Electrochemical Performance of SOFC Components Fabricated by Spray Pyrolysis Method

The method of Spray Pyrolysis (SP) was applied to prepare thin films of electrodes and electrolytes for Solid Oxide Fuel Cell (SOFC) components. These devices which operate typically above 500°C due to the adequateness of  ionic conductance requirement of their ceramic electrolyte, offer a great fuel flexibility and operation at high efficiencies with low environmental burdens. They typically operate with either an Yttria (8% mole)  Stabilized Zirconia (YSZ) which is suitable above 800°C or a Gadolinia (10-20% mole) doped Ceria electrolyte which exhibits better conductivities within an intermediate temperature range (i.e. 500-750°C).

  Various conventional fabrication methods such as tape casting and screen printing involve slurry preparation which includes ball milling, use of organic additives and sintering at high temperatures often above 1000 °C, all of which contribute to increasing the fabrication cost of a SOFC.  On the other hand, the spray pyrolysis technique is simple and it basically involves spraying of a solution of appropriate salts onto a heated substrate on which the desired film of either electrolyte or electrode is obtained after thermal decomposition of the precursors.  By appropriately tuning process parameters such as substrate temperature, solution flowrates and type and concentration of salts and solvents, thin films of the desired composition can be obtained.  In fact, if sintering is also done in situ, the process eliminates intermediate steps and even conduces to a lower sintering temperature due to its reliance on a molecular rather than a conventional particulate approach for thin film production. The process also offers the possibility of producing all cell components in situ and cell operation at lower temperatures due to the lower ohmic resistances  exhibited by thin ceramic films.

The aim of this work was to operate a SOFC with all its components fabricated in situ by SP and exhibiting an electrochemical performance at least equivalent with that obtained by a conventionally made SOFC. Anodic electrodes were chosen for their performance as suitable electrocatalysts for the direct oxidation of hydrocarbons in these high temperature fuel cells. The strategy, here, is to use a good electronic conductor like Cu for enhanced conductivity but inert to undesired reactions  such as coking in conjunction with a good catalyst such as CeO₂ for hydrocarbon oxidation.

 As a cathodic electrode the perovskite  (La_0.8Sr_0.2)_0.95MnO₃ was used and deposited for all the experiments due to its excellent thermal and chemical compatibility with YSZ electrolyte. Thus, results will be presented of the application of this method for the production of composites consisting of Cu-CeO₂/YSZ/LSM and Cu-La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM) /YSZ/LSM deposited by SP on porous support substrates of lanthanum strontium manganite (LSM) of 2 mm thickness. Furthermore, the same type of anodic and cathodic electrodes were deposited on dense YSZ substrates and compared in terms of morphology and electrochemical performance  with the previous composite structures.

Film microstructure was analyzed by scanning electron microscopy (SEM) and was found that thin defect-free films could be produced on porous and dense substrates. X-ray diffraction (XRD) analysis showed that the films were crystalline after the deposition at relatively low  (i.e. 700°C) post-deposition sintering temperatures. Electrochemical tests of composite structures consisting of Cu-CeO₂/YSZ/LSM and Cu-LSCM/YSZ/LSM showed that it is possible to obtain a SOFC with sufficient electrochemical performance fabricated by spray pyrolysis.