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Nanocrystalline oxides for selective oxidative electrocatalysis

Final Report Summary - NOSOE (Nanocrystalline oxides for selective oxidative electrocatalysis)

The goal of the proposed project was to combine the latest developments in the fields of materials chemistry and fundamental electrocatalysis to:
a) elucidate the mechanisms and active sites of the anodic gas evolution reactions on oxide electrocatalysts; and
b) use advanced synthetic approaches to synthesise a new class of oxide based electrocatalytic materials with controlled activity and selectivity.

Working on the project we achieved the following results. We have optimised the synthetic procedures for preparation of RuO2 based electrocatalysts heterostaticaly doped with Ni, Co and Fe by co-precipitation method with particle size ranging between 3 and 40 nm. The newly implemented supercritical fluid synthesis and spray -freezing freeze-drying approach were used for synthesis of RuO2 with Cr and Zn substitution respectively. The characterisation by X-ray diffraction method shows that the substation range extends to 20 at.% of doping for the systems under consideration.

The heterostaticaly doped electrocatalytic materials were tested with respect to their electrocatalytic activity towards oxygen and chlorine evolution in acid media. As a rule, the substitution changes the selectivity towards the oxygen evolution, this trend enhances at more positive potentials. The observed trend may be associated to structural changes forced by the doping process. The importance of the structural effects is experimentally reflected in the behavior of Zn doped RuO2, which shows unusually high selectivity for oxygen evolution even at significant chloride concentrations.

Heterostaticaly doped ruthenium oxides were characterised by static (ex-situ) X-ray absorption spectroscopy. The EXAFS spectra were used to formulate local structure model describing distribution and bonding arrangements of the cations in the oxide electrocatalysts. The Ru oxidation state in doped electrocatalysts exceeds that in non-doped RuO2 (IV); the observed change of the oxidation change, however, indicates a pronounced oxygen non-stoichiometry. The EXAFS spectra of Fe, Co, Ni and Zn doped ruthenium dioxide samples revealed that local structures of the materials in the vicinity of doping elements differ despite the anticipate similarities in the structural chemistry of 3d metals.

The key trend traced in the cationic sub-lattice arrangements is the tendency of the metal cation to maintain maximum possible coordination. This leads to agglomeration of the doping cations into clusters of variable size coherent with the RuO2 host with face sharing of the coordination polyhedra. Detailed structural models were formulated for electrocatalysts in Ru-Ni-O Ru-Co(Fe)-O and Ru-Zn-O systems.

We have used in-situ X-ray absorption spectroscopy measurements to identify the prospective active sites on the oxide catalyst surface. It was found that during oxygen evolution process, the dominating surface species are oxo- and hydroxo- groups bound to the coordination unsaturated sites of Ru atoms on surface. The formation of the bridging peroxo groups predicted by theory has not been observed. It indicates that formation of oxo groups is a rate limiting step of the whole process of oxygen evolution and the fast release of peroxo species from the surface does not allow for experimental detection.

The results obtained within the project were disseminated in the form of four research papers in the high profile international journals and nine oral presentations at the international conferences. Three more scientific papers are expected to be submitted / published during 2011. It may be expected that the results of the will be directly affecting the development of catalysts for reversible fuel cells needed to mitigate poor controllability of renewable source based on wind and solar energy.