Phase-inversion Fabrication And Characterization Of Solid Oxide Cells

Solid oxide cell (SOC) is a clean and efficient all-solid-state energy conversion device which can be operated in fuel cell (SOFC) and electrolysis cell (SOEC) modes. Thus, SOC can not only produce electricity by electrochemically combining fuel and oxidant gases but also electrolyze CO2and H2O by renewable energy such as wind and solar power. The syngas (CO and H2mixture) can be further converted into hydrocarbon fuels by Fischer-Tropsch synthesis for efficient and feasible transportation and storage by existing infrastructures. Finally carbon-neutral energy technologies can be realized. Therefore, SOC is very important for the development of clean energy and alleviation of energy crisis. However, there are still many issues such as concentration polarization loss and electrode materials stability to overcome, which requires further optimization of electrode structure and development of key materials. In this dissertation, we develop a graphite-assistant phase inversion method to fabricate fuel electrode with graded porosity and further apply this structure to SOC. Moreover, double-perovskite material was employed as electrode material for the asymmetrical SOC operated without using H2as carrier. Three parts are involved:(1) the fabrication and characterization of Ni-YSZ electrode with graded porosity;(2) SOC with asymmetric structure for direct synthesis of CH4from CO2-H2O co-electrolysis;(3) Perovskite Sr2Fe1.5Mo0.5O6-δ materials based symmetrical solid oxide cells for CO2-H2O co-electrolysis.At first, the development and principle of oxide ion conducting SOC and relevant thermodynamic parameters were briefly introduced. Based on highlighting the polarization losses, key material for the fuel electrode and phase inversion, Proposal on this dissertation work was thus presented.In chapter2, graphite-assisted phase inversion technology is employed to control the dynamic process of phase separation. The obtained porosity gradient shows an increasing porosity and pore size from the electrode-electrolyte interface to the electrode surface. Compared to the conventional structure, substrates with porosity gradient exhibit continuous pore size distribution and better N2permeability. In SOFC tests, sample shows a peak power density of0.64Wcm-2with the fuel utilization of38.6%at850℃, which is higher than that of samples with normal structure (0.50Wcm-2,34.8%). The concentration polarization loss at high current density is eliminated with the graded structure. When it’s used as SOEC subtract, the novel asymmetric structure also exhibits improved electrochemical performance when the SOECs are operated with lower steam humidity and higher external operation voltage. For example, the H2production rate can reach3.58mL min-1cm-2at20vol%humidity (1.3V,800℃), which is higher than that of samples with normal structure with skin layer (2.39mL min-1cm-2). Thus, the optimized fuel electrode structure is promising in SOC for better performance.In chapter3, tubular units with8cm long and0.42cm in diameter were fabricated based on controlled phase inversion method. The tubular design can integrate high temperature SOC and low temperature Fischer-Tropsch synthesis in single chamber for directly converting CO2to CH4. In SOFC test, the cell shows a peak power density of0.48Wcm-2with the total resistance of0.70Ωcm2at800℃. The cell performance is comparable with the result in chapter2. In co-electrolysis tests, a current density of0.47Acm-2is observed for40vol%humidity (1.3V,800℃) with the hydrogen production rate of3.24mL min-1cm-2. The tubular unit can be operated in different models when placed in the different positions of mini-mite furnace. When the unit was integrated the high-temperature CO2-H2O co-electrolysis and low temperature methanation synthesis, CH4can be directly synthesized in single chamber with improved CH4yield ratio from17.1%(single methanation process) to41.0%. During the24h short term process, the co-electrolysis current density is stabilized at0.42Acm-2and the average CH4yield is11.40%(0.84mL min-1) with the total CO2conversion ratio of64.1%. The novel design presents a significant advancement in the carbon neutral renewable energy cycle.In chapter4, the phase inversion combined tape casting and impregnation were employed to fabricate all ceramic based asymmetric Sr2Fe1.5Mo0.5O6-δ(SFM)-YSZ|YSZ|SFM-YSZ cells. The asymmetric cell shows a peak power density of0.84Wcm-2with the ohmic resistance of0.16Ωcm2at850℃. In SOEC tests, the steam electrolysis current density of2.14Acm-2is observed for60vol%humidity (1.5V,800℃) when H2was used as steam carrier and the electrolysis current density of CO2can reach0.97Acm-2. Co-electrolysis test was carried without using H2as carrier. The co-electrolysis current density can reach1.18Acm-2(1.5V,800℃,20vol.%H2O), which increased21.6%compared to that of dry CO2electrolysis. However, the degradation rate shows a0.2%/h during the short term test. Thus, the performance of the all ceramic based asymmetric cell need to be further improved by optimizing the electrode microstructure.In chapter5, hybrid Pt/a-Fe2O3nanorods were prepared. Based on the polyol reduction condition for Pt deposition, subsequent TEM characterization of Pt deposition position and particle size, the Pt grow mechanism was discussed. XPS were employed to clarify the chemisorbed oxygen species and the change of chemical state of Pt nanoparticles from electron deficient state to metallic state. We quantitatively analyzed the specific periphery density and revealed the increased efficiency of the separation of the hole/electron excitation pairs, then the methylene blue (MB) degradation mechanism was proposed. The photocatalytic activity can be optimized by varying the surface amount Pt. For example, the degradability of20mg Pt(1.79%)/a-Fe2O3achieves86%after being irradiated for150min (20W fluorescent lamp with400<λ<760nm as light source,100mL of10mgL-1MB solution as pollution).