Unraveling the Synergistic Effect of Heteroatomic Substitution and Vacancy Engineering in CoFe 2 O 4 for Superior Electrocatalysis Performance.

Metal ion substitution and anion exchange are two effective strategies for regulating the electronic and geometric structure of spinel. However, the optimal location of foreign metallic cations and the exact role of these metals and anions remain elusive. Herein, CoFe 2 O 4 -based hollow nanospheres with outstanding oxygen evolution reaction activity are prepared by Cr 3+ substitution and S 2- exchange. X-ray absorption spectra and theoretical calculations reveal that Cr 3+ can be precisely doped into octahedral (Oh) Fe sites and simultaneously induce Co vacancy, which can activate adjacent tetrahedral (Td) Fe 3+ . Furthermore, S 2- exchange results in structure distortion of Td-Fe due to compressive strain effect. The change in the local geometry of Td-Fe causes the *OOH intermediate to deviate from the y -axis plane, thus enhancing the adsorption of the *OOH. The Co vacancy and S 2- exchange can adjust the geometric and electronic structure of Td-Fe, thus activating the inert Td-Fe and improving the electrochemical performance.