high performance of sdc and gdc core shell type composite electrolytes using methane as a fuel for low temperature sofc

High performance of SDC and GDC core shell type composite electrolytes using methane as a fuel for low temperature SOFC , , Muneeb Irshad, Khurram Siraj , Rizwan Raza , Fayyaz Javed, Muhammad Ahsan, Imran Shakir, and Muhammad Shahid Rafique Citation: AIP Advances 6, 025202 (2016); doi: 10.1063/1.4941676 View online: http://dx.doi.org/10.1063/1.4941676 View Table of Contents: http://aip.scitation.org/toc/adv/6/2 Published by the American Institute of Physics AIP ADVANCES 6, 025202 (2016) High performance of SDC and GDC core shell type composite electrolytes using methane as a fuel for low temperature SOFC Muneeb Irshad,1 Khurram Siraj,1,a Rizwan Raza,2,a Fayyaz Javed,1 Muhammad Ahsan,1 Imran Shakir,3 and Muhammad Shahid Rafique1 Department of Physics, University of Engineering and Technology, Lahore, Pakistan Department of Physics, COMSATS Institute of Information Technology, Lahore, Pakistan Deanship of scientific research, College of Engineering, PO Box 800, King Saud University, Riyadh 11421, Saudi Arabia (Received October 2015; accepted 20 January 2016; published online February 2016) Nanocomposites Samarium doped Ceria (SDC), Gadolinium doped Ceria (GDC), core shell SDC amorphous Na2CO3 (SDCC) and GDC amorphous Na2CO3 (GDCC) were synthesized using co-precipitation method and then compared to obtain better solid oxide electrolytes materials for low temperature Solid Oxide Fuel Cell (SOFCs) The comparison is done in terms of structure, crystallanity, thermal stability, conductivity and cell performance In present work, XRD analysis confirmed proper doping of Sm and Gd in both single phase (SDC, GDC) and dual phase core shell (SDCC, GDCC) electrolyte materials EDX analysis validated the presence of Sm and Gd in both single and dual phase electrolyte materials; also confirming the presence of amorphous Na2CO3 in SDCC and GDCC From TGA analysis a steep weight loss is observed in case of SDCC and GDCC when temperature rises above 725 ◦C while SDC and GDC not show any loss The ionic conductivity and cell performance of single phase SDC and GDC nanocomposite were compared with core shell GDC/amorphous Na2CO3 and SDC/ amorphous Na2CO3 nanocomposites using methane fuel It is observed that dual phase core shell electrolytes materials (SDCC, GDCC) show better performance in low temperature range than their corresponding single phase electrolyte materials (SDC, GDC) with methane fuel C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4941676] I INTRODUCTION Electric power has become the essential part of present life Economical and sustainable energy resources are the backbone of current humanity For industrial progress and financial growth of any country, environmental friendly and steadfast power resources are compulsory to enhance the living standards of the human being.1,2 Decrease in the resources of fossil fuels strained the researchers to consider the alternative energy resources The fuel cell technology amply fulfills to execute the demands of electricity production with high efficiency, lofty stability and environment friendly Fuel Cells have three major components i.e., electrolyte (ion conductor), anode and cathode (electronic and ionic conductor), which can generate electric power as far as incessant fuels (air/oxygen at cathode and fuel at anode) are supplied.3 Among all types of fuel cells, Solid Oxide Fuel Cells (SOFCs) are most attractive due to their higher efficiency, versatility in fuels and low environmental pollution Scientists and engineers are endeavoring hard to lower SOFCs working temperature Low temperature SOFCs can be obtained by increasing ionic conductivity of electrolytes and improving a Corresponding author Tel.: 00923338299000 Email Address: razahussaini786@gmail.com and khurram.uet@gmail.com 2158-3226/2016/6(2)/025202/9 6, 025202-1 © Author(s) 2016 025202-2 Irshad et al AIP Advances 6, 025202 (2016) reaction rate at electrodes,4,5 which is realizable by introducing new materials and/or different fuels (like H2, C3H8, H2S, CH4 etc) along with reduced manufacturing cost and improved life time of cell components Many materials have been used to improve the ionic conductivity of electrolytes such as Scandia stabilized Zirconia (ScSZ), Yttria stabilized Zirconia (YSZ), Yttria doped-Ceria (YDC), Samarium doped Ceria (SDC), Gadolinium doped Ceria (GDC) and many other ceria composites.3–11 At low temperatures (< 700 ◦C), ionic conductivity of ceria based electrolytes is higher than the zirconia based electrolytes12 making ceria based electrolytes more attractive The ionic conductivity depends upon the crystalline quality and composition of the materials The crystalline structure, crystallite size, grain boundaries and composition of materials can be optimized to get higher ionic conductivity.12,13 Pure CeO2 fluorite type structure shows both electronic and ionic conductivities but its oxygen ion conductance is poor Ionic conductivity of CeO2 can be enhanced by adding low valence dopant cations of rare earth metals e.g Sm+3 or Gd+3, which increase oxygen vacancies and augment both ion and electronic conductivities.6,12 Although doped Ceria based electrolytes show better performance at temperatures ranging from 450 to 700 ◦C but their efficiency remains lower due to mixed conduction The performance of the cell can significantly increased by increasing the ionic conductivity of the electrolyte In order to improve the ionic conductivity of ceria based electrolytes, different approaches have been taken by different researchers.6,12,13 One way to reduce the electronic conductivity and further enhance the ionic conductivity of doped ceria based materials is the use of alkaline salts The addition of the alkaline salts in the doped ceria creates a second phase of salts along with the matrix phase doped ceria.12,13 The flexibility of SOFC in terms of its fuel makes it most cost effective due to removal of the need of reformers needed for conversion of hydrocarbon into hydrogen The most widely available hydrocarbon in the world is the methane It is not only main component of natural gas and coal gas but also of biogas and therefore can be used as renewable energy resource A lot of attention is focused on using methane as fuel for SOFC since it does not need any reformer.14–16 The routes available for using methane as fuel for SOFC are; external reforming, direct electrochemical conversion and internal reforming.17–19 The direct use of hydrocarbon for the generation of energy can result into deposition of carbon at high temperatures.20–22 In present work, doped ceria and carbonate ceria composite electrolytes are synthesized; their structure, thermal stability, ionic conductivity and cell performance are investigated and compared using methane gas as fuel II EXPERIMENTATION Gadolinium doped Ceria (GDC), Samarium doped Ceria (SDC), Gadolinium doped Ceria carbonate (GDCC) and Samarium doped Ceria carbonate (SDCC) were synthesized using one step carbonate co-precipitation method.8 The molar ratio of Samarium and Gadolinium with Ceria is taken as 1:4 each for (Ce0.8Sm0.2O1.9) and (Ce0.8Gd0.2O1.9) stoichiometry SDC and GDC powders were synthesized by dissolving Sm(NO3)3.6H2O (Sigma-Aldrich) and Gd(NO3)3.6H2O (Sigma-Aldrich) in deionized water with above mentioned stoichiometric ratio through magnetic stirrer Na2CO3 (Sigma-Aldrich) is slowly mixed as a precipitation agent with SDC and GDC with molar ratio 2:1 The mixture obtained in each case is filtered and dried at 100 ◦C in an oven for hours The powder obtained is grinded using motor pestle and sintered at 650 ◦C for hours Pellets were prepared using hydraulic press at 300 MPa pressure for cell performance The scheme of cell fabrication is shown in Table I Weight ratio of anode, cathode and electrolyte was taken 0.30 g, 0.30 g and 0.25 g respectively for trilayer cell The pellets of 13mm diameter and 2mm thickness were prepared and silver paste was applied to both sides of pellets for electric measurements The synthesized powders were analyzed using X-ray Diffractrometer (Philips X’pert pro super Diffractrometer 00-001-800), Thermo gravimetric analysis (TA TGA Q500), Energy Dispersive analysis and Scanning Electron Microscopy (HITACHI S-3000H) for crystal structure, surface analysis, physical properties and elemental analysis respectively The ionic conductivity at different temperatures was measured using two probe method 025202-3 Irshad et al AIP Advances 6, 025202 (2016) TABLE I The scheme of anode, cathode and electrolyte for SOFC Cell No Cell Cell Cell Cell Anode Electrolyte Cathode LiNCZ + GDC LiNCZ + GDCC LiNCZ + SDC LiNCZ + SDCC Sample (GDC) Sample (GDCC) Sample (SDC) Sample (SDCC) BCCF + GDC BCCF + GDCC LSZF + SDC LSZF + SDCC III RESULTS AND DISCUSSION Fig shows the XRD patterns of GDC, GDCC, SDC and SDCC containing similar cubic fluorite structure No plane of gadolinium and samarium is observed that is due to their proper doping in ceria lattice The absence of Na2CO3 plane in XRD pattern shows its amorphous nature Therefore, it may be inferred that SDC and GDC electrolytes are in crystalline phase and composite electrolytes (GDCC, SDCC) are in two distinct phases (crystalline and amorphous phases) In composite core shell electrolytes, crystalline phase of GDC and SDC signify core and amorphous phase of Na2CO3 represents shell.23,25 The results based on XRD patterns depict that neither new compound formation take place nor chemical reaction occurred between crystalline and amorphous phase The average crystallite sizes of all samples based on CeO2 (111) reflection peaks were determined by the Scherer’s formula given below; D= 0.89λ β cos θ (1) Where λ is X-ray wavelength, θ is the angle between sample and X-ray beam, β represents full width at half maxima position and D is the crystallite size Table II represents the calculated crystallite size and lattice constant values The lattice constant of pure CeO2 is 5.41 Å, which is smaller than the intended lattice constant (5.44 Å) that is conformity to Vegard’s rule.24 It can be also be observed that lattice constants of SDC and SDCC are greater than GDC and GDCC respectively because ionic radii of Sm+3 (1.22Å) is larger than Gd+3 (1.19Å).26 Fig 2(a)-2(d)) represents the EDX analysis of SDC, core shell SDCC, GDC and core shell GDCC Fig 2(a), 2(c) show the presence of samarium and gadolinium within the CeO2 while XRD in Fig confirms their doping Fig 2(b), 2(d) shows presence of samarium and gadolinium along FIG XRD patterns of GDC, GDCC, SDC and SDCC 025202-4 Irshad et al AIP Advances 6, 025202 (2016) TABLE II The crystallites size and lattice constant of doped ceria and ceria carbonate composite electrolytes Name SDC SDCC GDC GDCC Crystallite size (nm) Lattice constant (Å) 57 5.44 56 5.44 60 5.42 59 5.42 with the Na2CO3 over the SDC and GDC particles in amorphous phase as confirmed by no plane in XRD pattern and by earlier reported results The surface morphology of GDC, GDCC, SDC and SDCC electrolytes were analyzed using SEM as shown in Fig Fig 3(a), 3(c) shows that agglomerates were present in the GDC and SDC electrolyte materials, which were formed due to the high temperature calcinations While in case of GDCC and SDCC composite electrolyte samples, agglomerates were reduced due to covering of molten carbonate over the surface of samples during composition process (Fig 3(b) and 3(d)) Moreover, when composite electrolytes were directly taken out from oven, these carbonates froze rapidly on the core of GDC and SDC The frozen layer of carbonate generates a core-shell on the composite particles.27 The thermal analysis of electrolytes samples was carried out with heating rate of 15 ◦C min−1 in air The permanence/stability of sodium carbonate phase in high temperature environment of SOFC is also serious issue because ambient environment might results into decomposition of carbonates that would eventually affect the performance of SOFC Fig represents the TGA curves of GDC, GDCC, SDC, and SDCC composite electrolytes The weight loss can be divided into three regions, region A starts from room temperature to 370 ◦C, region B lies between 370 ◦C to 725 ◦C and region C starts above 725 ◦C In region A, all materials show weight loss upto 370o C which is due to evaporation and decomposition of physisorbed water, oxygen and hydroxide present on the surface of SDC, GDC, SDCC and GDCC powders (since synthesis was done in water by co-precipitation) In region B, no weight loss is observed which shows that no decomposition reaction for all four materials is occurring in this region As temperature increases from 725 ◦C, TGA curves show a steep weight loss occuring only for SDCC and GDCC indicating the volatilization of sodium carbonate as reported in previous studies too.28 These results verify the presence of Na2CO3 in SDCC and GDCC composite electrolytes These TGA curves designate that neither intermediate compound nor chemical reaction take place between SDC and GDC with Na2CO3 salt It can also be concluded that best operating temperature for the core shell composite electrolyte in SOFC lies between 400- 650 o C otherwise carbonate will evaporate and decompose at higher temperature FIG The EDX analysis of (a) GDC, (b) GDCC, (c) SDC and (d) SDCC 025202-5 Irshad et al AIP Advances 6, 025202 (2016) FIG SEM micrograph of (a) GDC, (b) GDCC, (c) SDC and (d) SDCC Fig shows the ionic conductivity of single phase and dual phase doped ceria composite electrolytes with respect to temperature The dual phase doped ceria materials show higher conductivity than corresponding single phase doped ceria materials It is evident from the Fig that the conductivity of GDC and GDCC with methane varies from 0.002- 0.018 Scm−1 and 0.002-0.022 Scm−1 respectively in temperature range of 250-580 ◦C However, conductivity of SDC and SDCC with methane varies from 0.003 to 0.025 Scm−1 and 0.004 to 0.028 Scm−1 respectively In present work the conductivity of SDCC (0.028 Scm−1) with CH4 is considerably high The solid state ionic theory may facilitate to know the conduction mechanism of composite ceria based electrolytes In GDC and SDC doped electrolytes, single phase is conscientious for ionic conduction while in SDCC and GDCC the addition of Na2CO3 creates another phase Thus co-existence of dual phase can produce a lot of interfacial sites in composite electrolytes, causing increased ionic conduction by FIG TGA of ceria composite electrolyte materials 025202-6 Irshad et al AIP Advances 6, 025202 (2016) FIG Arrhenius Plot of GDC, GDCC, SDC and SDCC in methane atmosphere FIG Fuel Cell performance at 580 ◦C of (a) GDC, GDCC and (b) SDC, SDCC 025202-7 Irshad et al AIP Advances 6, 025202 (2016) FIG Stability of core shell SDCC electrolyte supplying high conductive transportation paths Thus addition of minor carbonate can increase the ionic conduction of doped ceria electrolyte by reducing its electronic conduction Fig 6(a), 6(b) depicts the measured performances of trilayer fuel cells having GDC, GDCC, SDC and SDCC, as electrolytes with Lithium Nickel Copper Zinc-oxide (LNCZ) as anode and Lanthanum Strontium Zinc Iron-oxide (LSZF) as cathode Silver paste was used on both outer surfaces of the trilayer fuel cells for improved electrical contact With methane as a fuel and oxygen as an oxidizing agent, power densities of 359 mWcm−2 and 400 mWcm−2 are obtained in case of GDC and GDCC at temperature of 580 ◦C with open circuit voltage (OCV) 0.643 V and 0.752 V respectively Fig 6(b) shows the power densities of 472 mWcm−2 with OCV 0.812 V for SDC and 520 mWcm−2 with OCV 0.917 V for SDCC with same conditions In distinct phase composites the mechanism of interfacial super ionic conduction can be attributed for the transportation path provided to protons, ions and carbonate.29,30 Due to dual phase existence by addition of alkaline salts, ceria carbonate composite materials have shown excellent performance with methane The better conductivity of core shell electrolytes is due to is the result of amorphous nature of Na2Co3 shell It not only forms a shield over the SDC active surface and interfaces at nano-scale to increase the stability of nano material but also increase the transportation of oxygen ion through interfacial mechanism.31 In addition Na2CO3 core shell also surpasses the electronic conductivity of SDC and GDC electrolyte.32 As a result core shell based electrolytes have better OCV Figure shows the stability of the core shell SDCC electrolyte for 12 hours The core shell electrolytes based cell are considered as thermally stable due to activation of electrodes catalyst and the interfaces of electrolyte and electrodes.33,34 The core shell electrolytes based cell are considered as thermally stable due to activation of electrodes catalyst and the interfaces of electrolyte and electrodes as reported by the other researchers.33,34 IV CONCLUSIONS Composite powders GDC, SDC and core shell powders GDCC, SDCC were synthesized and compared in aspect of their properties and performances in SOFC cell as an electrolyte It is revealed that layer of carbonate shell over the composite particle is responsible for better performance at low temperature compared to SDC and GDC electrolyte The core shell SDCC and GDCC have shown better performance than SDC and GDC respectively due to second alkaline salt phase Excellent performance of 520 mWcm−2 is achieved at 580 ◦C in case of SDCC using methane as a fuel The SDCC is better option for the electrolyte due to its higher electrical conductivity, mechanical strength, chemical stability, low electronic conductivity and high ionic conductivity Hence these composites can be used efficiently as electrolytes in low temperature SOFCs using methane as a 025202-8 Irshad et al AIP Advances 6, 025202 (2016) fuel The power densities and OCVs with methane for core shell electrolytes in present work are excellent ACKNOWLEDGMENT Dr Imran Shakir would like to extend his sincere appreciation to the Deanship of Scientific Research at the King Saudi University for its funding of this research work through the Prolific Research Group PRG-1436-25 One of the authors, also acknowledges the financial support by the Higher Education Commission, Pakistan under International Research Support Initiative Program ISRIP A.B Awan and Z.A Khan, “Recent progress in renewable energy – Remedy of energy crisis in Pakistan,” J Renew Sust Ener 33, 236–253 (2014) H.B Khalil and S.J.H Zaidi, “Energy crisis and potential of solar energy in Pakistan,” J Renew Sust Ener Rev 31, 194–201 (2014) V.V Kharton, F.M.B Marques, and A Atkinson, “Properties of solid oxide electrolyte ceramics: a brief review,” J Solid Stat Ion 174, 135–149 (2004) M Ali, S.A, A Muchtar, N Muhamad, and A.B Sulong, “A review on preparation of 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(2006)  ...AIP ADVANCES 6, 025202 (2016) High performance of SDC and GDC core shell type composite electrolytes using methane as a fuel for low temperature SOFC Muneeb Irshad,1 Khurram Siraj,1 ,a Rizwan... methane It is not only main component of natural gas and coal gas but also of biogas and therefore can be used as renewable energy resource A lot of attention is focused on using methane as fuel. .. conductivity and cell performance of single phase SDC and GDC nanocomposite were compared with core shell GDC/ amorphous Na2CO3 and SDC/ amorphous Na2CO3 nanocomposites using methane fuel It is