The effect of ionomer/carbon (I/C) ratio on proton exchange membrane (PEM) fuel cell cold start is investigated experimentally with theoretical water transport analysis. The scanning electron microscope (SEM) images show larger agglomerates and smaller effective reaction area by increasing the I/C ratio from 0.7 to 1.7. For normal operation, increasing the I/C ratio can improve the humidity tolerance, especially in the cathode. For cold start >-10 C-omicron, a lower I/C ratio leads to better performance because the core reaction area is shifted towards the membrane, leading to more membrane water absorption and slower ice formation. For <-15 C-omicron, the total water production is low and almost the same for the different I/C ratios because the ice formation takes place before effective membrane water absorption; and although the cathode catalyst layer (CL) and micro-porous layer (MPL) can provide sufficient space to store all the ice, higher I/C ratios (e.g. 1.2) still cause more ice formation in GDL and flow channel because the core reaction area becomes closer to GDL. The results show that the CL design has significant effect on the cold start performance, and there is a potential for further improvement. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.