Metals are typically used as active materials for negative electrodes in batteries but redox-active organic molecules, including quinone- and amine-based molecules, have been used as negative electrodes in rechargeable metal–air batteries with oxygen-reducing positive electrodes. Here, protons and hydroxide ions participate in the redox reactions.
These batteries exhibit high performance close to the maximum capacity that is theoretically possible. Furthermore, using redox-active organic molecules in rechargeable air batteries overcomes problems associated with metals, including the formation of dendrites. These batteries use liquid electrolytes - like metal-based batteries – that pose safety concerns.
Now, in a new study published in Angewandte Chemie International Edition Japanese researchers have developed an all-solid-state rechargeable air battery (SSAB) and investigated its capacity and durability. The study was led by Professor Kenji Miyatake from Waseda University and the University of Yamanashi, and co-authored by Professor Kenichi Oyaizu from Waseda University.
The researchers chose 2,5-dihydroxy-1,4-benzoquinone (DHBQ) and its polymer poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) (PDBM) as active materials for the negative electrode due to their stable and reversible redox reactions in acidic conditions. In addition, they utilised Nafion, a proton-conductive polymer, as the solid electrolyte.
“To the best of my knowledge, no air batteries based on organic electrodes and solid polymer electrolyte have been developed yet,” Miyatake said in a statement.
After the SSAB was in place, the researchers assessed its charge–discharge performance, rate characteristics, and cyclability. They found that unlike typical air batteries that use a metallic negative electrode and an organic liquid electrolyte, the SSAB did not deteriorate in the presence of water and oxygen. Furthermore, replacing the redox-active molecule DHBQ with its polymeric counterpart PDBM formed a better negative electrode. While the per gram-discharge capacity of the SSAB-DHBQ was 29.7mAh, the corresponding value of the SSAB-PDBM was 176.1mAh, at a constant current density of 1mAcm-2.
The researchers also found that the coulombic efficiency of SSAB-PDBM was 84 per cent at four C rate, which gradually decreased to 66 per cent at 101 C rate. While the discharge capacity of SSAB-PDBM reduced to 44 per cent after 30 cycles, by increasing the proton-conductive polymer content of the negative electrode, the researchers improved it to 78 per cent. Electron microscopic images confirmed that the addition of Nafion improved the performance and durability of the PDBM-based electrode.
This study demonstrated the operation of an SSAB comprising redox-active organic molecules as the negative electrode, a proton-conductive polymer as the solid electrolyte, and an oxygen-reducing, diffusion type positive electrode. The researchers hope that it will lead to further advancements.
“This technology can extend the battery life of small electronic gadgets such as smartphones and eventually contribute to realizing a carbon-free society,” said Miyatake.