Effect of Dispersion of Platinum Nanoparticles on the Proton-Conducting Properties of Strontium Cerate and Zirconate
Authors: Takamura Yasuhiro , Leonard Kwati , Matsumoto Hiroshige 
Published: February 2020

Introduction

Ionic properties of metal oxides possibly changes in the vicinity of the interface with a hetero-phase material due to the formation of space charge layer and/or the strain originated from different coefficients of thermal expansion between the two materials, followed by the rearrangement of charge carriers [1]. In particular, the former case can be examined in a material with well dispersed second phase particles as an intriguing approach to alter the macroscopic properties. [2, 3]. This so-called nanoionics composite materials can be applied in energy conversion devices such as solid oxide fuel cells that operate over low to intermediate temperature ranges if the effect enhances the ionic conductivity. In this work, we have examined and clarified the effects of homogeneously dispersed platinum nanoparticles on the proton conductivity of strontium cerate and zirconate perovskites. The work demonstrates that interfaces engineering can either enhance or reduce the conductivity of proton perovskites type oxides.
2. Experimental
SrCe0.95Yb0.05O3
−

δ

(SCYb), SrZr0.9Y0.1O3
−

δ

(SZY) and their composites with platinum were prepared by combustion synthesis using EDTA, citric acid, and ammonium nitrate as polymerization agents [2, 3]. The amount of platinum was adjusted so that the amount of platinum metal in the composite oxide was 0.5 vol % to the original metal oxide. Phase purity of the samples were confirmed by X-ray diffraction (XRD). The conductivity was measured using a four-probe AC impedance method with changing the surrounding atmospheres in turn, wet Air, wet Ar, wet 1%-H2- wet Ar, wet Air, which are all saturated with water vapor at 17°C: p(H2O)=1.9 kPa.
3. Results and discussion
Single phase perovskite type structures were confirmed for both platinum doped and undoped SZY (Pt-SZY) and SCYb (Pt-SCYb) sintered at 1350 and 1100 °C, respectively. These temperatures were optimal for complete solid dissolution of platinum, and no metallic platinum phases were detected. The highest relative density obtained for both specimens were 83.7 and 70.2 % respectively. The electrical conductivity of Pt-SZY and Pt-SCYb measured at 800°C under reducing and oxidizing atmospheres revealed a reversible nonionic phenomenon as a result of precipitation and dissolution of platinum nanoparticles. Fig.1 shows the electrical conductivity of Pt-SZY and Pt-SCYb as a function of time in the respective atmospheres. As observed, the electrical conductivity of both specimens are higher in air mainly due to hole conduction within this range. The electrical conductivity then decreases significantly when the atmosphere is change to hydrogen as shown in Fig. 1(a) for Pt-SZY. In the case of Pt-SCYb an opposite change takes place, i.e. an increase in the conductivity can be seen for the same changes of the atmosphere (Fig. 1(b)). Comparing these electrical conductivities with the un-doped electrolytes, the introduction of platinum is understood to have an effect that lead to the decrease in conductivity for Pt-SZY. This change in electrical conductivity has been explained in terms of a percolation model [2, 3] in which precipitated platinum nanoparticles are assumed to form a resistive skin in the oxide phase due to band bending, thereby blocking the electrical conduction pathways in SZY. The electrical conductivity behavior of Pt-SCYb in 1% H2 seems to suggest an accumulation of positive charges at the vicinity of the interface between Pt-SCYb with respect to the band bending phenomenon. Band bending usually occurs at the hetero-interface of proton conductors with metal due to the differences in the work function and the corresponding generation of space charges [2, 3]. This work function mismatch can either lead to an increase in the concentration of protons that are in equilibrium with the holes and electrons. However, the conductivity of Pt-SCYb, even though indicated an initial increase in 1% H2, the conductivity value is similar to the conductivity of SCYb in the same atmosphere. Thus the conductivity of Pt-SCYb decreases in air relative to the conductivity of SCYb.
References
1) J. Maier, Nature Materials, 4 (2005), p. 805
2) Matsumoto, H., Tanji, T., Amezawa, K., Kawada, T., Uchihisa, Y., Sakai, T., Ishihara, T., Solid State Ionics, 182 (2011), p. 13
3) Matsumoto, H., Furuya, Y., Okada, S., Tanji, T., Ishihara T., Science and Technology of Advanced Materials, 8 (2007), p. 531

Figure 1