Mn Oxide / IrO2 / Ti Anodes Prepared By Calcination for Oxygen Evolution in Seawater Electrolysis
Authors: Takahashi Soma , Monma Yu , Sato Kise , Ito Seira , Ono Yuki , Abdel-Galeil Mohamed , Maruo Yasuko Yamada , Kato Zenta 
Published: December 2024

INTRODUCTION
In the future, hydrogen production will be performed by direct electrolysis of seawater. However, every time hydrogen is produced, it is necessary to avoid generating chlorine because it has a negative impact on the ecosystem. We have been developing an oxygen evolution anode for seawater electrolysis that produces only oxygen without chlorine. It has been found that anodically deposited g-MnO2-type Mn1-xMoxSnyO2+xanodes showed the oxygen evolution efficiency for more than 4000 hours in the electrolysis of 0.5 M NaCl [1]. The anodic deposition method has required many steps and further cleaning of the electrolytic diaphragm. For these reasons, in order to commercialize oxygen evolution anodes for seawater electrolysis, it is essential to establish a simple manufacturing method. We are considering fabricating Mn oxide as an active material on an IrO2 intermediate layer formed on a titanium substrate using calcination method.
In this study, Mn oxide electrodes were fabricated by coating various concentrations of manganese nitrate butanol solutions on the IrO2intermediate layer and at various calcination temperatures. We have investigated the crystal structure and Mn oxide weight that would have the highest activity for oxygen evolution efficiency.
EXPERIMENTAL
Titanium metal substrates were immersed to roughen enhancing the anchor effect of the substrate on the electrocatalyst layer in concentrated H2SO4. IrO2 intermediate layer was formed by coating on one side of the titanium substrate in 0.26 M Chloroiridic acid with a brush, dried at 363 K for 10 min in air. The other side was coated in the same procedure and dried. And then these substrates was calcined at 723 K for 10 min in air. This procedure was repeated three times but the final calcination of the specimen was continued for 60 min at 723 K in air for calcination.
Mn oxide electrocatalyst for oxygen evolution was formed by coating on the IrO2 / Ti in 0.073～0.52 M Mn(NO3)2 butanolsolution with a brush and drying at 353 K for 90 min in the same way as when forming the IrO2 intermediate layer. Calcination was performed at 473, 573, and 723 K. Impurities formed without becoming manganese oxides were electrolytically cleaned in 0.5 M NaCl until no dissolution of the impurities was observed.
The performance of the electrode was examined by electrolysis of 0.5 M NaCl solution at 1000Am-2. The oxygen evolution efficiency was estimated by the difference between the total charge passed during electrolysis and the formation charge of chlorine analyzed by iodometric titration. Polarization curves were measured galvanostatically. Correction for IR drop was made with the electrochemical impedance spectroscopy (EIS) method.
The characterization of the electrode was carried out by XRD and EPMA.
RESULTS AND DISCUSSION
XRD diffraction clarified that Mn₂O₃ were consisted at 723 K, Ramsdellite-type MnO₂ at 673 K, and IrO2 at 573 K.
Figure 1 shows the relationship between Mn oxide weight and oxygen evolution efficiency. The oxygen evolution efficiency of the anode consisted of only IrO2 was about 8%. It found that the oxygen evolution efficiency was different due to the difference in calcination temperature, that is, the difference in crystal structure to Mn oxide weight of about 2.5 mgcm-2. The oxygen evolution efficiency of the anode with Ramsdellite-type MnO₂ formed at 573 K was about 57 % at 1.8 mgcm-2. The oxygen evolution efficiency of anodes formed at 473 and 723 K with the equivalent weight was about 52 and 32 % respectively. Previous research has shown that anodes containing the γ-MnO2 formed by anodic deposition have high catalytic activity for oxygen evolution [2]. The γ-MnO2 contains stacking faults in which β-MnO2 is mixed into the ramsdellite-MnO2[3]. The electrode with 4.5 mgcm-2 of Mn₂O₃ at 723 K shows the maximum oxygen generation efficiency, which is about 83%.
A future challenge is to add substances such as molybdenum as additional elements to obtain higher oxygen evolution efficiency. In conventional electrode fabrication using anodic deposition, the addition of molybdenum significantly has improved oxygen generation efficiency.
REFERENSE
[1]Z. Kato, J. Bhattarai , N. Kumagai, K. Izumiya, and K. Hashimoto, Applied. Surface. Science. 257 (2011) 8230.
[2] A. A. El-Moneim, N. Kumagai, K. Asami, and K. Hashimoto , Materials Transactions, Vol. 46, No. 2 (2005) pp. 309
[3] Jian-Bao LI, K. Koumoto and H. Yanagida, Journal of the Ceramic Society of Japan, 96 [1] (1988)74

Figure 1