課題番号 ：S-18-KU-0012
利用形態 ：機器利用
利用課題名（日本語） ：効率的な非貴金属ハイブリッド触媒の開発
Program Title (English) ：Development of efficient and low-cost non-noble metal hybrid catalysts
利用者名（日本語） ：Junfang Cheng, 中嶋　直敏
Username (English) ：Junfang Cheng, N. Nakashima,
所属名（日本語） ：九州大学カーボンニュートラル・エネルギー国際研究所
Affiliation (English) ：International Institute for Carbon-Neutral Research, Kyushu University

１．概要（Summary ）
Iridium oxide (IrOx)-based materials are the most suitable oxygen evolution reaction (OER) catalysts for water electrolysis in acidic media. A strong demand from industry is the improved performance and reduction of the Ir amount. Here, we report a novel catalyst, IrOx-TiO2-Ti (ITOT) with a high concentration of active OH species on its surface. The obtained ITOT catalyst shows an excellent OER activity (1.43 V vs. RHE at 10 mA cm-2), which is the best performance in acidic media. No apparent potential increase was observed even after a chronopotentiometry test at 10 mA cm-2 for 100 h and cyclic voltammetry for 700 cycles. We proposed a detailed OER mechanism based on the analysis of the in situ electrochemical X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements as well as Density Functional Theory (DFT) calculations. All together, we have concluded that controllable Ir-valence and the high OH concentration in the catalyst is highly important for the obtained high OER performance.

２．実験（Experimental）
SEM images were acquired using an SU9000 (Hitachi High-Technologies) operated at 30 kV. The high-resolution TEM and EDX-STEM images were taken by a JEM-ARM200F instrument (JEOL Ltd.) at 200 kV.
X-ray photoelectron spectroscopy (XPS) spectra were obtained by an AXIS-ULTRA DLD (Shimadzu, Co., Japan). X-ray diffraction analysis (XRD) was conducted using a SmartLab (Rigaku Corp.) instrument.

３．結果と考察（Results and Discussion）
In the catalyst, uniform nanoparticles were homogeneously deposited on the surfaces of the support nanotubes. Based on a high-resolution transmission electron microscopy (HRTEM) image, the lattice spacing of the nanotubes was determined to be 0.35 nm, which corresponds to the (101) crystal face fringes of the anatase TiO2. High-angle annular dark field STEM (HAADF-STEM) images show that the lattice space of 0.22 nm and 0.23 nm correspond to the (111) crystal face of Ir and (200) crystal face of IrO2, respectively, suggesting the coexistence of the Ir metal and IrOx clusters in the ITOT catalyst.
The electrocatalytic activity for the OER was evaluated by cyclic voltammetry (CV). We first measured the CV curves of the ITOT with different IrOx loadings. The OER activity was significantly improved with the increase in the amounts of IrOx. The catalyst with the loading of IrOx = 0.58 mg cm-2 exhibited a potential of EOER = 1.43 V vs. RHE at 10 mA cm-2, which is the lowest potential, to the best of our knowledge, when compared to the state-of-the-art Ir(Ru)-based catalysts, indicating a remarkably high OER performance. The Tafel plot of the ITOT catalyst shows that the slope is ~48 and 66 mV dec-1in the higher and lower potential regions, respectively.
In summary, we designed and synthesized a novel IrOx-TiO2-Ti (ITOT) catalyst on which IrOx nanoparticles with ~5 nm size were uniformly distributed. The obtained catalyst with a reduced IrOx amount exhibits an outstanding OER activity compared to the state-of-the-art Ir(Ru)-based catalysts as well as the high cyclic stability in acidic media.

４．その他・特記事項（Others）
なし

５．論文・学会発表（Publication/Presentation）
論文投稿中
学会発表：i) J. Cheng, N. Nakashima, “Reaction Mechanism of a Novel IrOx-Based OER Catalyst in Acidic Media”, AiMES 2018, Cancun, Mexico.2018.10.3.
ii) J. Cheng, 山内美穂、北野翔、福島貢、N. Nakashima, “Tiナノチューブ上に形成された高性能Irベース水分解触媒のデザイン、合成”,56回フラーレンナノチューブグラフェンシンポジウム, 東京大学、2019.3.2.

６．関連特許（Patent）
山口美穂、J. Cheng, 北野翔、福島貢、中嶋直敏、”Catalyst and electrod”, 特願2018-112139。