PEM燃料電池進氣流場設計與陰極觸媒量子化學計算及電化學光電子量測

Air-side Flow Field Design and Computational Quantum Chemistry with Electrochemical Photoelectron Measurements on the Cathodic Catalyst in PEMFCs

劉泓孝(Hung-Hsiao Liu)

指導教授 ： 洪哲文

國立清華大學/工學院/動力機械工程學系/博士(2017年)

摘要

本研究針對質子交換膜燃料電池(proton exchange membrane fuel cell, PEMFC)陰極端反應氣體的均勻性分佈與觸媒的氧還原反應(oxygen reduction reaction, ORR)這兩種關鍵重要步驟做相關模擬實驗與細節探討。
首先，本研究建立一工業級67 cell 5-kW PEMFC三維陰極端實體模型，利用有限體積法來研究罩板與檔板設計對於壓力與質量流率分佈均勻性的影響。結果證明在上罩板增加多孔檔板，可以有效改善流場的均勻性。質量流率的變化由39%降低到2%，可以產生較高的壓力與較均勻的質量流率。研究經光學輔助流體可視化實驗驗證理論模擬的正確性。
在觸媒的理論研究方面，本研究使用第一原理(first principles) 計算配合密度泛函理論 (density functional theory, DFT) 研究ORR反應機制，探討燃料電池陰極觸媒ORR的詳細過程，並計算出反應步驟的吸附能。此外，本研究以 Sabatier principle為基礎，使用Brønsted–Evans–Polanyi relation (BEP)關係來計算活化能，最後調整白金參雜在單臂奈米碳管(SWCNT)與石墨烯(GR)上的重量百分比，以得出具有較高反應活性的組合，最後使用Sabatier Analysis來量化觸媒的活性。本研究發現，Pt/GR白金的重量百分比為85.3wt%時，有良好的反應活性，而Pt/SWCNT參雜白金的重量百分比為18.5 wt%時，有最好的反應活性，後者可大幅降低白金的使用量，並維持良好的反應活性。
觸媒實驗使用旋轉圓盤電極與X光光電子能譜儀，兩種儀器用來量測商用Ni/CNTs的活化能與束縛能，結果發現束縛能與活化能有相同的趨勢性。在活化能實驗與模擬部分，藉由BEP關係式得到的活化能與實驗值相比趨勢相同且誤差皆在同一量級。由實驗的氧1s (O 1s)窄能譜得知，添加Ni金屬觸媒，會使O-Ni與O-H2的化學鍵位移有所增加。在模擬方面，當Ni比例持續增加到20.1wt%，鎳觸媒還是無法達到火山圖的尖點，不過離Au觸媒距離不甚遠。總而言之，使用本研究的理論可以預測觸媒比例及奈米碳材支架架構，對氧還原反應活性之影響，趨勢可完全準確預測，而數值結果在工程可接受範圍，另外電腦模擬可以大量省下觸媒調配之昂貴經費與時間。

關鍵字

質子交換膜燃料電池 ； 氣體的均勻性分佈 ； 氧還原反應 ； 密度泛函理論 ； 束縛能 ； 活化能 ； d-band中心

並列摘要

This thesis carries out simulations and experiments to study the flow distribution in the inlet air manifold and oxygen reduction reaction (ORR) at the cathode catalyst of a proton exchange membrane fuel cell (PEMFC).
The simulation establishes a real-size 3D fluid dynamics model of the air-side manifold of an industrial 5-kW PEMFC stack containing 67 cells. Finite volume method was employed to study the effect of manifold and baffle designs on the pressure and mass flow rate uniformity. Adding a baffle below the inlet air stream and a porous baffle on top of the channel engenders a highly uniform mass flow rate and pressure distribution. Adding a baffle in the inlet manifold reduces the mass flow variation in the inlet air channels from 39% to 2%. Through optical flow visualization experiments, the flow field simulation was proved correct.
The 2nd part of this thesis studies the mechanisms of the reaction by first principles calculation using density functional theory (DFT). The adsorption energy of the system, binding energy and activation energy of the ORR are all evaluated, and Brønsted–Evans–Polanyi relation (BEP) is used to calculate the catalyst activity. In order to find out the highest reaction activity, the weight percentage of Pt doped on nano-frames is tuned. Simulation results show that the 85.3 wt% of Pt on graphene (GR) and 18.5 wt% of Pt doped on single wall carbon nano-tubes (SWCNTs) have the best reaction activities.
Commercial Ni / CNTs catalysts have also been investigated using rotating disk electrode measurement and X-ray photoelectron spectroscopy. Simulation and experimental results revealed that binding energy and activation energy have similar trends. In conclusion, catalyst weight percentages can be predicted using the simulation and experimental results from this study. The effect of nano carbon frame to oxygen reduction reaction rate can also be predicted. The predicted trends are in good agreement with experiments. Moreover, computer simulation can save the cost and time compared with trial and error experiments in catalyst composition tuning.

並列關鍵字

proton exchange membrane fuel cell ； flow distribution ； oxygen reduction reaction ； density functional theory ； binding energy ； activation energy ； d-band center

參考文獻

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