Study Of Surface Reaction Process Of Solid Oxide Electrode

The solid oxide fuel cell (SOFC) is an energy conversion device that can efficiently convert the chemical energy in fuels to electricity. The performance is sensitive to the materials selection and structure design. In this thesis, efforts are made on the surface reactionprocess and synergistic effect between solid oxides under the decided microstructure and materials, quantificationally. Furthermore, the relationship between surface reaction and mechanical behavior as well as the measurement of mechanical property in micro-scale are also discussed.A new elementary step for theoxygen electrochemical reduction is proposed in theory and experimentally confirmed. Oxygen electrochemical reduction at the cathode is studied owing to itsstrong contribution to the polarization losses of SOFC. In chapter two, an elementary step of oxygen vacancy transport across the electrode-electrolyte interface is proposed to demonstrate the electrolyte effect on electrode performance besides a series of elementary steps occurring on the electrode surface. According to the reaction kinetics, the electrode interfacial polarization resistance, Rp, can be theoretically related to the electrolyte conductivity, σ, with a general formula, Rp∝σ1Po2n, where pO2is the oxygen partial pressure at the cathode,l and n are the controlling parameters corresponding to various elementary steps occurred at the electrode-electrolyte interface as well as on the electrode. The oxygen vacancy transport step is experimentally confirmed by analyzing the electrochemical impedance spectra of symmetric cells of porous La0.6Sr0.4Co0.2Fe0.8O3electrodes on samaria-doped ceria electrolytes with different conductivities as a result of various dopant contents. The high frequency resistance, which can be fitted to a Warburg-type element, increases linearly with the electrolyte resistivity, clearly demonstrating that this process corresponds to the transport of oxygen vacancy at the electrode-electrolyte interface, from the electrolyte to the electrode.Electrical conductivity relaxation (ECR) technique is proposed to study the surface process of anode and measure the rate at three-phase boundary (TPB). In chapter three, the surface process of doped ceria reduction, i.e. the chemical surface exchange process in reduced atmospheres is studied to characterize their catalytic activity for fuel oxidation. The oxygen surface reaction coefficient of Gd0.1Ce0.902-δ is comparable to that obtained by thermogravimetric measurement, demonstrating the feasibility of ECR method. Usually, when the smallest diffusion thickness of the samples is as low as0.3mm, the ECR process is limited by the surface exchange step and almost independent on the bulk oxygen ion diffusion kinetics. Among various materials of R1.2Ce0.8O2-δ (R=Y, Gd, Sm, La) and SmxCe1-xO2-δ (x=0,0.05,0.1,0.2,0.3), Sm0.2Ce0.802-δ (SDC) exhibits the highest surface exchange coefficient, thus should be promise as the anode component. Moreover, it is found that, at temperature below700℃, surface exchange kinetics at the grain boundary is significantly faster than on the grain, suggesting additional advantage of developing SOFCs by low-temperature sintering. In chapter four, the electrochemistry performance of SDC surface-modified by the metal Pt or Au is studied using the similar method. By introducing Pt particles to the surface, the surface exchange kinetics can be remarkably improved. When Au is also deposited as a contrast, the re-equilibration time slightly increased contrast to SDC substrate caused by the decrease of exchange surface. Moreover, the increased catalytic surface exchange coefficient is linked to the surface microstructure, suggesting that the active site of metal support interaction is the Pt-SDC boundary.ECR techniqueis proposed to study the surface process of composites, quantificationally. In chapter five, the oxygen reaction kinetics of Sr2Fe1.5Mo0.5O6-δ-Sm0.2Ce0.8O1.9(SFM-SDC) dual-phase composites has been investigated as a function of SDC phase volume fraction. It is shown that the surface reaction kinetics of SFM could generally be enhanced by SDC. The enhancement is theoretically analyzed to quantitatively reveal the synergic effect between SFM and SDC on surface reaction. When the oxygen partial pressure is step increased in the range from0.01to1atm, the oxygen incorporation reaction take place at the surface of composites like the oxygen electrochemical reduction in cathode. The synergic effect contributes up to92%of the total amount of oxygen that is incorporated. The synergic rate is affected by the composition as well as the surface microstructure, suggesting the synergic reaction occurs on SDC surface rather than at SFM-SDC boundaries. Moreover, the synergic contribution and rate can be easily calculated with the apparent oxygen surface reaction coefficients (ka) and oxygen surface exchange coefficient for pure SFM (k). When the atmosphere is changed from humidified H2/Ar (60:40) to pure H2, as well as from CO/CO2(1:1) to CO/CO2(2:1), oxygen release from oxide like the anode reaction, the oxygen content of released from SDC and SFM through the synergic route can be calculated from the relaxation curves. The synergic effect contributes as high as70%of the total amount of oxygen that releases form SDC. With the increase of SDC content, the release oxygen through the interaction route increases. Furthermore, the rate of extra oxygen released from the oxides is also calculated. The initial release rates of the interaction between SDC-SFM are related to the surface microstructure parameters. When the atmosphere changes from humidified H2/Ar (60:40) to pure H2, the surface synergic rate is affected by the TPB length, linearly. So, the electriferous oxygen species reacted with the fuel is the rate-limiting step. When the atmosphere is abruptly changed from CO/CO2(1:1) to CO/CO2(2:1), the rate is affected by the size of SDC. That is, the electriferous oxygen species transfer is the rate-limiting step.The relationship between surface reaction and mechanical behavior as well as the measurement of mechanical property in micro-scale are discussed in chapter six. A novel method is presented to detect the mechanical stresses by combining the Fick’s second law, oxygen surface exchange and oxygen-ion diffusion properties. The surface tensile stress is weak for the smallstructural dimensions due to the short diffusion length. When the surface exchange kinetics is increased by means such as surface modification, the improved surface exchange rate may result in largemechanical stress and the stress loading rate, and consequently, reduce the mechanical stability. A new mechanicalmodulus (ω) is introduced to predict the stress, and larger co means higher mechanical stress. The predictionis experimentally confirmed with (La0.75Sr0.25)0.95Cr0.5Mn0.5O3-δ (LSCM) samples, where the fracture is related to its conductivity. It is found that porous LSCM has excellent stability while fractures are observed with Ni impregnated porous LSCM. Furthermore, a novel method is presented to determine the relationship between micro-fracture mechanics and conductivity, quantificationally. By the measurement of the conductivity change of YSZ-Al2O3composites in thermal cycles, the fracture between YSZ particles caused by thermal stress can be statistically "counted" using the Weibull or normal distribution. And then the parameters in fracture statistics distributions can be calculated with a statistical principle. The method offers a possible way to understand the fracture in microscale.