The chemical identity, state and structure of catalytically active centers during the electrochemical CO₂ reduction on porous Fe–nitrogen–carbon (Fe–N–C) materials

Abstract

We report novel structure-activity relationships and explore the chemical state and structure of catalytically active sites under <i>operando</i> conditions during the electrochemical CO₂ reduction reaction (CO₂RR) catalyzed by a series of porous iron-nitrogen-carbon (FeNC) catalysts. The FeNC catalysts were synthesized from different nitrogen precursors and, as a result of this, exhibited quite distinct physical properties, such as BET surface areas and distinct chemical N-functionalities in varying ratios. The chemical diversity of the FeNC catalysts was harnessed to set up correlations between the catalytic CO₂RR activity and their chemical nitrogen-functionalities, which provided a deeper understanding between catalyst chemistry and function. XPS measurements revealed a dominant role of porphyrin-like Fe-N _<i>x</i> motifs and pyridinic nitrogen species in catalyzing the overall reaction process. <i>Operando</i> EXAFS measurements revealed an unexpected change in the Fe oxidation state and associated coordination from Fe²⁺ to Fe¹⁺. This redox change coincides with the onset of catalytic CH₄ production around -0.9 V_RHE. The ability of the solid state coordinative Fe¹⁺-N _<i>x</i> moiety to form hydrocarbons from CO₂ is remarkable, as it represents the solid-state analogue to molecular Fe¹⁺ coordination compounds with the same catalytic capability under homogeneous catalytic environments. This finding highlights a conceptual bridge between heterogeneous and homogenous catalysis and contributes significantly to our fundamental understanding of the FeNC catalyst function in the CO₂RR.