OPTIMIZATION OF THE CATHODE LONG TERM STABILITY IN MOLTEN CARBONATE FUEL CELLS: EXPERIMENTAL STUDY AND MATHEMATICAL MODELING

Abstract

The cathode materials for molten carbonate fuel cells (MCFCs) must have low dissolution rate, high structural strength and good electrical conductivity. Currently available cathodes are made of lithiated NiO which have acceptable structural strength and conductivity. However a study carried out by Orfeld et al. and Shores et al. indicated that the nickel cathodes dissolved, then precipitated and reformed as dendrites across the electrolyte matrix. This results in a decrease in cell utilization and eventually leads to shorting of the cell. The solubility of NiO was found to depend upon the acidity/basicity of the melt (basicity is directly proportional to log P{sub CO2}), carbonate composition, H{sub 2}O partial pressure and temperature. Urushibata et al. found that the dissolution of the cathode is a primary life limiting constraint of MCFCs, particularly in pressurized operation. With currently available NiO cathodes, the goal of 40,000 hours for the lifetime of MCFC appears achievable with cell operation at atmospheric pressure. However, the cell life at 10 atm and higher cell pressures is in the range between 5,000 to 10,000 hours. The overall objective of this research is to develop a superior cathode for MCFC's with improved catalytic ability, enhanced corrosion resistance with low ohmic losses, improved electronic conductivity. We also plan to understand the corrosion processes occurring at the cathode/molten carbonate interface. The following cathode materials will be subjected to detailed electrochemical, performance, structural and corrosion studies. (i) Passivated NiO alloys using chemical treatment with yttrium ion implantation and anodic yttrium molybdate treatment; (ii) Novel composite materials based on NiO and nanosized Ce, Yt, Mo; (iii) Co doped LiNiO{sub 2} LiNiO{sub 2} doped with 10 to 20% Co (LiCo{sub 0.2}NiO{sub 2}) and NiO cathodes; and (iv) CoO as a replacement for NiO. Passivation treatments will inhibit corrosion and increase the stability of the cathode at high temperatures. Deposition of refractory metals (Mo, W, Li{sub 2}NiCrO{sub 4}) will impart stability to the cathode at high temperatures. Further it will also increase the electrocatalytic activity and corrosion resistance of the cathode. Doping with Co will decrease the alloy dissolution and increase the cycle life of the cathode. In the reporting period the oxidation behavior of Ni and Co in Li + Na carbonate eutectic was investigated under oxidizing environment using cyclic voltammetry, electrochemical impedance spectroscopy and potentiodynamic technique. The open circuit potential was monitored as a function of time in order to evaluate the material's reactivity in the melt.