Enhanced Hydrogen Spillover on Carbon Surfaces Modified by Oxygen Plasma

Hydrogen storage capacity by hydrogen spillover on metal-doped carbons can be significantly enhanced by introducing surface oxygen functional groups to. the carbon. It is shown that the enhancement is greater by oxygen plasma treatment compared to air oxidation because different surface groups were formed. Oxygen plasma treatment generated mainly semiquinone (C=O) groups while air oxidation formed mainly hydroxyl (C-OH) groups. Experimental heats of adsorption, X-ray photoelectron spectroscopy (XPS) analyses, and ab initio molecular orbital calculations showed that the binding energies between the spiltover hydrogen and the different groups followed the following order: lactone > semiquinone > carboxyl > basal plane. Thus, the H-2 storage capacity at 298 K and 10 MPa was increased from 1.17 wt % (without O-2 treatment) to 1.74 wt % on Pt-doped on a templated carbon that was pretreated with OZ plasma. Similar enhancements were seen on Pt doped on a superactivated carbon, AX-21. However, there was a decrease in storage capacity during the first three adsorption-desorption cycles (at 298 K and 10 MPa). The capacity decreased from 1.74 wt % to 1.30 wt % and remained unchanged after three cycles. XPS results showed that this decrease was caused by the very strong (and irreversible) binding of the spiltover hydrogen with the lactone groups (HO-C=O). Nonetheless, the main groups of semiquinone remain functional as receptor sites upon cycling, and the 1.3 wt % storage capacity is among the highest reversible capacities reported in the literature.