Oxidation States Of Transition Metals Research Articles

Modulating Electronic Interaction in Electrocatalysts for Boosting Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a crucial half-reaction in numerous electrochemical processes including water electrolysis, CO2 reduction, and rechargeable metal-air batteries. While precious metal-based catalysts such as IrO2 and RuO2 demonstrate effectiveness in OER, their high cost and instability hinder widespread adoption. Consequently, there is an urgent demand to develop low-cost transition metal-based electrocatalysts, satisfying both low overpotentials as well as robust stability. Given this perspective, research has concentrated on employing multi-metal systems as a catalyst design strategy to engineer effective transition metal-based catalysts for the OER. This approach facilitates the fine-tuning of catalytic properties by harnessing the synergistic interactions inherent within the constituent elements. Achieving this goal necessitates a profound comprehension of electronic structural modulations based on composition, coupled with rigorous analysis of the intricate redox mechanisms underlying the catalytic activity of these materials. In the course of our investigation, we synthesized high-entropy oxide in a spinel phase with a nano-sheet morphology. In addition to the flexible structure of spinel oxide accommodating various coordination environments and oxidation states, the unique physicochemical properties of high-entropy materials arising from their multi-elemental configuration make it possible to tailor the catalytic properties. We utilized OER-active metals Fe, Co, and Ni, complemented by the strategic introduction of non-3d elements Cr and Mo in their high-valence states, to significantly enhance the intrinsic activity of the catalysts by adjusting the oxide electronic structure. The resulting high-entropy spinel oxide (CrFeCoNiMo)3O4 displayed promising OER activity and stability outperforming the IrO2 benchmark, demonstrating its feasibility for practical applications. In particular, the sequential incorporation of Cr and Mo in (FeCoNi)3O4 led to performance enhancements, showcasing the tunable nature of high-entropy oxides. The prepared catalysts were further extended to a lab-scale anion exchange membrane water electrolyzer (AEMWE) to test the reliability, achieving lower overpotential than required by the IrO2 benchmark. For elucidation of catalytic activity enhancement, careful X-ray analysis was conducted. The presence of highly electronegative species such as Cr 6+ and Mo 6+ is expected to modulate the electronic structure of the other metal sites by inducing partial electron transfer through the bridging oxygen. Upon the inclusion of high-valence elements, a decrease in the electron density of the existing elements is observed, indicating the electron-withdrawing effect of high-valence elements. This decrease in electron density correlates with an increase in the oxidation state of transition metals, which in turn has been associated with enhanced oxygen activity. Consequently, the metal-oxygen bond weakened increasing the electron density at the O site, potentially enabling the detachment of oxygen from the catalysts. In conclusion, our study unveils the potential of integrating high-valence elements into high-entropy spinel oxide as a novel catalyst design approach. This optimizes the electronic structure of the catalyst and fine-tunes the metal-oxygen bond properties, thereby activating the water oxidation reaction pathway towards oxygen redox chemistry. By shedding light on this methodology, our study contributes valuable insights to the design strategy of effective electrocatalysts, emphasizing the significance of considering interactions among the constituent elements. Moreover, our research presents a diverse array of analytical approaches for elucidating elemental interactions, enriching our understanding of catalyst behavior. Additionally, experimental tests confirm the feasibility of applying these catalysts in practical electrochemical devices. Overall, our findings underscore the versability of high-entropy spinel oxides as catalysts for advancing electrochemical applications.

(Invited) Hydroflux Process as a Low-Temperature Synthesis of Layered Cathodes

Layered LiCoO2 (LCO) is usually synthesized at high temperatures ≥ 800 °C by conventional solid-state processes. It also has another polymorph with a spinel phase, which is synthesized around 400 °C. Hence, the layered LCO and the spinel LCO have been called HT-LCO and LT-LCO, respectively. Conversely, first-principles calculations predicted that the layered LCO is thermodynamically stable even at low temperatures. Furthermore, an in situ XRD study revealed that the spinel LCO formation is the topotactic displacement of the Co2+ or Co3+ ion at the 8a site in the Co3O4 spinel with two Li+ ions. Therefore, layered LCO should be obtained even at low temperatures. Here, we propose an effective synthesis approach called the “hydroflux process” to synthesize a highly crystalline layered LCO at low temperatures of ≤ 300 °C within a short duration of ≤ 1 h. Our synthetic strategy uses a eutectic LiOH-NaOH binary alkaline hydroxide as the lithium source and the flux for crystal growth. Since the eutectic point of the mixed hydroxide is 220 °C, the obvious crystal growth of LCO was observed at 250 °C and above. We also found that the crystal water of the LiOH monohydrate showed obvious crystal growth of the layered LCO even at low temperatures. We also investigated the phase evolution of the starting materials during the hydroflux process at 300 °C. The series of the XRD patterns showed that the starting material of Co(OH)2 transformed to a rocksalt CoO after 10-15 minutes of heating. Subsequently, the layered LCO formation was observed during the following 15 minutes. The crystal growth of the layered LCO was completed within 30 minutes of the heating duration. The observed reaction pathway shows that the water molecules significantly accelerate the crystal growth of the layered LCO in the molten hydroxide. The formation of soluble cobalt species of HCoO2 – ion at high pH conditions enables the crystal growth of the LCO at low temperatures. We also attempted to synthesize LiNiO2 and ortho-LiMnO2 at 300 °C. Controlling the oxidation state of transition metals becomes a key parameter for the development of our synthetic strategy. We believe that the hydroflux process has the potential to revolutionize the synthesis of layered cathode active materials, offering a rapid and scalable solution.

An insight into the electrochemical performance of nanostructured V2O5 in aqueous neutral electrolytes and fabrication of 2V, high energy density, symmetric supercapacitor

The exceptional redox characteristics, simple methods of synthesis, low cost and wide range of oxidation states of transition metal oxides like V2O5 have made them highly sought after electrode materials for supercapacitors. In the present study, V2O5 nanostructures prepared via a facile microwave-assisted method is used as the electrode material. The electrochemical performances of this electrode material is analysed in four different aqueous neutral electrolytes. The nature of the ions present in the electrolytes are found to have a significant influence on the performance of the electrode, achieving the highest specific capacitance values in KCl electrolyte while highest potential window and stability in Na2SO4 electrolyte. The symmetric supercapacitor fabricated using the Na2SO4 electrolyte exhibits a specific capacitance value of 82.35 F g-1 and a specific energy of 45.75 Wh kg-1 at a specific power of 1000 W kg-1. Moreover, the device retains 70 % of its initial capacitance even after 10,000 charge-discharge cycles at a current density of 5 A g-1. An asymmetric supercapacitor is also fabricated using KCl electrolyte after optimizing the mass-ratio of positive and negative electrodes using Ragone plot simulator. The asymmetric supercapacitor exhibits a specific capacitance of 34 F g-1 with an energy density and power density of 11 Wh kg-1 and 775 W kg-1 respectively. The present study summarizes the effect of different neutral electrolytes on the electrochemical performance of V2O5 nanostructured electrodes. The physiochemical properties of ions such as solvation, diffusion and mobility, and their influence on the electrochemical behaviour are also taken in to account to explain the observed electrode performance.

(Invited) Extended Hubbard Functionals for Accurate Modeling of Li-Ion Battery Cathode Materials

Designing novel cathode materials for Li-ion batteries necessitates accurate first-principles predictions of their properties. Density-functional theory (DFT) employing standard (semi-)local functionals encounters challenges due to pronounced self-interaction errors within the partially filled d shells of transition-metal (TM) elements. Here, we demonstrate the efficacy of DFT with extended Hubbard functionals in accurately predicting the "digital" change in oxidation states of TM ions for mixed-valence phases at intermediate Li concentrations in phospho-olivine and spinel cathode materials. The resulting voltages exhibit remarkable agreement with experimental data [1,2].Our approach involves the use of onsite U and intersite V Hubbard parameters computed from density-functional perturbation theory with Lowdin-orthogonalized atomic orbitals, thus circumventing empirical factors [3,4]. Notably, the incorporation of intersite Hubbard V interactions proves indispensable for the precise prediction of thermodynamic quantities, particularly when electronic localization occurs amid inter-atomic orbital hybridization.Extending our investigation to vibrational spectroscopy, we calculate Raman spectra for these materials and find very good agreement with available experimental data [5]. Leveraging the robust and accurate computational framework based on extended Hubbard functionals, we embark on a high-throughput screening of Li-containing compounds, aiming to unveil novel and promising candidates for cathodes [6]. Our findings underscore the pivotal role of advanced DFT methodologies in advancing the discovery and design of high-performance battery materials.[1] I. Timrov, F. Aquilante, M. Cococcioni, N. Marzari, PRX Energy 1, 033003 (2022).[2] I. Timrov, M. Kotiuga, N. Marzari, Phys. Chem. Chem. Phys. 25, 9061 (2023).[3] I. Timrov, N. Marzari, M. Cococcioni, Phys. Rev. B 98, 045141 (2021).[4] I. Timrov, N. Marzari, M. Cococcioni, Phys. Rev. B 103, 085127 (2018).[5] L. Bastonero, N. Marzari, I. Timrov, in preparation.[6] C. Malica, L. Bastonero, M. Bercx, N. Marzari, I. Timrov, in preparation.

BaCoO2 with Tetrahedral Cobalt Coordination: The Missing Element to Understand Energy Storage and Conversion Applications in BaCoO3-δ-Based Materials.

Barium-cobaltate-based perovskite (BaCoO3-δ) and barium-cobaltate-based nanocomposites have been intensively studied in energy storage and conversion devices mainly due to flexible oxygen stoichiometry and tunable nonprecious transition metal oxidation states. Although a rich and complex family of structural polymorphs has already been reported for these perovskites in the literature, the potential structural evolution that may occur during the oxygen reduction reaction and the oxygen evolution reaction has not been investigated so far. In this study, we synthesized and characterized the lowest Co-oxidation state possible in the compound, BaCoO2, which exhibits a quartz-derived, trigonal structure with a helicoidally corner-sharing, CoO4-tetrahedral-framework as already proposed by Spitsbergen et al. Oxygen can reversibly be inserted in such a crystal structure to form BaCoO3-δ, i.e., with 0 ≤ δ ≤ 1, based on the results of an in situ coupled thermogravimetric - neutron diffraction study and which presents therefore giant oxygen capacity storage due to the extreme tunability of the electronic configuration of the cobalt cations which defines the fundamental origins of the materials performance. The reversible conversion of BaCoO2 to BaCoO3-δ associated with a similar electronic conductivity above 900 K permits to clarify the high potential of BaCoO3-δ-based energy storage and conversion devices.

Enhanced photocatalytic CO2 reduction to methanol by modulating valence states of copper in CuSrTiO3

Manipulating the oxidation states of transition metals is an ideal solution to achieve selective generation of different products and copper is widely used in CO2 photocatalytic reduction to methanol. However, the understanding on the effects of valence states of copper species in SrTiO3 on its structure and the selectivity of the corresponding product in CO2 photocatalysis are currently still limited. Here, copper-doped SrTiO3 nanofibers with different controllable ratios of Cu+/Cu2+ and Cu+/Cu0 were prepared by the combination of the electrospinning with the in-situ reduction method. The proportion of copper with different valence states (Cu+/Cu2+ and Cu+/Cu0) was changed by rationally adjusting the time of hydrogen reduction. The presence of monovalent copper facilitated the generation of reactive intermediates, thereby enhancing its photocatalytic activity. The copper-doped SrTiO3 nanofibers obtained after 3 h of hydrogen reduction (STCu0.08-H-3 h) had the highest proportion of Cu+/Cu2+ and Cu+/Cu0 with the highest methanol yield (6.96 μmol·g−1·h−1). Methanol generation rate and Cu+/Cu2+ ratio of STCu0.08-H-xh (x = 1, 2, 3, 4,5) with different hydrogen reduction time were proven to be linearly positively correlated. Furthermore, theoretical investigation and in-situ CO2 infrared spectrum deeply understood the difference of CO2 adsorption behavior and CO2 activation over Cu2+ and Cu+ of STCu0.08-H-3 h photocatalyst and the reaction intermediates involved in the photocatalytic CO2 reduction to methanol.

M-Ge-Si thermolytic molecular precursors and models for germanium-doped transition metal sites on silica.

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, -OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the implications of which are discussed in the context of low-coordinate and low oxidation state thermolytic molecular precursors.

Understanding LiNiO2 Electronic Structure and Redox Mechanism by Raman and X-Ray Techniques

LiNiO2 (LNO) is considered a very promising and high energy density alternative to Co-containing Li-ion battery cathodes. It is also the benchmarked cathode material of the European BIGMAP project, which aim is to combine advance operando characterization, computational modelling and artificial intelligence in order to accelerate the research and development of new generation batteries.However, the material suffers from several degradation issues resulting in fading cell performance. In particular, the presence of highly reactive Ni4+ species at the electrode/electrolyte interface1, Ni dissolution from the cathode leading to the loss of active material 2, and O2 release at high voltage are the major drawbacks precluding commercial applications. A recent in situ study1 has shown that O2 release compromises the structural stability and triggers reactions with ethylene carbonate (EC), prompting CO2evolution3,4.In parallel, solid state physics community is also largely interested into LiNiO2 in the framework of a larger investigation around the electronic and atomic structure of nickelates, in particular rare-earth nickelates. This interest rises from the fact that there is still no consensus on the atomic/electronic structure of LiNiO2, while different descriptions exist in literature. Local probe techniques demonstrate the distortion of NiO6 octahedra, and there are two on-going theories for explaining the presence of this distortion: (i) Ni3+ (t2g6 eg1) Jahn-Teller (JT) distortion, (ii) 2 Ni3+ → Ni2+ + Ni4+ bond disproportionation (BD)5. Surprisingly, the local order distortion does not result in the long-range distortion, the mechanism behind this phenomenon is not completely understood.In this contribution we present a detailed study of LNO cathode material by operando Raman and X-ray absorption spectroscopy (XAS) at the Ni K-edge, ex situ X-ray emission spectroscopy (XES) and resonant Inelastic X-ray scattering (RIXS). Distinguishing between JT and BD models is challenging, and therefore NaNiO2 reference is used, which is isoelectronic material to LiNiO2, and is known to feature long-range JT distortion.While XAS is widely established technique to probe local structure and transition metal oxidation state, RIXS is useful for assessing d-d transitions and bond disproportionation. Operando Raman spectroscopy is an excellent tool for studying phase transitions in Li-ion battery cathodes and is therefore complementary to the employed X-ray techniques6. It provides information on two characteristic for layered oxides crystalline phonon modes A1g and Eg, and sheds light on both short-range order and cationic/anionic redox. The penetration depth is estimated as top few hundred nanometers of around five micrometres sized secondary particle, therefore near-surface area is probed. The results show a complex behaviour of band positions and intensities during cycling (figure 1), corresponding to four phases transformations. Notably the peaks intensities increase considerably, while the widths are decreased, which is evidence of increased order upon material delithiation, and is in agreement with the Extended X-ray Absorption Fine Structure (EXAFS). Raman is also used to evaluate the contribution of anionic redox, since all the reduced oxygen species (superoxide, peroxide and molecular oxygen) have well-known Raman-active modes.To conclude, we demonstrate how a combination of multiple vibrational and X-rays based spectroscopies provides a better understanding of both LNO ground state electronic structure and redox mechanism. This increased fundamental understanding will stimulate the design of better strategies for prevention of the material degradation and improved cycle retention.

A Drive Towards Elemental Simplicity in Cathode Design by Controlling Grain Growth Behavior

Sodium-ion battery technologies are emerging as an alternative to lithium-ion batteries by utilizing the earth abundant, widely distributed, and low-cost sodium resources. Among different types of sodium ion cathodes, transition metal based layered oxide cathodes have a superior edge due to their comparatively higher energy density, lower cost, environment friendly raw materials, and similar synthesis chemistry as the already commercialized lithium-ion battery cathode materials. They also enable the incorporation of iron, an abundant, low-cost, and environmentally benign element, replacing the toxic and expensive cobalt.In such layered oxide-based cathodes, transition metals exhibit heterogenous elemental and oxidation state distribution at different depths of the particles (e.g., from the surface to bulk) which can influence the stability, mechanical integrity, and electrochemical performance of these particles to a significant extent. The inherent local structural and electronic state heterogeneity developed in these materials during synthesis can modulate the charge distribution heterogeneity and consequent phase transitions during cycling, ultimately controlling the battery performance.In this work, a sodium transition metal layered oxide cathode is synthesized using a co-precipitation method followed by high temperature calcination under different calcination durations. Calcining the material at different conditions lead to the formation of particles with different morphologies and grain growth behavior. Using electron microscopy and Xray diffraction techniques, the grain growth behavior at different stages of calcination is studied. This will guide the design of particles with better morphologies and grain orientation which demonstrate improved electrochemical performance. Transition metal distribution and oxidation state change at different depths of the particles are probed using synchrotron based Xray absorption spectroscopy and three-dimensional Transmission Xray microscopy. The particles are probed at the surface, subsurface and deep into the bulk, which show heterogeneity in the distribution of transition metal oxidation state and provide insight into the phase transitions during calcination. This study can pave the way for better understanding of phase evolution during synthesis and improved design of cathode particle morphology and composition by identifying and eliminating the detrimental phases occurring during different stages of calcination.

Comparative of machine learning classification strategies for electron energy loss spectroscopy: Support vector machines and artificial neural networks

Machine Learning (ML) strategies applied to Scanning and conventional Transmission Electron Microscopy have become a valuable tool for analyzing the large volumes of data generated by various S/TEM techniques. In this work, we focus on Electron Energy Loss Spectroscopy (EELS) and study two ML techniques for classifying spectra in detail: Support Vector Machines (SVM) and Artificial Neural Networks (ANN). Firstly, we systematically analyze the optimal configurations and architectures for ANN classifiers using random search and the tree-structured Parzen estimator methods. Secondly, a new kernel strategy is introduced for the soft-margin SVMs, the cosine kernel, which offers a significant advantage over the previously studied kernels and other ML classification strategies. This kernel allows us to bypass the normalization of EEL spectra, achieving accurate classification. This result is highly relevant for the EELS community since we also assess the impact of common normalization techniques on our spectra using Uniform Manifold Approximation and Projection (UMAP), revealing a strong bias introduced in the spectra once normalized. In order to evaluate and study both classification strategies, we focus on determining the oxidation state of transition metals through their EEL spectra, examining which feature is more suitable for oxidation state classification: the oxygen K peak or the transition metal white lines. Subsequently, we compare the resistance to energy loss shifts for both classifiers and present a strategy to improve their resistance. The results of this study suggest the use of soft-margin SVMs for simpler EELS classification tasks with a limited number of spectra, as they provide performance comparable to ANNs while requiring lower computational resources and reduced training times. Conversely, ANNs are better suited for handling complex classification problems with extensive training data.

Electronic Structure of Orthorhombic Bi4V2O11 and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxides shows promise as an excellent material for photoanodes in water splitting. However, the efficiency of Bi4V2O11 in water splitting is not satisfactory compared to BiVO4. In this study, we investigated the electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotron soft X-ray spectroscopy techniques to clarify the causes of inefficiency. Our DFT calculations of the two-fold superlattice, as confirmed by near-edge X-ray absorption fine structure measurements, revealed its indirect-band-gap nature. An angle-resolved photoelectron spectroscopy study identified the states within the band gap, and the resonant photoemission technique and DFT simulation clarified that the origin of the gap states was anisotropic localization of electrons around V–O tetrahedra. Water absorption enhances the gap states, which dramatically impedes the kinetics of water oxidation reactions. Our results reveal the role of gap states in water oxidation reactions and offer new insights into manipulating gap states in 3d transition metal oxides.

Automated Active Space Selection with Dipole Moments.

Multireference calculations can provide accurate information of systems with strong correlation, which have increasing importance in the development of new molecules and materials. However, selecting a suitable active space for multireference calculations is nontrivial, and the selection of an unsuitable active space can sometimes lead to results that are not physically meaningful. Active space selection often requires significant human input, and the selection that leads to reasonable results often goes beyond chemical intuition. In this work, we have developed and evaluated two protocols for automated selection of the active space for multireference calculations based on a simple physical observable, the dipole moment, for molecules with nonzero ground-state dipole moments. One protocol is based on the ground-state dipole moment, and the other is based on the excited-state dipole moments. To evaluate the protocols, we constructed a dataset of 1275 active spaces from 25 molecules, each with 51 active space sizes considered, and have mapped out the relationship between the active space, dipole moments, and vertical excitation energies. We have demonstrated that, within this dataset, our protocols allow one to choose among a number of accessible active spaces one that is likely to give reasonable vertical excitation energies, especially for the first three excitations, with no parameters manually decided by the user. We show that, with large active spaces removed from consideration, the accuracy is similar and the time-to-solution can be reduced by more than 10 fold. We also show that the protocols can be applied to potential energy surface scans and determining the spin states of transition metal oxides.

Theoretical Understanding of Reactions of Rhenium and Ruthenium Tris(thiolate) Complexes with Unsaturated Hydrocarbons: Noninnocent Nature of the Ligand, Mechanism, and Origin of Differential Reactivity

In retrospect to the complexity induced by the noninnocent ligands in identifying the transition metal's oxidation state and correlating the ligand's noninnocence with reactivity, the reactions of alkene/alkyne addition to rhenium/ruthenium tris(thiolate) complexes are particularly good cases for shedding light on the chemistry of the dppbt ligand, including its noninnocent nature, ligand-centered mechanism, and origin of differential reactivity. Density functional theory (DFT) combined with the high-level ab initio calculations performed herein demonstrates that, upon alkene/alkyne addition, the orbital symmetry properly regulates the reaction to form ligand-centered cis-interligand dithioethers as the most favorable pathway. The neutral and cationic Re and Ru dithioethers are revealed via DFT calculations to be in a low-spin ground state; on the contrary, high-level ab initio methods confirm that the dicationic Re-dithioethers exhibit obvious multireference character with antiferromagnetic coupling between Re-dyz and S1-py. The metal-stabilized thiyl radicals play a pivotal role in delivering the reactivity of [RuL3]+ and [ReL3]+/2+ toward alkene/alkyne rather than [ReL3], where [RuL3]+ and [ReL3]+/2+ present significant radical characters on ligand S2, yet neutral [ReL3] has little such feature, from which differential reactivity arises. Faster styrene addition to Ru tris(thiolate) in contrast to Re tris(thiolate) has been properly interpreted using DFT calculations with major products assigned. The deeper understanding gained in this work would illuminate further experimental exploration in adding alkene/alkyne to other metal-stabilized thiyl radicals.

Bandgap and defects regulation of La2−xAxNi1−yByO4+δ (A = K, Sr, B = Co, Mn) Ruddlesden-Popper type perovskites for efficient photocatalytic hydrogen evolution

The doping of certain exotic metal ions in photocatalysts may produce surface defects (such as oxygen vacancies) as well as bandgap changes, which may play an important role for their photocatalytic performance. In this work, bandgap modulation by precise control of defect concentration and transition metal oxidation state mainly owning to A-site doping and the B-site cations were achieved for the Ruddlesden-Popper structure (La2NiO4). The effects of K+, Sr2+, Co2+, Mn2+ single-mental doping and co-doping on the photocatalytic activity of La2NiO4 photocatalysts were investigated. It was found that the co-doping samples had a significant synergistic effect on the optical properties and photocatalytic activity of La2NiO4 photocatalysts. All the single-metal-doped La2NiO4 photocatalysts exhibited higher photocatalytic activity than their pristine counterpart. Moreover, the Sr2+, Co2+ co-doped La2NiO4 exhibited the best optical properties, photocatalytic activity and a the hydrogen production rate of La1.9Sr0.1Ni0.8Co0.2O4+δ more than 29 times of pure La2NiO4, has been achieved. Further analysis indicates that the surface oxygen vacancies as active sites promote efficient separation and migration of photogenerated carriers. The co-doped photocatalyst not only has a narrower bandgap, but also owns a more favorable conduction band position for photoinduced electron reduction. Overall, this work provides a simple method for introducing oxygen defects and bandgap modulation in Ruddlesden-Popper structure through metal ion doping. It is also expected to enlarge the understanding of the structure-dependence of the photocatalytic performance of perovskite materials.

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn,Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

Elucidating Catalytic Sites Governing the Performance toward the Hydrogen Evolution Reaction in Ternary Nitride Electrocatalysts

Proton exchange membrane electrolyzers are considered the most advanced devices for producing green hydrogen by water electrolysis. Their development requires catalytic materials that are stable under acidic conditions and drive the hydrogen evolution reaction (HER) forward efficiently which makes research into the identification of the catalytic sites important. We report that free-standing Co2Mo3N and Ni2Mo3N achieve overpotentials of 149 ± 8 and 158 ± 10 mV (in 0.5 M H2SO4) at a benchmark current density of 10 mA cm–2. Both nitrides remained stable and consistently deliver current densities >500 mA cm–2 at a potential as low as 308 ± 22 mV when they were immobilized on nickel foam. Replacing Ni for Fe in Ni2Mo3N leads to FexNi2–xMo3N (0.5 ≤ x ≤ 1.25) that show a decrease in catalytic activity as the value of x increases which confirms that Ni (rather than Mo and N) sites are catalytically active. The X-ray photoelectron spectroscopy data additionally suggests that preserving the low oxidation states of transition metals in the nitrides is important for achieving good catalytic performance toward the HER in acidic electrolytes.

Porous Structural Properties of K or Na-Co Hexacyanoferrates as Efficient Materials for CO2 Capture.

The stoichiometry of the components of hexacyanoferrate materials affecting their final porosity properties and applications in CO2 capture is an issue that is rarely studied. In this work, the effect that stoichiometry of all element components and oxidation states of transition metals has on the structures of mesoporous K or Na-cobalt hexacyanoferrates (CoHCFs) and CO2 removal is reported. A series of CoHCFs model systems are synthesized using the co-precipitation method with varying amounts of Co ions. CoHCFs are characterized by N2 adsorption, TGA, FTIR-ATR, XRD, and XPS. N2 adsorption results reveal a more developed external surface area (72.69-172.18 m2/g) generated in samples containing mixtures of K+/Fe2+/Fe3+ ions (system III) compared to samples with Na+/Fe2+ ions (systems I, II). TGA results show that the porous structure of CoHCFs is affected by Fe and Co ions oxidation states, the number of water molecules, and alkali ions. The formation of two crystalline cells (FCC and triclinic) is confirmed by XRD results. Fe and Co oxidation states are authenticated by XPS and allow for the confirmation of charges involved in the stabilization of CoCHFs. CO2 removal capacities (3.04 mmol/g) are comparable with other materials reported. CO2 adsorption kinetics is fast (3-6 s), making CoHCFs attractive for continuous operations. Qst (24.3 kJ/mol) reveals a physical adsorption process. Regeneration effectiveness for adsorption/desorption cycles indicates ~1.6% loss and selectivity (~47) for gas mixtures (CO2:N2 = 15:85). The results of this study demonstrate that the CoHCFs have practical implications in the potential use of CO2 capture and flue gas separations.

Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core–hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe–Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides─TiO2, α-Cr2O3, β-MnO2, α-Fe2O3, Co3O4, NiO, CuO, and ZnO─are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core–valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Influence of Co:Fe:Ni ratio on cobalt Pentlandite’s electronic structure and surface speciation

X-ray photoemission spectroscopy (XPS) was used to investigate the electronic structure and oxidation state of transition metals in synthetic cobalt pentlandites (Fe,Co,Ni)9S8 with measured stoichiometries of Fe4.85Ni4.64S8, Co0.13Fe4.68Ni4.71S8, Co2.73Fe3.18Ni3.16S8, Co5.80Fe1.63Ni1.59S8 and Co8.70S8. The addition of cobalt was found to decrease the bond length and hence decrease the unit cell dimensions of the cobalt pentlandite crystal structure by up to 0.18 Å. High resolution XPS S 2p spectra show increases in the binding energies of bulk 5-coordinated (162.1 eV) and surface 3-coordinated (160.9 eV) sulfur (≈0.2 eV) with increasing Co concentration and decreasing bond lengths. As the Co concentration increases, variation in metal site occupation decreases, resulting in smaller S 2p FWHMs due to a dominant single Co-S state rather than mixed Fe-S, Ni-S and Co-S states with similar binding energies. The S 2p high binding energy tail, previously identified as a S 3p- Fe 3d ligand to metal transfer in Fe chalcogenides, shows a marked decrease in intensity as the concentration of Co increases, that is attributed to a decreased probability of ligand-to-metal charge transfer as the eg orbitals are filled. The transition metal XPS 2p spectra were modelled using CTM4XAS to investigate metal site occupation and ligand-to-metal charge transfer. Fe, Co, and Ni were all best simulated using a tetrahedral symmetry and 2+ oxidation state, their 2p3/2 and 2p1/2 peaks occurred at 706.9 and 719.9 eV, 778.2 and 793.1 eV, and 852.8 and 870.0 eV, respectively. A negative charge transfer energy confirms the high binding energy tail results from S3p-Fe3d ligand to metal charge transfer. This increased understanding of the pentlandite electronic structure will provide a basis for the refinement of mineral processing techniques and allow for a reduced environmental impact from limited efficiency.

Method development for the investigation of Mn2+/3+ , Cu2+ , Co2+ , and Ni2+ with capillary electrophoresis hyphenated to inductively coupled plasma-mass spectrometry.

The lifetime of lithium ion batteries (LIBs) decreases under continuous cycling due to various degradation processes, such as dissolution of transition metals (TMs) from the electrodes. Therefore, suitable methods to analyze the oxidation states of TMs are mandatory to better understand the dissolution mechanisms of TMs from positive and negative electrodes (LIBs). To investigate the dissolution of Mn2+ and Mn3+ in electrolytes of LIBs, a previously implemented capillary electrophoresis (CE) method with UV/Vis spectroscopy detection was further developed with the aim of higher sensitivities and additional detection of other dissolved divalent TMs such as Co2+ , Ni2+ , and Cu2+ . Therefore, inductively coupled plasma-mass spectrometry was applied instead of UV/Vis for detection. This also allows the use of Ga3+ instead of the previously used Cu2+ as an internal standard, which solves the limitation of this method for cycled LIBs due to copper dissolution from the copper-based current collector. The CE buffer based on sodium diphosphate as complexing agent for the stabilization of Mn3+ and cetyltrimethylammonium bromide as dynamic capillary wall modifier was optimized in terms of concentrations and pH. Finally, both manganese species and Co2+ , Ni2+ , and Cu2+ could be analyzed within 15min. With this improved method, the dissolution of TMs in LIBs for positive electrode materials such as LiNi0.5 Mn1.5 O4 (LNMO) or LiNix Coy Mnz O2 (NCM, x+y+z=1) can be studied in future in more detail.