This paper presents a 2D, fully coupled and comprehensive transient model that accounts for micro-structural features of various cell layers. The model benefits from state of the art sub-models for reaction kinetics and incorporates the polymer relaxation dynamics. Furthermore, a mixed wettability model is utilized to simulate the transient two phase conditions in the porous layers. The model is validated with transient experimental data under various conditions. A comprehensive simulation study is presented to investigate the impact of operating temperature and relative humidity on the transient response. The effects of cathode Pt loading and operation mode, i.e., current control versus voltage control, are also studied. The cell response is found to be dominated by water transport through its thickness. Additionally, it is found that reducing the Pt loading can influence the performance by changing the water balance in the cell, which has rarely been highlighted in the literature. In particular, at low temperature more water is transported toward the anode when the cathode Pt loading is reduced, since the resistance to water back diffusion is lowered with reduced thickness of the cathode catalyst layer. This trend is reversed at a higher temperature due to increased volumetric heat generation with reduced thickness. The model can help in understanding various transport phenomena and is expected to be useful for inspecting spatio-temporal temperature, potential, and species distributions across the cell's thickness and optimizing the cell design and choice of materials. (c) The Author(s) 2019. Published by ECS.