Gallic acid-assisted synthesis of nitrogen-doped carbon microspheres as efficient bifunctional materials for oxygen reduction and volumetric lithium storage

Although various carbons are widely used in energy conversion and storage, the efficient optimization of the components and structures to further improve their performances is still a great challenge. Herein, N-doped carbon microspheres (NCM) were prepared using low-cost chitosan as a nitrogen and carbon source by a simple one-step solvothermal method with the assistance of gallic acid as a "molecular nucleating agent". The NCM features uniform N-doping, abundant defects, and high packing density with a large surface area, thereby demonstrating efficient performances as a bifunctional material for the oxygen reduction reaction (ORR) and volumetric lithium storage. As a metal-free ORR electrocatalyst, NCM delivers an excellent ORR activity (0.996 V (vs. reversible hydrogen electrode) for onset potential and 0.818 V for half-wave potential), a 4e pathway (3.73-3.91), and long-term stability (93.5% retention over 100 h) in an alkaline medium. As a lithium storage electrode, NCM achieves an outstanding volumetric capacity (1373 mA h cm^-3@0.1 A g^-1), rate capability (247 mA h cm^-3@5.0 A g^-1) and long-life cycling stability. The impressive performances could be attributed to the unique microsphere structure involving uniform N-doping and defects inducing more available ORR-active sites, a suitable surface area and pore volume providing high packing electrode density and more Li⁺-accessible areas, the multiple porous channel offering an unimpeded ion pathway, and the graphitized carbon layers facilitating electronic transport. This study is a promising case to prepare high-efficiency energy-related materials without using toxic chemical reagents or harsh conditions.