Room Temperature Ionic Liquid Electrolyte Research Articles

Ionic Liquid Enabled High Energy-Density Lithium-Ion Batteries

The automotive segment of the global lithium-ion battery market is slated for exceedingly high growth in the near future, stoking a demand for next-generation electrode materials capable of delivering the 400 Wh/kg benchmark at low-costs. While most of the recent research efforts to enable such materials focus on complex material modifications and nano-architectured electrode design, developing alternative electrolyte chemistries presents another avenue towards creating the next generation Li-ion battery. Previous work at the University of Colorado Boulder demonstrated the viability of this approach. By pairing a simple, scalable, yet robust Si electrode architecture with an imide-based room temperature ionic liquid (RTIL) electrolyte, this work enabled a Si/L333 full-cell capable of long-term cycling (>1000 cycles) at a charge-discharge rate of 1C.1 Recently an initial feasibility study has revealed the impressive compatibility of the same imide-based RTIL electrolyte with lithium-manganese-rich (LMR) layered oxides. The newly developed LMR material had the potential to revolutionize the transportation industry, especially if the material could be paired with a high capacity anode material such as silicon (Si). The resulting full-cell was proposed to truly enable the electric vehicle (EV), driving down battery costs to less than $200/kWh while supplying double the drive range of state-of-the-art Li-ion technology. The material, formulated as xLi2MnO3Ÿ(1-x)LiMO2 or Li[LixM1-x]O2 (M = Ni, Mn, Co), is known as the lithium-manganese-rich (LMR) oxide. The beauty of the LMR material lies in the activation process undergone at >4.4 V vs. Li/Li+ during initial charging, resulting in an unprecedentedly high operating voltage and capacities of ~260 mAh g-1.2-8 Despite the potential for massive technological impact, worldwide research has struggled to enable the LMR material. Early work impressively laid the foundation for widespread efforts targeting this material and its signature drawback: the gradual lowering of cell operating voltage over cycling life as the originally layered crystal structure transforms to a spinel phase, accompanied by oxygen evolution during activation of the Li2MnO3 component and transition metal dissolution.In this work, we have focused our efforts on the electrode-electrolyte interactions known to accelerate phase change in the LMR system. Leveraging the understandings of LMR interfacial behavior built by decades of research, we employ a unique electrolyte composition to form a cathode-electrolyte interface (CEI) that allows for the improved long-term voltage stability of the LMR cathode. Our novel CEI is formed in situ through the oxidative decomposition of a room temperature ionic liquid (RTIL) electrolyte doped with a sacrificial fluorinated salt additive. For the first time, we demonstrate an LMR system capable of 1000 high capacity cycles with minimal voltage decay, shedding light on the importance of the LMR CEI and elucidating the complex interplay between the electrolyte and the atomic scale transformations of an unstable crystal lattice. In this talk, these results will be presented in more detail. D. Molina Piper, D. et al. Nat. Commun. 2015, 6, 6230. H. Yu, H. Zhou, J. Phys. Chem. Lett. 2013, 4, 1268. M.M. Thackeray, S.-H Kang, C.S. Johnson, J.T. Vaughey, R. Benedek, S.A. Hackney, J. Mater. Chem. 2007, 17, 3112. G.G. Amatucci, N. Pereira, T. Zheng, J.M. Tarascon, J. Electrochem. Soc. 2001, 148, A171. S.H. Kang, K. Amine, J. Power Sources 2005, 146, 654. M.H. Roussow, M.M. Thackeray, Mater. Res. Bull. 1991, 26, 463. P. Kalyani, S. Chitra, T. Mohan, S. Gopukumar, J. Power Sources 1999, 80, 103. J. Croy, J.S. Park, F. Dogan, C.S. Johnson, B. Key, M. Balasubramanian, Chem. Mater. 2014, 26, 7091.

Silver Electrodeposition from Room Temperature Ionic Liquid Electrolytes

Reversible deposition of silver thin films has numerous industrial applications including, auto-dimming mirrors for the automotive industry, electrochromic windows for the aviation industry, smart glass for the architectural industry and mirrors or thermal emitters for deployment on orbital platforms. Room temperature ionic liquids possess a number of unique properties that make them attractive for reversible silver systems, such as their relatively high conductivity, large electrochemical and liquid phase window, and their ability to solvate high concentrations of metal ions of interest, making them an attractive alternative to conventional electrolytes such as cyanide-based electrolytes, which in addition pose toxicity issues. Use of RTILs in these systems is anticipated to enable deposition or stripping of thin films with specific optical, emissive and electrical properties that may be tuned as needed for specific commercial applications. Furthermore, use of RTILs is anticipated to be beneficial in space applications compared to conventional electrolytes due to their low volatility, preventing evaporation if exposed to vacuum, and their excellent chemical and thermal stability, which may enhance tolerance to orbital radiation. Electrodeposition of electrochromic materials should be a less expensive alternative to achieve thin metallic films when compared with other deposition technologies, such as thermal spray or vapor deposition. Furthermore, pulse and pulse reverse processes have the ability to improve the deposition process of reversible silver systems as compared to operation under constant voltage or constant current electric fields, namely the ability to enhance mass transport properties of the system as well as driving nucleation of the silver thin films over grain growth, which is anticipated to help in the stripping of the film from the substrate. Operation under pulse/pulse reverse electric fields is expected to achieve highly reflective metallic mirror films while maximizing the cycling lifetimes and preventing operational degradation of the electrolyte and cell components. The present work explores the use of RTIL as electrolytes for reversible silver deposition systems. The first studies utilizing ionic liquid electrolytes for silver deposition focused on chloroaluminate melts, which have a high sensitivity to water, including atmospheric moisture. The present work focuses on deposition and stripping of silver thin films from RTIL with relative low moisture sensitivity. Simple cells composed of transparent electrically conductive electrodes were used and evaluated as a function of the different doping elements (i.e. tin oxide, platinum and silver). In this work, the system is evaluated in terms of RTIL electrolyte, substrate material and operating conditions including pulse/pulse reverse electric fields using cyclic voltammetry and chronoamperometry techniques. These studies were conducted in terms of cycling efficiency and reproducibility. Careful study of the applied voltage was conducted as voltages that are too high damage the transparent electrically conductive electrodes and limit device lifetime and voltages that are too low result in poor mirror formation and low stripping efficiencies. Deposits plated under different conditions were analyzed and compared with an optical surface profilometer, specifically to assess the continuity and smoothness of the resulting silver deposits as a function of deposition conditions. By optimizing the deposit properties, it is anticipated that device lifetime will be extended increasing device performance and associated cost metrics over conventional operation based on constant voltage or constant current electric fields. Acknowledgements – Funding for this work is gratefully acknowledged from Air Force SBIR Grant Number FA9453-16-M-0466. Platinum coated ITO electrodes were prepared by Dr. John Bryan Plumley.

Outstanding room-temperature capacitance of biomass-derived microporous carbons in ionic liquid electrolyte

A remarkable capacitance of 180F·g−1 (at 5mV·s−1) in solvent-free room-temperature ionic liquid electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was achieved in symmetric supercapacitors using microporous carbons with a specific surface area of ca. 2000m2·g−1 calculated from gas sorption by the 2D-NLDFT method. The efficient capacitive charge storage was ascribed to textural properties: unlike most activated carbons, high specific surface area was made accessible to the bulky ions of the ionic liquid electrolyte thanks to micropores (1–2nm) enabled by fine-tuning chemical activation. From the industrial perspective, a high volumetric capacitance of ca. 80F·cm−3 was reached in neat ionic liquid due to the absence of mesopores. The use of microporous carbons from biomass waste represents an important advantage for large-scale production of high energy density supercapacitors.

Studies of electrochemical interfaces by broadband sum frequency generation

We present a perspective on the use of potential-dependent broadband multiplex vibrational sum-frequency generation spectroscopy (hereafter SFG) to study electrochemical systems. In SFG, a broadband mid-infrared (IR) pulse is combined with a narrowband visible pulse, generating a pulse at the sum frequency, which contains a spectrum that, due to the principles of nonlinear optics, originates solely from the electrified interface. Our SFG spectrometer can obtain one hundred or more spectra during a routine cyclic voltammetry (CV) measurement. We used SFG to study a model for a Li-ion battery anode and a low-overpotential CO2 reduction reactor based on a room-temperature ionic liquid (RTIL). SFG spectra from these complex systems were difficult to interpret, in part due to the organic electrolytes that have a forest of infrared vibrational transitions. Here we discuss a rubric for experimental measurement and interpretation with such systems. We describe the electrified interface, or double layer, as consisting of the electrode surface, a region of adsorbed molecules and an outer diffuse electric double layer. We combined resonant SFG, where the IR pulses were tuned to vibrational transitions of adsorbates, with nonresonant SFG where the IR pulses were tuned away from all vibrational transitions. The former provides information about chemistry on the electrode surface and the latter about the potential-dependent response of the double layer. We show how this rubric can be used to understand solid-electrolyte interphase formation on a model for the Li-ion electrode, and how low-overpotential CO2 reduction on Ag is controlled by potential-driven structural transitions of the RTIL electrolyte.

Sensing Behavior of Room Temperature Amperometric H2 Sensor with Pd Electrodeposited from Ionic Liquid Electrolyte as Sensing Electrode

A fuel cell based amperometric room temperature hydrogen sensor with Nafion as proton conducting electrolyte, Pd as sensing electrode, Pt as both counter and reference electrodes has been developed. Sensing electrode was prepared by electrodeposition of Pd on carbon gas diffusion electrode using room temperature ionic liquid electrolyte. Pt spray coated on gas diffusion electrode forms the counter electrode in the sensor. Pd electrodeposition was carried out at an applied potential of −1.0 V vs Pd wire and resulted in the formation of Pd clusters. The microstructure of electrodeposited Pd was investigated using field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The crystal structure of the Pd nanoparticles was indexed to face centered cubic by X-ray diffraction (XRD) and selected area electron diffraction (SAED). X-ray photoelectron spectroscopy (XPS) confirmed metallic form of the deposited Pd nanoparticles. The scratch test endorsed the fact that Pd nanoparticles adhered firmly to the surface of the gas diffusion electrode and it is found to be stable up to the load of 2 N. Range of response of the sensor to H2 was in the concentration range 1% to 5% and found to be linear.

Rate limiting activity of charge transfer during lithiation from ionic liquids

Abstract Given the increased use of room temperature ionic liquid electrolytes in Li-ion batteries, due to their non-flammability and negligible volatility, this study evaluates the lithiation kinetics to understand and improve the rate performance of Li-ion batteries. Lithium titanate spinel is used as a model electrode and the electrolyte is composed of LiTFSI and TFSI-coordinated alkoxy-modified phosphonium ionic liquid. Based on the analysis of activation energies for each process, we report that the charge-transfer reaction at the electrode/electrolyte interface is the rate-limiting step for cell operation. This finding is further supported by the observation that a 50-fold decrease in charge-transfer resistance at higher temperatures leads to a significant performance improvement over that of a traditional organic electrolyte at room temperature. Charge-transfer resistance and electrolyte wetting on the electrode surface are critical processes for optimal battery performance, and such processes need to be included when designing new ionic liquids in order to exceed the power density obtained with the use of current carbonate-based electrolytes.

Electroless Deposition of Aluminum from a Room Temperature Ionic Liquid Electrolyte

The development of the technology to produce the aluminum thin film and aluminum wiring with low cost has been earnestly desired. Such a technology has profound consequence across many electronic machinery industries including semiconductor, electronic circuit, etc. Since aluminum is quite less noble metal, it is impossible to obtain the electrodeposition from aqueous solution. On the other hand, we could developed aluminum deposition technology from the room temperature ionic liquid. Moreover, we developed first the revolutionary technique of the electroless deposition of aluminum based on using the ionic liquid as electrolyte. There are many use of aluminum thin films including aluminum wiring as stated above, but it is necessary to use large instruments such as sputtering and glow plasma methods in order to deposit aluminum on the insulating substrate of silicon and glass. If the technique of electroless deposition of aluminum is established, it will be able to obtain the thin and thick film coating on the substrates that are insulating material and have quite complicated structure, without electricity. At our laboratory, electrodeposition of aluminum with the room temperature ionic liquid composed of aluminum chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (EMIC) have been developed. Therefore, the purpose of this study is to develop electroless deposition method of aluminum with room temperature ionic liquid. In this study, various substrates were attemted electroless deposition of aluminum and deposits were evaluated. In addition, the reaction mechanism of the aluminum electroless deposition was figured out by the electrochemical consideration. First, LiH was added as the reducing agent into the AlCl3-EMIC (2:1) melt. At the same time, various substrates (glass, silicon, etc.) were immersed in the electrolyte, then the electroless deposition of aluminum started. The plated specimen was qualitatively identified by X-ray diffractmeter and its composition was determined by inductively coupled plasma atomic emission spectrometry. The observed diffraction peaks of the plated film agreed with those of aluminum in the literatures. The data from ICP measurement also showed that the purity of the deposited aluminum was more than 99%. These indicated that the deposited film from the electrolyte with LiH was aluminum. The elemental mapping result by SEM-EDX also indicated that the plating film consisted of aluminum only, and the aluminum was homogeneously distributed on the surface of the film. Next, the mechanism of the aluminum electroless plating was tried to find out. The change of the ionic apecies was analyzed by using laser Raman spectrophotometer. It suggested that Al2Cl7 -, which is ionic species for aluminum deposition in the acidic AlCl3-EMIC melt, had such a reaction shown below and the aluminum was deposited from the electrolyte. 4Al2Cl7 - + 3e- → 7AlCl4 -+ Al (1) It was also considered that the reducing agent, LiH, had a reaction indicated below in order to cause the reaction (1). LiH → Li+ + 1/2H2+ e- (2) Hydride composits such as LiH are good reducing agent, however, those also have some dangerous of explosion. We tried to find other reducing agent instead of hydride. In this meeting, we will show the results of electroless deposition of aluminum from room temperature ionic liquid with other reducing agents.

Anion effect on lithium electrodeposition from N‐propyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid electrolytes

The electrodeposition of lithium metal from room temperature ionic liquid (RTIL) electrolytes consisting of N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr13[FSI]) with LiFSI, LiTFSI, LiBF4, LiPF6 or LiAsF6 salts onto Pt and Li electrodes was undertaken to identify mechanistic details. Cyclic voltammetry at both Pt and Li electrodes is complicated by the chemical reaction between fresh/electrodeposited Li metal and electrolyte to form a solid-electrolyte interphase (SEI). As such, all electrolyte systems depict quasi-reversible cycling, owing to the concomitant chemical breakdown of the electrolyte and deposition of a passivation product onto the electrode surface. The rate at which the SEI forms can be observed through cyclic voltammetric scan rate studies. Chronoamperometry data supports the cyclic voltammetry studies and indicates that an instantaneous nucleation and growth type mechanism is taking place at all potentials as determined through modelling the current-time transients utilising the Hills-Scharifker theory. We show herein that these RTIL based electrolytes can be cycled effectively in an order of stability of salt inclusion as follows: LiBF4>LiFSI>LiAsF6>LiTFSI>LiPF6.

A higher performance dye-sensitized solar cell based on the modified PMII/EMIMBF4 binary room temperature ionic liquid electrolyte

Additives and iodine (I2) are used to modify the binary room temperature ionic liquid (RTIL) electrolyte to enhance the photovoltaic performance of dye-sensitized solar cells (DSSCs). The short-circuit current density (JSC) of 17.89 mA/cm2, open circuit voltage (VOC) of 0.71 V and fill factor (FF) of 0.50 are achieved in the optimal device. An average photoelectric conversion efficiency (PCE) of 6.35% is achieved by optimization, which is over two times larger than that of the parent device before optimization (2.06%), while the maximum PCE can reach up to 6.63%.

A review of developments in the electrodeposition of tin-copper alloys

The deposition of Sn–Cu binary alloys, mainly from aqueous electrolytes, is considered. The importance of tin–copper electrodeposits is summarised and the scope for plating them is highlighted. Sn–Cu (bronze) deposits are important in corrosion protection, decorative finishes, electronics and tribology. The past 20 years have seen a series of developments in the science and technology of bronze plating, including nanostructured deposits, environmentally friendly baths, such as methanesulfonic acid ones, and more ambitious coatings including multi-layers and composites. Among non-aqueous baths, room temperature ionic liquid electrolytes have received increasing attention. Our ability to realise controlled deposit morphology, composition and structure has been improved by newer electrolytes, improved electrolyte additives and pulse plating. The diversity of Sn–Cu deposit applications has extended to lithium batteries by newer layer structures such as composites, multi-layers and nanostructures), electrical control and relative bath/electrode movement. Electrochemical aspects of modern tin and bronze alloy deposition are illustrated by data from the authors' laboratory and elsewhere, which highlights the use of methanesulfonic acid electrolytes in tin alloy deposition due to their versatility and lack of environmental impact. A wide range of deposit composition, colour and surface finish are possible using suitable addition agents and by control of electrolyte flow and operating conditions. This review focuses on electrochemical voltammetry techniques together with morphological information from SEM imaging. Subject areas deserving further research and development are identified.

Lithium Molten Salt Battery at Near Room Temperature Using Low-Melting Alkali Metal Melts

As an electrolyte of lithium ion battery (LIB), a lithium salt as ion source, such as lithium hexafluorophosphate, is usually dissolved into an organic solvent, such as mixture of ethylene carbonate and dimethyl carbonate because the lithium salt is solid phase at room temperature and its melting point is above 200 ºC. We have focused on the electrolyte lithium salts themselves and attempt to use a “lithium molten salt” as electrolyte with no solvent. Firstly, molten salt has low-vapor pressure and non-flammability. A low-melting lithium melt enables a molten salt battery, which has been practically used as rugged storage,[1] to be used for broad application similar to LIB. On the other hand, the lithium melt contain no organic solvents, organic cations, which are composed in room temperature ionic liquid (RTIL). Therefore, discussion about alkali metal melt is free from complicated effects of the solvents and organic cations. The lithium melt is expected to be a base on all of the electrolytes containing the lithium salt. We found that lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide (Li[fTfN]) possesses a significantly low melting point (100 ºC) among the lithium salts due to asymmetric structure of fTfN anion.[2] This melt allows stable charge-discharge of composite LiCoO2 or LiFePO4 positive electrode[3] and graphite negative electrode,[4] in half-cell. The Li[fTfN] can be used as a single lithium molten salt electrolyte without any organic solvents or RTILs. A cation mixture of the Li[fTfN] and its corresponding cesium salt (Li0.4Cs0.6[fTfN]) shows another interesting phenomena.[5] The Li0.4Cs0.6[fTfN] has 102~103 times higher viscosity, and 10– 2~10– 3 times lower specific conductivity than the organic electrolyte. However, its rate performance in LiFePO4composite electrode was equal to those of the organic electrolyte. In this study, we investigated charge-transfer resistance with these composite electrode and lithium metal in the Li[fTfN] and Li0.4Cs0.6[fTfN] by electrochemical methods, such as ac impedance, in order to discuss about factors of the stable charge-discharge without solvents and specific rate performance against its electrolyte property. Based on the single Li[fTfN] melt, we prepared the cation mixture added the other alkali metal fTfN salts, the RTIL electrolyte added the organic cation and the organic electrolyte solvated in the organic solvent. They will be compared for the physicochemical properties of electrolyte itself, the charge-transfer resistance with each electrode, and charge-discharge property.

Effect of Ambient Conditions on Ionic-Liquid-Electrolyte Aluminium-Air Batteries

Increasing energy demand and environmental concerns dictate the utilization of renewable energy sources into the electricity networks globally. Commonly used renewable sources, such as solar and wind power supply a fluctuating power output both daily and annually that can create excess electricity when it is not demanded. Therefore, this excess energy has to be stored to supply future demands. However, such storage systems should be based on cheaper and more abundant materials considering the amount of energy will be stored. One option to provide cheap and sustainable energy storage is the aluminium-air battery due to its high theoretical energy density (2980 Ah kg-1 for Al) and high natural abundance of aluminium (8.1% in earth’s crust). Research on aluminium anodes with alkaline electrolytes has been conducted since 1960s1. In these systems, high corrosion rates, surface passivation and evaporation of the aqueous electrolyte resulted in low discharge capacities and low anode mass utilizations. A recent work about Al-Air batteries with room temperature ionic liquid electrolyte showed, that high anode mass utilization up to 70% could be obtained2. This work investigates the battery performance of Al-Air batteries with oligo-fluorinate ionic liquid electrolytes under different operating and ambient conditions. The objective was to identify the influence of the temperature on the performance, corrosion rates and mass utilizations of the anode. The methods applied in the study contain the characterization of electrochemical properties of the cells, such as open circuit potentials (OCP) and the potentials at various discharge current densities as a function of temperature. In the scope of the investigations, variations of the water content of the room temperature ionic liquid electrolytes and its effect on the cell characteristics were analysed. The microstructures of the anode surfaces were investigated at the different stages of the battery operation via microscopy techniques. Room temperature electrochemical experiments showed that, at high current densities (1.5 mA/cm2) relatively high discharge capacities up to 120 mAh can be achieved and high mass utilizations can be accomplished with such battery systems. These mass utilizations are lower than those of the alkaline electrolyte based Al-Air battery systems. Low current density discharge experiments indicated that, rising temperatures increased both OCP and discharge potentials. However, the corrosion rate is increased and the mass utilization is substantially decreased at low current densities. Higher temperature promotes corrosion rate of the anodes and reduces anode mass utilization as well as the discharge capacity. Therefore, a better understanding of the temperature driven processes on the cell performance of the battery is necessary. 1. Zaromb, S. The Use and Behavior of Aluminum Anodes in Alkaline Primary Batteries. J. Electrochem. Soc. 109, 1125 (1962). 2. Gelman, D., Shvartsev, B. & Ein-Eli, Y. Aluminum–air battery based on an ionic liquid electrolyte. J. Mater. Chem. A 2, 20237–20242 (2014).

Effects of Operating Temperature on the Electrical Performance of a Li-air Battery operated with Ionic Liquid Electrolyte

Li-air cell operated with ionic liquid electrolytes is a very promising energy storage technology for electric vehicle and plug-in hybrid electric vehicle due to several favorable characteristics of ionic liquids. However, Li-air cells that employ room temperature ionic liquid (RTIL) electrolytes exhibit poor performance due to limited oxygen solubility and low reactant species mobility. To circumvent these aforementioned drawbacks, we investigated the electrical performance of a Li-air cell with ionic liquid electrolytes operating at high temperature. A continuum based model is used to quantify the performance of the Li-air cell, with an ionic liquid (MPPY-TFSI) electrolyte, as a function of operating temperature. Simulations at the atomistic scale, such as molecular dynamics (MD) and density functional theory (DFT) calculations are used to obtain key properties for the continuum model. These properties include electron transfer reaction rate constant, species diffusivity and oxygen solubility. The MD simulations indicate that oxygen solubility in ionic liquid increases with temperature, which is very favorable for high temperature operation. The continuum based cell level simulation results show that the battery performance can be improved significantly by increasing operating temperature. For instance, specific capacity as high as 3000mAh/g can be achieved at 110°C operating temperature, which is almost 25 times higher than its counterpart at room temperature. Simulation results also reveal that by increasing the operating temperature, the specific capacity can be improved significantly for high load current density, which is one of the most critical drawbacks in RTIL based Li-air battery. We also studied the effect of cathode thickness on the performance of Li-air battery at different operating temperatures. The transport limitation of oxygen and lithium ions can be alleviated at higher operating temperatures, suggesting that even thicker cathode materials can be used to enhance the cell capacity at elevated temperature.

Electrochemical Preparation of Porous Poly(3,4-ethylenedioxythiophene) Electrodes from Room Temperature Ionic Liquids for Supercapacitors

Poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes were prepared using current pulse method from different room temperature ionic liquids electrolytes (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMI][TFSI]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMI][BF4]), 1-allyl-3-(2-methoxyethyl)-imidazolium bis(trifluoromethylsulfonyl)imide ([AMO][TFSI]), and organic solution (0.1 M LiClO4 in acetonitrile)). The SEM and XPS studies show that the PEDOT films electropolymerized in [BMI][BF4] are highly porous and have a higher anion doping level than those electropolymerized in [BMI][TFSI], [AMO][TFSI], and LiClO4/ACN electrolytes. The electrochemical performance of the symmetrical supercapacitor based on PEDOT electrodes were investigated in 0.1 M KClO4 aqueous electrolyte by cyclic voltammetry and galvanostatic charge-discharge test. The PEDOT electrodes prepared from ionic liquids exhibit higher capacitative performance than those prepared from conventional solvent (ACN). Especially, the PEDOT electrodes prepared from a [BMI][BF4] ionic liquid exhibited high specific capacitance (191 F/g) with an obvious improvement in energy density and power density as comparison to the PEDOT electrodes prepared from ACN solution (100 F/g) in aqueous electrolyte. The symmetrical supercapacitor based on PEDOT electrodes prepared from [BMI][BF4] ionic liquid shows maximum specific energy of 5.37 Wh/Kg and good cycle durability. This study shows using ionic liquids as electrolytes for preparing PEDOT electrodes is an efficient way to improve performance of supercapacitors.

Tailoring graphene-based electrodes from semiconducting to metallic to increase the energy density in supercapacitors

The semiconducting character of graphene and some carbon-based electrodes can lead to noticeably lower total capacitances and stored energy densities in electric double layer (EDL) capacitors. This paper discusses the chemical and electronic structure modifications that enhance the available energy bands, density of states and quantum capacitance of graphene substrates near the Fermi level, therefore restoring the conducting character of these materials. The doping of graphene with p or n dopants, such as boron and nitrogen atoms, or the introduction of vacancy defects that introduce zigzag edges, can significantly increase the quantum capacitance within the potential range of interest for the energy storage applications by either shifting the Dirac point away from the Fermi level or by eliminating the Dirac point. We show that a combination of doping and vacancies at realistic concentrations is sufficient to increase the capacitance of a graphene-based electrode to within 1 μF cm−2 from that of a metallic surface. Using a combination of ab initio calculations and classical molecular dynamics simulations we estimate how the changes in the quantum capacitance of these electrode materials affect the total capacitance stored by the open structure EDL capacitors containing room temperature ionic liquid electrolytes.

Structural Transition in an Ionic Liquid Controls CO2 Electrochemical Reduction

Broad-band multiplex vibrational sum-frequency generation spectroscopy (SFG) was used to study CO2 reduction on a polycrystalline Ag electrode with a room-temperature ionic liquid (RTIL) electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4), with 0.3 mol % water. The Ag/RTIL/H2O system has been shown to reduce CO2 with low overpotential and, depending on water concentration, with Faradaic efficiency of nearly 100% (Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I. Science 2011, 334, 643–644). The adsorbed CO created by CO2 reduction was probed with infrared (IR) pulses tuned to the CO stretch. Nonresonant (NR) SFG was used to probe the double layer. SFG showed that CO binds weakly to Ag at the CO2 reduction threshold of −1.33 V (vs Ag/AgCl), so CO does not poison the surface. At potentials equal to or more negative than the threshold, the curvature of the parabolic potential-dependent NR intensity significantly increased, and the Stark ...

Interrogating a Deeply Buried Electrode by Vibrational Sum Frequency Spectroscopy. Towards Understanding the Electroreduction at Ionic Liquid-Metal Interfaces

We used vibrational sum frequency generation spectroscopy (SFG) synchronized with cyclic voltammetry (CV) to study CO2 electroreduction on polycrystalline Ag, in a room-temperature ionic liquid (RTIL) electrolyte, 1-ethyl-3-methylimidazolium tetrafluorborate (EMIM-BF4). CO was produced from CO2 at -1.33V (vs. Ag/AgCl). SFG measurements showed CO adsorbed on Ag, but at -1.33V adsorption was weak, so CO did not poison Ag. At this same -1.33 V potential, the nonresonant SFG intensity was a minimum and the curvature of the SFG intensity vs. potential relation changed dramatically. This was true whether or not CO2 was present. The SFG results indicate the RTIL undergoes a potential-driven structural transition in the double layer, and this transition controls the reduction of CO2. Adding water to the RTIL increases the efficiency of CO2 reduction and shifted the structural transition 80 mV to lower potential.

Study and Development of Al-Air Battery Applying Room Temperature Ionic Liquid Electrolyte

Metal – air battery chemistry defers by the electrolyte of choice. Zn – Air, Al – Air and Fe – Air batteries were already reported utilizing aqueous alkaline and brine solutions as electrolytes. The main advantage of these specific electrolyte of choice is theirs high ionic conductivity and the ability to efficiently reduce oxygen into hydroxide anions. Nevertheless, such batteries systems suffers from major drawbacks, such as comparably low voltage and specific energy, for Zn and Fe, and extremely high corrosion rates under open circuit condition, for Al. To deal with such challenges a fairly new approach has been taken. The approach is based on Room Temperature Ionic Liquids (RTIL) utilization as electrolyte in Metal – air batteries research. Among these, one could find Na –and Si – air battery systems. RTILs offer unique properties, while serving as electrolytic media. Among the properties one can count their low vapor pressure, even at considerably elevated temperatures, the high intrinsic ionic conductivity, non-flammability, thermal and chemical stability with considerably large electrochemical windows (in some cases as much as 5 V) what makes them remarkable electrolytes for high-end electrochemical applications. In our recent report[1], a new Al – air battery was presented applying 1-ethyl-3-methylimidazolium oligoflourohydrogenat (EMIm(HF)2.3F) RTIL. The specific conductivity of this exact stoichiometry is 100 mS cm-1 at 298 K, one the highest of all RTIL family, comparable to that of 0.5 M KCl aqueous solution. The presented novel Al – air battery can sustain current densities up to 1.5 mA cm-2, producing capacities above 140 mA h cm-2, and thus utilizing above 70% of the theoretical Al capacity. This is equivalent to outstanding energy densities of 2300 W h kg-1 and 6200 W h L-1. In the current work a further investigation of the RTIL based Al – air battery is presented. Combining surface characterization with electrochemical experiments, both in half cell and also a full battery discharge, an ongoing progress in the study of this unique battery is shown. One of the main achievements in the study is related to the understanding of the operation voltage drop (deep). Conditions for elimination of this voltage deep were established and will be reported. [1] D. Gelman, B. Shvartsev, Y. Ein-Eli, J. Mater. Chem. A. 2.47 (2014) 20237-20242.

Ionic Liquid Structure and Dynamics at Charged Graphene Interface

Supercapacitors with room temperature ionic liquid (RTIL) electrolytes and graphene-based electrodes are promising candidates for small electronic devices, especially flexible and wearable ones. The theoretical upper limit for the electric double layer (EDL) capacitance of graphene is the highest in all carbon materials. Also, graphene-based electrodes open up novel designs that are free of binders and inert conductive additives [1]. Furthermore, RTILs stand out as safe, non-volatile electrolytes with a wide potential window. However, our knowledge of these highly dense ionic plasmas at electrified interfaces is still at its infancy due to the lack of in situ experimental data about their potential-dependent EDL structures and dynamics [2]. Here, we use in situ real-time X-ray reflectivity integrated with fully atomistic molecular dynamics (MD) simulations to elucidate the interfacial ionic liquid structure and dynamics at epitaxial graphene electrode during cyclic voltammetry and potential steps [3]. Our results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by MD simulations at various static potentials [4]. The potential-driven transition between the two structures is characterized by an energy barrier (~0.15 eV) that is independent of temperature. The model nicely explains the coexistence of distinct anion and cation adsorbed structures and provides further insights to ultra-slow response of the interfacial structure to potential steps. References 1- Y. Shao et al. Chemical Society Reviews DOI:10.1039/C4CS00316K (2015) 2- M. Fedorov and A. Kornyshev. Chemical Reviews 114, 2978-3036 (2014) 3- A. Uysal et al. Journal of Physical Chemistry C, 118, 569-574 (2014) 4- A. Uysal et al. Journal of Physics: Condensed Matter, 27, 032101 (2015) * This effort was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The research described here was done in collaboration with S. S. Lee, H. Zhou, P. Fenter (Argonne National Laboratory), G. Feng, S. Li, P. Cummings (Vanderbilt University), P. Fulvio, P. Zhang, S. Dai (Oak Ridge National Laboratory), J. McDonough, Y. Gogotsi (Drexel University)

A nonlinear mechanical model for the fatigue life of thin-film carbon nanotube supercapacitors

The effects of cyclic loading on the mechanical performance and fatigue life of a novel carbon nanotube supercapacitor are investigated. The highly flexible supercapacitor is a monolithic, pre-fabricated, fully functional film made of a nanostructured free-standing layer in which ions are stored within two vertically aligned multi-walled carbon nanotube (MWCNs) electrodes that are monolithically interspaced by a solution of microcrystalline cellulose in a room temperature ionic liquid electrolyte. To study the cyclic mechanical response of such nanostructured multilayer composite, an original framework is adopted by combining the equivalent continuum approach of Eshelby–Mory–Tanaka and a Weibull-like approach for the evolution of debonding carbon nanotubes electrodes. One- and three-layer models of the supercapacitor are proposed. Cyclic tests are numerically carried out in strain control. A fatigue life limit is determined by considering a confidence interval for the number of cycles corresponding to the states at which the effective elastic modulus of the partially debonded nanostructured portion of the supercapacitor is reduced by a percentage between 20% and 30%. The simulated cyclic tests yield Wholer-type fatigue curves showing the fatigue life limit as the maximum number of cycles N for each strain amplitude.The sensitivity of the fatigue life with respect to meaningful parameters such as the interfacial strength between the MWCNs and cellulose is investigated. Frequency-response functions of the multilayer nanostructured composite are further computed as function of the strain amplitude during cyclic tests.