Ultrahigh Coloration Efficiency and Long-Life Nickel Oxide Electrochromic Devices for Sustainable Applications

In the context of electrochromic devices (ECDs), sustainability is highly important and relates to material stability, low energy of operation, and long run of service cycles. Among various EC materials, nickel oxide (NiO) thin films are particularly attractive for their energy efficient operations. However, NiO-based ECDs that rely on ion transport during operation often show long-term degradation because repeated cycling can cause irreversible ion trapping or accumulation in the host, ultimately limiting device lifetime. In this study, we address this challenge by engineering the crystallinity and morphology of reactive sputter-deposited NiO on indium tin oxide (ITO)-coated glass substrates through systematic post-annealing at 400 °C (between 30 to 120 min) under ambient conditions. The electrochemical and EC behaviors of the annealed films are examined in a standard three-electrode configuration using a 1 M KOH electrolyte solution, allowing for the identification of the desired microstructure that supports rapid and reversible ion insertion. The optimized microstructure has an ultrahigh coloration efficiency of 99.3 cm2 C−1 with fast switching, coloration, and bleaching times of 1.6 and 1.5 s, respectively, retaining an optical modulation of 81.1% with an excellent cyclic stability over 40,000 cycles. To fabricate the ECD, this optimized NiO was used as the active electrode, a polyvinyl alcohol (PVA)−KOH gel as the electrolyte, and a plain ITO glass as the counter electrode. The fabricated device also delivers fast switching with outstanding durability and demonstrates stable operation beyond 40,000 cycles while retaining an optical modulation of 70.8% (93% of its initial value of 76.8% at 10,000 cycles). These results demonstrate that controlled microstructural tuning can suppress irreversible ion accumulation and enable sustainable, long-life NiO-based complementary ECDs for practical smart-window and display applications.