Electrolyte and Electrode Design for Dynamic Windows Based on Reversible Metal Electrodeposition

Dynamic windows, which allow electronic control over light and heat flux, enable responsive environments free of glare and unwanted heat. Electronic control over traditionally static performance characteristics like visible light transmittance and solar heat gain coefficients will allow dynamic windows to be integrated with algorithms to optimize for energy efficiency and comfort. Dynamic windows based on reversible metal electrodeposition (RME) represent an exciting, new class of electrochromic devices. Metals are ideal light-modulators for dynamic windows because they can be color neutral, inert, photostable, and opaque at 20-50 nanometer thicknesses. Our team recently demonstrated metal-based dynamic windows that boast cheap processing, fast switching, and stable performance over thousands of cycles on the 25 cm2 scale. Several technical challenges hinder further scalability of the technology. Indium tin oxide (ITO), the most ubiquitous transparent conductor, has a relatively high sheet resistance. By Ohm’s Law, maintaining a uniform current density across ITO electrodes necessitates a voltage drop proportional to the electrode area. Large-scale RME windows must be engineered with a tolerance to the voltage drop across the transparent electrodes in order to maintain optical uniformity. Additionally, undesired side reactions like the hydrogen evolution reaction (HER) set strict limits on the potentials that may be applied in aqueous electrolytes. The ideal system will enable uniform nucleation and growth of metal films over the potential range set by side reactions inherent to aqueous electrolytes, and the voltage drop inherent to transparent conductors. Prudent design of electrode structure and electrolyte composition enable uniform and reversible electrodeposition over large areas. First, we demonstrate a scalable, self-assembly process to fabricate transparent electrodes functionalized with Pt nanoparticles. The nanoparticles enable site-selective control over nucleation of metal deposits. Second, we establish selection criteria for the use of polymeric levelling agents in reversible metal electrodeposition systems. By understanding the binding affinity between levelling agents and metal ions in our system, we access a range of electrodeposit morphologies that offer tunable control over optical properties. Finally, we demonstrate 100 cm2 RME-based dynamic windows that switch uniformly from a clear state (60% transmission) to an opaque state (<5% transmission) in less than one minute. Figure 1