中孔洞碳材做為儲氫材料之應用研究 Potential Applications of Mesoporous Carbon Materials for Hydrogen Fuel Storage

Li-Ren Chang
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中文摘要 氫氣被公認為是新世代的主要替代能源,因為它兼具乾淨及高燃燒效率等優點,亦是解決現今天然石化燃料逐漸枯竭以及其所衍生日趨嚴重的環境大氣污染等問題之有效且重要的出口。然而,針對未來氫能源的研發與應用,吾人所面臨的挑戰除了氫氣的產生、純化及尖端燃料電池等汲汲可行的應用之外,另一重要環節即在於氫氣的儲存與運輸。就此而論,尋求輕質量、低成本並且高儲存量的有效氫氣儲存載體,是一個值得探討的重要課題。本論文之主要目的,係對一系列新穎中孔洞碳材料CMK-n (n = 1, 3, 5)做為氫氣儲存載體之應用評估研究,並與其他商用碳材如石墨(graphite fine powder; GFP)與石墨奈米纖維(graphite nano-fiber; GNF)等加以比較。吾人利用不同光譜實驗技術,配合傳統分析方法,對各種碳材之結構物性及化性加以鑑定,並了解其吸附與動力行為。 在碳材組成結構鑑定方面,Raman 圖譜顯示各樣品均有sp2與sp3 混成軌域的鍵結模式。由粉末X-光譜繞射(PXRD)圖譜亦得知所有中孔洞碳材均存在類似石墨的特徵峰。在孔洞結構特性方面,吾人透過氮氣等溫吸附脫附曲線測量(77 K),分別得到各碳材比表面積(BET)、孔徑(pore diameter)和孔體積(pore volume)等相關資訊。以比表面積(單位:m2/g)而言,其大小依序為:CMK-5 (1690) > CMK-1 (1263) > CMK-3 (772) >> GNF (65) > GFP (18)。而其孔徑(單位:nm)依序為:CMK-3 (3.6) > CMK-5 (3.2) > CMK-1 (2.3);其中,GFP與GNF並無中孔結構。 經由2D NMR實驗發現:純氘氣之化學位移(6.8 ppm)與譜寬並不隨壓力與溫度而改變。然而,當氘氣吸附在碳材料時,其2D NMR化學位移值則有明顯下降趨勢,吾人將此結果歸因於各類碳材中sp2混成軌域致使之金屬特性所造成的Knight shift現象。另外,相較於GFP及GNF,氘氣吸附在CMK-n中所測得之2D NMR化學位移值亦均隨溫度及氘氣吸附量之增加而增加,所測得之光譜譜寬均有明顯窄化現象,顯示氘氣在中孔洞性碳材中進行快速交換,因此偵測到均一的吸附環境。反之,氘氣吸附在GFP及GNF時,其譜寬則明顯寬化,尤以後者為甚。然而,訊號譜寬之貢獻項可由氫核與氘核gyromagnetic ratio(γ)值成正比,吾人分析出H2與D2吸附在中孔洞碳材的NMR訊號貢獻度主要來自於材料的磁化率。 在氫氣儲存能力方面,發現各類碳材之氫氣儲存能力依序為:CMK-5 > GNF > CMK-3 > CMK-1> GFP。其中,以CMK-5之儲存量較優,在273 K、80 atm下可達約3.5 wt%,比GFP(1.5 wt%)及GNF(1.0 wt%)高出許多。此外,由於CMK-5具有有序中空的孔洞結構、輕質量、高比表面積等優點,在77 K、80 atm下,其吸附量高達約75 wt%,是一般商用石墨的三倍以上。 根據動力學模型的計算,吾人得到中孔洞碳材之氫氣吸附熱約為5 kJ/mol,此外,從單層飽和吸附(Xm)的計算中中得到吸附在各碳材表面之氘氣莫耳數與化學位移的關係。 此一研究除能增進吾人對新穎奈米結構碳材料基本物化特性之了解外,並將有利於提昇類似碳材料在燃料儲存之開發、製備與其在能源相關議題之應用。
ABSTRACT Hydrogen, which can be produced from renewable sources, has emerged as one of the most promising candidates for the replacement of the current carbon-based energy carriers in the twenty-first century. The advantages of hydrogen over other fossil fuels are two folds: first, it is a clean combustion of a nontoxic fuel. Secondly, its high delivered energy per mass. Thus, the utilization of hydrogen fuel is believed to be the solution for today’s continuous shortages in fossil fuel supplies and increasing demands in environmental control issues. Although many advances in hydrogen production, purification and utilization technologies have been made since 1990’s, hydrogen storage technologies must be significantly advanced if a hydrogen-based energy system, particularly in the transportation sector, is to be established. In this context, the search for ideal technologies for hydrogen storage, which requires major advances in storage capacity, energy efficiency, safety and cost, so that the uptake can be rationally optimized to commercially attractive levels is a demanding and challenging task. The objective of this research is to evaluate the potential applications of novel carbon mesoporous materials, CMK-n (n = 1, 3, 5), as supports for hydrogen fuel storage. For comparison, commercially available carbon materials, namely graphite fine powder (GFP) and graphite nano-fiber (GNF), were also examined. The structural and physical-chemical properties of the carbon materials as well as the adsorption dynamics of hydrogen were characterized by a variety of different analytical and spectroscopic techniques. All carbon materials exhibit typical graphitized diamond-like structure with sp2 and sp3 bands, as also confirmed by powdered x-ray diffraction (PXRD) studies. N2 adsorption/desorption isotherm measurements (at 77 K) revealed the textual properties of various carbon materials; their BET surface areas (in unit of m2/g) follow the trend: CMK-5 (1690) > CMK-1 (1263) > CMK-3 (772) >> GNF (65) > GFP (18), whereas their pore diameter (in nm) follow the trend: CMK-3 (3.6) > CMK-5 (3.2) > CMK-1 (2.3), among them, GFP and GNF carbons were found to possess null porosities. Furthermore, unlike deuterium (2D) NMR of pure D2 gas, for which the observed chemical shift (6.2 ppm) was found independent of temperature and pressure, a notable decrease in 2D NMR chemical shift was observed for D2 adsorbed on carbon materials, mainly due to Knight shift effect. The observed chemical shift was also found to increase with increasing D2 loading as well as temperature. However, the linewidths observed for D2 adsorbed in the CMK-n appeared to be much narrower than GFP and GNF, revealing that D2 are in fast exchange in the ordered mesoporous carbon materials. In terms of hydrogen storage capability, various carbons show the following trend for adsorption capacities: follow the following trend: CMK-5 > GNF > CMK-3 > CMK-1> GFP. CMK-5, which has a tubular hollow carbon structure and highest surface area, revealed a hydrogen storage capacity of ca. 3.5 wt% at 273 K, 80 atm, substantially higher than GFP (1.5 wt%) and GNF (1.0 wt%). As expected, the storage capacity increase with decreasing temperature, for example, CMK-5 exhibited a hydrogen storage capacity of ca. 75 wt% at 77 K, 80 atm; a value greater than conventional graphite materials by at least three folds. The results obtained from the present study should promote fundamental understanding and development of novel nanostructured carbon materials and their potential applications in fuel storage and energy related issues.
中孔洞碳材, 儲氫材料