Low-Overpotential Rechargeable Na-CO 2 Batteries Enabled by an Oxygen-Vacancy-Rich Cobalt Oxide Catalyst.

Rechargeable sodium-carbon dioxide (Na-CO 2 ) batteries have been proposed as a promising CO 2 utilization technique, which could realize CO 2 reduction and generate electricity at the same time. They suffer, however, from several daunting problems, including sluggish CO 2 reduction and evolution kinetics, large polarization, and poor cycling stability. In this study, a rambutan-like Co 3 O 4 hollow sphere catalyst with abundant oxygen vacancies was synthesized and employed as an air cathode for Na-CO 2 batteries. Density functional theory calculations reveal that the abundant oxygen vacancies on Co 3 O 4 possess superior CO 2 binding capability, accelerating CO 2 electroreduction, and thereby improving the discharge capacity. In addition, the oxygen vacancies also contribute to decrease the CO 2 decomposition free energy barrier, which is beneficial for reducing the overpotential further and improving round-trip efficiency. Benefiting from the excellent catalytic ability of rambutan-like Co 3 O 4 hollow spheres with abundant oxygen vacancies, the fabricated Na-CO 2 batteries exhibit extraordinary electrochemical performance with a large discharge capacity of 8371.3 mA h g -1 , a small overpotential of 1.53 V at a current density of 50 mA g -1 , and good cycling stability over 85 cycles. These results provide new insights into the rational design of air cathode catalysts to accelerate practical applications of rechargeable Na-CO 2 batteries and potentially Na-air batteries.