具介孔洞奈米材料於染料敏化太陽能電池與光催化水分解產氫之應用

Applications of Mesoporous Nanomaterials in Dye-Sensitized Solar cells and Hydrogen Production through Photocatalytic Water Splitting

鄭為允(Wei-Yun Cheng)

指導教授 ： 呂世源

國立清華大學/工學院/化學工程學系/博士(2012年)

https://doi.org/10.6843/NTHU.2012.00201

摘要

本研究利用多種化學合成方法，製備具介孔洞之奈米孔隙材料，並將其應用於染料敏化太陽能電池與光催化水分解產氫。染料敏化太陽能電池方面，本研究製備具高比表面積與次微米尺寸二氧化鈦孔隙材料作為染料敏化太陽能電池之光電極；另外，高比表面積與高導電性石墨烯氣凝膠則應用於染料敏化太陽能電池之對電極研究，評估其取代白金作為對電極的可行性。光催化水分解為一異相反應，因此材料具備高比面積可增加活性位置，適當的孔洞尺寸將助於反應溶液滲入孔隙材料內部進行催化反應，充分發揮材料實際特性。本研究以高孔隙二氧化鈦材料為擔體，負載氧化亞銅，藉以提升電荷分離效果，提升產氫效率；另外，本研究亦製備不同結構之硫化鎘，搭配氧化亞銅應用於可見光光催化水分解產氫。
光電極於染料敏化太陽能電池主要作為染料吸附與電子傳遞的角色，本研究發展一單一步驟水熱法，成功製備具介孔洞、高比表面積與次微米尺寸之二氧化鈦多孔球，將此材料與二氧化鈦氣凝膠結合形成複合結構之光電極，其光電轉換效率可達8.41 %，較P25組成之光電極，大幅提升38 %。此光電效率的改善主要來自於:高比表面積提升染料吸附量、次微米尺寸增加了散射效果，改善對入射光的利用性、二氧化鈦多孔球與氣凝膠具良好連結特性，有助於電子傳遞。此外二氧化鈦氣凝膠可填充於孔隙間，增加染料吸附與多孔球間的連結性，並且於導電玻璃表面生成二氧化鈦粒子，降低電子再結合率。
本研究利用溶膠凝膠法製備石墨烯氣凝膠，其比表面積可達814 m2/g，此外，石墨烯氣凝膠展現極佳導電性。將石墨烯氣凝膠作為染料敏化太陽能電池之對電極，研究結果發現太陽能電池之光電轉換效率主要受石墨烯氣凝膠厚度影響，較厚之石墨烯氣凝膠可增加較多活性位置進行催化反應，然而，太厚之對電極亦會增加電荷與質傳阻力，進而使轉換效果降低。此外，本研究負載微量白金於石墨烯氣凝膠，可獲得較薄、透光性較佳之對電極。兩種對電極的光電轉換效果分別為白金對電極的96%與98%。本研究利用電化學阻抗分析與循環伏安法進一步進行了解石墨烯氣凝膠的相關特性，其優異的催化能力與優異的導電性，說明石墨烯氣凝膠為具潛力的染料敏化太陽能電池對電極材料。
光催化水分解產氫部分，本研究將所製備之二氧化鈦多孔球，利用簡單步驟化學浴法負載氧化亞銅於材料表面與孔隙中，藉由調控氧化亞銅前驅物的濃度，可獲得氧化亞銅的最佳負載量。由於能階位置的匹配，二氧化鈦與氧化亞銅的複合材料較單純二氧化鈦材料展現較佳的電荷分離效果。此外，氧化亞銅/二氧化鈦複合材料於二氧化鈦激發波長範圍展現較佳之光穿透性，可減少材料造成的屏蔽效應。氧化亞銅/二氧化鈦具高空隙複合材料，其優異的電荷分離效應與較佳的光利用性，展現極佳的光催化水分解產氫效率。
太陽光為一簡單獲得的光源，其可見光部分約佔45%，因此可見光應答光觸媒長久以來一直備受重視。本研究利用水熱法及溶熱法製備硫化鎘奈米球、奈米粒子與奈米線，其能隙約介於2.1至2.3 eV，可應用於可見光光觸媒。於光催化水分解產氫中，硫化鎘奈米粒子較奈米球具有較大比表面積，因此可獲得較佳產氫效率；而硫化鎘奈米線，其一維與單晶結構有助於電荷傳遞，因此可將產氫效率提升至12.4 μmol/hr。藉由負載氧化亞銅於硫化鎘材料上，可改善其電荷分離效應並提升產氫率。然而，受到氧化亞銅完全包覆的硫化鎘，其光催化效果將受到抑制，本研究顯示硫化鎘奈米球為一較佳的載體，氧化亞銅/硫化鎘奈米球複合材料，其產氫效果可達238.3μmol/hr。

關鍵字

染料敏化太陽能電池 ； 二氧化鈦 ； 對電極 ； 水分解

並列摘要

Abstract
In this dissertation, TiO2 nanoparticle aggregates (NPGs), TiO2 xerogels, graphene aerogels (GAs), Cu2O/TiO2, and Cu2O/CdS composites with mesoporous structure are produced and applied in dye-sensitized solar cells (DSSCs) and hydrogen production through photocatalytic water splitting. For the DSSC, TiO2 NPGs and GAs are used as the photoanode and counter electrode, respectively. The Cu2O decorated composites, possessing excellent charge separation and suitable band structures, are applied in hydrogen generation through photocatalytic water splitting. The relevant properties of the synthesized mesoporous materials and the performance of DSSCs and photocatalytic water splitting are investigated and discussed in this study.
TiO2 NPGs with high specific surface areas and of sub-micron sized are produce by a one-step, template-less, surfactant-free hydrothermal process. With these NPGs, a new form of composite photoanode, consisting of the mesoporous TiO2 NPGs and xerogels, is proposed for high efficiency dye-sensitized solar cells (DSSCs). TiO2 xerogels are incorporated into the TiO2 NPGs layer with an impregnation process to form the TiO2 NPGs/xerogels composite. A high power conversion efficiency of 8.41% is achieved for the DSSCs based on the TiO2 NPGs/xerogels composite photoanode, representing a 38% efficiency boost over the efficiency of 6.11 % achieved with a P25 TiO2 based cell.
GAs prepared with an organic sol-gel process, possessing a high specific surface area of 814 m2/g and a high electric conductivity of 850 S/m, are applied as a counter electrode (CE) material for DSSCs. The performance of the GA as the CE material is found to be dependent on its film thickness. At an optimum GA film thickness of 4.9 μm, a power conversion efficiency of 96% of that achieved with a Pt CE based DSSC is obtained. In addition, a thinner GA film of 1.7 μm, when loaded with Pt of 1 mol.% through a photo-reduction process, achieves a power conversion efficiency of 98% of that obtained with a Pt CE based DSSC. The excellent performances of the GA-based CEs are manifested with electrochemical impedance analyses and cyclic voltammetry catalytic activity analyses.
Mesoporous TiO2 NPGs with a high specific surface area are decorated with nontoxic, band structure matched Cu2O nanocrystals through a simple, fast, and low cost chemical bath deposition process. The Cu2O nanocrystals serve as an electron-hole separation center to promote hydrogen productions. By tuning the concentration of the Cu2O precursor, the loading of Cu2O can be controlled. At preferred operation conditions, an ultrahigh specific hydrogen production rate of 223 mmol/hr．g is achieved. The Cu2O decorated TiO2 NPGs are found to possess high transmittances at low wavelengths where the TiO2 materials are photocatalytically active. With the advantages of high specific surface areas, improved electron-hole separations, and better light utilization of the Cu2O decorated TiO2 NPGs, the hydrogen production rate achieved is one order of magnitude higher than that by commercial P25 TiO2.
Crystalline CdS nanobeads (NBs), nanoparticles (NPs), and nanowires (NWs) are prepared with hydrothermal and solvothermal methods. These three CdS nanostructures are applied for hydrogen productions under visible light illumination. CdS NPs show a hydrogen production rate of 9.6 μmol/hr which is higher than that of CdS NBs, attributable to the higher specific surface area of the CdS NPs. The one dimensional structure and single crystallinity of CdS NWs, both beneficial for charge transport, lead to a higher hydrogen production rate of 12.4 μmol/hr. In order to boost the hydrogen production rate, these CdS nanostructures are modified with p-type Cu2O using a simple, fast, and low cost chemical bath deposition process. The composite photocatalysts of Cu2O decorated CdS nanostructures acquire significant improvements on the hydrogen production rate, resulting from the enhancements in charge separation. A high hydrogen production rate of 238.3 μmol/hr is achieved with Cu2O decorated CdS NBs, which is 74 folds of that of plain CdS NBs.

並列關鍵字

dye-sensitized solar cells ； TiO2 ； counter electrode ； water splitting

參考文獻

23. 蔡宗祐,“二氧化鈦光電極結構對染料敏化太陽能電池效率之影響”, 國立清華大學化學工程學系碩士班論文,2009

1. A. L. Dicks, J Power Sources, 1996, 61, 113.

2. M. Ni, M. K. H. Leung, D. Y. C. Leung, K. Sumathy, Renew. Sust. Energ. Rev., 2007, 11, 401.

4. J. K. Rath, Sol. Energ. Mat. Sol. C., 2003, 76, 431.

6. J. Britt, C. Ferekides, Appl. Phys. Lett., 1993, 62, 2851.