Fluoride electrolyte based galvanic cell: Stability of the hollandite BaRu6O12(s)

Abstract

BaRu6O12(s), a quasi one dimensional hollandite with a tetragonal crystal structure, exhibits interesting electronic properties. The solid state reaction route was used for the synthesis of BaRu6O12(s), and was identified by X-ray diffraction analysis, its structural parameters determined and its hollandite structure illustrated. A fluoride ion conducting solid electrolyte based Galvanic cell was constructed and the standard molar Gibbs energy of formation of BaRu6O12(s) was determined. This cell was standardized using phase mixtures of {Ni(s) + NiF2(s)} and {Fe(s) + FeF2(s)} as the two electrodes and CaF2 as the solid electrolyte prior to the measurement of the cell electromotive force, given by:-Pt/O2(g),{CaO(s)+CaF2(s)}//CaF2//{BaF2(s)+RuO2(s)+Ru(s)+BaRu6O12(s)},O2(g)/Pt+.The measured electromotive cell measurements for the above cell along with known Gibbs energy values from literature were used to calculate the standard molar Gibbs energy of formation of BaRu6O12(s) from elements in their standard state and the equation of ΔfGo(T) for BaRu6O12(s) can be given by: ΔfGo(BaRu6O12(s))/kJ⋅ mol−1(±2.94) = - 2447.7 + 1.165 ⋅ (T/K).The slope and intercept of the above equation gives the standard molar entropy and enthalpy of formation respectively, at the average experimental temperature. Thermodynamic stability was determined of this important one dimensional hollandite BaRu6O12(s) from these studies for the first time.