Abstract
With increasing global environmental problems, hydrogen, produced by renewable energies, has been considered a sustainable and clean fuel for next-generation energy sources[. Among the many proposed methods to produce hydrogen, the most renewable method is water electrolysis. However, the four electrons/protons needed for the oxygen evolution reaction (OER) in water electrolysis result in sluggish kinetics and low efficiency, representing the biggest obstacle for widespread applications. Polymer electrolysis membrane (PEM) electrolysis is expected to be commercialized due to its high current density, compact system, and high gas purity. Nevertheless, catalyst corrosion in acidic environments and high maintenance costs limit their general application. In contrast, alkaline water electrolysis (AWE) has been considered an alternative method for hydrogen production because earth-abundant transition-metal oxides (TM = Fe, Co, and Ni) can be used as catalysts, and certain states of TM cations accelerate the OER. Clarification of the electronic and local structure of catalysts during water electrolysis is important for understanding the activity and durability of catalysts. We have developed an operando X-ray Absorption Fine Structure (XAS) method that can be used to measure at high current densities under actual water electrolysis conditions. In this presentation, we will present examples of catalyst development using the established operando XAS method.
Previous studies have focused on Ni-based oxides, including Ni(OH)2 and doped NiOOH, due to their low material cost, easily mediated structure, and high OER activity originating from various dopants, making them promising catalysts to replace noble metal oxides. Compared to Co-containing perovskite-type oxides, relatively few studies correlating the electronic states of Ni-based oxides with their OER activities have been reported. Recent studies on nickel oxides have elaborated the doping effect of lithium on catalyst activity, such as in LiNiO2 where lithium doping changed the local electronic structure of nickel, improving OER activity. LiNiO2 has a nominal oxidation state of Ni3+ and exhibits excellent OER activity considering the eg state descriptor. Moreover, because the energy levels of the Ni 3d orbitals of LiNiO2 are close to those of the O 2p orbitals, they can strongly hybridize. Therefore, LiNiO2 offers a suitable model to study the relationship between electronic structure and OER activity. However, for practical catalysts, the durability is also important and the deterioration mechanism of the LiNiO2 catalyst has not yet been clarified. LiNiO2, having an ordered rock salt structure, is a well-known cathode material for lithium-ion batteries, and exhibits extremely high lithium-ion mobility. Thus, catalyst degradation may be caused by de-intercalation of lithium ions during anodic polarization. In the Li-ion battery field, many studies have shown that the electrochemical properties of LiNiO2 cathodes are extremely dependent on the degree of cation mixing, primarily due to the presence of Ni2+ at Li+ and Ni3+ sites because of the similarity in their ionic radii. Cation mixing is a disadvantage for cathode materials because the presence of Ni2+ at Li+ sites hinders Li+ diffusion. Inspired by this, hindrance of the lithium diffusion path by cation mixing of Li and Ni is expected to be an effective solution when LiNiO2 is applied as an OER catalyst to improve stability. Herein, the cation mixing effect on OER activity and stability was studied using LixNi2-xO2. A combination of transmission electron microscopy (TEM), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) investigations were conducted to probe the electronic structure of the materials. The valence changes of Ni during OER were further examined using operando XAS using a home-made flow-type cell. The degradation process was also investigated in detail using operando XAS.