Development of Iron-Based Rechargeable Batteries with Sintered Porous Iron Electrodes

Iron-based rechargeable batteries such as iron-air and nickel-iron batteries are a promising battery system which could meet the future demands in energy storage systems for stationary and automotive applications as iron is a cost effective, highly safe, abundant, and environmental-friendly material. 1-3) The iron-based electrodes have a technological advantage over the other metal electrodes like zinc; iron does not form dendrites in the charge–discharge cycles. A recent state-of-the-art nanotechnology for fabricating nanostructured iron electrodes provided higher specific energies than those of currently-used lithium-ion batteries, indicating the high potential of iron-based batteries for various applications. 4,5) One of the issues which spoil their high performances is formation of iron hydroxide passivation layers that prevent from successive electrochemical reaction at the electrode surfaces. A use of nanometer-size iron particles is effective to increase the specific capacity; however, loading of the adequate iron material may be problematic. In this study, we have developed rechargeable iron-air batteries consisting of sintered porous iron electrodes. Atomized steel powders formed by a water atomization process were used as a raw material in order to realize cost effective, high performance iron-electrodes. The steel powders with a diameter of a several ten micrometers, were mixed with PVA that acted as pore former, and sintered at 1120 °C for 20 min. Figure 1 shows the cross sectional images of (a) the sintered porous iron electrodes and (b) the normally sintered electrodes. A solution of 8M KOH was used as an electrolyte. The properties were evaluated by three electrodes cell. The iron-electrode was set as the working electrode and platinum ribbon acts as a counter electrode. Mercury oxide electrode serves as the reference electrode. Charge-discharge rates were 5mA. For metal-air battery, the carbon paper with manganese dioxide (MnO 2 ; Tosoh Corp.) was employed as the air-electrodes with catalyst. Figure 2 shows the charge–discharge curves of (a) the sintered porous iron electrodes and (b) the normally sintered electrodes. Two distinct plateaus were observed in the discharge curve obtained from the sintered porous electrodes. The specific capacity of the sintered porous iron electrodes was about 150 mAh/g (Fe) at the first cycle and about 100 mAh/g (Fe) even after the third cycle, which was much higher than that of the normally sintered ones. The operation as the iron-air battery using MnO 2 -loaded carbon paper as an air electrode was also confirmed. These results demonstrate the high potentiality of porous iron electrodes obtained from atomized steel powders for high performance iron-air batteries. 1) J. Christensen, et al. J. Electrochem. Soc. 159, R1 (2012), 2) Md. A. Rahman, et al. J. Electrochem. Soc. 160, A1759 (2013), 3) R. D. McKerracher, et al. ChemPlusChem , 80, 323 (2015), 4) A. K. Manohar, et al. J. Electrochem. Soc. 159, A1209 (2012), 5) Y. Maeda, et al. Extended Abstract of The 55th Battery Symposium in Japan, p.470 (2014) (in Japanese). Figure 1