DEVELOPMENT AND CHARACTERIZATION OF NOVEL CATHODE MATERIALS FOR MOLTEN-CARBONATE FUEL-CELL

In the development of molten carbonate fuel cell (MCFC) technology, th e corrosion of materials is a serious problem for long-term operation. Indeed, slow dissolution of lithiated-NiO cathode in molten carbonate s is the main obstacle for the commercialization of MCFCs. In the sear ch of new, more stable, cathode materials, alternative compounds such as LiFeO2, Li2MnO3, and La1-xSrCoO3 are presently under investigation to replace the currently used lithiated-NiO. The aim of the present wo rk was to investigate the possibility to produce electrode based on Li CoO2, a promising cathode material. At first, LixCoO2 powder samples ( 0.8<x<1.1) were made by thermal decomposition of carbonate precursors in air. The synthesis processes were monitored by thermal analysis (TG A, DTA). The calcined and sintered powder samples were characterized b y X-ray diffraction (XRD) and atomic absorption spectrophotometry (F-A AS). A single phase was detected in all the samples, without any chang e in crystal structure as a function of lithium content. Porous sinter ed electrodes were prepared starting from lithium cobaltite powders mi xed with different pore-formers by cold pressing and sintering. A bimo dal pore-size distribution with a mean pore diameter in the range of 0 .15 to 8 mum, a surface area of 2 to 12 m/g and a porosity of 10 to 65 %, determined by the Hg-intrusion technique, were observed in all the materials. Conductivity measurements were carried out in the temperatu re range of 500-700-degrees-C, in air. The influence of the deviations from stoichiometry on the electronic properties was determined, the c onductivity value of the stoichiometric compound being the lowest. A l inear relationship between the electronic conductivity and the sample porosity was found. Solubility testing of the materials was carried ou t to evaluate their chemical stability in the electrolyte. The samplin g method (F-AAS) and square wave voltammetry (SWV) were used to determ ine the concentration of dissolved cobalt in carbonates melt (Li2CO3/K 2CO3 = 63/38 mol%, pO2 = 0.33 atm, PCO2 = 0.67 atm, T = 650-degrees-C) . The Co content under steady-state conditions, was 6 ppm, i.e., lower than that for Ni from NiO(Li). To test the cathodic performance of th e materials, electrochemical impedance spectroscopy (EIS) measurements were carried out to investigate the porous electrode/molten carbonate s interface. For this purpose a laboratory-scale MCFC with the same el ectrolyte as that for the solubility tests was used. The cell was asse mbled with two identical porous cathodes (fabricated by cold pressing) to avoid any influence from an anode and a reference electrode. With this cell it was possible to determine the influence of atmospheric co mposition on electrode performance and the long-term stability.