Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells

The electrochemical oxidation of BH4- in 2 M NaOH on Pt and An (i.e. catalytic and non-catalytic electrodes, respectively, for BH4- hydrolysis accompanied by H-2 evolution) has been studied by cyclic voltammetry, chrono-techniques (i.e., potentiometry, amperometry, coulometry) and electrochemical impedance spectroscopy. In the case of Pt the cyclic voltammetry behaviour of BH4- is influenced by both, the catalytic hydrolysis of BH4- yielding H-2 (followed by electrooxidation of the latter at peak potentials between -0.7 and -0.9 V versus Ag/AgCl, KClstd) and direct oxidation of BH4- at more positive potentials, i.e., between -0.15 and -0.05 V. Thiourea (TU, 1.5 x 1 0(-3) M) was an effective inhibitor of the catalytic hydrolysis associated with BH4- electrooxidation on Pt. Therefore, in the presence of TU, only the direct oxidation of BH4- has been detected, with peak potentials between -0.2 and 0 V. It is proposed that TU could improve the BH4- utilization efficiency and the coulombic efficiency of direct borohydride fuel cells using catalytic anodes. The electrooxidation of BH4- on Pt/TU is an overall four-electron process, instead of the maximum eight electrons reported for Au, and it is affected by adsorbed species such as BH4- (fractional surface coverage similar to0.3), TU and possibly reaction intermediates. (C) 2003 Elsevier Ltd. All rights reserved.