Unravelling H$_2$ chemisorption and physisorption on metal decorated graphene using quantum Monte Carlo

TL;DR

This study uses advanced quantum Monte Carlo calculations to accurately analyze hydrogen adsorption on metal-decorated graphene, revealing chemisorption possibilities and highlighting discrepancies with common DFT methods, which impacts hydrogen storage material development.

Contribution

It provides the most accurate adsorption energies for hydrogen on metal-decorated graphene using quantum Monte Carlo, clarifying chemisorption feasibility and benchmarking DFT methods.

Findings

Chemisorption of H₂ on Ca-decorated graphene is feasible according to DMC.

Discrepancies exist between DMC and DFT in predicting adsorption energies.

Reference energies can improve large-scale modeling of hydrogen storage.

Abstract

Molecular hydrogen is at the core of hydrogen energy applications and has the potential to significantly reduce the use of carbon dioxide emitting energy processes. However, hydrogen gas storage is a major bottleneck for its large-scale use as current storage methods are energy intensive. Among different storage methods, physisorbing molecular hydrogen at ambient pressure and temperatures is a promising alternative - particularly thanks to tuneable lightweight nanomaterials and high throughput screening methods. Nonetheless, understanding hydrogen adsorption in well-defined nanomaterials remains experimentally challenging and reference information is scarce despite the proliferation of works predicting hydrogen adsorption. In this work, we focus on Li, Na, Ca, and K, decorated graphene sheets as substrates for hydrogen adsorption and compute the most accurate adsorption energies available to date using quantum diffusion Monte Carlo (DMC). Building on our previous insights at the density functional theory (DFT) level, we find that a weak covalent chemisorption of molecular hydrogen, known as Kubas binding, is feasible on Ca decorated graphene according to DMC, in agreement with DFT. This finding is in contrast to previous DMC predictions of the 4H$_2$/Ca$^+$ gas cluster where chemisorption is not favoured. However, we find that the adsorption energy of hydrogen on metal decorated graphene according to a widely-used DFT method is not fully consistent with DMC and the discrepancies are not systematic. The reference adsorption energies reported herein can be used to find better work-horse methods for application in large-scale modelling of hydrogen adsorption. Furthermore, the implications of this work affect strategies for finding suitable hydrogen storage materials and high-throughput methods.