Mechanisms of Color Switching in Electrochromic Materials: A Comprehensive Review of Inorganic and Organic Systems

Electrochromic materials, capable of reversible color changes upon electrical stimulation, have garnered significant attention for applications in smart windows, displays, and energy storage devices. This comprehensive review delves into the underlying mechanisms of color switching in both inorganic and organic electrochromic systems. Inorganic materials, such as transition metal oxides (e.g., tungsten oxide and nickel oxide), exhibit electrochromism primarily through intercalation processes where ions like Li⁺ reversibly insert into the material's lattice, altering its optical properties. Recent advancements have introduced multicolored inorganic electrochromic materials, expanding their application potential. Organic electrochromic materials, including conjugated polymers like polyaniline (PANI) and polythiophene derivatives, undergo color changes via redox reactions that modulate their conjugation length and electronic structure. For instance, PANI transitions from a yellow reduced state to a green oxidized state upon voltage application. Additionally, innovations in organic systems have led to devices capable of modulating between primary colors, covering the entire visible spectrum. Hybrid materials, such as MXenes, have emerged as promising candidates by combining the advantageous properties of both inorganic and organic systems. Notably, Nb₁.₃₃C MXene-based devices demonstrate colorless-to-black switching with significant transmittance modulation across a broad wavelength range, attributed to reversible ion insertion mechanisms. This review synthesizes recent progress in understanding the color-switching mechanisms of diverse electrochromic materials, highlighting their structural and compositional influences on optical behavior. By elucidating these mechanisms, we aim to inform the design and development of next-generation electrochromic devices with enhanced performance and expanded color palettes.