Systematic Design of Electrochromic Energy Storage Devices Based on Metal–Organic Monolayers

Electrochromic materials (ECMs) change their colored state with changes in potential. When applied as an energy storage device, they reveal the current state of charge using intrinsic properties already within the ECM. This is possible because supercapacitors of the faradaic type, called pseudocapacitors, rely on the same redox reactions that induce the color change in ECMs. In this work, molecularly defined metal complexes were covalently attached to a surface-enhanced indium tin oxide nanoparticle-based screen-printed support to form a monolayer. We have investigated these monolayers for their ability to store and release a charge while retaining their electrochromic properties. Iron(II), cobalt(II), and osmium(II) complexes were deposited separately or simultaneously leading to corresponding single- or triple-metal materials. Spectroscopic and galvanostatic charge–discharge measurements confirm the dual electrochromic and energy storage nature of the devices in a range of currents. The introduction of a transparent charge storing TiO2 layer at the counter electrode improves the current density, capacitance, response time, and long-term performance of the triple-metal electrochromic device. This work adds to a limited number of chromophoric metal coordination complexes applied within energy storage devices.