Understanding multiphase transport in proton-exchange-membrane water electrolyzer (PEMWE) is critical to improving electrolyzer performance and optimizing the material.1-4 Mathematical modeling is ideally suited to tackle such issues, especially due to the inherent divergent time and lengthscales involved during operation. In a PEMWE, the multiphase transport mainly involves: a) Liquid water transport in the porous electrode; b) Interaction between liquid water and product gas; and c) Gas transport, i.e., removing product gas in the liquid water through the porous transport layer (PTL) and into the channel. For a), as liquid water is the reactant, its distribution in the PTL and catalyst layer is of great importance to the reaction distribution. For b), the gas bubble formation on the electrochemically active surface area (ECSA) blocks the water access and leads to a dynamic redistribution of current, which affects the local and perhaps global cell performance and may lead to enhanced degradation. Lastly, understanding the mechanism of gas bubble detachment is necessary in order to minimize the loss of ECSA due to gas bubble coverage. In this talk, a multiscale, multiphysics PEMWE model will be presented. The model includes a two-dimensional, nonisothermal, multiphase, macrohomogeneous model combined with a local microscale model that accounts for the fluid flow interaction between a gas bubble and liquid water. The bubble coverage is introduced as a function of current density,5 and its impact on current redistribution discussed in terms of allowable coverage and dynamic cycling gas evaluation at the local reaction site. The structural information such as pore-size distribution and tortuosity is obtained from X-ray computed tomography (CT) data, which is also used to validate some of the model predictions under operando conditions. Also, based on the X-ray data, the microstructure of the PTL has been reconstructed and two-phase fluid simulations have been carried out to predict effective transport properties that are incorporated into the marcoscale model. Lastly, the porous electrode parameter optimization such as wettability, porosity, and pore size, will be introduced to provide guidance in material and structural design for better electrolyzer performance. M. Carmo, D. L. Fritz, J. Mergel and D. Stolten, International Journal of Hydrogen Energy, 2013, 38, 4901-4934.U. Babic, M. Suermann, F. N. Büchi, L. Gubler and T. J. Schmidt, Journal of The Electrochemical Society, 2017, 164, F387-F399.E. Leonard, A. D. Shum, S. Normile, D. C. Sabarirajan, D. G. Yared, X. Xiao and I. V. Zenyuk, Electrochimica Acta, 2018, 276, 424-433.T. Kadyk, D. Bruce and M. Eikerling, Scientific reports, 2016, 6, 38780.R. Balzer and H. Vogt, Journal of the Electrochemical Society, 2003, 150, E11-E16.