Cobalt hexacyanoferrate nanoparticles for wet-processed brown-bleached electrochromic devices with hybridization of high-spin/low-spin phasesElectronic supplementary information (ESI) available. See DOI: 10.1039/c7tc02576a

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. A brown-bleached electrochromic device with inorganic printed nanoparticles and an appropriate mixture of a low-spin and a high spin state.