Qiong Sun,Keqing Zheng,Meng Ni*

Energy,Resources and Environmental Technology

Thermodynamic Analysis of Methane-fueled Solid Oxide Fuel Cells Considering CO Electrochemical Oxidation☆

Qiong Sun,Keqing Zheng,Meng Ni*

Building Energy Research Group,Department of Building and Real Estate,Hong Kong Polytechnic University,Hung Hom,Kowloon,Hong Kong,China

A R T I C L EI N F O

Article history:

Solid oxide fuel cell

Thermodynamics

Proton conductor

Oxygen ion conductor

Hydrocarbon fuels

Thermodynamic analysesinthe literaturehave shown that solidoxide fuelcells(SOFCs)with protonconducting electrolyte(H-SOFC)exhibited higherperformance than SOFC with oxygen ion conducting electrolyte(O-SOFC). However,these studies only consider H2electrochemical oxidation and totally neglect the contribution of CO electrochemical oxidation in O-SOFC.In this short communication,a thermodynamic model is developed to compare the theoretically maximum eff i ciencies of H-SOFC and O-SOFC,considering the electrochemical oxidation of CO in O-SOFC anode.It is found that O-SOFC exhibits a higher maximum eff i ciency than H-SOFC due to the contribution from CO electrochemical oxidation,which is contrary to the common understanding of electrolyte effect on SOFC performance.The effects of operating temperature and fuel utilization factor on the theoretical eff i ciency of SOFC are also analyzed and discussed.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

Solid oxide fuel cells(SOFCs)have been identif i ed as eff i cient and environmental-friendly electrochemical devices for power generation. Compared with low temperature fuel cells(such as proton exchange membrane fuel cells),one attraction of SOFCs is their fuel f l exibility.At typical working temperatures(i.e.1073 K),hydrogen fuel or hydrocarbon fuels,such as methane and ethanol,can be utilized in SOFC for power generation as internal reforming of hydrocarbon fuels can occur in theporous anode of SOFC[1-4].Electrolyte isa key component of SOFC and can be built with oxygen ion conducting ceramics (i.e.yttria-stabilized zirconia,YSZ)or proton conducting materials, such as BaCeO3doped with Gd or Nd.The use of different electrolytes not only yields different ohmic losses,but also inf l uences the mass transfer in porous electrodes,as steam is produced in the cathode of H-SOFC,which in turn impedes the diffusion of oxygen[5,6].

InordertoclarifythedifferencesbetweenH-SOFCandO-SOFCandto identify suitable electrolyte materials for SOFC operation,several thermodynamic analyses have been performed for both H2and hydrocarbon fueledSOFCs.DeminandTsiakaraswerethef i rstwhothermodynamically compared the maximum eff i ciencies of H-SOFC and O-SOFC fed with hydrogen fuel[7].It was found that the hydrogen fed H-SOFC had an essential advantage as compared to O-SOFC,due to a higher hydrogen concentration in the anode of H-SOFC[7].In a subsequent study, the model was extended to investigate the methane fed H-SOFC and O-SOFC[8].It was found that the maximum eff i ciency of methane fed H-SOFC was evidently higher than that of O-SOFC[8,9].In other thermodynamic analyses of SOFCs fed with ethanol or ammonia fuels, it was also observed that H-SOFC performed better than O-SOFC in terms of maximum eff i ciency due to a higher hydrogen concentration in the anode of H-SOFC[10-12].

However,intheabovementionedthermodynamicanalysesonmethane or ethanol fed SOFCs,only H2electrochemical oxidation was considered while the contribution of CO electrochemical oxidation to O-SOFC power generation was totally neglected.From the experiments,it has been conf i rmed that CO electrochemical oxidation could occur in the anode of O-SOFC,although its reaction kinetics was slower than that of H2electrochemical oxidation[13].Since CO electrochemical oxidation in H-SOFC would not occur,it is still unclear whether the methane fed H-SOFC is superior to O-SOFC in terms of maximum eff i ciency if H2and CO electrochemical oxidation reactions are considered for O-SOFC.In this short communication,a simple thermodynamic model is developed to compare the maximum eff i ciencies of methane fed H-SOFC and O-SOFC,considering the CO electrochemical oxidation in the anode of O-SOFC.Dueto a lack of experimental data,comparison of thepresent simulation results with literature data is not provided.However,comparison can be easily made once the relevant data are available.

2.The Model

The present thermodynamic analyses are based on the assumption of chemical and electrochemical reactions.The working principles of the methane fed H-SOFC and O-SOFC are shown in Fig.1(a)and(b), respectively.In operation,H2O-CH4mixture at a molar ratio of 2:1 issupplied to the anode channel of SOFC while air is supplied to the cathode.In both H-SOFC and O-SOFC,direct internal reforming(DIR) and water gas shift reaction(WGSR)take place in the porous anode, as shown by Reactions(1)and(2)respectively:

In the H-SOFC[Fig.1(a)],H2molecules produced from DIR and WGSR diffuse through theporous anodetothe anode-electrolyteinterface,wherethey areoxidized to proton andelectrons(Reaction 3).Subsequently,protons are transported to the cathode through the dense electrolyte while the electrons f l ow through the external circuit to the cathode side,where they react with oxygen molecules and protons to form steam(Reaction 4).

To evaluate the theoretically maximum ef fi ciency of CH4fed SOFCs, the equilibrium potential is calculated.According to thermodynamics of fuel cells,the equilibrium potential of SOFC can be expressed as

where E0is the voltage at standard pressures andcan be calculated from the Gibbs free energy change.PH2,PH2O,andPO2are the partial pressures (0.1 MPa)of hydrogen(anode),steam(cathode),and oxygen(cathode), respectively,R is the ideal gas constant(8.3145 J·mol?1·K?1),T is temperature(K),and F is the Faraday constant(96485 C?mol?1).

In order to calculate the equilibrium potential,the partial pressures of gas species must be obtained.The calculation procedures developed by Demin et al.[8]and Assabumrungrat et al.[11]are employed in this study.Both DIR and WGSR are assumed to be in equilibrium. Thus,the number of moles of gases reacted in DIR and WGSR can be determined by using the equilibrium constants[8,11].Considering DIR,WGSR and electrochemical reaction,the number of moles(n)of each component at the SOFC outlet can be calculated as follows.

In the anode,

Fig.1.Working principle of CH4-fueled SOFC:(a)H-SOFC and(b)O-SOFC.

wherea=1/3,b=2/3,andd=0.21arethemolenumbersofCH4,H2O, and O2at two inlets of SOFC,respectively.c is the mole of H2electrochemicallyconsumed(relatedtocurrentgeneratedandthusthefuelutilization).x and y are the numbers of moles of CH4and CO reacted in the DIR and WGSR reactions.The equilibrium constant of Reactions(1)and (2)are respectively:

where ΔGfis the change of Gibbs free energy between products and reactants of chemical reaction at a standard state.

Basedon themassbalanceandtheequilibrium constants,thepartial pressures of gaseous species can be determined.Detailed information can be found in Refs.[8,11,12].Then the equilibrium potential can be calculated,which is used in turn for subsequent calculation of the theoretical work output(W)as

where q is the electrical charge generated from SOFC.

The maximum eff i ciency(η)of CH4fed SOFC can thus be calculated as

where ΔH0is theformation enthalpy of CH4ata standard condition.For H-SOFC,theeff i ciency is noted asηH-SOFC.Thefuelutilization is def i ned as the ratio of consumed fuel(CH4)to the feeding fuel,x/a,and the oxygen utilization similarly as 0.5c/d.

2.2.O-SOFC

In the O-SOFC,O2molecules diffuse through the porous cathode to the cathode-electrolyte interface and react with electrons to produce oxygen ions(Reaction 18),which are transported through dense electrolyte to the anode side.At the anode,H2and CO molecules diffuse through the porous anode layer to the anode-electrolyte interface where they react with oxygen ions to produce electrons,H2O and CO2

In the O-SOFC,both H2and CO are involved in the electrochemical reactions for power generation.The Nernst potential for H2electrochemical oxidation can be determined by Eq.(5),exceptthat thepartial pressure of H2O in the anode should be used.The Nernst potential for CO electrochemical oxidation can be determined by where qcois the electric current generated from CO electrochemical oxidation in O-SOFC.The maximum eff i ciency of O-SOFC(ηO-SOFC)can be determined with Eq.(17)but W should be calculated from Eq.(22).

TherateofCOelectrochemicaloxidationinO-SOFCcanbeevaluated as rc,where r is the ratio of CO oxidation rate to H2oxidation rate(c). According to experimental measurements by Matsuzaki and Yasuda [13],the rate of H2electrochemical oxidation is about 2-3 times that of CO electrochemical oxidation.Thus,the value of r is set to be 0 to 1/3 in the present study.When r is 0,the contribution from CO is neglected and the present study is reduced to the previous studies[11].

To determine the partial pressures of gaseous species in the O-SOFC, the similar approach for H-SOFC is adopted.The effects of electrochemical reaction,DIR and WGSR(Reactions(1)and(2))on the molar fractions are all considered by using the parameters of x and y,as can be seenfromEqs.(6)to(10)andEqs.(23)to(27).Sincesteamisproduced in the anode,the number of moles of gas species should be modif i ed as follows.

2.3.Solution of models

The model is based on previous thermodynamic analyses[8,11,12]. The detailed methodology description can be found in Refs.[11,12]. The number of moles of H2O(b),CH4(a),and O2(d)are specif i ed at the inlet.The unknowns x and y are dependent on the extent of steam reforming and water gas shift reactions,which are calculated based on reaction equilibrium.In the simulation,an oxygen utilization factor ((0.5c+0.5rc)/d for O-SOFC and 0.5c/d for H-SOFC)of 20%is used whilethefuelutilizationfactorx/aisvariedtocalculatethetheoretically maximum eff i ciency[8,11,12].

3.Results and Discussion

Fig.2.Eff i ciencies of H-SOFC and O-SOFC with different reaction rates of CO.

Inthissection,parametricsimulationsareperformedtocomparethe maximum eff i ciencies of H-SOFC and O-SOFC running on methane fuel. In thisstudy,theoperatingtemperature(T)andthevalueofrarevaried to examine their effects on the theoretically maximum eff i ciency of SOFC running on CH4.

3.1.Effect of CO electrochemical oxidation rate

The value of r is varied from 0 to 1/3 to examine the effect of CO electrochemical oxidation rate on O-SOFC eff i ciency.Simulations are performed at an operating temperature of 873 K and an oxygen utilization factor of 20%.As can beseenfrom Fig.2,themaximum eff i ciency of O-SOFCis lower than that of H-SOFC at r=0,when theelectrochemical oxidation of CO is excluded.The result is consistent with the previous thermodynamic analysis by Demin et al.[8].The higher eff i ciency of H-SOFC than O-SOFC is due to the fact that the fuel-diluting steam is produced in the cathode of H-SOFC,thus the hydrogen molar fraction in the anode of H-SOFC is higher than that of O-SOFC.This results in ahigher Nernst potential of H-SOFC,leading to a higher maximum eff i ciency of H-SOFC than O-SOFC.However,when CO electrochemical oxidation is included in O-SOFC,more electrochemical power can be produced from O-SOFC,as can be seen from Eq.(22).Thus,the maximum eff i ciency of O-SOFC is considerably higher than that of H-SOFC, even at a low r of only 1/6.In addition,this difference increases with increasingr,as more CO is involved in electrochemical reaction for power generation from O-SOFC.This f i nding is different from our common understanding that H-SOFC is always better than O-SOFC in terms of maximum eff i ciency[7-12].At a higher fuel utilization,the difference betweendifferentscenarios becomesmoreobvious,whichis consistent with thepreviousthermodynamic analyses[8,12].Thelargeradvantage ofO-SOFCoverH-SOFCatahighfuelutilizationiscausedbythefactthat more CO is involved in electrochemical reaction in O-SOFC.

Fig.3.Eff i ciencies of H-SOFC and O-SOFC at different temperatures.

Fig.4.Eff i ciencydifference betweenH-SOFC and O-SOFC:(a)ηH-SOFC?ηO-SOFCat r=0; and(b)ηO-SOFC?ηH-SOFCat r=1/3.

3.2.Effect of operating temperature

The effect of operating temperature on SOFC maximum eff i ciency is simulated andtheresults areshowninFig.3.Themaximum eff i ciencies of both O-SOFC and H-SOFC decrease considerably astemperatureis increased from 873 K to 1273 K,especially at high fuel utilization factors. This is mainly caused by a decrease in equilibrium potential at standard pressure(EH20and ECO0)with increasing temperature,which in turn tends to decrease the power generation and eff i ciency at a given fuel utilization factor.

Tofurtherelucidatethedifference betweenO-SOFC and H-SOFC,the eff i ciency differences between H-SOFC and O-SOFC are shown in Fig.4. As can be seen from Fig.4(a),the value of(ηH-SOFC?ηO-SOFC)at r=0 increases with increasing fuel utilization and operating temperature.At afuelutilizationof70%,themaximumeff i ciencydifferenceisabout13%, whichisconsistentwithDemin'sdatainRefs.[7]and[14].However,the value of(ηO-SOFC?ηH-SOFC)at r=1/3 increases with increasing fuel utilization but decreases with increasing temperature[Fig.4(b)].This behavior may be caused by the fact that the H-SOFC produces a higher current density from H2electrochemical oxidation than O-SOFC at a higher temperature[see Fig.4(a)],which tends to narrow down the difference between O-SOFC and H-SOFC[Fig.4(b)].

4.Conclusions

A simple thermodynamic model is developed to compare the maximum eff i ciencies of methane fed H-SOFC and O-SOFC with consideration of CO electrochemical oxidation in O-SOFC anode.It is found that H-SOFC shows a higher maximum eff i ciency than O-SOFC when CO electrochemical oxidation is excluded,which is consistent with the results in the literature.When CO electrochemical oxidation in O-SOFC is considered,the maximum eff i ciency of O-SOFC is obviously higher thanthatofH-SOFC,andthiseff i ciencydifferenceincreasessignif i cantly withincreasingrateofCOelectrochemicaloxidationandfuelutilization. The maximum eff i ciencies of both H-SOFC and O-SOFC decrease with increasingtemperature.It is alsofound thatwith anincrease in temperature,the value of(ηH-SOFC?ηO-SOFC)at r=0 increases while the value of(ηO-SOFC?ηH-SOFC)at r=1/3 decreases.

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30 October 2012

☆Supported by Hong Kong Research Grant Council(PolyU 5238/11E).

*Corresponding author.

E-mail address:bsmengni@polyu.edu.hk(M.Ni).

http://dx.doi.org/10.1016/j.cjche.2014.06.018

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 29 April 2013

Accepted 10 August 2013

Available online 30 June 2014