Researchers at the CEA, CNRS and Joseph Fourier University, in Grenoble and Saclay, have developed two new cobalt-based materials that may be used to replace platinum, a rare and expensive metal, in producing hydrogen from water (electrolysis). One of them works in aqueous solutions with neutral pH . The other is the first catalytic material ever created that is 'commutable' , contains no noble metals and can be used in the two chemical reactions essential for water electrolysis: the production of hydrogen and the production of oxygen.
The results are published in Nature Materials and Nature Chemistry.
Renewable energies (sun and wind, etc.) are primary sources of energy which are irregularly distributed across regions and available only intermittently. For these reasons, it is crucial to develop ways to store the energy produced. Producing hydrogen by water electrolysis is a promising solution, but one that requires catalysts containing 'noble' metals, such as platinum. The fact that such metals are rare and expensive has blocked the sustainable economic development of hydrogen technology.
The bio-inspired chemistry is based on chemical processes that take place in certain living organisms. Such organisms have enzyme systems, called hydrogenases, which make exclusive use of cheap metals found in abundance in nature, which enable them to use hydrogen as a source of energy or to produce hydrogen from water. For many years, the researchers have drawn inspiration from such enzymes to develop new molecular catalysts, without platinum, but based on metals that are both cheap and found in abundance in nature (including iron, nickel, cobalt and manganese).
However, to be able to use these synthetic catalysts in technological developments, they must, just as for platinum, be fixed on electrodes in very large quantities. In 2009, the research teams at the CEA, CNRS and Joseph Fourier University successfully immobilized one of these bio-inspired catalysts, which was nickel-based, on carbon nanotubes. However, this material is only active in very acidic environments. In addition, two reactions are involved in electrolysis: hydrogen production and oxygen production, and, to do without platinum for either of these reactions it is necessary to work in neutral to basic pH conditions. Using the same approach as in 2009, but this time developing a new bio-inspired, cobalt-based catalyst, the same teams have made a major breakthrough thanks to the material capable of working in pH neutral aqueous solutions. The catalytic activity achieved is extremely stable in the long-term, with an incredibly strong bond between catalyst and nanotube.
The researchers have gone even further than this and, at the same time, have developed a second material, also cobalt-based. This is made up of cobalt nanoparticles coated in a cobalt oxo-phosphate. This material, which works in pH neutral water, is quite remarkable since it exists in two forms is able to commute between the two and acts as catalyst in either the production of hydrogen (H2), or in the other essential electrolysis reaction, the production of oxygen (O2) from water. This makes it the first ever "commutable" or "Janus" catalytic material not based on noble metals. As well as replacing platinum, it can also act as an extremely stable catalyst in the production of hydrogen from pH neutral water.
These new cobalt-based materials may potentially be used in developing stable and inexpensive hydrogen production technologies, as a storage "solution" for renewable energies. The researchers are now looking at ways to integrate them into an overall artificial photosynthesis system, enabling the totally renewable production of hydrogen from water and solar power.
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