Indium doping in SrCeO3 proton-conducting perovskites

doi:10.1016/j.jssc.2020.121210

Journal of Solid State Chemistry

Volume 284, April 2020, 121210

https://doi.org/10.1016/j.jssc.2020.121210 Get rights and content

Highlights

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Addition of 1 wt% of NiO allows to obtain dense sinters of SrCe_1-xIn_xO_3-a (x = 0.1, 0.2 and 0.3).
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Pmna crystal structure of SrCe_1-xIn_xO_3-a materials is stable up to 900 °C in air.
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With the increasing In doping oxygen conductivity of SrCe_1-xIn_xO_3-a decreases, while the activation energy increases.
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Proton and deuterium conductivity also decrease with In content in SrCe_1-xIn_xO_3-a.
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Highest proton/deuterium transference number are observed for increase with the indium content in SrCe_0.7In_0.3O_3-a.

Abstract

In this work we present results of studies of In³⁺ doping in strontium cerate, comprising structural aspects, and oxygen as well as proton conductivity. Crystal structure analysis of single-phase SrCe_1-xIn_xO_3-a (x = 0.1, 0.2 and 0.3) materials in 25–900 °C temperature range indicates presence of strong orthorhombic distortion of the perovskite-type structure, similar as for the undoped SrCeO₃. Limited sinterability of the obtained powders was mitigated by addition of 1 wt% of NiO, which allowed to manufacture dense sinters at 1400 °C. Electrochemical impedance spectroscopy measurements done in dry synthetic air show decrease of the ionic (oxygen) conductivity with the increase of In content, as well as associated increase of the activation energy. This indicates that formed oxygen vacancies are trapped in the structure. Overall, electrical conductivity for SrCe_1-xIn_xO_3-a in H₂O- and D₂O-containing atmospheres decreases with In content, but respective H⁺ and D⁺ transference numbers are larger for samples with higher indium doping. At 500 °C the highest proton and deuterium conductivity was recorded for SrCe_0.9In_0.1O_3-a, reaching up to 0.70·10⁻⁴ S cm⁻¹ and 0.26·10⁻⁴ S cm⁻¹, respectively. Derived diffusion and surface exchange coefficients are 10⁻⁷-10⁻⁶ cm² s⁻¹ and 10⁻⁶-10⁻⁵ cm s⁻¹, respectively in 500–700 °C temperature range.

Graphical abstract

Pnma symmetry for SrCe_0.7In_0.3O_3-a remains stable up to 900 °C in air. The hydrated material exhibits high proton transference numbers, which remain above 0.5 up to ca. 500 °C. Compound is characterized by low thermal expansion coefficient and relatively high values of transport coefficients, as derived from electrical conductivity relaxation experiments.

Introduction

Proton-conducting electrolyte materials are of great interest, which is due to their numerous applications, especially considering gas sensors, fuel cells, hydrogen separation membranes or high-temperature electrolyzes [[1], [2], [3], [4], [5]]. Application of proton-conducting perovskite-type oxides as solid electrolytes for Solid Oxide Fuel Cells (SOFC) seems to be an interesting alternative to the well-known oxygen ion-conducting electrolytes. Opposite direction of H⁺ flow in the electrolyte (in relation to movement of O²⁻) changes nature of the electrochemical reactions taking place at respective electrodes, which from the practical point of view is very beneficial. As water is produced at the air-electrode (cathodic) side, hydrogen fuel is not diluted, allowing for its complete utilization [2]. Moreover, for such the cell working in electrolysis mode, pure hydrogen can be produced (and compressed), without any additional gas separation or purification processes. Finally, usage of proton-conducting electrolytes allows to maintain high values of Nernst voltage in the cell, and it eliminates instability problem at the anode, since no water vapor is present there [6].

Since pioneering works about proton conductivity in (Ba,Sr)(Zr,Ce)O_3-δ oxides by Iwahara et al. [[7], [8], [9]], numerous proton-conducting perovskite have been studied, showing wide range of properties regarding easiness of incorporation of water into the structure and different values of proton conductivity at elevated temperatures [2,[10], [11], [12], [13], [14]]. Among them, acceptor type-doped cerium-based oxides present relatively high proton conductivity in atmospheres containing water vapor, with high H⁺ transference numbers at high temperatures [7,15]. Ce-site doping with (typically) Ln³⁺ selected lanthanides is necessary in order to induce presence of the oxygen nonstoichiometry, which play essential role for proton conductivity to occur. Present oxygen vacancies are indispensable for water incorporation into the lattice, during which process $O H_{O}^{•}$ defects are being reversibly formed in such materials [16].

Numerous studies are available regarding modification of SrCeO₃ parent material by doping with Eu, Ho, Mg, Sc, Sm, Tm, Y, La, Gd, Nd, Yb, Tb [2,[17], [18], [19], [20], [21], [22]]. Sr-deficient Sr_1-xCe_1-yM_yO_3-a (M = Gd, Yb) materials have also been investigated [20]. Generally, good proton conductivity has been observed, with e.g. results for terbium-doped SrCe_0.95Tb_0.05O_3-a exhibiting high values on the order of 10⁻³-10⁻² S cm⁻¹ in 500–900 °C range in hydrogen or methane-containing atmosphere [22]. Considering crystal structure, most of the papers report unmodified orthorhombic symmetry with Pnma space group (GdFeO₃-type structure), which is also observed for the undoped SrCeO₃ material [[23], [24], [25]].

Up to our best knowledge In-doping in SrCeO₃ has been very rarely investigated, despite that smaller indium (r_In3+ = 0.8 Å, r_Ce4+ = 0.87 Å at the 6-fold coordination) is expected to influence crystal structure by increase of the tolerance factor t. Reports for In-rich end member, i.e. Sr₂In₂O₅, are very scarce [26], suggesting difficulties in preparation of the compound. On the other hand, Ba₂In₂O₅ brownmillerite is very-well known [27]. Furthermore, if Ba₂In₂O₅ is doped appropriately to induce disordering of the oxygen vacancies, apart from the improved oxygen conduction, it also shows enhanced proton conductivity, due to the facilitated hopping of H⁺ between two adjacent oxygen sites [[28], [29], [30]]. In this work, systematic studies of In-doped SrCe_1-xIn_xO_3-a (x = 0.1, 0.2 and 0.3) perovskite oxides are reported, including crystal structure, hydration-related properties, as well as electrical conductivity in dry and wet atmospheres.

Section snippets

Experimental

All samples were prepared by high-temperature solid state route with respective oxides and strontium carbonate used as starting chemicals (all with ≥99.9% purity). After milling in a high-efficiency mill in propanol, the mixtures were dried and annealed at 1250 °C in order to decompose the carbonate. After several trials with different additives, in order to obtain dense sinters, the calcined powders were mixed with 1 wt% of polyvinyl butyral (PVB) in order to improve compressibility during

Crystal structure

All studied oxides SrCe_1-xIn_xO_3-a (x = 0.1, 0.2 and 0.3) could be obtained as single-phase materials, with no secondary phases visible (Fig. 1a-c). Structure of all three compounds can be refined with orthorhombic symmetry (Pnma space group) with good statistics, as presented in Table 1. This corresponds to the $\sqrt$ 2a_p × 2a_p × $\sqrt$ 2a_p multiplication of the simple cubic perovskite unit cell (with a_p parameter) in a, b and c axes, respectively. As mentioned above, the same GdFeO₃-type structure

Conclusions

All of the materials from SrCe_1-xIn_xO_3-a (x = 0.1, 0.2 and 0.3) group could be successfully synthesized by solid state method as single-phase materials, as well as dense sinters suitable for electrical conductivity studies were obtained with addition of 1 wt% of NiO. All compounds show orthorhombic Pnma symmetry at RT, which does not change considerable up to 900 °C in air. Similarly, like reported before in literature, structural parameters of the doped materials do not change linearly

Author contributions

Wojciech Skubida: Methodology, Data curation, Conceptualization. Kun Zheng: Conceptualization, Data curation, Validation, Writing - review & editing. Konrad Świerczek: Conceptualization, Reviewing. Mateusz Michna: materials synthesis. Łukasz Kondracki: TG data collection

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This project was funded by the National Science Centre, Poland, on the basis of the decision number UMO-2016/21/N/ST8/00268.

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Citation Excerpt :
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Indium doping in SrCeO3 proton-conducting perovskites

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Crystal structure

Conclusions

Author contributions

Declaration of competing interest

Acknowledgments

Solid State Ionics

J. Power Sources

Solid State Ionics

Solid State Ionics

Int. J. Hydrogen Energy

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

J. Alloys Compd.

Solid State Ionics

Mater. Res. Bull.

Solid State Ionics

J. Solid State Chem.

Solid State Ionics

J. Eur. Ceram. Soc.

J. Alloys Compd.

Thermochim. Acta

J. Alloys Compd.

Solid State Ionics

Int. J. Hydrogen Energy

Solid State lonics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Indium doping in SrCeO₃ proton-conducting perovskites