Unveiling the Aggregation of M−N−C Single Atoms into Highly Efficient MOOH Nanoclusters during Alkaline Water Oxidation

M−N−C‐type single‐atom catalysts (SACs) are highly efficient for the electrocatalytic oxygen evolution reaction (OER). And the isolated metal atoms are usually considered real active sites. However, the oxidative structural evolution of coordinated N during the OER will probably damage the structure of M−N−C, hence resulting in a completely different reaction mechanism. Here, we reveal the aggregation of M−N−C materials during the alkaline OER. Taking Ni−N−C as an example, multiple characterizations show that the coordinated N on the surface of Ni−N−C is almost completely dissolved in the form of NO3−, accompanied by the generation of abundant O functional groups on the surface of the carbon support. Accordingly, the Ni−N bonds are broken. Through a dissolution‐redeposition mechanism and further oxidation, the isolated Ni atoms are finally converted to NiOOH nanoclusters supported by carbon as the real active sites for the enhanced OER. Fe−N−C and Co−N−C also have similar aggregation mechanism. Our findings provide unique insight into the structural evolution and activity origin of M−N−C‐based catalysts under electrooxidative conditions. The activation and aggregation of M−N−C (M=Ni, Fe, and Co) single‐atom electrocatalysts during the oxygen evolution reaction (OER) are demonstrated. In this process, both N and supported carbon are severely oxidized, leading to the destruction of M−N bonds. Through a dissolution‐redeposition mechanism, the isolated M atoms finally aggregate to form MOOH nanoclusters. MOOH nanoclusters with a carbon support are responsible for the enhanced OER activity.