Regulating the coordination environment of Fe/Co–N/S–C to enhance ORR and OER bifunctional performance

Abstract

Single-atom dual-metal catalysts with enhanced binding ability of oxygen and facilitated cleavage of OO bonds have demonstrated superior electrocatalytic activity. However, the rational design and preparation of single-atom dual-metal electrocatalysts with various coordination environments to make them have bifunctional electrochemical catalytic activities for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are still challenging. In this work, dual-metal Fe/Co–N/S_x–C catalysts with different doping amounts of sulfur are designed and synthesized. The influence of N/S local coordination on the electrochemical catalytic oxygen reaction is studied. With the doping of S atoms, the Fe/Co–N/S_x–C active centre structure is changed into a three-dimensional structure, which leads to a localized strain and electron rearrangement on the surface of the catalysts. By increasing the coordination number of S, the locality of 3d orbital electrons of Fe/Co gradually decreases and the spin polarization of Fe/Co is enhanced. Furthermore, S atoms modulate the moderate orbital hybridization between the Fe–Co–N/S₂–C active centre and *OH, lowering the energy barrier of the *OH + H⁺ + e⁻ → H₂O reaction step and speeding up the ORR. As a result, the theoretical thermodynamic limit potential difference of Fe–Co–N/S₂–C between both the OER and ORR and the electrochemical experimental results reveal that Fe–Co–N/S₂–C can achieve good ORR and OER bifunctional electrocatalytic performance, which implies that it is favorable for application in zinc–air batteries. This study provides a new approach for the design and synthesis of single-atom dual-metal site electrocatalysts with a controllable coordination environment.