Electrolytes-relevant cyclic durability of nickel oxide thin films as an ion-storage layer in an all-solid-state complementary electrochromic device

NiOx thin films approximately 300 nm thick were deposited on indium tin oxide (ITO)-coated glass substrates by magnetron sputtering. Their electrochromic properties were tested in a 1 M potassium hydroxide (KOH) aqueous solution and in a 1 M lithium perchlorate dissolved in propylene carbonate (LiClO4-PC) anhydrous electrolyte, respectively. The evolution of the lattice structure, surface morphology, chemical composition, optical transmittance modulation, cyclic voltammograms, inserted and extracted charge capacities of NiOx thin films was evaluated as a function of the number of cycles. Even though the cyclic voltammograms and the charge capacity present a similar variation tendency during 1000 cycles, significant differences between cycling in 1 M KOH and 1 M LiClO4-PC were observed. In 1 M KOH, X-ray diffraction (XRD) results showed that the lattice planes (111) and (200) had a larger shift toward higher diffraction angles, and SEM images exhibited that cracks emerged more quickly from the surface of NiOx thin film during the cycling process. More importantly, the optical modulation varying from 58.5% to 32.9% presented an obvious degradation after 1000 cycles. On the contrary, in 1 M LiClO4-PC, NiOx thin films presented a better cyclic durability and a stable optical modulation approximating 41%. In a second step, an all-solid-state complementary electrochromic device with an ITO/WO3/LiClO4-PC-PMMA/NiOx/ITO structure was assembled. The device had an average optical modulation of 51.7% in the visible region and an excellent cyclic durability (over 50,000 times). In situ optical transmittance spectra showed that, in the initial cycles, the response time was 4.5 s for the coloring process and 1.7 s for the bleaching process. After 1000 cycles, both were delayed to 5.0 s and 2.9 s, respectively. In addition, open circuit memory of the device was characterized by the optical transmittance variation plotted against time after disconnection of the external circuit.