The interface of two materials having different work function leads to the development of space charge, resulting in the change of defect equivalence and hence the change in the charge carrier concentration. Another possibility lies in the formation of strain at the interface resulting in the change of the mobility of ionic charge carriers. If we can use these effects for the enhancement of ionic conductivity, introduction of hetero-interface is a potential guideline for designing new ion conduction in solids.

We have reported previously that when Pt nanoparticles precipitate in proton conducting SrZr_0.9Y_0.1O_3-δ (SZY), the electrical conductivity decreases markedly due to nanoionics effects; small amount of Pt can be dissolved into the Zr site and becomes zero-valent upon exposure to hydrogen [1]. Such a change in the macroscopic electrochemical properties is considered as a result of the unique microscopic electrochemical properties of the nanoscale space charge layer generated in the vicinity of the interface when the Pt nanoparticles are precipitated [2]. Using electrochemical spectroscopy, SrCe_0.95Yb_0.05O_3-δ (SCYb) and Sr(Zr,Ce,Y)O₃ proton conducting oxide thus doped with Pt were investigated with the aim to clarify the effects and mechanism of the Pt/oxide interface on the electrical properties of proton conducting oxide.

Pt-doped proton conducting oxides were prepared by combustion synthesis method. The electrical conductivity of Pt-SZY and Pt-SCYb measured at 800 °C under 1% H₂ and air atmospheres revealed a reversible nanoionics phenomenon as a result of precipitation and dissolution of platinum nanoparticles (Fig.1). The electrical conductivity then decreases significantly when the atmosphere is change to hydrogen for Pt-SZY. In the case of Pt-SCYb an increase in the conductivity can be seen for the same changes of the atmosphere. This change in electrical conductivity has been explained by the effect of precipitated Pt particles. In other words, a proton deficient region is formed in the vicinity of the interface between Pt and SZY whereas at the interface between Pt and SCYb, there is no such influence.

Also, this result can be applied to deepen understanding of the reaction at the electrodes of the electrochemical cell. Comparing the electrode overvoltage when SZY and SCYb are used for the electrolyte in a cell using Pt as the electrode, SZY is several orders of magnitude larger than SCYb [3]. The reason for this can be explained if it is thought that the region where protons are not present is formed near the interface of Pt / SZY like as the case of Pt nanoparticles disperse in the bulk of SZY. On the other hand, the reason why the overvoltage at Pt/SCYb interface is smaller than that at SZY is considered to be a reasonable result of the loss of protons not taking place significantly near the interface.

References

[1] H. Matsumoto et. al., Solid State Ionics, 182 (2011), p. 13

[2] J. Maier, Nature Materials, 4 (2005), p. 805

[3] H. Matsumoto et. al., J. Alloys Compd. 408–412 (2006), p. 456