Effect of Operation Strategies on Phosphoric Acid Loss in HT-PEM Fuel Cells

Abstract

One of the most important degradation mechanisms, which cause a reduction of lifetime and performance of high temperature (HT) polymer electrolyte membrane (PEM) fuel cells, is phosphoric acid loss. This paper presents the effect of different operation strategies on acid loss of HT-PEM FCs compared with fresh membrane electrode assemblies (MEA). These investigations have been executed during the run-time of the European Project CISTEM (Construction of Improved HT-PEM MEAs and Stacks for Long Term Stable Modular CHP Units, GA-No. 325262). The vision of this project is the development of a new HT-PEM FC based CHP (combined heat and power) technology with high efficiency and long lifetime. Degradation investigations of single components, MEAs, FC stacks and complete CHP units are main objectives within the project. With this paper we provide a more detailed insight to the phosphoric acid loss driven degradation process which causes shorter lifetimes and lower performances [1-2]. In dependence on the typical operational occurrences of the HT-PEM CHP-systems, the following operational strategies have been tested: Fuel switching between pure H2 and synthetic reformate (Test 1 - grey)Start-stop-cycling with an idling temperature of 25 °C (Test 2 - red)Long term tests at constant current density (0.3 A/cm²) with different reactant gas compositions: Synthetic reformate/pure O2 (Test 3 - blue)Synthetic wet reformate/O2 enriched air (Test 4 - black) All tests have been performed with Dapozol®-G55 MEAs, which consist of thermally cured polybenzimidazole (PBI) membranes and Pt/C based electrodes. During operation, the MEAs have been electrochemically characterized in-situ at Beginning of Life (BoL), once per week and End of Test (EoT) via polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV). In addition, product water samples have been weekly collected from both cathode and anode gas outlets, and their acid content has been determined with ion chromatography (IC). The results of the total acid leaching in product water for every operation strategy are shown in Figure 1. After EoT, the remaining H3PO4within the tested MEAs was identified by titration with sodium hydroxide [3]. By ex-situ ante- and post-mortem micro-computed tomography (µ-CT) investigations, mechanical transformations can be visualized and the thickness changes of all layers can be verified. Fresh MEAs are characterized by mirror symmetry through all layers, while MEAs, operated under dry reactant gases, disclose a reduction of catalyst layer thicknesses on anode sides. The operation with pure O2results into increased water production, which therefore causes higher acid-leaching. The MEA tested with synthetic reformate and pure oxygen has revealed the highest phosphoric acid loss (Figure 1) on one hand and lowest degradation rates (-6.4 µV/h) on the other hand. Figure 1: Total H3PO4 loss in product water after EoT and degradation rates (DR) of each operation strategy