Publication date: 10th April 2024
Understanding underlying processes and mechanisms in electrochemical cells is a crucial step towards their optimization and development. A promising material for high temperature solid oxide cells is SrTiO3 (STO). Although it is known that the space charge regions between STO and different mixed ionic and electronic conductors (MIECs) lead to measurable photovoltages [1], models describing such MIEC|MIEC interfaces are not developed yet. In particular, the interplay between ionic and electronic charge carriers is far from being understood.
The aim of this work is to investigate the interfacial space charge region between STO and different MIECs concerning the effects of the MIEC thin film thickness. La0.6Sr0.4FeO3-δ (LSF), La0.65Sr0.35MnO3-δ (LSM) and La0.9Sr0.1CrO3-δ (LSCr) thin films (0.5 – 10 nm) were grown on (nominally) undoped STO single crystals by pulsed laser deposition. The LSM and LSCr thin films were subsequently covered with a 50 nm layer of LSF to achieve the stack STO|LSM|LSF and STO|LSCr|LSF, respectively. The LSF thin film was coated with a 50 nm LSM layer to obtain the stack STO|LSF|LSM. The interfaces between STO and the MIEC stacks were investigated by impedance spectroscopy at 500°C in various oxygen partial pressures. Comparison with impedance measurements on solo thin films (50 nm) revealed that 10 nm interlayers exhibit a space charge very similar, if not identical, to much thicker films. The behavior of much thinner interlayers, however, seems to depend on the space charge resistances deduced from impedance data. MIECs can be clustered into materials leading to very pronounced space charges, such as LSM and LSCr, and moderate space charges, such as LSF. Interestingly, already LSM interlayer thicknesses of a single unit cell (0.5 nm) displayed a space charge akin to much thicker LSM films and thus have almost the same effect as several 10 nm LSM. However, the space charge for the STO|LSF|LSM stack differed from thicker LSF space charges for interlayers <2 nm. Ultimately, a model is proposed to describe and predict the space charges between STO and the different MIECs. This model is used in a further approach to interpret and describe the different effects interlayers may exert on the space charge region.
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