Electrochemical Potential Pulses Steer the CO2 Reduction Reaction Selectivity by Surface Reconstruction on Single-Atom Centers of Ti3C2Tx MXene

The electrocatalytic CO2 reduction reaction (CO2RR) is an attractive process to remediate greenhouse gas emissions while simultaneously producing value-added chemicals. However, it suffers from a selectivity problem, as several carbon products can be formed, and the competing hydrogen evolution reaction (HER) occurs under cathodic polarization. While experimental work has shown that CO2RR selectivity can be manipulated by pulsing the electrochemical potential, in the present work we describe the effects of alternating electrochemical anodic and cathodic potential pulses on the Ti3C2Tx MXene using density functional theory (DFT) calculations in combination with a descriptor-based analysis. Under anodic polarization, water-mediated surface reconstruction leads to the formation of single-atom centers with interfacial oxygen sites that control the CO2RR selectivity by suppressing HER. The combination of the anodic pulse with a subsequent cathodic pulse stimulates a second surface reconstruction, leading to a structure with the highest activity and selectivity toward HCOOH. However, excessively high cathodic potentials trigger the formation of a surface with low stability and high HER selectivity. Our results provide insights into the structure-performance relations of reconstructed Ti3C2Tx phases under cathodic polarization for the CO2RR and reveal how CO2RR selectivity can be manipulated by precise control of potential pulse protocols.