Hydrodynamics and Mass Transport in Wall Tube and Microjet Electrodes. Simulation and Experiment for Micrometer-Scale Electrodes

Abstract

Mass transport to micrometer-sized electrodes in a microjet (wall-tube) electrode configuration is examined experimentally and through finite element modeling. Electrochemical imaging experiments reveal that local mass transport is highly sensitive to the lateral position of the nozzle with respect to the electrode. When these two components are arranged coaxially, there is a pronounced minimum in the mass transfer rate to the electrode, as determined from transport-limited current measurements. Small lateral displacements of the nozzle from the coaxial position lead first to an increase in mass transport, with the current reaching a maximum at a displacement of around one nozzle radius (50 μm). For larger lateral displacements of the nozzle from the coaxial position, the limiting current gradually decreases with increasing distance. The implications of these observations for practical applications of the microjet electrode are considered. Voltammetric measurements on the oxidation of IrCl6 3- in aqueous solution, with the electrode and nozzle coaxial are shown to be in good agreement with simulation of mass transport. Increasing the solution viscosity dramatically decreases mass transport to the electrode, with the reduction in the diffusion coefficient of the redox species as the major factor.

The role of adsorbed hydroxyl species in the electrocatalytic carbon monoxide oxidation reaction on platinum

“…This would then mean that the formation of OH ads on polycrystalline platinum in sulphuric acid, although fast and reversible at the initial adsorption sites must undergo a relatively slow diffusion process away from that initial site. 90 There are supporting arguments in the literature for this. Davies et al 63 studied the oxidation of CO ads on ruthenium modified Pt(111) and suggested that the second peak in the CO ads oxidation transients was associated with the spill-over and diffusion of the oxidant produced at the ruthenium sites via a surface Grotthus-like mechanism.…”

“…It is perhaps because of this underlying assumption that the analysis of Pt oxide growth has often been limited to potentials above 0.8 V [18][19][20][21][22][23] . In the paper by Conway et al 22 the typical two anodic peaks and shoulder in the Pt i-V profile 90 are considered as occurring before a monolayer of hydroxyl has been formed, even at a single crystal surface, and based on this assumption they showed that the hydroxyl monolayer became irreversibly adsorbed above about 0.95 V, with a coverage of approximately 0.25, which was well below the 95 monolayer limit of one hydroxyl per Pt, which occurred at 1.1 V. However, it is unlikely that the formation of an adsorbed layer of hydroxyl could be anything other than reversible unless it results in a reorganisation of the Pt surface with migration of hydroxyl into the bulk, as suggested by Conway 100 et al (see Fig. 21.…”

“…The species involved in the reaction with 85 CO ads is generally regarded to be OH ads formed by the dissociation of adsorbed water 1, 6, 15-17, 29, 35, 48-50 and L-H kinetics are often assumed 2, 12, 51-55 . An interesting alternative theory has been proposed by Markovic et al 11 who explained the early onset of CO ads 90 oxidation for adlayers prepared below 150 mV in terms of a weakly adsorbed adlayer (different from individual weakly adsorbed CO ads species). They suggest that the more compressed CO ads adlayers are less strongly adsorbed because of increased lateral repulsive interactions, and that this 95 reduced adsorption enthalpy enables some OH ads to be nucleated at potentials above 150 mV oxidising some CO ads .…”

“…It also provides the minimum current density seen before the CO oxidation 85 peak. It was not possible to accurately resolve the peak position or current density at oxidation potentials above 850 mV, or minimum current density above 800 mV due to extremely rapid oxidation compared to the resolution of the experiment, or at oxidation potentials below 600 mV where 90 both the timescales of the experiments were not sufficient and the noise was proportionally too great. For the potential range 600-850 mV both the oxidation peak time and the oxidation peak current density show an exponential response with potential.…”

“…Upon polarisation to higher potentials, oxidative removal of the adsorbed hydrogen forms many small CO free regions 90 distributed throughout the CO ads adlayer. These CO free regions then allow the adsorption and/or formation of oxygen containing species, which react with the CO ads to form CO 2 .…”