The increase in the concentration of the metal centers is restricted by the limited number of defects that could anchor the metal atoms on the substrate and the sintering of the concentrated single metal sites at the high synthesis temperature.Owing to the low concentration of metal centers, high SACs mass loadings on the electrode are required to achieve con siderable current densities. However, the thick films formed at high mass loadings increase the mass transport and ion transport resistance in MEA systems, and lead to extra voltage losses. Intensive efforts have been devoted to increasing the concentration of the metal centers in SACs to decrease the total mass loading.