Low Molecular Mass Organogelator Based Gel Electrolyte with Effective Charge Transport Property for Long-Term Stable Quasi-Solid-State Dye-Sensitized Solar Cells

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

The gelation influence on diffusion and conductivity enhancement effect in renewable ionic gels based on a LMWG

An enhanced ionic conductivity effect (EICE) in renewable organic ionic gels used to monitor the gel state condition and quality.

Novel Composite Gel Electrolytes with Enhanced Electrical Conductivity and Thermal Stability Prepared Using Self-Assembled Nanofibrillar Networks

Novel composite gel electrolytes were prepared using self-assembled organogels as scaffolds. Mixing silica with the low-molecular-weight poly(ethylene glycol) (PEG)-based electrolytes resulted in precipitation due to significant aggregation of silica. However, clear and transparent PEG-silica composite gel electrolytes were obtained with 1,3:2,4-dibenzylidene-d-sorbitol (DBS) organogels. The organogels resulted from the formation of DBS nanofibrillar networks in which the diameter sizes of the nanofibrils ranged from 10 to 100 nm, as observed by transmission electron microscopy. These three-dimensional nanofibrillar networks entrapped the silica and prevented its aggregation. The thermal properties, such as gel dissolution and thermal degradation temperatures, of the composite gels significantly increased with increasing silica content, as determined by polarizing optical microscopy and thermogravimetric analysis. The conductivity of the prepared composite gel electrolytes was clearly enhanced by increasing the silica content. The silica was well dispersed along the DBS nanofibrillar networks, establishing homogeneous microstructures and effective contact with other components of the electrolytes, leading to an increase in the conductivity.

Induced alignment of a reactive mesogen-based polymer electrolyte for dye-sensitised solar cells

Controlling the morphology of liquid crystal-directed polymer templates at the micrometer scale using external alignment layers and electric fields.

Functional liquid crystalline gels through multi-scale hierarchical self-assembly of LAPONITE® and amidodiol

Energy storage devices accomplished with efficient LAPONITE® liquid crystalline gel electrolytes.

The Effect of the Scattering Layer in Dye-Sensitized Solar Cells Employing a Cobalt-Based Aqueous Gel Electrolyte

We developed an aqueous gel electrolyte based on the [Co(bpy)3](2+/3+) (bpy=2,2'-bipyridine) redox mediator for use in n-type dye-sensitized solar cells. Application of this electrolyte in combination with titania nanoparticle-based photoanodes and mesoporous titania bead scattering layers resulted in devices with efficiencies of 4.1% under simulated AM 1.5 sunlight, compared with 3.5% for devices using scattering layers made from a commercial paste. This difference was largely a result of a 25% improvement in the short-circuit current density and was ascribed to lower diffusion resistance for the gel electrolyte within mesoporous TiO2 beads, as confirmed by electrochemical impedance spectroscopy. Transient photocurrent measurements identified mass-diffusion problems for DSCs employing the commercial TiO2 paste. Moreover, under continuous 1 sun illumination, the devices based on an aqueous gel electrolyte displayed higher stability relative to those assembled with the corresponding liquid electrolyte.

Maximum bubble pressure rheology of low molecular mass organogels

Maximum bubble pressure rheology is used to characterize organogels of 0.25 wt % 12-hydroxystearic acid (12-HSA) in mineral oil, 3 wt % (1,3:2,4) dibenzylidene sorbitol (DBS) in poly(ethylene glycol), and 1 wt % 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) in poly(ethylene glycol). The maximum pressure required to inflate a bubble at the end of capillary inserted in a gel is measured. This pressure is related to the gel modulus in the case of elastic cavitation and the gel modulus and toughness in the case of irreversible fracture. The 12-HSA/mineral oil gels are used to demonstrate that this is a facile technique useful for studying time-dependent gel formation and aging and the thermal transition from a gel to a solution. Comparison is made to both qualitative gel tilting measurements and quantitative oscillatory shear rheology to highlight the utility of this measurement and its complementary nature to oscillatory shear rheology. The DBS and DMDBS demonstrate the generality of this measurement to measure gel transition temperatures.

Ionic liquid with a dual-redox couple for efficient dye-sensitized solar cells

A new type of ionic liquid that contains a nitroxide radical (N-O(.)) and iodide as two redox couples, JC-IL, has been successfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I2 in the wavelength region of 350-500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electrolyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electrolyte show promising cell efficiencies if coupled with dyes CR147 (8.12%) or D149 (6.76%). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electrolyte alone.

Phthaloylchitosan-Based Gel Polymer Electrolytes for Efficient Dye-Sensitized Solar Cells

Phthaloylchitosan-based gel polymer electrolytes were prepared with tetrapropylammonium iodide, Pr4NI, as the salt and optimized for conductivity. The electrolyte with the composition of 15.7 wt.% phthaloylchitosan, 31.7 wt.% ethylene carbonate (EC), 3.17 wt.% propylene carbonate (PC), 19.0 wt.% of Pr4NI, and 1.9 wt.% iodine exhibits the highest room temperature ionic conductivity of 5.27 × 10−3 S cm−1. The dye-sensitized solar cell (DSSC) fabricated with this electrolyte exhibits an efficiency of 3.5% withJSCof 7.38 mA cm−2,VOCof 0.72 V, and fill factor of 0.66. When various amounts of lithium iodide (LiI) were added to the optimized gel electrolyte, the overall conductivity is observed to decrease. However, the efficiency of the DSSC increases to a maximum value of 3.71% when salt ratio of Pr4NI : LiI is 2 : 1. This cell hasJSC,VOCand fill factor of 7.25 mA cm−2, 0.77 V and 0.67, respectively.

Open-circuit voltage enhancement on the basis of polymer gel electrolyte for a highly stable dye-sensitized solar cell

Dye-sensitized solar cells (DSSC) have received considerable attention owing to their low preparation cost and easy fabrication process. However, one of the drawbacks that limits the further application of DSSC is their poor stability, arising from the leakage and volatilization of the liquid organic solvent in the electrolyte. Therefore, to improve the long-term stability of DSSC, polymer gel electrolyte was studied to replace the conventional liquid electrolyte in this work. The results show that compared to liquid electrolyte, DSSC with polymer gel electrolyte has a smaller short-circuit current (Jsc), which decreases with the increase of the polymer gelator. Nevertheless, with the employment of the polymer gel electrolyte, there is a significant enhancement of open-circuit voltage (Voc), and it increases with the increase of the polymer gelator content. The highest Voc, up to 0.873 V, can be obtained for DSSC with a 30% polymer gelator content. The impact of the polymer gel electrolyte on the photovoltaic performance of DSSC, especially on Voc, was studied by analyzing the charge-transfer kinetics in the polymer gel electrolyte. Furthermore, the influence of the polymer gel electrolyte on the long-term stability of DSSC was also investigated.

Synthesis and photophysical properties of self-assembled metallogels of platinum(II) acetylide complexes with elaborate long-chain pyridine-2,6-dicarboxamides

A series of platinum(II) acetylide complexes with elaborate long-chain pyridine-2,6-dicarboxamides was synthesized. These metal complexes are capable of immobilizing organic solvents to form luminescent metallogels through a combination of intermolecular hydrogen bonding, aromatic π-π, and van der Waals interactions. Fibrillar morphologies were identified by TEM for these metallogels. Unique photophysical properties associated with the sol-to-gel transition have been disclosed with luminescence enhancement at elevated temperatures, which is in sharp contrast to typical thermotropic organogels or metallogels reported in the literature. Such unusual luminescence enhancement is attributed to the increased degree of freedom at higher temperatures that results in the formation of favorable molecular aggregates in the excited state through enhanced aromatic π-π and metallophilic Pt(II)···Pt(II) interactions. Structurally similar Pt-bp3 is not able to gel any common organic solvents. The inability of Pt-bp3 to form gels illustrates the importance of gelation to the macroscopic photophysical properties; Pt-bp3 does not show emission enhancement at elevated temperatures due to its low tendency to form strong aggregates in the ground state.

Crystallinity enhancement of Nafion electrolyte membranes assisted by a molecular gelator

Nanocrystallites, acting as physical cross-links in Nafion membranes, play a crucial role in building blocks for improving mechanical durability and stopping fuel crossover. However, Nafion membranes suffer from low crystallinity due to the irregular pendent side chains, which hinder self-aggregation of the poly(tetrafluoroethylene) (PTFE) backbones. For the first time, a molecular gelator was introduced in the membrane casting process to enhance the rate of self-assembly of PTFE backbones so as to increase the membrane's crystallinity as well as proton conductivity without sacrificing the purity of Nafion. The molecular gelator used was 3,4-dimethylbenzaldehyde (DMBA). Addition of 0.5 wt % DMBA led to a 42% increase in crystallinity, a 32% increase in yield strength, a 22% increase in tensile modulus and an 18% increase in proton conductivity at 60 °C and 90% relative humidity. Additionally, the membrane electrode assembly (MEA) prepared from the membranes cast from the solution containing 0.5 wt % DMBA also showed an increase of 17% in maximum power density in comparison to the MEA prepared from pure Nafion membrane in a single cell polarization test without any external humidification. Transmission electron microscopy (TEM) and molecular dynamics simulation were used to elucidate the structural changes in Nafion membrane due to the introduction of DMBA. It was observed that the presence of DMBA gives wider crystalline regions under TEM. The molecular dynamics simulation at 500 K shows that the PTFE backbones become elongated in the presence of DMBA due to the enhanced mobility. This is consistent with the observed increase in crystallinity in the membrane as it means reduced entropic change upon crystallization.

High-efficiency, solid-state, dye-sensitized solar cells using hierarchically structured TiO₂ nanofibers

High-performance, room-temperature (RT), solid-state dye-sensitized solar cells (DSSCs) were fabricated using hierarchically structured TiO₂ nanofiber (HS-NF) electrodes and plastic crystal (PC)-based solid-state electrolytes. The electrospun HS-NF photoelectrodes possessed a unique morphology in which submicrometer-scale core fibers are interconnected and the nanorods are dendrited onto the fibers. This nanorod-in-nanofiber morphology yielded porosity at both the mesopore and macropore level. The macropores, steming from the interfiber space, afforded high pore volumes to facilitate the infiltration of the PC electrolytes, whereas the mesoporous nanorod dendrites offered high surface area for enhanced dye loading. The solid-state DSSCs using HS-NFs (DSSC-NF) demonstrated improved power conversion efficiency (PCE) compared to conventional TiO₂ nanoparticle (NP) based DSSCs (DSSC-NP). The improved performance (>2-fold) of the DSSC-NFs was due to the reduced internal series resistance (R(s)) and the enhanced charge recombination lifetime (τ(r)) determined by electrochemical impedance spectroscopy and intensity modulated photocurrent/photovoltage spectroscopy. The easy penetration of the PC electrolytes into HS-NF layers via the macropores reduces R(s) significantly, improving the fill factor (FF) of the resulting DSSC-NFs. The τ(r) difference between the DSSC-NF and DSSC-NP in the PC electrolytes was extraordinary (~14 times) compared to reported results in conventional organic liquid electrolytes. The optimized PCE of DSSC-NF using the PC electrolytes was 6.54, 7.69, and 7.93% at the light intensity of 100, 50, and 30 mW cm⁻², respectively, with increased charge collection efficiency (>40%). This is the best performing RT solid-state DSSC using a PC electrolyte. Considering the fact that most reported quasi-solid state or nonvolatile electrolytes require higher iodine contents for efficient ion transport, our HS-NFs are a promising morphology for such electrolytes that have limited ion mass transport.

Synthesis, optical, and mesomorphic properties of self-assembled organogels featuring phenylethynyl framework with elaborated long-chain pyridine-2,6-dicarboxamides

A series of new organogelators with pi-conjugated phenylethynyl framework featuring long-chain carboxamides have been synthesized. These organgelators have shown great ability to gel a variety of organic solvents to form stable organogels with the minimum gelation concentration as low as 0.1 wt %. Gelation is completely thermoreversible, and it occurs due to the aggregation of the organogelators resulting in the formation of a fibrous network via a combination of intermolecular hydrogen bonding, pi-pi stacking, and van der Waals interactions that is observed for the xerogels by TEM. The influence of sol-to-gel transition has been explored in detail by variable-temperature 1H NMR, UV-vis absorption, and fluorescence spectroscopy. Aggregation-induced enhanced emission has been observed in these organogelator molecules with an order of higher fluorescence quantum yields from solution to gels. In addition, some molecules also exhibit unique liquid crystalline properties over a large temperature range as revealed by DSC and POM studies.