New materials to replace reference electrolytes for PCFC and SOFC

Olivier Joubert (MC), Yves Piffard (DR), Samuel NOIRAULT (Thesis)

Electrolytes are ionically conducting oxides (with an ionic transport number close to unity) in the form of membranes, dense enough to be impermeable to gases and thin enough to minimize ohmic losses, that is a demanding set of electrical (conductivity level), chemical (redox stability, no reactivity with anode and cathode materials) and mechanical (stable microstructure) properties.

Fig. 1 : Schematic representation of SOFC and PCFC fuel Cells

Challenges

• SOFC : reducing the operating temperature of YSZ (e.g. 8% yttria stabilized zirconia : σ700°C= 0.01 S cm-1) whilst keeping the conductivity level, that is reducing the thickness of the membrane or finding NEW materials.
• PCFC : replacing BCY (BaCe0.9Y0.1O3-δ : σ600°C= 0.01 S cm-1) by NEW materials with a long-term stability under CO2-containing atmospheres

Our strategies to identify new electrolytes

• Creating disorder within an ordered arrangement of oxygen vacancies,
• Metal substitution with lower valent cations to form or increase charge-compensating oxygen vacancies,
• For PCFC : systematic testing of the incorporation of protons by equilibration with water vapor → TGA and/or impedance measurements.
• Chemical and crystallo-chemical requirements for fuel cell application:
  • Redox stability,
  • Chemical stability with respect to hydrolysis, CO2 containing atmospheres,
  • Compounds with some rather weak M-O bonds.

Some examples of new electrolytes

By creating disorder within an ordered arrangement of oxygen vacancies :

The oxygen-deficient Ba₂In_2(1-x)Ti_2xO₅x₁-x (0 ≤ x ≤ 1) compounds [1] :
Substitution of In by Ti in Ba₂In₂O₅ : for 0.15<x<1, all ba₂in_2(1-x) ti_2xO_5+x1-x members adopt a disordered cubic perovskite structure at RT (Fig 2) → Improvement of the anionic conductivity (Fig. 3).

The Cuspidine-like compounds Ln₄[Ga_2(1-x)Ge₂xO_7+x1-x]O₂ (Ln = La, Nd, Gd; x ≤ 0.4) [2]
Nd₄(Ga₂O₇)O₂ is a group member of oxo-digallates with the Cuspidine structure (Fig. 4).
Upon substituting Ge₄+ for Ga₃+, the Cuspidine structure is maintained up to x = 0.4. The Ge₄+ substitution for Ga₃+ with concomitant filling of oxygen vacancies is beneficial to the anionic conductivity which increases rather regularly with x to reach 10-3 S/cm at 800°C for x = 0.4 and Ln = Nd.

	Fig. 2 : thermal evolution of Ba2In2O5: structure and conductivity
Fig. 3 : conductivity of  Ba2In2(1-x)Ti2xO5x1-x
	Fig. 4: [010] view of a fragment of the Nd4(Ga2O7)O2 structure showing chains of Ga2O7 groups, Nd atoms and O atoms bonded to Nd only. Empty sites between Ga2O2 groups can be considered as anion vacancies.
By forming or increasing charge-compensating oxygen vacancies (e.g. by Nd, Ge and Ga substitutions with lower valent cations): New Oxygen Ion Conductors based on Nd4GeO8 and Nd3GaO6 [3]. New anionic conductors Nd4Ge1-xGaxO8-x/2 (x≤0.1), Nd3Ga1-xMxO6-x/2 (M=Zn,Mg; x≤0.03) and Nd3(1-x)M'3xGaO6-3x/2 (M'=Ca; x≤0.03 and M'=Sr; x≤0.015) have been prepared by a combustion technique. These small substitution rates induce a significant increase in anionic conductivity (σ800°C = 0.2 to 0.7 10-2 S/cm) (Fig. 5) with respect to pure Nd4GeO8 and Nd3GaO6 (σ800°C≈3.10-4 S/cm). Fig. 5 : Conductivity data for : Nd3O2GaO4 (squares), Nd2.91Ca0.09GaO5.955 (triangles), Nd2.955Sr0.045GaO5.9775 (circles) ->
	In the search for new electrolyte for PCFC : example of fast proton conduction in La26O27(BO3)8 The monoclinic oxy-borate La26O27(BO3)8 is an inherently oxygen deficient compound. It can react with water vapor (Fig. 6) and incorporate protons leading to the oxy-hydroxy-borate La26O26(OH)2(BO3)8. This reaction is reversible. <- Fig. 6 : TG analysis of the as prepared La-phase heated at 1K min-1 in ambient air
As an alkali-earth free material La26O27(BO3)8 is an interesting candidate electrolyte for a proton ceramic fuel cell  (Fig. 7) that would operate in a CO2-containing atmosphere. To the best of our knowledge La26O27(BO3)8 is the first example of inherently oxygen deficient material with a crystal structure of low symmetry exhibiting fast proton conduction Fig. 7 Total conductivity for La26O27(BO3)8under dry O2 (open squares), wet atmospheres N2 (open triangles), H2 (Ar/H2(5%), full triangles) and N2/D2O (open circles)

participants

Olivier JOUBERT (MC), Yves PIFFARD (DR), Samuel NOIRAULT (Thèse)