porous MOFs can be transformed to the derived catalysts by self‐sacrifice methods. These MOFs‐derived cata‐ lysts usually possess improved electronic conductivity and remarkable HER and/or OER

As a class of the coordination polymers, MOFs are crystalline porous materials. MOFs are formed through self‐assembling using the inorganic metals as nodes and organic ligands as linkers [28, 29]. They possess unique structure properties, such as ultrahigh specific surface area and high porosity, tunable pore structure, and easy functionalization. The specific surface area of MOFs are much higher than that of other porous materials such as activated carbon and zeolites. For example, the synthesized MOF‐201 possesses a high BET specific surface area of ∼6240 m2 g−1 [30], while zeolites possess specific surface area less than 600 m2 g−1. High porosity of MOFs also brings up a very low density (∼0.13 g cm−3) [23], which is important to the storage of fuels or the applications in energy conversion and storage. The pore structure of MOFs can be tuned by choosing different organic ligands or altering the length of organic ligands. It was reported that Deng et al. prepared a novel IRMOF (IRMOF‐74 XI) with large apertures of ∼9.8 nm, by expanding the length of its organic linker to ∼5 nm [31]. Apart from the tunable pore structure, MOFs can be easily functionalized by decorating the pores, surfaces, or introducing guests into MOFs, thus resulting in a large amount of MOFs with various physicochemical properties [32]. Due to these unique properties, MOFs have been widely developed and applied in gas adsorption/separation [24], magnetism [25], optoelectronics [26], and catalysis [27] in recent years.

According to topological structure features, MOFs can be generally divided into four catego‐ ries: isoreticular metal‐organic frameworks (IRMOFs), zeoliticimidazolate frameworks (ZIFs), materials of institute Lavoisier frameworks (MILs) and pocket‐channel frameworks (PCNs) [33, 34].

IRMOFs are a large family of MOFs, which are built up by [Zn4O]6+ and organic hydroxyl acid as metal nodes and ligands, respectively. In the IRMOFs family, MOF‐5 is the typical one which was prepared by Li et al. as early as in 1999 [35]. MOF‐5 is highly porous, and the specific surface area and the density are about 2900 m<sup>2</sup> g−1 and 0.59 g cm−3, respectively. Specially, the space access to guest substance reaches up to 55–61%.

The topological structures of pores of ZIFs are similar to aluminosilicate zeolite. Generally, imidazole or its derivatives and Zn or Co ions are used as ligands and metal nodes, respec‐ tively. The N atoms in imidazole and its derivatives can coordinate with Zn, Co, or other transitional metal ions. Compared with other kinds of MOFs, ZIFs show higher thermal and chemical stability. For example, as a typical ZIF material, ZIF‐8 can sustain its structures in inorganic or organic solvents or even in boiling water for 1–7 days [36].

Generally, there are two categories of MILs. One is formed by using lanthanides or transi‐ tion metals as metal nodes and dicarboxylic acids as organic ligands. The other is obtained using trivalent metals such as aluminum or vanadium as metal nodes, and terephthalic acids or trimesic acids as organic ligands. MILs are highly porous and possess ultrahigh specific surface area. MIL‐100 and MIL‐101 are typical MILs. In MIL‐100, Cr3+ ions are metal nodes and BTC (1,3,5‐benzenetricarboxylate) is used as organic ligands. It possesses a high specific surface area of ∼3100 m2 /g and mesopores with the pore size of ∼2.9 nm [37]. In addition, MIL‐100 exhibits excellent thermal stability below 275°C [37].

During the synthesis of PCNs, Cu ions or oxo‐clusters are generally used as metal nodes, and tricarboxylic acids, 4,4′,4″‐s‐triazine‐2,4,6‐triyltribenzoate (H3TATB) or s‐heptazine triben‐ zoate (HTB) is usually used as an organic ligands. PCNs have pocket and three‐dimensional orthogonal channels which are connected through small sized windows. As a typical PCN, Cu‐BTC is formed by using Cu2(COO)4 as metal nodes and BTC as organic ligands. Two typical channel structures can be observed in Cu‐BTC. One is a small octahedron pocket, while the other is the three-dimensional orthogonal channels (∼1 nm) [38].

To prepare MOFs, various synthetic methods have been proposed, such as solvothermal (hydrothermal) method, diffusion method, microwave‐assisted method and ionothermal method. During the solvothermal (hydrothermal) process, water or organic solvents are usually used as reaction medium in an airtight reactor. High temperature and pressure condition can be achieved through heating the airtight reactor. Thus, some compounds with poor solubility at room temperature and atmospheric pressure can be dissolved and re‐ crystallized to obtain the desirable MOFs. In addition, high crystallinity and controllable particle size for the resulting MOFs can be obtained through the solvothermal (hydrother‐ mal) process. Therefore, the solvothermal (hydrothermal) methodis effective to produceMOFs with good orientation and perfect crystals [19, 39].

The diffusion methods include vapor diffusion, liquid diffusion, and gel diffusion methods. The products are obtained in the two‐phase interface through slow crystallization [20]. The diffusion methods have two advantages, including mild synthesis conditions and high quality crystal MOFs. However, the efficiency of this method is usually low, which requires a long synthesis time, up to 1 week. Furthermore, this method also requires precursors with good solubility.

In the microwave‐assisted process, the charging particles are placed in electromagnetic fields. These charging particles collide with high speed, forming the final products. Due to the high frequency of microwave, the microwave‐assisted method is considered as a high energy efficiency method [21]. It has many advantages, such as uniform heating, high reaction rate, selective heating, and no hysteresis effects.

For the ionothermal synthesis, ionic liquids and eutectic mixture are usually used as sol‐ vents [22]. Ionothermal synthesis can avoid the high‐pressure reaction condition, and produce final MOFs in an open condition, because ionic liquids have almost no vapor pressure.

In MOFs, the organic ligands cannot only be used as linkers, but also adsorb gas molecules through Van der Waals force. This is the reason that MOFs materials can be used for gas adsorption/separation [24]. Till date, most of the studies on MOFs still focus on gas adsorp‐ tion/separation. Because some metals such as group VIII metals possess excellent magnetic properties, MOFs are also popular in magnetism field by using magnetic metal ions as nodes, and they possess paramagnetic or antiferromagnetic properties [25]. Similar regularity was observed of MOFs in the optoelectronics area. Luminescent lanthanide(III) ions which are utilized as nodes for MOFs are always designed as luminescent centers [26]. Due to the high specific surface area and tunable pore structure, MOFs have also been applied in electrocatal‐ ysis involving energy conversion and storage in recent years [32]. For instance, MOFs can be directly used as electrocatalysts in the applications for HER and/or OER. However, the electric conductivity of most MOFs is poor. The combination of MOFs and highly conductive substrates (such as graphene, carbon nanotubes, Ni foams and conductive glasses) is an efficient protocol to enhance the electrocatalytic performance of MOFs [27]. In addition, MOFs also can serve as precursors to form derived catalysts such as metal oxides, metal (oxides or carbides or sulfides or phosphides)/nanocarbon hybrids or porous nanocarbons, which is another effective way to take advantage of MOFs for HER and/or OER [40].
