Hydrogen Technology

The potential for hydrogen use for transportation was brought to national attention in President Bush's 2003 State of the Union Address:

“A simple chemical reaction between hydrogen and oxygen generates energy, which can be used to power a car producing only water, not exhaust fumes. With a new national commitment, our scientists and engineers will overcome obstacles to taking these cars from laboratory to showroom so that the first car driven by a child born today could be powered by hydrogen, and pollution-free. Join me in this important innovation to make our air significantly cleaner, and our country much less dependent on foreign sources of energy.”

Figure 1: Click to enlarge

Table 1: Click to enlarge

Implementation of the hydrogen economy requires considerable progress in three key areas of research: renewable production of hydrogen, hydrogen storage, and PEM fuel cells. The hydrogen storage problem is thought to be the most difficult challenge as materials that store hydrogen at a volume density much greater than highly compressed or even liquid hydrogen are required to meet the DOE targets1 as illustrated in Figure 1. Potential storage media are described in Table 2.

Complex hydrides (such as NaAlH4) have the advantages of reversibility, high storage density, and acceptable release pressure, although they suffer from slow kinetics and thermal management problems. Our projects focus on investigating the size effects on the thermodynamics and kinetics, hydrogen diffusion, and heat transfer of hydrogen absorption by both first principles investigations and numerical simulations.

Nanostructures for Hydrogen

Nanostructures form an important component in developing hydrogen storage materials since size effects have a direct effect on key properties such as the storage capacity, sorption / desorption thermodynamics and kinetics, and mass and heat diffusion rates. For example, ball milling hydride media can lead to increased absorption kinetics and decreased desorption temperature2.

Figure 2: Click to enlarge

Thermal management is a significant issue since the hydriding reaction due to enormous heat generation (~1 MW), low effective thermal conductivity (~0.1 W/m-K), and stifling of the hydriding reaction with increased temperature3. First principle investigations based on thermodynamics and surface chemistry at the molecular level will be used to model the effect of the nanostructure on the energetics of absorption and stability of hydride phase. Density functional theory will then be used to validate these models as well as to study the effect of catalysts and lattice impurities on the hydrogen bond strength and heat of absorption. Simulations will include reaction kinetics, and heat and mass transfer as shown in Figure 2.

Mass diffusion will be investigated using molecular dynamics simulations while Monte Carlo simulations will be used for heat transfer. Complicated interactions and optimization tradeoffs exist between modeled phenomena: the sorption/desorption kinetics are strongly influenced by the temperature response, the hydriding reaction may be diffusion limited4, while the effect of varying the nanostructure of mass and heat diffusion have very different trends. These tools are anticipated to lend a fundamental understanding to the hydrogen storage problem to enable the discovery of storage materials with high hydrogen density, fast absorption kinetics, and other desirable properties.

References

  1. U.S. Department of Energy, Office of Basic Energy Sciences, 2003, Basic Research Needs for the Hydrogen Economy
  2. Applied Physics A 72 (2001) 157165
  3. J. Heat Transfer, 127 (2005) 1391-1399
  4. Int. J. Hydrogen Energy, 28 (2003) 529-536