Nanostructured palladium hydride electrodes: from the potentiometric mode in SECM to the measure of local pH during carbonation

Abstract

The detection of local variations of the proton activity is of interest in many fields such as corrosion, sedimentology, biology and electrochemistry. Using nanostructured palladium microelectrodes Imokawa et al. fabricated for the first time a reliable and miniaturized sensor with high accuracy and reproducibility of the potentiometric-pH response. In absence of oxygen, the nanostructured palladium hydride tips are sensitive only to the activity of the protons close to their local environment and they have an almost Nernstian theoretical response with a slope of -58.7 mV/pH (25C) from pH 2 to 14. In the bulk, the lifetime of the palladium hydride sensor is 60 times longer when the solution is saturated with argon than with oxygen. Besides, the open circuit potential (OCP) recorded during the discharge of the hydride is more positive in an oxygenated solution. To unravel the influence of oxygen on the potentiometric response of these tips, we carried out a series of potentiometric and amperometric scanning electrochemical microscopy (SECM) experiments over a range of tip-substrate distances against an inert substrate. Potentiometric SECM experiments in aerated solutions demonstrate that the duration of the hydrogen discharge and tip potential depend on the tip-substrate distance: the closer the tip is to an inert substrate, the longer the lifetime of the sensor is, and the more cathodic the open circuit potentials are. Linear sweep voltammetry (LSV) near the OCP values reveals that the polarization resistance decreases when the tip approaches the substrate. These trends are confirmed by Tafel plots recorded over a range of tip-substrate distances. Potentiometric and amperometric measurements are found to be in good agreement. These results can be analysed in terms of a mixed potential theory as used in corrosion. They reveal that in the potentiometric mode, despite being held at zero current, the tips promote the reduction of oxygen which in turns leads to the rapid discharge of hydrogen from the palladium hydride. The closer the tip is to the substrate, the smaller is the flux of oxygen, the longer is the duration of the discharge and the more negative is the OCP. This dissertation will therefore show that even in a potentiometric SECM experiment where the tip is supposed to be a passive probe, hindered diffusion can affect the tip potential and produce a dependence on the tip-substrate distance. In aerated solutions, a simple correction can be made to bulk experiments. In this study the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes are reported to demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their properties were validated by detecting pH transients during the carbonation of Ca(OH)2 within a fibrous mesh. Experimental pHs recorded in situ were in excellent agreement with theoretical calculations for the CO2 partial pressures considered. Results also showed that the electrodes were sufficiently sensitive to differentiate between the formation of vaterite and calcite, two polymorphs of CaCO3. These nanostructured microelectrodes are uniquely suited to the determination of pH in highly alkaline solutions, particularly those arising from interfacial reactions at solid and porous surfaces.

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

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