| dc.description.abstract | This thesis investigates the effects of adding ammonia to the syngas fuel on the performance, electrochemical impedance spectra (EIS), and operational stability of the button anode-supported solid oxide fuel cells (SOFCs) under different experimental conditions by using the SOFC test platform established in our laboratory. Current-voltage curves and EIS are measured under three different operating temperatures (650°C, 700°C, 750°C) using five different fuels: (1) 200 H2, (2) 70 H2 + 130 CO, (3) 56 H2 + 104 CO + 20 NH3, (4) 47 H2 + 86 CO + 34 NH3, and (5) 35 H2 + 65 CO + 50 NH3 (units in standard cubic centimeter per minute, sccm). The cathode is supplied with 200-sccm air for all five cases. Results show that the cell performance increases with increasing temperature. As expected, the hydrogen-fueled SOFC has the best performance. At 650°C, the performance of the syngas-fueled SOFC is higher than the syngas-ammonia-fueled SOFC, but at 750°C the performance of the syngas-ammonia-fueled SOFC is higher than the syngas-fueled SOFC. This is because the ammonia decomposition rate decreases at low temperature (650°C), resulting in a significant reduction in performance. On the other hand, the ammonia decomposition rate at 750°C is almost 100%.
The stability tests of the cells are conducted at both 650°C and 750°C where the current density is fixed at 350 mA cm-2 using H2/CO (35/65 sccm) and H2/CO/NH3 (35/65/50 sccm) as the fuels. Results show that the cells fueled by syngas and ammonia (H2/CO/NH3) can operate stably for at least 120 hours at both temperatures. At 650°C, the H2/CO/NH3 cell performance does not decrease at all, while at 750°C, there is only a 3.22% decrease in the cell performance after 120-hour operation. Conversely, when using syngas (H2/CO) as the fuel, the cell voltages drop below 0.2 V after only 2 hours and 20.5 hours at 650°C and 750°C, respectively. The reason why the addition of ammonia can inhibit carbon deposition is because NH3 molecule has a lone pair of electrons. Compared with CO molecule, it is easier to occupy the acidic sites of the Ni catalyst and then reduce carbon deposition. This also explains that the cell performance in the stability test does not decrease owing to sufficient ammonia molecules occupying the acidic sites of Ni catalyst under the low ammonia decomposition rate at 650°C.
The aforementioned results show that the addition of ammonia has a positive effect on suppressing carbon deposition in syngas SOFC when it is operated at low temperature (650°C), and it should be useful for the promotion of the practical application of syngas SOFCs. | en_US |