Borohydride oxidation over Pt/C, Au/C and AuPt/C thin-film electrodes studied by rotating disk electrode and differential electrochemical mass spectrometry flow cell measurements

We report results of a systematic study of the borohydride oxidation reaction (BOR) in borohydride containing 0.5 M NaOH electrolyte over Pt/C, Au/C and AuPt/C thin-film catalyst electrodes, performed under enforced mass transport conditions. Employing rotating disk electrode (RDE) and thin-layer flow cell differential electrochemical mass spectrometry (DEMS) measurements, we identify kinetic limitations over a wide range of transport conditions. Together with the highly sensitive detection of evolved hydrogen as a function of potential, due to the use of a cold trap at the mass spectrometer inlet, this allows us to separate changes in the reaction selectivity, from complete to incomplete borohydride oxidation, from other kinetic limitations. Evaluation of the (apparent) number of electrons transferred per borohydride ion, both from the RDE measurements via the Koutecky-Levich formalism and from the DEMS measurements via the H$_2$ formation current, further supports the identification of complete borohydride oxidation (8 electron transfer) and incomplete oxidation (< 8 electrons transfer) reaction conditions. Using data on isotope labeled BD$_4$$^-$ oxidation that we had published earlier, we identify weak secondary kinetic isotope effects for all catalysts, which indicate that B-H bond breaking does not represent the rate limiting step.