Correlation between microstructure and electrical conductivity in composite electrolytes containing Gd-doped ceria and Gd-doped barium cerate

Composite electrolytes with nominal compositions, Ce_0.8Gd_0.2O_1.9 + xBaO (x = 0.2 and 0.3), have been synthesized through the citrate route. Formation of two phases, namely Gd-doped ceria and Gd-doped barium cerate, has been confirmed through XRD and SEM studies. The impedance spectra show three distinct semi-circles, all originating from the composite electrolytes. In the temperature range 175-350 °C, the activation energies for the conductivity values extracted from the high frequency and intermediate frequency parts of the impedance spectra remains the same, irrespective of compositional and micro-structural variation. On the other hand, the activation energies for the conductivity values associated with the low frequency impedance spectra show a significant change with micro-structural variation. Solid oxide fuel cells constructed using these composite electrolytes exhibit a higher open circuit voltage compared to those based on single phase 20 mol% Gd-doped ceria.     

Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells

The electrochemical oxidation of BH₄⁻ in 2 M NaOH on Pt and Au (i.e. catalytic and non-catalytic electrodes, respectively, for BH₄⁻ hydrolysis accompanied by H₂ evolution) has been studied by cyclic voltammetry, chrono-techniques (i.e., potentiometry, amperometry, coulometry) and electrochemical impedance spectroscopy. In the case of Pt the cyclic voltammetry behaviour of BH₄⁻ is influenced by both, the catalytic hydrolysis of BH₄⁻ yielding H₂ (followed by electrooxidation of the latter at peak potentials between −0.7 and −0.9 V versus Ag/AgCl, KCl_std) and direct oxidation of BH₄⁻ at more positive potentials, i.e., between −0.15 and −0.05 V. Thiourea (TU, 1.5×10⁻³ M) was an effective inhibitor of the catalytic hydrolysis associated with BH₄⁻ electrooxidation on Pt. Therefore, in the presence of TU, only the direct oxidation of BH₄⁻ has been detected, with peak potentials between −0.2 and 0 V. It is proposed that TU could improve the BH₄⁻ utilization efficiency and the coulombic efficiency of direct borohydride fuel cells using catalytic anodes. The electrooxidation of BH₄⁻ on Pt/TU is an overall four-electron process, instead of the maximum eight electrons reported for Au, and it is affected by adsorbed species such as BH₄⁻ (fractional surface coverage ∼0.3), TU and possibly reaction intermediates.     

The effects of PVP surfactant in the direct and indirect hydrothermal synthesis processes of ceria nanostructures

CeO₂ nanostructures with completely different morphologies were successfully prepared using the same cerium source and mineralizer through the direct and indirect hydrothermal methods with different introducing strategies of PVP surfactant. The CeO₂ nanostructures tend to form the morphologies of nano-flowers and nano-cubes through the indirect and direct hydrothermal methods, respectively. X-ray diffraction (XRD) analyses revealed that both as-prepared nanostructures are composed of CeO₂ with a standard fluorite structure. The different synthesis mechanisms and corresponding chemical evolutions of the as-prepared CeO₂ nanostructures are discussed based on the different introducing strategies of PVP surfactant in the direct and indirect hydrothermal processes. Investigation of the UV-shielding ability of both CeO₂ nanostructures suggested that the UV absorbance of the nano-flowers is much higher than that of the nano-cubes.     

Electrochemical oxidation of borohydride on platinum electrodes: The influence of thiourea in direct fuel cells

The electrochemical behaviour of sodium borohydride on a platinum electrode in the absence and presence of thiourea (TU) was investigated by cyclic voltammetry. In the absence of thiourea, several overlapping peaks associated with the hydrolysis of BH₄⁻ appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 to −0.50 V versus Ag/AgCl. As a consequence of secondary reactions, the borohydride oxidation in 3 M NaOH solution shows a four to six-electron process, according to its concentration, in direct fuel cells. A conveyable TU/NaBH₄ concentration ratio of 0.6 inhibits the delivery of hydrogen simultaneously with catalytic hydrolysis of BH₄⁻. Thus, the coulombic efficiency in direct fuel cell discharge was increased showing an about eight-electron process for the oxidation of BH₄⁻.     

Modelling of alcohol and water diffusion in fuel cell membranes—Experimental validation by means of in situ Raman spectroscopy

In this work a multi-component transport model has been set up to describe the diffusion driven mass transport of water and methanol in fuel cell membranes. For a membrane in contact with liquid methanol and water on one side and conditioned air on the other, the corresponding differential equations and boundary conditions were derived in a polymer-related coordinate system taking into account the polymers three-dimensional swelling. Phase equilibrium parameters and unknown diffusion coefficients for Nafion^® 117 were obtained by comparing the simulation results to water and methanol concentration profiles measured with confocal Raman spectroscopy. The influence of methanol concentration, temperature and air flow rate was predicted by the model with a maximum relative mean deviation between measurement and simulation of 8.6% for methanol and 3.4% for water.     

Noble metal catalysts supported on gadolinium doped ceria used for natural gas reforming in fuel cell applications

An attractive possibility to simplify a fuel cell system would be the use of a sulfur-tolerant reforming catalyst. In an effort to find such a catalyst, platinum, rhodium and ruthenium catalysts supported on ceria doped with 20% gadolinium and on pure ceria were synthesized and characterized. A temperature-programmed reduction study of the reduction behavior of the catalysts showed that the doping of ceria with gadolinium enhances the low temperature reduction, while the high temperature reduction is suppressed. The activity as well as the stability of the catalysts can be correlated with the reducibility of the materials. The most stable catalyst, rhodium supported on gadolinium doped ceria, shows promising sulfur-tolerance.     

Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions

The mechanism and structure sensitivity of the electrocatalytic oxidation of hydrazine on platinum in alkaline solutions were investigated using cyclic voltammetry, steady-state current measurements, and on-line electrochemical mass spectrometry. The voltammetry of hydrazine oxidation on platinum in alkaline media is characterized by a single diffusion-controlled wave. The on-line electrochemical mass spectrometry measurements of hydrazine oxidation on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) surfaces indicated the formation of molecular nitrogen. No oxygen-containing nitrogen compounds were detected under the given experimental conditions. A comparative analysis of voltammetric data for hydrazine oxidation in alkaline solutions on the three surfaces at low overpotentials points to a structure sensitivity of the reaction. The electrocatalytic activity of basal planes increases in the order Pt(1 1 0) > Pt(1 0 0) > Pt(1 1 1), as deduced from the onset of the oxidation wave. The structure of the electrocatalyst surface affects the mechanism of the reaction, although without affecting the selectivity. At low overpotentials the hydrazine oxidation on the Pt(1 1 0) and Pt(1 1 1) surfaces is limited by the rate of electrochemical steps, whereas on Pt(1 0 0) a chemical step that probably involves N₂H₂ adsorbed intermediate is the rate-determining step. Platinum is more active in hydrazine oxidation in alkaline solution than in acidic solutions. In contrast to hydrazine oxidation on platinum in acidic media, the stabilization of chemisorbed hydrazine does not occur to a significant extent in alkaline media.     

直接甲醇燃料电池(DMFC)改性阳极催化剂的研究进展

介绍了国内外直接甲醇燃料电池（DMFC）的研究状况及其工作原理,阐述了DMFC阳极改性催化剂的作用机理,重点对目前国内外研究的各种改性催化剂体系进行了比较和评价,探讨了电催化剂的发展方向。     

Recent advances in platinum monolayer electrocatalysts for oxygen reduction reaction: Scale-up synthesis, structure and activity of Pt shells on Pd cores

We have established a scale-up synthesis method to produce gram-quantities of Pt monolayer electrocatalysts. The core-shell structure of the Pt/Pd/C electrocatalyst has been verified using the HAADF-STEM Z-contrast images, STEM/EELS, and STEM/EDS line profile analysis. The atomic structure of this electrocatalyst and formation of a Pt monolayer on Pd nanoparticle surfaces were examined using in situ EXAFS. The Pt mass activity of the Pt/Pd/C electrocatalyst for ORR is considerably higher than that of commercial Pt/C electrocatalysts. The results with Pt monolayer electrocatalysts may significantly impact science of electrocatalysis and fuel-cell technology, as they have demonstrated an exceptionally effective way of using Pt that can resolve problems of other approaches, including electrocatalysts’ inadequate activity and high Pt content.     

Promotion effects of ceria in partial oxidation of methane over Ni-calcium hydroxyapatite

Effects of ceria added as a promoter to a nickel-calcium hydroxyapatite catalyst, which has recently been reported to exhibit high activity and selectivity in partial oxidation of methane, were investigated. The ceria-promoted catalyst exhibited higher activity and stability than the unpromoted one. This is considered due to the oxygen storage capacity of ceria, which promotes easier removal of the deposited carbon. The optimum content of ceria was determined to lie in the range of the Ce/Ni ratio from 0.1/2.5 to 0.2/2.5.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

An experimental system for evaluation of well-defined catalysts on nonporous electrodes in realistic DMFC environment

This paper reports on an experimental setup wich enables us to investigate planar model catalysts in an environment closely resembling the environment found in an actual direct methanol fuel cell. The working electrodes were nano-structured catalyst particles immobilised on planar supports, reducing many of the commonly present non-catalyst related effects in conventional porous electrodes. Colloidal lithography was used for nano-structuring the samples. Nafion was used as electrolyte. Results are presented for the oxidation of methanol, formaldehyde, formic acid and carbon monoxide at temperatures between 30 and 70 ° C on Pt particles supported on glassy carbon disks.     

Comparative studies of configurations and preparation methods for direct methanol fuel cell electrodes

Catalyst-coated membrane (CCM) and catalyzed diffusion medium (CDM) prepared either by brush painting method or by spraying method were compared for direct methanol fuel cell (DMFC) anode and cathode. The pore structure and the morphology of the electrodes were characterized by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Internal resistance corrected polarization curves were employed to separate the contribution of each compartment of the membrane electrode assembly (MEA) to the overall polarization. It was shown that the increased mass transport resistance in the anode diffusion layer made the anode in CDM form act as the methanol barrier. The CCM configuration and the increased pores in micron scales in the catalyst layer were in favor of improving the performance of both anode and cathode. Accounting for the effect of methanol permeation, the combination of the anode in CDM form prepared by brush painting method and the cathode in CCM form prepared by spraying method was finally selected as the optimized configuration for MEA, which had the highest DMFC performance under near-ambient conditions.     

Effect of operating conditions on energy efficiency for a small passive direct methanol fuel cell

Energy conversion efficiency was studied in a direct methanol fuel cell (DMFC) with an air-breathing cathode using Nafion 117 as electrolyte membrane. The effect of operating conditions, such as methanol concentration, discharge voltage and temperature, on Faradic and energy conversion efficiencies was analyzed under constant voltage discharge with quantitative amount of fuel. Both of Faradic and energy conversion efficiencies decrease significantly with increasing methanol concentration and environmental temperature. The Faradic conversion efficiency can be as high as 94.8%, and the energy conversion efficiency can be as high as 23.9% if the environmental temperature is low enough (10 °C) under constant voltage discharge at 0.6 V with 3 M methanol for a DMFC bi-cell. Although higher temperature and higher methanol concentration can achieve higher discharge power, it will result in considerable losses of Faradic and energy conversion efficiencies for using Nafion electrolyte membrane. Development of alternative highly conductive membranes with significantly lower methanol crossover is necessary to avoid loss of Faradic conversion efficiency with temperature and with fuel concentration.     

Catalytic Influence of Oxide Component in Ni‐Based Cermet Anodes for Ammonia‐Fueled Solid Oxide Fuel Cells

Recently, the promising prospect of ammonia as a hydrogen carrier for solid oxide fuel cells (SOFCs) has attracted significant interests. In this work, the effects of temperature, fuel content, and total flow rate of anode gas on the performance of Ni/yttria‐stabilized zirconia (Ni/YSZ) anode for ammonia‐fueled SOFCs were investigated. Based on obtained results, the utilization route of ammonia on Ni/YSZ anode was discussed; the results of electrochemical experiments were related with the catalytic decomposition bahavior of ammonia over Ni/YSZ. Moreover, the catalytic activity for ammonia decomposition and anode performance of Ni/samarium‐doped ceria (Ni/SDC) and Ni/yttrium‐doped barium cerate (Ni/BCY) were also investigated. Among these anode materials, Ni/BCY exhibited the highest ammonia decomposition activity and anode performance for ammonia‐fueled SOFCs at intermediate temperatures.     

Performance and Evaluation of Cu‐based Nano‐composite Anodes for Direct Utilisation of Hydrocarbon Fuels in SOFCs

The anodes for direct utilisation of hydrocarbon fuels have been developed by using Cu/Ceria‐based nano‐composite powders. The CuO/GDC/YSZ–YSZ or CuO/GDC‐GDC nano‐composite powders were synthesised by coating nano‐sized CuO and CeO₂ particles on the YSZ or GDC core particles selectively by the Pechini process. Their microstructures and electrical properties have been investigated with long‐term stability in reactive gases of dry methane and air. The anodes fabricated using Cu‐based nano‐composite anodes showed almost no carbon deposition until 500 h in dry CH₄ atmosphere. The type of an electrolyte‐supported single cell in conjunction with the Cu/Ceria‐based anode must be selected together for the low melting temperature of Cu/CuO. The GDC electrolyte supported unit cell with the Cu/GDC–GDC anode showed the maximum power density of 0.1 Wcm^–2 and long‐term stability for more than 500 h under electronic load of 0.05 Acm^–2 at 650 °C in dry methane atmosphere.     

Evidence for high performances of low Pt loading electrodes based on capped platinum electrocatalyst and carbon nanotubes in fuel cell devices

Recently we reported the preparation and electrochemical behaviour of porous electrodes based on the controlled combination of carbon nanotubes and capped platinum nanoparticles towards oxygen reduction. Due to the organic crown of the nanoparticles, the electrodes exhibited low hydrogen underpotential deposition (H upd) electroactive surface areas but significant activity towards oxygen reduction was recorded down to very low platinum loadings of few μg/cm². While the presence of organic stabilizing material, at the surface of the electrocatalyst synthesized by wet chemistry, may be considered as a potential drawback in fuel cell community, we present in this paper results showing that our capped electrocatalyst associated with carbon nanotubes can be used without any pre-treatment and exhibit high performances in fuel cell devices, in spite of low platinum loadings. Beyond the practical interest of such capped nanoparticles in fuel cell technology demonstrated here, fundamental question related to the high performances of the capped electrocatalyst are still opened and are currently under investigation.     

Electrocatalytic Oxidation of Formaldehyde onto Carbon Paste Electrode Modified with Nickel Decorated Nanoporous Cobalt‐Nickel Phosphate Molecular Sieve for Fuel Cell

Nanoporous cobalt‐nickel phosphate VSB‐5 molecular sieve (CoVSB‐5) was synthesized by conventional heating for 48 h in the presence of (2‐hydroxyethyl) trimethylammonium hydroxide as template. Then, a novel, cheap and efficient catalyst was developed for formaldehyde electrooxidation by decorating Ni²⁺ ions on the surface of CoVSB‐5 modified carbon paste electrode (CoVSB‐5/CPE). The electrochemical behavior of the Ni‐CoVSB‐5/CPE electrode towards the formaldehyde oxidation was evaluated by cyclic voltammetry (CV) as well as chronoamperometry methods. An oxidation peak was observed at 0.60 V in 0.1M NaOH solution for electrocatalytic oxidation of formaldehyde with EC′ mechanism. It has been observed that CoVSB‐5 at the surface of CPE can improve catalytic efficiency of the dispersed nickel ions toward oxidation of formaldehyde. The values of electron transfer coefficient, the mean value of catalytic rate constant and diffusion coefficient for formaldehyde and redox sites were obtained to be 0.66, 1.80 × 10⁵ cm³ mol⁻¹ s⁻¹ and 3.62 × 10⁻⁴ cm² s⁻¹, respectively. Obtained results from cyclic voltammetry (CV) and chronoamperometry techniques specified that the electrode reaction is a diffusion‐controlled process. The good catalytic activity, high sensitivity, good selectivity and stability and easy in preparation rendered the Ni‐CoVSB‐5/CPE to be a capable electrode for formaldehyde electrooxidation.     

Catalytic reduction of hydrogen peroxide at metal hexacyanoferrate composite electrodes and applications in enzymatic analysis

The electrocatalytic activity of various metal hexacyanoferrates (Mhcfs) (i) immobilized on graphite electrodes, and (ii) as components of a composite electrode was investigated with respect to the reduction of hydrogen peroxide. The flow-through working electrode was a thin layer consisting of a composite of Mhcf, graphite, and polymethylmetacrylate (PMMA) as a binder, sandwiched between two Plexiglas plates. Among the pure Mhcfs immobilized on a graphite electrode, iron(III) hexacyanoferrate (Prussian blue) exhibits the highest electrocatalytic effect, whereas in the composite electrodes chromium(III) hexacyanoferrate (Crhcf) shows the highest activity and best performance and reproducibility for the electrochemical reduction of H₂O₂. The Crhcf electrode provides a linear dependence on H₂O₂ concentration in the range 2.5 × 10⁻⁶ mol L⁻¹ (LOD) to 1 × 10⁻⁴ mol L⁻¹ (phosphate buffer, pH 7). The sensor was applied for the detection of H₂O₂ enzymatically produced by glucose oxidase. The optimal conditions for the peroxide injection were 2 min after the beginning of the reaction and 25 °C with a detection limit of 7.0 × 10⁻⁶ mol L⁻¹ for glucose.     

Nanocomposite electrodes based on pre-synthesized organically capped platinum nanoparticles and carbon nanotubes. Part II: Determination of diffusion area for oxygen reduction reflects platinum accessibility

In this paper we report the determination of the diffusion area for oxygen reduction in porous electrode structure having a controlled platinum loading and based on capped platinum electrocatalysts and carbon nanotubes. Such a parameter is expected to be higher than the macroscopic geometrical area of the active porous layer. The oxygen diffusion area is determined by cyclic voltammetry after impregnation of the electrode structure by the electrolyte, and using the equations available for peak potential and peak current as a function of scan speed for irreversible redox couple. First it is found first that the oxygen diffusion area is dependent on the total amount of platinum in the electrode. Second, for a given platinum loading, the diffusion area is higher when the mass ratio of platinum to carbon nanotube decreases. This point indicates that the accessibility of platinum capped electrocatalyst is better in such cases. It is thus concluded that the oxygen diffusion area determination in porous electrode structures may be used to characterize the accessibility of the capped electrocatalysts for oxygen reduction. Even if this area is different in nature from the one calculated by Hydrogen Underpotential Deposition, we believe that its determination might be of interest for the characterization of porous electrodes structures in which the electrocatalyst is combined with a finely divided carbon support.