3.1 Performance of the Button LAA-SOFC
Figure 3 (a) and (b) show the polarization curves and the EIS results, respectively, of the LSCF cathode button fuel cells with different amounts of pore former. The LSCF10 cell reached the peak power density of 70 mW cm-2, and the peak power densities of the LSCF5 and LSCF0 cells were only 62 and 46 mW cm-2, respectively. The ohmic resistance of the LSCF5 cell and the LSCF0 cell were both approximately 1.2 Ω cm2, and the ohmic resistance of the LSCF10 was 1.3 Ω cm2. The polarization impedance of the LSCF10 cell was approximately 0.63 Ω cm2, which was smaller than that of the LSCF5 cell (1.1 Ω cm2) and LSCF0 cell (1.6 Ω cm2). According to previous research (Jayakumar et al. 2011), the polarization impedance of the LAA was approximately 0.06 Ω cm2. Therefore, in the Nyquist diagrams, the two semicircles of each cell both come from the cathodes. The different performances of the button fuel cells indicated that the different amounts of pore former in the spraying powder influenced the polarization impedance of the cathode. The impedance data of LSCF0 show an obvious Warburg-type impedance, which suggests a scarcity of mass-transport channels in the cathode. The significant difference in the impedance data among the three cells is a result of the different amount of pore former in the sprayed powder. The sufficient mass transport in the LSCF10 cathode provides it with better performance than that of LSCF5 and LSCF0.
The cathodes of the button fuel cells were characterized by SEM after the electrochemical tests. Figure 4 (a) shows an SEM image of the cathode in the LSCF0 cell. The porosity of the cathode was very low, and it was difficult for air to diffuse through the cathode layer, resulting in a relatively significant Nernst diffusion impedance. Figure 4 (b) shows an SEM image of the cathode in the LSCF10 cell. The porosity of the cathode was increased by adding pore former to the spraying powder, and the diffusion impedance decreased accordingly. However, the high content of pore former in the spraying powder resulted in relatively poor contact between the cathode particles and led to an increase in ohmic impedance.
Figure 5 (a) and (b) show the polarization curves and EIS results, respectively, of the button fuel cells with the LSCF10 cathode and LSM10 cathode. The peak power density of the LSM10 cell was only 29 mW cm-2, which was far less than that of the LSCF10 cell. However, the ohmic impedance of the LSM10 cell was much larger than that of the LSCF10 cell. This result can be attributed to the mixed electronic and ionic conductivities of the LSCF material (Zhang et al. 2016), and these mixed conductivities greatly expanded the reaction interface for oxygen reduction at the cathode of the cell. within contrast, the LSM material (T.Miruszewski et al. 2016) only conducts electrons at the working temperatures of SOFCs
The present LAA-SOFC with the LSCF10 cathode was compared with the plasma-sprayed SOFCs with different electrode arrangement type in Table 2. The polarization impedance of the cathode-down cells was much larger than that of the anode-down cells. In previous studies, the cathodes of the anode-down cells were prepared by a low torch power and a long spray distance, so the cathode particles were semi-molten when reaching the substrates. This made the plasma sprayed cathode layers porous but also decreased the bond strength of the cathode. For the cathodes of the anode-down cells, a low bond strength is generally acceptable because they were prepared at the outermost layer of the fuel cells and there wouldn’t be any other spraying process after the cathodes were prepared. Considering to the extra complexity, the pore former was rarely used in cathode fabrication of the anode-down plasma-sprayed cells. However, for the cathode-down cells, if the bond strength of the cathodes was low, the heat and the impact causing by the plasma torch would make the coating crack or break off. Therefore, the cathodes were prepared with a relatively high torch power, which would result in a lower porosity and a larger polarization impedance. In the present research, with the pore former addition, the porosity and the performance of the cathode was improved.
Table 2
Comparison of the plasma-sprayed SOFCs with different electrode arrangement type.
Authors
|
Cathode Materials
|
Spraying Method
|
Type
|
Pore Former
|
Polarization Impedance at 750℃
|
Tsai et al. (2014)
|
LSCF-LSGM
|
Atmospheric plasma spraying
|
Anode-down
|
15 wt.% carbon black
|
0.128 Ω cm2
|
Wang et al. (2020)
|
LSCF
|
Atmospheric plasma spraying
|
Anode-down
|
None
|
0.15 Ω cm2
|
Harris et al. (2017)
|
LSCF-SDC
|
Atmospheric plasma spraying
|
Anode-down
|
25 wt.% carbon black
|
0.37 Ω cm2
|
None
|
0.41 Ω cm2
|
Fan et al. (2016)
|
LSCF
|
Suspension plasma spraying
|
Symmetrical cell
|
0.3 wt.% carbon black
|
0.062 Ω cm2
|
Waldbillig and Kesler (2011)
|
LSM-YSZ
|
Suspension plasma spraying
|
Cathode-down
|
None
|
0.75 Ω cm2
|
Metcalfe et al. (2013)
|
LSCF-SDC
|
Atmospheric plasma spraying
|
Cathode-down
|
None
|
4.1 Ω cm2
|
Anode-down
|
None
|
0.13 Ω cm2
|
Present Study
|
LSCF
|
Atmospheric plasma spraying
|
Cathode-down
|
10 wt.% carbon black
|
0.63 Ω cm2
|
3.2 Performance of the Tubular LAA-SOFC
The tubular LAA-SOFC was fabricated with an LSCF10 cathode because of its respectable electrochemical performance in the button cell experiments. Figure 6 shows a cross-sectional SEM image of the tubular LAA-SOFC. The calculated reaction area of the tubular cell was 59.7 cm2. Figure 7 shows the voltage of the tubular fuel cell during the 20-hour constant current test. The voltage decreased from 0.62 to approximately 0.55 V for the first 8 hours and remained stable for the following 12 hours. Figure 8 (a) and (b) show the polarization curves and EIS results, respectively, of the tubular cell measured before and after the test. The peak power of the tubular cell before the discharging test was approximately 2.5 W and decreased to approximately 1.8 W after the test. The decrease in the output power was mainly due to the increase in the ohmic impedance. The ohmic impedance increased significantly from 36 to 53 mΩ, and the polarization impedance increased slightly from 21 to 25 mΩ. According to the ex situ analysis, the increase in ohmic impedance was mainly caused by the oxidation of the threaded joint between the tubular cell and the air channel. The ohmic impedance of the tubular cell could be stabilized by optimizing the connection methods, such as welding.
The operating power density of the current tubular LAA-SOFC is relatively low (approximately 20.9 mW cm-2). However, the LAA-SOFC showed an energy efficiency of 54.3% with the calculation method provided by Cao et al.(2019). In their calculation method, the energy efficiency of the LAA-SOFC is the product of the fuel efficiency, the theoretical efficiency and the voltage efficiency. Benefit from the solid state of the coal and the balance of the coal oxidation, the fuel efficiency of the Taixi coal in LAA was approximately 97% (Wang et al. 2014a). In regard to the theoretical efficiency, the heat requirement of the Sb2O3 reduction reaction could be provided by the Sb oxidation because of the large thermal capacity and the high thermal conductivity of the liquid antimony bath. Hence, the theoretical efficiency of the LAA-SOFC could be calculated by dividing the Gibbs free energy change of Sb oxidation (ΔG, −74.25 kJ per mol electron transfer at 1023 K) into the enthalpy change (ΔH, −98.66 kJ per mol electron transfer at 1023 K) of carbon oxidation in air, result in 75.3%. The cell operating voltage in the end of the 20-h constant current test was 0.55 V so the voltage efficiency was 74.3%. Considering the 1-kW LAA-SOFC stack with tubular cells in the present research, the size of the stack could be estimated. We assumed that the spacings between two adjacent cells were 1 cm and the utilized lengths of each tubular cell were 30 cm (the utilized length of the tubular cell was 7.6 cm in the present research). Thus, the size of the stack was 50×50×20 cm (196 tubular cells). Compared to that of other 1-kW fuel cell stacks by the APS method (approximately 15×15×30 cm, 25 planar cells, (Tsai et al. 2018)), the size of the as-prepared 1-kW fuel cell stack was relatively large. However, the tubular configuration could simplify the sealing of the stack. In addition, considering the space and cost saved by removing the purification equipment and the prospects of improving the LAA-SOFC performance, tubular LAA-SOFC stacks could be an attractive choice for converting solid carbon fuels.