TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: The supplementary power is a capacitor or a battery. It is connected to an external power of the fuel cell system to be charged by the external power at the time of being completely discharged.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Compound: A fuel supplied to the DBFC is sodium borohydride (NaBH4).    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: The DBFC includes a fuel cell stack where an electrolyte membrane is disposed between an anode and a cathode; a fuel tank for storing a fuel; a fuel pump for pumping a fuel stored in the fuel tank to the anode of the fuel cell stack; an air supply unit connected to the anode of the fuel cell stack by an air supply line, for supplying e.g. oxygen to the cathode; a gas/liquid separator for separating a fuel, air, and a byproduct remaining at the fuel cell stack after a reaction into gas and liquid; and a hydrogen supply unit for supplying hydrogen separated by the gas/liquid separator to the PEMFC. The air supply unit includes an air compressor for supplying air in the atmosphere to the cathode of the fuel cell stack; an air filter for filtering air supplied to the fuel cell stack; a humidifier for humidifying air supplied to the fuel cell stack; and a water tank for supplying moisture to the humidifier. The PEMFC includes a fuel cell stack where an electrolyte membrane is disposed between an anode that receives hydrogen generated at the DBFC and a cathode; an air compressor for supplying air in the atmosphere to the cathode of the fuel cell stack; an air filter for filtering air supplied to the fuel cell stack; a heat exchanger for humidifying and heating air supplied to the fuel cell stack; and an evaporator for evaporating a material remaining at the cathode after a reaction. The load sensing unit further includes a second sensing unit connected to a unit cell of the fuel cell stack that the anode and the cathode are stacked, for sensing a voltage of the unit cell. The control unit further includes a second control unit connected to the second sensing unit to receive a signal for complementing a voltage using the supplementary power when a voltage of the unit cell is less than a preset voltage. The control unit includes boosters respectively connected to the DBFC, the PEMFC, and the supplementary power in series, for boosting a voltage; and an inverter connected to the boosters, for converting a direct current into an alternating current. It further includes a buck converter connected to the boosters for converting a direct current into a direct current.