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Study on Electrodes Performance for Fuel Cell Efficiency Enhancement
|Authors: ||Chen, Chien-Chih|
|Contributors: ||NTOU:Department of Marine Engineering|
ceramic catalyst electrode;TPB;semi-organic solution evaporation;hydrothermal;spray pyrolysis
|Issue Date: ||2020-07-02T08:20:57Z
|Abstract: ||燃料電池的電極使用貴重金屬可以有效提升電池特性；以貴金屬純Pt做為電極，由於純導體的三相界面點較少，容易造成燃料電池中阻抗的增大。Pt價錢極高，大量使用將造成成本上的負擔。本研究利用陶瓷觸媒結合Pt貴金屬促進燃料電池陰、陽電極之電觸媒活性，改善燃料電池效率。使用PtCl2為起始原料在200C-4h還原氣氛的條件可得到分散性良好之具備足夠的觸媒能力純Pt金屬粒子。採用半有機溶液蒸發(SV)法將鈰基電解質披覆在還原製備得到的Pt顆粒表面，在800C熱處理下，可以形成Pt@CeO2結構粉體。使用水熱法將H2PtCl6披覆在CeO2顆粒上，僅需500C後熱處理製備得到CeO2@Pt核殼陽極粉體，Pt殼層與CeO2核之間無產生交互反應。SV法與水熱法製備的殼層披覆量與披覆厚度都隨披覆濃度而增加，但太高濃度如15mol%易造成殼層不均勻與凝聚現象。進一步也使用SV法與水熱法製作陰極核殼粉體，分別為SV法Pt@PNO，水熱法PNO@Pt，熱處理後之核殼陰極粉體，水熱法二次相，因此水熱法中之Pt與PNO間將產生反應。各種製備的陰陽極粉體調成漿料網印在LSBC電解質上共燒成半電池，陽極通燃料與陰極通氧化劑，量測半電池電化學功能，分析阻抗，其陽極端使用Pt@10CeO2與陰極端使用Pt@5PNO時，得到最佳之核殼電極披覆量與最佳之功率密度。將此兩者與LSBC結合為全電池，電池功率密度可達到110mW/cm2，約為Pt||LSBC||Pt電池的10倍，證明Pt與陶瓷觸媒結合的結構能夠延伸三相界面(TPB)至電極內部，增加載子交換反應面積與減少氣體擴散深度，以致降低電池中的活化阻抗與擴散阻抗，提高整體的功率密度。以預燒結之La0.3Sr0.7TiO3 (LST)多孔陽極為基材，利用超音波噴霧熱裂解法沉積滲透電解質LSBC至多孔陽極成為薄層電解質(iLSBC)，陰極使用披覆20mol%CeO2之Ba0.5Sr0.5FeO3 (BSF)材料(BSF@20CeO2)，所得薄電解質的LST||iLSBC||(BSF@20CeO2)全陶瓷電池比(Pt@5PNO)||LSBC||(Pt@10CeO2)電池高五倍的功率密度。因此薄電解質與陶瓷核殼電極結合可大大增進燃料電池效率。|
The noble metals as the electrodes of fuel cells can promote the cell efficiency effectively. The noble metallic electrode of pure platinum (Pt) provides less triple phase boundaries (TPB) due to continuous metal electrode coverage. Thus increases activation and diffusion impedances in fuel cells. The high price of Pt also loads the cost of fuel cells. In this study, ceramic electrocatalyst in accompanied with Pt are designed to enhance the electrocatalytic activities of cell electrodes and then to improve the fuel cell efficiency. The Pt particles with well dispersed and crystallized distribution size are produced by using PtCl2 reduced in 95%Ar/5%H2 atmosphere at 200C for 4h to obtain enough electrocatalytic ability. A semi-organic solution evaporation (SV) method successes to coat the ceria-based electrolyte shell on Pt particle surface after 800C heat-treatment, then forming Pt@CeO2 core-shell anode powders. The CeO2@Pt core-shell anode powders prepared from hydrothermal process which using H2PtCl6 as the coating source also no secondary phase is found. The formaton of CeO2@Pt powders only need 500C heat-treatment after hydrothermal process. The amount of coating and thickness of shell are dependent on the coating concentration observed from the analyses of FESEM and EDS-mapping for SV and hydrothermal processed samples. Too high concentration such as 15mol% will result in inhomogeneous shell thickness and exhibit aggregation. The Pt@PNO core-shell cathode powders prepared by SV method will form at 800C calcination. The PNO@Pt core-shell cathode powders are prepared by hydrothermal method in accompany with 500C heat-treatment. The second phases are found by the XRD patterns from hydrothermal processed samples. It indicates that the inter-reaction happened between Pt and PNO. The electrochemical performances of cells can be measured using test fixture under stable reduction-oxidation atmosphere at different temperatures. The optimal compositions for electrodes iares Pt@10CeO2 for anode and Pt@5PNO for cathode, respectively. They are individually cofired with LSBC electrolyte to form half-cells. Then, the half-cell power density and AC impedance are analyzed. Combining the above optimal electrodes to build the (Pt@5PNO)||LSBC||(Pt@10CeO2) full cell, then the power density of a cell is up to 110mW/cm2, which is about ten times of Pt||LSBC||Pt full cell. The results prove combining Pt and ceramic electrocatalyst core-shell structures can extend the triple phase boundaries into the electrode interior to enhance the exchange areas of charge carriers and reduce the gas transportation depth. Therefore, the total efficiency of a cell can be promoted significantly due to the reduction of activation and diffusion impedances. Thin layer of electrolyte (iLSBC) is futher deposited into porous La0.3Sr0.7TiO3 (LST) anode support substrate by ultrasonic spray pyrolysis. Such an anode supported assembly is coated by Ba0.5Sr0.5FeO3 (BSF)-20mol%CeO2 (BSF@20CeO2) core-shell cathode. The cofired LST||iLSBC||(BSF@20CeO2) full cell gets five times of power density of (Pt@5PNO)||LSBC||(Pt@10CeO2) full cell. Therefore, thin layer of electrolyte combining with ceramic core-shell electrodes enhances fuel cell power efficiency.
|Appears in Collections:||[輪機工程學系] 博碩士論文|
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