Modelling of Supercapacitors: Factors Influencing Performance

The utilizable capacitance of Electrochemical Double Layer Capacitors (EDLCs) is a function of the frequency at which they are operated and this is strongly dependent on the construction and physical parameters of the device. We simulate the dynamic behavior of an EDLC using a spatially resolved model based on the porous electrode theory. The model of Verbrugge and Liu (J. Electrochem. Soc. 152, D79 (2005)) was extended with a dimension describing the transport into the carbon particle pores. Our results show a large influence of the electrode thickness (Le), separator thickness (Ls) and electrolyte conductivity (κ) on the performance of EDLCs. In agreement with experimental data, the time constant was an increasing function of Le and Ls and a decreasing function of κ. The main limitation was found to be on the scale of the whole cell, while transport into the particles became a limiting factor only if the particle size was unrealistically large. The results were generalized into a simplified relation allowing for a quick evaluation of performance for the design of new devices. This work provides an insight into the performance limitation of EDLCs and identifies the critical parameters to consider for both systems engineers and material scientists

A zero dimensional model of lithium-sulfur batteries during charge and discharge

Lithium-sulfur cells present an attractive alternative to Li-ion batteries due to their large energy density, safety, and possible low cost. Their successful commercialisation is dependent on improving their performance, but also on acquiring sufficient understanding of the underlying mechanisms to allow for the development of predictive models for operational cells. To address the latter, we present a zero dimensional model that predicts many observed features in the behaviour of a lithium-sulfur cell during charge and discharge. The model accounts for two electrochemical reactions via the Nernst formulation, power limitations through Butler-Volmer kinetics, and precipitation/dissolution of one species, including nucleation. It is shown that the precipitation/dissolution causes the flat shape of the low voltage plateau, typical of the lithium-sulfur cell discharge. During charge, it is predicted that the dissolution can act as a bottleneck, as for large enough currents smaller amounts dissolve. This results in reduced charge capacity and an earlier onset of the high plateau reaction, such that the two plateaus merge. By including these effects, the model improves on the existing zero dimensional models, while requiring considerably fewer input parameters and computational resources. The model also predicts that, due to precipitation, the customary way of experimentally measuring the open circuit voltage from a low rate discharge might not be suitable for lithium-sulfur. This model can provide the basis for mechanistic studies, identification of dominant effects in a real cell, predictions of operational behaviour under realistic loads, and control algorithms for applications

Valid Asymptotic Expansions for the Maximum Likelihood Estimator of the Parameter of a Stationary, Gaussian, Strongly Dependent Process

We establish the validity of an Edgeworth expansion to the distribution of the maximum likelihood estimator of the parameter of a stationary, Gaussian, strongly dependent process. The result covers ARFIMA type models, including fractional Gaussian noise. The method of proof consists of three main ingredients: (i) verification of a suitably modified version of Durbin's (1980) general conditions for the validity of the Edgeworth expansion to the joint density of the log-likelihood derivatives; (ii) appeal to a simple result of Skovgaard (1986) to obtain from this an Edgeworth expansion for the joint distribution of the log-likelihood derivatives; (iii) appeal to and extension of arguments of Bhattacharya and Ghosh (1978) to accomplish the passage from the result on the log-likelihood derivatives to the result for the maximum likelihood estimators. We develop and make extensive use of a uniform version of Dahlhaus's (1989) Theorem 5.1 on products of Toeplitz matrices; the extension of Dahlhaus's result is of interest in its own right. A small numerical study of the efficacy of the Edgeworth expansion is presented for the case of fractional Gaussian noise.Edgeworth expansions, long memory processes, ARFIMA models

Additive manufacturing for solid oxide cell electrode fabrication

© The Electrochemical Society.Additive manufacturing can potentially offer a highly-defined electrode microstructure, as well as fast and reproducible electrode fabrication. Selective laser sintering is an additive manufacturing technique in which three-dimensional structures are created by bonding subsequent layers of powder using a laser. Although selective laser sintering can be applied to a wide range of materials, including metals and ceramics, the scientific and technical aspects of the manufacturing parameters and their impact on microstructural evolution during the process are not well understood. In the present study, a novel approach for electrode fabrication using selective laser sintering was evaluated by conducting a proof of concept study. A Ni-patterned fuel electrode was laser sintered on an yttria-stabilized zirconia substrate. The optimization process of laser parameters (laser sintering rate and laser power) and the electrochemical results of a full cell with a laser sintered electrode are presented. The challenges and prospects of using selective laser sintering for solid oxide cell fabrication are discussed

Happiness and the Human Development Index : the paradox of Australia

According to the well-being measure known as the U.N. Human Development Index, Australia now ranks 3rd in the world and higher than all other English-speaking nations. This paper questions that assessment. It reviews work on the economics of happiness, considers implications for policymakers, and explores where Australia lies in international subjective well-being rankings. Using new data on approximately 50,000 randomly sampled individuals from 35 nations, the paper shows that Australians have some of the lowest levels of job satisfaction in the world. Moreover, among the sub-sample of English-speaking nations, where a common language should help subjective measures to be reliable, Australia performs poorly on a range of happiness indicators. The paper discusses this paradox. Our purpose is not to reject HDI methods, but rather to argue that much remains to be understood in this area

Probing the band structure of InAs/GaAs quantum dots by capacitance-voltage and photoluminescence spectroscopy

The band structure of self-assembled InAs quantum dots, embedded in a GaAs matrix, is probed with capacitance-voltage spectroscopy and photoluminescence(PL)spectroscopy. The electron energy levels in the quantum dots with respect to the electron ground state of the wetting layer (WL) are determined from the capacitance-voltage measurements with a linear lever arm approximation. In the region where the linear lever arm approximation is not valid anymore (after the charging of the WL), the energetic distance from the electron ground state of the WL to the GaAs conduction band edge can be indirectly inferred from a numerical simulation of the conduction band under different gate voltages. In combination with PL measurements, the complete energy band diagram of the quantum dot sample is extracted

Structures of smooth muscle myosin and heavy meromyosin in the folded, shutdown state

Remodelling of the contractile apparatus within smooth muscle cells is an essential process that allows effective contractile activity over a wide range of cell lengths. The thick filaments may be redistributed via depolymerisation into inactive myosin monomers that have been detected in vitro, in which the long tail has a folded conformation. The structure of this folded molecule has been controversial. Using negative stain electron microscopy of individual folded molecules from turkey gizzard we show they are more compact than previously described, with heads and the three segments of the folded tail closely packed. Smooth muscle heavy meromyosin (HMM), which lacks two-thirds of the tail, closely resembles the equivalent parts of whole myosin. Image processing reveals a characteristic head region morphology for both HMM and myosin whose features are identifiable by comparison with less compact molecules. The two heads associate asymmetrically: the tip of one motor domain touches the base of the other, resembling the blocked and free heads of this HMM when it forms 2-D crystals on lipid. The tail of HMM lies between the heads, contacting the blocked motor domain, unlike in the 2-D crystal. The tail of the intact myosin is bent sharply and consistently at two positions close to residues 1175 and 1535. The first bend position correlates with a skip in the coiled coil sequence, the second does not. The first segment runs between the heads from the head-tail junction. Unexpectedly, the other segments associate only with the blocked head rather than both heads, such that the second bend lies at a specific position near the C-lobe of the blocked head regulatory light chain. Quantitative analysis of tail flexibility shows that the single coiled coil of HMM has an apparent Young’s modulus of about 0.5 GPa. The folded tail of the intact molecule is less flexible indicating interactions between the segments. The folded tail does not modify the compact head arrangement but stabilises it, indicating a structural mechanism for the very low ATPase activity of the folded molecule

The Regeneration Games: Commodities, Gifts and the Economics of London 2012

This paper considers contradictions between two concurrent and tacit conceptions of the Olympic ‘legacy’, setting out one conception that understands the games and their legacies as gifts alongside and as counterpoint to the prevailing discourse, which conceives Olympic assets as commodities. The paper critically examines press and governmental discussion of legacy, in order to locate these in the context of a wider perspective contrasting ‘gift’ and ‘commodity’ Olympics – setting anthropological conceptions of gift-based sociality as a necessary supplement to contractual and dis-embedded socioeconomic organizational assumptions underpinning the commodity Olympics. Costbenefit planning is central to modern city building and mega-event delivery. The paper considers the insufficiency of this approach as the exclusive paradigm within which to frame and manage a dynamic socio-economic and cultural legacy arising from the 2012 games