High temperature supercapacitors

The scientific objective of this research program was to determine the feasibility of manufacturing an ionic liquid-based supercapacitor that could operate at temperatures up to 220 °C. A secondary objective was to determine the compatibility of ionic liquids with other cell components (e.g. current collectors) at high temperature and, if required, consider means of mitigating any problems. The industrial motivation for the present work was to develop a supercapacitor capable of working in the harsh environment of deep offshore boreholes. If successful, this technology would allow down-hole telemetry under conditions of mechanical vibration and high temperature. The obstacles, however, were many. All supercapacitor components had to be stable against thermal decomposition up to T ≥ 220 °C. Volatile components had to be eliminated. If possible, the finished device should be able to withstand voltages greater than 4 V, in order to maximise the amount of stored energy. The internal resistance should be as low as possible. Side reactions, particularly faradaic reactions, should be eliminated or suppressed. All liquid components should be gelled to minimise leakage in the event of cell damage. Finally, any emergent problems should be identified. [Continues.

Poly(bisphenol) polymers as passivating agents for carbon electrodes in ionic liquids

Poly(bisphenol) polymers are identified as a new class of passivating agents for carbon electrodes in ionic liquids. They are inert and can readily be deposited as thin, conformal films by electropolymerization. Unlike conventional poly(monophenol) polymers, a single voltammetric scan is sufficient to accomplish their deposition. This is seen, for example, in the cases of poly(bisphenol A) and poly(bisphenol P). In each case, the thickness of the electropolymerized films is determined by the quantum tunneling distance of the faradaic electrons. Thus, film growth terminates when the faradaic electrons can no longer transit the film at a measurable rate. At that point, all the faradaic reactions cease, while the capacitive charging processes continue unabated. Experimentally, film thicknesses are observed in the range 4–30 nm. A challenging test for the poly(bisphenol) polymers is to coat them onto arrays of microelectrodes (RAM electrodes). Normally, microelectrodes are difficult to coat by electropolymerization due to the intense flux of soluble intermediates away from their surfaces. In the present work, however, coating is facile due to the extreme insolubility of the intermediates. This same property makes the films strongly adherent. Such remarkable behavior suggest that poly(bisphenol) films may have an important role to play as passivating agents in supercapacitors. They may also find application in other areas of technology that require thin-film passivity, such as nanostructural engineering and device physics

A universal equivalent circuit for carbon-based supercapacitors

A universal equivalent circuit is proposed for carbon-based supercapacitors. The circuit, which actually applies to all porous electrodes having non-branching pores, consists of a single vertical ladder network in series with an RC parallel network. This elegant arrangement explains the three most important shortcomings of present-day supercapacitors, namely open circuit voltage decay, capacitance loss at high frequency, and voltammetric distortion at high scan rate. It also explains the shape of the complex plane impedance plots of commercial devices and reveals why the equivalent series capacitance increases with temperature. Finally, the construction of a solid-state supercapacitor simulator is described. This device is based on a truncated version of the universal equivalent circuit, and it allows experimenters to explore the responses of different supercapacitor designs without having to modify real supercapacitors

Quantum design of ionic liquids for extreme chemical inertness and a new theory of the glass transition

In many modern technologies (such as batteries and supercapacitors), there is a strong need for redox-stable ionic liquids. Experimentally, the stability of ionic liquids can be quantified by the voltage range over which electron tunneling does not occur, but so far, quantum theory has not been applied systematically to this problem. Here, we report the electrochemical reduction of a series of quaternary ammonium cations in the presence of bis(trifluoromethylsulfonyl)imide (TFSI) anions and use nonadiabatic electron transfer theory to explicate the results. We find that increasing the chain length of the alkyl groups confers improved chemical inertness at all accessible temperatures. Simultaneously, decreasing the symmetry of the quaternary ammonium cations lowers the melting points of the corresponding ionic liquids, in two cases yielding highly inert solvents at room temperature. These are called hexyltriethylammonium TFSI (HTE-TFSI) and butyltrimethylammonium TFSI (BTM-TFSI). Indeed, the latter are two of the most redox-stable solvents in the history of electrochemistry. To gain insight into their properties, very high precision electrical conductivity measurements have been carried out in the range +20 °C to +190 °C. In both cases, the data conform to the Vogel-Tammann-Fulcher (VTF) equation with “six nines” precision (R 2 > 0.999999). The critical temperature for the onset of conductivity coincides with the glass transition temperature T g. This is compelling evidence that ionic liquids are, in fact, softened glasses. Finally, by focusing on the previously unsuspected connection between the molecular degrees of freedom of ionic liquids and their bulk conductivities, we are able to propose a new theory of the glass transition. This should have utility far beyond ionic liquids, in areas as diverse as glassy metals and polymer science

Ternary mixtures of sulfolanes and ionic liquids for use in high-temperature supercapacitors

Ionic liquids are a natural choice for supercapacitor electrolytes. However, their cost is currently high. In the present work, we report the use of ternary mixtures of sulfolane, 3-methyl sulfolane, and quaternary ammonium salts (quats) as low-cost alternatives. Sulfolane was chosen because it has a high Hildebrand solubility parameter (δ H = 27.2 MPa 1/2 ) and an exceptionally high dipole moment (μ = 4.7 D), which means that it mixes readily with ionic liquids. It also has a high flash point (165 °C), a high boiling point (285 °C), and a wide two-electrode (full-cell) voltage stability window ( > 7 V). The only problem is its high freezing point (27 °C). However, by using a eutectic mixture of sulfolane with 3-methyl sulfolane, we could depress the freezing point to -17 °C. A second goal of the present work was to increase the electrical conductivity of the electrolyte beyond its present-day value of 2.1 mS cm -1 at 25 °C, currently provided by butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (BTM-TFSI). We explored two methods of doing this: (1) mixing the ionic liquid with the sulfolane eutectic and (2) replacing the low-mobility TFSI anion with the high-mobility MTC anion (methanetricarbonitrile). At the optimum composition, the conductivity reached 12.2 mS cm -1 at 25 °C

High temperature supercapacitator

A supercapacitor device having a housing and at least one supercapacitor cell disposed in the housing, where the supercapacitor cell may include: two working electrode layers separated by an electrode separator wherein each working electrode layer is electrically connected to a current collector supported upon an inert substrate layer; an electrolyte-impervious layer disposed between each working electrode layer and each conducting layer to protect the conducting layer; and an ionic liquid electrolyte disposed within the area occupied by the working electrode layers and the electrode separator, wherein the ionic liquid electrolyte has at least one cationic component comprising a central cation with an asymmetric arrangement of substituents bonded thereto. The ionic liquid electrolyte may be gelled by a silica gellant to inhibit electrolyte flow. The operating range of the supercapacitor device is at least 20-220.degree. C