The pressing need for carbon emission reduction calls for a rapid move toward electrified mobility and expanded deployment of solar and wind on the electric grid. As a result, high-performance electrical energy storage systems are highly demanded. Rechargeable lithium-ion batteries (LIBs) are the most widely used battery system in portable electronics and electric vehicles nowadays because of their high energy per unit mass, power-to-weight ratio, and high-temperature performance. However, concerns about their high cost (250 US$ kW h⁻¹), inherent safety hazards, and limited cathode capacity have motivated the investigation beyond Li-ion technology. In particular, Zn-air batteries (ZABs) have emerged as a much more sustainable option than LIBs with high energy densities of 1218 Wh kg^-1 (gravimetric) and 6136 Wh L^-1 (volumetric) benefiting from the earth-abundant, low-cost, and environmentally friendly nature of Zn metal and the ample oxygen supply in ambient air.

One of the major challenges in ZABs research is the inefficient oxygen reaction kinetics at the air cathode. Among various proposed bifunctional oxygen catalysts, metal-free carbon-based catalysts have drawn tremendous attention due to their potential in reducing the costs and environmental impacts of noble and transition metal-based catalysts. In particular, nitrogen-doped carbonaceous materials have been recognized as one of the most promising catalysts because of their increased electrical conductivity via stimulating the delocalization of electrons and induced electron depletion on carbon atoms which optimize valence orbital energy for active sites. Nevertheless, conventional post-synthesis doping methods not only involve complicated experimental setups but also offer limited nitrogen doping (1 - 20%) levels along with poor control over C-N configurations. Accordingly, a facile synthesis method enabling high nitrogen doping content with the C-N configuration controllability is highly demanded for high-performance ZABs.

In this report, we successfully synthesized high nitrogen-content hollow carbon spheres (H-C_xN_y) via a novel metal-assisted denitrification (MAD) process. Specifically, we employed Zn metal and low-cost graphitic carbon nitride (g-C₃N₄) as catalyst and precursor, respectively, to construct the H-C_xN_y microstructures. During the annealing process, the Zn metal reacts with nitrogen during pyrolysis of the g-C₃N₄ and converts it into heat-stable Zn₃N₂ intermediates, which not only avoids the direct volatilization of nitrogen content but also displaces carbon atoms and subsequently rearranges carbon and nitrogen atoms into H-C_xN_y spherical structures. The content and the configuration of the nitrogen species on the hollow spherical skeleton were successfully modulated by controlling the synthesis conditions. To investigate the structure-property-activity relationship, scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Raman spectroscopy, and various electrochemical characterizations were conducted. The optimized H-C_xN_y sample demonstrated excellent bifunctional catalysis performance with a stable cyclic performance when employed as an air electrode in ZABs.