Showing papers in "Journal of Solid State Electrochemistry in 2004"

Oxygen transport in La 0.6Sr 0.4Co 1- yFe yO 3-d

Abstract: The surface exchange coefficient and chemical diffusion coefficient of oxygen for the perovskites La0.6Sr0.4Co1–yFeyO3–delta (y=0.2, 0.5 and 0.8) were measured using the conductivity relaxation technique. Measurements were performed between 600 and 800 °C in an oxygen partial pressure range between 10–4 and 1 bar. Both transport coefficients decrease markedly with decreasing oxygen partial pressure below about 10–2 bar at all temperatures. This is attributed to ordering of oxygen vacancies. Implications for using La0.6Sr0.4Co1–yFeyO3–delta as an oxygen separation membrane are discussed.

Electrochemical and structural study of LiCoPO4-based electrodes

Abstract: LiCoPO4 samples were synthesized by two different techniques (high-temperature solid-state reaction and lower-temperature synthesis using NH4CoPO4·H2O as precursor) and tested as cathode materials for 5-V lithium batteries. An irreversible lithium deinsertion was observed for the high-temperature sample. In contrast, the application of lower-temperature synthesis led to a significant improvement of the lithium storage reversibility. Different delithiation mechanisms in LiCoPO4 were found for the samples obtained by different synthetic techniques. The nature of capacity fading during cycling of the cells is discussed.

Cathode materials for lithium ion batteries prepared by sol-gel methods

Abstract: Improving the preparation technology and electrochemical performance of cathode materials for lithium ion batteries is a current major focus of research and development in the areas of materials, power sources and chemistry. Sol-gel methods are promising candidates to prepare cathode materials owing to their evident advantages over traditional methods. In this paper, the latest progress on the preparation of cathode materials such as lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, vanadium oxides and other compounds by sol-gel methods is reviewed, and further directions are pointed out. The prepared products provide better electrochemical performance, including reversible capacity, cycling behavior and rate capability in comparison with those from traditional solid-state reactions. The main reasons are due to the following several factors: homogeneous mixing at the atomic or molecular level, lower synthesis temperature, shorter heating time, better crystallinity, uniform particle distribution and smaller particle size at the nanometer level. As a result, the structural stability of the cathode materials and lithium intercalation and deintercalation behavior are much improved. These methods can also be used to prepare novel types of cathode materials such as nanowires of LiCoO2 and nanotubes of V2O5, which cannot be easily obtained by traditional methods. With further development and application of sol-gel methods, better and new cathode materials will become available and the advance of lithium ion batteries will be greatly promoted.

Electrochemical synthesis of Fe 3 O 4 nanoparticles in alkaline aqueous solutions containing complexing agents

Abstract: Ultrafine magnetite particles are prepared through an electrochemical process, at room temperature, from an iron-based electrode immersed in an alkaline aqueous medium containing complexing compounds. XRD and chemical analysis indicate that the product is pure magnetite, Fe3O4. The size and morphology of the particles are studied by SEM. The magnetite nanoparticles present a magnetoresistance of almost 3%, at 300 K, under a magnetic field of 1 T. A reactive mechanism for the electrochemical process is proposed.

Hydrous manganese oxide/carbon nanotube composite electrodes for electrochemical capacitors

Abstract: A novel type of composite electrode based on hydrous manganese oxide and a single-walled carbon nanotube has been prepared and used in electrochemical capacitors. Cyclic voltammetry, galvanostatic charging/discharging tests and electrochemical impedance measurements were applied to investigate the performance of the composite electrodes with different ratios of hydrous manganese oxide and single-walled carbon nanotube. For comparison, the performance of pure hydrous manganese oxide and pure carbon nanotubes was also studied. In this way, the composite electrode with a 6:4 ratio of hydrous manganese oxide to carbon nanotube was found to be the most promising active material for an electrochemical capacitor, which shows both good capacitance and power characteristics.

Microstructure and corrosion resistance of electrodeposited zinc alloy coatings

Abstract: A heat treatment effect on the microstructure and corrosion properties of electrodeposited Zn, Zn-Co, Zn-Fe and Zn-Ni alloy coatings was studied. Surface morphology examinations were carried with AFM, while XRD was used to determine metal lattice parameters, texture and phase composition. Low-temperature annealing (at 225 °C) caused the formation of intermetallic Fe/Zn compounds, a transformation of amorphous oxide inclusions to the crystalline form and a decrease in the Zn lattice parameter for Zn-Co and Zn-Fe alloys. The mentioned structural modifications were not accompanied, however, by corrosion behavior changes of these coatings. On the Zn-Ni alloy, the annealing caused a significant reduction in the diffraction peak width and simultaneous considerable augmentation of the corrosion current. This effect is related to the formation of a less disordered lattice for this alloy.

Capacitive and textural characteristics of manganese oxide prepared by anodic deposition: effects of manganese precursors and oxide thickness

Abstract: The influence of manganese precursors on the deposition rate of hydrous manganese oxide in the amorphous form (denoted as a-MnO x ·nH2O) and the effect of oxide thickness on the electrochemical properties of a-MnO x ·nH2O, for application as electrochemical supercapacitors, were systematically investigated in this work. The results showed that Mn(CH3COO)2·4H2O is a more promising precursor because of its high deposition rate at much lower potentials in comparison with MnSO4·5H2O, MnCl2·4H2O, and Mn(NO3)2·4H2O. The capacitive characteristics of a-MnO x ·nH2O were found to be independent of precursors, probably due to the fact that the mean oxidation state of Mn is not significantly affected by changing the anions of manganese precursors (from the XPS results). The capacity of oxide deposits was found to be proportional to the charge density of deposition (i.e., loading) of a-MnO x ·nH2O when it was equal to or less than a critical value (ca. 3.5 C cm−2), while poorer capacitive behavior with a lower capacity was clearly found beyond this critical value. The a-MnO x ·nH2O deposit with 3.5 C cm−2, exhibiting an acceptable capacitive performance, showed the highest capacity of energy storage for supercapacitors.

PEDOT films: multifunctional membranes for electrochemical ion sensing

Abstract: Properties of electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied from the point of view of direct use as ion-sensing membranes in potentiometric or amperometric sensors. Stable and reproducible potentiometric characteristics were obtained for PEDOT doped by poly(4-styrenesulfonate) ions, PEDOT(PSS) (cationic characteristics), and PEDOT doped by hexacyanoferrate(II) anions, PEDOT(HCF) (anionic characteristics). As shown by voltammetric and EDAX results, the anion exchange properties of the latter polymer result from gradual replacement of HCF ions by Cl− anions from solution. The zero-current potentiometric detection limit of PEDOT(PSS), equal to 3×10−6 M, can be shifted to 7×10−7 M by polarization using a cathodic current density of 3×10−7 A cm−2. PEDOT films doped by Cl− or PSS− ions can be used as membranes for sensing anions or cations, respectively, under pulse amperometric conditions, within the range from 10−4 to 1 M, comparable with that accessible by zero-current potentiometry. Dissolved oxygen (redox interferent of low charge transfer rate) exerts a minor influence on the slope of the potentiometric and amperometric characteristics of PEDOT films. Although the presence of redox reactants characterized by a high rate of charge transfer [Fe(CN)6 3−/4−] results in the disappearance of the potential dependence on KCl concentration, this disadvantageous effect is much less significant under pulse amperometric conditions.

Exploration of electrodeposited platinum alloy catalysts for methanol electro-oxidation in 0.5 M H2SO4: Pt-Ni system

Abstract: In this work, we examine the electrocatalytic activity of electrodeposited Platinum (Pt)-Nickel (Ni) alloy layers on an inert substrate electrode for methanol oxidation reaction. Analyses using energy-dispersive fluorescent X-ray analysis and powder X-ray diffractometry confirm alloying of Pt with Ni in a range of compositions. Steady-state polarisation measurements in 0.5 M methanol+0.5 M H2SO4 solutions clearly show that the onset of electro-oxidation shifts to less anodic potential values (approximately 160 mV), while also exhibiting current enhancements up to ~15 times the currents obtained for the pure Pt electrodeposit. A linear relationship between the cyclic voltammetric peak (oxidation) current and [MeOH] is observed at a scan rate of 50 mVs−1, thus indicating reduced influence of adsorbed CO (COads) surface poison. A critical composition, Pt (92%)/Ni (8%) [denoted Pt-Ni(3) alloy] is found to exhibit maximum electrocatalytic activity, beyond which the activity drops, whereas pure Ni does not catalyse the reaction. While the promotion of electro-oxidation is understood to be largely due to the alloy catalyst, surface redox species of Ni oxide formed during the electro-oxidation process may also contribute to the oxygenation of COads, thereby enhancing the oxidation current. Plausible mechanisms of methanol oxidation on Pt/ transition metal alloy electrocatalysts are discussed in terms of electron transfer (in the alloy) and the role of Ni oxide species.

Electrochromic and structural characteristics of mesoporous WO3 films prepared by a sol-gel method

Abstract: The preparation of electrochromic films of mesoporous tungsten trioxide from tungstic acid and tungstic hexaethoxide precursors with the addition of an organic stabiliser via a sol-gel method is reported. These films have been structurally characterised and both the film morphology and crystalline composition of the films were found to be significantly dependent on the temperature at which the films were annealed and upon the choice of precursor. Films annealed at lower temperatures consisted of amorphous and hexagonal tungsten trioxide, whereas films annealed above 500 °C comprised solely of monoclinic WO3. The electrochromic activity of the films was found to be equally dependent on method of preparation, and both the composition and the structure of the WO3 films were shown to clearly influence the colouration efficiency of the films.

Effect of polarization on the electrode behavior and microstructure of (La,Sr)MnO3 electrodes of solid oxide fuel cells

Abstract: The electrode behavior and microstructure of freshly prepared (La0.8Sr0.2)0.9MnO3 (LSM) electrodes were investigated under various polarization conditions. The original, large agglomerates in freshly prepared LSM electrodes were broken down into sphere-shaped grains when exposed to cathodic or anodic current passage of 200 mA cm−2 at 800 °C in air for 3 h. Microstructural changes under cathodic polarization could be related to the pronounced diffusion and migration of oxygen vacancies and Mn ions on the LSM surface and lattice expansion, while lattice shrinkage under oxidation conditions most likely contributes to the structural changes under anodic polarization. Such morphological changes were irreversible and were found to be beneficial to the performance of freshly prepared LSM electrodes. Freshly prepared LSM electrodes behaved very differently with respect to the cathodic and anodic current passage treatment.

Application of AC impedance techniques to Scanning Electrochemical Microscopy

Abstract: Fundamental aspects of alternating current Scanning Electrochemical Microscopy (AC-SECM) have been studied. In particular, the flow of AC currents in the cell and simplified equivalent circuit diagrams are discussed. In the case of conductive substrates, the local (1st and 2nd order) impedance characteristic of the substrate is presented as a function of DC potential. 3-D imaging of mixed substrates (containing both conductive and non-conductive areas) was performed in the supporting electrolyte alone, without an added redox mediator.

Electrochemical synthesis and characterization of poly(3,4-ethylenedioxythiophene) in ionic liquids with bulky organic anions

Abstract: The electrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT) was studied in two ionic liquids with bulky organic anions, ie, 1-butyl-3-methylimidazolium (BMIM) diethylene glycol monomethyl ether sulfate (MDEGSO4) and BMIM octyl sulfate (OctSO4) BMIM-MDEGSO4 is a liquid, while BMIM-OctSO4 is in solid form at room temperature Electrosynthesis of PEDOT in BMIM-MDEGSO4 with an EDOT concentration of 01 M and in BMIM-MDEGSO4/EDOT 1/1 (w/w) solution resulted in no polymer at all or a very limited amount of polymer on the electrode surface, as determined by cyclic voltammetry in 01 M KCl(aq) solution In contrast, electrosynthesis of PEDOT in BMIM-OctSO4/EDOT 1/1 (w/w) resulted in a high yield of electroactive material on the electrode surface Furthermore, electrosynthesis of PEDOT in ionic liquid–water solution (Cionic liquid=15 M) containing 01 M EDOT was also found to give a relatively high yield of electroactive material on the electrode surface, both for 15 M BMIM-MDEGSO4(aq) and 15 M BMIM-OctSO4(aq) The PEDOT electrodes showed an anionic potentiometric response in 10−5–10−1 M KCl(aq) solution, indicating a predominant anion transfer at the polymer–solution interface despite the relatively bulky anions (MDEGSO4− or OctSO4−) incorporated as counterions in PEDOT during electropolymerization On the basis of electrochemical impedance spectroscopy, the charge (ion) transport properties of the polymer film were strongly influenced by the water content of the ionic liquid (Cionic liquid=005–20 M)

Study of the fluorite–pyrochlore–fluorite phase transitions in Ln2Ti2O7 (Ln=Lu, Yb, Tm)

Abstract: The ordering processes in Ln2Ti2O7 (Ln=Lu, Yb, Tm) are studied by X-ray diffraction, thermal analysis, infrared absorption (IR) spectroscopy, and electrical conductivity measurements. The coprecipitation method followed by freeze-drying was used for Ln2Ti2O7 synthesis. The region of low-temperature fluorite phase existence is 600 °C 1,600 °C. The conductivity values are 5·10−3 S/cm for Tm2Ti2O7, 6·10−3 S/cm for Yb2Ti2O7, and 10−2 S/cm for Lu2Ti2O7 at 740 °C. This order–disorder transition leads to a 2 orders of magnitude conductivity growth and a 10–30 times permittivity increase in Ln2Ti2O7 samples obtained at 1,700 °C.

A sensitive reagentless pH probe with a ca. 120 mV/pH unit response

Abstract: The use of a basal plane pyrolitic graphite electrode immobilised with two redox active species each capable of undergoing a two-electron, two-proton redox process has allowed the development of a sensitive pH probe over a wide pH and temperature range. When the values of the peak potentials of the two processes are combined they shift by ca. 120 mV/pH unit at 25 °C, as measured against two independent, defined reference electrodes.

Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Abstract: Electrochemical characteristics for the nanoporous carbon|Et4NBF4+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et4NBF4+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths in the solid and liquid phases are interconnected by the double-layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.

Properties of Co-Mo coatings obtained by electrodeposition at pH 6.6

Abstract: Cobalt-molybdenum coatings were prepared by electrodeposition in a sulfate-citrate bath and their morphology, structure and magnetic properties were analysed. Concentrations of 0.1 mol dm−3 CoSO4 and 0.005 mol dm−3 Na2MoO4 at pH 6.6 led to Co-Mo deposits of 20–23% Mo that can be grown to several microns over graphite or copper substrates. At low deposition potentials or current densities, the deposits presented a close-packed hexagonal structure (hcp) that evolved to a (100)+(110) preferred orientation and acicular morphology as the deposit thickness increased. When the deposition potential or the current density was made more negative, a mixed crystalline+amorphous structure was obtained. The degree of crystallinity depended on the thickness: thin films were more amorphous than the thicker ones. Co-Mo deposits showed lower saturation magnetization (M s) and coercivity (H c) than the pure cobalt deposits. The crystalline+amorphous films showed the lowest H c values (around 40 Oe).

Electrical properties of gadolinia-doped ceria thin films deposited by sputtering in view of SOFC application

Abstract: Gadolinia-doped ceria (GDC) remains, up to now, the most promising candidate for replacing yttria-stabilised zirconia (YSZ) as electrolyte for solid oxide fuel cells (SOFC) operating at intermediate temperature. Literature data point out that GDC could be used as electrolyte, anode material, or interlayers for avoiding the chemical interactions occurring at the interfaces. In the present work, GDC thin layers were produced by d.c. reactive magnetron sputtering and deposited over a thickness domain between 450 nm and 5.5 µm. According to our knowledge, the deposition of GDC sputtered layers has never been reported. The physicochemical features of these thin films have been characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Impedance measurements have been carried out in order to determine the electrical properties of electrolyte thin films and in particular their ionic conductivity.

Influence of electrolyte characteristics on the electrochemical parameters of electrical double layer capacitors

Abstract: Electrical double layer capacitors based on ideally polarizable nanoporous carbon electrodes in propylene carbonate with the addition of different 1 M Me3EtNBF4, Me2Et2NBF4, MeEt3NBF4, Et4NBF4, Et3PrNBF4 and Et3BuNBF4 electrolytes have been tested by cyclic voltammetry, chronoamperometry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, Ragone plots (energy density vs. power density dependencies) and other characteristics have been discussed. The influence of the electrolyte molar mass on the electrochemical characteristics of the nanoporous carbon electrode cells has been established. The applicability limits of the Srinivasan and Weidner model have been tested.

Oxygen evolution on electrodes of different roughness: an electrochemical noise study

Abstract: Rough and porous Ni layers have been obtained by cathodic deposition from a NiCl2, NH4Cl solution, at high current density. Characterisation by SEM has shown that they consisted of micro-dendrites separated by pores with a typical diameter of 1 μm. In addition, circular hollows (10–100 μm in diameter) were found on the deposit surface; their density varied with the deposition current density and deposition charge. The surface roughness of the Ni deposits, measured by EIS, was found to increase roughly linearly with the deposition charge, and to be little dependent on current density, provided a threshold value was exceeded. The oxygen evolution reaction has been studied on these electrodes by simultaneous real-time measurements of potential and electrolyte resistance fluctuations. The analysis of the electrochemical noise indicated that the dimensions of oxygen bubbles detaching from the electrodes slightly increased with the deposit surface roughness. It is not clear, however, whether or not this increase was associated with the effect of the small (1 μm) or the large (10–100 μm) features on the electrode-bubble interactions.

Thiol- and disulfide-modified oligonucleotide monolayer structures on polycrystalline and single-crystal Au(111) surfaces

Abstract: We provide a comprehensive study of single- (ss) and double-strand (ds) oligonucleotides with either 25 or 10 bases or base pairs (bp) immobilized on polycrystalline and single-crystal Au(111) surfaces. The study is based on X-ray photoelectron spectroscopy, cyclic and differential pulse voltammetry, interfacial capacitance data, and electrochemical scanning tunnelling microscopy (in situ STM). The sequences used were the 25-bp sequence from the BRCA1 gene (25-mer), while the 10-bp oligonucleotides contained solely linear adenine and thymine sequences. The oligonucleotides were modified by the dimethoxytrityl group (DMT) via a disulfide group [DMT-S-S-ss25-mer and DMT-S-S-ds(AT)10], a pure disulfide group (A10-S-S-T10), or a thiol group [HS-ss25-mer and HS-ds-(AT)10], all via a hexamethylene linker. The overall pattern suggests strategies for controlled adsorption of DNA-based molecules and recognition of complementary strands or other molecules.

Nucleation at three-phase junction lines: in situ atomic force microscopy of the electrochemical reduction of sub-micrometer size silver and mercury(I) halide crystals immobilized on solid electrodes

Abstract: The electrochemical reduction of sub-micrometer size silver halide crystals immobilized on the surface of gold and platinum electrodes starts at the three-phase junction line where the three phases “metal”, “silver halide” and “electrolyte solution” meet. Following nucleation at this line the reaction advances within seconds on the surface of the silver halide crystals until the entire surface is covered with about 20 atomic layers of silver and the reduction is terminated. The silver layer can be oxidized anodically and the next layer of the silver halide crystals becomes accessible for further reduction. This sequence of reductions and oxidations can be repeated. The nucleation of silver at the three-phase junction line can be detected by atomic force microscopy (AFM) measurements when, after a short reduction pulse and dissolution of the remaining silver halide, a thin ring of silver is observed at the place where the three-phase junction line was situated. The entire scenario of electrochemical reduction of immobilized silver halide crystals depends on the crystal size. Large crystals (about 100 µm edge-length) immobilized on the surface of optically transparent indium tin oxide electrodes show the growing of silver whiskers on the crystal surface, similar to what is known for the reduction of silver halides with photographic developers. However, also in the case of the large crystals, the reduction starts at the three-phase junction line. The electrochemical reduction of immobilized sub-micrometer size crystals of Hg2Cl2 and Hg2Br2 starts also at the three-phase junction. In the case of gold electrodes the formation of liquid mercury is followed by the formation of a solid crystalline gold amalgam. In the case of platinum electrodes the liquid mercury wets the platinum surface but does not destroy it.

High-temperature ion transport in La1−xSrxFeO3−δ

Abstract: The conductivity of the entire solid solution La1−xSrxFeO3−δ, where x=0.2, 0.4, 0.5, 0.7 and 0.9, in the oxygen partial pressure range 10−19 to 0.5 atm and temperatures between 750 and 950 °C is reported. The analysis of the isothermal pressure dependences of the conductivity reveal that the lanthanum-strontium ferrites can be characterized as mixed ion-electron conductors in the low-oxygen pressure/high-oxygen deficiency limit. The partial contribution to conductivity from oxygen ions increases with strontium content and attains a maximal value at x=0.5. Further increase in doping results in the development of oxygen vacancy ordering phenomena and deterioration of the conducting properties.

Planar and tubular perovskite-type membrane reactors for the partial oxidation of methane to syngas

Abstract: Dense planar and tubular oxygen separation membranes of La0.6Ca0.4Fe0.75Co0.25O3−δ were investigated as reactors for the partial oxidation (POX) of methane to syngas. Their permeation properties were measured in an air/argon pO2 gradient as a function of temperature. At 900 °C, the oxygen flux through a 1.26-mm-thick membrane was 0.075 μmol/cm2·s and through a 0.25-mm-thick tube, 0.24 μmol/cm2·s. For the POX measurements, a catalyst was added to the membrane and methane was introduced on the argon side. This resulted in a gradual increase of the oxygen flux with increasing concentration of methane, reaching 2 μmol/cm2·s at 900 °C with pure methane. For the planar reactor, the CO selectivity reached 99% and the CH4 conversion 75% at 918 °C with pure methane. For the tubular reactor, the CO selectivity and CH4 conversion were 83 and 99%, respectively, under the same conditions. After 1,400 h of operation in a tubular POX reactor, the membrane was examined revealing phase demixing and local decomposition.

Binding energy of ruthenium submonolayers deposited on a Pt(111) electrode

Abstract: We investigated the 3d5/2 core-level binding energy of Ru in Ru nanoislands spontaneously deposited on a Pt(111) electrode [Pt(111)/Ru], and the binding energies of 3d5/2 iodine and 1s CO adsorbed on Pt(111)/Ru by the use of X-ray photoelectron spectroscopy. Both iodine and CO were used as surface probes of the electronic properties of Pt(111)/Ru. Little difference was found in the binding energy of Ru in Pt(111)/Ru and in Ru(0001). However, the addition of Ru to Pt(111) induces major changes in the core-level binding energies of chemisorbed iodine and CO as referenced to those adsorbed on Ru(0001). We conclude that the iodine 3d5/2 and CO 1s C core levels experience higher electronic charge on Pt(111)/Ru than on Ru(0001), suggesting a charge transfer from Pt to Ru, or to a Ru-I “surface molecule” within the deposit. The charge transfer from Pt to Ru is in agreement with the result of previous in situ electrochemical NMR investigations [P.K. Babu, H.S. Kim, A. Wieckowski, E. Oldfield (2003) J. Phys. Chem. B 107:7595] and confirms the general trend of reduction in the density of states of Pt due to alloying with Ru [J. McBreen, S. Mukerjee (1995) J. Electrochem. Soc. 142:3399]. Theoretical calculations are in progress to further interpret the origin of the binding-energy shifts observed in this study.

Kinetics of double-layer charging/discharging of the activated carbon fiber cloth electrode: effects of pore length distribution and solution resistance

Abstract: The effects of pore length distribution (PLD) and solution resistance, R sol, on the kinetics of double-layer charging/discharging of the activated carbon fiber cloth electrode (ACFCE) were investigated in a 30 wt% H2SO4 solution using nitrogen gas adsorption, a.c. impedance spectroscopy, the current transient technique, and cyclic voltammetry. The impedance spectra of the ACFCE were theoretically calculated based upon the transmission line model in consideration of the pore size distribution (PSD) and the PLD. From comparison of both the experimental and theoretical impedance spectra of the ACFCE, it is suggested that the deviation from the ideal impedance behavior of a cylindrical pore in the experimental impedance spectrum of the ACFCE is mainly ascribed to PLD, rather than to PSD. The cathodic current transients and cyclic voltammograms were theoretically calculated based upon the transmission line model as functions of the standard deviation σ of the PLD and R sol. From the results, it is concluded that ion penetration into the pores is closely related to both σ and R sol during double-layer charging/discharging of the ACFCE, that is, the larger σ and R sol, the lower is the rate capability, thus causing higher retardation of ion penetration into the pores.

Transport in ceria electrolytes modified with sintering aids: effects on oxygen reduction kinetics

Abstract: Small (2 mol%) cobalt oxide additions to ceria-gadolinia (CGO) materials considerably improve sinterability, making it possible to obtain ceramics with 95–99% density and sub-micrometre grain sizes at 1,170–1,370 K. The addition of Co causes a significant shift of the electrolytic domain to lower pO2. This modification to the minor electronic conductivity of the electrolyte material has influence on the cathodic oxygen reduction reaction. The impedance technique is shown to provide information not only about polarisation resistance, but also about the active electrode area from analysis of the current constriction resistance. It is demonstrated that this current constriction resistance can be related to the minor electronic contributions to total conductivity in these materials. A simple imbedded grid approach gives control of the contact area allowing the properties of the electrolyte materials to be studied. A much lower polarisation resistance for the Co-containing CGO electrolyte is observed, which can be clearly attributed to an increased three-phase reaction area in the Co-containing material, as a consequence of elevated p-type conductivity.

Silver incorporation into cathodes for solid oxide fuel cells operating at intermediate temperature

Abstract: Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La0.65Sr0.3MnO3−δ perovskite phase is already formed at 900 °C. The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen.

Stability and transport properties of La 2 Mo 2 O 9

Abstract: La2Mo2O9 samples were prepared from freeze-dried powder precursors and characterized by XRD, TG/DTA, SEM, electrical and electrochemical measurements. Pellets with different density were obtained by sintering at temperatures between 900 and 1100 °C to obtain nearly dense samples with grain sizes in the range 1–8 μm. The electrical conductivity was measured using impedance spectroscopy. The capacitance and relaxation frequencies of the main contributions to the spectra were used to ascribe the contributions of grain interiors and internal interfaces, and their temperature dependence. A coulometric titration technique was used to evaluate the change of oxygen stoichiometry under moderately reducing conditions, and to estimate the stability limits under strongly reducing conditions. An ion-blocking method was used to evaluate the onset of n-type conductivity, and a combination of these results with total conductivity measurements was used to obtain the ionic transport number. A combination of oxygen stoichiometry changes and ion-blocking results was used to obtain estimates of mobility.

Modification of a Pt surface by spontaneous Sn deposition for electrocatalytic applications. 2. Oxidation of CO, formaldehyde, formic acid, and methanol

Abstract: The electrocatalytic activity of a spontaneously tin-modified Pt catalyst, fabricated through a simple “dip-coating” method under open-circuit conditions and characterized using surface analysis methods, was studied in electrooxidation reactions of a preadsorbed CO monolayer and continuous oxidation of methanol, formic acid, and formaldehyde in the potentiodynamic and potentiostatic modes. The catalytic activity of the tin-modified Pt surface is compared with that of a polycrystalline Pt electrode. Spontaneously Sn-modified Pt catalyst shows a superior activity toward adsorbed CO oxidation and thus can be promising for PEFC applications. The methanol oxidation rate is not enhanced on the Sn-modified Pt surface, compared to the Pt electrode. Formic acid oxidation is enhanced in the low potential region on the Sn-modified surface, compared to the Pt electrode. The formaldehyde oxidation rate is dramatically increased by modifying tin species at the most negative potentials, where anodic formaldehyde oxidation is completely suppressed on the pure Pt electrode. The results are discussed in terms of poisoning CO intermediate formation resulting from dehydrogenation of organic molecules on Pt sites, and oxidation of poisoning adsorbed CO species via the surface reaction with OH adsorbed on neighboring Sn sites.