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. The concept of zero-emission requires a rapid shift from traditional fuels to next generation of clean technologies 1 to mitigate local pollution in urban areas and to mitigate the emission of carbon dioxide. Polymer electrolyte water electrolyzers for the hydrogen production (from solar and wind power) and polymer electrolyte fuel cells (PEFC) for electricity production from hydrogen are considered one of the most promising clean power technologies. Both electrochemical components are equipped with membrane electrode assemblies (MEAs) consisting either of gas diffusion electrodes (GDEs) assembled with membranes or catalyst coated membranes (CCMs) combined with gas diffusion layers (GDLs). 2,3 In the high-temperature polymer electrolyte fuel cell (HT-PEFC), which is considered here, GDEs are commonly used. In order to apply the catalyst dispersion onto the GDL, various coating methods can be used such as blade coating, spraying and screen printing. [4][5][6] During the subsequent drying process of the dispersion, cracks in the catalyst layer may occur, 4,7 which segment the catalyst layer into parts of a few hundred micrometers extension. The cracks, which are clearly visible, are not electrochemically active. A review about the formation and analysis of the crack structure as well as an ex-situ analysis of the crack width distribution in GDEs of HT-PEFCs was given by Froning et al. 8,9 However, it is not yet clear how the crack structures within the catalyst layers affect the performance of the fuel cell. Furthermore, the influence of the morphol...