Toward viable H2 storage in Ca decorated low-dimensional materials with insights from reference quantum Monte Carlo.

Hydrogen technology is set to be a key energy alternative for mitigating pollution and reducing CO2 emissions. However, the current storage mechanism of hydrogen molecules in carbon fiber tanks detracts from the fuel economy of hydrogen in mobile applications, necessitating the development of alternative storage mechanisms. Adsorbing hydrogen in its molecular form (H2) under typical operating conditions of proton exchange membranes can potentially meet storage requirements. However, H2 is the smallest molecule with only two electrons and, therefore, it has very limited propensity to physisorb in a material within the binding energy window of -0.2 to -0.4 eV that is suitable for storage. Calcium atom decorators on graphene have previously shown promise for tunable H2 binding, but the system is thermodynamically unstable toward the formation of calcium hydride. Moreover, the absolute adsorption of H2 is challenging to predict accurately and is typically overestimated with van der Waals inclusive density functional approximations. In this work, we perform state-of-the-art fixed-node diffusion Monte Carlo alongside a selection of density functional approximations for two strategies of anchoring Ca: (i) Ca on boron-doped graphene and (ii) Ca inside carbon nanotubes. We predict reliable Ca and H2 binding energies and establish that Ca is anchored inside carbon nanotubes and on boron-doped graphene, while boosting the H2 adsorption energy. Importantly, the H2 adsorption energy is found to be improved by the anchoring strategies, with the energy inside a Ca decorated carbon nanotube reaching the viable storage window. The reference DMC binding energies provide much-needed benchmarks for developing data-driven methods and guiding experiment in the systematic design of hydrogen storage materials.