The paper deals with controlling an electric power system comprised of a Hydrogen Fuel Cell (HFC), boost and boost/buck DC-DC power converter and the ultracapacitor(UC) for an auxiliary power supply in order to control servomotor speed within a vehicle application. Relative degree approach is applied for direct control of the servomotor input voltage and speed, as well as the HFC and UC current in the presence of the model uncertainties. The non-minimum phase property of the DC-DC boost converter is eliminated by controlling the HFC and UC current based on the power balance approach. The adaptive-gain second order sliding mode controllers (2-ASMC) control the current in HFC and the servomotor speed. The conventional Sliding Mode Controllers (SMC) is designed for controlling the output voltage of the converter and the load current of the UC. The efficiency and robustness of the proposed SMC and 2-ASMC are confirmed via computer simulations.