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9 1 I H I G H L I G H T S 2 0 2 2

Chemical and structural compatibility in materials for protonic solid-oxide cells

Micro X-ray fluorescence and micro X-ray absorption spectroscopy techniques allowed to investigate the solid-state compatibility between calcium- doped lanthanum niobate and three perovskite cathode materials. The results reveal that the cation interdiffusion is facilitated by the structural flexibility of the perovskite structure, which is able to accommodate a variety of foreign cations in different oxidation states.

Solid-oxide cells (SOC) represent fundamental devices in the envisioned hydrogen economy due to their capability of converting hydrogen to electricity or vice versa when working in fuel cell and electrolyser modes. Proton conductors have significant design and efficiency advantages over oxygen conductors as electrolytes for SOC. High proton conductivity is usually found in perovskites, whose crystallo-chemical and electrochemical properties have been extensively investigated over the past decades [1]. One of the most appealing alternatives to perovskite-based proton conductors is LaNbO4, whose compatibility with ceramic cathode materials has seldom been explored [2-3].

A deeper, more comprehensive understanding of the chemical compatibility in the solid-state (cathode/ electrolyte interface) at high temperatures is necessary in order to maximise material compatibility and preserve performances in the long-term [4]. In this context, an investigation of the solid-state compatibility was undertaken using the scanning X-ray microprobe at ID21, comprising a number of protonic and anionic electrolytes and perovskite cathodes [3,5-6].

This work studied the interface of a Ca-doped lanthanum niobate (Ca:LaNbO4, LNC) dense ceramic after prolonged contact at high temperature (1150°C) with three different perovskites: La0.6Sr0.4Fe0.8Cu0.2O3 (LSFCu),

Fig. 82: Annealed LSC/LNC at the La L3-edge. Left: map of lanthanum (red) and strontium (green). Right: map of calcium (red) and strontium (green).

Fig. 83: Annealed LSC/LNC at the Nb L3-edge. Nb L3-edge micro-XANES spectra measured

at different points (top to bottom in the inset); inset: map of niobium (blue).

La0.6Sr0.4Fe0.85Co0.15O3 (LSCF) and La0.6Sr0.4CoO3 (LSC). The X-ray absorption spectra at the different absorption edges were modeled ab initio, which allowed atomic-scale chemical and structural information to be resolved with sub-micron resolution.

The cathode/electrolyte reactivity at high temperatures resulted in the formation of several secondary phases with variable compositions and morphologies arranged in layers parallel to the cathode/electrolyte interface. Two major secondary phases were observed: one at the cathode side, with a structure and composition similar to the cathode but with a strong strontium enrichment; the other at the electrolyte side, with a perovskite-like (Ca, Sr)2Nb2O7 structure.

The extent of the reactivity zone at the LSFCu/LNC interface is about threefold larger than in the other two cathode/electrolyte couples, suggesting that the presence of Cu plays a role in triggering a faster interdiffusion at the interface zone. The cation distribution at the LSFCu/LNC and LSCF/LNC interface is homogeneous, whereas a needle-like secondary growth identified as (Ca, Sr)2Nb2O7 was found in the LSC/LNC couple (Figure 82).