Effect of Fluorine Doping on the Performance of Reversible Solid Oxide Cells and Related Kinetic Studies
Reversible solid oxide cell(RSOC)exhibits excellent thermodynamic and kinetic properties and is considered as a promising energy conversion device.In this work,two types of RSOC electrode materials,La0.6Sr0.4Fe0.8Co0.2O3(LSFC)and La0.6Sr0.4Fe0.8Co0.2F0.1O2.9(F0.1-LSFC)are synthesized using the solid-state reaction method,the effects of fluorine dop-ing on the cell discharge and electrolysis performance are discussed.Meanwhile,the electrode surface kinetic reactions are also explored through construction of equivalent symmetrical cells.After fluorine doping,SrF2 precipitates in F0.1-LSFC ma-terial,resulting in defects in perovskite La sites,which further leads to Coulomb cation exclusion and lattice disorder of FeO6 octahedron,resulting in expansion of unit cell volume.Moreover,the results also show that fluorine doping can reduce the valence state of B-site elements,increase the concentration of oxygen vacancies,and thereby improve cell performance.The maximum power density of RSOC composed of F0.1-LSFC at 700 ℃ with 30％H2O/H2 as the fuel is 234.3 mW·cm-2,ap-proximately 1.7 times higher than that of RSOC composed of LSFC.And at 1.3 V,the current densities of the RSOC com-posed of LSFC and F0.1-LSFC are-245.6 and-417.9 mA·cm-2,respectively.The effect of fluorine doping on the catalyt-ic activity of perovskite materials for hydrogen oxidation reaction(HOR)is investigated.The results show that the catalytic activity of perovskite materials for HOR can be improved by fluorine doping.In addition,the electrochemical impedance spectra(EIS)under different hydrogen partial pressures were measured by adjusting the hydrogen partial pressure(pH2)on both sides of the equivalent symmetrical cells,so as to explore the rate determining step(RDS)of electrode reaction.The results show that the RDS of electrode reaction changes after fluorine doping.For LSFC electrode,the RDS of reaction is mainly hydrogen adsorption and dissociation reaction,while for F0.1-LSFC electrode,the RDS of reaction is mainly charge transfer reaction.Similarly,the test method of equivalent symmetrical cells is still used for oxygen reduction reaction(ORR)dynamics research.The difference is that the atmosphere on both sides of the cells becomes O2.From EIS test results,it can be seen that with the increase of temperature,the polarization resistance(RP)value of the equivalent symmetrical cell gradu-ally decreases,and the RP value of F0.1-LSFC is lower than LSFC,indicating that fluorine doping and increasing the operating temperature can reduce the RP value,thereby improving the catalytic activity of ORR.It is worth noting that the RP value of the equivalent symmetrical cell composed of LSFC and F0.1-LSFC in O2 atmosphere is lower than that in H2 atmosphere,indicating that LSFC and F0.1-LSFC electrode materials are more prone to ORR.On the other hand,the RDS of ORR process can be studied from the effect of oxygen partial pressure(po2)on EIS of equivalent symmetrical cells.The results show that the RDS of electrode for ORR process is the reduction of oxygen atoms to O-.
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