Hybrid Fuel Cells with Carbonate/Oxide Composite Electrolytes: An Electrochemical and Theoretical Insight

In view of increasing the lifetime of high temperature fuel cells, lowering the working temperature is probably the most convenient solution. The targeted temperature range is 400-600°C, low enough to minimize the degradations kinetics while keeping rather good electrochemical performances and good conversion efficiencies. Carbonate/oxide composites are very promising electrolyte materials in hybrid fuel cells which could operate at lower temperature than the usual Molten Carbonate Fuel Cells (MCFC) or Solid Oxide Fuel Cells (SOFC). Suggested first by B. Zhu several years ago[1], the origin of the elevated performances of these materials is not fully understood, even though more research groups are involved in this thematic. Although the role of oxide ions and carbonate ions in the electrolyte performances is clear, a deeper understanding of these cells is needed. In the literature, R. Raza suggested in 2010 the role of the carbonate/oxide interface as a “superionic conduction pathway”, without explaining the transport mechanisms involved [2].

The aim of the current presentation is to get a deeper understanding on the origins of such improved performances by combining both experimental and modeling approaches. Experimentally, systematic studies dealing with the impact on conductivity of molten salts composition, oxide phase conductivity, environmental parameters (reducing and oxidizing atmospheres, temperature, cycling) were performed by impedance spectroscopy [3]. Moreover, Density Functional Theory calculations were carried out to provide a better understanding on the transport mechanisms and the species involved, by first determining the most stable surface structures for both phases, separately, before building different carbonate/oxide interfaces and investigating the operating principles of these cells. Regarding the theoretical calculations, periodic DFT calculations were performed on the bulk structure of ZrO₂, LiKCO₃, LiNaCO₃, evaluating the performances of different Gaussian-type basis sets and various exchange-correlation functionals, in order to select a reliable computational protocol that accurately describes the basic components of hybrid fuel cells. Considering the importance of the interface phenomena in composite materials, this protocol has then been used to examine the surface chemistry of the oxide and carbonate phases. The study of the electronic and structural properties of the most stable (001) and (110) surfaces of (LiK)CO₃ and (LiNa)CO₃ and the systematic investigation of the reducibility properties and of the stabilization of the cubic (111) of ZrO₂ through doping with Y₂O₃ (8 mol %) allowed us to identify a suitable surface model relative to the two phases that can be used to further simulate the interface of the composite material [4,5]. As far as we know, the combination of both approaches has never been reported in the literature and the first results dealing with YSZ and Li-Na or Li-K carbonates eutectics will be presented.

1. B. Zhu, Journal of Power Sources, 114 (2003) 1-9

2. R. Raza, X. Wang, Y. Ma, X. Liu, B. Zhu , International Journal of Hydrogen Energy, 35 (2010) 2684-2688

3. B. Medina-Lott, Ph-D thesis (2012) Paris6, France

4. C. Ricca, A. Ringuedé, M. Cassir, C. Adamo, F. Labat, Journal of Computational Chemistry 1 (2015) 9-21

5. C. Ricca, A. Ringuedé, M. Cassir, C. Adamo, F. Labat, RSC Advances 5 (2015) 13941-13951