Investigation and development of cuprous delafossites for solid oxide fuel cell cathodes

Abstract

The research into materials for use as cathode materials for solid oxide fuel cells (SOFC) is ongoing, with many different avenues being investigated. Copper based delafossites were studied for cathode side applications in SOFCs, as a novel and comparatively cheap material. The aim was to identify suitable materials with appropriate electrical conductivity, thermal, chemical and mechanical stability in air. Furthermore, understanding the behaviour of the delafossites during the thermal oxidation to spinel and copper oxide would be beneficial to further development of the materials.

The structure and properties of the copper based delafossites CuFeO₂, CuAlO₂ and CuCrO₂ were studied, alongside several doped compositions for each parent composition. The electronic conductivity of the CuFeO₂ family was improved by doping fluorine into the structure, with 1 atomic % doping producing ~3.8 S cm⁻¹ at 800 °C. However, as reported in literature the structure is vulnerable to oxidation at higher temperatures. In contrast, CuAlO₂ was stable over the SOFC temperature range, and therefore had appropriate thermal expansion coefficients (TEC) of ~11 x 10⁻⁶ K⁻¹, but relatively low electronic conductivity. CuCrO₂ compositions had good overall TECs, but aliovalent doping of Mg²⁺ improved the conductivity to ~17.1 S cm⁻¹ at 800°C for 2.5 atomic % doped CuCrO₂.

Neutron diffraction was utilised to study members of the solid solution CuFe₁₋ₓCrₓO₂ (x = 0, 0.25 and 0.5) during in-situ oxidation at high temperature. Points of positive scattering density were identified within the CuFeO₂ structure, which were attributed to the location of the intercalated oxygen ions before the transformation proceeded. Additionally, the cation distribution between the tetrahedral and octahedral sites within the developing spinel were characterised for x = 0, and partially for the x = 0.25 and 0.5 compositions using complimentary XRD patterns.

Finally, magnesium doped CuCrO₂ delafossites were used in several different preliminary symmetrical cells for study using electrochemical impedance spectroscopy (EIS). Pure delafossite inks gave relatively large area specific resistance (ASR) values, 1.29 – 2.69 Ω cm² at 800 °C. It was attempted to improve upon these values through infiltration of CeO₂ and through change in microstructure using composite type inks, without much success. Inks using CuCr₀.₈Fe₀.₂O₂ were also tested as both a single phase electrode and as a composite type electrode. The pure delafossite electrode still had a large ASR value, (~33.4 Ω cm² at 800 °C) while composite electrodes obtained much more respectable ASR values ~0.75 Ω cm² at 800 °C.