Journal of Applied Electrochemistry

Cells with cupric oxalate cathodes bonded with polystyrene and magnesium alloy AZ61 anodes were discharged at room temperature. The cathode current efficiency was approximately 75% to a 0.8 V cut-off at an apparent current density of 10 mA cm−2. Cells with sulphur-cupric oxalate cathodes showed discharge performance at higher voltages with higher efficiences than those without sulphur. The use of different oxalate samples appeared to result in no significant difference in performance. The samples were either obtained commercially or prepared by precipitation. The cathodic behaviour of cupric oxalate was also investigated by potential sweep voltammetry with a slow sweep rate. In sodium chloride solutions a double peak was obtained on the voltammogram. When the stoichiometric quantity of sulphur was added to the cupric oxalate active material, it gave a main and two smaller peaks. The possible reactions are discussed.

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 μm thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot filament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na2SO4 + 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] solution at rotation frequencies 0 < f rot < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.

The impedance characteristics of undischarged alkaline manganese cells of various sizes (LR20, LR6 and LR03) have been investigated over the frequency range 100 kHz to 1 mHz. Specially constructed reference electrode probes have been inserted into the cells in two basic positions such as to obtain a complete analysis of the contribution to the total impedance from the individual cell components. It is shown that the impedance of standard cells is determined by the cathode-can assembly, the anode-separator impedance being negligible by comparison. The cathode-can impedance is further resolved into two components. Firstly, a component due to a nickel oxide layer present on the surface of the nickel-plated steel can. This takes the form of a high frequency semicircle in the complex plane. Secondly, a component due to the cathode itself (manganese dioxide + graphite mixture) which takes the form of a higher frequency semicircle and a low-frequency straight line of slope ∼32°. The latter is interpreted in terms of porous electrode behaviour involving slow proton diffusion in the solid state. Some evidence for a porous-planar transition has also been obtained at a critical frequency of ∼3 mHz.

The electrochemical behaviour of cyclic polysilanes, namely, hexamesitylcyclotrisilane (Mes2Si)3 (I), 1,2,3,4-tetra-tert-butyltetramethyl-cyclotetrasilane [t-Bu(Me)Si]4 (II), decaethylcyclopentasilane (Et2Si)5 (III), decapropylcyclopentasilane (Pr2Si)5 (IV), dodecamethylcyclohexasilane (Me2Si)6 (V), tetradecaethylcycloheptasilane (Et2Si)7 (VI), hexadecamethylcyclooctasilane (Me2Si)8 (VII) and octadecamethylcyclononasilane (Me2Si)9 (VIII), have been studied by cyclic voltammetry and preparative scale electrolysis. The effects of electrolyte, ring size and nature of substitutes both on oxidation peak potential and upon reactivity as well as style of product are discussed.

The synthesis of propylene oxide by electrolysis of dilute sodium bromide solution with propylene gas was investigated in an electrochemical ‘flow-by’ bipolar reactor consisting of six parallel fixed beds of graphite particles separated by polypropylene felt diaphragms. The reactor was operated in a single pass mode with a two-phase co-current flow of propylene and sodium bromide solution through the beds of graphite particles. The maximum pressure in the system was 2.2 atm absolute. The effects of superficial current density (0.4–2.7 kAm−2), sodium bromide concentration (0.2 and 0.5 M), electrolyte load (4.4–13.2 kg m−2s−1), propylene gas load (0.08–1.66 kg m−2s−1), reactor outlet temperature (30 and 60°C), bed thickness (5–15 mm) and different carbon types (Union Carbide and Ultra Carbon) on the space-time yield and selectivity for propylene oxide were measured. Depending on conditions, the space-time yield for propylene oxide was between 5.5 and 97 kg m−3h−1, and the selectivity was between 55 and 87%. The current efficiency and the specific energy consumption varied from 14 to 58% and from 6 to 60 kWh kg−1 of propylene oxide. The space-time yield for propylene oxide increased with increasing current density, increasing gas flow and decreasing bed thickness. The current efficiencies for hydrogen, oxygen, dibromopropane, hypobromite, bromite and bromate were also determined.

Thermally prepared ternary Ir–Pt–Au electrodes were investigated experimentally over their entire compositional range using the mixture design method. The results, involving voltammetric charges from electrode redox (q*), oxygen reduction current densities (i), and their correspondingly electrocatalytic activities (i/q*), are examined through regression models and response surface contour plots. Using mixture experimental design, the empirical models are fitted and plotted as contour diagrams which facilitate comparisons with experimental trends noted by other investigators and reveal the synergistic/antagonistic effects between the mixed oxides. The material characterization was performed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) to assist in interpreting the electrocatalytic activity for oxygen reduction.

This research article presents an overview of the hydrothermal synthesis of nanostructured tungsten oxide (WO3) and its electrochromic (EC) performance. A remarkable evolution in the past few years of producing pure and fine WO3 nanostructures using mild hydrothermal synthesis has received great attention. The hydrothermal process is highly suited for producing monodispersed nanoparticles with control over size and morphology, low processing temperature, and easy synthesis. In this article, we developed a facile seed layer-free hydrothermal approach for preparing WO3 thin films with improved EC performance. Structural and morphological properties were studied using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The electrochemical stability of the propylene glycol-assisted nanostructured WO3 film was examined in lithium per chlorate-propylene carbonate (LiClO4-PC) electrolyte for prolonged color/bleach cycles. The results showed an improvement in electrochemical stability with fast response time.

The kinetics of the electrodeposition and electrocrystallisation of titanium were studied in alkali chloride melts. Voltammograms showed two distinct anodic peaks for the oxidation steps Ti/Ti(II) and Ti(II)/Ti(III) at 70 mV and 300 mV, respectively, referred to titanium. The cathodic reduction Ti(III)/Ti(II) was very irreversible, showing an extended cathodic wave and a shoulder on the Ti(II)/Ti reduction peak. The Ti(II)/Ti reduction was found to be quasi reversible. The rate constant at 450'C was ∼ 5 × 10−3 cm s−1 . The diffusion coefficient in KCl-LiCl eutectic was ∼ 1 × 10−5 cm2s−1 at 450° C rising to 3.5 × 10−5 cm2s−1 at 650° C. Potential step measurements gave curves indicating slow instantaneous nucleation and growth (I against t 1/2) followed by slow diffusion control (I against t −1/2). During constant current steady state deposition the nature of the deposit depended on the composition of the melt, the temperature and the cd. In KCl-LiCl melts coherent deposits were obtained for cds of 60–120 mA cm2 while higher current densities gave dendritic and spongy deposits.