The oxygen evolution reaction (OER) is one of the essential energy conversion reactions for hydrogen production. In quadruple perovskite oxides AA’3B4O12 (A=Ca, Sr, A’=Cu, Mn and B=3d metals), the new reaction mechanism of O−O bond formation between adsorbed oxygen species at adjacent A’‐B sites has been proposed in recent studies. This idea of multiple transition metals working together to form an active site, rather than a single active site, is appealing, but has not been systematically investigated so far. This study examined catalytic OER performances of CaMn7O12 quadruple perovskite oxides with precisely controlled Mn(A′)−Mn(B) distance by doping with Sr, Cu and Al. The crystal structure and the state of cations and O were investigated through XRD with Rietveld refinement and X‐ray absorption spectroscopy. The OER catalytic activity is clearly correlated with the distance of Mn(A’)−Mn(B) while both eg state of Mn and O 2p band center unchanged. The shrinkage of the A’‐B distance affects the stability of O−O bond on A’‐B site and accelerates OER kinetics.

Recent developments in hydrogen energy devices have furthered the research on sustainable hydrogen production methods. Among these, the water splitting process has been considered a promising hydrogen production method, particularly, in alkaline media. The lack of information on the reaction active sites under the conditions of the oxygen evolution reaction (OER) hinders establishing guidelines for catalyst development. In the case of powder catalysts, many operando techniques also measure bulk information, and therefore, extracting information on the reaction active sites is challenging. Accordingly, film electrodes were used in this study, and the electrochemical performance and reaction kinetics of perovskite-type La1–x Sr x CoO3−δ films as OER catalysts synthesized by pulsed laser deposition were investigated. By combining ex situ X-ray absorption spectroscopy (XAS) and operando total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), we succeeded in observing a significant oxidation state change on the surface of La0.6Sr0.4CoO3−δ, which indicated that the active surface sites were formed upon applying the OER potential. This surface reconstruction resulted in numerous active sites at the reaction interface, thereby enhancing the OER activity. This study provides definitive evidence for the surface reconstruction of OER catalysts, which enhances the fundamental understanding of OER catalyst behaviors, and can inspire the design of active OER catalysts by suitable surface modulation.

Oxide catalysts containing tetravalent nickel exhibit high oxygen-active catalytic activity. We investigated the effects of lithium content (x) on the electrocatalytic activity of Li x Ni 0.5 Mn 1.5 O 4 , a spinel that provides only divalent and tetravalent nickel ions in a KOH aqueous electrolyte for the oxygen evolution reaction (OER). The lithium content was controlled by the chemical delithiation of Li 0.96 Ni 0.49 Mn 1.51 O 4 using NO 2 BF 4 . Upon employing this approach, nickel changed from the divalent to tetravalent state. The OER activity increased with decreasing x, and a higher activity than that of standard LaNiO 3 was observed at x = 0.00. Comprehensive Tafel analyses and X-ray spectroscopic investigations revealed a downshift in the Ni conduction band, indicating that high OER activity correlated with strong hybridization of the Ni 3d and O 2p orbitals. These insights into the role of Ni 4+ in high OER activity are expected to facilitate the development of other highly active Ni-based electrocatalysts.

Nickel‐based oxide catalysts are widely used for the oxygen evolution reaction (OER) in alkaline water electrolysis because of their low cost and high activity. In particular, the LiNiO2 catalyst shows high activity. Therefore, to elucidate the fundamental relationship between the local structure, catalyst activity, and stability of LiNiO2, we investigated the cation mixing effect by mixing sites of lithium and nickel ions in the LiNiO2‐based catalysts. Lower degrees of cation mixing lead to higher intrinsic OER activity but lower long‐term stability. The X‐ray absorption spectra (XAS) displayed a strong hybridization state of the Ni 3d and O 2p orbitals, which is the origin of the different catalytic activity behaviors. Meanwhile, operando XAS studies combined with potentiostatic stability tests and inductively coupled plasma optical emission spectrometry (ICP‐OES) demonstrated the Li ion loss during the OER process. Thus, the instability of LiNiO2 originates from de‐intercalation of Li ions and this irreversible structure change deteriorates the performance. Hindering the lithium diffusion path by cation mixing is a useful strategy for maintaining performance. This strategy could provide a novel design principle for compatible high activity and long‐lasting catalysts by reasonable structure mediation.

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, ...