A comparison of key parameters of seven different gel electrolytes for use in electrochromic devices (ECD) is reported. The ionic conductivity, transmittance, and stability of the gel electrolytes are important considerations for smart window applications. The gel electrolytes were prepared by combining polymethylmethacrylate (PMMA) with a salt and a solvent combination. Two different salts, lithium perchlorate (LiClO4) and trifluorosulfonimide (LiN(CF3SO2)2), and three solvent combinations, acetonitrile and propylene carbonate (ACN and PC), ethylene carbonate and propylene carbonate (EC and PC), and Gamma-butyrolactone and propylene carbonate (GBL and PC) were investigated. Results show that gel electrolytes composed of a LiClO4 and GBL+PC combination and a LiClO4 and EC+PC combination are the best candidates for a smart window device based on its high conductivity over time and various temperatures, as well as its electrochemical stability and high transmittance.
A large contrast ratio and rapid switching EC polymer device which consists of a laminated two-layer structure between two electrodes was prepared. The new design consists of an ITO glass electrode, a cathodic EC polymer film, a gel electrolyte and a counter-electrode that replaces the anodic EC polymer and ITO electrode. Several types of EC polymers, such as, poly[3,3-dimethyl-3,4-dihydro-2H-thieno(3,4-b)(1,4)dioxepine] (PProDOT-(CH3)2) and
poly[3,4-(2,2-dimethylpropylenedioxy)-pyrrole] (PProDOP-(CH3)2) were synthesized as cathodic EC polymers. A carbon-based counter-electrode was prepared for comparison with an Au-based counter-electrode. Screen-printing was utilized for the carbon-based counter-electrode. Lithography and sputtering were used for the Au patterned glass counter-electrodes. Several kinds of polymer gel electrolytes were prepared for solid-state applications. Color change of high contrast ratio of visible light transmittance (>ΔΤ55%)of the device is rapidly obtained (0.5-1s) when even less than 2.5V is applied. The repeatability of color changeable EC polymer windows was estimated by the method of electrochemistry and spectrophotometry. This "smart window" technology can be used in many applications where a rapid
color change between transparent and color states is required.
A microwave switch based on EAP presents several advantages. A switch based on Flemion is studied. Flemion a perfluorinated carboxylic acid membrane shows improved performance as actuator material compared with Nafion (perfluorinated sulfonic acid). Flemion has a higher ion exchange capacity and good mechanical strength. In order to get a good Flemion actuator, highly conductive soft gold electrodes with large fractal structure have to be deposited on the membrane. The impregnation reduction technique used for plating requires exchange of a gold complex and reduction by gradual sodium sulfite additions. K+ shows the highest exchange ratio with the gold complex and reducing bath temperatures around 60°C with enough reducing agent present are shown to promote the growth of a gold fractal structure. The resulting material shows an actuation displacement with no relaxation, a key feature for switch applications. A simple mechanical switch based on a flemion actuator is prepared and tested as a microwave switch.
A new actuator system has been developed. This actuator uses Nafion, a solid electrolyte, in combination with Platinum Copper (Pt-Cu) electrodes and mobile ions of Cu2+ to create much larger actuation displacement at smaller levels of applied voltage (1V or less). This actuator provides bending deformation. Large deformation is provided by electrode reaction of copper. Since this reaction is reversible, Cu electrode is not consumed by using polarity change of applied voltage. This actuation mechanism is different from others. Because the induction of the large deflection of Nafion, the large number of the mobile cations is essential. Although it is possible to induce a large deflection by applying a higher electric field as alternative way, this would introduce the electrolysis of water that is not desired unless the device is always submerged in water. To convert bending deformation to liner actuation, we designed a device using a pair of Nafion actuator, which is termed as loop actuator. This loop actuator can be designed into the device with large force by making parallel array. Solid polymer electrolyte-metal composite actuator contains water inside. Therefore coating that prevents water from evaporation is needed for its use in dry condition.
To answer the need for a dense array of actuators, we tried to integrate several actuators in a single Nafion membrane by simply patterning the soft metal electrodes deposited on the membrane. We designed several independent octagonal electrodes on the same membrane. The experiments showed that we could actuate each cell independently. The use of gold electrodes and tetraethyl ammonium ion instead of platinum copper electrodes and copper ion improves the repeatability but at the cost of the large deformation. We studied the influence of the membrane thickness on actuation. Nafion 115 (125-micron thick) showed a similar behavior than the classic Nafion 117 (180-micron thick). We also investigated Nafion 112 (50-micron thick) as an actuator. We did 6 gold plating cycles to deposit the same amount of gold as on the other membranes. This membrane showed a unique actuation behavior, due its high flexibility. We prepared a 3x3 array of disk shaped actuators using Nafion 112. We used hydrophobic masks to create the patterning. Upon actuation the cell quickly reached a quite large deformation and relaxed to an equilibrium position. To get actuation between two stable positions we are currently trying to create symmetric equilibrium positions for the cell.