Ni K-edge X-ray Absorption Spectroscopy Research Articles

Structural and physical properties of Ni 1−x V x alloys around and away from quantum critical point

We investigate the room temperature structure (global and local), temperature dependent magnetic and transport behaviour of Ni 1−x V x ( 0⩽x⩽0.13 ) alloys. Our Energy Dispersive Analysis of x-rays results show that the prepared compositions are stoichiometric. With increase in V doping, the compounds exhibit a quantum phase transition around x c = 0.12, where the ferromagnetic phase is suppressed. Our results show that all the compounds stabilize in face centred cubic structure at RT and the lattice parameter shows unusual behaviour close to x c . The magnetic and heat capacity studies show signature of Griffiths phase on either side of x c . From 25 K to the lowest collected temperature, we observe a linear T dependence of resistivity at x = 0.1 and around x c , which is separated by a Fermi-liquid region around x = 0.106. This suggests that the origin of the transport behaviour is different around the quantum critical point and away from it. Our Ni K-edge x-ray Absorption Spectroscopy results show that there is a significant reduction in the first coordination number around Ni central atom on doping. Further, with doping, there is distortion in the first coordination shell around Ni. This suggests, with V doping, the local structure around Ni is different from the global structure as obtained from the x-ray Diffraction results. Interestingly, with doping, we observe a direct connection between the extent of distortion at RT and the magnetic disorder obtained at 2 K. We believe our results will motivate the scientific community to further study the interplay between the structural disorder and quantum fluctuations with temperature at the local level.

Probing the Effect of Environments on the Stability of Ni2P Nanoparticle Electrocatalysts

Ni2P has been shown to be an effective catalyst for the hydrogen evolution reaction (HER), making it a promising replacement for Pt which is the current state of the art HER catalyst.1,2 H2 is a clean-energy critical fuel and chemical highlighted by the US Department of Energy’s “Hydrogen Shot,” which seeks to reduce “the cost of carbon-neutral hydrogen by 80% to $1 per 1 kg in 1 decade, highlighting the key role of hydrogen in implementing carbon-neutral solutions.”3 Ni2P is also a good or promising catalyst for other reactions, such as alcohol oxidation, hydrodesulfurization,4,5 nitrate reduction, and oxygen evolution reaction.6 However, Ni2P and other TMPs are susceptible to oxidation, which can lead to catalyst degradation and loss in certain conditions.7,8 Stability is an important part of catalysis, especially for achieving the goals of the Hydrogen Shot. Therefore, it is important to understand complex degradation mechanisms in order to better understand structure-activity relationships in these materials. In the present work, in situ Ni K-edge X-ray absorption spectroscopy (XAS) and ex situ P Kα and Kβ X-ray emission spectroscopy (XES) couple together to understand the degradation of Ni2P and show the effects of electrolyte pH, ligands, and air on Ni2P nanoparticles.5 nm Ni2P nanoparticles were synthesized by heating NiCl2 and tris(diethylamino)phosphine in oleylamine. The nanoparticles were drop cast onto carbon fiber paper electrodes and Si wafers or Au-coated wafers for P Kα and Kβ XES. Oleylamine ligands are removed by thermal annealing. Electrochemical experiments were conducted with a three-electrode cell using carbon fiber paper or Au-coated wafers deposited with Ni2P nanoparticles as working electrode, Ag/AgCl calibrated to the H2 redox couple as reference electrode, and graphite rod as counter electrode. P Kα and Kβ XES experiments were conducted inside a N2 glovebox to prevent unintentional air oxidation. Transmission-mode Ni K-edge XAS was conducted on a benchtop spectrometer.Upon exposure to air, as-synthesized Ni2P nanoparticles were studied via P Kα and Kβ XES. Results showed that the phosphide species converted to phosphate with analysis of intermediates well fit by a simple two-phase mixture. Phosphide:phosphate ratios were quantified using linear combination analysis, providing a powerful tool to analyze P speciation on various samples in an inert environment and with minimal loss of material, both advantages of this system over 31P magic angle spinning solid state nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy. Continued work looks at analyzing the effects of the ligand environment on the stability of nanoparticles in electrolytes spanning from acidic to basic pH. We hypothesize that ligands will significantly stabilize the nanoparticles against corrosion and that without ligands Ni2P readily converts to a nickel phosphate species upon exposure to electrolyte.References(1) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135 (25), 9267–9270. https://doi.org/10.1021/ja403440e.(2) Liu, P.; Rodriguez, J. A. Catalysts for Hydrogen Evolution from the [NiFe] Hydrogenase to the Ni2P(001) Surface: The Importance of Ensemble Effect. J. Am. Chem. Soc. 2005, 127 (42), 14871–14878. https://doi.org/10.1021/ja0540019.(3) Bullock, M.; More, K. Basic Energy Sciences Roundtable: Foundational Science for Carbon-Neutral Hydrogen Technologies (Report); DOESC Office of Basic Energy Sciences, 2022. https://doi.org/10.2172/1834317.(4) Layan Savithra, G. H.; Muthuswamy, E.; Bowker, R. H.; Carrillo, B. A.; Bussell, M. E.; Brock, S. L. Rational Design of Nickel Phosphide Hydrodesulfurization Catalysts: Controlling Particle Size and Preventing Sintering. Chem. Mater. 2013, 25 (6), 825–833. https://doi.org/10.1021/cm302680j.(5) Rodriguez, J. A.; Kim, J.-Y.; Hanson, J. C.; Sawhill, S. J.; Bussell, M. E. Physical and Chemical Properties of MoP, Ni2P, and MoNiP Hydrodesulfurization Catalysts: Time-Resolved X-Ray Diffraction, Density Functional, and Hydrodesulfurization Activity Studies. J. Phys. Chem. B 2003, 107 (26), 6276–6285. https://doi.org/10.1021/jp022639q.(6) Menezes, P. W.; Indra, A.; Das, C.; Walter, C.; Göbel, C.; Gutkin, V.; Schmeiβer, D.; Driess, M. Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting. ACS Catal. 2017, 7 (1), 103–109. https://doi.org/10.1021/acscatal.6b02666.(7) Ha, D.-H.; Han, B.; Risch, M.; Giordano, L.; Yao, K. P. C.; Karayaylali, P.; Shao-Horn, Y. Activity and Stability of Cobalt Phosphides for Hydrogen Evolution upon Water Splitting. Nano Energy 2016, 29, 37–45. https://doi.org/10.1016/j.nanoen.2016.04.034.(8) Parra-Puerto, A.; Ng, K. L.; Fahy, K.; Goode, A. E.; Ryan, M. P.; Kucernak, A. Supported Transition Metal Phosphides: Activity Survey for HER, ORR, OER, and Corrosion Resistance in Acid and Alkaline Electrolytes. ACS Catal. 2019, 11515–11529. https://doi.org/10.1021/acscatal.9b03359.

Operando High-Temperature X-Ray Absorption Spectroscopy of LiNiO2 Cathode Material during Synthesis

The increasing demand for higher energy density in batteries for automotive applications has led to a trend toward increasing the Ni content in layered oxide cathode materials to increase their reversible capacity. However, LiNiO2, which has the highest Ni content, shows a faster capacity drop during charge-discharge cycling, which is attributed to its inherent structural instability in both the pristine and delithiated states. Therefore, to address the former issue and to better understand the appropriate synthesis conditions for LiNiO2, we have studied the changes in the local structure of LiNiO2 during the synthesis process using operando analysis techniques. Recently, high temperature in situ X-ray diffraction during LiNiO2 synthesis was reported by Bianchini [1] and Weber [2], which confirmed the loss of lithium and oxygen and the migration of Ni to Li layers above 700 °C. To further understand the mechanism of Li/Ni mixing and oxygen loss, operando high-temperature X-ray absorption spectroscopy (XAS) was performed during the synthesis of LiNiO2 to clarify the electrical structure of Ni at high temperatures. Pre-heated Li x NiO2 (x = 0.76, 1.01, 1.26, and 1.51) precursors were prepared by heating LiOH·H2O and Ni(OH)2 at 480 °C for 24 h in air. The precursor was pelletized and set in a quartz sample holder. Operando high-temperature Ni K-edge XAS spectra of the precursor were obtained at BL9A and BL12C in Photon Factory (PF), High Energy Accelerator Research Organization (KEK) by a transmission mode. The operando spectra of all samples except Li/Ni = 0.76 revealed a clear edge shift to the lower energy side corresponding to nickel reduction above 700 °C. The Li/Ni = 0.76 sample has a lithium-deficient composition and Ni is not oxidized to Ni3+ below 700 °C, so the Li/Ni = 0.76 sample has almost no Ni K-edge energy shift above 700 °C. These results suggest that Ni in LiNiO2 is reduced above 700 °C even in an oxygen atmosphere, regardless of whether the Li/Ni ratio is greater than 1. The Ni reduction above 700 °C is probably the cause of the Li/Ni mixture.Reference[1] M. Bianchini, J. Janek, et al., Angew. Chem., Int. Ed., 58, 10434 (2019).[2] R. Weber, J. R. Dahn, et al., J. Electrochem. Soc., 167, 100501 (2020).

Nano-sized Metallic Nickel Clusters Stabilized on Dealuminated beta-Zeolite: A Highly Active and Stable Ethylene Hydrogenation Catalyst

Supported Ni catalysts were synthesized using the beta-zeolite framework, with and without the framework Al, as a platform for dispersing Ni. The silanol nest sites of dealuminated zeolite beta provide isolated cationic Ni sites that can be reduced under relatively mild conditions to create highly dispersed metal clusters. Compared to the Ni sites present in Ni-[Al]-beta-19, Ni-[DeAl]-beta exhibit a 20-fold increase in the apparent reaction rate for C2H4 hydrogenation and is stable, with little deactivation over 16 h of catalysis. Ni K-edge X-ray absorption spectroscopy (XAS), as well as CO adsorption monitored with Fourier transform infrared spectroscopy, shows that in the oxidized Ni-[DeAl]-beta catalyst Ni reoccupies vacant silanol nests produced from dealumination. After reductive treatment, XAS shows that approximately 50% of Ni is reduced to metallic Ni, forming clusters that are approximately 1 nm in size. Scanning transmission electron microscopy images are consistent with the absence of large (>1 nm) metallic Ni clusters. These results indicate that [DeAl]-beta can be used to synthesize isolated cationic Ni sites as well as stabilize highly dispersed metal clusters that can be used as a highly active and stable C2H4 hydrogenation catalyst.

Mechanistic Study of Ni(II) Sorption by Green Rust Sulfate.

The sorption of Ni(II) by green rust sulfate (GR-sulfate) was studied in anoxic pre-equilibrated suspensions at pH 7.0 and pH 7.8 with combined batch kinetic experiments, X-ray diffraction measurements, and Ni K-edge X-ray absorption spectroscopy (XAS) analyses. Continuous removal of aqueous Ni(II) was observed over the course of the reaction (1-2.5 weeks) at both pH values, with no concurrent changes in aqueous Fe(II) levels or detectable mineralogical modifications of the GR sorbent. XAS results indicate that Ni(II) is not retained as mononuclear adsorption complexes on the GR surface but rather incorporated in the octahedral layers of an FeII0.67-xNiIIxFeIII0.33(OH)2-layered double hydroxide (LDH) phase with 0 < x < 0.67. The combined macroscopic and spectroscopic data suggest that Ni(II) substitutes into the GR lattice during Fe(II)-catalyzed recrystallization of the sorbent and/or forms secondary Ni(II)/Fe(II)-Fe(III)-LDH phases with a higher stability than that of GR, complemented likely by Ni(II)-Fe(II) exchange at GR particle edges. The results of this study reveal GR to be a dynamic sorbent that engages in dissolution-reprecipitation and exchange reactions, causing extensive incorporation of trace metal Ni(II)aq. Additional work is needed to further define the mechanisms involved and to assess the sorptive reactivity of GR with other trace metal species.

Coordination of the Co2+ and Ni2+ Ions in Tf2N- Based Ionic Liquids: A Combined X-ray Absorption and Molecular Dynamics Study.

Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) have been combined to study the coordination of the Co2+ and Ni2+ ions in ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide ([Tf2N]−) anion and having different organic cations, namely, 1-butyl-3-methylimidazolium ([C4mim]+), 1,8-bis(3-methylimidazolium-1-yl)octane ([C8(mim)2]2+), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([choline]+), and butyltrimethylammonium ([BTMA]+). Co and Ni K-edge XAS data have been collected on 0.1 mol L–1 Co(Tf2N)2 and Ni(Tf2N)2 solutions and on the metallic salts. MD simulations have been carried out to obtain structural information on the metal ion coordination. The analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the atomistic description provided by MD, and the studied ILs have been found to be able to dissolve both the Co(Tf2N)2 and Ni(Tf2N)2 salts giving rise to a different structural arrangement around the metal ions as compared to the solid state. The combined EXAFS and MD results showed that the Co2+ and Ni2+ ions are surrounded by a first solvation shell formed by six [Tf2N]− anions, each coordinating in a monodentate fashion by means of the oxygen atoms. The nature of the IL organic cation has little or no influence on the overall spatial arrangement of the [Tf2N]− anions, so that stable octahedral complexes of the type [M(Tf2N)6]4– (M = Co, Ni) have been observed in all the investigated ILs.

Estimation of the average oxidation number of nickel in a nickel oxide based on local structural information

We developed the methods to estimate the average oxidation number of Ni in a nickel oxide from the local structural information on the first nearest neighbor Ni–O shell obtained by extended X-ray absorption fine structure curve fitting analysis. The estimation method utilizes the Jahn-Teller distortion around Ni3+ sites, enabling the evaluation of local structural parameters and mole fractions for different Ni valence states. The average oxidation numbers of Ni in nickel oxides estimated by the methods proposed in this work exhibit good agreement with those reported or evaluated by a conventional method using instrumental and solution chemical analyses. It is demonstrated that the estimation methods also work well in an in situ X-ray absorption spectroscopy (XAS) study on the charge-compensating behavior of Ni in a Ni-containing battery electrode material. The estimation methods are based on Ni K-edge XAS using highly penetrating hard X-ray, and thus applicable to in situ XAS studies. The methods proposed in this work are expected to work as substitutes for the conventional empirical method using X-ray absorption near edge structure. The estimation methods would be important choices when the local environment around Ni could dramatically be changed during charge/discharge cycles, like in an overcharge condition.

Degradation Mechanism of Ni-Rich Layered Cathode Materials during High Voltage Cycling Revealed By Multi-Model Synchrotron X-Ray Imaging and Spectroscopy Techniques

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, these materials are suffering severe capacity fade caused by surface reconstruction, unstable cathode-electrolyte interface (CEI) formation and micro crack formation during cycling, especially when a high cut off charge voltage is applied (such as 4.6 and 4.8V). To understand the origin of this capacity fade, state-of-the-art diagnostic tools are applied to understand their structure, chemical and morphological properties of LiNi0.94Co0.06O2 (NC) and LiNi0.92Co0.06Al0.02O2 (NCA) cathode materials during cycling. Electrochemical measurement results show that the NCA cathode delivered higher capacity and stable cyclic performance even cycled at a high cut of voltage of 4.8V. In situ X-ray absorption spectroscopy (XAS) results of NC and NCA electrodes measured during the first and 51st charge/discharge cycling between the voltage range of 2.8-4.4V showed that both Ni and Co contribute to the capacity by going through the Ni3+/Ni4+ and Co3+/Co4+ redox couples. The Ni-K edge XAS spectra of NC cathode charged to 4.8V showed slightly reduction of Ni4+ during the high voltage charging. While, XAS spectra of NCA cathode did not showed reduction of Ni4+ when the cell charged to the higher voltage. More interestingly, C, F, O, Ni and Co soft XAS spectroscopy of NC and NCA electrodes at the different state of charge provided more comprehensive insight into surface and bulk chemical properties. The results showed that much thicker CEI layer formed on the NC surface compared with the NCA cathode, indicated that Al-doping effectively reduced the side reaction of cathode materials with the electrolyte. We have also compared the morphological and chemical evolution of NC and NCA secondary particles using full-filed in-situ transmission X-ray microscopy (TXM) during the initial cycle. Results will be presented at the meeting. Acknowledgement This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704.

Nickel incorporation in perovskite-type metal oxides – Implications on reducibility

Nickel is often applied in heterogeneous catalysis for its high catalytic activity towards a large variety of reactions at affordable price. Nickel reduction from perovskite-type mixed oxides is increasingly exploited to generate active and stable Ni catalysts. To investigate implications of the host perovskite structure on Ni reducibility, Ni was incorporated on the B-site of three perovskite-type mixed metal oxides of different lattice symmetries (LaFeO3 – orthorhombic, LaCoO3 – rhombohedral and La0.3Sr0.55TiO3 – cubic). Structural parameters of the phase pure undoped and Ni-doped perovskites were determined using synchrotron X-ray diffraction (XRD) and Ni K-edge X-ray absorption spectroscopy (XAS). Rietveld refinement and extended X-ray absorption fine structure (EXAFS) data fitting were used to verify that at this substitution level Ni enters the B-site and adopts its coordination environment. Expansion of the unit cell was found in LaCoO3 and La0.3Sr0.55TiO3, whereas contraction was observed in the case of LaFeO3. X-ray absorption spectra showed that Ni-containing unit cells exhibit the same symmetry as the host perovskite-type mixed oxides. The mean oxidation state of Ni was found to be equal in all three cases irrespective of the host lattice (+2.5). Lattice symmetry had significant effect on lowering the Ni reduction temperature determined by hydrogen temperature programmed reduction and a correlation between reduction temperature and crystal tolerance factor was found.

Elucidation of reaction mechanisms of Ni2SnP in Li-ion and Na-ion systems

Electrochemical performance of Ni2SnP was assessed in Li-ion and Na-ion battery systems. When cycled versus Li, Ni2SnP exhibited a reversible specific charge of 700 mAh.g−1 (theoretical specific charge: 742 mAh.g−1). In the Na system, the specific observed charge was ca. 200 mAh.g−1 (theoretical specific charge: 676 mAh.g−1). X-ray diffraction, Ni K-edge X-ray absorption spectroscopy, and 31P and 7Li/23Na nuclear magnetic resonance spectroscopy were used to elucidate the electrochemical mechanisms in both systems. Versus Li, Ni2SnP undergoes a conversion reaction resulting in the extrusion of Ni and the alloying of Li-Sn and Li-P. On delithiation, the material partially recombines into a Sn- and Ni-deficient form. In the Na system, Ni2SnP reacts through the conversion of P into Na3P. These results indicate that the recombination of the pristine material (even partially) increases cycling stability.

Synthesis, characterization, and olefin polymerization of half-sandwich Ir, Rh, Ru metallacycle enclosing a nickel in the center

Reactions of binuclear [Cp*Ir(μ-Cl)Cl]2 (Cp* = pentamethylcyclopentadienyl), [Cp*Rh(μ-Cl)Cl]2 and [(p-cymene)Ru(μ-Cl)Cl]2 with 2 equiv. AgOTf (OTf = O3SCF3) followed by addition of one equiv. (m-pyridyl)N=C(C10H6)C=N(m-pyridyl) (mPy-bian) linker and NiCl2·DME (DME = 1,2-dimethoxyethane) in methanol gave heterometallic cationic metallacycles [Cp*4Ir4(μ-Cl)4(μ-mPy-bian)2NiCl2](OTf)4 (1a), [Cp*4Rh4(μ-Cl)4(μ-mPy-bian)2NiCl2](OTf)4 (1b), and [(p-cymene)4Ru4(μ-Cl)4(μ-mPybian)2NiCl2](OTf)4 (1c), respectively. All the complexes are characterized by IR, NMR spectroscopy, and elemental analysis. Ni K-edge X-ray absorption spectroscopy (XAS) studies on 1a–1c reveal that nickel is six-coordinate with four nitrogens and two chlorides. Upon activation with MAO, 1a–1c showed moderate to good catalytic activity for ethylene and norbornene polymerization.

Site-Selective and Element-Selective Analysis of Nickel Substituted Li-Rich Layered Material Using By X-Ray Diffraction and Absorption Spectroscopy

1. Introduction Li-rich type (manganese) oxides are one of the most featured cathodes for lithium ion batteries in recent years, owing to their high capacity and cyclability. In these cathodes, partial substitution of manganese by other transition metals such as Ni and Co has been proposed and shown to be effective in improving the performance, [1] however, the role of such metals in the battery performance has not been clarified in detail.We have synthesized Ni ion substituted Li excess manganese oxides (Li2Mn1-xNixO3) as a simple model of Li2MeO3 phase in solid solution cathode to understand the effect of introduced Ni by using the combination of in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction spectroscopy (XDS) analysis. Diffraction anomalous fine structure (DAFS) spectra are achieved by XDS analysis, that there were quantitatively showed potential-dependent transbilayer migration of nickel and manganese and their reversibility. 2. Experimental procedure The active material of nickel substituted Li2MnO3 (Li2Mn1-x NixO3, x = 0 ~ 0.25) was prepared from solid phase calcination at 1173 K for 12 h in air. The working electrode consisted of active material (80 wt.%), acetylene black (10 wt.%) and polyvinylidene difluoride (PVdF) binder (10 wt.%), coated on aluminum current foil.The electrochemical measurements of the electrode were employed using aluminum pouch type cells, metallic lithium as counter and reference electrodes, 1 mol dm-3 LiPF6 in a 3:7 mixture of EC/DMC, and a polyolefin film as a separator. The electrochemical charge and discharge tests were performed at room temperature and the potential range was between 2.0 and 4.8 V.XAS measurements were performed by the three-electrode cell with charging and discharging processes (in Operando). The XAS measurements of the Ni-K and Mn-K edge region were performed using a solid-state detector at beamlines BL28XU in SPring-8 (Hyogo, Japan).The XDS measurements were conducted at beamline BL28XU, SPring-8, Japan. A two-dimensional Pilatus detector (Dectris Co., Ltd.) was mounted onto an arm of a multi-axis goniometer and was used to measure the diffraction profile, and two ionization chamber was used for the absorption correction of the DAFS spectra. 3. Results and Discussion The obtained X-ray absorption near edge structure (XANES) region spectra at N-K energy region of Li2Mn1-x NixO3 (x = 0.25) in the 1st charging and discharging processes are shown in Fig. 1. The edge energy was shifted to high energy, which is suggested that Ni element function as a charge compensation species. However the end of charging region around 4.7 V to 4.8 V was shifted to low energy as seen in previous XAS report [2], which means the reduction of nickel on average in this region.As our group have published before,[3] XANES-like spectra only from selected sites (i.e. DAFS spectra) can be extracted from the XDS analysis. Fig. 2 shows the DAFS spectra of initial sample. The Ni-K edge XANES-like spectra shows that there was cation mixing in the lithium layer of the pristine material. In contrast, Mn-K edge XANES-like spectra shows that manganese mostly occupied in transition metal layer. XDS analysis of the fully charged sample indicated that Ni element further migrated to the lithium layer.These analytical approaches are important to understand the metal migration behavior as a function of operation potential together with the charge compensation mechanism during electrochemical charging and discharging.

An Operando Fluorescence XAS Study of a Ni/YSZ Anode in a Solid Oxide Fuel Cell

Introduction Solid oxide fuel cells (SOFC) have high energy conversion efficiencies because of their ability to directly convert the chemical energy stored in fuels into electrical energy. Ni/YSZ (yttria-stabilized zirconia) cermet is commonly used for SOFC anodes, because Ni provides excellent catalytic activity and current collectability and YSZ provides good ionic conductivity and structural support. However, Ni/YSZ facies several problems such as instability upon redox cycling, nickel coarsening, carbon deposition and so on. Therefore, designing anode materials with high durability is a key objective in the development of SOFC technology. Especially, it is essential to understand the redox behavior of the Ni electrocatalysts at work. Operando X-ray absorption spectroscopy (XAS) is a powerful tool to investigate the relationship between the oxidation state of active site and cell performance under an operating condition. Hence in this work, we developed an operando fluorescence XAS measurement system for SOFC to study the oxidation state of the working Ni electrocatalyst at temperatures ranging from 973 to 1073 K. Experimental Figure 1a depicts a two-chamber SOFC for operando fluorescence XAS measurement developed in the present work. An anode-supported cell having a groove for the investigation of the interfacial region between the Ni/YSZ anode and the YSZ electrolyte was installed to the operando XAS measurement system. The lidded XAS instrument was heated up to 1073 K in the air to melt the seal material (Pyrex glass), and subsequently NiO in the anode was reduced in a gas flow consisting of 4% H2, 3% H2O, and N2 balance. After completing gas seal and NiO reduction, the reactor temperature was lowered to 973 K for operation. The Ni K-edge XAS measurement was implemented at the BL33XU (the Toyota beamline) of SPring-8 in fluorescence mode by irradiating the bottom center of the groove at the anode and detecting the emitted X-ray with a four-element silicon drift detector. The anode potential was measured or adjusted with reference to the potential of lanthanum strontium cobalt ferrite cathode. Results The operando XAS technique was applied to the reduction of NiO at the Ni/YSZ anode at 1073 K. Figure 1b shows a chronopotentiogram of the Ni/YSZ anode and corresponding change in average Ni valence estimated by the height of the main peak at the Ni K-edge. The average Ni valence and the anode potential exhibited different changing trends in the reduction process. There seemed to exist four regions for anode potential (i.e., two potential transition and two plateau regions) and two regions for average Ni valence. In situ observation with a CCD camera clarified that the reduction of NiO around the anode–electrolyte interface started in region 2 and continued until region 4. Operando XAS measurement was also conducted to examine the oxidation state of Ni in a high-power operation at 973 K. As a time-averaged oxidation state, Ni at the Ni/YSZ anode kept its metallic state even though the anode potential was so high as −0.4 V. The oxidation behavior of Ni at the anode as mentioned above and its relationship with degradation in the high-power operation will be further discussed in the conference. Acknowledgements The authors are grateful to Prof. Koji Amezawa of Tohoku University for his fruitful advices and discussion on the development of the operando fluorescence XAS measurement system for SOFC. All the XAFS measurements were performed at the BL33XU (the Toyota beamline) of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) with the proposal numbers of 2014B7024, 2015A7024, and 2015B7024. Figure 1

Understanding the Anomalously High Capacity of Li- and Mn-Rich Cathodes for Li-Ion Batteries

The reaction mechanism of a high capacity lithium- and manganese-rich metal oxide has been investigated. High-resolution synchrotron X-ray powder diffraction (HRPD) and X-ray absorption spectroscopy (XAS) were used, respectively, to evaluate the electrochemical charge and discharge reactions in terms of local and bulk structural changes, and variations in the oxidation states of the transition metal ions. Ni K-edge XAS data indicate the participation of nickel in reversible redox reactions, whereas Mn K-edge absorption spectra show that the manganese ions do not participate in the electrochemical reactions. Rietveld refinements of the oxygen occupancy during charge and discharge provide evidence of reversible oxygen contribution by the host structure; this unique oxygen participation is likely the main reason for the anomalously high capacity of these electrodes. The HRPD data also show that during the early cycles, characteristic peaks of the Li2MnO3 component disappear when charged to 4.7 V, but reappear on discharge to 2.5 V, consistent with a reversible lithium and oxygen extraction process. The results provide new insights into the charge compensation mechanisms that occur when high capacity, lithium- and manganese-rich electrode materials are electrochemically cycled – a topic that is currently being hotly debated in the literature.

Evidence of reversible oxygen participation in anomalously high capacity Li- and Mn-rich cathodes for Li-ion batteries

The reaction mechanism of a high capacity lithium- and manganese-rich metal oxide, 0.4Li2MnO3–0.6LiMn0.5Ni0.5O2, has been investigated at the atomic level. High-resolution synchrotron X-ray powder diffraction (HRPD) and X-ray absorption spectroscopy (XAS) were used, respectively, to evaluate the electrochemical charge and discharge reactions in terms of local and bulk structural changes, and variations in the oxidation states of the transition metal ions. Ni K-edge XAS data indicate the participation of nickel in reversible redox reactions, whereas Mn K-edge absorption spectra show that the manganese ions do not participate in the electrochemical reactions. Rietveld refinements of the oxygen occupancy during charge and discharge provide evidence of reversible oxygen release and re-accommodation by the host structure; this unique oxygen participation is likely the main reason for the anomalously high capacity of these electrodes. The HRPD data also show that during the early cycles, characteristic peaks of the Li2MnO3 component disappear when charged to 4.7V, but reappear on discharge to 2.5V, consistent with a reversible lithium and oxygen extraction process. The results provide new insights into the charge compensation mechanisms that occur when high capacity, lithium- and manganese-rich electrode materials are electrochemically cycled – a topic that is currently being hotly debated in the literature.

At the Heart of a Conversion Reaction: An Operando X-ray Absorption Spectroscopy Investigation of NiSb2, a Negative Electrode Material for Li-Ion Batteries

The reaction mechanism of NiSb2 with lithium was studied in detail by operando Ni K-edge X-ray absorption spectroscopy (XAS). The redox activity of the Ni centers was followed during the first two lithiation/delithiation cycles, showing that a reversible conversion takes place, with the formation of Ni metal nanoparticles at the end of lithiation and the reversible formation of a Ni–Sb phase close to pristine NiSb2 at the end of the delithiation process. The comparison of operando results with ex situ measurements on cycled NiSb2 materials emphasizes the importance of real-time investigation tools for the study of battery materials, in particular in the case conversion reactions. These results can be considered as a reference for the study of other compounds of this family of reactions, where the formation of very reactive and/or unstable species is observed during electrochemical cycling. In fact, such reactive species, which can be identified only by using operando techniques, are one of the keys of the good cycling properties of such materials together with an appropriate formulation strategy.

Local structure relaxation in nanocrystalline Ni1 − xO thin films

Non-stoichiometric nickel oxide (Ni1−xO) thin films were prepared by DC magnetron sputtering technique in mixed Ar/O2 atmosphere and studied by synchrotron radiation Ni K-edge x-ray absorption spectroscopy, x-ray diffraction and scanning electron microscopy. The use of advanced modelling technique, combining classical molecular dynamics with ab initio multiple-scattering extended x-ray absorption fine structure calculations, allowed us to describe the structure relaxation and dynamics in nanocrystallites and to estimate their size and the concentration of nickel vacancies.

Origin of Electrochromism in High-Performing Nanocomposite Nickel Oxide

Electrochromic effects of transition metal oxides provide a great platform for studying lithium intercalation chemistry in solids. Herein, we report on an electronically modified nanocomposite nickel oxide (i.e., Li2.34NiZr0.28Ox) that exhibits significantly improved electrochromic performance relative to the state-of-the-art inorganic electrochromic metal oxides in terms of charge/discharge kinetics, bleached-state transparency, and optical modulation. The knowledge obtained from O K-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that the internally grown lithium peroxide (i.e., Li2O2) species plays a major role in facilitating charge transfer thus enabling optimal electrochromic performance. This understanding is relevant to recent theoretical studies concerning conductivity in Li2O2 for lithium-air batteries (as cited in the main text). Furthermore, we elucidate the electrochromism in modified nickel oxide in lithium ion electrolyte with the aid of Ni K-edge XAS and Ni L-edge XAS studies. The electrochromism in the nickel oxide materials arises from the reversible formation of hole states on the NiO6 units, which then impacts the Ni oxidation state through the Ni3d-O2p hybridization states. This study sheds light on the lithium intercalation chemistry for general energy storage and semiconductor applications.

X-ray absorption spectroscopy studies of the Ni ion of Li(Ni0.8Co0.15Al0.05)0.8(Ni0.5Mn0.5)0.2O2 with a core–shell structure and LiNi0.8Co0.15Al0.05O2 as cathode materials

Core–shell materials have attracted a great deal of interest since core–shell particles have superior physical and chemical properties compared to their single-component counterparts. The cathode material Li(Ni0.8Co0.15Al0.05)0.8(Ni0.5Mn0.5)0.2O2 (LNCANMO) with a core–shell structure was synthesized via a co-precipitation method and investigated as the cathode material for lithium ion batteries. The core–shell particle consisted of LiNi0.8Co0.15Al0.05O2 (LNCAO) as the core and LiNi0.5Mn0.5O2 as the shell. The cycling behavior between 2.8 and 4.3V at a current of 0.1 C-rate showed a reversible capacity of ∼195mAhg−1 with little capacity loss after 50 cycles. Extensive assessment of the electronic structures of the LNCAO and LNCANMO cathode materials was carried out using X-ray absorption spectroscopy (XAS). XAS has been used for structure refinement on the transition metal ion of the cathode. In particular, XAS studies of electrochemical reactions have been done from the viewpoint of the transition metal ion. In this study, Ni K-edge XAS spectra of the charge and discharge processes of LNCAO and LNCANMO were investigated.

Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba8−δNix□ySi46–x–y

Type-I clathrate phase Ba(8)Ni(x)□(y)Si(46-x-y) (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba(8-δ)Ni(x)□(y)Si(46-x-y) crystallizes in the space group Pm ̅3n (No. 223) with lattice parameter ranging from a = 10.3088(1) Å for Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) to a = 10.2896(1) Å for Ba(8.00(3))Ni(3.82(4))Si(41.33(6)). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) and Ba(7.9(1))Ni(1.6(1))Si(44.4(1)) are superconducting with T(c) values of 6.0 and 5.5 K, respectively.