Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Pathway for Water Transport through Breathable Nanocomposite Membranes of PEBAX with Ionic Liquid [C12C1im]Cl

Water transport through membranes is an attractive topic among the research dedicated to dehydration processes, microenvironment regulation, or more simply, recovery of freshwater. Herein, an atomistic computer simulation is proposed to provide new insights about a water vapor transport mechanism through PEBAX membranes filled with ionic liquid (IL) [C12C1im]Cl. Starting from experimental evidence that indicates an effective increase in water permeation as the IL is added to the polymer matrix (e.g., up to 85·10-3 (g·m)/(m2·day) at 318.15 K for PEBAX@2533 membranes loaded with 70% of IL), molecular dynamics simulations are proposed to explore the key role of IL in water transport inside membranes. The polar region composed of anions and cationic head groups of the IL is demonstrated to serve as the pathway for water transport through the membrane. Water molecules always stay near the pathway, which becomes wider and thus has a larger water-accessible area with increasing IL concentration. Hence, the diffusion coefficients of water molecules and ions increase as the IL concentration increases. The simulation provides useful indications about a microscopic mechanism that regulates the transport of water vapor through a kind of PEBAX/IL membrane, resulting in full agreement with the experimental evidence.

Redox Shuttle-Based Electrolytes for Dye-Sensitized Solar Cells: Comprehensive Guidance, Recent Progress, and Future Perspective

A redox electrolyte is a crucial part of dye-sensitized solar cells (DSSCs), which plays a significant role in the photovoltage and photocurrent of the DSSCs through efficient dye regeneration and minimization of charge recombination. An I^-/I₃ ^- redox shuttle has been mostly utilized, but it limits the open-circuit voltage (V _oc) to 0.7-0.8 V. To improve the V _oc value, an alternative redox shuttle with more positive redox potential is required. Thus, by utilizing cobalt complexes with polypyridyl ligands, a significant power conversion efficiency (PCE) of above 14% with a high V _oc of up to 1 V under 1-sun illumination was achieved. Recently, the V _oc of a DSSC has exceeded 1 V with a PCE of around 15% by using Cu-complex-based redox shuttles. The PCE of over 34% in DSSCs under ambient light by using these Cu-complex-based redox shuttles also proves the potential for the commercialization of DSSCs in indoor applications. However, most of the developed highly efficient porphyrin and organic dyes cannot be used for the Cu-complex-based redox shuttles due to their higher positive redox potentials. Therefore, the replacement of suitable ligands in Cu complexes or an alternative redox shuttle with a redox potential of 0.45-0.65 V has been required to utilize the highly efficient porphyrin and organic dyes. As a consequence, for the first time, the proposed strategy for a PCE enhancement of over 16% in DSSCs with a suitable redox shuttle is made by finding a superior counter electrode to enhance the fill factor and a suitable near-infrared (NIR)-absorbing dye for cosensitization with the existing dyes to further broaden the light absorption and enhance the short-circuit current density (J _sc) value. This review comprehensively analyzes the redox shuttles and redox-shuttle-based liquid electrolytes for DSSCs and gives recent progress and perspectives.

Influence of interfacial water and cations on the oxidation of CO at the platinum/ionic liquid interface

CO oxidation is fundamental to the development of new catalyst materials for fuel cells and key for complete oxidation of small alcohols like methanol or ethanol on Pt catalysts. So far, room-temperature ionic liquids (RTIL) have been used to modify the selectivity and activity in electrocatalysis. In order to understand the mechanism of CO oxidation in RTIL in more detail we have investigated this reaction at the Pt(111)/1-butyl-3-methylimidazolium trifluorosulfonylimide [BMIM][NTf₂] electrode/electrolyte interface as a function of H₂O concentration and electrode potential with in situ sum-frequency generation (SFG) spectroscopy and infrared absorption spectroscopy (IRAS). Using SFG spectroscopy, we address the changes of linearly bonded CO molecules on Pt(111), while we monitor the changes in the bulk electrolyte with IRAS through vibrational bands from H₂O, CO₂ and CO. The presence of water in [BMIM][NTf₂] shifts the onset potential for CO oxidation by more than 200 mV when the water concentration is increased from 0.01 to 1.5 M, which we relate to the incorporation and the availability of water at the electrode/electrolyte interface. The nature of the RTIL cation has also a large effect on the surface excess of H₂O since RTILs like [BMMIM][NTf₂] and [BMPyrr][NTf₂] which are prone to form closed-packed structures, can block the incorporation of water and lead to more sluggish CO oxidation with larger overpotentials and oxidation in a much wider potential range for which we provide evidence by additional SFG measurements. These results clearly show that the choice of the RTIL is important for CO oxidation on Pt(111) electrode surfaces - an observation that is likely highly relevant also to other catalysts and catalytic reactions that require the presence of interfacial water.

Infrared and Terahertz Spectroscopic Investigation of Imidazolium, Pyridinium, and Tetraalkylammonium Tetrafluoroborate Ionic Liquids

We performed terahertz time-domain spectroscopy and infrared spectroscopy of imidazolium-based, pyridinium-based, and tetraalkylammonium-based tetrafluoroborate ionic liquids to study their characteristic intermolecular and intramolecular vibrational modes to clarify interactions between various cations and the tetrafluoroborate anion. It was found that the central frequency of the intermolecular vibrational band for these ionic liquids has a relatively high frequency, ranging from 90 to 100 cm^-1. In the 900-1150 cm^-1 range, the intramolecular vibrational absorption band of the 3-fold degenerate mode of tetrafluoroborate anions in the ionic liquids was observed. Although the tetrafluoroborate anion is attributable to one of the weakly coordinated anions, the spectroscopic splitting behavior of the 3-fold degenerate mode differs depending on the cation species. It was revealed that the degenerate mode is very sensitive to local interactions between the tetrafluoroborate anion and each cation.

Photovoltaic Performances of Dye-Sensitized Solar Cells Based on Modified Polybutadiene Matrix Electrolytes by Sol-Gel Process

A new type of polymer matrix electrolyte based on modified polybutadiene (modified PB) was developed for dye-sensitized solar cells (DSSCs) to improve their stability. The modified PB was fabricated by cross-linking the reaction of polybutadiene with siloxane groups as a substitute sol-gel process. A DSSC device using the modified PB matrix electrolyte showed an open-circuit voltage of 0.64 V, a short-circuit current density of 15.00 mA/cm², and a fill factor of 0.58 under photointensity of 100 mW/cm² at AM 1.5, consequently leading to an overall solar energy conversion efficiency of 5.49%. The DSSC device using the modified PB matrix electrolyte improved the conductivity, and the charge transfer ability showed the outstanding stability of the device.

Review on fabrication methodologies and its impacts on performance of dye-sensitized solar cells

This review highlights and summarizes the impact of different fabrication processes on the efficiency of dye-sensitized solar cells (DSSCs). Energy conversion efficiency of cell depends upon semiconductor, sensitizer, electrolyte, and counter electrode. Efficiency of DSSCs can be enhanced by properly selecting the optimum significance of various parameters of fabrications process. Major challenges of these solar cells are non-vegetal, noxious, extreme sensitizers. Application of natural dyes in this field plays a significant role. An optimized CdSe-TiO₂ photoanode showed a power conversion efficiency (PCE) of 13.29% and short circuit current density of 15.30 mA cm^-2 for the DSSC. Power conversion efficiency of 3.26% was achieved by using TTO electrode for DSSC device that is ascribed to the improved electrical and optical properties due to doping with Ta element. Absorbance of betalain was shown in the visible range of 530-535 nm for betanin while 450-559 nm for anthocyanin pigment. The natural dyes are economical, readily available, and environmentally friendly. This compilation would be beneficial for researchers working on dye solar cell.

Synthesis and Physical Properties of Tunable Aryl Alkyl Ionic Liquids (TAAILs)

Tunable aryl alkyl ionic liquids (TAAILs) based on the imidazolium cation were first reported in 2009. Since then, a series of TAAILs with different properties due to the electron‐donating or ‐withdrawing effect of the substituents at the aryl ring has been developed. Herein, a wide variety of those ionic liquids (ILs) is presented in terms of their cation structure. The authors synthesized ILs containing the bromide or bis(trifluoromethane)sulfonimide anion and 1‐aryl‐3‐alkyl imidazolium cations with various substituents in the ortho and/ or para position of the phenyl ring and alkyl chains of different lengths varying from butyl to dodecyl. The differences of their physical properties (melting point, thermal decomposition, viscosity, electro‐chemical window) of these ILs are reported according to their structure.

Evaluation of canonical choline chloride based deep eutectic solvents as dye-sensitized solar cell electrolytes

Deep eutectic solvents (DESs) are beginning to attract interest as electrolyte alternatives to conventional organic solvents and ionic liquids within dye-sensitized solar cells (DSSCs). The precise roles played by DES components and whether they simply represent a benign medium for mobilizing charge carriers or present beneficial functionality that impacts device performance remain unclear. To begin to address this deficiency in understanding, we performed a comprehensive characterization of the three "canonical" choline chloride-based DESs (i.e., reline, ethaline, and glyceline) as DSSC electrolytes hosting the iodide-triiodide (I^-/I₃ ^-) redox couple. The measurement of electrolyte viscosities, determination of triiodide diffusion coefficients, and photovoltaic performances assessed for water contents up to 40 wt. % allow the emergence of several important insights. A comparison to the observed photovoltaic performance arising from the individual components aids in further clarifying the impact of DES chemistry and solution viscosity on photovoltaic and charge carrier diffusion characteristics. Finally, we introduce the DES guaniline-consisting of a 1:1 molar ratio mixture of choline chloride with guanidinium thiocyanate-demonstrating it to be a superior DSSC electrolyte over those formulated from the three most widely studied canonical DESs at all water contents investigated.

Theoretical investigation of the stability, reactivity, and the interaction of methyl-substituted peridinium-based ionic liquids

This research work focuses on the reactivity, stability, and electronic interaction of pyridinium hydrogen nitrate (PHN)-based ionic liquids and the influence of methyl substituent on this class of ionic liquids: Ortho- (O-MPHN), meta- (M-MPHN), and para- (P-MPHN) substitution. Natural bond orbital (NBO) calculations were performed at the density functional theory (DFT) with Becke’s Lee Yang and Parr functional (B3LYP) methods and DFT/B3LYP/6-311++G(d,p) as basis set using GAUSSIAN 09W and GAUSSVIEW 6.0 software and the most important interaction between donor (Filled Lewis-type NBO’s) and the acceptor (vacant non-Lewis NBOs) were observed. From our natural bond orbital (NBO) result, it could be deduced that the higher the stabilization energy value, the greater the interaction between the donor and acceptor NBOs. The stability of the studied compounds is said to follow the order from O-MPHN > PHN > P-MPHN > M-MPHN based on the hyperconjugative interaction (stabilization energy) of the most significant interaction. The result of the highest occupied molecular orbital (HOMO), shows that PHN has the highest HOMO while the substituted derivatives have similar HOMO values between −7.70 and −7.98 eV thus PHN complex is the best electron donor while the substituted derivatives act as electron acceptors due to the presence of methyl group substituent which is observed to be electron deficient as a result of its withdrawal effect from the aromatic ring. Furthermore, the electron density, real space functions such as energy density and Laplacian of electron density at bond critical point (BCP) of the hydrogen bond interaction of the studied compounds were analyzed using Multifunctional Wavefunction analyzer software version 3.7 and it was observed that the hydrogen at position 6 and oxygen at position 11 (H6–O11) of M-methyl pyridinium nitrate with bond distance of 4.59 (Å) gave binding energy with the strongest electrostatic interaction between the cation and anion of the compounds under investigation. We also observed from our results that, substitution at the ortho position enhances the stability and strengthen the extent of charge transfer. This therefore implies that substitution at ortho position is more favorable for inter- and intramolecular interactions resulting to stabilization of the studied molecules.

Investigation of the Ionic Liquid Graphene Electric Double Layer in Supercapacitors Using Constant Potential Simulations

In this work, we investigate the effect of the cation structure on the structure and dynamics of the electrode–electrolyte interface using molecular dynamics simulations. A constant potential method is used to capture the behaviour of 1-ethyl-3-methylimidazolium bis (trifluoromethane)sulfonimide ([C2mim][NTf2]) and butyltrimethylammonium bis(trifluoromethane) sulfonimide ([N4,1,1,1][NTf2]) ionic liquids at varying potential differences applied across the supercapacitor. We find that the details of the structure in the electric double layer and the dynamics differ significantly, yet the charge profile and capacitance do not vary greatly. For the systems considered, charging results in the rearrangement and reorientation of ions within ∼1 nm of the electrode rather than the diffusion of ions to/from the bulk region. This occurs on timescales of O(10 ns) for the ionic liquids considered, and depends on the viscosity of the fluid.

Ionic Liquids Roles and Perspectives in Electrolyte for Dye-Sensitized Solar Cells

Exploration of renewable energy, such as solar energy, is imminent not only to cater to the escalating energy demand but also to address the uprising environmental issues due to heavy usage of non-renewable fossil fuel. The dye-sensitized solar cells (DSSCs) which are considered as the third-generation solar cells, have a huge potential to be commercialized due to their low cost, simplicity in fabrication, and promising photon-to-electrical energy conversion efficiency. Nevertheless, a high cell efficiency can only be achieved when an organic solvent is incorporated into the formulation of the electrolyte, which is prone to evaporation and leakage. As a result, DSSCs become unsuitable for long-run usage due to thermal instability in the electrolyte. The early intention of incorporating ionic liquids (ILs) into the electrolyte was to curb the abovementioned problem and to enable the DSSCs to function as a sustainable energy device. As such, this article briefly reviews how ILs have been incorporated into the electrolyte formulation and the extent of how the ILs can affect the cell efficiency in various electrolyte states. The role of the ILs in a range of electrolytes is also highlighted. This sheds light on the true purpose of introducing ILs into DSSC electrolyte, which is to enhance the ionicity of the electrolyte.

Elucidating structure–property relationships in imidazolium-based halide ionic liquids: crystal structures and thermal behavior

A set of imidazolium-based ionic liquids (ILs), 1-(2-hydroxyethyl)-3-methylimidazolium chloride (1), 1,3-bis-(2-hydroxyethyl)-imidazolium chloride (2), and 1-butyl-2,3,4,5-tetramethylimidazolium bromide (3), has been synthesized and their structural and thermal behavior studied. Organic halides are well-known IL formers with imidazolium halides being the most prominent ones. Functionalization of the imidazolium cation by enhancing its hydrogen bonding capacity, i.e. through introduction of –OH groups or by diminishing it, i.e. through substitution of the ring hydrogen atoms by methyl groups is expected to change the inter-ionic interactions. Consequently, the solid-state structures of 1–3 have been characterized with means of single X-ray diffraction to shed light on preferential inter-ionic interactions for obtaining valuable information on anti-crystal engineering, i.e. designing ion combinations that favor a low melting point and exhibit a low tendency for crystallization. The study reveals that endowing IL forming ions with an enhanced hydrogen bonding capacity leads to a depression in melting points and kinetically hinders crystallization. This study provides hints towards new design concepts for IL design, similar to the common strategy of employing conformationally flexible ions.

Freezing the Motion in Hydroxy-Functionalized Ionic Liquids-Temperature Dependent NMR Deuteron Quadrupole Coupling Constants for Two Types of Hydrogen Bonds Far below the Glass Transition

We measured the deuteron quadrupole coupling constants (DQCCs) for hydroxy-functionalized ionic liquids (ILs) with varying alkyl chain length over the temperature range between 60 and 200 K by means of solid-state NMR spectroscopy. For all temperatures, the ²H spectra show two DQCCs representing different types of hydrogen bonds. Higher values, ranging from 220 to 250 kHz, indicate weaker hydrogen bonds between cation and anion (c-a), and lower values varying from 165 to 210 kHz result from stronger hydrogen bonds between the OD groups of cations (c-c), in agreement with recent observations in infrared, neutron diffraction, and NMR studies. We observed different temperature dependencies for (c-a) and (c-c) hydrogen bonding. From the static pattern of the ²H spectra at the lowest temperatures, we derived the true DQCCs being up to 20 kHz larger than recently reported values measured at the glass transition temperature. We were able to freeze the librational motions of the hydrogen bonds in the ILs. The temperature dependence of the (c-a) and (c-c) cluster populations in the glassy state is opposite to that observed in the liquid state, partly anticipating the behavior of ILs tending to crystallize.

Formation of ion gels by polymerization of block copolymer/ionic liquid/oil mesophases

In this study, we introduce a new method of developing ion gels through polymerization of lyotropic liquid crystal (LLC) templates of monomer (styrene), cross-linker (divinylbenzene), ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate), and amphiphilic block copolymers (Pluronic F127). The polymerization of the oil phase boosts the mechanical properties of the ion-conducting electrolytes. We discuss the effect of tortuosity induced by crystalline domains and LLC structure on the conductivity of ion gels. The ion transport in polymerized LLCs (polyLLCs) can be controlled by changing the composition of the mesophases. Increasing the block copolymer concentration enhances the crystallinity of PEO blocks in the conductive domains, which slows down the dynamics of PEO chain and ion transport. We show that by adjusting the composition of LLC mesophases, the mechanical strength of ion gels can be increased one order of magnitude without compromising the ionic conductivity. The polyLLCs with 45/25/30 wt% (block copolymer/IL/oil) composition has storage modulus and ionic conductivity higher than 1 MPa and 3 mS cm-1 at 70 °C, respectively. The results suggest that LLC templating is a promising method to develop highly conductive ion gels, which provides advantages in terms of variety and processing.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Boron: Its Role in Energy-Related Processes and Applications

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications.

Above 800 mV Open-Circuit Voltage in Solid-State Photovoltaic Devices Using Phosphonium Cation-Based Solid Ionic Conductors

Here, we report phosphonium-based two solid ionic conductors (SICs), namely, triphenylphosphonium methyl iodide (TPPMeI) and triphenylphosphonium iodide (TPPHI), prepared via simple protocol at room temperature and were used as an electrolyte for solid-state photovoltaic devices (ss-PVDs) with open-circuit voltage (V_oc) exceeding 800 mV. Here, for the first time, detailed electrochemical investigations with theoretical aspects of phosphonium electrolytes were conducted, where PVDs prepared from these SICs, TPPMeI, showed the highest power conversion efficiency (PCE) of 4.08% with a V_oc of 810 mV. However, this performance was further improved up to the PCE of 6.71% with 824 mV of V_oc in the presence of additives like LiI and tert-butyl pyridine. This work leads to find the best alternative of liquid and quaternary ammonium ion-based electrolytes that suffers from problems like lower V_oc (<800 mV) and stability, leakage, etc.

Copper-based redox shuttles supported by preorganized tetradentate ligands for dye-sensitized solar cells

Three copper redox shuttles ([Cu(1)]2+/1+, [Cu(2)]2+/1+, and [Cu(3)]2+/1+) featuring tetradentate ligands were synthesized and evaluated computationally, electrochemically, and in dye-sensitized solar cell (DSC) devices using a benchmark organic dye, Y123. Neutral polyaromatic ligands with limited flexibility were targeted as a strategy to improve solar-to-electrical energy conversion by reducing voltage losses associated with redox shuttle electron transfer events. Inner-sphere electron transfer reorganization energies (λ) were computed quantum chemically and compared to the commonly used [Co(bpy)3]3+/2+ redox shuttle which has a reported λ value of 0.61 eV. The geometrically constrained biphenyl-based Cu redox shuttles investigated here have lower reorganization energies (0.34-0.53 eV) and thus can potentially operate with lower driving forces for dye regeneration (ΔGreg) in DSC devices when compared to [Co(bpy)3]3+/2+-based devices. The rigid tetradentate ligand design promotes more efficient electron transfer reactions leading to an improved JSC (14.1 mA cm-2), higher stability due to the chelate effect, and a decrease in VlossOC for one of the copper redox shuttle-based devices.

API ionic liquids: probing the effect of counterion structure on physical form and lipid solubility

Structure/property relationship of API IL counterions and salt physicochemical properties are investigated, the results highlight the complex interplay involved.

Expanding the Electrochemical Window: New Tunable Aryl Alkyl Ionic Liquids (TAAILs) with Dicyanamide Anions

A set of new tunable aryl alkyl ionic liquids (TAAILs) based on the 1-aryl-3-alkyl imidazolium motif has been synthesized, in which the following variables were systematically changed: alkyl chain length, aryl substitution (group and position), and counter ion. TAAILs with dicyanamide (DCA) and bis(trifluoromethylsulfonyl)imide (N(SO₂ CF₃ )₂ , NTf₂ ) anions showed remarkable differences of their physical properties: NTf₂ ionic liquids were found to have high decomposition temperatures and viscosities well below those of the DCA TAAILs. In contrast, the dicyanamide anion increased the electrochemical stability leading to TAAILs with an extremely wide electrochemical window of up to 7.17 V. Additionally, both classes of TAAILs extract transition metals from aqueous solutions: TAAILs with the DCA anion extract both platinum and copper while TAAILs with the NTf₂ anion are selective towards platinum. This demonstrates that minor changes of the molecular structure lead to TAAILs with drastically changed physicochemical properties.

Synthesis of Low-Viscosity Ionic Liquids for Application in Dye-Sensitized Solar Cells

Two types of ionic liquids (ILs), 1-(3-hexenyl)-3-methyl imidazolium iodide and 1-(3-butenyl)-3-methyl imidazolium iodide, are synthesized by introducing an unsaturated bond into the side alkyl chain of the imidazolium cation. These new ionic liquids exhibit high thermal stability and low viscosity (104 cP and 80 cP, respectively). The molecular dynamics simulation shows that the double bond introduced in the alkane chain greatly changes the molecular system space arrangement and diminishes the packing efficiency, leading to low viscosity. The low viscosity of the synthesized ionic liquids would enhance the diffusion of redox couples. This enhancement is detected by fabricating dye-sensitized solar cells (DSSCs) with electrolytes containing the two ILs and I₂ . The highest efficiency of DSSCs is 6.85 % for 1-(3-hexenyl)-3-methyl imidazolium iodide and 5.93 % for 1-(3-butenyl)-3-methyl imidazolium iodide electrolyte, which is much higher than that of 5.17 % with the counterpart 1-hexyl-3-methyl imidazolium iodide electrolyte.

Progress on Electrolytes Development in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have been intensely researched for more than two decades. Electrolyte formulations are one of the bottlenecks to their successful commercialization, since these result in trade-offs between the photovoltaic performance and long-term performance stability. The corrosive nature of the redox shuttles in the electrolytes is an additional limitation for industrial-scale production of DSSCs, especially with low cost metallic electrodes. Numerous electrolyte formulations have been developed and tested in various DSSC configurations to address the aforementioned challenges. Here, we comprehensively review the progress on the development and application of electrolytes for DSSCs. We particularly focus on the improvements that have been made in different types of electrolytes, which result in enhanced photovoltaic performance and long-term device stability of DSSCs. Several recently introduced electrolyte materials are reviewed, and the role of electrolytes in different DSSC device designs is critically assessed. To sum up, we provide an overview of recent trends in research on electrolytes for DSSCs and highlight the advantages and limitations of recently reported novel electrolyte compositions for producing low-cost and industrially scalable solar cell technology.

Reactions of a Polyhalide Ionic Liquid with Copper, Silver, and Gold

The reactions of copper, silver, and gold with the imidazolium‐based polyhalide ionic liquid (IL) [C₆C₁Im][Br₂I] were investigated by using X‐ray photoelectron spectroscopy (XPS), weight‐loss measurements, and gas‐phase mass spectrometry. All three Group 11 metals are strongly corroded by the IL at moderate temperatures to give a very high content of dissolved Cu^I, Ag^I, and Au^I species. The IL–metal solutions are stable against contact with water and air. The replacement of imidazolium with inorganic sodium cations decreased metal corrosion rates by orders of magnitude. Our results clearly indicate metal oxidation by iodide from dibromoiodide anions to form molecular iodine and anionic [Br‐M^I‐Br]⁻ (M=Cu, Ag, Au) complexes stabilized by imidazolium counterions. From experiments with a trihalide IL with imidazolium methylated at the 2‐position, we ruled out the formation of imidazole–carbene as a cause of the observed corrosion. In contrast to Group 11 metals, molybdenum is inert against the trihalide IL, which is attributed to surface passivation.

Cation influence on heterocyclic ammonium ionic liquids: a molecular dynamics study

Four different ionic liquids (ILs) consisting of the bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻) anion, with structurally similar systematically varying cations, are investigated herein through classical molecular dynamics.

A Dual Ionic Liquid-Based Low-Temperature Electrolyte System

Ionic liquids (ILs) show a promising future as electrolytes in electrochemical devices. In particular, IL-based electrolytes bring operations at extreme temperatures to realization that conventional electrolytes fail to accomplish. Although IL electrolytes demonstrate considerable progress in high-temperature applications, their breakthroughs in devices operating at low temperatures are still very limited due to undesirable phase transitions and unsatisfying transport properties. In this study, we present an approach where, by tuning molecular interactions in the system, the designed electrolyte of an IL-based mixture can reach a lower operating temperature with improved transport properties. We have discovered that the incorporation of the IL, ethylammonium nitrate ([EA][N]), can contribute to reforming the molecular interactions within the system, which effectively resolve the crystallization accompanied with the excess of water and retain a low glass transition temperature. The reported liquid electrolyte systems based on a mixture of 1-butyl-3-methylimidazolium iodide ([BMIM][I]), [EA][N], water, and lithium iodide exhibit a glass transition temperature below -105 °C. Furthermore, the optimized electrolyte system shows significant viscosity reduction and ionic conductivity enhancement from 25 to -75 °C. The influence is also noticeable on the increased ionicity, which made the developed electrolyte comparable with other good ILs under the Walden rule. The electrochemical stability of the electrolyte system is revealed by a steady and reproducible profile of iodide/triiodide redox reactions at room temperature over a proper potential window via cyclic voltammetry. The results from this work not only provide a potential solution to applications of the iodide/triiodide redox couple-based electrochemical devices at low temperatures but also show a practical approach to obtain tailored properties of a mixture system via modifying molecular interactions.

Research Progress on Photosensitizers for DSSC

Dye sensitized solar cells (DSSC) are considered one of the most promising photovoltaic technologies as an alternative to traditional silicon-based solar cells, for their compatibility with low-cost production methods, their peculiar optical and mechanical properties and the high indoor efficiency. Photosensitizers represent one of the most important components of a DSSC device and probably the most thoroughly investigated in the last twenty years, with thousands of dyes that have been proposed and tested for this kind of application. In this review we aimed to provide an overview of the three main classes of DSSC photosensitizers, namely ruthenium(II) polypyridyl complexes, Zn-porphyrin derivatives and metal-free organic dyes. After a brief introduction about the architecture and operational principles of a DSSC and the state of the art of the other main components of this type of device, we focused our discussion on photosensitizers. We have defined the numerous requirements DSSC photosensitizers should satisfy and have provided an overview of their historical development over the years; by examining specific dyes reported in the literature, we attempted to highlight the molecular design strategies that have been established for the optimization of their performance in real devices both in terms of efficiency (which recently reaches an outstanding 14.3%) and operational stability. Finally, we discussed, in the last section, the possible future developments of this intriguing technology.

Characteristic Spectroscopic Features because of Cation-Anion Interactions Observed in the 700-950 cm-1 Range of Infrared Spectroscopy for Various Imidazolium-Based Ionic Liquids

The 700-950 cm^-1 range in infrared spectroscopy was investigated for various imidazolium-based ionic liquids (ILs). Two main vibrational modes of the methylimidazolium cation exist in this region. At 700-800 cm^-1, there is an out-of-plane ⁺C(2)-H and ⁺C(4,5)-H bending mode with a larger motion of ⁺C(4,5)-H in the imidazolium ring, whereas at 800-950 cm^-1, there is an out-of-plane ⁺C(2)-H and ⁺C(4,5)-H bending mode with a larger motion of ⁺C(2)-H in the imidazolium ring. The molar-concentration-normalized absorbance spectrum of the former band at 700-800 cm^-1, which is related to the bending mode with a large ⁺C(4,5)-H motion in the imidazolium ring, is not particularly sensitive to the interactions with anions. The molar-concentration-normalized absorbance spectrum of the latter band at 800-950 cm^-1 was nearly identical for ILs that have methylimidazolium cations with carbon chains of different lengths and the same anions. The band shape of the latter band, which is related to the bending mode with a large out-of-plane ⁺C(2)-H bending motion, was highly sensitive to the interactions with anions and, interestingly, both blue- and red-shifted tendencies in the spectrum for each system were observed.

Electrical, thermal, and dielectric studies of ionic liquid-based polymer electrolyte for photoelectrochemical device

In this work, solution cast method was adapted for the preparation of 1-ethyl-3-methylimidazolium dicyanamide (EMImDCN)-doped solid polymer electrolyte. Optimum composition of polymer electrolyte (polyethylene oxide + sodium iodide) was treated as the host polymer. The ionic conductivity was further enhanced by adding low-viscosity ionic liquid (IL) EMImDCN. Electrical, thermal, dielectric, and photoelectrochemical properties of polymer host and IL-doped solid polymer electrolyte (ILDPE) are presented in detail. An electrochemical device, that is, dye-sensitized solar cell was fabricated using maximum conducting ILDPE film, which shows short-circuit current density of 0.118 mA/cm2, open-circuit voltage of 0.71 V, and overall efficiency of 0.061% at 1 sun condition.

Fickian yet non-Gaussian behaviour: A dominant role of the intermittent dynamics

We present a study of the dynamics of small solute particles in a solvent medium where the solute is much smaller in size, mimicking the diffusion of small particles in crowded environment. The solute exhibits Fickian diffusion arising from non-Gaussian Van Hove correlation function. Our study shows that there are at least two possible origins of this non-Gaussian behaviour: the decoupling of the solute-solvent dynamics and the intermittency in the solute motion, the latter playing a dominant role. In the former scenario when averaged over time long enough to explore different solvent environments, the dynamics recovers the Gaussian nature. In the case of intermittent dynamics the non-Gaussianity remains even after long averaging and the Gaussian behaviour is obtained at a much longer time. Our study further shows that only for an intermediate attractive solute-solvent interaction the dynamics of the solute is intermittent. The intermittency disappears for weaker or stronger attractions.