Fuel Cell Diagnostics
Fuel cell characterization by various diagnostic methods and understanding of the function and lifetime behavior of fuel cell components for PEFC and HTPEFC's is the vision and the mission of the fuel cell diagnostic activity.
This encloses the ex-situ characterization, the in-situ performance and durability testing, as well as the numerical modeling of the processes. The diagnostic methods comprise impedance spectroscopy, local gas phase analytics, in-situ tomographic characterization, as well as locally resolved electrochemical experiments on various scales. Active area of sub cm² single cells up to several 100 cm² stacks for mobile applications can be tested.
X-Ray tomography of water in operating fuel cells
Polymer electrolyte fuel cells (PEFC) convert the chemical energy of hydrogen with a high efficiency (40-70 %) directly into electricity. The product of the overall reaction is water, produced at the cathode of the cell.
The interaction of liquid water with the porous structures of the cell is one of the mechanisms in the PEFC that are commonly believed to be key for further optimization with regard to performance, durability and cost.
Synchrotron based X-ray tomographic microscopy (XTM) allows for the simultaneous in situ visualization of the water and carbonaceous structures in the gas diffusion layer (GDL) on the pore scale level [1, 2]. In-situ XTM scans of operating PEFC are performed within a few seconds per scan and pixel sizes of 2 - 3 µm. Experiments are made at the TOMCAT beamline of the Swiss Light Source (SLS).
The figure shows XTM surface renderings of the cathode channel with flow field plate, GDL, liquid water and catalyst layer.
 R. Flueckiger, F. Marone, M. Stampanoni, A. Wokaun, F.N. Buechi, Electrochim. Acta, 56, 2254 (2011)  J. Eller, T. Rosen, F. Marone, M. Stampanoni, A. Wokaun, and F.N. Buechi J. Electrochem. Soc., 158, B963 (2011)
Further publications: Electrochemistry Laboratory
Flow modeling in gas diffusion layers of PEFCs at the micro- and mesoscale
The optimization of thermochemical and electrochemical conversion systems is of high importance for a sustainable energy future society. Of particular interest regarding the performance of polymer electrolyte fuel cells (PEFCs) is the study of the gas flow in the gas diffusion layers (GDL). More specifically, permeability and diffusivity measurements in a model PEFC under normal operating conditions are highly desirable. As laboratory-measurements of these quantities under such conditions are very demanding, an alternative is the use of computer-based simulations. For this, two key elements are needed: a) an advanced numerical tool capable of modeling key microscale processes, and b) in-situ X-ray tomographic microscopy (XTM) scans of the GDL material. Physical modeling of 3D gas flows is accomplished through novel mesoscale computational algorithms based on the lattice Boltzmann method (LBM).
The provided figure illustrates computed flow streamlines through the GDL porous structure (carbon fiber paper Toray TGPH 060, domain size: 444x222x160 microns). The GDL microstructures, wherein the produced liquid water can be distinguished from the solid material, are obtained at the TOMCAT beamline of the Swiss Light Source (SLS). The results show that permeability and relative effective diffusivities of dry and partially liquid saturated GDL samples follow a relation proportional to (1-s)x, where (s) is the saturation level and the exponent x is approximately 3.
M. Kenzelmann, S. Gerber, N. Egetenmeyer, J.L. Gavilano, Th. Strässle, A. D. Bianchi, E. Ressouche, R. Movshovich, E.D. Bauer, J. L. Sarrao, and J.D. Thompson, Physical Review Letters 104, 127001 (2010)
 N. I. Prasianakis, T. Rosen, J. Kang, J. Eller, J. Mantzaras, F. N. Büchi, Simulation of 3D porous media flows with application to polymer electrolyte fuel cells, Comm. in Comp. Phys. (in press) (2012).  T. Rosén, J. Eller, J. Kang, N. I. Prasianakis, J. Mantzaras, F. N. Büchi, Saturation dependent effective transport properties of PEFC gas diffusion layers, (submitted) (2012)
Further publications: Fuel Cell Systems and Diagnostics Group
Local current measurement in PEFCs
Major barriers for a successful commercialization of Polymer Electrolyte Fuel Cells (PEFCs) are insufficient lifetime and high cost of platinum catalyst. A comprehensive understanding of aging and transport phenomena on all relevant length scales is a key to improve durability and to reduce precious metal loading.
Flow fields as used in PEFCs for the distribution of the reactant gases over the electrode area cause inhomogeneities. The importance of down the channel inhomogeneities has been realized. Inhomogeneities in the perpendicular to the flow channel direction, however, have not received adequate attention to date, possibly due to the lack of direct experimental evidence. A novel approach allows for the first time the direct measurement of the local cell current in channel and land areas of PEFCs with sub-millimeter resolution. The high potential of our method is demonstrated here in the evaluation of in-plane current transients during start-up of a PEFC and in transient flooding experiments in combination with neutron radiography for liquid water detection. The method provides key information that is badly needed for the understanding of transport and degradation phenomena and for the assessment of mitigation strategies.
I.A. Schneider, G.G. Scherer, Handbook of Fuel Cells – Fundamentals, Technology and Applications. Edited by Wolf Vielstich, Hubert A. Gasteiger, Harumi Yokokawa.Volumes 5&6, Part 4, Chapter 45, 2009: Advances in Electrocatalysis, Materials, Diagnostics and Durability.
Further publications: Electrochemistry Laboratory