Electrochemical hydrogen-storage capacity of graphene can achieve a carbon-hydrogen atomic ratio of 1:1

Abstract

As a promising hydrogen-storage material, graphene is expected to have a theoretical capacity of 7.7 wt%, which means a carbon-hydrogen atomic ratio of 1:1. However, it hasn’t been demonstrated yet by experiment, and the aim of the U.S. Department of Energy is to achieve 5.5 wt% in 2025. We designed a spatially-confined electrochemical system and found the storage capacity of hydrogen adatoms on single layer graphene (SLG) is as high as 7.3 wt%, which indicates a carbon-hydrogen atomic ratio of 1:1 by considering the sp3 defects of SLG. First, SLG was deposited on a large-area polycrystalline platinum (Pt) foil by chemical vapor deposition (CVD); then, a micropipette with reference electrode, counter electrode and electrolyte solution inside was impacted on the SLG/Pt foil (the working electrode) to construct spatially-confined electrochemical system. The SLG-uncovered Pt atoms act as the catalytic sites to convert protons (H⁺) to hydrogen adatoms (H_ad), which then spill over and are chemically adsorbed on SLG through surface diffusion during the cathodic scan. Because the electrode processes are reversible, the H_ad amount can be measured by the anodic stripping charge. This is the first experimental evidence for the theoretically expected hydrogen-storage capacity on graphene at ambient environment, especially by using H⁺ rather than hydrogen gas (H₂) as hydrogen source, which is of significance for the practical utilization of hydrogen energy.