# 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  
Hydrogen energy [2; 3; 135; 136] is known to have the highest weight-energy density among all the chemical fuels, and among all the methods to produce hydrogen energy, the electrolysis of water for hydrogen production [8; 10; 11; 137; 138; 139] is the most promising in green chemistry because it is an environmentally clean process. In an acidic solution  
\[\text{[PERSON]: H}^{+}+{}^{*}+\text{e}^{-}\rightarrow\text{H}^{*} \tag{1}\]  
\[\text{[PERSON]: H}^{*}+\text{H}^{+}+\text{e}^{-}\rightarrow\text{H}_{2}+{}^{*} \tag{2}\]  
\[\text{[PERSON]: H}^{*}+\text{H}^{*}\rightarrow\text{H}_{2}+2{}^{*} \tag{3}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**2D MOF** & **Synthesis Method** & **Synthesis Strategy** & **Topography** & **Thickness** & **Ref.** \\ \hline [Cu\({}_{\text{o}}\)Br(IN)\({}_{2}\)In & Top-down Synthesis & Ultrasonic exfoliation & uniform & \(0.5\pm 0.015\) nm & [103] \\ ZSB-1 & Top-down Synthesis & Wet sphere milling/ultrasonic & uniform & \(11.8\pm 2.3\) nm & [104] \\ vdW MOF-2 & Top-down Synthesis & Micro-mechanical peel-off & uneven & \(10.0\) nm & [107] \\ Zn\({}_{2}\)(bim)\({}_{3}\) & Top-down Synthesis & Wet sphere milling/ultrasonic & uneven & \(1.6\) nm & [108] \\ Zn\({}_{2}\)(PdTCP) & Top-down Synthesis & Intercalation and chemical exfoliation & uneven & \(1.0\) nm & [109] \\ [Co(Ni-H/TTPP)\({}_{2}\)]-8H\({}_{2}\)O & Top-down Synthesis & Chemical exfoliation & uneven & \(3.9\pm 0.2\) nm & [110] \\ Co\({}_{6}\)O(dhbdc)\({}_{2}\) & Top-down Synthesis & Electrochemical/chemical exfoliation & wrinkled & \(2\) nm & [111] \\ ZigI-L & Bottom-up Synthesis & Template assisted & uneven & \(5.5\) μm & [112] \\ Meso-CuBDC & Bottom-up Synthesis & Sacrificial templates & small square shape & \(8.0\) nm & [113] \\ Ag\({}_{2}\)BHT\({}_{2}\) & Bottom-up Synthesis & liquid–liquid interfacial growth & disordered & \(200\) nm & [115] \\ Ni-MOF & Bottom-up Synthesis & liquid–liquid interfacial growth & uniform & \(5\) nm & [116] \\ CuBDC & Bottom-up Synthesis & Interfacial synthesis & uniform & \(5\)-\(2\) nm & [117] \\ Zn-TCP & Bottom-up Synthesis & Surface-active agent-assisted & uniform & \(7.6\pm 2.6\) nm & [118] \\ Co/Zn-porphyrin & Bottom-up Synthesis & Surfactant-assisted & wrinkled & \(4\)-\(5\) nm & [119] \\ HXP & Bottom-up Synthesis & Molecular modulation assisted & uniform & \(20\) nm & [123] \\ [Cu(2,5-pydc)(H\({}_{2}\)O)]\({}_{1-2}\)H\({}_{2}\)O & Bottom-up Synthesis & Ultrasonic exfoliation & uniform & \(126\) nm & [132] \\ Ni\({}_{3}\)S\({}_{2}\)@2D Co-MOF & Bottom-up Synthesis & Self-sacrificial template & rough & \(60\) nm & [133] \\ NiCoSe & Bottom-up Synthesis & ion-exchange & uniform & \(230\) nm & [134] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Synthesis Methods for MOFs and their Composite-Based Electrocatalysts. In an alkaline or neutral medium  
\[\text{[PERSON]: H}_{2}\text{O}+\text{*}+\text{e}^{-}\rightarrow\text{H}^{\text{*}}+ \text{OH}^{-} \tag{4}\]  
\[\text{[PERSON]: H}^{\text{*}}+\text{H}_{2}\text{O}+\text{e}^{-}\rightarrow\text{* }+\text{OH}^{-}+\text{H}_{2} \tag{5}\]  
\[\text{[PERSON]: H}^{\text{*}}+\text{H}^{\text{*}}\rightarrow\text{H}_{2}+2^{\text{*}} \tag{6}\]  
Here, H\({}^{\text{*}}\) represents the hydrogen atom adsorbed on the catalyst surface, and * represents the active site of the catalyst. Under acidic conditions, hydrogen ions undergo reduction to form hydrogen, with the equilibrium half-cell potential being 0 V. Meanwhile, under alkaline conditions, water molecules are reduced to produce hydrogen and hydroxyl groups, resulting in an equilibrium half-cell potential of 0.83 V. The HER is a cathodic process involving a two-electron transfer. It has been observed that HER primarily follows either the Volmer-Tafel path or the Volmer-Heyrovsky path, as shown in Equations (1)-(6) [140; 141; 142; 143; 144; 145]. HER tends to occur via the Volmer-Tafel path or the Volmer-Heyrovsky path, given in Equations (1)-(6).