Oxygen Evolution Reaction of Titanium Oxide-Based Materials with and without Doping in Alkaline Solution

Abstract

Introduction Water electrolysis technology has paid attention to recent developing with Power-to-gas (PtG). Especially, technologies of alkaline water electrolysis (AWE) is one of essential component for PtG. Ni of the AWE anode is already reported that it is afraid to occur the degradation by the fluctuating power produced from renewable energy1, so that it should be required alternative anode with high durability. From our previous research, it was found that group 4, 5 transition metal oxides has catalytic activity for not only oxygen reduction reaction2 but also oxygen evolution reaction (OER) in alkaline solution3. In this study, we have focused on the titanium oxide-based material4 and the catalytic activity and durability of Ti oxide-based electrocatalyst with and without doping for the OER have investigated in the alkaline solution. Experimental Four types of TiOx doping with 1 mol% of Zr, Ce, and Ta (Zr-TiOx, Ce-TiOx, and Ta-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 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 7 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. For the simulation durability of fluctuation power supply, the potential cycling was performed for 5000 cycles with 500 mV s-1 in the range of 0 to 2.0 V by triangular wave, and then SSV measurement was demonstrated to evaluate the OER activity of TiOx samples. Results and discussion Each electrical resistivity of the sample was around 1 to 5 Ω cm, and no significant difference was observed depending on the presence or absence of doping other elements. In addition, although the resistance after electrochemical measurement has increased for all samples, the order of magnitude has not changed. Fig. 1 shows Tafel plots of OER on TiOx before the potential cycling test. The vertical axis uses the pseudo current density, i*, that was normalized by the electric double layer capacity. Since Tafel region of each sample was observed from 1.5 to 1.6 V, Tafel slope was calculated in range of these potential. In addition, the i* at 1.6 V was used as an index to estimate the OER activity in this study. The i* at 1.6 V of Zr-TiOx was 1.4×10-1 A F-1 and it was higher than that of initial TiOx of 3.9×10-2 A F-1. The Tafel slope of Zr-TiOx was 94 mV dec-1 while that of TiOx was 203 mV dec-1. As a result, the OER activity was improved by doping with Zr. In the case of Ce-TiOx and Ta-TiOx, both i*s at 1.6 V was similar to that of TiOx, and no significant change of Tafel slope were observed. According to both results, the effect of doping Ce and Ta was not found. The durability of sample was evaluated by the comparison between initial OER activity and that after potential cycling. We also used i* at 1.6 V as an index of OER activity. The i* at 1.6 V after potential cycling was obtained as following; TiOx: 2.7 × 10-2 A F-1, Zr-TiOx : 6.3 × 10-2 A F-1; Ce-TiOx : 6.1 ×10-3 A F-1, Ta-TiOx : 1.3 ×10-2 A F-1. The reduction rate from the initial activity was 31% for TiOx, 55% for Zr-TiOx, 74% for Ce-TiOx, and 47% for Ta-TiOx, respectively. Acknowledgment This work is partially supported by Toyota Mobility Foundation and the Suzuki Foundation. References 1) H. Ichikawa. K. Matsuzawa, Y. Kohno, I. Nagashima, Y. Sunada, Y. Nishiki, A. Manabe, and S. Mitsushima, ECS Trans., 58 (33), 9 (2014). 2) K. Matsuzawa, A. Ishihara, Y. Kohno, K. Ota, S. Mitsushima, and G. Jerkiewicz, Abst. 65th Annual Meeting of ISE, s04-001, Lausanne, Switzerland (2014). 3) A. Oishi, K. Matsuzawa, Y. Kohno, A. Ishihara, and S. Mitsushima, Abst. ECS 228th Meeting, #1890, Phoenix, Az (2015). 4) R. Suzuki, K. Matsuzawa, and A. Ishihara, Proc. 38th Meeting of Hydro. Ener. Sys. Soc. Jpn, p.95 (2018) (in Japanese). Figure 1