A fully baked idea: Hydrogen storage

Baking up hydrogen

Many technology pundits suggest that hydrogen produced from sustainable sources could lead us into a low-carbon economy that derives its energy from renewable s rather than fossil fuels. NMR spectroscopy has played a part in experiments that demonstrate how so-called "baking soda" might help researchers cook up a safe way to store this hazardous gas.

Although the incentives for developing safe and useful hydrogen storage materials are enormous and the efforts immense, there have so far only been a few approaches that appear viable in the laboratory. Cost and ease of use on a commercial scale often stymie the progression from benchtop test to real-world prototypes. However, writing in the Wiley journal Angewandte Chemie, Matthias Beller and his colleagues at the Leibnitz Institute for Catalysis in Rostock, Germany, suggest that they have found a novel approach to hydrogen storage that uses simple salts of formic acid and carbonic acid rather than sophisticated porous structural materials.

The team working on this novel approach comprises Beller, Albert Boddien, Felix Gaertner, Christopher Federsel, Peter Sponholz, Doerthe Mellmann, Ralf Jackstell and Henrik Junge. They explain that for hydrogen storage materials to be practical they must be able to adsorb, or take up, hydrogen at standard pressure and room temperature and to release their energy-laden payload under the same conditions. The material must also be able to hold as great a volume of the gas as a conventional pressurised cylinder would contain but without the attendant hazards. Until now, metal hydride tanks have fit some of this set of criteria but with the major disadvantages that they are very heavy and expensive and operate only at high temperatures and/or are far too slow to take up and release hydrogen.

Parallel chemistry

In parallel research to the development of hydrogen storage, chemists have looked at molecular storage in the form of methane gas and liquid methanol, which offer a chemical storage for hydrogen atoms that can be tapped into. Similarly, formic acid (HCO2H) and its salts, so-called formates, have been considered as materials that can "store" hydrogen and from which the gas might be generated on demand. This approach thus acts as a chemical store rather than a physical transport medium for hydrogen. There is an intrinsic problem with using such a molecular approach and that is to separate the carbon dioxide released when the hydrogen is chemically extracted from a formate.

In 2008, the Rostock team and that of Laurenczy et al independently demonstrated smooth hydrogen release fromformic acid/amine adducts under mild conditions with a ruthenium catalyst. Other groups have since experimented with other homogeneous and heterogeneous catalysts for hydrogen release from formic acid. Most of the experimental catalysts have used noble metals, but even iron complexes seem suitable in some instances.

The Rostock team believe they have now successfully used a ruthenium catalyst to accelerate the both the uptake and the release of hydrogen from formats but with the benefit of establishing a reversible, hydrogen storage cycle free of the carbon dioxide burden. In their system, the hydrogen is released from non-corrosive and non-irritating formates and the carbon dioxide produced as a byproduct of this process is trapped in the form of a bicarbonate.

Hydrogen storage cycle

"Our new concept has a number of advantages," explains Beller, "in comparison to carbon dioxide, solid bicarbonate is easy to handle and is very soluble in water. The resulting bicarbonate solution can be catalytically converted to a formate solution under much milder conditions than those required for the reactions to form methane or methanol." The hydrogen gas can be retrieved at room temperature or even cooler. "Most important is that a closed carbon cycle is now possible because the resulting bicarbonate can simply be loaded up with hydrogen again," adds Beller. The team points out that the essentially harmless bicarbonate solid can be stored or transported easily until needed.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.