Thin carbon layer covered Co₄N cubes encapsulated in N-doped porous carbon nanocage as tri-functional catalysts with enhanced charge-transfer efficiency for Zn-air battery and overall water-splitting

High-efficiency bifunctional catalysts with high energy conversion efficiency and high cycling performance are urgently needed for traditional zinc-air batteries (ZABs). Herein, a composite of highly active carbon layer covered Co4N cubes encapsulated in a nitrogen-doped porous carbon nanocage (Co4NC@NC) as a cathode catalyst was prepared by a hydrothermal acid-assisted strategy combined with subsequent calcination. The precursor is treated with carbon coating and calcination to obtain a double-carbon-layer-coated structure of Co4NC@NC. The inner carbon layer is tightly wrapped on the surface of the Co4N particles, and the outer carbon layer presents a porous structure. Co4NC@NC exhibits significantly enhanced electrocatalytic activities for ORR, OER and HER. In particular, the overpotential is only 166 mV for OER at a current density of 10 mA cm(-2). Density functional theory calculations reveal that the synergistic effect between the N-doped graphite carbon layer and Co4N can improve the adsorption efficiency of intermediate products, accelerate the reaction kinetics, and reduce the free energy of rate determining steps. ZABs assembled with Co4NC@NC showed excellent stability during 1050 cycles at 5 mA cm(-2). Furthermore, Co4NC@CN can also be used as an HER catalyst for overall water splitting, driven by Co4NC@CN-ZABs. The above results indicate that Co4NC@CN can be used as a high-performance three-function catalyst for ZABs and overall water splitting.