Investigating the Surface Segregation of Perovskite Thin Films to Optimise the Redox Behaviour for Application in Metal-Air Batteries

It is essential to improve energy storage in tandem with developing renewable energy alternatives as an intrinsic challenge of renewable energy sources is irregular power production. Currently oxygen evolution and oxygen reduction are two key electrochemical reactions which limit the efficiency of energy storage devices including metal-air batteries. Hence, improving these reactions would greatly benefit the development of a renewable energy economy.

It has previously been demonstrated that the perovskite La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ (LSCM) [1] can reversibly accommodate both a fully oxidised phase (δ = 0) and a reduced phase (δ = 0.62), making this the most hypo-stoichiometric simple perovskite reported to date (Figure 1). The oxygen vacancies introduced as a result of this variable stoichiometry as well as the potential range of transition metal oxidation states make this material an ideal candidate to investigate as a redox mediator for oxygen reduction and evolution reactions.

The ability to tune and control the defect structure of this system will allow optimisation of the electrochemical behaviour. With this is mind, LSCM thin films of 10 – 75 nm thick have been prepared by pulsed laser deposition with initial x-ray diffraction results indicating a change in lattice structure depending on the substrate the films are deposited on likely due to strain effects [2].

Since the surface characteristics are fundamental factors that governs the activity of these materials, further characterisation of the films included low energy ion scattering (LEIS) to consider segregation effects (Figure 2) [3], atomic force microscopy and x-ray photoelectron spectroscopy to determine changes in transition metal oxidation state. The redox activity of LSCM under different polarisation conditions has been investigated with isotopic labelling through secondary ion mass spectroscopy [4].

A systematic study of the La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ thin films has shown that we can control the cell parameters of LSCM by growing thin films on different substrates. The LEIS measurement revealed that the induced strain may be affecting the surface termination and hence, may be instrumental in optimising the electrochemical behaviour of the perovskite.

Figure 1 – Thermogravimetric analysis of La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ

Figure 2 – Comparison of the elemental composition of La_0.5Sr_0.5Co_0.5Mn_0.5O_3-δ thin films on lanthanum aluminate (LAO, left) where in-plane compressive strain is expected and magnesium oxides (MgO, right) substrates where tensile strain of the thin films is expected

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

[2] M. Kubicek, et. al., ACS Nano, 2013, 7, 3276–86.

[3] J. Druce, et. al., Solid State Ionics, 2014, 262, 893–896.

[4] J. A. Kilner, et. al., J. Solid State Electrochem., 2011, 15, 861–876.