Evaluation of Ta and Zr compounds for oxygen evolution reaction in sulfuric acid

Introduction Since a polymer electrolyte water electrolysis (PEWE) has a compact and highly efficient system compared to an alkaline water electrolysis, A PEWE is expected to be a hydrogen station for the fuel cell vehicle. However, there are some problems for PEWE such as high cost and overpotential of anode electrocatalyst consisting of precious metal oxide such as IrO2. In order to overcome these problems, an alternative anode electrocatalyst with low cost such as non-precious metal oxide is required. Since PEWE anode operates under high potential in a strong acidic environment, the alternative material should have not only the high catalytic activity for the oxygen evolution reaction (OER) but also high stability in an acidic electrolyte. From these points of view, we focused on the Ta and Zr compounds thin film which showed the high stability in an acidic solution. In this study, the OER activities of Zr and Ta compounds film have been investigated. Experimental Ta or Zr compound thin film an etched Ti rod (φ = 5.0 mm) as a base material was prepared by the reactive sputtering. TaC and ZrC were used as target materials. Base material temperature (θBase.) was controlled from 50 to 800C and partial pressure of oxygen (PO2) was from 0 to 20 mPa during sputtering to the preparation the film. Other conditions were constant. Electrochemical measurements were examined in 0.1 M (= mol dm) H2SO4 at 30C under atmospheric condition using a conventional 3-electrode cell. The working electrode was Ta or Zr compound thin film prepared by the sputtering, and IrO2 powder on an etched Ti rod was used as the working electrode for comparison. A reversible hydrogen electrode (RHE) was used as a reference electrode. A carbon plate was used as a counter electrode. Cyclic voltammometry (CV) was performed in the potential range from 0.3 to 0.8 V vs. RHE with 50 mVs of scan rate. Linear sweep voltammetry (LSV) was performed in the potential range from 1 to 2 V vs. RHE with 5 mVs of scan rate under air atmosphere to obtain the current for the OER. The chronoamperometry (CA) was also measured at 1.6 V vs. RHE for 0.5 h. Results and discussion Since the electrochemically active surface area (ECA) of thin a film seemed to be very small, compared to that of IrO2 powder electrode, the real surface area should be determined in order to evaluate the real OER activity on a thin film of Ta or Zr compound in order to compare to that of IrO2 powder. However it is difficult to evaluate the ECA of oxide and oxide-base compound such as a thin film in this study. In this study, the parameter of current density which was related to the active area was defined as i*(= i⋅QA), where QA was the anodic electric charge calculated from cyclic voltammogram. i* at 1.6 V of LSV applied for the initial OER specific activity described as iinit.*, and i* at 0.5 h of a CA applied for the steady state OER described as iSS.* to evaluate the OER activity of Ta and Zr compound thin film compared with that of IrO2 powder. Figure 1 shows the dependence of the iinit.* and the iSS.* for Ta compound film on θBase. of sputtering at PO2 = 20 mPa. Both the iinit.* and the iSS.* of the IrO2 were also plotted in Fig. 1. Both the iinit.* and the iSS.* of Ta compound films were increased with the decrease of PO2 of sputtering. Both the iinit.* and the iSS.* at θBase. = 50C were found to be the maximum of all in the range of these temperatures. At that temperature, iinit.* of Ta compound was larger than that of IrO2, and iSS.* of Ta compound was almost same to that of IrO2. Ta compound prepared at 50C under 20 mPa of PO2 was might have the possibility of higher OER specific activity than that of IrO2. Figure 2 shows the dependence of the iinit.* and the iSS.* for Zr compound film on PO2 of sputtering at θBase. = 200C. Both the iinit.* and the iSS.* of the IrO2 were also plotted in Fig. 2. Both iinit.* and iSS.* of Zr compound film had maximum region from 5 to 10 mPa of sputtering. At that region, both the iinit.* and the iSS.* of Zr compound film was more than 2 times larger than that of IrO2. These facts indicates that the OER specific activity of Zr compound thin film prepared at 200C under 5-10 mPa of PO2 might be higher than that of IrO2. Reference 1. J. Turner, G. Sverdrup, M.K. Mann, P-C. Maness, B. Kroposki, M. Ghirardi, R.J. Evans and D. Blake, Int. J. Energy Res., 32, 379 (2008). 2. K. Matsuzawa, C. Igarashi, S. Mitsushima, and K. Ota, ECS Trans, 25(23), 119 (2010). 3. Y. Ohgi, A. Ishihara, K. Matsuzawa, S. Mitsushima, and K. Ota, J. Electrochem. Soc., 157, B885 (2010). 4. M. Tamura, A. Ishihara, T. Tada, K. Matsuzawa, S. Mitsushima, and K. Ota, ECS Trans., 16(24), 125 (2009).