Oxygen Electrode Reaction of Titanium Oxide-Based Materials in Alkaline Solution

Abstract

1. Introduction In order to expand usage of renewable energies, it should be required for improving electricity storage technology from the viewpoint of power supply in the disaster such as earthquake in Japan and power sources for remote islands where it is hard to establish infatuation of power grids. For a large scale, it is expected to develop for hydrogen storage such as Power-to-Gas including alkaline water electrolysis (AWE). For a small scale, electric power storage such as batteries including metal-air secondary batteries (MASB). In both devices, the oxygen electrode plays an important role. However, it's already reported that the degradation of Ni in the AWE anode was occurred by potential cycling simulated as fluctuating power derived from renewable energy 1), and the durability and activity for positive electrode (air electrode) of MASB must be improved significantly 2). In our previous study, we focused on Ti oxide-based material because of its abundant resources and stability and tried to apply it with and without doping Zr or Ce as the oxygen electrode for AWE and MASB 3-4). In this study, we have investigated the oxygen electrode reaction of Zr and Ce-doped titanium oxide-based material in alkaline solution. 2. Experimental Two types of TiOx doping with 1 mol% of Zr and Ce (ZrCe-TiOx as shown in the following section) and without doping were prepared under oxygen starvation atmosphere, and then formed as rod shape (φ=5.0 mm, Toshima manufacturing Co., Ltd.). Electrochemical measurements were performed at 30 ± 0.5°C using a TiOx or ZrCe-TiOx rod described in above, reversible hydrogen electrode (RHE) and glassy carbon plate as working, reference and counter electrodes in a three-electrode cell filled with 0.1 mol dm-3 KOH solution. After the pretreatment, cyclic voltammetry (CV) was carried out for the scan rate of 50 mV s-1, and the electric double layer capacity was calculated from the current data. In order to evaluate the catalytic activity for the OER, slow scan voltammetry (SSV) was performed in the potential range of 1.2 to 2.0 V for the scan rate of 5 mV s-1. As a durability simulation against potential fluctuations, a potential cycling test was performed at 1 V s-1 and a triangular wave of 0 to 2.0 V for 5,000 cycles, then the electric double layer capacity and SSV were measured again under the same conditions, and this sequence was repeated 4 times. This means that potential cycling test is completed for 20,000 cycles. 3. Results and discussion Each electrical resistivity of the sample was around 1 to 5 Ω cm, and no significant difference was observed with and without doping. Although the resistance after electrochemical measurement was increased for all samples, the resistance values kept in same digit. Fig. 1 shows Tafel plots of OER on TiOx before the potential cycling test. The horizontal axis is the common logarithm of the current density, i geo,normalized by the geometric area. Tafel region of each sample was observed from 1.6 to 1.7 V. In addition, the i geo at 1.7 V was used as an index of the OER activity in this study. The Tafel slope of ZrCe-TiOx was larger than that of TiOx, and the OER activity of TiOx was higher than that of ZrCe-TiOx. As a result, doping Zr and Ce didn't contribute to enhance the OER activity of TiOx. The durability of sample was evaluated by the comparison between initial OER activity and that after potential cycling. We also used the i geo at 1.7 V as an index of OER activity. Figure 2 shows the i geo of ZrCe-TiOx at 1.7 V during the potential cycling test. From Fig. 2, the OER activity after 20,000 cycle was 40% lower than initial OER activity. Though the OER activity was decreased 40% in 5,000 cycle, the i geo at 1.7 V was almost same from 5,000 to 20,000 cycle. It is suggested that the degradation of OER activity on ZrCe-TiOx was suppressed after 5,000 cycle in this study.Acknowledgment This work is partially supported by Toyota Mobility Foundation and the Suzuki Foundation. References1) H. Ichikawa. K. Matsuzawa, Y. Kohno, I. Nagashima, Y. Sunada, Y. Nishiki, A. Manabe, and S. Mitsushima, ECS Trans., 58 (33), 9 (2014).2) X. Li, A.L. Zhu, W. Qu, H. Wang, R. Hui, L. Zhang, and J. Zhang, Electrochim. Acta, 55, 5891 (2010).3) R. Suzuki, A. Ishihara, K. Ota and K. Matsuzawa, Proc. ECSJ Fall Meeting 2019, 1L10 (2019) (in Japanese).4) R. Suzuki, A. Ishihara, K. Ota and K. Matsuzawa, Proc. 26th FCDIC Fuel cell symp., p.95 (2019) (in Japanese). Figure 1