Solidification of liquid electrolyte by inorganic nanoparticles

Room-temperature ionic liquids (RTILs) such as imidazolium iodide have been widely used in DSCs as a solvent and a source of I – or other ions, because of their favorable properties such as thermal stability, nonflammability, high ionic conductivity, negligible vapor pressure, and a possible wide electrochemical window. However, the fluidity of RTIL-based electrolytes, resulting in difficulty in seal, is still an obstacle for long-term operation. To reduce its fluidity, combination of RTILs with a framework material including small – molecular organogels, inorganic nanoparticles, and polymer, has been attempted by many groups.

Among these framework materials, inorganic nanoparticles have drawn more attention. In 2003, silica nanoparticles were used for the first time to solidify ionic liquids by Prof. Gratzel group (Wang et al., 2003a). The presence of silica nanoparticles has no adverse effect on the conversion efficiency, and the ionic liquid-based quasi-solid-state electrolytes are successfully employed for fabricating DSC with a conversion efficiency of 7%. This means that quasi-solid-state electrolytes offer specific benefits over the ionic liquids and will enable the fabrication of flexible, compact, laminated quasi-solid-state devices free of leakage and available in varied geometries. In addition, for their pore structures (2-50 nm) and large surface area, mesoporous materials may solidify liquid electrolytes and provide favourable channels for the triiodide/iodide diffusion. By using the mesoporous SiO2 material (SBA-15) as the framework material, Yang et al (Yang et al., 2005) fabricated a quasi-solid-state electrolyte and then fabricated DSC with a energy conversion efficiency of 4.34%. ZnO nanoparticle also can be used to solidify the liquid electrolyte. For example, Huang group (Xia et al., 2007) used ZnO nanoparticles as a framework to form a quasi-solid-state electrolyte for DSC. The quasi-solid-state DSC with the quasi-solid-state electrolyte showed higher stability in comparison with that of the liquid device, and gave a comparable overall efficiency of 6.8% under AM 1.5 illumination.

Thermal stability is an urgent concern for quasi-solid DSCs based on RTIL gel electrolytes. Some room-temperature quasi-solid-state electrolytes usually become liquid at high temperature (40-80 °C), for example, 3-methoxypropionitrile (MPN)-based polymer gel electrolyte (viscosity: 4.34 MPa s at 80 °C) (Wang et al., 2003b) and plastic crystal electrolytes (m. p. 40-45 °C) (Wang et al., 2004). Since the working temperature of DSCs may reach 60 °C under full sunlight, it is necessary that at high temperature (60-80 °C), the electrolytes are still in the quasi-solid or solid state and that the DSCs maintain high overall energy-

conversion efficiency (>4%). We (Chen et al., 2007b) have developed a succinonitrile-based gel electrolyte by introducing a hydrogen bond (O-H…F) network upon addition of silica nanoparticles and BMI-BF4 (1-Butyl-3-methylimidazolium tetrafluoroborate) to

succinonitrile. When the content of fumed silica nanoparticles was over 5 wt%, the succinonitrile-BMI-BF4-silica system became a gel, and the succinonitrile-BMI-BF4-silica (7 wt%) system still remained in the gel state even at 80 °C, as shown in the inset of Fig. 10, which confirms that the addition of silica nanoparticles and BMI-BF4 is critical for the gelation and thermostability of succinonitrile-based electrolytes. The appropriate addition of BMI-BF4 and silica nanoparticles in this gel electrolyte can greatly improve the thermostability but has no adverse effects on the conductivity, ionic diffusion coefficients and the cell performance. Moreover, the relatively high succinonitrile content in the electrolyte is also very important because the electrolyte without succinonitrile has very low conductivity and results in poor cell performance. Herein, the obtained succinonitrile-based gel electrolyte satisfies the need for both thermostability and high conductivity in electrolytes. DSCs with this gel electrolyte showed power conversion efficiencies of 5.0-5.3% over a wide temperature range (20-80 °C). Furthermore, the aging test revealed that the cell still maintained 93% of its initial value for the conversion efficiency after being stored at 60 °C for 1000 h, indicating an excellent long-time durability.

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Fig. 10. Photos of succinontrile-BMI-BF4-silica (silica: 7 wt% of succinontrile-BMI-BF4) at 80 °C (Chen et al., 2007b).