Investigating the Nature of the Active Sites for the CO₂ Reduction Reaction on Carbon-Based Electrocatalysts

Abstract

To achieve a better understanding of the CO2 reduction reaction on carbon-based electrocatalysts, we synthesized a library of nitrogen-doped carbonaceous materials with atomically dispersed 3d transition metals and corresponding metal-free electrocatalysts. The sacrificial support method was used yielding catalyst materials of high dispersity and high graphitic content. The resulting electrocatalysts were impurity free, hence allowing a better understanding of the mechanism of CO2 reduction. By combining the electrochemical results with density functional theory, we were able to separate the electrocatalysts into several categories, based on their CO2 → COOHads free energy and their COads binding strength. The “strong-CO binder” electrocatalysts (e.g., Cr, Mn and Fe–N–C) achieved a Faradaic efficiency up to 50% at −0.35 V vs. RHE (at pH = 7.5, in 0.1 M phosphate buffer). Such Faradaic efficiency was also achieved for a metal-free electrocatalyst, therefore showing the high activity of the metal-free, N-containing, moieties toward the CO2 reduction reaction. This was confirmed by near ambient pressure X-ray photoelectron spectroscopy that confirmed pyridinic and hydrogenated (pyrrolic) nitrogen moieties act as preferential adsorption sites for the CO2 on the Fe–N–C catalyst surface.