Synthesis, structure and conductivity of sulfate and phosphate doped SrCoO3
Graphical abstract
Phosphate/sulfate doping in SrCoO3−y leads to a structural change to a 3C-perovskite framework, with an accompanying large increase in conductivity.
Highlights
► Sulfate and phosphate are successfully doped into SrCoO3. ► The doping stabilises the 3C-perovskite framework. ► The doped samples show high conductivities.
Introduction
Solid oxide fuel cells (SOFCs) have attracted considerable interest as an energy generation technology due to their high efficiencies and accompanying low greenhouse gas emissions. For the cathodes of SOFCs, research into new materials has been dominated by materials based on the perovskite structure, due to their generally high electronic conductivities and catalytic activity [1], [2], [3]. Traditional doping strategies for such materials have involved the partial substitution of aliovalent cations with similar size, e.g. Sr doping for La in LaMnO3. Recently we have proposed an alternative doping strategy for perovskite systems with potential applications in SOFCs, namely the introduction of oxyanions (e.g. phosphate, sulfate). This work was inspired by prior work on cuprate perovskites, related to high temperature superconductors, which showed that the perovskite structure could incorporate significant levels of oxyanions (carbonate, nitrate, sulfate, phosphate) [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. In order to investigate the potential of SOFC related materials to accommodate such oxyanions, we initially investigated doping into the ionic conductor Ba2In2O5. This work showed that phosphate and sulfate could be introduced into Ba2In2O5, leading to a structural change from brownmillerite, containing ordered oxide ion vacancies, to a cubic perovskite, where the oxide ion vacancies are disordered [22], [23]. As a result of the higher symmetry, an increase in the ionic conductivity below 800 °C was observed. In addition an improvement in the stability of the compounds towards CO2 containing atmospheres was observed [24]. In this paper we extend this work to materials with potential as electrode materials, investigating incorporation of phosphate and sulfate into SrCoO3−y. The SrCoO3−y system has attracted significant interest as a promising SOFC cathode material. However, at temperatures below 900 °C the undoped phase adopts a hexagonal perovskite polymorph with low conductivity [25], [26], [27]. Detailed structural studies have indicated that this hexagonal phase is Co deficient, composition Sr6Co5O15, containing face sharing of CoO6 octahedra, similar to that of 2H-BaNiO3 [27]. The formation of a hexagonal perovskite can be related to the high (>1.0) tolerance factor for undoped SrCoO3−y. Doping SrCoO3−y on the Co site with a range of cations, e.g. Nb, Sb, Si has been shown to alter the stacking arrangement of the close packed oxygen layers from purely hexagonal to purely cubic (referred to as 2H and 3C, respectively). The consequent change from face-shared to corner linked CoO6 octahedra, leads to a substantial enhancement in the electronic conductivity [28], [29], [30], [31], [32]. This stabilization of the 3C-perovskite framework on Nb, Sb, Si doping can be attributed to a partial reduction of Co4+ to Co3+, thus increasing the perovskite B cation size, and hence reducing the tolerance factor. In this paper, we examine the effect of phosphate and sulfate doping on the stabilization of the 3C-perovskite polymorph.
Section snippets
Experimental
High purity SrCO3, Co3O4, and NH4H2PO4, and (NH4)2SO4 were used to prepare SrCo1−x(S/P)xO3−y samples (considering the incorporation as PO43−/SO42− the formula could also be written as SrCo1−x(PO4/SO4)xO3−4x−y). For the subsequently prepared Fe doped samples, Fe2O3 was used as the Fe source. The powders were intimately ground (agate mortar and pestle) and heated initially to 1100 °C for 12 h. They were then ball-milled (350 rpm for 1 h, Fritsch Pulverisette 7 Planetary Mill, zirconia balls and
X-ray diffraction data
The X-ray diffraction data showed that without any phosphate doping (x=0), the sample was a hexagonal perovskite, Sr6Co5O15, plus small concentrations of Co3O4 oxide impurities, as previously reported [27]. On the incorporation of a small amount of phosphate (x=0.03), the 3C-perovskite phase was obtained, and single phase samples could be achieved for x≤0.07 (Fig. 1). For higher phosphate levels, small concentrations of Sr10(PO4)6(OH)2 impurities were observed. Similar results were observed for
Conclusions
In this work, phosphate and sulfate were successfully incorporated into the perovskite structure of SrCoO3−y leading to an enhancement of the electronic conductivity, attributed to a change from a 2H- to a 3C-perovskite. The 3C-perovskite was, however, shown to be metastable and converted back to the hexagonal perovskite on annealing at intermediate temperatures (≈750 °C). The thermodynamic stability was improved by co-doping with Fe, with high conductivity maintained, suggesting that these
Acknowledgments
We would like to express thanks to the EPSRC for funding (Grant EP/G009929/2—studentship for CAH). The Bruker D8 diffractometer used in this research were obtained through the Science City Advanced Materials project: Creating and Characterising Next generation Advanced Materials project, with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF). We would like to thank SINQ for neutron diffraction time and Vladimir Pomjakushin for help with
References (36)
- et al.
Physica C
(1991) - et al.
Physica C
(1993) - et al.
Physica C
(1992) - et al.
Physica C
(1993) - et al.
J. Solid State Chem.
(1997) - et al.
J. Solid State Chem.
(2000) - et al.
Physica C
(1996) - et al.
Physica C
(1995) - et al.
Physica C
(1994) - et al.
Physica C
(1994)
Physica C
Solid State Commun.
Physica C
J. Power Sour.
Mater. Res. Bull.
J. Power Sour.
J. Membr. Sci.
Solid State Ionics
Cited by (46)
Cr deposition and poisoning on SrCo<inf>0.9</inf>Ta<inf>0.1</inf>O<inf>3-δ</inf> cathode of solid oxide fuel cells
2023, International Journal of Hydrogen EnergyCitation Excerpt :SrCoO3-δ exhibits excellent oxygen reduction activity but poor structure stability. Rational doping with higher valence cations at B-site, such as P, Ta, Nb and Sb [23–27] can significantly stabilize the structure with a cubic structure. It is reported that 10 mol% Nb- or Ta-doping yields good thermal stability with low polarization resistances [28,29].
The action mechanisms and structures designs of F-containing functional materials for high performance oxygen electrocatalysis
2023, Journal of Energy ChemistryCitation Excerpt :By applying density functional theory (DFT) calculations, it is demonstrated that the cubic SrCoO3 parent oxide would deliver the highest OER activity [84]. But it is very difficult to prepare an ideal SrCoO3 perovskite cubic structure [85]. For lattice modulation, Wang et al. prepared a stable cubic perovskite SrCoO3-δ particles by anion F-doping instead of traditional A and/or B site doping (ABX3 is the default perovskite composition, A and B are cations while X is anion) [39].
Modulation of electronic structure and oxygen vacancies of perovskites SrCoO<inf>3-δ</inf> by sulfur doping enables highly active and stable oxygen evolution reaction
2021, Electrochimica ActaCitation Excerpt :An initial investigation on nonmetal doping was about ionic conductor Ba2In2O5, in which phosphate and sulfate were introduced, resulting in a structural change from brownmillerite to cubic and disordered oxide ion vacancies [29]. The substitution of sulfate and phosphate in SrCoO3-δ lead to crystal phase change from hexagonal to cubic structure, which was linked to CoO6 octahedron variation from face-sharing to corner-sharing, and resulted in substantial promotion in electronic conductivity [30]. Besides, SrFeO3-δ perovskite materials incorporated with silicon was also considered as potential electrode materials for solid oxide fuel cells at high temperature [31].