Surface Analytical Investigation of Inner and Outer Surfaces of Porous Components for Polymer Electrolyte Membrane Fuel Cells
Authors: Schulze Mathias , Biswas Indro 
Published: February 2020

Fuel cells allow the direct conversion of chemical energy (typically hydrogen and oxygen or air) to electrical energy and therefore are an important high efficiency option for future energy systems. The surface composition is related to the surface properties, which influence the catalytic and hydrophobic behavior. In polymer electrolyte membrane fuel cells, several components are porous, namely the gas diffusion layer consisting of carbon structure as support and a micro porous layer (MPL) and the catalyst layer (CL). The porous structure is important for the transport of the reactants (hydrogen and air) and products (water) as well as for the electrochemical reactivity at the interfaces. Caused by the porous structure not only the properties of the outer surfaces but also the properties of the inner surfaces are important.

The combination of two different depth profiling techniques, ion etching and peel-off by an adhesive tape, and a surface sensitive analytic method, here X-ray photoelectron spectroscopy (XPS), allows to investigate the inner and outer surfaces. XPS allows to determine the chemical composition of the surfaces, which allows to derive information about the hydrophobic/hydrophilic properties. With the ion etching the structure of the surface layer will be investigated with a depth resolution in the range of nanometers in order to determine gradients in the surface compositions, e.g. thin films covering surface of the catalyst particles. The tape peel-off preparation allows to remove a film from the surface and gives the access to the inner surfaces.

The surface compositions of the different surfaces prepared with and without tape peel-off of various porous fuel cell components were determined by XPS depth profiles with ion etching. The MPL and the CL consist of a fluorine rich polymer (PTFE or Nafion with a PTFE backbone) and carbon black. In the case of the catalyst layer the carbon black is used as support for the platinum catalyst and in case of MPL as porous conductive material. The fluorine rich compound is used to tailor the hydrophobicity of the components. XPS profiles measured by ion etching of the outer surface and an inner surface of a catalyst layer prepared by an inkjet printing process [1] will be presented. In the C 1s spectra two different carbon species can be distinguished, the signal from carbon black and the carbon signal from the carbon in a fluorine rich compound.

The outer and inner surface show a very similar composition and depth profile. The concentrations of the fluorine rich carbon compound and of fluorine itself strongly decrease during the ion etching while the concentrations of the carbon black as well as of the platinum increase. This means that the outer surface as well as the inner surface have a higher concentration of polymer on the surfaces than in the bulk material, whereas the thin polymer film partially covers the Pt catalyst. This behavior is frequently observed for catalyst layers prepared from an ink mixed of the polymer, carbon supported catalyst and solvents [2].

Acknowledgement

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 303452 (IMPACT).

References

[1]    S. Shukla, K. Domican, K. Karan, S. Bhattacharjee, M. Secanell; Electrochimica Acta 156 (2015) 289

[2]    M. Schulze, C. Christenn; Appl. Surf. Sci. 252 (2005) 148