Zwitterionic Polymer Ionogel Electrolytes Supported by Coulombic Cross-Links: Impacts of Alkali Metal Cation Identity.

Zwitterionic (ZI) polymers enable the formation of noncovalent cross-links within ionic liquid electrolytes (ILEs) to create nonflammable, mechanically robust, and highly conductive ionogel electrolytes. In this study, ZI homopolymer poly(2-methacryloyloxyethyl phosphorylcholine) [poly(MPC)] scaffolds are synthesized in situ within lithium and/or sodium salt-based ILEs to construct a series of ionogels that contain between 3 and 15 wt % poly(MPC). Room-temperature ionic conductivity values of these ionogels are found to vary between approximately 1.3 and 2.2 mS cm-1. For sodium only and 1:1 lithium/sodium equimolar mixed salt ionogels containing 6 wt % poly(MPC), the ionic conductivity is found to improve by 14% compared to the neat ILE due to the presence of the ZI scaffold. Moreover, comparing the elastic modulus values of lithium- versus sodium-containing ionogels revealed a difference of up to 1 order of magnitude [10.6 vs 111 kPa, respectively, for 3 wt % poly(MPC)]. Molecular dynamics simulations of ionogel precursor solutions corroborate the experimental results by demonstrating differences in the lithium/ZI monomer and sodium/ZI monomer cluster size distributions formed, which is hypothesized to influence the scaffold network cross-link density obtained upon photopolymerization. This work provides insights into why ZI polymer-supported ionogel properties that are relevant for the development of safer electrolytes for lithium-ion and sodium-ion batteries depend upon the chemical identity of the alkali metal cation.

Zwitterionic Materials for Enhanced Battery Electrolytes.

Zwitterions (ZIs), which are molecules bearing an equal number of positive and negative charges and typically possessing large dipole moments, can play an important role in improving the characteristics of a wide variety of novel battery electrolytes. Significant Coulombic interactions among ZI charged groups and any mobile ions present can lead to several beneficial phenomena within electrolytes, such as increased salt dissociation, the formation of ordered pathways for ion transport, and enhanced mechanical robustness. In some cases, ZI additives can also boost electrochemical stability at the electrolyte/electrode interface and enable longer battery cycling. Here, a brief summary of selected key historical and recent advances in the use of ZI materials to enrich the performance of three distinct classes of battery electrolytes is presented. These include: ionic liquid-based, conventional solvent-based, and solid matrix-based (non-ceramic) electrolytes. Exploring a greater chemical diversity of ZI types and electrolyte pairings will likely lead to more discoveries that can empower next-generation battery designs in the years to come.

A quantitative thermodynamic metric for identifying deep eutectic solvents.

Deep eutectic solvents (DESs) are a rapidly expanding class of liquid phase mixtures that offer many useful features. However, there currently exists no widely accepted criterion to identify whether or not a particular mixture is, in fact, a DES. This study introduces a quantitative metric based on the molar excess Gibbs energy of a eutectic mixture and proposes a threshold value in order to classify a eutectic system as a DES.

Understanding Lithium Dendrite Suppression by Hybrid Composite Separators: Indentation Measurements Informed by Operando X-ray Computed Tomography.

This study investigates the significance of the mechanics of hybrid particle-polymer separators in the stabilization of lithium metal interfaces by probing these properties in realistic conditions informed by X-ray microcomputed tomography (micro-CT). Elastic properties and viscoelastic behavior of inorganic microparticle-filled poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films are characterized using a nanoindentation experiment whose displacement simulates the interfacial response seen in operando micro-CT. It is determined that the dominating mechanical behavior in this hybrid separator relevant to lithium metal cell conditions is comprised of viscoelasticity. Consistent with this finding, along with correlations across other physicochemical properties, a mechanism describing the improvement of lithium metal cycling performance according to inorganic filler type and content is proposed.

Influence of Hydrogen Bond Donor Identity and Intentional Water Addition on the Properties of Gelatin-Supported Deep Eutectic Solvent Gels.

Deep eutectic solvent (DES) gel electrolytes have recently emerged as promising alternatives to ionic liquid- or water-based gels for "ionic skin" sensor applications. Researchers have also been exploring the effects that varying amounts of water may have on the local hydrogen bonding environment within a few model DES systems. In this study, the physical properties and ionic conductivities of biopolymer (gelatin)-supported gels featuring two established DESs and three DES/water mixture formulations are investigated and compared. The DES/water mixtures are formed by combining choline chloride with one of three organic hydrogen bond donors (HBDs), ethylene glycol, glycerol, or 1,2-propanediol, in a 1:2 molar ratio, together with a controlled amount of water, 25 mol % (approximately 5-6 wt % water). For the same fixed gelatin content (20 wt %), DES/water mixture gel Young's modulus values are found to be tunable based on the organic HBD identity, increasing 6-fold from 7 (1,2-propanediol) to 42 (glycerol) kPa. Furthermore, large differences are observed in the resulting gel properties when water has been intentionally added to well-studied DESs. Coformulation with water is found to increase ethylene glycol-based DES gel toughness, measured via tensile testing, from 23 to 68 kJ/m3 while simultaneously boosting gel room temperature ionic conductivity from 3.3 to 5.2 mS/cm. These results highlight the multiple roles that controlled amounts of water in DES can play within gelatin-supported DES/mixture gel electrolytes, such as influencing gelatin self-assembly and reducing local viscosity to promote facile ion transport.

Turning on solid-state phosphorescence of platinum acetylides with aromatic stacking.

Neat solids that phosphoresce under ambient conditions are rare due to aggregation-caused quenching. This communication describes a platinum acetylide (PtPE) that phosphoresces as a solid due to programmed aromatic stacking interactions of pendant groups that prevent intermolecular aggregation.

Highly Flexible Transistor Threads for All-Thread Based Integrated Circuits and Multiplexed Diagnostics.

Physically intimate, real-time monitoring of human biomarkers is becoming increasingly important to modern medicine and patient wellness. Such monitoring is possible due to advances in soft and flexible materials, devices and bioelectronics systems. Compared to other flexible platforms, multifilament textile fibers or threads offer superior flexibility, material diversity, and simple ambient processing to realize a wide range of flexible devices such as sensors, electronics, and microfluidics. In this paper, we realize unique flexible transistors on threads and interconnect them to realize logic gates and small-scale integrated circuits. Compared to prior textile-based transistors, the proposed thread-based transistors (TBTs) are realized with a readily shaped, colloidally dispersed gel consisting of silica nanoparticles and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI) ionic liquid for all-around electrolyte gating of a carbon nanotube (CNT) semiconducting network assembled on the thread. We interconnect TBTs with thread-based electrochemical sensors (TBEs) to realize an all-thread based multiplexed diagnostic device. All-thread based platforms are thin, highly flexible and conformal, allowing them to be worn directly on the skin without any polymeric substrate, or sutured transdermally using a needle.

Colorimetric Gas Sensing Washable Threads for Smart Textiles.

A fabrication method for a stable entrapment of optically responsive dyes on a thread substrate is proposed to move towards a detection system that can be integrated into clothing. We use the dyes 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnTPP), methyl red (MR), and bromothymol blue (BTB), for a proof-of-concept. Our optical approach utilizes a smartphone to extract and track changes in the red (R), green (G) and blue (B) channel of the acquired images of the thread to detect the presence of an analyte. We demonstrate sensing of 50-1000 ppm of vapors of ammonia and hydrogen chloride, components commonly found in cleaning supplies, fertilizer, and the production of materials, as well as dissolved gas sensing of ammonia. The devices are shown to be stable over time and with agitation in a centrifuge. This is attributed to the unique dual step fabrication process that entraps the dye in a stable manner. The facile fabrication of colorimetric gas sensing washable threads is ideal for the next generation of smart textile and intelligent clothing.

Fully-Zwitterionic Polymer-Supported Ionogel Electrolytes Featuring a Hydrophobic Ionic Liquid.

In this report, fully-zwitterionic (ZI) copolymer scaffolds for ionogel electrolytes have been synthesized via in situ photopolymerization using various molar ratios of 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine vinylimidazole (SBVI) within the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI). Depending on the chemical composition of the ZI scaffold, ionogel room temperature ionic conductivities are found to vary between 2.5 and 6.7 mS cm-1 at a fixed 20 mol % total polymer content. Compressive elastic moduli also exhibit a strong dependence on the co-monomer ratio, with values between 23 kPa and 11 MPa observed because of different degrees of ZI physical cross-linking. These results, together with NMR chemical shift analysis, suggest that the phosphorylcholine ZI group of MPC interacts more strongly with EMI TFSI, while SBVI prefers to self-aggregate and form dipole-dipole cross-links in the ionic liquid (IL). Self-diffusivity measurements of the EMI+ cations and TFSI- anions in both ionogel and ZI solution samples confirm that slower ion diffusion in MPC-containing systems is due to attractive zwitterion/IL interactions, and not merely reduced mobility in the presence of a polymeric scaffold. This work highlights the importance of relative zwitterion/IL and ZI dipole-dipole interactions on the properties of a novel class of fully-ZI polymer-supported ionogel electrolytes containing a hydrophobic IL suitable for future electrical energy storage applications.

Enhanced Lithium Ion Transport in Poly(ethylene glycol) Diacrylate-Supported Solvate Ionogel Electrolytes via Chemically Cross-linked Ethylene Oxide Pathways.

Lithium-ion solvate ionic liquids (SILs), consisting of complexed Li+ cations and a weakly basic anion, represent an emergent class of nonvolatile liquid electrolytes suitable for lithium-based electrochemical energy storage. In this report, solid-state, flexible solvate ionogel electrolytes are synthesized via UV-initiated free radical polymerization/cross-linking of poly(ethylene glycol) diacrylate (PEGDA) in situ within the [Li(G4)][TFSI] electrolyte, which is formed by an equimolar mixture of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and tetraglyme (G4). Ion diffusivity measurements reveal enhanced Li+ diffusion in PEGDA-supported solvate ionogels, as compared to poly(methyl methacrylate)-supported gels that lack ethylene oxide chains. At 21 vol% PEGDA, a maximum Li+ transport number of 0.58 and a room temperature ionic conductivity of 0.43 mS/cm have been achieved in a solvate ionogel electrolyte that exhibits an elastic modulus of 0.47 MPa. These results demonstrate the importance of polymer scaffold selection on solvate ionogel electrolyte performance for advanced lithium-based batteries.

Etching of electrodeposited Cu2O films using ammonia solution for photovoltaic applications.

Impurities at the surface of electrodeposited p-Cu2O films have been efficiently removed through the use of concentrated aqueous ammonia solution as a wet etching agent. The performance of the Cu2O homojunction photovoltaic devices incorporating etched p-Cu2O as the bottom layer is higher compared to devices with as-deposited p-Cu2O layers due to an improvement of the homojunction interface quality. Reducing the density of defect states that act as carrier recombination centers led to larger open circuit voltages. The ammonia etchant has been found to preferentially interact with the {100} facets of Cu2O and expose a greater number of {111} facets, resulting in increased interface area for etched homojunction devices, which also improved short circuit current density values.

Deciphering Physical versus Chemical Contributions to the Ionic Conductivity of Functionalized Poly(methacrylate)-Based Ionogel Electrolytes.

Polymer-supported ionic liquids (ionogels) are emergent, nonvolatile electrolytes for electrochemical energy storage applications. Here, chemical and physical interactions between the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI) and three different cross-linked polymer scaffolds with varying chemical functional groups have been investigated in ionogels fabricated via in situ UV-initiated radical polymerization of methyl methacrylate (MMA), 2,2,2-trifluoroethyl methacrylate (TFEMA), or 2-(dimethylamino)ethyl methacrylate (DMAEMA) and a small amount of the cross-linker pentaerythritol tetraacrylate. Experimental findings demonstrate that the chemical functionality of the polymer side groups can significantly affect the degree of ion dissociation within the ionic liquid component of the ionogel and that the fraction of dissociated ions is the dominant factor in determining relative ionic conductivity in these materials, rather than any large differences in ion diffusivity. The MMA-based polymer scaffold exhibits a stronger attractive interaction with EMI TFSI (as evidenced by a higher activation energy of ionic conductivity) compared to the TFEMA- and DMAEMA-based scaffolds, resulting in consistently lower ionic conductivity values for MMA-based ionogels. These results may offer guidance toward the rational selection of future polymer-ionic liquid pairings in order to maximize the fraction of dissociated ions, thereby yielding highly conductive ionogel electrolytes.

Spectroscopic determination of relative Brønsted acidity as a predictor of reactivity in aprotic ionic liquids.

A simple titration technique utilizing pyridine as a FTIR spectroscopy probe is demonstrated to successfully predict relative Brønsted acid-limited reaction rates in different ionic liquid (IL) environments. Relative acidity is shown to vary across three aprotic ILs in a manner that is specific to the particular acid-IL pairing.

Synthesis of Zn:Cu2O thin films using a single step electrodeposition for photovoltaic applications.

Zinc-doped cuprous oxide (Zn:Cu2O) thin films have been prepared via single step electrodeposition from an aqueous solution containing sodium perchlorate. The Zn/Cu molar ratio in the Zn:Cu2O films can be tuned between 0.006 and 0.236 by adjusting the magnitude of the applied potential and the sodium perchlorate concentration. Electrical characterization reveals that zinc dopants increase the Fermi level in Zn:Cu2O films, enabling a 3-fold improvement in the power conversion efficiency of a fully electrodeposited Cu2O homojunction photovoltaic device.

Poly(dimethylsiloxane)-supported ionogels with a high ionic liquid loading.

The immiscibility of poly(dimethylsiloxane) (PDMS) and ionic liquids (ILs) was overcome to create PDMS-supported IL gels (ionogels) with IL loadings of up to 80% by mass through a simple sol-gel reaction at room temperature. By stirring a mixture of a functionalized PDMS oligomer, formic acid, and an IL (or lithium-in-IL solution), a resin was formed that could be cast to create a freestanding, flexible ionogel. PDMS-supported ionogels exhibited favorable ionic conductivity (ca. 3 mS cm(-1)) and excellent mechanical behavior (elastic modulus: ca. 60 kPa; fatigue life: >5000 cycles; mechanically stable at temperatures up to 200 °C). The activation energy of ionic conductivity was shown to be nearly identical for the ionogel and the neat IL, in contrast to ionogel systems wherein the scaffold material is miscible with the IL. This similarity indicates that IL/scaffold chemical interactions are key to the understanding of ionogel electrical performance, especially at elevated temperatures.

Acene-doped polymer films: singlet oxygen dosimetry and protein sensing.

This paper describes thin films comprising acenes dispersed in a conjugated polymeric host that have a ratiometric photoluminescence response to singlet oxygen. These films also respond to irradiation of protein-bound sensitizers, which represents a solution to the problem of protein-conjugated polymer non-specific interactions.

Ion Electrodiffusion Governs Silk Electrogelation.

Silk electrogelation involves the transition of an aqueous silk fibroin solution to a gel state (E-gel) in the presence of an electric current. The process is based on local pH changes as a result of water electrolysis - generating H(+) and OH(-) ions at the (+) and (-) electrodes, respectively. Silk fibroin has a pI=4.2 and when local pH

Poly(ethylene glycol) diacrylate-supported ionogels with consistent capacitive behavior and tunable elastic response.

Harnessing the many favorable properties of ionic liquids in a solid electrolyte thin film form is desirable for a host of electrical energy storage applications, including electrochemical double layer capacitors. Using a cross-linked polymer matrix to provide structural support, freestanding ionogel materials can be achieved with a wide range of polymer weight fractions. Compression testing and impedance spectroscopy have been used to characterize the mechanical and electrical responses of ionogels containing between 4.9 and 44.7 wt % poly(ethylene glycol) diacrylate. Although the elastic modulus of these solid electrolyte materials is observed to vary by more than 4 orders of magnitude within the composition range studied, concomitant changes in gel ionic conductivity and double layer capacitance were much less dramatic.

Contact printing of colloidal nanocrystal thin films for hybrid organic/quantum dot optoelectronic devices.

Novel thin film optoelectronic devices containing both inorganic colloidal semiconductor quantum dots (QDs) and organic semiconductor thin films have been widely investigated in recent years for a variety of applications. Here, we review one of the most versatile and successful methods developed to integrate these two dissimilar material classes into a functional multilayered device: contact printing of colloidal QD films. Experimental details regarding the contact printing process are outlined, and the key advantages of this QD deposition method over other commonly encountered techniques are discussed. The use of tapping mode atomic force microscopy (AFM) to effectively characterize QD film morphology both on an elastomeric stamp (before contact printing) and as-transferred to the organic semiconductor receiving film (after contact printing) is also described. Finally, we offer suggestions for future efforts directed toward the goal of rapid, continuous QD deposition over larger substrates for the advancement of hybrid optoelectronic thin film devices.

Electroluminescence from nanoscale materials via field-driven ionization.

The high degree of morphological and energetic disorder inherent to many nanosized materials places limitations on charge injection into and transport rates through thin films of these materials. We demonstrate electroluminescence achieved by local generation of charge that eliminates the need for injection of charge carriers from the device electrodes. We show electroluminescence from thin films of nanoscale materials that do not support direct current excitation and suggest a mechanism for the charge generation and electroluminescence that is consistent with our time-averaged and time-resolved observations.