Direct Fuel Oxidation Alkaline Fuel Cells; The Kinetics of Borohydride Oxidation

Abstract

Alkaline based fuel cells are among the most efficient due to the enhanced kinetics of oxygen reduction in alkaline media. In space applications, they have demonstrated efficiencies near 60%, with the potential to generate electricity with efficiencies at nearly 70%. One of the largest hurdles to overcome in the general uptake of hydrogen fuel cells is the ability to store hydrogen fuel in a form with sufficient energy density to allow for mobile systems to be truly viable. This often means that hydrogen has to be stored at very high pressure (around 70 MPa) in bulky tanks to provide sufficient capacity. Clearly this is not suitable for smaller vehicles or portable systems. Partly to address this, there has been considerable interest in direct oxidation liquid fuelled cells due to the very high specific energy density of liquid fuels. A very compact variant of the liquid fuelled cell which has received little attention is the alkaline dissolved fuel cell where the fuel is dissolved in the electrolyte and the system relies on a selective cathode for efficient operation.
Borohydrides present a particularly good option for an alkaline dissolved fuel system, having high energy densities, a low standard potential for the oxidation to borax (an 8 electron process) and good stability in alkaline conditions. A key area in the success of a fuel cell utilizing borohydride is the development of an anode which can make use of the full 8 electron oxidation to borax directly oxidising the borohydride with as little of the hydrolysis reaction occurring as possible. This is in addition to common requirements of high activity, high stability, good electronic conductivity and transport of reactants and products. Here we present investigations into the oxidation of borohydride in alkaline media under various conditions for a selection of candidate materials in different forms using an RDE (rotating disc electrode) based procedure. Results demonstrate that hydrolysis or an indirect oxidation mechanism reduces the coulombic efficiency of the oxidation reaction observed for the materials investigated so far. There is also a significant difference in behaviour between candidate materials. The best performing materials from the RDE studies will be used to develop suitably active and durable functional fuel cell electrodes using scalable processes such as screen printing.