An investigation of electrochemical techniques in acoustic environments

Abstract

A new system for the generation of hydrodynamic modulation voltammetry (HMV) is presented. This system consists of an oscillating jet produced through the mechanical vibration of a large membrane/piston. The structure of the cell is such that a relatively small vibration is transferred to a large (~ 1 m s-1) fluid flow at the jet outlet. High-speed imaging of the system shows vortex behaviour of the flow at the exit of the jet. Positioning of an electrode over the exit of this jet enables the detection of the modulated flow of liquid. The periodic character of the signal recorded at the electrode allows a “lock-in” approach to be employed. This enables discrimination of the background processes signal from the mass transport component. This is demonstrated for Fe(CN)6 3-/4- 3-/4-. Here “lock-in” to the modulated hydrodynamic signal is achieved through the deployment of bespoke software. The apparatus and procedure is shown to produce a simple and efficient way to obtain the desired signal. In addition the spatial variation of the HMV signal, phase correction and time averaged current with respect to the jet orifice is presented. The detection limit for the analysing system is shown to be 45 × 10-9 mol dm-3.
The HMV method is employed to study the reduction of molecular oxygen at high surface area (HI-Pt) modified electrodes. The successful elimination of background signals is achieved for the 0.5 mm diameter nanostructured Pt electrode with roughness factor (RF) of 42.4. Employment of higher roughness factors (>50) HIPt electrodes revealed an anomalous “drop off” effect characterising these electrodes. It is demonstrated that the “drop off” is not caused by the hydrogen peroxide production at the electrode or pH change near the electrode surface. However, a clear dependence of the current deflection on the roughness factor of the electrodes is observed. The shape of the “drop off” followed the shape of the hydrogen adsorption region. It is suggested that the surface characteristics of the electrodes are important in these investigations. It is proposed here, that the capacitance of the electrode influences the HMV signal.
Another type of hydrodynamic modulation method, specifically a vibrating 50 ?m diameter Pt or Au wire or “tight-rope” electrode was studied. High frequency modulation (80 Hz) is employed. FFT data processing was employed to extract the desired signal from the total current. This technique was applied to study reduction of molecular oxygen at the modulated electrodes. A current “drop off” in hydride region was again observed. This is shown to be related to the uncompensated resistance of the cell. In particular, the resistance of the reference electrode is demonstrated to contribute to this effect

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

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