Copper-Based Reversible Metal Electrodeposition Devices with Transparent, Blue, Red, and Mirror States

Reversible metal electrodeposition devices (RMEDs) operate through electrochemically controlled metal deposition and dissolution to modulate optical properties, making them a promising platform for smart windows and adaptive displays. Unlike traditional electrochromic devices that rely on ion insertion or organic redox processes, this reversible metal deposition/dissolution mechanism eliminates the need for complex multilayer structures, offering superior optical and thermal management through reflection-based light control. Silver-based RMEDs have successfully demonstrated multicolor switching capabilities; however, their high material costs limit their practical use. Copper, being abundant and having lower electrodeposition potentials, offers a compelling alternative; yet, previous copper-based devices achieved only monochromatic operation due to limited control over nucleation and growth. Here, we present a breakthrough copper-based RMED that overcomes these limitations with a dual-electrode design combining indium tin oxide (ITO) nanoparticle-decorated ITO (NITO) and smooth flat ITO (FITO) electrodes. This approach enables reversible switching among four distinct optical states: transparent, blue, red, and mirror, marking the first demonstration of achieving both blue and red coloration on a single electrode in a copper-based system. Adjusted voltage protocols (constant-voltage for blue, step-voltage for red) induce wavelength-specific localized surface plasmon resonance (LSPR) absorption at 700 and 550 nm, respectively, while anodic deposition on the smooth electrode produces broadband reflection for the mirror state. The device exhibits high optical modulation across all states (Δ<i>T</i> from 55.8 to 72.2%), with substantial color differences (Δ<i>E</i> values of 33.7 and 42.8 for the blue and red states, respectively) that ensure distinct visual perception. Consistent coloration times (∼10 s) and practical bleaching times (9.8-17.9 s) support responsive switching, making it suitable for smart window applications. The device demonstrates moderate cycling stability with retention exceeding 80% after 100 cycles across all optical states. These results establish copper-based RMEDs as a viable and cost-effective approach for multicolor electrochromic technologies in energy-efficient buildings and adaptive optical systems.