Immobilization of Polyiodide Redox Species in Porous Carbon for Battery-Like Electrodes in Eco-Friendly Hybrid Electrochemical Capacitors

Hybrid electrochemical capacitors have emerged as attractive energy storage option, which perfectly fill the gap between electric double-layer capacitors (EDLCs) and batteries, combining in one device the high power of the former and the high energy of the latter. We show that the charging characteristics of the positive carbon electrode are transformed to behave like a battery operating at nearly constant potential after it is polarized in aqueous iodide electrolyte (1 mol L−1 NaI). Thermogravimetric analysis of the positive carbon electrode confirms the decomposition of iodides trapped inside the carbon pores in a wide temperature range from 190 ◦C to 425 ◦C, while Raman spectra of the positive electrode show characteristic peaks of I3 − and I5 − at 110 and 160 cm−1 , respectively. After entrapment of polyiodides in the carbon pores by polarization in 1 mol L−1 NaI, the positive electrode retains the battery-like behavior in another cell, where it is coupled with a carbon-based negative electrode in aqueous NaNO3 electrolyte without any redox species. This new cell (the iodide-ion capacitor) demonstrates the charging characteristics of a hybrid capacitor with capacitance values comparable to the one using 1 mol L−1 NaI. The constant capacitance profile of the new hybrid cell in aqueous NaNO3 for 5000 galvanostatic charge/discharge cycles at 0.5 A g−1 shows that iodide species are confined to the positive battery-like electrode exhibiting negligible potential decay during self-discharge tests, and their shuttling to the negative electrode is prevented in this syste

An instrument for simultaneous EQCM impedance and SECM measurements

A novel combination of an electrochemical quartz crystal microbalance (EQCM) and a scanning electrochemical microscope (SECM) has been built. Unlike conventional EQCMs, the instrument described here allows rapid in situ measurement of the modulus of the quartz crystal's transfer function. Data analysis in the complex plane for the Butterworth-Van Dyke (BVD) equivalent circuit yields the real and the imaginary components R (damping resistance) and XL (reactive inductance) of the crystal's electroacoustic impedance around its resonant frequency of 10 MHz, The influence of different tip shapes of an approaching microelectrode on the electroacoustic impedance of the quartz crystal was studied and found to be minimal for certain geometries, The capability of the EQCM/SECM instrument was tested in cyclic voltammetric plating/stripping experiments using a copper(I) chloride solution of high concentration in 1 M HCl, Four parameters, XL, R, the substrate, and the tip current, can be recorded simultaneously. Depletion layer effects were observed and could be corrected for to yield accurate current efficiencies for potentiodynamic and potentiostatic copper plating. The amperometric response of the SECM tip positioned closely to the substrate reflects the concentration changes of electroactive ions in the diffusion layer of the substrate electrode

Electrodeposition and properties of nanostructured platinum films studied by quartz crystal impedance measurements at 10 MHz

The electrodeposition of platinum films from aqueous solutions and lyotropic liquid crystalline mixtures of the non-ionic surfactant octaethyleneglycol monohexadecyl ether, water, and hexachloroplatinic acid (HCPA) has been studied by rapid in-situ measurements of the quartz crystal electroacoustic impedance with an electrochemical quartz crystal microbalance (EQCM). For solutions and liquid crystalline template mixtures of high HCPA content large changes of the damping resistance, R have been observed at the initial stages and the end of the plating process as a result of composition changes in the depletion layer. They were corrected forte yield accurate in-situ mass data for the electroplating process. For the measured film thicknesses (< 200 nm) the faradayic efficiency depends inversely on the concentration of HCPA in the aqueous solutions and the template mixtures. This was attributed mainly to migrational transport of the intermediate [PtCl4](2-) anion away from the electrode and to the concentration dependence of the disproportionation/comproportionation reaction in the depletion layer between the Pt(0), Pt(II), and Pt(IV) species involved in the electrode reaction. A 65% increase in specific surface area was measured for the template deposited material compared to Pt deposits from aqueous solutions. This difference can be regarded as a minimum value for the thin films studied and becomes much larger for thicker films. Transmission electron microscopy (TEM) analysis of the former show the films to be mesoporous with a hexagonal nanostructure identical to that produced by template depositions onto non-oscillating substrates. The high specific surface area of the Pt films from lyotropic liquid crystalline templates results in a sensitivity enhancement of the in-situ electroacoustic impedance EQCM technique. It could hence be applied to mono- and sub-monolayer adsorption studies for polycrystalline Pt in sulfuric acid. An increase in damping resistance was found when stepping the potential from the double layer region into the hydrogen adsorption region and attributed to a higher degree of acoustic shear wave coupling to the bulk liquid. It is believed that the adsorbed hydrogen forms hydronium species, which others have observed earlier by IR spectroscopy

The preparation and characterisation of H-1-e palladium films with a regular hexagonal nanostructure formed by electrochemical deposition from lyotropic liquid crystalline phases

The hexagonal (H-1) lyotropic liquid crystalline phases of C16EO8 (octaethyleneglycol monohexadecyl ether) and Brij® 56 non-ionic surfactants have been used to template the electrochemical deposition of nanostructured palladium films. The resulting H-1-e palladium films were characterised by SEM, TEM and X-ray. The films contain regular hexagonal arrays of cylindrical pores separated by palladium walls with a centre to centre distance of 5.8 nm. Electrochemical studies show that these films have very high surface areas of the order of 91 m(2) g(-1). Studies of the hydrogen evolution reaction on these H-1-e palladium films in acid show that the formation of adsorbed hydrogen can be readily distinguished because of the high surface area to volume ratio of the films (of the order of 10(7) cm(2) cm(-3)). Hydrogen insertion into the palladium films is fast and the formation of both the alpha and beta-hydride phases is observed in the voltammetry at potentials which are similar to those reported for bulk palladium. The electrodes are stable towards repeated cycling to form the beta-hydride phase showing that the hydrogen insertion and concomitant lattice expansion does not destroy the H-1 nanostructure