Analysis of Bipolar Plate Material Properties for HT-PEM Fuel Cell Application

Abstract

To meet the world’s energy requirements, advanced technologies for energy storage and provision have to be developed. Hydrogen offers a promising way to solve fluctuating electricity demands as well as the possibility of energy storage. When fed to a fuel cell, the chemical energy of the hydrogen molecule can be converted to electrical energy, which can be consumed in stationary and mobile applications.A Hydrogen-based fuel cell consists of various components, such as the membrane electrode assembly, where oxygen and hydrogen gas are reacting. To ensure a controlled conduction of electrons via an external pathway, so called bipolar plates (BPPs) are necessary. The bipolar plates have several functions inside the fuel cell as they provide the gases to the electrodes and separate the individual cells in a fuel cell stack from each other. The material itself has to be gas-impermeable and must ensure heat conductivity for heating and cooling of the cells. Moreover, it has to withstand acidic conditions under potentials ranging from 0-1.5 V and elevated temperatures up to 160 °C present in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs).In this work, we present a detailed analysis of composite-based bipolar plate materials with application in HT-PEM fuel cells. Such bipolar plates mostly contain graphite and varying low amounts of a polymeric binder. While graphite has a low mechanical stability, it provides the required electrical conductivity. It is mixed with a binder polymer to enable mechanic flexibility and stability as well as gas tightness. From previous studies it was shown that fundamental knowledge about BPPs electrical and thermo-mechanical properties and their stability is crucial for the development of fuel cells with high power outputs1-3.Our study includes the investigation of the BPP materials properties in correlation to the nature of the different components and to their fabrication processes. Therefore, a series of different characterisation methods are applied to the composite material provided by the manufacturer. The surface morphology is analysed by scanning electron microscopy (SEM), confocal microscopy and atomic force microscopy (AFM), while X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) in combination with SEM will give insight in the surface composition. In addition, µ-computed tomography (µ-CT) is used to reveal inhomogeneities in the compounds volume. Those methods will enable us to analyse surface properties such as roughness and elemental composition as well as bulk composition.The electrical conductivity of the different BPPs is addressed by conductive-AFM and correlated to the materials composition. In this regard, Figure 1 shows the current distribution of two different BPP (PPS 1 and PPS 2) materials. The comparison shows significant differences in the current responses when applying a potential to the sample. While at one type of BPP already at 0.5 V high currents are observed locally, the overall current distribution for the other type of BPP is much lower (even at 1.0 V) but shows a very homogeneous distribution (Figure 1b). We will correlate such differences in current distribution to the surface morphology analysed by REM and confocal microscopy.Furthermore, the corrosion behaviour is analysed by extended chemical treatment in concentrated phosphoric acid at 160 °C and physicochemical changes of the composite material are tracked. This comprehensive study will enable us to gain new insights into the behaviour of carbon-based BPPs for HT-PEM fuel cells. Lee, D.; Lee, D. G., Journal of Power Sources 2016, 327, 119-126.Pilinski, N.; Nagappan, N. K.; Satola, B.; Rastedt, M.; Dyck, A.; Wagner, P., ECS Transactions 2018, 86, (13).Hartnig, C.; Schmidt, T. J., Electrochimica Acta 2011, 56, (11), 4237-4242. Figure 1