Optimizing Surface Segregation and Defect Structure of a Perovskite through Strain for Improving Oxygen Reduction and Evolution Catalysis

Renewable energy sources are intrinsically plagued by irregular power production. In order for alternative energy sources to become viable it is imperative to improve energy storage in tandem with source development. Currently oxygen evolution and oxygen reduction are two key electrochemical reactions which often limit the efficiency of energy storage devices. Identifying and developing a catalyst for these reactions would greatly benefit the development of a renewable energy economy.

It has previously been demonstrated that La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ¹ can accommodate both a fully oxidised phase (δ = 0) and a reduced phase (δ = 0.62), making it one of only a few reported perovskite oxides that is stable with δ > 0.5, illustrated in Figure a. The oxygen vacancies introduced as a result of this variable oxygen stoichiometry as well as the possibility of the transition metals to have a range of oxidation states make this material an ideal candidate to investigate as a catalyst for oxygen reduction and evolution reactions.

Thin films of the La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ perovskite have been prepared by pulsed laser deposition on a range of substrates. Thin films are an ideal system for conducting a fundamental study of catalysis as bulk effects become negligible and surface effects can be measured. Further, there is less variation of the microstructure between samples, allowing for a more controlled comparison.

Initial x-ray diffraction (XRD) results indicates a change in the lattice parameter consistent with a change in oxidation states, see Figure b. This is due to the strain effects² caused by the lattice mismatch between the perovskite and the substrate materials, strontium titanate (STO), magnesium oxide (MgO) and lanthanum aluminate (LAO). As the bulk material is able to reversibly interchange between a fully oxidised rhombohedra phase and a hypostoichiometric reduced orthorhombic phase, it is strongly suspected that changes in lattice parameter are indicative of the overall oxidation state of the thin film layer. Hence it is suggested that the number of vacancies and the metallic oxidation states can be tuned through lattice mismatch selection. X-ray photoelectron spectroscopy is used to further investigate the changes in oxidation state caused by straining the films.

Atomic force microscopy has been used to identify both the surface topology as well as film thickness (10 – 300 nm). Different morphology is observed depending on growth parameters including deposition temperature and substrate. Low energy ion scattering (LEIS) and secondary ion mass spectroscopy are employed to identify the composition of surface regions. The effects of post-growth treatment, including heating and cleaning processes, on these measurements are also considered. 

In addition to considering the transition metal oxidation states, surface segregation studies³ suggest that perovskite materials are often A-site terminated. LEIS studies allow for investigation of the segregation that occurs at both the surface and at the sub surface levels. This is critical as segregation is one of the main causes of ceramic degradation and will greatly influence the electrocatalytic properties. Strain is therefore being investigated as a method to control the surface segregation. 

A systematic study of the La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ thin films at room temperature delivers the groundwork characterisation that is required for further investigation of this perovskite as an oxygen reduction and oxygen evolution reaction catalyst, for future applications ranging from metal air batteries to fuel cells and electrolysers.

Figure a – Thermogravimetric analysis of La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ

Figure b – La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ lattice parameter from XRD measurements as a function of the substrate, and a comparison to lattice parameters of the bulk material determined by neutron diffraction data

References

1. A. Aguadero et al., Angew. Chem. Int. Ed. Engl., 50, 6557–61 (2011).

2. M. Kubicek et al., ACS Nano, 7, 3276–86 (2013).

3. J. Druce, T. Ishihara, and J. Kilner, Solid State Ionics, 262, 893–896 (2014).