Enhanced Coloration for Hybrid Niobium-Based Electrochromic Devices

Niobium pentoxide, Nb2O5, exists in many polymorphs with diverse properties and morphologies that are dependent on the synthesis route. In this work, we design a new approach to fabricate and stabilize the metastable polymorph hexagonal nano-niobium pentoxide using soft chemistry and in particular the polyol synthesis. The electrochromic properties of homogeneous niobium pentoxide thin films deposited by dip coating, from as-synthesized hexagonal powder, are studied in lithium trifluoromethanesulfonimide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) ionic liquid electrolyte versus Pt as counter electrode. The structure and morphology of the Nb2O5 thin films before and after cycling are characterized by ex-situ XRD and SEM while their optical and electrochemical properties are investigated by in-situ transmittance and cyclic voltammetry (CV), respectively. Thin films made from hexagonal Nb2O5 with a sphere architecture exhibit good cycling stability, high transmittance modulations (ΔT ≈ 33% at 550 nm), fast switching speeds (2–3 s for coloration and bleaching at 550 nm), and high coloration efficiency (CE ≈ 26 cm2 C–1) with a good quality factor G(λ) = 10.4 cm2 C–1 s–1. Finally, novel electrochromic devices based on hybrid thin films combining poly(ethylene-3,4-dioxythiophene):poly(styrenesulfonicacid) (PEDOT:PSS) and oxides are assembled showing enhanced coloration and simultaneous color change due to their double-sided configuration.