(Invited) Tuning Triblock Co-Polymer Silver Interactions on the Nanoscale to Enhance Transport in Electrodes for Electrochemical Devices Based on Anion Exchange Membranes

Abstract

Anion exchange membranes (AEMs) have recently attracted interest for applications in fuel cells, electrolyzers, redox flow batteries, reverse electrodialysis, and water purification. In contrast to proton exchange membranes (PEMs), AEMs operate in base instead of acid and facilitate the transport of anionic species. In fuel cells and electrolyzers, this allows for more facile kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) respectively. Therefore, AEM-based devices provide the opportunity to utilize non-precious metal catalysts to achieve similar performance to Pt-based catalysts and a substantially reduced cost. The electrodes of these devices are comprised of a heterogeneous mixture of conductive carbon support, catalyst particles, and ionomer, which form a triple-phase boundary where the electrochemical reactions take place. The ionomer is an alkaline polymer electrolyte similar to the AEM which transports OH- and water to/from the catalyst surface and promotes reactant transport.The electrode layers in PEM-based fuel cells were developed empirically, and it was not until recently that research efforts were dedicated to investigating the heterogeneous microstructure and triple-phase boundary that form. Currently, the catalyst layer as it exists in an operating device is not well understood due to its complex nature; however, model interfaces between ionomers and catalyst particles have been used to provide valuable insight on ionomer-catalyst interfacial interactions. It is known from studies with Nafion® that restructuring of the polymer morphology can occur when it exists as an ionomer thin film at a catalyst interface. The behavior of the thin film ionomer changes compared to its bulk characteristics, which ultimately affects electrode kinetics by altering the nature of its microstructure, leading to shifts in morphology, water uptake properties, and ionic and water transport networks.Initial work was driven by the overarching hypothesis that the polymer-catalyst interfacial interactions could be modified by changing the ionomer chemistry. This was investigated by studying interactions between silver nanoparticles and anion exchange ionomers functionalized with two different quaternary ammonium cations, using our previously reported block copolymer backbone polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene (PCMS-b-PCOE-b-PCMS) functionalized with either trimethylammonium (TMA) or methylpiperidinium (MPRD) quaternary ammonium cations. Our early findings indicated there were interactions between silver nanoparticles and C=C groups in the polycyclooctene midblock of the polymer backbone. This motivated the development of a modified anion exchange polymer by hydrogenating the PCOE midblock in PCMS-b-PCOE-b-PCMS to synthesize a polyethylene (PE) midblock. Hydrogenation was achieved through a reaction with p-toluenesulfonyl hydrazide. The improved characteristics of the PE-based membrane have prompted studies investigating the comparison of saturated and unsaturated ionomers.A uniquely tunable polymer system was developed which has enabled further investigations of ionomer thin films on silver substrates. The thin film geometry allows for the use of grazing incidence small angle x-ray scattering (GISAXS) and atomic force microscopy (AFM) to study the interfacial morphology as a function of ionomer chemistry. Restructuring of the polymer morphology compared to its bulk properties has been confirmed, and our initial findings on the contribution of different cationic moieties will be discussed in the results of this report.