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  Paul, S.; Kao, Y.-L.; Ni, L.; Ehnert, R.; Herrmann-Geppert, I.; van de Krol, R.; Stark, R.W.; Jaegermann, W.; Kramm, U.I.; Bogdanoff, P.: Influence of the Metal Center in M-N-C Catalysts on the CO₂ Reduction Reaction on Gas Diffusion Electrodes. ACS Catalysis 11 (2021), p. 5850–5864

10.1021/acscatal.0c05596
Open Access Version

Abstract:
In this work, the influences of various transition metal ions as active sites in high purity metal- and nitrogen-doped carbon catalysts (in short M–N–C), where M: Mn3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, or Sn4+ in the catalyst powders, were systematically investigated for the electrochemical reduction of CO2 in the aqueous electrolyte. The almost exclusive presence of isolated M–N4 centers as catalytic sites was determined by X-ray photoelectron spectroscopy (XPS). The catalysts were electrochemically investigated in a gas diffusion electrode arrangement in bypass mode coupled in-line to a mass spectrometer. This allowed for the nearly simultaneous detection of products and current densities in linear sweep voltammetry experiments, from which potential-dependent specific production rates and faradaic efficiencies could be derived. Postmortem XPS analyses were performed after various stages of operation on the Cu–N–C catalyst, which was the only catalyst to produce hydrocarbons (CH4 and C2H4) in significant amounts. The data provided insights into the potential-induced electronic changes of the Cu–N–C catalyst occurring under operating conditions. Our work further experimentally revealed the high affinity of M–N–C catalysts to convert CO2 to industrially relevant carbonaceous raw materials, while effectively suppressing the competing hydrogen evolution reaction. These results led to a better understanding of the role of the active sites, especially the central metal ion, in M–N–C and could contribute significantly to the improvement of selectivities and activities for the CO2RR in this catalyst class through tailor-made optimization strategies.