Micro-solid oxide fuel cells (µ-SOFC) are multi-layered thin films which can be used in electronic devices and have the potential to replace batteries due to their high efficiency. However, these fuel cells are limited due to their high operating temperatures. It is critically important to lower the operating temperature of these devices to reduce the operational costs, the degradation of the materials and improve the lifespan of these devices. Ruddlesden-Popper phase materials such as La2NiO4+δ (L2NO4) are a promising candidate for use as intermediate temperature cathode material. L2NO4 is a mixed ionic and electronic conducting oxide which that is thermally and mechanically compatible with the commonly used electrolytes and exhibits high electrochemical activity at intermediate temperatures. L2NO4has an oxygen over-stoichiometry in the range of 0 ≤ δ ≤ 0.18 due to its ability to accommodate excess oxygen as interstitials. The contribution from the interstitials is significant for the ionic conductivity of L2NO4 and the mobility of oxygen ions can be affected by the crystal structure.
This study shows an innovative strategy to tune intrinsic and apparent surface activity by tailoring the nanostructure of L2NO4 thin films deposited by pulsed injection-MOCVD. Our results demonstrate that the increased surface area, in combination with the exposure of different surface terminations, leads to a significant enhancement of the total exchange activity in the films.
Below ca 450°C, several phases exist and/or coexist over the over-stoichiometry range. These phase transitions were studied between room temperature and 450 °C using Raman spectroscopy in a specific high temperature cell. This temperature range corresponds to the one targeted for the use of this material in prospective fuel cells designed to work at quite low temperatures, and also corresponds to the interesting zone of the O-doping - temperature phase diagram. This study also shows the feasibility of using in situ Raman spectroscopy to study the phase diagram of bulk and thin film L2NO4.
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