Cobalt hexacyanoferrate nanoparticles for wet-processed brown–bleached electrochromic devices with hybridization of high-spin/low-spin phases

Abstract

To prepare electrically colour-switching brown and bleached devices, we investigated the electrochromism of electrodes with cobalt hexacyanoferrate (CoHCF) fabricated with a coating of ink composed of water-dispersible CoHCF nanoparticles (CoHCF-NPs). The CoHCF-NPs with surface modification with hexacyanoferrate anions were well dispersed in aqueous solution. A film shows a clear brown–bleached colour change in the presence of an applied potential switching between +1.4 V and −0.4 V. To exhibit the brown colour during electrochromism, the co-existence of a Co(III)–Fe(II) blue low spin and a Co(II)–Fe(III) red high spin state is expected to be crucially important. Hybridization of the spin states was confirmed by the shift of the infrared spectrum corresponding to the CN-vibration mode. CoHCF-NP incorporated electrochromic devices exhibiting the brown–bleached colour change were also demonstrated in combination with a counter-electrode of zinc hexacyanoferrate nanoparticles having no colour in either redox state.