Perovskite electrocatalyst surfaces stabilized by admetals

Efficient energy conversion and storage are indispensable for a sustainable energy strategy based on renewable sources, since wind and solar power, for example, are highly dependent on external conditions. Improving the efficiency of energy conversion requires high- performance materials. Perovskite oxides present an economical class of functional materials for energy conversion applications such as solid oxide fuel cells (SOFCs). In SOFCs, doped perovskites are employed as cathode materials, responsible for the interaction with oxygen gas molecules. Under operating conditions the cathodes suffer from deactivation phenomena, such as the segregation of dopant atoms, partially driven by the electrostatic attraction between dopants and surface oxygen vacancies. Thus, detailed knowledge of the surface properties is crucial for attempts to enhance the stability of SOFC cathodes. However, the surfaces of perovskite oxide electrocatalyst materials remain significantly under-investigated at the atomic level. The goal of the proposed project is to provide atomic-scale understanding of the surface properties of state-of-the-art cathode materials with Sr-doped LaMnO3, LaCoO3, LaFeO3 (LSM, LSC, LSF) and (Ba,Sr)(Co,Fe)O3 as model systems. We will test the central hypothesis of the project that deposited metal cations can modify the surface oxygen vacancy concentration and thus influence the stability of the cathode surface. Preliminary experiments on LSC demonstrate that the long-term performance can be enhanced by up to a factor of 30. In a combined surface science and electrochemistry approach, the surface properties will be assessed before and after metal deposition, giving insights into the mechanisms underlying the improved performance. The surface structure and composition will be characterized using low-energy electron diffraction and photoelectron spectroscopy. On single crystals of the model systems, their surfaces will be imaged atom by atom using scanning tunneling microscopy. Application-oriented properties such as the oxygen exchange resistance will be studied using electrochemical impedance spectroscopy of the perovskite thin films. The pressure regimes of surface science experiments in ultrahigh vacuum and electrochemical measurements in air will be connected using in-operando x-ray spectroscopy techniques at synchrotron facilities. This comprehensive approach has the potential to provide deep, atomic-scale understanding of the surfaces of SOFC cathode materials and their interaction with deposited metals. The results are expected to stimulate research on perovskite surfaces and create broad interest in their fields of application, such as catalysis, electrolysis, syn-gas production, and membranes for ion transport and gas separation. Regarding SOFCs, the project has the potential to pave the way for improved cathode surface layers enhancing the long-term performance of future devices.

The demand for more efficient energy conversion and storage devices for a sustainable energy strategy poses enormous expectations to the performance of materials in these applications. Perovskite oxides present an economical class of functional materials for energy conversion applications such as solid oxide fuel cells (SOFCs). In SOFCs, doped perovskites are employed as cathode materials, responsible for the interaction with oxygen. Under operating conditions the cathodes suffer from deactivation phenomena, such as surface segregation of dopant species, partially driven by their electrostatic attraction to surface oxygen vacancies. The present project provides insights into the effects of surface composition and electrochemical polarization on this deactivation, as well as atomic-scale information on the surface properties of a state-of-the-art high-temperature cathode material: Sr-doped LaMnO3. The role of surface composition is explored via the deposition of metals of different chemical characteristics to modify the surface stability. Using in-situ photoelectron spectroscopy at high temperature in oxygen environment, we show a clear stabilization in the presence of Hf. This behavior is attributed to a decrease in the density of surface oxygen vacancies, which become energetically less favorable if the surface is enriched in metals that are difficult to reduce. Our observation establishes that the enhanced stability previously observed for Hf-modified (La,Sr)CoO3 is not specific to one material or deposition method but that the deposition of selected metals has general potential to stabilize related oxides suffering from segregation. Moreover, we tested the effect of applying polarization to Ca-, Sr-, and Ba-doped LaMnO3. This approach allows us to simultaneously probe the effects of dopant size and effective oxygen pressure, which is determined by the voltage applied between the model cathode and a counterelectrode in a well-defined oxygen environment. The point of minimum segregation is observed to shift to more reducing conditions for increasing dopant size (Ca < Sr < Ba), in line with a stronger response of larger dopants to changes in the lattice parameter. In addition to surface chemistry, the atomic-scale surface properties of La0.8Sr0.2MnO3 were studied using scanning tunneling microscopy. First images on La0.8Sr0.2MnO3(001) single crystals show differences in the surface structure compared to thin films on SrTiO3, which are commonly used as model systems in surface studies. Low-energy electron diffraction results, however, indicate the same type of surface reconstruction for thin films and single crystals. In summary, the present project provides new insights into the surface properties of doped perovskite manganites ranging from atomic-scale structural information to the effects of polarization and metal deposition on their stability. The observed stability enhancement of surfaces modified by metals shows the general applicability of this approach and is considered relevant for technological applications of this class of oxides, while the results on the surface structure of La0.8Sr0.2MnO3 and the effects of polarization enhance the fundamental understanding of the stability of these materials and the mechanisms underlying degradation.