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Singapore, Singapore

Maher K.,TUM CREATE | Yazami R.,Nanyang Technological University
Journal of Power Sources | Year: 2014

Lithium ion batteries (LiB) are cycled under a galvanostatic regime (∼C/2-rate) between 2.75 V and 4.2 V for up to 1000 cycles. After each completed 100 cycles, the discharge capacity, capacity loss, average discharge potential were determined under the same C/2 rate. Then cells undergo an additional charge and discharge cycle at C/6 rate followed by a thermodynamics measurements test. This enables open-circuit potential (OCP), entropy (ΔS) and enthalpy (ΔH) data to be assessed. It is found that with increasing cycle number, the entropy and enthalpy profiles show more significant changes than those observed in the discharge and the OCP curves especially at particular SOC and OCP values. These differences are attributed to the higher sensitivity of entropy and enthalpy state functions to changes in the crystal structure of the graphite anode and the lithiated cobalt oxide (LCO) induced by cycle aging compared to the free energy ΔG (OCP) alone. The thermodynamics data are supported by post-mortem X-ray diffractometry (XRD) and Raman scattering (RS) analyses on the electrode materials. The results show important LCO crystal structure degradation, whereas, surprisingly, the graphite anode remains almost unaffected by heavy cycling, if not improved. © 2013 Elsevier B.V. All rights reserved.

Subiantoro A.,TUM CREATE | Ooi K.T.,Nanyang Technological University
International Journal of Refrigeration | Year: 2013

Expanders have been shown to improve the energy efficiencies of refrigeration systems. The current technology is also adequate to manufacture and integrate expanders to practical air-conditioners. In this paper, an economic analysis of the installation of expanders on to existing vapor compression cooling systems, particularly medium scale air-conditioners, is presented. Various refrigerants, including the established and the newly proposed varieties, are considered. From the investigations, it was found that when the expander efficiency is 50%, the payback periods of most conventional systems are below 3 years in high temperature countries with high electricity tariffs and are above 5 years in other countries. Expanders are especially attractive for the transcritical CO2 and the R404A systems. The payback periods are shorter for systems with highly efficient expanders, high cooling loads, high ambient temperatures and for low refrigerating temperature applications. © 2013 Elsevier Ltd and IIR. All rights reserved.

Mason C.W.,TUM CREATE | Gocheva I.,TUM CREATE | Hoster H.E.,TUM CREATE | Yu D.Y.W.,TUM CREATE | And 2 more authors.
Chemical Communications | Year: 2014

Iron(III) sulfate, a rhombohedral NASICON compound, has been demonstrated as a sodium intercalation host. This cost-effective material is attractive, as it can be slurry processed in bulk with ball-milling, while utilizing the iron 2+/3+ redox couple, offering stable 3.2 V performance for over 400 cycles. © The Royal Society of Chemistry 2014.

Riemann R.,TUM CREATE | Wang D.Z.W.,Nanyang Technological University | Busch F.,TU Munich
Transportation Research Part C: Emerging Technologies | Year: 2015

In this study, the optimal locations of a specific type of charging facilities for electric vehicles (EVs), wireless power transfer facilities, are investigated. A mathematical model has been developed to address this problem. The objective of the model is to locate a given number of wireless charging facilities for EVs out of a set of candidate facility locations for capturing the maximum traffic flow on a network. The interaction between traffic flow patterns and the location of the charging facilities is incorporated explicitly by applying the stochastic user equilibrium principle to describe electric vehicle drivers' routing choice behavior. Firstly, the problem is formulated into a mixed-integer nonlinear program, secondly a solution method is developed to obtain the global optimal solution of the linearized program. Numerical experiments are presented to demonstrate the model validity. © 2015 Elsevier Ltd.

Zhu J.,Rice University | Cao L.,Rice University | Cao L.,Chongqing University | Wu Y.,Rice University | And 10 more authors.
Nano Letters | Year: 2013

Various two-dimensional (2D) materials have recently attracted great attention owing to their unique properties and wide application potential in electronics, catalysis, energy storage, and conversion. However, large-scale production of ultrathin sheets and functional nanosheets remains a scientific and engineering challenge. Here we demonstrate an efficient approach for large-scale production of V2O5 nanosheets having a thickness of 4 nm and utilization as building blocks for constructing 3D architectures via a freeze-drying process. The resulting highly flexible V 2O5 structures possess a surface area of 133 m2 g-1, ultrathin walls, and multilevel pores. Such unique features are favorable for providing easy access of the electrolyte to the structure when they are used as a supercapacitor electrode, and they also provide a large electroactive surface that advantageous in energy storage applications. As a consequence, a high specific capacitance of 451 F g-1 is achieved in a neutral aqueous Na2SO4 electrolyte as the 3D architectures are utilized for energy storage. Remarkably, the capacitance retention after 4000 cycles is more than 90%, and the energy density is up to 107 W·h·kg-1 at a high power density of 9.4 kW kg -1. © 2013 American Chemical Society.

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