crystals Article Investigation into the Effect of Sulfate and Borate Incorporation on the Structure and Properties of SrFeO3-δ Abbey Jarvis and Peter Raymond Slater * School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-121-414-8906 Academic Editor: Stevin Pramana Received: 28 April 2017; Accepted: 3 June 2017; Published: 7 June 2017 Abstract: In this paper, we demonstrate the successful incorporation of sulfate and borate into SrFeO3-δ, and characterise the effect on the structure and conductivity, with a view to possible utilisation as a cathode material in Solid Oxide Fuel Cells. The incorporation of low levels of sulfate/borate is sufficient to cause a change from a tetragonal to a cubic cell. Moreover, whereas heat treatment of undoped SrFeO3-δ under N2 leads to a transformation to brownmillerite Sr2Fe2O5 with oxygen vacancy ordering, the sulfate/borate-doped samples remain cubic under the same conditions. Thus, sulfate/borate doping appears to be successful in introducing oxide ion vacancy disorder in this system. Keywords: solid oxide fuel cell; cathode; perovskite; sulfate; borate 1. Introduction Research into solid oxide fuel cells (SOFCs) as alternate energy materials has grown due to their high efficiency and consequent reduction in greenhouse gas emissions. Specifically, research into perovskite materials has attracted significant interest for potential SOFC materials including cathode, electrolyte and anode materials (see, for example, the review articles [1,2]). Traditionally, doping strategies for perovskite materials has involved the introduction of cations of a similar size e.g., Sr2+ for 3+ 2+ 3+ La , Mg for Ga to give the oxide ion conducting electrolyte La1−xSrxGa1−yMgyO3−x/2−y/2 [3]. More recently, we have applied oxyanion (silicate, phosphate, sulfate, borate) doping strategies to improve the properties of solid oxide fuel cell materials, initially demonstrating the successful incorporation into Ba2In2O5 [4]. Research on oxyanion incorporation into perovskites was initially reported for superconducting cuprate materials. This work showed that a wide range of oxyanions (carbonate, borate, nitrate, sulfate, and phosphate) could be incorporated into the perovskite structure [5–12]. In this doping strategy, the central ion of the oxyanion group is located on the perovskite B cation site, with the oxygens of this group occupying either three (borate, carbonate, nitrate) or four (sulfate, phosphate) of the available six anion positions around this site, albeit suitably displaced to achieve the correct coordination for the oxyanion group. As an extension to this work on cuprate superconductors, we have demonstrated successful oxyanion doping of perovskite materials for SOFC applications. This has included oxyanion (sulfate, silicate, phosphate) doping of Ba2(In/Sc)2O5 electrolyte materials [13–16]. Doping with these oxyanions introduces disorder on the oxygen sublattice, leading to improvements in the ionic conductivity. This work has also been extended to potential solid oxide fuel cell electrode materials, with successful oxyanion doping in the perovskite systems SrCoO3 [17–20], SrCo0.85Fe0.15O3 [21], SrMnO3 [18], CaMnO3 [22] and SrFeO3-δ [23], with the results suggesting improved performance/stability. Crystals 2017, 7, 169; doi:10.3390/cryst7060169 www.mdpi.com/journal/crystals Crystals 2017, 7, 169 2 of 13 Following the prior work demonstrating the successful incorporation of silicate into SrFeO3-δ [23],Crystals we 2017 report, 7, 169 in this paper an investigation into the effect of sulfate and borate doping.2 of 13 The Crystalsundoped2017 ,system7, 169 SrFeO3-δ has attracted interest due to its high ionic and electronic conductivity,2 of 13 howeverFollowing under lowthe priorp(O2 )work the oxide demonstrating ion (and electronicthe successful conductivity) incorporation are significantlyof silicate into reduced. SrFeO3- δThis is[23], due we to reportoxygen in loss this leading paper anto investigationa transformati intoon tothe the effect oxygen of sulfate vacancy and ordered borate doping.brownmillerite The Following the prior work demonstrating the successful incorporation of silicate into SrFeO [23], 3-δ 3-δ structure,undoped Srsystem2Fe2O 5SrFeO (Figure has1). Weattracted have thereforeinterest due investigated to its high the ionic effect and of electronic borate/sulfate conductivity, doping on we report in this paper an investigation into the effect of sulfate and borate doping. The undoped thishowever transformation. under low p(O2) the oxide ion (and electronic conductivity) are significantly reduced. This systemis due SrFeO to oxygen3-δ has loss attracted leading interest to a transformati due to its highon to ionic the andoxygen electronic vacancy conductivity, ordered brownmillerite however under lowstructure, p(O2) the Sr2 oxideFe2O5 (Figure ion (and 1). electronic We have conductivity)therefore investigated are significantly the effect reduced. of borate/sulfate This is due doping to oxygen on lossthis leading transformation. to a transformation to the oxygen vacancy ordered brownmillerite structure, Sr2Fe2O5 (Figure1). We have therefore investigated the effect of borate/sulfate doping on this transformation. Figure 1. Structure of Sr2Fe2O5 (showing oxygen vacancy ordering) (left) and SrFeO3 (right). Figure 1. Structure of Sr2Fe2O5 (showing oxygen vacancy ordering) (left) and SrFeO3 (right). 2. ResultsFigure and Discussion 1. Structure of Sr2Fe2O5 (showing oxygen vacancy ordering) (left) and SrFeO3 (right). 2. Results and Discussion 2. Results and Discussion 2.1. SrFe1−xSxO3-δ 2.1. SrFe1−xSxO3-δ 2.1. SrFe1−xSxO3-δ 2.1.1. X-ray Diffraction Data 2.1.1. X-ray Diffraction Data 2.1.1.A X-ray range Diffraction of samples Data of SrFe1−xSxO3-δ with increasing sulfate content (0 ≤ x ≤ 0.1) were prepared. A range of samples of SrFe1−xSxO3-δ with increasing sulfate content (0 ≤ x ≤ 0.1) were prepared. X-ray Adiffraction range of samplesanalysis of showed SrFe1−x SthatxO3- δwithout with increasing sulfate doping sulfate contentSrFeO3- δ(0 forms ≤ x ≤ a0.1) tetragonal were prepared. perovskite X-ray diffraction analysis showed that without sulfate doping SrFeO3-δ forms a tetragonal perovskite in line with prior reports. This is illustrated in Figure 2 (expanded region 2θ = 45 to 60°) where peak inX-ray line with diffraction prior reports.analysis Thisshowed is illustrated that without in Figuresulfate2 doping (expanded SrFeO region3-δ forms 2 θ a= tetragonal 45◦ to 60◦ perovskite) where peak splitting can clearly be observed. Upon doping with sulfate, there is a transformationθ to a cubic cell splittingin line with can clearlyprior reports. be observed. This is illustrated Upon doping in Figure with 2 sulfate, (expanded there region is a transformation 2 = 45 to 60°) where to a cubic peak cell (0.025splitting ≤ x can≤ 0.075), clearly where be observed. no peak Up splittingon doping is now with ob sulfate,served there (Figure is a 2).transformation Above x = 0.075, to a cubic small cell SrSO 4 (0.025 ≤ x ≤ 0.075), where no peak splitting is now observed (Figure2). Above x = 0.075, small SrSO 4 impurities(0.025 ≤ x ≤appear, 0.075), wheresuggesting no peak that splitting the solubility is now limitobserved of sulfate (Figure in 2).SrFe Above1−xSxO x3- =δ is0.075, x ≈ 0.075. small SrSO4 impurities appear, suggesting that the solubility limit of sulfate in SrFe1−xSxO3-δ is x ≈ 0.075. impurities appear, suggesting that the solubility limit of sulfate in SrFe1−xSxO3-δ is x ≈ 0.075. Figure 2. Cont. CrystalsCrystals 2017 2017, ,7 7, ,169 169 33 of of 13 13 Crystals 2017, 7, 169 3 of 13 Figure 2. a b c FigureFigure 2.2. X-rayX-ray diffractiondiffraction patternspatternspatterns for forfor ( (a()a) ) SrFeO SrFeOSrFeO3-3-δ3-δ,(,δ , (b(b)) ) SrFe SrFeSrFe0.9750.9750.975SSS0.0250.0250.025OOO3-3-δ3-,δ ,δ (,(c(c) ) SrFe)SrFe SrFe0.950.950.95SS0.050.05SO0.05O3-3-δO,δ , 3-(d(δd), ) (d) SrFe S O ,(e) SrFe S O , and (f) SrFe S O . The data show a tetragonal cell for SrFeSrFe0.9250.925S0.925S0.0750.075OO0.0753-3-δ,δ ,( e(e)3- )SrFeδ SrFe0.90.9SS0.10.1OO0.93-3-δ,δ 0.1,and and3- ( fδ(f) )SrFe SrFe0.850.85SS0.150.15OO0.853-3-δ.δ .The The0.15 data data3-δ show show a a tetragonal tetragonal cell cell for for SrFeO SrFeO3-3-δ,δ , ◦ ◦ SrFeO(as(as highlightedhighlighted3-δ, (as highlighted byby thethe peakpeak by the splittisplitti peakng:ng: splitting: seesee expandedexpanded see expanded regionregion region FigureFigure Figure 22θθ ==2 45θ45 = toto 45 60°),60°),to 60 whilewhile), while thethe the sulfate-doped samples are cubic. SrSO impurities (highlighted by an asterisk) are observed for sulfate-dopedsulfate-doped samplessamples areare cubic.cubic. SrSOSrSO4 4 impurities4impurities (highlighted(highlighted byby anan asterisk)asterisk) areare observedobserved forfor samplessamples withwith with higherhigher higher sulfatesulfate sulfate contentscontents contents (x(x (x == = 0.1 0.1 and and 0.15). 0.15). InIn order orderorder to toto provide provideprovide further furtherfurther support supportsupport for the forfor incorporation thethe incorporationincorporation of sulfate, ofof sulfate, equivalentsulfate, equivalentequivalent Fe-deficient Fe-deficientFe-deficient samples 0.95 3-δ weresamplessamples prepared werewere withoutpreparedprepared addition withoutwithout of addition sulfate.addition The ofof sulfate. X-raysulfate. diffraction TheThe X-rayX-ray data diffractiondiffraction for SrFe 0.95 datadataO3- δforfor(Fe-deficient, SrFeSrFe0.95OO3-δ 0.95 0.05 3-δ no(Fe-deficient,(Fe-deficient, sulfate) and no no SrFe sulfate) sulfate)0.95S0.05 and andO3- SrFe δSrFeare0.95 comparedSS0.05OO3-δ are are in compared Figurecompared3. These in in Figure Figure data 3. show3. These These the data data presence show show of the the impurities presence presence 0.95 3-δ forofof impurities SrFeimpurities0.95O3- δ forforalong SrFeSrFe with0.95OO3- peakδ alongalong splitting, withwith consistentpeakpeak splitting,splitting, with a consistentconsistent tetragonal withwith cell, asaa tetragonal fortetragonal the undoped cell,cell, asas SrFeO forfor the3-theδ 3-δ parentundopedundoped phase. SrFeO SrFeO In3- contrast,δ parentparent impurities phase.