Synthesis and Applications of N-Heterocyclic Carbene Based Ionomers in the Catalyst Layer of CO2 Electrolysers

Abstract

With the rise in global warming, carbon dioxide conversion into valuable chemicals and fuels becomes a higher priority. CO₂ electrolysers are able to produce carbon monoxide, which can in turn be used in the synthesis of various hydrocarbons or alcohols. A key component of zero-gap CO₂ electrolysers is the triple phase boundary (TPB) formed at the catalyst layer. This is where the gaseous phase (CO₂), ion conducting phase (electrolyte), and the electron conducting phase (Ag/C catalyst) come into contact, allowing the reduction of CO₂ into CO, known as the CO₂ reduction reaction (CO2RR). The problem is, a competing reaction at the Ag/C electrocatalyst is the hydrogen evolution reaction (HER), producing H₂. As it is more thermodynamically favourable at the overpotential used for the CO2RR, this side reaction reduces the overall Faradaic efficiency (FE) of the cell. It is noteworthy that, N-heterocyclic carbenes (NHCs) can form stable covalent bonds with various metals and would be suitable ligands in electrolysers. Through affecting the TPB by binding the NHCs to the Ag/C catalyst, a favourable environment for key intermediates in the CO2RR can be achieved. Corollary, novel NHC-ionomers synthesized will be incorporated into the catalyst layer as binders in order to increase the Faradaic efficiency of CO₂ electrolysers.