Two-dimensional IrN2 monolayer: An efficient bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

The development of bifunctional electrocatalysts with good stability and high efficiency for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for renewable energy conversion and storage. Herein, by means of swarm-intelligence structure search and density functional theory (DFT) computations, we proposed a novel kind of two-dimensional (2D) monolayer with hypercoordinate structure as electrocatalysts for ORR/OER, namely, transition dinitride (TMN2, TM = V, Co, Rh, Pd, W, Re, and Ir) monolayer. Our result revealed that these TMN2 monolayers have excellent thermal, dynamic and chemical stability, as well as inherent metallic nature for their practical applications in electrocatalysis. More interestingly, among all 2D TMN2 materials, the IrN2 monolayer was suggested to perform as an ideal bifunctional electrocatalyst for ORR/OER with a low overpotential of 0.47 and 0.27 V, respectively, which is comparable to Pt and Ir- or Ru-based oxides. Furthermore, by examining the d-band centers of the active sites in different TMN2 monolayers, we well rationalized the superior catalytic activity of IrN2 monolayer for ORR/OER. Our findings not only further enrich 2D nanomaterials with hypercoordinate structure, but also open a new door to develop bifunctional oxygen electrocatalysts with high efficiency.