Investigations On Dissolution Mechanisms Of NiO Cathode Under Scattering Load And On Preparation And Properties Of Ternary Composite-based Cathode Materials For Molten Carbonate Fuel Cell

Molten Carbonate Fuel Cell （MCFC） is believed to be one of the most promisingenergy conversion devices converting chemical energy into electricity directly. It hasseveral advantages, such as the high efficiency, the ability to utilize a wide variety offuels such as hydrogen, natural gas and coal gas, the avoidance of precious metals as acatalyst and so on. It has been applied preliminarily to power plants on a commercialscale. However, the dissolution of the state of the art cathode material NiO into theelectrolyte is one of the major technical obstacles to the further commercialization ofthe MCFC. Thus, investigations on dissolution mechanisms of traditional cathode anddevelopment of candidate cathodes or novel composite cathodic materials to prolonglife and improve performance of the cell are very necessary.In this work, a home-made deformation-testing system was applied to investigatethe deformation/dissolution mechanism of Ni-based cathode material under conditionsof simulated MCFC startup and working stages. On the base of thedeformation/dissolution mechanism, a method in preparing novel MCFC ternarycomposite-based cathode materials has been proposed. Some promising candidatematerials, such as LiCoO₂, CeO₂and LiFeO₂, were prepared by Pechini method underdifferent calcined temperatures. The electrophoretic deposition （EPD） technique wasexploited to prepare novel MCFC ternary composite-based cathodes, by simultaneousmodification on Ni surface with nanoparticles of LiCoO₂and one of LiFeO₂and CeO₂.Some ternary composite-based cathodes were prepared by EPD on the control ofdeposition voltage, deposition time, the concentration and pH value of suspension.The deformation/dissolution and the electrochemical behaviors of the ternary composite-based cathodes were also evaluated. Thereinto, considerable efforts weremade to screen out the preferable preparation parameters for the composite materials.The results show that porous Ni-based cathode was inclined to deform in a simulatedstartup and running stages of MCFC stack. The deformation occurred apparently inthe first20hours at the startup stage. The stage in which the Ni-based cathodedeformed seriously was also the stage in which the Ni-based cathode dissolvedheavily. Both the deformation and dissolution decreased while the Ni-based cathodehad transformed into lithium-doped NiO working cathode. The thickness of theloaded area significantly less than that of unloaded area, and the surface of loadedarea was covered with more re-deposited needle NiO than the surface of the unloadedarea. The unloaded surface was oxidated and lithiated more adequately due to acontact with the atmosphere and less re-deposited needle NiO on its surface wasobserved.The particles （LiCoO₂, LiFeO₂and CeO₂） calcined at650℃possess goodcrystallinity, purity, and in nano scale （30-50nm、30-60nm and10-20nmrespectively）, making them well-deposited on the surface of porous nickel. Suitabledispersant （isopropanol）, in which nanoparticles can be well simultaneously dispersed,were chosen on basis of transparency experiments. Two kinds of ternarycomposite-based cathodes, LiCoO₂-LiFeO₂-Ni and LiCoO₂-CeO₂-Ni, are inclined topossess good morphology and homogeneous elements distribution when preparedunder the optimal EPD conditions （EPD voltage:60V, current:12mA, depositiontime:2min, concentration of the suspension:0.10g/25mL, and pH value of thesuspension:3.00）. Compared with Ni-based cathode, the new type of composite-basedcathodes manifested excellent performances on anti-deformation/anti-dissolutionunder the conditions of simulated startup stage of MCFC，since the compact ceramiclayer modified on the surface of porous nickel can avoid a direct contact betweenporous nickel and molten carbonates hence effectively precluding the earlier NiOlayer from spalling into melts during oxidation/lithiation of nickel; moreover, the newphase of LiCoyNi1-yO forming on the surface of the composites is quite stable in meltsand can further inhibit the dissolution of nickel. The electrochemical impedance spectroscopy （EIS） of lithium-doped LiCoO₂-CeO₂-NiO and LiCoO₂-LiFeO₂-NiOworking cathodes suggested that these two novel composite-based cathodes ownedgood electrochemical performance in the simulated operating conditions of MCFC.The dissolution mechanisms of the traditional MCFC Ni-based cathode underscattering load were clarified. And two ternary composite-based cathode materials formolten carbonate fuel cells, LiCoO₂-CeO₂-Ni, and LiCoO₂-LiFeO₂-Ni, which possessgood performances of anti-deformation, anti-dissolution and good electrochemicalproperty, were successfully prepared by EPD technique. Because of their attractiveadvantages, the ternary composite-based cathodes possess the potential for practicalapplication in the MCFC industry.