Understanding Br− transfer into electrochemically generated discrete quaternary ammonium polybromide droplet on Pt ultramicroelectrode

In this report, we present the finite element analysis for the electrolysis of Br− in electrochemically generated quaternary ammonium polybromide (QBr2n+1) droplets, and revealed that the Br− transfer from aqueous phase into QBr2n+1 (Br− transfer-water│QBr2n+1) as the preceding step limits the rate of the Br−-electrolysis. At first, the theoretical dimensionless i-t and the polarization curves based on the EC (electrochemical-chemical) and CEC (chemical-electrochemical-chemical) mechanisms were studied. The simulation based on the EC pathway indicates the only limiting factor for the total electrolysis time in the droplet regime was the rate of the charge transfer in the EC reaction, while the kinetics of the proceeding chemical reaction did not have any effects on the total rate of the Br−-electrolysis. Compared with the simulation by the EC mechanism, the results by the CEC elucidated the importance of not only the rate of the charge transfer, but also the kinetics of its preceding process for the total electrolysis time in the droplet. We adopted the theoretical simulation models based on the EC and CEC mechanisms to the Br− electro-oxidation occurring in a discrete N-methyl-N-ethyl pyrrolidinium polybromide (MEPBr2n+1) droplet. We confirmed the CECC (chemical-electrochemical-chemical-chemical) mechanism well explained the electrolysis behavior of Br− in a MEPBr2n+1 droplet, implicating the existence of the Br− transfer-water│MEPBr2n+1 as the preceding process for the Br−-electrolysis in a MEPBr2n+1 droplet. We also attempted to explain the experimental data by the simulation results based on the ECC (electrochemical-chemical-chemical) mechanism without the preceding Br− transfer-water│MEPBr2n+1 step. In this case, the charge transfer rate of Br·/Br− redox reaction should be unrealistically slow for the explanation of current spikes from the Br− electro-oxidation in a MEPBr2n+1 droplet. However, the simulation results gave the significant disagreement with the experimentally obtained polarization curve, which indicates that the Br−-electrolysis in MEPBr2n+1 cannot be explained without the Br− transfer-water│MEPBr2n+1. We further estimated the rate of the Br− transfer from aqueous phase to an ethylpyridinium polybromide (EPyBr2n+1) droplet based on the CECC mechanism, and the rate of the Br− transfer-water│EPyBr2n+1 is three times higher than that into a MEPBr2n+1 droplet. We believe the discrepancy of the rate of the Br− transfer-water│QBr2n+1 with different Q+ mainly results from the different degree of interaction of Br− with Q+.