Sodium borohydride as an additive to enhance the performance of direct ethanol fuel cells

The effect of adding small quantities (0.1-1 wt.%) of sodium borohydride (NaBH₄) to the anolyte solution of direct ethanol fuel cells (DEFCs) with membrane-electrode assemblies constituted by nanosized Pd/C anode, Fe-Co cathode and anion-exchange membrane (Tokuyama A006) was investigated by means of various techniques. These include cyclic voltammetry, in situ FTIR spectroelectrochemistry, a study of the performance of monoplanar fuel cells and an analysis of the ethanol oxidation products. A comparison with fuel cells fed with aqueous solutions of ethanol proved unambiguously the existence of a promoting effect of NaBH₄ on the ethanol oxidation. Indeed, the potentiodynamic curves of the ethanol-NaBH₄ mixtures showed higher power and current densities, accompanied by a remarkable increase in the fuel consumption at comparable working time of the cell. A ¹³C and ¹¹B {¹H}NMR analysis of the cell exhausts and an in situ FTIR spectroelectrochemical study showed that ethanol is converted selectively to acetate while the oxidation product of NaBH₄ is sodium metaborate (NaBO₂). The enhancement of the overall cell performance has been explained in terms of the ability of NaBH₄ to reduce the PdO layer on the catalyst surface.