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High Temperature Electrolyser/Fuel Cell Systems

C. Zhang, B. Yildiz, B. Yu and P.D. Bristowe

Sponsors: Low Carbon Energy University Alliance (LCEUA)

  • Electrode stability and degradation of HTEFC materials
  • Collaboration between Tsinghua University, MIT and University of Cambridge

Research Highlight

First principles calculations of oxygen vacancy formation in barium-strontium-cobalt-ferrite

C. Zhang and P. D. Bristowe, RSC Advances, 3. pp. 12267-12274 (2013)

Perovskite-type barium-strontium-cobalt-ferrite (BSCF) is a potentially significant material in the development of electrodes for solid oxide fuel cells and electrolysis cells, primarily because of its large oxygen vacancy concentration and mobility. Using density functional theory (DFT) with the DFT+U approach, we perform first principles calculations of oxygen vacancy formation in bulk BSCF with the composition Ba0.5Sr0.5Co0.75Fe0.25O3-δ, in which a range of oxygen deficiencies (0.125 ≤ δ ≤ 0.875) is investigated. Contrary to previous DFT studies for δ = 0.125, DFT+U predicts that the non-stoichiometric structure is more stable than its stoichiometric counterpart with δ = 0, and that this originates from a significantly different atomic and electronic structure following the introduction of the Hubbard U termin the calculations. As more vacancies are created there is no tendency for ordering and the total vacancy formation energy becomes positive and increases as the oxygen deficiency increases. Using ab initio thermodynamics, the hat of formation of BSCF as a function of oxygen partial pressure and temperature is determined and the results are in broad agreement with available stoichiometry measurements.

fuelcell.jpg
DFT-optimized local structure surrounding an oxygen vacancy in BSCF, represented by a dashed circle, between two Co ions (light blue). Ions labeled nn are indicative of the nearest neighbors to the vacancy.