Increasing Polymer Electrolyte Fuel Cells' (PEFCs) operating temperature has benefits on the performance and the ease of utilisation of the heat generated; however, efforts for high temperature PEFCs have resulted in high degradation and reduced life time. In the literature, conventional low temperature (T < 80 degrees C) and high temperature (140-200 degrees C) regimes have been extensively studied, while the gap of operating at intermediate temperature (IT) (100-120 degrees C) has been scarcely explored. The main bottleneck for operating at IT conditions is the development of a suitable proton exchange membrane with comparable performance and lifetime to the commercially used Nafion operating at conventional conditions. In this work, composite membranes of Graphene Oxide (GO) and Nafion of varied thickness were fabricated, characterised and assessed for in-situ single cell performance under automotive operating conditions at conventional and intermediate temperatures. The material characterisation confirmed that a composite GO-Nafion structure was achieved. The composite membrane demonstrated higher mechanical strength, enhanced water uptake, and higher performance. It was demonstrated that by utilising GO-Nafion composite membranes, an up to 20% increase in the maximum power density at all operating temperatures can be achieved, with the optimum performance is obtained at 100 degrees C. Moreover, the GO-Nafion membrane was able to maintain its open circuit voltage values at increased temperature and reduced thickness, indicating better durability and potentially higher lifetime. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.