The freezing site of super-cooled water and its effects on the triple-phase interface and pathway during PEFC cold start remain a subject of controversy. To address this deficiency, dynamic electrochemical characterizations, such as cyclic voltammetry (H-2/N-2), galvanostatic (H-2/Air) and potentiostatic EIS (H-2/N-2, at OCV and 0.2 V), are attempted during the isothermal sub-zero operation of a fuel cell from the start till the failure. The EIS data are fitted using transmission line models, revealing a significant rise in mass transfer resistance and rather constant ohmic, activation resistance and proton conductivity of catalyst layer. Consecutive CVs before, during and after the cell failure show no change. These results show that ice does not cover the surface of the catalysts, but rather blocks the pores of cathode catalyst layer, resulting in the suffocation of oxygen supply to the reaction sites. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.