Fuel cell power plants are electrochemical alternative power sources for both stationary and mobile applications. The fuel cell, which is the heart of such power plant, consists of an anode, a cathode and an electrolyte that separates the two. Anode shall mean a positive electrode, and cathode shall mean a negative electrode. In the operation of a fuel cell, fuel reactant gas, which is typically a hydrogen rich stream, enters a support plate that is adjacent to the anode. Such a support plate is, therefore, referred to as an anode support plate. Oxidant reactant gas, which is commonly air, enters a support plate adjacent to the cathode. This support plate is, therefore, referred to as a cathode support plate. As the hydrogen rich stream passes through the anode support plate, a catalyst located between anode support plate and the electrolyte causes the hydrogen to oxidize, thereby resulting in the creation of hydrogen ions and electrons. While the hydrogen ions migrate through the electrolyte to the cathode, the electrons migrate through an external circuit to the cathode. Another catalyst on the cathode side of the electrolyte causes the oxygen to react with the hydrogen ions and electrons released at the anode, thereby forming water. The occurrence of these reactions near the catalysts and electrolyte creates an electric potential across the fuel cell. The flow of electrons through an external circuit that is connected to the fuel cell, therefore, produces useful work, such as powering an electric motor in a vehicle.
There are various types of fuel cells, which vary according to their electrolyte. The electrolyte is the ionic conducting substance between the anode and the cathode. One type of fuel cell includes a solid polymer electrolyte, otherwise referred to as a proton exchange membrane (PEM). Fuel cells incorporating a solid polymer membrane or proton exchange membrane will hereinafter be referred to as a PEM fuel cell. The catalyst layers within a PEM fuel cell are typically attached to both sides of the membrane, thereby forming a membrane electrode assembly (MEA). As noted above, while hydrogen ions pass through the MEA, the electrochemical reaction between the hydrogen ions, electrons, and oxidant reactant gas forms water within the cathode. This water is commonly referred to as “product water.” In addition, water may also accumulate in the cathode, due to the drag of water molecules, which pass from the anode and through the MEA along with the hydrogen ions during the operation of the fuel cell. This water is commonly referred to as “proton drag water.” The proton drag from the anode to the cathode results in a lower water content on the anode side of the PEM compared to the cathode side. This difference in water content between the anode and cathode sides results in an osmotic force, which fosters the flow of water from the cathode side of the PEM towards the anode side. However, if the PEM (i.e., electrolyte) doesn't remain highly saturated with water, the PEM resistance increases, and the useful power obtained from the fuel cell decreases. Additionally, if product water and drag water accumulate in the cathode, the accumulated water may impede and could prevent oxygen from reacting with the hydrogen ions and electrons. Accumulation of water in the cathode will thus reduce the electric potential created across the fuel cell, thereby limiting the fuel cell's performance. Furthermore, if the cathode water content fails to decrease, the cathode will flood, and the fuel cell will eventually cease to produce power and shut down.
In order to assist the oxidant reactant gas in reaching the catalyst on the MEA, the cathode support plate typically comprises a diffusion layer and a substrate layer. Both the diffusion layer and the substrate layer are typically constructed of porous carbon layers that are rendered hydrophobic. Hydrophobic means antagonistic to water and is therefore often referred to as wet-proofed. It is known, however, to utilize a hydrophilic substrate in lieu of a hydrophobic substrate. Hydrophilic means capable of absorbing water and therefore, is often referred to as wettable. U.S. Pat. No. 5,641,586, for example, describes a cathode support plate comprising a hydrophilic substrate layer and a hydrophobic diffusion layer. Objects of U.S. Pat. No. 5,641,586 included providing a porous support plate which reduced the pressure drop of the oxidant gas as it passed through such support plate, minimizing water accumulation within such support plate and maximizing access of the oxidant reactant gas to the catalyst. Although a hydrophilic substrate may reduce the pressure drop of the oxidant reactant gas through the cathode, the hydrophilic substrate, by its inherent nature, absorbs more water than a hydrophobic substrate. Therefore, unless the water is properly removed from the cathode support plate, the hydrophilic substrate will absorb the water, which, in turn, will eventually flood the cathode support plate. Flooding the cathode support plate would, therefore, negate one of the objects of U.S. Pat. No. 5,641,586: namely, the object relating to minimizing water accumulation.
Flooding the cathode support plate would also prevent the oxidant reactant gas from reaching the catalyst. U.S. Pat. No. 5,641,586 describes a cell operating at elevated pressure, and product water within such a cell typically exits the cell via the oxidant reactant gas exhaust stream as a combination of water vapor and entrained liquid water. Entrained liquid water moves along a reactant flow channel from the interior of the cell to the oxidant reactant gas exhaust stream. This concept is well accepted for cell configurations which utilize a hydrophobic substrate and a solid reactant support plate. U.S. Pat. No. 5,641,586, however, describes a cell with a hydrophilic substrate, which will absorb the liquid water and flood the substrate, thereby impeding transport of oxygen to the cathode catalyst.
What is needed in the art is a fuel cell power plant that ensures proper removal of the product and proton drag water from the cathode, thereby ensuring that the maximum amount of oxygen from the oxidant reactant gas stream reaches and reacts with the catalyst on the cathode side of the MEA.