Abstract
Protonic ceramic fuel cells (PCFCs) have great potential in applications compared with oxygen-ion-conducting-electrolyte based cells due to their relatively high ionic conductivity under intermediate and low temperature operating conditions. In addition, water is produced at the air electrode side of a proton-conducting fuel cell without diluting the fuel in the fuel electrode side, which provides higher operating voltage than an oxygen ion conducting SOFC. Among the proton-conducting materials, the most traditionally and widely used one is barium cerate-zirconate based perovskite oxide. It is simple to tailor its proton conductivity and stability by adjusting the doping concentration of Ce⁴⁺ and Zr⁴⁺. In a highly performed cell, the fuel electrode plays a vital role. The conventional fuel electrode is fabricated by metal-electrolyte composites, where the metal particles provide the catalytic activity and electronic conductivity and the electrolyte material ensures the proton conductivity. The main drawbacks of this cermet electrode are metal sintering and carbon coking, which degrades the cell performance in a long run.In past decades, exsolution has been proposed as an effective way for in situ nanoparticles growth from perovskite oxide by a controlled phase decomposition process. Exsolved nanoparticles are socketed strongly in parent perovskite and show better resistance towards coarsening and coking compared with conventional electrodes. The nanoparticles exsolution from perovskite can be simply accomplished by doping the catalytically active transition metal into the perovskite structure, followed by chemical or electrochemical reduction.
The exsolution phenomenon has been well investigated on perovskite titanate oxide. However, little study has been focused on the exsolution from protonic conducting oxides and their applications in PCFCs. This thesis explores the exsolution behavior from the doped barium cerate zirconate oxide. Generally, nicely distributed particles are obtained on the BCZY perovskite through the in situ exsolution approach. Protonic ceramic fuel cells (PCFCs) have great potential in applications compared with oxygen-ion-conducting-electrolyte based cells due to their relatively high ionic conductivity under intermediate and low temperature operating conditions. In addition, water is produced at the air electrode side of a proton-conducting fuel cell without diluting the fuel in the fuel electrode side, which provides higher operating voltage than an oxygen ion conducting SOFC. Among the proton-conducting materials, the most traditionally and widely used one is barium cerate-zirconate based perovskite oxide. It is simple to tailor its proton conductivity and stability by adjusting the doping concentration of Ce⁴⁺ and Zr⁴⁺. In a highly performed cell, the fuel electrode plays a vital role. The conventional fuel electrode is fabricated by metal-electrolyte composites, where the metal particles provide the catalytic activity and electronic conductivity and the electrolyte material ensures the proton conductivity. The main drawbacks of this cermet electrode are metal sintering and carbon coking, which degrades the cell performance in a long run.
In past decades, exsolution has been proposed as an effective way for in situ nanoparticles growth from perovskite oxide by a controlled phase decomposition process. Exsolved nanoparticles are socketed strongly in parent perovskite and show better resistance towards coarsening and coking compared with conventional electrodes. The nanoparticles exsolution from perovskite can be simply accomplished by doping the catalytically active transition metal into the perovskite structure, followed by chemical or electrochemical reduction.
The exsolution phenomenon has been well investigated on perovskite titanate oxide. However, little study has been focused on the exsolution from protonic conducting oxides and their applications in PCFCs. This thesis explores the exsolution behavior from the doped barium cerate zirconate oxide. Generally, nicely distributed particles are obtained on the BCZY perovskite through the in situ exsolution approach. Different from the previously observed process, where the B-site cations diffuse from bulk to surface for nucleation, here, the exsolution from BCZY perovskite competes with a phase segregation/decomposition process. In reduction, Ba-M-O phases (M is transition metal) are firstly formed on the perovskite surface, followed by the exsolution of transition metal ions from the Ba-M-O phases. This mechanism determines that for the B-site exsolutions from barium cerate zirconate oxide, A-site deficiency is unnecessary. Moreover, the cell parameters are found to have a significant effect on the electronic conductivity and stability of the materials with exsolution, which should draw the attention when optimizing the material properties in order to obtain a good performance.
By doping two transition metals on B-site, alloy particles are able to be achieved by exsolution. Compared with the single metal exsolution, the migration tendency of the metal ions to the surface is hindered by the other one. In this work, the co-exsolved Ni-Cu alloy particles show a much tidier nanoparticles distribution than the single particles, with a smaller particle size and higher population. Moreover, the doping of bimetal promotes their solubility in the perovskite structure. While the addition limit of sintering aids (Ni, Co, Cu etc.) is generally believed to be within 1 wt% (about 4 mol%) and sintering aids-rich impurities are constantly observed during the perovskite sintering, the nickel and copper bimetal doped BCZY perovskite shows a single pure phase, with a solubility of at least 5 mol% of the total doping concentration.
More importantly, the Cu particles exsolved from the doped BCZY oxide show a strong catalytic activity towards CO oxidation reaction. Furthermore, the morphology of Cu particles is reconstructed by oxygen plasma, exhibiting even higher catalytic activity. This gives a great view in plasma modifying catalyst morphology, hence enhancing performance. The recent work on the catalytic tests of BCZY supported Cu particles shows a very high activity and promising performance, which draws a lot of value on the exsolution from BCZY.
| Date of Award | 30 Jun 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | John Irvine (Supervisor) |
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