Journal
ACS CATALYSIS
Volume 11, Issue 9, Pages 5850-5864Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.0c05596
Keywords
electrocatalysis; CO2 reduction reaction; M-N-C; M-N-4 active centers; electrochemistry coupled to mass spectrometry; CO formation; hydrocarbon formation
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Funding
- Darmstadt Graduate School of Excellence Energy Science and Engineering for the GSC [1070]
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This study investigated the influences of transition metal ions in high purity metal- and nitrogen-doped carbon catalysts for the electrochemical reduction of CO2. It was found that the Cu-N-C catalyst showed high selectivity and productivity in converting CO2 to hydrocarbons while suppressing hydrogen evolution reaction. Postmortem XPS analyses provided insights into electronic changes of the catalyst during operation, contributing to a better understanding of the role of active sites in M-N-C for CO2 reduction.
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-N-4 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.
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