The paper deals with controlling an electric power system comprised of a Proton Exchange Membrane Fuel Cell (PEMFC) and boost DC-DC power converter in order to control servomotor speed within a vehicle application. Relative degree approach is applied for direct control of the output load voltage and servomotor speed, as well as the PEMFC current in the presence of the model uncertainties. The system's zero dynamics are analyzed and appear to be stable. The adaptive-gain twisting sliding mode controllers control the current in PEMFC and the servomotor speed. The conventional Sliding Mode Controller (SMC) is designed for controlling the output voltage of the converter. The efficiency and robustness of the proposed SMC and 2-adaptive-gain twisting SMC are confirmed via computer simulations.