Water electrolysis system for hydrogen production utilizing renewable energy is required to operate stably over
a long period and in unstable energy supplying condition. Analysis of degradation mechanism of catalysis under
electrochemical potential cycling will be useful for stable operation of water electrolysis. In this study, soft X-ray
absorption spectroscopy (XAS) was utilized for the analysis of nickel catalyst used in a conventional alkaline water
electrolysis system. Ni L-edge and O K-edge XAS were measured for the initial and electrochemical cycled nickel
metal. In the initial state, the nickel(Ⅱ) oxide phase was present on the nickel metal surface. The electrochemical
cycling caused the formation of nickel(Ⅲ) oxyhydroxide on the surface of the nickel metal. It was shown that the
degradation mechanism can be analyzed at various depths by using XAS method for the catalyst of water
Materials using multiple anion compounds are expected to be a new direction for realizing functional materials.
In this study, compounds using fluoride and sulfide ions as anions were synthesized, and the electronic structures
were analyzed by F K-edge and O K-edge X-ray absorption spectra. The analytical results of the spectra with the
total electron yield mode providing the surface information and the fluorescence mode providing the bulk
information show the difficulty for the removal of oxygen sources contained in the starting material during the
synthesis process and uniformly distribution internally by a calcination process.
Cathode materials for lithium-ion batteries utilize oxidation/reduction reaction of metal-ion for charge storage.
However, little analysis of the electronic state of lithium carrier ions has been performed. This is because there are not many methods for observing the light element lithium, and some surface films on active materials deposited after charging and discharging make the analysis of bulk structure difficult. In this study, the electronic structure change of lithium was analyzed by soft X-ray absorption spectroscopy which can directly observe the electronic state of lithium. To prevent the surface film formation, lithium was chemically extracted from LiCoO2, LiMn2O4, Li1.2Ni0.13Co0.13Mn0.53O2 of representative electrode materials.
The effect of the calcination and reduction treatment on the chemical state of the Ni species supported on
TiO2 was revealed by the XRD and XAFS method. The TiO2-supported Ni catalyst calcined at various temperatures was characterized by the XRD measurements. It is revealed that the NiO species was incorporated into TiO2 support as the species of NiTiO3, when the calcination temperature was higher than 600 °C. The chemical state conversion of the Ni species during the temperature programmed reduction process was investigated by in situ XAFS measurements. The reduction of NiTiO3 proceeded below 700 °C, and the metallic Ni(0) species was generated on TiO2. The reduction temperature was higher than that of NiO on TiO2.
The chemical state conversion of the Ni species supported on SiO2 has been analyzed by in situ XAFS measurements. The composition change by the gas switch from H2 to O2 for the partially reduced NiO particles indicates that the metallic Ni(0) species exists at both the particle surface and the inner core. In addition, the analysis on the reduction property suggests that the oxidized Ni species is located at the adjacent place to the metallic Ni(0) species. It is concluded that the metallic Ni(0) and NiO species are heterogeneously distributed in the Ni particle under the reaction condition. The reduction of the NiO particle was initiated at the highly reactive spots and was expanded peripherally. This study achieved to find out the detailed reduction mechanism of the NiO particle supported on SiO2.
Conversion reaction for NiO nanoparticles supported on acetylene black (AB) was analyzed using an in situ XAFS technique. The NiO particle supported on AB was synthesized by the impregnation method. For the first discharge process for a test cell of an NiO/AB cathode with a Li anode, it was revealed that the reactions of the Ni species proceeded at two different potentials. This indicates that the reactions in the surface layer and the bulk of the NiO particle have different reduction potentials. In addition, it was clarified that the Ni particles became smaller after the first discharge process and the fine size was kept in the following cycles.
Three geometrical factors, a tilt angle of the first crystal, an offset distance of the crystal surface, and a tilt angle of the horizontal guide rail, cause the drift of the exit beam height of the monochromatic X-ray for the Golovchenko-type monochromator. We have formulated the geometrical factors and established the technique to adjust the alignment of the monochromator to restrain the beam drift.