Electrochemical quartz crystal microbalance study of borohydride electro-oxidation on pt: The effect of borohydride concentration and thiourea adsorption

V. W.S. Lam, D. C.W. Kannangara, A. Alfantazi, E. L. Gyenge

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Abstract

A systematic investigation of BH4- electro-oxidation in 2 M NaOH on Pt was carried out using the electrochemical quartz crystal microbalance technique (EQCM). Four sets of experiments were conducted: (i) Pt in NaOH, (ii) BH4- (between 10 and 60 mM) on Pt, (iii) thiourea (TU) on Pt, and (iv) BH4- in the presence of TU (between 0.01 and 2.1 mM) on Pt. The BH4- electro-oxidation mechanism was developed by correlating the EQCM results with density functional theory (DFT) calculations from the literature revealing the energetically most favorable adsorbates. The surface coverage by key intermediates was estimated using the van der Waals molecular areas. On the anodic scan, the BH4- electrosorption was followed by dissociation of BH4,ad generating BHy,ad and (4 - y)H ad, with y between 1 and 3, depending on the available surface sites. BHy,ad is further oxidized as a function of electrode potential in Eley-Rideal and Langmuir-Hinshelwood type mechanisms with the participation of OH- and OHad, respectively. The oxidative desorption on the cathodic scan at potentials between 0.1 and 0.5 V of strongly adsorbed intermediates such as BOHad and BH2OHad is essential for recovering the Pt electrocatalytic activity. TU adsorption on Pt produces a characteristic potential dependent adsorption-desorption hysteresis. Furthermore, at TU concentrations above 0.045 mM for 30 mM BH4 - a bilayer is formed on the surface, which is stabilized by Lewis acid-base interactions between TU and BH4-. As a result, the BH4- oxidation overpotential is increased leading to incomplete oxidation, whereas the BH4- thermocatalytic hydrolysis is inhibited.

Original languageBritish English
Pages (from-to)2727-2737
Number of pages11
JournalJournal of Physical Chemistry C
Volume115
Issue number6
DOIs
StatePublished - 17 Feb 2011

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