Cover of the March 2016 Advanced Materials journal. (A.M. El-Sawy et al., “Oxygen Reactions: Controlling the Active Sites of Sulfur-Doped Carbon Nanotube–Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis,” Advanced Energy Materials, 2016, Vol. 6, no. 5. © Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)
A new material could make H capture more commercially viable and provide a key element for a new generation of cheaper, light-weight hydrogen fuel cells. The new metal-free catalyst uses carbon graphene nanotubes infused with sulfur. Producing high-grade hydrogen is an expensive and energy-consuming process. So a cheaper and more efficient way of capturing hydrogen would go a long way toward the creation of a sustainable hydrogen economy, and would help reduce the world’s reliance on fossil fuels.
CurrentH production uses intense heat to separate hydrogen from hydrocarbons found in crude oil. But the resulting hydrogen isn’t very pure, and byproducts must be scrubbed out.
An alternate process, capturing hydrogen in water, is cleaner and more sustainable, but it too has limitations. Electrocatalysts involved in this process are usually made of rare earth metals like platinum and iridium. But they are very expensive, making the commercialization of pure hydrogen fuels difficult.
Finding a non-metal catalyst that has all of the electrochemical properties of the rare earth metals but can be made at a much reduced cost and still remain stable has been a goal of scientists for years. Suib and Rusling knew that sulfur-infused carbon graphene nanotubes were a potentially efficient non-metal catalyst for an oxygen reduction reaction. An ORR happens when O2 and H2 are converted to water. The reaction is a key component of hydrogen-based fuel cells. H2 used to power the cells passes through a catalyst, currently a corrosive-resistant metal like platinum, causing an oxygen reduction electrochemical reaction that creates energy and, as a byproduct- water.
But reversing that process starting with water and extracting pure hydrogen from it, a procedure known as an oxygen evolution reaction is much more of a challenge electrochemically. The key was manipulating the sulfur and carbon atoms to create stable bonds and structures within the nanotubes, while also maintaining or improving the tubes’ electrochemical potential so that it mirrored those found in the rare metals.
The new process uses a dual doping procedure involving sulfur and benzyl disulfide treated at high heat. The researchers had to carefully add heteroatoms of sulfur at extremely low levels to strike the delicate balance needed to maintain usability and stability. Add too much sulfur and the sample would be unstable; not enough and it would be ineffective.
In the end, the S-doped nanotubes used much less energy in the chemical reaction process than other known processes, and were much more active and efficient catalysts than other known products. They are efficient for both separating hydrogen from water and reducing oxygen into water. Materials with those dual properties are rare, he notes.
http://today.uconn.edu/2016/05/new-catalyst-found-clean-energy-fuel/
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