Elsevier

Solid State Ionics

Volume 272, April 2015, Pages 9-17
Solid State Ionics

Electrical conductivity and grain boundary composition of Gd-doped and Gd/Pr co-doped ceria

https://doi.org/10.1016/j.ssi.2014.12.006Get rights and content

Highlights

  • We employed impedance spectroscopy and transmission electron microscopy.

  • Pr/Gd co-doped ceria had greater grain boundary conductivity than Gd-doped ceria.

  • We employed electron energy-loss spectroscopy in a scanning electron microscope.

  • Gd and Pr segregation was characterized for grain boundaries.

  • Grain boundaries are suspected to play role in conductivity enhancement.

Abstract

We characterize electrical conductivity, microstructure, nano-scale grain boundary structure and chemistry of ceria electrolytes with nominal compositions of Gd0.2Ce0.8O2-δ (GDC) and Gd0.11Pr0.04Ce0.85O2-δ (GPDC). The electrolytes are fabricated using mixed oxide nanopowders synthesized by spray drying. AC impedance spectroscopy was performed from 150 °C to 700 °C in air to determine grain-interior electrical conductivity. Grain-boundary conductivity was determined below 300 °C. The grain-interior conductivity of the GPDC was higher than that of GDC by as much as 10 times, depending on the temperature. The GPDC specific grain-boundary conductivity was measured to be approximately 100 times higher than that of GDC. Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) confirmed the grain-to-grain compositional uniformity of both materials following heat treatments. Grain boundaries were free of glassy intergranular phases; dopant concentration and Ce oxidation state were found to vary significantly near grain boundaries. Boundary core composition was estimated from STEM EELS to be Gd0.62Ce0.38O2-δ, and Gd0.29Pr0.16Ce0.55O2-δ in GDC and GPDC, respectively. Pr segregation to grain boundaries in the GPDC is hypothesized to enhance conductivity by both decreasing oxygen vacancy migration energy, and inducing mixed ionic–electronic conductivity in the near-boundary region.

Introduction

Intermediate temperature (350 °C to 550 °C) oxygen ion conductors and mixed ionic and electronic conductors have received considerable attention in recent years due to their potential applications in devices such as oxygen sensors, oxygen generators, separation membranes and solid oxide fuel cells (SOFCs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. Owing to its relatively high ionic conductivity under non-reducing conditions and at intermediate temperatures, ceria doped with trivalent cations such as Gd3+ or Sm3+ has emerged as a promising candidate material to provide the desired performance in this operating temperature range. Other dopants such as mixed-valence Pr3+/4+ induce electronic conductivity yielding mixed ionic and electronic conductivity, an attractive possibility for applications which require both electronic and ionic currents. Furthermore, doubly-doping with two cation species has been explored by experiment [2], [5] and simulation [3]. For instance Lubke et al. demonstrated increased electronic conductivity, and total ionic conductivity as the result of decreased grain boundary resistance in Gd-doped ceria upon the addition of Pr [5]. This result is in accordance with theoretical work based on density functional theory and Monte Carlo simulations conducted by Dholobai et al. who predicted increased ionic conductivity in ceria doped with both Pr and Gd [3].

The conductivity in a polycrystalline fluorite based oxide arises from conductivity through grains and across grain boundaries. The grain boundaries are typically orders of magnitude less conducting than the grain interior at low and intermediate temperatures. This reduced conductivity originates from space-charge effects which hinder the transport of ions across the boundary [10]. Changes in the structure and composition may also have a substantial influence on the grain-boundary conductivity. For example, formation of thin silica layers due to impurity segregation to the grain boundary during heat treatment constricts the ion migration pathway, reducing the electrical conductivity [10]. Conversely, segregation of some transition metals may reduce the magnitude of the electrostatic potential barrier at the grain boundary and enhance grain-boundary conductivity [12]. The complex interplay between composition, structure and electrical properties of grain boundaries is still not well understood. Understanding the role of nanoscale structure and composition on grain-boundary electrical properties requires the correlation of advanced transmission electron microscopy (TEM) with techniques such as impedance spectroscopy. In this work, we employ AC impedance spectroscopy and high spatial resolution TEM to investigate the electrical properties and nanoscale structure and chemistry of bulk ceramics fabricated with spray-dried ceria doped with Gd, and doubly-doped with Gd and Pr.

A number of approaches have been employed to synthesize starting powders, or to simultaneously synthesize starting materials and fabricate practical structures for the study of solid ceramic electrolytes and electrodes. The overarching goal of these techniques is to fabricate powders or device components with tunable chemistry and predictable microstructures which provide the desired properties of the final component (e.g. high sintered density, ionic conductivity and ionic transference for SOFC electrolytes). Researchers have reported using a diverse set of synthesis approaches including co-precipitation [8], solid-state reactions [1], [11], [12], spray pyrolysis [13], pulsed-laser deposition [14], DC sputtering [15], electrostatic spray deposition [16], combustion and microwave syntheses [17] and aerosol deposition [18]. Due to its simplicity, low cost, high yield, and ability to continuously produce nanoscale powders and deposit layers of tunable composition, spray drying has also garnered attention for use in the synthesis of several materials including rare-earth doped ceria [6], [19], [20], [21], [22], [23]. During spray drying, pressurized gas atomizes a liquid solution of precursor ions into a fine mist which enters a reaction vessel where it is rapidly heated to produce solid solution particles that are compositionally mixed at the nanometer scale.

An essential prerequisite of this work is to demonstrate that the spray drying method is an effective approach for fabricating doped ceria model electrolytes. The ability to employ a wide range of different nitrate salts in the spray drying approach makes it possible to synthesize electrolytes with a wide range of different composition. We first show that a conventional Gd-doped ceria (GDC) electrolyte can be easily fabricated and the resulting electrical properties (measured with impedance spectroscopy) are comparable to GDC electrolytes synthesized with other approaches. We then demonstrate that it is easy to introduce multiple dopants into the formulation by preparing an electrolyte co-doped with Gd and Pr. Finally, we investigate the effect of single and double doping on the grain boundary structure, chemistry and electrical conductivity. Analysis of electrical properties shows that the grain boundaries in the co-doped sample are almost 100 times more conducting than the singly doped boundaries. The enhanced conductivity may be the result of strong Pr segregation to the grain boundaries that we observe using aberration corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS).

Section snippets

Experimental

We fabricated doped ceria powders using a spray-drying synthesis technique developed by the authors and described elsewhere [6]. In this technique, an aqueous solution is sprayed as a fine mist into a reaction vessel where it is introduced to a stream of air heated to approximately 300 °C. The hot air rapidly dries the solution droplets and initiates the decomposition of the nitrate precursors into oxide. Precursors of 99.998% purity Ce(NO3)3·6H2O, Gd(NO3)3·6H2O, and Pr(NO3)3·6H2O (Alpha Aesar,

Electrical properties

Fig. 1 displays representative impedance data as Nyquist plots acquired at 200 °C from pellets fabricated from spray dried GDC and GPDC powders. Both spectra exhibit two arcs corresponding to the grain interior and grain boundary polarizations. An arc corresponding to the electrode polarization was also visible in the GPDC spectrum; however, it was not included in the curve fitting procedure described below. Nyquist plots were interpreted by fitting to an equivalent circuit model comprised of a

Conclusions

Gd0.2Ce0.8O2-δ (GDC) and Gd0.11Pr0.04Ce0.85O2-δ (GPDC) powders were successfully synthesized with spray drying, and used to fabricate sintered pellets for bulk electrical characterization by AC impedance spectroscopy performed between 150 °C and 700 °C. Following electrical characterization, specimens were analyzed via scanning transmission electron microscopy (STEM) using high resolution imaging, energy dispersive X-ray spectroscopy (EDX) and electron-energy-loss spectroscopy (EELS) in a number

Disclaimer

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

The full description of the procedures used in this manuscript includes identification of certain commercial products and their suppliers. The inclusion of such information should in no way be construed as indicating that such products or suppliers are endorsed by NIST or are recommended by NIST or that they

Acknowledgments

The authors wish to thank the Fulton Undergraduate Research Initiative at Arizona State University, the National Institute of Standards and Technology's Summer Undergraduate Research Fellowship, Arizona State University's Doctoral Enrichment Fellowship, and the National Science Foundation's Graduate Research Fellowship Program for their gracious support without which this work would not have been possible. We also gratefully acknowledge the staff that assisted this work, as well as the use of

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