Comparison of Space-Charge Formation at Grain Boundaries in Proton-Conducting BaZrO₃ and BaCeO₃

Abstract

Acceptor-doped BaZrO 3 (BZO) and BaCeO 3 (BCO) both exhibit considerable bulk proton conductivity, which makes them suitable as electrolytes in electrochemical devices. However, these materials display high grain-boundary (GB) resistance that severely limits the overall proton transport in polycrystalline samples. This effect has been attributed to the presence of space charges at the GBs, which form because of segregation of protons and charged oxygen vacancies. This blocking behavior is less prominent in BCO, but in contrast to BZO, BCO suffers from poor chemical stability. The aim with the present work is to elucidate why GBs in BZO are more resistive than GBs in BCO. We use density-functional theory (DFT) calculations to study proton and oxygen vacancy segregation to several GBs and find that the oxygen vacancy segregation energy is quite similar in both materials while the tendency for proton segregation is larger in BZO compared with that in BCO. This is not related to the GBs, which display similar proton formation energies in both materials, but because of different formation energies for protons in the bulk regions. This can be understood from a stronger hydrogen bond formation in bulk BCO compared with that in bulk BZO. Furthermore, segregation energies are evaluated in a space-charge model, and the resulting space-charge potentials provide a consistent explanation of the experimentally observed difference in GB conductivity.

Influence of Ce3+ polarons on grain boundary space-charge in proton conducting Y-doped BaCeO3

“…Oxygen vacancies exhibited the most exothermic segregation energy of À1.66 eV, which is similar to the (1 0 2)[0 0 1] boundary, À1.74 eV. 32 The vacancy segregation energies were essentially unchanged with accompanying polarons in the form of Ce…”

“…29,30 In comparison to cubic BaZrO 3 , proton segregation energies obtained for orthorhombic BaCeO 3 have been found to be about half as exothermic for similar grain boundary structures. 31,32 Lindman et al related the lower segregation energies in BaCeO 3 to a higher stability of protons in the bulk region due to stronger hydrogen bond formation, rather than a property of the grain boundary structure. 32 In the present work, we further investigate space-charge formation at a (0 2 1)[1 0 0] tilt boundary in Y-doped BaCeO 3 .…”

Gas Humidification Impact on the Properties and Performance of Perovskite‐Type Functional Materials in Proton‐Conducting Solid Oxide Cells

“…Second, some introduced dopants possessed refractory nature in comparison with cerium ions. In this situation, the grain boundary conductivity considerably dropped due to the smaller grain size and higher grain boundary density (Figure c) . Third, the oxygen vacancy concentration decreased when the donor‐type cations were used as the dopants.…”

“…c) bulk and grain boundary conductivity of doped BaCeO 3 and BaZrO 3 materials. The graph is presented in work based on the published data in literature . Reproduced with permission .…”

Samples of Ba1−xSrxCe0.9Y0.1O3−δ, 0 < x < 0.1, with Improved Chemical Stability in CO2-H2 Gas-Involving Atmospheres as Potential Electrolytes for a Proton Ceramic Fuel Cell

“…Grain boundary conductivity is also an important factor in determining the total electrical conductivity of ceramic material. The high electrical resistance of intergranular areas in a sintered ceramic sample is very often a factor preventing the acquisition of high values of ionic conductivity in proton-conducting electrolytes containing BaZr 0.9 Y 0.1 O 3−δ (BZY) or BaCe 0.9 Y 0.1 O 3−δ (BCY) [51,52]. Lindman presented an analysis of the potential for reducing the value of resistance of grain boundaries for BZY and BCY.…”