F2: Solid-Oxide Fuel Cells and Membranes

Nanomaterials for Fuel Cells and Membranes

Fuel cells convert chemical energy directly into electrical energy (or vice versa). This conversion occurs by a cascade of electrochemical processes. The corresponding exchange of electronic and ionic charges at the gas/electrode/electrolyte interface takes place at length scales in the nanometer range. In particular, solid-oxide fuel cells (SOFC, subproject F2.2) allow high energy-conversion efficiencies for fossil fuels or hydrogen without the need for noble-metal catalysts. Aiming at even faster reaction kinetics at surfaces and charge-transfer processes at electrode/electrolyte interfaces, it is most effective to enlarge the lateral extension of the active triple-phase boundary (TPB). This can be achieved, e.g., by using mixed-ionic-electronic conducting (MIEC) nanoporous thin films.

Regarding oxygen-permeation membranes (subproject F2.1), the focus is on the enhancement of oxygen surface-exchange and oxygenion transport by nanostructured functional layers on a dense, mixed ionic-electronic conducting thin-film membrane, thus increasing the oxygen flux through such a device. Perovskite-type MIEC oxides are again the material of choice due to their high oxygen-permeability and -selectivity for oxygen/air separation in the temperature range of technical interest (around 800 °C).

Goals of the Project

The specific goals of F2 include the preparation of perovskite-type (ABO₃, A: La, Sr, Ba; B: Co, Fe) thin-films over a broad range of grain sizes, porosity, and film thickness. Furthermore, structural stability, surface exchange kinetics and electrochemical efficiency of the resulting thin-films are investigated experimentally and by microstructure modeling.

Experiments and Models

As even in the best MIEC thin-film structures, electronic and ionic conduction differ by orders of magnitude, the contributions of surface-exchange reactions, charge-transfer processes, and Ohmic losses have to be carefully analyzed and separated by, e.g., electrochemical in-situ measurements.

Furthermore, the combination of elaborate electron-microscopy techniques and microstructure modeling/simulation methods is required to clarify the processing-structure-property relationship, e.g., for optimizing the lateral extension of the TPB area.

Visions and Perspectives

The results of project F2 will provide valuable insight into the fundamental process of oxygen incorporation into mixed ionic-electronic conducting perovskite-type materials. From a technological point of view, the performance of solid-oxide fuel cells and oxygen-separation membranes will be improved substantially by custom-tailored nanostructures.