In situ monitoring of pH changes in concrete and lime mortars

This paper describes how a new type of solid state microelectrode based on nanostructured palladium-hydride (PdH) can be used to monitor pH variations during carbonation of calcium hydroxide (Ca(OH)2). The experiments described were carried out under conditions similar to those typical of common porous substrates such as cement and lime mortars. In these tests PdH microelectrodes were employed to record pH transients during the carbonation within a humid fibrous mesh that simulated the porous substrate. To control the carbonation rate, experiments were carried out at different carbon dioxide partial pressures (pCO2) and in each case the experimental results were compared to theoretical calculations obtained using PHREEQC (pH-REdox-EQuilibrium) software. Scanning electron microscopy (SEM) observations and X-ray diffraction (XRD) analysis of the calcium carbonate (CaCO3) crystals deposited on the fibrous mesh were used to evaluate characteristics of the solid phases precipitated during the reaction. Results demonstrate that these pH microsensors can operate reliably in very alkaline environments (pH>12) such as those produced by the dissolution of Ca(OH)2

Production of Graphene Stably Dispersible in Ethanol by Microwave Reaction

Graphene is a 2D carbon material with peculiar features such as high electrical conductivity, high thermal conductivity, mechanical stability, and a high ratio between surface and thickness. Applications are continuously growing, and the possibility of dispersing graphene in low-boiling green solvents could reduce its global environmental impact. Pristine graphene can be dispersed in high concentration only in polar aprotic solvents that usually have high boiling points and high toxicity. For this reason, the oxidized form of graphene is always used, as it is easier to disperse and to subsequently reduce to reduced graphene oxide. However, compared to pristine graphene, reduced graphene oxide has more defects and has inferior properties respect to graphene. In this work, the polymerization of (diethyl maleate derivate) on graphene obtained by sonication was performed in a microwave reactor. The obtained material has good stability in ethanol even after a long period of time, therefore, it can be used to deposit graphene by mass production of inks or by casting and easy removal of the solvent. The thermal annealing by heating at 300–400 ◦C in inert atmosphere allows the removal of the polymer to obtain pristine graphene with a low number of defects

A Facile and Green Synthesis of a MoO2-Reduced Graphene Oxide Aerogel for Energy Storage Devices

A simple, low cost, and "green" method of hydrothermal synthesis, based on the addition of l-ascorbic acid (l-AA) as a reducing agent, is presented in order to obtain reduced graphene oxide (rGO) and hybrid rGO-MoO2 aerogels for the fabrication of supercapacitors. The resulting high degree of chemical reduction of graphene oxide (GO), confirmed by X-Ray Photoelectron Spectroscopy (XPS) analysis, is shown to produce a better electrical double layer (EDL) capacitance, as shown by cyclic voltammetric (CV) measurements. Moreover, a good reduction yield of the carbonaceous 3D-scaffold seems to be achievable even when the precursor of molybdenum oxide is added to the pristine slurry in order to get the hybrid rGO-MoO2 compound. The pseudocapacitance contribution from the resulting embedded MoO2 microstructures, was then studied by means of CV and electrochemical impedance spectroscopy (EIS). The oxidation state of the molybdenum in the MoO2 particles embedded in the rGO aerogel was deeply studied by means of XPS analysis and valuable information on the electrochemical behavior, according to the involved redox reactions, was obtained. Finally, the increased stability of the aerogels prepared with l-AA, after charge-discharge cycling, was demonstrated and confirmed by means of Field Emission Scanning Electron Microscopy (FESEM) characterization

Decoration of laser induced graphene with MXene and manganese oxide for fabrication of a hybrid supercapacitor

During the last years, Internet of Things has become a prominent topic of technical, social, and economic importance. One of the main consequences is the high demand for energy and power density from small energy storage devices. In this field the laser induced graphene (LIG) has become a promising material to produce flexible micro-supercapacitors. The issue with this material is that the performances are strongly restrained by its limited surface area and the relatively low conductivity. In this work we improve the performance of a LIG supercapacitor by decorating its surface through electrophoresis: one electrode will be decorated with metal nitrides and metal carbides (MXenes), the other with manganese oxide. These two materials have appreciable conductivity and pseudocapacitance. Electrochemical measurements have been carried out on the two electrodes separately. After a charge balancing, the device has been sealed in pouch and tested

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

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

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

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

Nanostructured Pd hydride microelectrodes: in situ monitoring of pH variations in a porous medium

In this study we report the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes and demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their potentiometric response was found to be reproducible and stable over several hours but primarily Nernstian over a remarkably wide pH range, including alkaline conditions up to pH 14. Continuous operation was demonstrated by reloading hydrogen at regular intervals to maintain the correct hydride composition thereby alleviating the need for calibration. These 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 and are thus highly appropriate as pH sensing tips in scanning electrochemical microscopy