碳氣凝膠金屬複合材料之合成、特性鑑定及其儲氫能力之研究

Synthesis and Characterization of Carbon Aerogels Metal Composites and Their Hydrogen Adsorption Abilities

邱思圍(Su-Wei Chiu)

指導教授 ： 林錕松

元智大學/工程學院/化學工程與材料科學學系/碩士(2007年)

https://doi.org/10.6838/YZU.2007.00278

摘要

近年來，由於使用石化燃料以及對能源的過度依賴，造成環境污染的問題日益嚴重。氫氣是種理想的潔淨燃料，其燃燒過程並不會產生污染產物，但對於氫氣使用而言，氫氣的儲存技術更是相當重要。因此，本研究之主要目的在於開發氣凝膠碳材(Carbon aerogels, CAs)或多壁奈米碳管(Multiwall carbon nanotubes, MWNTs)，摻雜金屬氫化物(NaAlH4、MgH2)、金屬合金(Fe-Ti-Mg)及吸氫金屬(Mg、Co及Pd)，相互混合成為多元之儲氫材料，來探討其對於提升儲氫能力之影響。所合成之氣凝膠碳材為一種由球狀奈米粒子相互而成的輕質多孔材料，網狀顆粒直徑約為3~10 nm，比表面積大約500~700 m2/g，孔隙率大約為80~98%。本實驗採用溶膠-凝膠法製備氣凝膠，其中乃將間苯二酚(Resorcinol)與甲醛(Formaldhyde)均勻混合，隨即加入Na2CO3與水作為觸媒與稀釋劑，再經由烘乾、乾燥、熱解等步驟後，獲致所要之中孔洞及高表面積氣凝膠碳材。
在氣凝膠碳材之改質實驗中，碳材在熱解階段會因高溫產生焦油沉澱而堵塞碳材孔洞，形成所謂的焦結現象；故避免此結果，乃藉由水蒸氣物理活化方法，利用水蒸氣與碳發生氣化反應，取代堵塞孔洞之焦油，將氣凝膠碳材之比表面積予以提升。隨著活化時間增長，碳材之比表面積與總孔體積均有提升，經由BET表面積測定儀量測可知，目前所合成之氣凝膠碳材最高比表面積可達1402 m2/g，後續再利用醇還原法或無電鍍方式，沉積奈米金屬粒子(Pd, Co)以提升材料之儲氫能力。另外，在本實驗中所使用提升儲氫之改質材料，可分為四大類：分別為鋁氫化鈉(NaAlH4)、氫化鎂(MgH2)、多壁奈米碳管與氣凝膠碳材。在NaAlH4方面，由於其可逆反應無法輕易達到，因此嘗試加入Ti來提升NaAlH4之氫氣吸/放能力，以降低所需的吸附能；在MgH2方面，則是先將Mg氫化成MgH2後，隨即加入Fe-Ti合金來提升MgH2的吸/放氫氣能力。最後，其金屬氫化物與奈米碳管以及氣凝膠碳材以高能機械研磨方式混合形成金屬複合材料。
由TEM、FE-SEM分析，除了暸解儲氫材料外觀之外，亦可觀察摻雜於碳材中之Pd與Co顆粒尺寸，其大小為5~10及20~25 nm；由EDS分析得知，儲氫材料含有Pd、Fe及Ti元素，再經由ESCA之分析，可知吸氫Pd金屬因氫氣還原成零價而發生向左偏移的現象；而XRD分析得知吸氫金屬與金屬合金為Pd、Co及Fe-Ti晶相；FT-IR可知氣凝膠碳材之官能基含有C-O以及-CH2-，可證實氣凝膠碳材的合成為此官能基所交聯而成；TGA分析得知在氮氣環境下熱解溫度介於200~600℃之間，氣凝膠碳材之重量損失約為40%，而於20~80℃因溶劑揮發之緣故而造成有10%之重量損失。經Raman分析可知，氣凝膠碳材於1580 cm-1 (G-band)有ㄧ吸收峰，表示為C=C之結構；而於1350 cm-1 (D-band)之位置，因石墨結構不完整而產生此吸收峰。XANES與EXAFS分析獲得吸氫金屬Pd之價數為Pd(0)而第一層結構Pd-Pd之鍵長約為2.70 Å，因Pd為一體心立方結構，故其配位數為8；在本實驗中所使用重量法之高壓微量天平裝置作為儲氫量之量測，利用儲氫合金在不同壓力(1-30 stm)下，於適當溫度(30-150℃)下進行吸氫實驗，測得真實之儲氫量。結果得知儲氫量依序如下：在Pd方面為CAB-20Pd (1.6 wt%)＞CAA-20Pd (1.2 wt%)＞CNT-SP-20Pd (1.1 wt%)＞CNT-Na-20Pd (0.2 wt%)；而在MgH2方面為MgH2-5FeTi-5CAB (4.0 wt%)＞MgH2-5FeTi-5CNTSP (3.5 wt%)＞MgH2-5FeTi-5CNTNa (2.8 wt%)＞MgH2-5FeTi-6h (2.7 wt%)＞MgH2-6h (0.6 wt%)＞MgH2-3h (0.3 wt%)。由於氣凝膠碳材具有極高之比表面積與中孔洞結構，且對於氫氣亦有極佳之吸附性質以及增加氫氣於儲氫過程中之擴散速率。因此，藉由添加金屬氫化物或金屬合金，於氣凝膠碳材此結構上能有效提升吸/脫氫氣之能力。

關鍵字

儲氫材料；氣凝膠碳材；水蒸氣物理活化；多壁奈米碳管；金屬氫化物；金屬合金；金屬鈀；吸附

並列摘要

In recent years, the excessive use of fossil fuels to satisfy the over-reliance of energy resulted in growing concerns for environmental pollution. Hydrogen is an ideal fuel because of its zero or low pollutant emission during combustion. In order to utilize hydrogen effectively, hydrogen storage technology is quite important. Therefore, the main objectives of this study were to investigate the synthesis and characterization of carbon aerogels (CAs) and multi-wall carbon nanotubes (MWNTs) for H2 storage, and combined with three kinds of hydrogen storage materials (metal hydrides, metal alloys, and hydrogen storage metals) to form multi-nanocomposite materials. The enhancement of hydrogen storage capacity of these products was also investigated. CAs were prepared through the sol-gel method by mixing resorcinol and formaldehyde, with Na2CO3 as the basic catalyst, water as the diluent, and then dried and pyrolysed. CAs are light multi-porous material connected by several spheric nanoparticles. The diameter of those nanoparticles were 3-10 nm, BET surface area were 500-700 m2/g, and pore ratio were 80-98 %.
During pyrolysis, coke was produced at high temperature which blocked the pores of CAs, thus steam-activation was used to prevent such behavior.The coke that obstructed in the pores of carbon materials was removed by steam gasifying reaction and increased the BET surface areas of carbon aerogels. BET surface area and total pore volume increased with activated time. The highest BET surface area reached up to 1402 m2/g. The amount of hydrogen storage capacity was increased by the ethylene glycol and the electroless plating methods where nano-metal particles were loaded. Four kinds of materials (NaAlH4, MgH2, MWNTs, and CAs) were used to investigate the hydrogen storage properties. For NaAlH4, Ti was added to improve the ability of hydrogen adsorption/desorption and to decrease the energy of adsorption. In case of MgH2, first MgH2 was obtained by hydrogenation of Mg and then Fe-Ti alloy was added to increase the ability of hydrogen adsorption/desorption. Finally, the metal composites were formed by mixing the metal hydride with CAs (or MWNTs) by high-energy mechanical ball-milling method.
As observed from the TEM and FE-SEM, the diameter of Pd and Co nano-particles doped in these carbon materials were found to be within 5-10 nm and 20-25 nm respectively. The EDS data indicated that these carbon materials were composed of Pd, Fe, and Ti chemical components. Since the hydrogen storage metal (Pd) was reduced by flowing hydrogen gas, the valency of Pd was zero; the ESCA spectra showed that the Pd peak shifted to left after hydrogenation. The XRD patterns indicated that the Pd and Co doped carbon materials was crystalline and consisted of Pd, Co, and Fe-Ti. From the FTIR spectra, CAs synthesized via the linking of the functional groups were found to be CAs with C-O and -CH2- functional groups. TGA curves of these materials indicated that there was a significant weight loss of 40% in the pyrolysis temperature range of 200-600℃ under nitrogen atmosphere, suggesting that the organic residues were removed at this temperature range. A preliminary weight loss of 10% at 20-80℃ was due to the residual solvent evaporation. The Raman spectra also showed that there were two peaks from carbon aerogels at 1580 and 1350 cm-1. The peak that located at 1580 cm-1 represents carbon double bond and peak at 1350 cm-1 was due to incomplete structure of graphite.
The XANES/EXAFS spectra showed that the valency of the Pd was zero and a first shell of Pd-Pd bonding with bond distances of 2.70 Å. Coordination numbers of Pd with BCC structures were close to 8. In addition, the real amount of hydrogen storage capacities of the materials in different pressures (up to 30 atm) and temperatures (30-150 ℃) were measured by a high-pressure TGA microbalance system. The amount of hydrogen storage capacities were CAB-20Pd (1.6 wt%) > CAA-20Pd (1.2 wt%) > CNT-SP-20Pd (1.1 wt%) > CNT-Na-20Pd (0.2 wt%) in Pd alloys materials. Similarly, in MgH2 alloys, the amount of hydrogen storage capacities were MgH2-5FeTi-5CAB (4.0 wt%) > MgH2-5FeTi-5CNTSP (3.5 wt%) > MgH2-5FeTi-5CNTNa (2.8 wt%) > MgH2-5FeTi-6h (2.7 wt%) > MgH2-6h (0.6 wt%) > MgH2-3h (0.3 wt%). Due to the inherent properties of CAs (very high BET surface area and mesopore structures), thus CAs has good adsorption behavior and diffusion rate. Notably, the CAs relied on addition of metal hydrides or alloys to increase the abilities of hydrogen adsorption/desorption.

並列關鍵字

Hydrogen storage material ； Carbon aerogels ； Steam-activation ； Mutliwall carbon nanotubes ； Metal hydride ； Metal alloy ； Palladium ； Adsorption

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