Hydrogen Storage Materials
EMSL Project ID
10491a
Abstract
The increasing demands for clean energy sources that do not add more carbon dioxide and other pollutants to the environment have resulted in increased attention worldwide to the possibilities of a "hydrogen economy" as a long-term solution for a secure energy future based on potentially renewable resources. Some of the greatest challenges are the discovery and development of new on-board hydrogen storage materials and catalysts for fuel cell powered vehicles. New materials that store both high gravimetric high volumetric densities of hydrogen that release H2 at temperatures <100 C and uptake H2 at pressures < 100 bar are highly desired. The volumetric constraints eliminate from consideration pressurized hydrogen systems and guide towards the development of solid storage materials. There are no currently known materials that meet these requirements. As such, there is a need for fundamental understanding of the chemical and physical properties of hydrogen rich materials (HRM). What molecular attributes facilitate the release and uptake of molecular hydrogen?We recently suggested that efficient storage of hydrogen might be accomplished in compounds that have alternating electron rich and electron deficient sites capable of covalently binding H+ and H-, respectively. There are two fundamental premises that will guide us towards the discovery of novel HRM that are operational at temperatures between ambient and 100˚C: (1) binding of hydrogen requires formation of chemical bonds, as physisorption of H2 is too weak (2) inherent polarity of low molecular weight species bearing electron rich and electron deficient sites will likely result in the formation of molecular solids.
These guidelines led us to initially consider the ammonia borane (AB = NH3BH3). AB is isoelectronic with ethane. This inorganic analog of ethane yields far more favorable volumetric densities, as it is a solid (m.p. 115 C) rather than gas. This molecular crystal is formed from the reaction of ammonia with diborane to form a dative bond between the electron deficient B and the electron rich N (NH3-->BH3). The molecular crystalline solid is further composed of a network of dihydrogen bonds formed between the protic H+ attached to nitrogen and hydridic H- attached to boron.
Preliminary experimental results showed the rates of hydrogen release from the bulk phase solid AB follows an apparent nucleation and growth kinetic model. However, little is known about the nucleation events and the role of the intermolecular dihydrogen bonding in the formation of molecular hydrogen. Solid 11B{1H} NMR spectra of these reactions taken at 300 MHz 1H frequency are shown in Figure 1. These studies have aided in determining reaction mechanisms, however, some products remain unidentified due to spectral overlap. Higher field is necessary to enhance resolution and reduce the quadrupolar coupling (see Figure 2) to positively identify all reaction products as a result of hydrogen release.
In addition to investigating the reaction utilizing the 11B nucleus, we have incorporated 2H into AB, both ND3BH3 and NH3BD3 for kinetic analysis, and we have reacted NH3BH3 with 2H2 (rather than 1H2) to determine the reversibility of the H2 reaction (Figure 3). Our current NMR experiments have utilized a 300 MHz instrument to great advantage, though obvious limitations have arisen. The spinning 2H spectrum is very broad due to the quadrupolar interaction (Figure 3), further complicated by spin-spin interactions, and would greatly benefit from investigation at higher field. The higher resolution at 900 would enhance the ability to interpret the spectra and elucidate mechanisms.
Thus, we propose to investigate the reaction products of the NH3BH3 family of compounds with 1H2 and 2H2, using the 900 MHz instrument to provide maximum sensitivity and resolution. Our initial efforts will focus primarily on completing our product identification using 11B NMR. If additional time is available, 2H NMR will be utilized to investigate the reversibility of hydrogen release and the structure of the crystal. The development of hydrogen storage materials is identified as a Grand Challenge and understanding the fundamental mechanisms is a necessary step in their development. Obtaining NMR data of ammonia borane at high field will allow us to further quantify the mechanism used in hydrogen release and uptake, providing critical data to assess the fundamental chemistry of this potentially important hydrogen storage material.
Project Details
Project type
Capability Research
Start Date
2005-06-01
End Date
2006-04-24
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members