The performance of proton exchange membrane fuel cells (PEMFCs) depends on the intricate interplay between fluid dynamics, electrochemical reactions, heat transfer and mechanical stresses. This study presents a fully coupled multi-physics numerical model that simultaneously accounts for these phenomena to evaluate and optimize PEMFC performance. The model integrates thermo-fluid transport, electrochemical kinetics and mechanical deformation using the finite element method. A detailed analysis is conducted to investigate the effects of current density, temperature distribution, reactant consumption and mechanical stress across the membrane-electrode assembly. The simulation results demonstrate that optimized reactant flow and improved heat management significantly reduce energy losses and temperature gradients, while adequate clamping pressure minimizes mechanical degradation. The model predictions are validated against experimental data, showing strong agreement. This work provides a comprehensive framework for improving the durability and efficiency of PEMFC and offers a valuable numerical tool for the design of next-generation hydrogen fuel cell systems.