Effect of Co substitution on microstructure and electrochemical kinetics of La–Y–Ni superlattice alloys

In this study, based on the previously optimized La–Y–Ni superlattice hydrogen-storage alloy matrix La₁₋ₓYₓNi₃.₇₅Mn₀.₂Al₀.₁₅, a series of La₀.₄Y₀.₆Ni₃.₇₅₋ₓMn₀.₂Al₀.₁₅Coₓ(x = 0,0.3,0.6,0.9) alloys was prepared by partially substituting Co for Ni at the B-site to investigate how B-site electronic structure and local volume regulation influence the stability of the superlattice structure and its electrochemical properties. Previous studies have shown that Co preferentially enters the AB₅ subunit, reducing the A₂B₄/AB₅ volume ratio from 1.0179(1.0185) to around 1.0159(1.01) at x = 0.9, thereby improving substructure volume matching. The electrochemical tests show that with the increase in Co content, the cycle stability continuously improves, showing a significant enhancement. The discharge capacity and high-rate performance generally exhibit a “low content retention, excessive decay” trend, with x = 0.3 performing the most balanced across several performance indicators. Electrochemical P–C–T measurements indicate that Co addition lowers the plateau pressure, but when x ≥ 0.6, the enhanced hydride phase stability leads to reduced reversible dehydrogenation capacity. Kinetic analysis demonstrates that increasing Co content slightly decreases the exchange current density I₀, continuously reduces the diffusion coefficient D, and increases both R_ct and diffusion impedance in EIS; the kinetics are dominated by charge-transfer control for x ≤ 0.3, whereas bulk diffusion becomes the limiting factor for x ≥ 0.6. Post-cycling SEM observations show that moderate Co contents (x = 0.3,0.6) promote the formation of relatively uniform corrosion layers, helping alleviate pulverization and maintain electrode structural integrity, while high Co content (x = 0.9) further enhances surface corrosion resistance but produces a dense film that increases interfacial impedance and suppresses hydrogen diffusion. Overall, Co in this system exhibits coupled effects of “modulating substructure matching” and “altering kinetic behavior,” establishing a necessary balance between structural stability and kinetic performance, with x = 0.3 being the optimal doping level in terms of comprehensive properties.