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Introduction Alkaline water electrolysis (AWE) is one of hydrogen production methods without any carbon dioxide emission with renewable electric power supply. Anode of AWE is usually used Ni based material which is stable under steady electrolysis1). Although fluctuating electricity from renewable energy enhances deterioration of Ni anode, the lithium doped NiO coated Ni anode (LixNi2-xO2/Ni) has high stability, conductivity, and activity because of increase of stabilized Ni3+ 2, 3). Our previous method, which is oxidation of LiOH coated Ni was difficult to control composition and thickness of the lithium doped NiO film. In this study, we have investigated the OER activity and durability with thermal decomposition method to control the lithium doped NiO film using various precursors. Experimental Working electrodes were the lithium doped NiO coated Ni prepared with acetate or nitrate precursor hereinafter referred to as Ni_A, Ni_N, respectively. The precursor was aqueous solution of LiNO3 ( Wako Pure Chemical Industries Ltd., 99+% ) , Ni(NO3)2・6H2O ( Junsei Chemical Co. Ltd., 98.0% ) and Ni(CH3COO)2・4H2O ( Junsei Chemical Co. Ltd., 98.0% ) with nominal composition of Li : Ni = 0.1 : 1.9 in mole fraction. The precursor was coated on Ni plate (The Nilaco corp., 99.+%) which was previously etched in boiling 20 % hydrochloric acid for 6 min and dried at 80oC for 15 min. Then it was thermally decomposed at 550oC for 15 min. The process of the coating, drying, and thermal decomposition were repeated several times to get fixed amount of oxide film. Finally, it was baked at 550oC for 1 h to get a Ni_A or a Ni_N. Counter and Reference electrode were a Ni coil and a reversible hydrogen electrode (RHE), respectively. All measurements were performed with a three-electrode electrochemical cell at 30±1oC. The electrolytes were in 7.0 M (=moldm-3) of KOH. Cyclic voltammetry was applied for 100 cycles between 0 and 1.5 V vs. RHE with the scan rate of 100 mVs-1 as electrochemical pretreatment. After that, the cyclic voltammetry between 0.5 and 1.8 V vs. RHE with the scan rate of 1 Vs-1 was applied for the duration protocol. The catalytic activity of the OER was evaluated by slow scan voltammetry between 0.5 and 1.8 V vs. RHE with the scan rate of 5 mVs-1 during the deterioration protocol. The resistance of oxide film (R f) and electric double layer capacitance (C dl) were also evaluated by AC impedance spectroscopy with higher frequency arc for the R f and lower frequency arc of electrochemical process for the C dl at bias potential from 1.6 V vs. RHE during the duration protocol. Results and discussion Figure 1 shows slow scan voltammograms of the Ni, Ni_A and Ni_N electrode before and after durability test in 7.0 M KOH. The onset potential of Ni and the Ni_N, which is onset potential of OER, and the redox charge around 1.3 V vs. RHE increased with the duration. On the other hand, the onset potential of the Ni_A did not increase after the duration test, although OER current of the Ni_A increased in the initial period of durability protocol, accompanied with the increase of redox charge around 1.4 V vs. RHE. The oxide film of the Ni_A would be denser and thinner than that of the Ni_N to protect Ni corrosion and the redox site on the Ni_A would contribute to OER compared with the others. Figure 2 shows the R f, C dl, and the dependence of the potential at 10 mAcm-2 of the OER (E @10mA) as a function of the potential cycle number in 7.0 M KOH at 30oC. The E @10mA corresponds to the anode overpotential of the OER. The E @10mA of Ni_A decreased in initial period and became same to that of Ni at the initial period of the durability test. The C dl of Ni_A was increased a little, but was the smallest in these electrodes. The E @10mA and the C dl of Ni_N were the largest in these electrodes. The R fs were constant in all electrodes. Therefore, the oxide film of Ni_N was low activity, with porous oxide structure. The Ni_A has dense and thin oxide surface and high activity. Reference 1)K. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010). 2)H. Ichikawa, K. Matsuzawa, Y. Kohno, I. Nagashima, Y. Sunada, Y. Nishiki, A. Manabe, and S. Mitsushima, ECS Trans, 58(33), 9 (2014). 3)J. C. Botejue and A. C. C. Tseung, J. Electrochem. Soc., 132, 2957(1985). Figure 1 |
