Multicolor electrochromic thin films and devices based on the Prussian blue family nanoparticles

• Nanoparticles and spin-coated films of Prussian blue analogues (PBAs) are reported.
• Electrochromic PB, Co-PBA, Ni-PBA, and Cu-PBA films are prepared and studied.
• Reversible PB's yellow state and high-contrast PBA electrochromism is attained.
• Multicolor PB/Ni-PBA composite electrochromic film is demonstrated.
• Multicolor PB/Ni-PBA//Cu-PBA electrochromic device is demonstrated.

Here we report a color palette of electrochromic (EC) nanoparticles (NPs) based on the Prussian blue (PB) family and present its promising multicolor thin film and device applications. We synthesized PB, cobalt-PB analogue (Co-PBA), nickel (Ni)-PBA, and copper (Cu)-PBA NPs and prepared the corresponding aqueous NP inks with or without 10 vol% PEDOT:PSS additive. Uniform EC films with high visual contrast were coated onto ITO electrodes by repeated spin-coating of the NP inks. The EC films were characterized with SEM, cyclic voltammetry, Li+ diffusion speed, and in situ spectroelectrochemical methods. The EC colors of PB, Co-PBA, Ni-PBA, and Cu-PBA were achieved with these NP films. In particular, the PB NP film showed reversible green and yellow electrochromism that were not possible for an electrodeposited PB film, and Co-PBA, Ni-PBA and Cu-PBA films with sufficient thickness to exhibit visualized EC colors were also difficult to attain by electrodeposition. We also proved that a multicolor EC film could be prepared by a simple layer-by-layer interchanged coating of two different PBA NPs (e.g., PB and Ni-PBA). Finally, we assembled a new, multicolor, and full-PBA EC window prototype based on a PB/Ni-PBA working electrode and a Cu-PBA counterelectrode.

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