(Invited) Novel Strategies in the Design of the Oxygen Evolving Catalysts for Water Electrolysis

Abstract

Hydrogen producing electrolytic water splitting is one of the most important electrochemical technologies supporting the de-fossilization of the society and enabling integration of renewable energy sources into electric grid. The water electrolysis in fact combines two electrocatalytic reactions – a two electron hydrogen evolution and four electron oxygen evolution, which in most cases represents the kinetically limiting process determining the efficiency of the green hydrogen production. Given the harsh conditions at which the oxygen evolution is driven in water electrolysers a design of practical oxygen evolving catalysts needs to reflect not only the intrinsic activity, but also stability and feasibility. To reflect these additional factors a specific challenges connected with the pH at which the intended water electrolysis ought to operate need to be included into catalyst design. The development of the anode materials for alkaline water electrolyzers faces less restrictions given reasonable stability of the non-noble transition metal oxides/oxihydroxides at operando conditions. The development of novel anode materials for polymer electrolyte water electrolyzers is far more challenging due the low stability of transition metals at OER potentials in acid media which restricts the available catalysts to those based on scarce Ru, or Mn.The presented paper will describe two approaches attempting development of novel OER catalysts for acid media. The first approach aims at following the high entropy concept pioneered theoretically by Svane and Rossmeisl [1]. The synergetic effects encountered in high entropy oxides will be demonstrated on mixed Ru-Ir pyrochlores stabilized by lanthanides prepared by spray freeze freeze drying technique. While the high entropy oxides are highly active in OER the need to explore noble metal oxides may still represent a stumbling block in large scale deployment. This restriction may be removed by targeted activation of wide band semiconducting oxides by simultaneous n- and p- co-doping [2]. The co-doping strategy alters the electronic structure of the host oxide generating a pseudo-band in the materials band gap facilitating an electrocatalytic activity (in dark). The concept predicts that a wide band semiconductors based on , e.g. Ti, may be activated into a active OER catalysts potentially compensating a lower intrinsic activity with low price. The potential of the approach will be demonstrated on two systems – co-doped anatase polymorph of TiO2 and on co-doped cubic titanate based perovskites.