Ralaiarisoa, M.; Krishnamurti, S.S.; Gu, W.; Ampelli, C.; van de Krol, R.; Abdi, F.F.; Favaro, M.: In situ investigation of ion exchange membranes reveals that ion transfer in hybrid liquid/gas electrolyzers is mediated by diffusion, not electromigration. Journal of Materials Chemistry A 11 (2023), p. 13570-13587
10.1039/d3ta02050a
Open Access Version
Abstract:
Ion-exchange membranes (IEMs) play a crucial role in (photo)electrochemical conversion and storage devices such as (photo)electrolyzers, electrodialysis cells, fuel cells, and batteries. These devices require media separation and selective mass transfer to sustain targeted electrochemical processes. To optimize IEM properties, a thorough understanding of the molecular-level processes under operating conditions is essential. This is particularly important for ion transfer and rejection across polymer membranes. Currently, it is widely accepted that ion transfer occurs through diffusion across the membrane mediated by the local electric field generated by the ionized functional groups and via electromigration induced by the presence of an applied bias between two electrodes placed on each side of the membrane. However, a direct investigation of such processes under working conditions using a chemically-sensitive technique is still missing. In this study, commercially available cation- and anion-exchange membranes were investigated in hybrid liquid/gas electrolyzers by coupling in situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) with finite element analysis (FEA). Our findings show that selective ion transport across the membrane separating the liquid from the gas side is driven by diffusion mediated by the ionized functional groups present in the membranes, rather than electromigration. Additionally, we were able to directly detect unwanted polarization fields at the interface between the liquid electrolyte and the polymer membrane. This occurred when the polarity of the applied potential was the same as the charge of the ionized functional groups present in the IEMs. The generation of such polarization fields was attributed to the accumulation of co-ions at the liquid electrolyte/membrane interface, resulting from their rejection and consequent inability to diffuse through the membrane. This leads to a loss in the cell voltage, with detrimental effects on the overall performance and reaction selectivity of hybrid liquid/gas (photo)electrolyzers. This study serves as a proof-of-concept for the use of in situ AP-HAXPES to investigate polymer membranes during operation and sheds new light on the ionic dynamics within the IEMs employed in hybrid liquid/gas electrolyzers.