| 摘要: | 本研究採用恆溫揮發熔煉鑄造製程(Isothermal Evaporation Casting Process)專利合成的Mg2Ni1-xCux合金(x=0,0.2,0.4,0.6,0.8,1.0)為儲氫材料，研究其在6M 氫氧化鉀電解液中之液態電化學吸、放氫之行為；材料中添加Cu至Mg2Ni合金中取代Ni的目的，在於嘗試改善此合金的液態吸放氫性能。 在6M 氫氧化鉀電解液中，控制電位(本論文均以標準氫電位SHE表示)在-0.05V~-1.05V 範圍中，以循環伏安法(Cyclic Voltammmetry) 研究Mg2Ni1-x Cux合金之電化學吸、放氫行為，由CV圖譜中結果顯示: Mg2Ni 於- 0.8V 出現一特性氧化峰(推測為氫化物的氧化，可評估其放氫能力)，而Mg2Ni1-x Cux合金則在- 0.7 V (推測為氫化鎂之氧化)與 - 0.4V(推測屬於Cu?CuO22-的氧化反應)處各出現一特性氧化峰。量測Mg2Ni1-x Cux合金在-0.7V(與- 0.8V附近)氧化峰之高度與並積分其面積值，顯示合金中以Mg2Ni0.6Cu0.4之面積最大，放氫能力最佳，釋氫量最大。圖譜中陰極區電流(還原)積分後之大小，會隨著合金中Cu成分之增加而降低，亦即:添加銅會降低合金的吸氫(還原氫)能力。若將CV實驗控制在較小電位範圍 (-0.5V~-1.05V)進行，不僅可避免- 0.4V(Cu氧化峰)之出現，合金在-0.7V處的電流較無衰退之趨勢。 以定電流(10mA/g)測試Mg2Ni1-xCux合金的放電實驗，量測其電壓(至- 0.10V結束)對時間之關係，結果顯示: 放電量大小依序為x= 0.4 > 0.2 > 0.6 > 0.8 > 1.0 > 0.0，同時在含Cu合金的放電曲線上 -0.4V~-0.5V處發現多出一平原區 (對應於CV圖譜- 0.4V處銅的氧化峰)，屬於銅氧化之不可逆放電。若終止放電實驗於- 0.5V，則可減緩放電量隨放電週次增加而衰減之現象。此結果與循環伏安法的研究結果相符。 合金試片在充放電前後分別進行X光結晶繞射分析，結果發現:含Cu合金在充放電後，Mg2Ni (003)與(101)晶面繞射峰(2θ=20.1和20.8度)的強度大幅減少，顯示合金有不可逆的結構的變化(放氫能力衰退)；純Mg2Ni之XRD則無此現象。 比較極化阻抗之量測結果，顯示Mg2Ni0.6Cu0.4較其他含銅合金之極化阻抗低(約低65 %)，亦即反應活性增高約65 %。在定電位放電下量測合金中氫的擴散係數，結果得知：Mg2Ni0.6Cu0.4合金的擴散係數(7.11*10-10 cm2/s)，比純Mg2Ni (3.54*10-10 cm2/s)增快約1倍。 The electrochemical hydrogenation/dehydrogenation of the Mg2Ni1-xCux alloys (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) in 6 M KOH solution has been investigated. Copper was added in the alloy, through a patent process named isothermal evaporation casting process, in an attempt to improve the hydrogenation capacity and the kinetics of hydrogenation/dehydrogenation. Cyclic voltammmetry (CV) of the alloys in 6 M KOH solution was studied in -1.05V~-0.05V (All potentials reported with respect to standard hydrogen electrode, SHE) to estimate their capacity of hydrogenation/dehydrogenation. The CV curve of Mg2Ni revealed a simple characteristic oxidation peak at - 0.8V (supposedly due to oxidation of the hydride that is useful for estimation the capability of dehydrogenation). In contrast, the curve of Mg2Ni1-x Cux indicated an oxidation peak at - 0.7 V (perhaps arisen from oxidation of MgH2) with another one at - 0.4V (may be ascribed to the reaction Cu?CuO22-). The area of each peak could be estimated by integration and compared. The capability of dehydrogenation is proportional the area of the peak. Alloy Mg2Ni0.6Cu0.4 has the highest peak area to reveal the highest capability of dehydrogenation. The integration of the cathodic current loop on CV exhibits that the area decreases with increasing the copper content in the alloy. It implies that the addition of copper in the alloy decreases the capacity of hydrogenation. The enhancement on the dehydrogenation but diminishment on the hydrogenation may be due to alkaline dissolution of copper in the alloy. The performance of CV in a constraint range (i.e., -1.05V~ -0.5V) can significantly avoid the loss of hydrogenation capacity. This discharge at lower potential (i.e. -0.5V) prevents the occurrence of irreversible oxidation (i.e., -0.4 V) of copper in the alloy. Thus, there is no prominent depression on the discharging current at -0.7V. The capacity of dehydrogenation was also measured by constant current discharging (at 10mA/g) of the alloys. The variation of voltage versus time was monitored. The electrical quantity discharged was estimated and it decreases in the order x = 0.4 > 0.2 > 0.6 > 0.8 > 1.0 > 0.0 Cu. There arises a plateau at -0.5V~-0.4V for the alloy containing Cu > 0.2 that is consistent with the result from CV (-0.4 V on CV responsible for irreversible oxidation of copper in the alloy). If the discharging measurement was terminated at -0.5V, the irreversible oxidation of copper could be avoided so that the current depression at -0.7 V becomes less significant. This result agrees with that from CV. X-ray diffraction (XRD) patterns of the specimens prior to and post electrochemical hydrogenation/dehydrogenation were compared. There is no obvious change for the Mg2Ni but with a depression of the intensity at 2θ=20.1 and 20.8o, for Mg2Ni (003) and (101) in the Mg2Ni1-xCux alloys, revealed an irreversible change of the crystal in the dehydrogenation that is responsible for a decay of their capacity of dehydrogenation The polarization resistance of the system was measured and compared for the alloys. The resistance is roughly 25 % lower for Mg2Ni0.6Cu0.4 than Mg2Ni. This infers an 25 %-enhancement on the kinetics. The diffusion coefficient of hydrogen in the alloy was determined under constant current discharging, the coefficient is one-time higher in Mg2Ni0.6Cu0.4 (7.11*10-10 cm2/s) than in Mg2Ni (3.54*10-10 cm2/s). |
