Alkaline water electrolysis (AWE) produces hydrogen without any carbon dioxide emission with renewable electric power supply. Anode of AWE is usually used Ni based material which is stable under steady electrolysis. Although fluctuating electricity from renewable energy enhances deterioration of Ni anode, high temperature prepared (Li)NiO/Ni anode has high stability under fluctuated power supply1). LaNiO3 is known as high electric conductivity and activity of oxygen evolution reaction (OER) in alkaline electrolyte2), however durability of LaNiO3 has never investigated under potential cycling. In this study, we have investigated the OER activity and durability of a lanthanum nickel oxide coated on Ni (La-Ni-O/Ni) using various precursors under potential cycling.
Working electrodes were the La-Ni-O/Nis prepared with thermal decomposition coating of various composition precursors. The precursors were aqueous solutions of La(CH3COO)3・1.5H2O (Junsei Chemical Co. Ltd., 99.9%), Ni(NO3)2・6H2O (Junsei Chemical Co. Ltd., 98.0%) with nominal composition of La: Ni = 1.7: 1.0, 1.4: 1.0, 1.2: 1.0 and 1.0: 1.0 in mole fraction to get intermediate composition of La2NiO4, La3Ni2O7, La4Ni3O10 and LaNiO3, and the La-Ni-O/Nis were referred to as La/Ni_1.7, 1.4, 1.2 and 1.0, respectively. The precursor was coated on a Ni plate (The Nilaco corp., 99. +%) with etching to clean up its surface. The process of the coating, drying, and 873 K for 10 min of thermal decomposition were repeated 20 times. Finally, it was baked at 1173 or 873 K for 1 h in air to get a La/Ni_1.7~1.2 or La/Ni_1.0, respectively. 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 303±1 K in 7.0 M (= mol dm-3) of KOH.
Cyclic voltammetry was applied for 100 cycles between 0 and 1.0 V vs. RHE with the scan rate of 100 mVs-1 as electrochemical pretreatment. The catalytic activity of the OER and the resistance of surface oxide film (Rf) were evaluated by slow scan voltammetry between 0.5 and 1.8 V vs. RHE with the scan rate of 5 mVs-1 and the AC impedance spectroscopy with higher frequency arc at 1.6 V vs. RHE during the duration protocol of potential cycling between 0.5 and 1.8 V vs. RHE with the scan rate of 1 Vs-1. The electrodes were analyzed by XRD and SEM before and after electrochemical measurements.
Results and discussion
Figure 1 shows dependence of the Rf as a function of the potential cycle number. The Rfs were much larger than expected from the resistivity La-Ni-Os and thickness of the oxide with the SEM images, therefore the Rfs were affected by formation of NiO, which was confirmed with XRD. The Rf of La/Ni_1.0 was much smaller than others, which were baked at higher temperature than La/Ni_1.0. These values were almost the same during potential cycling. Therefore, the Rfs would be able to reduce by improvement of preparation procedure.
Figure 2 shows the iR free polarization curves of the La/Ni_1.7~1.0 and Ni before (a) and after (b) potential cycling. Here, the R is the higher frequency intercept on the real axis, which correspond to the electrolyte resistance. The tails of Ni(II)/Ni(III) redox were observed below 1.45 V vs. RHE, and the Rfs seems to be affect to the difference from the liner region around 1.55 V vs. RHE at higher potential region. The Tafel slope of the La/Ni_1.7~1.0 were almost same around 1.55 V vs. RHE. On the other hand, the current in the Tafel region, which corresponds to the OER activity, of La/Ni_1.0 was much larger than others. According to XRD, the coating of the La/Ni_1.2 and 1.0 was LaNiO3, that of the La/Ni_1.4 was mixture of LaNiO3 and La2NiO4 and that of the La/Ni_1.7 was mixture of LaNiO3, La4Ni3O10 and La2NiO4. OER currents of the La/Ni_1.7~1.0 before potential cycling were almost the same, and they increased during the potential cycling until 4000 cycles at least, although OER currents of Ni dramatically decreased with potential cycling. Therefore, La-Ni-Os are much higher durability than Ni, and LaNiO3 would be the highest OER activity in La-Ni-Os because of the largest current around 1.55 V vs. RHE.
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