| Summary: | 碩士 === 國立中央大學 === 材料科學與工程研究所 === 97 === 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).
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