氮、硫-摻雜之石墨烯中空球電觸媒於染料敏化太陽能電池之應用

Abstract

現今世界對能源的高度需求，造成化石燃料的大量使用以及其醞藏量快速的遞減，進而引發嚴重的環境汙染與能源危機。因此，再生能源是人類現在極需努力開發的能源，其中最具發展潛力和應用價值的太陽能目前已經成為各國積極開發的方向。染料敏化太陽能電池(dye-sensitized solar cell, DSSC)是一種薄膜太陽能電池，其製程容易、成本低，具備可撓性、多彩性與可透光性等特性，應用範圍廣泛。但染料敏化太陽能電池的光電轉換效率尚低，且其材料仍需使用到一些貴重金屬，例如鉑(platinum)、釕(ruthenium)等。因此開發具有更高的光電轉換效率且不使用貴重金屬的材料仍是研究焦點之一。本研究利用二維材料石墨烯(graphene)建構成球型結構，並開發出三維石墨烯中空球(graphene hollow nanoballs, GHBs)、氮-摻雜石墨烯中空球(nitrogen-doped graphene hollow nanoballs, N-GHBs)、硫-摻雜石墨烯中空球(sulfur-doped graphene hollow nanoballs, S-GHBs)以及氮、硫-摻雜石墨烯中空球(nitrogen/sulfur-codoped graphene hollow nanoballs, N,S-GHBs)等材料，將其應用於染料敏化太陽能電池。 本研究第一部份利用化學氣相沉積法(chemical vapor deposition, CVD)，將石墨烯球沉積於各種基板上，例如矽晶片(silicon wafer, Si)、碳布(carbon cloth, CC)。並於上述CVD方法中再加入氮前驅物或硫前驅物來合成氮-摻雜、硫-摻雜以及氮、硫-摻雜的石墨烯中空球。第二部分將上述材料做為無金屬之催化觸媒，應用於染料敏化太陽能之對電極(counter electrode)，進行碘還原催化反應(triiodide reduction)。石墨烯本身已具有高載子遷移率、高導電性、可饒性、高機械強度等特性，而本研究所開發的石墨烯中空球更解決了平面石墨烯相互堆疊的問題，且可提供更高的表面積。此外，異原子的摻雜可降低電荷轉移之電阻(charge-transfer resistance)，因而提升石墨烯中空球作為染料敏化太陽能電池電極的催化活性。故本研究探討了氮、硫兩種異原子摻雜的石墨烯中空球，對碘還原反應具有不同的催化效應，並發現氮、硫共同摻雜時具有協同效應，可進一步提升催化活性。將氮、硫-摻雜的石墨烯中空球作為對電極觸媒之染料敏化太陽能電池元件效率可達9.02 %，其效率可匹配於使用標準白金對電極製作之電池元件(8.90 %)。

A huge amount of fossil fuels, such as coal, petroleum, and gas, has been consumed in order to meet the high demand of energy in the world. However, the combustion of these fossil fuels results in not only detrimentally environmental pollution, but also the rapid reduction of fossil resources on the Earth. Recently, several kinds of renewable energy, e.g., fuel cells, wind power, and solar energy, have drawn tremendous attention in academic studies and industrial applications. Among them, solar energy is the most attractive renewable energy; in particular, dye-sensitized solar cells (DSSCs) have the advantages of simple fabrication processes, low cost, flexibility, and semi-transparency. However, if a DSSC possesses low power conversion efficiency and utilizes noble metals, e.g., platinum (Pt) or ruthenium (Ru), as a counter electrode (CE), these disadvantages would hinder this DSSC from wide applications. Therefore, it is an urgent challenge to develop a noble metal-free CE with high power conversion efficiency in DSSCs. Graphene has high carrier mobility, high electrical conductivity, high mechanical strength and flexiblility. In this study, we took advantage of the unique chracteristics of graphene to fabricate high-performance DSSCs by employing different graphene-based CEs, such as graphene hollow nanoballs (GHBs), nitrogen-doped graphene hollow nanoballs (N-GHBs), sulfur-doped graphene hollow nanoballs (S-GHBs), and nitrogen/sulfur-codoped graphene hollow nanoballs (N,S-GHBs). First, we synthesized GHBs on silicon wafers (Si) or carbon cloth (CC) substrates with a chemical vapor deposition (CVD) method. A nitrogen or sulfur precursor, or both, was incorporated in the CVD rection to from N-GHBs, S-GHBs, and N,S-GHBs, respectively. Second, the as-synthesized doped GHBs were used as metal-free CEs to investigate their power conversion efficiencies in DSSCs. The highly curved GHBs could avoid the self-assembly restacking of planar graphene sheets and provide high surface area. In addition, the heteroatomic incorporation in GHBs can reduce the charge-transfer resistance and enhance the catalytic activity of GHBs. We found that pristine GHB (with ∆EP of 698 mV) and heteroatom-doped GHBs (∆EP of 530 mV for N-GHBs and ∆EP of 498 mV for S-GHBs) have different catalytic activities on the I-/I3- reduction reaction and the N,S-GHBs (∆EP of 459 mV) shows the best catalytic performance due to the synergistic effect of electronic and geometric changes. Consequently, the power conversion efficiency of a DSSC with N,S-GHBs as a CE reaches to 9.02 %, comparable to that (8.90 %) of a standard sputtered Pt CE-based cell.