Understanding electrochemical switchability of perovskite-type exsolution catalysts

Abstract

Exsolution of metal nanoparticles from perovskite-type oxides is a very promising approach to obtain catalysts with superior properties. One particularly interesting property of exsolution catalysts is the possibility of electrochemical switching between different activity states. In this work, synchrotron-based in-situ X-ray diffraction experiments on electrochemically polarized La0.6Sr0.4FeO3-δ thin film electrodes are performed, in order to simultaneously obtain insights into the phase composition and the catalytic activity of the electrode surface. This shows that reversible electrochemical switching between a high and low activity state is accompanied by a phase change of exsolved particles between metallic α-­Fe and Fe-oxides. Reintegration of iron into the perovskite lattice is thus not required for obtaining a switchable catalyst, making this process especially interesting for intermediate temperature applications. These measurements also reveal how metallic particles on La0.6Sr0.4FeO3-δ electrodes affect the H2 oxidation and H2O splitting mechanism and why the particle size plays a minor role.

Fabrication and Characterization of a Tubular Solid Oxide Fuel Cell with Impregnated Perovskite Electrodes

“…However, Ni metal nanoparticles have a very high catalytic activity towards H2 adsorption and dissociation (7), making the perovskite with Ni exsolution exhibit improved performance. In this case, hydrogen will absorb and dissociate to hydrogen species on the Ni and diffuse to the LCNT surface, bypassing the step described by reaction 3 (15). The open-circuit voltage (OCV) of the tubular cell after switching at 700-800 °C was above 1.1 V at the whole temperature range, suggesting a good tightness of the cell (Figure 3a).…”

“…Hydrogen could chemisorb on the MIEC absorption vacant site, which number among others depends on the concentration of oxygen-terminated surface sites, as during chemisorption, hydrogen bonds with oxygen (8). Likely, the process involves the two-step oxidative hydrogen dissociation to surface hydroxyl and water formation in the last step (15). Assuming that only Ti cation takes part in charge transfer reaction; the elementary steps on LCNT may look as follows:…”

Metal Exsolution to Enhance the Catalytic Activity of Electrodes in Solid Oxide Fuel Cells

“…However, it is worth noting that it is much easier to change the effective P(O 2 ) in SOCs by simply applying a reducing overpotential on the electrode [31]. This very interesting concept of "electrochemical switching" has been demonstrated by several authors in different electrochemical systems [32][33][34][35].…”

Fabrication and Characterization of a Tubular Solid Oxide Fuel Cell with Impregnated Perovskite Electrodes

“…The oxidation mechanism of H2 on switched perovskite electrodes. On MIEC perovskite surface, the H2 oxidation process involves both lattice oxygen (O O X ) and B-site cation (B B X ), producing H2O, oxygen vacancies (V O •• ) and electrons associated with B-site cation (B B ′ ), the reaction can be written in Kröger-Vink notation (8,15):…”