Yttrium-doped Barium Zirconate-Barium Cerate Proton Conductor Electrolyte And Its Application In Fuel Cells Using Containing Carbon Fuels

Solid oxide fuel cells(SOFC),which consist of porous electrodes and a dense electrolyte,are devices that continuously and efficiently convert the chemical energy of fuel into electricity.Compared with oxygen ion conductor electrolytes,proton conductor electrolytes have lower activation energy and higher ionic conductivity at medium and low temperatures.Therefore,proton conductor solid oxide fuel cells(PCFCs)are easier to operate at medium and low temperatures.Existing proton conductor oxide materials,BaZr0.8Y0.2O3-δ(BZY)and BaZr0.1Ce0.7Y0.2O3-δ(BZCY)are the most promising materials for application,but there are still some problems that need to be solved.BZY has good stability,but its sintering activity is poor and grain boundary conductivity is low.Although BZCY has excellent ionic conductivity,its chemical stability is poor.These problems hinder the development and application of PCFCs.This paper focuses on the problem of medium and low temperature proton conductor BZY and BZCY electrolyte materials,aiming at improving the sintering activity and preparation process of BZY,as well as the practical application and chemical stability of BZCY.The specific research contents are as follows:(1)In order to solve the problem of poor sintering activity of BZY,the sintering activity of BZY was improved by adding the additives NiO and Fe2O3.It was found that the electrolyte could be densified after sintering at 1450℃ for 5 h when the conventional dosage of NiO was reduced to 2 mol%.The anode-supported cells of NiO and Fe2O3 sintering agents were prepared respectively.The open-circuit voltages of the two are similar,which indicates that Fe2O3 sintering agent can densitize the electrolyte.The power density of the cell using NiO as sintering aid is higher than that using Fe2O3 as sintering aid at medium and high temperature,but the performance of the cell using Fe2O3 as sintering aid is higher at low temperature.This is because in the process of co-firing,Ni diffuses into the electrolyte with Fe2O3 as sintering aid,which promotes sintering.With the decrease of temperature,the ohmic impedance of the cell with Fe2O3 as sintering aid becomes smaller.(2)BZY cell can not only use hydrogen as fuel,but also use organic compounds containing hydrocarbons as fuel.BZY powder and anode-supported BZY cells were prepared by high temperature solid state method.The electrolyte BZY and cathode La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)have good chemical stability.Using humid H2,CH4 and C3H8 as fuel,the performance of cell is reduced,respectively and the maximum power densities at 700℃ are 356 mW cm-2,309 mW cm-2 and 85 mW cm-2,respectively.When C3H8 is used as fuel gas,the power density of the cell is low,and the phenomenon of anodic carbon deposition is found by XRD,and the anodic powder falls off.When using methane fuel,the performance of the cell was 87%of that when using hydrogen,and no significant carbon accumulation was observed.(3)In the above experimental process,it was found that the sintering activity of BZY synthesized by solid phase method was poor,and the cell was prone to crack in the process of preparing anode-supported cell.To solve this problem,BZY powder was prepared by gel sol method(GS)and compared with solid phase method(SP).It is found that the powder particles prepared by GS method are smaller and the linear shrinkage of the bulk materials is larger in the sintering process.The thermal matching performance was better between BZY electrolyte prepared by GS method and NiO-BZY anode prepared by solid phase method,which can avoid the phenomenon of cell cracking caused by inconsistent thermal matching between electrolyte and anode during co-firing,and significantly improve the yield of battery and stability during the cell testing.(4)At present,there are many studies on BZY and BZCY,but there is no comparative study on BZY and BZCY.Through systematic comparison of BZY and BZCY,the difference between the two materials and the corresponding cell performance is intuitively understood and evaluates the advantages and disadvantages of the two materials in different performance aspects.It was found that adding 2 mol.%NiO as sintering assistant could improve the sintering performance of BZY and BZCY After sintering at 1400℃ for 6 h,Ni is doped into the gap of BZY,resulting in lattice expansion.However,Ni enters the lattice position of BZCY and occupies the B position,resulting in small crystal cells.Sintered BZY has small grains,more grain boundaries and lower conductivity,while sintered BZCY has large grains,less grain boundaries and higher conductivity.The electrochemical performance of anode-supported BZY cell is lower than that of anode-supported BZCY cell,mainly because the ohmic resistance of BZY electrolyte is larger than that of BZCY electrolyte.With the increase of co-sintering temperature,the performance of PCFC with BZY electrolyte could not be improved due to the increase of anode polarization impedance resulting from excessive sintering.Although the performance of BZY is lower than that of BZCY,BZY is stable in an atmosphere containing CO2,while BZCY is unstable in Ar(650℃)containing 10%CO2.(5)Using BZCY electrolyte with higher electrochemical performance,the performance of the cell was studied using C and H2O as fuel.An anode-supported BZCY proton conductor fuel cell was prepared by solid-state method.The peak power density of the cell was 875 mW cm-2 at 700℃ using wet hydrogen as fuel gas and air as oxidizing gas.5mL/min of 75%H2O-25%Ar was passed into the anode C powder and C and H2O react to generate H2 in situ to realize the work of the cell.The power densities of the cell are 38 mW cm-2,7 mW cm-2 and 0.5 mW cm-2 at 700℃-600℃,respectively.As the product gas contains a large amount of CO2 gas,it can react with BZCY and leads to the damage of the anode structure of the cell.In order to reduce CO2 and increase H2 concentration in product gas,Ca(OH)2 was mixed into C fuel and the power densities of the cell at 700℃-600℃ are 300 mW cm-2,130 mW cm-2 and 37 mW cm-2,respectively.The reason for the obvious improvement of electrochemical performance is that the addition of Ca(OH)2 can not only absorb the CO2 produced by the reaction,but also promote the reaction of C and H2O to generate more H2,which improves the output performance of the cell.(6)In order to improve the chemical stability of BZCY on the basis of maintaining high conductivity.When the B site of BZCY is stoichiometric doped with 2 mol.%Fe(BZCYF2),the electrolyte was dense,the grain size is the largest,the number of grain boundaries is the least,the linear shrinkage rate is the best,and the sintering activity is the best.No enrichment of Y3+element occurred in BZCYF2 sintered at high temperature.It is found that BZCYF2 has the highest conductivity and the lowest activation energy under wet H2 and wet air.Anodesupported single cells were prepared and their electrochemical performance was tested.At 700℃ and 250℃,the peak output power densities of BZCYF2 and BZCY cells were 880 mW cm-2 and 850 mW cm-2,and 7 mW cm-2 and 2 mW cm-2,respectively.BZCYF2 was assembled into an electrolytic cell,and an electrolytic current of 1.3 A/cm2 was obtained at an electrolytic voltage of 1.3 V.Through discharge test at 500℃ and H2O resistance test,it can be found that the cell with BZCYF2 electrolyte has good electrochemical stability,and the introduction of Fe can improve the water resistance of BZCY.These studies show that PCFC is a high-performance fuel cell that can operate at relatively low temperatures.The yield of PCFC can be improved by using proper powder synthesis method and adding proper sintering agent.PCFC can be fueled not only by hydrogen,but also by methane,propane,water vapor and carbon.It was found that BZCY had better electrochemical performance and sintering performance than BZY,but BZY had stability.By adding 2 mol%Fe to BZCY,the stability of the material was significantly improved,and the excellent electrochemical performance of BZCY was retained.