Analysis of Cell Life and Performance of Molten Carbonate Fuel Cells with Li-Na and Li-K Carbonate Electrolytes

Abstract

This work has been focused on the effect of Li-K and Li-Na carbonate electrolytes in a coin type molten carbonate fuel cell (MCFC). The MCFC uses molten carbonate as an electrolyte. The performance of cells is affected by the electrolyte composition. An important factor to consider when monitoring the performance of MCFC is the type of electrolyte or the species of electrolyte to be used. In this experiment, Li-K and Li-Na carbonate electrolytes were used. Li-Na electrolyte has lower internal resistance (i.e. higher ionic conductivity) compared to Li-K electrolyte. This causes a better cell performance in Li-Na electrolyte than in Li-K electrolyte MCFC [1]. A coin type molten carbonate fuel cell was used in this experiment. The diameter of electrodes was 3 cm. The matrix was made of LiAlO2. The anode gas was made up of 72 mol% H2, 14 mol% CO2 and 14 mol% H2O. A gas mixture of 70 mol% air and 30 mol% CO2 served as the cathode gas. The Li-K carbonate electrolyte was made up of 62 mol% Li2CO3 and 38 mol% K2CO3 whiles the Li-Na carbonate electrolyte consisted of 52 mol% Li2CO3 and 48 mol% Na2CO3. All the cells were operated at a temperature of 927 K and a pressure of 1 atm. The cell performance of the electrolyte was analyzed via steady state polarization, step-chronopotentiometry, and impedance method. After the experiment, the cell was dissolved in 10 wt% acetic acid solution and the remaining amount of electrolyte was analyzed. Firstly, the remaining amount of electrolyte after operating for 12 hours was analyzed using 1.5, 2.0, 3.0, 4.0 g of electrolyte in order to investigate the reduction ratio of the electrolyte. The rate of reduction per weight of each electrolyte was similar. It revealed that each electrolyte had almost the same slope. Therefore, the relationship between the initial amount of electrolytes and weight reduction ratio can be expressed by a linear equation. Secondly, in order to investigate the correlation between the cell life and the amount of electrolyte, the cells were operated in 3 g of each electrolyte. A 300 mA/cm2 current density was applied to the fuel cells in order to compare the performances of the electrolytes. After the application of 300 mA/cm2 current density, It was observed that the cell using Li-Na electrolyte had a higher performance than the cell using Li-K electrolyte but the cell life of Li-Na electrolyte was shorter than the cell life of Li-K electrolyte at 650oC (923K). This implies that Li-Na has a faster electrolyte loss rate than Li-K electrolyte. Using cells at open circuit state, the impedance behavior of the two electrolytes was also observed. An AC signal of 5 mV RMS was applied at a frequency range of 1 kHz to 10 mHz. Two half circles similar to a bench scale cell at low and high frequencies were observed. The high-frequency half circle indicated cathodic overpotential, and low frequency one represented anodic overpotential [2]. It was also observed that total overpotential of all cells increased while cell performances decreased as time progresses. It was probably due to the electrolyte loss in the cells. Finally, in order to find the relationship between amount of electrolyte loss and cell life, a graph of initial amount of electrolyte was plotted against weight reduction ratio. A linear graph was observed in each plot. It was estimated that both cells showed a decrease in amount of electrolyte of about 20 wt%. It can be concluded that a permissible range of electrolyte loss is sensitive. Reference [1] H. Morita, M. Komoda, Y. Mugikura, Y. Izaki, T. Watanabe, Y. Masuda, T. Matsuyama, Journal of Power Sources 112 (2002) 509-518. [2] C.-G. Lee, Journal of Electroanalytical Chemistry 776 (2016) 162-169.