# Two-Dimensional Metal-Organic Frameworks and Their Derivative Electrocatalysts for Water Splitting
## 3 Application of 2D MOFs and Their Derivatives for Water Splitting
### Hydrogen Evolution Reaction
As demonstrated in Figure 8d,e, X-ray photoelectron spectroscopy (XPS) images revealed a slightly negative shift of 0.2 eV in the binding energy of Ni 2p 3/2 in NiFe-MOF-74 and a positive shift of 0.8 eV in the binding energy of Fe 2p 3/2 in NiFe-MOF-74, indicating that the transfer of electrons from Fe to Ni improved the catalytic activity of the catalyst. The synthesis of heterostructures in 2D MOF materials has emerged as an effective strategy for enhancing electrocatalytic activity while preserving the previous structure and morphology. For instance, [PERSON] et al. [133] successfully prepared Ni\({}_{3}\)S\({}_{2}\)@2D Co-MOF nanosheets by introducing Ni\({}_{3}\)S\({}_{2}\) on the surface of Co-MOFs. The resulting Ni\({}_{3}\)S\({}_{2}\)@2D Co-MOF/CP exhibited remarkable electrocatalytic performance, driving a current density of \(-\)10 mA cm\({}^{-2}\) with an overpotential of only 140 mV in a 1.0 M alkaline solution (Figure 8f). Moreover, the Tafel slope of the material was found to be only 90.3 mV dec\({}^{-1}\) (Figure 8g), indicating its high catalytic activity and fast electron transfer rate. As shown in Figure 8h,i, the combination of Co-MOFs and Ni\({}_{3}\)S\({}_{2}\) can prevent agglomeration on the surface of carbon fibers, and the formation of a honeycomb-like porous structure allows the enhancement of the number of active sites. The XPS pattern demonstrates that the charge transfer from Co-MOF to nickel sulfide enriches its surface with electrons, which promotes H\({}_{2}\) production. [PERSON] et al. [157] recently reported that Co-MOF nanosheets were synthesized by the hydrothermal process. Then, 2D CoP/Co-MOF heterostructure nanosheets were prepared by low-temperature phosphorylation on a carbon paper substrate. The prepared CoP/Co-MOF has a similar 2D nanosheet structure as the pristine Co-MOF. The TEM and SEM outcomes indicate that the 2D CoP/MOF heterostructure was fabricated successfully by partly phosphorylation, which has a larger surface area and exposes more active sites. The electrostatic interactions between the 2D CoP nanosheets and 2D Co-MOF nanosheets play a crucial role in modulating the surface chemistry of the resulting electrocatalysts. Specifically, the 2D CoP/Co-MOF nanosheet array exhibits exceptional electrocatalytic performance, requiring only an overpotential of 52 mV to drive a current density of \(-\)10 mA cm\({}^{-2}\) in 0.5 M H\({}_{2}\)SO\({}_{4}\). CoP/Co-MOF heterostructures can thoroughly combine the superior electrical conductivity and porosity of Co-based nanoparticles and the original MOF's advantage of dispersed active sites. In catalysts that form heterojunctions, the activity of the catalyst is improved due to the modification of the electronic morphology. In addition, [PERSON] et al. [134] fabricated 2D MOF-derived multiporous NiCoSe nanosheet arrays based on ZIF-67. NiCoSe nanosheet arrays drive a current density of \(-\)10 mA cm\({}^{-2}\), requiring only 170 mV low overpotential in alkaline solutions; the outstanding HER electrocatalytic performance of NiCoSe was attributed to the porous structure, exposing more active sites. [PERSON] et al. [158] synthesized a novel mesoporous bimetallic sulfide combined CoS\({}_{2}\) nanoarray mixture FeS\({}_{2}\)-MoS\({}_{2}\)@CoS\({}_{2}\)-MOF by designing and utilizing 2D MOF-derived materials. This catalyst drove a current density of \(-\)10 mA cm\({}^{-2}\) requiring only 92 mV overpotential in 1.0 M alkaline solution. By combining MoS\({}_{2}\) with FeS\({}_{2}\) sheets, additional active sites were generated, and conductivity was enhanced, resulting in improved intrinsic performance for the HER. Table 2 summarizes the HER catalytic performance of various 2D MOFs and their derivatives.