A device used to supply a gas mixture including hydrogen and carbon monoxide at a concentration not exceeding 100 ppm to a main fuel cell which is supplied with air. The device includes a reformer, which is used to convert a fuel into a gas mixture including carbon monoxide and hydrogen, and a purification device to diminish the concentration of carbon monoxide in the gas mixture and that is connected to an anode inlet of the main fuel cell. The supply device also includes an auxiliary fuel cell supplied with air and that receives the carbon monoxide produced by the reformer to convert part of the chemical energy of the carbon monoxide into electrical energy.