Design Of High Ionic Conduction In Semiconductor And Heterostructure Electrolytes For Low-temperature Solid Oxide Fuel Cells

Low-temperature solid oxide fuel cell(LT-SOFC)is widely regarded as a promising energy conversion technology to fulfill the word-wide need of energy,due to its capability of converting chemical energy into electricity with high efficiency,low pollution and less cost than conventional SOFC.To develop LT-SOFC,the new materials to enable fast ionic conductivity at low operating temperatures are essentially required.In this thesis work,several promising semiconductor oxide materials have been developed as electrolyte by doping and heterostructure approaches to obtain good ionic conductivity and successful demonstration of the LT-SOFCs.In the first part of the thesis,single-phase semiconductors Ba Co_0.2Fe_0.3-xCe_xTm_0.1Zr_0.3Y_0.1O_3-?[x=0.1-0.2],Sr_0.5Pr_0.5Fe_0.4-xMg_xTi_0.6O_3-?[x=0,0.1-0.2],and Sr_0.1Tm_xCe_0.9-xO_2-?[x=0.1]were prepared and investigated in terms of phase study,surface morphology,microstructure characterization and X-ray Photoelectron spectroscopy(XPS)analysis.Further electrical and electrochemical characterizations,e.g.the electrolyte function in terms of oxide ions and protonic conduction,electrochemical impedance spectroscopy analysis and fuel cell performance were carried out in the LT-SOFC environments.It was found that Ba Co_0.2Fe_0.1Ce_0.2Tm_0.1Zr_0.3Y_0.10_3-?,Sr_0.5Pr_0.5Fe_0.2Mg_0.2Ti_0.6O_3-?,and Sr_0.1Tm_0.1Ce_0.8O_2-?gained high ionic conductivity of 0.193,0.123,and 0.13 S cm^-1 at 530,520,and 550?,respectively.The generation of large number of oxygen vacancies through doping enabled the high ionic conductivity.Moreover,careful doping enhanced the fuel cell power output significantly to0.873,0.8,and 0.682 W cm^-2 of Ba Co_0.2Fe_0.1Ce_0.2Tm_0.1Zr_0.3Y_0.1O_3-?,Sr_0.5Pr_0.5Fe_0.2Mg_0.2Ti_0.1O_3-?,and Sr_0.1Tm_0.1Ce_0.8O_2-?at 530,520,and 550?,respectively.On the basis of these results,the energy band structure and built-in-field mechanism was proposed to interpret the charge transport behavior in Ba Co_0.2Fe_0.1Ce_0.2Tm_0.1Zr_0.3Y_0.1O_3-?,Sr_0.5Pr_0.5Fe_0.2Mg_0.2Ti_0.6O_3-?,and Sr_0.1Tm_0.1Ce_0.8O_2-?.The formation of Schottky junction(SJ)was successfully verified that helped in the charge separation to prevent electrons passing through the anode/electrolyte interface and promoted the transport of protons(H⁺)or oxide(O^2-)ions from the electrolyte to the electrodes.These findings revealed that single-phase semiconductor oxides can be tuned to a good ion-conducting electrolyte function applied for LT-SOFCs by doping method.In the second part of the thesis,the heterostructure materials have further developed by two semiconductor materials of Co-doped Zn O and Ba_0.5Sr_0.5Co_0.1Fe_0.7Zr_0.1Y_0.1O_3-?with typical ionic conductors Sm_0.2Ce_0.8O_2-?(SDC)and Ca_0.04Ce_0.80Sm_0.16O_2-?(SCDC),respectively.The Co_0.2Zn_0.8O-SDC and Ba_0.5Sr_0.5Co_0.1Fe_0.7Zr_0.1Y_0.1O_3-?-SCDC heterostructures were utilized as electrolytes in LT-SOFCs and exhibited attractive fuel cell power outputs of 0.928 and 0.9 Wcm^-2 at 550 and 520?,respectively.Moreover,the formation of heterostructure Co_0.2Zn_0.8O-SDC and Ba_0.5Sr_0.5Co_0.1Fe_0.7Zr_0.1Y_0.1O_3-?-SCDC achieved even higher ionic conductivity of 0.24 and0.22 S cm^-1 at 550 and 520?.The synthesized materials were characterized for crystallographic,morphology,elemental mapping,microstructure and XPS analysis.The XPS analysis provided a detail study of charge transfer mechanism and illustrated the formation of oxygen vacancies at interface.Moreover,the investigation of electrochemical properties reveals that the formation of interfacial conduction in heterostructure plays a crucial role in the fast transport of ions conduction.Based on the physical analysis,the tuning of energy band structure and the Schottky junction were studied to deeply illustrate the mechanism of charge separation and blocking of electronic conduction in the constructed heterostructure electrolyte that could also enhance the fuel cell device performance.It should be pointed out that the prepared semiconducting oxides and heterostructure materials can achieve higher ionic conductivity than that of the conventional electrolyte,e.g.>0.1 S cm^-1 for single phase and>0.2 S cm^-1 for the heterostructure materials compared to conventional YSZ(yttrium stabilized zirconia>0.1 S cm^-1 at 1000?and SDC(samarium doped ceria)at 800?.The purpose to develop doped semiconductor and semiconductor heterostructure are the creation of high content of oxygen vacancies,tuning the energy band structure and various kinds of junction formation such as SJ,np-bulk heterojunction(BHJ)and semi-ionic junction.Following the proposed mechanisms,the oxygen vacancies have been found in the doped semiconductor and heterostructure to be beneficial to the fast and high ions conduction.Meanwhile,the prepared materials employed as the electrolyte were verified for the formation of np-BHJ at particles in the electrolyte membrane,and SJ formation in the device at anode/electrolyte(semiconductor).These junctions'formation in doped semiconductor and heterostructure mainly helped to avoid short-circuiting in the device and promote ionic transport as well at low operational temperature.The results showed in the present thesis work have successfully demonstrated that the doping and construction of semiconductor heterostructure can tune effectively the band structure and alignment,based on which,the build-in electric field(BIEF)can promote the fast transport of ions conduction at interface of particles/grains.Therefore,this thesis work of single-phase semiconductor doped materials and their heterostructure presents novel strategy to improve the ionic properties of the electrolyte and to demonstrate the optimal performance of advanced LT-SOFCs.