Electrochemical Behavior in Calcium Aqueous Electrolyte of Spinach, Malabar Spinach and Komatsuna

Abstract

Multivalent-ion rechargeable batteries using calcium ions have an attracted attention as one of next generation batteries. Calcium-ion batteries using a metal negative electrode that does not form dendrites can contribute the twice number of electrons per cation compared with lithium ion, which increase the charge capacity for the specific insertion host. In recent years, the electrochemical stripping and plating of calcium metal has been a report for anode. [1] On the other hand, the calcium insertion hast for cathode active materials such as NiFe(CN)6 [2] and MnFe(CN)6 [3] have been reported. However, their capacity and potential are far from those of lithium-ion battery cathode materials, and there are not so many candidates for cathode active materials. This is because the diffusion of multivalent ions in the inorganic compound is disturbed by a large interaction with cations and anions in the host structure. It is still challenge to improve ionic conduction drastically by the conventional concept and novel material concept is needed. We focused on the fact that, among the calcium compounds using natural products, calcium is contained relatively in green vegetables. Since green vegetables such as Spinach, Malabar spinach, and Komatsuna contain a relatively large amount of calcium and a metal cation, electrochemical insertion / deinsertion of calcium would be realized. Then, new material guideline can be established by analyze the chemical structure for calcium insertion / deinsertion. In this study, we report the electrochemical behavior of Spinach, Malabar spinach, and Komatsuna as active materials. Spinach, Malabar spinach, and Komatsuna were vacuum dried at 75 ℃ and grind in a mortar. The vegetable powder was mixed with acetylene black and PTFE at weight ratio of 75:15:10. The pelletized electrode was pressed between two Ti meshes. The three-electrode cell using Ag / AgCl reference electrode and an activated carbon counter electrode was assembled with 2.5 M Ca(NO3) aqueous electrolyte.CV measurement was performed at the scan rate of 10 mV s-1 with the potential range of -0.8 V to 0.9 V. Figure 1 shows the CV of Spinach, Malabar spinach, and Komatsuna in aqueous Ca(NO3) electrolyte. All samples are swept to cathodic first and then anodic. No distinct reduction peaks were observed, whereas the anodic sweep confirmed the distinct peaks. The anodic peaks were observed around 0.3 V for spinach and malabar spinach, and around 0.5 V for komatsuna. Higher anodic current is observed, particularly in komatsuna which has different chemical species of calcium, and the peak position is shifted towards positive direction. Among the three vegetables, the calcium content is in the order of komatsuna, malabar spinach, spinach, which consists with the order in the peak currents. When this anodic current is assumed to be derived from electrochemical deinsertion of calcium ions, it is estimated that the approximately 30 to 50% of the calcium content is extracted in this reaction. Reference. [1] A. Ponrouch, C. Frontera, F. Bardé and M. R. Palacín, Nature. Mater., 15, 169-172 (2016). [2] T. Tojo et al, Electrochem. Acta., 207, 22-27, (2016) [3] A. L. Lipson et al., Chem. Mater., 27, 8442-47, (2015) Figure 1