Lalit Kumari, Here's the answer to both your questions regarding hydrogen adsorption and decorated monolayers:

1) US DOE Criteria for Hydrogen Binding Energies

The U.S. Department of Energy (DOE) has outlined targets for hydrogen storage materials to ensure reversible adsorption/desorption under ambient or near-ambient conditions.

• Ideal hydrogen binding energy: 0.2–0.6 eV per H₂ molecule < 0.2 eV: Too weak — hydrogen desorbs too easily > 0.6 eV: Too strong — hydrogen is difficult to release

This range ensures efficient adsorption at moderate pressures and desorption near room temperature, aligning with the practical requirements of hydrogen storage applications (e.g., in fuel cells).

2) Bond Length Criteria for Decorated Monolayers

For decorated monolayers (e.g., graphene or borophene functionalized with metal atoms):

• Metal–H bond length: Typically in the range of 1.5–2.5 Å (depends on the metal species and coordination environment)
• Metal–substrate bond length: Usually around 1.8–3.0 Å, depending on the monolayer (C, B, N-based) and metal

These ranges support moderate binding and maintain structural stability. Bond lengths outside this range often indicate too weak or too strong binding, leading to desorption issues or clustering.