Co₃O_4-δ Quantum Dots As a Highly Efficient Oxygen Evolution Reaction Catalyst for Water Splitting

Co₃O_4-δ quantum dots (Co₃O_4-δ-QDs) with a crystallite size of approximately 2 nm and oxygen vacancies were fabricated through multicycle lithiation/delithiation of mesoporous Co₃O₄ nanosheets. Used as an oxygen evolution reaction (OER) electrocatalyst for water splitting, the catalytic performance (an overpotential of 270 mV@10 mA cm^-2 and no decay within 30 h) of Co₃O_4-δ-QDs is superior to that of previously reported Co-based catalysts and the state-of-the-art IrO₂. Compared to that of the Co₃O₄ nanosheets, the enhanced OER activity of Co₃O_4-δ-QDs is attributed to two factors: one is the increased quantity of the Faradaic active sites, including the total active sites (q*_Total), the most accessible active sites (q*_Outer), and their ratio (q*_Outer/q*_Total); the other is the enhanced intrinsic electroactivity per active site evaluated by the turnover frequency and the current density normalized by the most accessible active sites (j/q*_Outer) related to the OER. This multicycle lithiation/delithiation method can be applied to other transition metal oxides as well, offering a general approach to develop high-performance electrocatalysts for water splitting.