A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and a fuel such as hydrogen or a hydrocarbon-based material such as methanol, ethanol, or natural gas.
A fuel cell can be classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type depending upon the kind of electrolyte used. Although each of these different types of fuel cells operates in accordance with the same basic principles, they may differ from one another in the kind of fuel, the operating temperature, the catalyst, and the electrolyte used.
Recently, a polymer electrolyte membrane fuel cell (PEMFC) has been developed. The PEMFC has power characteristics that are superior to conventional fuel cells, as well as a lower operating temperature and faster start and response characteristics. Because of this, the PEMFC can be applied to a wide array of fields such as for transportable electrical sources for automobiles, distributed power sources such as for houses and public buildings, and small electrical sources for electronic devices.
A PEMFC is essentially composed of a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a body of the PEMFC, and the fuel pump provides fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies the hydrogen gas to the stack. Fuel stored in the fuel tank is pumped to the reformer using power which can be provided by the PEMFC. Then, the reformer reforms the fuel to generate the hydrogen gas, and the hydrogen gas is electrochemically oxidized and the oxidant is electrochemically reduced in the stack to generate the electrical energy.
Alternatively, a fuel cell may include a direct oxidation fuel cell (DOFC) in which a liquid fuel is directly introduced to the stack. Unlike a PEMFC, a DOFC does not require a reformer.
In the above-mentioned fuel cell system, the stack for generating the electricity has a structure in which several unit cells, each having a membrane electrode assembly (MEA) and a separator (also referred to as a “bipolar plate”), are stacked adjacent to one another. The MEA is composed of an anode (also referred to as a “fuel electrode” or “oxidation electrode”) and a cathode (also referred to as an “air electrode” or “reduction electrode”) that are separated by a polymer electrolyte membrane.
The polymer electrolyte membrane can be fabricated using a perfluorosulfonic acid ionomer membrane such as Nafion® (by DuPont), Flemion® (by Asahi Glass), Asiplex® (by Asahi Chemical), and Dow XUS® (by Dow Chemical). The electrodes including the catalysts supported on the carbon can be fabricated by binding electrode substrates such as porous carbon paper or carbon cloth with a carbon powder carrying pulverized catalyst particles such as platinum (Pt) or ruthenium (Ru) using a water-repellent binder.
What is needed is a high power membrane electrode assembly and a fuel cell system where the transferring rate of the reactant is fast and a high concentration of reactants can be present on the surface of a catalyst.