Insights Onto the Active Sites and Reactivity of Metal-Doped Carbonaceous Electrocatalysts for the CO2 Reduction Reaction

The CO 2 valorization, i.e. its use in various electrochemical processes resulting in the production of carbon-based fuels and value-added chemicals, is of critical importance. Hence, the CO 2 reduction reaction (CO 2 RR) has been widely investigated on electrocatalysts ranging from Cu-based nanostructures to carbonaceous materials. The latter often consist of carbon with atomically dispersed nitrogen and metals atoms (M-N-C, see Figure 1c ). Such materials have been discussed in the literature, mostly using Fe as the metallic element 1 (but also Mn, Ni, Co and Cu 2–4 ), thus providing insights onto this family of electrocatalysts reactivity. But most of those electrocatalysts, as a result of their synthesis process, presents unwanted metallic contaminants 2 . Here, we synthesized M-N-C electrocatalysts (with M = Cr, Mn, Fe, Co, Ni, Cu & Zn) using the sacrificial support method, that resulted into non-contaminated M-N-C materials. We then combined electrochemistry and density functional theory to separate the electrocatalysts in several categories, based on their CO ads binding strength. The strong CO ads 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), as well as a metal-free electrocatalyst synthesized by the same method. This phenomenon was further investigated using near-ambient pressure XPS, which evidenced that the preferential adsorption site for CO 2 was dependent of the metallic element nature: for Fe-N-C, the CO 2 preferentially adsorb on pyridinic and hydrogenated (pyrrolic) nitrogen moieties (see Figure 1 ), hence evidencing the key role played by said moieties in the reactivity of the M-N-C electrocatalysts for the CO 2 RR. (1) Varela, A. S.; Ju, W.; Strasser, P. Adv. Energy Mater. 2018 , 8 (30), 1–35; (2) Ju, W.; Bagger, A.; Hao, G. P.; Varela, A. S.; Sinev, I.; Bon, V.; Roldan Cuenya, B.; Kaskel, S.; Rossmeisl, J.; Strasser, P. Nat. Commun. 2017 , 8 (1), 1–9; (3) Varela, A. S.; Ranjbar Sahraie, N.; Steinberg, J.; Ju, W.; Oh, H. S.; Strasser, P. Angew. Chemie - Int. Ed. 2015 , 54 (37), 10758–10762; (4) Hu, X. M.; Hval, H. H.; Bjerglund, E. T.; Dalgaard, K. J.; Madsen, M. R.; Pohl, M. M.; Welter, E.; Lamagni, P.; Buhl, K. B.; Bremholm, M.; Beller, M.; Pedersen, S. U.; Skrydstrup, T.; Daasbjerg, K. ACS Catal. 2018 , 8 (7), 6255–6264. Figure 1