System Integration Issues for a Direct Sodium Borohydride Fuel Cell (DNBFC)

A fuel cell for air independent mobile applications using Direct Sodium Borohydride/Hydrogen Peroxide fuels in a low temperature PEM configuration is under development [1, 2]. As part of the development of this unique all liquid fuel cell, we have been studying methods for system integration, including methods for water management, stacking issues involving fluid conductivity control, and the design of a composite catalysis-diffusion layers. [3, 4] The goal is to find optimal conditions (minimum activation, ohmic and transport losses plus maximum run time per fuel loading) in this unique all liquid fuel cell. In contrast to conventional H₂/O₂ cells, the high electron and ion conductivity of the aqueous solution based fuels introduces special design considerations. For example, in stack design, the path length of flow channels connecting cells must be lengthened to increase the electric resistance which would otherwise introduce serious electrical shorting. With the catalyst coated throughout the diffusion layer, increasing ion conductivity from reaction sites to the PEM region also becomes a key design consideration, involving the porosity and entanglement of catalyst materials. Water management in this type of cell involves unique issues beyond humidification of the PEM which is automatically wetted by the liquid fuels. Here the issue is recirculation of product water from the cathode side back to the borohydride side to prevent reaction product NaBO₂ from exceeding its solubility limit. These system integration issues are studied by a coordinated experimental approach which will be described in the presentation.