L.G.J. de Haart

Reaction of Hydrogen/Water Mixtures on Nickel‐Zirconia Cermet Electrodes: II. AC Polarization Characteristics

The reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes has been investigated as a function of potential and temperature at three different partial pressures of the reactants. Apparent kinetic parameters, namely, reaction orders, activation enthalpies, and pre-exponential factors, were determined as a function of potential in low-temperature (725--845 C) and high-temperature (845--950 C) regions for both the hydrogen oxidation and the hydrogen evolution reactions. In the low-temperature region, the charge-transfer coefficient, {alpha}{sub a}, calculated according to absolute rate theory was 0.7, independent of temperature. The apparent reaction order of the hydrogen oxidation reaction is in the region of 0.5 at 725 C, from which the authors conclude that at low temperatures, charge transfer involving atomically adsorbed hydrogen species determines the rate of this reaction. The results for the hydrogen oxidation reaction at temperatures above 845 C as well as for the hydrogen evolution reaction are not consistent with a charge-transfer-controlled electrode reaction. Adsorption and chemical reaction between adsorbed species are considered to be a dominant for the hydrogen oxidation reaction at temperatures above 845 C and the hydrogen evolution reaction over the entire temperature range studied.

Reaction of CO/CO2 gas mixtures on Ni–YSZ cermet electrodes

The reaction of carbon monoxide/carbon dioxide mixtures on Ni–YSZ cermet electrodes was investigated as a function of the electrode potential and the partial pressures of the reactants at 1273 K. Time-dependent reaction rates are observed for the CO oxidation reaction for oxygen activities corresponding to open circuit potentials in the range from −750 to −1010 mV. The electrode changes between a passive state and several active states for the CO/CO2 reaction. Periodic changes of the reaction rate for the CO oxidation are observed every 30 and 80 s. The impedance spectra recorded at the rest potential and the overpotential dependence of the CO oxidation rate indicate a change in the number of active sites in the reaction zone. In the active state, the CO oxidation reaction is more than one order of magnitude slower than the hydrogen oxidation reaction on these Ni–YSZ cermet electrodes. These results indicate clear differences in the kinetics of the CO and H2 oxidation reaction.

Modeling of Mass and Heat Transport in Planar Substrate Type SOFCs

A mathematical model is presented that incorporates the mass transport by diffusion in the porous structure of thick substrate type solid oxide fuel cells ~SOFCs!. On the basis of the mean transport pore model a multidimensional study allows for an optimization of the structural parameters of the substrates with respect to cell performance. Next to the mass transport in the porous substrates the electrochemical kinetics, methane/steam reforming and shift reaction, and energy equations are integrated in the model and boundary as well as operation conditions can be varied. Two-dimensional simulations for both anode as well as cathode substrate describe the extension of the existing model to two dimensions. The analysis emphasizes mass transport in porous electrodes, especially in the problematical zone below the interconnect ribs. Furthermore, the cathode substrate concept is considered as well, which has not been done before. Model The model was developed for a SOFC based on a planar sub- strate type concept. The fuel cell is considered to operate on either hydrogen or a ~prereformed! methane/steam mixture as fuel and oxygen from air as oxidant. Next to the description of the electro- chemical reactions, the methane/steam reforming and shift reactions, the heat-transfer and the mass-transfer processes are included in the model. Distinct from previous models, the fuel and oxidant flow, i.e., multicomponent transport, were developed for a realistic fuel cell comprising porous elements, based on a finite-volume-based computational fluid dynamics ~CFD! approach. For this purpose the model calculations were performed using the commercial CFD- package FLUENT. Model assumptions.—Ideal gas mixtures, incompressible and laminar flow due to small gas velocities, and pressure gradients are assumed. The electrodes have a homogeneous, isotropic structure and no gradients within mechanical properties. The electrolyte is regarded as an infinite thin layer between anode and cathode. In the model the mass and heat sources according to the electrochemical conversion arise in the boundary cells of the anode next to the elec- trolyte. The thickness of these boundary cells depends on the trun- cation grade of the anode.

A Kinetic Study of the Electrochemical Vapor Deposition of Solid Oxide Electrolyte Films on Porous Substrates

The electrochemical vapor deposition (EVD) method is a very promising technique for making gas-tight dense solidelectrolyte films on porous substrates. In this paper, theoretical and experimental studies on the kinetics of the depositionof dense yttria-stabilized zirconia films on porous ceramic substrates by the EVD method are presented. The more systematictheoretical analysis is based on a model which takes into account pore diffusion, bulk electrochemical transport, andsurface charge-transfer reactions in the film growing process. The experimental work is focused on examining the effectsof the oxygen partial pressure and substrate pore dimension on the EVD film growth rates. In accordance with thetheoretical prediction, the pressure of oxygen source reactant (e.g., water vapor), the partial pressure of oxygen and substratepore dimension are very important in affecting the rate-limiting step and film growth rate of the EVD process. In thepresent experimental conditions (e.g., low pressure of oxygen source reactant and small substrate pore-size/thicknessratio), the diffusion of the oxygen source reactant in the substrate pore is found to be the rate-limiting step for the EVDprocess.

Operation of anode-supported thin electrolyte film solid oxide fuel cells at 800°C and below

Abstract The influence of the fuel composition and fuel utilisation on the performance of an anode-supported solid oxide fuel cell (SOFC) have been studied as function of time by DC-methods both on single cells and on short stacks. For a single cell the influence of the water/hydrogen ratio in the fuel can mainly be attributed to the change in the open cell voltage. The performance of a single cell fed with a mixture of methane and water (1:2) (internal reforming) is in the current density range up to 0.3 A/cm 2 comparable to the performance of the cell under hydrogen/water (2:1) having the same open cell voltage. The influence of the water content in the hydrogen on the performance of a short stack is also mainly caused by the difference in open cell voltage. The influence of a prolonged galvanostatic load on the performance of a short stack was measured. A short stack driven at 375 mA/cm 2 and 800°C for 1000 h (fuel H 2 /H 2 O=99:1 fuel utilisation 9%) showed an ageing of 64 μ Ω ·cm 2 /h. This value is typical for these anode-supported cells under these conditions. A stack running under high humidity (H 2 /H 2 O=1:1) lost 53 mV during the first 100 h of operation. After this period the stack voltage increased to values comparable to the starting values.

The sensitization of SrTiO3 photoanodes by doping with various transition metal ions

The influence that various dopants exert on the behaviour of single crystalline SrTiO3 electrodes in a photoelectrochemical cell for water decomposition has been investigated. The electrodes were surface doped with Sr3MeNb2O9 (Me2+ = Mn, Fe, Co, Ni) and LaMeO3 (Me3+ = V, Cr, Mn, Fe, Co, Rh). The results have been compared with those for the sensitization of single crystalline SrTiO3 photoanodes homogeneously doped with LaCrO3 or the oxides of the transition metals Cr, Mn, Fe, Co, Ni, Nb, Mo, Ta and W. The response to visible light is greatest for Cr3+ and decreases in the sequence Co2+, Ni2+, Mn2+, Rh3+. The dopant ions V3+, Mn3+, Fe2+, Fe3+, Co3+ and the oxides of Nb, Mo, Ta and W reveal hardly any or no photosensitization. This is discussed in terms of a simple model proposed recently by members of our group.

The kinetics of electrochemical reactions on high temperature fuel cell electrodes

The rates of electrochemical reactions relevant for use in high-temperature solid oxide fuel cells (SOFC) has been investigated as a function of electrode potential, temperature and composition of the gas mixture. From Arrhenius plots, apparent activation energies, Ea, and apparent pre-exponential factors, A, were calculated for the oxygen-reduction and oxygen-evolution reactions at La0.84Sr0.16MnO3 cathodes. At low overpotentials (|η| ⩽ 0.2 V), both apparent activation energies and apparent pre-exponential factors are much higher in the temperature range T = 800−1000 °C (Ea ≈ 160−210 kJ/mol, log A ≈ 6−9) compared with those in the range T = 500−800 °C (Ea ≈ 80−110 kJ/mol, log A ≈ 2−4). For oxygen reduction, reaction orders of ze = 1 at pO2 > 0.2 bar and ze = 0.5 at pO2 < 0.2 bar were obtained. These values may be related to either oxygen adsorbed as molecules or atoms as the reacting species. From impedance spectroscopy, it follows that the rate of the oxygen-exchange reaction is determined not only by charge transfer, but also by another process, possibly the adsorption or surface diffusion of intermediates. For the nickel zirconia cermet anode fabricated by wet powder spraying (WPS), an increase in sintering temperature to 1400 °C results in an increase in current density. A current density of 0.27 A cm−2 at an overvoltage of 0.1 V may be achieved. From Arrhenius plots, an energy of activation of 130 ± 10 kJ mol−1 was determined.

Deposition and Electrical Properties of Thin Porous Ceramic Electrode Layers for Solid Oxide Fuel Cell Application

The influence of the microstructure on the electrical properties and polarization behavior of thin porous ceramic electrodelayers used in solid oxide fuel cells has been investigated. Thin layers (2–10 µm) of the cathode materialSr0.15La0.85MnO3 (15SLM) were film coated on YSZ substrates from classified suspensions. Narrow particle-size distributionsin the suspension resulted in close-packed layers with a very homogeneous porosity and pore-size distribution. Thespecific conductivity of the layers decreased significantly with increasing porosity and mean pore size. A specific conductivityof 109 S · cm–1 was obtained at 1000°C for a 2.9 µm thick layer from a suspension with particles in the range 0.10–0.25 µm. The current-overvoltage behavior of the film-coated layers presented in this study did not, however, show anysignificant dependence on the thickness and the microstructure of the porous layers. Overvoltages (eta) at a current densityof 0.1 A/cm2 at 898°C were quite low, i.e., in the range 60–70 mV. In comparison with other studies it is shown that filmcoating improves the microstructure of the ceramic electrode layers, which in turn lowers the cathodic overvoltages forthe oxygen reduction reaction.

Real-SOFC - A Joint European Effort to Improve SOFC Durability

The Integrated Project Real-SOFC joined 26 partners from throughout Europe active in SOFC technology. The project was funded by the European Commission within the 6th Framework Programme and aimed at improving the durability of planar SOFC stacks to degradation rates of well below 1% per 1000 hours of operation. This is an essential requirement in gaining access to the market for stationary applications. The underlying idea was to improve materials and materials processing on the basis of extensive test results identifying degradation mechanisms, and then to supply industrial components of enhanced quality for repeated testing analysis. This ‘feedback loop’ resulted in ‘2 nd ’ and ‘3 rd ’ Generations of SOFC components. This paper summarises the project approach, shows examples of the major results and of longterm durability testing.

On the kinetic study of electrochemical vapour deposition

A theoretical analysis is presented which quantitatively describes the transition behavior of the kinetics of the electrochemical vapour deposition of yttria-stabilized zirconia on porous substrates. It is shown that up to a certain deposition time and corresponding film thickness the rate limiting step is oxygen diffusion through the substrate pores, giving a linear dependence of the film thickness on the deposition time. For longer deposition times, i.e. thicker films, a transition of the rate limiting step to bulk electrochemical diffusion in the film occurs, resulting in a parabolic dependence of the film thickness on the deposition time. Simulation results are presented to show the effects of the experimental conditions on this transition time.