In dimensionally stable electrodes (DSE^® type electrodes) technology, reduction of precious metal loading is significant issue to reduce cost of system. As improvements of the DSE^® type electrode, many efforts of preparation conditions have been reported. Higher preparation temperature reduces electrocatalyst consumption, but corrosion of the base substrate/electrocatalyst interface makes limitation of service lifetime [1-4]. Here, intermediate layer should be important to improve durability. As the intermediate layer, 4 and 5^th group transition metal oxide or composite with the transition metal oxide and precious metal was proposed, but their property has not be clarified. In addition, these material technology would be important basic for polymer electrolyte membrane water electrolysis, too.

In this study, corrosion resistance of low oxygen partial pressure oxidized titanium electrodes and oxygen evolution reaction activity and durability of low IrO₂ loaded electrodes of the low oxygen partial pressure oxidized titanium have been investigated as basic study of intermediate layer of DSE^® type electrode in industrial electrolysis and electroconductive oxide supported precious metal oxide electrocatalyst for polymer electrolyte membrane water electrolysis.

Experimental

Titanium electrode was oxidized in H₂ – O₂ – Ar of estimated p(O₂) of 1.84×10^-17 atm at 1050°C for 10 or 20 h, H₂ – O₂ – Ar of the p(O₂) of 1.43×10^-17 atm at 1050°C for 10 h, air at 500°C for 1 h. For comparison, Titanium electrode was also used. Surface oxide was determined with X-ray diffraction for thin – film of 2.0° of incidence angle.

0.02 mg cm^-2 of IrO₂, which is ca. 1/50 level of usual industrial electrode, was loaded on the titanium electrodes by thermal decomposition of hydrogen hexachloroiridate(IV) hydrate butanol solution at 400°C for 1 h in air

Electrochemical measurement was performed in 0.1 or 1.0 M (= mol dm^-3) of H₂SO₄ at 30°C. Pretreatment was 300 cycles of cyclic voltammetry between 0.05 to 1.2 V vs. RHE with 200 mV s^-1 for the all electrodes. The electrochemical stability of the titanium electrodes and the electrocatalytic activity for oxygen evolution reaction for the IrO₂ coated titanium electrodes were determined with cyclic voltammetry of 5 mV s^-1 of sweep rate. Resistances of the electrolyte and of surface oxide were determined with the electrochemical impedance spectroscopy (EIS).

Results and discussion

The surface of the titanium electrode oxidized in H₂ – O₂ – Ar of the p(O₂) of 1.84×10^-17 atm for 20 h and that of the 1.43×10^-17 atm for 10 h at 1050°C was mixture of Ti₄O₇ and TiO₂, and that of the p(O₂) of 1.84×10^-17 at 1050°C for 10 h and air at 500°C was TiO₂. So, the notations of the titanium electrodes are Ti₄O₇_1050_20h/Ti, Ti₄O₇_1050_10h/Ti, TiO₂_1050_10h/Ti, and TiO₂_500/Ti, respectively.

Figure 1 shows the surface oxide layer resistance of the titanium electrodes determined by the higher frequency region arc of the EIS measurement as a function of higher limit potential of the slow scan cyclic voltammetry. The resistances of the high temperature (1050°C) oxidized electrodes were constant, while that of the Ti and the 500°C oxidation Ti increased with higher limit potential of the slow scan voltammetry. In addition, the Ti₄O₇ containing oxide layer showed lower resistance than the TiO₂ Therefore, high temperature prepared electroconductive oxidation layer has low resistance with stability as a support material.

Figure 2 shows the surface oxide layer resistance of the IrO₂ loaded titanium electrodes determined by higher frequency region arc of the EIS. Basically, the surface oxide layer resistance decreased with the IrO₂ loading, and especially, the surface oxide layer resistance of the TiO₂_1050_10h/Ti significantly decreased rather than others.

Figure 3 and 4 shows the IR corrected initial performance and the lifetime test at 150 mA cm^-2 for the IrO₂ loaded titanium electrodes for OER. The IrO₂/Ti, which has no intermediate layer, has the highest initial OER activity, but the shortest lifetime in these electrodes, and TiO₂_1050_10h/Ti has relatively high OER activity and the longest lifetime in these electrodes. In the design of the intermediate layer, not only the property of the intermediate material itself but also synergy effect of 4 and 5^th group metal oxide and precious metal oxide should be taken into account.

This research was supported by the JSPS Grant-in-Aid for Scientific Research (C) JP 16K12345. The authors want to thank the person concern with.

Reference

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