| Reversible solid oxide cell(RSOC)is a chemical-electric energy conversion device that operates alternately in solid oxide fuel cell(SOFC)and solid oxide electrolysis cell(SOEC)modes.It can be used both as a power generator as well as a high-efficiency hydrogen production and electricity storage device.The oxygen electrode of RSOC needs to have both oxygen reduction reaction(ORR)activity,oxygen evolution reaction(OER)activity and excellent reversibility.The development of oxygen electrodes with high activity and high stability is one of the main challenges currently.Simple perovskite materials such as La0.8Sr0.2Mn O3-?(LSM)and La0.6Sr0.4Co0.2Fe0.8O3-?(LSCF)have been widely studied.LSM material has been widely used in high temperature SOFC due to its excellent conductivity and compatibility with YSZ electrolyte.However,when the temperature drops to a medium or low level,its performance drops sharply due to its poor ionic conductivity.LSCF has good electron and ionic conductivity and catalytic activity for ORR.However,the stability of LSCF electrode is reduced by Sr segregation during long-term operation.Meanwhile,the OER activity of LSCF also needs to be further strengthened.Based on the traditional LSM and LSCF materials,different modification and enhancement methods were used in this paper.The composite electrodes of LSM/ESB and LSM-ESB were fabricated by co-synthesis and mechanical mixing methods.Ruddlesden-Popper(RP)structure Pr2Ni0.8Cu0.2O4+?(PNCO)was selected as surface modification material for LSCF.The PNCO-LSCF composite electrode was prepared by the solution infiltration.The electrochemical performance of these RSOCs was studied in detail,and the reaction process and enhancement mechanism of oxygen electrode were also investigated.The main findings are as follows:(1)The co-synthesized LSM/ESB and the mechanically mixed LSM-ESB have similar thermal expansion coefficients.LSM/ESB has more weight loss at high temperature,higher conductivity and specific surface area.The LSM/ESB single cell has better electrochemical performance in SOFC mode.The polarization resistance in half cell is only 0.029?·cm2 at750?,the maximum power density is 1.747 W/cm2,which is?2.6 times that of the LSM-ESB electrode(0.667 W/cm2);(2)In the SOEC mode,the performance of the LSM/ESB single cell is also more prominent.At 750?,30%AH and 1.5 V operating conditions,the current density is 1.43A/cm2.The investigation of the enhancement mechanism found that the co-synthesized LSM/ESB particles more uniform with rounded particles.The better active state on surface and tighter connecting between electronic conductor LSM and oxygen ion conductor ESB of LSM/ESB can enlarge the three-phase reaction boundary,thereby improving the electrochemical performance;(3)LSCF and PNCO materials have good chemical compatibility after being calcined at 950?for 3 h.Results have shown that PNCO can form phases on the surface of LSCF,and the PNCO forms a continuous film layer that completely covers the LSCF skeleton.The maximum power density of PNCO-LSCF single cell reaches 1.358 W/cm2 at 750?.Compared with the 0.907 W/cm2 of LSCF single cell,the performance is significantly improved.In the SOEC mode,the current density of the PNCO-LSCF single cell is 1.953A/cm2 at 50%AH and 1.5 V at 750?.The Rp after infiltration is reduced from 0.141?·cm2to 0.105?·cm2;(4)The DRT analysis of the EIS indicates that PNCO infiltration can improve the surface exchange of O2 and O2-bulk diffusion.The surface modification takes full advantage of the PNCO and LSCF materials.The ORR/OER performance,reversibility and stability of the PNCO-LSCF single cell are significantly enhanced.The enhancing mechanism can be attributed to the accelerated surface oxygen exchange and transport kinetics by the infiltration of PNCO,and PNCO thin film stabilizes the electrode by inhibiting surface segregation of Sr.The synergy of these reasons greatly improves the overall performance of PNCO-LSCF single cell. |
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