Effects of Reactant Gases on HT-PEM Fuel Cells

Within the European project CISTEM (Construction of Improved HT-PEM MEAs and Stacks for Long Term Stable Modular CHP Units), a new HT-PEM fuel cell based Combined Heat and Power (CHP) technology will be developed. This CHP system will have an electrical output of up to 100 kW_el, a modular set-up and will allow a flexible fuel input.

One important task of the CISTEM project is the investigation of degradation processes, which may be caused by different operation conditions. The possibility to use different fuels and oxidants makes a HT-PEM fuel cell much more flexible and versatility applicable. Therefore, the degradation effects induced by varied reactant gases will be described in this work.

Three long term tests (1000 hours or end-of-life) have been carried out with Dapozol^®-MEAs and different reactant gas compositions:

• Hydrogen and air;
• Synthetic reformate and air;
• Hydrogen and oxygen.

The test conditions were identical for all three experiments; the temperature was set to 160°C after break-in procedure and the MEAs have been operated with a constant load of 0.3 A/cm².

The MEAs have been characterized periodically with in-situ electrochemical measurements like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV).

Next to the electrochemical analysis, the MEAs will be investigated post-mortem by using ex-situ imaging techniques like Scanning Electron Microscopy (SEM) and micro-computed tomography (µ-CT). With help of the µ-CT-investigations, a 3D-modell/image of each MEA will be constructed. Amongst others, the reconstructed and binary µ-CT datasets will be used for the analysis of the GDL porosity.

Voltage and thickness changes of the MEAs under different fuel and oxidants combinations are presented in Figure 1. The MEA operated with hydrogen and air shows the most stable behavior but the voltage loss is quite similar for all three MEAs. The degradation rates of the three long term test with different reactant gases emphasize this assumption; the degradation rates are listed in Table 1.

The high stability of the MEA operated with hydrogen and air is also reflected in the MEA thickness changes. The largest MEA thickness changes are observed for the MEA operated with pure hydrogen and oxygen.

Table 1: Averaged degradation rates of long term tests with different reactant gases

Reactant gases	Degradation rate
H2/Air	-56.5 µV/h
Syn. Reformate/Air	-63.2 µV/h
H2/O2	-70.0 µV/h

Thus, these reductions on the MEA thicknesses in long-term operation can be related to a summarized oxidation of the MEA materials as membrane, catalytic layer, microporous layer and gas diffusion layer [1-3]. But compared to the voltage loss the changes in thickness are a lot more distinctive. The reasons for such high degradation rates will be discussed accompanied by post-mortem imaging investigations to reveal possible explanations for the different MEA thickness changes.

Figure 1: Voltage and MEA thickness changes as function of time under long term test conditions with different reactant gas compositions, 160°C, 0.3 A/cm², Dapozol^®-G55-MEAs.

A holistic examination will be realized by the combination of electrochemical and imaging investigations and will make a contribution to discover the most suitable reactant compositions for HT-PEM fuel cell operation.

References:

1.            G. Liu, H. Zhang, J. Hu, Y. Zhai, D. Xu, Z. Shao; J. Power Sources, 162, 547-552 (2006).

2.            Y. Zhai, H. Zhang, G. Liu, J. Hu, B. Yi J. Electrochem Soc 154, 1, B72-B76 (2007).

3.            D.C. Seel, B.C. Benicewicz, L. Xiao, T.J. Schmidt, in: Vielstich W, Gasteiger HA, Yokokawa H, eds. Handbook of Fuel Cells - Fundamentals, Technology and Applications. Vol 5, John Wiley & Sons, Ltd. 300-312 (2009).