Using a Tunable Triblock Cationic Polymer to Unravel Performance and Durability Effects in Anion Exchange Membrane Based Electrolysis

Abstract

The use of anion exchange membranes (AEMs) as the polymer electrolyte membrane and related cationic polymers as ionomers in the electrodes of water splitting electrolyzers could lead to significant advantages including the elimination of precious metal electrocatalysts. Because oxygenated radicals in base have a very much lower half life than in acid these materials can be based on hydrocarbon backbones. This leads to an enormous possible variation in polymer chemistry and, while exciting from the point of view of the creation of new polymer chemistries, has led to a lack of detail in determining exactly how all of these many polymers really behave in terms of performance and durability. All of the new materials that are currently being commercialized are designed to be free standing films that are chemically stable to attack by the strongly nucleophilic hydroxide anion. Often these polymers have excessive swelling and poor mechanical properties and lack easy variation enabling their modification to the properties required for an ionomer. Studies where the effect of film processing on performance or durability are also lacking. There is a chronic lack of knowledge in how cationic polymers interact with catalyst particles, charge carrier and the AEM in electrodes. Further complicating these studies is whether the device is operated in water, carbonate or hydroxide solution. The first of which give unexpectedly low performance but may be more amenable to long durability and latter of which gives high performance and low durability.We have developed a triblock co-polymer, with polychlorostyrene blocks that can be quaternized to give a hydrophilic phase and a central hydrophobic polyethylene (pE) block that gives the material good mechanical properties. This seemingly simple system can show tremendous variations. We can control the polymer block lengths and overall molecular weight of the polymer. The material can be suspension cast or melt processed to obtain uniform films. The films are post-quaternized and we can control the degree of functionalization and the nature of the cation. What is interesting about these materials is that for all anions that we have studied their conductivity behavior indicates that they are fully dissociated. We hypothesize that the pE content of the films localizes the water, this has the effect of both promoting anion conduction and also not wetting much of the polymer and so improving the durability and mechanical properties. In a series of films where we systematically increase the pE content, the materials swell less but do not necessarily loose performance. They do however show durability improvements, where a material that is 50% pE will thin in an electrolysis test giving rise to a degradation rate of -23 mV/h, a material with 80% pE will show a degradation rate of +400mV/h, at 500 μA/cm2. Further understanding of these materials is likely to result in further durability improvements. Performance and durability can also be varied by the method by which the film is processed.By using an unsaturated version of the cationic triblock co-polymer in electrode formulations and post hydrogenating we can insolubilize the polymer (by creating an insoluble pE containing polymer insitu) as the ionomer in the electrode. This allows us to study, ionomer loading and to begin to control the positioning of the ionomer. We have baselined these materials using Ni supports, and studied Membrane electrode assemblies on our optimized membranes. These ionomers are surprisingly durable as they contain aromatic groups, leading to further speculation that the localization of the water in the material again gives performance and durability advantages. In this talk we will also show how ionomer chemistry can be used to improve the performance and durability of electrolysis electrodes with either silver, cobalt oxide, or manganese oxide.