Suppression of Oxide Growth on Ti Substrate of Ir1-XSnxO2/Ti Anodes in Electrolytic Oxygen Evolution

Abstract

INTRODUCTION In order to prevent global warming and to supply renewable energy in the form of methane, Hashimoto et al. have been proposing global CO2 recycling and have tailored the necessary key materials, that is, the anodes and cathodes for seawater electrolysis and catalysts for CO2 methanation by the reaction with hydrogen. The most difficult was the anode for seawater electrolysis, because the anode should evolve only oxygen without forming chlorine even in seawater electrolysis. Titanium is generally used as the electroconductive substrate. We usually covered the titanium substrate with IrO2 for prevention of oxidation of titanium. In chloride containing electrolytes, chlorine evolution occurs preferentially on the IrO2-covered titanium. Thus, we needed to prepare g-MnO2-type electrocatalysts covering the IrO2 layer for exclusive oxygen evolution in seawater electrolysis. We revealed that the Ir1-xSnxO2 double oxides have better protectiveness against oxidation of titanium than IrO2. The Mn1-x-yMoxSnyO2+x/Ir1-xSnxO2/Ti anode showed more than 99.9% oxygen evolution efficiency for 4200 h in the electrolysis of 0.5 M NaCl solution of pH 1 at 1000 Am-2. Although the oxygen evolution efficiency was close to 100%, the oxide growth on the titanium substrate was unavoidable due to the inward diffusion of oxygen through electrocatalyst and intermediate layers during oxygen evolution. The present work aimed to improve the protectiveness of Ir1-xSnxO2 double oxide. Since the Ir1-xSnxO2/Ti anodes themselves are oxygen evolution anodes in chloride free aqueous solutions, the protectiveness of Ir1-xSnxO2 double oxide during oxygen evolution at high current densities was examined using the Ir1-xSnxO2/Ti anodes in H2SO4 solutions. EXPERIMENTAL After surface roughening by immersion in 11.5 M H2SO4 the titanium substrate was coated with butanol solutions of H2IrCl6 and SnCl4, in which the sum of metallic cations was 0.1, 0.26 or 0.52 M. The specimens were dried at 80oC for 10 min and calcined at 250-450oC for 10 min. This procedure was repeated three times and the final calcination was continued for 0.5-24 h. The anodes were previously used for the electrolysis at 13,333 Am-2 in 3 M H2SO4 solution at room temperature for various periods of time, and then characterization of the used anodes was carried out by electrochemical measurements in 1 M H2SO4 solution at 25°C. Galvanostatic polarization curves were measured by the current density rise every 1 min. The potential rise due to the solution resistance at high current densities was corrected by the ordinary current interruption technique. Two more different current interruption techniques were applied at higher current densities using a digital oscilloscope for the measurement of the potential decays at every 0.0625 ms and 0.4 ms, respectively. RESULTS AND DISCUSSION The oxide growth on the titanium substrate occurred during electrolysis at high current densities for long periods. The oxide growth rate was estimated with anodic polarization curves and current interruption technique by the assumption of equivalent circuit of oxide covered metal electrode. When the sum of Ir4+ and Sn4+ was 0.52 M, the solely IrO2-coated anode showed the lowest rate of resistance growth. However, when the sum of Ir4+ and Sn4+ was 0.26 M, an increase in Ir4+ higher than 0.13 M was not effective to decrease the oxide growth. When the sum of Ir4+ and Sn4+ was 0.1 M, the minimum oxide growth rate appeared at 0.04 M Ir4+, where the oxide growth rate was similar to that of the anode prepared in 0.13 M Ir4+ - 0.13 M Sn4+ solution. Consequently, in order to save iridium, lower concentrations of Ir4+ and Sn4+ will be useful. On the other hand, an increase in calcination temperature led to incorporation of oxidized titanium in Ir1-xSnxO2. This also led to a resistance increase. Furthermore, the increase in calcination temperature resulted in potential increase at low current densities. This implied a decrease in the oxygen evolution activity due to inclusion of Ti2+ in Ir1-xSnxO2. However, the formation of triple oxide of Ir1-x-ySnxTiyO2 was not always detrimental for the elongation of the life of the anode. The growth rate of titanium oxide during anodic polarization at 13,333 Am-2 in 3 M H2SO4 solution was lowest when the anode was formed by calcination at 450°C. Consequently, the protectiveness of Ir1-x-ySnxTiyO2 formed by calcination at 450°C was highest even if oxidized titanium was included in the active anode for oxygen evolution. Consequently, the optimum preparation condition of Ir1-xSnxO2/Ti anodes was coating of Ti substrate with butanol solution of 0.13 M Ir4+ and 0.13 M Sn4+ and calcination at 450°C for 1 hour.