Addition of Multi‐walled Carbon Nanotubes to Fe‐ and Ru‐Metallopolymer Electrodes Enhances Response Time and Cycling Stability in Electrochromic Cells

Abstract

Metallopolymers (MEPEs) show a strong absorption band attributed to a metal-to-ligand charge transfer (MLCT) transition. Fe-MEPE and Ru-MEPE thin films switch reversibly from blue-purple (λ max = 584 nm, Fe 2 + ) to colorless (Fe 3 + ) or orange (λ max = 518 nm, Ru 2 + ) to pale green (Ru 3 + ). The addition of multi-walled carbon nanotubes (MWCNTs) enhances the electrochromic (EC) properties of Fe-and Ru-MEPE electrodes in several ways: (1) faster response for bleaching/coloring; (2) enhanced Coulombic efficiency; and (3) improved cycling stability, particularly for Ru-MEPE. Moreover, the charge density of the thin film electrodes can be increased upon the addition of the MWCNTs. This performance improvement is demonstrated in electrochromic devices (ECDs) with titanium-doped vanadium oxide (TiVO x ) as the optically-passive ion storage layer. The visible light transmittance τ v values of the ECDs are improved by the addition of the MWCNTs from 20 %/56 % to 17 %/56 % and from 31 %/47 % and 34 %/55 % in the dark/bright state, for Fe-MEPE and Ru-MEPE, respectively. Thus, the addition of MWCNTs to Fe-or Ru-MEPE is a very promising route for future fast-switching EC applications, e. g., displays, sunroofs, and rear-view mirrors.

A black electrochromic device based on a CuI film for energy-efficient applications

“…Owing to the diversity of material energy levels in metal ions and ligands, electrochromic devices based on metal–organic materials can display different colors in a single device, 15 which has attracted considerable research attention. The mechanism responsible for electrochromism in metal–organic electrochromic materials is the metal–ligand charge transfer (MLCT), 16–19 and their optical modulation ability can be adjusted through the exchange of functional groups. Currently, the most commonly used ligands are pyridines, 20–22 phthalocyanine 23,24 and salen, 25–27 some of which show undesirable properties such as unwanted film forming ability and brightness of color.…”