Experimental and Modelling Insights into the Borohydride Electrooxidation Reaction Mechanism at Platinum Electrodes

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 BH_x 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 BH_x 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 OH_ad species was further modelled, based on experimental and calculated ⁹ finding of the literature (Figure 1 and 2) ¹⁰.

 

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