Maintaining precise output voltage is critical for optimal performance in Proton Exchange Membrane (PEM) fuel cell systems, a cornerstone technology in hydrogen-based vehicles. This research introduces a robust adaptive fixed-time integral sliding mode control (FxT ISMC) strategy to address voltage regulation challenges caused by uncertainties in load and input voltage. FxT ISMC eliminates the need for pre-defined disturbance bounds through adaptive gains and therefore simplifies the implementation. The ISMC design guarantees robust and rapid convergence to the desired voltage, regardless of initial conditions, ensuring smooth power delivery for the electric motor in a hydrogen vehicle. The benefits include faster voltage regulation, lower voltage spikes, reduced control oscillations, and exceptional resilience to fluctuations, ultimately contributing to a more efficient and reliable method of propulsion in hydrogen-powered vehicles.