Engineering metal-nitrogen-carbon catalysts for electrochemical oxygen reduction reaction

Abstract

The critical issues of fossil fuel consumption and greenhouse gas emission urge people to develop green and sustainable techniques to achieve carbon-neutral targets. Electrochemical devices, such as fuel cells, metal-air batteries and electrolysers, are promising energy conversion and storage devices. Oxygen reduction reaction (ORR) is the key cathode reaction in these electrochemical energy applications, which can activate O2 molecules to electrical power by a four-electron pathway, or hydrogen peroxide (H2O2), a value-added product by a two-electron pathway. ORR electrocatalysts play the most important role, as the catalysts can promote reaction kinetics, efficiency and selectivity for the products. Existing commercial catalysts are based on platinum-group metals, which suffer from high costs and earth scarcity. Metal-nitrogen-carbon (M-N-C) catalysts have been one of the most promising candidates among non-platinum-group metal catalysts, featuring earth-abundant metal precursors, good utilisation of single-atom centres, versatile carbon structures and high electrical conductivity. However, there are still crucial challenges for M-N-C catalysts for their further applications: (1) insufficient intrinsic activity and/or selectivity associated with the electronic properties of metal active centres; (2) sluggish reaction kinetics due to insufficient amount of M-N active sites, mediocre microenvironment at the O2-electrolyte-active site interface, and poor active site accessibility and mass transportation; (3) unsatisfactory catalyst durability due to the deficient carbon structure. Targeting on these challenges, this thesis develops multiscale engineering strategies for M-N-C catalysts, improving their intrinsic properties as well as extrinsic hierarchical structures, achieving efficient ORR for electricity generation and H2O2 production. The proposed engineering strategies can offer the universality of scalable devices and more electrochemical processes.