Synthesis of CaMn 2 O 4 -related electrocatalyst for oxygen evolution electrode of water-splitting
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Synthesis of CaMn2O4-related electrocatalyst for oxygen evolution electrode of watersplitting Yuya Taki1, Zhenquan Tan2, Satoshi Ohara2, Takashi Itoh4, Yoshiaki Nakano1, Katsushi Fujii1,3, Masakazu Sugiyama1 1 School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, JAPAN 2 Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka, 567-0047, JAPAN 3 Global Solar plus Initiative, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538904, JAPAN 4 Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, 6-3, Aramaki aza Aoba, Aoba-ku, Sendai 980-8578, JAPAN
ABSTRACT Water-splitting by using electric power produced by solar cells is promising system to produce hydrogen without fossil fuels. Oxygen evolving catalyst is, however, major problem to prevent using this system widely because precious materials are used in the catalyst. Considering from the photosynthesis II of plants, the compound of Ca-Mn-O is one of the candidates for the oxygen evolving catalyst. In this study, the synthesis condition and the oxygen evolving electrocatalytic activity of CaMn2O4•xH2O are investigated. The overpotential at 0.1 mA/cm2 was 0.28 V when using the electrode of carbon paste and CaMn2O4•H2O with the weight ratio of 3:1.
INTRODUCTION Depletion of fossil resources is one of the problems that human being have to solve to survive for a long time on the earth. Hydrogen is promising material which solves the global energy problem if it generates without fossil resources. Water-splitting by using electric power produced by solar cells is a candidate system to generate hydrogen without fossil fuels and emitting carbon dioxide [1]. Using this system, the solar to hydrogen efficiency was reported to be around 15% [2]. This efficiency is much higher than those of photosynthesis or photoelectrode which are less than a few % [3]. Therefore, this technique is thought to be a prevailing system to generate renewable hydrogen. However, there are several problems which prevent the spread of this system. One of them is that precious materials are used as electrocatalysts such as Pt, RuO2 or IrOx [4-6]. In addition, the oxygen evolving overpotential is also a major problem, that is, even if a precious metal oxide such as RuO2 is used as the catalyst, the overpotential (0.1 V at 0.1 mA/cm2) is much higher than that of hydrogen evolution (< 0.05 V at 0.1 mA/cm2) [4]. Therefore, a new oxygen evolving catalyst, which reduces overpotential and consists of earth-abundant materials, is highly required. Several researches have examined the electrochemical oxidation of water by earth-abundant materials such as MnOx, Co-Pi and Co3O4 [4,7-12]. The mimetic of oxygen evolving center (OEC) of photosynthesis II (PSII) has been paid attention to realize the decrease of oxygen evolving overpotential with earth-abundant materials.
PSII is the first step of photosynthesis of plants where water is split to oxygen and proton emitting electrons by using light energy (2H2O → O2 +
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