Characterization Of The Key Material And Electrochemical Performance Of Solid Oxide CO₂-Electrolysis Cells

Solid oxide electrolysis cell(SOEC)is an all-solid-state device for energy conversion,which has received extensive attentions because of high-efficiency and no pollution.Combined with the electricity from renewable energy sources,SOEC can electrolyze CO2 and H2O to produce H2 and CO,nevertheless,the development and application has been hampered mainly due to the sluggish CO2/H2O reduction kinetics at the cathode and the instability(oxidation,coking and agglomeration)of Ni-based cathodes at high concentration of CO2/H2O.In this dissertation,the primary goal is to design and develop alternative and catalytically active cathode materials,while tailoring cell configurations and electrode microstructures,and obtain high-performing SOECs for direct CO2 electrolysis.Based on perovskite oxide Sr2Fe1.5Mo0.5O6-?(SFM),we will improve CO2 reduction kinetics through a series of engineering methods including composite,nano,surface and bulk doping.Meanwhile,the electrochemical performance and stability are investigated.Chapter 1 introduces the importance and development history of SOEC,as well as the operation principle,thermodynamic/kinetic parameters and factors controlling SOEC performance.At the same time,recent advance of critical materials and characterization methods are summarized.Moreover,the main research basis and contents of this dissertation are presented.In chapter 2,electrochemical properties of SFM as the cathode for CO2 reduction are investigated,as well as the structures.SFM shows a cubic structure in 1:1 CO-CO2 and has the conductivity of 8.5-25 Scm-1 at 500-850 0C.For a LSGM electrolyte-supported SOEC with single phase SFM cathode,a current density of 0.71 Acm-2 is obtained at 800 ? and 1.5 V.The performance is further improved by using SFM-SDC composite,and the current density increases to 1.09 Acm-2.Durability test at 800 ? for 100 h demonstrates a relatively stable performance for CO2 electrolysis.In chapter 3,the electrochemical performance of nano-structured SFM electrode and SOEC is comprehensively studied.The apparent conductivity and conductivity of the infiltrated SFM are investigated at 600-750 ?,which increase with the SFM loading and temperature.When the loading increases to 5.46 vol%,SFM nano-particles can connect with each other to form a continuous conductive network.A SFM-YSZ supported SOEC is fabricated by phase-inversion tape-casting and infiltration methods,where SFM-YSZ has a unique porous structure facilitating gas delivery and electrode reactions.The current density of 1.10 A cm-2 is obtained at 800 ? and 1.5 V for CO2 electrolysis,and increases to 1.27 A cm-2 when 20vol.%H2O is added.It is very feasible to control the H2/CO ratio through the voltage,e.g.,exactly being 2 at 1.3 V.A novel symmetrical cell is further prepared by facile tape-casting and infiltration methods,which consists of a thin dense LSGM electrolyte and two symmetrical porous LSGM layers deposited with SFM nanoparticles.The cells achieves a current density of 1.24 A cm-2 at 800 ? and 1.5V for CO2 electrolysis,because of extended active sites resulting from novel architectures.The EIS under a series of voltages indicates that CO2 electrolysis reaction is limited by the charge transfer process when the voltage is below 1.2 V,while the rate-determining step above 1.2 V is changed to ohmic resistance.In Chapter 4,the enhancement effect of in-situ exsolved NiFe alloy nanoparticles on CO2 reduction at the SFM cathode is investigated.TPR analysis indicates that A-site deficiency facilitates the reduction and exsolution of Ni and Fe in the Sr,.9Fe1.5Mo0.4Nio iO6-? perovskite lattice.EIS combined with DRT method proves that NiFe nanoparticles enhance the surface reaction kinetics,including CO2 adsorption,dissociation and charge transfer processes,while Tafel fitting suggests that the first electron reduction of CO2 to CO2-is the rate-determining step.The SOEC with NiFe@SFM-SDC cathode achieves a current density of 2.16 A cm-2 at 800 ? and 1.5V for CO2 electrolysis,and the surface reaction rate constant increases from 7.15× 10-5 to 1.01×10-4cm s-1.The cell also demonstrates superior durability and coking-resistance,which can attributed to a strong interaction between NiFe nanoparticles and SFM support.In Chapter 5,fluorine anion is doped into SFM,forming perovskite oxyfluoride Sr2Fe1.5Mo0.5O6-?F0.1(F-SFM),to evaluate its potential use for CO2 reduction.XRD indicates that fluorine doping reduces the lattice constant from 3.916 to 3.910 A.Meanwhile,fluorine doping weakens the Coulombic force between B site metal ions and oxygen,resulting in much more oxygen vacancies and electrons,and consequently enhances the surface reaction rate constant and chemical bulk diffusion coefficient by factors of 2-3.The faster kinetics is also reflected by a lower polarization resistance of 0.656 ? cm2 for F-SFM than 1.130 ? cm2 for SFM at 800 ?.Furthermore,the single cell with F-SFM cathode achieves a current density of 1.36 A cm-2 at 1.5 V and excellent stability over 120 hours at 800 ? under harsh conditions.The theoretical computations confirm that fluorine doping is energetically favorable to CO2 adsorption and dissociation.In Chapter 6,the innovation points and research conclusions of this dissertation are both summarized.Furthermore,the future research directions in SOEC systems are proposed.