(Invited) Interactions of Block Co-Polymer Ionomers on Metal Surfaces for Tuned Electrocatalysis

Abstract

In acidic fuel cells, the ionomer and catalyst layer were developed empirically, and only recently studies have begun investigating the success of the heterogeneous microstructure and triple-phase boundary. Currently, the catalyst layer is not well understood due to its complexity; 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 ionic and water transport networks. In the field of alkaline fuel cells, there is limited research published on ionomer developments in general or on specific interactions between ionomers and catalysts. The few fuel cell studies that compare the effects of ionomer chemistry have conflicting conclusions, which likely arise because the ionomer chemistries interact with Pt in different ways. Overall, this provides the motivation for our work to study idealized structures in the electrode, specifically to elucidate and modify interactions occurring at the interface between ionomers and non-PGM catalysts. This insight will enable the rational and systematic development of ionomer chemistry with the ultimate goal of improving electrode kinetics for a variety of important electrochemical reactions.We have synthesized a large number of model cationic block co-polymers, diblocks, triblocks and pentablocks (AB, ABA, ABABA). Where the hydrophobic block is either polyisoprene or polycyclooctadiene, or their hydrogenated analogues, polymethylbutylene or polyethylene. The other block is polychloromethylstyrene that can be quaternized with a variety of amines to vary the interaction with the surface, inmost of work we use the relatively simple benzyltrimethylammonium (TMA) cation or the reportedly more stable and more bulky benzylmethylpyrolidinium (MPRD) cation. Our initial studies using commercial Ag nano powders showed that the olefinic groups of the polymers interacted wit the Ag and that the interaction of the cations with the Ag surface could completely remove the crystallinity from the entire sample in the case of the polyethylene block. Probing the effects of these interactions on electrocatalysis of the oxen reduction reaction is on going and we will report our progress in this paper.To further constrain the polymers we have also studied them as thin films on flat Si wafers and the same substrate coated with thin metallic films and studied by GISAXS. When block co-polymers are spin coated on plain silicon wafers we almost always see no scattering as there is no alignment of the polymer on the silicon surface. When a thin layer of silver is coated onto the silicon, discrete alignment of features perpendicular to the surface are observed for many of the polymers we have synthesized on the nanoscale. The size of these features in the thin films is not always correlated with the block sizes observed in the bulk films by SAXS. AFM imaging of the polymer air interface suggests that these vertically aligned features persist throughout the film, a property presumably needed for transport, but that the size can vary. The data shown below is typical of aligned vertical triblock ABA polymers on Ag. We have also investigated these thin films as a function of temperature and hummidity and observed transients of the changes in the morphology of these materials. In this paper we will show how these interactions can be utilized for the beneficial development of electrocatalysts. Figure 1