Local Evaluation of Processed Membrane Electrode Assemblies by Scanning Electrochemical Microscopy

Gas diffusion electrodes (GDEs) for high-temperature polymer electrolyte fuel cells with different sizes of the used binder particles were evaluated by scanning electrochemical microscopy (SECM) with shear force (SF) supplement. The SF data provide means of checking the substrate morphology with respect to cracks formed during the drying process and with respect to aggregates from used binder of poly(fluoroethylene) (PTFE) simultaneously to the electrochemical data. Electron microscopy results show that a GDE prepared with smaller PTFE particles exhibits less PTFE aggregation and more regular cracks. The SECM images show a more homogeneous distribution and higher level of oxygen reduction reaction activity for the GDE prepared with smaller PTFE particles. The quantitative comparison is enabled by the SF setup that maintains a constant working distance toward the sample in the variant of the redox competition mode, in which a cyclic voltammogram was recorded for every grid position of the microelectrode probe. Mass transport limitations of oxygen during the experiment are avoided by dedicated shape of the microelectrode body. Images of microelectrode currents at specific potentials were extracted to map the local electrocatalytic activity of the GDE. The GDEs were processed to membrane electrode assemblies and applied in HT-PEFC single cell tests. The polarization curve agree with the SECM results that GDEs produced with smaller PTFE particles favor the MEA performance