Investigation of The Role of Dielectric Relaxation in Resulting Superior Conductivities in Oxide Ion Electrolyte for Solid Oxide Fuel Cells

Abstract

Fuel cells are the electrochemical devices capable of continuously converting the chemical energy stored in a fuel such as hydrogen or methane to electricity. Fuel cell technologies can be employed to gradually reduce fossil fuel dependency and environmental impact compared to conventional combustion-based power generation technologies. Solid oxide fuel cells (SOFCs) use ceramic oxide-ion or proton-conducting electrolytes as separators to electrochemically connect the fuel (Hydrogen or Methane) to oxidant (air or oxygen) to produce electricity. SOFC operates at higher temperatures to avoid the ohmic losses generated due to lower ionic conductivities of electrolyte separators. One of the key approaches to lowering the operating temperature of solid oxide fuel cells (SOFCs) is to develop novel electrolyte materials with superior ionic conductivities, thermo-mechanical properties, and low processing temperature compared to the well-known ones perovskite and fluorite based oxide ion conductors. This thesis focuses on the synthesis of oxide-ion conductors and establishing the role of high κ dielectric in accelerating the motion of oxide ion vacancies. Novel materials were synthesized employing the solid-state and sol-gel auto combustion method. Samples were characterized structurally (XRD), morphologically (SEM, TEM-EDAX), and thermally (TGA, DSC) thoroughly. Spectroscopy studies (Impedance, Raman, FTIR) were also deliberated to understand the transport properties of the samples. Perovskite oxides with anionic vacancies (inherent or created by doping) perform as oxide ion conductors in a dry atmosphere at higher temperatures and proton conductors in a hydrated atmosphere at lower operating temperatures. Novel high κ dielectric/ferroelectric KTaO3 perovskite structure is envisaged here as a host lattice to develop a superior oxide-ion electrolyte. The simultaneous substitution of Ti and Ge developed lossy nature of high κ relaxor dielectricity or polarization in KTa0.4Ti0.3Ge0.3O2.7, resulting in high oxide-ion xix conductivity at elevated temperatures (Oxide-ion conductivity σo> 10-2 s/cm, T ≥ 550oC). The maxima of the dielectric constant or highest dielectric constant were found at ~ 5300 at applied frequency 20 kHz at 650oC, and Tm was varying or decreasing with increasing applied frequency. This linear decrease of Tm from 650oC to 550oC from 20 kHz to 100 kHz of applied frequency confirms the relaxor/leaking nature of high κ dielectric behaviour of KTa0.4Ti0.3Ge0.3O2.7. The high oxide-ion conductivity of the samples was observed in correlation with the Tm variation with the applied frequency establishing the role of high κ dielectric in accelerating the motion of oxide-ion vacancies within the percolation limit inside the host crystalline structure. Similarly, doped BaZrO3 and SrTiO3 are the other perovskites explored in this thesis. To overcome the low ionic conductivity issues of BaZrO3 & SrTiO3 based materials at elevated temperatures, the simultaneous doping of smaller Ga on Zr and Ti site and K on Ba and Sr site respectively in BaZrO3 and SrTiO3 was employed here to create a higher concentration of oxide-ion vacancies to realize superior oxide-ion conductivities. The simultaneous substitution of K and Ga created the oxygen vacancies type point defects resulting in higher ionic conductivity ~10-2 S/cm above 650oC. The conductivity represented here for Ba0.8K0.2Zr0.8Ga0.2O2.8 and Sr0.8K0.2Ti0.8Ga0.2O2.8 sample is superior or equivalent to the conductivity obtained for the yttria-stabilized zirconia; a well known ceramic oxide-ion electrolyte. To develop a superior alternative to YSZ, cubic phase of ZrO2 was stabilized with simultaneous doping of Bi and Gd in the form of Zr1-x-yBixGdyO2-δ and was proposed as an oxide-ion conductor to be an electrolyte for IT-SOFC. ZrO2 is a key constituent in the perovskite structure, and ZrO6 octahedra act as a central point in providing high dielectric and ferroelectric constant in PZT-based perovskite structures. With the incorporation of Bi3+ and Gd3+ ions, in conjunction with lone pairs of Bi, the cubic Zr1-x-yBixGdyO2-𝜹 phase showed relaxor type high κ dielectric behaviour (έ= 9725 at 600 oC at applied frequency 20 kHz) with xx Tm approaching to 600oC. Synergistic interaction of Bi and Gd in the host ZrO2 lattice also seems to create the more labile or movable oxide ion vacancies that enable superior oxide-ion transport resulting in high oxide ion conductivity (σo> 10-2 s/cm, T > 500oC) at relatively lower temperatures. Therefore, in this thesis, I conclude that dielectric relaxation is the crucial fundamental feature that plays important role in the oxide-ion conductivities of the sample. I have already shown that as Tm approaches, the conductivity maximizes in the samples. These unique features can be used as directive principles to develop new superior oxide-ion electrolytes for SOFCs.