Redox Electrolytes for Hybrid Type II Electrochromic Devices with Fe−MEPE or Ni1−xO as Electrode Materials

Abstract The use of redox electrolytes in electrochromic devices (ECDs) based on cathodically coloring metallopolymers (MEPEs) and various redox mediators, including potassium hexacyanoferrate(II)/(III) (KHCF(II)/(III)), ferrocene/ferrocenium (Fc 0/+ ) and tetramethyl thiourea/tetramethylformaminium disulfide (TMTU/TMFDS 2+ ), was investigated. By decreasing the concentration of KHCF(III) in the electrolyte, it was possible to significantly reduce the loss current. Although the unpleasant yellow tint could be diminished with lower concentrations, Prussian blue (PB, Fe 4 [Fe(CN) 6 ] 3 ) was formed during cycling. Fc 0/+ and TMTU/TMFDS 2+ were studied as the basis of alternative redox electrolytes and characterized in terms of their electrochemical and optical properties. The maximum visible light transmittance changes (Δ τ v ) of 50 % and 48 % were reached with the MEPE reference ECD and the MEPE‐ECD with 0.001 M TMTU/TMFDS 2+ , respectively. Finally, an ECD combining an anodically coloring non‐stoichiometric nickel oxide (Ni 1− x O) electrode with a TMTU/TMFDS 2+ (molar ratio: 1/0.1) containing electrolyte exhibits a maximum Δ τ v of 32 %, with τ v ranging from 38 % in the dark state to 70 % in the bright state. These results imply that incorporating redox mediators into the electrolyte provides a straightforward and effective means of simplifying device structures and improving the performance of ECDs based on MEPEs and metal oxides.