Product Selectivity Switching between CO and Formate in Electrocatalytic CO2 Reduction Driven by Ionic Liquid-Induced Reorientation of CO2 over Cathode

Abstract

Rationally switching reaction pathways to selectively produce diverse value-added chemicals is crucial for electrochemical CO2 reduction reactions (CO2RRs). However, such a goal is usually achieved by catalyst design, and it remains a challenge for electrolyte engineering. Herein, two structurally similar ionic liquids (ILs) differing only in the C2 substituent (H or –CH3) of the imidazolium cation, namely, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([BMMIM][PF6]), were employed as electrolytes to modulate the product selectivity of CO2RR over a bimetallic Fe5Sn1NC electrocatalyst. It was found that CO2 reduction product was significantly switched between CO (Faradaic efficiency (FE) = 98.9%) and HCOOH (FE = 99.7%) via changing the IL cation from [BMMIM]+ to [BMIM]+ in the electrolyte, and the current density in the H-cell reached up to 93.7 mA·cm–2 at −2.2 V and 181.4 mA·cm–2 at −2.6 V (vs Ag/Ag+), respectively. Linear sweep voltammetry curves, H/D exchange, in situ/ex situ infrared spectroscopy, and density functional theory calculations suggest that the significant product switching between CO and HCOOH originates from the CO2 reorientation on the catalyst surface driven by different active sites in the imidazolium cations of the ILs, which leads to different reaction pathways in CO2RR.