Exsolution has gained attention as a versatile electrode functionalization method for solid oxide electrochemical cells ¹. One of the key advantages of exsolution is the possibility to exsolve alloys or multielement nanoparticles, which can unlock unprecedented electrocatalytic functionalities. However, controlling exsolution of several elements aiming at equimolar metallic concentrations in the nanoparticles entails several mechanistic hurdles, governed by the kinetics of cation reduction. In order to understand in deeper detail the governing principles of multielemental nanoparticle exsolution, Sr₂Fe_1.2Co_0.1Ni_0.1Cu_0.1Mo_0.5O_6-δ, perovskite is selected as a model.

In this work, we have evaluated the impact of temperature and time on (1) the size and the population of the nanoparticles and (2) the composition of the exsolved nanoparticles. Short exsolution times (2-6 h) enabled the formation of nanoparticles mainly composed of Cu-Ni metals, while longer times (24 h) generate Janus ² type nanoparticles at similar treatment temperatures. Janus nanoparticles have the special feature of having two distinguished compositional zones, one mainly composed by Cu while the other side has Co, Ni and Fe. Lower exsolution temperatures (700 ºC) give nanoparticles composed mainly of Cu and Ni, while higher temperatures (900 ºC), for the same time and atmosphere, lead to nanoparticles composed of the four metals but mainly of Fe. These results could be a great step when tuning the nanoparticle composition since Fe has been reported to have the most energy-demanding exsolution metal among the studied ³. Other relevant tests on this material help to comprehend how oxidation steps, wet exsolution, or electrode exsolution work, with encouraging results such as morphological changes and optimized nanoparticle population when in situ electrode exsolution is performed.

Finally, the studied material has been characterized as electrode in solid-oxide fuel cells in a symmetrical configuration, where the polarization resistance was 2.94 Ω cm² after 24 h of exsolution, characterising their electrochemical behaviour in both cathodic and anodic operation. When tested in non-symmetrical fuel cells, as anode, polarization resistances as low as 0.85 Ω · cm² were reached  700 ºC.

This work shows the high tunability that could be achieved in multielemental nanoparticle exsolution, mainly in terms of composition but also in terms of size, shape and exsolution conditions. These results highlight the influence of processing parameters in the composition of the multielemental nanoparticles, providing guidelines for compositional fine tuning, with high influence in electrocatalytic activity and selectivity.