One of the biggest barriers towards establishment of a "hydrogen economy" is the availability of hydrogen gas. Although it is the most abundant element in the universe, on earth it is always present as part of a compound, which must be broken apart to retrieve it. This requires energy, and if hydrogen is to be a low carbon energy carrier, that energy must itself be low carbon. Currently, most hydrogen is produced from hydrocarbons or by electrolysis of water, both of which generally use energy generated with the production of carbon dioxide.
The Newcastle team, led by chemical engineer Prof Iain Metcalfe, has developed a process which avoids mixing reactant gases by transferring oxygen between reactant streams via a solid-state oxygen reservoir. Reacting water and carbon dioxide to generate hydrogen and carbon monoxide, the process operates close to an equilibrium state, resulting in a pure stream of hydrogen which does not require additional separation processes.
The oxygen reservoir retains a "chemical memory" of the conditions to which has been exposed, and therefore Metcalfe has dubbed it a hydrogen memory reactor. In a paper in Nature Chemistry, he and colleagues from the Universities of Durham and Edinburgh, along with European synchrotron research facility in France, describe how the reactor can be powered by renewable energy, making it a source of zero carbon hydrogen.
“Chemical changes are usually performed via mixed reactions whereby multiple reactants are mixed together and heated. But this leads to losses, incomplete conversion of reactants and a final mixture of products that need to be separated," Metcalfe said. “With our Hydrogen Memory Reactor we can produce pure, separated products. You could call it the perfect reactor.”
The oxygen reservoir is composed of a perovskite, a type of transition metal-containing mineral more often associated with solar cells. “This reservoir is able to remain close to equilibrium with the reacting gas streams because of its variable degree of non-stoichiometry and thus develops a ‘chemical memory’ that we employ to approach reversibility” the researchers explain.
Another advantage of the system is that it accomplishes all the stages of the reaction in a single unit, unlike conventional hydrogen production which requires two reactors and a separation stage, Metcalfe explained. This is particularly significant because the purification stage of traditional processes is often costly and energy intensive. Because the gaseous reactants never mix, carbon cannot be carried over into the hydrogen stream, avoiding contamination of the product which is a serious problem if the hydrogen is to be used, for example, in a fuel cell. The system also produces an unusually high conversion rate, unlike other processes where only a relatively small percentage of the hydrogen in the water stream is separated.
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