Multi‐Cation (High‐Entropy) Doping of Mesoporous Co 3 O 4 Thin Films With Cr, Mn, Fe, Ni, Cu, and Ag: Assessment of the Oxygen Evolution Reaction Performance

Co 3 O 4 is a promising earth‐abundant electrocatalyst for the oxygen evolution reaction (OER), yet its intrinsic activity remains limited by high charge–transfer resistance and restricted active‐site availability. Here, we systematically investigate the effect of single‐cation (Cr, Mn, Fe, Ni, Cu, and Ag) and multi‐cation (Cr–Mn–Fe–Ni–Cu–Ag) doping in sol‐gel‐derived mesoporous Co 3 O 4 thin films to identify composition–structure–activity correlations relevant for alkaline water electrolysis. All doped films crystallize in the cubic spinel phase, composed of ultrasmall nanocrystallites (∼4 nm) and form structurally intact mesoporous networks, as confirmed by SEM, XRD, and Raman spectroscopy. UV–vis analysis reveals enhanced visible‐light absorption, consistent with the formation of dopant‐induced sub‐band states. XPS verifies the presence and oxidation states of the incorporated metals at the surface, whereas silver doping results in partial surface segregation. Electrochemical measurements in 1 M KOH demonstrate that all dopants enhance the OER activity relative to pristine Co 3 O 4 . Fe‐doped Co 3 O 4 exhibits the best geometric OER performance, achieving an overpotential of 344 mV at 10 mA/cm 2 , the lowest charge–transfer resistance (9.3 Ω), and good electrochemical stability over 24 h. In contrast, multi‐cation doping does not outperform the best single‐element dopants, indicating the absence of a synergistic high‐entropy effect under the investigated conditions. These results identify Fe, Cu, and Mn as the most effective dopants for enhancing the alkaline OER stability of mesoporous Co 3 O 4 thin films while preserving structural integrity, thereby providing design guidelines for compositionally optimized transition metal oxide electrocatalysts.