IN SITU STUDIES OF FUEL CELL INTERFACES USING AMBIENT-PRESSURE XPS AND NEUTRON REFLECTOMETRY

Steven C. DeCaluwe, Joseph A. Dura, Charles F. Majkrzak, Gregory S. Jackson, Chunjuan Zhang, Bryan W. Eichhorn 

Hydrogen fuel cells, such as Solid Oxide Fuel Cells (SOFCs) and Polymer Electrolyte Fuel Cells (PEMFCs), are among the most promising technologies for clean, efficient power conversion, both from traditional and renewable fuel sources.  Currently, a host of technical challenges prevent the commercialization of fuel cells as reliable, cost-competitive technologies for widespread applications such as electric vehicles or auxiliary power units for remote power.  Part of the difficulty in optimizing fuel cell performance stems from a lack of direct knowledge regarding limiting reactions at the electrode interfaces with electrolyte and air phases. 

            This work presents two approaches to in situ measurement of operating fuel cell interfaces from two studies: Ambient-Pressure XPS (APXPS) measurements of SOFC gas/electrode interfaces, and the proposed use of Neutron Reflectometry (NR) to investigate hydration phenomena at PEMFC catalyst/electrolyte interfaces.  APXPS measurements were taken at the Advanced Light Source at Lawrence Berkeley National Labs.  Measurements probe the near-surface oxidation state and electric potential of thin-film ceria electrodes in an operating SOFC.  Results demonstrate the limiting role of heterogeneous surface reactions in ceria-based electrodes.  Furthermore, results show that this reactive region extends orders of magnitude farther than for non-MIEC catalyst materials (e.g. Ni).  Lastly, the results are used to validate a numerical model for ceria-based SOFC anodes.

            Similarly, in situ NR measurements will probe the operating state of an important fuel cell material, namely Nafion, which is the basic structural unit for state-of-the-art PEMFC electrolytes.  NR measurements taken in a humidified environment in the absence of electrochemistry have detected lamellar water layers at Nafion interfaces.  This work demonstrates the progress in extending these results to an operating PEMFC system, which will allow for simultaneous electrochemical and NR measurements at the NCNR.  Taken together, the two studies demonstrate the challenges and advantages of collecting in situ measurements of operating fuel cell interfaces.