Lateral-Size-Mediated Efficient Oxygen Evolution Reaction: Insights into the Atomically Thin Quantum Dot Structure of NiFe2O4

The study of high-performance electrocatalysts for driving the oxygen evolution reaction (OER) is important for energy storage and conversion systems. As a representative of inverse-spinel-structured oxide catalysts, nickel ferrite (NiFe 2 O 4 ) has recently gained interest because of its earth abundance and environmental friendliness. However, the gained electrocatalytic performance of NiFe 2 O 4 for the OER is still far from the state-of-the-art requirements because of its poor reactivity and finite number of surface active sites. Here, we prepared a series of atomically thin NiFe 2 O 4 catalysts with different lateral sizes through a mild and controllable method. We found that the atomically thin NiFe 2 O 4 quantum dots (AT NiFe 2 O 4 QDs) show the highest OER performance with a current density of 10 mA cm –2 at a low overpotential of 262 mV and a small Tafel slope of 37 mV decade –1 . The outstanding OER performance of AT NiFe 2 O 4 QDs is even comparable to that of commercial RuO 2 catalyst, which can be attributed to its high reactivity and the high fraction of active edge sites resulting from the synergetic effect between the atomically thin thickness and the small lateral size of the atomically thin quantum dot (AT QD) structural motif. The experimental results reveal a negative correlation between lateral size and OER performance in alkaline media. Specifically speaking, the number of low-coordinated oxygen atoms increases with decreasing lateral size, and this leads to significantly more oxygen vacancies that can lower the adsorption energy of H 2 O, increasing the catalytic OER efficiency of AT NiFe 2 O 4 QDs.