Activation of Transition Metal (Fe, Co and Ni)-Oxide Nanoclusters by Nitrogen Defects in Carbon Nanotube for Selective CO₂ Reduction Reaction

The electrochemical carbon dioxide reduction reaction (CO2RR), which can produce value-added chemical feedstocks, is a proton-coupled-electron process with sluggish kinetics. Thus, highly efficient, cheap catalysts are urgently required. Transition metal oxides such as CoO_x, FeO_x, and NiO_x are low-cost, low toxicity, and abundant materials for a wide range of electrochemical reactions, but are almost inert for CO2RR. Here, we report for the first time that nitrogen doped carbon nanotubes (N-CNT) have a surprising activation effect on the activity and selectivity of transition metal-oxide (MO_x where M = Fe, Ni, and Co) nanoclusters for CO2RR. MO_x supported on N-CNT, MO_x/N-CNT, achieves a CO yield of 2.6–2.8 mmol cm⁻² min⁻¹ at an overpotential of −0.55 V, which is two orders of magnitude higher than MO_x supported on acid treated CNTs (MO_x/O-CNT) and four times higher than pristine N-CNT. The faraday efficiency for electrochemical CO₂-to-CO conversion is as high as 90.3% at overpotential of 0.44 V. Both in-situ XAS measurements and DFT calculations disclose that MO_x nanoclusters can be hydrated in CO₂ saturated KHCO₃, and the N defects of N-CNT effectively stabilize these metal hydroxyl species under carbon dioxide reduction reaction conditions, which can split the water molecules and provide local protons to inhibit the poisoning of active sites under carbon dioxide reduction reaction conditions.