Ionic Nanosystems: Large Space‐Charge Effects in a Nanostructured Proton Conductor (Adv. Funct. Mater. 23/2010)

Decreasing the dimensions of heterogeneous mixtures of ionic conductors towards the nanoscale results in ionic conduction enhancements, caused by the increased influence of the interfacial space‐charge regions. For a composite of TiO₂ anatase and solid acid CsHSO₄, the strong enhancement of the ionic conductivity at the nanoscale also can be assigned to this space‐charge effect. Surprisingly high hydrogen concentrations in the order of 10²¹ cm^?3 in TiO₂ are measured, which means that about 10% of the available sites for H⁺ ions are filled on average. Such high concentrations require a specific elaboration of the space‐charge model that is explicitly performed here, by taking account of the large occupation numbers on the exhaustible sites. It is shown that ionic defects with negative formation enthalpy reach extremely high concentrations near the interfaces and throughout the material. By performing first‐principles density functional theory calculations, it is found that proton insertion from CsHSO₄ into the TiO₂ particles is preferred compared to neutral hydrogen atom insertion and indeed that the formation enthalpy is negative. Moreover, the average proton fractions in TiO₂, estimated by the theoretical ionic density profiles, are in good agreement with the experimental observations.     

Ionic Current Rectification in Soft‐Matter Diodes with Liquid‐Metal Electrodes

A soft‐matter‐based diode composed of hydrogel and liquid metal (eutectic gallium indium, EGaIn) is presented. The ability to control the thickness, and thus resistivity, of an oxide skin on the metal enables rectification. First, a simple model system with liquid‐metal/electrolyte‐solution/Pt interfaces is characterized. The electrically insulating oxide skin on the EGaIn electrode is reduced or oxidized further depending on the direction of the bias, thereby allowing unidirectional ionic current. The forward current of the diode increases as the conductivity of the electrolyte increases, whereas backward current depends on the pH of the medium in contact with the insulating oxide layer on the EGaIn electrode. As a result, the diode shows a higher rectification ratio (defined as the ratio of forward to backward current measured at the same absolute bias) with more conductive electrolyte at neutral pH. Replacement of the liquid electrolyte solution with a hydrogel improves the structural stability of the soft diode. The rectification performance also improves due to the increased ionic conductivity by the gel. Finally, a diode composed entirely of soft materials by replacing the platinum electrode with a second liquid‐metal electrode is presented. Contacting each liquid metal with a polyelectrolyte gel featuring different pH values provided asymmetry in the device, which is necessary for rectification. A hydrogel layer infused with a strong basic polyelectrolyte removes the insulating oxide layer, allowing one interface with the EGaIn electrode to be conductive regardless of the direction of bias. Thus, the oxide layer at the other interface rectifies the current.     

Solvent‐Free Ionic Liquid Electrolytes for Mesoscopic Dye‐Sensitized Solar Cells

Ionic liquids have been identified as a new class of solvent that offers opportunities to move away from the traditional solvents. The physical‐chemical properties of ionic liquids can be tuned and controlled by tailoring their structures. The typical properties of ionic liquids, such as non‐volatility, electrochemical stability and high conductivity, render them attractive as electrolytes for dye‐sensitized solar cells. However, the high viscosity of ionic liquids leads to mass transport limitations on the photocurrents in the solar cells at full sunlight intensity, but the contribution of a Grotthous‐type exchange mechanism in these viscous electrolytes helps to alleviate these diffusion problems. This article discusses recent developments in the field of high‐performance dye‐sensitized solar cells with ionic liquid‐based electrolytes and their characterization by electrochemical impedance analysis.     

One‐Pot Synthesis of Luminescent Polymer‐Nanoparticle Composites from Task‐Specific Ionic Liquids

A multifunctional polymerizable ionic liquid, diallyldimethylammonium tetrafluoroborate (DADMA BF₄), is used in a one‐pot synthesis of novel luminescent polymer‐nanoparticle composites. First, small monodisperse lanthanide fluoride nanoparticles are formed by microwave irradiation in the presence of Ln(OAc)₃·xH₂O (Ln = Gd, Eu, Tb; OAc = acetate) in the ionic liquid. The nanoparticles can be precipitated for structural characterization or kept in the solution, which yields after irradition by high intensity UV light colorless, processable polymer materials with good photophysical properties. Both green‐emitting Tb‐containing and red‐emitting Eu‐containing IL‐ polymers are described.     

All‐Inorganic Ionic Polymer‐Based Memristor for High‐Performance and Flexible Artificial Synapse

The implementation of memristors that are wearable and transparent has attracted significant attention. However, the development of high‐performance memristors that simultaneously possess high flexibility and environmental stability has remained a tremendous challenge suffering from limited choice of materials with both good ion‐electron mobility and structural flexibility. Inspired by the unique poly‐ionic nature of ammonium polyphosphate (APP), a novel Au/APP/ITO memristor with favorable flexibility and stability is prepared. Synaptic behaviors can be stimulated by voltage pulses that are 20 ns in width, 0.1 V in amplitude, and repeatable under 10⁴ pulse cycles, thereby outperforming several other benchmark memristors. Further, the device, prepared on conductive silicone, can sustain its synaptic performance even under 360° bending. Furthermore, the device can sustain its synaptic behaviors even after exposure to fire for 60 s and 5.6 kGy of ionic irradiation. Additionally, APP is determined to be nontoxic, biodegradable, and transparent when compared with all the organics and inorganics used in previous memristors. The results of this study will inspire the development of more inorganic polymers for their utilization in future environmentally stable and flexible electronics.     

Effects of the Ionic Currents in Electrolyte‐gated Organic Field‐Effect Transistors

Polyelectrolytes are promising materials as gate dielectrics in organic field‐effect transistors (OFETs). Upon gate bias, their polarization induces an ionic charging current, which generates a large double layer capacitor (10–500 µF cm^?2) at the semiconductor/electrolyte interface. The resulting transistor operates at low voltages (<1 V) and its conducting channel is formed in ～50 µs. The effect of ionic currents on the performance of the OFETs is investigated by varying the relative humidity of the device ambience. Within defined humidity levels and potential values, the water electrolysis is negligible and the OFETs performances are optimum.     

Macroscopically Aligned Ionic Self‐Assembled Perylene‐Surfactant Complexes within a Polymer Matrix

Ionic self‐assembled (ISA) surfactant complexes present a facile concept for self‐assembly of various functional materials. However, no general scheme has been shown to allow their overall alignment beyond local polydomain‐like order. Here we demonstrate that ionic complexes forming a columnar liquid‐crystalline phase in bulk can be aligned within polymer blends upon shearing, taken that the matrix polymers have sufficiently high molecular weight. We use an ISA complex of N,N′‐bis(ethylenetrimethylammonium)perylenediimide/bis(2‐ethylhexyl) phosphate (Pery‐BEHP) blended with different molecular weight polystyrenes (PS). Based on X‐ray scattering studies and transmission electron microscopy the pure Pery‐BEHP complex was found to form a two‐dimensional oblique columnar phase where the perylene units stack within the columns. Blending the complex with PS lead to high aspect ratio Pery‐BEHP aggregates with lateral dimension in the mesoscale, having internal columnar liquid‐crystalline order similar to the pure Pery‐BEHP complex. When the Pery‐BEHP/PS blend was subjected to a shear flow field, the alignment of perylenes can be achieved but requires sufficiently high molecular weight of the polystyrene matrix. The concept also suggests a simple route for macroscopically aligned nanocomposites with conjugated columnar liquid‐crystalline functional additives.     

Polyoxometalate‐coupled Graphene via Polymeric Ionic Liquid Linker for Supercapacitors

The integration of electrical double‐layer capacitive and pseudocapacitive materials into novel hybrid materials is crucial to realize supercapacitors with high energy and power densities. Here, high levels of energy and power densities are demonstrated in supercapacitors based on a new type of nanohybrid electrode consisting of polyoxometalate (POM)‐coupled graphene in which a polymeric ionic liquid (henceforth simply PIL) serves as an interfacial linker. The adoption of PIL in the construction of nanohybrids enables a uniform distribution of discrete POM molecules along with a large surface area of graphene sheets. When testing electrochemical characteristics under a two‐electrode system, as‐prepared supercapacitors exhibit a high specific capacitance (408 F g^?1 at 0.5 A g^?1), rapid rate capability (92% retention at 10 A g^?1), a long cycling life (98% retention during 2000 cycles), and high energy (56 Wh kg^?1) and power (52 kW kg^?1) densities. First‐principles calculations and impedance spectroscopy analysis reveal that the PILs enhance the redox reactions of POMs by providing efficient ion transfer channels and facilitating the charge transfer in the nanohybrids.     

Solid Polymer Electrolytes Based on Ionic Graft Polymers: Effect of Graft Chain Length on Nano‐Structured,Ionic Networks

The length of graft chains in graft polymers is controlled in order to dictate the formation of a nanochannel network of ions in a non‐ionic matrix. Graft polymers were prepared by copolymerization of styrene with poly(sodium styrene sulfonate) (PSSNa) macromonomers. The latter were prepared with controlled molecular weight and narrow polydispersity by stable free radical polymerization. Phase separation of ionic aggregates occurs to a greater extent in films prepared from amphiphilic polymers possessing longer graft chains. Films prepared from polymers containing low ion content comprise of isolated ionic domains and exhibit low ionic conductivity. Increasing the ion content with the membrane, by increasing the number density of ionic graft chains in the polymer, results in ionic domains that coalesce into a network of nanochannels, and a dramatic increase in ion conductivity is observed. The ionic network is developed to a greater extent for films based on longer ionic graft chain polymers; an observation explained on the basis of phase separation.     

Tuning the Viscosity Profile of Ionic Vitrimers Incorporating 1,2,3‐Triazolium Cross‐Links

Vitrimers are dynamic polymer networks with unique viscoelastic behavior combining the best attributes of thermosets and thermoplastics. Ionic vitrimers are a recent class of dynamic materials, where 1,2,3‐triazolium cross‐links are reshuffled by trans‐N‐alkylation exchange reactions. Comparison of dynamic properties with a selection of vitrimers relying on different exchange reactions highlights the particularly high viscous flow activation energies of trans‐N‐alkylation reactions, thus providing an enhanced compromise between fast reprocessing at moderately high temperatures and low creep at service temperature. Varying the [monomer]/[cross‐linker] ratio in the initial formulation of these 1,2,3‐triazolium‐based networks affords a fine tuning of their viscosity profiles. Confrontation of rheometry and X‐ray photoelectron spectroscopy data allows the correlation of variations in chemical composition with changes in the covalent exchange dynamics. This unprecedented approach enables the proposition of a dissociative two‐step mechanism for the trans‐N‐alkylation of 1,2,3‐triazoliums initiated by a nucleophilic attack of the 1,2,3‐triazolium cross‐links by the iodide counteranion, yielding uncrosslinking by de‐N‐alkylation. Subsequent rapid re‐N‐alkylation of the formed 1,2,3‐triazole by surrounding iodide‐functionalized dangling chains affords exchange of the cross‐link position. This study highlights that strictly associative exchange reactions are not compulsory to induce vitrimer behavior, and may pave the way to a much wider variety of vitrimers relying on conventional reversible covalent reactions.     

Dynamic Doping in Planar Ionic Transition Metal Complex‐Based Light‐Emitting Electrochemical Cells

Using a planar electrode geometry, the operational mechanism of iridium(III) ionic transition metal complex (iTMC)‐based light‐emitting electrochemical cells (LECs) is studied by a combination of fluorescence microscopy and scanning Kelvin probe microscopy (SKPM). Applying a bias to the LECs leads to the quenching of the photoluminescence (PL) in between the electrodes and to a sharp drop of the electrostatic potential in the middle of the device, far away from the contacts. The results shed light on the operational mechanism of iTMC‐LECs and demonstrate that these devices work essentially the same as LECs based on conjugated polymers do, i.e., according to an electrochemical doping mechanism. Moreover, with proceeding operation time the potential drop shifts towards the cathode coincident with the onset of light emission. During prolonged operation the emission zone and the potential drop both migrate towards the anode. This event is accompanied by a continuous quenching of the PL in two distinct regions separated by the emission line.     

Tough Transient Ionic Junctions Printed with Ionic Microgels

Emerging soft ionotronics better match the human body mechanically and electrically compared to conventional rigid electronics. They hold great potential for human-machine interfaces, wearable and implantable devices, and soft machines. Among various ionotronic devices, ionic junctions play critical roles in rectifying currents as electrical p–n junctions. Existing ionic junctions, however, are limited in electrical and mechanical performance, and are difficult to fabricate and degrade. Herein, the design, fabrication, and characterization of tough transient ionic junctions fabricated via 3D ionic microgel printing is reported. The 3D printing method demonstrates excellent printability and allows one to fabricate ionic junctions of various configurations with high fidelity. By combining ionic microgels, degradable networks, and highly charged biopolymers, the ionic junctions feature high stretchability (stretch limit 27), high fracture energy (>1000 Jm⁻²), excellent electrical performance (current rectification ratio >100), and transient stability (degrade in 1 week). A variety of ionotronic devices, including ionic diodes, ionic bipolar junction transistors, ionic full-wave rectifiers, and ionic touchpads are further demonstrated. This study merges ionotronics, 3D printing, and degradable hydrogels, and will motivate the future development of high-performance transient ionotronics.     

Polypyrrole‐Derived Activated Carbons for High‐Performance Electrical Double‐Layer Capacitors with Ionic Liquid Electrolyte

As electrical energy storage and delivery devices, carbon‐based electrical double‐layer capacitors (EDLCs) have attracted much attention for advancing the energy‐efficient economy. Conventional methods for activated carbon (AC) synthesis offer limited control of their surface area and porosity, which results in a typical specific capacitance of 70–120 F g^?1 in commercial EDLCs based on organic electrolytes and ionic liquids (ILs). Additionally, typical ACs produced from natural precursors suffer from the significant variation of their properties, which is detrimental for EDLC use in automotive applications. A novel method for AC synthesis for EDLCs is proposed. This method is based on direct activation of synthetic polymers. The proposed procedure allowed us to produce ACs with ultrahigh specific surface area of up to 3432 m² g^?1 and volume of 0.5–4 nm pores up to 2.39 cm³ g^?1. The application of the produced carbons in EDLCs based on IL electrolyte showed specific capacitance approaching 300 F g^?1, which is unprecedented for carbon materials, and 5–8% performance improvement after 10 000 charge–discharge cycles at the very high current density of 10 A g^?1. The remarkable characteristics of the produced materials and the capability of the fabricated EDLCs to operate safely in a wide electrochemical window at elevated temperatures, suggest that the proposed synthesis route offers excellent potential for large‐scale material production for EDLC use in electric vehicles and industrial applications.     

Balancing Ionic and Electronic Conduction for High‐Performance Organic Electrochemical Transistors

Conjugated polymers that support mixed (electronic and ionic) conduction are in demand for applications spanning from bioelectronics to energy harvesting and storage. To design polymer mixed conductors for high‐performance electrochemical devices, relationships between the chemical structure, charge transport, and morphology must be established. A polymer series bearing the same p‐type conjugated backbone with increasing percentage of hydrophilic, ethylene glycol side chains is synthesized, and their performance in aqueous electrolyte gated organic electrochemical transistors (OECTs) is studied. By using device physics principles and electrochemical analyses, a direct relationship is found between the OECT performance and the balanced mixed conduction. While hydrophilic side chains are required to facilitate ion transport—thus enabling OECT operation—swelling of the polymer is not de facto beneficial for balancing mixed conduction. It is shown that heterogeneous water uptake disrupts the electronic conductivity of the film, leading to OECTs with lower transconductance and slower response times. The combination of in situ electrochemical and structural techniques shown here contributes to the establishment of the structure–property relations necessary to improve the performance of polymer mixed conductors and subsequently of OECTs.     

All‐Solid‐State Asymmetric Supercapacitors with Metal Selenides Electrodes and Ionic Conductive Composites Electrolytes

All‐solid‐state flexible asymmetric supercapacitors (ASCs) are developed by utilization of graphene nanoribbon (GNR)/Co_0.85Se composites as the positive electrode, GNR/Bi₂Se₃ composites as the negative electrode, and polymer‐grafted‐graphene oxide membranes as solid‐state electrolytes. Both GNR/Co_0.85Se and GNR/Bi₂Se₃ composite electrodes are developed by a facile one‐step hydrothermal growth method from graphene oxide nanoribbons as the nucleation framework. The GNR/Co_0.85Se composite electrode exhibits a specific capacity of 76.4 mAh g^?1 at a current density of 1 A g^?1 and the GNR/Bi₂Se₃ composite electrode exhibits a specific capacity of 100.2 mAh g^?1 at a current density of 0.5 A g^?1. Moreover, the stretchable membrane solid‐state electrolytes exhibit superior ionic conductivity of 108.7 mS cm^?1. As a result, the flexible ASCs demonstrate an operating voltage of 1.6 V, an energy density of 30.9 Wh kg^?1 at the power density of 559 W kg^?1, and excellent cycling stability with 89% capacitance retention after 5000 cycles. All these results demonstrate that this study provides a simple, scalable, and efficient approach to fabricate high performance flexible all‐solid‐state ASCs for energy storage.     

Monolithic Dual‐Material 3D Printing of Ionic Skins with Long‐Term Performance Stability

Artificial “ionic skin” is of great interest for mimicking the functionality of human skin, such as subtle pressure sensing. However, the development of ionic skin is hindered by the strict requirements of device integration and the need for devices with satisfactory performance. Here, a dual‐material printing strategy for ionic skin fabrication to eliminate signal drift and performance degradation during long‐term use is proposed, while endowing the ionic skins with high sensitivity by 3D printing of ionic hydrogel electrodes with microstructures. The ionic skins are fabricated by alternative digital light processing 3D printing of two photocurable precursors: hydrogel and water‐dilutable polyurethane acrylate (WPUA), in which the ionically conductive hydrogel layers serve as soft, transparent electrodes and the electrically insulated WPUA as flexible, transparent dielectric layers. This novel dual‐material printing strategy enables strong chemical bonding between the hydrogel and the WPUA, endowing the device with designed characteristics. The resulting device has high sensitivity, minimal hysteresis, a response time in the millisecond range, and excellent repetition durability for pressure sensing. The results demonstrate the potential of the dual‐material 3D printing strategy as a pathway to realize highly stable and high‐performance ionic skin fabrication to monitor human physiological signals and human–machine interactions.     

A Novel Moisture‐Insensitive and Low‐Corrosivity Ionic Liquid Electrolyte for Rechargeable Aluminum Batteries

Rechargeable aluminum batteries (RABs) are extensively developed due to their cost‐effectiveness, eco‐friendliness, and low flammability and the earth abundance of their electrode materials. However, the commonly used RAB ionic liquid (IL) electrolyte is highly moisture‐sensitive and corrosive. To address these problems, a 4‐ethylpyridine/AlCl₃ IL is proposed. The effects of the AlCl₃ to 4‐ethylpyridine molar ratio on the electrode charge–discharge properties are systematically examined. A maximum graphite capacity of 95 mAh g^?1 is obtained at 25 mA g^?1. After 1000 charge–discharge cycles, ≈85% of the initial capacity can be retained. In situ synchrotron X‐ray diffraction is employed to examine the electrode reaction mechanism. In addition, low corrosion rates of Al, Cu, Ni, and carbon‐fiber paper electrodes are confirmed in the 4‐ethylpyridine/AlCl₃ IL. When opened to the ambient atmosphere, the measured capacity of the graphite cathode is only slightly lower than that found in a N₂‐filled glove box; moreover, the capacity retention upon 100 cycles is as high as 75%. The results clearly indicate the great potential of this electrolyte for practical RAB applications.