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

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

Opportunities for disruptive advances through engineering for next generation energy storage

Throughout human history, major economic disruption has been due to technological breakthroughs. Since 1990 the energy density of lithium-ion cells has increased by a factor of four and the cost has dropped by a factor of 10. This has caused disruption to the energy industry, but advances are slowing. The manufacturing and supply chain complexity means that the next big technology will take 15 years to dominate. The academic literature charts this process of development and can be used to show what is in the pipeline. Three candidates that have had a large increase in publication count are: lithium sulphur, solid-state, and sodium-ion technology. From the level of investments in start-ups and academic publication counts, solid‑state cells are closest to maturity. To identify disruption potential, look at uncertainty in performance. Cell lifetime in lithium-ion cells indicates room for improvement. Define a new disruption metric: . Look for areas of industry that lower this metric. Thermal management is a lucrative area for improvement. Cooling the cell tabs of a 5Ah cell reduces the lifetime cost by 66%, compared to 8%/pa for 13 years relying on cost reduction. Second life applications lower the lifetime cost by using the remaining 75% of energy throughput available in a cell after use in an electric vehicle. Drop-in changes to standard manufacturing processes enable huge disruption. Electrolyte additives can increase cell life by 10 times, lowering lifetime cost by 90% in a simple manufacturing intervention

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

What Limits the Rate Capability of Li-S Batteries during Discharge: Charge Transfer or Mass Transfer?

Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li2S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li2S precipitation