Steven McIntosh

Evidence for Two Activation Mechanisms in LSM SOFC Cathodes

Dense La 0.8 Sr 0.2 MnO 3 (LSM) film electrodes with an average thickness of 600 nm were fabricated on yttria-stabilized zirconia and cerium gadolinium oxide by ultrasonic spray pyrolysis. LSM was studied for initial nonstationary behavior by activating with current density for short duration (5 min) and long duration (16 h). The polarization resistance at zero dc bias was reduced upon activation irrespective of the electrolyte, with the reduction more significant after long-duration activation. Short-duration activation was removed by deliberate introduction of La 2 Zr 2 O 7 impurities into the LSM phase or by surface doping with La 0.6 Sr 0.4 FeO 3 nanoparticles. However, long-duration activation still occurred in these samples. Scanning electron micrographs of short-duration-activated films showed no changes in morphology while long-duration activation resulted in a significant bulk pore formation in the LSM phase. Two distinct mechanisms for LSM activation in a solid oxide fuel cell (SOFC) are proposed. Short-duration activation results in changes in the film surface chemistry while long-duration activation leads to the reconstruction of the LSM phase.

Performance and Activation Behavior of Surface-Doped Thin-Film La0.8Sr0.2MnO3 − δ Cathodes

La 0.8 Sr 0.2 MnO 3-δ (LSM) film electrodes of thickness 100, 450, 650, and 700 nm were fabricated on yttria-stabilized zirconia (YSZ) electrolyte pellets by ultrasonic spray pyrolysis. In addition, 650 nm thick LSM electrodes with a surface doped with La-, Sr-, or Mn-nitrate were studied. Electrodes were deposited on both sides of the YSZ pellets in a symmetric arrangement. The individual electrode impedance spectra were measured at 973 K in laboratory air using a reference electrode placed on the free electrolyte surface. The open-circuit polarization resistance of the pure LSM and Sr and Mn surface-doped electrodes decreased significantly after initial application of cathodic current. In contrast, the La-doped sample showed no activation, with an initial polarization resistance similar to the activated, undoped LSM film. This indicates that surface La species play an important role in LSM electrode activation. All of the electrodes showed a strong, nonlinear decrease in polarization resistance accompanied by a linear decrease in ohmic resistance with increasing applied cathodic current. These changes are attributed to the introduction of a bulk-ion-transport path in the LSM film due to reduction of LSM under polarization. A minimum polarization resistance of 0.17 Ω cm 2 was measured for the La surface-doped 650 nm thick electrode at 250 mA/cm 2 .

The Influence of Current Density on the Electrocatalytic Activity of Oxide-Based Direct Hydrocarbon SOFC Anodes

We have studied the performance of Lao 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3-δ (LSCM )-Cu-yttria-stabilized zirconia anodes for the direct utilization of hydrocarbon fuels in a solid oxide fuel cell (SOFC). A significant decrease in anode polarization resistance and an increase in impedance peak frequency was observed with increasing the cell current density for H 2 , CH 4 , and C 4 H 10 fuel at both 973 and 1073 K. This was interpreted by considering an increase in LSCM lattice oxygen stoichiometry and a corresponding increase in electrocatalytic oxidation activity with increasing oxygen flux to the anode. A series of catalytic measurements indicates that, at a low current density (low oxygen stoichiometry), the anode reaction mechanism is dominated by hydrocarbon cracking. At a higher current density (higher oxygen stoichiometry), the reaction mechanism is dominated by total oxidation of hydrocarbon fuels on the LSCM surface to form CO 2 and H 2 O. This change in mechanism is confirmed by measurement of cell open-circuit voltage as a function of fuel, CO 2 , and H 2 O partial pressure and by analysis of the SOFC product stream. The increase in oxidation activity is attributed to an increase in nonequilibrium oxygen, relative to the anode gas, within the LSCM phase.

Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situ neutron diffraction

The layered perovskite NdBaCo2O5+δ (NBCO) was characterized using neutron powder diffraction under in situ conditions from 577–852 °C and in 10−1 to 10−4 atm oxygen. The best fit to the data was obtained in the tetragonal (P4/mmm) space group. No oxygen atom vacancy ordering was observed that warranted a lowering of the symmetry to orthorhombic (Pmmm). Two P4/mmm structural models were investigated: Model 1 (no split sites) and Model 2 (split Nd and O2 sites). Transport of oxygen through the material via the vacancy hopping mechanism likely involves the nearest-neighbor oxygen atom sites in the Nd layer. Total oxygen stoichiometry values were in the range 5.51 ≤ δ ≤ 5.11. The tetragonal lattice parameters increased with temperature as expected. However, the a-axis expands while the c-axis contracts with decreasing pO2 at a given temperature due to increasing vacancy concentration in the Nd layer.

On the methane oxidation activity of Sr2(MgMo)2O6-δ: a potential anode material for direct hydrocarbon solid oxide fuel cells

Sr2(MgMo)2O6−δ and materials of nominal compositions with increased and decreased Mo content were synthesized and their stability and catalytic activity towards CH4 oxidation characterized under solid oxide fuel cell (SOFC) anode conditions. Sr2(MgMo)2O6−δ was synthesized as a double perovskite. Materials with increased Mo content formed a mixture of the double perovskite Sr2MgMoO6−δ and the single perovskite SrMoO3, after synthesis in H2, with the amount of SrMoO3 increasing with increasing Mo content. SrMoO3 is unstable in air and slowly oxidized to the scheelite-structured impurity SrMoO4. All compositions decomposed to Mo, MgO and SrO in dry 20% CH4/N2 at temperatures ≥ 850 °C. CH4 reaction rates were catalytically limited with measured rates significantly lower than for La0.75Sr0.25Cr0.5Mn0.5O3-δ. Rates increased by 1–2 orders of magnitude when 2 wt-% of Pt was added to the surface.

Properties and Performance of Anode-Supported Proton-Conducting BaCe0.48Zr0.4Yb0.1Co0.02O3-δ Solid Oxide Fuel Cells

The anode-supported BaCe 0.48 Zr 0.4 Yb 0.1 Co 0.02 O 3―δ (BCZYC2)-based proton-conducting solid oxide fuel cells (H + -SOFCs) were fabricated using dual layer tape casting technique. BCZYC2 is a phase pure cubic perovskite and is stable in both humidified H 2 and CO 2 at 973 K. The total dc conductivity of BCZYC2 measured in humidified H 2 was 2.1 × 10 ―3 S/cm at 873 K. The cell performance was measured with humidified H 2 fuel and methanol/water fuel [with varying molar steam:carbon (S:C) ratios] at 873 K. The maximum open-circuit potential (OCP) of 1.04 V was achieved at 873 K in 3% humidified H 2 fuel with an accompanying peak power density of 89 mW/cm 2 . The total cell polarization resistance was 0.83 Ω cm 2 . Impedance measurements showed decreasing polarization and ohmic resistance with increasing current density. The cell operation via internal steam reforming of methanol was demonstrated using a methanol/water feed with 3:1 and 2:1 S:C ratios; however, the OCP was reduced to 0.97 and 0.95 V and the peak power density decreased to 65 and 58 mW/cm 2 , respectively. This was primarily attributed to a lower H 2 concentration in the feed stream.