Revealing Indirect Electro-Reduction of Quaternary Ammonium Polybromide Droplets in Acidic Aqueous Solutions

Abstract

In this article, we account for the reduction pathway of quaternary ammonium polybromide (QBr(2n +/- 1)) droplets on Pt ultramicroelectrode (UME). In previous work, we electrochemically analyzed stochastic current spikes from Br -electrolysis in QBr(2n +/- 1) droplets by their particle impact on Pt UME. For electro-reduction of QBr 2n +/- 1 droplets through chronoamperometry, we found no evidence of stochastic current spikes, which are supposed to be evidence for direct electro-reduction (DR) of QBr(2n +/- 1) droplets to Br- and Q(+) through their particle impact on Pt UME. Instead, we suggest an indirect electro-reduction (IDR) mechanism for QBr(3) droplets via the following pathway based on a finite element analysis. The electrochemically generated QBr(3) droplets are mostly distributed in the vicinity of Pt UME within our experimental time scale due to their sizes. QBr(3) equilibrated with Q(+) and Bra(-) and the liberated Br3(-) from QBr(3) droplets are electrochemically reduced to Br until all the QBr(3) droplets in the vicinity of the Pt UME are depleted. We further found that QBr(2n +/- 1) species (n = 2 and 3) are not electro-reduced through IDR but are precipitated on Pt UME. In the chronoamperogram at each forward step of the fifty potential cycles, CA(forward-50 cycles), the potential was applied for 0.4 s where Br is oxidized on the Pt UME in an acidic solution containing QBr. In the CA(forward-50 cycles), the current spikes attributed to Br- -electrolysis in QBr(2n +/- 1) droplets by their particle-impact on Pt UME were observed. In the CA(reverse-50 cycles), the potential was applied for another 0.4 s, which is enough to drive the electro-reduction of Br3(-) on Pt UME. The frequency of the current spikes in the CA(forward-50 cycles) significantly declined as the potential-cycles increased. The chronoamperometric analyses indicated that the Pt surface is partially blocked by the reduction-inactive QBr(2n +/- 1 )species, which was confirmed to be mostly QBr(5) and QBr(7) by Raman spectroscopy.