Understanding current amplification by quaternary ammonium polybromides droplets on Pt ultramicroelectrode

In this article, we report that electrochemically generated quaternary ammonium polybromide (QBr2n+1) droplets can act not only as electrochemical reactors for the electro-oxidation of Br−, but also as tiny reductants for Br2 dissolved in an aqueous phase. We suggest two different theoretic models: Cloud and Droplet. In the Cloud model, we consider a cloud composed of small droplets located in the vicinity of a Pt ultramicroelectrode (UME). The positive feedback loop of the redox reaction is derived in the gap between the Cloud and the Pt UME, which leads to catalytic current enhancement, like the positive feedback mode of scanning electrochemical microscopy (SECM). In the Droplet model, a droplet adsorbed on the center of a Pt UME drives the catalytic feedback loop of the redox reaction. Next, we adopted the two theoretical models to explain the current amplification by QBr2n+1 observed in our experimental systems. In the early potential region for electro-oxidation of Br−, we found the QBr2n+1 droplets-Cloud model was a more reliable scenario for the catalytic current amplification. As the potential became more positively biased, stochastic collisions of QBr2n+1 droplets occurred on the Pt UME, and in this stage, we determined that the QBr2n+1-Droplet model was the main catalytic mechanism for Br− electro-oxidation in the presence of QBr in the solution.