Operating heterogeneities within a direct borohydride fuel cell

Abstract

This paper presents an analysis of the operation of a direct borohydride fuel cell fed with an alkaline solution of sodium borohydride as the fuel and pure oxygen as the oxidant. The membrane-electrode assembly is derived from proton-exchange membrane fuel cells, i.e. consists of a Nafion® cation-exchange membrane and carbon-supported platinum electrodes deposited on gas diffusing layers. The average performance is observed to be quite sensitive to the cell temperature. The experimental set-up makes it possible to measure the quantity of hydrogen produced by the cell, the local current density and the local potentials relative to hydrogen reference electrodes. The behavior of the cell is heterogeneous between the inlet and outlet of the reagents, due to the progressive production of hydrogen by hydrolysis down the channel direction. The direct oxidation of borohydride is effective at low current densities, but it is the oxidation of hydrogen that achieves maximum power densities. Finally, the cathode overvoltage is limiting, because the production of soda at the cathode restricts oxygen transfer (this issue can be overcome by using alkaline or bipolar membranes).