Proton conductivity of lanthanum and barium zirconate: Microscale aspects on first-principles basis
Doktorsavhandling, 2012

Fuel cells are devices which convert chemical energy into electrical energy cleanly and efficiently. Development of fuel cells compatible with hydrocarbon fuels would make more efficient use of present fossil and renewable fuels, and also enable progress towards a future hydrogen economy. One of the major hindrances to commercially viable fuel-cell technologies is the lack of materials with properties appropriate for devices operating at temperatures high enough to allow carbon-containing fuels, while low enough to suppress the negative side effects of high temperatures. Part in a direct route towards intermediate temperature fuel cell technologies is the optimization of the proton conductivity of solid, ceramic, oxide materials for use as electrolytes in so-called solid oxide fuel cells. This thesis puts forward theoretical investigations into atomic and microscopic mechanisms which directly influence the proton conductivity of solid oxide materials posing as candidates for proton-conducting electrolyte materials. The foundation of this work is the description of the atomic and electronic structure of materials offered by methods based on density-functional theory. Combined with thermodynamic and electrostatic theory, the pressing issue of grain boundary resistivity in the otherwise promising proton-conducting solid oxide material barium zirconate (BaZrO3), is addressed. Furthermore, fundamental aspects related to the optimization of proton conductivity by means of acceptor-doping are examined in the not as frequently studied material lanthanum zirconate (La2Zr2O7). Acceptor-doping is intended to increase proton concentration by causing vacant oxygen positions, which, by incorporation of water molecules, can be filled with hydroxide ions. The most important work and results can be summarized as follows: (i) By examining several different dopant species in La2Zr2O7 it is shown that a poor choice of dopant can not only lead to inefficient concentration improvement but also to trapping of both protons and oxygen vacancies. In consistency with experimental observations, Ca and Sr are pointed to as the most promising dopants out of the twelve investigated species. (ii) By calculating the energy of oxygen vacancies in the vicinity of different grain boundary structures of BaZrO3 it is demonstrated that accumulation of oxygen vacancies at the core of the grain boundary interfaces can significantly hamper the effective proton conductivity in the material. This accumulation leads to charged grain boundary cores and gives rise to a depletion of protons in the surrounding region. The magnitude of the effect corresponds well with experimental conductivity data.

first principles

trapping

conductivity

grain boundary

segregation

pyrochlore

space charge

solid oxide fuel cell

depletion

electrolyte

La2Zr2O7

oxygen vacancy

BaZrO3

proton

DFT

point defect

perovskite

acceptor

A423 (Kollektorn), Kemivägen 9, MC2, Chalmers
Opponent: Senior Scientist Tejs Vegge, Department of Energy Conversion and Storage, Technical University of Denmark

Författare

Joakim Nyman

Chalmers, Teknisk fysik, Material- och ytteori

Protonic defects in pure and doped La2Zr2O7 pyrochlore oxide

Solid State Ionics,; Vol. 178(2009)p. 1642-

Artikel i vetenskaplig tidskrift

Oxygen vacancy segregation and space-charge effects in grain boundaries of dry and hydrated BaZrO3

Applied Physics Letters,; Vol. 100(2012)p. 061903-

Artikel i vetenskaplig tidskrift

Oxygen vacancy segregation in grain boundaries of BaZrO3 using interatomic potentials

Solid State Ionics,; Vol. 230(2013)p. 27-31

Artikel i vetenskaplig tidskrift

Substitutional doping and oxygen vacancies in La2Zr2O7 pyrochlore oxide

Solid State Ionics,; Vol. 189(2011)p. 19-28

Artikel i vetenskaplig tidskrift

Drivkrafter

Hållbar utveckling

Ämneskategorier

Övrig annan teknik

Den kondenserade materiens fysik

Styrkeområden

Energi

Materialvetenskap

ISBN

978-91-7385-629-4

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 3310

A423 (Kollektorn), Kemivägen 9, MC2, Chalmers

Opponent: Senior Scientist Tejs Vegge, Department of Energy Conversion and Storage, Technical University of Denmark

Mer information

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2017-10-07