Graphene Oxide-Nafion Multilayer Membrane: Influence of Preparation Method

Abstract

Intermediate Temperature PEFC (IT-PEFC) operating between 100 and 120 °C benefit from faster chemical reactions and more facile water and heat management as compared to conventional PEFC operating below 100 °C. As the operating temperature is increased, the temperature difference between external environment and the fuel cell also increases and heat rejection and removal by heat exchangers is enhanced. Smaller cooling systems can therefore be used reducing the weight and economic costs of PEFC systems. Moreover, the specific operating conditions of IT-PEFC enable water to remain purely in the vapour phase. Thus, the water balance is easier to maintain unlike in ordinary PEFC, which contain water in dual phase. As a consequence, water will not condensate in the flow channels, minimising flooding and interference with the gas flow. Despite these advantages, the main issue in IT-PEFCs is the durability and performance of the polymer electrolyte membrane (PEM), which lies in the heart of the fuel cell. Commercial Nafion membranes and equivalent perfluorinated sulphonic acid (PFSA) membranes are not able to hold enough water at 120 °C and low humidification conditions will eventually cause them to dry out, resulting in poor IT-PEFC performance. This is due to the dependence of the proton transport mechanisms (hopping and vehicular) and hence proton conductivity on the water content of the membrane. Due to various oxygen groups on the basal and edge planes, graphene oxide (GO) is highly hydrophilic and a good electronic insulator. These properties led GO to be used in PEMs. In the last ten years, GO and functionalised GO have been used as additives in composite membranes to improve the membrane properties. These composite membranes have been fabricated with varied polymers such as Nafion, PBI and PSU. In recent work, GO has also been used in multilayer membranes. In composite membranes water retention due to GO can be extremely high leading to irregular expansion and affecting the membrane dimensionality. In multilayer membrane (MM) systems, however, the external layers can limit the expansion. Besides, the external layers of Nafion can avoid water retained in the inner layer of GO being transported towards the electrodes. GO is able to hold water at higher temperatures trapping the molecules in the structure even at intermediate temperature conditions, thus making it ideal for use in IT-PEFC. This work investigates a MM with three layers composed of Nafion in the external layers and GO in the inner layer for application in an IT-PEFC. GO multilayer membranes were prepared by two different routes, 1) hot pressing, and 2) casting. Hot pressing is a faster process and minimises solvent issues. However, the interlayer-interaction in this case is only mechanical and hence relatively weak. This may result in delamination. Casting is a more involved route, but the interaction between the layers is chemical in nature and stronger. In order to compare the influence of the preparation method on the interfacial properties these two membrane types were evaluated for water retention and ion exchange capability. Proton conductivity and single cell IT-PEFC testing were carried out to evaluate the overall performance of the IT-PEFC. Preliminary results revealed that the cast N/GO/N MM presented better results, especially when comparing the proton conductivity. In both cases, the adherence between the inner layer of inorganic GO and the external layers of organic Nafion was not ideal. As such, a small percentage of Nafion was mixed into the inner GO layer. For both methods, hot pressing and casting, optimisation of the Nafion and GO content in the inner layer was performed in order to enhance interlayer interaction.