**4. MOFs‐based catalysts for HER**

are seen as promising ways to produce electricity in the future and may effectively reduce

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‐

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

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

our dependence on traditional fossil fuels and relieve environmental pollution.

mercialization of HER/OER‐related devices.

needed.

118 120Metal-Organic Frameworks

needed.

In recent years, MOFs have been widely explored to store hydrogen fuels due to high specific surface area, tunable pore structures, and various functions [45]. Meanwhile, they also have been developed as electrocatalysts for the HER through electrochemical splitting of water. Generally, MOFs can be directly used as catalysts or served as precursors for the derived catalysts for HER. Recently, Gong et al. synthesized Cu3(Mo8O26)(H2O)2(OH)2(L1)4 (L1 = 4H‐4‐ amino‐1,2,4‐triazole) and Ag4(Mo8O26)‐(L2)2.5(H2O) (L2 = 3,5‐dimethyl‐4‐amino‐4H‐1,2,4‐ triazole) MOFs via a hydrothermal process [46]. The former had a chain‐like structure, while the latter possessed a 3D structure. Both of them showed electrocatalytic activity for HER in 0.5 M H2SO4 with low overpotentials. Cu3(Mo8O26)(H2O)2(OH)2(L1)4 had better HER activity than Ag4(Mo8O26)‐(L2)2.5(H2O). Their HER activities were related to the redox of [Mo8O26] 4− polymolybdate anions. Qin et al. developed another polymolybdate (POM)‐based MOF for HER [47]. This novel 3D open structure was formed by connecting POM fragments as nodes and H3BTB (H3BTB = benzene tribenzoate) or H3BPT (H3BPT = [1,1′‐biphenyl]‐3,4′,5‐tricarbox‐ ylic acid) as ligands. POM‐based MOFs had excellent HER activity likely due to the combina‐ tion of the redox of POM and the porosity of MOFs. Noticeably, the POM‐based MOFs were stable not only in air but also in acid or basic solutions. Among these catalysts, NENU‐500 ([TBA]3[ε‐PMo<sup>V</sup> 8MoVI4O36(OH)4Zn4] [BTB]4/3·18H2O, TBA+ = tetrabutylammonium ion) exhibit‐ ed excellent HER activity with an onset overpotential of about 180 mV and Tafel slope of 96 mV dec−1 in 0.5 M H2SO4. What's more, this catalyst could maintain its catalytic activity after 2000th cycles in 0.5 M H2SO4 solution.

Cobalt dithiolene species are highly efficient molecular catalysts for HER. Cloigh et al. integrated cobalt dithiolene into metal‐organic surface (MOS, [Co3(BHT)2] 3+and [Co3(THT)2] 3+ (BHT= benzenehexathiol, THT=triphenylene‐2,3,6,7,10,11‐hexathiol) materials to obtain effective catalysts for HER [48]. The prepared catalysts Co/MOS possessed a two‐dimension‐ al layer structure with high surface/volume ratio, resulting in high charge transfer efficiency and high surface active site concentration. The catalysts, especially [Co3(BHT)2] 3+, showed not only remarkable stability in acidic solutions, but also excellent HER catalytic activity under wide pH conditions, and the current densities at 0.5 V (vs. Standard Hydrogen Electrode, RHE) increased with decreasing electrolyte pH. The mechanism of HER of the MOS involved Co3+/ Co2+ redox reactions, following protonation of the S sites on the ligands.

Recently, MOFs also have been combined with 2D materials, such as graphene and metal dichalcogenide nanosheets, to form nanocomposites with unique properties and wide applications [49]. A Cu‐MOF/graphene oxide (GO) nanocomposite catalyst for HER was successfully synthesized via a solvothermal process [27]. The Cu‐MOF was prepared using Cu(NO3)2, 1,4‐benzenedicarboxylic acid and triethylene‐diamine as precursors. The integra‐ tion of Cu‐MOF with graphene oxide (GO) can effectively enhance the electron transfer, which further significantly improve HER activity. It was also found that the GO content affected the HER activity of the nanocomposite catalysts. The optimized GO content was about 8%. The HER current density of the (GO 8 wt%) Cu‐MOF was high up to -30 mA cm−2 at an overpo‐ tential of -0.2 V in N2‐saturated 0.5 M H2SO4, whereas the overpotential of 20 wt% Pt was -0.06 V at the current density of -30 mA cm−2.

**Figure 1.** Schematic illustration of (A) synthesis of Co‐P/NC [50]; (B) formation of CoP NPCs and CoP NRCs derived from ZIF‐67‐Co [51]; (C) the space‐confined synthesis of MoS2/3D‐NPC composites and its application in HER [53].

Because MOFs possess structural diversity, high specific surface area, and large pore vol‐ ume, MOFs are regarded as ideal precursors to prepare various inorganic nanomaterials as electrocatalysts, such as porous carbon, metal‐oxide nanoparticles/porous carbon or metallic nanoparticles/carbon. It has been reported that cobalt species are active for HER. Among them, cobalt phosphides as HER electrocatalysts have been widely investigated in recent years. You et al. prepared a Co‐P/NC catalyst by embedding CoP<sup>x</sup> nanoparticles into N‐doped carbon

through carbonizing Co‐ZIF‐67 and a subsequent phosphidation (**Figure 1A**) [50]. After carbonization, the obtained Co‐ZIF‐67 inherited the morphology of polyhedron‐like ZIF‐67, in which the metallic Co nanoparticles were wrapped by the porous N‐doped carbon shells. Through the following phosphidation, the products also preserved the polyhedron‐like morphology of ZIF‐67, while the metallic Co nanoparticles were transformed into CoPx nanoparticles. The optimal Co‐P/NC catalyst afforded a high specific surface area of 183 m<sup>2</sup> g −1and a high pore volume of 0.276 cm3 g−1. The overpotentials of the catalyst at 10, 20, and 100 mA cm−2 were ∼154, 173, and 234 mV respectively in 1 M KOH solution, indicating an outstanding HER activity. Such high HER activity can be attributed to its unique structures, such as abundant HER active species (CoPx and N‐doped carbon), 3D interconnected meso‐ pores, and the porous N‐doped carbon shells. Jiang et al. synthesized several cobalt phos‐ phides by directly phosphating Co‐ZIF‐67 under mild conditions [51]. The CoP nanorod assemblies (NRAs) were obtained by calcined precursor in N2 atmosphere, while the CoP nanoparticle assemblies (NPAs) were obtained by thermally treated precursor air atmos‐ phere (**Figure 1B**). The CoP NRAs showed better HER electrocatalytic performance than CoP NPAs. The CoP NRAs only needed overpotential of about 181 mV at 10 mA cm−2 in 0.5 M H2SO4, whereas the overpotential for CoP NPAs was 393 mV at 10 mA cm−2. After 1000th CV cycles, CoP NRAs and CoP NPAs both have slight loss of initial current density. The high efficiency of HER for CoP might be related to the charge transfer from Co to P, similar to the charge transfer process between hydride‐acceptor and proton‐acceptor of hydrogenase.

tential of -0.2 V in N2‐saturated 0.5 M H2SO4, whereas the overpotential of 20 wt% Pt was -0.06

**Figure 1.** Schematic illustration of (A) synthesis of Co‐P/NC [50]; (B) formation of CoP NPCs and CoP NRCs derived from ZIF‐67‐Co [51]; (C) the space‐confined synthesis of MoS2/3D‐NPC composites and its application in HER [53].

Because MOFs possess structural diversity, high specific surface area, and large pore vol‐ ume, MOFs are regarded as ideal precursors to prepare various inorganic nanomaterials as electrocatalysts, such as porous carbon, metal‐oxide nanoparticles/porous carbon or metallic nanoparticles/carbon. It has been reported that cobalt species are active for HER. Among them, cobalt phosphides as HER electrocatalysts have been widely investigated in recent years. You et al. prepared a Co‐P/NC catalyst by embedding CoP<sup>x</sup> nanoparticles into N‐doped carbon

V at the current density of -30 mA cm−2.

120 122Metal-Organic Frameworks

In addition to Co species, Ni species are also considered to be active for HER. Nickel phos‐ phides (Ni2P and Ni12P5) were prepared by phosphatizing Ni‐MOF (Ni‐BTC) in mild condi‐ tions [52]. During the preparation of Ni‐BTC, nickel nitrates and BTC (benzene‐1,3,5‐ tricarboxylic acid) were used as Ni sources and organic ligands, respectively. Then Ni2P and Ni12P5 were obtained by phosphating from sodium hypophosphite at 275 and 325°C, respec‐ tively. The prepared Ni12P5 nanoparticles show similar morphology to the Ni2P nanoparti‐ cles. However, the average diameter of the Ni2P (25 nm) was smaller than that of the Ni12P5 (80 nm). The HER activity of the Ni2P was better than that of the Ni12P5 nanoparticles in 0.5 M H2SO4, due to the similar effects of CoP on the HER activity [51]. The durability of the Ni2P nanoparticles was further evaluated by chronoamperometric durability test, and current density was reduced to 75% of the original value after 6 h test.

The two‐step synthesis method was also utilized by Liu et al. to prepare MoS2‐based HER catalyst with 3D hierarchical structure, in which MoS2 nanosheets grew in the nanopores of MOFs‐derived 3D carbons (MoS2/3D‐NPC) (**Figure 1C**) [53]. An Al‐PCP was formed first by a solvothermal process, and a 3D‐NPC was obtained through calcination. The 3D‐NPC pos‐ sessed randomly assembling nanopores, benefiting the intercalating of Mo precursors. After a solvothermal process, The MoS2/3D‐NPC catalyst was finally obtained, where MoS2 nanosheets were dispersed well in nanoporous 3D‐NPC. This unique 3D hierarchical struc‐ ture can provide exposed active site and enhanced conductivity, thus resulting in the high performance for HER. The MoS2/3D‐NPC catalyst requires overpotentials of 180 and 210 mV to achieve current densities of 1 and 10 mA cm−2 in 0.5 M H2SO4 solution, respectively.

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

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.
