Weak and strong hydrogen interactions on porous carbon materials in high-temperature systems

Abstract

The adsorption mechanisms of hydrogen on porous carbon materials were examined using constant volume chemisorption at 700 °C, the operating temperature of various advanced fission devices. It is recognized that hydrogen diffuses and adsorbs into carbon-based materials through various processes, making it difficult to interpret experimental data. Further, previous hydrogen-carbon interaction studies have predominately focused on storage applications at high pressures and cryogenic temperatures or on specific types of structural or neutron-moderating materials at atmospheric pressures and temperatures exceeding 1000 °C. Meanwhile, it has recently been shown that different carbon allotropes may also provide the ability to select and design for desired tritium transport behavior at high-temperature reactor conditions, motivating a need for deeper and more general understanding of these interactions across a range of adsorbents.In this study, it was found that total chemisorption consists of different adsorption modes that vary in binding energy. These interactions were grouped into strong interactions, which require significant energy and temperature to promote desorption, and weak interactions, which have a low desorption activation barrier. The weak interactions were thermodynamically modeled and they were found to follow a dissociative Langmuir isotherm, implying dissociative adsorption and therefore desorption that is driven by recombination of atomic hydrogen. Further, it was shown that strong adsorption can be modeled accurately by difference between total and weak adsorption isotherm models. Both the weak and strong adsorption was found to increase with increasing microporosity (<2 nm). The results suggest that weak chemisorption is a combination of in-pore confinement and hydrogen dissociation on the carbon surface while strong chemisorption occurred due to chemical entrapment in sites made more accessible by increased access to the bulk of carbon materials.