Cation Doping Research Articles

Mechanism of p-Type Heteroatom Doping of Lithium Stannate for the Photodegradation of 2,4-Dichlorophenol: Enhanced Hole Oxidative Capability and Concentrations.

A systematic evaluation of enhancing photocatalysis via aliovalent cation doping is conducted. Cation In3+, being p-type-doped, was chosen to substitute the Sn site (Sn4+) in Li2SnO3, and the photodegradation of 2,4-dichlorophenol was applied as a model reaction. Specifically, Li2Sn0.90In0.10O3 exhibited superior catalytic performance; the photodegradation efficiency reached about 100% within only 12 min. This efficiency is far greater than that of pure Li2SnO3 under identical conditions. Density functional theory calculations reveal that introducing In3+ increased the electron mobility, yet decreased the hole mobility, leading to photogenerated carrier separation. However, photoluminescence and time-resolved photoluminescence suggest that In3+ induced nonradiative coupling in the matrix, reducing the photogenerated carrier separation ratio compared with that of Li2SnO3. The optical band gap of Li2Sn0.90In0.10O3 was almost unchanged compared with that of Li2SnO3 via ultraviolet-visible absorption. The increased photocatalytic efficiency was ascribed to the lower valence band position and enhanced hole concentrations by valence band X-ray photoelectron spectroscopy and electrochemical measurements. Finally, a 2,4-dichlorophenol degradation pathway, an intermediate toxicity assessment, and a photocatalytic mechanism were proposed. This work offers insights into designing and optimizing semiconductor photocatalysts with high performance.

On the origin of the improved hydrogen evolution reaction in Mn- and Co-doped MoS2.

In the field of hydrogen production, MoS2 demonstrates good catalytic properties for the hydrogen evolution reaction (HER) which improve when doped with metal cations. However, while the role of sulfur atoms as active sites in the HER is largely reported, the role of metal atoms (i.e. molybdenum or the dopant cations) has yet to be studied in depth. To understand the role of the metal dopant, we study MoS2 thin films doped with Co and Mn ions. We identify the contribution of the electronic bands of the Mn and Co dopants to the integral valence band of the material using in situ resonant photoemission measurements. We demonstrate that Mn and Co dopants act differently: Mn doping favors the shift of the S-Mo hybridized band towards the Fermi level, while in the case of Co doping it is the less hybridized Co band that shifts closer to the Fermi level. Doping with Mn increases the effectiveness of S as the active site, thus improving the HER, while doping with Co introduces the metallic site of Co as the active site, which is less effective in improving HER properties. We therefore clarify the role of the dopant cation in the electronic structure determining the active site for hydrogen adsorption/desorption. Our results pave the way for the design of efficient materials for hydrogen production via the doping route, which can be extended to different catalytic reactions in the field of energy applications.

Halogen doping of p-type inorganic hole transport layer: electronic nature-based dopant engineering for modulating hole selectivity in inverted planar perovskite solar cells

The role of metal cation and halide anion dopants in regulating the properties of NiOx hole transport layer (HTL) was explored. This study confirms that halide dopants also increase the Ni3+ defect density and work function of p-type inorganic HTL.

Doping strategies for inorganic lead-free halide perovskite solar cells: progress and challenges.

In recent years, remarkable advancements have been achieved in the field of halide perovskite solar cells (PSCs). However, the commercialization of PSCs has been impeded by challenges such as Pb leakage and the instability of hybrid organic-inorganic perovskites (HOIPs). Hence, the future lies in the development of environmentally friendly inorganic lead-free halide perovskites (LFHPs) based on elements like Sn, Ge, Bi, Sb, and Cu, which show great promise for photovoltaic applications. However, LFHP photovoltaic cells still face challenges such as low efficiency, poor film quality, and stability in comparison to HOIPs. These limitations significantly hinder their further development. To address these issues, element doping strategies, including cationic and anionic doping, as well as the use of additives, are frequently employed. These strategies aim to improve film quality, passivate defects, reduce the band gap, and enhance device performance and stability. In this paper, we aim to provide a comprehensive review of the recent research progress in doping strategies for LFHPs.

Substituted Bi3.5Mg1.8Nb2.7O13.8 pyrochlores with transition metals (Zn, Ni and Cd): Doping mechanism, structure and electrical properties

The influence of transition metals (Zn, Cd and Ni) on bismuth magnesium niobate (Bi3.5Mg1.8Nb2.7O13.8, BMN) pyrochlores was investigated. Considering the difference in their ionic radii, Cd was postulated to substitute Mg at the eight-fold coordinated A-site whereas Zn and Ni were to replace Mg at the six-fold coordinated B-sites of the host structure. Solubility limits in these substitutional pyrochlore solid solutions were determined to be 0.4, 0.1 and 0.3 for Zn, Ni and Cd, respectively. Upon introducing these divalent cation dopants, notable changes in the lattice parameters and grain sizes were observed with increasing dopant concentration. The high dielectric constants, ?? of Zn, Cd and Ni doped BMN pyrochlores were discernible with their bulk values of 195-216, 173-195 and 153-195, respectively. The highest ?? of 216 was recorded for the composition with Zn = 0.1 in contrast to the lowest value of 153 found in the composition with Ni = 0.1. Meanwhile, these highly insulating divalent cation doped BMN pyrochlores also exhibit low dielectric losses, tan _ whose values are in the order of 10?3. The excellent dielectric properties allow these novel pyrochlore phases to be potentially applied for the energy storage applications.

Revealing the evolution of local structures in the formation process of alkaline earth metal cation-containing zeolites from glasses.

Alkaline earth metal cations are ubiquitously present in natural zeolites but less exploited in synthetic zeolites due to their low solubility in water, and hence it remains elusive how they contribute to zeolite formation. Herein, harmotome, a PHI-type zeolite with Ba2+, is readily synthesized from a Ba-containing aluminosilicate glass. This glass-to-zeolite transformation process, in particular the structure-regulating role of Ba2+, is investigated by anomalous X-ray scattering and high-energy X-ray total scattering techniques. The results demonstrate that the steady Ba2+-aluminosilicate interactions not only help prevent the precipitation of barium species under alkaline synthetic conditions, but also dictate the local structures with distinct interatomic distances between the Ba2+ and the surrounding aluminosilicate species throughout the transformation process, which lead to the successful formation of harmotome without detectable impurities. This study highlights the usefulness of the comprehensive X-ray scattering techniques in revealing the formation scheme of the zeolites containing specific metal species. In addition, a promising alternative approach to design and synthesize zeolites with unique compositions and topologies by using well-crafted glasses with suitable metal cation dopants is demonstrated.

Tailoring the Electrophilicity of Co Surface for Favorable toward Alkaline Oxygen Evolution Reaction by Metal Cation Doping

The chemical coupling of molybdenum carbide (Mo2C) to cobalt (Co) promotes oxygen evolution reaction (OER) kinetics on the Co surface by making the surface more electrophilic. Here, to gain a deeper understanding of the effects of the surface electrophilic properties on the OER kinetics of Co and to obtain high OER activity, Fe and Ni are additionally incorporated into Co nanoparticles that are coupled with Mo2C nanoparticles (Co‐Mo2C). Considering the oxidation states of Fe (Fe3+), Co (Co2+/Co3+), and Ni (Ni2+) ions, Fe and Ni are expected to affect the electronic structure of Co in the opposite direction. Lewis acidic Fe3+ doping makes the Co surface oxide more electrophilic, promoting the formation of OER‐active CoOOH by strongly attracting hydroxide ions (OH−). Thus, the OER kinetics is facilitated on the Co surface of Fe‐doped Co‐Mo2C, resulting in a significantly lower overpotential for the OER. On the other hand, the Ni2+ doping makes the Co surface oxide less electrophilic, leading to an increase in the overpotential for the OER. Tailoring the electrophilic properties of the Co surface is presented as a key parameter in the design of a Co‐based OER catalyst for alkaline water electrolysis.

Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery

P2-type Na0.67Ni0.33Mn0.67O2 has attracted considerable attraction as a cathode material for sodium-ion batteries owing to its high operating voltage and theoretical specific capacity. However, when the charging voltage is higher than 4.2 V, the Na0.67Ni0.33Mn0.67O2 cathode undergoes a detrimental irreversible phase transition of P2-O2, leading to a drastic decrease in specific capacity. To address this challenge, we implemented a Cu-doping strategy (Na0.67Ni0.23Cu0.1Mn0.67O2) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal–oxygen (TMO) bonds, particularly the MnO bond and inhibiting interlayer slip during deep desodiation. As a result, the irreversible phase transition voltage is delayed, with the threshold voltage increasing from 4.2 to 4.4 V. Ex-situ X-ray diffraction measurements revealed that the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode maintains the P2 phase within the voltage window of 2.5–4.3 V, whereas the P2-Na0.67Ni0.33Mn0.67O2 cathode transforms entirely into O2-type Na0.67Ni0.33Mn0.67O2 when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode occurred at 4.6 V, indicating that Cu2+ doping enhances the stability of the layer structure and increases the threshold voltage. The resulting Na0.67Ni0.23Cu0.1Mn0.67O2 cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g−1) and retaining more than 78 % of its capacity after 500 cycles.

Effective approach to improve photocatalytic dye degradation ability of ZnO based nanopowder: simultaneous cationic and anionic doping, and enzyme coupling

Effective approach to improve photocatalytic dye degradation ability of ZnO based nanopowder: simultaneous cationic and anionic doping, and enzyme coupling

Modulate the Strong Exciton Effect by Na+ Coordination-Induced Trap States: Efficient Photocatalytic H2O2 Production.

Due to the strong Coulomb interaction, in most polymer photocatalysts, electron-hole pairs exist in the form of excitons rather than free charge carriers. The giant excitonic effect is a key obstacle to generating free charge carriers. Therefore, effectively regulating the exciton effect is the first step to achieving optimized carrier separation. Here, we used C-ring/g-C3N4 as the prototypical model system to design a photocatalyst with a Na-coordination-induced trap state. We demonstrate that the excitons can be effectively dissociated into charge carriers by combining with the trap state formed by Na doping sites. Encouragingly, signals from the dissociation of excitons into carriers were observed by ultrafast transient spectroscopy. Benefiting from the enhanced exciton dissociation, Na-C/CN displayed a H2O2 production rate of 17.4 mmol·L-1·h-1 with an apparent quantum efficiency up to 26.9% at 380 nm, which is much higher than many other g-C3N4-based photocatalysts. This work explains the effect of cation doping on the exciton-carrier behavior in polymers. Also, it provides a new way to regulate the exciton effect.

(Invited) Oxide Ion Conductivity and Oxide Vacancy Diffusion Coefficient of Stabilized ZrO2 and Doped CeO2 Discussed Based on Classical Defect Chemistry with Particular Focus on Valence State (2+, 3+, and 4+) of Doped Cations

So far tremendous works have been reported on the oxide-ion conductivity of fluorite-type oxides typically stabilized zirconia and doped ceria. It is well known that the oxide-ion conductivity vs. dopant concentration plot shows maximum around 10 mol percent dopant concentration and the maximum conductivity varies with the dopant species. Interests tend to be focused on the higher and the stabler conductivity. For the aim, explorations are made on the dopant survey and the phase stability of the solid solution.Generally, conductivity is considered proportional to the product of carrier concentration and carrier mobility. For the fluorite-type oxide ion conductors, it is considered that carrier is oxygen vacancy, V O ··, and mobility is proportional to oxygen vacancy diffusion coefficient, DV . However, such defect chemical approach to the fluorite-type oxides is not popular. When we assume that the mobility is constant following the classical hypothesis, we cannot explain the conductivity maximum.The author has been trying to discuss the oxide-ion conduction in fluorite-type oxide solid solutions in terms of oxygen vacancy concentration, cV ,and DV .Using the literature data on conductivity and composition relationship, the author has already calculated DV for some compositions of stabilized zirconia and doped ceria as a function of composition. The data have been published in a short review [1] and books [2,3]. However, the published graphical displays were so crowded that the physical meanings seem still unclear.In this presentation, discussion is made on the conductivity of stabilized zirconia and doped ceria in the following order to make clear the characteristic feature of DV with the variation of valence state and chemical species of dopant oxides.1: zirconia base solid solutions 1a: with divalent cation oxides 1b: with trivalent cation oxides2: ceria base solid solutions 2a: with divalent cation oxides 2b: with trivalent cation oxides3: stabilized zirconia – ceria solid solutions 3a: stabilized zirconia base solid compositions 3b: ceria base solid compositions 3c: trivalent cerium formation by partial reductionIt is clarified that DV of fluorite type oxide is affected by the dopant cation not only by different valent but also by the same valent cations.

(Invited) High-Nickel Layered Oxide Cathodes: Crack Vs. Surface Reactivity

The appetite for long driving range and high energy density is pushing the nickel content in layered oxide cathodes for lithium-ion batteries. The lying of Ni3+/4+:3d band slightly above the Co3+/4+:3d band enables to extract more lithium during charge without encountering oxygen release from the lattice in high-nickel cathodes compared to that in LiCoO2. However, as the nickel content increases in the lattice, the cells suffer from cycle and thermal instability. The cycle instability has largely been attributed in the literature to the formation of cracks caused by the occurrence of H2-H3 phase transition with large lattice parameter changes during high degree of charge. Several approaches, such as doping with other cations and microstructural control of the primary and secondary particles, have been extensively pursued to overcome the challenges and improve the cycle life.With a systematic investigation of a number of cathode compositions and their characterization after extensive cycling, this presentation will illustrate that surface reactivity of high-nickel cathodes with electrolyte plays a dominant and decisive role on cycle life. Even cathodes like LiNiO2 with 100% Ni without any cation doping or surface coating could be cycled with high capacities over a large number of cycles with an appropriate choice of electrolyte that suppresses surface reactivity. Electrolytes that are stable to slightly higher voltages, even by as low as 100 mV, compared to the conventional electrolytes alter the surface reconstruction pathway on cathode surface in contact with electrolyte. Electrolytes with better high-voltage stability favor the formation of mildly reduced LiNi2O4 spinel on the surface without much oxygen release from the surface. In contrast, electrolytes that have inferior stability to higher voltages lead to a drastic reduction of the cathode surface to highly reduced Ni3O4 spinel and NiO rock salt phases with significant oxygen loss from and volume change on the surface, which could cause crack initiation on the surface, followed by their propagation into the bulk. Interestingly, the fast 3-dimensional lithium-ion and electron transport in spinel LiNi2O4 supports a facile charge transfer through the cathode-electrolyte interphase (CEI), resulting in better cycle life, in contrast to the poor lithium-ion transport in both spinel Ni3O4 and rock salt NiO phases, resulting in poor charge-transfer through the CEI and inferior cycle life. With robust electrolyte formulations with better high-voltage stability, long cycle life could be achieved even with the repeated occurrence of the H2-H3 phase transitions over a large number of cycles.

Inhibiting Oxygen Redox to Boost Ni Activity Enables Highly Stable Properties in Na0.67Ni0.33Mn0.67O2

Cation doping is a conventional and effective strategy for promoting favorable properties in layered oxides. Here, Mg and Sc were used as dopants in Na0.67Ni0.33Mn0.67O2 to significantly improve the cycling stability in the high voltage range. It was found that both dopants could successfully suppress the P2 to O2 phase transition, but induced a sloping curve as well as a short high voltage plateau above 4.0 V, respectively. We demonstrate that these different electrochemical behaviors are indicative of the suppression of oxygen redox by means of resonant inelastic X-ray scattering (RIXS) measurements. Furthermore, both X-ray absorption spectroscopy (XAS) and density functional theory show that Mg doping could enhance Ni activity while simultaneously suppressing O activity. However, these results are in contrast to other Mn-based layered oxides where Mg is known to effectively trigger the oxygen redox. We address this contradiction by proposing a competing mechanism between Ni and Mg that impacts differences in oxygen activity in Na0.67MgxNi0.33-xMn0.67O2 (x≤0≤0.33). These findings will provide a new direction in understanding the effects of cation doping on the electrochemical behavior of layered oxide materials. Figure 1

Conductivity Increase As a Result of the Disappearance of Short-Range Ordering in a Scandia-Zirconia Electrolyte

Solid oxide cell electrolytes and structures have been greatly optimised to offer high performance and good durability for commercial implementation. This has highlighted an underlying issue where the slow, long-term conductivity degradation in the generally used fluorite structured oxide-ion conducting doped zirconias known to be related to reorganisation of their defect short-range structures. Among the doped zirconia systems, scandia stabilised zirconia (SSZ) gives the highest ionic conductivity due to the smallest mismatch between the dopant and host cations. However, owing to these similar and small sizes, the system is more prone to defect short-range ordering. This leads to a complex phase diagram of the Sc2O3-ZrO2 system and, depending on the composition, its ionic conductivity ages more significantly than other zirconia systems on high temperature annealing. Here, we report a magnesia and india co-doped scandia stabilised zirconia (IMSSZ) composition that offers high ionic conductivity, 0.14 at 850 ℃ and 0.4 at 1000 ℃, improved phase stability in both macro- and micro scales, and long-term conductivity stability. The conductivity after annealing the sample at 850 ℃, for different times, 0 (as-sintered), 167 and 426 hours, is correlated to the microdomain structural evolution. We show the initial of conductivity increase over time as a result of eliminating the defect short-range ordering using electron diffraction and powder neutron diffraction, resulting in a long-term stable disordered structure with improved conductivity. We will discuss here the thermodynamic driving forces for the breakdown of defect short-range ordering, and how right co-dopant sizes may play a role. Figure 1

Progress and Prospects with Cationic and Anionic Doping of Solution-Processed Kesterite Solar Absorbers: A Mini-Review

Progress and Prospects with Cationic and Anionic Doping of Solution-Processed Kesterite Solar Absorbers: A Mini-Review

Enhancement in the Rate of CO2 Photoreduction Using Divalent Strontium Cation (Sr2+) Doped TiO2 Nanotube Arrays

The sunlight-assisted conversion of CO2 into portable value-added products has the potential to address both the energy and environmental challenges faced by the world. Therefore, we currently witness intense research activity and substantial progress in the field of CO2 photoreduction. Anatase TiO2 nanotube arrays (TNTAs) formed by electrochemical anodization constitute a well studied semiconductor photocatalyst with excellent optoelectronic properties and photocatalytic behavior [1, 2]. Yet, many aspects related to structure, surface composition, adsorbates, reaction intermediates, rate of reaction, multi-step electron transfer steps, etc. remain poorly understood even for TiO2. This led to our interest in forming surface coatings and bulk nanocomposites of TNTAs with metal oxides of Sr, Mg, Ca, Zn etc, which provide basic sites on the surface of the catalyst for the chemisorption of CO2 molecules. The higher basicity of hydroxyl groups associated with alkaline earth metals generate carbonate and bicarbonate adsorbates which enhance the reactivity of CO2 [3]. Divalent metal cations are also known to influence and modify the crystallographic texture of TiO2 during the annealing treatment step at elevated temperature [4].Here we performed divalent strontium cation (Sr2+) doping of anodically grown TNTAs using two different techniques – an electrochemical cathodic treatment in Sr2+-containing electrolytes and a wet impregnation technique using an aqueoussolution of strontium salts. The X-ray diffraction pattern and Raman spectra indicate the formation of Sr compounds at the TNTA surface. Sr-doping results in modification of the crystallographic texture of anatase phase TNTAs, increased lattice strain, reduced crystal size, and phonon confinement. Subtle changes are observed in the infrared spectra of the CO2 adsorbed on the Sr-doped TNTAs specifying a larger prevalence of monodentate and bidentate carbonate species on the surface. This attributes to the higher alkalinity of surface hydroxyls bound to Sr in comparison to the Ti-bound hydroxyls. In addition to linearly adsorbed CO2, a smaller population of bent CO2 molecules on the surface is observed, which suggests an enhanced stabilization of the carbon dioxide anion radical on the Sr-doped TNTA surfaces. Finally, under AM1.5G one sun illumination, the cathodized TNTAs produce 25 µmolg-1hr-1 whilethe wet impregnated Sr-doped TNTAs produce 17 µmolg-1hr-1 of CO which are nearly 3 and 2 times the amount of CO generated by bare TNTAs (7.7 µmolg-1hr-1) respectively.REFERENCES: Vahidzadeh, Ehsan, et al. "Asymmetric multipole plasmon-mediated catalysis shifts the product selectivity of CO2 photoreduction toward C2+ products." ACS Applied Materials & Interfaces13.6 (2021): 7248-7258.Zeng, Sheng, et al. "Optical control of selectivity of high rate CO2 photoreduction via interband-or hot electron Z-scheme reaction pathways in Au-TiO2 plasmonic photonic crystal photocatalyst." Applied Catalysis B: Environmental267 (2020): 118644.Kwon, Stephanie, et al. "Alkaline-earth metal-oxide overlayers on TiO 2: application toward CO 2 photoreduction." Catalysis Science & Technology6.21 (2016): 7885-7895.Kisslinger, Ryan, et al. "Preferentially oriented TiO2 nanotube arrays on non-native substrates and their improved performance as electron transporting layer in halide perovskite solar cells." Nanotechnology30.20 (2019): 204003.

Doping Strategy to Improve the Electrochemical Behavior of Lithium Rich Transition Metal Oxides As Cathodes for Lithium-Ion Batteries

Lithium-rich transition metal oxides are considered a promising cathodic solutions in Li-ion batteries to meet the growing energetic demand, thanks to their high specific capacity and operational voltage [1-2]. Li-rich layered materials have a general stoichiometry Li1+xTM1-xO2, in which TM is a blend of transition metals, as Ni, Mn, Co [2]. They stand out due to their high lithium concentration and specific capacity [1-2]. The higher capacity of Lithium-Rich can be attributed to the specific anionic redox reaction in the bulk at above 4.5 V. Nevertheless, large irreversible capacity loss in the first cycle, poor rate capability, mean working potential decay upon cycling and cobalt content are still major drawbacks that are hindering commercialization. In order to make these materials up scalable to industrial production, it is important to consider the reduction of costs and the toxicity of raw materials, in particular cobalt. Cobalt reduction has been identified as major driver to improve the environmental benignity of batteries and the sustainability of the overall production-consumption-recycling lifecycle. Several efforts to reduce these problems have been made, including the development of different synthetic strategies, doping and surface modification. The most, widely explored, chemical strategy to mitigate the voltage decay and structural degradation in Lithium Rich cathodes is the doping. Elemental doping can be divided into cation and anion doping. As an example, incorporation of redox inactive metals, such as Al, Zr, Ti, has been proposed in order to stabilize the lattice as well as the partial replacement of lithium ions with other alkali cations, e.g. K and Na; or doping the oxygen anion sublattice. In this communication, we present our doping approaches. In particular, the simultaneous replacement of cobalt in the layered structure with aluminum and lithium (over-lithiation). Moreover, we present over-lithiated Co-free materials where the introduction of different amount of iron has been investigated. The characterization of these novel layered materials is reported in terms of composition, structure, morphology and electrochemical performances. These approaches can be applied as a strategy to mitigate the voltage decay and reduce the Cobalt content.[1] Ji, X. et al. Journal of Power Sources 487, (2021).[2] Zhao, S. et al. Angewandte Chemie - International Edition 60, 2208–2220 (2021).

(Invited) An Overview of the Behavior of the LixNiO2 Electrode at High Voltage

Nickel-rich layered oxides Li(Ni,M)O2 (M = Co,Mn, ..) are widely adopted as electrode materials for electric vehicles. They suffer from a loss of capacity when the cells are charged above 4.15 V vs Li/Li+. This limitation is generally attributed to: (i) sharp decrease of the interslab distance at the end of charge which induces cracks in the particles, (ii) slab gliding leading to the formation of the H4 phase, (iii) reaction with the liquid electrolyte, which leads to formation of a rocksalt type structure. The cycling behavior can be improved by texture optimization (concentration gradient, structural orientation). Doping, coating with various cations lead, in some cases, to significant improvement of the cycling properties when the cells are cycled at high voltage. However, the true mechanisms involved at high voltage are not fully understood. In order to overcome the effect of M cations our work was focused on LiNiO2.GITT and in situ XRD experiments were performed at very low rate in various voltage range (3.8 -4.3 V). On the "4.2-4.3 V plateau" the R2 phase is transformed simultaneously in R3, R3 with H4 stacking faults and H4. When the cell voltage is extended to 4.6V the H4 phase (NiO2) is formed. Cells were charged up to 4.3, 4.5 and 4.6V with a long potentiostatic step (144 hours).The recovered materials were characterized by S-XRD and HAADF electron microscopy. In all cases a mixture of R3 and H4 phases were obtained. The electron microscopy show that both phase R3 an H4 are mixed inside the whole particles. NiO2 is expected to be an ionic insulator (no Li+) and an electronic insulator (d6 low spin). Since there is no formation of a shell of NiO2 around the particles, the difficulty to deintercalation Li suggests that at high voltage another mechanism must be considered.To try to have a better understanding of the involved process several electrochemical studies were performed in various experimental conditions. As the charge proceeds above 4.17 V the cell polarization dramatically increases, hindering further Li deintercalation. In discharge, such polarization decreases immediately. Also and upon cycling, the polarization increases at each charge above 4.17 V. In discharge, the capacity and dQ/dV features below 4.1 V remain constant and unaffected, suggesting that the bulk of the material did not undergo significant structural defect.This study shows that the change in polarization between the charge and the discharge results from the electrochemical behavior of the grain surface rocksalt type phase having very low conductivity above 4.17 V and high conductivity below this threshold. The results presented here open a new approach to monitor the electrochemical behavior of the rocksalt type shell and manage its electrochemical properties, at high voltage, by cationic doping of the surface layer. It can explain the behavior observed with some dopants (usually referred as “infused” coatings) like tungsten.

Tuning CO adsorption states via enhanced metal-support interaction for boosting CO2 methanation activity of Ru/rutile-TiO2

Understanding the correlation between metal–support interactions (MSIs) and the adsorption states of reaction intermediates is critical for developing heterogeneous catalysts. To modulate MSIs and CO adsorption states, various Ru/Ti1-xCrxO2 catalysts were prepared by Cr3+ cation doping and used for CO2 methanation compared to the rutile TiO2-supported Ru catalyst (Ru/r-TiO2). The activity followed the order: Ru/Ti0.95Cr0.05O2 > Ru/Ti0.97Cr0.03O2 > Ru/Ti0.87Cr0.13O2 ≫ Ru/r-TiO2. The best Ru/Ti0.95Cr0.05O2 catalyst possessed CO2 turnover frequency (TOF) value at 200 °C 10.2 times higher than that on the Ru/r-TiO2. Cr3+ doping in the lattice of r-TiO2 support led to the Ru/Ti0.95Cr0.05O2 with higher dispersion of Ru and enhanced MSIs. A linear-CO*-II intermediate formed on the Ru/Ti0.95Cr0.05O2 with higher C-O vibrational frequency (2003 cm−1) than the linear-CO*-I at 1972 cm−1 on the Ru/r-TiO2, and the blue-shifted CO possessed higher activity for CO hydrogenation to CH4. The higher activity of the linear-CO* were accounting for the much higher TOF value of the Ru/Ti0.95Cr0.05O2. The strength of MSIs was proved to be highly correlated with CO adsorpton states including the balance of the strength of Ru-C and C-O bonds, which were found to be a crucial factor for the activity of supported Ru-based catalysts for CO hydrogenation.

Synthesis and Structural Characterization of Layered Ni+1/+2 Oxides Obtained by Topotactic Oxygen Release on Nd2−xSrxNiO4−δ Single Crystals

Layered nickelate oxides containing Ni1+/Ni2+ are isoelectronic to Cu2+/Cu3+ compounds and of present interest with respect to recent findings of superconductivity in a series of different compositions. It is thereby questionable why superconductivity is still rare to find in nickelates, compared to the much larger amount of superconducting cuprates. Anisotropic dz2 vs. dx2−y2 orbital occupation as well as interface-induced superconductivity are two of the main advanced arguments. We are here interested in investigating the feasibility of synthesizing layered nickelate-type oxides, where the Ni1+/Ni2+ ratio can be tuned by oxygen and/or cation doping. Our strategy is to synthesize Sr-doped n = 1 Ruddlesden–Popper type Nd2−xSrxNiO4+δ single crystals, which are then reduced by H2 gas, forming Nd2−xSrxNiO4−δ via a topotactic oxygen release at moderate temperatures. We report here on structural studies carried out on single crystals by laboratory and synchrotron diffraction using pixel detectors. We evidence the general possibility to obtain reduced single crystals despite their increased orthorhombicity. This must be regarded as a milestone to obtain single crystalline nickelate oxides, which further on contain charge-ordering of Ni1+/Ni2+, opening the access towards anisotropic properties.