Invited: HT-PEFC: From Fundamentals to Stacks and Systems

High Temperature Polymer Electrolyte Fuel Cells (HT-PEFC) are commonly based on polybenzimidazole-type membranes doped with phosphoric acid and have a typical operating temperature of 160 °C. Due to this temperature they possess a higher CO tolerance compared to classic PEFCs and are therefore particularly suitable for the use of hydrogen rich reformate gases. Further, due to the properties of phosphoric acid, external humidification is not necessary which results in simplified layouts of complete fuel cell systems in comparison to PEFCs. In the past several HT-PEFC stacks and systems were presented but still fundamental research is necessary to improve the power density and lifetime according to the required application.

The proton conductivity of the membrane and of the electrodes inside a HT-PEFC is ensured by phosphoric acid. The most common polymer membranes used are Poly[2,2-(m-phenylen)-5,5-bibenzimidazol] (PBI) and Poly(2,5-benzimidazol) (ABPBI) [1], where the different polymer backbone units result in different phosphoric acid uptake per repeat unit. It was demonstrated that the performance of HT-PEFCs is nearly independent of the way of acid introduction into the membrane and electrode but strongly depends on the amount inserted into it [2]. Inside the MEA phosphoric acid undergoes concentration changes accompanied by changes of the proton conductivity under varying operating conditions. Further, the distribution of the phosphoric acid inside the membrane electrode assembly (MEA) depends on the operating conditions and the electrodes used [3,4,5]. These effects have to be taken into account when designing and operating cells and stacks.

To achieve the desired lifetime of cells and stacks the current distribution in the cells needs to be as homogeneous as possible. The main factors influencing this distribution are the flowfields, the operating conditions (like stoichiometry and humidity of the gases) and the temperature distribution in the cell area [6]. Several cell and stack concepts have been examined experimentally and by simulation techniques. The best performance and the highest lifetime for stacks operated on reformate gas were achieved with liquid cooled stacks. The active area was 200 cm² per cell and counter-flow of the gases was applied, whereas the cooling liquid was in co-flow with the anode gas stream [7].  After scaling up, stacks with an active area of 320 cm² were operated in an APU-like diesel reformate system. The maximum power achieved with the system was 5 kW_el [8].

During the talk we will present an overview of recent challenges for the cell and stack materials with special emphasis on the phosphoric acid. Further we will summarise the impact of the stack layout including cooling system, the operating conditions and lifetime. Finally we will report about our experience with a 5 kW diesel based system.

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[2] Ch. Wannek, I. Konradi, J. Mergel and W. Lehnert, J. Power Sources, 192, 258-266 (2009)

[3] W. Maier, T. Arlt, K. Wippermann, C. Wannek, I. Manke, W. Lehnert and D. Stolten, J. Electrochem. Soc, 159 F398-F404 (2012)

[4] F. Liu, S. Mohajeri, Y. Di, K. Wippermann and W. Lehnert, Fuel Cells, DOI: 10.1002/fuce.201300272 (2014)

[5] T. Arlt, W. Maier, Ch. Tötzke, Ch. Wannek, H. Markötter, F. Wieder, J. Banhart, W. Lehnert and I. Manke, J. Power Sources, 246, 290-298 (2014)

[6] L. Lüke, H. Janßen, M. Kvesić, W. Lehnert and D. Stolten, Int. J. Hydrogen Energy, 37,  9171-9181 (2012)

[7] J. Supra, H. Janßen, W. Lehnert and D. Stolten, Proc. ASME. 45226; Volume 6: Energy, Parts A and B:855-864 (2012)

[8] R. C. Samsun, J. Pasel, H. Janßen, W. Lehnert, R. Peters and D. Stolten, Applied Energy, 114, 238–249 (2014)