Effect of Electrolyte Amount on the Performance in a Molten Carbonate Fuel Cell
Authors: Kim Yu-Jeong , Kim Tae-Kyun , Lee Sang-Woo , Lee Ki-Jeong , Lee Choong-Gon 
Published: February 2020

This work has been focused on the effect of electrolyte amount in a coin type molten carbonate fuel cell (MCFC). The MCFC uses molten carbonate as an electrolyte. The carbonates are distributed at the anode and cathode electrodes to provide three phase boundaries where the electrochemical reactions occur.

One factor which controls polarization is the degree of filling in electrodes. MCFC cathodes consist of in-situ oxidized NiO. The cathode is wetted completely by molten carbonate, so the cathode is entirely covered with a thin film of electrolyte. This causes some diffusion resistance even under optimal operating conditions, i.e., at 15-30% filling of the total pore volume by electrolyte. However, when the pores become filled with excess amount of electrolyte, calling as flooding, diffusion increases steeply. On the other hand, the anode is much less sensitive to the filling than the cathode. The reason is that electrode kinetics and mass transport are relatively rapid at the anode. Moreover, the level of polarization is not high because of the rapid kinetics. Therefore the anode can serve as an electrolyte reservoir. Thus, electrolyte amount in the electrodes are very important factor in the cell performance.

Four different amounts of electrolyte, 1.5, 2, 3, and 4 g, were used. In general, the carbonate amount is expressed by pore filling ratio (pfr) which is the ratio of electrolyte filling in the void volume of electrodes. The electrolyte amounts of 1.5, 2, 3, and 4 g are corresponding to 0, 10, 60, and 110% of pfr, respectively.

A coin type molten carbonate fuel cell was used in this work. The diameter of electrodes was ca. 3 cm. The matrix was made of LiAlO2. The anode gas was a mixture of 69 mol% H2, 17 mol% CO2 and 14 mol% H2O. A gas mixture of 70 mol% air and 30 mol% CO2 served as the cathode gas. The electrolyte was 62 mol% Li2CO3 and 38 mol% K2CO3. All the cells were operated at 923K under atmospheric condition.

The cell performance of the electrolyte was analyzed via steady state polarization, step- chronopotentiometry, and impedance method.

First is steady state polarization. The current density was applied from 0 to 150 mA/cm2. In general, the voltage of MCFC is the difference between the open circuit voltage (OCV) and total voltage loss. At pfr 60%, the voltage decreases linearly and slightly from the OCV, indicating that pfr 60% has negligible activation polarization. The reason is that pfr 60% has a large active surface area. When the electrolyte amounts were increase, total voltage loss decreased. However, the pfr of 110% has a severe voltage loss at high current density, representing large mass transfer resistance takes place at the electrode due to the flooding phenomenon.

Second is step-chronopotentiometry. The current steps were applied at every 60 seconds and following voltage relaxation was observed. All the cells showed step-voltage relaxation according to the current step. In particular, voltage relaxation was reduced by the increase of electrolyte amount up to pfr 60%. However, voltage relaxation tended to increase at pfr 0% and 110%. The reason is that too much or too small amounts of electrolyte reduced surface area and it resulted in large mass transfer resistance and voltage relaxations.

Third is impedance behavior with various amounts of electrolyte at open circuit state. The AC signal of 5 mV RMS was applied from the high frequency (10 kHz) to low frequency (10 mHz).  Similar to the bench scale cell, two half circles at high and low frequency regions are observed. The high-frequency half circle reflected cathodic overpotential, and low frequency one represented anodic overpotential [2]. The starting point of half circuit represented internal resistance. In this work, internal resistance and electrode overpotential are decreased by enlarging electrolyte amount up to pfr 60%. The enlarged amount of electrolyte provides larger gas-liq-solid three phase boundary, thus the active surface would increase. The pfr 0% and 110% showed larger high frequency half circle than other pfrs. It is plausible that the large mass transfer resistance at the electrodes is resulted from high or lower electrolyte amounts. Moreover, a large resistance gap was observed between pfr 0% and 10%. In the case the difference of electrolyte amount was only 0.5 g. It is estimated that a permissible range of electrolyte is limited and electrode is very sensitive to the degree of filling.