Planar Heterojunction Structure Electrolyte Design And The Performance Optimization Of SOFC

The green sustainable hydrogen energy will replace traditional fossil energy.Solid oxide fuel cell（SOFC）is a device that converts chemical energy of fuel into electrical energy efficiently.However,the expensive production cost and high operating temperature limit the commercialization of SOFCs.The major development direction of SOFCs is that designing a low-temperature device with high-performance.Electrolyte as the key part of SOFC determines the electrical performance.Thin film electrolyte and new crystal materials are effective ways to improve the electrical performance and reduce the operating temperature.In this paper,a planar hetero-structured electrolyte was designed,and the semi-conductors were used as the basis material.The energy band arrangement of the planar structure electrolyte optimized the fuel cell performance.The thin film novel electrolyte was explored to provide a research basis for low temperature SOFC（LT-SOFC）.The innovations of this paper included:novel planar structure instead of widely used semiconductor composite electrolytes,which increased the contact interface between different semiconductor materials;the introduction of new Ba₅Nb₄O₁₅,Al₂O₃,TiO₂,and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF）materials as fuel cell electrolytes,replacing the traditional high-temperature dense ceramic electrolytes,effectively reducing the device operating temperature;the thin-film treatment of electrolyte,reducing the ohmic resistance of the electrolyte;the performance of SOFC optimized by constructing multilayer planar heterojunction electrolytes,which provided methodology for SOFC.The main work and results are as follows:1.The in-situ generation method to form heterojunction delivered unique electrochemical properties compared with the complex process of mechanical mixing,thus the in-situ generation method was widely applied in the electrochemical field.In this work,Co ions diffused from the anode NCAL into the Ba₅Nb₄O₁₅electrolyte during the fuel cell operating process,and the heterostructure Ba₃Co Nb₂O₉/Ba₅Nb₄O₁₅ was in situ generated.The cell based on Ba₃Co Nb₂O₉/Ba₅Nb₄O₁₅ electrolyte membrane delivered better power density of703 m W/cm²at 510 ^oC due to electron inhibition by the strong heterogeneous barrier.The X-ray photoelectron spectroscopy（XPS）and X-ray Diffraction（XRD）characterization verified that the tested temperature influenced the Co doping concentration to determine the electron conductivity,proton conductivity and activation energy of Ba₃Co Nb₂O₉/Ba₅Nb₄O₁₅electrolyte.Furthermore,due to the formation of a built-in electric field,it restricted the electron migration through the electrolyte leading to avoiding the short circuit problem.The results indicated that less amount of Co-doped composite Ba₃Co Nb₂O₉/Ba₅Nb₄O₁₅ was a promising candidate as a proton-conducting electrolyte for low-temperature solid oxide fuel cells（LT-SOFCs）.2.Perovskite LSCF as a promising cathode material possessed overwhelming electronic conduction along with certain ionic conductivity.Its strong electron conduction capability hindered the application of pure-phase LSCF as electrolyte in SOFC.In order to constrain the electron transport and take advantage of the decent ion conduction of LSCF,a thin layer ofγ-Al₂O₃ with insulating property combined with LSCF as an electron barrier layer to form a two-layer structure electrolyte.Through adjusting the weight ratio of LSCF/γ-Al₂O₃ to optimize the thickness of double layers,an open circuit voltage（OCV）of 0.98 V and a maximum power density(P_max)of 690 m W/cm² was received at 550 ^oC.At the same time,SEM,EIS and other characterization technology had proven that the LSCF/γ-Al₂O₃bi-layer electrolyte can work efficiently at low temperature.The advantage of this work was the application of double-layer（γ-Al₂O₃/LSCF）structure electrolyte to instead of mixed material electrolyte in low-temperature solid oxide fuel cells.Structural innovation and the using of insulating materials provided clues for the further development of SOFC.3.The reverse blocking state of the pn junction can prevent the electron from conducting,which considered the design requirement of electrolytes in solid oxide fuel cells.It was a new attempt to apply two semiconductors to form a pn junction for a fuel cell electrolyte.LSCF with P-type semiconductor nature and a typical N-type semiconductor TiO₂were selected.The LSCF/TiO₂ pn planar junction electrolyte with a double-layer structure contributed the performance in the range of low temperature（550 ^oC～475 ^oC）.Through an innovative process of pulp film,the thickness of LSCF and TiO₂ materials optimized.LSCF（powder）/TiO₂（film）fuel cell had higher performance than LSCF（film）/TiO₂（powder）,which achieved a high OCV of 1.00 V and a superior P_max of 769 m W/cm² at 550 ^oC.Scanning transmission electron microscope（STEM）,spectroscopy,kelvin probe force microscopy（KPFM）and other approaches were used to observe the changes of the interface and confirm the interfacial electric field.The LSCF/TiO₂ pn planar junction can block electron conduction and accelerate ion transport simultaneously.The application of pn junction electrolytes in fuel cells will became a new theoretical guidance method.4.The p-i-n junction had a wider space charge region compared with pn junction,which can make the charge carriers separating rapidly,and had a more pronounced barrier effect on electrons.Therefore,p-i-n-type planar heterojunction electrolytes can effectively optimize the electrical performance of SOFC.Ultra-wide band gap（～7.6 e V）of Al₂O₃ as I-layer introduced between bi-layer structure of pn-type planar electrolytes LSCF/TiO₂,which formed p-i-n-type planar heterojunction electrolytes.The electrical performance of SOFC based on p-i-n-type planar heterojunction electrolyte material enhanced,and the TiO₂（film）/Al₂O₃（film）/LSCF（powder）device achieved a P_max of 1005 m W/cm² at 550°C with an OCV of 1.08 V.Meanwhile,the thin film materials were also explored,and the bi-layer Al₂O₃films effectively enhanced the electron barrier effect,and the TiO₂（film）/2Al₂O₃（film）/LSCF（film）cell with an OCV of 1.06 V and a P_max of 927 m W/cm² at 550°C.Planar hetero-structured electrolytes were optimized based on semiconductor theory.The space charge region and energy band arrangement for the operation of SOFC were used to analyze by considerable cell performance,electron barrier and charge separation capability.The electrochemical approach explained the ionic conduction properties of the electrolyte.The p-i-n planar heterojunction of TiO₂/Al₂O₃/LSCF successfully optimized the electrical performance of SOFC,and the pulp film process made the ohmic resistance of the electrolyte reduction.This work provided directions for the application of planar heterojunction electrolyte in LT-SOFC.This work explores the application of planar heterojunction electrolytes in LT-SOFC.The reliable results reveal the feasibility of planar heterojunction electrolytes for SOFCs.And the planar heterojunction electrolyte effectively reduces the operating temperature of SOFC and improves the fuel cell performance.This work also provides methodology and directions for the thin film electrolytes of the commercialized SOFC.