Researchers at the California Institute of Technology (Caltech) recently split water in a nontoxic and non-corrosive way. This series of chemical reactions could provide a possible new route to hydrogen-gas production.
A research group led by Mark Davis, the Warren and Katharine Schlinger Professor of Chemical Engineering at Caltech, describes the new, four-reaction process in the early edition of the Proceedings of the National Academy of Sciences (PNAS).
Since hydrogen’s combustion does not emit carbon dioxide into the atmosphere, there is some belief that it could even fuel a potential “hydrogen economy” an energy delivery system based entirely on this one gas. But, since there is no abundant supply of hydrogen gas that can be simply tapped into, this lighter-than-air gas has to be mass produced.
Hydrogen gas is most often produced by using excess heat (above 1,000°C). This process is difficult to mass produce and creates a number of toxic and corrosive liquid intermediates.
Hydrogen is a coveted gas. The industry uses it for everything from removing sulfur from crude oil to manufacturing vitamins. Credit: National Renewable Energy Laboratory
“We wanted to combine the best of both worlds,” Davis said. “We wanted to use solids, as they do in the high-temperature cycles, so we could avoid these toxicity and corrosion issues. But we also wanted to learn how to lower the temperature.”
The first step of the process is a chemical reaction between manganese oxide and sodium carbonate, activated through the addition of water. When water is introduced to the system it is split into hydrogen and oxygen molecules. In this cycle none of the hydrogen or oxygen is lost and the cycle can continue splitting water into the gasses. In the current paper, the researchers ran their newly created cycle five times to show reproducibility. It will be needed to show that the cycle can run thousands of times in order to be practical. Experiments of this type are beyond the capabilities currently in the Davis lab.
“We’re excited about this new cycle because the chemistry works, and it allows you to do real thermochemical water splitting with temperatures of 850°C without producing any of the halides or other types of corrosive acids that have been problems in the past,” Davis noted. Still, he is careful to point out that the implementation of the cycle as a functioning water-splitting system will require clever engineering. For example, for practical purposes, engineers will want some of the reactions to go faster, and they would also need to build processing reactors that have efficient-energy flows and recycling amongst the different stages of the cycle.
Figuring out ways to decrease the operating temperatures is at the heart of Davis’s interest in this project. “What we’re trying to ask is, ‘Where are the places around the world where people are just throwing away energy in the form of heat?’” he said.
Davis speculated that there could be a day when water-splitting plants are able to run on the heat given off by a variety of manufacturing industries such as the steel- and aluminum-making industries and the petrochemicals industries, and by the more traditional power-generation industries.
“The lower the temperature that we can use for driving these types of water-splitting processes,” he added, “the more we can make use of energy that people are currently just wasting.”