Hydrogen adsorption of iron/carbon nanocomposite

鐵/碳奈米複合材料之氫氣吸附

謝承恩(Cheng-En Hsieh)

指導教授 ： 陳建瑞 ； 鄧希平

國立清華大學/工學院/材料科學工程學系/碩士(2009年)

https://doi.org/10.6843/NTHU.2009.00391

摘要

奈米碳粉本身具有高比表面積與微小孔洞的特性，對於氫氣的吸附是有利的特性。在這個研究中，利用鐵氧化物對於奈米碳粉的表面進行改質，而製得鐵/碳奈米複合材料。在實驗中我們利用PCT在氫氣壓37atm溫度300K下進行鐵碳複合材料氫氣吸附的量測、利用N2/77K BET進行比表面積與孔徑分布分析、利用SEM探討表面形貌、利用XRD分析氧化鐵晶相。
在實驗結果中發現添加14%的γ-Fe2O3/碳奈米複合材料有最大的氫氣吸附量(0.48wt%)，相較於原始碳粉提升1.9倍(0.256wt%)，原因之一是表面微小孔洞因氧化鐵在奈米碳粉表面生成時大量形成。而其他添加鐵量(7wt%、28wt%、56wt%)的γ-Fe2O3/碳奈米複合材料氫氣吸附量也獲得提升，這是由於γ-Fe2O3對於奈米碳粉表面提供了極性，使得有利於氫氣分子被吸附在表面，再進而藉由氫溢流效應儲存到奈米碳粉上的孔洞。
但我們所製造出另一種氧化鐵複合材料－α-Fe2O3/碳奈米複合材料的氫氣吸附量皆小於原始碳粉，造成這結果的原因之一是表面微小孔洞的消失。為了瞭解α-Fe2O3(赤鐵礦)與γ-Fe2O3(磁赤鐵礦)對於氫氣吸附貢獻的差異，我們利用相轉變將α-Fe2O3/碳奈米複合材料中的α-Fe2O3(赤鐵礦)轉變為γ-Fe2O3(磁赤鐵礦)，發現氫氣吸附量得到提升，證明γ-Fe2O3(磁赤鐵礦)對於提升奈米碳粉室溫下吸氫量優於α-Fe2O3(赤鐵礦)。
根據實驗結果，適當的鐵氧化物能為奈米碳粉表面提升極性以增加氫氣吸附能力，而表面微小孔洞的存在也能使氫氣吸附量提升。

關鍵字

儲氫 ； 氧化鐵 ； 奈米碳

並列摘要

Carbon nanopowder intrinsically has the characteristics of high specific surface area and micropores, which are favorable for hydrogen adsorption. In this study, we obtained Fe/CNP nanocomposite materials by modifying the surface of the carbon nanopowder (CNP) with different iron oxides. To measure hydrogen storage behaviors of Fe/CNP nanocomposite, we used PCT apparatus at hydrogen pressure of 37atm and 300K. The specific surface areas and pore diameter were analyzed by N2/77K BET, and the surface morphologies were studied by SEM. Further, XRD analysis clearly indicated the presence of different iron oxides in these composite materials.
From the experimental result, the hydrogen adsorption capacity of γ-Fe2O3 / CNP composite increased with different iron contents (7wt%、28wt%、56wt%). Especially, γ-Fe2O3 / CNP composite with 14 wt% of iron-spiked showed maximum hydrogen adsorption of 0.48wt%, which is 1.9 times of the original CNP(0.256wt%). For one reason, massive micropores appeared when iron oxide formed on the surface of CNP. For another, it was postulated that γ-Fe2O3 crystals promoted the access of hydrogen molecules by polarizing the surface, which benefited the migration of hydrogen molecules to the carbon nanopowder (CNP) surface. In this case, γ-Fe2O3 played as a promoter to attract hydrogen molecules whereas CNP skeleton performed as the principal absorbent for hydrogen storage.
The hydrogen adsorption capacity of the other composite α-Fe2O3 / CNP decreased with all iron contents compared to original CNP. It was result from the decrease of micopores on surface. In order to investigate the contribution to hydrogen adsorption capacity between two different iron oxides, we transformed α-Fe2O3/CNP composite to γ-Fe2O3 / CNP composite under hydrogen atmosphere of 20atm and 633K. We found out that the hydrogen adsorption increased after phase transformation. This phenomenon proved that the presence of γ-Fe2O3 made the surface of composite more attractable for hydrogen than α-Fe2O3.
Based upon our experimental results, a high hydrogen uptake could be achieved by appropriately adjusting the surface polarity of CNP with well dispersed iron oxides crystals and more micropores on surface.

參考文獻

〔8〕P. Chen, X. Wu, J. Lin, K. L. Tan, High H2 Uptake by Alkali-Doped Carbon Nanotubes Under Ambient Pressure and Moderate Temperatures, Science, New Series, Vol. 285, No. 5424 (Jul. 2, 1999), 91-93。

〔11〕Rzepka M, Lamp P, de la Casa-Lillo MA. Physisorption of Hydrogen on Microporous Carbon and Carbon Nanotubes. J Phys Chem B 1998;102:10894-8.。

〔12〕Soo-Jin Park , Byung-Joo Kim, Young-Seak Lee, Min-Jun Cho. Influence of copper electroplating on high pressure hydrogen-storage behaviors of activated carbon fibers Int J Hydrogen Energy. 2008,33,1706-10

〔1〕A.C. Dillon, K. M. Jones, T. A. Bekkedahl, C. H.Kiang, D. S. Bethune, and M. J. Heben, Nature (London) ,1997,386, 377 。

〔2〕F. H. Yang, R. T. Yang, Carbon,2002,40, 437-444。