**3. HER/OER**

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

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

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

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

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,

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

/g and mesopores with the pore size of ∼2.9 nm [37]. In addition,

surface area of ∼3100 m2

116 118Metal-Organic Frameworks

solubility.

MIL‐100 exhibits excellent thermal stability below 275°C [37].

other is the three-dimensional orthogonal channels (∼1 nm) [38].

with good orientation and perfect crystals [19, 39].

selective heating, and no hysteresis effects.

The growing concern of energy crisis and environment pollution promotes the development of highly effective clean energy storage and conversion systems, such as water splitting [1], metal‐air batteries [2], and fuel cells [3]. In such systems, oxygen‐ and hydrogen‐involving electrocatalytic processes, including oxygen reduction reaction (ORR), hydrogen oxidization reaction (HOR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER) are the most mentioned key components in determining the practical performance of these energy storage and conversion systems. The former two reactions consume oxygen and hydrogen to provide power, but the premise is that the available oxygen and hydrogen are generated from water splitting through OER and HER, respectively. The OER/HER involved in water splitting can be given in the following equations [9, 41]:

Hydrogen evolution reaction:

2H+ + 2e → H2 in acid solution

2H2O + 2e → H2 + 2OH in alkaline solution

Oxygen evolution reaction:

2H2O - 4e → O2 + 4H+ in acid solution

4OH‐ - 4e → O2 + 2H2O in alkaline solution

Water splitting can produce pure hydrogen and oxygen which can be directly utilized as raw materials for fuel cells or for other industrial processes individually. Apart from water splitting, HER or OER can be associated with other electrochemical processes. For instance, OER is also a key reaction for rechargeable metal‐air batteries [2] and regenerative fuel cells [3] when being charged. Rechargeable metal‐air batteries and regenerative fuel cells are seen as promising ways to produce electricity in the future and may effectively reduce our dependence on traditional fossil fuels and relieve environmental pollution.

Unlike HOR/ORR, HER/OER don't maintain a constant value at high overpotential. In other words, HER/OER are not limited by the mass transferrate and they obey Butler‐Volmer model even in high overpotential. Generally, the HER/OER catalytic mechanisms differ from each other. Even so, the descriptor (Δ*G*H\*, hydrogen binding energy/adsorption free energy) is widely used to reflect HER activity for catalysts, while the descriptor (Δ*G*O\*−Δ*G*OH\*) is used to evaluate the OER activities [5]. HER involves the two‐electron transfer processes, while OER includes more complex processes with the multistep charge‐/proton‐transfer. These complex processes result in the low efficiencies of HER/ORR and make it difficult for them to satisfy practical applications. Therefore, the development of efficient electrocatalysts for HER/OER to accelerate the kinetics and reduce the overpotentials is of great significance for the com‐ mercialization of HER/OER‐related devices.

Currently, the HER/OER electrocatalysts mainly include precious metal catalysts and non‐ precious metal catalysts [5, 42].The precious metal catalysts have been widely developed due to their extremely excellent catalytic performance. For instance, precious metal cata‐ lysts for HER are mainly dependent on Pt‐based catalysts since they exhibit the best HER electrocatalytic performance in basic solution with a low onset overpotential. IrO2 and RuO2 have been demonstrated as the benchmark OER electrocatalysts (Pt‐based catalysts barely show any OER catalytic activity), and even show remarkable stability in acid solu‐ tion. However, although the precious metal catalysts possess excellent catalytic perform‐ ance, their scarcity and high price hamper their large‐scale applications. Thus, non‐ precious metal catalysts with low‐cost and earth‐abundance for HER/OER are urgently needed.

The non‐precious metal catalysts for HER mainly include transition‐metal sulfides, phos‐ phides, alloys, etc. Through element selection, Co sulfides or phosphides are seen as the most promising HER catalysts, but they suffer from low stability. The alloys have been developed by integrating Ni with Mo or Cu [12, 13]. Diverse structures and composition are the advan‐ tages to those alloyed catalysts, but their restriction of activity limits their applications. The non‐precious metal catalysts for the OER mainly contain transition metal oxides [14–16], sulfides [17, 18], hydroxides [43], and carbons [44], etc. Among them, metal oxides in single phase or in mixed phases are the most developed ones as efficient OER electrocatalysts due to their diverse compositions, superior stability, and remarkable OER performance. Thus, metal oxides are widely explored as promising OER catalysts, but the mechanism of OER is very sensitive to the compositions and structures of the catalysts, which requires further design, preparation, and optimization of OER catalysts. In spite ofrapid development of the HER/OER electrocatalysts with relatively good performance, to date, no one catalyst can satisfy the practical demands. More researches for highly efficient HER and OER electrocatalysts are needed.
