|Institution:||University of St. Andrews|
|Keywords:||187 ; Solid oxide fuel cell ; LiFeO₂ ; Cogeneration of chemical and electrical power ; Hybrid direct carbon fuel cell ; Solid oxide fuel cells ; Anodes – Materials ; Lithium compounds ; Iron oxides|
|Full text PDF:||http://hdl.handle.net/10023/6307|
In this study, Lithium iron oxide (LiFeO₂ – LFO) was investigated as a new anode material for the high temperature SOFCs. From the DC conductivity measurement in argon containing 5% H₂, LFO exhibits good electronic conductivity of 5.08 Scm⁻¹ at 650 °C. LFO poses a high TEC value of 19.5 x 10⁻⁶ K⁻¹ in air. However, the TEC values of the commonly used 8YSZ and CGO electrolytes are much lower, 10.5 x 10⁻⁶ K⁻¹ and 12.5 x 10⁻⁶ K⁻¹ respectively. In order to resolve the mismatch in the TEC values between the electrode and the electrolyte, button fuel cells were fabricated via tape casting. LFO was infiltrated onto the porous and stable scaffold. Presently, the predominant electrolyte material used for the high temperature SOFC is 8YSZ. Due to this reason, the initial performance of LFO as an anode material was tested on tape-cast 8YSZ electrolyte-supported cell. The 8YSZ electrolyte-supported infiltrated with 30 wt% LFO for the anode and 40 wt% LSF for the cathode achieved a maximum power density of 50 mWcm⁻² at 700 °C in humidified H₂. Increasing the weight loading of LFO to 40 wt% worsen the performance. XRD pattern of the sintered powder containing 50 wt% LFO and 50 wt% 8YSZ confirmed that LFO and 8YSZ react with each other. CGO was considered as an alternative electrolyte material to 8YSZ. XRD pattern of the sintered powder containing 50 wt% LFO and 50 wt% CGO confirmed that they are compatible with each other. The CGO electrolyte supported cell infiltrated with 40 wt% LFO for the anode and 40 wt% LSC for the cathode achieved a maximum power density of 180 mWcm⁻² at 650 °C in humidified H₂. The addition of 10 wt% ceria to the LFO anode enhances the electrochemical activities of the cell. However, the overall performance of the cell decreased due to a larger increase in the series resistance. Since CGO electrolyte is easily reduced when testing at temperature higher than 550 °C, LSGM was used to increase the testing temperature. The 245 µm thick LSGM electrolyte-supported cell infiltrated with 40 wt% LSC and 30 wt% LFO obtained a maximum power density of 227 mWcm⁻² at 700 °C in humidified H₂. Decreasing the electrolyte thickness from 245 µm to 130 µm increased the performance of the cell. The 130 µm LSGM electrolyte-supported cell infiltrated with 40 wt% LSC and 30 wt% LFO was tested with the carbon/carbonate fuel as a HDCFC. Performance measurements of the cell was conducted at 650 °C and 700 °C with N₂ flowing at 20 ml/min. The cell performed better when testing at higher temperature. Recently, there has been great interest in developing a SOFC system for the cogeneration of electricity and valuable C₂ chemicals. The catalytic testing for oxidative methane coupling of methane revealed a high C₂ selectivity for the LFO powder. Cell testing of a sample infiltrated with 40 wt% LSC and 30 wt% LFO also achieved a methane conversion of ~3% and a C₂ selectivity of ~80% in methane at 700 °C.