Electrochemical Properties and Dissolution Mechanism of A₂Ni₇ Hydrides (A=Y, Gd, La–Sm)

Abstract

The electrochemical behavior of hydride‐forming A2B7 alloys (Y2Ni7, LaSmNi7, and Gd2Ni7) is investigated in KOH solution. The evolution of the material performance is shown to strongly depend on the A elements. Electrochemical impedance spectroscopy (EIS), SEM observations, XRD, and Raman spectroscopy showed that Y2Ni7 is mainly sensitive to calendar corrosion and decrepitation, whereas LaSmNi7 and Gd2Ni7 are sensitive to the concomitant effect of both cycling and calendar corrosion. It is also shown that the capacity loss of Gd2Ni7 can be ascribed to amorphization on cycling, which is not the case for Y2Ni7. The formation of both an thin oxide film (a few tens of nanometers thick) at the material surface, measured by EIS, and the dissolution products formed during long‐term immersion, analyzed by Raman spectroscopy, is in agreement with the decrease in material activity evaluated by EIS during cycling at different depths of charge.

Correlations between stacked structures and weak itinerant magnetic properties of La2−xYxNi7compounds

“…A 2 Ni 7 compounds (A = rare earth) have raised interest for their fundamental physical properties [1][2][3] as well as their applications as hydrogen storage materials [4][5][6] and negative electrodes in Ni-MH batteries [7][8][9][10][11]. A 2 Ni 7 intermetallic compounds crystallize in two polymorphic crystal structures which are either hexagonal (Ce 2 Ni 7 -type, P6 3 /mmc) or rhombohedral (Gd 2 Co 7 -type, R-3m) depending on the stacking of the [A 2 B 4 ] and [AB 5 ] units along the c axis (B = Ni) [1].…”

Anisotropic Nanoporous Nickel Obtained through the Chemical Dealloying of Y2Ni7 for the Comprehension of Anode Surface Chemistry of Ni‐MH Batteries

“…This layer crystallizes in Ni cubic structure (space group Fm m ) and is highly nanoporous . It is also hypothesized that this metallic Ni is further passivated by forming a NiO thin film at its surface . Though this nanoporous Ni (np‐Ni) layer is a corrosion product, it plays a key role in the hydrogen absorption kinetics as a competition exists between proton absorption in the metallic host structure and recombination into gaseous H 2 at its surface.…”

Relationship between H2 sorption properties and aqueous corrosion mechanisms in A2Ni7 hydride forming alloys (A = Y, Gd or Sm)

“…For Gd 2 Ni 7 and Sm 2 Ni 7 , nickel hydroxide was identified (Figure S2, Supporting Information) whereas for Y 2 Ni 7 , no nickel oxide or hydroxide was found. However, Puga et al[30] report by impedance spectroscopy measurements that an oxide forms upon corrosion for Y 2 Ni 7 and Gd 2 Ni 7 . It is probably some nickel oxide embedded into the porous nickel layer.A 3+ diffuses from the A 2 Ni 7 compound through the nickel rich layer and finally participates to the growth of hydroxide needles.…”