optimizing the manufacturing method of half-cell fuel cell

Iranica Journal of Energy & Environment 3 (4): 341-346, 2012

ISSN 2079-2115

IJEE an Official Peer Reviewed Journal of Babol Noshirvani University of Technology

DOI: 10.5829/idosi.ijee.2012.03.04.08

Optimizing the Manufacturing Method of Half-Cell Fuel Cell Based onSolid Electrolyte with Hydrogen Ion Conductivity

Naghmeh Mirab, Zahra Sherafat and Mohammad Hossein Paydar

Abstract: Barium cerate-based perovskite oxides are protonic conductor candidates for intermediate temperaturesolid oxide fuel cells due to their high ionic conductivity and good sinterability. The aim of the present study is tofabricate a half-cell single-cell includes substrate, anode and electrolyte layers. The exact composition ofBaZr0.1Ce0.7Y0.2O3─δ (BZCY7) has been selected as a proton conducting electrolyte. The fabrication process of adense electrolyte membrane on a NiO- BaZr0.1Ce0.7Y0.2O3─δ (NiO-BZCY7) anode substrate has been studied by a co-pressing process after co-firing at 1400ºC. BZCY7 powders were synthesized by solid-state reaction method aftercalcinations at 1150ºC. A single phase was obtained at this low temperature. The phase composition of the resultingspecimens was investigated using X-ray diffraction (XRD) analysis. Scanning electron microscope (SEM) was usedto evaluate the features of the synthesized powders and also the condition of connected layers in half-cell.

Key words: Solid oxide fuel cell; Proton conducting electrolyte; Solid-state synthesis; Dry pressing.

INTRODUCTION

There is an increasing worldwide interest for energysupply due to population growth and industrializationin developing countries. But fossil fuels that are themain energy source cannot properly respond to thisgrowing demand. Moreover, excessive fuelconsumption will lead to increasing pollution andglobal warming. Therefore, discovering of analternative energy source is very important and manyresearches have been done in this field [1]. In thisscenario, solid oxide fuel cells (SOFCs), which areelectrochemical devices that are able to directlyconvert chemical into electrical energy, are promisingcandidates. They have high-energy conversionefficiency, wide application range (stationary andmobile applications are both possible), and fuelflexibility [2].

Most SOFCs are based on ceramic electrolytematerials with oxygen ion conductivity, which requireoperation temperatures in the range of 800–1000 °C[3, 4]. These conditions lead to high costs due to theneed for using ceramic interconnects, expensive rawmaterials and materials degradation [5]. In addition,this high operating temperature represents the maindrawback for SOFC practical applications, because oflong start-up time, large energy input to warm up thecell, and the occurrence of thermal stresses that reducethe cell lifetime. Lowering the operating temperatureof SOFCs is required not only to reduce the costs, butalso to develop miniaturized SOFCs for portable-power supply [2].

At the intermediate temperature range, the ionicconductivity of proton conductors is more than oxygenion conductors. Because of the activation energy forproton transfer with smaller ionic radius is generallylower than that for oxygen-ion conducting electrolytes[5]. Therefore proton conducting ceramics have thecapability to be used as an electrolyte in IT fuel cells[3, 4]. These compounds show proton conductivitywhen exposed to hydrogen or water-vapor containingatmospheres, achieving suitable conductivity values inthe IT range [2]. Furthermore, the use of protonconductors in fuel cells presents other advantages withrespect to oxygen ion conductors. For instance in theconventional oxygen ion conductor electrolytes, thefuel concentration is unduly diluted at the anode bythe formation of oxidation products such as watervapor and carbon dioxide.

The Nernst potential difference falls reducing thecell efficiency, while for protonic electrolytes the fuelcan be fully depleted in the anode reaction, and thesystem operates ‘dead-ended’ maintaining the fuelpressure to compensate for hydrogen flux as ionsthrough the electrolyte. Therefore, the use of protonconductors as electrolytes is a promising alternativethat might solve the limits of SOFCs [5].

BUT

In the attempt to develop SOFCs with the abilityof operating at intermediate-temperature (IT) range,500–700 °C, the ohmic resistance of the electrolyteneeds to be reduced. This can be achieved with the aim of using alternative materials as electrolyte that have a higher ionic conductivity than that of oxygen ionconductors in the IT range [2].

Department of Materials Science and Engineering, Faculty of Engineering, Shiraz University, Shiraz, Iran

*Corresponding Author: Department of Materials Science and Engineering, Faculty of Engineering, Shiraz

University, Shiraz, Iran.E-mail: [email protected]

Iranica J. Energy & Environ., 3 (4): 341-346, 2012

Many acceptor doped perovskites exhibitproton conductivity in an intermediate temperaturerange between 300 and 700 °C and thus are goodcandidate electrolyte materials for intermediate-temperature SOFCs. In these compounds, presence ofdopant is necessary. In fact oxygen vacanciesgenerated by doping, play an important role in theformation of proton conducting properties includingthe formation and transport of protons [6, 7].However, these compounds usually do not combinethe three important requirements for electrolytematerials including high ionic conductivity, chemicalstability, and good sinter ability [8]. Doped bariumcerate electrolytes represent high protonicconductivity and satisfactory sinterability, but theyreact with acidic gases (e.g., CO2 and SO2) and steam[9, 10]. On the other hand, chemically stable dopedbarium zirconate sintered pellets generally have a lowtotal proton conductivity because of the large volumecontent of poorly conducting grain boundaries [11,12].

A proposed solution that is taken by theresult of extensive research of a group of researchersincluding Badwal, is using mixed solid solutions ofperovskite structures to achieve a compromisebetween the properties of both materials [13]. Sinceboth compounds, barium zirconate and barium cerate,are soluble in any proportion, replacement ofappropriate amount of Zr into barium cerate couldmake a compound with suitable chemical stability andhigh proton conductivity. Between these chemicalcompositions, BZCY7 is an appropriate choice forusing as an intermediate temperature electrolytebecause it is shown the desired properties in a widerange of working conditions of the fuel cell [14].

According to the subjects mentioned above, thecomposition of BZCY7 has been selected as anelectrolyte. But up to now, it is still a challenge toprepare fuel cell by a cost-effective and efficienttechnology. In this work, experiments conducted on anall-solid-state process, which is possible to furthersimplify the cell preparation. Also dry-pressingmethod was applied to decrease the cost of cellfabrication. Therefore, the main purpose of this studyis to fabricate the layers of fuel cell (includessubstrate, anode and electrolyte) with co-pressingmethod and then optimize fabrication process.

EXPERIMENTAL

Preparation of powders: The BZCY7 powders weresynthesized by a solid-state reaction method. Bariumcarbonate (BaCO3), zirconium oxide (ZrO2), ceriumoxide (CeO2) and yttrium oxide (Y2O3) powders withthe minimum purity of 99.9% and the average grainsize of about 1 micron were used as raw materials.The powders were weighed in the stoichiometric ratiosand then mixed using an attrition mill with 5 mm indiameter stabilized zirconia ball media in ethanol for 5h. The resultant mixture was then dried at 100 ˚С for 1h, followed by calcination at 1150 ˚С for 5 h. The

attrition and calcination process under the samementioned conditions, were repeated twice to obtainpure phase.

These pre-BZCY7 powders were added topure nickel oxide (NiO), with grain size of less than10 micron, in a weight ratio of 40:60 to prepare anodepowder. This mixture was well mixed by the attritionmill in ethanol for 1 h. To produce a final sample withenhanced strength, due to the low thickness of theanode and the electrolyte, these layers were fabricatedon the anode substrate. This substrate was produced ina similar way of the anode production and just tocreate sufficient porosity in this layer; 20wt% cornstarch was added to the anode material as the poreformer.

Fabrication of tri-layer anode-supported half-cells:After producing powders of NiO-BZCY7-Starch/NiO-BZCY7/BZCY7 (substrate/anode/electrolyte), the tri-layer anode-supported half-cells were fabricated byuniaxial dry-pressing method. The thickness of thelayers was controlled by varying the amounts ofpowders used.

First, substrate powders were pressed under65 MPa pressure with 20 mm in diameter and 0.5 mmin thickness to form the anode substrate with a flatsurface and acceptable mechanical strength. Then theanode layer with thickness of 0.1 mm was pressedunder 160 MPa on the pressed surface. At the end, athin and dense electrolyte layer with a thickness ofabout 0.07 mm was pressed together with the pre-pressed two layers of anode and substrate under 200MPa pressure. To enhance green strength ofelectrolyte layer, exact amount of 3wt% of a 2wt%PVA solution was used as a binder.

Finally, a green tri-layer sample from thepress, were sintered under a suitable process. The finalsintering temperature was selected at 1400 ˚С for 5 h.The green half-cells were heated to the targettemperature, step by step to remove binder and starchat 600 and 800 ˚С, respectively.

Characterization of the phase composition andmicrostructure of the cell component: Phasestructure and composition of the produced powderswere identified by X-ray diffraction (XRD) analysisusing CuKα radiation. The microstructures of theproduced powders were investigated by scanningelectron microscopy (SEM) and also the fracturesurface of a produced half-cell was imaged by SEM toensure the accuracy of manufacturing process andconnectivity of various layers together and alsodetermine the size and percentage of remainingporosities.


In order to examine the densificationbehavior of various layers in half-cell(substrate/anode/ electrolyte), single-layer sampleswere prepared and then sintered at differenttemperatures of 800, 1000, 1200, and 1400 ˚С for 5 h.Then the changes in volume and the relative density ofthe samples were investigated. Shrinkage variationsduring sintering of each layer were calculated withmeasuring the dimensions of the single-layer samplesthat were pressed separately, before and aftersintering, by caliper. The relative density of thesamples was obtained based on Archimedes method.

RESULTS AND DISCUSSION

Fig. 1 shows the XRD patterns of BZCY7 andBZCY7+NiO powders. As it can be seen in Fig. 1(a),only the peaks corresponding to BZCY7 withperovskite structure can be found and there are not anypeaks attributable to impurities or unwanted phases. Inthe pattern of anode powder shown in Fig. 1(b), thepeaks related to NiO were clearly detectable so itconfirms the presence of nickel oxide in anodecomposition.

Fig. 1: XRD patterns of BZCY7 (a) and NiO+BZCY7(b), electrolyte and anode powders respectively.

Fig. 2 shows the morphology of the BZCY7electrolyte and NiO-BZCY7 anode powders obtainingby scanning electron microscope. The powders,consisting of particles about 0.5 microns in size,agglomerate together. This structure is typical forpowders prepared via a solid state reaction method.

Fig. 3 shows variations of shrinkage versussintering temperature, and indicates the shrinkagebehavior of each layer.

It is obvious that the shrinkage of anodesamples are more than others. Difference in shrinkagebehavior of layers will cause disintegration of tri-layersamples, because of the sintering of the thin-filmelectrolyte on the anode substrate is actually aconstraint sintering process.

Fig. 2: SEM images of the BZCY7 electrolyte (a) andNiO-BZCY7 anode powders (b)

Since anode and substrate layers together are morethan 6 times thicker than electrolyte layer, sintering ofelectrolye layer is more affected by anode layer. Asthe presence of porosity in the anode layer willincrease cell efficiency, so introducing porosity in theanode layer in so much that equalize shrinkagebehavior of anode and electrolyte layers can helpfabrication of tri-layer samples with high quality withno undesirable effect on cell performance. For thispurpose, an appropriate percentage of pore formermaterials (such as starch or graphite) was added to theanode powder. It should be noted that due to thepresence of porosity in the substrate layer, itsshrinkage will be affected by the anode layer.Therefore, the difference in the sintering behavior ofthe anode and the substrate layers will not cause aproblem. Variations of relative density versussintering temperature presented in Fig. 4, showssimilar trend as shrinkage variations.

To investigate the microstructure and to beensure about the accuracy of manufacturing process,the fracture surface of a produced half-cell wasimaged after sintering using SEM. Fig. 5 shows thecross-sectional view of the tri-layer half-single-cellafter co-firing at 1150 ˚С.

As shown in Fig. 5, the three distinct layers(the anode substrate, the active anode layer and theelectrolyte), can be easily distinguished. The smoothand recognizable interfaces between the layers

(a)

(b)


represent no inter diffusion between the differentlayers.

Fig. 3: Shrinkage variations in separate layers ofelectrolyte, anode and anode-substrate during sinteringat different temperatures for 5 h

Fig. 4: Relative density variations in separate layers ofelectrolyte, anode and anode-substrate during sinteringat different temperatures for 5 h

Fig. 5: SEM images of the fracture surface of the tri-layer sample

Figs. 6 and 7 show the fracture surface images of thesample at higher magnifications. Therefore, only twolayers of them are visible and as the result, theconnection of layers can be investigated moreaccurately.

Fig. 6: The fracture surface SEM images of theinterface between the anode active layer and substrate

As these images show clearly,the layers connected toeach other very well and no porosity observed in theinterfaces. Fig. 7 indicates that the BZCY7 membraneis quite dense, without any obvious cracks or pores,which points that the BZCY7 membrane can reachdense after sintering at the relatively low temperatureof 1150 ˚С.

Fig. 7: The fracture surface SEM images of theinterface between the anode active layer andelectrolyte

The microstructure of anode observed by SEM wasnot porous enough. Since the porosity of the activeanode layer leading to increase the single cellefficiency. Therefore, a material which producesporosity can be added but with far less percentage ofwhat added to substrate. Fig. 8 shows SEM image of asingle-layer anode in which 5wt.% of 2wt.% PVAsolution was added in order to create porosity.


Fig. 8: SEM image of fracture surface of the single-layer anode sample

Fig. 9: SEM images of fracture surface of (a) the tri-layer sample, (b) the interface of substrate-anode (c)the interface of anode-electrolyte

As it is clear in the image and it wasexpected, the addition of PVA solution lead toincrease the percentage of porosity in the anode layer.In this condition, not only a tri-layer sample was notbroken during sintering, but also the connection of thelayers improved. Due to increasing porosity, the cellwill produce a better performance. The image of thetri-layer sample produced with this composition ofanode, which is more porous, is shown in Fig. 9. Inthis sample, due to better pressability and sinterability,the connectivity of layers improved which is visible athigher magnifications in Fig. 9 (b) and (c).

CONCLUSION

Based on the XRD analysis, BZCY7 and NiO-BZCY7 compositions were producedsuccessfully by using solid-state synthesis as asimple and cost-effective method.

By the SEM images, it can be distinguished thatsolid-state synthesis, to produce powders, andthe uniaxial dry-pressing method can be used tofabricate half of the single-cell and thus theproduction costs can be significantly reduced.

By adding the appropriate amounts of PVA asthe pore former materials to the anode powder,the shrinkage of this layer was controled anddisintegration of a tri-layer sample duringsintering was avoided.

By increasing the percentage of PVA to theanode powder, in addition to preventing thedisintegration of three layers, pressability andsinterability also increased and thus theconnectivity of layers improved.

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optimizing the manufacturing method of half-cell fuel cell

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