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Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

Reducing the energy loss associated with Li2O2 electrochemical oxidation is paramount to the development of efficient rechargeable lithium-oxygen (Li-O2) batteries for practical use. The influence of a series of perovskites with different eg filling on the kinetics of Li2O2 oxidation was examined using Li2O2-prefilled electrodes. While LaCrO3 is inactive for oxygen evolution upon water oxidation in alkaline solution, it was found to provide the highest specific current towards Li2O2 oxidation among all the perovskites examined. Further exploration of Cr-based catalysts showed that Cr nanoparticles (Cr NP) with an average particle size of 40 nm, having oxidized surfaces, had comparable surface area activities to LaCrO3 but much greater mass activities. Unlike Pt/C and Ru/C that promote electrolyte oxidation in addition to Li2O2 oxidation, no evidence of enhanced electrolyte oxidation was found for Cr NP relative to Vulcan carbon. X-ray absorption spectroscopy at the O K and Cr L edge revealed a redox process of Cr(3+) ↔ Cr(6+) on the surface of Cr NP upon Li2O2 oxidation, which might be responsible for the enhanced oxidation kinetics of Li2O2 and the reduced charging voltages of Li-O2 batteries.

Compact solid discharge products enable energy storage devices with high gravimetric and volumetric energy densities, but solid deposits on active surfaces can disturb charge transport and induce mechanical stress. In this Letter, we develop a nanoscale continuum model for the growth of Li2O2 crystals in lithium-oxygen batteries with organic electrolytes, based on a theory of electrochemical nonequilibrium thermodynamics originally applied to Li-ion batteries. As in the case of lithium insertion in phase-separating LiFePO4 nanoparticles, the theory predicts a transition from complex to uniform morphologies of Li2O2 with increasing current. Discrete particle growth at low discharge rates becomes suppressed at high rates, resulting in a film of electronically insulating Li2O2 that limits cell performance. We predict that the transition between these surface growth modes occurs at current densities close to the exchange current density of the cathode reaction, consistent with experimental observations.

Unfortunately, the practical applications of Li-O2 batteries are impeded by poor rechargeability. Here, for the first time we show that superoxide radicals generated at the cathode during discharge react with carbon that contains activated double bonds or aromatics to form epoxy groups and carbonates, which limits the rechargeability of Li-O2 cells. Carbon materials with a low amount of functional groups and defects demonstrate better stability thus keeping the carbon will-o'-the-wisp lit for lithium-air batteries.

Next-generation electrochemical energy storage for integrated microsystems and consumer electronic devices requires novel electrode materials with engineered architectures to meet the requirements of high performance, low cost, and robustness. However, conventional electrode fabrication processes such as doctor blading afford limited control over the electrode thickness and structure at the nanoscale and require the incorporation of insulating binder and other additives, which can promote agglomeration and reduce active surface area, limiting the inherent advantages attainable from nanoscale materials. We have engineered a route for the synthesis of highly stable, sub-8 nm TiO2 nanoparticles and their subsequent incorporation with acid-functionalized multiwalled carbon nanotubes (MWNTs) into nanostructured electrodes using aqueous-based layer-by-layer electrostatic self-assembly. Using this approach, binder-free thin film electrodes with highly controllable thicknesses up to the micrometer scale were developed with well-dispersed, nonagglomerated TiO2 nanoparticles on MWNTs. Upon testing in an Li electrochemical half-cell, these electrodes demonstrate high capacity (>150 mAh/gel(ectrode) at 0.1 A/gel(ectrode)), good rate capability (>100 mAh/gel(ectrode) up to 1 A/g(electrode)) and nearly no capacity loss up to 200 cycles for electrodes with thicknesses up to 1480 nm, indicating their promise as thin-film negative electrodes for future Li storage applications.

The slow kinetics of oxygen surface exchange hinders the efficiency of high-temperature oxygen electrocatalytic devices such as solid oxide fuel cells and oxygen separation membranes. Systematic investigations of material properties that link to catalytic activity can aid in the rational design of highly active cathode materials. Here, we explore LaCoO 3 thin films as a model system for tuning catalytic activity through strain-induced changes in the Co spin state. We demonstrate that Raman spectroscopy can be used to probe the Co-O bond strength at different temperatures to determine the relative spin occupancies of LaCoO 3 . We find that strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co-O bond. The decrease in Co-O bond strength and increased spin moment of the thin films result in significant enhancements of the oxygen surface exchange kinetics by up to 2 orders of magnitude.

For many electrochemical reactions such as oxygen reduction, catalysts are of critical importance, as they are often necessary to reduce reaction overpotentials. To fulfill the promises held by catalysts, a well-defined charge transport pathway is indispensable. Presently, porous carbon is most commonly used for this purpose, the application of which has been recently recognized to be a potential source of concern. To meet this challenge, here we present the development of a catalyst system without the need for carbon. Instead, we focused on a conductive, two-dimensional material of a TiSi2 nanonet, which is also of high surface area. As a proof-of-concept, we grew Pt nanoparticles onto TiSi2 by atomic layer deposition. Surprisingly, the growth exhibited a unique selectivity, with Pt deposited only on the top/bottom surfaces of the nanonets at the nanoscale without mask or patterning. Pt {111} surfaces are preferably exposed as a result of a multiple-twinning effect. The materials showed great promise in catalyzing oxygen reduction reactions, which is one of the key challenges in both fuel cells and metal air batteries.

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

The ability to direct bimetallic nanoparticles to express desirable surface composition is a crucial step toward effective heterogeneous catalysis, sensing, and bionanotechnology applications. Here we report surface composition tuning of bimetallic Au-Pt electrocatalysts for carbon monoxide and methanol oxidation reactions. We establish a direct correlation between the surface composition of Au-Pt nanoparticles and their catalytic activities. We find that the intrinsic activities of Au-Pt nanoparticles with the same bulk composition of Au0.5Pt0.5 can be enhanced by orders of magnitude by simply controlling the surface composition. We attribute this enhancement to the weakened CO binding on Pt in discrete Pt or Pt-rich clusters surrounded by surface Au atoms. Our finding demonstrates the importance of surface composition control at the nanoscale in harnessing the true electrocatalytic potential of bimetallic nanoparticles and opens up strategies for the development of highly active bimetallic nanoparticles for electrochemical energy conversion.

Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied at the sub-10 nm level. Comparison between electrochemical strain microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggests that small-angle grain boundaries act as regions with enhanced electrochemical activity. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study potentially paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single extended structural defects such as grain boundaries and dislocations.

In this Letter, we report the first in situ transmission electron microscopy observation of electrochemical oxidation of Li2O2, providing insights into the rate limiting processes that govern charge in Li-O2 cells. In these studies, oxidation of electrochemically formed Li2O2 particles, supported on multiwall carbon nanotutubes (MWCNTs), was found to occur preferentially at the MWCNT/Li2O2 interface, suggesting that electron transport in Li2O2 ultimately limits the oxidation kinetics at high rates or overpotentials.

Perovskites are used to promote the kinetics of oxygen electrocatalysis in solid oxide fuel cells and oxygen permeation membranes. Little is known about the surface structure and chemistry of perovskites at high temperatures and partial oxygen pressures. Combining in situ X-ray reflectivity (XRR) and in situ ambient pressure X-ray photoelectron spectroscopy (APXPS), we report, for the first time, the evolution of the surface structure and chemistry of (001)-oriented perovskite La0.8Sr0.2CoO3-δ (LSC113) and (La0.5Sr0.5)2CoO4+δ (LSC214)-decorated LSC113 (LSC113/214) thin films as a function of temperature. Heating the (001)-oriented LSC113 surface leads to the formation of surface LSC214-like particles, which is further confirmed by ex situ Auger electron spectroscopy (AES). In contrast, the LSC113/214 surface, with activities much higher than that of LSC113, is stable upon heating. Combined in situ XRR and APXPS measurements support that Sr enrichment may occur at the LSC113 and LSC214 interface, which can be responsible for its markedly enhanced activities.

Li-O2 batteries, wherein solid Li2O2 is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kgLi2O2) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li2O2 particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li-O2 batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li2O2 particles form first as stacked thin plates, ∼10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li2O2 crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li2O2 particles precedes growth of discs and toroids.

Understanding the reaction mechanism of nonaqueous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to increase the low round-trip efficiency and power capability of rechargeable Li-air batteries. Here we show that the ORR kinetics are much faster than OER kinetics and OER occurs in two distinct stages upon Li-air battery charging. The first OER stage occurs at low overpotentials (<400 mV) with a slopping voltage profile, whose kinetics are relatively insensitive to charge rates and catalysts. This OER stage could be attributed to the delithiation of the outer part of Li2O2 forming lithium-deficient Li2-xO2, which is chemically disproportionate to evolve O2. The second stage takes place at high overpotentials (400-1200 mV), whose kinetics are sensitive to discharge/charge rates and catalysts, which can be attributed to the oxidation of bulk Li2O2 particles. Our study provides insights into bridging current two schools of thought on the OER mechanism.

The electronic structure of transition metal oxides governs the catalysis of many central reactions for energy storage applications such as oxygen electrocatalysis. Here we exploit the versatility of the perovskite structure to search for oxide catalysts that are both active and stable. We report double perovskites (Ln₀.₅Ba₀.₅)CoO(₃-δ) (Ln=Pr, Sm, Gd and Ho) as a family of highly active catalysts for the oxygen evolution reaction upon water oxidation in alkaline solution. These double perovskites are stable unlike pseudocubic perovskites with comparable activities such as Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O(₃-δ) which readily amorphize during the oxygen evolution reaction. The high activity and stability of these double perovskites can be explained by having the O p-band centre neither too close nor too far from the Fermi level, which is computed from ab initio studies.

Lithium-oxygen batteries have a great potential to enhance the gravimetric energy density of fully packaged batteries by two to three times that of lithium ion cells. Recent studies have focused on finding stable electrolytes to address poor cycling capability and improve practical limitations of current lithium-oxygen batteries. In this study, the catalyst electrode, where discharge products are deposited and decomposed, was investigated as it has a critical role in the operation of rechargeable lithium-oxygen batteries. Here we report the electrode design principle to improve specific capacity and cycling performance of lithium-oxygen batteries by utilizing high-efficiency nanocatalysts assembled by M13 virus with earth-abundant elements such as manganese oxides. By incorporating only 3-5 wt% of palladium nanoparticles in the electrode, this hybrid nanocatalyst achieves 13,350 mAh g(-1)(c) (7,340 mAh g(-1)(c+catalyst)) of specific capacity at 0.4 A g(-1)(c) and a stable cycle life up to 50 cycles (4,000 mAh g(-1)(c), 400 mAh g(-1)(c+catalyst)) at 1 A g(-1)(c).

Transition-metal oxide and phosphate materials, commonly used for lithium battery devices, are active as oxygen evolution reaction (OER) catalysts under alkaline and neutral solution conditions. Electrodes composed of LiCoO(2) and LiCoPO(4) exhibit progressive deactivation and activation for OER catalysis, respectively, upon potential cycling at neutral pH. The deactivation of LiCoO(2) and activation of LiCoPO(4) are coincident with changes in surface morphology and composition giving rise to spinel-like and amorphous surface structures, respectively. The amorphous surface structure of the activated LiCoPO(4) is compositionally similar to that obtained from the electrodeposition of cobalt oxide materials from phosphate-buffered electrolyte solutions. These results highlight the importance of a combined structural and electrochemical analysis of the materials surface when assessing the true nature of the OER catalyst.

The oxidation kinetics of Li(2)O(2) was studied in a carbonate-free electrolyte using electrodes consisting of non-catalyzed and catalyzed Vulcan carbon (VC) and chemically synthesized Li(2)O(2) particles. VC and Au nanoparticles supported on VC (Au/C) were fairly inactive for catalyzing the oxidation of Li(2)O(2), where oxidation currents greater than 10 mA g(carbon)(-1) were found only at voltages equal to and greater than 4.0 V vs. Li (V(Li)). Pt and Ru nanoparticles supported on VC (Pt/C and Ru/C) could significantly increase the kinetics of Li(2)O(2) oxidation, where Li(2)O(2) could be removed largely at voltages below 4 V(Li). In addition, Pt/C and Ru/C showed quick initiation of Li(2)O(2) oxidation in contrast to VC and Au/C.

A facile synthetic route was developed to make Au nanowires (NWs) from surfactant-mediated bio-mineralization of a genetically engineered M13 phage with specific Au binding peptides. From the selective interaction between Au binding M13 phage and Au ions in aqueous solution, Au NWs with uniform diameter were synthesized at room temperature with yields greater than 98 % without the need for size selection. The diameters of Au NWs were controlled from 10 nm to 50 nm. The Au NWs were found to be active for electrocatalytic oxidation of CO molecules for all sizes, where the activity was highly dependent on the surface facets of Au NWs. This low-temperature high yield method of preparing Au NWs was further extended to the synthesis of Au/Pt core/shell NWs with controlled coverage of Pt shell layers. Electro-catalytic studies of ethanol oxidation with different Pt loading showed enhanced activity relative to a commercial supported Pt catalyst, indicative of the dual functionality of Pt for the ethanol oxidation and Au for the anti-poisoning component of Pt. These new one-dimensional noble metal NWs with controlled compositions could facilitate the design of new alloy materials with tunable properties.

The oxidation of AuCu nanoparticles was studied as a function of composition and temperature. Oxidation rates at 110 °C were higher for NPs with higher Cu content, showing that Au stabilized the Cu. Electrochemistry measurements show that AuCu could be a promising catalyst for lowering the over potential of CO(2) reduction.

The activities of the oxygen evolution reaction (OER) on iridium-oxide- and ruthenium-oxide-based catalysts are among the highest known to date. However, the OER activities of thermodynamically stable rutile iridium oxide (r-IrO2) and rutile iridium oxide (r-RuO2), normalized to catalyst mass or true surface area are not well-defined. Here we report a synthesis of r-IrO2 and r-RuO2 nanoparticles (NPs) of ∼6 nm, and examine their OER activities in acid and alkaline solutions. Both r-IrO2 and r-RuO2 NPs were highly active for OER, with r-RuO2 exhibiting up to 10 A/goxide at 1.48 V versus reversible hydrogen electrode. When comparing the two, r-RuO2 NPs were found to have slightly higher intrinsic and mass OER activities than r-IrO2 in both acid and basic solutions. Interestingly, these oxide NPs showed higher stability under OER conditions than commercial Ru/C and Ir/C catalysts. Our study shows that these r-RuO2 and r-IrO2 NPs can serve as a benchmark in the development of active OER catalysts for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications.

Generation of soluble sources of peroxide dianion (O(2)(2-)) is a challenge in dioxygen chemistry. The oxidizing nature of this anion renders its stabilization in organic media difficult. This Report describes the chemically reversible reduction of oxygen (O(2)) to cryptand-encapsulated O(2)(2-). The dianion is stabilized by strong hydrogen bonds to N-H groups from the hexacarboxamide cryptand. Analogous stabilization of peroxide by hydrogen bonding has been invoked recently in crystalline saccharide and protein systems. The present peroxide adducts are stable at room temperature in dimethyl sulfoxide (DMSO) and N,N'-dimethylformamide (DMF). These adducts can be obtained in gram quantities from the cryptand-driven disproportionation reaction of potassium superoxide (KO(2)) at room temperature.

The lack of fundamental understanding of the oxygen reduction and oxygen evolution in nonaqueous electrolytes significantly hinders the development of rechargeable lithium-air batteries. Here we employ a solid-state Li(4+x)Ti(5)O(12)/LiPON/Li(x)V(2)O(5) cell and examine in situ the chemistry of Li-O(2) reaction products on Li(x)V(2)O(5) as a function of applied voltage under ultra high vacuum (UHV) and at 500 mtorr of oxygen pressure using ambient pressure X-ray photoelectron spectroscopy (APXPS). Under UHV, lithium intercalated into Li(x)V(2)O(5) while molecular oxygen was reduced to form lithium peroxide on Li(x)V(2)O(5) in the presence of oxygen upon discharge. Interestingly, the oxidation of Li(2)O(2) began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O(2) cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O(2) chemistry.

We report a facile synthesis of Fe-N-C catalysts based on the surface functionalization of multi-walled carbon nanotubes (MWCNTs), which show high activity and stability for oxygen reduction reaction (ORR) in acid. Fe-N-MWCNT catalysts, whose ORR mass activities could vary by 3-4 times depending on the choice of Fe precursors, were found to have considerably higher ORR mass activity and higher stability than N-modified MWCNTs (N-MWCNTs). The Fe-N-MWCNT catalyst with a dominant Fe-N(x) moiety (with x ≈ 4) and a surface Fe/C ratio of ∼0.004 exhibits the highest ORR mass activity in acid (∼0.7 mA mg(-1)(Fe-N-MWCNT) at 0.8 V vs. RHE), where the lower mass activity of other Fe-N-MWCNT catalysts can be attributed to lower Fe/C ratios and Fe-N(x) moieties (with x smaller than 4) as revealed from X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Moreover, the enhanced stability of Fe-N-MWCNTs in comparison to N-MWCNTs can be attributed to less H(2)O(2) production during ORR as determined from rotating ring disk electrode (RRDE) measurements, and higher activity for H(2)O(2) electro-reduction by rotating disk electrode (RDE) measurements. The large surface Fe/C ratio and Fe-N(x) moiety corresponding to high ORR activity and stability of Fe-N-MWCNTs demonstrate that surface functionalization can be very helpful to graft active catalytic sites onto carbon nanostructures, and to provide insights into the ORR mechanism of non-noble metal catalysts (NNMCs) for proton exchange membrane fuel cells (PEMFCs).

The efficiency of many energy storage technologies, such as rechargeable metal-air batteries and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen evolution reaction (OER). We found that Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) (BSCF) catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than that of the state-of-the-art iridium oxide catalyst in alkaline media. The high activity of BSCF was predicted from a design principle established by systematic examination of more than 10 transition metal oxides, which showed that the intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an e(g) symmetry of surface transition metal cations in an oxide. The peak OER activity was predicted to be at an e(g) occupancy close to unity, with high covalency of transition metal-oxygen bonds.

We report the intrinsic oxygen reduction reaction (ORR) activity of polycrystalline palladium, platinum, ruthenium, gold, and glassy carbon surfaces in 0.1 M LiClO(4) 1,2-dimethoxyethane via rotating disk electrode measurements. The nonaqueous Li(+)-ORR activity of these surfaces primarily correlates to oxygen adsorption energy, forming a "volcano-type" trend. The activity trend found on the polycrystalline surfaces was in good agreement with the trend in the discharge voltage of Li-O(2) cells catalyzed by nanoparticle catalysts. Our findings provide insights into Li(+)-ORR mechanisms in nonaqueous media and design of efficient air electrodes for Li-air battery applications.

Thin film electrodes of polyaniline (PANi) nanofibers and functionalized multiwall carbon nanotubes (MWNTs) are created by layer-by-layer (LbL) assembly for microbatteries or -electrochemical capacitors. Highly stable cationic PANi nanofibers, synthesized from the rapid aqueous phase polymerization of aniline, are assembled with carboxylic acid functionalized MWNT into LbL films. The pH-dependent surface charge of PANi nanofibers and MWNTs allows the system to behave like weak polyelectrolytes with controllable LbL film thickness and morphology by varying the number of bilayers. The LbL-PANi/MWNT films consist of a nanoscale interpenetrating network structure with well developed nanopores that yield excellent electrochemical performance for energy storage applications. These LbL-PANi/MWNT films in lithium cell can store high volumetric capacitance (~238 ± 32 F/cm(3)) and high volumetric capacity (~210 mAh/cm(3)). In addition, rate-dependent galvanostatic tests show LbL-PANi/MWNT films can deliver both high power and high energy density (~220 Wh/L(electrode) at ~100 kW/L(electrode)) and could be promising positive electrode materials for thin film microbatteries or electrochemical capacitors., (© 2011 American Chemical Society)

The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate that the ORR activity for oxide catalysts primarily correlates to σ-orbital (e(g)) occupation and the extent of B-site transition-metal-oxygen covalency, which serves as a secondary activity descriptor. Our findings reflect the critical influences of the σ orbital and metal-oxygen covalency on the competition between O(2)(2-)/OH(-) displacement and OH(-) regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.

Recently one-dimensonal (1-D) Pt nanostructures have shown greatly enhanced intrinsic oxygen reduction reaction (ORR) activity (ORR kinetic current normalized to Pt surface area) and/or improved durability relative to conventional supported Pt catalysts. In this study, we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in proton exchange membrane fuel cells (PEMFCs). The average ORR intrinsic activity of Pt NPs/MWNTs is ∼0.95 mA/cm(2) Pt at 0.9 ViR-corrected versus reversible hydrogen electrode (RHE), ∼3-fold higher than a commercial catalyst -46 wt % Pt/C (Tanaka Kikinzoku Kogyo) in 0.1 M HClO4 at room temperature. More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mgPt at 0.9 ViR-corrected vs RHE) is higher than other 1-D nanostructured catalysts and TKK catalysts. The enhanced intrinsic activity of 1-D Pt NPs/MWNTs could be attributed to the weak chemical adsorption energy of OHads-species on the surface Pt NPs covering MWNTs.

PtAu nanoparticles (NPs) were shown to strongly enhance the kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Li-O(2) cells. Li-O(2) cells with PtAu/C were found to exhibit the highest round-trip efficiency reported to date. During ORR via xLi(+) + O(2) + xe(-) --> Li(x)O(2), the discharge voltage with PtAu/C was considerably higher than that of pure carbon and comparable to that of Au/C. During OER via Li(x)O(2) --> xLi(+) + O(2) + xe(-), the charge voltages with PtAu/C fell in the range from 3.4 to 3.8 V(Li), which is slightly lower than obtained with Pt. It is hypothesized that PtAu NPs exhibit bifunctional catalytic activity, having surface Au and Pt atoms primarily responsible for ORR and OER kinetics in Li-O(2) cells, respectively.

Multiwall carbon nanotube (MWNT)/manganese oxide (MnO2) nanocomposite ultrathin film electrodes have been created via redox deposition of MnO2 on layer-by-layer (LbL)-assembled MWNT films. We demonstrate that these LbL-assembled MWNT (LbL-MWNT)/MnO2 thin films consist of a uniform coating of nanosized MnO2 on the MWNT network structure using SEM and TEM, which is a promising structure for electrochemical capacitor applications. LbL-MWNT/MnO2 electrodes yield a significantly higher volumetric capacitance of 246 F/cm3 with good capacity retention up to 1000 mV/s due to rapid transport of electrons and ions within the electrodes. The electrodes are generated with two simple aqueous deposition processes: the layer-by-layer assembly of MWNTs followed by redox deposition of MnO2 at ambient conditions, thus providing a straightforward approach to the fabrication of high-power and -energy electrochemical capacitors with precise control of electrode thickness at nanometer scales.

Energy storage devices that can deliver high powers have many applications, including hybrid vehicles and renewable energy. Much research has focused on increasing the power output of lithium batteries by reducing lithium-ion diffusion distances, but outputs remain far below those of electrochemical capacitors and below the levels required for many applications. Here, we report an alternative approach based on the redox reactions of functional groups on the surfaces of carbon nanotubes. Layer-by-layer techniques are used to assemble an electrode that consists of additive-free, densely packed and functionalized multiwalled carbon nanotubes. The electrode, which is several micrometres thick, can store lithium up to a reversible gravimetric capacity of approximately 200 mA h g(-1)(electrode) while also delivering 100 kW kg(electrode)(-1) of power and providing lifetimes in excess of thousands of cycles, both of which are comparable to electrochemical capacitor electrodes. A device using the nanotube electrode as the positive electrode and lithium titanium oxide as a negative electrode had a gravimetric energy approximately 5 times higher than conventional electrochemical capacitors and power delivery approximately 10 times higher than conventional lithium-ion batteries.

An aberration-corrected JEOL 2200FS scanning-transmission electron microscope (STEM), equipped with a high-angle annular dark-field detector (HAADF), is used to monitor the coalescence and sintering of Pt nanoparticles with an average diameter of 2.8 nm. This in situ STEM capability is combined with an analysis methodology that together allows direct measurements of mass transport phenomena that are important in understanding how particle size influences coalescence and sintering at the nanoscale. To demonstrate the feasibility of this methodology, the surface diffusivity is determined from measurements obtained from STEM images acquired during the initial stages of sintering. The measured surface diffusivities are in reasonable agreement with measurements made on the surface of nanoparticles, using other techniques. In addition, the grain boundary mobility is determined from measurements made during the latter stages of sintering.

Design of highly active nanoscale catalysts for electro-oxidation of small organic molecules is of great importance to the development of efficient fuel cells. Increasing steps on single-crystal Pt surfaces is shown to enhance the activity of CO and methanol electro-oxidation up to several orders of magnitude. However, little is known about the surface atomic structure of nanoparticles with sizes of practical relevance, which limits the application of fundamental understanding in the reaction mechanisms established on single-crystal surfaces to the development of active, nanoscale catalysts. In this study, we reveal the surface atomic structure of Pt nanoparticles supported on multiwall carbon nanotubes, from which the amount of high-index surface facets on Pt nanoparticles is quantified. Correlating the surface steps on Pt nanoparticles with the electrochemical activity and stability clearly shows the significant role of surface steps in enhancing intrinsic activity for CO and methanol electro-oxidation. Here, we show that increasing surface steps on Pt nanoparticles of approximately 2 nm can lead to enhanced intrinsic activity up to approximately 200% (current normalized to Pt surface area) for electro-oxidation of methanol.

We have developed a system that uses two 1D interferometric phase projections for reconstruction of 2D water content changes over time in situ in a proton exchange membrane (PEM) fuel cell system. By modifying the filtered backprojection tomographic algorithm, we are able to incorporate a priori information about the object distribution into a fast reconstruction algorithm which is suitable for real-time monitoring.

All multiwall carbon nanotube (MWNT) thin films are created by layer-by-layer (LBL) assembly of surface functionalized MWNTs. Negatively and positively charged MWNTs were prepared by surface functionalization, allowing the incorporation of MWNTs into highly tunable thin films via the LBL technique. The pH dependent surface charge on the MWNTs gives this system the unique characteristics of LBL assembly of weak polyelectrolytes, controlling thickness and morphology with assembly pH conditions. We demonstrate that these MWNT thin films have randomly oriented interpenetrating network structure with well developed nanopores using AFM and SEM, which is an ideal structure of functional materials for various applications. In particular, electrochemical measurements of these all-MWNT thin film electrodes show high electronic conductivity in comparison with polymer composites with single wall nanotubes, and high capacitive behavior with precise control of capacity.

Atomically resolved structures and compositions of Pt alloy nanoparticles were obtained using aberration-corrected high-angle dark field imaging, which was correlated to specific ORR activity based on a Pt surface area. The enhanced specific ORR activity (approximately 2 times relative to Pt) of acid-treated "Pt3Co" nanoparticles can be related to composition variations at the atomic scale and the formation of percolated Pt-rich and Pt-poor regions within individual particles. Upon annealing, we show direct evidence of surface Pt sandwich-segregation structures, which correspond to a specific ORR activity approximately 4 times relative to Pt.

The metastable O2-LiCoO(2) phase undergoes several reversible phase transitions upon lithium deintercalation. The first transition leads to an unusual oxygen stacking in such layered compounds. This stacking is found to be stable for 0.52 < x < or = 0.72 in Li(x)()CoO(2) and is called T(#)2. We studied this phase from a structural viewpoint using X-ray and neutron diffraction (ab initio method). The new stacking derives from the O2 one by gliding every second CoO(2) slab by (1/3, 1/6, 0). The lithium ions are found to occupy very distorted tetrahedral sites in this structure. We also discuss the possibility of this T(#)2 phase to exhibit stacking faults, whose amount depends on the method used to prepare this deintercalated phase.

John Cowley and his group at Arizona State University pioneered the use of transmission electron microscopy (TEM) for high-resolution imaging. Three decades ago they achieved images showing the crystal unit cell content at better than 4 angstroms resolution. Over the years, this achievement has inspired improvements in resolution that have enabled researchers to pinpoint the positions of heavy atom columns within the cell. More recently, this ability has been extended to light atoms as resolution has improved. Sub-angstrom resolution has enabled researchers to image the columns of light atoms (carbon, oxygen, and nitrogen) that are present in many complex structures. By using sub-angstrom focal-series reconstruction of the specimen exit surface wave to image columns of cobalt, oxygen, and lithium atoms in a transition metal oxide structure commonly used as positive electrodes in lithium rechargeable batteries, we show that the range of detectable light atoms extends to lithium. HRTEM at sub-angstrom resolution will provide the essential role of experimental verification for the emergent nanotech revolution. Our results foreshadow those to be expected from next-generation TEMs with CS-corrected lenses and monochromated electron beams.

LiCoO2 is the most common lithium storage material for lithium rechargeable batteries, used widely to power portable electronic devices such as laptop computers. Operation of lithium rechargeable batteries is dependent on reversible lithium insertion and extraction processes into and from the host materials of lithium storage. Ordering of lithium and vacancies has a profound effect on the physical properties of the host materials and the electrochemical performance of lithium batteries. However, probing lithium ions has been difficult when using traditional X-ray and neutron powder diffraction techniques due to lithium's relatively low scattering power when compared with those of oxygen and transition metals. In the work presented here, we have succeeded in simultaneously resolving columns of cobalt, oxygen and lithium atoms in layered LiCoO2 battery material, using experimental focal series of LiCoO2 images obtained at sub-ångstrom resolution in a mid-voltage transmission electron microscope. Lithium atoms are the smallest and lightest metal atoms, and scatter electrons only very weakly. We believe our observations of lithium to be the first by electron microscopy, and that they show promise for direct visualization of the ordering of lithium and vacancies in transition metal oxides.