Accurate zero-dimensional models of proton exchange membrane water electrolysis are valuable for stack design, technoeconomic analysis, and system level simulation, but their predictive capability is often limited by the use of parameters obtained from full-cell curve fitting. This work presents a multiphysics lumped model of a single PEM water electrolyzer cell that combines thermodynamics, electrode polarization, mass transport, ohmic losses, and bubble-related effects. A three-electrode configuration incorporating a palladium hydride wire reference electrode was used to resolve the anodic and cathodic polarization responses independently, while full-cell and membrane-free electrochemical impedance spectroscopy were employed to estimate the membrane and lumped electronic/contact resistances. The electrode specific Tafel parameters were introduced into the model as effective descriptors of the porous-electrode response rather than as intrinsic elementary kinetic constants, particularly for the cathode, where the measured slopes were influenced by the coupled effects of HER/HOR kinetics, local hydrogen accumulation, and catalyst-layer transport. Using a single parameter set, the model reproduced the measured polarization curves at 25, 40, and 70 °C without temperaturespecific adjustment, with mean relative absolute deviations below 1%. Anodic activation losses dominate at low current density (up to 70% of the total overpotential), while ohmic and bubble-induced losses become increasingly significant at high current density, with anodic bubble coverage reaching approximately 31%. A global sensitivity analysis identifies membrane hydration and exchange current densities as the parameters governing model uncertainty. The framework provides a validated, physically grounded kernel for predictive PEM electrolyzer design and system level studies.