Fe acting as functionalized auxiliary agents in nitrogen-doped carbon supported Ni-based catalysts for electrocatalytic CO2 Reduction: Effect of valence state

Converting CO2 into useful chemicals is an effective measure to address the energy crisis and the greenhouse effect. CO2RR has become a research hotspot due to its mild conditions. Ni-N-C catalysts are known as promising electrocatalysts, while achieving atomic level dispersion of Ni on the surface of the carrier is still facing challenges. In this work, various N-doped carbon supported Ni-based catalysts were synthesized by selecting Fe2+/ Fe3+ as functionalized auxiliary agents in precursors. Different coordination situations of Fe(III)/Ni@NC and Fe (II)/Ni@NC are formed which derived from the valence state of Fe. During the synthesis process of Fe(III)/ Ni@NC, Ni2+ replaced Fe3+ through competitive complexation, thereby achieving atomic level dispersion of Ni atoms on the catalyst surface. XAS confirms that Fe(III)/Ni@NC is a Ni-single-atom catalyst with Ni-N4 coordination. In contrast, Fe(II)/Ni@NC contains Ni3C/Ni nanoparticles coated with carbon layer, which shows a characteristic peak approaching to 2.12 angstrom related to the metallic Ni-Ni bonds. In the H-type cell, the FECO of Fe (III)/Ni@NC achieves 97.8 % at -0.83 V and maintains over 95 % in a wide potential window. The in-situ Fourier transform infrared (FTIR) spectroscopy confirms that the excellent product CO selectivity in CO2RR is due to the accumulation of *COOH intermediates at the active sites. Significantly, Fe(III)/Ni@NC shows excellent performance in the flow cell with FECO over 96 % from -40 to -140 mA cm-2. This work reveals the impact of Fe valence state of functionalized auxiliary agents on the electrocatalytic performance of materials, which provides a new approach for designing high-performance atomically dispersed electrocatalysts.