A Simple Surface Modification of NiO Cathode with TiO₂ Nano-Particles for Molten Carbonate Fuel Cells (MCFCs)

Experimental

TiO2-modified Ni Powder.TiO2-modified Ni powder was prepared by ball-milling of TiO2 powder (particle size ≤ 25 nm, Aldrich) and Ni powder (particle size = 2-3 mm, Inco nickel 255). The sample powders were prepared with various mole ratios of TiO2 (1.00, 1.75, 2.50, and 2.75 mol %). The ball-milled powder samples were annealed at various temperatures (200-650 °C) for 10 h under ambient condition. In order to set the annealing temperature, the dispersion test4 was performed. About 0.020 g of Ni powder and TiO2-modified Ni powder with oxidation for annealing or without, are immersed into 5 mL of solvent which is same composition of slurry (ethanol: toluene = 70:30 wt %). The mixture is ultra-sonicated for one minute. After the soni-cation, the status of powder in solvent is observed.4

TiO2-modified Ni Cathode. The TiO2-modified cathode was prepared by tape casting with TiO2-modified Ni powder and slurry after ball-milling for 5 h. The slurry was composed binder (polyvinyl butyral, Monsanto), plasticizer (dibutyl phthalate, Junsei), dispersant (Disper BYK-110), defoamer (Dappo D354) and solvent (ethanol:toluene = 70:30 wt %, Junsei).4,9,12,13 After ball-milling and degassing using a rotary evaporator with stirring for mixing of slurry with TiO2-modi-fied Ni powder, the green sheet with 0.7-0.8 mm thickness was obtained by tape casting using double doctor blade. The green sheet dried at room temperature for 24 h was sintered under the reducing atmosphere (Ar:H2 = 70:30%).

Lithiation and Solubility Tests. The degree of lithiation of the cathode and the concentration of Ni in the electrolyte were investigated in the equivalent condition as MCFC operation.4,5,9,12-14 For lithiation test, 1.5 g of modified powder and 100 g of (Li/K)2CO3 (62/38 mol %) was transferred to alumina crucible, and heated to 650 °C under CO2:O2 (67:33%) atmosphere. After 300 h, the powder sample was collected and cleaned with 1.0 M acetic acid solution to get rid of carbonates on the surface, and then washed with distilled water and dried. The structure and the surface morphology of the samples were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). For solubility test, the experimental procedure is identical to that of lithiation test except for the sample which is not powder but sintered electrode.4,5,9,12-14 Periodically ~0.30 g of molten carbonate was collected using alumina pipette to measure the concentration of Ni in the molten carbonate. The solidified carbonates were dis-solved in 1.0 M HNO3 solution. After the solvent was evaporated, it was diluted with 20 mL of distilled water. Concentration of Ni dissolved into the molten carbonates was measured with inductively coupled plasma atomic emi-ssion spectroscopy (ICP-AES, JY Ultima2C (Jobin Yvon, France) or inductively coupled plasma mass spectroscopy (ICP-MS, Elan DRC II / Perkin Elmer, USA).

Unit Cell Operation. The performance of unit cell was operated using 3 × 3 cm2 cell at 650 °C for 300 h. The open circuit and the closed circuit voltages at current density of 150 mA/cm2 were measured. Except the cathodes, other components of cell were commercial products.4,5,9,12-14 The anode gas was H2/CO2 (67:33%) humidified at 50 °C, and the cathode gas was CO2:O2 (67:33%) or air:CO2 (70:30%). To investigate the electrochemical properties of prepared cathode materials, electrochemical impedance spectroscopy (EIS) was employed in with an IM6 electrochemical analyzer (ZAHNER elektrik, Germany). The frequency range was 10 MHz to 100 kHz with a rate of 20 point per decade. The amplitude of sinusoidal voltage signal for the impedance analyzer was 5 mV.