Sol-gel Electrolyte Research Articles

Integrating High-Sensitivity Photodetector and High-Energy Aqueous Battery in All-in-One Triple-Twisted Fiber.

Fiber-shaped photodetectors (FPDs) have attracted special attention to wearable health monitoring due to their 3D absorption capabilities. However, the practical application of traditional FPDs is severely limited by the irreversible degradation of performance caused by vulnerable interface compatibility on complex deformation and a single function. Here, an integrated photoelectrochemical FPD/battery device (FPDB) is designed, consisting of a common electrode, photoanode, anode, and sol-gel electrolyte as an isolation layer, which not only effectively avoids the short circuit problem of FPD but also endows high-efficiency energy storage capacity. As expected, the resulting all-in-one triple-twisted fiber-shaped FPDB simultaneously achieves high responsiveness of 151.45 mA W-1 and excellent volume capacity of 18.75 mAh cm-3. Such a stable architectural design and multifunctional integration of functional fibers accelerate the development of next-generation wearable fabrics.

Development of a Printed Cathode and Catalyst Layer for Printed Zinc-Air Batteries

Printed electronics provide a means for realizing large area, flexible, and low cost electronic systems such as wireless sensors and wearable devices. While significant progress in printed transistor technology has been made, advances in printed power sources and energy storage are necessary to create fully integrated devices. Designing a thin, low cost, high energy density printed battery would enable seamless additive manufacturing of fully powered systems. Several demonstrations of printed batteries have been reported, but have typically relied upon high cost silver-oxide cathodes, required inert processing conditions with Li-ion chemistries, or demonstrated low performance at current densities above 1 mA cm- 2. Zinc-air batteries are an ideal alternative to silver-oxide and Li-ion systems given its low materials cost, high energy and power densities, and inherent air stability. This work shows the development of a printable oxygen cathode and catalyst layer integrated in a packaged, fully printed zinc-air battery. To assemble the battery, stencil printing was chosen based on its low cost and compatibility with various substrates including flexible plastics. Stencil printing is also better suited than other printing methods such as inkjet or gravure in order to print thick active layers (10s-100s µm) necessary to achieve high areal capacities. Processing temperatures of each battery component were below 150°C to remain compatible with low cost, flexible substrates. The anode ink was composed of zinc, zinc oxide, bismuth (III) oxide, and polyethylene glycol in a polyethylene oxide binder. A photopolymerizable polyacrylic acid (PAA) sol-gel was previously designed and implemented as the electrolyte and separator. The cathode ink was fabricated by combining Vulcan XC 72 carbon powder with a platinum catalyst in a polyvinylidene fluoride (PVDF) binder. PVDF was chosen based on its hydrophobicity to prevent the electrolyte solution from flooding the cathode and limiting the reduction of oxygen. The carbon-binder ratio was varied to tune the ink viscosity for stencil printing and to optimize the sheet resistance of the printed cathodes. Oxygen half-cells were designed to determine the effect of platinum concentration on the oxygen reduction reaction (ORR) overpotential and discharge performance. In addition, the ORR overpotential was studied as a function of potassium hydroxide concentration and volume used in the printed sol-gel electrolyte. The fully printed zinc-air batteries demonstrated a steady discharge voltage of 1.25 to 1.30 V. High areal capacities of 2-5 mAh cm-2 were achieved with current densities between 1-5 mA cm-2, exceeding the performance of previously reported printed zinc-air batteries. Self-discharge lifetimes of one month were obtained with the use of a PDMS encapsulation layer. The internal resistance of these cells was typically less than 200 Ω while using low cost carbon based current collectors and less than 100 Ω with silver-carbon bilayer collectors. This work represents the first demonstration of a packaged, fully printed zinc-air battery with a printed cathode and catalyst layer. The batteries exhibit low internal resistances (< 100 Ω) and high areal capacities (> 2 mAh cm-2) at peak current requirements for printed electronics applications (> 1 mA cm-2), a step towards realizing integrated energy storage for printed electronics systems.

Sol–Gel Electrolytes Incorporated by Lanthanide Luminescent Materials and Their Photophysical Properties

A group of silica gel electrolytes with lanthanide luminescent hybrid materials were assembled and investigated. Photophysical studies showed that terbium and europium hybrids displayed characteristic green and red emissions within the electrolytes. The influence of different concentration of the lanthanide hybrids on the electrochemical behavior of a gelled electrolyte valve-regulated lead-acid battery were studied through cyclic voltammograms, electrochemical impedance spectroscopy, water holding experiments and mobility tests. The morphology and particle size were analyzed by scanning electron microscopy. The results proved that lanthanide (Tb3+/Eu3+) luminescent materials are effective additives which will significantly improve the electrochemical properties of lead-acid batteries.

Redox-Stable High-Performance Thin-Film Solid Oxide Fuel Cell

An anode-supported Solid Oxide Fuel Cell (ASC) consists of a mechanically supporting, thick, and highly porous anode substrate, coated on top with thin films of a fine-structured porous anode, a dense electrolyte, and a fine-structured cathode. Both, the anode substrate and the anode are made of a cermet of nickel and yttria-stabilized zirconia (YSZ). An increase of the oxygen partial pressure at the anode side will lead to a reoxidation of the nickel and to microstructural changes. This may result in a macroscopic expansion of the anode substrate and in a complete cell failure, due to cracking of the YSZ electrolyte. In this work, a redox-stable SOFC with a thin-film sol-gel electrolyte is presented. Using tailored suspensions and polymeric sols of varying target particle size, a 1 µm thick, three layer sol-gel electrolyte is coated on top of a tape cast anode substrate and a screen printed anode by spin-coating and dip-coating techniques. With this sol-gel electrolyte a power output larger than 1.25 W/cm2 at 0.7 V and an operating temperature of 600 °C could be demonstrated. In this study, half cells (anode substrate, anode, and electrolyte) are reoxidized in excess air, in order to test the redox stability of these SOFCs. Afterwards the electrolyte is investigated ex situ using optical microscopy and SEM. No cracks are found in the sol-gel electrolyte after reoxidation for 4 hours at 600 °C and 15 minutes at 800 °C, respectively, whereas a lot of cracks are visible in a standard ASC with a 10 µm thick, screen printed electrolyte for both reoxidation conditions. This means the sol-gel electrolyte can tolerate higher degrees of oxidation (DoO) of the anode substrate than the screen printed electrolyte. The residual compressive stresses of both electrolytes are measured to about 600 MPa using XRD and are therefore not responsible for the higher stability against reoxidation of the sol-gel electrolyte. In contrast, the energy release rate has to be larger than a critical value for the creation of cracks in a thin-film on a thick substrate. Due to the fact, that the energy release rate is proportional to the thickness of the thin-film, a thinner film is more stable against cracking than a thicker film at constant tensile stresses. After reoxidation of 8 hours at 600 °C and 60 minutes at 800 °C, respectively, cracks are also found in the sol-gel electrolyte. However, the SOFC with the thin-film sol-gel electrolyte can be considered as stable against reoxidation, because the long reoxidation time of 4 hours at an operating temperature of 600 °C is unlikely to happen under real conditions.

Chromium-Related Degradation of Thin-Film Electrolyte Solid Oxide Fuel Cell Stacks

Solid oxide fuel cell short stacks for dezentralized stationary power delivery have been operated with special thin-film electrolyte anode-supported cells. After operation times of up to 3,000h the cells were post-test characterized. The cells have besides very thin electrolyte films (≤ 3µm) either LSCF or LSC cathodes (LSCF: La-Sr-Co-Fe oxide, LSC: La-Sr-Co oxide). Typical operation parameters were temperatures between 600 and 750°C, a current density of 500mA/cm², hydrogen with 20% water vapor and a fuel utilization of approx. 40%. Typical chromium evaporation protection layers like wet powder sprayed manganese oxide and cathode contact layers (a perovskite based on La-Mn-Co-Cu oxide) were used. Microstructural post-test characterization reveals in the case of using a 1µm 8YSZ electrolyte prepared via sol-gel route and on top a sputtered diffusion barrier layer composed of GDC, the existence of SrCrO4 either on top of the LSCF cathode as expected or, unexpectedly the formation of the spinel phase at the borderline cathode / barrier (electrolyte) layer. In case of using a 3µm thin screen printed 8YSZ electrolyte and an LSC cathode, the chromite phase has not been detected at the cathode / barrier border. SEM and TEM post-test analysis of the cells show in case of the sol-gel electrolyte and sputtered GDC also the formation of micro-pores within the GDC layer. Theoretical calculations reveal that the chosen current density may lead to either preferred oxygen reduction or preferred chromium reduction depending on the real local current density within the cathode / barrier (electrolyte) interface.

Redox-Stable High-Performance Thin-Film Solid Oxide Fuel Cell

In this work, a mechanically redox-stable SOFC with a 1 µm thin-film sol-gel electrolyte is presented. With this electrolyte a power output larger than 1.25 W/cm2 at 0.7 V and an operating temperature of 600 °C could be demonstrated. Half cells were re-oxidized in excess air, in order to test the redox stability of these SOFCs. No cracks were found in the sol-gel electrolyte after re-oxidation for 4 hours at 600 °C and 30 minutes at 800 °C, respectively. Due to the fact, that the energy release rate is proportional to the thickness of the thin-film, a thinner film is more stable against cracking than a thicker film at constant tensile stresses. The SOFC with the thin-film sol-gel electrolyte can be considered as stable against re-oxidation, because the long re-oxidation time of 4 hours at an operating temperature of 600 °C is unlikely to happen under real conditions.

Volume fraction of ether is a significant factor in controlling conductivity in proton conducting polyether based polymer sol-gel electrolytes.

We have synthesized several copolymers of methyl polyethylene glycol siloxane (MePEG7SiO3)m and methyl polypropylene glycol siloxane (MePPGnSiO3)m as hydrogen ion (H(+)) conducting polymer electrolytes. These copolymers were prepared by a sol-gel polymerization of mixtures of the MePEG and MePPG monomers. We synthesized these H(+) conducting polymer electrolytes in order to study the relationship between observed ionic conductivity and structural properties such as viscosity, fractional free volume, and volume fraction of ether. We found that viscosity increased as the fraction of the smaller comonomer increased. For the MePPG2/MePPG3 copolymer, an increase in fractional free volume increased the fluidity. The heterogeneous copolymers (PEG/PPG copolymers) obeyed the Doolittle equation, while the homogeneous (PEG/PEG and PPG/PPG) copolymers did not. The increase of FFV did not, however, correspond to an increase in conductivity, as would have been predicted by the Forsythe equation. The conductivity data did correspond to a modified Forsythe equation substituting Volume Fraction of Ether (V(f,ether)) for FFV. We conclude that the proton conductivity of MePEG copolymers is more dependent on the volume fraction of ether than on the fractional free volume.

Comparison of electrochromic devices with V- and Sn/Mo-oxide counter electrodes and (3-glycidoxypropyl)trimethoxysilane-based ormolytes with three different lithium salts

Electrochromic (EC) devices represent an effective way to modulate the intensity of incoming solar radiation in buildings or through the sun roofs of cars, but can also be used in helmets, sunglasses and home appliances. Herein we report on the development of all sol–gel WO3-based EC devices of a battery type, in which all three internal layers were prepared via sol–gel processes. A comparison of the performance of EC devices assembled with two different counter electrode films, i.e. V-oxide (150°C) and Sn/Mo-oxide (500°C) films deposited on fluorine dopped tin oxide (FTO) glass, is presented. The applicability of both types of counter electrodes for flexible polymeric substrates is also demonstrated. The sol–gel electrolyte (ormolyte) based on (3-glycidoxypropyl)trimethoxysilane was applied in EC devices; additionally, three different lithium salts were introduced, i.e. lithium perclorate (LiClO4), lithium trifluoromethanesulfonate (LiCF3SO3) and lithium bis(oxalato)borate (LiBOB). The spectroelectrochemical characterisation of the EC devices revealed that the optical modulation at 634nm increased when electrolytes with LiClO4<LiCF3SO3<LiBOB were used.

Dye-sensitized solar cell based on blood mimetic thixotropy sol–gel electrolyte

Blood mimetic thixotropy sol-gel electrolytes were prepared and sandwiched between dye-coated porous TiO(2) and a platinum counter-electrode to form a dye-sensitized solar cell. The highest overall conversion efficiency of 6.45%, and the fill factor of 0.74 under the white light irradiation AM 1.5 (100 mW cm(-2)) were obtained.

Anion and cation transport in composite films of polypyrrole with a sulphonated silica (ormosil) hydrogel

Composite films of polypyrrole with a sulphonated organically modified silica (ormosil) have been prepared on electrodes by the electrochemical oxidation of pyrrole in a liquid sol–gel electrolyte. The ormosil is incorporated into the polypyrrole matrix as an immobile polymeric counterion in addition to mobile Cl − counterions from the sol–gel electrolyte. Ion transport in the composites was investigated by impedance spectroscopy and electron microprobe analysis. The presence of both mobile and immobile counterions allows potential control of the mode of ion transport, with anion transport dominating at high potentials and cation transport dominating at low potentials. As the polymer is un-doped (reduced), its ionic conductivity first decreases as mobile anions are expelled, and then increases as cations and excess electrolyte are incorporated.

Dye-sensitized photoelectrochemical solar cells based on nanocomposite organic–inorganic materials

Dye-sensitized photoelectrochemical solar cells have been constructed by using nanocomposite organic–inorganic sol–gel electrolytes and a titania nanocrystalline film also based on a sol–gel nanocomposite material. Among other advantages connected with nanocomposite electrolytes is the balance between hydrophilic and hydrophobic domains that allows reducing polarity-connected repulsive forces developing between the titania-dye system and the electrolyte. The overall efficiencies of these cells range between 5 and 6% under 100 mW/cm 2 illumination (equivalent to 1 sun at AM1.5).

In situ resonance micro-Raman and UV–visible spectroelectrochemical studies of an electrochromic device with an I 3−/I − redox sol–gel electrolyte

An all-sol–gel electrochromic (EC) device was constructed of electrochromic WO 3 and transparent Pt films, both deposited on conductive glass electrodes encapsulating a gel electrolyte with an I 3 −/I − redox couple. WO 3 films (200–400 nm) were deposited by the dip-coating technique from peroxopolytungstic acid sols. The films consisted of nanocrystalline WO 3 grains of dimensions 20–30 nm having monoclinic structure. The gel redox I 3 −/I − electrolyte, which was made via the sol–gel route, consisted of an organic phase (poly(propyleneglycol), PPG, M W=4000) covalently bonded via urea bridging groups to a silica phase. Gelation was performed with acetic or valeric acid. The functioning of the device was studied by the micro-resonance Raman spectroscopic technique, focusing the laser excitation line ( λ=647 nm) on the electrolyte encapsulated in the EC device. Variation in the potential from −2 to 2 V (scan rate 5 mV s −1) resulted in changes of the intensity of the symmetric stretching resonance Raman mode of I 3 − ions ( ν s(I 3 −)) and was accompanied by variation in the background Rayleigh scattering. From this, we concluded that the intercalation/deintercalation of K + ions in the WO 3 film was accompanied by an increase/decrease in the I 3 − ion concentrations and reversible dissociation/association of the KI microcrystallites existing in the redox electrolyte.

Electrochemistry of Prussian Blue in silica sol-gel electrolytes doped with polyamidoamine dendrimers

The inclusion of a generation-4 polyamidoamine (G4-PAMAM) dendrimer in a silica sol-gel yielded a solid electrolyte that was used to encapsulate Prussian Blue (PB), iron(III) hexacyanoferrate(II), and cobalt hexacyanoferrate. The PB was synthesized in the doped silica by sequential immersion of a monolith in 0.1 M K4Fe(CN)6, water, and 0.1 M FeCl3. Inclusion of G4-PAMAM resulted in a nanoporous anion-exchange material with a capacity of 10.1 mmol g–1, which is about four times greater than the capacity of silica alone. Relative to its G0 counterpart, the G4-PAMAM doped silica increased the rate of formation of PB by a factor of ca. 20. The solid state voltammetry of PB in the doped silica had the usual features for this compound. At 0.1 V vs. a Ag quasi-reference electrode, a reversible reduction was seen; the relationship between current and scan rate was that for a surface-confined redox couple. The quasi-reversible oxidation of PB was observed at 0.85 V. Inclusion of G4-PAMAM increased the lifetime of silica as a solid electrolyte from a few days to at least three months. Raman microprobe mapping analysis demonstrated that PB was homogeneously distributed across the entire width (ca. 1 mm) of the G4-doped monolith with 20-h immersions.

Electrochemical oxidation of aniline in a silica sol–gel matrix

Electrochemical oxidation of aniline encapsulated in a silica solid electrolyte prepared by a sol–gel process yielded products that were dependent on the pore size. An acid-catalyzed process that used tetramethyl orthosilicate as the precursor and aniline as a dopant yielded the silica. When the aging time was limited to one day so that a mesoporous solid was obtained, the potentiodynamic oxidation of aniline at a carbon fiber electrode resulted in the formation of polyaniline. With aging times of 3–5 days, microporous silica was obtained. In this electrolyte, the formation of dimers and other oligomers was observed by cyclic voltammetry. Evidence for these products was the presence of a quasi-reversible redox couple at 0.2 V vs Ag/AgCl that was previously related to oligomeric aniline by Raman spectroscopy. The results supported the hypothesis that the pore structure of sol–gel electrolytes can influence the pathways of electrode reactions therein.

Mediated Oxidation and Determination of Gaseous Monomethyl Hydrazine in a Solid-State Voltammetric Cell Employing a Sol-Gel Electrolyte

Mesoporous silica that was prepared by a sol-gel process was used as the electrolyte for the amperometric determination of vapor-phase monomethyl hydrazine (MMH). Three detection strategies were tested, namely cyclic voltammetry at a Pt working electrode with the oxidation mediated by FeII,III that was hosted by the silica, pulsed electrochemical detection with a gold working electrode, and potentiostatic oxidation at a mixed-valence ruthenium oxide (mvRuOx) film that was deposited on a Pt working electrode prior to casting the solid electrolyte overlayer. The sensitivities (and linear dynamic ranges) for the respective methods were 1.4 nA/ppm (500–1400 ppm), 0.49 nA/ppm (70–1050 ppm), and 0.30 nA/ppm (1–1050 ppm). With pulsed electrochemical detection and mvRuOx, respective detection limits of 20 ppm and 0.3 ppm were calculated on the basis of a concentration that yielded a response three times the standard deviation of the blank. Because of high uncertainty of the intercept of the FeII,III-mediated method, a detection limit was not calculated for this technique. A cell design that provided a three-phase boundary at a mvRuOx-modified electrode yielded a response of 90 % of the steady-state current at 11 s.

Mediated Oxidation and Determination of Gaseous Monomethyl Hydrazine in a Solid-State Voltammetric Cell Employing a Sol-Gel Electrolyte

Mesoporous silica that was prepared by a sol-gel process was used as the electrolyte for the amperometric determination of vapor-phase monomethyl hydrazine (MMH). Three detection strategies were tested, namely cyclic voltammetry at a Pt working electrode with the oxidation mediated by FeII,III that was hosted by the silica, pulsed electrochemical detection with a gold working electrode, and potentiostatic oxidation at a mixed-valence ruthenium oxide (mvRuOx) film that was deposited on a Pt working electrode prior to casting the solid electrolyte overlayer. The sensitivities (and linear dynamic ranges) for the respective methods were 1.4 nA/ppm (500–1400 ppm), 0.49 nA/ppm (70–1050 ppm), and 0.30 nA/ppm (1–1050 ppm). With pulsed electrochemical detection and mvRuOx, respective detection limits of 20 ppm and 0.3 ppm were calculated on the basis of a concentration that yielded a response three times the standard deviation of the blank. Because of high uncertainty of the intercept of the FeII,III-mediated method, a detection limit was not calculated for this technique. A cell design that provided a three-phase boundary at a mvRuOx-modified electrode yielded a response of 90 % of the steady-state current at 11 s.

Sol-gel electrolytes in lithium batteries

This paper presents the results of the thermodynamic calculations of material compatibility along with the results of the experimental studies using lithium aluminosilicate gel electrolyte in lithium batteries. Initially, there were problems with gel monoliths and porous cathodes in the Li solid electrolyte batteries. Better results were obtained through the direct application of thin films of the lithium aluminosilicate gels to the surfaces of dense, sintered oxide cathodes. It was important to maintain extremely low moisture and oxygen levels in the dry glove box during the assembly and testing of the battery, especially when it came to achieving good contact between the sol-gel electrolyte and the lithium metal. Suggestions are given about procedures for further development of the sol-gel electrolyte batteries.