Valence State Evolution Research Articles

Improving hydrogen production activity of bifunctional CuCoFe-layered double hydroxides for electrocatalytic water splitting: Effects of valence state evolution and oxygen vacancies

The development of noble metal-free electrocatalysts is crucial for efficient water splitting via both hydrogen (HER) and oxygen evolution reaction (OER) in alkaline electrolyte. In this study, bifunctional CuCoFe layered double hydroxides grown in-situ on Ni foam (NF) have been prepared to achieve high-efficient water splitting. The CuCoFe LDHs need a low overpotential of 49 mV for HER and 255 mV for OER to deliver a current density of 10 mA·cm−2, which is superior to many reported LDH-based electrocatalysts. On the one hand, the Cu2+ ions induce distortions of structure units, which promotes the formation of oxygen vacancies and facilitates electron migration as well as charge transfer. On the other hand, excessive addition of Cu2+ ions can lead to a reduction in active sites due to the acceleration of Co2+ oxidation during preparation. This work presents an opportunity to regulate the structure of LDH-based electrocatalysts by the new approach of Cu-doping, resulting in enhanced control over valence state evolution and oxygen vacancies.

Morphology and valence state evolution of Cu: Unraveling the impact on nitric oxide electroreduction

Ammonia (NH3) serves as a critical component in the fertilizer industry and fume gas denitrification. However, the conventional NH3 production process, namely the Haber-Bosch process, leads to considerable energy consumption and waste gas emissions. To address this, electrocatalytic nitric oxide reduction reaction (NORR) has emerged as a promising strategy to bridge NH3 consumption to NH3 production, harnessing renewable electricity for a sustainable future. Copper (Cu) stands out as a prominent electrocatalyst for NO reduction, given its exceptional NH3 yield and selectivity. However, a crucial aspect that remains insufficiently explored is the effects of morphology and valence states of Cu on the NORR performance. In this investigation, we synthesized CuO nanowires (CuO-NF) and Cu nanocubes (Cu-NF) as cathodes through an in situ growth method. Remarkably, CuO-NF exhibited an impressive NH3 yield of 0.50 ± 0.02 mg cm−2 h−1 at −0.6 V vs. reversible hydrogen electrode (RHE) with faradaic efficiency of 29.68% ± 1.35%, surpassing that of Cu-NF (0.17 ± 0.01 mg cm−2 h−1, 16.18% ± 1.40%). Throughout the electroreduction process, secondary cubes were generated on the CuO-NF surface, preserving their nanosheet cluster morphology, sustained by an abundant supply of subsurface oxygen (s-O) even after an extended duration of 10 h, until s-O depletion ensued. Conversely, Cu-NF exhibited inadequate s-O content, leading to rapid crystal collapse within the same timeframe. The distinctive current–potential relationship, akin to a volcano-type curve, was attributed to distinct NO hydrogenation mechanisms. Further Tafel analysis revealed the exchange current density (i0) and standard heterogeneous rate constant (k0) for CuO-NF, yielding 3.44 × 10−6 A cm−2 and 3.77 × 10−6 cm−2 s−1 when NORR was driven by overpotentials. These findings revealed the potential of CuO-NF for NO reduction and provided insights into the intricate interplay between crystal morphology, valence states, and electrochemical performance.

Synthesis of Oxide Interface-Based Two-Dimensional Electron Gas on Si.

Two-dimensional electron gas (2DEG) at the interface of amorphous Al2O3/SrTiO3 (aAO/STO) heterostructures has received considerable attention owing to its convenience of fabrication and relatively high mobility. The integration of these 2DEG heterostructures on a silicon wafer is highly desired for electronic applications but remains challanging up to date. Here, conductive aAO/STO heterostructures have been synthesized on a silicon wafer via a growth-and-transfer method. A scanning transmission electron microscopy image shows flat and close contact between STO membranes and a Si wafer. Electron energy loss spectroscopic measurements reveal the interfacial Ti valence state evolution, which identifies the formation of 2D charge carriers confined at the interface of aAO/STO. This work provides a feasible strategy for the integration of 2DEG on a silicon wafer and other desired substrates for potential functional and flexible electronic devices.

Iron valence state evolution and hydrochar properties under hydrothermal carbonization of dyeing sludge

Iron (Fe) migration mechanisms and hydrochar properties in dyeing sludge hydrothermal carbonization (HTC) are important topics in wastewater treatment. HTC treatment of sludge produces wastewater containing Fe so it is necessary to study the migration behavior of Fe during HTC treatment. This study investigated the basic properties and Fe migration behavior of hydrochar during HTC treatment supplemented with nitric acid (HNO3). The results showed that the carbonization degree and yield of hydrochar treated with the HNO3 solution (HHC) were much lower than those of hydrochar treated with ultrapure water (WHC). The variation of total Fe (TF) concentration indicated that the decomposition of organic material and dissolution of minerals in the aqueous release of Fe during the liquid phase, led to much lower TF concentrations compared to the original dyeing sludge. Fe release was further enhanced with the addition of HNO3 and increase of temperature, rendering a much lower TF concentration of the HHC compared to the WHC. The variations of Fe3+ and Fe2+ concentrations indicated that the HTC-treated hydrochar contained more Fe2+, caused by Fe3+ reduction with hydroxyl methyl-furfural and glucose in the liquid and subsequent Fe2+/Fe3+ transferral to the solid hydrochar phase. X-ray diffraction (XRD) showed that the main Fe content in WHC was FeO(OH), while HHC contained mainly Fe(SO4)(OH)•2H2O and Fe3O4. XPS and XRF showed that Fe could more easily enter the internal pores of the hydrochar instead of being deposited on the surface. This study provided more insights on Fe migration behavior during HTC treatment.

Mesoporous VO2(B) nanorods deposited onto graphene architectures for enhanced rate capability and cycle life of Li ion battery cathodes

Monoclinic vanadium dioxide (VO2(B)) is considered as a promising cathode material for lithium ion batteries (LIBs) owing to its high capacity, short ion diffusion channel, and multiple oxidation states. However, VO2(B) is technically limited due to its low electronic conductivity and large volume expansion. In order to resolve these drawbacks, we develop a mesoporous composite of reduced graphene oxide (rGO) and VO2(B) for enhanced rate capability and cycle life of LIB cathode. The uniform deposition of VO2(B) nanorods onto the mesoporous surface of the rGO architecture provides a facile access of Li ions to the storage site, large accessible area, short diffusion pathway, and chemical and mechanical stabilities. The VO2(B)/rGO composite electrode achieves a high capacity of 226 mA h g−1 at current density of 50 mA g−1 and superior capacity retention of 67.5% even at 40 times increase in current density up to 2000 mA g−1. In addition, this composite electrode demonstrates long cyclic stability of 88.5% after 500 cycles at 1000 mA g−1. In situ synchronous X-ray absorption spectroscopic data confirms a reversible change of the local structure and valence state evolution during a charging and discharging process.

Electrochemical and structural evolution of structured V2O5 microspheres during Li-ion intercalation

Abstract With the development of stable alkali metal anodes, V2O5 is gaining traction as a cathode material due to its high theoretical capacity and the ability to intercalate Li, Na and K ions. Herein, we report a method for synthesizing structured orthorhombic V2O5 microspheres and investigate Li intercalation/de-intercalation into this material. For industry adoption, the electrochemical behavior of V2O5 as well as structural and phase transformation attributing to Li intercalation reaction must be further investigated. Our synthesized V2O5 microspheres consisted of small primary particles that were strongly joined together and exhibited good cycle stability and rate capability, triggered by reversible volume change and rapid Li ion diffusion. In addition, the reversibility of phase transformation (α, e, δ, γ and ω-LixV2O5) and valence state evolution (5+, 4+, and 3.5+ ) during intercalation/de-intercalation were studied via in-situ X-ray powder diffraction and X-ray absorption near edge structure analyses.

Probing the Crystal and Electronic Structures of Molybdenum Oxide in Redox Process: Implications for Energy Applications

Knowledge regarding the phase and valence state evolution of molybdenum (Mo) and its oxides during the redox reaction is essential for advancing their energy applications (e.g., electrocatalysis), which unfortunately remains largely unexplored. Herein, the effects of atomic and electronic structures on the electrocatalytic performance of Mo/oxides core–shell structures are investigated on the basis of the combination of ex situ and in situ experiments. First, a two-step reaction pathway is revealed during the oxidation of nanoscale Mo: the formation of amorphous MoO3 (A-MoO3) shells followed by the nucleation of crystalline α-MoO3. It is shown that the electrocatalytic performance of A-MoO3 is superior to that of α-MoO3, mainly due to more catalytically active sites in the former material. Furthermore, in situ transmission electron microscopy observations show that the A-MoO3 shell can be rapidly reduced into metallic MoO2 under reductive environment, which is likely to occur during the hydrogen evolution...

Unraveling bulk and grain boundary electrical properties in La0.8Sr0.2Mn1−yO3±δ thin films

Grain boundaries in Sr-doped LaMnO3±δ thin films have been shown to strongly influence the electronic and oxygen mass transport properties, being able to profoundly modify the nature of the material. The unique behavior of the grain boundaries can be correlated with substantial modifications of the cation concentration at the interfaces, which can be tuned by changing the overall cationic ratio in the films. In this work, we study the electronic properties of La0.8Sr0.2Mn1−yO3±δ thin films with variable Mn content. The influence of the cationic composition on the grain boundary and grain bulk electronic properties is elucidated by studying the manganese valence state evolution using spectroscopy techniques and by confronting the electronic properties of epitaxial and polycrystalline films. Substantial differences in the electronic conduction mechanism are found in the presence of grain boundaries and depending on the manganese content. Moreover, the unique defect chemistry of the nanomaterial is elucidated by measuring the electrical resistance of the thin films as a function of oxygen partial pressure, disclosing the importance of the cationic local non-stoichiometry on the thin film behavior.

Red-emitting Lu3Al5O12: Mn transparent ceramic phosphors: valence state evolution studies of Mn ions

A red-emitting Lu3Al5O12: Mn (LuAG: Mn) transparent ceramic phosphor was successfully prepared via a two-step process: vacuum sintering and further high-temperature annealing in air. The LuAG: Mn ceramics with a transmittance of ~40% in visible light were obtained after vacuum sintering. Through the transmittance spectra, electron spin resonance measurement, cathodoluminescence technology and photoluminescence spectra, valence state evolution of Mn ions in LuAG ceramics before and after annealing were elaborately demonstrated. It's proved that part of Mn2+ were oxidized to Mn4+ during the annealing process, which is responsible for the bright red emission of the annealed sample under ultraviolet light excitation. The red-emitting LuAG: Mn transparent ceramic phosphor is a promising candidate to help make white light-emitting diodes (WLEDs) with high color rendering index.

The behavior of the manganese-cerium loaded metal-organic framework in elemental mercury and NO removal from flue gas

Mn-Ce loaded metal-organic framework (MnCe@MOF) catalyst was obtained by incipient wetness impregnation of MOF with manganese-cerium nitrate. The obtained materials were investigated as potential catalysts for elemental mercury (Hg0) and NO removal from simulate flue gas and the results indicated that MnCe@MOF exhibited superior activity in comparison with the conventional catalysts Mn-Ce loaded ZrO2, especially at low temperatures (<250°C). It was established that the crystalline structure of MOF support influenced the catalytic behavior in a complex way. The MOF support played an important role in promoting dispersion of active components, improving the Hg0 adsorption and enhancing oxygen utilization, which resulted in a desirable catalytic activity. To understand the mechanism of the Mn-Ce catalyst better, dynamic valence state evolution during the Hg0 oxidation for the active components was investigated. The ratio of Mn3+/Mn4+ remained constant during the reaction at various temperatures, indicating the stability of Mn in the catalytic process. However, the ratio of Ce3+/Ce4+ gradually decreased with extended periods of reaction time, which most likely lead to the decreased of chemical adsorbed oxygen on the catalyst.

Bifunctional Ni1−xFex layered double hydroxides/Ni foam electrodes for high-efficient overall water splitting: A study on compositional tuning and valence state evolution

Toward both hydrogen (HER) and oxygen evolution reaction (OER) in alkaline electrolyte, bifunctional Ni1−xFex layered double hydroxides (LDHs) on Ni foam (NF) have been systematically developed for high-efficient water splitting by changing Fe content. The interlaced NiFe LDHs nanosheets consist of ultra-small γ-NiOOH-like nanoparticles with size of 2–5 nm interconnected by amorphous domains. By tuning Fe content from 15% to 25% and 36%, optimized μ10 values (μ10 is the overpotential required to reach a current density of 10 mA/cm2) of 169 mV (HER) and 140 mV (OER), and Tafel slope of 31 mV/dec have been achieved at x = 0.25. A hydrogen generation rate of 170 μmol/h/cm2 with H2/O2 ratio nearly to 2:1 and Faradic efficiency of 93% have been realized at 1.7 V by using two Ni0.75Fe0.25 LDHs/NF as respective anode and cathode in a 2-electrode system. Based on the valence state evolution of the electrodes, undergoing respectively H2 and O2 generation, tremendous transformation of NiFe(OH)2 to NiFeOOH is deduced, which reaches a Ni3+/Ni2+ ratio of ∼2.35 for the Fe content of 25%. Further or less Fe incorporation will depress the electrocatalytic property. The effective Ni3+ active sites on the NiFe-based nanosheets endow the Ni0.75Fe0.25 LDHs/NF with high bifunctional catalytic property.

In situ X-ray near-edge absorption spectroscopy investigation of the state of charge of all-vanadium redox flow batteries.

Synchrotron-based in situ X-ray near-edge absorption spectroscopy (XANES) has been used to study the valence state evolution of the vanadium ion for both the catholyte and anolyte in all-vanadium redox flow batteries (VRB) under realistic cycling conditions. The results indicate that, when using the widely used charge-discharge profile during the first charge process (charging the VRB cell to 1.65 V under a constant current mode), the vanadium ion valence did not reach V(V) in the catholyte and did not reach V(II) in the anolyte. Consequently, the state of charge (SOC) for the VRB cell was only 82%, far below the desired 100% SOC. Thus, such incompletely charged mix electrolytes results in not only wasting the electrolytes but also decreasing the cell performance in the following cycles. On the basis of our study, we proposed a new charge-discharge profile (first charged at a constant current mode up to 1.65 V and then continuously charged at a constant voltage mode until the capacity was close to the theoretical value) for the first charge process that achieved 100% SOC after the initial charge process. Utilizing this new charge-discharge profile, the theoretical charge capacity and the full utilization of electrolytes has been achieved, thus having a significant impact on the cost reduction of the electrolytes in VRB.

High-resolution EELS study of the vacancy-doped metal/insulator system, Nd 1−xTiO 3, [formula omitted] to 0.33.

High-resolution electron energy loss spectroscopy (EELS) spectra have been obtained for several members of the vacancy-doped metal/insulator perovskite system Nd 1− x TiO 3 where NdTiO 3 ( x = 0 ) is a Mott–Hubbard insulator and Nd 2/3TiO 3 ( x = 0.33 ), a band insulator. The insulator to correlated-metal transitions occur at x ≈ 0.20 and x ≈ 0.10 . Both O K- and Ti L 2,3-edge data were obtained on the title materials and the related d 0 perovskites, CaTiO 3, SrTiO 3 and BaTiO 3. The crystal structure of Nd 0.70TiO 3 ( x = 0.30 ) was refined from powder neutron data in Cmmm which is taken as a model for the structure of Nd 2/3TiO 3 ( x = 0.33 ) as well. The L-edge spectra for Nd 1− x TiO 3 show systematic changes consistent with the valence state evolution from all Ti 3+ ( x = 0.0 ) to all Ti 4+ ( x = 0.33 ). Octahedral crystal fields of 1.5 and 2.0 eV were inferred from the L-edge data for NdTiO 3 and Nd 2/3TiO 3 by comparison with calculated spectra from the literature. Broader L 3 and L 2 peak widths for Nd 2/3TiO 3 than for the other d 0 perovskites are attributed to the more complex crystal structure of this material which includes two Nd sites with partial occupation and a more distorted Ti–O environment with low site symmetry. A markedly different O K-edge spectrum for Nd 2/3TiO 3 is attributed to the involvement of Nd 5 d levels.

Valence state evolution of C60 deposited on Sm film

Ultraviolet photoemission spectra ofC60 deposited on Sm film were measured. The results indicate thatC60 and Sm can combine with each other to form fulleride in the interfaceat room temperature. The bonding is mainly ionic with some covalentcontribution. Both the lowest unoccupied molecular orbital (LUMO) and theLUMO+1 orbitalof C60 are occupied at coverages less than about one monolayer. As for thesecond overlayer, only the LUMO orbital is occupied. The first and secondC60 overlayers are metallic and semiconducting, respectively.

Chemically driven diffusion of Pd in the surface region of Zn(0001): An ultraviolet photoemission investigation

The diffusion of deposited Pd through the (0001) surface region of zinc has been studied with photoemission at hν = 21.2 eV by following the time evolution of the Zn 3d and Pd 4d peaks for a Pd initial coverage of 1, 3, 10 and 15 monolayers. The time decay of the Pd 4d signal is explained with a model where the diffusion coefficient D is not constant; it is (4.6 ± 0.5) × 10 −19 cm 2 s −1 for t≲7000s, then decreases to (5.5 ± 1) × 10 −20 cm 2 s −1 for t ≳ 15,000 s. The D values correlate well with the spectroscopic results on the valence state evolution during diffusion. At short times (higher D) the spectra show an electron energy gain of Pd atoms during diffusion while at higher time (lower D) this gain is negligible. The initial diffusion is chemically driven while at longer times the diffusion becomes gradually entropic.