We investigated the effect of process conditions on the electrochromic (EC) properties of tungsten trioxide (WO3) films. When WO3 films deposited using a sol-gel method were thermally treated in air at 150 °C, the majority of tungsten species in the films became W6+, which is important for the realization of an optically transparent bleached state. On the other hand, annealing in a vacuum required only 60 °C to induce a similar level of W6+ in the WO3 films. However, a cracked film morphology was observed at higher temperatures, regardless of whether the films were annealed in air or vacuum. Using the WO3 films prepared under various conditions, EC devices (ECDs) were fabricated to evaluate EC properties. We concluded that the optimal annealing conditions for WO3 films for ECDs are 60 °C in vacuum, at which the highest coloration efficiency, largest transmittance difference, and fastest bleaching/coloration dynamics were obtained. These mild fabrication conditions at a low temperature (60 °C) provide the opportunity to utilize flexible electrodes on plastic. Therefore, we successfully demonstrated a flexible WO3-based ECD.
Vanadium redox flow batteries (VRFBs) have been researched as large energy storage systems due to their long cycle life, high energy efficiency, low cost, and flexible design. However, cation exchange membranes are permeable to the vanadium ions in aqueous acidic electrolyte, and vanadium ions crossover reduces the efficiency and capacity of VRFBs. To improve membrane selectivity, proton conducting inorganic materials are proposed for the modification of conventional membranes, e.g., Nafion. Clusters inside Nafion membrane are filled with inorganic materials using in situ sol-gel processes, and this results in homogeneous distribution of inorganic materials. Hybrid membranes with Nafion 115 (coded as HN115) exhibit comparable ionic conductivity and a 70% reduced permeability to vanadium ions compared with pristine Nafion 115 (coded as N115). The columbic and energy efficiencies of VRFBs with HN115 at 20 mA•cm −2 exhibit higher values of 95% and 80% in their columbic and energy efficiencies, respectively; VRFBs with N115 exhibit 78% and 70%, respectively. The capacity performance is also improved when HN115 is used in VRFBs. The VRFBs with hybrid membranes (lower permeable membrane) show higher columbic efficiency than the VRFB with N115. HN115 exhibit similar columbic efficiency values of 95% over entire current ranges, which are almost unrelated to the current density. However, N115 shows a fluctuating and lower columbic efficiency of 75%, 88%, 93% at 20 mA•cm −2 , 40 mA•cm −2 , 80 mA•cm −2 , respectively. VRFB with N115 (high conductive membrane) exhibits lower voltage drops for discharging and higher energy efficiency at high current ranges. With these results, it is proposed that the energy efficiencies of VRFBs are compromised with membrane conductivity and permeability. The columbic efficiencies are more contributed by membrane permeability. The permeability properties are more dominant in low current density and the ionic conductivity is more effective in high current ranges. To obtain higher performance of VRFBs, the membrane design for selectivity should be considered according to the operation conditions.
With the expanding need for large electrical energy storage systems in connection with renewable energy sources, flow batteries, have been enormously considered due to their high flexibility in upgrade and low cost associated with scale-up. Of all the flow batteries, vanadium redox flow battery (VRFB) use of same element in both half-cell solutions that overcomes the inherent issue of cross contamination by diffusion of different ions across the membrane. Together with the absence of any toxic emissions, the vanadium redox flow battery has demonstrated its uniqueness in terms of safety and long life cycle. Typical charge-discharge reactions of a VRFB involve two vanadium redox couples, V(II)/V(III) and V(IV)/V(V), in the negative and positive half-cells, respectively. In a fashion similar to most batteries, electrons are transferred between the two electrodes through the external circuit during the charge and discharge processes. In a VRFB, the ion exchange membrane is a key component as an ionic conductor and separator: it not only provides an ionic conduction pathway between the two electrolytes but also prevents mixing of the negative and positive electrolytes. The crossover of ions through the membrane, with the diffusion of vanadium ions from one half-cell to the other due to the concentration gradients between the two electrolytes, will result in self-discharge and thus the loss of the chemical energy. In this study, the blended membrane of hydrocarbon polymer with perfluorinated organic membrane were fabricated and characterized in terms of ionic conductivity and permeability. The ionic conductivity was measured with four point probe method and the permeability was measured with UV spectroscopy. The blended membrane exhibited comparable ionic conductivity with perfluorinated organic membrane and 30 – 40 % lower permeability than perfluorinated organic membrane. With performances of VRFBs, the energy efficiencies of VRFBs will be discussed.
Vanadium redox flow battery (VRFB) is a form of rechargeable battery which converts chemical energy directly to electrical energy. VRFB have been investigated for their potential utility as large energy storage systems due to their advantageous performances in terms of long cycle life, high energy efficiency, low cost, and flexible design. Catholyte and anolyte, in the negative and positive half-cells respectively, containing dissolved electro-active species flows through an electrochemical cell and generate cell potential. During operation of VRFB, ion-exchange membrane separate the catholyte and anolyte for preventing cross contamination which is induced by diffusion of different ions across from one half-cell to the other, will result in self-discharge and thus the loss of the chemical energy. Therefore, the ion-exchange is one of the most important components in the VRFB system and it should have low permeability and high ionic conductivity. A perfluorosulfonic acid (PSFA) membrane, such as Nafion (DuPont) for example, is the most commonly used membrane due to its high chemical-mechanical stability and high ionic conductivity. However, Nafion is permeable to electrolyte and expensive. The major focus is to modify the water-filled channels of the Nafion membrane by introducing non-conducting metal oxides. For example, SiO2 and TiO2 have been used to prepare Nafion-metal oxide composite membranes. The inorganic fillers, such as SiO2, act as a physical barrier for diffusing vanadium cation through the membrane channels, while allowing proton transport. In this study, composite membranes based on Nafion and nanoclay were fabricated and characterized in terms of ionic conductivity and permeability. The ionic conductivity was measured with four point prove method and the permeability was measured with UV spectroscopy. For fabrication of the composite membranes, nanoclay solution was blended with Nafion solution according to the nanoclay composition in the dry membrane (5, 3 and 1 wt.% : identified as NF-NC 5, 3 or 1 wt.%, respectively). Ionic conductivity and the permeability were decreased with the increasing of the nanoclay composition in the composite membrane. With performances of VRFBs, the energy efficiencies of VRFBs and optimum composition of the composite membrane will be discussed.
Electrochromic devices, which dynamically change colour under applied potential, are widely studied for use in energy-efficient smart windows. To improve the viability of smart windows, many researchers are utilizing nanomaterials, which can provide electrochromic devices with improved coloration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. The flexible electrochromic films are promising film for applications in automobiles, buildings, etc. Here, we synthesized nanostructured tungsten trioxide (WO3) particles for electrochromic materials. The thin layers with nanostructured materials of WO3 inks were coated on ITO glass at room temperature, solution-processed method. For comparing the effects of electrolyte, liquid electrolytes were used with/without a porous substrate. Porous substrates can be helpful in leakage issues of liquid electrolytes. The porosity and pore size of porous substrate were parameters for characterization of electrolyte. The coloration efficiencies and cycle performance will be discussed with characteristics of porous substrate.
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