Electrochemical performance of mixed ionic–electronic conducting oxides as anodes for solid oxide...

Solid State Ionics 120 (1999) 75–84

Electrochemical performance of mixed ionic–electronic conductingoxides as anodes for solid oxide fuel cell

*Shizhong Wang, Yi Jiang , Yahong Zhang, Wenzhao Li, Jingwang Yan, Zigui LuDalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China

Received 4 November 1998; accepted 4 December 1998

Abstract

Mixed ionic–electronic conducting (MIEC) oxides, SrFeCo O , SrCo Fe O and La Sr Fe Co O have0.5 x 0.8 0.2 32d 0.6 0.4 0.8 0.2 32d

been synthesized and prepared on yttria-stabilized zirconia as anodes for solid oxide fuel cells. Power output measurementsshow that the anodes composed of such kinds of oxides exhibit modest electrochemical activities to both H and CH fuels,2 4

2giving maximum power densities of around 0.1 W/cm at 9508C. Polarization and AC impedance measurements found thatlarge activation overpotentials and ohmic resistance drops were the main causes for the relative inferior performance to theNi-YSZ anode. While interlayered with an Ni-YSZ anode, a significant improvement in the electrochemical performance wasobserved. In particular, for the SrFeCo O oxide interlayered Ni-YSZ anode, the maximum power output reaches 0.250.5 x

2W/cm on CH , exceeding those of both SrFeCo O and the Ni-YSZ, as anodes alone. A synergetic effect of SrFeCo O4 0.5 x 0.5 x

and the Ni-YSZ has been observed. Future work is needed to examine the long-term stability of MIEC oxide electrodesunder a very reducing environment. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Mixed ionic–electronic conducting oxides; Anodes; Methane oxidation; Solid oxide fuel cell

1. Introduction stationary purposes. Since it works at temperatures( $ 7008C) of catalytic reaction interest, it is a

The solid oxide fuel cell (SOFC) is one of the potential catalytic co-generator for the simultaneousmost advanced fuel cell systems for generating production of electricity and valuable chemicals.electricity in an efficient and environmentally friend- Several cases of cogeneration processes such asly way. The principal advantages of SOFC over partial oxidation of CH to syngas [1–3], NH to NO4 3

other types of fuel cells are fuel adaptability and [4] have been demonstrated.long-term stability. It can be easily adapted to In practice, nickel yttria-stabilized zirconia (Ni-operate on a wide variety of fuels including gasoline, YSZ) and strontium-doped lanthanum manganitediesel, and natural gas fuels, so that it can find a (LSM) or LSM 1 YSZ composite electrodes arebroad range of applications from transportation to widely used anodes and cathodes for SOFC, due to

their superior electrochemical performances as wellas other essential properties for SOFC such as

* thermal and chemical stability and compatibility withCorresponding author. Fax: 186-411-4691570.E-mail address: [email protected] (Y. Jiang) YSZ [5–12]. Tremendous work has been devoted to

0167-2738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 98 )00558-X

76 S. Wang et al. / Solid State Ionics 120 (1999) 75 –84

study the electrochemical properties of the LSM characteristic of both high ionic and electronicelectrode and the mechanism of oxygen reduction conductivities at elevated temperatures. They arereaction [5–10]. attractive candidate materials for several important

Although the Ni-YSZ anode has also a long applications, including SOFC electrodes, oxygenhistory for SOFC, and has been studied extensively. separation membranes, and membrane reactors forThere are still a number of problems associated with syngas production and the partial oxidation of hydro-the Ni-YSZ as SOFC anodes. First, the sintering of carbons. Teraoka et al. studied the electronic andnickel particles at high operation temperatures is one ionic conductivities and permeability of perovskiteof the main problems leading to the degradation of oxide, SrCo Fe O , and reported that12y y 32d

the anode performance. Second, the electrochemical SrCo Fe O exhibited an electronic and ionic0.8 0.2 32d21activity is still not sufficiently high for hydrocarbons, conductivities of around 100 S ? cm and 10 S ?

21in particular, for methane as a fuel. Third, carbon cm in air at 8008C [17] and the highest permeabili-deposition is a vital problem related to the significant ty among this series of oxides with a differentdeterioration of the Ni-YSZ anode when SOFC is amount of Fe substitution [18].fuelled by a hydrocarbon, e.g. methane. In the recent years, another kind of mixed ionic–

A great deal of work has been aimed at solving the electronic conducting oxide, SrFeCo O , was re-0.5 x

above mentioned problems. Lee and coworkers [13] ported having both high electronic and ionic conduc-21correlated the Ni-YSZ anode performance with the tivities (10 and 7 S ? cm ) [19]. This kind of oxide

anode microstructure. They found that the Ni-YSZ has been successfully tested as a membrane reactoranode prepared from NiO and YSZ with similar for partial oxidation of methane to syngas, showingsized starting powders and nearly equal population high oxygen permeability. Moreover, it can stand theprovided the largest reaction sites with a well harsh conditions of extremely oxidizing and reducingdeveloped channel structure, thus, a high stability atmospheres and retain its structure integrity [20].

¨against Ni sintering. Wildenhoner reported that a fine It is well recognized that mixed conducting oxidesYSZ framework in Ni-YSZ anodes formed from can be potentially promising electrodes, in particular,small starting YSZ powder could improve the anode as a cathode for SOFC applications, because theperformance and durability [14]. electrochemical reaction zone will not be confined to

In order to improve the performance of Ni anodes, the three phase boundary area (TPB) due to the ionicThampi and coworkers [15] modified the YSZ conductivity. Theoretical analysis of porous mixedsurface with reducible oxides such as MnO , CeO , conductors as cathodes has been reported by severalx x

etc., which they referred to as a domain. They found authors [21,22], revealing the extension of the elec-that the significant improvement in power output was trochemical area over the electrode surface, whichachieved after such a modification. The authors leads to a higher electrochemical performance than aattributed this improvement to the redox mechanism pure electronic conductor as a cathode.taking place in the domain. Tsai and Barnett [16] In this paper, three mixed ionic–electronic con-found a significant increase in the power output of ducting (MIEC) oxides, SrCo Fe O ,0.8 0.2 32d

SOFCs when thin porous yttria-doped ceria (YDC) SrFeCo O and La Sr Fe Co O were syn-0.5 x 0.6 0.4 0.8 0.2 32d

layers were deposited on either side of YSZ beneath thesized and prepared on YSZ plates as anodes forthe Ni-YSZ anode and LSM cathode. The cells of H and CH oxidation. Anodes made of MIEC2 4

such kind provided a power density up to 480 mW/ oxides displayed modest electrochemical activities2cm at 6508C running on H . The authors explained for H and CH and the maximum power density2 2 4

2that the extension of reaction area due to the mixed around 0.1 W/cm at 9508C. A synergetic effect hasconductivity of the YDC on the anode side and an been observed when such MIEC oxides were inter-increased oxygen surface-exchange rate on the layered with a Ni-YSZ anode. In particular, for acathode side contributed to the improvement. SrFeCo O oxide interlayered Ni-YSZ anode, the0.5 x

Mixed ionic–electronic conductor (MIEC) oxides, maximum power density and the current densityeither perovskite type, (LaSr)(CoFe)O , or non- significantly exceed those of both Ni-YSZ and32d

perovskite type, SrFeCo O exhibit the peculiar SrFeCo O as anodes alone.0.5 x 0.5 x

S. Wang et al. / Solid State Ionics 120 (1999) 75 –84 77

2. Experimental was described elsewhere [10]. The Pt referenceelectrodes were deposited and calcined on the same

2.1. Mixed ionic–electronic conductors (MIECs) sides of the cathodes with a few millimeter apartfrom the latter. The areas of anodes, cathodes and Pt

2 2La Sr Fe Co O , SrFeCo O , reference electrodes were 1.0 cm , 1.5 cm and 0.10.6 0.4 0.8 0.2 32d 0.5 x2SrCo Fe O , below referred as to LSFC, SFC cm .0.8 0.2 32d 0.5

and SF C respectively, were all prepared by a0.2 0.8

solid-state reaction. The source materials were 2.3. Single cell preparationLa O , SrCO , Co(NO ) ? 6H O and Fe O , which2 3 3 3 2 2 2 3

were mixed stoichiometrically and ground thorough- The YSZ plate was sealed with a glass-sealingly in ethanol into a homogeneous mixture. Then, the material to a two-open-ends YSZ tube (16 cm inmixture was dried and fired in air at 9008C for 16 h. length, 2 cm in diameter). The anode sealed insideThe resulting powders after firing were ground with the tube was exposed to the fuel, either H or CH ,2 4

an agate mortar and pestle into fine particles and which was introduced from an inlet tube intrudingwere used as green materials for the preparation of from an appropriately designed stainless steel tubeanodes. X-ray diffraction (XRD) confirmed that cap. The cap was gas tightened to the other end ofLSFC and SF C were perovskite structures, the YSZ tube and was water cooled during high0.2 0.8

while SFC was brownmillerite structure, similar to temperature operation. The cathode as well as the0.5

that reported by Ma and Balachandran [23]. reference electrode was exposed to the ambient air,which was the oxygen source for H or CH2 4

2.2. Electrode preparation oxidation. Three gold grids were attached to thethree electrodes and spring pressed as current collec-

LSFC, SF C and SFCo anodes were pre- tors. Three gold wires were connected to the anode,0.2 0.8 0.5

pared by screen printing the respective paste on the cathode and reference electrode respectively andYSZ plate and calcined in air for 2 h. The pastes were led to a potentiostat /galvanostat. The effluentwere prepared from mixing the respective MIEC gas from the outlet tube was analyzed by on-line gasoxide powders with an appropriate amount of binder chromatography equipped with a TCD when CH4

and solvent. The final calcination temperatures for was fed as fuel.LSFC, SF C and SFC were determined by0.2 0.8 0.5

trial and error, so as to obtain a good adhesion to 2.4. Electrochemical and scanning electronYSZ and appropriate porosity. They were 12408C, microscopy measurements11408C, and 11708C for LSFC, SF C and SFC0.2 0.8 0.5

respectively. Ni-YSZ anodes, consisting of 56 wt% The electrochemical measurements, includingNiO 1 YSZ (referred to Ni-YSZ), were also prepared power output and overpotential measurements, wereby mixing proper amounts of NiO and YSZ powders carried out with a potentiostat /galvanostat (EG andwith a suitable amount of binder, followed by screen G 173) in conjunction with an EG and G 376printing (120 mesh) on one side of YSZ electrolyte interface and 175 universal programmer. IR dropsplates, then fired in air at 13508C for 2 h. were determined by AC impedance measurements

The interlayered Ni-YSZ anode was prepared by and were substracted from the overpotential.depositing an outer layer of an Ni-YSZ paste on the The Ni-YSZ, oxide and interlayered samples wereoxide layer and fired in situ in air at 9808C for 2 h. examined by SEM. The SEM observation wasThe preparation procedure was similar to that of a performed by Scanning Electron Microscopesingle Ni-YSZ anode described above. (HITACHI, S-3200N). Typical and representative

The cathodes were all La Sr MnO 1 20 wt% pictures of the cross microstructures were taken by a0.8 0.2 3

YSZ composite electrodes, which were deposited on digital camera. Both the Ni-YSZ and oxide anodesthe other side of the YSZ plates, facing the anodes were porous and were intimately sintered on YSZby the screen printing method (80 mesh) and cal- plates. The thickness of the Ni-YSZ and oxidecined at 13008C for 2.5 h. The detailed procedure anodes were found to be around 8 mm and 30 mm.


2 23. Results and discussion W/cm and 0.03 W/cm , much lower than that ofSF C . In order to clarify the main factors respon-0.2 0.8

3.1. The electrochemical performances of mixed sible for the loss of power density, overpotential andconducting oxide anodes on H AC impedance analyses have been performed. Fig. 22

shows IR free overpotentials of three electrodes atThe mixed ionic–electronic conductors possess various current densities. The overpotentials of

both ionic and electronic conductivities. They are, in SFC are significantly larger than those of SF C0.5 0.2 0.8

principle, considered to be promising electrode and LSFC, which is one of the main factors attribut-candidates for SOFCs. Since the high ionic con- able to the poor performance of SFC anode in H .0.5 2

ductivity can facilitate oxygen ion transportation and Although the overpotential of both LSFC andthus result in a spatial extension of electrochemical SF C are similar, their ohmic resistances are0.2 0.8

active zone. Theoretical work has been reported by conspicuously different as shown in Table 1. TheLiu recently [22], showing the correlation between ohmic resistances of LSFC and SFC are sig-0.5

the electrochemical area extension with the prop- nificantly larger than that of SF C . As a result0 0.2 0.8

erties of mixed ionic–electronic conductor such as of the small overpotential and ohmic resistance, thethe rate of surface reaction, ionic and electronic best performance is obtained for SF C anode.0.2 0.8

conductivities. SrFeCo O , SrCo Fe O and The poorest is found for SFC due to the large0.5 x 0.8 0.2 32d 0.5

La Sr Fe Co O , among those with both overpotential and ohmic resistance. Since the calcu-0.6 0.4 0.8 0.2 32d2high ionic and electronic conductivities have been lated ohmic resistance is about 0.1 V.cm for the

used as membranes for separation of O from air due YSZ electrolyte (150 mm) at 9508C, the main2

to simultaneous conducting of electrons and oxygenions. To this end, SFC , SF C and LSFC have0.5 0.2 0.8

been investigated as SOFC anodes for H and CH .2 4

The electrochemical performances of the oxideanodes on H at 9508C are given in Fig. 1. The2

electrochemical performances and power outputs areremarkably different among three anodes. SF C0.2 0.8

gives the best performance, i.e. the terminal potentialdrops most slowly as the current density drawn fromthe cell increases, resulting in a highest maximum

2power density of about 0.08 W/cm . The maximumpower outputs of LSFC and SFC are around 0.050.5

Fig. 2. IR free overpotentials of three mixed conducting oxideanodes at various current densities at 9508C in H .2

Table 12Ohmic resistances (V?cm ) of three oxide anodes at 9508C in H2

and CH4

Ohmic Resistance LSFC SFC SF C0.5 0.2 0.8

R(H ) 2.2 2.2 0.702

R(CH ) 0.40 1.0 0.504aR (CH ) 0.27 0.16 0.374Fig. 1. Cell performances of three SOFCs with SFC , SF C0.5 0.2 0.8

aand LSFC oxides as anodes at 9508C on H . From mixed conducting oxide interlayered Ni-YSz anodes.2


contributions for such a large ohmic resistance mayarise from the contact resistance (between elec-trolyte, electrode and current collector) and anoderesistance itself.

The total and ionic conductivities of these denseoxides have been studied in separate experimentsusing AC impedance and electron blocking elec-trodes. The conductivities at 9508C are listed inTable 2. For both SFC and SF C oxides, the0.5 0.2 0.8

5total and ionic conductivities in 1–10 Pa oxygenpartial pressure range have been measured, decreas-ing with decreasing P [24]. Under CH reducingO 42

gas, the dense bars of SFC and SF C for0.5 0.2 0.8

Fig. 3. Cell performances of three SOFCs with SFC , SF Cconductivity measurement were more or less stable 0.5 0.2 0.8

and LSFC oxides as anodes at 9508C on CH .4and the total ionic conductivities are also shown inTable 2. As can be seen from the table, the totalconductivities of three materials are around few S? SFC anode. Oppositely, the performance of the0.5

21 2cm . Considering the thickness of the anode, SF C anode was decreased to ca. 0.05 W/cm on0.2 0.8

around |30 mm, and taking into account the effect CH . Analysis of electrochemical polarization be-4

of porosity, the anode resistance itself can be negli- havior revealed that the SF C anode shows the0.2 0.8

gible for all anodes. Therefore, the large ohmic largest overpotential at current densities below 150resistances must be caused mainly by contact re- mA/cm (Fig. 4). For the SFC anode, the over-0.5

sistances. potential is the lowest at low current densities whileit increases drastically with further increasing current

3.2. The electrochemical performances of mixed density. This sharp increase in overpotential can notconductors anodes on CH be simply related to the mass transfer limitation4

behavior, as generally referred to, according to theIn general, perovskite oxides and mixed conduct- current-overpotential pattern because, as shown

ing oxides are catalytically active for oxidation of below, the SFC /Ni-YSZ interlayered anode did0.5

hydrocarbons. Investigation on the anodic perform- not behave the same. We preliminarily attribute thisance of SFC , SF C and LSFC toward dry CH0.5 0.2 0.8 4

have been conducted. Fig. 3 shows the power outputcurves of three oxide anodes running on dry CH .4

The performance order is LSFC.SFC .SF C ,0.5 0.2 0.8

which is different from the cell performance orderrunning on H . In the case of CH , the maximum2 4

power output of the LSFC anode was improved,2 2reaching 0.09 W/cm compared to 0.05 W/cm on

H . A similar improvement was also found for the2

Table 2Total and ionic conductivities of respective dense oxides at 9508C

21Conductivity /S?cm LSFC SFC SF C0.5 0.2 0.8

s (air) 110 10 61.7t

s (air) 0.3 5.3 6.4I

s (CH ) – |3.0 |2.0t 4 Fig. 4. IR free overpotentials of three mixed conducting oxides (CH ) – |1.8 |0.3I 4 anodes at various current densities at 9508C in CH .4


rapid overpotential increase to the slowness of the 3.3. Electrochemical performance of mixed oxideCH activation step or /and to the low total and ionic interlayered Ni-YSZ anodes4

conductivities of SFC , in particular, at low P .0.5 O2

The above results show that these kinds of mixedThe activation step, which involves the adsorption ofconducting oxides, when used as anodes, give mod-CH and breaking up of the C–H bond, is generally4

est electrochemical activities for both H and CH asunaffected by the electrode potential. When the 2 4

fuels. In particular, the cell performance on CH iselectrochemical oxidation of CH is limited by this 44

comparable to that on H , showing the perspectivestep, the anodic current will be hardly increased with 2

for anodes of SOFCs running on dry CH , that inoverpotential. The overpotential of the LSFC anode 4

fact they possess the high ionic conductivity, oxygenincreases slowly over a relatively wide current2 mobility and activity toward methane. However, thedensity range and rises obviously after 200 mA/cm .

electrochemical performances of mixed oxide anodesComparing three anodes, SFC exhibits the highest0.5

are, in general, inferior to the Ni-YSZ anode. Ni-electrochemical activity toward CH , resulting in the4

based catalysts are active catalysts for methanelowest activation overpotential at low current den-activation as well as conversion, and have beensities. However, due to the activation barrier or to aapplied for methane steam reforming traditionallylower conductivity (Table 2), which gives rise to aand the partial oxidation of methane or auto-reform-large overpotential at high current densities, theing recently [25]. Therefore, an idea has been putpower output of the SFC anode is dwarfed.0.5

forward to couple a mixed ionic–electronic conduct-The main products evolving from the electro-ing oxide with Ni-YSZ to form an interlayeredchemical oxidation over three oxide anodes whenanode. The mixed conducting oxide was sandwichedCH is fed at a flow-rate of 60 ml /min are generally4

between the YSZ electrolyte and the Ni-YSZ anode,same, CO and H . There is a trace of CO formed.2 2

hopefully functioning as a spatially extended electro-The results show that these oxides are not only activechemically active interlayer. The Ni-YSZ anodebut also selective as anodes for SOFCs for producingserves both as a catalyst for CH activation (to breaksyngas with the spontaneous generation of electrical 4

down C–H band) and as a current collector to reduceenergy. In the literature, SFC oxide has been used0.5

contact resistance. Fig. 5 shows the examples of theas a membrane in conjunction with a Rh catalyst forcross section SEM micrographs of the Ni-YSZ,partial oxidation of methane. Both high activity andSFC oxide and the interlayered anodes. Fig. 5aselectivity to CO and H were achieved [20]. 0.52

Fig. 5. The cross section SEM micrographs of the Ni-YSZ (7a), SFC (7b) and the SFC interlayered Ni-YSZ (7c) anodes.0.5 0.5


illustrates the porous Ni-YSZ anode structure with a the microstructure of Ni-YSZ on the top of thethickness of around of 8 mm. While the SFC anode SFC layer (Fig. 5c) is not identical to the Ni-YSZ0.5 0.5

displays about a 30-mm porous thick structure (Fig. electrode directly sintered on YSZ (Fig. 5a) due to5b). Both anodes have intimately sintered interfaces different sintering temperature. In order to make surewith YSZ electrolytes. The interlayered anode micro- that the improvement was not caused by the changestructure shows a small particle sized Ni-YSZ layer of the microstructure, an Ni-YSZ electrode preparedsintered on the top of the larger particle sized SFC on YSZ plate in an identical way was examined and0.5

oxide layer with a distinguishable interface (Fig. 5c). found very poor in electrochemical performance. It isThis configuration of the anode is different from the interesting to note that the performance of suchelectrochemical domain proposed by Thampi and interlayered anodes were pretty much poor whilecoworkers [15]. In their case, a thin layer of low pure H was used as fuel. As shown in Fig. 7, the2

conductivity, reducible oxides, such as MnO , CoO , cell performance of the SFC oxide interlayeredx x 0.5

etc., was beneath an Ni anode, playing a role of Ni-YSZ anode declines when even a mixture of CH4

charge relay so as to facilitate the redox process of and H is used as fuel. As the concentration of H2 2

the reaction on the anode. The results of our configu- increases; the overpotential increases rapidly at highration of the MIEC oxide /Ni-YSZ anodes on CH current densities. The understanding into this prob-4

are given in Fig. 6. The power outputs of three lem is not clear and is currently under investigation.MIEC interlayered anodes were improved signifi- Polarization analysis shows that the overpotentials ofcantly comparing to the power outputs of the respec- the interlayered anodes are all substantially reducedtive MIEC oxides as anodes alone(see Fig. 3). In comparing to the corresponding MIEC oxide anodesparticular, the SFC interlayered anode gave a (Fig. 8). The polarization overpotential order is0.5

much improved power output both in terms of the SFC /Ni-YSZ,LSFC/Ni-YSZ,SF C /Ni-0.5 0.2 0.82maximum power density and the closed circuit YSZ over the current range of 0–600 mA/cm ,

current density. The maximum power density was parallel to the order of MIEC oxides as anodes alone2 2remarkably improved and reached 0.25 W/cm . In at current densities below 100 mA/cm . This indi-

the other two cases, although there were improve- cates that the interlayer of MIEC oxide dominatesments in power output for MIEC oxide anodes, the the electrode performance. In other words, thepower outputs were lower or close to that of the electrode performance is largely affected by theNi-YSZ. As can be seen from the SEM micrographs, mixed oxide. Among three MIEC oxides, the SFC0.5

Fig. 6. Cell performances of three SOFCs with SFC , SF C0.5 0.2 0.8

and LSFC oxides interlayered Ni-YSZ anodes running on CH at Fig. 7. Effect of H concentration in CH fuel on the cell4 2 4

9508C. performance of the SFC interlayered Ni-YSZ anode at 9508C.0.5


electrochemical reaction zone, while the Ni-YSZanode mainly reduces the CH activation barrier and4

the ohmic resistances (contacting resistance). Asshown in Table 1, the ohmic resistances are indeedsignificantly reduced, indicating the improvement incontact between the MIEC layer and the currentcollector. The reason for the good performance ofSrFeC over the other two oxides is most likely0.5

associated with its superior mixed conductivities andstructure stability even at a very reduced oxygenenvironment, which has been demonstrated elegantlyin several studies [19,20,23]. Authors found that theionic conductivity remained significantly large, atleast two orders of magnitude higher than that of

216YSZ, at very low P down to 10 atm [19] andO2

the structure retained integrity when exposed toFig. 8. IR free overpotential-current density curves for both oxideanodes and oxide interlayered anodes at 9508C on CH . reducing gas such as CH [20]. In our experiments,4 4

SFC showed to be more stable than the other two0.5

oxides. The total and ionic conductivities were foundto be high enough under reducing atmosphere.

interlayered anode turns out to be the most effective The main products of the electrochemical reactionas shown in Fig. 9, which is replotted from the of methane over the interlayered Ni–YSZ anodesNi-YSZ, SFC and the interlayered anodes. Both were found to be CO1H , products of partial0.5 2

activation overpotential and ohmic resistance were oxidation. The reaction mechanism could be similarfound significantly reduced, resulting in a remark- to the direct partial oxidation, proposed by Schmit

2ably high output power density of 0.25 W/cm , [25] and Cao [26] for the catalytic partial oxidationexceeding those of both SFC and the Ni–YSZ as of methane over Rh or Ni-based catalysts at compar-0.5

anodes alone. The synergetic effect has been ob- able temperature range. The reaction involvesserved in this case. The results show that the methane decomposition to carbon and H over the2

existence of the MIEC oxide layer may extend the catalyst:

CH → C 1 2H (1)4 2

followed by the oxidation of adsorbed carbon speciesto CO.

C 1 1/2O → CO (2)2

This mechanism can be adapted to the electrochemi-cal oxidation of methane in SOFC, where the onlydifference is that oxygen is electrochemically sup-plied through the electrolyte instead of being co-fedfrom the gas phase. In principle, oxygen suppliedfrom the electrolyte is more favorable for selectiveoxidation to syngas since the deep oxidation of COwith gaseous O can be avoided. In this paper,2Fig. 9. Comparison of cell performances of SOFCs with theselectivities to CO and H are very high, close to2Ni-YSZ, SFC and SFC interlayered Ni-YSZ anodes at 9508C0.5 0.5

100% although methane conversion is still low,on CH .4


around 10% even at a significantly increased current Acknowledgementsdensity over the Ni-YSZ anode. It could be expected

Financial support from the Natural Sciencethat selectivities to CO and H can remain high for2

Foundation of China (Contract No. 29676044) andthe electrochemical oxidation of methane in SOFC asthe State Science and Technology Commission islong as the ratio of CH to O supplied from the4 2

gratefully acknowledged. Sincere thanks are ex-electrolyte keeps higher than the stoichiometric valuepressed to Simens AG, Germany, for providing theof two.YSZ plates.

4. ConclusionReferences

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[17] Y. Teraoka, H. Zhang, K. Okamoto, N. Yamazeo, Mat. Res.thickness needs detailed investigation.Bull. 5 (1998) 1.In the cases of CH as fuel, CH is partially4 4 [18] Y. Teraoka, H. Zhang, S. Furukawa, Yamazeo, Chem. Lett.

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[20] U. Balachandran, J.T. Dusek, R.L. Mieville, R.B. Poeppel, [23] B. Ma, J.-H. Park, U. Balachandran, Ibid 144(8) (1997)M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, A.C. 2816.Bose, Appl. Catal. A: General 133 (1995) 19. [24] Y.Z., Y. Jiang, S.Z. Wang, J.W. Yan, W.Z. Li, in preparation.

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Electrochemical performance of mixed ionic–electronic conducting oxides as anodes for solid oxide...

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