S.P Jiang

Origin of the initial polarization behavior of Sr-doped LaMnO3 for O2 reduction in solid oxide fuel cells

Abstract The initial behavior of O 2 reduction reactions was studied on freshly-prepared Sr-doped LaMnO 3 (LSM) electrodes with and without dilute acid etching at 900°C in air. LSM electrodes without acid etching showed very high polarization potential for the O 2 reduction and the electrode performance improved significantly with the cathodic current passage. On the other hand, for the reaction on LSM electrode after acid etching, the change in the polarization potential with the current passage time was much smaller and cathodic current passage had little enhancing or activation effect on the electrode performance. The results indicate that the dilute acid etching effectively replaced the activation process of the cathodic polarization on LSM electrodes and the initially very high polarization losses for the reaction on LSM electrodes are most likely originated from the passivation species such as MnO x and in particular SrO species originally enriched on the LSM electrode surface.

Hydrogen Oxidation at the Nickel and Platinum Electrodes on Yttria‐Tetragonal Zirconia Electrolyte

H{sub 2} oxidation has been studied for Pt and Ni electrodes for different H{sub 2}/H{sub 2}O ratios at 1,000 C in solid oxide fuel cells using yttria-tetragonal zirconia electrolyte by galvanostatic current interruption and electrochemical impedance spectroscopy. The results clearly indicate that the mechanism and kinetics of the H{sub 2} oxidation reaction are strongly dependent on the catalytic activities of electrode materials, electronic conductivity of the electrolyte surface, and the water content in H{sub 2} gas. The effect of water vapor in H{sub 2} gas on the reaction kinetics is very much dependent on the electrode materials and is related to the partial pressure of oxygen. A reaction mechanism with two rate-limiting steps has been proposed and discussed.

The electrochemical performance of LSM/zirconia–yttria interface as a function of a-site non-stoichiometry and cathodic current treatment

Abstract The adhesion and the electrochemical performance of Sr doped LaMnO 3 (LSM) electrode with zirconia–yttria electrolyte has been investigated as a function of the A-site non-stoichiometry. In addition the effect of cathodic current treatment on the electrode resistance and the mechanism of oxygen reduction have been carefully studied. The LSM/electrolyte interface region was examined by SEM/EDS and XRD. The adhesion of porous LSM electrode coating is critically dependent on free La activity near the interface region and the formation of a pyrochlore phase, La 2 Zr 2 O 7 . A slight A-site deficiency (∼0.1) is effective in inhibiting the formation of such a pyrochlore phase and it greatly improves the adhesion to the electrolyte. The cathodic current treatment significantly enhances the electrochemical activity of LSM and also modifies the reaction mechanism for oxygen reduction. This effect appears to be associated with the surface composition of LSM, near the electrode/electrolyte interface, which changes with prevailing oxygen partial pressure near the three-phase boundary region under the condition of current flow.

Fabrication and performance of Ni/3 mol% Y2O3–ZrO2 cermet anodes for solid oxide fuel cells

Abstract A fabrication method for the preparation of Ni/3 mol% Y 2 O 3 –ZrO 2 (Ni/TZ3Y) cermet electrodes from various commercial powders has been described. The fabrication parameters investigated include: particle size and particle size distribution of NiO and TZ3Y powders, Ni content in the cermet, firing temperature, electrode thickness and the effect of adding pore-formers. Ni/TZ3Y cermet anodes were prepared by screen printing and the electrode performance was characterised by galvanostatic current interruption and electrochemical impedance spectroscopy techniques. The results showed that the electrochemical activity of Ni/TZ3Y cermet anodes for hydrogen oxidation is strongly dependent on the selection of fabrication parameters, and the effect of these processing parameters on the electrode microstructure is closely interlinked. The most critical parameters appear to be the particle size and particle size distribution of NiO and TZ3Y powders, which can be controlled by screening various commercial powders and by optimising the heat treatment conditions of these powders. For the best anode, the overpotential losses at 800°C were less than 90 mV at a current density of 350 mA cm −2 . The anode degradation, performed on 50 mm×50 mm×100 μm TZ3Y electrolyte cells, was less than 1.7% over the ∼4300 h test period under a current load of 250 mA cm −2 at 900°C.

H2 oxidation on Ni/Y-TZP cermet electrodes – a comparison of electrode behaviour by GCI and EIS techniques

Abstract Reaction order and activation energy for H2 oxidation on Ni/3 mol.% Y2O3–ZrO2 (Ni/Y-TZP) cermet electrodes have been evaluated by galvanostatic current interruption (GCI) and electrochemical impedance spectroscopy (EIS) techniques. GCI data show a reaction order in PH2O of 0.4 and activation energy of 76 kJ mol−1. In contrast, a first order with respect to PH2O and an activation energy of 169 kJ mol−1 are obtained from EIS data. During GCI measurements, the actual H2O content or PH2O/PH2 ratio at the electrode/electrolyte interface region could be significantly higher than that in the bulk electrode as H2O, the product of the reaction, remains in the gas phase. On the other hand, the H2O content at the interface region is very close to that in the bulk during EIS measurements under open circuit. It has been shown that H2O produced at the interface region during GCI measurements enhances the H2 oxidation reaction, resulting in the shifting of the reaction rate dependency on H2O and activation energy to lower values as compared to those measured by EIS.

An investigation of shelf-life of strontium doped LaMnO3 materials

The shelf-life of Sr doped LaMnO3 (LSM) materials of varying stoichiometric compositions prepared by wet chemical synthesis was investigated under storage conditions of low and high humidity at ambient temperature for ∼350 days. It has been found that the adhesion, chemical stability and stability of the electrode performance of LSM materials are critically dependent on the A-site stoichiometry (i.e., (La+Sr)/Mn ratio) of the LSM materials and much less dependent on storage conditions (humidity level and storage time) over the period investigated. In this respect, LSM materials with stoichiometric and sub-stoichiometric compositions where (La+Sr)/Mn≤1 are stable and LSM with A-site sub-stoichiometry ((La+Sr)/Mn≤∼0.9) is effective in achieving good adherence, high chemical stability and high stability of electrode performance.

Chemical diffusion in perovskite cathodes of solid oxide fuel cells: the Sr doped LaMn1−xMxO3 (MCo, Fe) systems

Abstract This paper reports the results of chemical diffusion coefficient measurements for oxygen in (La,Sr)(Mn,Co)O 3 and (La,Sr)(Mn,Fe)O 3 using a manometric method in the temperature range of 773 to 1023 K. It was observed that the addition of Fe or Co into (La 4 x Sr x ) 1− y MnO 3 results in a substantial increase of the chemical diffusion coefficient. In the range 25–100 mol% of Co or Fe substitution at B-site in (La,Sr)MnO 3 , the chemical diffusion coefficient exhibits much lower activation energy than that in (La,Sr)MnO 3 . The chemical diffusion data in (La,Sr)(Mn,Co)O 3 and (La,Sr)(Mn,Fe)O 3 system determined by the manometric method are generally in agreement with those determined by electrochemical relaxation methods reported in the literature. A comparison of the chemical diffusion data indicates that the oxygen mobility is lowest in (La,Sr)MnO 3 and highest in (La,Sr)CoO 3 .

A manometric method for the determination of chemical diffusion in perovskite-type cathode materials of the solid oxide fuel cell

This paper describes a manometric method for the determination of chemical diffusion coefficients from gas/solid equilibration kinetics for electrode materials of the type (La,Sr)(Mn,Fe,Co)O3. The equilibration kinetics was monitored by measurements of the p(O2) changes during a reaction between oxygen and the electrode material involving both oxidation and reduction experiments in the temperature range 773–1123 K. Activation energy of the chemical diffusion measurements at different temperatures was determined during both oxidation and reduction experiments. The chemical diffusion coefficients measured by the present manometric method on La0.8Sr0.2MnO3−δ have been compared with those reported in the literature.