N, O-diatomic dopants activate catalytic activity of 3D self-standing graphene carbon aerogel for long-cycle and high-efficiency Li-CO2 batteries

Abstract

Carbon-based metal-free materials are regarded as viable cathode catalysts for Li-CO2 batteries due to their low costs and lightweight. And heteroatom doping (such as N atom) has great potential to improve the catalytic activity of carbon-based catalysts. However, the underlying catalytic mechanism is yet unclear, which hinders the construction of high-efficiency catalysts and further improvements in electrochemical performance. Especially, the role of oxygen-containing groups prevalent in carbon-based catalysts has never been explored. In this work, guided by theoretical simulation, a self-standing N-doped graphene carbon aerogel with certain oxygenic groups was well-designed and synthesized by a straightforward, one-step thermal approach as the cathode catalyst. N dopant can effectively regulate the electronic structure of graphene and thus lower the free energy change of reactants/intermediate species. The intrinsic oxygen-containing functional groups still presented in graphene aerogel can further stabilize CO2-related intermediate species and improve the catalytic activity through a synergistic coupling effect with N dopant. This effect was originally discovered and clarified in the Li-CO2 battery system. Additionally, intriguing 3D hierarchical pores of as-obtained graphene carbon aerogel not only guarantee good conductivity but also offer a vast surface area to expose numerous accessible active sites. The resulting Li-CO2 batteries showed a significantly enhanced initial energy efficiency of approximately 78.46% and remarkable cyclic stability of more than 1500 h at 20 μA cm−2. This fundamental understanding of the structure-performance relationship gives new ideas for creating extremely effective carbon-based metal-free catalysts for Li-CO2 batteries.