Electrochromic devices performance improvement

The potential benefits of using ionic liquids as electrolytes in conducting polymer devices have been investigated by a number of authors in recent years, for applications such as actuators [8-17], supercapacitors [18-20], electrochromic devices [12, 21] and solar cells [22], with significant improvements in lifetimes and device performance reported. 

Although various electrochromic devices have been demonstrated, their performance still needs to be drastically improved. This will require a major research and development effort. On the other hand, the fiber optic metal hydride hydrogen sensor already shows that metal hydride applications may provide a clear advantage over competing systems. 

As naturally abundant and low-cost semiconductor, NiO is widely used in electrochromic windows [20], batteries [21], supercapacitors [22], and sensors [23], While all these applications benefit from an interconnected, three-dimensional NiO nanostructure that combines a high specific surface area with a good electric conductivity, the performance enhancement becomes vividly evident as an increased coloration contrast and improved switching behavior when applied in electrochromic devices. NiO nanomaterials recently employed in electrochromic studies include nanocomposites [24], inverse opals [25], macroporous [26] and mesoporous films [27-29],... 

To improve performance, many PEDT derivatives used either alone or in combination have been proposed. While the electro-optical properties of WO3 are fixed, the colors and hue of conducting polymers may be altered by modification of the monomers. For example, Reynolds has studied the electrochromic properties of a multitude of EDT derivatives [100,109], and recently reported an all-polymer electrochromic device based on two different PEDT derivatives [110]. Depending on their chemical structure, the various PEDT derivatives exhibit different colors upon switching from the oxidized to the reduced state. For example, poly(tetradecylethylenedioxythiophene) (C14-EDT) is similar in switching to PEDT from transparent to blue, but it has an enhanced optical contrast. 

Few reports on polypyrrole-based electrochromic devices have appeared due to air instability of the neutral undoped state of the polymer. A polypyrrole-based optical window performing in water has been proposed [423]. DePaoli et al. [424] have produced a device from electrochemically deposited polypyrrole dodecylsulfonate which results in improved properties over polypyrrole doped with perchlorate. 

Electrochromic devices using solid electrolytes have been produced more recently. A device [425] is based on poly(3-octylthiophene) or polypyrrole films with vanadium oxide as a counter-material and poly(ethylen-oxide) as a solid electrolyte. The poor performances (maximum 100 cycles of operation) of these early devices, due to the instability of the materials and the high operating temperature, have been greatly improved by the use of poly(3,4-ethylenedioxythiophene), provided with a low oxidation potential and optimum contrast, and of a poly(ethylenoxide)-poly(phospha-zene) solid electrolyte of high room temperature conductivity [153]. This device may be operated up to 1000 cycles without losses. 

Nanomaterials of undoped vanadia employed in electrochromic studies include nanowires [35-37], inverse opals [34], and mesoporous films [38]. Similar attempts were undertaken for NiO devices [39 2]. Although these recent attempts brought about an improvement in electrochromic performance, the reported switching times remained 1-2 orders of magnitude above the desired video rate of 24 frames per second [34-38]. This is mainly due to the use of sub-optimal MO morphologies in terms of their structural dimension, connectivity and integrity. 

To investigate the effect of the nanostructure on the electrochromic performance of NiO, transparent devices were assembled from nontemplated and DG-structured films with a FTO counter electrode and a 1M KOH(aq) electrolyte, see Fig. 6.9a. The active electrode material used was limited in area to 0.95mm. During the nickel electroplating process limiting the deposition area improve the control and quality of the deposit. 

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