Rojana Pornprasertsuk

Predicting ionic conductivity of solid oxide fuel cell electrolyte from first principles

First-principles quantum simulations complemented with kinetic Monte Carlo calculations were performed to gain insight into the oxygen vacancy diffusion mechanism and to explain the effect of dopant composition on ionic conductivity in yttria-stabilized zirconia (YSZ). Density-functional theory (DFT) within the local-density approximation with gradient correction was used to calculate a set of energy barriers that oxygen ions encounter during migration in YSZ by a vacancy mechanism. Kinetic Monte Carlo simulations were then performed using Boltzmann probabilities based on the calculated DFT barriers to determine the dopant concentration dependence of the oxygen self-diffusion coefficient in (Y2O3)x(ZrO2)(1−2x) with x increasing from 6% to 15%. The results from the simulations suggest that the maximum conductivity occurs at 7–9mol% Y2O3 at 600–1500K and that the effective activation energy increases at higher Y doping concentrations in good agreement with previously reported literature data. The increase i...

Interpretation of Low Temperature Solid Oxide Fuel Cell Electrochemical Impedance Spectra

Electrochemical impedance spectroscopy was performed on low temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes and different electrode materials and morphologies. Three loops are seen in a Nyquist plot; the high frequency loop is attributed to the electrolyte and series resistance. The intermediate and low frequency loops are influenced by the material and morphology of both electrodes. To clarify which elementary processes contribute to each loop, kinetic Monte Carlo simulations of a solid oxide fuel cell were performed to calculate the reaction rates for each elementary process. The rates fall into three groupings, allowing the identification of processes with corresponding features in the impedance spectra. Vacancy diffusion processes occur at the highest frequency, agreeing with the usual assignment of the high frequency loop with series resistance. Chemical reactions at the anode have an intermediate frequency, suggesting that the intermediate frequency loop is dominated by anode reactions. Low frequency reactions include electrochemical reactions, chemical reactions at the cathode, and water formation and desorption at the anode. This agrees with the experimental findings of the strong dependence of the low frequency loop on the bias voltage and the dominance of the cathode reactions in the low frequency regime.

Characteristics of Oxygen Reduction on Nanocrystalline YSZ

We present a series of experimental results indicating improved oxygen reduction characteristics on nanocrystalline yttria-stabilized zirconia (YSZ) surfaces. We compare (i) cathodic interface impedances on nanocrystalline and microcrystalline YSZ, (ii) oxygen exchange rate on and bulk diffusivity in nano- and single-crystal YSZ, and (iii) bulk and surface conductivities of nano-YSZ. The results collectively indicate that the oxygen reduction/exchange rate is faster on nanocrystalline YSZ than on micro- and single-crystal YSZ, which likely is related to fast oxygen or electronic transport on the surface.

Kinetic Monte Carlo Simulations of Solid Oxide Fuel Cell

The kinetic Monte Carlo technique was employed to simulate an entire solid oxide fuel cell (SOFC) during operation to gain insight into the electrode kinetics and rate-limiting steps in the intermediate temperature range. By combining the quantum simulation studies of oxide ion migration in the fuel cell electrolyte with the experimental studies of the cathode and anode reaction rates, a complete SOFC can be modeled. To study the effect of triple phase boundaries and the size of the catalyst, simulations were performed for different sizes of Pt clusters on the electrolyte surface. The results confirm that the charge-transfer reaction rates depend on the catalyst size. The fuel cell with smaller catalyst particles produces higher power density as expected. The reaction rates of each process were recorded as a function of time. The overpotentials were subsequently determined as a function of catalyst size. The results show that oxygen adsorption is the slowest step on the cathode, while water formation is the slowest step on the anode. The methodology can be used to optimize the catalyst size on both electrodes to reduce the activation loss in intermediate temperature SOFCs.

Effect of Doping Concentration on the Proton Conductivity of Y-doped BaZrO3 Thin Films

Y-doped BaZrO3 (BYZ) pellets and thin films were fabricated by solid state reaction and co-sputtering technique, respectively. The conductivity of BYZ samples was measured using electrochemical impedance spectroscopy technique in the atmospheres of air, dry H2, and wet H2. As Y content increases from 6 at.% to 20 at.%, the increase of bulk and grain boundary conductivity is observed, while at 30 at.%, the conductivity starts to degrade. The activation energies in bulk and grain boundary are in the range of 0.28-0.40 eV and 0.91-0.96 eV, respectively. BYZ pellets have the highest proton conductivity at the Y doping concentration of 20 at.%. Furthermore, the dense and uniform BYZ thin films of 130-140 nm in thickness were obtained by co-sputtering technique. The conductivities of the BYZ thin films are about 10^3 - 10^4 times those of the pellet samples. The reason of the observed high conductivity is still under further investigation.

Ion Conductivity Enhancement Effect by Introduction of Dislocations in Yttria-Stabilized Zirconia

High dislocation density in zirconia systems may significantly affect oxygen ion conductivity. Several processes for introduction of high dislocation density were investigated including ion irradiation. YSZ single crystals were irradiated with Ar+ ions and dislocations were observed by TEM. After heat treatment, high dislocation density was observed in a certain range of depth, and no dislocations were observed in the vicinity of the surface. Conductivity measurements by AC impedance spectroscopy indicated that conductivity improved by 55%.

Characterisation of NiO-YSZ Porous Anode-Support for Solid Oxide Fuel Cells Fabricated by Ceramic Injection Moulding

Ceramic injection moulding (CIM) has advantages for a cost effective fabrication of large-scale, near-net-shape products. In this work, CIM is carried out to prepare porous anode-support for solid oxide fuel cells (SOFC) applications. The CIM process started with a preparation of feedstocks by mixing powder with binder. The feedstock is then injected into the mould of desired shapes. The mouldings were subsequently undergo the removal of the binder (debinding) and, finally, sintering. It is shown that porous nickel oxide-yttria stabilized zirconia (NiO-YSZ) anode-support for SOFC were successfully prepared by CIM technique. In addition, a water-soluble based binder system, consisted mainly of polyethylene glycol (PEG), has been used in this work. This is to avoid the use of organic solvents when wax-based binder was used. Therefore, it can promote more environmentally friendly process. The removal of binder was carried out using water debinding technique. The porous anode for SOFC was subjected to systematic characterisation. The effect of processing parameters, such as powder characteristics and powder/binder ratio has been investigated. Rate of binder removal was also studied. The porous anode specimens were characterised for their properties and microstructure. It was also found that the porosity of the specimens can be controlled by adjusting the sintering temperatures and holding times.

Process Optimization and Characterization of YSZ Thin Film Electrolyte on Anode Substrate Prepared by Electrophoretic Deposition Technique

Thin film electrolyte made of 8-mol% yttria stabilized zirconia (8YSZ) was fabricated on porous NiO-8YSZ anode substrates using electrophoretic deposition (EPD). The porous NiO-8YSZ anode substrates were prepared by powder injection molding technique. The electrolyte suspensions containing 8YSZ nanoparticles and polyethylene glycol (PEG) as a dispersant (1-19 wt%) were formed in ethanol. The maximum zeta potential value was obtained from the 8YSZ suspension with 5 wt% PEG considered as an optimal content of PEG dispersant. The electrophoretic deposition of 8YSZ film was performed on the porous anode substrate using a constant voltage of 30 V for 150 sec prior to co-sintering at different temperatures in order to obtain dense 8YSZ electrolyte film on the porous anode substrate. Co-sintering at 1250°C for 1 h resulted in a formation of a dense 8YSZ thin-film electrolyte with a thickness of 6.35 mm. An open circuit voltage at 800°C of a single cell having 8YSZ thin-film electrolyte on porous NiO-8YSZ anode substrate was 1.09 V, indicating a gas-tightness of 8YSZ thin-film electrolyte fabricated by using EPD.

Preparation of Xylem-Imitating Porous Ceramic by Coconut Coir and Pottery Clay

The transport of water through xylems in tree is one of the most efficient transport mechanisms. Porous conventional ceramic is one way to mimic capillary force in xylem, which was operated by mixing pottery clay with volatile natural fiber (coconut coir) and extruded into 30 millimeter in diameter rod. The coconut coir content in the mixtures was varied from 0 to 8 wt% of dried clay. Dried samples were fired at 800 °C and 900 °C for 2 h to burn out the coconut coir. The water transport performance test of samples is performed by immersing 3 cm of the initial parts of samples in water and observe watermark at 1, 2, 4, 8 and 16 h under the humidity and temperature controls at 60% and 30 °C, respectively. The sample with the 8 wt% coconut coir after firing at 900 °C showed the highest watermark level at 45 cm from the initial point and the highest water transpiration rate of 2.275 g/h.

Ion Irradiation Effects in Solid Oxide Fuel Cell Electrolytes

Single crystal Ytrria-stabilized Zirconia was irradiated with Xe 2+ and Xe 3+ ions at 320 and 450 keV over a range of doses from 10 13 to 10 16 ions/cm 2 . Damage appears as a 150 nm surface layer with a dense dislocation network. The X-ray diffraction pattern shows an increasing lattice expansion with increasing dose that reaches a saturation point. Ion irradiation increases the surface conductance of the material; this effect is removed with certain post-treatments. Preliminary isotope depth profiling indicates enhanced ion diffusion in the damaged layer.