Controlled Generation of β-NiOOH by Sulfuric Acid Treatment Enables Active and Durable Oxygen Evolution Catalysis

Designing oxygen evolution reaction (OER) electrocatalysts that can reversibly operate as both oxidation and reduction electrodes is key to developing efficient and rechargeable electrochemical energy systems. However, the evolution of nickel active sites between hydroxide and oxyhydroxide phases, and how such transformations can be made reversible under dynamic operating conditions, remains poorly understood. Here, we report a sulfuric-acid-induced surface oxidation strategy that enables both the controlled formation and reversible regeneration of β-NiOOH active sites on free-standing nickel foam electrodes. Moderate H₂SO₄ treatment (1 M) produces an optimized electrode (1.0-Ni) exhibiting superior OER performance, requiring overpotentials (η) of only 218, 348, and 458 mV at 10, 500, and 1000 mA cm^{- 2}, respectively. The enhanced activity originates from sulfuric-acid-induced modulation of Ni sites, which promotes β-NiOOH formation as the catalytically active phase while maintaining structural integrity. Moreover, a buried β-NiOOH-rich layer serves as a structural-memory reservoir, allowing reversible reconstruction of surface β-NiOOH during redox cycling. This reversible behavior enables the same electrode to function as both anode and cathode, relevant to rechargeable zinc-air battery and other bifunctional electrochemical systems. The dual mechanism acid-induced active-site formation and electrochemical regeneration thus provides a scalable strategy for constructing reversible, heteroatom-free nickel electrodes for sustainable energy conversion.