research proposal by wody PCFC.doc

Why “proton conducting materials”?Fuel cell, a kind of electrochemical devices that can allow direct conversion from chemical energy to electrical energy, has been given increasingly more attention due to its high conversion efficiency and negligible pollutant emission1. Among the various types of fuel cells, SOFCs have their unique advantages, such as no corrosion, no leakage, being able to use kinds of hydrocarbon gas (such as methane and ethane) or syngas as fuel and their high conversion efficiency2. In recent decades, intense investigation has been performed into SOFC, especially those with oxide-ion conducting electrolyte. And some commercial manufactures3, represented by Siemens Westinghouse Co.4, emerged at the historic moment and contributed much to the practical application and commercialization of SOFC. However, one of the drawbacks that hinders its wide application is the high operating temperature (800-1000)5.Recently, an ever rising number of researchers6 have been attempting to reduce the operating temperature of SOFC and, meanwhile, guarantee the relatively high ionic conductivity. To achieve this goal, two main approaches have been taken: reduce the thickness of electrolyte or develop novel materials that have high ionic conductivity at intermediate temperature. As is known, development of thin-film preparation techniques, such as electrochemical deposition, plasma spraying, tape casting, tape calendaring and et al., makes it not that difficult to fabricate electrolyte with thickness of 10-30 m. However, the electrolyte can never be made too thin as, if so, it becomes easier to be broken down by local overheating or higher voltage and may also be too fragile due to the poor mechanical property7. Hence, developing alternative materials with high ionic conductivity at intermediate temperature appears more practical and potential. BaCeO3 based oxides, first reported by Iwahara8 that can efficiently conduct proton at relatively low temperature, have been paid growing interest as promising proton-conducting materials that can be used in intermediate temperature SOFC. At 600 , the highest proton conductivity of BaCeO3-based oxides is 10-2 S cm-1, larger than that of YSZ at the same temperature, and its electron conductivity is negligible. Compared with conventional oxygen-ion conducting SOFC, another advantage of proton conducting SOFC is the fact that water is produced at the cathode not at the anode, avoiding diluting the fuel gas flowing at the anode. However, a sever problem regarding BaCeO3 as proton-conducting electrolyte is its poor chemical stability9; that is BaCeO3 can react with CO2 and water vapor, which will badly corrode the electrolyte during continuous operation of SOFC. Many strategies, such as doping Zr10-13, which can enhance chemical stability of BaCeO3, always result in decrease of proton conductivity. Thus, more work, either developing novel proton conducting materials or further research into BaCeO3 based oxides, is in need to couple good chemical stability and high proton conductivity.What has been done to combine chemical stability and high proton conductivity?An obvious tendency of SOFC development is to use diversified fuel, such as syngas (containing CO, H2 and H2S) and hydrocarbon gas (methane, ethane)14, instead of pure H2 and this will generate CO2 at high concentration, which calls for adequate chemical stability of BaCeO3 based electrolyte. Recently, intense work has been done to reach this goal.The most common strategy to strengthen chemical stability of BaCeO3 is to partially replace Ce with Zr. Zuo et al.15 reported that chemical stability of BaCeO3, doped with 10 mol% Zr, was relatively improved, with a slight decrease of proton conductivity. At 600 , proton conductivity of BaCe0.7Zr0.1Y0.2O3- is 10-2 S cm-1 and at 550 , its ionic conductivity is higher than that of YSZ and DCO. Although BaCe0.7Zr0.1Y0.2O3- has been accepted and employed to fabricate proton conducting electrolyte in some studies16, 17, Zr doping concentration of 10 mol% seems not sufficient to guarantee adequate chemical stability of the proton conducting electrolyte. However, larger Zr doping concentration may lead to other problems including low proton conductivity and poor sinterability. Addition of sintering aids, e.g. ZnO, which can improve the sinterability of BaCeO3 doped by Zr to a certain scale, can not completely tackle the problems of low proton conductivity and large grain boundary resistance.To overcome the drawbacks of Zr doping into BaCeO3, some other quadrivalent cations, e.g. Ti4+, were employed as dopants to enhance chemical stability of BaCeO3. Xie et al.18 reported that BaCe0.7Ti0.1Y0.2O3- had remarkably improved chemical stability, although with slightly lower conductivity, compared with BaCeO3. As for sinterability, BaCe0.7Ti0.1Y0.2O3- acted better than BaCe0.7Zr0.1Y0.2O3-. Single fuel cell, with BaCe0.7Ti0.1Y0.2O3- thin film (20 m) as electrolyte, provided largest power density of 280 mW cm-2 at 700 and did not show remarkable attenuation within a continuous period of 18 h. However, chemical stability of BaCe0.7Ti0.1Y0.2O3- electrolyte in higher content CO2 and in a longer testing period remains to be investigated.Cations at other valence state, such as Ta5+ and In3+, have also been taken as dopants into BaCeO3 to improve its chemical stability. Bi et al.19 found that BaCe0.7Ta0.1Y0.2O3- was more stable than BaCe0.8Y0.2O3-, with humidified H2 as fuel. Whereas, when the fuel cell is fed by hydrocarbon gas, the chemical stability of BaCe0.7Ta0.1Y0.2O3- remains to be studied. BaCe0.7In0.3O3- was also proved to be more stable than BaCeO3 and have better sinterability; however, its proton conductivity was much lower20. Apart from those ABO3 perovskite-type oxides, some other novel perovskite composite oxides, e.g. Ba3Ca1+xNb2-xO9-, have also been developed and investigated as potential proton conducing materials used in SOFC. Du et al.21 reported that, at 800 , proton conductivity of Ba3Ca1.18Nb1.82O9- (BCN18) was higher than that of Nd doped BaCeO3, and its ionic mobility was as high as 1, even without electron conductivity. However, in several further reports, significant differences and even complete conflicts were found among the conclusions. Proton conductivity of BCN18 obtained by Nowick was 10-2 S cm-1, two orders of magnitude higher than that (5.5*10-4 S cm-1) reported by Norby22. Another problem of BCN18 is its bad sinterability, which results in great challenges for its membranization. Micronizing BCN18 into nanoparticles may help to improve its sinterability. Other solid oxides that have been studied as potential proton conductor include La0.9Sr0.1Ga0.8Mg0.2O3-, La0.99Ca0.01NbO4, La0.99Ca0.01TaO4, La1.95Ca0.06Zr2O7 and Eu2Zr2O7. These materials all show better chemical stability compared with conventional BaCeO3 based oxides; however their either much lower proton conductivity or poor sinterability severely impedes their practical application as electrolyte in SOFC. What should we do next?In conclusion, developing proton conducting thin-film electrolyte is a promising strategy to reduce operating temperature of SOFC. This strategy calls for adequate chemical stability of the proton conductor under operation condition, together with high proton conductivity. To achieve this goal, approaches can be followed: 1) doping conventional ABO3 perovskite-type oxides, e.g. BaCeO3; 2) developing novel proton conducting materials. However, progress in BaCeO3s chemical stability has not been significant enough to allow its practical application in SOFC; and these existing “novel proton conductors” does not seem to show many comparative advantages with BaCeO3 based materials. Hence, further work is still in severe need to optimize the chemical stability, ionic conductivity, sinterability and microstructure of BaCeO3 based proton conductor; and furthermore, significant efforts ought to be made to unravel novel proton conducting materials. My ideas about proton conducting materials1. Study novel BaCeO3 doping system, with Co, Y23-25, W, Ti, Sc, Sm, Mn, Sr, Nd26-28 and et al. or mixture of them as dopants2. Utilization of novel sintering techniques, such as Spark Plasma Sintering (SPS)29 and Selected Laser Sintering (SLS)30, 31, to cope with the problems of poor sinterability.3. Establish mathematical models32-35 to simulate the mechanism of proton conducting in various BaCeO3 based oxides, reconstruct the microstructure of the doped proton conductor and investigate the effect of its microstructure on the chemical stability and ionic conductivity.4. Employ efficient particle-engineering techniques, e.g. microfluid based techniques and supercritical CO2 based techniques (SAS process, SEDS process, SpEDS process)36 on which I have relatively rich experience, to generate ultrafine particles of ionic conducting oxides, improve the electrolytes sinterability and reduce its grain boundary resistance.5. Investigation of materials synthesis strategy6 and the detailed procedure37. Take close insight to novel materials-synthesis methods38-41. 6. Alter existence form of electrode catalyst into ultrafine particle, nanotube42, ultrafine fiber, porous particles and et al. to improve the catalytic activity and decrease activation loss. Reference:1 Bernay C, Marchand M, Cassir M. Prospects of different fuel cell technologies for vehicle applications. Journal of Power Sources. 2002;108:139-52.2 Raj NT, Iniyan S, Goic R. A review of renewable energy based cogeneration technologies. 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