Elucidating the characteristics of the Cu-Se, In-Se, and Ga-Se bonds in chalcopyrite-type Cu(In,Ga)Se-2-based semiconductors can aid the design and fabrication of highly efficient thin-film photovoltaic devices. In this study, we used extended X-ray absorption fine structure (EXAFS) analyses to evaluate the characteristics of chemical bonds in Cu(In,Ga)Se2 and Cu(In,Ga)S-2 powder samples at low temperatures (10-300 K). The temperature dependence of the structural and vibrational disorder (Debye-Waller factor) provides the Einstein temperature of an individual bond in Cu(In,Ga)Se-2-based materials. The analyzed Einstein temperature, which is proportional to the Einstein frequency, increased in the order of Cu-Se(S) < In-Se(S) <= Ga-Se(S), and the S-containing bond had a higher Einstein temperature than the corresponding Se-containing one. We also analyzed the effect of the Ga/ (Ga + In) ratio on the Einstein temperature of each chemical bond. The force constant of the oscillator (i.e., the bond) was determined from the Einstein frequency using the reduced mass of the constituent atoms. The obtained bond properties were found to correlate with the diffusion characteristics of the constituent atoms in CuInSe2-based solar cell materials.

Electrochemical nitrate reduction reaction (NO 3 RR) represents a sustainable, carbon‐neutral alternative to the Haber–Bosch process for ammonia synthesis. Time‐resolved operando X‐ray absorption spectroscopy reveals the chemical states and structural evolution of copper(I) oxide microcrystals deposited on carbon fiber (Cu 2 O/C) across a potential range of +0.6 to −0.7 V versus reversible hydrogen electrode (V RHE ), where nitrate reduction to nitrite and ammonia occurs. Without nitrate, Cu 2 O microcrystals are quickly reduced to metallic Cu(0) aggregates at low reduction potentials (≈0.1 V RHE ). In contrast, only 29% Cu(0) is observed in 0.1 M NaNO 3 at 0.1 V RHE , indicating that nitrate adsorption passivates the surface and promotes selective electron transfer to nitrate, thereby retarding the kinetics of Cu 2 O microcrystals transformation to Cu(0) particles. Ammonia formation initiates at −0.3 V RHE in 0.1 M NaNO 3 (pH 13) solution, accompanied by the formation of metallic copper particles for the hydrogenation of the intermediates. The Faradaic efficiency (FE) of ammonia is increased with more negative potential, accompanied by the formation of metallic Cu(0) particles. The fully reduced Cu particles exhibit superior NO 3 RR activity to produce nitrite at lower reduction potentials and ammonia at higher reduction potentials, achieving 89.7% ammonia FE at −0.7 V RHE .

Hydrogen (H-2) is a clean and promising energy carrier that can help develop sustainable energy solutions. Ammonia borane (AB) is considered an efficient hydrogen storage material because of its high hydrogen capacity, chemical stability, and controlled hydrogen release under ambient conditions. However, achieving efficient hydrogen release through AB hydrolysis requires effective catalysts. This study synthesized and evaluated Pd-deposited oxygen-deficient tungsten oxide (Pd/WO3-x) for its catalytic efficiency in producing hydrogen from AB hydrolysis. The introduction of oxygen vacancies in WO3 enhanced its electronic properties, improving charge separation and catalytic activity. Additionally, Pd nanoparticles facilitated faster hydrolysis by promoting efficient charge transfer and activation of AB molecules. The optimized Pd/WO3-x catalyst exhibited high catalytic efficiency with a reaction rate of 6.27 mu mol min(-1), demonstrating its potential for sustainable hydrogen production. This study highlights the significance of oxygen vacancies and noble metal deposition in designing advanced catalysts for hydrogen energy applications. The findings contribute to the advancement of high-performance and cost-effective photocatalysts, paving the way for practical hydrogen storage and utilization in clean energy technologies.

Hydrogen peroxide (H2O2) is a valuable chemical, and its eco-friendly electrochemical production has gained attention to obtain pH-neutral aqueous solutions without impurities. However, achieving H2O2 faradaic efficiencies (FEs) above 30 % has been a challenge with conventional proton exchange membrane (PEM) electrolyzers. To enhance H2O2 FE, efficient collection of H2O2 from the catalyst surface using liquid water is necessary, but oxygen diffusion becomes a limiting factor in aqueous-immersed systems. To overcome this, we designed a zero-gap electrolyzer, supplying oxygen gas from the anode side through the membrane to the cathode. A gas flow-through porous PEM was developed by embedding an acidic ionomer into a membrane filter, enabling the crossover oxygen supply to the cathode flooded with water. This porous PEM design facilitated the formation of a three-phase interface at the catalyst, where high-concentration oxygen gas and liquid water interact closely, achieving 79 % H2O2 FE at 5 mA cm-2. Continuous synthesis of pure H2O2 solution exceeding 5500 mg L-1 (0.55 wt%) was sustained for over 50 hours.

Photoelectrochemical (PEC) water splitting holds significant promise for sustainable hydrogen production. Conventional photoelectrodes typically consist of metal oxide semiconductors deposited on conductive substrates. Pelletized photoelectrodes, on the other hand, offer advantages such as facile and scalable fabrication, along with high thermal stability, but their potential and structure-property-activity relationships in PEC water splitting remain largely unexplored. In the present study, we address this gap by developing pelletized electrodes of rutile TiO2 using a powder pressing and sintering method. Increasing the sintering temperature resulted in larger grain sizes with fewer grain boundaries. The resulting TiO2 pellets were treated with a hydrogen flow at 973 K to enhance their electrical conductivity, leading to the formation of Ti3+ species. The TiO2 pellet with larger grain sizes achieved photocurrent densities of 3.8 mA cm-2 for water oxidation in neutral electrolyte (pH 6.8) under UV illumination (40 mW cm-2) at 1.23 V vs RHE, with an incident photon-to-current conversion efficiency (IPCE) of 32% at 365 nm. The analysis revealed that grain boundary density limits the performance of pelletized TiO2 electrodes, with photocurrent density increasing as electron transport resistance decreases. Fewer grain boundaries and higher donor (Ti3+) density after hydrogen treatment enhance charge transport and separation, resulting in superior PEC performance of the pelletized photoelectrode with large grains.

Understanding the diffusion properties of constituent atoms in chalcopyrite-type Ag-containing Cu(In,Ga)Se-2-based semiconductors is important for the design of highly efficient photovoltaic devices. Herein, we experimentally evaluated the vibrational energy (Einstein frequency and temperature) of individual bonds in (Ag,Cu)InSe2 and Ag(In,Ga)Se-2 powders using Debye-Waller factors derived from extended X-ray absorption fine structure (EXAFS) data at low temperatures. We found that the Einstein temperature and frequency increased in the order of Ag-Se < Cu-Se < In-Se < Ga-Se. This order correlates with the theoretical activation energy of atomic migration calculated for Ag(In,Ga)Se-2 and Cu(In,Ga)Se-2 with Ag or Cu defect sites. In the chalcopyrite compounds, the Ag atom is the easiest to diffuse, owing to the smallest force constant and the largest reduced mass of the Ag-Se bond. The bond length varied with the sample composition, and the force constant's dependence on bond length further suggests that activation energy for atomic diffusion can be modulated through compositional adjustments. The low-temperature EXAFS study provides beneficial information for the design of multicomponent materials including bond length variations and Einstein frequencies of individual bonds. The Einstein frequencies could be an indicator of the atomic diffusion properties, reflecting the influence of force constant and reduced mass, to understand the elemental gradients in chalcopyrite semiconductors for highly efficient photovoltaic devices.

Electrochemical nitrate reduction reaction (NO3RR) represents a sustainable, carbon-neutral alternative to the Haber-Bosch process for ammonia synthesis. Time-resolved operando X-ray absorption spectroscopy reveals the chemical states and structural evolution of copper(I) oxide microcrystals deposited on carbon fiber (Cu2O/C) across a potential range of +0.6 to -0.7 V versus reversible hydrogen electrode (VRHE), where nitrate reduction to nitrite and ammonia occurs. Without nitrate, Cu2O microcrystals are quickly reduced to metallic Cu(0) aggregates at low reduction potentials (≈0.1 VRHE). In contrast, only 29% Cu(0) is observed in 0.1 M NaNO3 at 0.1 VRHE, indicating that nitrate adsorption passivates the surface and promotes selective electron transfer to nitrate, thereby retarding the kinetics of Cu2O microcrystals transformation to Cu(0) particles. Ammonia formation initiates at -0.3 VRHE in 0.1 M NaNO3 (pH 13) solution, accompanied by the formation of metallic copper particles for the hydrogenation of the intermediates. The Faradaic efficiency (FE) of ammonia is increased with more negative potential, accompanied by the formation of metallic Cu(0) particles. The fully reduced Cu particles exhibit superior NO3RR activity to produce nitrite at lower reduction potentials and ammonia at higher reduction potentials, achieving 89.7% ammonia FE at -0.7 VRHE.

Electrocatalytic two-electron oxygen reduction reaction (2e(-) ORR) is a crucial process for on-site and on-demand H2O2 production. Evaluating the impact of the medium's pH is essential for achieving selective H2O2 production at higher current density through continuous O-2 supply to gas diffusion electrodes (GDEs) in membrane electrolyzers. We investigated the effect of electrolyte pH on both H2O2 production and the flooding behavior of GDEs loaded with a cobalt single-atom catalyst in a typical H-type cell. The electrocatalyst was prepared by heating cobalt(II) tetraphenylporphyrin loaded on Ketjen Black (CoTPP/KB) at 750 degrees C. Remarkably, the cobalt single-atom catalyst exhibited high current density when the top of the hydrophobic gas-diffusion layer was exposed to the gas phase, facilitating efficient O-2 diffusion within the GDE. Potential-time curves at -40 mA cm(-2) showed stable potentials and Faradaic efficiencies (>80%) over 5 h across a pH range from 1 to 10, with the H2O2 concentration reaching approximately 100 mmol L-1 at pH 1.0. In contrast, the potential at pH 13 decreased abruptly in 3 h due to the flooding of GDE. Long-term tests demonstrated stable electrocatalytic H2O2 production only in the acidic electrolytes for 24 h, attributed to reduced flooding with decreasing pH. These findings underscore the impact of electrolyte pH on GDE performance during H2O2 production via the 2e(-) ORR, with acidity favoring the mitigation of undesirable flooding behavior.

A formate faradaic efficiency of 84.6% at 300 mA cm −2 was achieved in a bicarbonate electrolyzer by incorporating a porous membrane between the proton exchange membrane and the cathode to enhance in situ CO 2 generation from 3.0 M KHCO 3 .

Photocatalytic transformation of methane to ethane and hydrogen (2CH(4) -> C2H6 + H-2) is enhanced over metallic cocatalyst-loaded Ga2O3 particles under gas-flow conditions with water vapor. We examined the photocatalytic activity of oxide materials (Ga2O3, NaTaO3:La, SrTiO3:Al, and AgTaO3) loaded with Pd or Rh-Cr oxides, which are hydrogen-evolving cocatalysts. Electron spin resonance analysis revealed that the hydroxyl radical ((OH)-O-center dot) generated through the single-electron oxidation of water is the active species for methane activation to produce C2H6 and carbon dioxide over Pd/Ga2O3 and Pd/NaTaO3:La photocatalysts. In contrast, the photocatalysts active for water vapor splitting rather than methane transformation generate a surface peroxyl radical (superoxo) moiety as an intermediate of oxygen evolution. For methane activation, the ionization potentials (valence band maximum and midgap surface states) of the photocatalysts should be sufficiently high to promote single-electron transfer from the water to generate (OH)-O-center dot since methane exhibits high oxidation potential. This study unravels the crucial role of surface energy levels of photocatalysts for the activation of methane with water vapor.

A Pd3Bi intermetallic compound (IMC) was photocatalytically deposited onto the gallium oxide (Ga2O3) surface at room temperature. Conventional impregnation and reduction methods were difficult for the formation of the Pd3Bi IMC on Ga2O3, highlighting the importance of the photodeposition approach. The Pd3Bi-loaded Ga2O3 photocatalyst exhibited 84% selectivity in methane-to-ethane conversion with hydrogen production in the presence of water vapour.

Intermediates in the overall water splitting on particulate photocatalysts were investigated by electron paramagnetic resonance (EPR) measurements using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap. In situ EPR spectra under ultraviolet light showed the exclusive formation of a hydroxyl radical adduct ((DMPO)-D-center dot-OH) over Ga2O3 and NaTaO3:La without a Rh-Cr oxide cocatalyst. In contrast, the Rh-Cr oxide cocatalyst-loaded photocatalysts active for water splitting formed a 5,5-dimethyl-2-oxo-pyrroline-1-oxyl radical ((DMPOX)-D-center dot). (DMPOX)-D-center dot is suggested as an indicator of the surface peroxyl radical intermediate (M-OO center dot) in the oxygen evolution reaction pathway.

Amorphous RuO 2 have high OER activity but are unstable during the OER in 0.1M H 2 SO 4 . RuO 2 nanoparticles with high crystallinity simultaneously achieved high OER activity and stability.

Determination of a reaction pathway is an important issue for the optimization of reactions. However, reactions in solid‐state compounds have remained poorly understood because of their complexity and technical limitations. Here, using state‐of‐the‐art high‐speed time‐resolved synchrotron X‐ray techniques, the topochemical solid‐gas reduction mechanisms in layered perovskite Sr 3 Fe 2 O 7− δ (from δ ∼ 0.4 to δ = 1.0), which is promising for an environmental catalyst material is revealed. Pristine Sr 3 Fe 2 O 7− δ shows a gradual single‐phase structural evolution during reduction, indicating that the reaction continuously proceeds through thermodynamically stable phases. In contrast, a nonequilibrium dynamically‐disordered phase emerges a few seconds before a first‐order transition during the reduction of a Pd‐loaded sample. This drastic change in the reaction pathway can be explained by a change in the rate‐determining step. The synchrotron X‐ray technique can be applied to various solid‐gas reactions and provides an opportunity for gaining a better understanding and optimizing reactions in solid‐state compounds.

Automotive exhaust gases containing harmful gases, such as hydrocarbons, CO, and NOx, are purified on a Pd-loaded catalyst. Since the purification efficiency depends on the oxygen concentration in the automotive exhaust gas, a material with a high oxygen storage performance is also essential for effectively purifying automotive exhaust gases. The present study demonstrates that the purification of the exhaust gases under fluctuating oxygen concentration proceeds efficiently on Pd/Ca2AlMnO5+δ which exhibits high oxygen release and storage capacities. Pd/Ca2AlMnO5+δ maintained reduction of NO to N2 for a longer time compared to conventional Pd/CeO2-ZrO2, under oxidative conditions. From the results obtained by combining operando and static X-ray absorption fine structure (XAFS) spectra, the valence state of the Mn species in the catalyst support was readily changed in response to the oxygen concentration in the exhaust gas, rather than that of the Pd species. In other words, the redox properties of the Mn species in Pd/Ca2AlMnO5+δ concern the NO reduction behavior under fluctuating oxygen concentration.

The requirement for liquid electrolytes to maintain their ionic conductivity hinders the practical implementation of photoelectrochemical (PEC) systems. Here, a membrane electrode assembly was fabricated by incorporating porous alpha-Fe2O3 (hematite)-based photoanodes with an anion-exchange membrane (AEM). AEM-PEC water splitting was demonstrated in pure water without supporting electrolytes over porous hematite photoanodes under visible-light irradiation. This PEC system was operational even under low conductivity and near-neutral conditions (similar to 1 mS/m and pH similar to 9). Action spectral analysis confirmed a visible-light response extending up to 600 nm, surpassing that of previously reported photoelectrodes with solid-electrolyte membranes.

We characterized the optical and electronic properties of chalcopyrite-type Cu(Al,In)Se 2 , which is a candidate for wide-bandgap solar cell materials. The bandgap energy was determined from diffuse reflectance spectra. The bandgap energy increased from 1.00 eV for CuInSe 2 to 2.61 eV for CuAlSe 2 with an increase in the Al content. The ionization energy corresponding to the energy levels of the valence band maximum (VBM) was determined using photoemission yield spectroscopy. The VBM level of the Cu(Al,In)Se 2 system stayed relatively constant, but the conduction band minimum level increased with increasing Al content. To analyze the local structures of Cu and In atoms in Cu(Al,In)Se 2 , Cu and In K-edge X-ray absorption fine structure (XAFS) spectra were measured at SPring-8. We discuss the crystallographic characteristics of Cu(Al,In)Se 2 based on the results of the XAFS analyses and a comparison of the phase diagrams of the Cu 2 Se–Al 2 Se 3 , Cu 2 Se–In 2 Se 3, and Cu 2 Se–Ga 2 Se 3 systems.

Chalcopyrite CuInSe2 (CISe)-based thin-film photovoltaic solar cells have been attracting attention since the 1970s. The technologies of CISe-based thin-film growth and device fabrication processes have already been put into practical applications and today commercial products are available. Nevertheless, there are numerous poorly understood areas in the physical and chemical aspects of the underlying materials science and interfacial and bulk defect physics in CISe-based thin-films and devices for further developments. In this paper, current issues in physical and chemical studies of CISe-based materials and devices are reviewed. Correlations between Cu-deficient phases and the effects of alkali-metals, applications to lightweight and flexible solar minimodules, single-crystalline epitaxial Cu(In,Ga)Se-2 films and devices, differences between Cu(In,Ga)Se-2 and Ag(In,Ga)Se-2 materials, wide-gap CuGaSe2 films and devices, all-dry processed CISe-based solar cells with high photovoltaic efficiencies, and also fundamental studies on open circuit voltage loss analysis and the energy band structure at the interface are among the main areas of discussion in this review.

The strong metal-support interaction (SMSI) prompts the reductive treatment-induced encapsulation of platinum group metal (PGM) nanoparticles by reducible oxide catalyst supports and therefore often leads to negative outcomes, such as a decrease in catalytic activity. Herein, the SMSI effect was found to benefit a model reaction of automotive exhaust gas purification, namely the catalytic reduction of NO with C3H6 and CO over a Pd/Ca(2)AMnO(5+delta). The reductive treatment of Pd/Ca2AlMnO5+delta induced epitaxial growth of the (111) plane of the MnO-CaO solid solution on the (111) plane of the Pd metal core to produce Mn-doped CaO shells. Experimental results and theoretical calculations indicated that the bimetallic oxide surface of the Mn-doped CaO(111) plane exhibited excellent catalytic activity for NO reduction despite the absence of a Pd metal surface. Thus, this study not only demonstrates that transition-metal sites can exhibit high catalytic activity, similar to that of a Pd metal surface, but also provides a design guideline for environmental catalysts.