Rasmus Barfod

Detailed Characterization of Anode-Supported SOFCs by Impedance Spectroscopy

Anode-supported thin electrolyte cells are studied by electrochemical impedance spectroscopy (EIS). The aim is to describe how the losses of this type of cells are distributed at low current density (around open-circuit voltage) as a function of temperature. An equivalent circuit consisting of an inductance, a serial resistance (R s ), and five arcs to describe the polarization resistance is suggested. This equivalent circuit is based on previous studies of single electrodes in three-electrode and two-electrode symmetric cell setups. The equivalent circuit components have been assigned to the electrode processes, and the assignments were verified by extensive full cell studies in which the partial pressure of reactant gases on both the electrodes as well as temperature was systematically varied with the aim to identify frequency regions which are dominated by an electrode specific process. Furthermore, the model is applied on a good performing cell with area specific resistance (ASR) = 0.15 Ω cm 2 at 850°C and a poor performing cell with ASR = 0.29 Ω cm 2 at the same temperature. Both cells were fabricated using nominally the same procedure. The EIS analysis indicated that the difference in performance originates from microstructural differences on the cathode. This is further supported by the observation of large differences in the cathode microstructure by scanning electron microscope.

Degradation of Anode Supported SOFCs as a Function of Temperature and Current Load

The degradation behavior of anode supported solid oxide fuel cells (SOFCs) was investigated as a function of operating temperature and current density. Degradation rates were defined and shown to be mainly dependent on the cell polarization. The combination of a detailed evaluation of electrochemical properties by impedance spectroscopy, in particular, and post-test microscopy revealed that cathode degradation was the dominant contribution to degradation at higher current densities and lower temperatures. The anode was found to contribute more to degradation at higher temperatures. Generally, the degradation rates obtained were lower at higher operating temperatures, even at higher current densities. A degradation rate as low as 2%/1000 h was observed at 1.7 A/cm 2 and 950°C over an operating period of 1500 h.

Assessment of the Cathode Contribution to the Degradation of Anode-Supported Solid Oxide Fuel Cells

The degradation of anode-supported cells was studied over 1500 h as a function of cell polarization either in air or oxygen on the cathode side. Based on impedance analysis, contributions of the anode and cathode to the increase of total resistance were assigned. Accordingly, the degradation rates of the cathode were strongly dependent on the pO 2 . Microstructural analysis of the cathode/electrolyte interface carried out after removal of the cathode showed craters on the electrolyte surface where the lanthanum strontium manganite (LSM) particles had been located. The changes of shape and size of these craters observed after testing correlated with the cell voltage degradation rates. The results can be interpreted in terms of element redistribution at the cathode/ electrolyte interface and formation of foreign phases giving rise to a weakening of local contact points of the LSM cathode and yttria-stabilized zirconia electrolyte and consequently a reduced three-phase boundary length.

Nanostructured Lanthanum Manganate Composite Cathode

Anode-supported cells were fabricated with optimized cathodes showing high power density of 1.2 W/cm2 at 800°C under a cell voltage of 0.7 V and an active area of 4 4 cm. A microstructure study was performed on such cell using a field-emission gun scanning electron microscope, which revealed that the La1−xSrx yMnO3± LSM composite cathodes consist of a network of homogenously distributed LSM, yttria-stabilized zirconia YSZ , and pores. The individual grain size of LSM or YSZ is approximately 100 nm. The degree of contact between cathode and electrolyte is 39% on average.

New efficient catalyst for ammonia synthesis: barium-promoted cobalt on carbon

Barium-promoted cobalt catalysts supported on carbon exhibit higher ammonia activities at synthesis temperatures than the commercial, multipromoted iron catalyst and also a lower ammonia inhibition.

Degradation of Conductivity and Microstructure under Thermal and Current Load in Ni-YSZ Cermets for SOFC Anodes

The degradation of electrical conductivity in porous nickel-yttria stabilized zirconia composite cermets in a H2/H2O atmosphere under high temperature treatments has been investigated. The parameters varied were: temperature, water partial pressure, and electrical current load. The microstructure was analyzed before and after the treatment by optical microscopy and field emission scanning electron microscopy (FE-SEM). From the optical images the particle size and total amount of Ni, as area fraction, in the sample were measured. By the use of charge contrast (CC) in the FE-SEM particle size and area fraction of percolated Ni was measured. Temperature proved to have the largest effect on the degradation. Samples tested at 1000°C, in contrast to 750°C, showed a severe decrease of conductivity and growth of Ni particles. Higher water partial pressure accelerated Ni particle growth at both temperatures, but the loss of percolation and conductivity at 1000°C was less severe under high water partial pressure. A possible explanation for this behavior is discussed.