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The extraordinary stability and cycle life performance of today’s electrochemical double-layer capacitors (EDLCs) are generally ascribed to the fact that charge storage in activated carbon (AC) is based on pure double-layer charging. In contrast, Faradaic charge-transfer reactions like those occurring in batteries are often connected with dimensional changes, which can affect the cycle life of these storage devices. Here we report the charge-induced height change of an AC electrode in an aprotic electrolyte solution, 1 mol/l (C2H5)4NBF4 (TEABF4) in acetonitrile. The results are compared with those obtained for a graphite electrode in the same electrolyte. For both electrodes, we observe an expansion/contraction of several percent for a potential window of ±2 V vs. the immersion potential (ip). For the EDLC electrode, significant expansion starts at about 1 V remote from the ip and hence is well within the normal EDLC operation range. For the graphite electrode, the height changes are unambiguously caused by intercalation/deintercalation of both anions and cations. The close analogies between the graphite and the EDLC electrode suggest that ion intercalation or insertion processes might play a major role for charge storage, self discharge, cyclability, and the voltage limitation of EDLCs.

International audience; This paper investigates double layer capacitor (or supercapacitor) thermal modelling. The originality of this work resides in the fact that 4 type K thermocouples are inserted inside a 350 F supercapacitor modified cell. These thermocouples allow supercapacitor internal temperature measurement. Experimental results have shown that the electrical storage device temperature increases in the phase of charge and decreases during the discharge. The developed model takes into account the effect which is due to the reversible heat generation in the supercapacitor. The model is based on heat equation solution and allows temperature distribution determination inside and outside the supercapacitor as a function of the position and of the time. The model is developed for double layer capacitor based on activated carbon and organic electrolyte technology.

Gas evolution and associated pressure build up in supercapacitor devices was monitored using commercial electrode rolls and 1 mol/l solutions of TEABF4 in acetonitrile (AN), propylene carbonate (PC), and γ-butyrolactone (GBL). During subsequent constant voltage periods (24 h) at 2.5 V, 2.75 V, 3.0 V and 3.25 V, significant gas evolution was observed in GBL already at 2.5 V. Under the same conditions, severe gas evolution in PC started only at 3.0 V, while in AN rate of gas evolution remained low even at 3.25 V. Keywords: Electrochemical double layer capacitor, Aprotic solvent, Electrolyte decomposition, Gas evolution

International audience; The main objective of this study is to investigate heat transfer inside and around a supercapacitor stack. The crossflow staggered arrangement with five rows was also studied for different transverse and longitudinal pitches. The mathematical model governing the problem is numerically solved in a three-dimensional computational domain using the commercial computational fluid dynamic package Fluent 6.3. The model is elliptic and takes into account orthotropic thermal conductivity in the active zone. The average Nusselt number and friction factor results of the present model are in good agreement with previously published experimental data of different authors for flow across tube banks. The results include evolution of the maximum temperature of each supercapacitor and contours of temperature distribution. For a definite ventilation power, the arrangement with the lowest spacing provides maximum cooling performance. Inside the supercapacitor the temperature distribution is not concentric around the axis and the maximum temperature is downstream. In the axial direction the temperature variation can be neglected.

Increasing the available cell voltage for electrochemical double layer capacitors (EDLC) is one route to simultaneously increase energy density and power density of the EDLC. Increased cell voltage may, however, introduce faradaic reactions such as ion insertion and electrolyte decomposition, which potentially limit the lifetime of the device. Using a purpose designed pressure cell, we have, for the first time, measured the pressure increase in capacitor cells based on real EDLC electrode coils in 1 M (C 2 H 5 ) 4 NBF 4 /propylene carbonate electrolyte during cycling between 0 and 2.5 V and for constant cell voltages up to 3 V. During cycling a reversible pressure decrease was observed upon charging. An irreversible pressure increase was monitored during load tests at constant cell voltages above 2.5 V. The absolute amount of gases evolved could be determined by means of simultaneous compressibility monitoring.

This article examines whether public-private partnerships are useful for financing federal capital, defined as land improvements and buildings. The point of view is that of a federal agency. Because relatively few public-private partnerships have financed federal capital, this article analyzes the issue through a conceptual approach. It finds that pursuing a public-private partnership to finance federal capital may not necessarily be less costly than the routine federal budgeting approach, but it can allow an agency to meet its capital needs quicker and at no significant additional (equivalent) cost. Faster acquisition would likely lead to a net social benefit to the American people equal to the value of the agency’s additional public services produced with that capital. Consequently, the article concludes that public-private partnerships can be a useful approach for federal agencies to acquire federal capital.

The strain induced by electrochemical charging was studied for a series of carbon materials in two different aprotic electrochemical double layer capacitor electrolytes: 1 M tetraethylammoniumtetrafluoroborate in propylene carbonate and in acetonitrile. Five different carbon samples were investigated: a powder of mesocarbon microbeads (MCMB), a high surface area graphite, an activated carbon (AC)-based proprietary electrode material, an AC black, and an AC fibre cloth. The charge specific expansion was determined by means of electrochemical dilatometry in combination with linear sweep voltammetry. A simple and fast semi-quantitative method is used to estimate the coulombic efficiency and the charge capacity of the different systems. For MCMB distinct intercalation and de-intercalation current peaks and simultaneous height changes are observed for either scan direction with the onset more than 1.5 V remote from zero charge (about 3.0 V vs. Li/Li+). The porous carbons show the typical capacitive charging behaviour within a potential window of at least ±1 V around zero charge. In this regime a small but noticeable height change in the order of 1% is seen with the onset close to zero charge. At more remote potentials a steep expansion and decrease of coulombic efficiency is observed. For all systems, the potential stability window appears to be asymmetric with respect to zero charge. The smaller anodic stability range is likely to be governed by the onset of solvent oxidation. The wider cathodic range is apparently limited by the strong expansion caused by cation insertion.

Electrochemical impedance spectroscopy (EIS) is one of the most important analytical tools for characterization of electrochemical double-layer capacitors (EDLC). As an example, we have characterized a commercial capacitor (BCAP0350 Maxwell Technologies) by EIS, and we will discuss the typical performance of an EDLC compared to an ideal capacitor. EIS was used to determine internal resistance and capacitance of the same capacitor as a function of temperature and as a function of time during constant voltage tests. In addition, the effect of the electrolyte on the temperature behavior was investigated. While the capacitance is a very weak function of temperature, the ESR increases significantly with reduced temperature. Temperature effects are much more pronounced for propylene carbonate (PC) than for acetonitrile (AN)-based electrolytes. From the data obtained at various temperatures and voltages, we could determine acceleration factors for the degradation. On the basis of an Arrhenius plot of the leakage current measured during load life tests at capacitor voltages between 2.5 V and 3.0 V and temperatures between −40 °C and +70 °C, we determined acceleration factors for capacitor degradation of about 2 for a temperature increase of 10 °C and also a factor of about 2 for a potential increase of 0.1 V.

For the first time gas evolution in a double-layer capacitor cell employing activated carbon electrodes and a solution of 1 M (C2H5)4NBF4 in propylene carbonate (PC) has been monitored by means of on-line mass spectrometry (DEMS). During slow scan voltammetry, CO2, propene, and H2 were detected as the major gaseous decomposition products. Probably propene and H2 are formed by solvent reduction at the negative electrode, while CO2 is formed by solvent oxidation at the positive electrode. A small amount of propene is already detected at a cell voltage of below 1 V. For all species the onset of significant gas evolution coincides roughly with the rise of faradaic currents. The increase of coulombic efficiency during subsequent cycling is attributed to passivation processes taking place at both electrodes. Keywords: On-line mass spectrometry, Head space analysis, Electrochemical double layer capacitor, Activated carbon, Propylene carbonate decomposition

This study examined the impact of public-private competitions on the costs of providing maintenance and repair services for administrative buildings at 104 Army installations from 1989 to 1999. The study’s purpose was to determine whether the Army achievedstatistically significant cost savings through competition. The study is significant because it challenges the Army’s policy of allowing its in-house workforce to compete with private contractors to provide commercial-type services as a means of reducing the costs of providing those services, instead of merely outsourcing its work andterminating the employment of the in-house workforce. The study found that the impact of competition depended on the category of the winner of the competition. Contracting out ledto significant cost efficiencies, whereas keeping the work in-house did not. Consequently, the Army policy of allowing the in-house workforce to compete for its work has been only partially effective in achieving intended cost savings.

A new method of supercapacitor online health diagnosis based on an electric model and on experimental results obtained by calendar ageing tests is proposed. The calendar ageing of supercapacitor is mainly related to thermal and voltage constraints from the application. This ageing causes the degradation of supercapacitor performances which can finally lead to failures. In vehicle applications, the goal is to avoid these failures which may have serious consequences and high non-quality costs. The device survey and ageing state detection are a priority for anticipating the problems. This paper presents the case of calendar ageing tests on supercapacitors made of activated carbon and organic electrolyte. The ageing is performed at constant temperature (65 °C) under a 2.9 Vdc bias voltage. These tests permit to highlight online and in real-time the measurable external indicators of ageing.

This paper presents supercapacitor ageing according to the voltage, the temperature and thermal shock tests. To investigate this effect, a test bench of accelerated supercapacitor calendar ageing was carried out. Experimental tests are realized at constant temperature when the supercapacitors are polarized at the maximum voltage. To quantify the supercapacitor ageing, the equivalent series resistance (ESR) and the equivalent capacitance (C) are measured using the DC and AC characterization. To lead to the determination of the supercapacitor lifetime, Arrhenius law, that describes the effect of temperature on the velocity of a chemical reaction, is considered. Finally, experimental results of supercapacitor thermal shock are presented.

Nanostructured carbon films have been grown at room temperature by supersonic cluster beam deposition. Due to a structure based on nanotube embryos and a porosity with grain sizes of a few tens of nanometers, these films have a highly accessible surface area needed for electrochemical applications such as supercapacitors. Films with a density of 1 g/cm3 show, in the dc regime, a specific capacitance per electrode of 75 F/g on a single-cell device with polycarbonate as the organic electrolyte. The resulting energy and power densities of cluster-assembled carbon electrodes are 76 Wh/kg and 506 kW/kg. The possibility of depositing nanostructured films over a large area on a variety of substrates makes cluster-beam deposition very interesting for the realization of supercapacitors.

A new thermal model which allows the temperature distribution determination inside the supercapacitor cell is developed. The model is tested for supercapacitor based on the activated carbon and organic electrolyte technology. In hybrid vehicle applications, supercapacitors are used as energy storage devices offering the possibility to provide peak power requirement. They are charged and discharged at a high current rate. The problematic of this operating mode is the big amount of heat produced in the device which can lead to its destruction. An accurate thermal modelling of the internal temperature is required to design a cooling system for supercapacitors module meeting safety and reliability of the power systems. The purpose of this paper is to study the supercapacitor temperature distribution in steady and transient states. A thermal model is developed; it is based on the finite-differential method which allows the supercapacitor thermal resistance determination. A test bench is realized. The cases of supercapacitor thermal distribution using natural and forced convection are studied. Simulations and experimental results are reported to validate the proposed model. The results obtained with this model may be used to determine the cooling system required for actual supercapacitor applications. The originality of this study is in the fact that a thermocouple is placed inside the supercapacitor.

International audience; A simple electrical model has been established to describe supercapacitor behaviour as a function of frequency, voltage and temperature for hybrid vehicle applications. The electrical model consists of 14 RLC elements, which have been determined from experimental data using electrochemical impedance spectroscopy (EIS) applied on a commercial supercapacitor.The frequency analysis has been extended for the first time to the millihertz range to take into account the leakage current and the charge redistribution on the electrode. Simulation and experimental results of supercapacitor charge and discharge have been compared and analysed. A good correlation between the model and the EIS results has been demonstrated from 1 mHz to 1 kHz, from −20 to 60 °C and from 0 to 2.5 V.

International audience; Reliability is the key issue concerning ultracapacitor applications. In several power applications ultracapacitors are charged and discharged at high current which causes ultracapacitors heating. This paper presents ultracapacitor ageing according to the temperature. To investigate this effect, a test bench of accelerated ageing of ultracapacitor is carried out. Experimental tests are realized at constant temperature when the ultracapacitors are polarized at the maximum voltage. To quantify the ultracapacitor ageing, the equivalent series resistance (ESR) and the equivalent capacitance (Cequi) are measured using Electrochemical Impedance Spectroscopy (EIS). The cause of this ageing is the degradation of the electrolyte and the activated carbon. The electrolyte decomposes during the oxydoreduction phenomena which creates by-products. This degradation leads to an increase of the Equivalent Series Resistance. These by-products deteriorate also the surface of the activated carbon which generates the reduction in the capacitance. After these tests, the ultracapacitor lifetime is obtained versus temperature for the fixed voltage. With the use of the law of Arrhenius the ultracapacitor lifetime is generalized at all the temperatures but always for an extreme voltage which is the nominal voltage of the component. Finally, other tests on various ultracapacitors are made to generalize the lifetime of the ultracapacitor according to the temperature.

This paper presents the supercapacitor characterization for hybrid vehicle applications. This characterization takes into account frequency, voltage and temperature variations. The frequencies dependence is mainly caused by the dynamics of the ions in the electrolyte. With high frequencies, the ions do not have time to reach the surfaces not easily accessible, located deeply in the pores which causes a reduction of the capacitance and equivalent series resistance with the frequency. To take in to account these effects in supercapacitor modeling, it is necessary to characterize them in experiments according to the electric, thermal and frequency constraints.

Located in Fredrick, MD, U.S. Army Garrison Fort Detrick serves 36 tenant organizations whose primary missions are biomedical research and development, military medical logistics, and global telecommunications. The Directorate of Installation Services at Fort Detrick provides operations and maintenance support for the buildings occupied by those tenants and charges its customers for facilities-related support based on the time and materials required. In the recent past, the Resource Manager at Fort Detrick has received numerous customer inquiries regarding the accuracy and relevancy of their facilities-related charges. This study investigated how financial data are captured at Fort Detrick. The report recommends several steps that Fort Detrick can take to improve its ability to effectively track charges for facilities-related work and thus improve customer satisfaction.

Since 1988, the Department of Defense (DoD) has closed 97 major military bases. DoD assists communities with reusing the bases to help with economic recovery. The Office of Economic Adjustment (OEA) coordinates the assistance and reviews the base reuse. OEA periodically reviews the progress in reusing the former military bases. This report summarizes the review of four bases. OEA used three economic and fiscal criteria to measure the progress of the reuse of the bases. The criteria are the level of public and private capital investment, an increase in new jobs and payroll, and fiscal well being of the local development authority. All four bases have converted successfully to civilian use and apparently will achieve their employment goals early. Although land use patterns at the bases are different, their success is based on common elements. The common elements are improved infrastructure, establishment of a quasi-public authority, and adequate financing to implement the reuse plans. The anticipation is that employment growth at the former bases will continue and, ultimately, exceed on-base civilian employment that existed before the base closed.

A method for testing patterned metallized film capacitors for DC applications is presented. Individual breakdowns in the capacitor can be detected by observing the leakage current. Capacitance of the charged capacitor and the amount of energy involved in clearings are obtained.

A novel absolute position optical encoder using two, pseudorandom and periodic, synchronously engraved tracks on a glass slab is presented. The encoded track is read with a GRIN lens and a linear CCD array together with the appropriate signal processing. The spatial coupling of both tracks gives submicrometre resolution when the periodic track is read with the standard Moire technique.

We present cw and pulsed EPR experiments on V4+ in the rutile and anatase phases of TiO2. For the rutile phase, the cw data confirm earlier results, but the relaxation data are different from those previously reported. No earlier results for V4+ in the unreduced pure anatase phase exist. We show that a simple point-charge model can be used to interpret the values of the g tensors, but that models considering only nearest neighbors will give erroneous results. We conclude that the observed V4+ is substitutional in the rutile, but interstitial in the anatase phase. We propose a relaxation mechanism through phonon-modulated hyperfine coupling to explain our T1 data in the rutile phase.

In a crystal of finite size, the frequency of the lowest normal-mode vibration is not arbitrarily close to zero. Well-known theoretical consequences of this gap in the phonon spectrum are an Einstein-like specific heat at extremely low temperatures and a vanishing direct-process electron-spin-lattice relaxation for paramagnetic impurities in small crystals. For the case of MgO we survey calorimetric data from the literature, and present detailed ESR experiments on ${\mathrm{Ni}}^{2+}$ impurities in loose aggregates of small particles, in search of experimental evidence for these effects. For unknown reasons, the result is negative.

Any resonator can be characterised intrinsically by two parameters, the filling factor eta and the quality factor Q. The authors have derived the relations that relate them to the parameters of both CW and pulsed magnetic resonance, i.e. the signal-to-noise ratio and the RF magnetic field amplitude B1.At X-band frequencies, they have compared a 'loop-gap' resonator to a cylindrical TE101 cavity both theoretically and experimentally. It was found that the former had distinct advantages over the latter in pulsed ESR, and retained some of them even in CW ESR. A new coupling configuration is proposed, based on the 'Gordon coupler', to connect the loop-gap to a waveguide. This resonator and coupling structure are well suited for spin-lattice relaxation experiments in solids at low temperatures.

The application of 2,2-dimethoxypropane to the in situ conversion of the free fatty acids of cod oil to methyl esters is successful in reducing the free fatty acid content to the order of 1%. This material, in the presence of a suitable acid catalyst, serves both as a water scavenger and a source of methanol for the esterification. Of the catalysts examined, methanolic HCl gives the best results. Purification procedures giving a stable product are outlined.

Ozonolysis of oleic acid in methanol, a reacting solvent, with subsequent decomposition of the ozonide products by hydrogen peroxide in formic acid, gives yields of principal dicarboxylic acid fission products exceeding 95% with a minimum of secondary acidic products. The method is highly reproducible and offers unique advantages in the total recovery of the dicarboxylic acids and the elimination of peroxidic materials. The principal non-acidic by-products were tentatively identified as the Csalcohols and their formyl esters.

An intrinsic quasi-distributed optical fibre sensor network composed of three intensity-modulated fibre sensors has been assembled to study the sensor response signal multiplexing performances. A frequency-modulated RF continuous wave Ft is used to modulate the intensity of the optical carrier emitted by a laser diode. The performances and operating conditions of two detection schemes, one digital and one analogue, have been presented and discussed. The first method makes use of three narrow parallel digital filters centred on each sensor signal frequency. The authors have obtained a signal-to-noise ratio of 40 dB and a cross-talk level lower than the noise for a signal frequency separation ratio of 1.2. The sensor modulation bandwidth is limited by the data acquisition time and the computing time to about 10 Hz. The analogue detection consists of an analogue filter and a linear phase-locked loop. The synchrone detection delivers a signal-to-noise ratio of 40 dB with an integration time constant of 100 ms. With sensor signals of equal intensity the cross-talk is lower than -40 dB. The sensor modulation bandwidth is limited by the sensor signal frequency separation to about a tenth of that value.

Well-aligned carbon nanotubes films were synthesized by a pyrolytic method with aluminum and silicon as substrates. The substrate was coated with a thin film of Fe(NO3)3. This film was transformed by subsequent heating into Fe2O clusters with a diameter of a few nanometers. Nanotubes were synthesized from acetylene at a temperature between 630°C and 750°C. The nanotubes observed are “multiwall” type with a length in the range of 1–10 μm and a diameter of 5–100 nm. The growth of the nanotubes is a function of the film thickness of deposited Fe(NO3)3 film as well as the temperature. The nanotubes deposited on aluminum exhibit excellent properties as electrode material in electrochemical double layer capacitors (ECDLs).

International audience; A new thermal model, which allows temperature distribution determination inside a supercapacitor cell, is developed. The model is tested for a supercapacitor based on the activated carbon and organic electrolyte technology. In hybrid vehicle applications, supercapacitors are used as energy-storage devices, offering the possibility of providing the peak-power requirement. They are charged and discharged at a high current rate. The problem with this operating mode is the large amount of heat produced in the device which can lead to its destruction. An accurate thermal modeling of the internal temperature is required to design a cooling system for supercapacitor module, meeting the safety and reliability of the power systems. The purpose of this paper is to study the supercapacitor temperature distribution in steady and transient states. A thermal model is developed; it is based on the finite-differential method which allows for the supercapacitor thermal resistance determination. The originality of this paper is in the fact that a thermocouple (type K) was placed inside the supercapacitor from Maxwell Technologies. A test bench is realized. The cases of supercapacitor thermal distribution using natural and forced convection are studied. Simulations and experimental results are reported to validate the proposed model. The results obtained with this model may be used to determine the cooling system required for actual supercapacitor applications.

The influence of impregnating rapeseed oil on the dielectric breakdown behaviour of thin metallized polypropylene (PP) foils used for high power capacitors was investigated. With oil impregnation the dielectric breakdown strength of the PP foil increases strongly. The breakdown behaviour of rapeseed oil impregnated PP foils was compared to silicon oil and pentaerythrit-tetrapelargonate oil impregnated PP foils. Based on these experiments a guideline for the selection of impregnating oil for high power capacitors can be given. The breakdown strength of PP capacitor foils decreases with increasing temperature. In addition to this behaviour, breakdown strength measurements performed as a function of temperature showed discontinuities for dry and impregnated PP foils at defined temperatures. DSC (Differential Scanning Calorimetry) measurements reveal structure modifications in the same temperature regions as the breakdown discontinuities occur. Investigations have been performed to find out if the structural phase transitions can be attributed to pure mechanical relaxations observed in heat treatment experiments.

The carbon nanotubes (CNT) show promising electrochemical characteristics particularly for electrochemical energy storage. The electrochemical double-layer (ECDL) capacitor is a new type of capacitor with features intermediate between those of a battery and a conventional capacitor. ECDL capacitors have been made using various types of CNT and activated carbon (a-C) as electrode material. The specific capacitance per surface area of the electrodes depends on the thickness and the specific surface area of the active material. The CNT electrodes show a specific capacitance from 0.8 and 280 mF cm −2 and 8t o 16 Fc m −3 , respectively. Increasing the mass density also helps to increase the capacitance. Commercially available activated carbon (a-C) electrodes were also tested in order to study their specific capacitance as a function of their physical properties. The various a-C electrodes have specific capacitance per surface area ranging from 0.4 to 3.1 F cm −2 and an average specific capacitance per volume of 40 F cm −3 due to their larger mass density.

The subtitutional doping of 120-A- sized TiO2 Particles with Fe(III)ions has a profound effect on the charge carrier recombination time in this colloidal semiconductor. In undoped particles, the mean lifetime of an electronhole pair is ca. 30 ± 15 ns. Doping with 0.5% Fe(III) drastically augments the charge-carrier lifetime which is extended to minutes or hours. The slow character of the recombination dynamics in Fe(III)-doped colloids was confirmed by laser photolysis using the characteristic optical of electrons in TiO2 to monitor the time course of the reaction. EPR studies showed the Fe(III) ions to enter the host lattice on Ti(IV) sites, charge compensation taking place through the formation of oxygen vacancies. Valence-band holes produced under band-gap excitation react with these centers it the bulk forming Fe(IV), the conduction band electrons being trapped by Ti(IV) at the particle surface. Presumably, the spatial separation of the trapped electron and hole sites inhibits their recombination.

In a previous paper (Gallay and van der Klink 1986, hereafter referred to as GK), the authors developed a simple reasoning for determination of the signal-to-noise ratio in a pulsed ESR spectrometer. For purposes of comparison, they mentioned the signal power in the case of continuous-wave ESR. However, equation concerned with the power in GK is not the power which reaches the detector, but the power which is dissipated in the paramagnetic sample. Expressions for the calculation of the power absorbed by the magnetisation of the sample and the power radiation are derived. It is pointed out that in a continuous wave ESR spectrometer the detected power is quadratic in the intrinsic resonator properties, while in a pulsed ESR spectrometer it is only linear.