Contruccion local Paucarpata

The LSGM cell with the 230 µm porous scaffold on either side was infiltrated with 40 wt% LSC for the cathode and 30 wt% LFO for the anode. This specimen was used to investigate the performance of the LFO anode as a cogeneration device. The cell was tested in dry and humidified CH4 flowing at a rate of 20 ml/min at 750 °C. The cathode was exposed to stagnant air. Impedance spectra of the cell presented in Figure 157 show that the cell obtained a series resistance of 0.245 Ωcm2 in humidified (3% H2O) CH4. The series resistance of the cell increased slightly to 0.262 Ωcm2 _{when the fuel} was switched to dry methane. In addition, the captured impedance spectra in both wet and dry CH4 show signs of diffusion limitations. However, the polarization of the cell in wet CH4 seems to be smaller compared to that in dry CH4.

155

Figure 157: Impedance spectra at 750°C of LSGM electrolyte-supported cell infiltrated with 30 wt% LFO for the anode and 40 wt% LSC for the cathode at OCV when wet or

dry CH4 was flown into the anode. The cathode was exposed to stagnant air. As shown in Figure 158, the OCV value of the cell was slightly higher in dry CH4 than wet CH4. The cell obtained an OCV value of 0.82 V in dry methane compared to 0.79 V in wet CH4. The performance of the cell was also better in dry methane than wet methane. The cell obtained a maximum power density of 29 mWcm-2 in dry CH4. In order to observe the amount of C2 chemicals produced by the cell, the cell was put under a bias voltage of 0.3 V. As shown in Figure 159, methane conversion was around 0.5% at OCV in humidified CH4. However, the methane conversion increased to 3.1% after applying 0.3 V to the cell for 30 minutes. At the highest methane conversion rate, the cell achieved a good C2 hydrocarbon selectivity of 85.2% and a C2 yield of 2.6% (Table 13).

When the fuel was changed to dry methane, the maximum power density of the cell increased to 32 mWcm-2_{. There was little change in the methane conversion. In dry} methane, the methane conversion was 3.4% compared to 3.1% in humidified CH4. However, the selectivity of methane decreased slightly to 83.4% in the dry fuel condition compared to 85.2% in wet methane.

-3 -2.5 -2 -1.5 -1 -0.5 0 0 1 2 3 Z" Z' (Ohmcm2) wet CH4 dry CH4 0.1 Hz

156

Figure 158: Performance curves at 750 °C of the LSGM cell with 30 wt% LFO when wet or dry CH4 was flown into the anode while the cathode was exposed to stagnant air.

Figure 159: Methane conversion and current density at 750 °C of the LSGM cell with 30 wt% LFO when humidified CH4 was flown into the anode. The cathode was exposed to

stagnant air. 0 5 10 15 20 25 30 35 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 25 50 75 100 125 Pow er D en si ty (mW cm -2) Vo lta ge (V)

Current Density (mAcm-2₎

750 oC dry CH4 750 oC wet CH4 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 0 20 40 60 80 100 120 140 160 Cur re nt De ns ity (m Ac m -2) CH4 c on ver si on (% ) Time (minute) XCH4 Current OCV OCV @ 0.3 V

157

CH4 XCH4 (%) SH2 (%) SC2 (%) SCO2 (%) YC2 (%) Carbon

Balance

Wet 3.1 14.1 85.2 9.5 2.6 94.7

Dry 3.4 8.3 83.4 9.1 2.8 92.5

Table 13: The catalytic performance of the LSGM cell infiltrated with 30 wt% LFO under 0.3V at 750 °C. Wet and dry CH4 was flown into the anode at 20ml/min while the

cathode was exposed to stagnant air. 8.4 Summary

In this chapter, the activity of LiFeO2 for OCM reaction was investigated. LFO powder tested in CH4 maintained a methane conversion rate of 15-17% and the C2 selectivity remained around 73-79% for 30 hours. LSGM cell infiltrated with 40 wt% LSC and 30 wt% LFO achieved a maximum power density of 30 mWcm-2 while obtaining the CH4 conversion of around 3%. The cell achieved a high C2 selectivity of around 80% in dry or wet methane. Captured impedance spectra of the cell in wet and dry methane suggest that there is limitation due to gas diffusion.

158

9

Conclusion

In this study, LiFeO2 was investigated as an anode material for high temperature fuel cells. Under reducing atmosphere, LFO has good electronic conductivity. The conductivity of LFO is dependent of temperature and time. When reduced in 5% H2, LFO decomposes and introduces two more phases, a metallic Fe and Li5Fe04. Interestingly, the crystal structure of the reduced LFO is reversibly attained.

Since the TEC value of LFO was much higher compared to the commonly used electrolytes, all tested button cell was fabricated via the tape casting and infiltration methods. The cathode was prepared by infiltrating LSF or LSC into the scaffold while LFO was infiltrated into the scaffold for the anode. The continuous formation of nano-particles on the scaffold also resolves the mismatch in TEC values between the electrode and the electrolyte.

Compatibility testing of 8YSZ and LFO showed that they react with each other after sintering both powder in air for 24 hours. In addition, LFO is unstable above 700 °C. Due to this reason, the cell performance of the cell decreased when tested at 700 °C and above.

Performance of the scaffold infiltrated with LFO improved as the temperature increased for cells with CGO or LSGM electrolyte in humidified H2. CGO cell infiltrated with 40 wt% LFO had a maximum power density of 180 mWcm-2 in humidified H2 at 650 °C. LSGM cell impregnated with 30 wt% LFO achieved a maximum power density of 460 mWcm-2 _{at 700 °C in humidified H2. Increasing the operating temperature to} 700 °C improve performance of the cell. However, there are some drawbacks when operating the cell at the edge of the stability of LFO. The cell degraded much faster when operating at higher temperature. In reducing atmosphere, LFO decomposes and poses more electronic conductivity with increasing temperature and reduction time. This benefit seemed to be negated by the degradation rate and particle growth. Particle growth reduces the surface area for the electrochemical activity. Since there are many components in the fuel cell, the agglomeration of LFO also affect the bonding of the functional material and the current collector. Due to these reasons, cell infiltrated with LFO never reached an equilibrium state. Operating temperature of the system should be below 700 °C to maintain better stability and avoid major decomposition under reducing atmosphere.

159

LSGM cell infiltrated with LFO seemed promising as a HDCFC. VI curves and impedance spectra showed sign of limitation due to high air utilization. However, the performance of the cell can be improved by flowing air into the cathode. Future investigation on improving the performance of the system should consider different type of carbon fuels and vary the ratio of carbon to carbonate.

In addition, LFO also poses some promising characteristics as a catalyst for OCM. LFO powder tested in CH4 maintained a methane conversion rate of 15-17% and the C2 selectivity remained around 73-79% for 30 hours. LSGM cell infiltrated with 40 wt% LSC and 30 wt% LFO achieved a maximum power density of 30 mWcm-2 while obtaining the CH4 conversion of around 3%. The cell achieved a high C2 selectivity of around 80%. From this study, LFO has proven to be a good candidate for HDCFC and cogeneration system that produces electricity and valuable chemicals such as ethane and ethylene.

160

10

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