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The invention provides a fuel cell system that is free from troubles due to contaminant ions by controlling the concentration of contaminant ions in cooling water. The fuel cell system comprises a fuel cell stack and a means for controlling the cell temperature by circulating a liquid coolant in the

The invention provides a fuel cell system that is free from troubles due to contaminant ions by controlling the concentration of contaminant ions in cooling water. The fuel cell system comprises a fuel cell stack and a means for controlling the cell temperature by circulating a liquid coolant in the fuel cell stack or bringing it in contact with the fuel cell stack, the fuel cell stack comprising a plurality of unit cells that are laid one upon another, each of the unit cells comprising a hydrogen ion-conductive electrolyte membrane, a pair of gas diffusion electrodes which sandwich the electrolyte membrane, an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to one of the electrodes, and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the other of the electrodes, wherein a material adsorbing or absorbing ions is provided on a portion of the fuel cell system to come in contact with the liquid coolant.

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1. A fuel cell system comprising a fuel cell stack and a means for controlling the cell temperature by circulating a liquid coolant in said fuel cell stack or bringing it in contact with said fuel cell stack, said fuel cell stack comprising a plurality of unit cells that are laid one upon another

1. A fuel cell system comprising a fuel cell stack and a means for controlling the cell temperature by circulating a liquid coolant in said fuel cell stack or bringing it in contact with said fuel cell stack, said fuel cell stack comprising a plurality of unit cells that are laid one upon another, each of said unit cells comprising a hydrogen ion-conductive electrolyte membrane, a pair of gas diffusion electrodes which sandwich said electrolyte membrane, an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to one of the electrodes, and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the other of the electrodes, wherein a material adsorbing or absorbing ions is provided on a portion of the fuel cell system to come in contact with said liquid coolant. 2. The fuel cell system in accordance with claim 1, wherein said material adsorbing or absorbing ions adsorbs or absorbs ions at a speed or in an amount which is dependent on a potential difference between said liquid coolant and said material. 3. The fuel cell system in accordance with claim 2, wherein said material adsorbing or absorbing ions is electrically connected to the fuel cell stack and has a potential which is dependent on a potential of the connected portion of the fuel cell stack with respect to said liquid coolant. 4. The fuel cell system in accordance with claim 1, wherein said material adsorbing or absorbing ions comprises carbon or a metal oxide. 5. The fuel cell system in accordance with claim 1, wherein said liquid coolant comprises water or an organic liquid having a hydroxyl group. 6. The fuel cell system in accordance with claim 1, further comprising a means which is disposed on a flow path of said liquid coolant for intermittently applying a potential to said material adsorbing or absorbing ions. 7. A fuel cell system comprising a fuel cell stack and a humidifying means for humidifying a reaction gas with steam, said fuel cell stack comprising a plurality of unit cells that are laid one upon another, each of said unit cells comprising a hydrogen ion-conductive electrolyte membrane, a pair of gas diffusion electrodes which sandwich said electrolyte membrane, an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to one of the electrodes, and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the other of the electrodes, wherein a material adsorbing or absorbing ions is disposed so as to be in contact with humidifying water of said humidifying means with a potential applied by said fuel cell stack.