Abstract
The increasing need in the development of storage devices is calling for the formulation of alternative electrolytes, electrochemically stable and safe over a wide range of conditions. To achieve this goal, electrolyte chemistry must be explored to propose alternative solvents and salts to the current acetonitrile (ACN) and tetraethylammonium tetrafluoroborate (Et4NBF4) benchmarks, respectively. Herein, phenylacetonitrile (Ph-ACN) has been proposed as a novel alternative solvent to ACN in supercapacitors. To establish the main advantages and drawbacks of such a substitution, Ph-ACN + Et4NBF4 blends were formulated and characterized prior to being compared with the benchmark electrolyte and another alternative electrolyte based on adiponitrile (ADN). While promising results were obtained, the low Et4NBF4 solubility in Ph-ACN seems to be the main limiting factor. To solve such an issue, an ionic liquid (IL), namely 1-ethyl-3-methylimidazolium bis [(trifluoromethyl)sulfonyl] imide (EmimTFSI), was proposed to replace Et4NBF4. Unsurprisingly, the Ph-ACN + EmimTFSI blend was found to be fully miscible over the whole range of composition giving thus the flexibility to optimize the electrolyte formulation over a large range of IL concentrations up to 4.0 M. The electrolyte containing 2.7 M of EmimTFSI in Ph-ACN was identified as the optimized blend thanks to its interesting transport properties. Furthermore, this blend possesses also the prerequisites of a safe electrolyte, with an operating liquid range from at least −60 °C to +130 °C, and operating window of 3.0 V and more importantly, a flash point of 125 °C. Finally, excellent electrochemical performances were observed by using this electrolyte in a symmetric supercapacitor configuration, showing another advantage of mixing an ionic liquid with Ph-ACN. We also supported key structural descriptors by density functional theory (DFT) and COnductor-like Screening Model for Real Solvents (COSMO-RS) calculations, which can be associated to physical and electrochemical properties of the resultant electrolytes.
Highlights
Supercapacitors have placed themselves in a unique position among all different emerging advanced energy storage systems because of their unique capability of delivering very high power, along with reasonably decent energy storage capabilities [1,2,3,4]
This salt solubility limit is lower than that reported value in ACN (i.e., 1.68 M at 20 ◦ C) [28], but this limit is very close to that determined for another alternative solvent, e.g., adiponitrile, ADN (0.7 M at 25 ◦ C), in similar conditions [22]
Before being able to visualize the 3D structure of each species using TmoleX, all structures were optimized in the gas phase with a convergence criterion of 10−8 Hartree, using HF-6 311G**++ basis set, followed by density functional theory (DFT) calculations combining the resolution of identity (RI) approximation, [37,38]
Summary
Supercapacitors have placed themselves in a unique position among all different emerging advanced energy storage systems because of their unique capability of delivering very high power, along with reasonably decent energy storage capabilities [1,2,3,4]. In order to achieve more applications for these devices, many efforts have been conducted during recent time to improve their energy density [5,6,7]. The energy density of a supercapacitor is expressed by the equation E = (1/2) CV2. C is the capacitance, and V is the operational voltage of the supercapacitor. From this equation, it is very straightforward that increasing the operating voltage of the supercapacitor is an effective. Molecules 2020, 25, 2697 way to achieve higher energy density. Extension of operating voltage can be made by selecting an electrolyte, which can provide a higher electrochemically stable potential window [8]
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