Study On Key Materials And Structural Properties Of Proton-Conducting Reversible Solid Oxide Cells

Reversible solid oxide cell?RSOC?is an energy conversion device which can achive direct conversion between chemical energy and electrical energy.It has the advantages like clean,highly efficient,and fuel flexibility,etc.At present,the development trend of RSOC is to reduce the working temperature from high temperatures?800¹000^oC?to intermediate and low temperatures?400⁷00^oC?,thereby slowing the sintering of electrode,mitigating the aging of materials,reducing thermal stress inside the cell,and enhance electrochemical stability.Meanwhile,when RSOC works at low temperatures,the sealing becomes easier,the starting speed becomes faster,and the cost becomes lower.However,the decrease in working temperature would cause the increase in ohmic and polarization resistances,resulting in severe degradation of cell performance.proton conductors?such as doped Ba?Ce,Zr?O₃?with a low activation energy,have good protonic conduction at intermediate and low temperatures.Therefore,the development of proton-conducting RSOC?PC-RSOC?with proton conductor electrolytes is an effective strategy to reduce the ohmic loss of RSOC at intermediate and low temperatures.Nevertheless,there are still some problems in the existing PC-RSOC systems,such as poor sintering activity,relatively low conductivity,and instability of electrolyte materials;rare types,unoptimizaed microstructure,and large polarization loss of electrode materials.In this study,the electrolyte and electrode materials were modified by doping method to improve their sintering activity,electrical conductivity,and stability.Besides,the microstructure of the electrode is optimized to increase the electrode reaction rate.The main research contents and results are as follows:1.Study on the doping modification of Ba?Ce,Zr?O₃-based proton conductor electrolyte materials.?1?The solid solubility of In in BaCe_0.8Zr_0.2O₃ is very large,the optimum doping concentration is 30 mol%.The BaCe_0.5Zr_0.2In_0.3O_3-??BCZI3?has an improved conductivity and stability.The cell with BCZI3 electrolyte shows an excellent stability in electrolytic cell operational mode.However,In-doping can not reduce the sintering temperature significantly.?2?The doping concentration of Cu in BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3-??BCZYY?is limited,2 mol%is an optimum doping concentration.BaCe_0.68Zr_0.1Y_0.1Yb_0.1Cu_0.02O_3-??BCZYYC2?sintered at 1200 ^oC for 5 h exhibits a high density and protonic conductivity.The RSOC with BCZYYC2 electrolyte has a MPD of 0.85 W?cm^-2,and an electrolysis current density?1.3 V?of 1.96 A?cm^-2 at 700 ^oC.In addition,no obvious degradation is found for the same cell tested for 60 h in reversible fuel cell?FC?/electrolytic cell?EC?operational mode.2.Study on microstructure optimization of hydrogen electrode support.Different addition amounts of carbon microspheres?CMSs?are used as pore formers to control the microstructure of the hydrogen electrode support.The porosity,line shrinkage,and thermal expansion coefficient?TEC?of the hydrogen electrode support simultaneously increase with the CMS content,and the optimum addition amount of CMS is 30 wt%.The support with 30 wt%CMS has a high porosity?39.27vol%?with a TEC of 7.74?10^-6 K^-1.Besides,the same support CMS has the largest three-phase boundary?TPB?length and the lowest polarization resistance,thus it shows the best electrochemical performance.3.Study on materials and microstructure optimization of A₂MO₄-type oxygen electrode.?1?Cobalt-free Ln_1.2Sr_0.8NiO₄?Ln=La and Pr?materials were synthesized by a modified Pechini method.The TEC of Ln_1.2Sr_0.8NiO₄ is close to most of Ba?Ce,Zr?O₃-based proton conductors.The polarization resistance of La_1.2Sr_0.8NiO₄?LSN?-based and Pr_1.2Sr_0.8NiO₄?PSN?-based sysmmetrical oxygen electrode cells is 0.15 and 0.23??cm² at 700 ^oC,respectively.Besides,LSN and PSN are stable in high temperature and high humidity contidions.In addition,both of two single cells present good electrochemical performance and stability in reversible FC/EC operational mode.?2?LSN-BCZYYC2 composite oxygen electrode was fabricated by infiltration method.By comparing the micro-morphology and polarization resistance of the LSN-BCZYYC2 oxygen electrode with different infiltration amounts,the optimal infiltration amount of 42.2 wt%was determined.The polarization resistance of the 42.2 wt%LSN-infiltrated oxygen electrode is as low as 0.039??cm² at 700 ^oC.The single cell with infiltrated oxygen electrode shows a MPD of 1.22 W?cm^-2,and an electrolysis current density?1.3 V?of-3.02 A?cm^-2 at 700 ^oC.4.Study on LSN oxygen electrode-supported PC-RSOC.Oxygen electrode-supported PC-RSOC was prepared by using LSN-BCZYYC2as oxygen electrode,BCZYYC2 as electrolyte,and Ni-BCZYYC2 as hydrogen electrode.The effect of pore former on the micro-morphology of the support was studied.It was found that the support obtained a high porosity after adding 20 wt%CMS-30 wt%starch mixed pore former,and the optimum sintering temperature was found to be 1300 ^oC.Ni-BCZYYC2 hydrogen electrode was prepared by infiltration,the corresponding single cell is stable in H₂ atmosphere,but unstable in CH₄ atmosphere.The Cu-CeO₂-based hydrogen electrode was used instead of the Ni-based hydrogen electrode,and the single cell exhibits excellent stability in the CH₄ atmosphere.