Year: 2026
Type: preprint
Abstract: Electrode interfaces with ionic polymers (ionomers) create local microenvironments in membrane electrolysis and other applications, yet the formation and dynamics of electrical double layers (EDLs) at ionomer|electrode interfaces are incompletely understood. Unlike traditional aqueous electrolytes with both mobile cations and anions, ionomer interfaces couple mobile counterion motion to the slower response of tethered polymer-backbone ionic groups. Studying these solid|ionomer interfaces, with pure water feed as in many technology applications, is more difficult than with conventional electrolytes due to the lack of a liquid supporting electrolyte for simple integration of counter and reference electrodes. Here, we use a membrane-electrode-assembly-inspired architecture with a lateral ionomer strip and a salt-bridged reference electrode to independently track anodic and cathodic potentials while sweeping the voltage between the two Pt-solid|ionomer interfaces in pure water. We quantify electrode-resolved non-faradaic charging and show that fixed polymer charges and mobile counterions contribute differently to interfacial capacitance. In anion-exchange ionomers, mobile Cl- responds rapidly, whereas fixed quaternary ammonium groups relax more slowly, producing a capacitance asymmetry. This asymmetry weakens when sufficient time is allowed for relaxation and is suppressed by supporting electrolyte. When Nafion cation-exchange ionomer is used instead, the results show a reversal in the sign of the asymmetry. Supporting molecular dynamics simulations establish coupled counterion motion and tethered-charge relaxation as governing features of pure-water ionomer EDLs. The work thus provides a framework for studying and ultimately designing ionomer microenvironments for membrane electrolysis and other applications. The Bigger Picture Pure-water-fed membrane electrolyzers rely on ionomers to carry charge to catalyst surfaces. Unlike soluble liquid electrolytes, ionomers contain mobile counterions and oppositely charged groups fixed to a polymer chain. The details of how these charges form electrical double layers under positive and negative polarization are not fully understood. We developed a model cell with a solid ionomer electrolyte that senses each electrode potential separately during cell operation. The measurements show asymmetric, time-dependent charging in pure water, with mobile counterions responding more rapidly than polymer-bound charge. Added electrolytes suppress this charging-rate imbalance; reversing the ionomer charge sign also reverses the asymmetry of the charging response, and a custom-synthesized more-flexible polymer backbone reduces the rate dependence of the apparent capacitance. The work here shows how the ionomer is an active part of the electrochemical interface, in addition to an ion conductor in the bulk. The platform and fundamental principles developed here are relevant to water electrolyzers, fuel cells, and other membrane-based electrochemical devices.
Source: ChemRxiv
Language: English
Subfield: Filtration and Separation
Field: Chemical Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7216567978