Showing papers in "Journal of The Electrochemical Society in 2008"

Improving the Gate Stability of ZnO Thin-Film Transistors with Aluminum Oxide Dielectric Layers

Abstract: We report on the fabrication of gate-stable ZnO thin-film transistors (TFTs) with aluminum oxide dielectric. When an off-stoichiometric AlO x was deposited at room temperature, the ZnO-TFT revealed unreliable transfer characteristics: a large drain current-gate bias (I D -V G ) hysteresis and a large amount of threshold voltage (V T ) shift under gate-bias stress. As rapid thermal annealing (RTA) in O 2 ambient was applied onto AIO X at 300°C prior to ZnO channel deposition, the gate-bias reliability of the ZnO device was improved. The RTA might cause our AlO x surface to be more stoichiometric and thus to be resistant against ZnO sputter-induced damage. When the bottom-gate ZnO-TFT was fabricated with a stoichiometric Al 2 O 3 dielectric grown by atomic layer deposition (ALD), our device showed much more stable electrical characteristics than with the sputter-deposited off-stoichiometric AlO x . Last, as an ultimate effort to improve the gate reliability, we fabricated a top-gate ZnO-TFT device adopting the same thick ALD-grown stoichiometric Al 2 O 3 as in the bottom-gate device. Our top-gate device with the Al 2 O 3 dielectric then showed no hysteresis and no V T shift after several times of gate bias sweep. We conclude that both the high quality dielectric and optimized device structure are necessary to realize electrically stable ZnO-TFTs.

Evaluation and Modeling of the Cell Resistance in Anode-Supported Solid Oxide Fuel Cells

Abstract: The impedance of anode-supported single cells [Ni/8 yttria-stabilized zirconia (YSZ) anode; La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ cathode; 8YSZ electrolyte; area 1 cm 2 ] was characterized in a broad measuring range of temperature and air/fuel gas composition. The data has been analyzed by calculating the distribution function of relaxation times (DRTs). DRT computations enabled us to separate five different loss mechanisms occurring inside the cathode and anode without the need of an equivalent circuit. Two processes exhibit a systematic dependency on changes in the oxygen partial pressure of the cathode gas and thus can be attributed to diffusional and electrochemical losses on the cathode side. The remaining three processes are very sensitive to changes in the fuel gas but are not affected by variations of the cathode gas. These resistances are classified as a gas diffusion polarization within the anode-substrate and as an electro-oxidation reaction at the triple-phase boundary, respectively.

Electrolyte Solutions with a Wide Electrochemical Window for Rechargeable Magnesium Batteries

Abstract: Electrolyte solutions for rechargeable Mg batteries were developed, based on reaction products of phenyl magnesium chloride (PhMgCl) Lewis base and Alcl 3 Lewis acid in ethers. The transmetallation of these ligands forms solutions with Mg x Cl + y and AlCl 4-n Ph n - ions as the major ionic species, as analyzed by multinuclei nuclear magnetic resonance spectroscopy. Tetrahydrofuran (THF) solutions of (PhMgCl) 2 -Alcl 3 exhibit optimal properties: highly reversible Mg deposition (100% cycling efficiency) with low overvoltage: <0.2 V and electrochemical windows wider than 3 V. A specific conductivity of 2-5 X 10 -3 Ω -1 cm -1 could be measured between -10 and 30°C for these solutions, similar to that of standard electrolyte solutions for Li batteries. Mg ions intercalate reversibly with Chevrel phase (Mg x Mo 6 S 8 ) cathodes in these solutions. These systems exhibit high thermal stability. The solutions may enable the use of high voltage, high-capacity Mg insertion materials as cathodes and hence open the door for research and development of high-energy density, rechargeable Mg batteries.

LnBaCo2O5+δ oxides as cathodes for intermediate-temperature solid oxide fuel cells

Abstract: LnBaCo 2 O 5+δ (Ln = Nd, Sm, Gd, and Y) oxides with a cation ordered perovskite structure have been investigated as cathode materials for intermediate-temperature solid oxide fuel cells (SOFCs) The oxygen content 5 + δ, thermal expansion coefficient (TEC), and electrical conductivity (metallic) decrease with decreasing size of the Ln 3+ ions from Ln = La to Y While the decrease in TEC is due to the decreasing ionicity of the Ln-O bond, the decrease in electrical conductivity is due to the increasing oxide ion vacancies and a bending of the O-Co-O bonds The power density of single-cell SOFCs fabricated with the LnBaCo 2 Ο 5+δ cathodes, La 08 Sr 02 Ga 08 Mg 02 Ο 28 electrolyte, and Ni-Ce 09 Gd 01 Ο 195 cermet anode decrease with decreasing size of the Ln 3+ ions, partly due to a decreasing electrical conductivity The LnBaCo 2 Ο 5+δ cathodes with an intermediate lanthanide ion such as Sm 3+ offer a trade-off between catalytic activity and TEC

Design of an Electrochemical Cell Making Syngas ( CO + H2 ) from CO2 and H2O Reduction at Room Temperature

Abstract: An electrolysis-cell design for simultaneous electrochemical reduction of CO 2 and H 2 O to make syngas (CO + H 2 ) at room temperature (25°C) was developed, based on a technology very close to that of proton-exchange-membrane fuel cells (PEMFC), i.e., based on the use of gas-diffusion electrodes so as to achieve high current densities. While a configuration involving a proton-exchange membrane (Nafion) as electrolyte was shown to be unfavorable for CO 2 reduction, a modified configuration based on the insertion of a pH-buffer layer (aqueous KHCO 3 ) between the silver-based cathode catalyst layer and the Nafion membrane allows for a great enhancement of the cathode selectivity for CO 2 reduction to CO [ca. 30 mA/cm 2 at a potential of -1.7 to -1.75 V vs SCE (saturated-calomel reference electrode)]. A CO/H 2 ratio of 1/2, suitable for methanol synthesis, is obtained at a potential of ca. -2 V vs SCE and a total current density of ca. 80 mA/cm 2 . An issue that has been identified is the change in product selectivity upon long-term electrolysis. Results obtained with two other cell designs are also presented and compared.

Si Electrodes for Li-Ion batteries- A new way to look at an old problem

Abstract: High-capacity Si-based electrodes could replace carbon-based electrodes in the next generation of Li-ion batteries. Although Si-based electrodes have large gravimetric capacities, they typically suffer from poor cyclability. One reason for the poor cyclability is large volume expansion associated with 3.75 mol of Li reacting with 1 mol of Si. A theoretical approach to design electrodes that can accommodate this large volume expansion is discussed. It is shown that experimental results agree well with the theoretical approach. We show that Si-based electrodes with a relatively low Si content (<33 wt %) and high binder content (33-56 wt %) cycle at large capacities (∼ 660 mAh/g) for hundreds of cycles. No special electrode processing or cycling procedures are required to achieve high capacities with good cyclability.

Thermal Model for a Li-Ion Cell

Abstract: A thermal model for a lithium-ion cell is presented and used to predict discharge performance at different operating temperatures. The results from the simulations are compared to experimental data obtained from lithium-ion pouch cells. The model includes a set of parameters (and their concentration and temperature dependencies) that has been obtained for a lithium-ion cell composed of a mesocarbon microbead anode, LiCoO 2 cathode in 1 M LiPF 6 salt, in a mixture of ethylene carbonate, propylene carbonate, ethyl-methyl carbonate, and diethyl carbonate electrolyte. The parameter set was obtained by comparing the model predictions to the experimental discharge profiles obtained at various temperatures and rates. The concentration and temperature dependence of the extracted parameters were correlated through empirical expressions. Also, the effect of including the thermal dependence of various parameters in the model on the simulated discharge profiles is discussed.

A Mathematical Model for a Lithium-Sulfur Cell

Abstract: A mathematical model is presented for a complete lithium-sulfur cell. The model includes various electrochemical and chemical (precipitation) reactions, multicomponent transport phenomena in the electrolyte, and the charge transfer within and between solid and liquid phases. A change in the porosity of the porous cathode and separator due to precipitation reactions is also included in the model. The model is used to explain the physical reasons for the two-stage discharge profiles that are typically obtained for lithium-sulfur cells.

Intercalation-Induced Stress and Heat Generation within Single Lithium-Ion Battery Cathode Particles

Abstract: Intercalation-induced stress and heat generation inside Li-ion battery cathode (LiMn 2 O 4 ) particles under potentiodynamic control are simulated in this paper. We combined analyses of transport and kinetics in determining resulting stresses, which arise from concentration gradients in cathode particles, and heat generation. Two peaks in boundary reaction flux, and resulting stresses, were determined from the modeling of electrochemical kinetics and diffusion, using intrinsic material properties (resulting in two plateaus in the open-circuit potential) and the applied potential. Resistive heating was identified as the most important heat generation source. To probe the impact of the particle shape (equivalent radius and aspect ratio of an ellipsoidal particle) and the potential sweep rate on stress and heat generation, a surrogate-based analysis was also conducted. The systematic study showed that both intercalation-induced stress and time-averaged resistive heat generation rate increase with particle radius and potential sweep rate. Intercalation-induced stress increases first, then decreases as the aspect ratio of an ellipsoidal particle increases, whereas time-averaged resistive heat generation rate decreases as aspect ratio increases. This surrogate-based analysis suggests that ellipsoidal particles with larger aspect ratios are preferred over spherical particles, in improving battery performance when stress and heat generation are the only factors considered.

The Effects of AlF3 Coating on the Performance of Li [ Li0.2Mn0.54Ni0.13Co0.13 ] O2 Positive Electrode Material for Lithium-Ion Battery

Abstract: National Basic Research Program of China (973 Program) [2007CB209702]; National Natural Science Foundation of China (NNSFC) [20433060, 20473068, 29925310]

Combined Deconvolution and CNLS Fitting Approach Applied on the Impedance Response of Technical Ni ∕ 8YSZ Cermet Electrodes

Abstract: A high-resolution impedance study of the hydrogen oxidation in Ni/8YSZ (yttria-stabilized zirconia) cermet anodes has been realized in consideration of a broad range of operating conditions (temperature and partial pressure of fuel gas components H 2 , H 2 O, N 2 , He). A major problem in this respect concerns the origin and physical interpretation of empirical equivalent circuits used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: (i) at first, by the deconvolution of a distribution function of relaxation times (DRT), four different processes and their characteristic relaxation times have been identified. Two processes at frequencies < 1 kHz represent a gas-conversion process or, respectively, a gas diffusion, whereas two processes at higher frequencies (2-30 kHz) are associated with the electro-oxidation of hydrogen at active sites, including the charge transfer reaction and the ionic transport. (ii) Subsequently, the last mentioned processes were fitted to a "transmission line" model describing the electronic and ionic transport properties of the Ni/8YSZ cermet. The high resolution of the DRT combined with the numeric accuracy of the complex nonlinear least square (CNLS) fit enabled us to determine (i) the effective ionic conductivity of the Ni/8YSZ cermet, (ii) the spatial extension of the electrochemically active area adjacent to the electrolyte/electrode interface, and (iii) the charge transfer resistance and its thermal activation energy.

Solid Oxide Electrolysis Cells: Microstructure and Degradation of the Ni/Yttria-Stabilized Zirconia Electrode

Abstract: Solid oxide fuel cells produced at Riso DTU have been tested as solid oxide electrolysis cells for steam electrolysis by applying an external voltage. Varying the sealing on the hydrogen electrode side of the setup verifies that the previously reported passivation over the first few hundred hours of electrolysis testing was an effect of the applied glass sealing. Degradation of the cells during long-term galvanostatic electrolysis testing [850°C, -1/2 A/cm 2 , p(H 2 O)/p(H 2 ) = 0.5/0.5] was analyzed by impedance spectroscopy and the degradation was found mainly to be caused by increasing polarization resistance associated with the hydrogen electrode. A cell voltage degradation of 2%/1000 h was obtained. Postmortem analysis of cells tested at these conditions showed that the electrode microstructure could withstand at least 1300 h of electrolysis testing, however, impurities were found in the hydrogen electrode of tested solid oxide electrolysis cells. Electrolysis testing at high current density, high temperature, and a high partial pressure of steam [-2 A/cm 2 , 950°C, p(H 2 O) = 0.9 atm] was observed to lead to significant microstructural changes at the hydrogen electrode-electrolyte interface.

Mn4 + -Activated Red Photoluminescence in K2SiF6 Phosphor

Abstract: The aim of this paper is to report the optical properties of K 2 SiF 6 :Mn 4+ phosphor prepared by wet chemical etching of Si wafers in a HF/KMnO 4 mixed solution. The luminescence centers of sharp red emission at ∼630 nm are ascribed to the Mn 4+ ions in the octahedral site of K 2 SiF 6 gained by the activation of local vibration modes ( 4 A 2 → 2 E). All the expected electronic and vibronic origins of the 2 E → 4 A 2 , 4 A 2 → 4 T 2 , and 4 A 2 → 4 T 1 transitions of the Mn 4+ ions are identified and depicted in the TanabeSugano diagram of a 3d 3 transition-metal system. The temperature-dependent photoluminescence properties suggest that the anti-Stokes/Stokes intensity ratio can be explained by the Maxwell-Boltzmann factor but only at low temperatures and low-excitation power intensities.

Effect of Electrolyte Composition on Lithium Dendrite Growth

Abstract: Lithium deposition is observed in situ using a microfluidic test cell. The microfluidic device rapidly sets up a steady concentration gradient and minimizes ohmic potential loss, minimizes electrolyte usage, and shows good repeatability. Dendrite growth is observed at different current densities for electrolytes containing lithium hexafluorophosphate or lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) in mixtures of propylene carbonate (PC) and dimethyl carbonate. Dendrites are formed at shorter times in electrolytes containing LiTFSI and high amounts of PC. The time to first observed dendrites increases linearly (for all electrolyte compositions) with a resistance given by the Tafel slope of the lithium reduction polarization curve.

In Situ High-Resolution Neutron Radiography of Cross-Sectional Liquid Water Profiles in Proton Exchange Membrane Fuel Cells

Abstract: High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The cross-sectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. Detailed information was obtained on the cross-sectional water content in the membrane electrode assembly and the gas flow channels. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate, where both water diffusion from the cathode and thermal gradients play a significant role in determining the water balance of the cell. Within the membrane electrode assembly itself, the cathode side was moderately more hydrated than the anode side of the assembly from 0.1 to 1.25 A cm -2 . The total liquid water content of the membrane electrode assembly was fairly stable between current densities of 0.25 and 1.25 A cm -2 , even though the water in the gas flow channels changed drastically over this current density range. At 60°C, the water content in the center of the gas diffusion layer was depleted compared to the membrane or gas flow channel interfaces. This phenomenon was not observed at 80°C where evaporative water removal is prevalent.

Surface Modification of High Capacity Layered Li [ Li0.2Mn0.54Ni0.13Co0.13 ] O2 Cathodes by AlPO4

Abstract: Layered Li[Li 0.2 Mn 0.54 Co 0.13 Ni 0.13 ]O 2 cathode, which is a solid solution between layered Li[Li 1/3 Mn 2/3 ]O 2 and Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 , has been surface modified with various amounts (0-4 wt %) of AlPO 4 and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical measurements in lithium cells. Annealing the surface-modified samples at 400 and 700°C leads to the formation of Li 3 PO 4 on the surface and an incorporation of some Al 3+ into the layered oxide lattice. More importantly, surface modification with AlPO 4 (<4 wt %) increases the discharge capacity and drastically reduces the irreversible capacity loss in the first cycle compared to the values observed with the pristine (unmodified) sample. For example, the irreversible capacity loss decreases from 75 to 27 mAh/g and the discharge capacity increases from 253 to 279 mAh/g on surface modification with 2 wt % AlPO 4 . The results are explained based on the retention of more oxide ion vacancies in the layered lattice after surface modification.

Germanium MOSFET Devices: Advances in Materials Understanding, Process Development, and Electrical Performance

Abstract: 7cm 2 ; however, only a 2 times reduction in junction leakage is observed and no benefit is seen in on-state current. Ge wet etch rates are reported in a variety of acidic, basic, oxidizing, and organic solutions, and modifications of the RCA clean suitable for Ge are discussed. Thin, strained epi-Si is examined as a passivation of the Ge/gate dielectric interface, with an optimized thickness found at 6 monolayers. Dopant species are overviewed. P and As halos are compared, with better short channel control observed for As. Area leakage currents are presented for p/n diodes, with the n-doping level varied over the range relevant for pMOS. Germanide options are discussed, with NiGe showing the most promise. A defect mode for NiGe is reported, along with a fix involving two anneal steps. Finally, the benefit of an end-of-process H2 anneal for device performance is shown.

Investigation and Discussion of Characteristics for Intermetallic Phases Common to Aluminum Alloys as a Function of Solution pH

Abstract: This paper presents results for corrosion potentials, pitting potentials, and electrochemical characteristics for intermetallic particles commonly present in high strength aluminum-based alloys, for tests conducted in a 01 M NaCl solution of varying pH via the use of a microcapillary electrochemical cell The intermetallics investigated were Mg2Si, MgZn2 ,A l7Cu2Fe, Al2Cu, Al2CuMg, and Al3Fe Elaboration of the results reveals that the electrochemical behavior of such compounds varies markedly with pH, with attendant ramifications for localized corrosion and protection in Al alloys Examples of this are shown for AA7075-T651, where it is shown that the localized corrosion morphology is drastically different upon the bulk alloy depending on the pH of the test environment A stochastic pitting is observed at an acid pH, near-neutral conditions result in a deterministic-type pitting, and a general corrosion is observed at an alkaline pH

Localized Corrosion of Magnesium in Chloride-Containing Electrolyte Studied by a Scanning Vibrating Electrode Technique

Abstract: An in situ scanning vibrating electrode technique (SVET) is used to investigate localized corrosion occurring on unpolarized magnesium (Mg) samples immersed in 5% w/v aqueous sodium chloride electrolyte. Corrosion is characterized by the appearance of circular, blackened areas which expand radially at a constant rate and evolve hydrogen vigorously. These are shown to consist of a cathodically active center surrounded by a 0.5 mm wide anodic ring. Any localized corrosion currents emerging from the intact (uncorroded) Mg surface are negligible by comparison. Local anodic current density is shown to be directly proportional to the radius of the local cathode, while corresponding local cathodic current density remains relatively constant with time. Estimates of time-dependent rates of total equivalent Mg loss and H 2 evolution, obtained by numerical integration of SVET-derived normal current density distributions, indicate that corrosion rate is controlled by the area of local cathodic activity. The empirical findings are consistent with a mechanism involving cathodic H 2 evolution on the dark, film-free region which is galvanically coupled with anodic attack of the intact Mg surface. It, is proposed that cathodic activation of the film-free, corroded "disk" is caused by a combination of elevated pH and enrichment in noble iron-containing impurity phases.

Synthesis and Characterization of Nano- MnO2 for Electrochemical Supercapacitor Studies

Abstract: Nanostructured MnO 2 was synthesized at ambient condition by reduction of potassium permanganate with aniline. Powder X-ray diffraction, thermal analysis (thermogravimetric and differential thermal analysis), Brunauer-Emmett-Teller surface area, and infrared spectroscopy studies were carried out for physical and chemical characterization. The as-prepared MnO 2 was amorphous and contained particles of 5-10 nm diameter. Upon annealing at temperatures >400°C, the amorphous MnO 2 attained crystalline α-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods is evident from scanning electron microscopy and transmission electron microscopy (TEM) studies. High-resolution TEM images suggested that nanoparticles and nanorods grow in different crystallographic planes. Capacitance behavior was studied by cyclic voltammetry and galvanostatic charge-discharge cycling in a potential range from -0.2 to 1.0 V vs SCE in 0.1 M sodium sulfate solution. Specific capacitance of about 250 F g -1 was obtained at a current density of 0.5 mA cm -2 (0.8 A g -1 ).

Theoretical Energy Density of Li–Air Batteries

Abstract: A model for predication of the gravimetric and volumetric energy densities of Li-air batteries using aqueous electrolytes is developed. The theoretical gravimetric/volumetric capacities and energy densities are calculated based on the minimum weight of the electrolyte and volume of air electrode needed for completion of the electrochemical reaction with Li metal as an anode electrode. It was determined that both theoretical gravimetric/volumetric capacities and energy densities are dependent on the porosity of the air electrode. For instance, at a porosity of 70%, the maximum theoretical cell capacities are 435 mAh/g and 509 mAh/cm 3 in basic electrolyte, and 378 mAh/g and 452 mAh/cm 3 in acidic electrolyte. The maximum theoretical cell energy densities are 1300 Wh/kg and 1520 Wh/L in basic electrolyte, and 1400 Wh/kg and 1680 Wh/L in acidic electrolyte. The significant deduction of cell capacity from specific capacity of Li metal is due to the bulky weight requirement from the electrolyte and air electrode materials. In contrast, the Li-air battery using a nonaqueous electrolyte does not consume electrolyte during the discharge process and has high cell energy density. For Li-air batteries using both aqueous and nonaqueous electrolytes, the weight increases by 8-13% and the volume decreases by 8-20% after the cell is fully discharged.

Nondestructive Reconstruction and Analysis of SOFC Anodes Using X-ray Computed Tomography at Sub-50 nm Resolution

Abstract: A high-resolution, nondestructive X-ray computed tomography (XCT) technique is applied to image the three-dimensional (3D) microstructure of a solid oxide fuel cell (SOFC) composed of a solid yttria-stabilized zirconia (YSZ) electrolyte and a porous nickel YSZ (Ni-YSZ) anode. The X-ray microscope uses the 8 keV Cu Kα line from a laboratory X-ray source, with a reflective condenser optic lens providing a spatial resolution of 42.7 nm. The reconstructed volume data is visualized as 3D images and further postprocessed in binary-image format to obtain structural parameters. The porosity is calculated using a voxel counting method, and tortuosity is evaluated by solving the Laplace equation. A 3D representation of the microstructure is used to calculate true structural parameters and carry out a detailed study of the gas transport within an SOFC electrode at the pore scale. Simulation of multicomponent mass transport and electrochemical reactions in the anode microstructure using the XCT data as geometric input illustrate the impact of this technique on SOFC modeling.

Li∕Polymer Electrolyte∕Water Stable Lithium-Conducting Glass Ceramics Composite for Lithium–Air Secondary Batteries with an Aqueous Electrolyte

Abstract: A water-stable Li metal anode with water-stable lithium-conducting glass ceramics, Li 1+x+y Ti 2-x Al x Si y P 3-y O 12 (LTAP), and a lithium-conducting polymer electrolyte, PEO 18 Li(CF 4 SO 2 ) 2 N (PEO 18 LiTFSI), was proposed as the lithium anode for lithium-air batteries with an aqueous solution at the air electrode. LTAP was unstable when in direct contact with Li metal, and the cell resistance of Li/LTAP/Li rapidly increased as a function of the contact time. The Li/PEO 18 LiTFSI/LTAP/PEO 18 LiTFSI/Li symmetrical cell showed no change in the total resistance (around 800 Ω cm 2 at 60°C) over a period of 1 month. The PEO 18 LiTFSI membrane served as a protective interlayer to suppress the reaction between the water-stable glass ceramics LTAP and Li metal effectively. The Li/PEO 18 LiTFSI/LTAP/aqueous LiCl/Pt air cell showed a stable open-circuit voltage of 3.70 V at 60°C for 2 months. The open-circuit voltage was comparable with that calculated from the cell reaction of 2Li + 1/2O 2 + H 2 O = 2LiOH. The cell exhibited a favorable discharge and charge performance at 0.25 mA cm -2 and 60°C.

Effect of Carbon Particle Size on Electrochemical Performance of EDLC

Abstract: The effect of particle size on the electrochemical performance of electrical double-layer capacitors (EDLCs) has been studied using carbon derived from silicon carbide powders with 20 nm to 20 μm grains at temperatures from 800 to 1200°C. For the same synthesis temperature, similar pore texture and microstructure of carbide-derived carbons produced from different powders have been observed. Nanoparticles exhibited a slight porosity modification with a larger pore volume (1.8 cc/g) and surface area (1300 m 2 /g) as compared to micrometer particles (0.4 cc/g and 1100 m 2 /g). Capacitances as high as 135 F/g associated with a small resistance and time constant have been reached for nano- and sub-micrometer particles synthesized at low temperatures and tested in a tetraethylammonium tetrafluoroborate in acetonitrile solution. This result suggests that small particles facilitate the migration of the ions inside the porous electrodes, allowing them to reach the whole pore volume due to a shorter transport distance within the particle.

Durability of Perfluorosulfonic Acid and Hydrocarbon Membranes: Effect of Humidity and Temperature

Abstract: The effect of humidity on the chemical stability of two types of membranes [i.e., perfluorosulfonic acid type (PFSA, Nafion 112) and biphenyl sulfone hydrocarbon type, (BPSH-35)] was studied by subjecting the membrane electrode assemblies (MEAs) to open-circuit voltage (OCV) decay and potential cycling tests at elevated temperatures and low inlet-gas relative humidities. The BPSH-35 membranes showed poor chemical stability in ex situ Fenton tests compared to that of Nafion membranes. However, under fuel cell conditions, BPSH-35 MEAs outperformed Nafion 112 MEAs in both the OCV decay and potential cycling tests. For both membranes, (i) at a given temperature, membrane degradation was more pronounced at lower humidities and (ii) at a given relative humidity operation, increasing the cell temperature accelerated membrane degradation. Mechanical stability of these two types of membranes was also studied using relative humidity (RH) cycling. Due to decreased swelling and contraction during wet-up and dry-out cycles, Nafion 112 lasted longer than BPSH-35 membranes in the RH cycling test.

Stability of Corrosion-Resistant Magnéli-Phase Ti4O7-Supported PEMFC Catalysts at High Potentials

Abstract: Substoichiometric titanium oxide (Ti 4 O 7 )-supported Pt catalysts were prepared and their electrochemical properties, particularly the effects of high-potential conditions on the activity and stability of Pt/Ti 4 O 7 catalysts, were compared to those of Pt/C catalyst. Polarization measurements using membrane electrode assemblies revealed that the Pt/Ti 4 O 7 cathode shows a similar activity for the oxygen reduction reaction as Pt/C catalyst at 80°C. A high-potential holding test (1 h holding at 1.0-1.5 V vs anode) demonstrated that the Pt/Ti 4 O 7 catalyst is quite stable against high potential up to 1.5 V. A single cell using a Pt/Ti 4 0 7 cathode was operated at 80°C, and voltage stability for >350 h with H 2 /O 2 was also demonstrated.

Roles of Shape and Size of Component Crystals in Semiconductor Gas Sensors I. Response to Oxygen

Abstract: An attempt to derive a comprehensive theory on the roles of shape and size of component crystals in semiconductor gas sensors is described. Based on Poisson's equations, the depth profiles of the electric potential inside the component crystals under electron-depleted conditions are solved for each shape of plate, sphere, or column. By combining this electronic equilibrium with the gas adsorption equilibrium taking place outside, the reduced resistance (R/R 0 ) of each sensor device can be expressed as a function of reduced adsorptive strength (y) of the adsorbing gas for crystals that are different in shape and are of a reduced size (n). As an important phenomenon typically associated with small crystals, a new type of depletion (volume depletion) appears following the conventional one (regional depletion) on increasing y, and the transition to volume depletion takes place at smaller y as n decreases. In the stage of volume depletion, R/R 0 is linear to y, with a linearity constant inversely proportional to n.

Hydrogen Peroxide Formation Rates in a PEMFC Anode and Cathode Effect of Humidity and Temperature

Abstract: Hydrogen peroxide (H 2 O 2 ) formation rates in a proton exchange membrane fuel cell (PEMFC) anode and cathode were estimated as a function of humidity and temperature by studying the oxygen reduction reaction (ORR) on a rotating ring disk electrode. Fuel cell conditions were replicated by depositing a film of Pt/Vulcan XC-72 catalyst onto the disk and by varying the temperature, dissolved O 2 concentration, and the acidity levels in hydrochloric acid (HClO 4 ). The HClO 4 acidity was correlated to ionomer water activity and hence fuel cell humidity. The H 2 O 2 formation rates showed a linear dependence on oxygen concentration and square dependence on water activity. The H 2 O 2 selectivity in ORR was independent of oxygen concentration but increased with the decrease in water activity (i.e., decreased humidity). Potential dependent activation energy for the H 2 O 2 formation reaction was estimated from data obtained at different temperatures.

Comparison Between Electrochemical Properties of Aligned Carbon Nanotube Array and Entangled Carbon Nanotube Electrodes

Abstract: Carbon nanotubes (CNTs) are promising electrochemical double-layer capacitor electrode materials because they have excellent conductivity and mesopores (2-50 nm) dominated pore structures Electrochemical properties of an ultralong (10 mm) aligned carbon nanotube array (ACNTA) electrode and an entangled CNT (ECNT) electrode in ionic liquid electrolyte were studied by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance The ACNTA electrode obtains higher specific capacitance, lower equivalent series resistance, and better rate capability than the ECNT electrode The reason is that ACNTA electrode possesses larger pore size and more regular pore structure and conductive paths, which are revealed by N 2 adsorption and scanning electron microscopy results, than ECNT electrode

Modeling Distributed Charge-Transfer Processes in SOFC Membrane Electrode Assemblies

Abstract: A model is developed to represent chemistry and transport in porous mixed ionic-and-electronic conducting composite electrode structures in solid oxide fuel cells (SOFC). The model considers the coupled behavior of a full membrane electrode assembly (MEA, i.e., cathode, electrolyte, and anode), which is an important advance compared to earlier models that consider only a single electrode structure. Within each electrode the model represents parallel conduction of electrons and ions, as well as porous-media, chemically reacting gas transport. The model predicts electric-potential distributions in both phases. Charge-transfer chemistry is handled via a modified Butler-Volmer formalism, which depends on the local electric-potential difference between phases. Heterogeneous chemistry (e.g., reforming or partial oxidation) is handled via a detailed surface-reaction mechanism. For typical composite-electrode structures the charge-transfer region extends about 10-20 μm from the dense electrolyte. The results show cell performance depends upon particle sizes within the porous electrodes. Smaller particles generally improve cell performance as a result of expanded three-phase-boundary length. However, smaller particle sizes impede gas transport. Cell performance can be optimized as a function of functional-layer thickness and particle sizes. A distributed charge-transfer formulation is especially important in advanced thin-film electrode structures (e.g., segmented-in-series architectures) where the entire MEA is only a few tens of micrometers thick. The model is formulated as continuum differential equations, which are solved computationally on a discrete mesh network. The paper illustrates the model with examples comparing alternative MEA structures.