以石墨及複合材料之無鉑對電極應用於染料敏化太陽能電池

Graphite and Composite Material Applied as the Platinum-Free Counter Electrode for Dye-Sensitized Solar Cells

利宇晏(Yu-Yan Li)

指導教授 ： 何國川

國立臺灣大學/工學院/化學工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00818

摘要

由於石油供應的短缺，太陽能電池的開發已經是綠色能源研究領域的重要一環。作為新一代的太陽能電池，染料敏化太陽能電池在近年來已被廣泛研究，因其具有一些優勢例如組裝簡易、可撓、價格低廉。
與其他種類的太陽能電池相比較，具有組裝簡易以及價格低廉特色的染料敏化太陽能電池已被研究具有超越12%的光電轉換效能。時至今日，白金作為對電極材料因具有良好的催化活性以及導電性，使得染料敏化太陽能電池光電轉換效率有良好的表現，因此被廣泛的研究。然而，白金為一種稀有的金屬材料，利用常見的製備白金膜的方式，像是真空濺鍍、熱裂解，其價格是相當昂貴的。因此若利用白金作為對電極材料，則會因其本身的高價位限制了染料敏化太陽能電池原本價格低廉的優勢。由近幾年發表的文獻報導可知，使用非白金材料以取代白金並運用於染料敏化太陽能電池的對電極催化層已是趨勢。一般常見的非白金對電極催化層材料大致上可分為三類，包括碳材、導電高分子，以及過渡金屬化合物。為了開發高效能的非白金對電極，本篇論文包含了兩部分研究。第一部分的研究是使用不同維度結構的石墨作為染料敏化太陽能電池對電極的催化層材料。第二部分的研究則是以鎳金屬奈米粒子以及導電高分子聚二氧乙基噻吩-聚苯乙烯磺酸(poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonate), PEDOT:PSS) 的複合材料作為對電極的催化層材料。
為了研究出於染料敏化太陽能電池較高效能的石墨對電極，本研究論文的第一部分將比較三種不同維度結構的石墨，包含一維結構的石墨奈米線(1-D-GNF)、二維結構的天然石墨(2-D-NtG)，以及三維結構的奈米石墨(3-D-NnG)，運用於對電極的催化還原層。運用nafionR117的分散效果，分別將三種石墨分散於乙醇中，並運用滴塗漿料於ITO導電玻璃的塗佈方式，將含有石墨的漿料滴製成膜。在此部分的研究將對三種石墨性質加以鑑定與討論，例如表面結構、結構缺陷的含量、比表面積，以及導電性。3-D-NnG擁有最好的效率表現，為7.88%，與白金的8.38%效能相比相當接近，主要原因是3-D-NnG擁有最多含量的結構缺陷，使其擁有較佳的催化還原能力。另一方面，比起3-D-NnG，1-D-GNF以及2-D-NtG所擁有的結構缺陷都較少，也因此較差的催化還原能力導致了較低的效能表現(1-D-GNF為3.60%，2-D-NtG為2.99%)。而在此部分研究，三種維度石墨的催化還原能力也被常見的電化學分析方式以及旋轉電極檢視。
為了開發出適合的材料運用於染料敏化太陽能電池的催化還原層，有相當多關於複合材料，通常是兩種材料複合並提供個別的材料本性優勢，進而達到整體元件較高的效能。本論文的第二部分主要是將金屬鎳的奈米粒子，以及PEDOT:PSS相混合製成漿料，並用刮刀式塗佈法，將此複合材料塗佈於FTO導電玻璃上，製成對電極的催化還原層。金屬鎳的奈米粒子已被報導具有優良的催化還原性以及導電性，然而其高密度以及易聚集的本性，金屬鎳的奈米粒子形成的薄膜容易龜裂和由基材脫落。而PEDOT:PSS作為染料敏化太陽能電池的對電極材料也已經被廣泛報導，然而此種高分子較差的導電性以及催化活性限制了應用範圍。金屬鎳的奈米粒子以及PEDOT:PSS組成的複合材料可發揮出較高的元件性能(複合材料光電轉換效率為7.01%，而白金效率為7.63%)，因為金屬鎳的奈米粒子可提供較佳的導電性以及催化還原能力，而PEDOT:PSS則可提供較佳的基材附著能力。因此導致金屬鎳的奈米粒子以及PEDOT:PSS低效率的因素(金屬鎳的奈米粒子效率為0.24%、PEDOT:PSS的效率為4.36%)在此論文已經被研究解決。不同材料組成的催化還原層的性質以及表面結構也被清楚分析。複合薄膜比白金組成的催化還原薄膜具有更好的長程穩定性，因此比起白金，此複合薄膜在商品化的應用上更具有價值潛力。

關鍵字

對電極 ； 染料敏化太陽能電池 ； 石墨 ； 聚二氧乙基噻吩-聚苯乙烯磺酸 ； 無鉑 ； 金屬鎳的奈米粒子 ； 旋轉電極

並列摘要

The development of solar energy is one of the most important studies in the field of green energy because of the deficiency of oil energy. Dye-sensitized solar cells (DSSCs), as a new generation solar cell, have been investigated widely for the recent years due to some advantages such as simple fabrication, flexibility, and low-cost.
Compared to the other types of solar cells, DSSCs with the properties of low-cost and easy fabrication have been developed to reach the high efficiency exceeding 12%. Up to date, platinum (Pt) has remained the widely used electro-catalytic material as the counter electrode (CE) because of the properties of superior electro-catalytic ability and conductivity which cause the good performance in DSSCs. However, as one of the rare metal materials, the cost of DSSCs is expensive by the conventional preparations such as sputtering method, thermal decomposition method. Pt with the high price will limit the application of DSSCs because the property of low-cost, one of the advantages of DSSCs, will be also limited. From the reports published for the recent years, there is a trend which is using Pt-free materials to replace Pt as the counter electrode for DSSCs. In general, there are three kind of materials to be used as the Pt-free electro-catalytic film, including carbon, conductive polymer, and transition metal compound. In order to develop the efficient Pt-free electro-catalytic film, this thesis includes two parts. The first part is about using different dimensional structures of graphite as the counter electrode for DSSCs. The second part is combining nickel nanoparticles and poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) into a composite film as the counter electrodes for DSSCs.
For investigating the better performance of graphite as the counter electrode for DSSCs, the first part of this thesis is comparing the three different dimensional structures of graphite, including one dimensional graphite nanofiber (1-D-GNF), two dimensional natural graphite (2-D-NtG), and three dimensional nanographite (3-D-NnG), used as the electro-catalytic film. The graphite were dispersed in the solution of ethanol mixed with nafionR117, and then coated on the ITO glass substrates by drop-casting. The characteristics of the three different dimensional structures of graphite are discussed, including the morphology, degree of defect, surface area, and conductivity. 3-D-NnG shows the best performance of 7.88% , which is close to the performance of Pt (8.38%), due to its higher degree of defect causing the more superior electro-catalytic ability . On the other hand , 1-D-GNF and 2-D-NtG show the lower degree of defect compared to 3-D-NnG that cause the poorer performance (1-D-GNF is 3.60% and 2-D-NtG is 2.99%). Moreover, the electro-catalytic ability of graphite was also investigated by some common electrochemical methods and quantified by rotating disk electrode (RDE).
In order to develop the suitable materials as the electro-catalytic film for DSSCs, there are many reports about using the composite film which the two materials can provide each their advantages to obtain the higher efficiency for DSSCs. The second part of this thesis is about blending nickel nanoparticles and PEDOT:PSS into the slurry, and coating the slurry on FTO glass substrates by the doctor-blade method as the composite film. The superior electro-catalytic ability and conductivity of nickel nanoparticles has been widely reported, but the high density and aggregation of nickel nanoparticles will let the film crack and detach from the substrate. PEDOT:PSS has been also widely reported and applied as the counter electrode for DSSCs, but its poor conductivity and electro-catalytic ability will limit its application. The composite film of nickel nanoparticles and PEDOT:PSS distributes the good performance (7.01%, the performance of Pt is 7.63%) for DSSCs because the good performance can be attributed to the good catalytic ability, conductivity of nickel nanoparticles and the good adhesion of PEDOT:PSS. The disadvantages of nickel nanoparticles and PEDOT:PSS which cause the lower performance (nickel nanoparticles is 0.24%, PEDOT:PSS is 4.36%) compared to the composite film have been solved in this study. The morphologies and characteristics of the films containing the various materials were also specified. The long-term stability of the composite film is better than the Pt film that means the composite film is more suitable for the commercial purpose than the Pt film.

並列關鍵字

Counter electrode ； Dye-sensitized solar cell ； Graphite ； PEDOT:PSS ； Pt-free ； Nickel nanoparticle ； Rotating disk electrode

參考文獻

[11] Chidichimo, G.; Filippelli, L., Organic solar cells: problems and perspectives, International Journal of Photoenergy 2010.

[1] Gratzel, M., Photoelectrochemical cells, Nature 2001, 414, 338-344.

[2] Spitler, M. T., Dye photooxidation at semiconductor electrodes: A corollary to spectral sensitization in photography, Journal of Chemical Education 1983, 60, 330-332.

[3] West, W., First hundred years of special sensitization, Photogr. Sci. Eng. 1974, 18, 35-48.

[5] Chapin, D. M.; Fuller, C. S.; Pearson, G. L., A new silicon p-n junction photocell for converting solar radiation into electrical power, Journal of Applied Physics 1954, 25, 676-677.