Investigations on three kinds of interfaces with SECM and impact electrochemistry: Experiments and simulations

Abstract

This work utilizes scanning electrochemical microscopy (SECM) to investigate three different systems. The first is the liquid-liquid interface between a saturated FcMeOH (ferrocenemethanol) in decane solution and a saturated FcMeOH in aqueous electrolyte solution. Current approach curves were recorded and simulated to calculate the diffusion coefficient of FcMeOH in the decane phase. Unlike normal electrochemical experiments, where experiments are performed within a local phase to extract information, this SECM experiment was performed in the electrolyte next to the decane which is not ion-conductive, to extract information therein. By comparing with simulation results, the diffusion coefficient of FcMeOH in the decane phase was found to be around $3\times10^{-10}\;\mathrm{m^2\;s^{-1}}$. Also, since the interfaces between these two solutions are curved, which is common in many reported experimental situations, the influence of misalignment between the electrode and the centre of the decane drop was studied through simulation. The misalignment was found to be significant within a short distance, invisible to the naked eye. Impedance approach curves were recorded and simulated, but deviations between the experimental and simulated results were observed. These discrepancies are believed to be caused by the unique nature of FcMeOH oxidation, which transforms a neutral molecule into a cation. Further experiments and simulations are required to verify this interpretation.

The second system is the liquid-gas interface between an air bubble and an aerated electrolyte. The current approach curve for the oxygen reduction reaction (ORR) was investigated. The experimental approach curves were found to be in very good agreement with the simulated ones. Also, the "noise" observed on the chronoamperometric data was carefully analysed. A "noise" circa 20 Hz was found, which matches the signal recorded by an accelerometer from the vibration of the building. Such noise has been widely and constantly suffered by most of the lab users. It was found that sensitivity to the "noise" increased as the electrode approaches the liquid-gas interface. Moreover, during the experiment, the ORR underwent a complex process and produced hydrogen peroxide. It was found by cyclic voltammogram (CV) that when the region between the electrode and the bubble is small, hydrogen peroxide remain trapped in between, showing an increasing peak at around 0.16 V \vs SMSE (saturated mercury/mercury sulfate electrode) as the electrode approaches the bubble.

The third system is the particle impact electrochemistry, where impedance information was recorded while particles were fired at the surface of a disc electrode in the electrolyte. By comparing the impedance features with the shapes of particles recorded by the high-speed camera, it has been concluded that the approach of large and flat particles leads to a larger change in the uncompensated resistance and apparent capacitance, which was extracted from the impedance recorded by the electrode. Conversely, the approach of small and pointy particles results in a smaller change. The interpretation of such a relation can be subjective since only the front views of the particles were recorded and analysed. More impedance simulations were implemented with conical and ellipsoidal particles with different degrees of sharpness The former represents particles with a continuous end and the latter a singularity end. The simulation results agree well with the previous conclusion. Similar simulations were performed with a cube particle with different rotations, and the approach curves confirmed the conclusion once again. The particle fly-over simulation offers a novel perspective to predict the orientations of cube particles from their characteristic curves.

Additionally, a way of decoupling concentration and diffusion coefficient was developed from chronoamperometric data at two different sizes of electrodes. Organic synthesis of FcMeOH was performed as well as relevant characterisations. Electrode manufacturing methods were introduced with some 3D-printed gadgets to centre the metal core. A Python program connecting the potentiostat and the micropositioner is illustrated to automate the approach curve experiments in the Appendix.

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Identifiers

ORCID for Jiasheng Yi: orcid.org/0000-0002-8854-7849

ORCID for Guy Denuault: orcid.org/0000-0002-8630-9492

ORCID for Peter Birkin: orcid.org/0000-0002-6656-4074

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