Peter J. Hall

Energy storage in electrochemical capacitors: designing functional materials to improve performance

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomass-derived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.

Optimizing for Efficiency or Battery Life in a Battery/Supercapacitor Electric Vehicle

A novel energy control strategy for a battery/supercapacitor vehicle, which is designed to be tunable to achieve different goals, is described. Two possible goals for adding a pack of supercapacitors are examined for a test vehicle using lead-acid batteries: 1) improving the vehicles efficiency and range and 2) reducing the peak currents in the battery pack to increase battery life. The benefits of hybridization are compared with those achievable by increasing the size of the battery pack by a comparable mass to the supercapacitors. The availability of energy from regenerative braking and the characteristics of the supercapacitors are considered as impact factors. Supercapacitors were found to be effective at reducing peak battery currents; however, the benefits to range extension were found to be limited. A battery life extension of at least 50% is necessary to make supercapacitors cost effective for the test vehicle at current prices.

Effect of activated carbon xerogel pore size on the capacitance performance of ionic liquid electrolytes

The use of ionic liquid (IL) electrolytes promises to improve the energy density of electrochemical capacitors (ECs) by allowing for operation at higher voltages. Several studies have also shown that the pore size distribution of materials used to produce electrodes is an important factor in determining EC performance. In this research the capacitative, energy and power performance of ILs 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), 1-ethyl-3-methylimidazolium dicyanamide (EMImN(CN)2), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide (DMPImTFSI), and 1-butyl-3-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMPyT(F5Et)PF3) were studied and compared with the commercially utilised organic electrolyte 1 M tetraethylammonium tetrafluoroborate solution in anhydrous propylene carbonate (Et4NBF4–PC 1 M). To assess the effect of pore size on IL performance, controlled porosity carbons were produced from phenolic resins activated in CO2. The carbon samples were characterised by nitrogen adsorption–desorption at 77 K and the relevant electrochemical behaviour was characterised by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. The best capacitance performance was obtained for the activated carbon xerogel with average pore diameter 3.5 nm, whereas the optimum rate performance was obtained for the activated carbon xerogel with average pore diameter 6 nm. When combined in an EC with IL electrolyte EMImBF4 a specific capacitance of 210 F g−1 was obtained for activated carbon sample with average pore diameter 3.5 nm at an operating voltage of 3 V. The activated carbon sample with average pore diameter 6 nm allowed for maximum capacitance retention of approximately 70% at 64 mA cm−2.

Solvent induced swelling of coals to study macromolecular structure

Abstract The swelling properties of a rank range of eight coals were studied using two series of solvents, in terms of electron donor number and pK b values. It was found that swelling properties could be correlated well with pK b values, the electron donor number values were not related to coal behaviour. The swelling data are consistent with the conclusion that the mechanism of swelling of the macromolecular structure of coals in basic solvents is one of breakage of hydrogen bonds.

The copyrolysis of poly(vinylchloride) with cellulose derived materials as a model for municipal waste derived chars

The pyrolysis and gasification of poly(vinylchloride) (PVC) with wood and straw is considered as a simple model for municipal wastes of different composition. It is shown that during pyrolysis of mixtures of PVC with straw that char yields are greater than produced by pyrolysis of the individual components. This was believed to be caused by some interaction of HCl with the cellulose below 600 K. The reactivities of chars produced from mixtures of straw with PVC are significantly lower than the reactivities expected if the individual components were pyrolysed separately. It is argued that the presence of chrlorinated polymers in municipal waste may tend to increase char yields during pyrolysis and reduce the reactivity of the resulting chars.

An Improved Lead–Acid Battery Pack Model for Use in Power Simulations of Electric Vehicles

A new model for a lead-acid battery pack is proposed for use in power simulations of electric vehicles. A linear approximation using a constant voltage drop has been used to model the charge-transfer resistance of the battery pack, and an exponential voltage-recovery equation has been used to model the transient capacitance effects following a period of discharge. The new model is easy to implement with simple calculations and easily acquired parameters, combining speed of implementation with accuracy. The new model was found to have a peak error of 3.1% in drive cycle tests, thus comparing favorably to existing models of similar complexity. An initial assessment of the models suitability for use with a lithium-ion battery pack was also performed, finding a peak error of 5%.

Thermal analysis investigation of hydriding properties of nanocrystalline Mg–Ni- and Mg–Fe-based alloys prepared by high-energy ball milling

The hydrogen loading characteristics of nanocrystalline Mg, Mg–Ni (Ni from 0.1 to 10 at.%), and Mg–Fe (Fe from 1 to 10 at.%) alloys in 3 MPa H2 were examined using high pressure differential scanning calorimetry and thermogravimetric analysis. All samples showed rapid uptake of hydrogen. A decrease in the onset temperature for hydrogen absorption was observed with increasing Ni and Fe alloy content, but the thermal signatures obtained suggested that only Mg was involved in the hydriding reaction; i.e., no clear evidence was found for the intermetallic hydrides Mg2NiH4 and Mg2FeH6. Hydrogen loading capacity decreased with temperature cycling, and this was attributed to a sintering process in the alloy, leading to a reduction in the specific surface available for hydrogen absorption.

A thermal analysis investigation of the hydriding properties of nanocrystalline Mg-Ni based alloys prepared by high energy ball milling

A thermal analysis study of the hydrogen loading characteristics of nanocrystalline Mg-Ni alloys (Ni content ranging from 0.1 at% to 10 at%) has been carried out in 3 MPa hydrogen, employing the techniques of differential scanning calorimetry and thermogravimetric analysis (TGA). The measurements confirmed the nonequilibrium state of the samples as prepared by the mechanical alloying technique. An enthalpy associated with the stabilisation of the alloys on first heating in hydrogen was found for all the samples studied. The magnitude of this enthalpy increased with the nickel content of the alloy. All the samples showed rapid uptake of hydrogen at 3 MPa pressure, indicating that the nickel was thus playing a very active role at the alloy surface in dissociating hydrogen and so enabling more rapid hydride formation by the alloy. This catalytic activity of the nickel decreased with temperature cycling over the range 80°C to 500°C. Although TGA analysis, carried out at the end of the cycling period, gave the hydrogen content as 1.1 wt% to 1.7 wt% for the alloys, this is well short of the theoretical amounts expected (7.6 wt% for MgH2), indicating that the samples had become deactivated during cycling. No evidence was found of the intermetallic Mg,Ni prior to or after hydriding.

The effect of novel processing on hydrogen uptake in FeTi- and magnesium-based alloys

This paper discusses the production and initial evaluation of hydrogen storage alloys produced by physical vapour deposition (PVD) and mechanical alloying (MA). PVD is usually associated with the production of thin films and coatings. However, DERA Farnborough have developed a high rate vapour condensation process to produce bulk deposits, in some cases up to 44 mm thick. Vapour condensation using electron beam evaporation produces the ultimate in cooling rates with extended solid solubility and refinement of microstructure, which produce enhanced physical and mechanical properties. MA is a complimentary technique for processing hydrogen storage materials which has been developed within DERA over the past 3 years. These techniques have been applied to Mg and FeTi alloy systems and it is shown that both methods greatly enhance the amount of hydrogen uptake and the ease of activation.

Applications of Energetic Distributions of Oxygen Surface Complexes to Carbon and Char Reactivity and Characterization

Recently, in our laboratory, we have been concerned with the determination of probability density functions of desorption activation energies of oxygen surface complexes from oxidized carbons and chars via temperature programmed desorption/mass spectrometry (TPD/MS), and their potential applications. Here we present our results on: (1) identification of controlling phenomena and deconvolution and interpretation of resultant TPD spectra; (2) a technique to transform desorption rate data into energetic distributions, and their application to carbon and char diagnostics and characterization; and (3) application of energetic distributions to the a priori prediction of char reactivity.