Chemical engineers discover new way to split water
11 Jun 2012
Nontoxic, noncorrosive method provides possible new route to hydrogen-gas production
Researchers at the California Institute of Technology (Caltech) have, for the first time, discovered how to split water in a nontoxic, noncorrosive way at relatively low temperatures.
This could have significance in the production of hydrogen which, among other methods, can be created by splitting water.
Known as thermochemical water splitting, this method is appealing because it can take advantage of excess heat given off by other processes.
So far, it has been attempted in two ways: using two steps and taking advantage of high temperatures (above 1000°C) associated with solar collectors; or through multiple steps at “lower temperatures”.
Mark Davis, who led the project, was interested in the latter approach.
The four-reaction cycle begins with a manganese oxide and sodium carbonate, and is a completely closed system.
His first paper as a graduate student dealt with a low-temperature water-splitting cycle, called the sulphur-iodine system, which has since been piloted for use around the world.
Although that cycle operates at a maximum temperature of 850°C, it also produces a number of toxic and corrosive liquid intermediates that have to be dealt with.
The cycle’s high-temperature counterparts typically involve simpler reactions and solid intermediates—but there are very few processes that produce excess heat at such high temperatures.
“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 four-reaction cycle the team came up with begins with a manganese oxide and sodium carbonate, and is a completely closed system: the water that enters the system in the second step comes out completely converted into hydrogen and oxygen during each cycle.
This is important because it means that none of the hydrogen or oxygen is lost, and the cycle can run over and over, splitting water into the two gases.
There could be a day when water-splitting plants are able to run on the heat given off by a variety of manufacturing industries
“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,” said Davis.
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.
Davis believes 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 aluminium-making industries and the petrochemicals industries, and by the more traditional power-generation industries.
