Project

Discipline

thin films ; interfaces ; first-principles molecular dynamics ; high temperature proton conductors; pulsed laser deposition ; strain

Lead
Festoxidbrennstoffzellen (solid oxide fuel cells, “SOFCs”) sind hocheffiziente elektrochemische Energieumwandler mit einer potentiell bedeutsamen Rolle für die Entwicklung umweltfreundlicher Eneriequellen. Der Einsatz von alternativen Materialien im Design und in der Fabrikation von SOFCs könnte helfen die momentanen technologischen Beschränkungen zu umgehen, wodurch die schnelle und weitverbreitete Entwicklung dieser Technologie gefördert würde.
Lay summary
Hintergrund Aktuelle SOFC Technologien verwenden Sauerstoffionenleiter als Elektrolyten. Diese Materialien erfordern typischerweise Betriebstemperaturen weit über 800°C. Tiefere Betriebstemperaturen könnten enorme Vorteile bringen. Allerdings nimmt die Ionenleitfähigkeit von Standartelektrolyten mit sinkender Temperatur rapide ab. Protonenleitende Oxide sind alternative Elektrolytmaterialien welche den SOFC Betrieb bei tiefen Temperaturen erlauben würden, da eine tiefere thermische Energie für eine effiziente Ionenleitung erforderlich ist. Der Mechanismus der Protonenleitung in Oxiden ist allerdings noch nicht komplett verstanden, da es einen grossen Unterschied für die Werte der Aktivierungsenergie aus Theorie und Experiment gibt. Ziel Das Ziel dieses theoretischen und experimentellen Forschungsprojektes ist die Fabrikation und Charakterisierung von Modellproben. Diese werden durch Dünnschichttechnologien hergestellt und erlauben die Untersuchung des Effektes von Gitterverzerrungen (Dehnung) auf die Protonenleitfähigkeit. Streckende oder komprimierende  Gitterverzerrungen könnten zu unterschiedlichen Werten für die Migrationsenergie führen durch veränderte interatomare abstände, insbesondere den durchschnittlichen Abstand zwischen Protonen und Dotieratomen. Bedeutung Verdehnte Dünnfilme könnten “Superprotonenleitfähigkeit” besitzen dank der durch die Verdehnung gesenkten thermischen Energien für den Ladungstransport. Indem das Dotierungselement und die Verdehnung variiert werden, werden theoretische Simulationen und die Charakterisierung der Mikrostruktur es erlauben die lokale Mikrostrukturumgebung zu identifizieren die schnelle Protonenleitung bei tiefen Temperaturen erlaubt. Die Stabilisation metastabiler protonenleitender Phasen durch Gitterverzerrungen könnten in der Tat eine vielversprechende Forschungsrichtung sein: Die Entwicklung neuer Materialien für einen hoch effizienten Betrieb von SOFCs bei tiefen Temperaturen wird durch protonenleitende Oxide ermöglicht.

Direct link to Lay Summary

Last update: 05.08.2015

Number	Title	Start	Funding scheme

Proton-conducting oxides have been investigated for decades, especially for their application as electrolyte materials in solid-oxide fuel cells (SOFC). Current SOFC technology is based on oxygen-ion conducting electrolytes, such as Y2O3-stabilized ZrO2, and requires very high operating temperatures (above 800°C) to achieve suitable ionic conductivity. Such high temperatures require stringent and challenging material specifications that up to now have represented the key drawback for this technology. The use of proton-conducting electrolytes, as alternative to oxygen-ion conductors, shows great potential for a significant reduction of the SOFC operating temperature, due to the much smaller activation energies for charge transport. Among proton conducting oxides, Y-doped BaZrO3 (BZY) is a particularly interesting material due to its chemical stability in the fuel-cell operating environments and its good proton conductivity in the grains’ interior. However, difficult sintering and poor conducting properties for its grain-boundary regions have precluded so far the use of BZY in the form of sintered ceramic membranes. Only in 2009 a relatively large average-grain size was achieved for BZY sintered pellets, while in 2010 highly textured grain-boundary-free BZY thin films were grown, showing the largest proton conductivity ever reported for solid electrolytes. These recent results have revitalized the search for materials displaying fast protonic conduction pathways at low temperatures (below 500°C); in addition, very recent literature reports that the lattice strain of proton conductors seems to have a large influence on their conductivity. In particular, it has been observed that compressive strains significantly decrease the conductivity by increasing the activation energy for charge transfer, whereas the opposite effect, i.e. a lower activation energy, had been expected from theoretical considerations, with a tensile strain leading to enhanced conductivity in these lower-temperature range. In this project we will combine state-of-the-art experimental synthesis, thin-film growth and characterization, and first-principles calculations to determine the driving force and the microscopic origin for enhanced conductivities, and engineer optimal materials and microstructures for deployment in lower-temperature SOFCs. In particular, biaxially textured thin films of BZY will be fabricated by pulsed laser deposition, where different strain states of the films can be tuned by suitable insulating buffer layers with the same crystalline symmetry but different lattice parameter. Moreover, the growth of highly ordered epitaxial multi-layered heterostructures comprising a proton conducting and an insulating phase will allow investigating the potential contribution of the interfaces to the total conductivity. The conducting properties of the films will be measured and correlated with the microstructural characteristics and the residual strain state of the depositions. In close feedback with these experimental results, extensive simulations will be performed, to elucidate the mechanisms of proton migration in distorted lattices. These will involve the determination of reaction pathways, activation energies, and diffusion coefficients for the protons using a combination of static transition-state finding methods and first-principles Car-Parrinello molecular dynamics; these latter also able to take into account nuclear quantum effects via non-Markovian Langevin equations of motion. Last, an accelerated-dynamics framework will also be developed to rapidly screen diffusion of light nuclei into the solid matrix. The main target of this investigation is the discovery and identification of low-temperature superprotonic conductivity resulting from strain-engineering. The existence of such a superprotonic state has been strongly suggested as an extrapolation of reported experimental results, but has never been observed to date.