Mortaza Sahibzada

Pd-promoted La0.6Sr0.4Co0.2Fe0.8O3 cathodes

Porous La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) electrodes supported on Ce0.9Gd0.1O1.95 (CGO) electrolyte were impregnated with small amounts of Pd and the electrochemical impedances resolved by AC impedance spectroscopy. The optimum Pd loading resulted in 3–4 times lower cathodic impedance in the temperature range 400–750°C. Single cell Solid Oxide Fuel Cells (SOFCs) were fabricated by tape casting CGO electrolyte onto an anode support. Under d.c operation, the addition of Pd to the LSCF cathode was found to decrease the overall cell resistance by 15% at 650°C and 40% at 550°C. This suggests that the SOFC becomes more limited by the performance of the cathode at lower temperatures and hence the greater benefit of Pd promotion. Finally, Pd particles were deposited on dense smooth LSCF substrates and Isotope Exchange Depth Profiling (IEDP) and Secondary Ion Mass Spectrometry (SIMS) were carried out to determine the tracer diffusion (D*) and surface exchange (k) coefficients. D* was not significantly effected but k decreased by well over an order of magnitude with Pd loadings similar to those applied to the electrode.

Development of solid oxide fuel cells based on a Ce(Gd)O2−x electrolyte film for intermediate temperature operation

Initial tests have been carried out with the fuel cell arrangement La0.6Sr0.4Co0.2Fe0.8O3∥Ce0.9Gd0.1O1.95∥Ni/YSZ, incorporating dense film (5–10 μm) Ce0.9Gd0.1O1.95 electrolyte tape cast onto the supporting anode, to investigate the feasibility of intermediate temperature operation (500–700°C). A good open circuit voltage of approx. 0.8 V was obtained at 550°C using moist hydrogen as the fuel. Slightly lower open circuit voltages were found at higher temperatures, which may have been caused by minor gas leakage and the electronic conductivity of the electrolyte. Power outputs in excess of 100 mW/cm2 were obtained at 650°C, and the cell resistance was 0.8Ω cm2 at this temperature. This resistance, and the greater resistance at lower temperature, was predominantly due to the cathode according to AC impedance measurements. Experiments were also carried out at 600°C using direct methanol fuels at the anode; the maximum power output was approximately half of that obtained with hydrogen.

Oxygen stoichiometries in La1−xSrxCo1−yFeyO3−δ perovskites at reduced oxygen partial pressures

Abstract La 1− x Sr x Co 1− y Fe y O 3− δ perovskite materials have been considered as potential cathodes for solid oxide fuel cells operating at intermediate temperatures. The effect of temperature, ambient oxygen partial pressure and composition on the extent of oxygen deficiency was electrochemically investigated for various perovskites by means of solid electrolyte coulometry. Experiments were performed at reduced oxygen partial pressures between 1.3×10 −3 and 6×10 −5 atm since in a real fuel cell under cathodic bias, the material near the electrode/electrolyte interface is expected to be in a reduced state. It was found that the degree of oxygen non-stoichiometry ( δ ) increases with increasing temperature, decreasing oxygen partial pressure and increasing Sr or Co content. Transient re-equilibration experiments were also performed in an attempt to investigate the oxygen transport characteristics of the perovskite under various operating conditions. These experiments have shown that the time needed for re-equilibration increases with increasing oxygen deficiency.

La0.6Sr0.4Co0.2Fe0.8O3 as the anode and cathode for intermediate temperature solid oxide fuel cells

Abstract The perovskite material La0.6Sr0.4Co0.2Fe0.8O3 (LSCF-6428) has been considered as both the anode and cathode in solid oxide fuel cells (SOFCs) operating at intermediate temperatures (550–700°C). Solid electrolyte coulometry (SEC) has been used to measure the oxygen non-stoichiometry as a function of temperature and ambient oxygen partial pressure, thus enabling kinetic data relating to oxygen transport in cathodes to be correlated with the material oxygen vacancy concentration. The catalytic activity towards methane oxidation, and susceptibility to deactivation through carbon deposition have both been investigated by temperature programmed methods, and compared with data for the conventional Ni/YSZ anode material.

Wastewater treatment: wet air oxidation as a precursor to biological treatment

Abstract The use of wet air oxidation as a pretreatment step in the context of an integrated chemical/biological process was investigated for model organic-containing wastewaters. It was found that the selection of suitable wastewaters and pretreatment conditions (i.e. temperature, residence time, use of catalysts) was significant for the effective application of an integrated process. For the systems under consideration, an integrated process was found to be advantageous for treating a polymer-containing wastewater, while mild wet air oxidation alone may suffice to treat a polyphenol-containing wastewater.

Solid oxide fuel cells based on Ce(Gd)O2 − x electrolytes

The fuel cell arrangement air, La0.4Sr0.6Coo.2Fe0.8O3∥Ce0,Gd0.1O1.95∥Ni/YSZ, fuel, incorporating either thick film (5–10 yum) or self supported (∼300 μm) Ce0.9Gd0.1O1.95 electrolyte, was investigated at 650 °C. Scanning electron microscopy (SEM) analysis showed that a dense electrolyte layer in good contact with highly porous electrodes was achieved using either the thick film or self supported electrolyte. Measurements of the current-voltage characteristics, using H2CO2/H2O fuel to simulate reformed methanol, showed that a higher terminal voltage was obtainable with the self supported electrolyte, but that the voltage dropped off quickly with increasing currents. Hence the thick film based arrangement was more suitable for the fuel cell and power outputs exceeding 100 mW/cm2 were obtained.

Hydrogenation of carbon dioxide to methanol over palladium-promoted Cu/ZnO/A12O3 catalysts

Abstract The effect of Pd on a Cu/ZnO/A1 2 O 3 catalyst for methanol synthesis from CO 2 /H 2 has been investigated. Activities of impregnated catalysts and physical mixtures were studied in an internal recycle reactor under 5 MPa, 250°C and a range of conversions. In all cases, the promotion of methanol production was greater at higher flow rates (lower conversions). The promotion achieved by use of Pd/A1 2 O 3 + Cu/ZnO/Al 2 O 3 physical mixtures was found to increase with Pd content. Greater promotion was observed over the Pd impregnated Cu/ZnO/Al 2 O 3 catalysts, although this was insensitive to the particular Pd loadings used. The results are consistent with the proposal that hydrogen spillover is responsible for the observed promotion. The effectiveness of Pd as a promoter for the reduction of CuO in the catalysts was studied by TPR and was found to be related to the level of promotion in methanol production.

Effect of humidification at anode and cathode in proton-conducting SOFCs

Abstract Electrical conductivity measurements and hydrogen–oxygen fuel cell experiments were carried out at 600–800 °C using proton-conducting Yb-doped SrCeO 3 . The total conductivity increased with P O 2 higher than 10 −5 atm and decreased with respect to P H 2 O , suggesting that electron hole formation was suppressed in the presence of water. In fuel cell tests, the introduction of water to the oxygen cathode resulted in a lower OCV due to the converse hydrogen potential, but a higher ionic transport number in agreement with the conductivity measurements. The introduction of water to the hydrogen anode did not affect the OCV but improved (lowered) the area-specific resistance (ASR), probably by promotion of the uptake of protons through hydroxyl insertion.

A simple method for the determination of surface exchange and ionic transport kinetics in oxides

Abstract A method is proposed for obtaining the surface exchange coefficient ( k ) and chemical diffusion coefficient ( D ) for oxide materials under reducing conditions. Transient oxygen stoichiometry re-equilibration data is obtained from powdered samples using solid electrolyte coulometry where oxygen dosing/titration is performed by means of electrochemical cells. The data has been fitted to a suitable diffusion model using a non-linear parameter optimisation method from which the parameters k and D have been extracted. This method has been applied to the perovskite material La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ at intermediate temperatures and reduced oxygen pressures in the range 1×10 −3 –6.3×10 −5 atm. The results obtained in this study were found to be consistent with those obtained by means of other relaxation techniques.

Pd-Promoted Cu/ZnO Catalyst Systems for Methanol Synthesis From CO2/H2

Cu/ZnO-based catalysts are widely used for methanol production from CO/CO 2 /H 2 . In recent years there has been growing interest in methanol synthesis from CO 2 /H 2 . Cu/ZnO catalysts are less active for methanol production in CO 2 -rich feed gas, but the activity appears to be promoted by the addition of Pd to the catalyst. This paper presents a brief chronological overview of kinetic results in these catalyst systems including the work done by the author. Experiments at controlled levels of conversion, including microreactor studies, have provided an insight into the role of products in the kinetics of methanol synthesis. In particular, it has been shown that the product water inhibits CO 2 hydrogenation to methanol. Under these conditions, Pd appears to promote the activity, but in fact it plays a role in counteracting the inhibition by water. This could possibly be explained by hydrogen spillover which moderates the [Cu(metal)+H 2 O=Cu-O(ads)+H 2 ] redox. The paper goes on to discuss the mechanistic effect of water on Cu/ZnO-catalyst systems under CO 2 -rich and CO-rich atmospheres.