In this application, we request a total of 4,800,000 core hours, corresponding to an average of 400,000 core hours per month during one year. The requested computational resources will primarily be used for high-fidelity simulations of multiphase flow in fuel cells and electrolyzers. Our research activities are supported by the VINNOVA-funded Competence Center AdTherM (Dnr: 2023-00552), which provides a strong framework for collaboration between academic and industrial partners in the development of advanced energy conversion technologies. The overarching goal of this computational work is to investigate phase-change effects, transport phenomena, and electrochemical reactions in order to enhance the performance of Proton Exchange Membrane Fuel Cells (PEMFCs) and Proton Exchange Membrane Electrolyzers (PEMELs). Both technologies involve strongly coupled physical and electrochemical processes occurring over a wide range of spatial and temporal scales. A detailed understanding of these processes is therefore essential for improving efficiency, performance, and operational stability. A particular focus of the proposed work is multiphase transport and the associated phase-change phenomena. In PEMFCs, water is generated as a product of the electrochemical reactions and must be effectively transported away from the reaction sites. Accumulation of liquid water can obstruct the transport of reactant gases and lead to flooding, while insufficient water content can reduce membrane conductivity. In PEMECs, gas bubbles generated during operation interact with the liquid water phase and may influence local transport processes and electrochemical performance. Understanding the formation, transport, and removal of the different phases is consequently important for both technologies. The numerical work therefore addresses several fundamental and strongly coupled topics, including multiphase flow, heat and mass transfer, species transport, phase change, and electrochemical reactions. High-fidelity simulations require accurate modeling of complex thermal and reactive flows and the inclusion of detailed physical phenomena such as phase change, multicomponent species diffusion, surface roughness and wettability effects, and large local gradients in temperature and species concentration. These phenomena strongly interact and can result in highly transient and spatially heterogeneous flow conditions. Proper resolution of the relevant length and time scales requires large computational meshes and extensive simulation runtimes. Fine spatial resolution is particularly important near phase interfaces, solid surfaces, and regions characterized by steep concentration or thermal gradients. At the same time, sufficiently small time steps are required to accurately capture transient multiphase phenomena. The resulting computational requirements make access to large-scale parallel computing resources essential. The requested computational allocation will enable systematic simulations over a range of operating conditions, geometrical configurations, and material and surface properties. It will also allow mesh-resolution studies and model verification to ensure that the obtained results are numerically reliable. The simulations will provide detailed insight into local transport and phase-change mechanisms that are difficult to access experimentally and will contribute to the development and optimization of next-generation PEMFCs and PEMECs.