Dual-phase Ceramic Scaffolds Based Solid Oxide Cells For Electrolysis Of CO₂ And H₂O

Solid oxide electrolysis cells(SOEC)allow for efficient electrolysis of H₂O to H₂O,CO₂ to CO,or co-electrolysis of H₂O/CO₂ to syngas at high temperatures(600-1000?)using renewable electricity generated from solar,wind,hydro or geothermal power.This renewable energy cycle can not only help reduce annual greenhouse gas emissions,but also promote the large-scale integration of intermittent renewable energy into the grid,enabling a quick transition of the current energy regime to a"carbon neutral"society.Ni-based cermets have been the most widely used cathodes in SOEC,which require the presence of protective gases such as H₂ or CO so as to prevent oxidation of Ni metals.Moreover,Ni-based cermets have shown poor stability in redox cycling and poor sulfur resistance,which may deactivate the electrodes and thus reduce the lifetime of SOEC.Mixed ionic and electronic conducting ceramic oxides have been actively explored as an alternative SOEC cathode due to their superior redox stability and sulfur resistance.Generally,high-performance ceramic electrodes are prepared by the chemical impregnation method,which requires about 30 wt%of catalyst to be loaded into the porous electrode scaffold to form a conductive network.Repetitive impregnation makes the cell manufacturing process labor-intensive and time-consuming.Note that a large gradient in the oxygen partial pressure occurs between the oxidative atmosphere in anodes and the reductive atmosphere in cathodes for operating SOEC,resulting large consumption of electrctricity to pump oxide ions from the cathodes to the anodes.In addition,pure oxygen as generated in the anodes has yet to be economically utilized.The aim of this thesis is to adopt dual-phase ceramic composites as the electrode scaffolds for SOEC and to investigate the novel methane-assisted SOEC,with the main research contents and research results as follows:(1)Preparation and characterization of dual-phase ceramic scaffolds to catalyze the electrolysis of CO₂:Given the excellent physical and chemical compatibility between YSZ and LSCr F,the symmetric tri-layered structure of“porous YSZ-LSCr F|dense YSZ|porous YSZ-LSCr F”with varied YSZ/LSCr F ratios were prepared by the tape casting and lamination techniques.With increasing the LSCr F content from 20 to80 wt%,the conductivity of the mixed ionic-electronic conductive scaffolds increased dramatically,yielding continuously increased electrolysis performance in pure CO₂.Cells with the symmetric 80 wt%LSCr F-20 wt%YSZ electrodes showed the highest performance.The measured current densites at 1.5 V were 0.33,0.56 and 0.84 A cm^-2 at 750,800 and 850°C,respectively.(2)Enhanced catalysis of YSZ-LSCr F dual-phase scaffold for CO₂ electrolysis via impregnated SDC nanoparticulates:In order to improve the CO₂ electrolysis performance of the symmetric cells,nano-scale SDC catalysts were impregnated into both porous YSZ-LSCr F scaffolds.The electrolysis current densities measured at 1.5V in pure CO₂ increased to 1.30 A cm^-2 at 850?.Impedance analysis showed that coating of nano-scale SDC catalysts reduced the interface polarization resistance of the cell from 0.66 to 0.34?cm².Further DRT analysis of impedance data measured on the symmetrical cells in homogeneous environments reveals that these SDC catalysts effectively promoted the surface reaction and the charge transfer during CO₂ reduction.Preliminary short-term measurements showed good stability at varied applied voltage from 1.0 to 1.6 V.(3)Co-electrolysis of H₂O and CO₂ couppled with POM reactions to produce syngas:Thermodynamic calculations showed that CH₄-assisted co-electrolysis of H₂O/CO₂ can reduce the Gibbs free energy and thus reduce the electricity consumption when compared with the conventional co-electrolysis.The required voltages to achieve an electrolysis current density of-400 m A cm^-2 at 850?were 1.0 V for the conventional co-electrolysis and 0.3 V for the CH₄-assisted co-electrolysis,indicative of a 70%reduction in the electricity consumption.For an inlet of H₂O/CO₂(50/50vol),syngas with a H₂:CO ratio of?2 can be always produced from the cathode while the anode effluent strongly depended upon the operation conditions,with syngas favorably produced under the moderate current densities at higher temperatures.It was demonstrated that syngas with a H₂:CO ratio of?2 can be produced from the anode at a formation rate of 6.5 m L min^-1 cm^-2 when operated at 850°C with an electrolysis current density of-450 m A cm^-2.(4)Symmetric YSZ-LSCr F electrode-supported SOECs with straight open pores for H₂O electrolysis:YSZ-LSCr F electrode-supported SOECs with straight channels were prepared by the phase-inversion tape casting method.The YSZ-LSCr F scaffolds had an open pore structures with low tortuosity.The pore size was widely distributed between 0.4-25?m.SDC nanoparticles were impregnated into both electrodes.Such straight open pores helped to reduce the Knudsen diffusion resistance during the has transport process,enabling higher limiting current densities measured in H₂O electrolysis for the present cells than for the conventional counterpart with the tortuous pores,especially at low steam partial pressures.The rate-determing step was the charge transfer process across the electrolyte/electrode interface on the anode and the surface catalytic reduction process of H₂O on the cathode,with an activation energy of1.39 and 1.12 e V,respectively.No obvious degradation was observed for cells measured in 50%H₂O-50%N₂ for 80 h at 800?and 1.3 V.(5)CO₂ electrolysis in SOECs coupled with OCM reactions:LSFNM and Mn₂O₃-Na₂WO₄/TS-1 were impregnated into the YSZ-LSCr F scaffolds with the straight open pores,acting as the anode and cathode catalysts,respectively.The measured electrolysis current density at 850?and 1.5 V was 1.77 A cm^-2 under the CH₄-assisted CO₂ electrolysis mode.The Anode effluent consisted of CO,CO₂,C₂H₄ and C₂H₆with negligible CO₂.The selectivity of C₂(C₂H₄ and C₂H₆)was above 80%,and higher yields could be achieved at higher methane concentration.Neverthelss,low catalyst loadings and small OCM reaction rate constants resulted in a low methane conversion rate.