Abstract
We report novel structure-activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO(2) reduction reaction (CO(2)RR) 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 CO(2)RR 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 (x) motifs and pyridinic nitrogen species in catalyzing the overall reaction process. Operando EXAFS measurements revealed an unexpected change in the Fe oxidation state and associated coordination from Fe(2+) to Fe(1+). This redox change coincides with the onset of catalytic CH(4) production around -0.9 V(RHE). The ability of the solid state coordinative Fe(1+)-N (x) moiety to form hydrocarbons from CO(2) is remarkable, as it represents the solid-state analogue to molecular Fe(1+) 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 CO(2)RR.