The targeted variation of oxide material properties by changing their composition is an integral part of everyday scientific work in the field of solid state ionics. This approach can be used, for example, to change the conductivity of an oxide for a specific charge carrier species, to influence the space charge that forms at an interface, or to manipulate the surface reaction kinetics of an electrode. The latter in particular has attracted a great deal of attention in recent years in several respects. On the one hand, the enormous variability of the reaction kinetics of mixed-conducting oxide electrodes has been linked to small changes in their surface chemistry, which has also opened up the possibility of compensation strategies for degradation phenomena. On the other hand, the concept of exsolution (which involves the intentional partial reductive decomposition of perovskite-type materials forming catalytically active surface particles) has emerged as an exceptionally compelling tool for preparing novel heterogeneous catalysts with highly attractive properties. What can be learnt from the investigation of such highly surface-sensitive systems is that materials with completely new electro-catalytic properties can be generated by applying only small changes that have their effect primarily on the surface.

In this contribution a number of various possibilities of manipulating the surface chemistry of mixed conducting electrodes will be explored:

(i) The perovskite-type electrode material (La,Sr)FeO_3-δ (LSF) is modified by application of tiny amounts of platinum. First, as a dopant by introducing Pt during the synthesis procedure of the oxide. This yields the remarkable result that Pt in LSF has no extra catalytic activity, but nevertheless enhances the electrode’s oxygen exchange kinetics. This at first glance peculiar behavior, however, can be explained by an influence of the noble metal dopant on the available number of redox-active sites. Second, Pt is applied as nanosized, metallic surface particles. In this form, its effect is completely different with the p(O₂) dependence of the oxygen reaction being even inversed compared to bare LSF.

(ii) The complex behavior of exsolution particles under electrochemical polarization of the corresponding parent oxide is highlighted. More specifically, the oxidation state and the associated catalytic activity of particles exsolved from ferrite-based thin-film electrodes are selectively switched by applying a bias voltage. Interestingly, in case of iron particles, the oxidation state of the particles is almost exclusively governed by the parent oxide, while nickel particles are significantly more influenced by the atmosphere. We explain the behavior with a 'kinetic competition' between the gas atmosphere and the applied electrochemical driving force with different oxygen transport kinetics in Fe- and Ni-based particles as a decisive factor. Additionally, we demonstrate voltage-controlled sequential exsolution of Ni and Fe nanoparticles, and extend our exploration to bimetallic exsolution systems with three distinct activity states.