BODIPY-Sensitized Photocharging of Anthraquinone-Populated Polymer Layers for Organic Photorechargeable Air Battery

Photocharging of anthraquinone (AQ)-populated polymers was accomplished by incorporating chromophores with redox potentials suitable for the 2e^- reduction of the AQ pendants near -1 V versus Ag/AgCl in aqueous basic electrolytes. The photoactive polymers were designed to exhibit redox-isolated behaviors and reversible charge-discharge responses, either by employing a polyacetylene main chain with the π-π* absorption bands in a visible region, or with an aliphatic chain incorporating a BODIPY dye in the form of a multilayer. The charge storage density was maximized by minimizing the molecular weight of the compact repeating units, thus reducing the redox site-to-site distance to allow redox gradient-driven electron self-exchange reactions to proceed throughout the slab of the polymer layer. Homogeneous layers of poly(2-ethynylanthraquinone) and poly(2-vinylanthraquinone) were both sufficiently robust for charge storage application, swellable, and yet insoluble in aqueous basic electrolyte solutions. Such properties allowed the accommodation of external cations from the electrolyte to permeate through the layer for electroneutralization of the negative charge produced by the electroreduction, which led to the repeatable charging and discharging cycles of the photoanodes without degradation of the charge storage capabilities. By virtue of the swelling properties of the polymer layers in the basic aqueous electrolyte, exploration of the aqueous electrochemistry and photochemistry of AQ that had been inaccessible by the lack of the solubility in H₂O became feasible. A striking feature of the photoanodes was the capability of employing a hydroxide ion (OH^-) as the sacrificial reducing agent, which enabled the use of the O₂/OH^- redox couple for the cathode reaction during discharging. Combination of the photoanodes with the MnO₂/carbon composite cathode, sandwiching the electrolyte layer of aqueous KOH, gave rise to photorechargeable battery effect under irradiation and repeatable discharging with the consumption of O₂ at the cathode.