Engineering Low Temperature Solid Oxide Fuel Cells And Related Novel Electrodes

The major demand for solid oxide fuel cells?SOFC?commercialization is to lower its operating temperature to low temperatures?LTs,?650 ??while still maintaining a desirable cell performance.Among the developed electrolyte materials,doped ceria?DCO?and isovalent cation stabilized bismuth oxides?SBO?have higher oxygen ionic conductivity at LTs.However,either DCO or SBO has some disadvantages,which limits the LT applications.When compared with oxygen ion conductors,the electrolytes for proton conducting SOFC?H-SOFC?has lower activity energy?Ea?,which has the potential to achieve high electrochemical performance at LTs.In this thesis,related low cost fabrication techniques to obtain high quality DCO-SBO bilayer membranes,new cell structures and novel materials are developed.In addition,on account of limited low cost cobalt-free cathodes for LT high performance H-SOFC,novel cathode materials are developed to analyse the electrochemical reaction mechanism,which could provide the theory to design the LT high performance cobalt-free H-SOFC cathode materials.In Chapter 1,the problems existed in LT-SOFC and the development of particular materials for SOFC components are presented.Moreover,based on the high performance of proton blocking cathode?PBC?for H-SOFC,the cobalt-free PBC for H-SOFC is summarized.In Chapter 2,a simple low cost technique combining co-pressing with drop-coating is developed for Sm_0.075Nd_0.075Ce_0.85O_2-?-Er_0.4Bi_1.6O₃?SNDC-ESB?bilayer electrolyte cell,which showed the highest power output at 450-600 ? for DCO-SBO bilayer structured cells in the literature.The maximum power density?MPD?of SNDC-ESB bilayer membrane cell is 130 mW cm-2 at 450 ?,which is relatively high at LT operations.Compared with the advanced small-scale technique,such as the DC?direct current?magnetron sputtering and pulsed laser deposition?PLD?,this facile fabrication technique provide an effective and interesting way for the fabrication of DCO-SBO bilayer electrolyte films with encouraging performance at LTs and could be beneficial to the application of SBO materials for high performance LT-SOFC.In Chapter 3,different ceria-based materials GDC and SNDC with different ionic conductivity,were evaluated based on anode supported DCO-SBO bilayer electrolyte structured single cells,in order to compare and investigate the influence of different DCO materials on the performance of the DCO-SBO bilayer electrolyte films for high.performance LT-SOFC?HPLT-SOFC?.The results demonstrate it that SNDC with higher conductivity is beneficial to the efficiency improvement of the DCO-SBO bilayer structured cells.Furthermore,an interesting phenomenon that DCO-SBO bilayer films reveal higher ionic conductivity than both DCO and SBO at lower temperatures occurred.Then a plausible mechanism concerning the rational principle for the conductivity behavior of the DCO-SBO bilayer electrolyte in relatively low temperatures was proposed,which could give a theory explanation for this curious phenomenon.In Chapter 4,in the light of the high reactivity of SBO,limited cathodes for DCO-SBO bilayer structured HPLT-SOFC are developed,except for several noble metal materials,for example,Bi₂Ru₂O₇.A novel composite cathode consisting of A-site disordered perovskite material Pr_0.5Ba_0.5MnO_3-??PBM?and ESB is developed.The ESB-PBM is a nano structrure composite cathode,which has the particle size about 100-200 nm.When applied to the SNDC|ESB bilayer structured cell,it achieves an encouraging performance with the MPD of 994 mW cm-2 at 650 ?.The excellent performance indicates it that ESB-PBM is a superior LT high performance cathode material for DCO-SBO bilayer electrolyte HPLT-SOFC.In Chapter 5,a novel proton blocking composite cathode?PBCC?La₂NiO₍4+?-_LaNi_0.6Fe_0.4O_3-??LNO-LNF?with new interstitial oxygen transfer mechanism is designed for H-SOFC,which has different oxygen transfer mode with traditional PBCC materials.Two types of proton-blocking composites,LNO-LNF?1:1 volume ratio,the interstitial oxygen transfer mechanism?and Sm_0.2Ce_0.8O_2-?-LNF?1:1 volume ratio,SDC-LNF,the oxygen vacancy transfer mechanism?,were evaluated as cathode materials for H-SOFC based on the BaZr_0.1Ce_0.7Y_0.2O_3-??BZCY?electrolyte,in order to compare and investigate the influence of two different oxygen transfer mechanism on the performance of the cathode for H-SOFC,which could explain the transfer mechanism of oxygen ions and elctrons and the processes of the interface reaction,give the intrinsic factors of the total cell performance improvement and provide a reference for the design of proton blocking cathode?PBC?materials.Furthermore,based on the preferable PBCC LNO-LNF,different Ni content anode functional layers?AFL?were applied to the BZCY based H-SOFC,to optimize the cell performance output with all the other cell components being ensured with the fine structure.The optimized cell has the power output of 270 mW cm-2 at 500 ?,which makes it clear that LNO-LNF PBCC is a preeminent alternative for H-SOFC.In Chapter 6,a tetragonal layered structure material Pr_1.5Ba_1.5Cu₃O_7-??PBCu?,which composed with the ionic conductor SDC to form SDC-PBCu proton blocking composite cathode?PBCC?,is developed for H-SOFC.With 20 ?m BZCY as electrolyte,the cell with SDC-PBCu PBCC achieved sky-high performance,which has the MPD output of 1000 mW cm-2 at 700 ?.Apart from this,the highest power output at 500-600 ? compared with other results for H-SOFC cobalt-free PBCC reported in the literature indicates it that SDC-PBCu is a good potential LT cathode material for H-SOFC.In Chapter 7,a summary of the thesis and some recommendations for future research on DCO-SBO bilayer structured HPLT-SOFC and cobalt-free PBC for LT H-SOFC.