Rationalizing the Hydrogen Uptake–Release Balance in AB ₂ Alloys via d-Band Center Evolution

The inherent trade-off in AB 2 -type alloys between maximizing saturation capacity and minimizing residual hydrogen remains a critical bottleneck to complete desorption at practical back pressures. Here, we employ the d-band center ( ε d ) as a descriptor, revealing that its evolution follows an asymptotic saturation trajectory during continuous Zr substitution. Crucially, Zr-induced lattice strain dominates this electronic reconfiguration, overriding ligand effects to govern the ε d shift. This nonlinear behavior defines a Sabatier-type optimal design window that balances hydride stability and reversible capacity. First-principles calculations further show that Zr substitution reduces the energetic penalty for adjacent-site hydrogen occupancy by alleviating short-range H–H repulsion, thereby expanding the α + β plateau. Consequently, the optimized Ti 0.82 Zr 0.18 Mn 1.15 Cr 0.75 V 0.10 alloy was synthesized near the ε d saturation regime. Under temperature swing cycles (4.0 MPa saturation at 0 °C and 0.1 MPa desorption at 50 °C), the alloy sustains a 1.938 wt % effective capacity with 95.94% utilization, featuring an extensive α + β plateau capacity of 1.748 wt %, accounting for 86.32% of the saturation capacity. This performance ensures stable and efficient operation of proton exchange membrane fuel cells. Overall, this work offers a strategic design principle for developing high-performance hydrogen storage materials.