Defect-orchestrated Co3O4: unraveling the competing roles of Co3+ reducibility and oxygen vacancies in superior OER activity.

We report a tunable NaNO3-assisted thermal-chemical activation strategy that simultaneously modulates Co3+ reducibility and oxygen-vacancy density in spinel Co3O4. An optimal 1 : 4 Co3O4 : NaNO3 composition, followed by water washing and HNO3 etching, generates a defect-rich surface with enhanced Co3+ content, improved hydrophilicity, and enlarged mesoporosity. The resulting catalyst delivers an overpotential of 375 mV at 10 mA cm-2 with a low Tafel slope of 54 mV dec-1, approaching that of sol-gel derived RuO2 and remaining stable for 70 h. Spectroscopic analyses attribute the enhanced activity to Na+-induced surface reconstruction and selective lattice distortion, highlighting a simple and scalable route to vacancy-engineered cobalt oxides for efficient OER catalysis.