Showing papers by "Zongping Shao published in 2007"

Novel mixed conducting SrSc0.05Co0.95O3‐δ ceramic membrane for oxygen separation

Abstract: A novel perovskite-type mixed-conducting oxide of SrSc0.05Co0.95O3-delta (SSC) was synthesized by a combined EDTA-citrate complexing method. The 5 mol % of Sc3+ doping into the B-site of SrCoO3-delta (SC) through the sol-gel synthesis effectively stabilized the oxygen vacancy disordered cubic perovskite structure of the oxide, and simultaneously resulted in a substantial increase of the electrical conductivity. The oxide was fabricated into dense ceramic membrane for oxygen separation by pressing/sintering process. The oxygen permeation flux of the SSC membrane and the rate-determination step of the permeation process were investigated between 750 and 900 degrees C. The experimental results demonstrated that SSC is a promising membrane for oxygen separation with ultrahigh permeation fluxes, compared favorably with reported high oxygen semi-permeable Ba0.5Sr0.5Co0.8Fe0.2O3-delta and SrCo0.8Fe0.2O3-delta membranes under air/helium gradient. At the condition of reduced temperature and low oxygen partial pressure at the sweep side atmosphere, the permeation process was found to be rate-determined mainly by the slow oxygen surface exchange kinetics at the air (feed) side membrane surface based on the single cell oxygen permeation study. The activation energy for the oxygen surface exchange and oxygen bulk diffusion was found to be around 126 U mol(-1) and <= 62.1 kJ mol(-1), respectively. (c) 2007 American Institute of Chemical Engineers AIChE J, 53: 3116-3124, 2007.

Comparisons of different carbon conductive additives on the electrochemical performance of activated carbon

Abstract: Three kinds of carbon conductive additives, i.e. multi-walled carbon nanotubes (MWNTs), vapour grown carbon fibres (VGCFs) and acetylene carbon blacks (AB), were investigated to improve the electrochemical performance of activated carbon (AC) used as electrode materials for supercapacitors. Galvanostatic charge/discharge and cyclic voltammetric measurements demonstrate that MWNTs are the most effective additive to improve the electrochemical performance of AC under the same conditions. To get the same results for AC in a symmetric supercapacitor, the desired additive amount was 3.0 wt% MWNTs, ~5.0 wt% VGCFs and ~9.0 wt% AB, respectively. X-ray diffraction analysis results demonstrate MWNTs have the sharpest (002) peak and highest graphitic degree. Scanning electron microscopy images show MWNTs have a vimineous fibre shape and VGCFs have a stubbed virgate shape. MWNTs run across AC particles and VGCFs distribute discretionarily among AC particles. High-resolution transmission electron microscopy demonstrates the microstructural difference between MWNTs and VGCFs. Some mechanisms were then developed to explain the different performance of the three kinds of carbon conductive additives.

Tunability of Propane Conversion over Alumina Supported Pt and Rh Catalysts

Abstract: Propane conversion over alumina supported Pt and Rh (1 wt% metals loading) was examined under fuel rich conditions (C_3H_8:O_2:He = 1:2.25:9) over the temperature range 450–650 °C. Morphological characteristics of the catalyst materials were varied by calcining at selected temperatures between 500 and 1,200 °C. X-ray diffraction and BET analysis showed the treatment generated catalyts metals with particle sizes in the range of 500 nm, and support surface areas in the range of 20–240 m^2/g. Remarkably, both Rh and Pt yielded product compositions close to equilibrium values (with high H_2 and CO selectivity, complete oxygen conversion and almost complete propane conversion) so long as the metal particle size was sufficiently low, ≲10–15 nm. In cases where the particle size was large, primarily complete oxidation rather than partial oxidation products were observed, along with unreacted C_3H_8, indicative of a direct oxidation pathway in which gas-phase CO and H_2 are not present as intermediate species. It is proposed that the high resistance of Rh to coarsening is largely responsible for the observation of a higher selectivity of this material for syngas products when prepared by procedures similar to those for Pt. Overall, the tunability of the product composition obtained over Rh and Pt via processing steps has direct significance for the incorporation of such catalyts into the anodes of solid oxide fuel cells.

From Chelating Precursor to Perovskite Oxides and Hollow Fiber Membranes

Abstract: Perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) is a promising mixed-conducting ceramic membrane material in addition to being a good electrode catalyst for solid oxide fuel cells. In this study, BSCF powder was synthesized via a chelated water-soluble complex method at relatively low temperatures. The combined ethylenediaminetetraacetic acid and citric acid was used for the synthesis of a complex-based precursor, followed by thermal decomposition of the precursor at high temperatures. Thermal behavior, crystal phases, and structures of the prepared powders were characterized by thermogravimetric analysis/differential scanning calorimetry, XRD, and scanning electron microscopic (SEM) techniques, respectively. Pure and single-phase perovskite could be obtained after sintering at a temperature higher than 800 degrees C for 5 h. The soft precursor powder synthesized at lower temperatures, i.e., 600 degrees C, is water insoluble and more appropriate for use as a membrane material to prepare gas-tight tubular or hollow fiber ceramic membranes. By contrast, the hollow fibers prepared via the traditional techniques where the perovskite powder is used as the starting membrane materials display gas leakage. The fibers were characterized by SEM, XRD, and tested for air separation at ambient pressure and temperatures between 700 degrees and 950 degrees C. The oxygen flux measured in this work reached 3.90 mL.(min.cm(2))(-1) and compares favorably with any experimental values reported in the open literature.

Effect of HNO3 treatment on the La0.6Sr0.4Co0.2Fe0.8O3-delta obtained via combined EDTA-citrate complexing process

Abstract: The La0.6Sr0.4Co0.2Fe0.8O3-delta(LSCF) composite oxide was prepared via combined EDTA-citrate complexing process with concentrated nitric acid treatment. The treatment would result in the self-combustion of solid state precursors at low temperatures. The effect of preparing conditions on LSCF's catalytic properties was investigated by using decomposition of peroxide hydrogen as the model. The FT-IR results of the solid state precursor and the pH values of aqueous solution of it were studied to determine the mechanism of the thermal decomposition of organic in the precursor and of the self-combustion process. Moreover, XRD was employed to characterize the crystal structure of LSCF calcined at higher temperatures. The study shows that the treatment can depress the growth of crystallite and improve the catalysis for decomposition of peroxide hydrogen. Of the all samples, the LSCF-40-900 has the highest activity to the decomposition of peroxide hydrogen.

A thermally self-sustaining miniature solid oxide fuel cell

Abstract: A thermally self-sustaining miniature power generation device was developed utilizing a single-chamber solid oxide fuel cell (SOFC) placed in a controlled thermal environment provided by a spiral counterflow "Swiss roll" heat exchanger and combustor. With the single-chamber design, fuel/oxygen crossover due to cracking of seals via thermal cycling is irrelevant and coking on the anode is practically eliminated. Appropriate SOFC operating temperatures were maintained even at low Reynolds numbers (Re) via combustion of the fuel cell effluent at the center of the Swiss roll. Both propane and higher hydrocarbon fuels were examined. Extinction limits and thermal behavior of the integrated system were determined in equivalence ratio―Re parameter space and an optimal regime for SOFC operation were identified. SOFC power densities up to 420 mW/cm 2 were observed at low Re. These results suggest that single-chamber SOFCs integrated with heat-recirculating combustors may be a viable approach for small-scale power generation devices.

A thermally self-sustaining miniature solid oxide fuel cell

Abstract: A thermally self-sustaining miniature power generation device was developed utilizing a single-chamber solid oxide fuel cell (SOFC) placed in a controlled thermal environment provided by a spiral counterflow “Swiss roll” heat exchanger and combustor. With the single-chamber design, fuel/oxygen crossover due to cracking of seals via thermal cycling is irrelevant and coking on the anode is practically eliminated. Appropriate SOFC operating temperatures were maintained even at low Reynolds numbers (Re) via combustion of the fuel cell effluent at the center of the Swiss roll. Both propane and higher hydrocarbon fuels were examined. Extinction limits and thermal behavior of the integrated system were determined in equivalence ratio—Re parameter space and an optimal regime for SOFC operation were identified. SOFC power densities up to 420 mW/cm^2 were observed at low Re. These results suggest that single-chamber SOFCs integrated with heat-recirculating combustors may be a viable approach for small-scale power generation devices.