Universal domino reaction strategy for mass production of single-atom metal-nitrogen catalysts for boosting CO₂ electroreduction

Developing efficient electrocatalysts for CO₂ conversion into value-added chemicals is highly desirable, but it still remains a great challenge to achieve scalable production of these catalysts that displays simultaneously the high product selectivity and large current density at a low overpotential. Here, we develop a universal domino reaction strategy for mass production of metal single atoms anchored on N-doped carbon nanosheets (M-SA/NC), including Fe, Co, Ni, Mn, Mo, Pd and combinations thereof (among FeCo, FeNi, FeCoNi) SA/NC by ball-milling of polyaniline (PANI) with appropriate salts (e.g., NiCl₂, NaCl and NaNO₃), followed by pyrolysis. During pyrolysis, NaNO₃ is decomposed in-situ to release gases capable of blowing PANI, then carbon sheets from carbonized PANI is etched by O₂ to create microporous and the aggregated metal particles is etch by CO. The as-prepared Ni-SA/NC exhibits an extraordinary catalytic activity for CO₂ reduction to CO, yielding a large current density of 213.2 mA cm⁻² with CO Faradaic efficiency up to 96.9% at a low overpotential of 0.55 V in a flow cell. DFT calculations reveal that N atoms in NiN₄ species act as the active sites for CO₂RR, rather than conventional Ni atoms, since the neighboring pyrrolic-N induces electrons of Ni 4s orbitals shift to adjacent N 2s orbitals in NiN₄, leading to a high N 2s electron density for facilitating the COOH* formation. This work provides not only a rational design concept for mass preparation of M-SA/NC heterogeneous electrocatalyst for CO₂ reduction and beyond, but also a deep insight of an electron transfer mechanism for CO₂RR.