Kinetics of hydrogen storage on catalytically-modified porous silicon
Graphical abstract
Introduction
Efficient and reversible storage is the primary challenge to a Hydrogen Economy. Hydrogen is 6.9 times lighter than Lithium per unit electric charge providing a fundamental advantage over electrochemical battery technology. Of course hydrogen storage must be paired with a fuel cell to compare directly to a battery, so the ultimate theoretical advantage over lithium metal batteries will be moderated, perhaps an advantage of a factor of 3.
Ultra-high pressure storage of hydrogen in gaseous form incurs a heavy penalty in parasitic energy loss needed for compression. Widespread gaseous use implies large central hydrogen generation plants with vast networks of specialized pipelines. The enormous capital expense of such infrastructure, needed in advance of market adoption of hydrogen-fueled vehicles and home fuel cell applications presents a dilemma from which there is no obvious emergence. Cryogenic storage in liquid form requires energy-intensive refrigeration and substantial insulation to reduce boil-off. While useful for large rockets, liquid storage is impractical for vehicles or for diffuse distribution. Solid state storage in metal hydrides is handicapped by the mass of the metals required, and by the highly exothermic nature of the recharge process demanding either slow absorption or complex cooling mechanisms.
A novel method of solid state hydrogen storage using porous silicon has been proposed and studied [1], and is now the subject of four US patents. Silicon bonds with hydrogen atoms at energies lower than does carbon, and hydrogen has been shown to bond-hop along silicon surfaces. Porous silicon can be synthesized with very high surface area. The greatest energy barrier in such a system is the dissociation of gaseous dihydrogen () into atomic hydrogen, which can be mediated by the introduction of a catalyst. Strategic placement of a catalyst such as palladium at pore mouths provides a reversible pathway from dihydrogen gas to dissociated hydrogen on clusters of catalyst which transfers to the silicon surface via spillover and then bond-hops along the surface to form a solid state hydrogen storage reservoir. This system is charged by gas pressure and discharged by temperature. An additional discharge mechanism is available via infra red light which passes through lightly-doped silicon but is absorbed by the SiH bond.
Herein the kinetics of this reaction pathway are studied using Density Functional Theory (DFT). In this way the energy barriers for each of the three steps can be studied individually, namely: dissociation, spillover, and bond-hopping. In the composite the charging rate of catalytically-modified porous silicon can be estimated. These detailed calculations are compared with first-order macroscopic kinetic calculations to provide support for the feasibility of the results.
Section snippets
Chemical kinetics
In this proposed storage system gaseous hydrogen goes through a series of equilibrium reactions that form different types of compounds that bond silicon and hydrogen. At the beginning, molecular hydrogen (dihydrogen) is dissociated into atomic hydrogen by the palladium catalyst.
These hydrogen ions are mobile and can spillover onto the dangling or vacant bonds available on the silicon surface, creating the SiH.
The net chemical reaction, mediated by the catalyst, is as follows:
Catalyst
In as-synthesized nanoporous silicon (npSi) based solid state hydrogen storage systems, palladium is used as catalyst for its ability to reduce the substantial gaesous dihydrogen () dissociation energy barrier. In this npSi, SiH and Si are most prominent, with the presence of a small number of Si bonds [5] where the silicon atom is bonded to only one neighboring silicon atom and are therefore the structurally weakest spot. These weak spots are supposed to give away dihydrogen gas first
Discussion
Experimental results from TPD suggest that zero-point energy is not active in the surface diffusion of hydrogen on nanoporous silicon because the derived is 1.81 eV which is just 3.8 percent greater than the DFT computed first hop energy of 1.74 eV. Because both hops are needed for long-distance movement of hydrogen atoms the higher energy barrier will dominate the transport. Computing diffusion coefficients from Eq. (11) yields values of diffusivity in the range of at 376 C which
Conclusions
The kinetics of recharging hydrogen gas onto catalytically-modified nanoporous silicon (npSi) has been studied with a combination of Density Functional Theory (DFT) and laboratory testing. These results have been applied to the sequence of gaseous dissociation onto a catalyst cluster, spillover onto the npSi support or matrix, and bond-hopping along the interior surfaces of pores which have been created electrochemically within silicon crystal. The rate-limiting step is site-to-site diffusion
Acknowledgments
This work was funded in part by NSF grant 1648748.
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Current address: Electrical and Computer Engineering Department, Brigham Young University, Provo, UT 84602, USA.