M-N-C single-atom catalysts for electrocatalytic CO2 reduction: Design concept, advanced characterizations, pathways and challenges

Abstract

The growing demand for sustainable energy solutions has driven significant research into electrochemical CO₂ reduction (ECO₂R) for carbon recycling and energy storage. Single-atom catalysts (SACs), owing to their distinctive atomic-level distribution of metal sites, have exceptional catalytic features, including high activity and selectivity for ECO₂R. This review offers an in-depth overview of ECO₂R, utilizing SACs supported on carbon-based substrates. This review covers various approaches for fabricating carbon-supported SACs, including the regulation of metal loading, coordination environment, and support defects to enhance performance. Further, this review covers advanced characterization techniques, including aberration corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM), STEM, X-ray absorption near edge structure (XANES) and near edge X-ray absorption fine structure (EXAFS), synchrotron radiation photoelectron spectroscopy (SRPES), In-situ Raman, CO diffuse reflective infrared Fourier transform spectroscopy (CO-DRIFT), and Mössbauer spectroscopy. These techniques are explored for investigating the atomic structure, electronic features, and active sites of SACs. The review addresses key reaction pathways of ECO₂R, emphasizing the significance of intermediate adsorption energies and the influence of the second and third coordination shell on improving the catalytic performance. The challenges related to the stability and scalability of SACs in practical applications are also discussed. The review ultimately presents future perspectives for the improvement of highly efficient M-N-C SACs for ECO₂R, addressing advances in their long-term performance and commercial feasibility.