Large Enhancement of Oxygen Surface Exchange Kinetics in Ruddlesden–Popper Thin Films By Thickness Controlled Strain

Abstract

The design of highly active catalysts for the oxygen reduction reaction (ORR) that leads to the majority of efficiency loss is a critical requisite in the needs of intermediate temperature solid oxide fuel cells (SOFCs). Introducing the lattice strain of an epitaxial film induced by lattice mismatch with a substrate enables a new way to enhance the ORR at elevated temperatures. However, very limited available electrolytes to induce strains in high temperature electrochemical applications, in which yttria-stabilized zirconia (YSZ) uniquely satisfies both the growth requirements (i.e. lattice mismatch) and the electro-chemical requirements (i.e. ionic conductor and electronic insulator) impose a constraint on applying the lattice strain. The limited choice of substrates imposes a constraint on the range of lattice strains, which suggests the need to develop a new strategy to modulate the epitaxial strain for designing catalytically active materials. In this work, we explore the dramatically enhanced ORR kinetics in the Ruddlesden–Popper oxide, LaSrCoO4 (LSC214), which have been highlighted as alternative cathode materials for intermediate temperature SOFCs grown on (001) YSZ substrates using film thickness to control the epitaxial strain. Pulsed laser deposition (PLD) is employed to deposit LSC214 thin films with four different film thicknesses. Electrochemical impedance spectroscopy (EIS) results demonstrate that the oxygen surface exchange coefficient (k q) values of ~ 15 nm LSC214 thin film with only ~1 % in-plane tensile strain can be dramatically enhanced by up to two orders of magnitude compared to fully relaxed thicker films (~70 and 140 nm). We find that this enhancement is strongly associated with a change of the surface chemistry revealed from Auger electron spectroscopy (AES), which can lead to a change of the electronic structure, the oxygen 2p band center relative to the Fermi energy level. In addition, we also show no discernible change in the k q values at low oxygen partial pressures regardless of the film thickness. This result suggests that the rate-limiting step for the oxygen surface exchange in LSC214 thin films changes depending on the oxygen partial pressure. Our work illustrates that controlling thickness is a new strategy to design highly active oxide materials for applications of complex oxides in the solid-state electrochemistry.