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

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 (λmax=584 nm, Fe2+) to colorless (Fe3+) or orange (λmax=518 nm, Ru2+) to pale green (Ru3+). 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 (TiVOx) as the optically-passive ion storage layer. The visible light transmittance τv 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.