Physi-Sorption of H2 on Pure and Boron–Doped Graphene Monolayers: A Dispersion–Corrected DFT Study

Iffat Nayyar,Thomas Gennett,Thomas Autrey,Bojana Ginovska,Abhijeet Karkamkar

doi:10.3390/c6010015

Abstract

High-surface-area carbons are of interest as potential candidates to store H2 for fuel–cell power applications. Earlier work has been ambiguous and inconclusive on the effect of boron doping on H2 binding energy. Here, we describe a systematic dispersion–corrected density functional theory study to evaluate the effect of boron doping. We observe some enhancement in H2 binding, due to the presence of a defect, such as terminal hydrogen or distortion from planarity, introduced by the inclusion of boron into a graphene ring, which creates hydrogen adsorption sites with slightly increased binding energy. The increase is from −5 kJ/mol H2 for the pure carbon matrix to −7 kJ/mol H2 for the boron–doped system with the boron content of ~7%. The H2 binding sites have little direct interaction with boron. However, the largest enhancement in physi-sorption energy is seen for systems, where H2 is confined between layers at a distance of about 7 Å, where the H2 binding nearly doubles to −11 kJ/mol H2. These findings suggest that interplanar nanoconfinement might be more effective in enhancing H2 binding. Smaller coronene model is shown to be beneficial for understanding the dependence of interaction energy on the structural configurations and preferential H2 binding sites.

Highlights

Among hydrogen storage materials, activated carbons, graphene, carbon nanotubes, and metal-organic frameworks are of great interest due to a combination of large surface area, low mass density, and high structural stability [1]
We investigated the sensitivity of the binding energy on the density functional theory (DFT) exchange, using van der Waals (vdW)–DF2 with three other functionals: revPBE, optPBE, and RPBE (Table 6, bottom four entries)
We find that for ~2% B–doping, the enhancement in H2 binding energy increases to –6.6 kJ/mol H2 from that of –5.2 kJ/mol H2 for pure graphene (Figure 2, top and middle)

Summary

Introduction

Among hydrogen storage materials, activated carbons, graphene, carbon nanotubes, and metal-organic frameworks are of great interest due to a combination of large surface area, low mass density, and high structural stability [1]. The weak binding energy is attributed to physi-sorption dominated by van der Waals attraction, such that adsorbed molecules of H2 do not remain attached at ambient temperatures and require liquid nitrogen temperatures to achieve adsorption [3,4]. For practical applications, such as H2 fuel–cell electric vehicles, the operating temperatures are much higher, ranging from −40 to 85 ◦ C, and require binding energies on the order of 15 to 25 kJ/mol for pressures ranging from 5 to 100 bar [5,6]. Other approaches have involved templating porous carbons that can provide impressive H2 sorption properties and gravimetric and

Methods

Results

Conclusion