The structure of electrochemical reaction zone and the catalyst layer (CL) thickness affect the performance of high temperature polymer electrolyte membrane fuel cells. In this study, the physical structures and compositions of CL are investigated by electron microscopy and polarization curve techniques. The Pt concentration of Pt/C decides the thickness of CL when the Pt loading is fixed. A higher weight percentage Pt/C contains a lower amount of carbon powder results in a thin CL and limited space for electrochemical reaction. On the other hand, the lower weight percent Pt/C provides larger space and smaller size of platinum catalyst which engenders the electrochemical reaction in CL more easily. The ionomer binds electrocatalysts Pt/C particles together and offers the ion conducting phase. Two different ionomers, Polytetrafluoroethylene (PTFE) and Polyvinylidenedifluoride (PVDF), were tested. SEM results showed that PTFE forms a better uniform CL structure than PVDF. With 10 wt% Pt/C, PTFE ionomer possesses a higher gas permeability property which induces a higher reactant flow rate in CL, and consequently results in a 42.9% higher cell potential than the PVDF at 0.4 A/cm(2) current density output. A proper combination of 10 wt% Pt/C with PTFE ionomer is able to gain 0.62 A/cm(2) output at 0.3067 V for the HT-PEMFC. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.