Studies On Performance Of La_0.3Sr_0.7Fe_0.7Ti_0.3O_3-? Fuel Electrode For Solid Oxide Cells

Solid oxide cell?SOC?is an energy conversion device at high temperature,which could be operated separately in solid oxide electrolysis cell?SOEC?and solid oxide fuel cell?SOFC?mode.SOC can directly convert electric energy into chemical energy.In addition,SOC has advantages over traditional combustion of fossil fuel,such as environmental friendliness,high energy conversion efficiency and so on.The development of SOC can reduce the reliance to nonrenewable fossil fuels and reduce emissions of greenhouse gases,which are helpful to change the energy structure and achieve the recycling of CO₂.At present,the main problem of SOC is the low performance of fuel electrode and poor fuel adaptability in SOFC mode.Moreover,the low current efficiency of conversion CO₂ to CO is caused by the poor catalytic activity of CO₂ reduction in the SOEC mode.The aim for this paper is to develop a new type of fuel electrode material in SOC.La_0.3Sr_0.7Fe_0.7Ti_0.3O_3-??LSFT?is Fe based perovskite oxide and the electrochemical performance of LSFT used as fuel electrode working in two modes of SOC was systematically studied.LSFT electrode materials were prepared by the conventional solid-state reaction method.The phase sturcture and the calculation of the tolerance factor were carried out,and the chemical stability of the LSFT anode was investigated under the reducing condition.The influence of La doping in A-site of Sr Fe0.7Ti0.3O3?SFT?on structure,sintering property,electrical transport,thermal expansion and electrochemical properties were investigated through the comparison between LSFT and SFT.The applications of LSFT as the symmetrical electrode material have been investigated in detail.When using LSFT as anode and cathode at 800 oC respectively,the corresponding polarization resistance was 0.25 ? cm² and 0.11 ? cm²,respectively.Furthermore,a single cell consisting of LSFT symmetrical electrode and Yttria-Stabilized Zirconia?YSZ?electrolyte support?⁴00 ?m?shows a maximum power density?MPD?of 215 mW cm-2 at 800 oC when using wet H₂ as fuel.During 40 h operation,the cell based on LSFT electrodes has good stability in wet H₂ at 800 oC.In order to solve the difficult problems of carbon deposition and sulfur poisoning which is easy to appear at fuel electrode?anode?in SOFC mode,the fuel adaptability of LSFT electrode when using hydrocarbon fuel was investigated in detail.In addition,the carbon resistant ability and stability of LSFT anode when using wet CH4 as fuel was examined.The results show that the open circuit voltage?OCV?of the cell using LSFT as the anode was 0.89 V at 850 oC using wet CH4 fuel.During 40 h constant current operation,the voltage degradation ratio is only about 4.5%.It was found that the LSFT anode had a good ability to avoid sulfur poisoning,and the threshold value of H₂ S poisoning was very high?³00 ppm?when the LSFT anode used H₂S-H₂ fuel.Furthermore,it was found that LSFT can be used as the anode material of direct carbon solid oxide fuel cell?DCSOFC?.A new strategy was proposed to reduce the concentration polarization and slow down the decay rate of the DC-SOFC.In order to solve the low current efficiency and large cathodic polarization resistance in SOEC mode,the application of LSFT electrode which is Fe based perovskite oxide was carried out by symmetrical configuration.In this paper,the electrochemical properties of the symmetrical SOEC for direct electrolysis of CO₂ were systematically evaluated.The physical mechanism of the electrode reaction was described,and the reasons for the LSFT cathode with high activity were investigated.It was found that the electrochemical reduction?Fe4+?Fe3+?under low voltage and the CO₂ chemical adsorption at high temperature were the main reasons for the excellent reduction activity of CO₂ for LSFT cathode.The direct electrolysis of CO₂ was carried out using LSFT cathode,and a new method for the measurement of the current efficiency of direct electrolysis of CO₂ was presented.After carrying out the electrolysis experiments with different gas flow rates and electrolysis current densities,it was found that the oxygen leakage was the main reason that affects the Faraday current efficiency in SOEC mode.