Engineering of the bipolar stack of a direct NaBH4 fuel cell

When a fuel cell (FC) utilizes liquid fuels directly, a few complications arise due to the conductance or the potential conductance of the fuel. Fuel cell stacks are typically designed in a bipolar fashion so that the voltage of individual cells can be added up in series to give an adequate and convenient output voltage. The conductivity of fuels brings about two risks if the bipolar stack is not properly designed and engineered. On one hand, the conductive liquid fuel may short circuit the neighboring cells of a bipolar FC stack with traditional integrated fuel manifolds. On the other hand, the conductive fuel may pass a high voltage to some parts of the cell through an ordinary manifold, causing excessive corrosion. These issues need to be addressed through a cell-isolation fuel distribution network (FDN). The function of such an FDN is to increase the shunting resistance of neighboring cells, so as to maintain a reasonable open circuit voltage. Also, the presence of a gas phase in the liquid fuel during cell operation affects fuel circulation and therefore needs to be considered in the FDN design. On the plus side, a liquid fuel, in contact with high surface area FC electrodes, functions as a super-capacitor, giving the FC an excellent pulse overload capacity. Also the fuel itself is a fair coolant, enabling high power density at minimal increase in stack weight. These considerations are applied to a kilowatt NaBH4/H2O2 fuel cell stack to generate the desired operational characteristics. (c) 2008 Elsevier B.V. All rights reserved.