Pyridinic-N groups and ultramicropore nanoreactors enhance CO₂ electrochemical reduction on porous carbon catalysts

Wood-based activated carbons, as received and modified by introduction of nitrogen and/or oxidation, were studied as CO₂electrochemical reduction reaction (CO₂ERR) catalysts. The carbons have similar pore structures but they differ significantly in surface chemistry. An electrochemical reduction process applied to the surface before CO₂reduction increased their catalytic performance. On the carbons tested Faradaic efficiency for CO formation reached 40% and methane formation − 1.2% at −0.66 V vs. RHE. The high efficiency for the CO formation was linked mainly to positively charged carbon close to pyridinic nitrogen, which stabilizes CO₂ ^[rad]−intermediate in the pore system. On the other hand, the results indicate that quaternary nitrogen is less influential and it is less affected by the reduction process. N-oxides outside the ring (C[sbnd]N⁺[sbnd]O⁻) were also found as active sites for CO₂ERR. Hydrogen evolution reaction and CO₂ERR compete for these active sites. Owing to the specific texture of nanoporous carbon, Faradaic reactions might not be a unique mechanism of CH₄formation. It is also possible that CO, upon strong adsorption in ultramicropores of sizes less than 0.7 nm combines there with adsorbed H₂from water reduction resulting in the formation of methane. Thus, the ultramicropores can be considered as pseudo Fisher-Tropsch nanoreactors. The results also indicate that the acidic surface of the catalysts increases the overpotential of the maximum Faradaic efficiency of either CO or CH₄formation.