# Two-Dimensional Metal-Organic Frameworks and Their Derivative Electrocatalysts for Water Splitting
## 3 Application of 2D MOFs and Their Derivatives for Water Splitting
### Oxygen Evolution Reaction  
The water electrolysis reaction device mainly comprises two half-reactions, the anode [16; 17] OER, and the cathode HER. Compared with the HER, the OER involving four-electron transfer has slow kinetics. The standard electrode potential of the OER on  
Figure 8: (**a**) Ultrasound-assisted exfoliation process of [Cu(2.5-pydc)(H\({}_{2}\)O)]\({}_{n}\cdot\)2H\({}_{2}\)O. (**b**) SEM image of the exfoliated nanosheets. (**c**) SEM image of nanosheets. (**d**) XPS spectra of Fe 2p. (**e**) XPS spectra of Ni 2p. Reproduced with permission. [132; 156] Copyright 2021, Elsevier. (**f**) Ni\({}_{3}\)S\({}_{2}\)@2D Co-MOF/CP polarization curves. (**g**) Tafel plots of NiS\({}_{2}\)@2D Co-MOF/CP. (**h**) SEM images of Ni\({}_{3}\)S\({}_{2}\)/CP. (**i**) SEM images of Ni\({}_{3}\)S\({}_{2}\)@2D Co-MOF/CP. Reproduced with permission [133]. Copyright 2022, Elsevier. In an alkaline  
\[\mathrm{OH^{-}+^{*}\to HO^{*}+e^{-}} \tag{7}\]  
\[\mathrm{HO^{*}+OH^{-}\to O^{*}+H_{2}O+e^{-}} \tag{8}\]  
\[\mathrm{O^{*}+O^{*}\to O_{2}+2^{*}} \tag{9}\]  
\[\mathrm{O^{*}+OH^{-}\to HOO^{*}+e^{-}} \tag{10}\]  
\[\mathrm{HOO^{*}+OH^{-}\to*+H_{2}O+e^{-}+O_{2}} \tag{11}\]  
In an acidic solution  
\[\mathrm{H_{2}O+^{*}\to HO^{*}+H^{+}+e^{-}} \tag{12}\]  
\[\mathrm{HO^{*}\to O^{*}+H^{+}+e^{-}} \tag{13}\]  
\[\mathrm{O^{*}+O^{*}\to O_{2}+2^{*}} \tag{14}\]  
\[\mathrm{O^{*}+H_{2}O\to HOO^{*}+H^{+}+e^{-}} \tag{15}\]  
\[\mathrm{OH^{-}+^{*}\to HO^{*}+e^{-}} \tag{7}\]0  
where * represents the active site of the catalyst. OER is a crucial half-reaction that takes place at the anode during water splitting, and its mechanism can vary depending on whether the reaction occurs in acidic or alkaline conditions [169; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172]. Currently, the precise mechanism of OER is not fully understood, and researchers have proposed several potential mechanisms. Among them, the most widely accepted view is that the intermediate M-OH or M-O is formed during OER. Subsequently, these intermediates can follow two routes: the vertical path (shown in Figure 9), where the M-O intermediate directly combines to generate O\({}_{2}\), and the left semicircular route (also depicted in Figure 9), where the M-O intermediate further transforms into the M-OOH intermediate, which subsequently decomposes to release O\({}_{2}\). These pathways are illustrated by Equations (7)-(16). Traditionally, catalysts based on noble metals such as \(\mathrm{IrO_{x}}\) exhibit higher activity for oxygen production. However, due to their scarcity, high cost, and susceptibility to deactivation, water electrolysis technologies employing precious metal catalysts are not widely utilized. With the advancement of 2D MOF materials, an increasing number of 2D MOFs have shown promising activity in OER. The catalytic performance of 2D MOFmaterials in OER [173; 174; 175; 176] has garnered significant attention, offering potential alternatives to noble metal catalysts. The 2D MOFs are layered materials composed of organic ligands and metal ions, and the catalytic activity of bimetallic 2D MOFs can be enhanced due to the synergistic interaction between the bimetallic atoms. [PERSON] et al. [178] synthesized NiCo-MOF by a one-step hydrothermal procedure. Figure 10a,b shows that introducing Ni into 2D ZIF-67 not only reduces agglomeration but also promotes the formation of 2D lamellar structures. As demonstrated in Figure 10c,d, XPS analysis revealed that the 5% NiCo-MOF exhibited a higher Co 2p binding energy compared to the 2D ZIF-67, with peaks observed at 796.7 and 781.2 eV, respectively. This indicates that the addition of Ni can modulate the electronic arrangement of Co and change its chemical valence so that it can attract electrons more efficiently, thus improving the catalytic performance and accelerating the charge transfer. The electrocatalytic activity of the 5% NiCo-MOF can be optimized by adjusting the molar ratio of Ni/Co.