Utilizing a Tunable Triblock Copolymer for Design of Model Systems to Study Interactions at the Anion Exchange Polymer-Catalyst Interface

Abstract

The heterogeneous microstructure of the electrode in polymer electrolyte-based electrochemical devices is not well understood. Due to its complex nature, it is challenging to investigate in a meaningful way. Model systems need to be designed to study controlled interfaces between polymer electrolytes and catalyst particles or surfaces. It is known from studies on proton exchange membranes that restructuring can occur at the polymer-catalyst interface and propagate into the bulk morphology of the material. This is potentially to the detriment of reactant and product species transport in the electrode, and ultimately affects device performance. There is currently little to no work reported on interactions between anion exchange polymers and non-precious metal catalysts more relevant to alkaline systems, such as silver or nickel. This research utilizes a highly tunable ABA triblock copolymer (polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene) to investigate different parameters within the chemistry space and how they influence interactions between the polymer and a catalyst surface. The synthesis route for the triblock copolymer has been optimized to achieve a variety of block lengths and A:B ratios, which ultimately lead to a range of characteristics (ionic conductivity, water uptake, mechanical properties) and morphologies when they are post-quaternized to produce AEMs. Additionally, the polycyclooctene (PCOE) midblock contains C=C double bonds that allow for a variety of post-modifications, such as radical crosslinking and hydrogenation. This system has given us a way to make changes to the chemistry and morphology of the polymer and investigate the effect on interactions at a silver surface. The polymer-catalyst interface has been fabricated for study in two different model systems – as a thin film on a flat surface of silver and in a constrained environment of silver nanoparticles. The structure and properties of the anion exchange polymer in these systems will be representative of any changes that occur due to interactions with the catalyst. In the thin film geometry, the properties of the polymer are studied by comparing the ‘bulk’ properties (represented as a thin film on atomically flat silicon where there are no interactions) with those of a thin film on a silver surface. Samples of varying thickness and chemistry have been prepared and the morphology of each system has been investigated using grazing incidence small angle and wide angle x-ray scattering (GISWAXS) and atomic force microscopy (AFM). Clear differences were found between the bulk properties and those at the silver interface that indicate a strong interaction between the anion exchange polymer and a silver catalyst surface. These change as a function of the various parameters under investigation, including A:B block ratios, molecular weight, chemistry of the polymer backbone, and chemistry of the quaternary ammonia cation group. For AEMs in silver nanoparticle constrained environments, properties under investigation are compared to those of the bulk polymer. Areas of current work include studying changes in thermal characteristics (using thermogravimetric analysis and differential scanning calorimetry), morphology (using transmission electron spectroscopy and small angle x-ray scattering), backbone crystallinity (using wide angle x-ray scattering), ionic and electrical conductivity, alkaline stability, vapor and liquid water uptake, and mechanical properties. Interactions between specific chemical groups are also being investigated with Fourier-transform infrared spectroscopy (FTIR). Furthering the understanding of interactions and morphological changes at the polymer-catalyst interfaces will provide a framework for rational design of anion exchange polymers. Ultimately, improvements in polymer design can have significant impacts on the transport of species in the electrode layer and overall device performance.