二碲化鈷應用於電催化水分解

Cobalt Ditelluride for Electrocatalytic Water Splitting

盧子翔(Tzu-Hsiang Lu)

指導教授 ： 劉如熹

國立臺灣大學/理學院/化學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201600436

摘要

電催化水分解產氫為解決能源危機與環境汙染之有效途徑之一。貴重金屬催化劑具較高之催化水分解活性，但貴金屬低儲量與高成本之問題，限制其大規模之應用，因此尋找與改進非貴重金屬催化觸媒為近年來之研究重點。而電催化水分解具陰極析氫反應: 2H+ + 2e- → H2與陽極析氧反應: 2H2O → O2 + 4H+ + 4e-，於電催化水分解產氫時，不僅須提高陰極析氫反應之催化活性，同時亦須改進析氧反應之催化活性，故尋找與改進非貴重金屬催化劑不僅於陰極反應，且陽極反應亦相當重要。近年來，改進非貴重金屬之方向主要為降低過電位、增加反應電流密度、提高反應之動力學與提高穩定度。
本研究主要為藉水熱法配製二碲化鈷奈米顆粒(CoTe2 NP)與二碲化鈷/奈米碳管(CoTe2/CNT)複合材料並將其黏附於玻璃碳電極進行電化學測量，分別應用於陰極析氫反應與陽極析氧反應，了解其於陰極析氫反應與陽極析氧反應之活性及機制探討。藉X光繞射儀鑑定樣品之晶相，以穿透式電子顯微鏡與掃描式電子顯微鏡觀測樣品形貌。藉同步輻射產生之X光測量吸收光譜，分別量測樣品之金屬氧化價數及電子於軌域中之填充率(electron-filling rate)，最後藉電化學儀測量線性掃描伏安法、循環伏安法、電化學阻抗譜與安培法測量過電位、活性位表面積、電荷轉移電阻與穩定度。經上述鑑定發現於0.5 M硫酸(H2SO4)進行析氫反應時，證實顆粒較小之CoTe2具較多活性面積，故具較佳催化活性。於10 mA/cm2下，20至50 nm之CoTe2 NP過電位僅246 mV，且穩定度可達48小時。於1.0 M氫氧化鉀溶液(KOH)進行析氧反應時，證實CNT之複合可增進CoTe2之催化活性，其主要原因為於CoTe2及CNT之界面間有一電荷轉移之現象，電子由CoTe2轉移至CNT，改變CoTe2中Co之電子結構，達到協同效應之產生，改進析氧反應之活性及動力學。於10 mA/cm2下，CoTe2/CNT過電位僅291 mV。與純CoTe2相比，穩定度明顯提升至24小時。

關鍵字

過渡金屬 ； 碲化物 ； 奈米碳管 ； 電催化 ； 水分解

並列摘要

Electrocatalytic water splitting to produce hydrogen is one of the effective ways to solve the energy crisis and environmental pollution. Noble metal catalysts show high activity to carry out water splitting, but their problem of low abundance and high costs limits their large-scale application. Therefore, the research focuses on looking for and improving non-noble metal catalysts in recent years. Electrocatalytic water splitting involves two half reaction. One is hydrogen evolution reaction (HER) in cathode: 2H+ + 2e- → H2, and the other is oxygen evolution reaction (OER) in anode: 2H2O → O2 + 4H+ + 4e-. We must not only enhance the catalytic activity of cathodic HER but also that of anodic OER, so looking for and improving non-noble metal catalysts are important not only in HER but also OER. In recent years, the direction of improving non-noble metal catalyst is decrease in overpotential, increase in current density, increase in kinetics of reaction and enhancement of stability.
We synthesize cobalt ditelluride nanoparticles (CoTe2 NPs) and cobalt ditelluride/carbon nanotube (CoTe2/CNT) hybrid material by hydrothermal reaction in this study. And then we adhere these two catalysts on glass carbon electrode to perform electrochemical measurement. We apply them to HER and OER respectively to explore their activity and mechanism of HER and OER. X-ray diffraction was carried out to determine the crystallinity and the crystal structure. The morphology of the samples was investigated by using scanning electron microscopy and transmission electron microscopy. The X-ray absorption near edge structure of Co K- and L-edge was conducted to determine valence state and d orbital electron-filling rate of metal ion in samples. Finally, electrochemical measurements of linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy and amperometry were performed to determine overpotential, active surface area, charge transfer resistance and stability. During the hydrogen evolution reaction was in 0.5 M sulfuric acid (H2SO4), we confirmed that the CoTe2 NPs with smaller particle size generates more active site on the surface so they show better performance of HER. These nanoparticles can generate 10 mA/cm2 at an overpotential of 246 mV without any decay up to 48 h of continuous reaction. When oxygen evolution reaction was in 1.0 M potassium hydroxide (KOH), we confirmed that the CNT can enhance the catalytic activity of CoTe2, the main reason is the phenomenon of charge transfer at the interface between CoTe2 and CNT. The electron transfers from CoTe2 to CNT, which changes the electronic structure of Co in CoTe2. It achieves synergistic effect to improve activity and kinetics of OER. The hybrid material can generate 10 mA/cm2 at an overpotential of 291 mV with enhanced stability of 24 h compared with pure CoTe2.

並列關鍵字

transition metal ； telluride ； carbon nanotube ； electrocatalytic ； water splitting

參考文獻

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