RESEARCH SCHOLAR, IIT DELHI
Strontium Doped Tungsten Cobaltite as Cathode Material for Solid Oxide Fuel Cell
Solid oxide fuel cell (SOFC) is a ceramic power device which directly converts chemical energy into electrical energy. It operates between 400 -1000 degree Celsius. It is a multi-fuel (eg. Hydrogen, Methane, Methanol, Carbon Dioxide etc.) capable ceramic power device. It does not suffer from carbon mono-oxide poisoning. Overall efficiency of solid oxide fuel cell is greater than 70%. The major applications of SOFC are: Stationary electrical power generation, Marine power, Aircraft Auxiliary Power Units. Every fuel cell has two electrodes called, respectively, the anode and cathode. The reactions that produce electricity take place at the electrodes. Every fuel cell also has an electrolyte, which carries electrically charged particles from one electrode to the other, and a catalyst, which speeds the reactions at the electrodes. In conventional SOFC, 8 mol % Yttria stabilized zirconia (YSZ) is used as electrolyte, which is a pure ionic conductor. Anode is composite of Nickel-Yttria stabilized zirconia (Ni-YSZ) which provides electronic conductivity. Lanthanum strontium manganite, La0.8Sr0.2MnO3 (LSM) is used as cathode, which provides electronic conductivity. My research work is development of cathode materials for solid oxide fuel cells. Cathode must have: (1) mixed conductivity which provide ionic and electronic conductivity, (2) chemical compatible with other fuel cell components which avoids thermal expansion coefficient (TEC) mismatch, (3) long triple phase boundary (TPB) (where electrode, gas phase and electrolyte come in contact), (4) porosity so that oxygen can easily transferred through cathode and reached at electrode and electrolyte interface, (5) stable in a oxidizing environment, (6) catalytic activity for oxygen surface exchange, (7) fine particle size or better performance. Lanthanum strontium manganite (LSM) cathode and yttria-stabilized zirconia (YSZ) electrolyte react at high temperature sintering during fabrication and form insulating phases at the interface in solid oxide fuel cell (SOFC) which degrade the electrochemical performance. To overcome such loss in performance, mixed ionic and electronic conducting (MIEC) electrodes with a large surface area can be used in SOFC. Strontium doped tungsten cobaltite (Sr0.5W0.5CoO3) has been synthesized, using solid state route and used as cathode in SOFC. Strontium doped tungsten cobaltite shows ionic conductivity of 0.03 S/cm at 800 °C and activation energy 0.81 eV between 600-800°C.
Abstract: Structure, electrical conductivity and electrochemical performance for a new oxygen deficient perovskite material with composition SrCo0.85Fe0.15O2.62 is explored here. It crystallizes in a “314-type” oxygen-vacancy ordered structure (Space Group: I4/mmm) with 2ap × 2ap × 4ap unit cell, and contains alternate oxygen replete [Co2/Fe2-O] and oxygen deficient [Co1/Fe1-O] planes along c-direction. Room temperature Mössbauer spectroscopy and room temperature neutron powder diffraction confirm the presence of mixed valence states (3+ and 4+) for both Fe and Co. Polaronic behavior in the electrical conductivity is observed below 400 °C, and attributed to the presence of Co4+ and Fe3+ cations in the structure. The electrical conductivity at 600 °C is high enough (∼200 S cm−1) to act as a cathode material. At the same time, the polarization resistance measurements show an excellent value of 0.1 Ω cm2 at 600 °C. These results suggest that at 600 °C, this material is a mixed oxide-ion and electronic conductor exhibiting excellent activity for the oxygen reduction reaction, making it a promising cathode material for an intermediate temperature solid oxide fuel cell.
Pub.: 09 Jun '17, Pinned: 27 Jul '17
Abstract: Molecular and dissociative adsorption of oxygen on the surfaces of solid oxide fuel cell cathodes of La0.8Sr0.2CoO3, La0.6Sr0.4CoO3, La0.6Sr0.4Co0.2Fe0.8O3, and La0.8Sr0.2MnO3 were investigated using temperature-programmed desorption measurements for molecularly and atomically adsorbed oxygen. The desorption of molecularly adsorbed oxygen occurred at 140–150 K, and the adsorption sites for molecularly adsorbed oxygen were mostly oxygen vacancy sites that were formed by combinative desorption of surface oxygen atoms at 550–580 K. In the case of La1–xSrxCoO3, the recovery of oxygen vacancy sites required oxygen adsorption above 523 K, which indicates that the dissociative adsorption of oxygen at the oxygen vacancy sites was an activation process. Oxygen adsorption is the first step in the cathode reactions of fuel cells, and the oxygen adsorption properties of each cathode material are discussed with respect to the activity of the cathode.
Pub.: 14 Jun '17, Pinned: 27 Jul '17
Abstract: Design of optimal microstructures for infiltrated solid oxide fuel cell (SOFC) electrodes is a complicated task because of the multitude of electro-chemo-physical phenomena taking place simultaneously that directly affect working conditions (cell temperature, current density, and flow rates) of the SOFC electrode and therefore its performance. In this study, an innovative design paradigm is presented to obtain a part of geometry-related electrochemical and physical properties of an infiltrated SOFC electrode. A range of digitally realized microstructures with different backbone porosity and electrocatalyst particle loading under various deposition conditions are generated. Triple phase boundary (TPB), active surface density of particles, and gas transport factor are evaluated in realized models on the basis of selected infiltration strategy. On the basis of this database, a neural network is trained to relate desired range of input geometric parameters to a property hull. The effect of backbone porosity, loading, distribution, and aggregation behavior of particles is systematically investigated on the performance indicators. It is shown that from the microstructures with very high amount of TPB and particle contact surface density, a relatively low gas diffusion factor should be expected; meanwhile, increasing those parameters does not have sensible contradiction with each other. Excessive agglomerating of particles has a negative effect on TPB density, but the distribution of seeds always has a positive effect. Direct search and genetic algorithm optimization techniques are used finally to achieve optimal microstructures on the basis of assumed target functions for effective geometric properties.
Pub.: 04 Jul '17, Pinned: 27 Jul '17
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