On the mechanism of high product selectivity for HCOOH using Pb in CO2 electroreduction

Abstract

While achieving high product selectivity is one of the major challenges of the CO₂ electroreduction technology in general, Pb is one of the few examples with high selectivity that produces formic acid almost exclusively (versus H₂, CO, or other byproducts). In this work, we study the mechanism of CO₂ electroreduction reactions using Pb to understand the origin of high formic acid selectivity. In particular, we first assess the proton-assisted mechanism proposed in the literature using density functional calculations and find that it cannot fully explain the previous selectivity experiments for the Pb electrode. We then suggest an alternative proton-coupled-electron-transfer mechanism consistent with existing observations, and further validate a new mechanism by experimentally measuring and comparing the onset potentials for CO₂ reduction vs. H₂ production. We find that the origin of a high selectivity of the Pb catalyst for HCOOH production over CO and H₂ lies in the strong O-affinitive and weak C-, H-affinitive characteristics of Pb, leading to the involvement of the *OCHO species as a key intermediate to produce HCOOH exclusively and preventing unwanted H₂ production at the same time.