Svein Sunde

Monte Carlo Simulations of Polarization Resistance of Composite Electrodes for Solid Oxide Fuel Cells

The polarization resistance of composite electrodes for solid oxide fuel cells was modeled by three-dimensional random-resistor networks. These were generated on a computer by identifying neighbors in cubic lattices randomly occupied by electrolyte particles (ionic conductors) or electrode particles (electronic conductors), or in random packings generated by sequential deposition of such particles in random order. The polarization resistances between electrode and electrolyte particles were taken to be in parallel with interfacial capacitances, and the polarization resistance of the composite was calculated as the difference between high- and low-frequency resistance of the resistor networks. The volume fraction of electrode particles at which the minimum in polarization resistance occurs was found to increase with the ratio between electrode-particle radius and electrolyte-particle radius. This was rationalized by investigating the limits within which the composite may be expected to contain electrode-electrolyte interfaces in which both the participating clusters extend throughout the composite. If such interfaces are present, there will be a thickness dependence in the polarization resistance to a degree depending on the component conductivities and polarization conductances, otherwise not. The results are in reasonable agreement with experimental data.

Mathematical Modeling of Oxygen Exchange and Transport in Air‐Perovskite‐Yttria‐Stabilized Zirconia Interface Regions II. Direct Exchange of Oxygen Vacancies

The transport of oxygen in a porous perovskite solid oxide fuel cell cathode with a relatively high oxygen ion conductivity is modeled by taking into account exchange kinetics at the gas/electrode interface, bulk diffusion of oxygen vacancies, surface diffusion of adsorbed oxygen atoms, and electrochemical kinetics at the cathode/electrolyte interface. The electrochemical mechanism is assumed to be controlled by direct exchange of oxygen vacancies between the cathode and electrolyte phases. Simulated polarization curves typically exhibit Ta el-like behavior in the cathodic direction, which, however, is caused by concentration rather than activation polarization. In the anodic direction, a limiting current behavior is predicted, due to occupation of oxygen lattice sites on the cathode side of the interface. The effective polarization resistance either decreases or remains constant upon reduction of the oxygen partial pressures depending on prevailing kinetic and material parameters. Analytical expressions valid for the asymptotic case of a fast oxygen adsorption process at the gas/electrode interface are derived for the apparent Tafel slope, apparent exchange current density, anodic limiting current, and the effective polarization resistance. The theoretical results are consistent with experimental data in the literature for dense perovskite electrodes and for porous electrode materials with high oxygen nonstoichiometries. An overall assessment of the two parts of this study indicates that the catalytic properties of the perovskite surface, which enhances adsorption and surface diffusion of oxygen, is more significant than processes involving the bulk material, such as fast oxygen exchange with the bulk and vacancy diffusion, in determining cathode performance.

Electrochemical Oxidation of CO on Pt and Ni Point Electrodes in Contact with an Yttria-Stabilized Zirconia Electrolyte I. Modeling of Steady-State and Impedance Behavior

The steady-state and impedance response of a solid metal point electrode in contact with a solid, oxygen-ion conducting electrolyte in CO-CO 2 atmospheres was derived for three reaction mechanisms, involving gaseous species or species adsorbed on the surface of the electrode and/or the electrolyte. The overall electrochemical reaction was assumed to proceed in elementary steps, such as adsorption, diffusion of adsorbed species, and charge transfer. Eliciting the reaction mechanism from the steady-state response may require an extensive analysis in terms of temperature, gas-phase composition, and overpotential dependence. The impedance response, on the other hand, can in favorable cases be more distinctively dependent on the number of adsorbed species involved in the reaction and on the role of diffusion. Thus, if only one adsorbed intermediate species is involved, the impedance spectrum will always appear in the first quadrant of the impedance plane plot irrespective of experimental conditions. If two adsorbed intermediates are involved, however, this may lead to appearance of fourth-quadrant data.

Calculations of impedance of composite anodes for solid oxide fuel cells

The impedance of composite anodes for solid oxide fuel cells was modelled by three-dimensional impedance networks. These networks were generated by identifying neighbours in computer-generated random packings of electrode and electrolyte particles. The impedance of the networks was then calculated from Kirchhoffs current law, assuming for the impedance of a single electrode-electrolyte interface the impedance of a generic three-step reaction pattern. The shape of the impedance-plane plot for the composite will, in general, differ significantly from that of the single electrode-electrolyte interface. These differences appear as a distortion of the spectrum at high frequencies, in some cases even as clearly distinguishable extra arcs. The nature and strength of the distortions will depend on the volume fraction of electrode particles, impedance parameters, conductivities and geometrical factors. The results are explained by the presence of electrode-particle clusters entirely confined within electrolyte clusters. Since the interfacial impedance of the former occurs partly in parallel with the ohmic resistance of the latter, additional time constants may appear in the impedance of the composite.

Iridium oxide-based nanocrystalline particles as oxygen evolution electrocatalysts

Iridium-based oxides are highly active as oxygen evolving electrocatalysts in PEM water electrolyzers. In this work XRD reveals that Ir-Sn oxides contain a single rutile phase with lattice parameters between those of pure IrO2 and SnO2. Addition of Ru leads to the synthesis of a core-shell type material due to the strong agglomeration of Ru colloids during the preparation procedure. The shell of this material consists of an Ir-Sn-Ru oxide deficient in Ru relative to the bulk. This leads to a decrease in the surface noble metal concentration (as found by XPS), which in turn results in a significant reduction in electrochemically active surface area. Polarization analysis indicates that the addition of Ru can influence the rate-determining step or mechanism by which oxygen is evolved. In a PEM water electrolysis cell, small additions of Sn do not significantly reduce the operating performance, however larger additions cause a performance loss due to a reduction in active surface area and increased ohmic resistance. When a pure IrO2 anode is used, a cell voltage is 1.61 V at 1 A cm−2 and 90°C.

Electrochemical Oxidation of CO on Pt and Ni Point Electrodes in Contact with an Yttria-Stabilized Zirconia Electrolyte II. Steady-State and Impedance Measurements

Electrochemical measurements were performed with Pt and Ni electrodes forming an approximate (macroscopic) point contact with a solid electrolyte, (Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 , for mixtures of CO-CO 2 in the temperature range 827 to 950°C and 775 to 914°C, respectively. Steady-state current and impedance spectra vs. overpotential under the different experimental conditions were compared with the theoretical models derived in Part I of this paper. A low-frequency inductive loop in the fourth quadrant of the impedance spectra appearing under certain experimental conditions is consistent with a model for a reaction mechanism involving two adsorbed intermediates. At high anodic overpotentials, the data for Ni electrodes are consistent with the formation of an insulating layer of nickel oxide (NiO) in the electrode/electrolyte interface. Irrespective of the experimental conditions, no effects of diffusion limitations were seen in the impedance spectra. The electrode metal plays a significant role in the overall reaction. For Ni, the results indicate a lower specific elecirocatalytic activity for CO-CO 2 than for the H 2 -H 2 O reaction.

Calculation of Conductivity and Polarization Resistance of Composite SOFC Electrodes from Random Resistor Networks

A model for total conductivity and polarization resistance of composite electrodes in solid oxide fuel cells is proposed. The model is based on numerically solving Kirchoff`s current law for the resistor network resulting from scattering predetermined fractions of electrode and electrolyte particles at random on the sites of a three-dimensional cubic lattice. The model predicts an almost abrupt rise in total conductivity at an electrode-particle volume-fraction of 0.3, and a polarization resistance with a relatively broad minimum close to this percolation threshold, in good agreement with experimental results.

Towards a highly-efficient fuel-cell catalyst: optimization of Pt particle size, supports and surface-oxygen group concentration.

In the present work, methanol oxidation reaction was investigated on Pt particles of various diameters on carbon-nanofibers and carbon-black supports with different surface-oxygen concentrations, aiming for a better understanding of the relationship between the catalyst properties and the electrochemical performance. The pre-synthesized Pt nanoparticles in ethylene glycol, prepared by the polyol method without using any capping agents, were deposited on different carbon supports. Removal of oxygen-groups from the carbon supports had profound positive effects on not only the Pt dispersion but also the specific activity. The edge structures on the stacked graphene sheets in the platelet carbon-nanofibers provided a strong interaction with the Pt particles, significantly reconstructing them in the process. Such reconstruction resulted in the formation of more plated Pt particles on the CNF than on the carbon-black and exposure of more Pt atoms with relatively high co-ordination numbers, and thereby higher specific activity. Owing to the combined advantages of optimum Pt particle diameter, an oxygen-free surface and the unique properties of CNFs, Pt supported on heat-treated CNFs exhibited a higher mass activity twice of that of its commercial counterpart.

Critical Analysis of Potentiostatic Step Data for Oxygen Transport in Electronically Conducting Perovskites

The solid-state potentiostatic technique is a convenient and versatile tool for studying oxygen transport in electronically conducting perovskites. Furthermore, a systematic analysis of the relaxation data helps elucidate the mechanistic nature of the prevalent oxygen rate process. As a case in point, we describe potential step measurements on 90% dense samples of SrCo 1-x Fe x O 3-δ , focusing per se on the assumptions underlying the technique and the interpretation of results. Careful analysis of the current-relaxation data pointed to the presence of high-diffusivity paths in the samples. It also indicated that the overall transport rate is controlled by an oxygen exchange reaction at the grain boundaries. This complication prevented unambiguous assessment of the chemical i usion coefficient in this material. In addition, the analysis showed that wide disagreements in the oxygen chemical diffusion coefficients reported in the literature for doped perovskites may be attributed to differences in the sample quality (e.g., density, grain size, and distribution) and the measurement technique employed.

Materials for Electrocatalysis of Oxygen Evolution Process in PEM Water Electrolysis Cells

Proton exchange membrane (PEM) water electrolysis offers several advantages compared to the traditional alkaline technologies including higher energy efficiencies, considerably higher specific production rates leading to more compact design, and avoiding a liquid and corrosive electrolyte. The oxygen electrode is the critical part in the energy consumption of such cells. To obtain high performance, electrocatalytically very active anode materials have to be developed for oxygen evolution. The most promising electrocatalytic materials are based on IrO2 and RuO2, preferably in mixtures with other transition metal oxides with electronic conductivity. Excellent performance has been obtained by using nanocrystalline electrocatalysts based on iridium oxide with additions of ruthenium oxide and/or tin oxide, forming rutile structures, or mixed with tantalum pentoxide.This concept has been applied extensively in our work and has been successful in understanding oxygen evolution performance variations in IrO2-RuO2, IrO2-SnO2, IrO2-RuO2-SnO2 and IrO2-RuO2-Ta2O5 systems.