Jeongsuk Seo

Layered perovskite oxynitrides are attractive semiconductors for sunlight‐driven water splitting thanks to their tunable electronic structures and structural anisotropy. However, their synthesis remains challenging because nitridation of layered perovskites often causes structural collapse or direct transformation into 3D perovskites, obscuring the nitridation mechanism. Herein, we systematically investigate the nitridation of a robust (111)‐plane layered perovskite, La ₄ Ti ₃ O ₁₂ . Neutron diffraction and electron microscopy provide direct experimental evidence that nitridation proceeds topotactically in a gradual and spatially heterogeneous manner, preserving the layered framework over an extended time window and yielding an N‐substituted layered perovskite oxynitride before full conversion into the 3D perovskite LaTiO ₂ N. This previously unrecognized persistence of layered oxynitride enables formation of a layered/3D core−shell heterostructure, La ₄ Ti ₃ (O,N) ₁₂ @LaTiO ₂ N, in which the layered phase acts as a structural and electronic host framework for formation of favorable heterojunction band alignment. As a result, the core–shell photoanode improves charge carrier dynamics, thereby showing substantially enhanced photoelectrochemical water splitting activity compared with both the single‐phase 2D layered and the 3D oxynitrides. These findings identify layered oxynitrides as a key factor governing heterojunction formation and provide direct experimental insight into the nitridation mechanism of layered perovskites, providing a viable pathway toward efficient perovskite‐based oxynitrides.

Sustainable solar seawater splitting over a BaTaO ₂ N photoanode via Cl ⁻ regeneration and closed-loop operation.

The oxygen evolution reaction (OER) proceeds via a four-electron oxidation. However, in neutral aqueous solutions, its kinetics is typically sluggish, requiring a significant overpotential for effective catalysis. As an alternative to OER, the chlorine evolution reaction (CER) presents a promising approach for enhancing hydrogen production efficiency in neutral seawater electrolytes. ¹ Given that Cl ⁻ ions are more abundant in natural seawater compared to other anions such as Br ⁻ and SO ₄ ^{2 −} , photoelectrochemical (PEC) seawater splitting via CER can be an effective strategy for improving the hydrogen evolution rate. ^2,3 Thermodynamically, CER (E ⁰ =1.36 V _NHE ) is more oxidative than OER (E ⁰ =1.23 V _NHE ). However, because CER follows a two-electron oxidation pathway, it exhibits faster kinetics than OER in neutral electrolytes. In a previous study, highly crystalline, perovskite-type SrNbO ₂ N particles grown on an Nb substrate, synthesized via bottom-up fabrication, exhibited a threefold increase in photocurrent upon the addition of 0.5 M NaCl to a 0.2 M NaPi electrolyte at neutral pH 6.4. ⁴ Moreover, the onset potential for seawater splitting, including CER, shifted approximately 0.1 V _RHE lower than that for pure water splitting leading exclusively to OER. ⁵ These results clearly demonstrate that, despite its thermodynamically less favourable potential, CER kinetics was faster than OER. Despite the enhanced CER kinetics, quantitative analysis revealed that O₂ remained the dominant oxidation product, along with CER products such as HOCl/ClO⁻. Thus, improving selectivity for CER may be essential for further enhancing seawater splitting activity and increasing the hydrogen evolution rate. Herein, we present a novel MoO _x -CoO _y electrocatalyst designed to enhance selectivity for CER and improve PEC seawater splitting activity. The perovskite BaTaO₂N semiconductor, which absorbs visible light up to 660 nm approximately, was used as a photoanode, onto which the MoO _x -CoO _y catalyst layer was deposited via CoO _y impregnation, followed by radio frequency magnetron sputtering of MoO _x . PEC seawater splitting at the photoanode surface was systematically investigated under different electrolyte conditions, and the resulting products—H ₂ , O ₂ , and HOCl/ClO⁻—were quantitatively analyzed to evaluate selectivity for CER. The results indicate that the Faradaic efficiency for CER with the MoO _x -CoO _y catalyst was more than twice as high as that achieved with the CoO _y catalyst alone. The improved seawater splitting activity and enhanced selectivity for CER of the MoO _x -CoO _y /BaTaO ₂ N photoanodes in artificial seawater will be discussed in detail in the presentation. References J. Seo, H. Nishiyama, T. Yamada, K. Domen, Angew. Chem. Int. Ed., 2018 , 57 , 2. J. Seo, K. Domen, Mater. Chem. Front ., 2024 , 8, 1451. C. R. Lhermitte, K. Sivula, ACS Catal ., 2019 , 9, 2007 V-H. Trinh, J. Seo, ACS Sustain. Chem. Eng ., 2023 , 11, 1655 T. T. T. Tran, J. Seo, Mater. Chem. A , 2024 , 12, 17128

Photo-electrochemical (PEC) seawater splitting using semiconductors is a promising means of directly producing hydrogen from seawater occupying approximately 97% of Earth’s water. The n-type perovskite oxynitride semiconductors with a general formula AB(O,N)₃ (A=Ca, Sr, Ba or La; B=Ti, Nb or Ta) are capable of absorbing intensive visible light up to 750 nm wavelength, thermodynamically leading to high solar-to-hydrogen conversion efficiency of 10% or above.¹ Also, their band positions straddle various oxidation and reduction potentials including the water redox potential to produce value-added energy resources such as H₂, HCOOH, and NH₃.² Based on the favorable optical properties, the water splitting activity using the oxynitrides has been significantly increased in strong alkaline electrolytes, although it is still low in neutral electrolytes.^2-3 Oxygen evolution reaction (OER) is driven via four-electron oxidation. Its kinetics is commonly very sluggish in neutral aqueous solutions, which the significant overpotential is required to catalyze it. Thus, applying alternative oxidations to OER, such as chlorine evolution reaction (CER), may be a feasible approach to efficient hydrogen production in neutral seawater electrolytes.⁴ In terms of thermodynamic potential, OER (E ⁰ = 1.23 V_NHE) is more oxidative than CER (E ⁰ = 1.36 V_NHE). However, CER driven via two-electron oxidation can be catalyzed faster than OER kinetically in neutral electrolytes. Herein we first present sunlight-driven seawater splitting activity of perovskite AB(O,N)₃ largely improved in neutral environments and also discuss CER activity depending on various pH values as an alternative oxidation to OER. Highly-crystalline, porous SrNbO₂N particles grown on Nb substrate was prepared by bottom-up fabrication including oxidation of Nb substrate and flux-assisted nitridation.⁵ The advanced fabrication caused less-defective oxynitride with a large surface area. The resulting SrNbO₂N/Nb photoanode produced significant OER photocurrent in 0.2 M NaPi electrolyte at a neutral pH 6.4, and the photocurrent was three times increased by adding 0.5 M NaCl to catalyze CER simultaneously. Moreover, the onset potential for seawater splitting including CER was shifted to a lower potential by approximately 0.1 V_RHE than that for water splitting leading to OER exclusively. These results clearly demonstrate that CER was preferentially catalyzed during neutral seawater splitting compared with competitive OER. It also implies that CER kinetics over the SrNbO₂N was faster than OER kinetics despite its unfavorable thermodynamic potential. The BaTaO₂N/Ta photoanode, more photoactive and stable than the Nb-based oxynitrides, was synthesized in the similar manner. The seawater splitting of the oxynitride at different pH values was further investigated, and the quantitative analysis of the resulting products, namely, H₂, O₂, and ClO⁻ was also performed to determine the operating conditions suitable for efficient and stable seawater splitting. Moreover, different electro-catalysts to improve the selectivity to CER were evaluated in the artificial seawater. The enhanced seawater splitting activity and selectivity to CER of the AB(O,N)₃ in artificial seawater will be discussed in detail in a presentation. References J. Seo, H. Nishiyama, T. Yamada, K. Domen, Angew. Chem. Int. Ed., 2018, 57, 2. J. Seo, K. Domen, Mater. Chem. Front., 2024, 8, 1451. J. Seo, T. Hisatomi, M. Nakabayashi, N. Shibata, T. Minegishi, M. Katayama, K. Domen, Adv. Energy Mater., 2018, 1800094. C.R. Lhermitte, K. Sivula, ACS Catal., 2019, 9, 2007 V-H. Trinh, J. Seo, ACS Sustain. Chem. Eng., 2023, 11, 1655

Less-defective, two-dimensional LaTiO₂N crystals prepared from layered perovskite BaLa₄Ti₄O₁₅ highly improved sunlight-driven seawater-splitting activity at neutral pH.

Perovskite and layered perovskite oxynitrides are regarded as promising visible-light-responsive semiconductors for efficient artificial photosynthesis to produce renewable value-added energy resources, including H₂, formic acid, and ammonia.

Argyrodites, Li₆PS₅Cl (LPSCl), are considered one of the most promising solid electrolytes (SEs) for all-solid-state batteries (ASSBs). However, the use of petrochemical-based toxic solvents, such as dimethylformamide and tetrahydrofuran (THF), has limited their commercial applications in the mass production of SEs via liquid-phase synthesis both in the laboratory and industry. Herein, we aim to achieve the scalable production of cost-effective LPSCl-based sulfide SEs with high ionic conductivities (>2 mS cm⁻¹) based on various green and sustainable inert solvents. Our simple liquid-phase synthesis route was designed by using sulfur to form an intermediate polysulfide to completely dissolve precursors in common organic solvents without employing high-energy and high-pressure processes. Metastable polysulfides have high solubility in a broad range of solvents; they offer the use of greener and/or sustainable alternatives. The ASSBs fabricated with the LPSCl SEs using green solvents exhibited excellent electrochemical performance comparable to that of LPSCl wet-synthesized using conventional THF. LPSCl SEs fabricated using monoglyme, which has the most similar solvent properties to those of THF, exhibited the best ionic conductivity and contributed to the high electrochemical performance of ASSBs. The state-of-the-art bulk-type production of sulfide SEs using solution-phase synthesis should ultimately be replaced by green and/or sustainable solvents to reduce chemical-related effects on human health and eliminate environmental pollution. Therefore, this study will be of great interest to those who wish to mass-produce sulfide-based SEs cost-effectively for commercial applications with minimal unnecessary steps.