Operando resonant soft X-ray emission spectroscopy of the LiMn2O4 cathode using an aqueous electrolyte solution.

The Mn 3d electronic-structure change of the LiMn2O4 cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L3 RXES spectra for the charged state revealed the Mn4+ state with strong charge-transfer from the O 2p to Mn 3d orbitals dominates, while for the open-circuit-voltage and discharged states it is ascribed to the mixture of sites with Mn3+ and Mn4+ states. The degree of charge transfer is significantly different between the Mn3+ and Mn4+ states, indicating that the redox reaction takes place on the strongly-hybridized Mn 3d-O 2p orbital rather than the localized Mn 3d orbital.

In situ IR spectroscopy during oxidation process of cobalt Prussian blue analogues.

Cobalt Prussian blue analogues (Co-PBA; NaxCo[Fe(CN)6]y), consisting of cyano-bridged transition metal network, -Fe-CN-Co-NC-Fe-, are promising cathode materials for Na-ion secondary batteries. In the oxidation process, oxidization of Fe and/or Co are compensated by Na+ deintercalation. Here, we investigated the oxidization process of three Co-PBAs by means of in situ infrared absorption (IR) spectroscopy. With use of an empirical rule of the frequencies of the CN- stretching mode in ferrocyanide ([FeII(CN)6]4-) and ferricyanide ([FeIII(CN)6]3-), the oxidation processes of Co-PBAs were determined against the Fe concentration (y) and temperature (T). We will discuss the interrelation between the oxidation processes and Fe concentration (y).

Aggregation tendency of guest Fe in NaCo1-xFexO2 (x < 0.1) as investigated by systematic EXAFS analysis.

In transition metal (M) compounds, the partial substitution of the host transition metal (Mh) to guest one (Mg) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of Mg is crucial. Here, we propose that a systematic EXAFS analysis against the Mg concentration can reveal the spatial distribution of Mg. We chose NaCo1-xFexO2 as a prototypical M compound and investigated the local intermetal distance around the guest Fe [dFe-M(x)] against Fe concentration (x). dFe-M(x) steeply increased with x, reflecting the larger ionic radius of high-spin Fe3+. The x-dependence of dFe-M(x) was analyzed by an empirical equation, [Formula: see text], where dFe-Fe and dFe-Co are the Fe-Fe and Co-Fe distances, respectively. The parameter s represents degree of distribution of Fe; s = 1, > 1, < 1 are for random, attractive, and repulsive distribution, respectively. The obtained s value (= 4.8) indicates aggregation tendency of guest Fe.

Operando soft X-ray emission spectroscopy of the Fe2O3 anode to observe the conversion reaction.

Drastic electronic-structure changes in an Fe2O3 thin film anode for a Li-ion battery during discharge (lithiation) and charge (delithiation) processes were observed using operando Fe 2p soft X-ray emission spectroscopy (XES). The conversion reaction forming metallic iron due to the lithiation reaction was confirmed by operando XES in combination with the analysis using full-multiplet calculation. The valence of Fe at the open-circuit voltage (OCV) before the second cycle was not Fe3+, but Fe2+ with a weak p-d hybridization, suggesting a considerable irreversibility upon the first discharge-charge cycle and a weakened Fe-O bond after the first cycle. Moreover, we revealed that the Fe 3d electronic-structure change during the second cycle was to some extent reversible as Fe2+ (2.7 V vs. Li/Li+: open circuit voltage) → Fe0 (0.1 V vs. Li/Li+: discharged) → Fe(2+δ)+ (3.0 V vs. Li/Li+: charged). This operando Fe 2p XES in combination with the full-multiplet calculation provides detailed information for redox chemistry during a discharge-charge operation that cannot be obtained by other methods such as crystal-structure and morphology analyses. XES is thus very powerful for investigating the origin and limitation of the lithiation function of anodes involving conversion reactions.

Mn 2p resonant X-ray emission clarifies the redox reaction and charge-transfer effects in LiMn2O4.

High-energy-resolution soft X-ray emission spectroscopy (XES) was applied to understand the changes in the electronic structure of LiMn2O4 upon Li-ion extraction/insertion. Mn 2p-3d-2p resonant XES spectra were analyzed by configuration-interaction full-multiplet (CIFM) calculations, which reproduced both dd and charge-transfer (CT) excitations. From the resonant XES spectra it is found that Mn3+ and Mn4+ coexist in the initial state, while this changes into Mn4+ in the charged-state. For the discharged-state, the Mn3+ component appears again although the dd excitations are slightly modified from those for the initial state. Furthermore, negative CT energy is expected for the Mn4+ configuration, which suggests very strong hybridization between the Mn 3d and O 2p orbitals. The large difference in the CT effect between the Mn4+ and Mn3+ states should give mechanical stress to the Mn-O bond during charge-discharge cycling, leading to capacity fading.

Measurements of ultrafast dissociation in resonant inelastic x-ray scattering of water.

There has been a discussion on the interpretation of the resonant inelastic x-ray scattering (RIXS) spectra of liquid water in terms of either different structural environments or that core hole dynamics can generate well-resolved dissociative spectral components. We have used RIXS with high resolution in the OH stretch vibration energy part, at extremely high overtones going toward the continuum of full OH bond breakage, to identify the amount of dissociative contributions in the valence band RIXS spectra at different excitation energies. We observe that at low excitation energies, corresponding to population of states with strongly antibonding character, the valence band RIXS spectra have a large contribution from a well-resolved dissociative feature. Instead, at higher excitations, this spectral component diminishes and becomes a weak structure on the high-energy side of one of the spectral peaks related to the 1b1 state from tetrahedral configurations. This result brings both interpretations to be essential for the understanding of RIXS spectra of liquid water.

Strong localization of oxidized Co3+ state in cobalt-hexacyanoferrate.

Secondary batteries are important energy storage devices for a mobile equipment, an electric car, and a large-scale energy storage. Nevertheless, variation of the local electronic state of the battery materials in the charge (or oxidization) process are still unclear. Here, we investigated the local electronic state of cobalt-hexacyanoferrate (Na x Co[Fe(CN)6]0.9), by means of resonant inelastic X-ray scattering (RIXS) with high energy resolution (~100 meV). The L-edge RIXS is one of the most powerful spectroscopic technique with element- and valence-selectivity. We found that the local electronic state around Co2+ in the partially-charged Na1.1Co2+0.5Co3+0.5[Fe2+(CN)6]0.9 film (x = 1.1) is the same as that of the discharged Na1.6Co2+[Fe2+(CN)6]0.9 film (x = 1.6) within the energy resolution, indicating that the local electronic state around Co2+ is invariant against the partial oxidization. In addition, the local electronic state around the oxidized Co3+ is essentially the same as that of the fully-charged film Co3+[Fe2+(CN)6]0.3[Fe3+(CN)6]0.6 (x = 0.0) film. Such a strong localization of the oxidized Co3+ state is advantageous for the reversibility of the redox process, since the localization reduces extra reaction within the materials and resultant deterioration.

Invariant nature of substituted element in metal-hexacyanoferrate.

The chemical substitution of a transition metal (M) is an effective method to improve the functionality of materials. In order to design the highly functional materials, we first have to know the local structure and electronic state around the substituted element. Here, we systematically investigated the local structure and electronic state of the host (M h) and guest (M g) transition metals in metal-hexacyanoferrate (M-HCF), Na x (M h, M g)[Fe(CN)6] y (1.40 < x < 1.60 and 0.85 < y < 0.90), by means of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) analyses. The EXAFS and XANES analyses revealed that the local structure and electronic state around M g are essentially the same as those in the pure compound, i.e, M g-HCF. Such an invariant nature of M g in M-HCF is in sharp contrast with that in layered oxide, in which the M g valence changes so that local M g-O distance (d M-Og) approaches the M h-O distance (d M-Oh).

Wetting Induced Oxidation of Pt-based Nano Catalysts Revealed by In Situ High Energy Resolution X-ray Absorption Spectroscopy.

In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption

Local structures around the substituted elements in mixed layered oxides.

The chemical substitution of a transition metal (M) is an effective method to improve the functionality of a material, such as its electrochemical, magnetic, and dielectric properties. The substitution, however, causes local lattice distortion because the difference in the ionic radius (r) modifies the local interatomic distances. Here, we systematically investigated the local structures in the pure (x = 0.0) and mixed (x = 0.05 or 0.1) layered oxides, Na(M1-xM'x)O2 (M and M' are the majority and minority transition metals, respectively), by means of extended X-ray absorption fine structure (EXAFS) analysis. We found that the local interatomic distance (dM-O) around the minority element approaches that around the majority element to reduces the local lattice distortion. We further found that the valence of the minority Mn changes so that its ionic radius approaches that of the majority M.

A compact permanent-magnet system for measuring magnetic circular dichroism in resonant inelastic soft X-ray scattering.

A compact and portable magnet system for measuring magnetic dichroism in resonant inelastic soft X-ray scattering (SX-RIXS) has been developed at the beamline BL07LSU in SPring-8. A magnetic circuit composed of Nd-Fe-B permanent magnets, which realised ∼0.25 T at the center of an 11 mm gap, was rotatable around the axis perpendicular to the X-ray scattering plane. Using the system, a SX-RIXS spectrum was obtained under the application of the magnetic field at an angle parallel, nearly 45° or perpendicular to the incident X-rays. A dedicated sample stage was also designed to be as compact as possible, making it possible to perform SX-RIXS measurements at arbitrary incident angles by rotating the sample stage in the gap between the magnetic poles. This system enables facile studies of magnetic dichroism in SX-RIXS for various experimental geometries of the sample and the magnetic field. A brief demonstration of the application is presented.

Lewis Basicity of Nitrogen-Doped Graphite Observed by CO2 Chemisorption.

The characteristics of CO2 adsorption sites on a nitrogen-doped graphite model system (N-HOPG) were investigated by X-ray photoelectron and absorption spectroscopy and infrared reflection absorption spectroscopy. Adsorbed CO2 was observed lying flat on N-HOPG, stabilized by a charge transfer from the substrate. This demonstrated that Lewis base sites were formed by the incorporation of nitrogen via low-energy nitrogen-ion sputtering. The possible roles of twofold coordinated pyridinic N and threefold coordinated valley N (graphitic N) sites in Lewis base site formation on N-HOPG are discussed. The presence of these nitrogen species focused on the appropriate interaction strength of CO2 indicates the potential to fine-tune the Lewis basicity of carbon-based catalysts.

Pt-free carbon-based fuel cell catalyst prepared from spherical polyimide for enhanced oxygen diffusion.

The development of a non-precious metal (NPM) fuel cell catalyst is extremely important to achieve globalization of polymer electrolyte fuel cells due to the cost and scarcity of platinum. Here, we report on a NPM cathode catalyst prepared by the pyrolysis of spherical polyimide nanoparticles that contain small amounts of Fe additive. 60 nm diameter Fe-containing polyimide nanoparticles were successfully synthesized by the precipitation polymerization of pyromellitic acid dianhydride and 1,3,5-tris(4-aminophenyl)benzene with Fe(acac)3 (acac = acetylacetonate) as an additive. The particles were subsequently carbonized by multistep pyrolysis to obtain the NPM catalyst while retaining the small particle size. The catalyst has good performance and promising durability for fuel cell applications. The fuel cell performance under a 0.2 MPa air atmosphere at 80 °C of 1.0 A cm(-2) at 0.46 V is especially remarkable and better than that previously reported.

Characterization of nitrogen species incorporated into graphite using low energy nitrogen ion sputtering.

The electronic structures of nitrogen species incorporated into highly oriented pyrolytic graphite (HOPG), prepared by low energy (200 eV) nitrogen ion sputtering and subsequent annealing at 1000 K, were investigated by X-ray photoelectron spectroscopy (XPS), angle-dependent X-ray absorption spectroscopy (XAS), and Raman spectroscopy. An additional peak was observed at higher binding energy of 401.9 eV than 400.9 eV for graphitic1 N (graphitic N in the basal plane) in N 1s XPS, where graphitic2 N (graphitic N in the zigzag edge and/or vacancy sites) has been theoretically expected to appear. N 1s XPS showed that graphitic1 N and graphitic2 N were preferably incorporated under low nitrogen content doping conditions (8 × 10(13) ions cm(-2)), while pyridinic N and graphitic1 N were dominantly observed under high nitrogen content doping conditions. In addition, angle-dependent N 1s XAS showed that the graphitic N and pyridinic N atoms were incorporated into the basal plane of HOPG and thus were highly oriented. Furthermore, Raman spectroscopy revealed that low energy sputtering resulted in almost no fraction of the disturbed graphite surface layers under the lowest nitrogen doping condition. The suitable nitrogen doping condition was discovered for realizing the well-controlled nitrogen doped HOPG. The electrochemical properties for the oxygen reduction reaction of these samples in acidic solution were examined and discussed.

Active site formation mechanism of carbon-based oxygen reduction catalysts derived from a hyperbranched iron phthalocyanine polymer.

Carbon-based cathode catalysts derived from a hyperbranched iron phthalocyanine polymer (HB-FePc) were characterized, and their active-site formation mechanism was studied by synchrotron-based spectroscopy. The properties of the HB-FePc catalyst are compared with those of a catalyst with high oxygen reduction reaction (ORR) activity synthesized from a mixture of iron phthalocyanine and phenolic resin (FePc/PhRs). Electrochemical measurements demonstrate that the HB-FePc catalyst does not lose its ORR activity up to 900°C, whereas that of the FePc/PhRs catalyst decreases above 700°C. Hard X-ray photoemission spectra reveal that the HB-FePc catalysts retain more nitrogen components than the FePc/PhRs catalysts between pyrolysis temperatures of 600°C and 800°C. This is because the linked structure of the HB-FePc precursor has high thermostability against nitrogen desorption. Consequently, effective doping of active nitrogen species into the sp (2) carbon network of the HB-FePc catalysts may occur up to 900°C.

Enhancement in Kinetics of the Oxygen Reduction Reaction on a Nitrogen-Doped Carbon Catalyst by Introduction of Iron via Electrochemical Methods.

The iron (Fe) electrodeposition-electrochemical dissolution has been employed on nitrogen-doped carbon material (P-PI) prepared via multi-step pyrolysis of a polyimide precursor to achieve the introduction of Fe species, and its influence on the oxygen reduction reaction (ORR) is investigated by cyclic and rotating ring-disk electrode voltammetry in 0.5 M H2SO4. After the electrochemical treatment, the overpotential and H2O2 production percentage of ORR on the P-PI are decreased and the number of electrons transferred is increased in the meanwhile. In combination with the results of X-ray absorption fine structure spectra, the presence of Fe-Nx sites (Fe ions coordinated by nitrogen) is believed to be responsible for the improved ORR performance. Further kinetic analysis indicates that a two-electron reduction of O2 is predominant on the untreated P-PI with coexistence of a direct four-electron transformation of O2 to H2O, while the introduction of Fe species leads to a larger increase in the rate constant for the four-electron reduction than that for the two-electron process, being in good agreement with the view that Fe-Nx sites are active for four-electron ORR.

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X-ray Emission Spectroscopy.

We combine Mn L2,3-edge X-ray absorption, high resolution Mn 2p-3d-2p resonant X-ray emission, and configuration-interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn(2+) low-spin state separately from that of the Mn(2+) high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn(2+) low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t2g and CN π orbitals than the Mn(2+) high spin state. The d-d-excitation peak nearest to the elastic scattering was ascribed to the Mn(2+) LS state by the CIFM calculation, indicating that the Mn(2+) LS state with a hole on the t2g orbital locates near the Fermi level.

Selective probing of the OH or OD stretch vibration in liquid water using resonant inelastic soft-x-ray scattering.

High-resolution O 1s resonant inelastic x-ray scattering spectra of liquid H2O, D2O, and HDO, obtained by excitation near the preedge resonance show, in the elastic line region, well-separated multiple vibrational structures corresponding to the internal OH stretch vibration in the ground state of water. The energy of the first-order vibrational excitation is strongly blueshifted with respect to the main band in the infrared or Raman spectra of water, indicating that water molecules with a highly weakened or broken donating hydrogen bond are correlated with the preedge structure in the x-ray absorption spectrum. The vibrational profile of preedge excited HDO water is well fitted with 50%±20% greater OH-stretch contribution compared to OD, which strongly supports a preference for OH being the weakened or broken H-bond in agreement with the well-known picture that D2O makes stronger H-bonds than H2O. Accompanying path-integral molecular dynamics simulations show that this is particularly the case for strongly asymmetrically H-bonded molecules, i.e., those that are selected by preedge excitation.

Ultrahigh resolution soft x-ray emission spectrometer at BL07LSU in SPring-8.

An extremely high resolution flat field type slit less soft x-ray emission spectrometer has been designed and constructed for the long undulator beamline BL07LSU in SPring-8. By optimizing the ruling parameters of two cylindrical gratings, a high energy resolution ΔE < 100 meV and/or an E∕ΔE ~ 10 000 are expected for the energy range of 350 eV - 750 eV taking into account the broadening by the spatial resolution (25 µm) of a CCD detector. A coma-free operation mode proposed by Strocov et al., is also applied to eliminate both defocus and coma aberrations. The spectrometer demonstrated experimentally that E/ΔE = 10 050 and 8046 for N 1s (402.1 eV) and Mn 2p (641.8 eV) edges, respectively.

Co oxidation accompanied by degradation of Pt-Co alloy cathode catalysts in polymer electrolyte fuel cells.

The electronic structure of Co ions in Pt(3)Co alloy cathode catalysts for polymer electrolyte fuel cells (PEFCs) under different operation conditions has been investigated using X-ray absorption spectroscopy (XAS). The line shapes of the Co 2p XAS spectra indicate that the Co atoms in the cathode layer are predominantly metallic in all cells, and the spectrum of the first-operated cell is the most metallic among them. The Co 2p XAS spectra slightly change depending on the operation conditions, and the differences in the spectra can be explained by the presence of cobalt oxides. Decomposition analysis of the Co 2p XAS spectra reveals that the operation of Pt-Co PEFCs yields trivalently oxidised cobalt in the cathode layer.