Mechanisms of adsorbing hydrogen gas on metal decorated graphene

Hydrogen is a key player in global strategies to reduce greenhouse gas emissions. In order to make hydrogen a widely used fuel, we require more efficient methods of storing it than the current standard of pressurized cylinders. An alternative method is to adsorb H-2 in a material and avoid the use of high pressures. Among many potential materials, layered materials such as graphene present a practical advantage as they are lightweight. However, graphene and other 2D materials typically bind H-2 too weakly to store it at the typical operating conditions of a hydrogen fuel cell, meaning that high pressure would still be required. Modifying the material, for example by decorating graphene with adatoms, can strengthen the adsorption energy of H-2 molecules, but the underlying mechanisms are still not well understood. In this work, we systematically screen alkali and alkaline-earth metal decorated graphene sheets for the static thermodynamic adsorption of hydrogen gas from first principles and focus on the mechanisms of binding. We show that there are three mechanisms of adsorption on metal decorated graphene and each leads to distinctly different hydrogen adsorption structures. The three mechanisms can be described as weak van der Waals physisorption, metal adatom facilitated polarization, and Kubas adsorption. Among these mechanisms, we find that Kubas adsorption is easily perturbed by an external electric field, providing a way to tune H2 adsorption. This work is foundational and builds our understanding of H2 adsorption under idealized conditions.

Automated summary

Hydrogen plays a crucial role in global efforts to reduce greenhouse gas emissions. Efficient methods of storing hydrogen are needed to make it a widely used fuel. Graphene, a lightweight layered material, has potential for hydrogen storage but currently binds hydrogen too weakly. In this work, we systematically investigate the adsorption of hydrogen on metal-decorated graphene sheets and identify three different mechanisms of binding. We propose that Kubas adsorption, which can be easily manipulated by an external electric field, has potential for tuning hydrogen adsorption. This study enhances our understanding of hydrogen adsorption and lays the foundation for further research.