Fast-Switching Electrochromic\nDevice Enabled by Fe<sup>2+</sup>-Mediated MnO<sub>2</sub>/Mn<sup>2+</sup> Redox Reactions

Reversible electrodeposition and dissolution of manganese\ndioxide\n(MnO<sub>2</sub>) on transparent conducting substrates offer a promising\nstrategy for light modulation in electrochromic (EC) smart windows\nbut are limited by slow dissolution kinetics and limited reversibility.\nIncreasing electrolyte acidity speeds dissolution, yet reversibility\nremains constrained even with up to 1 M sulfuric acid. Notably, adding\n10 mM Fe<sup>2+</sup> to a less acidic 50 mM H<sub>2</sub>SO<sub>4</sub> electrolyte markedly improves the speed and reversibility of MnO<sub>2</sub>/Mn<sup>2+</sup> redox reaction. The enhancement is due to\nthe reduced acidity accelerating electrodeposition during coloration,\nwhile Fe<sup>2+</sup> aids the dissolution of MnOOH intermediate during\nbleaching. The setup achieves 99% transmittance contrast at shorter\nwavelengths within 25 s and remains stable over 500 cycles. Additionally,\na tandem EC device integrating MnO<sub>2</sub>/Mn<sup>2+</sup> and\nZn<sup>2+</sup>/Zn redox reactions offers selective or full-spectrum\nblocking, delivering 95% modulation contrast with fast switching.\nThis research provides critical insights for developing advanced EC\nsmart windows.