In this report, W⁶⁺ doping as a defect engineering strategy has been proposed to improve the electrochromic properties of NiO film. Further research was conducted to explore the electrochromic properties and the modified mechanism of W-doped NiO film. Compared to the pure NiO, W-doped NiO film exhibits improved electrochromic properties with significant optical modulation (61.56% at 550 nm), fast switching speed (4.42 s/1.40 s for coloring/bleaching), high coloration efficiency (45.41 cm²·C⁻¹) and outstanding cycling stability (no significant attenuation after 2000 cycles) in Li-based electrolytes. Density functional theory (DFT) calculations combined with the experimental results indicate that the improved electrochromic properties were due to enhanced the electronic conductivity and ion conductivity after the introduction of W⁶⁺. The charge capacity of W-doped NiO has also been improved, and it can function with WO₃ to achieve a high performance black electrochromic smart window (ECSW) by balancing charge. This work could advance the fundamental understanding of defect engineering as an effective strategy to boost the electrochromic properties of NiO anodic material, manifesting a significant development as a candidate counter electrode in high-performance black smart windows.

Prussian blue (PB) is an anodic coloring candidate in the wide area of electrochromic (EC) applications. However, the co-influence of weak adhesion and low electrical conductivity leads to the poor stability and slow switching speed. To tackle this bottleneck, a novel TiO₂/Au/PB nanorod array is designed through hydrothermal and electrodeposition approaches on fluorine-doped tin oxide (FTO) glass. Such a designed ternary array structure could not only increase reactive site and conductivity, but also improve ion storage capacity and promote charge transfer, attributed to the synergistic effect of TiO₂/Au/PB core–shell heterostructure and the localized surface plasmon resonance (LSPR) effect of Au nanoparticles. Besides, density functional theory (DFT) calculation confirms the strong interaction between rutile TiO₂ and FTO substrate, which contributes to the improvement of EC cycle stability. Benefiting from these effects, the TiO₂/Au/PB film shows a fast coloration/bleaching response of 1.08/2.01 s (2.17/4.48 s, PB film) and ultra-stable EC performance of 86.8% after 20,000 cycles (50% after 600 cycles, PB film). Furthermore, the high-intensity light source can be shot clearly by the designed and assembled EC iris device (ECID) with TiO₂/Au/PB film as an EC layer, while the photograph without an ECID is blurry, confirming the feasibility of the material in portable digital products.

Electrochromic devices (ECDs) can regulate the indoor solar radiation by adjusting optical transmissive properties, showing great commercial potential and important social value of green energy saving. However, the unsafety and high cost of Li⁺ based electrolyte hinder the large-scale and industrialized production of ECDs. Other metal ions have been used as electrolyte ions, but they are rarely reported in all solid state ECDs. In this study, MgF₂ film is used as the solid electrolyte to construct all solid state ECD with the structure of glass/ITO/WO₃/MgF₂/NiO/ITO. The ECD shows the large optical modulation (~83% at 820 nm, with 100 s durations) and fast response (19.2 s for bleaching and 8.3 s for coloring, with 25 s durations). Moreover, the ECD achieves the extreme transmittance value of colored states Tc ≈ 0%, which can give an absolute private state. This work not only indicates that MgF₂ film can be an alternative to Li⁺ based electrolyte in all solid state ECDs, but also broadens the applications of all solid state EC smart windows to private buildings.