Redox-Active Metal-Organic Framework As an Anode-Active Material for Rechargeable Air Batteries
Authors: Akai Ryota , Kasai Hitoshi , Oka Kouki 
Published: December 2024

[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-(OH2) 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-(OH2) did not form dendrites or metal oxides in H2SO4 aqueous electrolyte (pH 1) and exhibited charge storage with high cycle stability. A rechargeable acidic MOF–air battery was demonstrated for the first time. After use, UiO-66-(OH2) electrode was facilely decomposed and separated to the original raw chemicals by NH4HCO3 aqueous solution.
[Results and Discussion]
We synthesized UiO-66-(OH2) from zirconium(IV) chloride and 2,5-dihydroxyterephthalic acid by microwave irradiation. UiO-66-(OH2) was coated on a glassy carbon plate in the presence of single-walled carbon nanotube as the conductive additive. The UiO-66-(OH2) electrode exhibited charging-discharging curves of a plateau voltage at 0.2 V, and the ratio of discharging vs. charging capacity (i.e. coulombic efficiency) were nearly 96%. The high capacity of the UiO-66-(OH2) electrode suggested that almost all of the -OH moieties contributed to charge storage. Even after 300 cycles, UiO-66-(OH2) electrode exhibited high cycle performance, which was relatively robust compared to other n-type redox materials for rechargeable aqueous air batteries. These results demonstrated that UiO-66-(OH2) has high cycle stability as an anode-active material for rechargeable acidic MOF-air batteries.
A rechargeable acidic MOF-air battery was fabricated with a UiO-66-(OH2), Pt/C, and H2SO4 aqueous electrolyte (pH 1) as the anode material, cathode material, and electrolyte, respectively. The coulombic efficiencies of the battery were nearly 97%, demonstrating the reversible charge storage property of the cell. The high capacity for the anode suggested that almost all of the -OH moieties contributed to charge storage. Even after 100 charging-discharging cycles, the battery kept the high discharging capacity even at a rapid discharging. These results demonstrated the long-life ability of the rechargeable acidic MOF-air battery. UiO-66-(OH2) electrode in the battery was facilely decomposed and separated to the original raw chemicals by NH4HCO3 aqueous solution after charging and discharging.
[Conclusions]
We synthesized UiO-66-(OH2), which is stable in aqueous electrolyte, and applied it to an anode-active material. The rechargeable acidic MOF-air battery exhibited high cycle performance without the formation of dendrites or carbonates. In addition, we successfully recycled UiO-66-(OH2) electrode after charging and discharging. The current work presented a first step for developing electrode materials for the future energy storage.
[References]
[1] J. Kim, Y. Kim, J. Yoo, G. Kwon, K. Kang et al.,
Nat. Rev. Mater.
, 8, 54–70, 2023.
[2] K. Oka, S. Furukawa, S. Murao, H. Nishide, K. Oyaizu et al.,
Chem. Commun.
, 56, 4055–4058, 2020.

Figure 1