For some years, the storage and the deployment of renewable energies have been considered as one of the main ways to tackle climate change. However, the intermittent nature of these energy sources makes it necessary to develop solutions to store the energy produced. One way to do this is to store it in the form of a chemical vector: Hydrogen. Chemical transformation processes can valorize this element, as for fertilizer production.
To store energy in the form of hydrogen, scientists are working on the water electrocatalysis that requires the use of catalysts from noble metals (Ir, for example), which is expensive. Moreover, in catalysis, the activity improvement of a catalyzer often goes hand in hand with a decrease in its stability.
To solve this dilemma, a team of researchers, one of whose members, Alexis Grimaud, is part of the RS2E, proposes not to go against the activity/stability correlation, but to develop strategies to destabilize inactive systems and create new performing and robust catalyzers. Their works were published in the journal Nature Catalysis.
Destabilize an inactive phase to make it active
Previous studies have shown that some transition metal oxides, such as cobalt oxides, would reconfigure under certain conditions into oxyhydroxides, which are stable and very active in catalysis. The issue is to understand how to allow the surface reconstruction that occurs.
The article published in Nature Catalysis focus on CoAl2O4, which is inactive under its oxide form. The researchers propose substituting Al with a little amount of Fe in order to destabilize the material and facilitate its restructuration.
This idea pays off because this substitution leads to the formation of the oxyhydroxide. The researchers demonstrate, using several analytical techniques, that the surface reconstruction is eased by the pre-oxidation of cobalt and by the modification of the energy states of the oxygen atoms in the oxide. This allows for greater structural flexibility.
Figure 1: Reconfiguration of the CoAl204 oxide surface and role of Fe3 + cations in facilitating this phenomenon
Going further, the research team shows that the partial substitution of Al by Fe leads to a different reconstruction mechanism that allows stable surface chemistry once the oxyhydroxide has formed. Fe would allow an additional deprotonation reaction in the final phase making certain oxygen atoms active during the catalysis for a lower potential: This increases the efficiency of the hydrogen synthesis in water.
Those results open the way to alternative strategies to control the formation of new active sites in the oxides by playing on the different dynamics of reconfiguration. This method could permit the elaboration of more robust catalyzers.
Iron-facilitated dynamic active-site generation on spinel CoAl 2 O4 with self-termination of surface reconstruction for water oxid ation
Tianze Wu, Shengnan Sun, Jiajia Song, Shibo Xi, Yonghua Du, Bo Chen, Warhana Aji Sasangka, Hanbin Liao, Chee Lip Gan, Günther G. Scherer, Li Zeng, Haijiang Wang, Hui Li, Alexis Grimaud and Zhichuan J. Xu
Nature Catalysis, 05/08/19, DOI : 10.1038/s41929-019-0325-4