The Multi-phase Boundary Performance Of SOC Composite Electrodes And Chemically-induced Mechanics

Solid oxide fuel cells (SOFCs) and the reversed solid oxide electrolysis cells(SOECs), which are collectively called by a short name of SOC, are highly efficient and clean energy conversion system at high temperature. The SOFC can directly converse chemical energy to electric energy, while the SOEC can utilize electric energy from renewable energy system to reducing water and carbon dioxide. Syngas can be further synthesized and store electric energy from wind and solar power, etc.Thus, the development of SOC play very important role in alleviating the energy and environment issues, which are challenging humans severely. On the road of SOC development, reducing its polarization loss is a key way to improve SOC performance and reduce the operation temperature. And the fuel electrode polarization loss is an considerable part of the whole polarization. Electrode reactions occur normally on the mutiphase interface according to the nature of the electrochemical reactions.Composite electrodes extend the reaction active 3PB area through the whole electrode.For the mixed ionic and electrical conductor electrode, the composite electrodes own the each surface 2PB reaction sites and also the 3PB reaction sites. The reaction kinetics in the 3PB and 2PB of electrod is very important to be clarified. Based on the LSCM-SDC composite electrode, this dessertation analyzed the reaction kinetics at the 3PB and 2PB of electrod. And the chemically-induced mechanics in SOC electrolytes were disscussed.At first, in chapter one, the electrochemical reaction processes of SOCs at different parts were interpreted in thermodynamic and kinetic perspectives,introducing the concept of polarization loss. Then the foundation of mutiphase interface effect on SOC performance were investigated and the theoretical basis of chemically-induced mechanics were illustrated.In chapter two, the influence of composite anode on its performance in H2 was studied first. With the electrical conductivity relaxation (ECR) curves of LSCM-SDC composites, the oxygen releasing process was analyzed numerically. The 3PB length variation with the constituent of the dual-phase composite were calculated from the surface microstructure pictures. The 3PB length increases with SDC fraction increasing initially, and reach the maximum value of 1.57 ×106m-1 at 48 vol. % SDC.The 3PB effect exists in the inter-diffusion of oxygen ions between two phases through their interface, and also their synergic effect on surface exchange of oxygen.As for the inter-diffusion part, the ratio of the oxygen transport value from LSCM to SDC to the total releasing oxygen from LSCM is 69.6%. As for the surface exchange part, the contribution of synergic oxygen releasing volume trough 3PB to the total releasing value can reach 62% at most and share a same variation tendency with the relation between 3PB length and SDC fraction. Further analysis also indicated 3PB synergic effect on surface reaction rate played a dominant role.Based on this conclusion, samaria-doped ceria (SDC) was impregnated on LSCM to build nano-scale composites, which have much more 3PB length than mixed micro-scale composites. The electrode performance regarding the surface reaction and oxygen-ion transport in the electrode reaction were investigated. The results showed that the electrochemical performance had been promoted by incorporating with nano-SDC particles. The ECR of dense LSCM exhibited significant improvement in the surface exchange coefficient, from 5.34×10-6 cm s-1 at 850 ? for bare LSCM to 1.48×10-4 cm s-1 for LSCM coated with 0.9 mol L-1 SDC precursor solution. The surface reaction promotion factor was about 28 for the nano SDC particle coating,much higher than the factor of 6.5 for LSCM-SDC composite. It was demonstrated that the promotion in surface reaction rate are mainly contributed by LSCM-SDC-gas three-phase boundaries (3PB). Interfacial polarization resistances of SDC impregnated LSCM symmetrical cells presented over 1 order of magnitude reduction at 850 ?with a SDC loading of 35.8 wt.%. The electrode performance improvement was attributed to the connective oxygen ion conduction path and rich 3PB formed by the impregnated SDC particles.In chapter 3, the nano-scale composites were employed to electrolyze CO2 in SOEC. The SDC nano-particles were impregnated similarly on LSCM electrode to investigate its performance variation with SDC loading. The surface exchange coefficient was improved from 7.13×10-5 cm at 850 ? for bare LSCM, to 2.19×10-4 cm s -1 for impregnated LSCM, by a factor of 3.07 with CO-CO2 atmosphere. Tested in 50%CO-CO2 atmosphere, the minimum interfacial polarization resistances of SDC impregnated LSCM symmetrical cells was 0.24 ? cm2 with 37.4 wt.% SDC loading,with a promotion factor of 11.25. Besides, the higher CO content presented relatively lower polarization resistance. In SOEC mode with a voltage of 1.5 V, the full cell have a current density of 632 mA cm2 at 850 ? with pure LSCM cathode, and 786 mA cm2 with SDC impregnated LSCM.In chapter four, the influence of chemical stress in SDC electrolyte on its stability was studied. In addition to good performance, the reliable structural stability is also an significant precondition for normal long-term operation. Doped-ceria is an attractive electrolyte material for SOECs operated at intermediate temperatures. However, ceria is prone to break down under high applied voltages and low oxygen partial pressures at the fuel side. This phenomenon is analyzed here for the typical Sm0.2Ce0.8O19-?electrolyte based on the chemically-induced mechanics. The sensitivities of the maximum tensile stresses are explored under typical SOEC operating parameters such as temperature, applied voltage and oxygen partial pressure. Varying from short-circuit of SOFC mode to high voltage of SOEC conditions, the applied voltage sharpens the maximum tensile stress by seven times and raises the minimum permitted oxygen partial pressure at the cathode-electrolyte interface by a factor of 104.5 at most. The analysis results also indicated that the electrode porlarization at cathode-electrolyte interface make it easier to breakdown. This further explains the inapplicability of doped-ceria electrolyte in SOEC mode.