A non-aled solid oxide fuel cell micro-stack with two gas channels
Yanting Tian a ,Zhe Lu ¨a ,*,Bo Wei a ,Zhihong Wang a ,Mingliang Liu b ,Wenyuan Li a ,Xiqiang Huang a ,Wenhui Su a
a Center for Condend Matter Science and Technology,Department of Physics,Harbin Institute of Technology,Harbin 150001,China b
School of Science,Beijing University of Chemical Technology,Beijing 100029,China
a r t i c l e i n f o
Article history:
Received 20January 2011Received in revid form 6March 2011
Accepted 10March 2011Available online 13April 2011Keywords:
Non-aled solid oxide fuel cell Micro-stacks Two gas channels
a b s t r a c t
Non-aled solid oxide fuel cell (NS-SOFC)micro-stacks with two gas channels were fabricated and operated successfully under various CH 4/O 2gas mixtures in a box-like stainless-steel chamber.The cells with an anode-facing-cathode configuration were con-nected in rial by zigzag sliver sheets.Each cell consisted of the Ni/yttria-stabilized zirconia
(YSZ)
anode,
the
YSZ
electrolyte,
and
the
Sm 0.2Ce 0.8O 1.9-impregnated
(La 0.75Sr 0.25)0.95MnO 3cathode.In this configuration,to ensure the identical gas distribution over the electrode surfaces,two gas channels with small vents flanking the stacks were ud as gas channels of methane and oxygen for anodes and cathodes,respectively.The lectivity requirement of both the anode and cathode for the oxidation and reduction of CH 4and O 2was lowered and the sheets could extend the residence time of gas flow over the electrode surface.By the direct flame heat with a liquefied petroleum gas burner,the stacks prented a rapid start-up and full utilization of the exhaust gas.Eventually,an open-circuit voltage (OCV)of 1.8V and maximum power output of 276mW was produced by a two-cell stack.For a four-cell stack,a maximum power output of 373mW was obtained.Copyright ª2011,Hydrogen Energy Publications,LLC.Published by Elvier Ltd.All rights
rerved.
1.Introduction
In conventional dual-chamber solid oxide fuel cells (SOFCs),gas-tight aling together with a den electrolyte parates the cell into anode and cathode compartments.The oxidant is fed to the cathode and the fuel to the anode through parate gas supplies without previous intermixing of the t
wo reaction gas.As mentioned in the review article [1],the necessity of gas paration and aling has a vere impact on the mechanical and thermal shock resistance of SOFCs and their long-term stability.In order to avoid tho challenges,single-chamber solid oxide fuel cell (SC-SOFC),wherein the anode and
cathode are expod to the same mixture of fuel and oxidant,has been developed.The working principle of SC-SOFCs is bad on the lectively catalytic properties of the electrodes against the fuel/oxidant mixture [2].The ideal anode needs to be lectively active for the partial oxidation of the fuel (Eq.(1))and the electrochemical oxidation of the produced syngas (Eqs.(2)and (3)).The ideal cathode should not enable any fuel reactions but only the electrochemical reduction of oxygen (Eq.(4))on it.As a result,both the electro-catalytic activity and the lectivity of the electrodes lead to the generation of the elec-tromotive force (EMF)and electricity in SC-SOFCs.Typical reactions using hydrocarbon fuel as fuel are given as follows:
*Corresponding author .Tel.:þ8645186418420;fax:þ8645186412828.E-mail address:lvzhe@ (Z.Lu ¨
).
A v a i l a b l e a t w w w.s c i e n c e d i r e c t.c o m
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /h e
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)7251e 7256
0360-3199/$e e front matter Copyright ª2011,Hydrogen Energy Publications,LLC.Published by Elvier Ltd.All rights rerved.doi:10.1016/j.ijhydene.2011.03.057
At the anode:
C x H yþx/2O2/y/2H2þx CO(1) H2þO2À/H2Oþ2eÀ(2) COþO2À/CO2þ2eÀ(3) At the cathode:
1/2O2þ2eÀ/O2À(4)
Single-chamber operation provides a simple cell structure without gas aling,thereby enhancing the mechanical and thermal tolerance of the stacks with many cells.In the past decade,SC-SOFCs have been investigated intensively and higher power densities have been obtained[3e5].Recently, considerable efforts have also been devoted to the develop-ment of SC-SOFC stacks,and some stack designs with good results have been reported.Shao et al.[6]have propod a heat lf-sustaine
d micro-stack with an anode-facing-anode configuration,and a power output of350mW was obtained with a total cathode area of 1.42cm2.Yano et al.have successfully fabricated a tightly connected SC-SOFC stack, which generated the OCV and power output almost propor-tionally to the increa in the number of single cells[7].Wei et al.[8]have propod a star-shaped SC-SOFC micro-stack, which powered a USB fan stably.This symmetric stack design consists of a uniform gas distribution over the single cells.To utilize the heating effect of the SC-SOFC stack,Liu et al.[9] have asmbled four single cells with an annular configura-tion.The power output of the stack was380mW at CH4/O2¼1 and the average maximum power density of each cell was 190mW/cm2.They have also fabricated an anode-facing-cathode micro-stack with a novel cell-array for the purpo of improving the space utilization[10].All the studies posss effective improvement for the stack performance.However, vere constraints still exist on bulky cell designs,especially in terms of the gasflow management over the single cells. Meanwhile,SC-SOFCs have lower fuel efficiency due to the inherently differentflow geometry in which not only part of the fuel pass through the cathode side unreacted,but also the residence time of theflow over the cell is much shorter [11].At the same time,direct reaction between fuel and oxygen also occurs during the mixed gasflowing through the gas chamber with large cross-ction area slowly under high temperature and it caus fuel consumption and affects the cell performance far away from gas inlet.The constraints have prented a major barrier keeping SC-SOFCs from real application.
In order to supply a relatively independent fuel and oxidant distribution over the respective electrodes and additionally extend the resident time of the gasflow over the cell,herein we propo a non-aled solid oxide fuel cell micro-stack with two gas channels.In addition,a liquefied petroleum gas(LPG) burner was adopted to heat the cell stacks.Therefore,the design has not only combined the advantages of both the dual-and single-chamber SOFCs but also made a full utiliza-tion of the exhaust gas,and carried out a rapid start-up.2.Experimental
The anode-supported SOFCs were purchad from Ningbo Institute of Material Technology&Engineering,Chine Academy of Sciences.Conventional NiO/yttria-stabilized zirconia(YSZ)anode,YSZ and(La0.75Sr0.25)0.95MnO3(LSM)were ud as the anode,electrolyte and cathode,respectively. Fig.1a shows the schematic diagram of the cell configuration. The thickness of YSZfilm was12m m.The LSM cathodes were impregnated by3mol/L Sm0.2Ce0.8(NO3)x solution and heat-treated at850 C for1h to improve the cell performance [12,13].The active cathode area of each cell was0.7Â0.7cm2 in size.The anode substrates were reduced in hydrogen at 700 C before asmbling by using a conventional dual-chamber configuration to avoid the reduction of cathodes.
The schematic diagram of the two-cell stack is prented in Fig.1b.The cells were arranged in an anode-facing-cathode configuration and the distance between them was6mm.Sliver paste and silver
wires were attached to the electrode surfaces for current collection and the cells were connected in rial by zigzag sliver sheets.Fig.1c shows the unfold view of the sheets.The part of the sheets contacted with the electrodes was also0.7Â0.7cm2,the same size with that of the cathodes. Some small holes were uniformly distributed on this part of sheets for gas diffusion.Two half-open ceramic tubes,which the inner diameter and outer diameter were w2and3.4mm respectively,flanked by the cells were ud as gas channels to transport the reactant gas.One channel provided the passage for nitrogen and methane,and the other one provided the passage for oxygen.Several small gas vents were arranged at the side of the tubes at intervals of6mm for the gasflow to the electrodes.The temperature of the stack was monitored through K-type thermocouple placed beside the cells.
The stack and ceramic tubes werefixed on a micanite plate.The micanite plate was then arranged in a box-like stainless-steel gas chamber(thickness of0.5mm;outer length,width and height of100mm,30mm and15mm, respectively).A liquefied petroleum gas(LPG)burner was ud to heat the gas chamber and the stack.A few pinholes were designed at the end of the stainless-steel chamber to expel the exhaust gas,which was then directly burned in theflame to improve the operating temperature of the cells.The temper-ature of theflame was regulated by an air valve and aflow governor.Theflow rates of nitrogen,methane and oxygen were controlled by mass-flow controll
ers(MFCs,D08-4D/2M, Seven-Star Huachuang,China).The stack performances were measured by a Solartron SI1287electrochemical interface and a Solartron SI1260impedance gain/pha analyzer.The impedance spectra were measured in the frequency range from0.1Hz to91kHz with a signal amplitude of10mV.
3.Results and discussion
3.1.The rapid start-up of the two-cell stack
An LPG burner was applied to heat the cells in this study.The temperature variation curve during the start-up and test procedure of the two-cell stack is prented in Fig.2.The
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temperature of the stack ro to 650 C from 25 C in 4min during the start-up process.The temperature is esntial to meet the operating needs of stack under the CH 4/O 2gas mixtures [3,14].Furthermore,during the whole testing process,the flow rate of N 2was fixed at 50sccm,and the flow rates of CH 4and O 2were varied over a range of 10e 100sccm and 10e 50sccm,respectively.The operated temperature of stack was between 710and 750 C.Here the temperature ri
may came from two factors.On the one hand,it is well-known that the catalytic partial oxidation reactions of CH 4as shown in Eq.(1)supply thermal energy [15e 18].On the other hand,according to the design,the exhaust gas can directly burn on the pinholes located at the end of the stainless-steel gas chamber.Obvious combustion was obrved during the testing procedure.The combustion will further improve the operating temperature.
The results indicate that the experimental t-up for stack testing is feasible and practicable.The heat supply method by using an LPG burner allows rapid start-up and the additional heat could be collected.A minimum energy loss and the improved energy utilization will be expected if a combined heat and power (CHP)is further ud.
3.2.Performance of two-cell stack and each single cell
Bad on the reports [19e 21],the fuel/oxygen ratio is a very important factor for the stack performance.The cur-rent e voltage (I e V )and current e power (I e P )curves of the two-cell stack operated under various CH 4/O 2ratios (R )are shown in Fig.3.The flow rates of N 2and O 2were fixed at 50and 10sccm,respectively.An increa of the open-circuit voltage (OCV)occurs with increasing the CH 4/O 2ratio.The OCV of the stack reaches 1.82V at R ¼2and the average OCV for individual
Electrolyte
Small holes
Small holes
Fold line
Cell-2
Cell-1
Gas vent Gas vent
Pinholes
a
b
c
Fig.1e The schematic diagram of the cell configuration and the two-cell stack:(a)the single cell,(b)the perspective view of the two-cell stack and (c)the unfold view of the silver sheet.
Fig.2e The temperature variation curve during the start-up and test procedures of the two-cell stack.
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cell is 0.91V,indicating that every single cell has a good elec-trochemical catalytic property.The maximum power outputs of 58,85and 119mW were obtained at R ¼1,1.5and 2,respectively.Compared with the usual single-chamber mode,wherein oxygen and fuel are mixed before being introduced to the cells,the oxygen and fuel are directly transported to the surfaces of the cathodes and anodes parately by their own channels herein.The oxygen first reaches at the cathodes and then diffus to the anodes for the methane reaction.Thus,the lectivity requirement of both the anode and cathode for the oxidation and reduction of CH 4and O 2in mixed gas has been
lowered within this stack configuration.Especially,the SC-SOFC is so far not a truly mixed reactant fuel cell [22].At the same time,direct reaction between fuel and oxygen during the mixed gas flowing through the gas chamber will not occur in this study and the fuel consumption is thus lowered.The fuel utilization of the two-cell stack under a CH 4/O 2ratio of 2is about 2.8%at the maximum current condition according to Faraday’s law [23]:3¼I =I F
(5)
where I and I F are the actual current and the current calculated for 100%fuel conversion,respectively.I F can be determined by combining Eqs.(1)e (3)of methane,the gas flow rate and Faraday’s law.The residence time of gas flow over the electrode surfaces could be somewhat extended by the silver sheets and the fuel utilization will be further incread.
The significant influence of the flow rate of gas mixtures on the performance of the traditional SC-SOFCs has been obrved by many rearchers [16e 19,22,24].The impact of the gas flow rate on the stack performance has also been studied in this work.As shown in Fig.4,the OCV of the stack varies slightly with the flow rate and remains steady at 1.8V when the methane/oxygen ratio is 2.In contrast to OCV,the power output is significantly incread with the increasing flow rate.The maximu
m power output are 192,230and 276mW at CH 4flow rate of 50,70and 100sccm,respectively.This phenomenon is the same with the reported rearches.In almost all cas,the cell performance became better with increasing gas flow rate.According to Suzuki et al.[5],Nap-porn et al.[16,17],and Shao et al.[18],the heat evolved by methane oxidation at the anode was incread with
increasing gas flow rate.At the same time,the increasing flow rate could caus an incread concentration of the gas going upon the electrode surface and thus the output performance of the cells become better.
Fig.5a and b shows the electrochemical characteristics and impedance spectra of the single cells at the flow rates of CH 4¼100sccm and O 2¼50sccm,respectively.The maximum power output of cell-1is slightly higher than that of cell-2.The I e V curves of the cells exhibit obvious concentration polari-zation,especially for cell-1.The impedance spectra were tested under open-circuit conditions.Fig.5c shows the equivalent circuit for the impedance analysis.The inductance L is attributed to the measurement apparatus and silver wires.The resistance R 1corresponded to the ohmic resistance including electrolyte resistance,contact resistance and elec-trode ohmic resistance.R 2(left micircle at higher frequency)and R 3(right micircle at lower frequency)can be interpreted as charge transfer and gas diffusion resistance,respectively.For the thick anode substrate,the gas diffusi
on resistance is larger than that in the cathode.And the charge transfer resistance can be greatly reduced with abundant reaction zones in the anode.Accordingly,R 2at higher frequency is mainly attributed to the cathode resistance and R 3at lower frequency is mainly associated with anode resistance [25,26].We can e that both the ohmic resistance and cathode polarization resistance of cell-2are higher than that of cell-1.The differences between the two cells are mainly caud by the special construction of the stack.
The methane partial oxidation of the anode is an exothermic reaction.Such a reaction consumes not only the O 2near the anode but also that in the mixture gas far away from the anode via diffusion.As a result,the anodes have the competitions of the oxygen.On the cathode side,the oxygen vent of cell-2lies between the two cells and the efflux of oxygen from this vent is provided for the methane partial oxidations with great oxygen consumption over the two anodes,as shown in Fig.6.While the oxygen vent of cell-1is only clo to the anode of itlf.As a conquence,the oxygen reduction reaction at the cathode of cell-1is more complete than that of cell-2.It is evident from Fig.5b that the cathode resistance of cell-1is much smaller than that of cell-2.
0.000.020.040.060.080.100.120.140.160.180.20
0.00.2
0.40.60.81.01.21.4
1.61.8
2.0P o w e r (W )
V o l t a g e (V )
Current (A)
0.000.02
0.040.060.080.100.120.14Fig.3e I e V and I e P curves of the two-cell stack operated under various CH 4/O 2ratios.
P o w e r (W )
V o l t a g e (V )
Current(A)
0.000.05
0.100.150.200.250.300.35Fig.4e I e V and I e P curves of the two-cell stack for various CH 4and O 2flow rates with R [2.
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For the methane partial oxidation over the anode,the reaction of cell-1is more complete with adequate oxygen than that of cell-2due to the favorable gas transport pathways from both the oxygen vents.The heat effect of methane partial oxidation at the anodes leads to the difference of actual temperature among the single cells.The increa in cell temperature enhances the ion conductivity of the electrolyte and the catalytic activity of the electrodes,which leads to
a decrea in the ohmic resistance and an increa in the cell performance.As can be en from Fig.5b,the ohmic resis-tance of cell-1is smaller than cell-2and the output perfor-mance of cell-1is better than cell-2as well (Fig.5a).
The concentration polarization of the cells could be ascribed to the small number and size of the holes on the silver sheets which obstruct the reaction gas that pass through the porous electrodes,especially for the thick anode support.The concentration polarization of the single cells caus a negative curvature on the cell stack (e Fig.4).The porosity of the sheets is about 57.7%.Obviously,the number and size of the holes needs to be adjusted in order to eliminate the concentration polarization and smooth over the differ-ence between the single cells.For a scaled-up cell stack,the cells in the middle region posss nearly the same confor-mation and gas environment,and the differences between them would be negligible.The outer flank of both the margi
nal cells need adding silver sheets to create a balanced reaction environment for them and thus to provide a stable output property with a relatively uniform gas composition.
3.3.Scale-up of the stack
In order to verify the feasibility of the structure,a stack with four cells has been investigated.The flow rate of N 2is fixed at 50sccm as previously described and the CH 4/O 2ratio is a constant of 2.The output performance of the stack under various gas flow rates is shown in Fig.7.The OCV of the stack increas from 3.5V to 3.6V with the CH 4flow rate increasing from 60sccm to 100sccm.The maximum power output increas obviously with the increasing of gas flow rate,from 190mW at CH 4¼60sccm to 373mW at CH 4¼100sccm.The maximum power density is 186mW/cm 2at the flow rates of CH 4¼100sccm and O 2¼50sccm,which is lower than that of the two-cell stack (276mW/cm 2)under the same condition.The active electrode area of a four-cell stack is twice as big as that of a two-cell stack,which caus a large gas consuming.As a result,a four-cell stack with larger surface area of the total electrodes needs higher gas flow rate.A better perfor-mance is expected to be achieved through further optimiza-tion of the gas concentration.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.400.00.10.20.30.40.50.60.70.80.9
1.0Two-cell stack
P o w e r (W )
V o l t a g e (V )
Current (A)
cell-1 cell-2
0.000.020.04
0.060.080.10
0.12
0.140.00.20.40.60.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.0
0.30.60.91.21.5
1.8
2.1Two-cell stack
-Z "()
Z'()
cell-1 cell-2
b
a
ΩΩc
Fig.5e (a)I e V and I e P curves and (b)impedance spectra of the single cells at the flow rates of N 2[50sccm,
CH 4[100sccm and O 2[50sccm.(c)The equivalent circuit.
N 2+CH 4
O 2
Fuel vent of cell-2
Fuel vent of cell-1
Oxygen vent of cell-1
Oxygen vent of cell-2 Flow path of O 2
Flow path of O 2
Fig.6e The flow
paths of oxygen for the methane partial oxidations.
V o l t a g e (V )
Current (A)
P o w e r (W )
0.000.05
0.100.150.200.250.300.350.40Fig.7e I e V and I e P curves of four-cell stack at various gas flow rates with R [2.
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4.Conclusion
In this study,non-aled SOFC micro-stacks with two gas channels were fabricated and operated successfully in a box-like stainless-steel chamber.The novel stack design has the following advantag
es:(1)Uniform reaction environment for the scaled-up stacks.The gas vents are designed to maintain a uniform distribution of gas reactant over the anodes and cathodes and the silver sheets between the cells form a rela-tively independent reaction atmosphere for each cell,thus produce a stable output performance of the stack;(2)Low lectivity requirement of both the anode and cathode for the oxidation and reduction of CH4and O2within this stack configuration and the sheets could extend the residence time of gasflow over the electrode surface;(3)The CH4and O2are transported to the anode and cathode parately without a previous mixing and the direct reaction between fuel and oxygen during the mixed gasflowing through the gas chamber in usual SC-SOFC mode will not happened,the fuel consumption is thus lowered;and(4)Flexible output capa-bility and modularization.The number and the shape of single cells areflexible according to requirement and the unit stacks can be connected in ries or parallelflexibly among three-dimensional orientation to form a scale-up stack module.
The stack consisted of two cells generating an OCV of1.8V and a maximum power output of276mW at theflow rate of CH4¼100sccm and O2¼50sccm.Both the OCV and output performance tend to increa with CH4/O2ratio.A high power output of373mW was produced with a four-cell stack. Improvement can be obtained by the gasflow management and further adjusting of the porosity of th
e holes on the silver sheets for the gas diffusion.More importantly,this novel stack design offers great potential to form a bigger stack module for portable power generation.
Acknowledgement
This rearch was supported by the Ministry of Science and Technology of China(2007AA05Z139)and the National Natural Science Foundation of China(20901020).
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