Air-Stable, Self-Bleaching Electrochromic Device Based on Viologen- and Ferrocene-Containing Triflimide Redox Ionic Liquids

High-Entropy Metal Sulfide Nanocrystal Libraries for Highly Reversible Sodium Storage

Controlled synthesis of high-entropy materials offers a unique platform to explore unprecedented electrochemical properties. High-entropy metal sulfides (HEMSs) have recently emerged as promising electrodes in electrochemical energy storage applications. However, synthesizing HEMSs with a tunable number of components and composition is still challenging. Here, a HEMS library is built by using a general synthetic approach, enabling the synthesis of HEMS with arbitrary combinations of 5 to 12 out of 28 elements in the periodic table. The formation of a solid solution of HEMS is attributed to the two-step method that lowers the energy barrier and facilitates the sulfur diffusion during the synthesis. The hard soft acid base (HSAB) theory is used to precisely describe the conversion rates of the metal precursors during the synthesis. The HEMSs as cathodes in Na-ion batteries (SIBs) is investigated, where 7-component HEMS (7-HEMS) delivers a promising rate capability and an exceptional sodium storage performance with reversible a capacity of 230 mAh g-1 over 3000 cycles. This work paves the way for the multidisciplinary exploration of HEMSs and their potential in electrochemical energy storage.

Research Progress in Ionic Liquid-Based Electrolytes for Electrochromic Devices

Electrochromic (EC) technology has become one of the smart technologies with the most potential for development and application at this stage. Based on electrochromic devices (ECDs), this technology has shown extraordinary potential in the fields of smart windows, display devices, and sensing systems. With the optimization and iteration of various core components in ECDs, the electrolyte layer, a key component, evolved from its initial liquid state to a quasi-solid state and solid state. As driven by increasing application demands, the development trend indicates that all-solid-state, transparent electrolytes will likely become the future form of the electrolyte layer. Recently, the application of ionic liquid (IL)-based electrolytes in the field of electrochromism attracted a lot of attention due to their ability to bring outstanding EC cycling stability, thermal stability, and a wider operating voltage range to ECDs, and they are regarded as the new generation of electrolyte materials with the most potential for application. Although compared with conventional electrolytes, IL-based electrolytes have the characteristics of high price, high viscosity, and low conductivity, they are still considered the most promising electrolyte materials for applications. However, so far, there has been a lack of comprehensive analysis reports on "Research progress in ionic liquid-based electrolytes for electrochromic devices" within the EC field. In this article, the research progress of IL-based electrolytes in ECDs will be summarized from three perspectives: liquid, quasi-solid, and solid state. The future development directions of IL-based electrolytes for ECDs are discussed.

Synthesis and Electrochromic Properties of Ferrocene-Aryl Dicyanovinylene-Based Donor-Acceptor Systems

The favorable redox properties of ferrocene have led to the extensive development of ferrocene-based systems for several electrochemical applications but have scarcely been explored for electrochromism. Here, we report the synthesis and electrochromic properties of novel π-conjugated ferrocene-dicyanovinylene systems (M1-M5 and D1-D3). Monosubstituted (M1-M5) and disubstituted (D1-D3) compounds have been developed via Knoevenagel condensation of methyl-dicyanovinyl ferrocenes (4 or 5) with various aromatic aldehydes. The compounds' optical (λmax = 320-515 nm) and electronic properties (band gap = 2.6-3.1 eV) have been tuned by the appropriate choice of the aryl substituents. The strong D-A interactions between ferrocene and dicyanovinylene moieties rendered intense colors in the solution and thin-film state and demonstrated high-contrast electrochromism. The electrochromic behavior of these compounds is highly reversible (>48 cycles) and fast (0.98-1.06 s) in both solution and thin-film states. These are the first examples of stand-alone ferrocene systems explored for electrochromism.

A Multimode Dynamic Color-Changing Device for Smart Windows Based on Integrating Thermochromic and Electrochromic Properties

Electro- and thermochromic materials have been greatly applied in smart windows and displays due to the excellent properties of color variation and solar radiation. However, the mono color and single response to voltage and temperature hinder their application and development. Here, a multimode dynamic color-changing device (T/ECD) was developed by integrating the electrochromic property of synthetic viologen dyes and the thermochromic properties of hydroxypropyl acrylate (HPA). The T/ECD achieves four modes of optical regulation, namely, colorless transparent state, tinted transparent state, colorless opaque state, and tinted opaque state, which can be regulated independently/coordinately using heat and voltage. The optimized T/ECD switched color at 1.2 V with 15 s or adjusted the transparent/opaque state at >34 °C with 46 s. In addition, based on the red viologen (ViO-R), green viologen (ViO-G), and blue viologen (ViO-B) dyes, colorful T/ECDs were successfully designed and fabricated, and T/ECDs have excellent cycling properties, expanding the application requirement. Moreover, we demonstrated their application in smart windows and privacy protection. The design philosophy and successful exploration have great prospects for energy-saving buildings, displays, and information masking/storage systems.

Development of Electrochromic Devices, Based on Polymeric Gel, for Energy Saving Applications

In this work, the implementation of an electrochromic device (10 cm × 10 cm in size) for energy saving applications has been presented. As electrochromic system has been used with an electrochromic solution (ECsol) made by ethyl viologen diperchlorate [EV(ClO4)2], 1,1'-diethyl ferrocene (DEFc) and propylene carbonate (PC), as solvent. The final system has been obtained by mixing the ECsol, described above, with a polymeric system made by Bisphenol-A glycerolate (1 glycerol/phenol) diacrylate (BPA) and 2,2-Dimethoxy-2-phenylacetophenone (Irgacure 651) in a weight percentage equal to 60:40% w/w, respectively. Lithography has been used to make a spacer pattern with a thickness of about 15-20 µm between the two substrates. Micro-Raman spectroscopy confirmed the presence of the EV•+ as justified by the blue color of the electrochromic device in the ON state. Electrochemical and optical properties of the electrochromic device have been studied. The device shows reversible electrochromic behavior as confirmed by cyclic color variation due to the reduction and oxidation process of the EV2+/EV•+ couple. The electrochromic device shows a variation of the % transmittance in the visible region at 400 nm of 59.6% in the OFF state and 0.48% at 3.0 V. At 606 nm the transmittance in the bleached state is 84.58% in the OFF state and then decreases to 1.01% when it is fully colored at 3.0 V. In the NIR region at 890 nm, the device shows a transmittance of 74.3% in the OFF state and 23.7% at 3.0 V while at 1165 nm the values of the transmittance changed from 83.21% in the OFF state to 1.58% in the ON state at 3.0 V. The electrochromic device shows high values of CCR% and exhibits excellent values of CE in both visible and near-infrared regions when switched between OFF/ON states. In the NIR region at 890 nm, electrochromic devices can be used for the energy-saving of buildings with a promising CE of 120.9 cm2/C and 420.1 cm2/C at 1165 nm.

A Piezoelectric-Driven Electrochromic/Electrofluorochromic Dual-Modal Display Device

Electrochromic (EC) reflective displays offer great advantages in delivering information and providing visual data, but are limited in dark environments. Reflective/emissive dual-modal displays capable of electrochemically-induced color and fluorescence change simultaneously are highly desirable, especially possessing rapid response speed as well as long-term durability. Herein, an electroactive fluorescent ionic liquid based on triphenylamine and imidazole (EFIL-TPA) has been synthesized for reflective/emissive dual-modal display. The resultant device exhibits outstanding electrochromic/electrofluorochromic (EC/EFC) performance with low driving voltage (below 1.0 V), fast switching speed (0.57-1.8 s), and remarkable cycling durability (91% retention for 10 000 cycles). A piezoelectric nanogenerator (PENG) driven EC/EFC integrated system is fabricated to harvest energy from human motion and visually drive the color/fluorescence change for human motion indication in both bright and dark environments. This innovative EC/EFC dual-modal display device based on EFIL-TPA supports a huge space for the development of self-powered human motion visualized indication in all-light conditions.

Waste-Free Fully Electrically Fueled Dissipative Self-Assembly System

The importance and prevalence of energy-fueled active materials in living systems have inspired the design of synthetic active materials using various fuels. However, several major limitations of current designs remain to be addressed, such as the accumulation of chemical wastes during the process, unsustainable active behavior, and the lack of precise spatiotemporal control. Here, we demonstrate a fully electrically fueled (e-fueled) active self-assembly material that can overcome the aforementioned limitations. Using an electrochemical setup with dual electrocatalysts, the anodic oxidation of one electrocatalyst (ferrocyanide, [Fe(CN)₆]^4-) creates a positive fuel to activate the self-assembly, while simultaneously, the cathodic reduction of the other electrocatalyst (methyl viologen, [MV]²⁺) generates a negative fuel triggering fiber disassembly. Due to the fully catalytic nature for the reaction networks, this fully e-fueled active material system does not generate any chemical waste, can sustain active behavior for an extended period when the electrical potential is maintained, and provides spatiotemporal control.

Recent progress in electrochromic energy storage materials and devices: a minireview

Integration of several functionalities into one isolated electrochemical body is necessary to realize compact and tiny smart electronics. Recently, two different technologies, electrochromic (EC) materials and energy storage, were combined to create a single system that supports and drives both functions simultaneously. In EC energy storage devices, the characteristic feature of EC materials, their optical modulation depending on the applied voltage, is used to visually identify the stored energy level in real time. Moreover, combining energy-harvesting and EC storage systems by sharing one electrode facilitates the realization of further compact multifunction systems. In this minireview, we highlight recent groundbreaking achievements in EC multifunction systems where the stored energy levels can be visualized using the color of the device.

Polymeric Ion Conductors Based on Sono-Polymerized Zwitterionic Polymers for Electrochromic Supercapacitors with Improved Shelf-Life Stability

Monolithic electrochromic supercapacitors (ECSs) have attracted increasing interest in recent electrochemical electronics due to their simplicity and unique ability to visually indicate stored energy levels. One crucial challenge for practical use is the improvement of shelf-life. Herein, zwitterionic (ZI) ionogels are proposed as effective electrolytes to reduce the self-discharging decay of ECSs. All-in-one ZI electrochromic (EC) gels are produced by one-pot sono-polymerization. The presence of ZI moieties in the gel does not affect the EC characteristics of chromophores. In addition, excellent capacitive properties in areal capacitance and coulombic efficiency are presented owing to the alignment of ZI units under an electric field and the formation of ion migration channels where rapid ion transport is allowed. Furthermore, the shelf-life of the ZI gel-based ECS is significantly improved by adjusting the interaction between polymeric gelators and ion species. The ZI gel-based ECS is expected to be a key platform for future smart energy storage devices.

Redox of Viologen for Powering and Coloring

Viologen is among the most attractive and easiest-to-use organic redox active group in many functional molecular assemblies. It plays crucial roles as an electron transfer mediator in the artificial photo-energy conversion systems and electron-transfer protein assemblies and as a building block of supramolecules. Its features include electrochemically reversible redox activity and stability. Strong blue color and tendency to dimerization of the one-electron reduced form, viologen mono-radical mono-cation, are remarkable. In this Account, we describe the use of viologen to give a powered movement of small molecules and motion of millimetre-sized macroscopic soft-matters and the use of viologen ionic liquid as electrochromic materials. Attractivities of the use of viologen units for powering and coloring are demonstrated and discussed. In particular, we highlight driving of mechanical movements by π-π stacking dimerization, incorporation in a hydrogel to attain highly deformable material, induction of 2D phase transformation, and sharp color change of very thin ionic liquid layer in a compartment-less electrochromic display.

Synthesis of Ferrocenesulfonyl Chloride: Key Intermediate toward Ferrocenesulfonamides

Ferrocenesulfonyl chloride is the key intermediate in the synthesis of ferrocenesulfonamides, a family of underexplored derivatives. A one-pot synthesis of this compound, able to easily deliver multigram quantities of product, is reported. An original protocol for the synthesis of ferrocenesulfonamides is described along with highlighting the reactivity difference between arene and ferrocenesulfonyl chlorides. Finally, an example of diastereoselective deprotolithiation of chiral ferrocenesulfonamides is described.

A redox-active ionic liquid manifesting charge-transfer interaction between a viologen and carbazole and its effect on the viscosity, ionic conductivity, and redox process of the viologen

Redox-active ionic liquids (RAILs) are gaining attention as a material that can create a wide range of functions. We herein propose a charge-transfer (CT) RAIL by mixing two RAILs, specifically a carbazole-based ionic liquid ([CzC₄ImC₁][TFSI]) as a donor and a viologen-based ionic liquid ([C₄VC₇][TFSI]₂) as an acceptor. We investigated the effect of CT interaction on the physicochemical properties of the CT ionic liquid (CT-IL) using the results of temperature-dependent measurements of UV-vis absorption, viscosity, and ionic conductivity as well as cyclic voltammograms. We employed the Walden analysis and the Grunberg–Nissan model to elucidate the effect of the CT interaction on the viscosity and ionic conductivity. The CT interaction reduces the viscosity by reducing the electrostatic attraction between the dicationic viologen and TFSI anion. It also reduces the ionic conductivity by the CT association of the dicationic viologen and carbazole. The electrochemically reversible responses of the viologens in [C₄VC₇][TFSI]₂ and CT-IL are consistent with the Nernstian and the interacting two-redox site models. Notably, the transport and electrochemical properties are modulated by CT interaction, leading to unique features that are not present in individual component ILs. The inclusion of CT interaction in RAILs thus provides a powerful means to expand the scope of functionalized ionic liquids.

A redox-active ionic liquid (RAIL) consisting of a carbazole and viologen shows charge transfer (CT) interaction. The physicochemical properties are modulated by the CT interaction by comparison with the individual RAILs.

Automatic light-adjusting electrochromic device powered by perovskite solar cell

Electrochromic devices can modulate their light absorption under a small driving voltage, but the requirement for external electrical supplies causes response-lag. To address this problem, self-powered electrochromic devices have been studied recently. However, insensitivity to the surrounding light and unsatisfactory stability of electrochromic devices have hindered their critical applications. Herein, novel perovskite solar cell-powered all-in-one gel electrochromic devices have been assembled and studied in order to achieve automatic light adjustment. Two alkynyl-containing viologen derivatives are synthesized as electrochromic materials, the devices with very high stability (up to 70000 cycles) serves as the energy storage and smart window, while the perovskite solar cell with power-conversion-efficiency up to 18.3% serves as the light detector and power harvester. The combined devices can automatically switch between bleached and colored state to adjust light absorption with variable surrounding light intensity in real-time swiftly, which establish significant potentials for applications as modern all-day intelligent windows.

D–A dyads and A–D–A triads based on ferrocene: push–pull dyes with unusual behaviours in solution

Ferrocene has been extensively used for the design of chromophores with reversible electrochemical properties.

Bioinspired Color-Changeable Organogel Tactile Sensor with Excellent Overall Performance

Inspired by chameleons' structural color regulation capability, a simple, but effective, swelling method is proposed for the first time to prepare an ionic polyacrylamide (PAAm) organogel for simultaneous tactile sensing and interactive color changing. The PAAm organogel obtained by swelling the PAAm scaffold in the dimethyl sulfoxide solution of organic electrochromic material (OECM) shows an extremely large stretchability with an elongation of 1600%, a supersoftness with a compressive modulus of 7.2 kPa, an excellent transmittance up to 90%, and a very fast response time of 0.5 s combined with the characteristic of interactive color changing. The PAAm organogel also suggests a universal design ability to tailor coloration spectra for tactile sensors via simply changing the type and content of OECM. The tactile sensor based on a PAAm organogel is capable of serving as a wearable device for precisely tracing human body motion performance and directly visualizing the stress distribution via interactive color changing capability. It is demonstrated that the swelling method proposed here is a simple and practical strategy to prepare ionic organogels with both piezo-resistive and electrochromic effects.

Ultra-Low Power Electrochromic Heat Shutters Through Tailoring Diffusion-Controlled Behaviors

In this study, we propose low power consumption, all-in-one type electrochromic devices (ECDs) for effective heat shutters. Considering diffusion-controlled device operation, polymeric viologens (poly-viologens) are synthesized to lower the diffusivity of EC chromophores and to minimize self-bleaching. In comparison with devices based on mono-viologens corresponding to the monomer of poly-viologens, poly-viologen-containing ECDs exhibit advantages of lower coloration voltage (ca, -0.55 V) and higher coloration/bleaching cyclic stability (>1500 cycles). In particular, poly-viologen ECDs show remarkably reduced self-bleaching as designed, resulting in extremely low power consumption (∼8.3 μW/cm²) to maintain the colored state. Moreover, we successfully demonstrate solar heat shutters that suppress the increment of indoor temperature by taking the advantage of low-power operation and near-IR absorption of the colored poly-viologen-based ECDs. Overall, these results imply that the control of the diffusivity of EC chromophores is an effective methodology for achieving single-layered, low-power electrochemical heat shutters that can save indoor cooling energy when applied as smart windows for buildings or vehicles.

A Zr-cluster based thermostable, self-healing and adaptive metallogel with chromogenic properties responds to multiple stimuli with reversible radical interaction

A Zr-cluster based metallogel is synthesized via an unusual one-pot solvothermal method. The resulting metallogel is robust, adaptive, self-healing, highly thermostable and conductive. Moreover, the metallogel exhibits reversible stimuli-responsive properties. The gel could respond to at least four kinds of stimuli such as light, aliphatic amines, electricity and metals with color and fluorescence tunability. Importantly, the metallogel with electrochromic properties could be used as soft electrochromic devices for smart windows and electro display boards, and metalchromism provides a practical way for coating corrosion monitoring of metal materials.

A transparent-to-gray electrochromic device based on an asymmetric viologen

A transparent-to-gray electrochromic device (ECD) based on an asymmetric viologen was fabricated and characterized.

Viologen-Based Electrochromic Materials: From Small Molecules, Polymers and Composites to Their Applications

Organic materials have gained considerable attention for electrochromic (EC) applications owing to improved EC performance and good processability. As a class of well-recognized organic EC materials, viologens have received persistent attention due to the structural versatility and property tunability, and are major active EC components for most of the marketed EC devices. Over the past two decades, extensive efforts have been made to design and synthesize different types of viologen-based materials with enhanced EC properties. This review summarizes chemical structures, preparation and EC properties of various latest viologen-based electrochromes, including small viologen derivatives, main-chain viologen-based polymers, conjugated polymers with viologen side-chains and viologen-based organic/inorganic composites. The performance enhancement mechanisms are concisely discussed. The current marketed viologens-based electrochromic devices (ECDs) are briefly introduced and an outlook on the challenges and future exploration directions for viologen-based materials and their ECDs are also proposed.

X-ray and UV Dual Photochromism, Thermochromism, Electrochromism, and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster-Based 7-Fold Interpenetrated Framework

Smart materials are highly desirable over the recent decade due to the growing demand of complicated nature. Stable stimuli-responsive smart materials exhibit widespread potential for applications in smart windows, sensors, separators, chemical valves, and release platforms but are rare. Despite being good candidates, viologen-based multifunctional smart materials are still a challenging task for chemists. To obtain such materials, the judicious strategy is to introduce polynuclear metal-carboxylate clusters as electron donors into a stable framework to increase chromic sensitivity. Toward this endeavor, we have synthesized a novel viologen-based polymer with a unique Anderson-like metal-carboxylate cluster, [Zn₇(bpybc)₃(o-BDC)₆]·2NO₃·6H₂O (bpybc = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium, o-BDC = o-benzenedicarboylic acid) (1), which is a particular 7-fold interpenetrated framework with a 3D pcu network in which bpybc ligand as the linker and Zn₇O₃₀C₁₂ as the second building unit (Zn₇ SBU) were used as 6-connected nodes. More importantly, it shows excellent chromic behavior in response to multiple external stimuli especially soft X-ray and UV dual light, temperature, electricity, and organic amines, which stand out in the viologen-based polymers. Interestingly, the coloration process of 1 from "core" to "edge" is observed upon heating at the appropriate temperature, which has not yet been found in other reported thermochromic materials. Of particular interest for 1 is the couple of quaternary stimuli-sensitive abilities because it simultaneously meets the following conditions: (i) the capability of withstanding high light, higher temperature, extreme pH, and other harsh conditions; and (ii) the high sensitivity to external stimuli keeping away from photodegradation, thermal relaxation, side reactions, and so on. To be noted, 1 has high thermal stability and chemical stability, which are excellent advantages as smart materials. To further develop possible practical utilization, 1 has been doped into the polymer matrixes to construct a hybrid film, which not only keeps the response to external stimuli but also significantly improves the repeatability of the photochromic process, indicating that a new smart device with multi-stimuli-responsive functions will emerge successively in the future.

Electrochromism of Ferrocene- and Viologen-Based Redox-Active Ionic Liquids Composite

Redox-active ionic liquids (RAILs) require no other additional reagents such as solvent and supporting electrolyte for electrochemical reactions under undiluted condition. Viologen-based RAILs are one of the electrochromic (EC) ionic liquids with sharp color contrast and high chemical stability. An operation of an EC cell requires two electroactive elements, an EC material and a charge compensating material. In this study, an equimolar composite of a viologen-based RAIL as the EC material and a ferrocene-based RAIL as the charge compensation material, was synthesized and applied to an EC cell. The EC cell with the composite RAIL of as high concentration as 0.92 M each redox species showed good coloration efficiency (91.4 cm² C^-1 at 540 nm on 1.0 V). The coloration process of the EC cell was diffusion-limited process. The current and absorbance of the EC cell reached constant values at large enough bias voltage because of the charge recombination between reduced viologens and oxidized ferrocenes. The recombination affected rapid color erasing process. Almost no deterioration of the composite RAIL was found by ¹H NMR after 13 000 potential cycle durability experiment.

Ligand Impact on Monolayer Electrochromic Material Properties

In this study, we present a range of efficient highly durable electrochromic materials that demonstrate excellent redox and lifetime stability, sufficient coloration contrast ratios, and the best-in-class electron-transfer constants. The materials were formed by anchoring as little as a monolayer of predefined iron complexes on a surface-enhanced conductive solid support. The thickness of the substrate was optimized to maximize the change in optical density. We demonstrate that even a slight change in molecular sterics and electronics results in materials with sufficiently different properties. Thus, minor changes in the ligand design give access to materials with a wide range of color variations, including green, purple, and brown. Moreover, ligand architecture dictates either orthogonal or parallel alignment of corresponding metal complexes on the surface due to mono- or bis-quaternization. We demonstrate that monoquaternization of the complexes during anchoring to the surface-bound template layer results in redshifts of the photoabsorption peak. The results of in-solution bis-methylation supported by density functional theory calculations show that the second quaternization may lead to an opposite blueshift (in comparison with monomethylated analogs), depending on the ligand electronics and the environmental change. It is shown that the variations of the photoabsorption peak position for different ligands upon attachment to the surface can be related to the calculated charge distribution and excitation-induced redistribution. Overall, the work demonstrates a well-defined method of electrochromic material color tuning via manipulation of sterics and electronics of terpyridine-based ligands.

From Broadband to Electrochromic Notch Filters with Printed Monochiral Carbon Nanotubes

Dense layers of semiconducting single-walled carbon nanotubes (SWNTs) serve as electrochromic (EC) materials in the near-infrared with high optical density and high conductivity. EC cells with tunable notch filter properties instead of broadband absorption are created via highly selective dispersion of specific semiconducting SWNTs through polymer-wrapping followed by deposition of thick films by aerosol-jet printing. A simple planar geometry with spray-coated mixed SWNTs as the counter electrode renders transparent metal oxides redundant and facilitates complete bleaching within a few seconds through iongel electrolytes with high ionic conductivities. Monochiral (6,5) SWNT films as working electrodes exhibit a narrow absorption band at 997 nm (full width at half-maximum of 55-73 nm) with voltage-dependent optical densities between 0.2 and 4.5 and a modulation depth of up to 43 dB. These (6,5) SWNT notch filters can retain more than 95% of maximum bleaching for several hours under open-circuit conditions. In addition, different levels of transmission can be set by applying constant low voltage (1.5 V) pulses with modulated width or by a given number of fixed short pulses.

Reversible Silver Electrodeposition from Boron Cluster Ionic Liquid (BCIL) Electrolytes

Electrochemical systems offer a versatile means for creating adaptive devices. However, the utility of electrochemical deposition is inherently limited by the properties of the electrolyte. The development of ionic liquids enables electrodeposition in high-vacuum environments and presents opportunities for creating electrochemically adaptive and regenerative spacecraft components. In this work, we developed a silver-rich, boron cluster ionic liquid (BCIL) for reversible electrodeposition of silver films. This air and moisture stable electrolyte was used to deposit metallic films in an electrochemical cell to tune the emissivity of the cell in situ, demonstrating a proof-of-concept design for spacecraft thermal control.

Enhancing thermoelectrochemical properties by tethering ferrocene to the anion or cation of ionic liquids: altered thermodynamics and solubility

Entropic changes inherent within a redox process typically result in significant temperature sensitivity. This can be utilised positively or can be a detrimental process. This study has investigated the thermoelectrochemical properties (temperature-dependant electrochemistry) of the ferrocenium|ferrocene redox couple in an ionic liquid, and in particular the effect of covalently tethering this redox couple to fixed positive or negative charges. As such, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was employed to dissolve ferrocene, as well as cationic-tethered ferrocene (the 1-ethyl-3-(methylferrocenyl)imidazolium cation) and anionic-tethered ferrocene (the ferrocenylsulfonyl(trifluoromethylsulfonyl)imide anion). These systems were characterised in terms of their voltammetry (apparent formal potentials, diffusion coefficients and electron transfer rate constants) and thermoelectrochemistry (temperature coefficients of the cell potential or 'Seebeck coefficients', short circuit current densities and power density outputs). The oxidised cationic species behaved like a dicationic species and was thus 6-fold more effective at converting waste thermal energy to electrical power within a thermoelectrochemical cell than unmodified ferrocene. This was almost exclusively due to a significant boost in the Seebeck coefficient of this redox couple. Conversely, the oxidised anionic species was formally a zwitterion, but this zwitterionic species behaved thermodynamically like a neutral species. The inverted entropic change upon going from ferrocene to anion-tethered ferrocene allowed development of a largely temperature-insensitive reference potential based upon a mixture of acetylferrocene and ferricenyl(iii)sulfonyl(trifluoromethylsulfonyl)imide.