Evaluation of Neutron Imaging for Measuring Phosphoric Acid Distribution in High Temperature PEFCs

Abstract

The possibility of using neutron imaging for non-invasive investigation of the phosphoric acid distribution in high temperature polymer electrolyte fuel cells (HT-PEFC) was explored with a small scale test cell. In particular, the issue of providing a suitable reference -necessary for distinguishing the neutron attenuation due to the acid from the attenuation due to the structural components -was solved by using in situ deuteration/protonation of the phosphoric acid, a fully reversible process. Experiments with a nonoperating cell have shown that this isotope exchange can be performed in less than 20 minutes. The possibility of imaging the acid distribution either over the cell area (through-plane imaging) or across the cell structure (in-plane imaging) was demonstrated. Although some discrepancies between the two modes remain, quantitative analysis resulted in a good agreement with the amount of acid used in the cell. The durability of high temperature polymer electrolyte fuel cells (HT-PEFCs) based on phosphoric acid/polymer composites or gels is limited by several processes, mainly found in the degradation of the membrane electrodes assemblies (MEAs) and their components. Membrane thinning and pinhole formation, evaporation of phosphoric acid (both from membrane and catalyst layers), Pt dissolution and carbon corrosion from the catalyst layer have been identified as the main degradation modes.1 Although durability of several 10,000 hours have been demonstrated in laboratory environments, 2,3 operation under realistic conditions including high operating temperatures of up to 180• C, temperature cycles and start/stop cycling, combined with operation on highly impure reformates produced by fuel processors 4 remains challenging in order to reach the desired >40,000 hours required for stationary combined heat and power (CHP) HT fuel cell systems. One of the most important, but widely underestimated and least understood topics in the development of HT-PEFCs is the behavior of the phosphoric acid electrolyte during operation.5 A detailed picture of the initial acid distribution, its re-distribution and evaporation upon fuel cell operation is missing, although it is one of the key points to be addressed when it comes to the design of highly durable MEAs and cells for HT-PEFCs. Recently, the deleterious effect of acid movement from MEA into the pores of bipolar plates on the durability of HT-PEFC MEAs 6 could be demonstrated. In this study, only cells with completely non-porous plate materials suppressing quick acid distributions during operation showed high durability, whereas cells allowing for acid movement from MEA to plates could not be operated for much longer than 300 h, pointing out the importance of understanding the acid transport within the cell. A possibility to measure distribution of liquid electrolyte in alkaline, phosphoric acid and molten carbonate fuel cells by using an electrochemical impedance sensor located in the bipolar plate material has been described by Kunz. 7 In this work, e.g., the phosp...

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