Abstract
The effects of Fe partially replacing Ni in a misch metal-based superlattice hydrogen absorbing alloy (HAA) were studied. Addition of Fe increases the lattice constants and abundance of the main Ce2Ni7 phase, decreases the NdNi3 phase abundance, and increases the CaCu5 phase when the Fe content is above 2.3 at%. For the gaseous phase hydrogen storage (H-storage), Fe incorporation does not change the storage capacity or equilibrium pressure, but it does decrease the change in both entropy and enthalpy. With regard to electrochemistry, >2.3 at% Fe decreases both the full and high-rate discharge capacities due to the deterioration in both bulk transport (caused by decreased secondary phase abundance and consequent lower synergetic effect) and surface electrochemical reaction (caused by the lower volume of the surface metallic Ni inclusions). In a low-temperature environment (−40 °C), although Fe increases the reactive surface area, it also severely hinders the ability of the surface catalytic, leading to a net increase in surface charge-transfer resistance. Even though Fe increases the abundance of the beneficial Ce2Ni7 phase with a trade-off for the relatively unfavorable NdNi3 phase, it also deteriorates the electrochemical performance due to a less active surface. Therefore, further surface treatment methods that are able to increase the surface catalytic ability in Fe-containing superlattice alloys and potentially reveal the positive contributions that Fe provides structurally are worth investigating in the future.
Highlights
Misch metal-based superlattice hydrogen absorbing alloy (HAA) has become the mainstream negative electrode active material for commercial nickel/metal hydride (Ni/MH) batteries due to its higher capacity, improved high-rate capability, wider operating temperature range, and lower self-discharge compared to the conventional AB5 HAA [1,2,3,4]
We investigate the effects of Fe, another transition metal with an atomic number between Mn and Co, partially replacing Ni on the structural, gaseous phase storage, and electrochemical properties of HAA
If we only focus on the Fe-substituted alloys, we may reach the conclusion that with increasing Fe content, the unit cell volume of the main Ce2 Ni7 phase increases, resulting in a more stable hydride
Summary
Misch metal-based superlattice hydrogen absorbing alloy (HAA) has become the mainstream negative electrode active material for commercial nickel/metal hydride (Ni/MH) batteries due to its higher capacity, improved high-rate capability, wider operating temperature range, and lower self-discharge compared to the conventional AB5 HAA [1,2,3,4]. The addition of Fe to V-containing AB2 HAA deteriorated the low-temperature performance but increased the discharge capacity [21] and cycle stability [18]. Contradictory results from adding Fe to V-free AB2 HAA were reported (improved low-temperature performance but degraded capacity and cycle performance) [22]. Negative impact on low-temperature performance [30] and positive contribution to cycle stability [32] from Fe addition in AB5 HAA have been previously reported. While this paper (Part 1) summarizes the results of structural, gaseous phase, and electrochemical (in half-cell configuration) studies on the Fe-substituted misch metal-based superlattice alloys, the performance and failure analysis of the Ni/MH batteries made using these alloys will be discussed in another publication (Part 2, [44])
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