質子交換膜燃料電池白金觸媒-高分散白金奈米粉體於不同碳支撐材之製備與電化學特性

Synthesis and Electrochemical Characteristics of Highly Dispersed Platinum Nanoparticles on Different Carbon Supports for PEMFC

蔡天翔(Tien-Hsiang Leo Tsai)

指導教授 ： 彭宗平

國立清華大學/工學院/材料科學工程學系/碩士(2006年)

https://doi.org/10.6843/NTHU.2006.00590

摘要

本研究採用商用材XC-72碳黑、多層壁奈米碳管、以及竹炭作為白金觸媒的碳支撐材，此觸媒則應用在質子交換膜燃料電池(PEMFC)之電極。
由於奈米碳管的獨特形貌，比XC-72對白金沉積所造成的幾何阻礙較少，一般認為使用奈米碳管當做支撐材可減少白金顆粒的積聚現象。多層壁奈米碳管的表面處理已被研究多年，為了在多層壁奈米碳管的表面上得到更多的反應點，可利用硫酸與硝酸的超音波震盪化學處理，使奈米碳管的表面上產生如carbonyl (-CO)、 hydroxylic (-COH)、以及carboxylic (-COOH)之官能基。此外，竹炭擁有相當大的表面積，表面上有相當多的孔洞可使氣體更容易流動其中。
為使白金顆粒均勻分散於碳支撐材上，利用氯鉑酸為前驅物，而乙二醇則作為溶劑以及還原劑。
白金顆粒的尺寸大小以及其分散程度是決定觸媒效能的兩大關鍵因素。在此實驗中，藉由改變白金於觸媒中之含量可以控制白金顆粒的尺寸大小。利用X光繞射、熱重分析、以及穿透式電子顯微鏡分析白金觸媒的特性。可以觀察到白金顆粒均勻分散在所有碳支撐材上，當使用XC-72作為支撐物且觸媒含量控制在5%時，白金顆粒的粒徑大小可以降低至3奈米以下。使用分散於各種碳支撐材上的白金觸媒於質子交換膜燃料電池，測試單電池放電效率其結果分別說明如下︰
當使用XC-72碳黑作為支撐物，觸媒含量控制在5%，放電效率最高可達0.17瓦特，略高於觸媒含量控制在10%和20%的放電效率0.16瓦特，此係觸媒含量控制在5%時，白金奈米顆粒分散相當均勻，且其粒徑低於3奈米，使得反應總表面積增加所致，其放電效率也因相同原因略高於商用觸媒的放電效率0.15瓦特；當使用表面處理之奈米碳管作為支撐物，觸媒含量控制在5%、10%、20%所測得之放電效率都在0.12瓦特左右，是由於降低觸媒含量但其白金顆粒分散度沒有大幅改善之故，整體效率低於商用材原因來自奈米碳管純度不夠和較少的總反應面積；當使用竹炭作為支撐物，觸媒含量控制在20%，放電效率約為0.12瓦特，其效果低於商用材是因為竹炭本身的純度不夠且白金顆粒之粒徑將近10奈米所致。

關鍵字

白金觸媒 ； 質子交換膜燃料電池 ； 竹炭 ； 奈米碳管 ； 表面處裡 ； 乙二醇

並列摘要

Commercial XC-72 carbon black, multi-wall carbon nanotube (MWCNT), and bamboo charcoal were used as carbon supports for Pt catalyst for application in proton exchange membrane fuel cell (PEMFC).
Due to the unique morphology of carbon nanotube, there are less geometric barriers than XC-72 for deposition of Pt. Aggregation of platinum particles may be decreased by using carbon nanotube as the support. Surface treatment on MWCNTs has been studied for many years. In order to get more active sites on the surface of MWCNTs, a sonochemical treatment with sulfuric acid and nitric acid was made. By this process, functional groups such as carbonyl (-CO), hydroxylic (-COH), and carboxylic (-COOH) can be created on the surface of carbon nanotubes. Bamboo charcoal has a very large surface area, and is considered as a good candidate as the support. There are many pores on the surface to make easier gas diffusion.
Chloroplatinic acid was used as the precursor, and ethylene glycol was used as the solvent and reducing agent to get well-dispersed platinum nanoparticles on the carbon supports.
The particle size and dispersion of platinum particles are two key factors to determine the performance of catalysts. In this experiment, platinum particle size was controlled by changing the loading percentage of platinum. The deposition of Pt was characterized by X-ray diffraction, thermal gravimetric analysis, and transmission electron microscopy. It was found that platinum particles were well-dispersed on all carbon supports. The average diameter of platinum particles deposited on XC-72 with 5 wt % Pt loading was lower than 3 nm. The performance of single-cell PEMFC using the Pt deposited on various supports was evaluated, as follows:
When using XC-72 as the support and controlling the Pt loading to 5 wt%, the cell power was close to 0.17 watt, higher than that of the cell using Pt/XC-72 as catalyst with 10 wt% or 20 wt% Pt loading, 0.16 watt. The reason was that Pt particles could be well dispersed on XC-72 and the size of Pt particles was lower than 3 nm when the Pt loading was controlled to 5 wt%. The cell power was also higher than that using commercial E-TEK as catalyst, 0.15 watt, due to the same reason. When using M-CNT as the support and controlling the Pt loading to 5, 10, or 20 wt%, the power of the cell was close to 0.12 watt, because the dispersion of Pt particles could not be improved much, and because of residual impurities in the CNT sample and smaller surface area of Pt on the carbon materials. When using bamboo charcoal as the support and controlling the Pt loading to 20 wt%, the cell power was close to 0.12 watt, being possibly because of the impurities in the bamboo charcoal and the large size of Pt particles.

並列關鍵字

platinum catalyst ； proton exchange membrane fuel cell ； bamboo charcoal ； carbon nanotube ； surface treatment ； ethylene glycol

參考文獻

5.P. J. de Wild, M. J. F. M. Verhaak, Catal. Today, 60, 3 (2000).

11.V. S. Murthi, R. C. Urian, S. Mukerjee, J. Phys. Chem. B, 108, 11011 (2004).

12.P. Liu, A. Logadottir, J. K. Norskov, Electrochim. Acta, 48, 3731 (2003)

13.S. Litster, G. McLean, J. Power Sources, 130, 61 (2004).

14.L. Wang, A. Husar, T. H. Zhou, and H. Liu, Int. J. Hydrogen Energ., 28, 1263 (2003).