Fuel Cell Diagnostics through Neutron Imaging

The operation of polymer electrolyte fuel cells (PEFC) at high current density is of high importance in order to minimize the total stack area – an in consequence the system cost – for a given output power. At high current density, mass transport losses can become an important limitation, in particular in relation with the accumulation of water in the fuel cell porous layers. In this context, the visualization of liquid water in operating fuel cells has been extensively applied (e.g. (1-7)) in the last decade. Among the different visualization methods available, neutron imaging features the unique combination of being non-invasive (due to the transparency of fuel cell materials and the absence of radiation damage to the cell) and providing an excellent contrast for liquid water. However, unravelling the relation between water accumulation and mass transport losses cannot be done without combining visualization with advanced electrochemical characterization methods.

In a first part, the recent advances at PSI in neutron imaging and combined diagnostics methods will be presented. The advantages and limitations of mass transport characterization methods such as electrochemical impedance spectroscopy (EIS), limiting current density and our recently developed pulsed gas analysis (PGA) (5) will be presented. Additionally, the choice of test cell hardware and its impact on results from these methods will be discussed.

In a second part, a selection of experimental results will show how the combination of imaging and advanced electrochemical characterization can bring a new understanding to the mass transport limiting processes. A particular attention will be given to the choice of gas diffusion layer (GDL) material characteristics such as the amount of hydrophobic coating and the presence of a microporous layer (MPL).

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2.            J. P. Owejan, J. J. Gagliardo, J. M. Sergi, S. G. Kandlikar and T. A. Trabold, Int J Hydrogen Energ, 34 3436 (2009).

3.            D. Kramer, J. B. Zhang, R. Shimoi, E. Lehmann, A. Wokaun, K. Shinohara and G. G. Scherer, Electrochim Acta 50, 2603 (2005).

4.            A. B. Geiger, A. Tsukada, E. Lehmann, P. Vontobel, A. Wokaun and G. G. Scherer, Fuel Cells 2, 92 (2003).

5.            P. Boillat, P. Oberholzer, A. Kaestner, R. Siegrist, E. H. Lehmann, G. G. Scherer and A. Wokaun, J Electrochemical Soc 159, F210 (2012).

6.            P. Oberholzer, P. Boillat, R. Siegrist, R. Perego, A. Kastner, E. Lehmann, G. G. Scherer and A. Wokaun, J Electrochemical Soc 159, B235 (2012).

7.            J. Biesdorf, P. Oberholzer, F. Bernauer, A. Kaestner, P. Vontobel, E. H. Lehmann, T. J. Schmidt and P. Boillat, Phys Rev Lett 112, 248301 (2014).

Figure caption:

Figure 1 – Top: Neutron radiograms of a differential fuel cell operating at 1 A/cm2 with different humidities. Bottom: corresponding transport losses measured using our PGA method. Reproduced from ref. (5).