Influence of composition, thickness, and 2D layer coupling on electrochemical CO2 reduction

The electrochemical reduction of CO2 to value-added carbon products is hindered by low Faradaic efficiency and competing hydrogen evolution. Here, we address this challenge by investigating how metal and C–N composition, layer thickness, and 2D heterostructure coupling influence CO2 activation and selectivity in emerging 2D materials. A suite of Ti2NTx, Ti4N3Tx, Ti3CNTx, V2NTx, V2CTx, MoS2/Ti2NTₓ, and MoS2/Ti4N3Tx were synthesized via a top-down etching method and evaluated for CO2RR alongside hydrogen evolution. Ti-based nitride MXenes showed minimal CO2RR activity and strongly favored HER, with layer thickness and MoS₂ coupling showing little effect on CO2RR product distribution. Incorporating both carbon and nitrogen in Ti3CNTx moderately improved performance, yielding FE of ~25–30% for CO and ~8–10% for HCOOH. A more substantial enhancement was achieved by altering the metal center: V2NTx reached ~50% CO and ~18% HCOOH selectivity while suppressing HER at moderate current densities and maintaining stability over extended operation. DFT calculations reveal that Ti-based MXenes favor HER due to facile water dissociation and strong Ti–O interactions, whereas V-based MXenes stabilize HCOO* intermediates via termination-mediated hydrogen bonding, lowering the CO2RR overpotential. These results elucidate composition–structure–activity relationships for rational design of selective 2D electrocatalysts.