Impact of the Water Management on the Performance Stability of Polyimidazolium-based (Aemion+®) Anion Exchange Membrane Fuel Cells

Kurzfassung

The anion exchange membrane fuel cell (AEMFC) is a potentially low-cost alternative to the proton exchange membrane fuel cell (PEMFC) due to the possible utilisation of less expensive cell and stack components. To compete with PEMFC, the performance needs to be optimised in terms of its long-term stability. For this purpose, it is essential to understand the impact of the more complex water management. In AEMFC, water is not only generated at the anode, but also consumed at the cathode. This can easily lead to drastic water imbalances between both half cells. To avoid electrode flooding or dry out and thus performance losses during operation, a deeper understanding of the parameters influencing the water management is mandatory to bring AEMFCs closer to market maturity. The fuel cells water management can be tuned by either the material properties or the operating conditions. Due to partially contrary effects of the water management on the performance and the stability, a compromise must be found by optimisation of these parameters. This highlights the need to understand the interaction between material and operational parameters to be able to optimise the water management of AEMFCs and therefore maximise its performance and long-term stability. In this work, we study the impact of several parameters influencing the water management and therefore the performance stability of AEMFC. Membrane electrode assemblies (MEA) are based on commercialised Aemion+® membranes and ionomers and PtRu/C and Pt/C catalysts at the anode and cathode, respectively. First, the performance is optimised by varying the relative humidity of the feed gases in connection with different material parameters such as the utilisation of a microporous layer (MPL), the variation of PTFE content and porosity of the gas diffusion layer (GDL). We developed a test protocol to efficiently study the impact of the tuned water management on the performance. Subsequently, the performance stability was evaluated under defined current density operation. Electrochemical impedance spectroscopy (EIS) was utilised to obtain information about possible dehydration of the anion exchange polymer as a possible degradation pathway. Furthermore, analyses such as infrared spectroscopy (IR) and atomic force microscopy (AFM) are used to explore degradation pathways of individual components of the MEA. Thus, performance losses due to inappropriate water management and irreversible degradation can be elucidated. The combination of measurements under AEMFC conditions and comprehensive analytical tools brings a better understanding of the various influences on the degradation mechanisms in order to achieve an optimum of high performance and long-term stability.