Electrochemical Stability Enhancement of Electrochromic Tungsten Oxide by Self-Assembly of a Phosphonate Protection Layer

The presented work demonstrates an innovative method to overcome electrolyte restrictions for electrodeposited tungsten oxide (WO3) electrochromic electrodes. By self-assembly of a phosphonic acid protection layer on top of the WO3 electrode, the cycle life of a WO3 electrode in aqueous electrolytes of potassium (KCl) and lithium chloride (LiCl) is dramatically enhanced. Based on the hydrophobic nature of the self-assembled monolayer (SAM), the modification allows for ion intercalation while it prevents etching of the electrode. The cycle life of a WO3 electrode in 1 M KCl increased from under 100 to over 1000 cycles between -0.6 and 0.6 V versus Ag/AgCl. Furthermore, the current-voltage cycling and simultaneous optical transparency measurements show that a WO3 electrode having a self-assembled monolayer of an n-dodecylphosphonic acid exhibits no degradation through detachment of the electrochromic material. Our results suggest that SAM modification of electrochromic oxides is a promising new route toward long lifetime electrochromic devices even in hostile electrolyte environments.