Enhancing Li-ion capacity and rate capability in cation-defective vanadium ferrite aerogels via aluminum substitution.

Cation-defective iron oxides have proven to be effective Li-ion charge-storage hosts in nonaqueous electrolytes, particularly when expressed in disordered, nanoscale forms such as aerogels. Replacing a fraction of Fe sites in ferrites with high-valent cations such as V5+ introduces cation-vacancy defects that increase Li-ion capacity. Herein, we show that compositional substitution with electroinactive Al3+ further increases Li-ion capacity by 30% when incorporated within a disordered VFe2Ox aerogel, as verified by electrochemical tests in a two-terminal Li half-cell. We use electroanalytical techniques to show that both Al-VFe2Ox and VFe2Ox aerogels exhibit many of the hallmarks of pseudocapacitive materials, including fast charge-discharge and surface-controlled charge-storage kinetics. These disordered, substituted ferrites also provide the high specific capacity expected from battery-type electrode materials, up to 130 mA h g-1 for Al-VFe2Ox. Our findings are discussed in the context of related Li-insertion hosts that blur the distinctions between battery-like and capacitor-like behavior.

Zinc-Sponge Battery Electrodes that Suppress Dendrites.

We report two methods to create zinc-sponge electrodes that suppress dendrite formation and shape change for rechargeable zinc batteries. Both methods are characterized by creating a paste made of zinc particles, organic porogen, and viscosity-enhancing agent that is heated under an inert gas and then air. During heating under the inert gas, the zinc particles anneal together, and the porogen decomposes; under air, the zinc fuses and residual organic burns out, yielding an open-cell metal foam or sponge. We tune the mechanical and electrochemical properties of the zinc sponges by varying zinc-to-porogen mass ratio, heating time under inert gas and air, and size and shape of the zinc and porogen particles. An advantage of the reported methods is their ability to finely tune zinc-sponge architecture. The selected size and shape of the zinc and porogen particles influence the morphology of the pore structure. A limitation is that resulting sponges have disordered pore structures that result in low mechanical strength at low volume fractions of zinc (<30%). Applications for these zinc-sponge electrodes include batteries for grid-storage, personal electronics, electric vehicles, and electric aviation. Users can expect zinc-sponge electrodes to cycle up to 40% depth of discharge at technologically relevant rates and areal capacities without the formation of separator-piercing dendrites.

Oxidation-stable plasmonic copper nanoparticles in photocatalytic TiO2 nanoarchitectures.

Ultraporous copper/titanium dioxide (Cu/TiO2) aerogels supporting <5 nm diameter copper nanoparticles are active for surface plasmon resonance (SPR)-driven photocatalysis. The extended nanoscale Cu‖TiO2 junctions in Cu/TiO2 composite aerogels-which arise as a result of photodepositing copper at the surface of the nanoparticulate-bonded TiO2 aerogel architecture-stabilize Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after exposure to oxidizing conditions. The metallicity of the Cu nanoparticles within the TiO2 aerogel is verified by aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, and infrared spectroscopy using CO binding as a probe to distinguish Cu(0) from Cu(i). In contrast, photoreduction of Cu(ii) at a commercial nanoscale anatase TiO2 powder with primary particle sizes significantly larger than those in the aerogel results in a copper oxide/TiO2 composite that exhibits none of the plasmonic character of Cu nanoparticles. We attribute the persistence of plasmonic Cu nanoparticles without the use of ligand stabilizers to the arrangement of Cu and TiO2 within the aerogel architecture where each Cu nanoparticle is in contact with multiple nanoparticles of the reducing oxide. The wavelength dependence of the photoaction spectra for Cu/TiO2 aerogel films reveals visible-light photocatalytic oxidation activity initiated by an SPR-driven process-as opposed to photo-oxidation initiated by excitation of narrow-bandgap copper oxides.

Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide.

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)6]3-/4-) and notoriously slow ([Fe(H2O)6]3+/2+) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)6]3-/4-, the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO2. For the classically outer-sphere [Fe(H2O)6]3+/2+, electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.

Electroanalytical Assessment of the Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale NiyFe1-yOx.

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.

Rechargeable nickel-3D zinc batteries: An energy-dense, safer alternative to lithium-ion.

The next generation of high-performance batteries should include alternative chemistries that are inherently safer to operate than nonaqueous lithium-based batteries. Aqueous zinc-based batteries can answer that challenge because monolithic zinc sponge anodes can be cycled in nickel-zinc alkaline cells hundreds to thousands of times without undergoing passivation or macroscale dendrite formation. We demonstrate that the three-dimensional (3D) zinc form-factor elevates the performance of nickel-zinc alkaline cells in three fields of use: (i) >90% theoretical depth of discharge (DODZn) in primary (single-use) cells, (ii) >100 high-rate cycles at 40% DODZn at lithium-ion-commensurate specific energy, and (iii) the tens of thousands of power-demanding duty cycles required for start-stop microhybrid vehicles.

CAD/CAM-designed 3D-printed electroanalytical cell for the evaluation of nanostructured gas-diffusion electrodes.

The ability to effectively screen and validate gas-diffusion electrodes is critical to the development of next-generation metal-air batteries and regenerative fuel cells. The limiting electrode in a classic two-terminal device such as a battery or fuel cell is difficult to discern without an internal reference electrode, but the flooded electrolyte characteristic of three-electrode electroanalytical cells negates the prime function of an air electrode-a void volume freely accessible to gases. The nanostructured catalysts that drive the energy-conversion reactions (e.g., oxygen reduction and evolution in the air electrode of metal-air batteries) are best evaluated in the electrode structure as-used in the practical device. We have designed, 3D-printed, and characterized an air-breathing, thermodynamically referenced electroanalytical cell that allows us to mimic the Janus arrangement of the gas-diffusion electrode in a metal-air cell: one face freely exposed to gases, the other wetted by electrolyte.

Retaining the 3D framework of zinc sponge anodes upon deep discharge in Zn-air cells.

We fabricate three-dimensional zinc electrodes from emulsion-cast sponges of Zn powder that are thermally treated to produce rugged monoliths. This highly conductive, 3D-wired aperiodic scaffold achieves 740 mA h gZn(-1) when discharged in primary Zn-air cells (>90% of theoretical Zn capacity). We use scanning electron microscopy and X-ray diffraction to monitor the microstructural evolution of a series of Zn sponges when oxidized in Zn-air cells to specific depths-of-discharge (20, 40, 60, 80% DOD) at a technologically relevant rate (C/40; 4-6 mA cm(-2)). The Zn sponges maintain their 3D-monolithic form factor at all DOD. The cell resistance remains low under all test conditions, indicating that an inner core of metallic Zn persists that 3D-electrically wires the electrode, even to deep DOD.

Mass spectrometry of ligand exchange chelation of the nanoparticle [Au25(SCH2CH2C6H5)18]1- by CH3C6H3(SH)2.

Mass spectrally detected products of ligand exchange reactions of the nanoparticle [Au25(SC2H4C6H5)18](1-), (abbrev. Au25(SC2Ph)18), where the dithiol is toluene-3,4-dithiol, CH3C6H3(SH)2, include nanoparticles containing both doubly (bidentate, or chelating) and singly bonded dithiol. The bidentate binding displaces two of the original -SC2Ph ligands, and singly bonded dithiol displaces one -SC2Ph ligand, while maintaining, for mass spectrally detected species, occupancy of 18 ligation sites. Extended exchange reaction times result in an apparent maximum of six chelated dithiolates. In the Au25(SC2Ph)18 nanoparticle, six semi-rings of -S(R)-Au-S(R)-Au-S(R)- act as the protecting ligand shell surrounding a Au13 core; the chelation is suggested to involve binding of dithiolates to adjacent semi-rings, rather than to a single semi-ring. Both high resolution ESI and lower resolution MALDI spectra support the product assignments. A minor extent of bidentate ligand incorporation is sufficient to severely compromise the well-known Au25(SC2Ph)18 UV-vis fine structure and to alter its voltammetric pattern, reflecting either associated semi-ring distortion and/or decay of the exchange product.

Synthesis of monodisperse [Oct4N(+)][Au25(SR)18(-)] nanoparticles, with some mechanistic observations.

A single phase (THF) synthesis of monodisperse [Oct(4)N(+)][Au(25)(SR)(18)(-)] nanoparticles is described that yields insights into pathways by which it is formed from initially produced larger nanoparticles. Including the Oct(4)N(+)Br(-) salt in a reported single phase synthetic procedure enables production of reduced nanoparticles having a fully occupied HOMO molecular energy level (Au(25)(SR)(18)(-), as opposed to a partially oxidized state, Au(25)(SR)(18)(0)). The revised synthesis accommodates several (but not all) different thiolate ligands. The importance of acidity, bromide, and dioxygen on Au(25) formation was also assessed. The presence of excess acid in the reaction mixture steers the reaction toward making Au(25)(SR)(18); while bromide does not seem to affect Au(25) formation, but it may play a role in maintaining the -1 oxidation state. Conducting the nanoparticle synthesis and "aging" period in the absence of dioxygen (under Ar) does not produce small nanoparticles, providing insights into the pathway of reaction product "aging" in the synthesis solvent, THF. The "aging" process favors the Au(25)(-) moiety as an end point and possibly involves degradation of larger nanoparticles by hydroperoxides formed from THF and oxygen.

The story of a monodisperse gold nanoparticle: Au25L18.

Au nanoparticles (NPs) with protecting organothiolate ligands and core diameters smaller than 2 nm are interesting materials because their size-dependent properties range from metal-like to molecule-like. This Account focuses on the most thoroughly investigated of these NPs, Au(25)L(18). Future advances in nanocluster catalysis and electronic miniaturization and biological applications such as drug delivery will depend on a thorough understanding of nanoscale materials in which molecule-like characteristics appear. This Account tells the story of Au(25)L(18) and its associated synthetic, structural, mass spectrometric, electron transfer, optical spectroscopy, and magnetic resonance results. We also reference other Au NP studies to introduce helpful synthetic and measurement tools. Historically, nanoparticle sizes have been described by their diameters. Recently, researchers have reported actual molecular formulas for very small NPs, which is chemically preferable to solely reporting their size. Au(25)L(18) is a success story in this regard; however, researchers initially mislabeled this NP as Au(28)L(16) and as Au(38)L(24) before correctly identifying it by electrospray-ionization mass spectrometry. Because of its small size, this NP is amenable to theoretical investigations. In addition, Au(25)L(18)'s accessibility in pure form and molecule-like properties make it an attractive research target. The properties of this NP include a large energy gap readily seen in cyclic voltammetry (related to its HOMO-LUMO gap), a UV-vis absorbance spectrum with step-like fine structure, and NIR fluorescence emission. A single crystal structure and theoretical analysis have served as important steps in understanding the chemistry of Au(25)L(18). Researchers have determined the single crystal structure of both its "native" as-prepared form, a [N((CH(2))(7)CH(3))(4)(1+)][Au(25)(SCH(2)CH(2)Ph)(18)(1-)] salt, and of the neutral, oxidized form Au(25)(SCH(2)CH(2)Ph)(18)(0). A density functional theory (DFT) analysis correctly predicted essential elements of the structure. The NP is composed of a centered icosahedral Au(13) core stabilized by six Au(2)(SR)(3) semirings. These semirings present interesting implications regarding other small Au nanoparticle clusters. Many properties of the Au(25) NP result from these semiring structures. This overview of the identification, structure determination, and analytical properties of perhaps the best understood Au nanoparticle provides results that should be useful for further analyses and applications. We also hope that the story of this nanoparticle will be useful to those who teach about nanoparticle science.

Tandem mass spectrometry of thiolate-protected Au nanoparticles Na(x)Au25(SC2H4Ph)(18-y)(S(C2H4O)5CH3)(y).

We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer Na(x)Au(25)(SC(2)H(4)Ph)(18-y)(S(C(2)H(4)O)(5)CH(3))(y) centrally involve the semi-ring Au(2)L(3) coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticle's protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na(2)AuL(2)](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na(2)Au(2)L(3)](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu(3)L(3)](1+) and [NaAu(4)L(4)](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au(4)L(4) in ESI-TOF of a much larger thiolate-protected Au(144) nanoparticle under non-CID conditions.

Electrospray ionization mass spectrometry of uniform and mixed monolayer nanoparticles: Au25[S(CH2)2Ph]18 and Au25[S(CH2)2Ph]18-x(SR)x.

New approaches to electrospray ionization mass spectrometry (ESI-MS)-with exact compositional assignments-of small (Au25) nanoparticles with uniform and mixed protecting organothiolate monolayers are described. The results expand the scope of analysis and reveal a rich chemistry of ionization behavior. ESI-MS of solutions of phenylethanethiolate monolayer-protected gold clusters (MPCs), Au25(SC2Ph)18, containing alkali metal acetate salts (MOAc) produce spectra in which, for Na+, K+, Rb+, and Cs+ acetates, the dominant species are MAu25(SC2Ph)182+ and M2Au25(SC2Ph)182+. Li+ acetates caused ligand loss. This method was extended to the analysis of Au25 MPCs with mixed monolayers, where thiophenolate (-SPh), hexanethiolate (-SC6), or biotinylated (-S-PEG-biotin) ligands had been introduced by ligand exchange. In negative-mode ESI-MS, no added reagents were needed in order to observe Au25(SC2Ph)18- and to analyze mixed monolayer Au25 MPCs prepared by ligand exchange with 4-mercaptobenzoic acid, HSPhCOOH, which gave spectra through deprotonation of the carboxylic acids. Adducts of tetraoctylammonium (Oct4N+) with -SPhCOO- sites were also observed. Mass spectrometry is the only method that has demonstrated capacity for measuring the exact distribution of ligand-exchange products. The possible origins of the different Au25 core charges (1-, 0, 1+, 2+) observed during electrospray ionization are discussed.