It is of both scientific and societal importance to advance the sustainable energy technology that enables the conversion of CO2 into fuels and value-added chemicals via cost-effective electrocatalysis. Development of alternative catalysts based on earth-abundant elements (i.e., free of precious group metals (PGM-free)) is therefore highly demanded. In recent years, transition metal (TM=Fe, Co, or Ni) and/or nitrogen (N) doped carbon (denoted as TM-N-C) catalysts have been demonstrated to be capable of catalyzing CO2 reduction to CO with a reasonable activity and amazingly high selectivity. However, knowledge about how the activity and selectivity of TM-N-C electrocatalysts for CO2RR are dependent on their chemical composition, structure, morphology, and synthetic chemistry is seriously lacking. For a rational design of these TM-N-C catalysts, we have performed density functional theory (DFT) calculations to investigate CO2RR on various TM-N4 active sites (TM = Fe, Co, Ni) with different moieties. Specifically, three different TM-N4 moieties are studied, such as the TM-N4 moiety embedded in the graphene basal plane, the porphyrin-like moiety and/or the TM-N4 moiety bridging the edges of two adjacent graphene layers. We calculated the adsorption energies of all the possible chemical species and the activation energies for C-O bond dissociation reactions involved in CO2RR on the active sites. Our DFT calculations predicted that the CO2RR could happen at the proposed active sites under an electrode potential more positive than that for competing hydrogen evolution reaction (HER). Consequently, these TM-N-C catalysts exhibit good selectivity by diminishing parasitic HER. Moreover, the TM-N4 moiety bridging the edges of two adjacent graphene layers was predicted to be capable of breaking the C-O bond efficiently. These theoretical results are in agreement with experimental observations. Furthermore, we investigated how the doping of S would affect the CO2RR on Fe-N-C catalysts. Our DFT calculation reveals that the adjacent S atoms engineering increases the charge density of centered Fe in a Fe-N4 site, and thus predicts that endowing S-modified Fe−N4 with lower formation energy barriers for the key COOH* intermediate in CO2RR. These results suggest that adding S into the TM-N-C catalysts is an effective approach to enhance their catalytic activity for CO2RR.

Computational study of metal/nitrogen doped carbon electrocatalysts for CO2 reduction reaction

2019 Fall National Meeting

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