High temperature proton conductors (HTPCs) constitute a unique class of oxide materials possessing both protonic as well as oxygen-ion conductivity in H2 and H2O containing atmospheres [1]. Such exceptional properties underpin the potential application of HTPCs in a wide range of solid oxide electrochemical devices, such as hydrogen sensors, pumps, reactors, oxygen and hydrogen permeable membranes, electrolyzes and fuel cells [2].
Currently, proton conductors are being extensively used as electrolytes for solid oxide fuel cells (SOFCs). It has been theoretically predicted and experimentally confirmed that the efficiency of the SOFCs based on proton-conducting materials exceeds that of the SOFCs based on unipolar oxygen-conducting electrolytes and can amount to 80% [3].
Numerous research works have shown that a high level of protonic conductivity was achieved for BaCeO3 and BaZrO3 doped oxides. However, there are serious drawbacks limiting the potential use of such compounds [4]: i) the high level of grain boundary resistance as a part of total resistance prevailing for materials on the base of BaZrO3, ii) the extremely high sintering temperatures required for dense ceramic’s formation, and iii) the insufficient thermodynamic stability of BaCeO3-based systems against H2O, CO2 and H2S.
The first two problems can simultaneously be solved by adding small amount of sintering additives to the precursors, whereas a significant enhance of stability of barium cerate is realized through partial substitution of host ions with elements possessing more acidic properties (for example, indium, titanium, niobium) [1,4]. Unfortunately, such modification of BaCeO3 inevitably leads to a decrease in its ionic conductivity due to the structure changes of ABO3 perovskites (lattice free volume decreasing or narrowing of B–O bond length) [4].
Zirconium can be considered as one of the most suitable dopants for BaCeO3 systems, because Zr-introduction has a less dramatic effect on conductivity deterioration and, at the same time, a considerable enhancement of materials’ thermodynamic stability [1,5].
The present work focuses on the synthesis features of BaCe0.8–xZrxY0.2O3–δ ceramic samples and the thorough evaluation of zirconium substitution effect on their structure, phase stability properties and electrical characteristics in H2O-, CO2- and H2S-containg atmospheres.
The single-phase ceramic samples of BaCe0.8–xZrxY0.2O3–δ + 1 wt.% CuO or Co3O4 (BCZYx) were obtained by the modified citrate-nitrate synthesis method. The Zr effect and type of treatment (H2O, CO2 and H2S/Ar at 700°C for 10 h) on the phase structure, unit cell parameters and microstructure were investigated by XRD and SEM methods. Fresh ceramics is found to be single-phased in the whole range of x. However, BCZYx materials exhibited a different perovskite structure with the orthorhombic, rhombohedral and cubic syngony for 0 ≤ x ≤ 0.2, x = 0.3 and 0.4 ≤ x ≤ 0.8, respectively.
The decomposition of treated BCZYx was observed at x ≤ 0.2 when treated with CO2 and H2S; however no degradation was noted in the case of water vapor exposure. For the ceramic sample with x = 0.3 (BCZY0.3) the perovskite structure was maintained in the case of CO2- and H2S-exposures with the formation of small amount of impurity phases. A high density with close-packed and well-developed grains is achieved for BCZY0.3 ceramic. However for CO2- and H2S-treaded BCZY0.3 samples nanostructure impurity sediments localized on grain surface are observed. The analysis of crystal structure and transport characteristics of the treated BCZY0.3 samples showed a weak deviation of the unit cell parameters and no degradation in electrical conductivity.
The thermodynamic calculation of hydration, carbonization and sulfidation processes demonstrated that with x increasing in BaCe1–xZrxO3 system the stability of materials significantly increases against H2O, CO2 and H2S. In the case of H2O treatment, the Gibbs free energy change values of reaction between barium cerate and water vapor are positive at temperatures higher than 400°C. Under CO2 exposure, the carbonization of BaCeO3 suppress just at temperatures not lower than 950°C, whereas BaZrO3 is stable at wide range of working temperatures (550–1000°C). Hydrogen sulfide theoretically has a dramatically effect because of negative value of Gibbs free energy change for both BaCeO3 and BaZrO3 oxides between 20 – 1000°C. However, practically the Zr-substituted BaCeO3 materials can be considered as rather stable due to kinetic difficulties of impurity phase formation.
The electrical conductivity of fresh BCZY0.3 (measured by 4-probe DC method) was found to be increased with the growth of both the water vapor partial pressure due to increase of proton conductivity and the oxygen partial pressure due to increase of hole conductivity. For example, the fresh BCZY0.3 exhibits conductivity of 2.7, 4.0, 1.7 and 3.8 mS cm–1 at 600°C in dry air, wet air, hydrogen and wet hydrogen atmospheres, respectively. The analysis of energy activation values of electrical conductivity confirms the ionic character of transport properties in reducing atmospheres and the mixed ionic-electronic one in oxidizing conditions.
On the basis of phase stability results, transport characteristics compared with literature data, it can be considered such BCZY0.3 material as a perspective electrolyte for SOFC applications.
References
[1] K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333–359.
[2] K.-D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster, Chem. Rev. 104(10) (2004) 4637–4678.
[3] A. Demin, P. Tsiakaras, E. Gorbova, S. Hramova, J. Power Sources 131(1–2) (2004) 231–236.
[4] D. Medvedev, A. Murashkina, E. Pikalova, A. Demin, A. Podias, P. Tsiakaras, Prog. Mater. Sci. 60 (2014) 72–129.
[5] E. Fabbri, L. Bi, D. Pergolesi, E. Traversa, Adv. Mater. 24(2) (2012) 195–208.