SEM and XAS characterization at beginning of life of Pd-based cathode electrocatalysts in PEM fuel cells

• PEM fuel cells with Pd, PdCu or Pd5Cu4Pt cathodes were tested for 100 h.
• XAS reflects changes in the electronic/geometric environments of Pd5Cu4Pt.
• There is a change in electronic environment due to charge transfer Cu to Pd/Pt.
• Cu in the electrocatalyst is a nano-mixture of metallic Cu, alloyed Cu and CuO.
• The Cu/CuO acts as a promoter of the ORR.

The fuel cell performance of membrane electrode assemblies with a Pt anode and Pd, PdCu or Pd5Cu4Pt cathodes has been tested during 116 h (beginning of life). The incorporation of Cu to Pd increases the fuel cell performance. Incorporation of Pt leads to further improvement. SEM micrographs of the as-prepared and the fuel cell-tested assemblies show the effects of the 116 h of continuous operation. Nafion membranes were characterized by small angle X-ray scattering. The results show a reduction of the size of the lamellar domains in the perfluorinated matrix after fuel cell testing, but no correlation with the cathode electrocatalyst material. The cathode electrocatalysts were characterized by ex-situ synchrotron radiation X-ray diffraction and X-ray absorption spectroscopy at the Pd L3, Cu K and Pt L3 edges. Re-organization of Pd5Cu4Pt electrocatalyst after fuel cell testing was observed. The Cu in the electrocatalyst can be described as a nano-mixture of metallic Cu, alloyed Cu and CuO. The CuO acts as a promoter of the ORR.