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Kevinjeorjios Pellumbi, Mathias Smialkowski, Daniel Siegmund, Ulf-Peter Apfel

• The electrochemical reduction of CO\(_2\) is an attractive strategy towards the mitigation of environmental pollution and production of bulk chemicals as well as fuels by renewables. The bimetallic sulfide Fe\(_{4.5}\)Ni\(_{4.5}\)S\(_8\) (pentlandite) was recently reported as a cheap and robust catalyst for electrochemical water splitting, as well as for CO\(_2\) reduction with a solvent-dependent product selectivity. Inspired by numerous reports on monometallic sulfoselenides and selenides revealing higher catalytic activity for the CO\(_2\) reduction reaction (CO\(_2\)RR) than their sulfide counterparts, the authors investigated the influence of stepwise S/Se exchange in seleno-pentlandites Fe\(_{4.5}\)Ni\(_{4.5}\)S\(_{8-Y}\)Se\(_Y\) (Y=1–5) and their ability to act as CO\(_2\) reducing catalysts. It is demonstrated that the incorporation of higher equivalents of selenium favors the CO\(_2\)RR with Fe\(_{4.5}\)Ni\(_{4.5}\)S\(_4\)Se\(_4\) revealing the highest activity for CO formation. Under galvanostatic conditions in acetonitrile, Fe\(_{4.5}\)Ni\(_{4.5}\)S\(_4\)Se\(_4\) generates CO with a Faradaic Efficiency close to 100 % at applied current densities of −50 mA cm\(^{−2}\) and −100 mA cm\(^{−2}\). This work offers insight into the tunability of the pentlandite based electrocatalysts for the CO\(_2\) reduction reaction.