Fabrication Science And Performance Study Of IT-Ceramic Membrane Fuel Cells

As a promising energy conversion device,Solid Oxide Fuel Cell(SOFC)has attracted widespread attention due to its high efficiency,fuel flexibility and environmental benefits,and now it is on the way to practical application and commercialization.A new generation SOFC with high performance,mainly aims at the application for intermediate temperatures(500-800℃),were characterized with the anode supported thin electrolyte and prepared mainly by the ceramic membrane fabricating techniques,so ceramic membrane fuel cell(CMFC)could express it more exactly.In recently years,our groups have put great effect on key material research, essential fabrication techniques and cells structure design to promote the commercialization of CMFC in China.My dissertation work was focused on the fabrication,improvement and characterization of intermediate temperature CMFC (IT-CMFC).In the first chapter,this paper introduces the fundamental principles of SOFC/CMFC,the present research status and developing trend both in China and abroad,and pays attention to summarize the electrolyte membranes fabrication techniques,the problems about carbon coking on electrode and fuel choice for practical application of CMFC.On the base of analyzing the development of CMFC, we fabricate and investigate a new cathode material for YSZ-based cells for intermediate temperature application(in chapter 2);developed a cost effective and mass production process to fabricate YSZ,SDC and high temperature proton conductor membranes(in chapter 3 and chapter 5);reduce the operation temperature of YSZ based CMFC(in chapter 4);try to directly fueled CMFC with liquid methanol (in chapter 6)and also studied on the ceramic interconnect materials.The main achievements and innovations in this paper are summarized as follows:1.Research on high-performance cathode materialsThe current development trend of SOFC is to lower the operation temperature. However,a major issue produced by the reduced operating temperature is the decrease in electrical conductivity and catalytic activity of the cathode materials.For the YSZ-based electrolyte,the problems of cathode materials and cathode polarization are extraordinarily serious.The traditional cathode material La_1-xSr_xMnO_3-δ(LSM) perovskite oxides show high electrochemical properties which have been widely investigated as a promising cathodes for high temperatures,however,the catalytic activity of LSM was decreased rapidly as the temperature decreases.Other cathode materials developed in recent years,such as perovskite-type cobaltite oxide compounds,though they exhibit high catalytic activity at intermediate temperatures, shows high thermal expansion coefficient.What's more,the cobaltite cathode is very expensive and easy to react with YSZ to form the second phase of zirconate with high resistance,such as La₂Zr₂O₇,SrZrO₃ and so on.Therefore,YSZ based SOFC need to find alternative cathode materials for LSM.In this paper,we try to use Pr-Nd mixed (which is intermediate products of rare earth separation and is much cheaper)to substitute La in LSM-(Pr-Nd)_1-ySr_yMnO₃(PNSM)as the cathode material.The conductivity of(Pr-Nd)_0.7Sr_0.3MnO₃(PN3SM),synthesized by Glycine-Nitrate Process,could reach 216 S/cm at 800℃,only slightly lower than the PNSM synthesized using the pure Pr and Nd.And the activation energy of conductivity is only about 10kJ/mol,indicating the performance fluctuation with the temperature change is smooth.The XRD data shows that PNS3M does not react with YSZ during the high-temperature processing of preparing the cells,and the average thermal expansion coefficient is 11.1×10^-6K^-1,which is very close to that of YSZ,which is 10.8×10^-6K^-1,apparently suitable cathode material for SOFCs.With PN3SM/YSZ as cathode,the tubular CMFC get very high power output,and the maximum power densities are 415-282 m W cm^-2at 850-700℃,respectively,which has reached the industrialization level of Acumentrics Corporation of America.At corresponding temperatures,the interfacial polarization resistances are 0.14,0.21,0.31,0.42Ωcm². The high power densities and low interface polarization resistances indicate that Pr-Nd mixed oxide substituted LSM is very suitable candidate cathode material for YSZ electrolyte based SOFC.Besides,the application of low-cost rare earth in CMFC will not only further reduce the cost,but also benefit for the development of China's rare earth resources in the high-end application.2.Research on the cost effective process for electrolyte membranes fabricationThe solid electrolyte membrane is the core part in CMFC,for its thickness and density will directly influence the output power densities of the cell.Configuration of porous electrode supported thin electrolyte is the key technique route for CMFC to achieve high performance at intermediate temperature.Various techniques,such as chemical vapor deposition/electrochemical vapor deposition,sputtering,tape-casting, screen printing method,have been developed for electrolyte membranes fabrication. However,considered the high costs and technical complexity,there are few methods that can really achieve low-cost and high performance output.In our work,we developed a highly reliable technique of suspension coating process to prepare thin electrolyte membranes,through which dense electrolyte membranes were successfully fabricated on planar and tubular anode supports and planar cathode support. According to the results of SEM and single-cell open circuit voltage tests,the electrolyte films(YSZ and SDC)prepared by this method are very dense.Moreover, the single cells performance reach the international level:YSZ-based planar single cell(15μm)gets the maximum power density of 905,654,487 and 326 mW/cm2 at 850,800,750 and 700℃,respectively;while SDC based planar single cell(24μm)at 650,600 and 550℃,respectively reached 870,592 and 307 mW/cm²;and the maximum power density of the PNSM/SDC cathode supported single cell prepared using this method,achieved 657 mW/cm² at 800℃,even at 650℃,could still reach 166 mW/cm².Such suspension coating process,requiting simple devices,low cost, easy operation,and highly efficient will be one of the important technical supports for the development of CMFC industry in China.3.Research on IT-CMFC based on YSZYSZ is the preferred electrolyte material in the commercialization of SOFC for its high chemical and structural stability.In order to operate YSZ electrolyte based CMFC at intermediate temperatures,a large number of studies have been done to fabrication thin electrolyte film around 10μm and even less on porous anode support to reduce the electrolyte ohmic resistance to a negligible level.At 800℃,a higher power density of 500mW/cm² and even close to 2W/cm² has been reported.However, as temperatures decrease,there will be significant electrode polarization resulting in lower cell performances,which will not meet the practical requirements.In order to improve the performance of YSZ based cell at intermediate temperature,we introduced gradient anode transition layers and SDC function layer to improve the anode/electrolyte and cathode/electrolyte interface,respectively,thus greatly improved the cell performance at intermediate temperature.Through the introduction of a gradient composition of ultra-fine Ni/YSZ layer on the anode support,the interface polarization resistance reduced 42%and 32%than those without modification,at 600 and 650℃,respectively;while the maximum power densities were reached 144 and 353 mW/cm²,that are increased by 44%and 56%, correspondingly.By further introduction of SDC function layer to the YSZ electrolyte and cathode layer,the interface polarization resistance has been further reduced to 1.36 and 0.49Ωcm² 600 and 650℃,respectively.A fairly high maximum power densities of 187m W/cm² and 443mW/cm² were reached,showing an increase about 87%and 95%compared with the performance without any modification.The high performance achieved in our result could meet the practical requirements of IT-CMFC based on YSZ electrolyte.4.Research on proton conductor based CMFCFor their high proton conductivity and low conductivity energy activity,the BaCeO₃ based high temperature proton conductors have attracted wide attention and been applied as the electrolyte materials for IT-CMFC.However,the poor sinter ability and poor chemical stability under operation condition(to CO₂ and H₂O)have prevented the application of this kind of materials in CMFC.Recently,the chemical stability of BaCeO₃ based material has been greatly improved by Zr substitution, which shows potential application.In this work,we developed a situ reaction,by using the oxide and carbonates that be used to synthesize the electrolyte,combined with suspension coating process to prepare proton conductor,and dense electrolyte membranes have been successfully fabricated on porous anode supports.The Ba(Zr_0.1Ce_0.7Y_0.2)O₃(BYCZ)electrolyte membrane fabricated through this innovation technology(the thickness is 20μm),with Sm_0.5Sr_0.5CoO₃/BaY_0.2Ce_0.8O₃(SSC/BYC)as the cathode,could reach the open circuit voltage of 1.010,1.032,1.052 V at 700, 650 and 600℃,respectively,which are very close to the EMF,indicating that the electrolyte membrane is very dense.The maximum power densities of 582,426 and 311 mW/cm² have been achieved correspondingly.Through membrane fabrication process,the cell performance of BaCeO₃ based CMFC was achieved as high as that of oxide electrolyte(for example,SDC)CMFC.What's more,this process has applied to fabricate a more complex compound material(contain 6 metal components): Ba(Ce_0.4Pr_0.1Zr_0.3Y_0.16Zn_0.04)(BYCZPZ).With BYCZPZ electrolyte,the OCV were 0.947,0.974 and 0.986V,and the maximum power densities were 597,427 and 295 mW/cm²,respectively at 700,650 and 600℃.When the cell operated at a voltage of 0.5V at 600℃,the out-put current density increased 40%after 240h testing.However, the reason for such phenomenon still need to be further investigated.All the data achieved in our results indicated that this innovation technology not only could realize the thin electrolyte membrane fabrication,but also suitable for the exploration and research of new materials system,which provides a new technology route for the exploration of new proton conductor materials and low cost fabrication method for CMFC.5.Direct liquid methanol fueled CMFCCompared with other categories fuel cells,the outstanding advantage of SOFC/CMFC is its great fuel flexibility,because they could use cheap and easy get hydrocarbon fuels.However,CMFC with Ni-based anodes suffers a number of drawbacks while using hydrocarbon fuels due to the propensity of Ni to catalyze carbon formation,resulting in the rapid degradation of the cell performance.In recent years,the research efforts have been put on solving the coking problem,but no effective way has been found.At the other hand,the first-choice commercial application of CMFC technology is portable system,so the liquid state fuels would be a better choice.To solve the carbon coking problem,our group,for the first time,put forward the new energy route of fueled CMFC with ammonia and methanol.In the past three years,the cell performances ammonia fueled CMFC have been increased many times in our group and achieved practical use lever.My work was focused on directly fueled CMFC with methanol.With SDC electrolyte,maximum power densities of 698,430 and 223 mW/cm² have been achieved respectively at 650,600 and 550℃,by using methanol as fuel.To investigate the endurance of the cell with methanol as fuel,the cell was operated at a voltage of 0.5V at 550℃for about 75hrs, then at 600℃for 5hrs and finally at 650℃for more than 80hrs until the methanol was exhaust.No detectable decay in the cell current was observed within the operating duration.This result surely demonstrates no degradation of the cell performance,indicating no coking on anode.That result is a great break through for CMFC commercialization by directly using liquid hydrocarbon fuels6.High performance interconnect material study and membrane fabricationBoth key materials and fabrication techniques developed so far for single cell of SOFC/CMFC have achieved a rather high level.However,we need to use interconnect materials to connect the series of single cells to achieve higher output power density to meet the requirement for practice.The performance of the interconnect material would determine the output performance of the cell stack,which is the key material for CMFC commercialization.Though,the planar SOFC operated at intermediate temperatures could use much cheap alloy interconnect,the surface of alloy should be protected by a doped LaCrO₃ coating,which was difficult.For tubular SOFC,ceramic interconnect materials is the only choice.Now,the successfully used ceramic-based interconnect material is doped LaCrO₃,however,this kind of materials show poor sinter ability and low electric conductivity at intermediate temperatures.In recent years,we have devoted great effect to decrease its sintering temperature by using A-site substitution and/or adding doped CeO₂,and the bulk density could achieve as high as 97%of the theory density at around 1400℃.This work was aimed at further decreasing the sinter temperature of La_0.7Ca_0.3CrO₃(LCC)by doping Zn at B site:La_0.7Ca_0.3Cr_0.95Zn_0.05O₃(LCCZ).In our result,the density could reach 95.2%at 1200℃and 98%at 1350℃.The electric conductivity both in air and hydrogen increased notably,which were 47.5 and 6.15 Scm^-1at 800℃,respectively,even at 500℃,there still as high as 37.2 and 2.06Scm^-1.What's more,the thermal expansion coefficient of such material is 11.5×10^-6/K,which could meet the need for interconnect materials.Also we try to fabricate LCCZ membrane on NiO/YSZ anode supports by using suspension coating process.The LCCZ membrane with the thickness of about 40μm was adhered well with the porous anode support,however, much work should be done to make the interconnect membranes more dense.