Temperature Effect on the Cell Life of Molten Carbonate Fuel Cell
Authors: Kim Tae-Kyun , Kim Yu-Jeong , Lee Sang-Woo , Lee Ki-Jeong , Lee Choong-Gon 
Published: February 2020

This study proposes the temperature effect on the cell life and performance with coin type molten carbonate fuel cells (MCFC). The cells were operated at 600 oC, 700 oC and 800 oC under an atmospheric condition. The temperature effects were analyzed with electrochemical methods of steady state polarization, step chronopotentiometry, and electrochemical impedance spectroscopy.

 In general, the electrochemical reaction is accelerated by the temperature rising. Thus, higher exchange current is expected at higher temperature. The ionic conductivity in the carbonate electrolyte is also increased by the temperature increasing. Moreover, the gas solubility into molten carbonate has positive activation energies. Diffusion in the liquid and gas phases is also enhanced by the temperature. Therefore, higher performance is obtained at higher temperature.

 However, the carbonate has volatility, although it can be negligible below 600oC. The metal corrosion is also a function of temperature, higher temperature results in larger corrosion rate. Therefore, it could be guessed that reduction of electrolyte amount by the temperature rising makes higher internal resistance and mass transfer resistance.

  The OCV at 700 oC was higher than that at 800 oC because the absolute value of Gibbs energy is larger at 700 oC. The OCV at 700 oC continues for 1,200 h whereas 800oC continues only 250 h. Therefore, the cell life at 700 oC was much longer than that at 800 oC. At 150 mAcm-2, voltage of 700 oC was compared with that of 800 oC. It was comprehensible to know the temperature effect more clearly. The voltage at 800oC was initially higher than that at 700 oC. This is acceptable because the voltage at higher temperature has lower ionic and reaction resistances. However, the voltage at 800oC drops very quickly, indicating the electrolyte consumption was significant. On the other hand, the voltage at 700 oC is relatively stable and its internal resistance is slowly increasing. Therefore, lower temperature cell gives more stable and longer cell life.

  The cell performance of the electrolyte was analyzed at every 300 h of the cell at 700 oC and at every 60 h of the cell at 800 oC. From the steady state polarization, the current-voltage behaviors were measured while current density is applied from 0 mAcm-
2 to 150 mAcm-
2. The changes of the slope were increasing but not significant at 700 oC. However, the cell at 800oC shows severe slope change by the time. It means the resistance increasing becomes severe at higher temperature.

   At the step-chronopotentiometry, current steps were applied at every 60 second and following voltage relaxation was observed. All the cells showed step-voltage relaxation according to the current step. At early stage of operation, voltage relaxation was consistent. However at the cell of 800 oC, voltage relaxation was increased significantly at the end part of operation. It means mass transfer resistance becomes significant.

  The impedance behaviors were measured at the same time period. From the impedance results increasing internal resistance was observed at the cells. Since the internal resistance reflects ionic conductivity of the electrolytes, its enlargement represents electrolyte depletion. In particular, steeper internal resistance increase was observed at 800 oC than other temperature, representing significant electrolyte consumption existed at 800 oC. In addition, enlarging of the high-frequency half circle was obtained. Because the high frequency one represents cathodic mass transfer resistance, it means cathodic resistance becomes significant. It is plausible that the reduced electrolyte amount decreases active surface area of the cathode and enlarges its mass-transfer resistance. The anode has very high exchange current density, therefore the electrolyte reduction may not affect mass-transfer resistance significantly.