Yong-il Park

Characterization of LSM–YSZ composite electrode by ac impedance spectroscopy

The characteristics of La1−xSrxMnO3 perovskite (LSM)–yttria-stabilized zirconia (YSZ) composite electrodes were studied over a range of compositions by ac impedance spectroscopy. The transfer and surface diffusion of oxygen ions were found to be rate-determining steps. The polarization resistance of oxygen ion transfer was found to be independent of the partial pressure of oxygen and proportional to the length of the three-phase boundary. The capacitance of oxygen ion transfer was approximately 10−4 F/cm2. The polarization resistance of O− surface diffusion was determined to be proportional to PO21/4, with a minimum at 40–50 wt.% YSZ. The polarization resistance of oxygen ion transfer was observed to remain largely unaffected by variations in cathodic potential, whereas that of O− surface diffusion decreased with increasing cathodic potential. At low partial oxygen pressure, the arc due to gas phase diffusion was observed in the low frequency region, with proportion to PO2 and having low activation energy.

Proton exchange nanocomposite membranes based on 3-glycidoxypropyltrimethoxysilane, silicotungstic acid and α-zirconium phosphate hydrate

Abstract Novel fast proton-conducting GPTS–STA–SiO 2 and GPTS–STA–ZrP composites were successfully fabricated. The polymer matrix obtained through hydrolysis and condensation reaction of 3-glycidoxypropyltrimethoxysilane (GPTS) showed apparent proton conduction at high relative humidity with conductivity from 1.0×10 −7 to 3.6×10 −6 S/cm, although no proton donor was incorporated. The proton conductivities of the fabricated composites were high, and increased up to 1.9×10 −2 S/cm by addition of silicotungstic acid (STA). By incorporating α-zirconium phosphate (ZrP) into the GPTS–STA polymer matrix, the composite showed increased conductivity at low temperature (∼80°C), indicating weak dependence on humidity by molecular water in ZrP. The high proton conductivity of the composites is due to the proton conducting path through the GPTS-derived ‘pseudo-polyethylene oxide (pseudo-PEO)’ networks, which also contains a trapped solid acid (silicotungstic acid) as a proton donor.

Characterization of CO tolerance of PEMFC by ac impedance spectroscopy

Abstract The CO tolerance of a proton exchange membrane fuel cell (PEMFC) was investigated by ac impedance spectroscopy. The impedance of the fuel cell could be obtained by feeding oxygen into the cathode side and simulated gas into anode side. Furthermore, the anode impedance could be obtained by feeding hydrogen into the cathode side and simulated gas into anode side. The CO gas had a greater effect on the charge transfer reaction (high frequency arc) and hydrogen dissociative chemisorption (medium frequency arc) but little effect on the low frequency arc. Although the cathode impedance is a main part at high temperature, irrespective of CO concentration (≤100 ppm), the impedance of the full cell depends on anode impedance at low temperature and high CO concentration. It was found that CO gas has little effect on cathode impedance.

Thin-Film SOFCs Using Gastight YSZ Thin Films on Nanoporous Substrates

We fabricated yttria stabilized zirconia (YSZ) thin films having submicrometer thickness without gas leakage to be incorporated in low-temperature solid oxide fuel cells (SOFCs). We obtained 30-300-nm-thick YSZ films by oxidizing Y-Zr alloy thin films deposited onto anodic nanoporous alumina substrates having pore diameter of 20 and 200 nm using dc-magnetron sputtering at room temperature. During the thermal oxidation, the alloy films were successfully transformed to defect-free oxide thin films. Volume expansion induced from the oxidation of the alloy resulted in dense oxide thin films that are free from hydrogen permeation. Conductivity of YSZ thin films at room temperature was measured and compared with the reported conductivity of YSZ ceramics. And low-temperature operation testing was performed using the fabricated fuel cell.

Proton-Conducting Properties of Inorganic-Organic Nanocomposites Proton-Exchange Nanocomposite Membranes Based on 3-Glycidoxypropyltrimethoxysilane and Tetraethylorthosilicate

Novel fast proton-conducting organic-morganic hybrid nanocomposites were successfully fabricated. The polymer matrix obtained through a hydrolysis and condensation reaction of 3-glycidoxypropyltrimethoxysilane (GPTS) and tetraethylorthosilicate showed relatively high proton conductivity, over 10 -7 S/cm at 100% RH, at 20-100°C, although a proton donor was not incorporated. The proton conductivities of the fabricated composites were high and increased up to 1.0 × 10 1 S/cm by addition of silicotungstic acid at 100% RH and 100°C. The high proton conductivity of the composites is due to the proton-conducting path through the GPTS-derived pseudopolyethylene oxide (pseudo-PEO) networks in which the trapped solid acid (silicotungstie acid in this study) as a proton donor is contained. The molecular water absorbed in the polymer matrix is also presumed to provide high proton mobility, resulting in an increase of proton conductivity.

Fabrication of GDL microporous layer using PVDF for PEMFCs

The Gas Diffusion Layer(GDL) of fuel cell, are required to provide both delivery of reactant gases to the catalyst layer and removal of water in either vapor or liquid form in typical PEMFCs. In this study, the fabrication of GDL containing Micro Porous Layer(MPL) made of the slurry of PVDF mixed with carbon black is investigated in detail. Physical properties of GDL containing MPL, such as electrical resistance, gas permeability and microstructure were examined, and the performance of the cell using developed GDL with MPL was evaluated. The results show that MPL with PVDF binder demonstrated uniformly distributed microstructure without large cracks and pores, which resulted in better electrical conductivity. The fuel cell performance test demonstrates that the developed GDL with MPL has a great potential due to enhanced mass transport property due to its porous structure and small pore size.

Effect of heating temperature on dielectric properties of Pb(Zr,Ti)O3 [PZT] fibers

Ferroelectric Pb(Zr0.53Ti0.47)O3 fibers were reproducibly fabricated by sol-gel technique using triethanolamine (TEA) complexed alkoxide. The phase transition from pyrochlore to perovskite took place about 400°C and a stable single perovskite phase was obtained at 550°C. PZT gel fibers spun through nozzle were heat-treated at 700°C, and at 1000°C for 1 h to certify the effect of heat-treatment temperature on the electrical properties. The PZT fibers had elliptical cross sections with diameter of 72 μm–92 μm, and dense microstructure was obtained by heating at 1000°C. In the PZT fibers heat-treated at 1000°C, a distinguishable relative permittivity peak and a pyroelectric current peak were observed at their Curie temperature. The P-E hysteresis loops of the crystalline PZT fibers were also observed.

Effect of composition on ferroelectric properties of sol-gel derived lead bismuth titanate (PbBi4Ti4O15) thin films

Bismuth layer-structured ferroelectric PbBi4Ti4O15(PBT) thin films were fabricated on Pt/Ti/SiO2/Si substrates by sol-gel spin coating and the effect of lead and bismuth concentration on phase transition, microstructure and ferroelectric property was investigated. Especially, the effect of non-stoichiometric compositions, that is, deficient concentration of Pb and excess concentration of Bi with respect to the stoichiometric composition, was examined. With an increase of lead and bismuth concentration, the pyrochlore phase was suppressed and PBT phase was developed. Large increases of 2Pr and 2Ec were also observed as lead and bismuth concentration increased. An improved ferroelectric property could be obtained by inserting a Bi-rich buffer layer between Pt electrode and Pb-rich PBT film. The PBT thin films with Bi-rich buffer layer showed homogeneous grain size distribution, good fatigue endurance up to 1 × 109 switching cycles, low relative permittivity of 270 and sufficient 2Pr values of 15.7 μC/cm2.

Fast Proton Conductors from Inorganic-Organic Composites: II. Amorphous Phosphate-PTFE and ZrP-PTFE Composites

A high proton-conductivity was observed in the composite of amorphous phosphate and polytetafluoroethylene (PTFE). Incorporation of amorphous phosphate into PTFE emulsion caused a large increase of conductivity to about 4×10 −2 S/cm at 23°C. However, the conductivity decreased with increasing heat-treatment temperature, and the fabricated composite showed very low chemical stability. As a chemically stable composite, PTFE-based composite was also synthesized from α- or γ-zirconium phosphate crystalline powders dispersed in partially polymerized PTFE particles. By addition of zirconium phosphate powders, the proton conductivity jumped up to 2.2×10 −3 S/cm from 10 −13 S/cm of PTFE.

Characteristics of Stainless Steel Composites with Nano-sized TiCxNy

Titanium carbonitride is more perspective materials compared to titanium carbide. It can be used in tool industry and special products because of its higher strength, abrasive wear-resistance and especially its strong chemical stability at high temperatures. We produced STS+TiCxNy composite by the spark plasma sintering for higher strength and studied the characteristics. The planar and cross-sectional microstructures of the specimens were observed by scanning electron microscopy. Characterizations of the carbon and nitride phases on the surface of composite were carried out using an X-ray diffractometer. During annealing TiCxNy particles diffusion into STS 430 was observed. After annealing, sintering isolations between particles were formed. It causes decreasing of mechanical strength. In addition when annealing temperature was increased hardness increased. Heterogeneous distribution of alloying elements particles was observed. After annealing composites, highest value of hardness was 738.1 MHV.