Adsorption of molecular hydrogen on honeycomb ZnO monolayers: A quantum density-functional theory perspective

TL;DR

This study uses quantum density-functional theory to analyze hydrogen adsorption on ZnO monolayers, evaluating storage capacities and thermodynamic properties across various conditions, revealing tighter confinement compared to graphene.

Contribution

It introduces a detailed quantum theoretical approach to assess hydrogen storage on ZnO monolayers, including thermodynamic and microscopic analysis.

Findings

ZnO monolayers have higher hydrogen storage capacity than graphene.

The isosteric heat of adsorption approaches 3.2 kJ/mol at low densities.

Adsorption behavior varies significantly with temperature and density.

Abstract

We investigate the adsorption of molecular hydrogen on pristine zinc oxide (ZnO) platelets. The volumetric and gravimetric hydrogen storage capacities of the ZnO monolayers are evaluated in a broad range of thermodynamic conditions (i.e., for temperatures in the range 77 K < T < 450 K, and for external gas pressures up to 200 bar). The thermodynamic properties and the microscopic spatial distribution of the adsorbed hydrogen fluid are assessed within the density functional theory of liquids for quantum fluids at finite temperature (QLDFT), and the adsorption enthalphies are obtained by fitting the computed adsorption densities to the Toth model isotherm. Compared to graphene platelets, the ZnO sheets impose a rather tighter confinement to the motion of the hydrogen molecules parallel to the surface. The isosteric heat of adsorption approaches 3.2 kJ/mol in the low density regime. This quantity shows a fairly smooth dependence on the hydrogen uptake for temperatures below 100 K, while it is shown to depend quite sensitively on the adsorbate density above this temperature.