Structure, Selectivity and Electrocatalytic Activity of Iridium-Based Oxides for Oxygen Evolution
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Iridium dioxide-based electrocatalysts were chosen as main object of the study because these materials are well-known in electrochemical technology as active and durable in a number of anodic reactions. However, some challenges regarding utilization of these materials in PEM water electrolysis still exist. For example, the problem of formation of potentially corrosive intermediates during electrochemical reactions and poor selectivity towards oxygen evolution in chloride-containing media, which can lead to product contamination, need to be solved. The aim of this thesis is to improve the understanding of the relationship between the nature of the electrocatalysts (composition, morphology and local structure) and electrochemical processes proceeding on these materials. Therefore, extensive electrocatalyst characterization has been performed in connection with spectroscopic and electrochemical characterization of these materials. During oxygen evolution at large overpotentials and under dynamic conditions the formation of two reaction byproducts was detected. The results indicate that these byproducts are hydrogen peroxide and hydroperoxy species, which are potentially corrosive for the electrode and membrane materials. The formation of the unwanted OER byproducts can be minimized by rational choice of the electrode materials, based on an analysis of binding energy of the reaction intermediates, which should be neither too strong nor too weak or by operating the electrolysis process in steady-state rather than under dynamic conditions. Parallel oxygen and chlorine evolution reactions were studied on the Ni, Co and Zn – iridium dioxide synthesized by the hydrolysis and solvothermal methods. It was shown that the method of synthesis applied and the doping element and its amount in the iridium dioxide were all modifying the local structure of the materials, which was characterized by extended X-ray adsorption spectroscopy. The influence of the presence of chloride salts in the solution was investigated. Results indicate that oxygen and chlorine evolution are competitive reactions, the reactions proceeding on the same active catalytic sites, which can be modified by the heterovalent substitution of the iridium atoms in the rutile-type iridium dioxide. These changes in the local structure alter the electrochemical activity and selectivity of the iridium dioxide-based materials, which were studied by combining electrochemical techniques with differential electrochemical mass spectroscopy. In most cases it was observed that the introduction of Co and Zn shifts catalyst selectivity towards oxygen evolution, whereas at low chloride concentrations incorporation of Ni promotes chlorine evolution.