Colorimetric Voltmeter Using Colloidal Fe₃O₄@SiO₂ Nanoparticles as an Overpotential Alarm System for Zinc–Air Batteries

In rechargeable batteries, overpotential problems may occur especially when the battery is aging. An alarm system to prevent overpotential may be useful. Colloidal nanoparticles of Fe3O4@SiO2 can self-assemble into amorphous photonic crystals (APCs) with tunable interparticle distances as a function of applied voltage. Here, we fabricated a colloidal APC that changes its color from mahogany to dark blue at applied voltages from 1.0 to 4.0 V, respectively. Citrate-modified Fe3O4 was solvothermally synthesized, followed by a SiO2 shell coating by the Stöber method to achieve a stable APC dispersed in polycarbonate solution. The APC is then connected alongside a zinc–air battery to indicate operational events including discharging, charging, and overcharging by color change of the APC. We anticipate that this simple idea provides an intuitive way to monitor the status of rechargeable batteries.