Showing papers in "Electrochemical and Solid State Letters in 2006"

Luminescence Properties of a Red Phosphor, CaAlSiN3 : Eu2 + , for White Light-Emitting Diodes

Abstract: We developed a new red phosphor, CaAlSiN 3 :Eu 2+ , which has a broad excitation band extended from UV region to 590 nm. At the optimum Eu concentration of 1.6 mol %, quantum output is seven times higher than for a conventional red phosphor, La 2 O 2 S:Eu 3+ , under 405 nm excitation. The phosphor is also more efficient than CaSiN 2 :Eu 2+ or Ca 2 Si 5 N 8 :Eu 2+ at any excitation wavelength. One reason of the high room-temperature efficiency is small thermal quenching, which is probably related to a rigid network of [SiN 4 ] and [AlN 4 ] tetrahedra. The phosphor is chemically stable as well. Accordingly, it is a promising material for warm-white light-emitting diodes.

Electron Microscopy Study of the LiFePO4 to FePO4 Phase Transition

Abstract: The mechanism by which LiFePO 4 is transformed into isostructural FePO 4 has been elucidated using electron microscopy on large, hydrothermally grown LiFePO 4 crystals following chemical delithiation. Lithium is extracted at narrow, disordered transition zones on the ac crystal surface as the phase boundary progresses in the direction of the a-axis. The substantial lattice mismatch along a (ca. 5%) causes crack formation in the bc plane. Despite considerable disorder in the transition zone, the general structural arrangement is preserved, leading to good crystallinity in the newly created FePO 4 domains. Implications for improved electrode performance are discussed.

Size Effects on Carbon-Free LiFePO4 Powders The Key to Superior Energy Density

Abstract: C-free LiFePO 4 crystalline powders were prepared by a synthesis method based on direct precipitation under atmospheric pressure. The particle size distribution is extremely narrow, centered on ca. 140 nm. A soft thermal treatment, typically at 500°C for 3 h under slight reducing conditions was shown to be necessary to obtain satisfactory electrochemical Li + deinsertion/insertion properties. This thermal treatment does not lead to grain growth or sintering of the particles, and does not alter the surface of the particles. The electrochemical performances of the powders obtained by this synthesis method are excellent, in terms of specific capacity (147 mAh g -1 at 5C-rate) as well as in terms of cyclability (no significant capacity fade after more than 400 cycles), without the need of carbon coating.

Synthesis of LiFePO4 Nanoparticles in Polyol Medium and Their Electrochemical Properties

Abstract: LiFePO 4 nanoparticles were synthesized using the polyol process without any further heating as a postprocessing step. The X-ray diffraction patterns of the sample exhibited a good fit with the orthorhombic phase with no unwanted impurity phases. The LiFePO 4 nanoparticles showed a reversible capacity of 166 mAh/g, which amounts to a utilization efficiency of 98%, with an excellent reversibility in extended cycles. The electrode shows an excellent capacity retention at high-rate current densities due to its single-crystal-like and monodispersed uniform morphology with orthorhombic shape with an average width of 20 nm and length of 50 nm.

Degradation Mechanisms of La – Sr – Co – Fe – O3 SOFC Cathodes

Abstract: The long-term stability of anode-supported YSZ electrolyte SOFCs utilizing (La0.6Sr0.4)0.98Co0.2Fe0.8O3-? (LSCF-6428) cathodes was assessed. Samples tested for 500 hours at 750 C and 0.7V indicated ?50% degradation. While scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) analysis indicated no obvious microstructural or chemical phenomena that could explain the high degradation, x-ray photon spectroscopy (XPS) revealed that enrichment of Sr at the cathode-electrolyte and cathode-current collector interfaces was at least partially responsible for the observed degradation.

Incremental Capacity Analysis and Close-to-Equilibrium OCV Measurements to Quantify Capacity Fade in Commercial Rechargeable Lithium Batteries

Abstract: A quantitative approach is used to identify sources of contribution of capacity fade in commercial rechargeable lithium battery cells in laboratory evaluations. Our approach comprises measurements of close-to-equilibrium open-circuit voltage (cte-OCV) of the cell after relaxation at the end of the charging and discharging regimes and an incremental capacity analysis, in addition to conventional cycle-life test protocols using the dynamic stress test schedule. This approach allows us to separate attributes to capacity fade due to intrinsic and extrinsic origins.

Effect of voltage on platinum dissolution : Relevance to polymer electrolyte fuel cells

Abstract: One of the processes responsible for performance degradation of a polymer electrolyte fuel cell (PEFC) is the loss of the electrochemically active surface area of the platinum-based electrocatalysts, due in part to platinum dissolution. The long-term dissolution behavior of polycrystalline platinum and high-surface-area carbon-supported platinum particles was studied under potentiostatic conditions relevant to PEFC cathode conditions. The equilibrium concentration of dissolved Pt was found to increase monotonically from 0.65 to 1.1 V (vs SHE) and decrease at potentials >1.1 V. Dissolution rates measured at 0.9 V were comparable for the two types of electrodes (1.4 and 1.7 × 10 -14 g/cm 2 s).

High Capacity, Surface-Modified Layered Li [ Li ( 1 − x ) ∕ 3Mn ( 2 − x ) ∕ 3Nix ∕ 3Cox ∕ 3 ] O2 Cathodes with Low Irreversible Capacity Loss

Abstract: Layered Li[Li 0.2 Mn 0.54 Ni 0.13 Co 0.13 ]O 2 and Li[Li 0.1 Mn 0.43 Ni 0.23 Co 0.23 ]O 2 cathodes belonging to the Li[Li (1-x)/ 3Mn (2-x)/3 Ni x/3 Co x/3 ]O 2 solid solution series between Li[Li 1/3 Mn 2/3 ]O 2 and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 have been synthesized and characterized by charge-discharge measurements in lithium cells before and after modifying their surface with Al 2 O 3 . The surface-modified cathodes show significantly lower irreversible capacity loss (ICL) and higher discharge capacity with excellent cyclability compared to the unmodified counterparts. For example, surface modified Li[Li 0.2 Mn 0.54 Ni 0.13 Co 0.13 ]O 2 shows a remarkably high capacity of 285 mAh/g with an ICL of 41 mAh/g and good rate capability. This capacity value is much higher than that achieved with the previously known Li[Li 0.17 Mn 0.58 Ni 0.25 ]O 2 after similar surface modification (<255 mAh/g).

Quantification of Liquid Water Saturation in a PEM Fuel Cell Diffusion Medium Using X-ray Microtomography

Abstract: Quantification of liquid water saturation distribution in a diffusion medium is critical to establishing a basic understanding of the two-phase flow and flooding occurrence in proton exchange membrane (PEM) fuel cells. We have used X-ray microtomography to obtain high-resolution (10 X 10 X 13.4 μm), three-dimensional images of liquid water distribution in a gas diffusion layer (GDL) during gas purge. We report on temporally resolved liquid saturation profiles across the GDL thickness and demonstrate the feasibility of using X-ray microtomography to quantify liquid water distribution at the component level. The results show that the drying rate decreases exponentially with purge time and no significant liquid water removal takes place after 6 min of purge, at room temperature.

Damage to the Cathode Catalyst of a PEM Fuel Cell Caused by Localized Fuel Starvation

Abstract: This paper examines damage to the cathode catalyst layer of a polymer electrolyte membrane (PEM) fuel cell caused by restriction of hydrogen access to a portion of the anode catalyst layer. The results show severe damage to the majority of the cathode catalyst behind the obstruction after of operation. A portion of the cathode catalyst layer near the outside edge of the obstruction remains undamaged.

TiO2(B) nanotubes as negative electrodes for rechargeable lithium batteries

Abstract: TiO 2 (B) nanotubes were investigated as anodes for rechargeable lithium batteries. They can accommodate up to 338 mAh g -1 of charge, equivalent to Li 1.01 TiO 2 (B) at a potential of ∼1.5 V vs Li + (1 M)/Li. Rate capability is good with a capacity of 95 mAh g -1 at 2000 mA g -1 (21C). Capacity fade is 0.16% per cycle compared with 0.10% for the corresponding nanowires. There is an irreversible capacity loss of 29% on the first cycle but thereafter charge/discharge efficiency is close to 100%.

Existence of path-dependence in the LiFePO4 electrode

Abstract: This paper explores two unique features of the lithium iron phosphate (LiFePO 4 ) electrode that provide insight into the electrochemical behavior of this system. First, we show the existence of an asymmetric behavior between charge and discharge, whereby the utilization on charge is considerably larger than that on discharge under current densities where transport limitations are important. Second, we show the existence of a path-dependence in this system whereby the high-rate electrochemical behavior of the electrode at a particular state of charge (SOC) depends on the path by which the electrode was brought to that SOC. We qualitatively explain both these features using a shrinking-core model to account for the juxtaposition of the two phases. The path-dependence reported in this paper could have implications in batteries used in hybrid-electric-vehicles as the power capability of this chemistry will depend on its cycling history, thereby complicating predictions of power. In addition, the data reported here emphasizes the importance of ensuring consistency in defining the SOC in experiments on this electrode.

A Tracer Study of Porous Anodic Alumina

Abstract: The present study employs a tungsten tracer incorporated into the aluminum substrate to investigate the development of porosity in anodic alumina formed in phosphoric acid electrolyte. An unusual inversion of the tracer distribution is revealed as the tracer layer traverses the barrier region. Although initially incorporated into the barrier layer at locations beneath pore bases, associated with the scalloped metal/oxide interface, the tracer at these locations subsequently lags behind that found at the cell walls. The behavior is contrary to expectations of a field-assisted dissolution model of pore development, with usual migration behaviors of film species in the barrier layer. However, the findings are consistent with pore formation due mainly to flow of alumina from the barrier layer toward the cell walls, driven by film growth stresses. Flow of film material can also account for the presence of phosphorus species in the film and the increased thickness of the film relative to that of the oxidized metal. Anodic alumina films are used extensively in protection and functionalization of aluminum alloys, in electronics through aerospace to architecture. The films are usually formed in aqueous electrolytes, with two morphological types recognized that depend upon composition of the electrolyte, pH, current density, voltage, temperature, etc. Fig. 1. 1-4 Barrier films consist of compact, amorphous alumina of uniform thickness, up to a few hundred nanometers. Porous films comprise a thin barrier layer next to the metal and an outer layer of porous alumina, up to tens of m thick. 2,3 The pores are of approximately cylindrical section and extend from the film surface to the barrier layer. The thickness of the barrier layer and the diameter of the pores are related to the forming voltage, with ratios of 1n m V 1 , while the thickness of the porous layer depends primarily upon the anodizing charge for a particular current density. The porosity has often been explained by massively increased dissolution of the alumina at the pore base under the high electric field of the barrier layer. 5 An early suggestion was made of a role of oxide flow in the dissolution process, with flow occurring due to electrostriction stresses, estimated to be of the order 100 MPa and sufficient to deform oxides. 6

Room-temperature solid-state sodium/sulfur battery

Abstract: Solid-state sodium/sulfur batteries using polyvinylidene-fluoride-hexafluoropropene (PVDF) polymer electrolyte were prepared and tested at room temperature. Solid sodium/sulfur batteries may be composed of solid-composite-type sulfur electrodes, sodium metal electrodes, and PVDF gel polymer electrolyte. The PVDF gel polymer electrolyte with tetraglyme plasticizer and NaCF 3 SO 3 salt had a high sodium ion conductivity of 5.1 X 10 -4 S cm -1 at 25°C. During the first discharge, the sodium/sulfur battery showed two plateau potentials of 2.27 and 1.73 V, respectively. The first discharge capacity was 489 mAh/g sulfur at room temperature, which was similar to the high temperature battery. The discharge capacity drastically decreased by repeated charge-discharge cycling, and remained at 40 mAh/g after 20 cycles.

Enhanced Electrochemical Performance of Mesoparticulate LiMnPO4 for Lithium Ion Batteries

Abstract: LiMnPO 4 was synthesized using a sol-gel method and tested as a cathode material for lithium ion batteries. After calcination at temperatures between 520 and 570°C particle sizes in the range of 140 to 160 nm were achieved. Subsequent dry ballmilling reduced the diameter to 130 ± 10 nm. Reversible capacities of 156 mAh/g at C/100 and 134 mAh/g at C/10 were measured. At 92 and 79% of the theoretical values, respectively, these are the highest values reported to data for this material. At faster charging rates, the electrochemical performance was found to be improved when smaller particles were used.

Degradation Mechanism of PEMFC under Open Circuit Operation

Abstract: We present a durability test of polymer electrolyte membrane fuel cell (PEMFC) in open circuit condition. Such a condition enhances the deterioration of the membrane electrode assembly. ac impedance spectroscopy measurements and scanning electron microscopy observation suggest that the degradation occurred at the cathode. Direct gas mass spectroscopy of the cathode outlet gas indicated the formation of HF, H 2 O 2 , CO 2 , SO, SO 2 , H 2 SO 2 , and H 2 SO 3 . A kinetic model is presented assuming that the H 2 gas cross leakage from the anode caused the cathode degradation. The model determines the rate of degradation using the permeability across the electrolyte membrane measured for crossover H 2 gas.

Washing Effect of a LiNi0.83Co0.15Al0.02O2 Cathode in Water

Abstract: The formation of LiOH and Li 2 CO 3 impurities on high Ni content LiNi 0 . 8 3 Co 0 . 1 5 Al 0 . 0 2 O 2 powders due to H 2 O and CO 2 absorption from the air can be reduced without structural degradation by washing in water. Although the as-synthesized sample had a moisture content of 570 ppm immediately after firing, this level increased rapidly to 1270 ppm in air with a relative humidity of 50%. However, its content was decreased to 210 ppm after washing twice in water, followed by heat-treatment at 700°C. It is believed that this improvement was due to the decreased level of Li 2 CO 3 and LiOH impurities on the particles. This was highlighted by the decreasing swelling of the Li-ion cell at 90°C, and the thickness of the cell containing the washed samples was decreased by 50% compared with the bare sample.

Phase Diagram of Li x FePO4

Abstract: The phase diagram for LixFePO4 has been determined for different lithium concentrations and temperatures. The two lowtemperature phases, heterosite and triphylite, have previously been shown to transform to a disordered solid solution at elevated temperatures. This disordered phase allows for a continuous transition between the heterosite and triphylite phases and is stable at relatively low temperatures. At intermediate temperatures the proposed phase diagram resembles a eutectoid system, with eutectoid point at around x = 0.6 and 200°C. Kinetics of mixing and unmixing transformations are reported, including the hysteresis between heating and cooling. The enthalpy of this transition is at least 700 J/mol.

A proton-conducting In3+-doped SnP2O7 electrolyte for intermediate-temperature fuel cells

Abstract: We report proton conduction in In 3+ -doped SnP 2 O 7 in the temperature range from 100 to 300°C, and the performance of a H 2 -air fuel cell using this material as the electrolyte. The proton conductivity of In 3+ -doped SnP 2 O 7 was more than 10 -1 S cm -1 between 125 and 300°C, and a conductivity value of 1.95 × 10 -1 S cm -1 was achieved at 250°C. The resulting fuel cell exhibited a reasonable power density of 264 mW cm -2 at 250°C (electrolyte thickness = 0.35 mm), together with perfect tolerance toward 10% CO and good thermal stability in unhumidified conditions.

Is H2O2 Involved in the Membrane Degradation Mechanism in PEMFC

Abstract: The involvement of H 2 O 2 in the membrane degradation mechanism in a polymer electrolyte membrane fuel cell (PEMFC) was investigated. Measurement of fluoride concentration in the effluent water was used as an indicator of the membrane degradation rate. It was found that H 2 O 2 is formed in the fuel cell in small concentrations but is not the main source of harmful species, which degrade the membrane. H 2 O 2 decomposition due to impurities or the catalyst leading to the possible formation of radical species would only account for a small fraction of the membrane degradation rate in a fuel cell.

LSM-Infiltrated Solid Oxide Fuel Cell Cathodes

Abstract: A single-step infiltration method has been developed to incorporate the La 0.85 Sr 0.15 MnO 3-δ (LSM) oxide phase into a pre-sintered, porous yttria-stabilized zirconia (YSZ) network, forming an effective LSM-YSZ composite cathode. The LSM particles, with a size of ∼30 to ∼100 nm, deposit preferentially on the pore walls throughout the porous YSZ, forming a pathway for electron conduction, and creating a high density of active sites for the oxygen reduction reaction. The resulting composite cathode has been evaluated in a solid oxide fuel cell operating at 923 K, and demonstrated the effectiveness of the infiltration process. The single-step infiltration method enables the possibility of producing solid oxide fuel cell cathodes, based on pre-sintered porous YSZ networks, with other catalysts that are not compatible with the high temperature processing that co-firing may require.

An Advanced Hybrid Electrochemical Capacitor That Uses a Wide Potential Range at the Positive Electrode

Abstract: An advanced hybrid electrochemical capacitor (HEC) has been proposed that uses a wide potential range at the positive electrode (cathode). The conventional HEC uses Li-dopable carbon as the negative electrode (anode) and activated carbon as the cathode, where the potential ranges from 3 to 4.5 V vs Li/Li + . In the advanced HEC, the potential range at the cathode is extended to the whole potential window of activated carbon, specifically from 1.5 to 4.5 V vs Li/Li + . This gives higher energy and power densities as a result of a reduction in the amount of cathode material and electrolyte.

Electrochemical Performance of Co3O4–C Composite Anode Materials

Abstract: Co 3 O 4 -C composite powder has been synthesized via spray pyrolysis of cobalt nitrate-sugar solution at 600°C and assessed for application as anode materials in Li-ion batteries. Microstructural characterization by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy confirm an even distribution of carbon throughout particles, as well as the presence of a carbon-based surface sheath surrounding Co 3 O 4 -C particle agglomerates. Charge-discharge cycling of half-cells indicates a stable reversible discharge capacity above 800 mAh g -1 . Equivalent circuit modeling of Nyquist plots show the Co 3 O 4 -C electrode has significant kinetic advantages over non-composite transition metal oxide electrodes.

Surface Charge Density of Unpassivated and Passivated Metal-Catalyzed Silicon Nanowires

Abstract: Metal-catalyzed nanowires have previously been proposed as the active elements of field-effect devices, such as metal oxide field effect transistors and sensors. For these applications, the nanowire surface properties must be understood and controlled. Initial measurements of nanowire surface-charge density are presented here. The surface-charge density was obtained by measuring nanowires of different diameters grown between electrodes. The surface-charge density of a nanowire covered with native oxide is about 2 X 10 12 cm -2 ; the density appears to decrease by a factor of two to four when the native oxide is replaced by a high-quality thermally grown oxide, with further improvement possible.

Air-Breathing Laminar Flow-Based Direct Methanol Fuel Cell with Alkaline Electrolyte

Abstract: Microfuel cells have the potential to achieve higher energy densities than batteries and have thus received intense investigation as a power source for a wide range of portable applications. Extensive research efforts are focused on the development and miniaturization of promising fuel cell technologies, including direct methanol fuel cells DMFCs and polymer electrolyte membrane-based fuel cells PEMFCs, operated with hydrogen/oxygen. 1-3 In most fuel cells, a polymer electrolyte membrane such as Nafion allows protons to diffuse from the anode to the cathode, while trying to prevent fuel molecules from diffusing across and mixing with oxygen at the cathode. Poor performance or a lack of selectivity by the membrane leads to a key performance-limiting process called fuel crossover that has plagued the PEM-based fuel cells. In addition to fuel crossover, cathode flooding and anode dry-out water management due to osmotic drag of water molecules associated with protons diffusing from the anode to the cathode, as well as due to the formation and consumption of water at the cathode and anode, respectively, impedes the performance and commercial implementation of these fuel cells. 4

Carbon nanotubes reinforced nafion composite membrane for fuel cell applications

Abstract: A carbon nanotubes (CNTs) reinforced Nafion composite membrane for the H-2/O-2 fuel cell was developed. CNTs/Nafion composite membrane, in which CNTs were dispersed uniformly, was prepared by solution-casting. CNTs reinforced membranes, with the addition of a small amount of CNTs (1 wt %), showed excellent mechanical strength. The CNTs/Nafion composite membrane could also decrease dimensional change compared with the commercial Nafion membrane. The performance of the CNTs reinforced composite membrane (50 mu m) was almost the same as the cell prepared with commercial Nafion NRE-212 membrane. (c) 2006 The Electrochemical Society.

In Situ Imaging of Liquid Water and Ice Formation in an Operating PEFC during Cold Start

Abstract: Cold-start capability and survivability of polymer electrolyte fuel cells PEFCs in a subzero environment remain a major challenge for automotive applications. Its fundamental mechanisms are not fully determined, but it is recognized that product water becomes ice or frost upon startup when the PEFC internal temperature is below the freezing point of water. If the local pore volume of the cathode catalyst layer CL is insufficient to contain all of the accumulated water before the cell operating temperature rises above freezing, solid water may plug the catalyst layer and stop the electrochemical reaction by starving the reagent gases. In addition, ice formation may result in serious damage to the structure of a membrane electrode assembly MEA. In spite of the importance of PEFC cold-start capability and associated MEA durability, very few studies in the literature have focused on PEFC startup dynamics, freeze/thaw cycling,

Resistive Switching in Pt ∕ Al2O3 ∕ TiO2 ∕ Ru Stacked Structures

Abstract: The electric-pulse-induced resistive switching properties of TiO 2 , Al 2 O 3 , Al 2 O 3 /TiO 2 , and Al 2 O 3 /TiO 2 /Al 2 O 3 thin films were studied by current-voltage (I-V) measurements using Pt/insulator/Ru structures and conductive atomic force microscopy. The switching parameters of the TiO 2 film were stable, whereas those of the Al 2 O 3 films show random variations during repeated I-V measurements. Both films show resistive switching by a filamentary switching mechanism with linear conduction behavior in the low V region. The stacked film shows a bias polarity-dependent switching behavior. This suggests that the nucleation of the conducting filaments occurs at the interface where the electrons are injected.

Synthesis and characterization of Li2MnxFe1-xSiO4 as a cathode material for lithium-ion batteries

Abstract: The synthesis, structure, and performance of carbon-containing Li 2 Mn x Fe 1-x SiO 4 were studied in this work. A synthesis route has been developed for Li 2 Mn x Fe 1-x SiO 4 materials, i.e., the carbon-containing Li 2 Mn x Fe 1-x SiO 4 was synthesized by a solution route. The results of X-ray diffraction show that Li 2 Mn x Fe 1-x SiO 4 solid solutions can be achieved in a wide compositional range. A capacity of 214 mAh/g and energy density of 593 Wh/kg has been achieved for the Li 2 Mn x Fe 1-x SiO 4 (x = 0.5) sample.