Study On Eelectrochemical Performances Of LaFeO_3-?-Based Symmetric Electrode Materials For Solid Oxide Cells

Environment pollution is getting more and more serious due to the consumption of fossil fuel.Solid Oxide Cells(SOCs)are one of the most flexibility and high efficiency energy conversion devices which directly convert energy between the chemical energy of fuels and electricity.Symmetric solid oxide cells(SSOCs),which employ the same material for both the cathode and anode,have received significant attention owing to the reduced fabrication cost,improved compatibility between electrolyte and electrode materials,and enhanced resistance to coking.LaFeO3 exhibits the good performance for hydrocarbon fuel and suitable stability in both oxiding and reducing atmosphere.However,the catalytic activity in low temperature and the sability of LaFeO3-based perovskite under H2 are still not enough.In this work,we firstly develop some symmetric electrode materials to improve the catalytic activity and stability of LaFeO3 in reducing atmosphere and further investigate the oxygen reduction reaction(ORR)and the hydrogen oxidation reaction(HOR)mechanism.In the end we explored the electrochemical performance and stability of these materials under SOCs operation.Two new LaFeO3-based electrode materials(LSFNi and CLSFNi)as potential candidates for SSOFC application have been synthesized and tested.La0.6Sr0.3Ce0.1Feo.9Ni0.1O3-?(CLSFNi)exhibits better ORR and catalytic activity towards to CH4 compared to La0.7Sr0.3Fe0.9Ni0.1O3-?(LSFNi).Under H2 fuel operation,both LSFNi and CLSFNi-based SSOFCs give excellent peak power densities(e.g.-900 mW cm-2 at 850 ?).In wet CH4/air(3%H20),the CLSFNi electrode greatly outperforms the LSFNi electrode(522 mW cm-2 vs.221 mW cm-2 at 850 ?)due to the enhanced methane reforming activity imparted by the cerium doping.Ni-Fe nano particles with 35-55 nm can be in-situ exsolved from the LSFNi perovskite in suitable redcuting atmosphere and exhibit good catalytic activity for H2O-CO2 co-electrolysis.Under the applied voltage of 1.5 V,the current densities of the electrolysis cells are-2423,-1617 and-1027 mA cm-2 at 850,800 and 7500C,respectively.The ratios of H2/CO are affected by the temperatures and the fuel composition in fuel electrode and the applied current mainly changes the production rates of syngas.The incorporation of Nb5+ on B-site in La0.5Sr0.5FeO3-?(LSF)significantly improves the chemical and phase stability.La0.5Sr0.5Fe0.9Nb0.1O3-?(LSFNb)is synthesized via sol-gel methode with NbCls as Nb source.The ORR and HOR mechanisms are explored based on the symmetric cell under different atmosphere.The polarization resistances of LSFNb symmetric cell in air are 0.04,0.06,0.08 and 0.14? cm2 at 850,800,750 and 700 0C,respectively.The low-frequency process(R3)is likely related to the gas diffusion and/or oxygen surface adsorption-dissociation in cathode while the high-frequency process(Ri)is likely related to the oxygen exchange between the electrode and electrolyte.The Mid-frequency process(R2)is associated to the charge transference process in the electrode.The polarization resistances of LSFNb symmetric cell in wet H2 are 0.21,0.24 and 0.4? cm2 at 850,8 00 and 750 ?,respectively.At 850 ?,the peak power densities of LSGM-supported SSOFCs based on LSFNb electrodes reach 1000 and 630 mW cm-2 at 850 ? and 750 ?,respectively.Finally,the LSFNb-based SSOFCs demonstrate promising stability(?140 h)and good redox stability in H2 and air at 800 ?,indicating that LSFNb is a promising symmetrical electrode system for SOFCs.LSFNb symmetric cells also exhibit good catalytic activity under Solid Oxide Electrolysis Cells(SOECs)mode.The OCV of the symmetric cells under 20 vol.%H2O-H2 are 0.97,0.98 and 0.99 V at 850,800 and 750 ?,respectively.In the SOFC mode,the Peak Power Density(PPD)at the same conditions are 739,592 and 295 mW cm-2.In the SOEC mode,the current densities of the symmetric cell are-1239,-943 and-619 mA cm-2 at 850,800 and 750 ?,respectively under applied voltage of 1.3 V.For the directly CO2 electrolysis,the current densities of the symmetric cell are-1825,-1233 and-665 mA cm-2 at 850,800 and 750 ?,respectively under applied voltage of 1.5 V.The polarization resistances under 1.1 V are 0.20,0.33 and 0.79 ? cm2,respectively.For H2O-CO2 electrolysis,the current densities of the symmetric cells under 20 vol.%H2O-CO2 and applied voltage of 1.5 V are-2257,-1560 and-993 mA cm-2 at 850,800 and 750 ?,respectively.The addition of the steam improves the electrolysis cells performance.The good catalytic activity of the LSFNb symmetric cell is due to the addition of Chloride.Chloride is added into the perovskite lattice and improves the ORR and HOR activity.At 850 ??the PPD of symmetric cell are 970,808 and 278 mW cm-2 under wet H2,C3H8 and CH4,respectively.The symmetric cell can operate under CH4 for over 90 h.These results indicate the incorporation of Chloride in the perovskite is an effective method to improve the ORR and HOR.