The power of economy

Michael Ward   Omar Yaghi

New metal-organic frameworks could make the transport, storage, and delivery of hydrogen much easier and open the door to what has been described as a hydrogen-based economy.

As Michael Ward of the University of Minnesota in Minneapolis points out in a recent issue of Science, concerns about dwindling petroleum reserves and climate change have led to a resurgence of interest in alternative fuel technologies. Hydrogen-based fuel cells, for instance, could be a major component of a fuel mix for the future but before then the problems of safe transport and storage of hydrogen will have to be solved.

Hydrogen has many advantages over hydrocarbons, one being that it has three times the energy density. Additionally, when reacted with oxygen, either in burning or in a fuel cell, the only by-product is water, which is important for reducing pollution in urban areas. Omar Yaghi of the University of Michigan in Ann Arbor is on record as explaining that, "It is becoming increasingly urgent to produce materials designed to perform highly specific and cooperative functions for use in technologies such as fuel-cells on a chip, regio- and stereo-selective catalysis, porous materials for highly selective separations, nanosensors, and molecular electronics."

Porous materials could store hydrogen Click image to magnify

There have been many different types of materials investigated as possible safe hydrogen storage media, among them high-surface-area carbon materials, such as single-walled nanotubes. Capacities reported have varied wildly.

Now, Yaghi and colleagues at the University of Michigan in Ann Arbor, the University of California at Santa Barbara, Los Alamos National Laboratory, New Mexico, and Arizona State University in Tempe have developed a novel class of porous materials that can adsorb large quantities of hydrogen. Within the framework, hydrogen molecules perch themselves on the linkages within the scaffold-like structure. When the pressure drops, perched hydrogen can fly from the molecular coop. The materials are based on a metal-organic framework composed of Zn₄O(BDC)₃ where (BDC = 1,4-benzenedicarboxylate) and the material has a cubic three-dimensional extended porous structure.

Fuel cell cars could benefit from hydrogen storage materials

With further tweaking, for instance swapping the benzene perches for naphthalene ones, the researchers say that their uniform metal-organic frameworks could store and release hydrogen at concentrations suitable for powering hydrogen-fueled vehicles, portable electronics, and other power-hungry devices.

Science, 2003, 300, 1127-1129;
www.sciencemag.org/cgi/content/full/300/5622/1127