Redox-Active Metal-Organic Framework As an Anode-Active Material for Rechargeable Air Batteries

[Introduction] Rechargeable batteries for the storage of electrical energy are required for the introduction of renewable energy and compact electronic devices for the sustainable society. [1] Although lithium-ion batteries currently dominate the market, rechargeable metal–air batteries with very high energy densities have been attracting attention due to increasing power consumption and demand for low environmental burden. They are composed of oxygen from air as the cathode-active material, metal as the anode-active material and a strong alkaline solution as the electrolyte. They have the advantage of high energy density and low environmental burden due to the use of oxygen as the cathode-active material. While primary Zn–air batteries are commercialized, rechargeable metal–air batteries are under investigation due to their low cycle performance caused by dendrites and metal oxides formed on the metal anode, which leads to disposal of anode-active materials. In addition, strong basic electrolytes are used due to their high ionic conductivity, and the carbonate clogging by carbon dioxide in the air is also a problem. Therefore, rechargeable air batteries, which consist of acid or neutral electrolyte and anode-active materials which is recyclable and do not form dendrites or metal oxides, are required. Organic redox molecules, which are reversibly redox-active organic compounds, are composed of earth-abundant and relatively available building blocks (C, H, N, O, and S), and their functions can be tuned by molecular design, which has attracted attention as new electrode-active materials. We have previously reported that rechargeable polymer–air batteries using an organic redox polymer with organic redox molecules in the side chain of polymer as the anode-active material and acidic aqueous solution as the electrolyte charged and discharged without the formation of dendrites or carbonates. [2] However, organic materials suffer from low durability, and organic redox materials with structural stability in the electrolyte have been required. In the current work, we focused on UiO-66, a metal-organic framework with structural stability in acid and base electrolyte, and synthesized UiO-66-(OH 2 ) with 1,4-dihydroxybenzene, an organic redox molecule as a linker, which undergoes a reversible redox reaction even in acid electrolytes. An anode-active material composed of UiO-66-(OH 2 ) did not form dendrites or metal oxides in H 2 SO 4 aqueous electrolyte (pH 1) and ex