# Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges
## 2 Mechanisms for the OER in Acidic Media  
In the case of the OER in acid media, two possible mechanisms built on consecutive proton and electron transfers during the catalytic cycle, known as the adsorbate evolution mechanism (AEM) and the lattice oxygen participation mechanism (LOM), have been widely accepted [24; 26; 54; 55] (Figure 2). For the reaction path based on the AEM, a water molecule first adsorbs on a surface metal cation and decomposes into a proton (H\({}^{+}\)) to form HO\({}^{\bullet}\), which further dissociates the second proton to form O\({}^{\bullet}\) in the second step. After that, HO\({}^{\bullet}\) is formed by the nucleophilic attack from another water molecule on the O\({}^{\bullet}\). Finally, oxygen is released, accompanied by the desorbed proton. Another four-electron transfer mechanism, known as the LOM, has been proposed based on a series of in-situ isotopic labeling experiments. In contrast to the AEM, lattice O participates in the formation of oxygen for the LOM. Firstly, one water molecule is adsorbed on a surface lattice O and dissociates the first proton to form HO\({}^{\bullet}\), which further dissociates the second proton to form O\({}^{\bullet}\) in the second step. After that, oxygen is released via coupling absorbed O and a surface lattice O along with the presence of a surface oxygen vacancy. Finally, the surface lattice is restored as before via water adsorption and dissociation on the vacancy [24]. Although both of the mechanisms involve a four-electron transfer, there are still some differences between them [26]. The first one is that the AEM requires a higher reaction energy barrier (>320 mV) than the LOM theoretically [56]. In terms of active sites, the catalytic process predominantly involves a cationic redox (i.e., transition metal ions) in the AEM mechanism and an anion redox (i.e., lattice oxygen) in the LOM [57]. However, it varies according to actual conditions. [PERSON] et al. demonstrated that both mechanisms exist in the OER, which can be detected by the charge storage behavior via the applied bias [58]. At a low bias, it mainly involves the charge storage of metal centers; at a high bias, it involves the storage of oxygen in IrO\({}_{\text{x}}\). Moreover, strategies for increasing activity are different based on these two different mechanisms. On basis of the AEM, the active metal centers are always at a lower valence state, which can promote the nucleophilic attack of water molecules by increasing the covalence of metal and oxygen [43]. For example, [PERSON] et al. simulated under-coordinated Ru atoms on a well-crystallized RuO\({}_{2}\) surface with superior OER activity [59]. For the LOM, the metal center is often at a higher valence state, which is committed to promoting the generation of more electrophilic oxygen atoms and increasing the interaction between metal and oxygen [60]. [PERSON] et al. studied the lattice oxygen behavior of La\({}_{2}\)LiIrO\({}_{6}\). They believed that Ir was not the active site for the OER owing to the pH-dependent activity [61]. Despite the existing difference, some phenomena occurring in the process of an OER can still be explained by these theories. For example, excessive oxidation of metal sites for the AEM and lattice participation for the LOM generally lead to material instability [49]. Furthermore, the reason why the amorphous metal oxides exhibit better catalytic activity is that lattice oxygen can participate in the catalytic reaction easier than the well-crystallized ones [35]. It should be mentioned that, apart from increasing the site density, we can also optimize the composition to modify the intrinsic activity of the OER [33; 62]. Recent work has pointed out the superiority of bi-metal oxides as some of the most advanced electrocatalysts toward the OER in acidic media, in terms of features including decreased Ru/Ir contents and enhanced OER activity and selectivity [23; 63].