# Metal-Organic Frameworks Toward Electrocatalytic Applications
## 4 Recent Progress in Utilizing MOFs for Electrocatalysis and Relevant Applications  
### Hydrogen Evolution  
Photoelectrochemical water splitting has attracted great attention and been considered as a potential route to generate renewable energy [3]. Except the photosensizers that can render charge separation, electrocatalys are also required for both the anode and cathode to accelerate the rates and reduce the overpotentials for both half reactions [4]. Platinum is the typical electrocatalys on the cathode side to catalyze the half reaction that converts water into hydrogen gas. Besides, metal sulfides, metal selenides, and metal phosphides have been proposed as the highly active electrocatalys for hydrogen evolution to replace platinum [80]. In 2011, [PERSON] and coworkers reported the use of polyoxometalate-based MOFs for electrocatalytic hydrogen evolution [81]. By utilizing the redox hopping between those polyoxometalate-based units presented within the framework, the MOF thin film was found to exhibit significant electrocatalytic activity for hydrogen evolution. [PERSON] et al. further designed another polyoxometalate-based MOF (NENU-500) that possesses a much higher porosity and larger pore size compared to the previous one [82]. With the help of a better porosity, the resulting NENU-500 thin film achieved a current density of 10 mA/cm\({}^{2}\) for hydrogen evolution at a relatively low overpotential of 237 mV. On the other hand, the use of a zirconium-based MOF (NU-1000) thin film electrodeposited with a layer of nickel sulfide for electrocatalytic hydrogen evolution was demonstrated by [PERSON] and coworkers (see Figure 5) [83]. The presence of highly robust NU-1000 framework along with nickel sulfide was found to facilitate proton transport during the electrocatalysis process in acidic electrolytes, which resulted in a much better electrocatalytic activity compared to that of the pristine electrodeposited nickel sulfide. To further increase the density of active sites within the MOF thin films, [PERSON] et al. also utilized NU-1000 as a porous support to deposit the spatially well-separated molybdenum sulfide nanoclusters for hydrogen evolution [84]. With the help of redox mediators to facilitate charge transport between those catalytically active sites decorated in the framework, the resulting electrocatalytic activity for hydrogen evolution can be significantly improved. Recently, [PERSON] and coworkers demonstrated the use of carbon nanotube-supported hafnium-based MOF constructed from cobalt (III) porphyrinic linkers for electrocatalytic hydrogen evolution [85]. With the help of carbon nanotubes as the conductive support to facilitate charge transport between MOF crystals, the electrocatalytic activity of the composite material was found to be significantly enhanced compared to that of the pristine MOF. A recent finding also shows that an iron-based MOF, MIL-53(Fe), can exhibit electrocatalytic activity for hydrogen evolution [86].  
Figure 4: Crystal structures of (**a**) UiO-66 [28] and (**b**) PCN-601 and PCN-602 [78; 79]. Lime green: Zr; red: O; grey: C; malachite: Ni; light slate blue: N. Hydrogen atoms are not shown for clarity.