Improvement of electricity generating performance and life expectancy of MCFC stack by applying Li/Na carbonate electrolyte: Test results and analysis of 0.44 m2/10 kW- and 1.03 m2/10 kW-class stack

Following the development of a 10 kW-class MCFC stack with a reactive area of 0.44 and 1.03 m2, which applies a Li/Na carbonate electrolyte and a press stamping separator, many tests have now been carried out. In the installation tests, the observed cell voltages of the 0.44 m2/10 kW-class stack agreed with the voltage predicted from the test results of the 100 cm2 bench scale cell. This agreement proves that the installing procedure of the bench scale cell can be applied to the 0.44 m2/10 kW-class stacks. The temperature distribution analysis model applied to the 100 kW-class stack was modified to calculate the temperature distribution of the 0.44 m2/10 kW-class stack. Taking the heat loss and the heat transfer effect of the stack holder into account, the calculated temperature was close to the measured temperature; this result proves that the modification was adequate for the temperature analysis model. In the high current density operating tests on the 0.44 m2/10 kW-class stack, an electrical power density of 2.46 kW/m2 was recorded at an operating current density of 3000 A/m2. In the endurance test on the 0.44 m2/10 kW-class stack, however, unexpected Ni shortening occurred during the operating period 2500–4500 h, which had been caused by a defective formation of the electrolyte matrix. The shortening seems to have been caused by the crack, which appeared in the electrolyte matrix. The voltage degradation rate of the 0.44 m2/10 kW-class stack was 0.52% over 1000 h, which proves that the matrix was inadequate for a long life expectancy of the MCFC stack. A final endurance test was carried out on the 1.03 m2/10 kW-class stack, of which the matrix had been revised. The fuel utilisation and the leakage of anode gas never changed during the 10,000 h operating test. This result suggests that no shortening occurred during the 10,000 h endurance test. The cell voltage degradation rate was around 0.2–0.3% over 1000 h in the 1.03 m2/10 kW-class stack. According to a comparison of the stack electricity generating performance of the 0.44 m2 and the 1.03 m2/10 kW-class stack under the same operating conditions, the performance of the 1.03 m2 stack was lower at the beginning of the endurance test, however, its performance exceeded the performance of the 0.44 m2/10 kW-class stack during the 10,000 h operating test. By carrying out the high current density operating test and the 10,000-hour endurance test using commercial sized 10 kW-class stacks, the stability of the MCFC stack with a Li/Na carbonate electrolyte and a press stamping separator has been proven.