Experimental and Modelling Insights into the Borohydride Electrooxidation Reaction Mechanism at Platinum Electrodes
Authors: Chatenet Marian , Olu Pierre-Yves , Bonnefont Antoine , Job Nathalie , Savinova Elena R. 
Published: February 2020

The electrooxidation of BH4
- (BOR) is a complex reaction and competes with its heterogeneous hydrolysis on most catalytic surfaces 1. Gold and platinum are the most studied electrocatalysts for the BOR so far (see e.g. 2), but despite a decade of research effort, the reaction mechanisms at such electrodes are still unclear. For example, the supposed faradaic-efficiency (resp. inefficiency) of Au (resp. Pt) electrocatalysts was only proven wrong recently, from mass spectrometry on-line coupling to electrochemistry: Au promotes the BH4
- hydrolysis when the BOR proceeds (hydrogen is evolved and irreversibly lost, because Au is not an efficient hydrogen electrooxidation catalyst) 3, whereas platinum does not above the open-circuit potential 4. In addition, in situ FTIR spectroscopy enabled to detect some BOR intermediates at smooth polycristalline Au and Pt surfaces 5,6, but this did not enable to completely uncover the BOR mechanism at these electrode materials. Moreover, the electrode morphology largely affects the observed performances, as recently demonstrated for carbon-supported Pt nanoparticles 7 or smooth Pt surfaces 8.

In this contribution, we studied the BOR at Pt nanoparticles deposited on vertically-aligned carbon nanofilaments (VACNF). We demonstrated that the BOR is largely influenced by the density of Pt sites at the electrode: whereas electrodes with large Pt density are active below E = 0.6 V vs. RHE in quasi-stationary conditions (but this may depend on the NaBH4 concentration), electrodes with small density of Pt sites are easily “poisoned” by adsorbed BHx intermediates and therefore deactivate at low potential (Figure 1). Nevertheless, hydroxyl species formation on the Pt surface for E > 0.6 V vs. RHE enable the removal of the BHx adsorbed intermediates that blocked the Pt surface at low potential, in a Langmuir-Hinshelwood type electrooxidation, but this proceeds at the detriment of the fraction of free Pt sites and affects the BOR pathway. Such dual role of the OHad species was further modelled, based on experimental and calculated 9 finding of the literature (Figure 1 and 2) 10.

 

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Figure 1