A three-dimensional non-isothermal model for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with phosphoric acid-doped polybenzimidazole (PBI) membranes is developed to numerically investigate key thermal aspects that were not considered in previous HT-PEMFC modeling and simulation studies. Particular emphasis is placed on analyzing the effect of the thermal gradient along the coolant flow path, which represents a more realistic thermal environment for a typical large-scale HT-PEMFC stack for residential applications. Our simulation results reveal that a lower coolant flow rate and resultant higher temperature rise toward downstream enhance oxygen reduction reaction (ORR) kinetics and proton conductivity, leading to higher overall cell performance. In addition, location-specific heat source terms in the HT-PEMFCs and their contributions to the overall energy balance are clarified using a numerical study. We found that the irreversible reaction heat in the cathode catalyst layer is the major contributor to heat generation in a HT-PEMFC, and the effect of ohmic joule heating becomes significant under high current density operation. (C) 2012 Elsevier B.V. All rights reserved.