• Energy Research

Abstract The current response in a chronoamperometric experiment on the alkaline manganese dioxide electrode has been modeled as a composite of two individual discharge processes using a spherical grain diffusion model. This analysis has provided us with the capacity and A √ D values for each discharge process, as well as a quantitative means of comparing different EMD samples. This has led to the identification of the domains within the γ-MnO 2 structure in which structural defects such as cation vacancies and Mn(III) ions are most likely to be found.

Abstract A novel method examining the fundamental electrochemical behaviour of carbon is outlined here involving the use of a half cell set-up and solid sacrificial anode. Using this method, electrochemical oxidation of graphite is assessed using selective contamination of a graphite electrode with major coal contaminants identified in selected Australian black coals using X-ray diffraction. Contaminants identified include anatase, alumina, pyrite, quartz, kaolin and montmorillonite. From the systematic introduction of these contaminants it is shown that clay materials, such as kaolin and montmorillonite, act catalytically to increase the rate of graphite oxidation. Metal oxides and sulfides such as anatase, alumina and pyrite give a limited increase in the normalised current, whereas quartz gives a significant decrease in performance. This demonstrates a clear effect of the solid phase interaction of these contaminants on the electrochemical oxidation of graphite since the same effect is not observed when the contaminants are added instead to the molten carbonate electrolyte.

A combination of step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS) has been used to examine the effect of anatase titanium dioxide added to the alkaline manganese dioxide electrode. The inclusion of titanium dioxide was found to improve performance by lowering the resistance to charge transfer at the MnO 2 /electrolyte/graphite interface. Furthermore, a consideration of mass transport processes within the electrode indicates that titanium ions may become incorporated into the γ-MnO 2 lattice upon cycling, impacting H + /e − mass transport.

The electrochemical oxidation of graphite is investigated to determine the relative influence of the composition of a molten carbonate electrolyte on a solid anode. The binary eutectics of sodium, lithium and potassium carbonates were investigated as well as ternary mixtures including the eutectic and combinations of systematically varied lithium content. It was seen that the combination of cations included in the carbonate melt can influence the electrochemical performance of graphite with binary combinations performing better than ternary in all cases. Very little change in the mechanistic behavior was observed through tafel analysis; instead it is proposed that the observed kinetic effects are a combination of the catalytic role of all metal cations present as well as the intercalation of lithium into the solid graphite electrode. The effect of lithium intercalation is suggested to lead to a change in the graphite surface polarity which affects product gas bubble formation and dislodgement at the solid anode. Increased lithium concentration, and it is assumed intercalation, encourages smaller bubbles to form which are dislodged at a faster rate than surfaces with less lithium intercalation in the ternary electrolyte. A threshold lithium concentration is reached above approximately 65 % where the electrochemical behavior is significantly changed and bubble formation behavior is no longer observed.

Abstract The growth, structure and morphology of polyterthiophene films has been characterised using a range of electrochemical and structural techniques. The kinetics of the growth process, including the diffusion coefficient of the terthiophene monomer and the rate constant of the polymerization process has been measured. The electrochemical analysis is consistent with a Stranski–Krastanov growth mode involving the completion of a dense precursor layer prior to the growth of a more porous bulk overlayer. Fabricating polyterthiophene films under diffusion-controlled conditions appears to maximise the thickness of the dense precursor layer. The implication of these results on the photovoltaic energy conversion efficiency of photoelectrochemical cells fabricated from electropolymerized terthiophene is discussed.

An investigation has been conducted to examine the effect that ultrasonication has on the synthesis of manganese dioxide used as a pseudocapacitive electrode material in supercapacitors. Using the reaction between KMnO4 and MnSO4, the layered birnessite polymorph of manganese dioxide was produced with and without ultrasonication (control). X-ray diffraction analysis of the materials indicates that ultrasonication has a minimal effect on the crystallite size, although the crystallite dimensions are small anyway (8.6 vs 14.1 nm within the layers, and 9.4 vs 8.9 nm between the layers for the control and ultrasonicated samples, respectively). However, ultrasonication has had a profound effect on morphology, essentially fusing these small crystallites together to form much larger needles 100–200 nm in length. This fusion process has also dramatically decreased the Brunauer, Emmett, and Teller surface area (from 170 to 10 m2/g with ultrasonication), and furthermore electrochemical performance has also been diminished, although by not as much as the drop in surface area; e.g., 106 vs 64 F/g. These phenomena are discussed in terms of the pore volume available for charge storage.

Matched glassy carbon electrodes in aqueous K2SO4 electrolytes were used to examine the effects of opposing electrode spacing on capacitive performance. Planar non-porous glassy carbon electrodes were used to avoid complications with porosity and roughness. Electrode spacing effects were examined in terms of device and individual electrode performance, using cyclic voltammetry, coupled with its deconvolution into residual, diffusional, and capacitive processes. Decreasing the spacing between electrodes led to a decrease in capacitive contributions, and a relative increase in diffusional and residual contributions, implying that individual electrodes were influencing the behaviour of each other. This is also consistent with the use of more dilute electrolytes. Electrode behaviour was modelled using the Poisson-Boltzmann equation, together with its integrated outputs of electric field and potential difference. For electrodes with the same amount of charge and a similar diffuse layer thickness, the electric field and potential drop was diminished because of their charge interaction. Conversely, it is shown that for a similar potential drop across the electrodes, the variable controlled in a cyclic voltammetry experiment, more charge accumulation is needed at the electrode-electrolyte interface to compensate for the counter charge generate from the opposing electrode.

In this work a structural and porosity analysis of a series of chemically reduced electrolytic manganese dioxide (EMD) samples has been carried out. EMD, or γ-MnO 2 , for use in aqueous alkaline cathodes possesses an orthorhombic unit cell that progressively increases in size as reduction continues. This unit cell expansion is nonuniform, with dimensional changes being different in all three dimensions (by > a o > c 0 ). The unit cell expansion is the direct result of the reduction process, with the discharge products (Mn 3+ and OH - ) being larger than the corresponding ions in the host lattice (Mn 4+ and 0 2- ). Gas adsorption isotherms on the same series of reduced EMD samples demonstrated that the Brunauer-Emmett-Teller surface area decreased considerably over the entire discharge compositional range. The calculated pore size distribution showed that microporosity was eliminated with reduction, but the mesopore volume increased. The origin of these changes in porosity is discussed in terms of the structural expansion, as is their effect on electrochemical performance.

Deconvolution of the electrolytic manganese dioxide (EMD) discharge curve has indicated the presence of a number of energetically different reduction processes. This has been used to determine the contribution of each reduction process to the total discharge. Using step potential electrochemical spectroscopy (SPECS), the i-t data were modelled as the sum of the discharge of the individual reduction processes. From this, A√ D for each reduction process as a function of degree of discharge was determined. The maximum A√ D values for each process ranged from 2.3×10−2 to 4.0×10−4 cm3 s−1/2 g−1 values are consistent with previously reported values for A√ D, although in this case we have determined values for the entire compositional range.