Catalysis Today 170 (2011) 126–133 Contents lists available at ScienceDirect Catalysis Today j ourna l ho me p ag e: www.elsevier.com/lo cate/cattod Rotating disc electrode studies of borohydride oxidation at Pt and bimetallic Pt–Ni and Pt–Co electrodes A. Tegou a , S. Papadimitriou a , I. Mintsouli a , S. Armyanov b , E. Valova b , G. Kokkinidis a , S. Sotiropoulos a,∗ a Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece b Rostislaw Kaischew Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria a r t i c l e i n f o Article history: Received 27 October 2010 Received in revised form 4 January 2011 Accepted 5 January 2011 Available online 2 February 2011 Keywords: Platinum catalysts Bimetallic catalysts Galvanic replacement Borohydride oxidation a b s t r a c t The electrochemical oxidation of borohydride has been studied by means of voltammetry at Pt and Pt–Ni, Pt–Co bimetallic rotating disc electrodes (RDEs). The bimetallic catalysts are prepared by means of a gal- vanic replacement method (whereby electrodeposited Ni and Co layers are partially replaced by Pt when immersed in a chloroplatinic solution) and are shown to have a Pt shell–bimetallic alloy core. The effects of electrode history, potential scan direction, rotation speed and electrode material on borohydride oxi- dation have been investigated. For all Pt-based catalysts tested a gradual decrease of the voltammetric current from its initial value to a steady state response is observed in the kinetic control potential region, irrespective of scan direction and rotation speed. The initial deactivation of the catalyst at low overpo- tentials as well as the shape of the initial and steady-state voltammograms in that region point to the heterogeneous hydrolysis of borohydride and the subsequent oxidation of its products. As the catalytic activity for the hydrolysis reaction decreases the voltammograms shift to more positive potentials where direct borohydride oxidation dominates. The bimetallic Pt–Ni and Pt–Co catalysts exhibit a more negative open circuit potential and higher oxidation currents at low overpotentials than Pt, but lower apparent number of electrons transferred in the mass transport control region. These findings can be interpreted by the lowering of the d-band energy level of Pt in the presence of Ni and Co and the associated decrease in its adsorption affinity. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Although the electrochemical oxidation of borohydride has long been studied and identified as a potential source of energy in the electrochemical literature [1–5], it was not until the beginning of the last decade that it attracted interest again as the anode reaction of direct borohydride fuel cells, in particular for possible portable device applications [6–8]. Direct borohydride oxidation can occur via one of the following two overall reactions depending on the anode material [6–10]: BH 4 - + 8OH - - 8e - → BO 2 - + 6H 2 O (1) BH 4 - + 4OH - - 4e - → BO 2 - + 2H 2 O + 2H 2 (2) The complete-8e reaction occurs at materials that have a low hydrogen adsorption/evolution affinity such as Au and Ag with the first step speculated to be that of electron transfer and formation of ∗ Corresponding author. Tel.: +30 2310 997742; fax: +30 2310 443922. E-mail addresses: eczss@chem.auth.gr, eczss@otenet.gr (S. Sotiropoulos). a borohydride radical that rapidly hydrolyses and looses a second electron to form monoborane: BH 4 - - e - → BH 4 • (3) BH 4 • + OH - → BH 3 - + H 2 O (4) BH 3 - - e - → BH 3 (5) The latter hydrolyses further and dimerizes to diborane which then undergoes a stepwise 6e oxidation involving a number of interme- diates [5]. The partial-4e reaction is favoured at materials that have sig- nificant hydrogen adsorption affinity and/or hydrogen evolution activity such as Pt, Ni, Pd, etc. with the first step being that of dissociative chemisorption: BH 4 - + 2M → M–BH 3 - + M–H (6) Following that, and depending on the strength of the M–BH 3 - and M–H bonds as well as on electrocatalytic activity, either electro- chemical oxidation or heterogeneous hydrolysis may take place. In the former case the sequence of reactions involves electron loss by 0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.01.003