Hydrogen Spillover and Adsorption on Nanoporous Carbon-Based Materials for Storage Applications.

Abstract

Interest in hydrogen as an alternative fuel has grown recently due, in part, to increasing global energy demands and environmental concerns. A challenge for commercialization of hydrogen fuel cells is storage of an adequate quantity of hydrogen on-board to match the range of current internal combustion power plants. Among several options, storage by adsorption is attractive because it has the potential to lower the overall system pressure for an equivalent amount of hydrogen, yielding a safer operating condition. In addition, adsorption is kinetically favorable compared to the absorption phenomena employed by most high capacity intermetallics.

Enhanced capacity spillover adsorbents have been synthesized using ultrasound assisted solution impregnation and bridge building techniques between common catalytic materials and novel nanostructured carbons. The impregnation method generates a metal dispersion of nearly 40% on nanoporous carbons. Bridge-building techniques can be applied with varied receptors to quickly augment spillover behavior of new materials. Capacity enhancements of up to 70% over molecular hydrogen physisorption on carbons have been realized at 10 MPa and 298 K.

A highly accurate, volumetric adsorption apparatus has been constructed and validated as a cost-effective means of high throughput screening for hydrogen spillover adsorbents at ambient temperature. Kinetic data can be collected for adsorption and desorption to facilitate comparison of the rates for both processes. This capability has directly led to the theory of different forward and reverse spillover mechanisms.

Isotopic tracer studies have been developed to evaluate forward and reverse spillover on carbon-based nanomaterials. A sequential dosing procedure has proven that hydrogen spillover proceeds at ambient temperature and follows a radial diffusion mechanism from the source particle with a diffusion coefficient on the order of 10^-15 cm2/s. Equilibrium dosing procedures, building upon kinetic results obtained with volumetric techniques, have pointed toward a mechanism for desorption whereby a portion of the adsorbed species recombine before reaching the source particle. Using kinetic results, a maximum reachable distance of 800 Å has been calculated for spillover on bridged composite nanostructured carbons. This information is a key parameter for optimizing the dispersion of metal nanoparticles on new materials to enhance capacity and improve kinetic response.