Selectivity control in anodic gas evolving reactions on Ru based oxides

Abstract

Effective control of anodic gas evolving processes like, e.g. oxygen evolution and chlorine evolution is important from both fundamental as well as of applied point of view. The oxygen evolution process is of crucial importance in steering the water electrolysis needed for renewable energy storage. The chlorine evolution then is at core of chlor–alkali and chlorate applications. Both electrode processes take part at similar potentials and also are catalyzed by the same type of materials. It is generally known that kinetic hindrances of four electron oxygen evolution are not only limiting possible deployment of water electrolyzers in replacement of fossil fuels, but also are responsible for kinetic preference of chlorine evolution as prevailing anodic reaction in chloride containing solutions. An effective control of the catalyst selectivity in these competing reactions can be seen as the key enabler of the use of brackish or even sea water electrolytic hydrogen production. This workshop will discuss the selectivity tendencies of Ru based oxides in parallel chlorine and oxygen evolution reactions. on identification of key catalyst parameters needed to steer selectively gas evolving reaction on Ru oxides substituted with Mn and Ti prepared by the spray freeze freeze drying approach and their behavior in parallel oxygen and chlorine evolution reactions. The selectivity of these single phase materials with rutile structure based on RRDE and DEMS measurements will be rationalized in terms of chemical composition, local structure and prevailing mechanism of the oxygen evolving process. It will be shown that the overall electrocatalytic activity of Ru oxides increases with increasing content of the substituting transition metal. The increasing presence of Ti and Mn has, however an opposite effect on the selectivity. While the presence of Ti in substituted materials supports the selectivity towards CER the presence of Mn leads to an increase the selectivity towards oxygen evolution in acidic chloride containing solutions. The analysis of reaction orders for both competing reactions with respect to chloride concentration indicates that the overall selectivity is in fact controlled by the oxygen evolution process regardless of the chemical composition. The high selectivity towards oxygen evolution reaction coincides with low tendency of prepared phases to evolve oxygen via lattice oxygen activation (LOER). The experimental behaviour is further discussed from the theoretical point of view by DFT analysis of the reactivity of the Ru-Me-O surfaces where Me stands for Ti, Mn and Ni respectively.