| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 91 === The self-discharge behavior and high-rate dischargeability of Mm-based AB5 hydrogen storage alloy as a negative electrode is investigated in this work. Two different AB5 alloy powders as well as two different electrolytes are used, namely alloy V (La0.26 Ce0.44Pr0.1Nd0.2Ni3.55Co0.72Mn0.43Al0.3) and alloy N (La0.58Ce0.25Pr0.06 Nd0.11Ni3.66Co0.74Mn0.41Al0.18), with an original electrolyte (EO, 6M KOH +1wt% LiOH) and a commercial electrolyte (EN, with abundant Al). The electrochemical properties are studied through cross-examination between different [MH electrode-electrolyte] systems.
Alloy N with a larger La content resulted in a larger unit cell volume, a higher hydrogen absorbing capacity and a lower plateau pressure, as confirmed by PCI analysis. The maximum capacity is also higher for alloy N. Activation rate is higher for alloys in commercial electrolyte but the maximum capacity is unaffected by the type of electrolyte being used.
Step mode self-discharge tests show that the reversible capacity loss is inevitable in open cells and dominates the total amount of capacity loss at room temperature. At 60℃, all systems show larger amounts of capacity loss, both reversibly and irreversibly. It is attributed to the fact that under high temperature, the plateau pressure is higher and the oxidation rate of the alloy is faster.
EDX results confirm that alloys in commercial electrolyte (NEN and VEN) forms a layer of aluminum oxide on the surface which protects the alloy from being further oxidized, therefore reduces the irreversible capacity loss. It is also observed from SEM that corrosion products in the form of hexagonal platelet are more likely to be found on the alloys in original electrolyte. EDX analyses show that these platelets are rich in O, Ce and K. It is believed that these platelets are oxides or hydroxides with higher Mm content than the needle shape corrosion products. Thus these alloys degrade faster as more mischmetal becomes hydroxides and the loss of hydrogen is contributed to the irreversible capacity loss. The self-discharge capacity loss is in an order of: VEO > NEO > VEN > NEN.
This work shows that the self-discharge behavior is contradictory to the high rate capability. Alloy N in commercial electrolyte (NEN) shows the highest capacity retention but the discharge efficiency drops to zero when discharge current reaches 9C. On the other hand, alloy V in original electrolyte (VEO) has a capacity retention less than 50% after 16 days storage under 60℃, yet its discharge efficiency is superior to all others.
Results show that both self-discharge behavior and high-rate capability are predominated by the type of electrolyte, follow by the type of alloy. It is reasonable to believe that the composition of electrolyte changes the surface state of the alloy and thereby changes their electrochemical behavior such as activation rate and anti-corrosion ability.
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