Correlation of the crystal structure and ion storage behavior of MoO3 electrode materials for aluminum-ion energy storage studied using in situ X-ray spectroscopy.

To characterize the correlation of the crystal structure and Al-ion storage behavior, we prepared various crystal structures of MoO3 (α-MoO3, ß-MoO3 and h-MoO3) electrode materials and studied them via in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The α-MoO3 electrode material possesses a specific capacitance of 575.4 F g-1 and a gravimetric capacity of 207.8 mA h g-1 at a current density of 1 A g-1. From the in situ XRD results, the crystal structures of α-MoO3 and ß-MoO3 show a significant distortion, whereas that of h-MoO3 is minorly affected during the insertion or extraction of Al3+ ions. Based on the in situ XAS results, the MoO6 octahedral structure and Mo ion valence of α-MoO3 and ß-MoO3 also exhibit a strong variation, whereas those of h-MoO3 are nearly unchanged during the insertion or extraction of Al3+ ions. Notably, in situ XRD and XAS also clearly show a possible phase of AlxMoO3 during the Al3+ insertion and extraction cycles in the α-MoO3 and ß-MoO3 electrode materials, which may play a crucial role in the behavior of the residue of Al3+ ions and poor cycling stability. We provide clear evidence that the Al-ion energy storage performance of various MoO3 electrode materials is strongly associated with the corresponding tunnel space and the stability of their crystal structures. This work also provides new insight into a strong correlation between ion-storage efficiency and the corresponding crystal structure, which is greatly helpful for the development and improvement of new electrode materials for Al-ion energy storage.

1.8 V Aqueous Symmetric Carbon-Based Supercapacitors with Agarose-Bound Activated Carbons in an Acidic Electrolyte.

The specific energy of an aqueous carbon supercapacitor is generally small, resulting mainly from a narrow potential window of aqueous electrolytes. Here, we introduced agarose, an ecologically compatible polymer, as a novel binder to fabricate an activated carbon supercapacitor, enabling a wider potential window attributed to a high overpotential of the hydrogen-evolution reaction (HER) of agarose-bound activated carbons in sulfuric acid. Assembled symmetric aqueous cells can be galvanostatically cycled up to 1.8 V, attaining an enhanced energy density of 13.5 W h/kg (9.5 µW h/cm2) at 450 W/kg (315 µW/cm2). Furthermore, a great cycling behavior was obtained, with a 94.2% retention of capacitance after 10,000 cycles at 2 A/g. This work might guide the design of an alternative material for high-energy aqueous supercapacitors.

Electrochemical properties and mechanism of CoMoO4@NiWO4 core-shell nanoplates for high-performance supercapacitor electrode application studied via in situ X-ray absorption spectroscopy.

Binary transition metal oxide CoMoO4@NiWO4 core-shell nanoplates grown directly on a Ni foam substrate were synthesized via a facile two-step hydrothermal process. The core-shell nanoplates with high electrochemical surface area (2933 cm2) demonstrated excellent electrochemical properties (areal capacity as high as 0.464 mA h cm-2 at a current density of 5 mA cm-2) and great cycle stability (92.5% retention after 3000 cycles with a high current density of 40 mA cm-2). The mechanism of the electrochemical reactions based on the in situ X-ray absorption spectroscopy technique clearly shows that the Co and Ni elements simultaneously participate in the faradaic reactions with the electrolyte. These results indicate that the excellent electrochemical performance of CoMoO4@NiWO4 compared to that of CoMoO4 nanoplates is attributed to a large electrochemical surface area and synergistic effect between NiWO4 and CoMoO4. This combination of two binary transition metal oxides can hence provide an excellent route to develop a high-performance electrode material for supercapacitor applications.

High Durability of Pt3Sn/Graphene Electrocatalysts toward the Oxygen Reduction Reaction Studied with In Situ QEXAFS.

To prevent the corrosion of carbon and to enhance corrosion resistance, charge transfer, and mass transfer, graphene, which exhibits a high surface area and good conductivity, was used as an electrocatalyst support for a fuel cell. Pt3Sn/G electrocatalysts for the oxygen reduction reaction (ORR) were prepared with alcohol reduction. The characterization of synthesized catalysts was analyzed according to the energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and extended X-ray absorption fine structure (EXAFS). The electrochemical performance was analyzed with cyclic-voltammetry (CV), linear sweep voltammetry (LSV), and accelerated degradation test (ADT) measurements. The Pt3Sn/G electrocatalysts showed more positive onset potential and larger ORR mass activity than commercial Pt/C catalysts after 5000 cycles of ADT, indicating that in an acidic environment, Pt3Sn/G is more chemically stable than Pt/C. Graphene has effective acid tolerance, is more stable against corrosion, and shows increased stability through preventing PtSn nanoparticles from detaching from the surface. According to the in situ quick EXAFS (QEXAFS) under a CV test to clarify the potential-dependent state of the Pt3Sn/G electrocatalyst, the results show that the electrode surface is reproducible; there is no perceptible change in the oxidation state of the Pt3Sn/G electrocatalyst. The radial distribution function (RDF) of the EXAFS spectra shows that the adsorption and desorption of H+ and OH- cause no structural change in the Pt3Sn crystallites. This work provides insight into the reaction mechanism of proton electroreduction and hydrogen adsorption on a Pt3Sn/G electrocatalyst surface.

The fluctuating population of Sm 4f configurations in topological Kondo insulator SmB6 explored with high-resolution X-ray absorption and emission spectra.

High-resolution partial-fluorescence-yield X-ray absorption and resonant X-ray emission spectra were used to characterize the temperature dependence of Sm 4f configurations and orbital/charge degree of freedom in SmB6. The variation of Sm 4f configurations responds well to the formed Kondo gap, below 140 K, and an in-gap state, below 40 K. The topological in-gap state is correlated with the fluctuating population of Sm 4f configurations that arises via carrier transfer between 3d94f6 and 3d94f5 states; both states are partially delocalized, and the mediating 5d orbital plays the role of a transfer path. Complementary results shown in this work thus manifest the importance of configuration fluctuations and orbital delocalization in the topological surface state of SmB6.

Cheap, High-Performance, and Wearable Mn Oxide Supercapacitors with Urea-LiClO4 Based Gel Electrolytes.

Here we report a simple, scalable, and low-cost method to enhance the electrochemical properties of Mn oxide electrodes for highly efficient and flexible symmetrical supercapacitors. The method involving printing on a printer, pencil-drawing, and electrodeposition is established to fabricate Mn oxide/Ni-nanotube/graphite/paper hybrid electrodes operating with a low-cost, novel urea-LiClO4/PVA as gel electrolyte for flexible solid-state supercapacitor (FSSC) devices. The Mn oxide nanofiber/Ni-nanotube/graphite/paper (MNNGP) electrodes in urea-LiClO4/PVA gel electrolyte show specific capacitance (Csp) 960 F/g in voltage region 0.8 V at 5 mV/s and exhibit excellent rates of capacitance retention more than 85% after 5000 cycles. Moreover, the electrochemical behavior of the MNNGP electrodes in urea-LiClO4/PVA at operating temperatures 27-110 °C was investigated; the results show that the MNNGP electrodes in urea-LiClO4/PVA exhibit outstanding performance (1100 F/g), even at 90 °C. The assembled FSSC devices based on the MNNGP electrodes in urea-LiClO4/PVA exhibit great Csp (380 F/g in potential region of 2.0 V at 5 mV/s, exhibiting superior energy density 211.1 W h/kg) and great cycle stability (less than 15% loss after 5000 cycles at 25 mV/s). The oxidation-state change was examined by in situ X-ray absorption spectroscopy. FSSC devices would open new opportunities in developing novel portable, wearable, and roll-up electric devices owing to the cheap, high-performance, wide range of operating temperature, and simple procedures for large-area fabrication.

Influence of Fe substitution on the Jahn-Teller distortion and orbital anisotropy in orthorhombic Y(Mn1-xFex)O3 epitaxial films.

Multiferroic YMn1-xFexO3(020) (x = 0.125, 0.25, 0.50) epitaxial thin films with an orthorhombic structure (space group Pbnm) were prepared on a YAlO3(010) substrate by pulsed-laser deposition. Upon Fe substitution, the b-axis was clearly shortened, whereas the a- and c-axes were slightly lengthened based on XRD analysis. To understand the influence of orbital polarization and the Jahn-Teller effect of Mn(3+) on Fe substitution and also the local octahedral-site distortion of Fe(3+) in an environment of Jahn-Teller-active Mn(3+) ions in YMn1-xFexO3 films, we measured the polarization-dependent X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges, and also simulated the experimental spectra using configuration-interaction multiplet calculations. Although Δeg for the Mn(3+) ion decreased from 0.9 eV in pure YMnO3 to 0.6 eV in the half-Fe-substituted sample, a single eg electron was still strongly constrained to the d3y(2)-r(2) orbital for all the Fe concentrations tested. The largest Δeg, 0.5 eV, for the Fe(3+) ion was derived for a sample with 12.5% Fe substitution, and gradually decreased to 0.15 eV for the half-Fe-substituted sample. The local octahedral-site distortion of the Fe(3+) ion inside the YMnO3 lattice was similar to that of the Mn(3+) ion, whereas the Jahn-Teller distortion and GdFeO3-type distortion of the Mn(3+) ion were decreased by the spherical high-spin Fe(3+) ions. The combination of the experimental and theoretical data provides both profound insight into the variation of the Jahn-Teller distortion and orbital anisotropy and instructive information about the magnetic structures in these orthorhombic YMn1-xFexO3 thin films.

Local structure distortion induced by Ti dopants boosting the pseudocapacitance of RuO2-based supercapacitors.

Binary oxides with atomic ratios of Ru/Ti = 90/10, 70/30, and 50/50 were fabricated using H2O2-oxidative precipitation with the assistance of a cetyltrimethylammonium bromide (CTAB) template, followed by a thermal treatment at 200 °C. The characteristics of electron structure and local structure extracted from X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) analyses indicate that incorporation of Ti into the RuO2 lattice produces not only the local structural distortion of the RuO6 octahedra in (Ru-Ti)O2 with an increase in the central Ru-Ru distance but also a local crystallization of RuO2. Among the three binary oxides studied, (Ru70-Ti30)O2 exhibits a capacitance improvement of about 1.4-fold relative to the CTAB-modified RuO2, mainly due to the enhanced crystallinity of the distorted RuO6 structure rather than the surface area effect. Upon increasing the extent of Ti doping, the deteriorated supercapacitive performance of (Ru50-Ti50)O2 results from the formation of localized nano-clusters of TiO2 crystallites. These results provide insight into the important role of Ti doping in RuO2 that boosts the pseudocapacitive performance for RuO2-based supercapacitors. The present result is crucial for the design of new binary oxides for supercapacitor applications with extraordinary performance.

Low temperature structural anomalies arising from competing exchange interactions in pyrochlore Nd2Ru2O7 probed by XRD and EXAFS.

Quantitative structural parameters of pyrochlore Nd2Ru2O7, with temperature dependence, have been derived upon fitting XRD and EXAFS data. An anomalous expansion of the lattice parameter and the Ru-O bond length indicates a structural instability at low temperatures; in particular, an increase in the non-thermal term of the mean square fluctuation in the bond length is the evidence for a static disorder of Ru atoms. This static disorder is closely correlated with a decrease in the average Ru-O-Ru bond angle with decreasing temperature, favoring the short-range ferromagnetic coupling in the material. This ferromagnetic coupling formed thus triggered the spin frustration at low temperature when the contradictory constraints of antiferromagnetic interaction act upon the same Ru site in the corner-sharing tetrahedrons of pyrochlore Nd2Ru2O7. This study demonstrates that the spin frustration arising from the competition of ferromagnetic/antiferromagnetic interactions in pyrochlore Nd2Ru2O7 will cause structural instability especially on the atomic scale, which provides a new point of view to help understand its particular magnetic state.

Low cost facile synthesis of large-area cobalt hydroxide nanorods with remarkable pseudocapacitance.

Large-area Co(OH)2-based supercapacitor electrodes composed of nanotube arrays grown on a 3D nickel-foam (CONTA) electrode and sucker-like nanoporous films grown on a 3D nickel-foam (COSNP) electrode were prepared with a facile electrochemical method for applications in energy storage. These nanoporous Co(OH)2 electrodes were fabricated with the codeposition of Cu/Ni film on the nickel foam, then etching of Cu from the Cu/Ni layer to form Ni nanotube arrays and sucker-like Ni nanoporous layers, and further cathodic deposition of Co(OH)2 on the prepared nanoporous Ni substrates. The CONTA and COSNP electrodes exhibited specific capacitances of 2500 and 2900 F/g in a voltage range of 0.65 V (capacitance of the substrates deducted from the total) at 1 A/g in a three electrode cell, respectively. The COSNP electrode demonstrated an excellent supercapacitive performance with specific capacitances 1100 F/g at 1 A/g and 850 F/g at 20 A/g in a voltage range of 1.2 V in a two electrode cell. The remarkable performance of COSNP electrodes correlated with a large conversion of the Co oxidation state during the charge/discharge cycling were examined by in situ X-ray absorption near edge structure (XANES).

Atomic distribution and structural evolution of mesostructured PtRu nanoparticles electrodeposited on a microemulsion lyotropic liquid-crystalline template probed using EXAFS and XANES.

Mesostructured PtRu nanoparticles were electrochemically reduced from their metallic salts directed by a hexagonally packed microemulsion lyotropic liquid-crystalline (MLLC) template. We investigated the structural evolution and atomic distribution of the MLLC-templated mesoporous PtRu nanoparticles (NPs) after electroreduction for varied duration using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, complemented by energy-dispersive X-ray spectroscopy (EDS) and field-emission transmission electron microscopy (FE-TEM). The XANES data at the Ru L2,3 and Pt L3 edges show predominantly metallic states of Ru and Pt in the PtRu NPs upon electroreduction. The reduction of Ru(3+) ions in RuCl3 into Ru atoms involves intermediate RuCl-containing complexes. A more rapid reduction of Pt precursors and a release of Ru atoms from Ru precursors in two steps upon electroreduction resulted in aggregation into PtRu nanoparticles, featuring a Pt-rich core, a Ru-rich shell and a varied alloy extent of Ru, deduced from EXAFS data. The complementary results provide insight into the mechanism of growth and atomic distribution of mesostructured PtRu bimetallic nanoparticles from the use of the MLLC-type templates.

Three-dimensionally ordered macroporous Cu2O/Ni inverse opal electrodes for electrochemical supercapacitors.

With an ordered polystyrene (PS) template-assisted electrochemical approach we synthesized three-dimensional ordered macroporous (3DOM) Cu2O/Ni inverse opals as electrodes for supercapacitors. The 3DOM Cu2O/Ni electrodes display superior kinetic performance, and satisfactory rate capability and cycling performance.