Investigating the Nature of the Active Sites for the CO ₂ 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 mech-anism of CO₂ reduction. By combining the electrochemical results with density functional theo-ry, we were able to separate the electrocatalysts into several categories, based on their CO₂ → COOHads free energy and their CO_ads 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 photoe-lectron spectroscopy that confirmed pyridinic and hydrogenated (pyrrolic) nitrogen moieties act as preferential absorption sites for the CO₂ on the Fe – N – C catalyst surface.

Keywords: Nitrogen-doped carbonaceous materials; transition metal-nitrogen-carbon catalysts; atomically dispersed transition metal sites; CO2 reduction reaction; near ambient pressure X-ray photoelectron spectroscopy; density functional theory