Binary Ionic Liquid Electrolyte Research Articles

An Azo-Spiro Mixed Ionic Liquid Electrolyte for Lithium Metal– LiFePO4 Batteries

Using a binary ionic liquid (IL) electrolyte composed of -methyl--propylpyrrolidinium bis(trifluoromethanesulfonyl)imide and 2-oxo-3,9-dioxa-6-azonia-spiro[5.5]undecane bis(trifluoromethanesulfonyl)imide at an optimized ratio, we investigated battery cycling behavior at temperatures of and above. We achieved an extended cycle life using the binary electrolyte system as the problem of cell short-circuiting (stemming from lithium dendrite growth) was significantly reduced. We provide evidence that 2-oxo-3,9-dioxa-6-azonia-spiro[5.5]undecane bis(trifluoromethanesulfonyl)imide assists in short-circuit prevention through the suppression of lithium dendrites. For batteries employing a lithium metal anode and a cathode, we report capacities in excess of , and by utilizing a pulse charging technique we were able to charge a cell at a current density , which we were not able to achieve using a normal charging regime. We also present lithium cycling data for 2-oxo-3,9-dioxa-6-azonia-spiro[5.5]undecane bis(trifluoromethanesulfonyl)imide in the absence of another IL. We show that poor capacity and a necessarily high operating temperature make this system undesirable although we observed no short-circuiting. We conclude that the structure of the components of the IL and the nature of the charging regime employed can cause a significant reduction in dendrite-induced short-circuiting.

Electrochemical characterization and photovoltaic performance of the binary ionic liquid electrolyte of 1-methyl-3-propylimidazolium iodide and 1-ethyl-3-methylimidazolium tetrafluoroborate for dye-sensitized solar cells

Determination of triiodide diffusion coefficient, charge-transfer resistance and photovoltaic performance of the binary ionic liquid electrolyte of 1-methyl-3-propylimidazolium iodide (MPII) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4) for dye-sensitized solar cells (DSSCs) were performed at varying volume ratios of MPII/EMIMBF 4. Compared to pure MPII ionic liquid, the addition of EMIMBF 4 not only increased triiodide diffusion coefficient, but also decreased charge-transfer resistance at Pt/electrolyte interface, leading to the notably improved photovoltaic performance of DSSC. The highest photoelectric conversion efficiency of 4.99% was obtained under light intensity of 100 mW/cm 2 for the DSSC with the binary MPII/EMIMBF 4 ionic liquid electrolyte when the volume ratio of MPII/EMIMBF 4 was 50/50, which was much higher than that of pure MPII ionic liquid electrolyte (2.6%).

Preparation of 1-methyl-3-propylimidazolium acetate and its application in dye sensitized solar cells

In this paper, we reported the preparation of 1-methyl-3-propylimidazolium acetate (MPIAc), which proceeded via the metathesis of 1-methyl-3-propylimidazolium iodide (MPII) and lead acetate or potassium acetate. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, MPIAc and MPII with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide increased and those of iodide slightly increased with the weight ratio increase of MPIAc and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I 2, 0.10 M LiI, 0.50 M 4- tert-butylpyrdine in the binary ionic liquid electrolyte of MPIAc (employing potassium acetate) and MPII (weight ratio 0.2), gave short circuit photocurrent density of 9.40 mA cm −2, open circuit voltage of 0.62 V, and fill factor of 0.57, corresponding to the photoelectric conversion efficiency of 3.34% at the illumination (air mass 1.5, 100 mW cm −2).

Limiting Current Density in Ionic Liquid Electrolyte for Lithium Batteries

The relationship between Li(I) transport property and the limiting current density was investigated in N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide (P13TFSA) at 30°C. The limiting current density in a LiTFSA/P13TFSA binary ionic liquid electrolyte was evaluated by chronoamperometry for a symmetric cell of Li|ionic liquid|Li, where the ionic liquid was incorporated in a porous polyethylene separator that was sandwiched between two Li electrodes. The diffusion process of Li(I) in the ionic liquid dominates the limiting current density in the cell. In addition to the Li(I) transport property of the electrolyte, the porosity and tortuosity of the separator has a significant effect on the limiting current density.

High-performance organic field-effect transistors with binary ionic liquids

High-performance rubrene single-crystal field-effect transistors are developed with binary ionic liquid electrolytes used for gating. Inclusion of small amount of inorganic salts in the ionic liquids enhances the degree of dissociation for the organic ions and accelerates formation of the electric-double-layers in response to the gate voltage. High carrier mobility of 2.9 cm 2/Vs is achieved in the rubrene single-crystal transistors with the mixture ionic liquid. In addition to the advantage of the low-voltage operation due to concentrated field in ultra-thin electric-double-layers, drastically increased capacitance at above 100 Hz makes the technique of the ionic liquid gating more attractive for fast-switching devices.

On the addition of conducting ceramic nanoparticles in solvent-free ionic liquid electrolyte for dye-sensitized solar cells

Titanium carbide (TiC) is an extremely hard conducting ceramic material often used as a coating for titanium alloys as well as steel and aluminum components to improve their surface properties. In this study, conducting ceramic nanoparticles (CCNPs) have been used, for the first time, in dye-sensitized solar cells (DSSCs), and the incorporation of TiC nanoparticles in a binary ionic liquid electrolyte on the cell performance has been investigated. Cell conversion efficiency with 0.6 wt% TiC reached 1.68%, which was higher than that without adding TiC (1.18%); however, cell efficiency decreased when the TiC content reached 1.0 wt%. The electrochemical impedance spectroscopy (EIS) technique was employed to analyze the interfacial resistance in DSSCs, and it was found that the resistance of the charge-transfer process at the Pt counter electrode ( R ct1) decreased when up to 1.0 wt% TiC was added. Presumably, this was due to the formation of the extended electron transfer surface (EETS) which facilitates electron transfer to the bulk electrolyte, resulting in a decrease of the dark current, whereby the open-circuit potential ( V OC) could be improved. Furthermore, a significant increase in the fill factor (FF) for all TiC additions was related to the decrease in the series resistance ( R S) of the DSSCs. However, at 1.0 wt% TiC, the largest charge-transfer resistance at the TiO 2/dye/electrolyte interface was observed and resulted from the poor penetration of the electrolyte into the porous TiO 2. The long-term stability of DSSCs with a binary ionic liquid electrolyte, which is superior to that of an organic solvent-based electrolyte, was also studied.

An improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate and its application in dye sensitized solar cells

In this paper, we reported an improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate (EMITA), which proceeded via efficient reaction of 1-methylimidazole and ethyl trifluoroacetate under solvent-free conditions using Teflon-lined, stainless steel autoclaves. It was shown that the procedure was simple and eco-friendly. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, EMITA and 1-methyl-3-propylimidazolium iodide (MPII) with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide slightly increased and those of iodide decreased with the weight ratio increase of EMITA and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I 2, 0.10 M LiI, 0.50 M 4- tert-butylpyrdine in the binary ionic liquid electrolyte of EMITA and MPII (weight ratio 1:2), gave short circuit photocurrent density of 7.88 mA cm −2, open circuit voltage of 0.61 V, and fill factor of 0.67, corresponding to the photoelectric conversion efficiency of 3.22% at the illumination (Air Mass 1.5, 100 mW cm −2).

Material development for dye solar modules: results from an integrated approach

AbstractIn this paper, we report on the outcome of a German network project conducted with 12 partners from universities and research institutes on the material development of dye solar cells (DSC). We give an overview in the field and evaluate the concept of monolithic DSC further with respect to upscaling and producibility on glass substrates. We have developed a manufacturing process for monolithic DSC modules which is entirely based on screen printing. Similar to our previous experience gained in the sealing of standard DSC, the encapsulation of the modules is achieved in a fusing step by soldering of glass frit layers. For use in monolithic DSC, a platinum free, conductive counter electrode layer, showing a charge transfer resistance of RCT < 1·5 Ω cm2, has been realized by firing a graphite/carbon black composite under an inert atmosphere. Glass frit sealed monolithic test cells have been prepared using this platinum‐free material. A solar efficiency of 6% on a 2·0 cm2 active cell area has been achieved in this case. Various types of non‐volatile imidazolium‐based binary ionic liquid electrolytes have been synthesized and optimized with respect to diffusion‐limited currents and charge transfer resistances in DSC. In addition, quasi‐solid‐state electrolytes have been successfully tested by applying inorganic (SiO2) physical gelators. For the use in semi‐transparent DSC modules, a polyol process has been developed which resulted in the preparation of screen printed, transparent catalytic platinum layers showing an extremely low charge transfer resistance (0·25 Ω cm2). Copyright © 2008 John Wiley & Sons, Ltd.

Performance of dye-sensitized solar cells based on Ionic liquids: Effect of temperature and iodine concentration

The performance of dye-sensitized solar cells, based on pure and binary ionic liquid electrolyte with different iodine concentrations, have been evaluated under 1 sun illumination and different cells temperature between 5 and 55 °C. At lower temperatures a short-circuit current ( J SC) may be limited by the diffusion of tri-iodide. In this situation J SC increases with increasing temperature, while at higher temperatures the J SC decreases due to more pronounced recombinations. We find a maximum for J SC at intermediate temperatures. This maximum shifts to lower temperatures by increasing iodine concentration. At a certain temperature different iodine concentration is optimal for different ionic liquids employed in the electrolyte. The open circuit voltage monotonically decreases with temperature while the conversion efficiency is basically dominated by temperature dependence of J SC.

Stable, High‐Efficiency Ionic‐Liquid‐Based Mesoscopic Dye‐Sensitized Solar Cells

Efficient and stable mesoscopic dye-sensitized solar cells (DSCs) introducing a low-viscosity binary ionic liquid (1-propyl-3-methyl-imidazolium iodide (PMII) and 1-ethyl-3-methyl-imidazolium tetracyanoborate (EMIB(CN)(4))) electrolyte in combination with a new high-molar-extinction-coefficient ruthenium complex, Ru(2,2'-bipyridine-4,4'-dicarboxylic acid)(4,4'-bis(2-(4-tert-butyloxy -phenyl)ethenyl) -2,2'-bipyridine) (NCS)(2), are demonstrated. The dependence of photovoltaic performance, charge transport and electron lifetime on the composition of the binary ionic-liquid electrolyte with different ratios of PMII/EMIB(CN)(4) were investigated by electrochemical impedance and photovoltage transient techniques. A photovoltaic conversion efficiency of 7.6 % was obtained under simulated full sunlight illumination, which is a record for solvent-free DSCs. These devices exhibit excellent stability at 80 degrees C in the dark or under visible-light soaking at 60 degrees C during 1000 h of accelerated tests.

Co-sensitization of Organic Dyes for Efficient Ionic Liquid Electrolyte-Based Dye-Sensitized Solar Cells

The co-sensitization of two organic dyes (SQ1 and JK2), which are complementary in their spectral responses, shows enhanced photovoltaic performance compared with that of an individual organic dye-sensitized solar cell. The power conversion efficiency of the co-sensitized organic dye solar cell based on the newly developed binary ionic liquid (solvent-free) electrolyte gives 6.4% under AM 1.5 sunlight at 100 mW/cm2 irradiation, which is higher than that of individual dye-sensitized solar cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) of the co-sensitized solar cell shows typical absorption peaks at 530 and 650 nm corresponding to the two dyes and displays a broad spectral response over the entire visible spectrum with IPCE of >40% in the 400-700 nm wavelength domain.

On the Influence of Anions in Binary Ionic Liquid Electrolytes for Monolithic Dye-Sensitized Solar Cells

Five ion c liquids (ILs) of the general formula Im(+)A(-), where Im(+) = I -methyl-3-n-butyl-imidazolium, A(-) = I- (1), BF4- (2), SCN- (3), CF3CO2- (4), and CF(3)S0(3)(-) (5), were used in elect ...

Temperature Dependent Impedance Analysis of Binary Ionic Liquid Electrolytes for Dye-Sensitized Solar Cells

At platinum electrodes kinetics of the electrode reaction were studied at two potential electrolyte systems for dye-sensitized solar cells (DSSCs) based on binary ionic liquid (IL) blends, i.e., 1-ethyl-3-methylimidazolium dicyanamide ([EMIM])/1-methyl-3-propylimidazolium iodide ([PMIM]I) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM])/[PMIM]I, respectively. The charge transfer resistances of the electrode reaction were determined via impedance spectroscopy for electrolyte blends at a mixing ratio of ILs of [PMIM]I (for [EMIM]/[PMIM]I) and [PMIM]I (for [EMIM]/[PMIM]I) up to [PMIM]I. In addition, the influence of iodine concentration on the electrode reaction was investigated for both electrolyte systems. The measurements were taken in a temperature range of 25 to 60°C to analyze the electrolyte properties in view of thermal stress of the DSSC for later practical application. Furthermore, exchange current densities were determined: the expected Arrhenius behavior was observed. Activation energies were obtained by fitting linearized Arrhenius plots.

A new ion-coordinating ruthenium sensitizer for mesoscopic dye-sensitized solar cells

A new ion-coordinating ruthenium polypyridyl sensitizer, NaRu(4-carboxylic acid-4′-carboxylate)(4,4′-bis[(triethyleneglycolmethylether) heptylether]-2,2′-bipyridine)(NCS) 2 (coded as K68), has been synthesized and characterized by 1H NMR, FTIR, UV–Vis absorption and emission spectroscopy. A power conversion efficiency of 6.6% was obtained for dye-sensitized solar cells (DSCs) based on the K68 dye and a newly developed binary ionic liquid electrolyte containing 1-propyl-3-methyl-imidazolium iodide (PMII) and 1-ethyl-3-methyl-imidazolium tetracyanoborate (EMIB(CN) 4). For a non-volatile organic solvent based electrolyte, a photovoltaic power conversion efficiency of 7.7% was obtained under simulated full sun light and exhibited a good thermal stability during the accelerated test under 80 °C in the dark. Solid-state DSCs incorporating K68 also perform remarkably well, out-performing our previously best ruthenium complexes employed in this type of DSC.

Stable Mesoscopic Dye-Sensitized Solar Cells Based on Tetracyanoborate Ionic Liquid Electrolyte

A stable dye-sensitized solar cell has been obtained based on a new binary ionic liquid electrolyte system containing 1-propyl-3-methylimidazolium iodide and 1-ethyl-3-methylimidazolium tetracyanoborate.

High Molar Extinction Coefficient Heteroleptic Ruthenium Complexes for Thin Film Dye-Sensitized Solar Cells

Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-hexyloxystyryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-methoxystyryl)-2,2'-bipyridine) (NCS)2, coded as K-19 and K-73, respectively, have been synthesized and characterized by 1H NMR, FTIR, UV-vis absorption, and emission spectroscopy and excited-state lifetime and spectroelectrochemical measurements. The introduction of the alkoxystyryl group extends the conjugation of the bipyridine donor ligand increasing markedly their molar extinction coefficient and solar light harvesting capacity. The dynamics of photoinduced charge separation following electronic excitation of the K-19 dye was scrutinized by time-resolved laser spectroscopy. The electron transfer from K-19 to the conduction band of TiO2 is completed within 20 fs while charge recombination has a half-life time of 800 s. The high extinction coefficients of these sensitizers enable realization of a new generation of a thin film dye sensitized solar cell (DSC) yielding high conversion efficiency at full sunlight even with viscous electrolytes based on ionic liquids or nonvolatile solvents. An unprecedented yield of over 9% was obtained under standard reporting conditions (simulated global air mass 1.5 sunlight at 1000 W/m2 intensity) when the K-73 sensitizer was combined with a nonvolatile "robust" electrolyte. The K-19 dye gave a conversion yield of 7.1% when used in conjunction with the binary ionic liquid electrolyte. These devices exhibit excellent stability under light soaking at 60 degrees C. The effect of the mesoscopic TiO2 film thickness on photovoltaic performance has been analyzed by electrochemical impedance spectroscopy (EIS).

Charge Separation and Efficient Light Energy Conversion in Sensitized Mesoscopic Solar Cells Based on Binary Ionic Liquids

A 7.4% power conversion efficiency at air mass (AM) 1.5 full sunlight was reached with a mesoscopic solar cell employing a new binary ionic liquid electrolyte composed of 1-propyl-3-methylimidazolium iodide and 1-ethyl-3-methylimidazolium tricyanomethanide in conjunction with the amphiphilic ruthenium complex NaRu(4-carboxylic acid-4'-carboxylate)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2), coded as Z-907Na. Ultramicroelectrode voltammetric, nanosecond laser transient absorbance, and photovoltaic measurements show that a high iodide concentration is required for dye regeneration to compete efficiently with charge recombination. A surprisingly fast reductive quenching process is turned on in pure iodide melts. This channel is unproductive, explaining the lower photocurrents observed under these conditions.

A Binary Ionic Liquid Electrolyte to Achieve ≥7% Power Conversion Efficiencies in Dye-Sensitized Solar Cells

For the first time ≥7% power conversion efficiencies under AM 1.5 sunlight have been acheived for photovoltaic devices with a binary ionic liquid electrolyte composed of 1-propyl-3-methylimidazolium iodide and 1-ethyl-3-methylimidazolium thiocyanate.