Vanadium ferrite (VFe 2 O x ) is a defective spinel system that can incorporate substantial Li+ and exhibits a high charge–discharge rate, particularly when structured as a nanoscale aerogel.1 Cations such as Zr, Zn and Al,2 can readily enter the structure substitutionally and have strong, but differing effects on the charge-storage capacity of the material. These earth abundant, cost- effective constituent elements give this class of materials strong potential as future Li-ion battery cathodes but optimizing the stoichiometry for maximum capacity and stability will require understanding the redox sequence of the host cations (Fe, V) and role of intentional dopants. We use density functional theory calculations in concert with in situ and operando X-ray absorption near-edge spectroscopy (XANES) spectra obtained using an in-lab X-ray absorption spectrometer to uncover the quantum mechanical-level effects that underpin relevant energy- storage behaviors of doped and undoped VFe 2 O x . Our experimental V K-edge and Fe K-edge spectra indicate reduction of both species during discharge but cannot distinguish between tetrahedral and octahedral Fe redox sites or fully resolve the valency of each element as a function of state of charge. Using a hybrid form of density functional theory that accounts for the strong correlation present in 3d elements, we show that both V and Fe are indeed reduced, but that only tetrahedral Fe is redox active until fully converted to Fe 2+ . Furthermore, both octahedral and tetrahedral Fe 3+ are high spin configuration, but the 5μ B moments in the fully filled majority spin channel at each symmetry site are anti-aligned. This arrangement allows for easy exchange of electrons and facilitates conduction. We also calculate XANES spectra based on first principles calculations to be compared directly to those measured in the lab. This cross- check allows us to understand the effect of Al, Zr, and Zn dopants on the redox sequence and relate these results to site preference and capacity. Our combined experimental and calculational investigation sheds light on how these complex materials store Li ions and points toward future alterations that may further improve their properties.
Nanocrystalline ceria is under study to improve performance in high-temperature catalysis and fuel cells. We synthesize porous ceria monolithic nanoarchitectures by reacting Ce(III) salts and epoxide-based proton scavengers. Varying the means of pore-fluid removal yields nanoarchitectures with different pore−solid structures: aerogels, ambigels, and xerogels. The dried ceria gels are initially X-ray amorphous, high-surface-area materials, with the aerogel exhibiting 225 m2 g-1. Calcination produces nanocrystalline materials that, although moderately densified, still retain the desirable characteristics of high surface area, through-connected porosity in the mesopore size range and nanoscale particle sizes (∼10 nm). The electrical properties of calcined ceria ambigels are evaluated from 300 to 600 °C and compared to those of commercially available nanoscale CeO2. The pressed pellets of both ceria samples exhibit comparable surface areas and void volumes. The conductivity of the ceria ambigel is 5 times greater than the commercial sample and both materials exhibit an increase in conductivity in argon relative to oxygen at 600 °C, suggesting an electronic contribution to conductivity at low oxygen partial pressures. The ceria ambigel nanoarchitecture responds to changes in atmosphere at 600 °C faster than does the nanocrystalline, non-networked ceria. We attribute the higher relative conductivity of CeO2 ambigels to the bonded pathways inherent to the bicontinuous pore−solid networks of these nanoarchitectures.
We demonstrate that, when distributed as nanoscale coatings on the walls of carbon nanofoam substrates, manganese oxides exhibit voltammetric signatures in LiOH-containing alkaline electrolytes that are characteristic of either electrochemical capacitors or batteries, depending on the potential range investigated. Pseudocapacitance is observed for positive potential ranges, and ex-situ X-ray absorption spectroscopy confirms that the native layered birnessite MnOx structure is retained as the Mn oxidation state is toggled between 3.72 and 3.43. When the cycling range is extended to more negative potential limits, well-defined reduction and oxidation features are observed, with an associated reversible change in the Mn oxidation state of 0.71 after 25 cycles. For these deep-discharge conditions, high charge-storage capacities are facilitated by the reversible interconversion of birnessite and γ-MnOOH forms of the nanoscale MnOx coating. Solid-state 7Li NMR is used to investigate the role of Li+ from the alkaline electrolyte in enhancing the cycling stability of the MnOx−carbon nanofoam.
The Back Cover picture shows that activity for NiFe2Ox–catalyzed O2 evolution linearly tracks the surface area of the nanostructured oxide: aerogel > xerogel > precipitated nanopowder. More information can be found in the Full Paper by C. N. Chervin et al. on page 1369 in Issue 9, 2016 (DOI: 10.1002/celc.201600206).
Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (NiyFe1–yOx, where y = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350 °C/air was necessary to crystallize the respective NiyFe1–yOx nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), rock salt for the most Ni-rich material (y = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ y ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ y ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for Ni0.67Fe0.33Ox aerogel heat-treated at 300 °C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich NiyFe1–yOx materials; however, as the Ni-content increases beyond y = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size.
Protocols that express functional materials in a way that amplifies their surface-to-volume ratio offer a means to probe the structural ambiguity and surface-mediated reactivity of technologically important materials. We previously reported that three-dimensional (3D) ultraporous scaffolds, such as silica aerogels, silica fiber paper, and carbon nanofoam paper (CNF), provide a form factor that expresses energy-storing, catalytic ruthenium oxide (RuOx) as essentially all-surface—and a highly disordered one at that. To track the chemical state and solid-state structure of the 3D-expressed RuOx nanoskins as a function of thermal processing, we use X-ray near-edge structure (XANES), extended X-ray fine structure (EXAFS), and differential pair-distribution function (DPDF) analyses. We find that a Ru-centered ∼2.4 Å correlation present in the as-deposited oxide, also observed in PDF analysis of RuO2·nH2O but previously unassigned, fits the metastable corundum-like Ru2O3 structure. This corundum-like feature diminishes in concentration with increasing treatment temperature (25–200 °C), commensurate with an increase in relative rutile RuO2 content, electrical conductivity, and charge-storing capacitance of the oxide. Yet disorder persists beyond 8 Å, and a rutile nanocrystalline structure is not attained until >200 °C. The combination of synthetic amplification and total scattering analyses offers a viable approach to elucidate the structural ambiguity of practical, disordered nanomaterials.
Transition-metal oxides that exhibit “pseudocapacitance” are promising alternatives to high-surface-area carbons as charge-storing materials in next-generation electrochemical capacitors (ECs). Hydrous ruthenium oxides remain the state-of-the-art for pseudocapacitive materials due to their fortuitous combination of high electronic and ionic conductivity. Lower-cost alternatives are being vigorously pursued, yet the low electronic conductivity of most other metal oxides of interest (e.g., MnO x ) necessitates that they be thoughtfully incorporated with a conductive carbon support. We have developed an electrode design in which pseudocapacitive oxides, such as MnO x and FeO x , are applied as nanoscale coatings onto ultraporous carbon nanofoam substrates that define the macroscale-to-nanoscale structure of the resulting electrode architecture [1,2]. In addition to their practical advantages for device fabrication, these nanofoam paper-based materials have also provided a designer platform with which to investigate the interplay of pore structure and electrochemical performance [3] and for in situ analysis of the mechanisms responsible for pseudocapacitance [4]. In the background of these studies, we have accumulated evidence that the physicochemical nature of the carbon–metal oxide interface can have a significant impact on electrochemical performance. While continuing to develop and transition our 3D electrode architectures, we are refocusing our research efforts on investigating fundamental charge-transfer properties at nanoscale carbon–metal oxide interfaces. Shifting from the characterization complexities of the nanofoam-based architectures, we use planar pyrolytic-carbon substrates to mimic the surface properties of 3D carbon, but in forms that are more readily characterized by conventional surface spectroscopy and scanning-probe microcopy techniques. We apply nanoscale pseuodocapacitive oxides to these planar carbon films using redox-deposition protocols previously demonstrated at 3D carbons [5], and explore how the physical, chemical, and electronic structure of the resulting carbon–metal oxide interface impacts electrochemical properties. Lessons learned from these model interfaces are readily translated to improved performance in practical 3D electrode architectures. 1. Fischer, A.E.; Saunders, M.P.; Pettigrew, K.A.; Rolison, D.R.; Long, J.W. J. Electrochem. Soc . 2008 , 155 , A246. 2. Sassin, M.B.; Mansour, A.N.; Pettigrew, K.A.; Rolison, D.R.; Long, J.W. ACS Nano 2009 , 4 , 4505. 3. Sassin, M.B.; Hoag, C.P.; Willis, B.T.; Kucko, N.W.; Rolison, D.R.; Long, J.W. Nanoscale , 2013 , 5 , 1649. 4. Beasley, C.A.; Sassin, M.B.; Long, J.W. J. Electrochem. Soc. 2015 , 162 , A5060. 5. Sassin, M.B. Chervin, C.N.; Rolison, D.R.; Long, J.W. Acc. Chem. Res. 2013 , 46 , 1062.
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