# 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
Note that the only difference between the reaction mechanisms in acidic and basic solutions is the source of protons. The reaction kinetics [146] of HER is related to the experimental Tafel slope [147], where a small Tafel slope indicates that the current density of the hydrogen precipitation reaction increases rapidly as the overpotential increases. The electrochemical HER kinetics are also related to the H\({}_{\text{ad}}\) (surface-bonded H atoms) formed on the electrode surface. If the bonding force between H\({}_{\text{ad}}\) and the electrode is too strong, it will inhibit the Volmer reaction, and if it is too weak, it will inhibit the Heyrovsky reaction or the Tafel reaction. The adsorption Gibbs free energy [148] is usually used to represent the HER performance of the catalyst. As shown in Figure 7, the volcano plot of the HER reaction illustrates the correlation between the free energy of hydrogen binding and the corresponding HER activity of the catalyst. The platinum group metals (Pd [149], Pt [150], Rh [151], and Ir [152]) are positioned near the roof of the volcano and represent the inorganic materials with the best HER properties. The transition metals are positioned to the right of the platinum family metals (such as Ag and Au), which are weakly bonded to hydrogen and difficult to initiate the reaction; the metals located to the left (such as Ni and Co) are more strongly bonded to hydrogen and difficult to release products rapidly. Figure 7: HER Reaction Volcano Diagram [153]. Copyright 2018, Royal Society of Chemistry. To date, the most advanced HER catalysts are based on a costly precious metal such as Pt [150; 154; 155] because it has good electrical conductivity, high electrocatalytic activity, and its hydrogen adsorption Gibbs free energy (GH) is close to zero. However, the high cost and severe scarcity greatly impeded the large-scale employment of water electrolysis. In contrast, 2D MOF nanosheets [72; 73; 75] and their derivatives [82; 83; 86; 87] are novel catalytic materials for HER due to their particular physicochemical properties. 2D MOFs possess highly desirable pore structures with large specific surface areas and controllable morphologies. For example, [PERSON] et al. [132] successfully prepared thin nanosheets of [Cu(2,5-pydc)(H\({}_{2}\)O)]\({}_{n}\cdot\)2H\({}_{2}\)O (pydc = 2,5-pyridinedicarboxylic acid) using ultrasonic exfoliation (Figure 8a), resulting in highly uniform nanosheets with a thickness of approximately 126 nm (Figure 8b,c). Remarkably, the electrocatalytic performance of these nanosheets was significantly enhanced compared to that of the bulk sample, with an overpotential reduced from 530 mV to 340 mV. The improved performance of 2D MOF nanosheets can be attributed to their enhanced electron transfer between the current collector and catalytic sites, which is facilitated by the large surface area and efficient transport pathways in these materials. Using multiple metals in MOF preparation not only optimizes the electronic structure and increases the electrical conductivity but also improves electron transfer and electrocatalytic activity. [PERSON] et al. [156] employed a one-step solvothermal method to prepare NiFe-MOF-74, which exhibited excellent electrocatalytic activity for OER. The hybridized NiFe-MOF-74 showed a network of interconnected honeycomb-like sheets, in contrast to the flower morphology of monometallic Fe-MOF-74 and the spherical shape of Ni-MOF-74 with a diameter of 3-5 \(\upmu\)m. This structure provided a large specific surface area, exposure of more metal sites, and facilitated mass transport during the catalytic reaction, driving a current density of \(-\)10 mA cm\({}^{-2}\) requiring only 195 mV overpotential in 1.0 M alkaline solution.