Effect of Local Cell Reaction on γ-NiOOH Formation in Nickel Hydrogen Battery

Introduction Recently, nickel hydrogen battery is taking a great attention as the most promising candidate for the power sources of hybrid vehicles. Therefore, it is important to develop a property of nickel hydrogen battery. When a nickel hydrogen battery has been cycled at a shallow depth of discharge, the battery will produce a less capacity than that corresponding to normal discharge-charge cycles [1] . In the open circuit, local cell reaction between electrode material and current collector (CC) will occur. Till now, the effect of local cell reaction has not been studied enough. In this study, we focused on the local cell reaction between the cathode material (β-NiOOH) and CC for nickel hydrogen battery. In the previous study [2] , when using nickel mesh as CC, the critical reduction in cell capacity was observed and the peak characteristic to γ-NiOOH was observed between 10°and 15°in 2θ. On the other hand, when using gold or platinum mesh as CC, the critical reduction in cell capacity was not observed and the peak characteristic to γ-NiOOH was not observed between 10°and 15°in 2θ. In this study, another electrode structure (Carbon coated nickel mesh) to prevent the formation of γ-NiOOH was developed. Experiment The cathode was fabricated by mixing powder of the synthesized β-NiOOH as the active material, acetylene black as a conducting additive and PTFE as a binder at the ratio of 80:15:5 by weight. Platinum plate was used as counter electrode and Ag/AgCl reference electrode was used as reference. The electrolyte was an 8 M potassium hydroxide. Carbon was coated on the surface of nickel mesh by ion sputtering device. Carbon coated nickel mesh was used to investigate the local cell reaction between electrode material and CC. As reference, gold, platinum and nickel mesh were used as CC. We discharged the electrode material to the cutoff voltage of 0.19V(vs. SHE) at 30mA/g and charged for 6h at 30mA/g. Then, we discharged to the half of the cell capacity at 30mA/g. After that, we opened the circuit and set rest for 3d. X-ray diffraction analysis of cathode was conducted using RINT-TTR (Rigaku co., CuKα, 200mA, 30kV) . Results and Discussion Fig.1 shows the experimental results of XRD measurement. (A) shows XRD pattern when using carbon coated nickel mesh as CC. The peak characteristic to γ-NiOOH [ 3 ] was not observed between 10°and 15°in 2θ. (B)-(D) show XRD pattern when using (B) gold (C) platinum and (D) nickel mesh as CC. The peak characteristic to γ-NiOOH [ 3 ] was not observed for (B) gold and (C) platinum mesh and observed for (D) nickel mesh. It is considered that carbon coated nickel is inactive and no local cell reaction occur. It is concluded that γ-NiOOH was formed when β-NiOOH acts as anode and that γ-NiOOH was not formed when β-NiOOH acts as cathode and no local cell reaction occur. References [1] Yuichi Sato et al., J. Power Sources 93 (2001) 20-24. [2]T. Iwai and T. Yao, Study of Memory Effect of Nickel Hydrogen Battery, 225 th ECS Abst. [3] J. Pan et al., Electrochemica Acta 54 (2009) 3812-3818 [4] T. Yao, T. Iwai and H. Tagashira, PCT/JP2013/ 74165 (2013)