Studies On The Cathode Preparation And Performance Of Protonic Ceramic Membrane Fuel Cells

Solid oxide fuel cell（SOFC）is a high-efficiency energy conversion technique,and is considered as one of the key technologies in the China’14th Five Year Plan about Energy Technology Innovation.Proton ceramic membrane fuel cell（PCMFC）is a kind of SOFC using proton-conducting oxides as electrolytes.Compared with oxygen-ionic SOFC,there are two outstanding advantages for PCMFC.The first is the low activation energy of proton conduction which indicates a higher conductivity in the intermediate-low temperature.The second is the generation of water vapor in cathode side which avoids the dilution of fuel gas and improves the fuel utilization efficiency.The cathode is a key component of PCMFC where the catalytic reaction of oxygen reduction occurs and the water vapor generates.Cathode polarization is dominant in PCMFC power loss and the stability of cathode determines the long performance durability of cells.This dissertation focuses on the improvement in the polarization performance and stability against CO₂ and H₂O of the cobalt-free ferrite-based perovskite cathode,and the optimization of the composite cathode preparation method.The detailed contents are as follows:Section 1:A high-performance cobalt-free Ba Fe_0.8Ce_0.1Y_0.1O_3-δcathodeThis section designed a novel high-performance cobalt-free Ba Fe_0.8Ce_0.1Y_0.1O_3-δ（BFCY）cathode.Its properties as a PCMFC cathode were comprehensively studied.Ce and Y dopants can stabilize the cubic structure of BFCY in the range of temperatures from 25 ℃ to 900 ℃.XPS analysis indicates that Ce and Fe elements in BFCY exhibit Ce^+3/+4and Fe^+3/+4mixed-valence state.This is favor to the formation of oxygen vacancies and oxygen reduction catalysis.The oxygen nonstoichiometry at high temperature is determined by the iodometric titration combining with TG.High-concentration oxygen vacancies promote oxygen reduction catalysis.The maximum electrical conductivity of BFCY is 1.55 S cm^-1 in moist air.The average thermal expansion coefficient of BFCY is 22.08×10^-6 K^-1 in 100-1000 ℃,higher than that of BZCY electrolyte,and therefore BFCY-Ba Ce0.8Fe0.1Y0.1O3-δ（BCFY）composite cathode is built.The single cell with BFCY-BFCY cathode achieves a high power density of 750 m W cm^-2and a low polarization resistant of 0.05Ωcm² at 700 ℃,indicating BFCY is a promising cathode with tremendous developing potentiality.The polarization resistance is lower than the reported high-performance cathodes,which is attributed to the triple-conducting property of cathode.Section 2:A barium ferrite cathode with good resistance against CO₂ and H₂OIn this section,the effects of A-site La and B-site Zr doping on the resistance against CO2and H₂O were investigated,and a novel triple-conducting Ba0.95La0.05Fe0.8Zr0.1Y0.1O3_-δ（BLFZY）cathode was designed and prepared.The cathode properties including phase stability,electrical conductivity,chemical and thermophysical compatibilities with a BZCY electrolyte,and the single cell performance and long-term stability were investigated in detail.The BLFZY keeps a stable cubic perovskite from room temperature to 900 ℃.Zr and La doping increase the average strength of Metal-Oxygen bonds and thus improve the tolerance to CO₂ and H₂O.The substitution of La for Ba can decrease thermal expansion coefficient and promote the electrical conductivity and oxygen adsorption ability.The single cell with single-phase BLFZY cathode achieves the maximum power density of 305 m W cm^-2 and polarization resistance of 0.492Ωcm² at 600 ℃,and displays steady power output in long-term monitoring of 120 h.Section 3:Nickel-doping optimization for cobalt-free Sr Fe0.9Nb0.1O3_-δcathodeThe Sr Fe_0.9Nb_0.1O_3-δ（SFN）is a promising cobalt-free cathode with good tolerance to CO₂.This section studies the effect of Ni doping on SFN cathode performance to further optimize the cathode performance of SFN.10%Ni doping does not change the phase structure of SFN,but increase oxygen vacancies due to its low valence state.The Ni doping improves the electrical conductivity,from 57.9 S cm^-1for SFN to 104.3 for SFNN at 600 ℃.The study on the symmetrical cells with SFN and SFNN indicates that Ni doping decreases the polarization resistance,e.g.from 1.4Ωcm²for SFN to 0.53Ωcm² for SFNN at 700 ℃.The DRT analysis of EIS reveals that surface oxygen absorption at cathode is the main rate-limiting of cathode reaction.The single cell performance for SFNN is higher than that for SFN under wet H₂/air condition,e.g.457.5 VS 392.5 m W cm^-2 for peak power density and 0.27 VS 0.35Ωcm² for polarization resistance at 700 ℃.In conclusion,Ni doping is one kind of effective strategies for improving the performance of ferrite cathodes.Section 4:One-step synthesis of Sm Ba Co₂O_5+δ-Sm_0.8Ce_0.2O_2-δcomposite cathodeIn this section,Sm Ba Co₂O5+δ-Sm_0.8Ce_0.2O_2-δ（SBC-SDC）composite powders were prepared by a simple one-step synthesis method.In contrast with the SBC+SDC composite powders prepared by traditional physical mixing method,SBC-SDC shows more homogeneous two-phase distribution and smaller particle size.The decreasing particle size is contributed by the mutual inhibition of SBC and SDC grain growth due to an increasing two-phase homogeneity.The improved microstructure of SBC-SDC cathode provides the rich and fast paths for electronic and oxygen-ionic transportation and increases the number of SBC/SDC heterojunction,which favors to decrease the cathode polarization resistance.Compared with the single cell with SBC+SDC cathode,the single cell with SBC-SDC cathode shows a higher peak power density and lower polarization resistance,indicating that one-step synthesis method can effectively improve the double perovskite-fluorite composite cathode performance.