Fast-Switching Electrochromic Device Enabled by Fe2+-Mediated MnO2/Mn2+ Redox Reactions

Reversible electrodeposition and dissolution of manganese dioxide (MnO2) on transparent conducting substrates offer a promising strategy for light modulation in electrochromic (EC) smart windows but are limited by slow dissolution kinetics and limited reversibility. Increasing electrolyte acidity speeds dissolution, yet reversibility remains constrained even with up to 1 M sulfuric acid. Notably, adding 10 mM Fe2+ to a less acidic 50 mM H2SO4 electrolyte markedly improves the speed and reversibility of MnO2/Mn2+ redox reaction. The enhancement is due to the reduced acidity accelerating electrodeposition during coloration, while Fe2+ aids the dissolution of MnOOH intermediate during bleaching. The setup achieves 99% transmittance contrast at shorter wavelengths within 25 s and remains stable over 500 cycles. Additionally, a tandem EC device integrating MnO2/Mn2+ and Zn2+/Zn redox reactions offers selective or full-spectrum blocking, delivering 95% modulation contrast with fast switching. This research provides critical insights for developing advanced EC smart windows.