Nanostructured Electrodes For Solid Oxide Cells

Solid Oxide Cell（solid oxide cell,SOC）are high-temperature device that can allow for reversible conversion between the chemical energy in fuels and electricity,operating either in the power generation mode（solid oxide fuel cell,SOFC）or in the electrolysis mode（solid oxide electrolysis cell,SOEC）.SOFC convert fuels such as hydrogen and methane into electricity at high efficiencies while producing minimized emissions.As powered by the surplus renewable electricity,SOEC can split H₂O to H₂ or CO₂ to CO,with the resulting syngas as the feedstock for the synthesis of liquid fuels and value-added chemicals.Over the last two decades,significant efforts have been made to reduce the cell operating temperature.Nevertheless,both the ohmic resistances（R_Ω）and the interfacial polarization resistances（R_p）increased substantially as reduced temperatures,resulting in much smaller power densities.Utilizing new electrolytes with much higher ionic conductivities at comparable temperatures and reducing the electrolyte thickness help to reduce the R_Ωvalue.With an increase in the surface area,the catalytic activity of porous electrods increases and the R_p value decreases.Therefore,this thesis will explore new architectures of nanostructured electrodes,with the correlation among their compositions,morphologies,catalytic activities and the electrochemical behavior established.The resistances of nanostructured electrodes against coking formation will also be evaluated.Note that high thermodynamic stability and kinetic inertia result in poor CO₂ electrolysis performance in SOEC.It becomes imperative to develop novel SOEC cathodes that are economically inexpensive,catalytically active and highly durable when operated at high electrolysis current densities.This thesis will also design some novel micro-nano dual scale SOEC cathodes and elucidate the mechanism of CO₂ reduction reactions.The main research contents are as follows:1.Selective doping with moderate lanthanide ions is used to regulate surface oxygen vacancies and bonded adsorbates of ceria nanorods so as to finely tune their activities toward electro-oxidation of H₂ and C₃H₈ in reduced-temperature solid oxide fuel cells.Lanthanide doped ceria nanorods are hydrothermally synthesized,and electrochemically evaluated as the anode catalysts for reduced-temperature SOFC.Probing the surface structure with hydrogen temperature-programmed reduction,UV-Raman and XPS reveals the catalytic activity in the order of CeO_2-δ<Ce_0.8Pr_0.2O_2-δ<Ce_0.8Gd_0.2O_2-δ<Ce_0.8Sm_0.2O_2-δ.Functioning cathode-supported SOFC show maximum power densities of 0.46,0.52 and 0.63Wcm^-2 in hydrogen,or 0.17,0.19 and0.23Wcm^-2 in propane for Ce_0.8Pr_0.2O_2-δ,Ce_0.8Gd_0.2O_2-δ,and Ce_0.8Sm_0.2O_2-δnanorods at 600℃,respectively.Such catalytic activities of Ce_0.8Ln_0.2O_2-δare essentially determined by surface reducibility,availability of surface oxygen vacancies and strongly bonded hydroxyls.2.Nanorods of Ni²⁺-doped ceria(Ce_1-xNi_xO_2-δ)are synthesized via a modified hydrothermal method,and evaluated as the anode catalysts for reduced-temperature solid oxide fuel cell（SOFC）.Exposure of these powders in H₂ at 600℃ results in exsolution of some spherical Ni particles of 11 nm in diameter,facilitate extraction of the lattice oxygen on the surface by H₂.Distribution of relaxation time（DRT）analysis of impedance data indicates that doping Ni²⁺into CeO_2-δpromoted dissociative adsorption of H₂.Ce_0.9Ni_0.1O_2-δand CeO_2-δanodes yield the highest power densities among the investigated series of anodes,e.g.,820 and 442mWcm^-2 in 97%H₂-3%H₂O,and 598 and 331m W cm^-2 in 68%CH₃OH-32%N₂.Moreover,Ce_0.9Ni_0.1O_2-δcathode achieves current density of 0.42A cm^-2 at 1.3 V for H₂O electrolysis,working steadly between SOFC mode and SOEC mode.3.Double perovskite oxides of La³⁺-and Ni²⁺-co-doped Sr₂Fe_1.5Mo_0.5O_6-?（LSFNM）are proposed as potential cathodes in SOEC for electrolysis of pure CO₂.Thermal treatment in pure H₂ at 800℃ results in exsolution of nano-scale Ni Fe alloy nanospheres out of the LSFNM perovskite lattice,enabling substantial enhancement in the CO₂ reduction kinetics with higher CO₂ adsorption capacity and faster re-equilibration kinetics.Fitting of the measured ECR curves yield k values of 4.78×10^-4cms^-1 for LSFNM and 1.83×10^-4cms^-1 for SFM.A remarkably improved current density of 3.07Acm^-2 is achieved at 1.5V and 800°C,which is 96.8%higher than that of SFM.DRT analysis shows that the exsolution of Ni Fe particles improved the adsorption and activation capacity of CO₂ on the surface of LSFNM.Moreover,a preliminary 100-hour measurement shows good stability without any noticeable coking formation.The excellent catalytic activity toward CO₂ reduction and superior coking resistance of LSFNM cathodes are attributed to in-situ exsolved Ni Fe alloy nanoparticles and their interaction with the perovskite oxides.4.Perovskite oxides La_xSr_2-xFe_1.5Ni_0.1Mo_0.4O_6-δ（L_xSFNM）are prepared and evaluated as symmetrical electrodes in solid oxide electrolysis cells for electrochemical reduction of pure CO₂,using the electrolyte-supported configurations with impregnated L_xSFNM catalysts（i.e.,Lx SFNM@LSGM|LSGM|Lx SFNM@LSGM）.The electrical conductivies of L_xSFNM increase with increasing the La³⁺content for x≤0.3,and then start to decrease for x≥0.4.The polarization impedances of symmetrical SOEC in air and CO-CO₂ atmospheres all meet the following order:L_0.3SFNM<L_0.2SFNM<L_0.4SFNM<L_0.1SFNM.The polarization resistances are 0.07Ωcm² in air and 0.62Ωcm² in 50%CO₂-50%CO for L_0.3SFNM electrode at 800℃.An electrolysis current density of 1.17Acm^-2 at 800℃ and 1.5V is achieved for the symmetrical SOEC in pure CO₂.Furthermore,the symmetrical cell has demonstrated excellent stability during the preliminary 50-hour CO₂ electrolysis measurements.It was shown that doping La³⁺improves the surface oxygen exchange reaction rate and the surface adsorption and dissociation of CO₂ on the catalyst.These results demonstrate that L_0.3SFNM oxides are appealing as a potential symmetrical electrode.