| Growing consciousness of environmental issues and inadequate source of energy resources has forced the society to look for alternative clean source.Protonic ceramic fuel cell(PCFC),has attracted many researchers'attention in recent years because of its high energy conversion efficiency,no pollution and low cost.The application of PCFC could effectively solve the problems of rapid aging and high internal thermal stress,but there are still some problems to be resolved.In particular,the internal reaction of the air electrode in PCFC and the harsh external environment(the presence of steam in the EC mode)lead to high requirements for the air electrode materials.In this work,the difference between proton and oxygen ion in exchange kinetics is found by researching the transport behavior of the carrier in triple conductor materials.Then,the performance of PCFC is improved by optimizing the triple conductor materials.In the second chapter,the special two-fold diffusion behavior in triple-conductor BCFZY is analyzed by means of electronic conductivity relaxation method(ECR).At700°C,the water incorporation reaction would happen in the formation of both hydration and hydrogenation.The hydration reaction would happen at the sacrifice of oxygen vacancy,and then,the incorporated proton diffuses from outer layer to the inner bulk.But due to much higher chemical diffusion coefficient of proton than oxygen vacancy,the highly mobile protons are charge compensated by holes,instead of waiting for the less mobile oxygen vacancy,and thus causes the speeding increased hole concentration,i.e.,increased conductivity.So,in the hydration reaction,the decoupled diffusion of proton and oxygen vacancy would result in a non-monotonic conductivity relaxation curve,i.e.,two-fold diffusion.After that,the proton uptake reaction would transit to hydrogenation,as total decreased conductivity in a monotonic curve.In the third chapter,anion doping is applied to improve proton conductivity in BCFZY.The strongly alkaline F-was doped into the lattice of BCFZY by EDTA-citric acid method.It was confirmed that F-was successfully doped into the crystal lattice by XRD and XPS.The oxygen and proton surface exchange kinetics before and after doping was calculated by ECR.The results showed that the oxygen surface exchange kinetics Kchem and proton surface exchange kinetics K?of F-BCFZY were 1.14×10-5cm s-1and 3.80×10-5 cm s-1,while they were 5.5×10-6 cm s-1 and 2.17×10-6cm s-1 for BCFZY at 500?.It indicated that F-could effectively improve the oxygen and proton surface exchange kinetics of BCFZY.The F-BCFZY air electrode area specific resistance(ASR)was 0.21?cm2,maximum power density of the single cell was 620 m W cm-2,and 0.57?cm2 and 450 m W cm-2at 650?for BCFZY electrode and single cell.In fourth chapter,the LSCF113-214 triple conductor with core-shell structure was prepared based on LSCF113 through the solution coating method.Unexpectedly,after these two materials were applied to PCFC as air electrode,the maximum power density of the single cell achieved 450 m W cm-2(LSCF113-214)and 382 m W cm-2(LSCF113)at 700°C.At 600°C,3%p H2O and 1.5 V constant voltage,the electrolysis current densities of the LSCF113 and LSCF113-214 single cells were 801 m A cm-2and 893 m A cm-2 respectively.In addition,it's found that LSCF113-214 has more stable electrolysis performance.And the change of RL value of the LSCF113-214 air electrode single cell was even lower,which means the charge transfer related reaction in LSCF113-214 shows better stability when the p H2O changes.These benefit from higher proton conductivity and stability of LSCF214. |
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