Probing charge trapping sites in M-N-C electrocatalysts via time-resolved transient absorption spectroscopy

Among non-platinum group metal (non-PGM) electrocatalysts for the oxygen reduction reaction (ORR), the class of metal-decorated nitrogen-doped carbon (M–N–C) materials is most promising. The active site is generally accepted to be a metal coordinated with nitrogen within the carbon lattice and/or nanoparticles. But confusion remains around the catalytic mechanisms. Better and more fundamental insights into these materials allow the design of more efficient M–N–C catalysts. In this study, we report the first direct observation of charge-trapping sites induced by metal decoration in M–N–C electrocatalysts using ultrafast time-resolved transient absorption spectroscopy. Ultrafast time-resolved spectroscopy has long been used for molecular characterization and to study photocatalysts, but its application to probe the intrinsic properties of electrocatalysts can be extended to all materials. Such sites have been associated with modulated charge distribution and enhancement of electron transfer efficiency during catalytic processes. Our study not only provides new insights into the electronic behavior of M–N–C materials but also establishes ultrafast spectroscopy as a powerful tool to study charge dynamics in electrocatalysts broadly, paving the way for more informed and efficient catalyst design.