Electrochromic Properties of Hybrid Structure with Electrospun Tungsten Oxide Nanofibers

Abstract

Tungsten oxide is one of the most promising electrochromic materials due to their excellent electrochromic properties. However, their uncertain durability of amorphous tungsten oxide and poor electrochromic properties of crystalline tungsten oxide remains a constraint on commercialization. In order to solve these problems, the hybrid film in which crystalline and amorphous phase coexist was prepared through electrospinning and sol-gel method. By controlling viscosity of electrospinning, crystalline tungsten oxide nanofibers (WO3 NFs) were successfully prepared. The average diameter of electrospun WO¬3 NFs was measured for about 77 nm. After electrospinning, the WO3 NFs deposited on FTO glass were spin-coated using the peroxotungstic acid (PTA) solution. The prepared hybrid film with thickness of about 235 nm displays complementary electrochromic properties in electrochemical and optical analysis. Compared to amorphous film, the hybrid film exhibited slightly low electrochromic properties of switching speed (9.3 s for coloring and 5.6 s for bleaching), optical modulation (76.7%) and coloration efficiency (57.3 cm2·Q-1). But the evaluated electrochromic properties of the hybrid film are much better than the WO¬3 NFs having poor electrochromic performance. In addition, cycling stability test showed a noticeable improvement (91.1% of initial optical modulation after 1500 cycles). The enhanced cycling stability can be attributed to addition of the WO¬3 NFs, which have a rigid crystal structures that can endure a series of insertion/extraction of Li+ ions. Finally, the hybrid electrochromic device was evaluated to confirm possibility as practical device. The hybrid film prepared by two-step process proposes a feasible solution for electrochromic device having good performance and reliable long-term stability.