(Invited) Electrical Conductivity of Ceria-Based Oxide/Alkali Carbonate Eutectics Nanocomposites
Authors: Mizuhata Minoru , Kubo Hiroshi , Ichikawa Yudai , Maki Hideshi , Matsui Masaki 
Published: December 2020

Currently, the problem of global warming and exhaustion of fossil fuel has become a big problem on the global scale, therefore a highly efficient electric power generating device that does not emit greenhouse gases such as carbon dioxide is required. Especially, since high temperature fuel cell such as solid oxide fuel cell (SOFC) or molten carbonate fuel cell (MCFC) operate at high temperature, these have the advantages that efficiency is high and expensive noble metal catalysts are unnecessary for the electrode. However, the operation at high temperature leads to degeneration of the cell component, which makes it unsuitable for long lifetime. Thus, attempts to lower the operating temperature have been made.1

Ceria-based oxide doped with trivalent Sm and Gd (SDC or GDC) was noticed, it achieves sufficient oxygen ion conductivity even in the middle temperature range of 773-973 K. However, with only ceria-based oxide, it indicates electron conductivity, which makes open circuit voltage lower. Adding carbonate makes composite indicated good electron insulator, ceria-based oxide/molten carbonate composite electrolyte is one of the candidate of middle temperature fuel cells. In the composite material, the molten carbonate is impregnated in the oxide and the interfacial layer is formed between solid oxide and carbonate due to solid-liquid interaction.
In this study, we applied nano-ordered SDC powder prepared by the complex polymerization method to composite of SDC/molten carbonate eutectics.2 The ionic conductivity using the ac impedance measurement, the thermal properties and spectral properties of carbonate was measured in order to discuss the influence of the solid phase to the ionic conduction near the solid phase.
Ceria and SDC were prepared by the Pechini method using citric acid and ethylene glycol mixed with predetermined amounts of Ce(NO3)3 and Sm (NO3)3 aqueous solution. Obtained Sm3+ doped CeO2 samples indicate Sm ratios of 10 and 20 mol% (SDC10 and SDC20). As the liquid (melt) phase, alkali carbonates were dehydrated in CO2 at 473 K for 48 hours, and the prepared eutectics; (Li0.52Na0.48)2CO3 (LN) and (Li0.43Na0.32K0.25)2CO3 (LNK) were used. Oxide and eutectics were mixed so as to obtain predetermined liquid phase volume fraction to obtain CeO2-eutectics or SDCs- eutectics. The mixed sample was pressed at 60 MPa for 30 minutes and fired at eutectic temperature or higher to prepare pellets for AC impedance measurement in the temperature range of 623-773K in 25 Hz -1 MHz. The eutectic point of the bulk LN is seen at 771 K, and the eutectic point is shifted to lower temperature by mixing with oxide. When the liquid phase volume fraction is around 60-90 vol%, no remarkable change in the eutectic point of LN was confirmed. However, the significant decrease in the eutectic point was confirmed from the region of 60 vol% or less in any oxides. The endothermic peak associated with melting decreased as the amount of liquid phase decreased, and finally disappeared below 30 vol% or less. On the other hand, when compared with past results using CeO2 (1.5 m2 g-1) having a small specific surface area as solid phase, the melting enthalpy was confirmed even when the liquid phase was 5 vol%.
The temperature dependences of electrical conductivity in ceria-based oxide/LN and LNK coexisting system are shown in Fig.1. For using LN eutectics, the transition point due to melting of the eutectic molten salt was observed at around 720 K at the liquid phase of 25 vol% or more. The conductivity at the liquid phase volume fraction of 45 vol% is smaller than the value of 15-35 vol% in the region below the transition point. As the liquid phase amount increases, the bulk molten salt crystallizes, from which, it is considered that the crystallized molten salt inhibited the electrical conduction pathway. In addition, at 15 vol%, the conductivity did not change remarkably even before and after the transition point, and there was no melting enthalpy change at 25 vol% or less, so from these results, the molten salt existing at the solid interface is in molten state even below the transition point. For LNK system, the effect of solid phase is more effective than that for LN systems. The activation energy of conductivity increased with decreasing apparent average thickness. This phenomenon was remarkable where an abrupt increase was observed below the apparent average thickness of 1 nm.

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Figure 1