Towards rational design in electrochemical denitrification by analyzing pH dependence

A small fraction of NO (<1%) always exists in CO₂ feedstock (e.g. exhausted gas), which can significantly reduce the efficiency of CO₂ electroreduction by ∼30%. Hence, electrochemical denitrification is the precondition of CO₂ electroreduction. The pH effect is a key factor, and can be used to tune the selectivity between N₂ and N₂O production in electrochemical denitrification. However, there has been much controversy for many years about the origin of pH dependence in electrocatalysis. To this end, we present a new scheme to accurately model the pH dependence of the electrochemical mechanism. An extremely small pH variation from pH 12.7 to pH 14 can be accurately reproduced for N₂O production. More importantly, the obviously different pH dependence of N₂ production, compared to N₂O, can be attributed to a cascade path. In other words, the N₂ was produced from the secondary conversion of the as-produced N₂O molecule (the major product), instead of the original reactant NO. This is further supported by more than 35 experiments over varying catalysts (Fe, Ni, Pd, Cu, Co, Pt and Ag), partial pressures (20%, 50% and 100%) and potentials (from -0.2 to 0.2 V vs. reversible hydrogen electrode). All in all, the insights herein overturn long-lasting views in the field of NO electroreduction and suggest that rational design should steer away from catalyst engineering toward reactor optimization.