Prussian-blue (PB) film for electro-chromism can be electro-deposited on to an electrode (usually tin-doped indium oxide/glass) either directly from a PB colloid or from ferric ferricyanide in a two-electrode electro-chemical cell by applying a reductive potential. Alternatively, a "sacrificial" electron-producing silver flag electrode in the solution, when connected to the PB-receiving electrode, can effect the required reductive deposition. A silver spot, here innovatively applied as silver paint directly onto the deposition electrode, produces PB film on immersion in the iron(III)(III) solution, obviating the separate counter-electrode method.
Barton, R. T., Kellawi, H., Marken, F., Mortimer, R. J., & Rosseinsky, D. R. (2012). Simplest Prussian-blue deposition from ferric ferricyanide solution by a reducing Ag spot put onto an ITO substrate. Journal of Solid State Electrochemistry, 16(12), 3723-3724. https://doi.org/10.1007/s10008-012-1811-7