Publication date: 10th April 2024
Oxygen ion conductors, both ionic and mixed conductors, are critical materials for the development of the important devices needed to achieve net-zero. These devices include the solid oxide fuel cell, solid oxide electrolyser cells, gas sensors and oxygen transport membranes. Unfortunately, the oxygen ion (O2-) is the largest ion in oxide lattices with an ionic radius of ~1.4 Å compared to, for example, a zirconium ion (Zr4+) at ~0.7Å . Student texts show how, by close packing the oxygen sublattice, various oxide structures can be achieved by populating the interstices with the appropriate, and usually much smaller cations. Thus, in principle, moving oxygen ions through the oxide lattice is a difficult process. Atomic transport does occur however because of the presence of crystal imperfections, i.e. point and extended defects present through either intrinsic or extrinsic processes.
this contribution I will review the development of ceramic oxygen ion conductors starting with a review of the history of these materials, the present state of the art and then some pointers to future development. Particular focus will be on the fluorite structured electrolyte materials and the mixed conducting perovskite and perovskite related materials. At elevated temperatures lattice oxygen transport can reach quite extraordinary levels for the solid state, with tracer diffusion coefficients approaching ~ 10-6cm2 sec-1 at close to 1273K. An important question is can this be high oxygen transport level be bettered by novel materials approaches such as high entropy oxides?
Because of the polycrystalline nature of practical devices, the oxygen ion must also cross the many interfaces in the device such as the gas/solid interface, for oxygen reduction and evolution, homogeneous solid/solid interfaces, i.e. grain boundaries in the ceramic components and heterogeneous interfaces e.g electrode/electrolyte interfaces. All these interfaces need to be optimised for ion transport and remain optimised for the operating lifetime of the device. To achieve this goal, we need to take a more holistic view of ion transport in these materials and not only focus on the most mobile species, but look at the slower process that can dominate in both processing and during the long timescales needed for commercial operation.
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