Showing papers in "Journal of The Electrochemical Society in 2006"

Highly Efficient Dye-Sensitized Solar Cells Based on Carbon Black Counter Electrodes

Abstract: Carbon black was employed as the catalyst for triiodide reduction on fluorine-doped tin oxide glass substrates (FTO-glass) used as counter electrodes in platinum-free dye-sensitized solar cells The fill factors were strongly dependent on the thickness of the carbon layer, and the light energy conversion efficiency also increased up to a thickness of 10 μm The charge-transfer resistance (R ct ) of the carbon counter electrode decreased with the thickness of the carbon layer The R ct for the thicker carbon layer is less than three times that for the platinized FTO-glass The highest cell efficiency was 91% under 100 mW cm -2 light intensity (1 sun AM 15 light, J sc = 168 mA cm -2 , V oc = 7898 mV, fill factor = 0685)

Crystalline MnO2 as Possible Alternatives to Amorphous Compounds in Electrochemical Supercapacitors

Abstract: Manganese dioxide compounds with various structures were synthesized and tested as "bulk" composite electrodes for electrochemical capacitors. The capacitance of the set of MnO 2 compounds having Brunauer-Emmett-Teller (BET) surface areas larger than 125 in 2 g -1 reached a maximum value of about 150 F g -1 . The capacitance of all amorphous compounds (except one) is due to faradaic processes localized at the surface and subsurface regions of the electrode. Further increasing the surface area does not provide additional capacitance. The capacitance of the crystallized materials is clearly dependent upon the crystalline structure, especially with the size of the tunnels able to provide limited cations intercalation. Thus, the 2D structure of birnessite materials gives an advantage to obtain relatively high capacitance values (110 F g -1 ) considering their moderate BET surface area (17 m 2 g -1 ). ID tunnel structure such as γ or β-MnO 2 is characterized by only a pseudofaradic surface capacitance and therefore relies on the BET surface area of the crystalline materials. 3D tunnel structure such as λ-MnO 2 shows some intermediate behavior between bimessite and ID tunnel structures.

Liquid Water Removal from a Polymer Electrolyte Fuel Cell

Abstract: Liquid water transport and removal from the gas diffusion layer GDL and gas channel of a polymer electrolyte fuel cell PEFC are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate or stoichiometry with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell’s active area and overall performance.

Enhanced Graphical Representation of Electrochemical Impedance Data

Abstract: Bode plots, corrected for Ohmic resistance, logarithmic plots of the imaginary component of the impedance, and effective capacitance plots are shown to be useful complements to the more traditionally used complex-plane and Bode representations for electrochemical impedance data. The graphical methods are illustrated by synthetic data and by experimental data associated with corrosion in saline environments. Bode plots are shown, in particular, to be confounded by the influence of electrolyte resistance. The plots proposed here provide useful guides to model development for both reactive and blocking systems. The logarithmic plots of the imaginary component of the impedance and effective capacitance plots are useful for all impedance data, and the correction for Ohmic resistance in Bode plots is useful when the solution resistance is not negligible.

A mathematical model of stress generation and fracture in lithium manganese oxide

Abstract: Fracture of Li y Mn 2 O 4 is predicted with a numerical model that calculates the stress generated in spherical particles due to lithium intercalation along the 4-V plateau and phase change along the 3-V plateau. In the former case, fracture is probable at the rates typical of high-power applications, while in the latter case, the probability of fracture is linked not to the discharge rate or particle size, but to the LiMn 2 O 4 /Li 2 Mn 2 O 4 phase ratio. The two-phase material should fracture immediately upon lithium extraction. The effects of variation in thermodynamic factor, diffusion coefficient, and lattice parameter are examined in detail.

Durability Study of Pt ∕ C and Pt ∕ CNTs Catalysts under Simulated PEM Fuel Cell Conditions

Abstract: The durability of carbon black supported Pt (Pt/C) and multiwalled carbon nanotubes supported Pt (Pt/CNTs) catalysts for potential application in polymer electrolyte membrane fuel cells are investigated using an accelerated durability test. The electrochemical surface area of Pt/C degrades by 49.8% during the 192-h test time, compared with 26.1% for Pt/CNTs, which is due to Pt particle growth and Pt loss from the support in the form of Pt ions and Pt particles. Transmission electron microscopy and X-ray diffraction analysis show that Pt particles in Pt/CNTs present higher sintering resistance. X-ray photoelectron spectroscopy characterization indicates that CNTs in Pt/CNTs are more resistant to electrochemical oxidation than carbon black in Pt/C. It can be concluded that Pt/CNTs are more stable under electrochemical operation, which can be attributed to specific interaction between Pt and the support and the higher resistance of the support to electrochemical oxidation.

Model of Carbon Corrosion in PEM Fuel Cells

Abstract: This paper presents a mathematical model of the corrosion of carbon catalyst supports in polymer electrolyte membrane (PEM) fuel cells The model describes how a maldistribution of hydrogen across the fuel electrode can induce both oxygen evolution and carbon corrosion on the positive electrode of the fuel cell in the fuel-starved region Implications of this reverse-current mechanism are explored by simulating a cell with a nonuniform distribution of hydrogen along the fuel channel in both steady-state and transient operation

Determination of Catalyst Unique Parameters for the Oxygen Reduction Reaction in a PEMFC

Abstract: The oxygen reduction reaction (ORR) kinetics of a high-surface-area carbon-supported platinum catalyst (Pt/C) were measured in an operating proton exchange membrane fuel cell (PEMFC). The ORR kinetics of Pt/C can be described over a wide range of temperature, pressure, and current density using four catalyst-specific parameters: transfer coefficient, exchange current density, reaction order with respect to oxygen partial pressure, and activation energy. These parameters were extracted using a combined kinetic and thermodynamic model, either referenced to the reversible cell potential (i.e., using exchange current density as activity parameter) or referenced to a constant ohmic-resistance-corrected (i.e., iR-free) cell voltage. The latter has the advantage of using an activity parameter (activity at 0.9 V iR-free cell voltage) which can be measured explicitly without extrapolation, in contrast to the exchange current density required in the former model. It was found that much of the variation in the published values for these catalyst-specific kinetic parameters derives from applying the same parameter name (e.g., activation energy) without specifying which of its many possible definitions is being used. The obviously significant numerical differences both for "oxygen reaction order" and for "activation energy" due to different definitions (often tacitly assumed and rarely explicitly stated in the literature) are illustrated by the kinetic ORR parameters which we determined for Pt/C: (i) at zero overpotential, where reaction order and activation energy are ∼0.5 and 67 kJ/mol, respectively, and (ii) at 0.9 V iR-free cell voltage, where reaction order and activation energy are ∼0.75 and 10 kJ/mol, respectively.

Synthesis and Characterization of Nanostructured 4.7 V Li x Mn1.5Ni0.5O4 Spinels for High-Power Lithium-Ion Batteries

Abstract: Nanostructured Li x Mn 1.5 Ni 0.5 O 4 (x = 0.95,1.0,1.05) spinel powders were synthesized by a modified Pechini method. The powders were annealed at different temperatures between 500 and 800°C for 15 h. Depending on the ordering/disordering of transition metal ions on octahedral sites, spinels were assigned to either ordered P4 3 32 (P) or disordered Fd3m (F) space groups. The spinels of the two symmetry groups differed significantly in fast discharge rate capability. Extensive characterization was employed to identify the source of the difference. Vibrational spectroscopy techniques (FTIR and Raman), in situ and ex situ XRD and impedance spectroscopy did not reveal any sign of structural degradation for electrochemically inferior P4 3 32 spinels even after rigorous cycling. The poor performance was assigned to an intrinsic property, the lower electrical conductivity of the cation ordered samples. Arrhenius plots of sintered pellets revealed that the ordered spinels were shown to have two orders of magnitude lower electronic conductivity than disordered samples. The difference in electronic conductivity was assigned to the presence of a small amount of Mn 3+ in disordered samples.

Characterization of the Lithium Surface in N-Methyl-N-alkylpyrrolidinium Bis(trifluoromethanesulfonyl)amide Room-Temperature Ionic Liquid Electrolytes

Abstract: The solid electrolyte interphase SEI formed on a lithium electrode in an N-methyl-N-alkylpyrrolidinium bistrifluoromethanesulfonylamide p1,xTf2N room-temperature ionic liquid electrolyte was characterized using X-ray photoelectron spectroscopy, diffuse reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, and electrochemical impedance spectroscopy EIS. The SEI was found to be composed mainly of reduction products of the Tf2N  anion. A pronounced difference in composition was observed between the SEI formed on the lithium surface and that formed in situ during lithium deposition on a copper substrate. In the case of the lithium surface, native surface species e.g., Li2O, Li2CO3 persisted in the SEI and dominated the SEI composition. The surface film formed on lithium-deposited-on-copper did not contain species associated with the lithium native film. Instead, in addition to the anion reduction products, significant quantities of species associated with the cation were observed. EIS indicated varied lithium conduction pathways through the film and that the pathways were in series, suggesting a layered structure. Calculated activation energies, resistivity, and thickness values were comparable to literature values for the SEI formed in conventional liquid electrolytes.

Interfacial Properties of the a-Si ∕ Cu :Active–Inactive Thin-Film Anode System for Lithium-Ion Batteries

Abstract: Amorphous silicon thin films deposited on copper foil have been observed to exhibit near theoretical capacity for a limited number of cycles. The films, however, eventually delaminate, leading to failure of the anode. In order to better understand the mechanism of capacity retention and the ultimate failure mode of a model brittle active:elastic/plastic inactive anode system, the films were subjected to in situ adhesion tests while observing the film surface using scanning electron microscopy. Atomic force and transmission electron microscopy, and electrochemical cycling were conducted to analyze the emerging morphology of the films during cycling. The adhesion of the as-deposited Si film to the Cu substrate was measured to ∼7.7 J/m 2 , reflecting a weak interface adhesion strength. Plastic deformation of the underlying Cu substrate combined with a ratcheting mechanism is proposed to occur in the Si:Cu system, with delamination failure mode occurring only after the formation of an interface imperfection. From the analysis of slow rate cycling experiments, nucleation of a lithium compound based on the interdiffusion of Si and Cu is identified as the most probable cause of the ultimate delamination failure of the deposited film.

A Nonisothermal, Two-Phase Model for Polymer Electrolyte Fuel Cells

Abstract: A model fully coupling the two-phase flow, species transport, heat transfer, and electrochemical processes is developed to investigate liquid water distribution and flooding in polymer electrolyte fuel cells (PEFCs) under nonisothermal conditions. The thermal model accounts for irreversible heat and entropic heat generated due to electrochemical reactions, Joule heating arising from protonic/electronic resistance, and latent heat of water condensation and/or evaporation. A theoretical analysis is presented to show that in the two-phase zone, water transport via vapor-phase diffusion under the temperature gradient is not negligible, with a magnitude comparable to the water production rate in PEFCs. Detailed numerical results further reveal that the vapor-phase diffusion enhances water removal from the gas diffusion layer (GDL) under the channel and exacerbates GDL flooding under the land. Simultaneously, this vapor-phase diffusion provides a new mechanism for heat removal through a phase change process in which water evaporates at the hotter catalyst layer, diffuses through the interstitial spaces of the GDL, and condenses on the cooler land surface. This new heat removal mechanism resembles the heat pipe effect. Three-dimensional simulations for a full PEFC using this nonisothermal, two-phase model are presented for the first time. Separate velocity fields of gas and liquid phases are given, clearly illustrating that the vapor-phase diffusion and capillary-driven liquid water transport in a GDL aid each other in water removal along the through-plane direction under the channel area, but oppose each other along the in-plane direction between the channel area and land.

The Effects of Acid Treatment on the Electrochemical Properties of 0.5 Li2MnO3 ∙ 0.5 LiNi0.44Co0.25Mn0.31O2 Electrodes in Lithium Cells

Abstract: The electrochemical properties of 0.5 Li 2 MnO 3 ·0.5 LiNi 0.4 Co 0.25 Mn 0.31 O 2 electrodes, when preconditioned and activated with acid for 2-24 h, have been studied in lithium cells. Powder X-ray diffraction data and electrochemical measurements provide supporting evidence for an intergrown, composite electrode structure from which Li 2 O can be leached from the Li 2 MnO 3 (Li 2 O MnO 2 ) component with acid, thereby mimicking the electrochemical charge process at high potentials (>4.5 V). The MnO 2 -rich domains generated by acid treatment are reduced during electrochemical discharge at a lower potential than electrochemically generated MnO 2 -rich domains. With prolonged cycling between 4.6 and 2.0 V, dQ/dV plots of untreated and acid-treated electrodes develop similar, but not identical, character, suggesting a coalescence and redox interaction of the manganese ions in MnO 2 -rich and Ni 0.44 Co 0.25 Mn 0.31 O 2 regions of the structure. Acid treatment eliminates the first-cycle capacity loss of the electrodes, consistent with earlier reports for related systems, but it damages their cycling stability and rate capability.

Water Management in Cathode Catalyst Layers of PEM Fuel Cells A Structure-Based Model

Abstract: The cathode catalyst layer (CCL) is the major competitive ground for electrochemical reaction, reactant transport, and water and heat exchange in a polymer electrolyte fuel cell (PEFC). Nevertheless, it is often treated as a thin interface. Its pivotal role in the fuel cell water balance is unexplored. Here, the structural picture of CCLs forms the basis for a novel model that links spatial distributions of processes with water handling capabilities and current voltage performance. In the first step, the statistical theory of random composite media is used to relate composition, porous structure, wetting properties, and partial saturation to effective properties. In the second step, these effective properties are used in a macrohomogeneous model of CCL performance. A set of reasonable simplifications leads to a full analytical solution. Results demonstrate that the CCL acts like a watershed in the fuel cell, regulating the balance of opposite water fluxes toward membrane and cathode outlet. Due to a benign porous structure, the CCL represents the prime component for the conversion of liquid to vapor fluxes in PEFCs. Furthermore, the CCL is highlighted as a critical component in view of excessive flooding that could give rise to limiting current behavior.

Detection of Membrane Drying, Fuel Cell Flooding, and Anode Catalyst Poisoning on PEMFC Stacks by Electrochemical Impedance Spectroscopy

Abstract: Membrane drying, fuel cell flooding, and anode catalyst poisoning by carbon monoxide are investigated on Hydrogenics production-type proton exchange membrane fuel cell (PEMFC) stacks similar to the stacks used in Hydrogenics HyPM 10-kW fuel cell power modules. Changes in fuel cell voltage and impedance with time are presented for each type of fault, the fuel cell stacks being controlled in galvanostatic mode. This study shows that these PEMFC stack faults can be differentiated by their impedance responses while fuel cell voltage monitoring alone is insufficient to distinguish between failure types. Membrane drying leads to an increase in the fuel cell impedance magnitude and phase angle at all frequencies studied. Fuel cell flooding leads to an increase in the impedance magnitude at low frequencies (f < 10 Hz) and to a decrease in the impedance phase angle at frequencies less than 100 Hz. Anode catalyst poisoning by CO is characterized by an increase in the fuel cell impedance magnitude at frequencies less than a few hundred Hz. For this fault, the impedance phase angle decreases within a large frequency range and is characterized by a minimum value appearing at 20-25 Hz at moderate current density.

Ether-Based Electrolytes for the Lithium/Oxygen Organic Electrolyte Battery

Abstract: The practical operation of a lithium/oxygen organic electrolyte battery depends on a significant amount of dissolved oxygen transporting through the organic electrolyte permeating the carbon black cathode before its reduction occurs. The rate of oxygen transport directly influences rate capability and discharge capacity. The organic electrolyte can be tailored to maximize the transport of oxygen while still retaining the ability to form a stable solid electrolyte interface with the lithium anode, chemical stability towards the discharge products Li 2 O 2 and Li 2 O, and oxidative stability to over 3 V. We investigated an ether-based electrolyte containing four different electrolyte salts to determine how electrolyte properties such as oxygen solubility, dynamic viscosity, and conductivity change with each electrolyte salt, and how this directly affects rate capability and discharge capacity. The results indicate that discharge capacity at 0.5 mA/cm 2 is determined by dynamic viscosity alone for these electrolytes, while discharge capacity at 0.2 and 0.05 mA/cm 2 shows no correlation with either oxygen solubility, dynamic viscosity, or conductivity. Our results demonstrate that a substantial improvement in rate capability can be achieved by optimizing electrolyte viscosity.

The phase stability of cerium species in aqueous systems. III. The Ce(III/IV)-H2O-H2O2/O2 systems dimeric Ce(IV) species

Abstract: Equilibria and thermodynamic data for cerium dimer species were calculated for both the Ce-H 2 O-O 2 and Ce-H 2 O-H 2 O 2 systems. It was found that Ce(IV) has a tendency to form dimers, especially when the total cerium concentration is higher than 0.001 M. There are three major soluble dimer forms considered in this paper: (CeOCe) 6 + , (CeOCe)(OH) 5 + , and (CeOCe)(OH) 4 + 2 , all having very similar standard potentials (E 0 C e ( I V ) d i m e r / C e ( I I I ) .1.66 V 0.02 V). When they coexist in solution, the concentration ratio of the Ce(IV) dimer to Ce 3 + depends on the total cerium concentrations, the oxidizing agent, and in some cases the pH. Increasing the value of each individual factor favors the formation of dimers. It was also found that the direct oxidation of Ce 3 + to dimers (CeOCe)(OH) ( 6 - x ) + x by O 2 appeared to be insignificant, but the oxidation of Ce 3 + to the three dimers by H 2 O 2 would be much more favorable.

Comparison of Metal Ion Dissolutions from Lithium Ion Battery Cathodes

Abstract: With an aim to develop a better understanding of the capacity fade mechanisms of manganese spinel oxide cathodes, the amount of metal ion dissolution from various lithium ion battery cathodes (layered, orthorhombic LiMnO 2 , 4 V spinel, 5 V spinel, and olivine) is compared. Orthorhombic LiMnO 2 , layered LiMn 0.8 Cr 0.2 O 2 , and spinel LiMn 2 O 4 containing Mn 3+ exhibit much higher amounts of manganese dissolution irrespective of the structure compared to the total transition metal ion dissolution found from cathodes containing Mn 4+ , such as layered LiNi 1/3 Mn 1/3 Co 1/3 O 2 and 5 V spinel LiMn 1.5 Ni 0.5 O 4 or from layered LiCoO 2 and olivine LiFePO 4 . However, the manganese dissolution from the Mn 3+ -containing spinel oxides could be lowered significantly to the levels found from LiCoO 2 , LiFePO 4 , and LiMn 1.5 Ni 0.5 O 4 by appropriate cationic and anionic (fluorine) substitutions, which leads to excellent capacity retention at elevated temperatures. For example, LiMn 1.85 Li 0.075 Ni 0.075 O 4 and LiMn 1.85 Li 0.075 Ni 0.075 O 3.94 F 0.06 exhibit manganese dissolutions of 1.5 and 1.1%, respectively, compared to 3.2% for LiMn 2 O 4 . Furthermore, manganese dissolution is found to bear a clear relationship to the lattice parameter difference Aa between the two cubic phases formed during the charge-discharge process; Mn dissolution decreases with decreasing Aa.

Chromium Poisoning of Perovskite Cathodes by the ODS Alloy Cr5Fe1Y2O3 and the High Chromium Ferritic Steel Crofer22APU

Abstract: The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell (SOFC) interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened (ODS) alloy developed by Plansee, Reutte, Austria, for use at high temperature. A typical material for medium-temperature SOFC, is the high chromium ferritic steel Crofer22APU supplied by Thyssen Krupp VDM, Germany. The two alloys form different oxide scales which affect chromium poisoning. Chromium vaporization as source term and electrochemical degradation of La 1-x Sr x MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) describing the poisoning were studied for the two alloys. The dynamics of the chromium deposition in porous perovskite cathodes was studied by a dc method and impedance spectroscopy. Electrical degradation of the LSM cathode by alloy Cr5Fe1Y 2 O 3 was significantly higher than for Crofer22APU. The microstructure of the cells was studied after measurements by scanning and energy filtering transmission electron microscopy. Significant amounts of chromium were observed at the TPB in the functional layer of cells, with the LSM cathode giving insight into the degradation mechanism. Cells tested with the LSCF cathode clearly show Cr poisoning. Formation of large SrCrO 4 crystals was observed on the surface of the LSCF cathode.

Large-Scale Fabrication of Ordered Nanoporous Alumina Films with Arbitrary Pore Intervals by Critical-Potential Anodization

Abstract: Various ordered nanoporous alumina films with arbitrary pore intervals from 130 to 980 nm were fabricated on aluminum by a critical-potential anodization approach with sulfuric, phosphoric, oxalic, glycolic, tartaric, malic, and citric acid electrolytes under 70-450 V. The pore intervals of the porous alumina films were linearly proportional to applied potentials, with corresponding dominated territories to the electrolytes. In addition to pore interval, the self-ordering extent of pore arrangement was also improved with increasing anodizing potentials, leading to highly ordered porous alumina films at critical-high potentials. A cell separation phenomenon occurred for the films formed in sulfuric and glycolic acid solutions at the critical potentials, thus leading to the formation of highly ordered alumina nanotubule arrays. The critical-potential anodization in the other electrolytes produced self-organized porous alumina films with two-layered pore walls and pore bases. The basic principle for achieving porous alumina films with desired pore intervals is controlling the balance of the growth of barrier layer and the pore generation by adjusting the acidity, the concentration, and temperature of electrolytes. The porous alumina films formed in various electrolytes were transparent, and the transmittances of the films were inversely proportional to the applied potentials or the pore intervals.

Methanol-Tolerant Oxygen Reduction Electrocatalysts Based on Pd-3D Transition Metal Alloys for Direct Methanol Fuel Cells

Abstract: Palladium-based alloys, such as Pd-Co, Ni, and Cr, have been developed as a novel methanol-tolerant oxygen reduction electrocatalyst for direct methanol fuel cells. The Pd alloy electrocatalysts were fabricated by a rf sputtering method. Their electrochemical characteristics for the oxygen reduction reaction (ORR) were determined in sulfuric acid solution with and without methanol at 30°C. The Pd alloys showed a higher ORR electrocatalytic activity than Pd, although lower than Pt. The Pd alloys also had no electrocatalytic activity for methanol oxidation in the presence of methanol. The maximum electrocatalytic activities for ORR were observed for the alloy composition of ca. 60 atom % Pd in all the Pd alloys. Based on the X-ray photoelectron surface analysis, it was confirmed that the filling of the Pd d-band by alloying decreased the density of states (DOS) at the Fermi level. The decreased DOS inhibited the formation of Pd oxide on the surface of the electrocatalyst. This result should contribute to the improvement of the ORR activity of the Pd alloy electrocatalysts.

Degradation of Anode Supported SOFCs as a Function of Temperature and Current Load

Abstract: The degradation behavior of anode supported solid oxide fuel cells (SOFCs) was investigated as a function of operating temperature and current density. Degradation rates were defined and shown to be mainly dependent on the cell polarization. The combination of a detailed evaluation of electrochemical properties by impedance spectroscopy, in particular, and post-test microscopy revealed that cathode degradation was the dominant contribution to degradation at higher current densities and lower temperatures. The anode was found to contribute more to degradation at higher temperatures. Generally, the degradation rates obtained were lower at higher operating temperatures, even at higher current densities. A degradation rate as low as 2%/1000 h was observed at 1.7 A/cm 2 and 950°C over an operating period of 1500 h.

Fe-Based Catalysts for Oxygen Reduction in PEMFCs Importance of the Disordered Phase of the Carbon Support

Abstract: Fe-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) have been prepared with commercial and developmental carbon black powders containing initially between 0 and 0.8 atom % of nitrogen. The catalysts were obtained by adsorbing 0.2 wt % Fe from iron acetate on each carbon support, which is then pyrolyzed at 900°C for 1 h in a NH 4 /H 2 /Ar mixture. Under these conditions, N contents from 0 to 2.3 atom % are measured at the surface of the catalysts and increased N content leads to increased activity for the ORR. The N content correlates with the weight loss of the carbon support due to a reaction with NH 3 during pyrolysis. It was found that NH 3 reacts mainly with the disorganized carbon, leaving nitrogen at the surface of the support. The larger the amount of disorganized carbon in the pristine carbon black, the better the activity for ORR of the resulting catalyst. The most active non-noble catalyst was tested in fuel cells, where it was found that its specific activity (in A per cm 3 of electrode) is still about two orders of magnitude below the target of a non-noble catalyst for automotive applications. However, such catalysts could already compete with Pt in, e.g., methanol fuel cells because they are ORR-selective.

H2S Poisoning of Solid Oxide Fuel Cells

Abstract: The influence of H 2 S fuel impurity on power generation characteristics of solid oxide fuel cells (SOFCs) has been analyzed by measuring cell voltage at a constant current density, as a function of H 2 S concentration, operational temperature, and fuel gas composition. Reversible cell voltage change was observed around 1000°C, while fatal irreversible degradation occurred at a lower operational temperature, at a higher H 2 S concentration, and at a lower fuel H 2 /CO ratio. Sulfur tolerance of SOFCs was improved by using Sc 2 O 3 -doped ZrO 2 instead of Y 2 O 3 -doped ZrC 2 as electrolyte and/or as electrolyte component in the anode cermets. It has been found that H 2 S poisoning consists of at least two stages, i.e., an initial cell voltage drop within a short time period to a metastable cell voltage, followed by a gradual larger cell voltage drop. Possible H 2 S poisoning processes are discussed.

Investigation of Pseudocapacitive Charge-Storage Reaction of MnO2 ∙ nH2O Supercapacitors in Aqueous Electrolytes

Abstract: Pseudocapacitive charge-storage reaction of MnO 2 ·nH 2 O in several aqueous alkali and alkaline salts solutions, including LiCI, NaCI, KCI, CsCI, and CaCl 2 , has been studied on fine-grained MnO 2 ·nH 2 O thin films and particles which possess the e-MnO 2 -type crystal structure. In situ synchrotron X-ray diffraction analysis shows that charge transfer at Mn sites upon reduction/ oxidation of MnO 2 ·nH 2 O is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charge/discharge cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that H 3 O + plays the predominant (>60%) role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as: Mn(IV)O 2 ·nH 2 O + δe - + δ(1- f)H 3 O + + δfM + ⇄ (H 3 O) δ(1-f) M δf [Mn (III) δ Mn(IV) 1-δ ]O 2 ·nH 2 O, where M + is alkali cation.

Coupled Thermal and Water Management in Polymer Electrolyte Fuel Cells

Abstract: Thermal and water management are intricately coupled in polymer-electrolyte fuel cells. In this paper, we simulate fuel-cell performance and account for nonisothermal phenomena. The transport of water due to a temperature gradient and its associated effects on performance are described, with the increase of reactant dilution by the water-vapor partial pressure being the most dominant. In addition, simulations are undergone to find the optimum operating temperature and maximum power density as a function of external heat-transfer coefficient. The shape of the optimization curves and the magnitudes of the nonisothermal phenomena are also detailed and explained.

Hybrid Aqueous Energy Storage Cells Using Activated Carbon and Lithium-Intercalated Compounds: I. The System

Abstract: A hybrid aqueous electrochemical supercapacitor technology is presented in which activated carbon was used as a negative electrode and a lithium-ion intercalated compound LiMn 2 O 4 as a positive electrode in a neutral Li 2 SO 4 aqueous electrolyte. The charge/discharge process is associated with the transfer of a Li ion between two electrodes. It is quite different from the electrolyte behavior in conventional electrochemical double-layer supercapacitors and other reported hybrid supercapacitors, where the cations and anions separate and the electrolyte is consumed. By optimization of the positive/negative electrode mass load ratio, operating voltage window, and pH of the electrolyte solution, the cell exhibits a sloping voltage profile from 0.8 to 1.8 V and delivers an estimated specific energy of ca. 35 Wh/kg based on the total weight of the active electrode materials. The cell exhibits excellent cycling performance with less than 5% capacity loss over 20,000 cycles at 10 C charge/discharge rate.

ALD and Characterization of Aluminum Oxide Deposited on Si ( 100 ) using Tris(diethylamino) Aluminum and Water Vapor

Abstract: © The Electrochemical Society, Inc. 2006. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Katamreddy, R., R. Inman, G. Jursich, A. Soulet, and C. Takoudis, 2006, ALD and characterization of aluminum oxide deposited on Si (100) using tris(diethylamino) aluminum and water vapor: Journal of the Electrochemical Society, v. 153, no. 10, p. C701-C706.

Investigation on Capacitance Mechanisms of Fe3O4 Electrochemical Capacitors

Abstract: The capacitance mechanisms of magnetite (Fe 3 O 4 ) electrochemical capacitor in Na 2 SO 3 , Na 2 SO 4 , and KOH aqueous solutions have been investigated by electrochemical quartz-crystal microbalance analysis, along with cyclic voltammetry and X-ray photoelectron spectroscopy. The oxide thin-film electrode was prepared by an electroplating method, and exhibits a capacitance of ∼170, 25, and 3 F/g in 1.0 M Na 2 SO 3 (aq), Na 2 SO 4 (aq), and KOH(aq), respectively. Strong specific adsorption of the anion species was evidenced in all solutions. Experimental results indicate that, in Na 2 SO 3 (aq), the capacitive current of magnetite electrode originates from the combination of electric double-layer capacitance (EDLC) and the pseudocapacitance that involves successive reduction of the specifically adsorbed sulfite anions, from SO 2 - 3 through, e.g., S 2 - , and vice versa. In Na 2 SO 4 (aq), the current is due entirely to EDLC. Furthermore, due to the specific adsorption behavior, magnetite exhibits high EDLC, >30 μF/cm 2 , in both Na 2 SO 3 and Na 2 SO 4 solutions. The lowest capacitance of magnetite was observed in KOH, which is attributed to the formation of an insulating layer on the magnetite surface.