Page 6 - Thesis_Oprysk_Volodymyr

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impedance spectroscopy, enabling a thorough analysis of kinetic and diffusion processes

in the electrode materials studied.

Chapter 3 presents the results of a comprehensive study of the composition, crystal

structure, and electrochemical hydrogen sorption properties of AB 2 Laves phases in R-

Mg-d-transition metal systems. Three series of pseudobinary Laves phase alloys with the

general composition RMgNi 4-yCo y (R = Pr, Pr 0.5La 0.5, Pr 0.5Nd 0.5) were synthesized, and the

solid solution existence ranges were established. It is shown that an increase in the unit

cell volume due to isomorphic substitution of Pr/La and Ni/Co contributes to the

stabilization of γ-hydride phases, which are characterized by higher hydrogen sorption

capacity, which reaches ~6.7 at. H/f.u. for the Pr 0.5La 0.5MgNi 2Co 2 alloy. The highest

discharge capacity among the investigated AB 2-type alloys was demonstrated by the

Pr 0.5La 0.5MgNi 3.5Co 0.5 composition (305 mAh/g), while the maximum cyclic stability was

shown by the single-phase PrMgNi 4 alloy (S 30 = 95%).

Chapter 4 presents the results for AB 3-type phases. For the (La,Y) 3-xMg xNi 9-yCo y

system, an optimal range of unit cell volumes (522-535 ų) was established, within which

the discharge capacity consistently exceeds 350 mAh/g. The La 1.5Y 0.5MgNi 7Co 2 alloy

(C max = 390 mAh/g; S 50 = 84%) was identified as offering the best compromise between

high capacity and cycling stability. The LaYMgNi 7Co 2 alloy demonstrates exceptionally

high cyclic stability (S 50 = 99%), and the La 1.5Y 0.5MgNi 9 alloy shows the best kinetic

characteristics, in particular, high rate discharge (HRD 1000 = 86%) and the hydrogen

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diffusion coefficient (D H = 2.67 10 cm /s). For the Nd 2MgT 9 series, it is shown the
decisive role of the Ni content in the formation of the hydrogen sorption properties of

compounds of this type of alloys.

A systematic study of Mg-free AB 3-type phases with the composition

R xY 3-xNi 9-yMn y is reported for the first time. It was found that partial replacement of Ni

by Mn leads to an almost twofold increase in discharge capacity: compounds of the

RY 2Ni 8Mn type demonstrate values in the range of 311-370 mAh/g compared to 126-194

mAh/g for the original RY 2Ni 9 phases. The maximum discharge capacity of 370 mAh/g

was observed for the PrY 2Ni 8Mn alloy, which represents one of the highest values

reported for AB 3-type phases (PuNi 3 type of structure) in Mg-free systems.