These studies demonstrate that interfacial confinement and cluster modulation are effective approaches for improving the performance of non-noble metal catalysts in acidic OER. The FeO/Mn₃CoO_x system highlights the role of confined microcrystallites in regulating electronic structure and reaction pathways, whereas Mo/Co₃NiO_x catalyst illustrates how cluster engineering can influence reaction energetics and stabilize catalytic intermediates. A comparative summary of these two representative systems is provided in Table 1. Such strategies provide useful guidance for the rational design of advanced catalysts for proton exchange membrane water electrolysis.