Single cell testing is carried out using a sensitivity testing approach to evaluate the performance of GRAFCELL (R) materials used as macro-porous substrate for cathode GDLs under a range of flow, pressure, temperature and humidity conditions. Polarization curves and electrochemical impedance spectroscopy data are used to establish a correlation between the processing factors and the in-situ fuel cell performance, to identify the key characteristics required to optimize the fuel cell performance and to downselect the specimens with the best combination of properties.