低溫燃料電池陰極觸媒 量子化學計算分析

Computational Quantum Chemistry Analysis of the Cathodic Catalyst in Low Temperature Fuel Cells

許湶琳(Chuan-Lin Hsu)

指導教授 ： 洪哲文

國立清華大學/工學院/動力機械工程學系/碩士(2014年)

摘要

提升燃料電池的效率，一直都是學術界與工業界研究追求目標，而陰極部分的氧還原反應 (Oxygen Reduction Reaction, ORR) 為速率決定步驟(Rate-Limited Step)，有效地提升其反應速率便能大幅提升燃料電池的效率，而目前提升的方式以添加觸媒為主，常用的觸媒有白金觸媒或是白金合金觸媒，但由於其數量稀少且價格昂貴，一直是使燃料電池無法普及的最大原因。
本研究為了降低白金觸媒的使用量，利用單層奈米碳管與石墨烯等導電性良好，且高比表面積材料作為陰極奈米支架，並將白金參雜於其中，分析其氧還原反應的機制。由於實驗上極難量測其氧還原反應的詳細過程，因此本論文以第一原理計算 (First Principles) 配合密度泛函理論 (Density Functional Theory, DFT) 研究反應機制，同時探討燃料電池陰極觸媒氧還原反應的詳細過程，並計算出吸附能、系統總能、反應能與活化能。
氧還原反應的第一個步驟即為氧分子 (O2) 的吸附，因此藉由分析不同初始氧吸附形式的吸附能與各步驟相對能，可以發現石墨烯參雜白金提供較穩定的反應環境，但穩定的反應環境並不代表有較良好的反應活性，因此在另一方面，本研究也利用反應能與Sabatier Analysis計算反應活性 (activity)，最後調整白金參雜在奈米支架上的的重量百分比，以利得出具有較高反應活性的組合。本研究發現，石墨烯參雜白金的重量百分比為94.2 wt%時，有良好的反應活性，而奈米碳管參雜白金的重量百分比為14.62 wt%時，有最好的反應活性，兩者都可大幅降低白金的使用量，增加反應面積，並維持良好的反應活性。

關鍵字

燃料電池 ； 奈米碳管 ； 石墨烯 ； 觸媒活性 ； 氧還原反應

並列摘要

Enhancing the conversion efficiency from hydrogen to electricity is always the target of fuel cell research. The oxygen reduction reaction (ORR) in the cathode is regarded the rate-limited step. Platinum alloys are common methods to increase the catalyst activity. However, platinum is rare and expansive, therefore it’s the reason that fuel cell can’t be commercialized.
In this research, platinum doped single wall carbon nanotube and graphene that have good electrical conductivity and high specific surface area are used as nano-frames for reducing the usage amount of platinum in cathode. It is difficult to study the whole process of ORR by experiments. Therefore, this thesis studies the mechanisms of the reaction by First Principles calculation using Density Functional Theory (DFT). The adsorption energy, total energy of the system, reaction energy and activation energy of the ORR are all evaluated.
Since oxygen adsorption is the first step of the ORR, the adsorption energy and relative energy are calculated for different initial adsorption forms. It is found that Pt doped graphene can offer a stable reaction environment. However, a stable reaction environment does not promise a good reaction activity. This research uses Sabatier analysis methodology to calculate the reaction activity. In order to find out the highest reaction activity, the weight percentage of Pt doped nano-frames is tuned. It is found that the 94.2 wt% has its best reaction activity for Pt doped graphene and the 14.62 wt% has its best reaction activity for Pt doped single wall carbon nanotubes. Both of these two nano-frames can reduce the usage amount of platinum, increase the reaction area and maintain an excellent reaction activity.

參考文獻

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