1.  Field of the Invention
This invention relates generally to a fuel cell stack and, more particularly, to a fuel cell stack that includes a non-permeable, low contact resistance shim plate at both ends of the stack for preventing cooling fluid that permeates through a composite unipolar plate from corroding a terminal plate at the ends of the stack. 
2.  Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell.  A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween.  The anode receives hydrogen gas and the cathode receives oxygen or air.  The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons.  The hydrogen protons pass through the electrolyte to the cathode.  The hydrogen protons react with the oxygen and the electrons in the cathode to generate water.  The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. 
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles.  The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane.  The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer.  The catalytic mixture is deposited on opposing sides of the membrane.  The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).  MEAs are relatively expensive to manufacture and require certain conditions for effective operation. 
Several fuel cells are typically combined in a fuel cell stack to generate the desired power.  For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells.  The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor.  Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product.  The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. 
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack.  The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack.  Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA.  Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA.  The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. 
The bipolar plates also include flow channels through which a cooling fluid flows.  In one known fuel cell stack design, the bipolar plates are composite bipolar plates, such as ester based compression molded plates, that absorb the cooling fluid, causing it to leak into adjacent flow fields. 
Unipolar plates are provided at both ends of the fuel cell stack where the unipolar plate includes the flow channels at a cathode side or anode side of the last fuel cell in the stack.  A terminal plate is positioned at an opposite side of the unipolar plate from the stack of fuel cells, and acts as a current collector for the current generated by the stack.  In certain designs, the terminal plate is a copper plate that is coated with nickel or tin.  An insulator plate is positioned at the end of the stack adjacent to the terminal plate.  For those stack plates made of a composite material, the cooling fluid flowing through the flow channels in the unipolar plate leaks through the unipolar plate and collects in the cavity between the unipolar plate and the insulator plate where the terminal plate is located.  The cooling fluid corrodes the coating on the terminal plate making it less conductive, which in turn leads to a significant loss in stack performance.