Low-Concentration Electrolytes toward High-Performance Aqueous Ammonium-Ion Electrochromic Devices

Ammonium-ion electrochromic devices (AECDs) have emerged as a promising candidate for next-generation intelligent electronics with visible working states. However, developing electrodes exhibiting remarkable electrochromic performance and exceptional long-term cycling stability within suitable ammonium-ion electrolytes is highly desirable yet challenging. Herein, the electrochemical and electrochromic performances of Prussian blue (PB) electrodes were systematically investigated in various aqueous electrolytes and different concentrations of (NH₄)₂SO₄ electrolyte. According to experimental exploration and molecular dynamics simulations, a low-concentration electrolyte is conducive to high-performance AECDs. A 0.25 M (NH₄)₂SO₄ solution was identified as yielding optimal fast diffusion kinetics, remarkable electrochromic performance of PB electrodes, and excellent long-term cycling stability over 5000 cycles with a retention of 82.3%. Furthermore, by coupling it with a transparent electrochromic negative electrode composed of Nb₁₈W₁₆O₉₃ (NbWO) and ammonium-ion hydrogel electrolytes, a quasi-solid-state AECD is assembled. It demonstrates remarkable optical modulation (61.1% at 630 nm) and long-term cycling stability (more than 3000 cycles). Significantly, based on the discovery of the quantitative relationship between the optical characteristics and electrochemical reactions in devices, it becomes feasible to assess the operational state of the AECD in real time. Considering that the development of AECDs is still in the infancy stage, this work provides insights and perspectives for the next generation of intelligent ammonium-ion electrochemical devices.