A Dual-Mode Anode-Free Zinc-Prussian Blue Electrochromic Device

Conventional Prussian blue (PB)-based electrochromic devices (ECDs) suffer from a narrow light modulation range due to their single absorption band. Herein, an anode-free Zn-PB electrochromic device is reported, utilizing a platinum (Pt) layer-modified ITO glass (denoted as Pt/ITO glass) counter electrode with a hybrid electrolyte containing propylene carbonate (PC). This device compensated for the charge released or consumed during the bleaching/coloring process of the PB electrode (i.e., ion-insertion/extraction) through a reversible Zn electrodeposition occurring on the surface of the Pt/ITO glass. The Pt layer ensured a uniformly distributed electric field across the electrode surface, leading to uniform Zn deposition. Concurrently, PC molecules modified the solvation structures of ions, engendering uniform Zn deposition and suppressing the “ion trapping” effect of PB. Meanwhile, PC suppressed water activity by changing the H-bonding network of electrolytes, thereby limiting the formation of by-products, the occurrence of side reactions, and the destruction of the PB structure. As a result, the optimized anode-free Zn-PB ECDs demonstrated high transmittance modulation ability (60.3% at 700 nm) and exceptional cycling durability (71.7% capacity retention and 69.1% of its initial ΔT after 1000 cycles). Finally, a dual-mode electrochromic device is developed with five color states to expand the light modulation range.