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*Corresponding author. Tel.: #44-131-650-7254; fax: #44-131-

650-6551.

E-mail address: [email protected] (M. Sahibzada).

Chemical Engineering Science 55 (2000) 3077}3083

Intermediate temperature solid oxide fuel cells operatedwith methanol fuels

Mortaza Sahibzada*, Brian C. H. Steele, Klaus Hellgardt, Dieter Barth, Astrid E! endi,Dionissios Mantzavinos, Ian S. Metcalfe

School of Chemical Engineering, University of Edinburgh, King ' s Buildings, Edinburgh EH9 3JL, UK

Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK

Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK

Abstract

Solid oxide fuel cells were fabricated with a 5 m thick "lm of Ce

Gd

O

electrolyte, supported on a NiO}YSZ cermet anode

with a La

Sr

Co

Fe

O

cathode, for operation at temperatures below 7003C. The performance for power generation was

studied with externally reformed methanol fuel or with methanol/water vapour fed directly to the anode. The open-circuit voltages of

0.7}0.8 V were lower than the theoretical potential as a result of electronic permeation across the electrolyte "lm. Maximum power

outputs of 126, 65 and 32 mW/cm were obtained at 650, 600 and 5503C, respectively, using the externally reformed fuel. With direct

methanol fuel the maximum power output was lower at each temperature by 16, 38 and 53%, respectively. The latter result at 550 3C

was due to the poor internal reforming of methanol to hydrogen at the anode. The addition of Pd to the anode promoted the internal

reforming at 5503C: the maximum power output was similar to the previous result with externally reformed fuel and 45% lower with

direct methanol fuel. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Reaction engineering; Electrochemistry; Ceramic "lm; Fuel; Solid electrolyte; Methanol fuel; Internal reforming; Power generation; Ceria;

Ni anode

1. Introduction

Solid oxide fuel cells (SOFCs) o! er the potential for

e$cient power generation in distributed power systems

and vehicular applications, resulting in lower CO

emis-

sions. One advantage of SOFCs over other types of fuel

cells is the easier use and durability of solid (rather than

liquid) electrolyte materials. SOFCs are operated at

higher temperatures than other fuel cells, so providing

#exibility in the type of fuel (e.g. light hydrocarbons and

oxygenates may be used). In addition, the reforming of

the fuel to hydrogen can be integrated with the SOFC, i.e.

internal reforming can be carried out directly at the

anode of the fuel cell.

Conventional SOFCs incorporating yttria-stabilised

zirconia (YSZ) as the electrolyte are operated in the

temperature range of 900}10003C (Bevc, 1997). This

places considerable constraints on the materials which

can be used for interconnections and balance-of-plant

(e.g. the interconnect material LaCrO

is prone to degra-

dation during long-term operation) (Steele, 1994). Lower-

ing the operating temperature would relax the materials

constraints. At intermediate temperatures of 500}7003C

it would be possible to use stainless steels for the inter-

connects and balance-of-plant (e.g. ferritic stainless steel

is compatible with the ceramic components we use below

in terms of its thermal expansivity). This would make the

fabrication of SOFCs much more cost-e! ective, parti-

cularly for vehicular applications.

We have been investigating SOFCs fabricated with

Ce(Gd)O

electrolyte "lms tape cast onto the sup-

porting anode with the overall fuel cell arrangement

La

Sr

Co

Fe

O

(cathode)/Ce

Gd

O

(electrolyte)/NiO}YSZ cermet (anode) (Sahibzada et al.,

1996; Sahibzada, Steele, Zheng, Rudkin & Metcalfe,

1997; Sahibzada, Benson, Rudkin & Kilner, 1998;

Sahibzada, Steele, Barth, Rudkin & Metcalfe, 1999).Other groups are investigating similar systems

(Godickemeier, Sasaki, Gauckler & Riess, 1996; Doshi,

0009-2509/00/$- see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 09 - 2 5 0 9 (9 9 ) 0 0 5 69 - 2

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Richards, Carter, Wang & Krumpelt, 1999).

Ce

Gd

O

(CGO) exhibits over an order of mag-

nitude higher ionic conductivity than YSZ. The use of

this novel electrolyte in place of YSZ allows the temper-

ature to be lowered whilst maintaining a su$ciently high

ionic conductivity. In addition, preparing the electrolyte

in the form of a dense ceramic "lm of say 10 m thickness

reduces the resistance to ionic transport, enabling aneven lower operating temperature. However, lowering

the operating temperature retards the kinetics of reac-

tions associated with the fuel}electrode (anode) and oxy-

gen}electrode (cathode) (Weston & Metcalfe, 1998;

Sahibzada et al., 1998; Hartley, Sahibzada, Weston

& Metcalfe, 1999). Where the fuel is fed directly to the

SOFC, the internal reforming of the fuel at the Ni-based

anode is one such reaction.

In the present study we investigated SOFCs operated

at 550}6503C with methanol-steam fuel. This fuel was

provided via an external reformer or alternatively fed

directly to the anode. A gas chromatograph was used to

measure the products of reforming and combustion of

fuel at the anode. The current}voltage performance of the

cell with the two fuel types was characterised. Hence, we

were able to evaluate the e! ect of the internal reforming

reaction at the anode on the power output of the cell.

Subsequently, the anode was modi"ed by the impregna-

tion of Pd with the aim of promoting the internal reform-

ing catalytically. For comparison, experiments were also

carried out with moist hydrogen and with dry methanol

fuels fed to the anode. (Note that in the external reformer

we used a Cu-Zn based catalyst which is the industriallow temperature (2503C) methanol synthesis catalyst and

therefore was considered to be suitable for the reverse

reaction, i.e. reforming. On the other hand, Cu or Zn is

not stable at the operating temperature of the SOFC and

so we required an alternative metal catalyst, Pd, to

promote the direct reforming at the anode.)

2. Experimental

A porous solid #at disc of NiO}YSZ cermet (1 mm

thick25 mm diameter, provided by E.C.N., Holland,

pre-sintered at 14003C) was used as the anode as well as

a physical support for the Ce

Gd

O

(CGO) elec-

trolyte "lm. A suspension of CGO (supplied by Seattle

Speciality Ceramics), containing an organic solvent and

a plasticiser, was tape cast onto one side of the anode

support. The assembly was sintered at 14003C for 3 h to

form a dense impermeable CGO "lm of approximately

5 m in thickness. After this, the powdered cathode

material, La

Sr

Co

Fe

O

(LSCF, prepared in

house), suspended in an organic solvent, was painted on

top of the CGO "lm. Two pieces of Pt mesh were appliedto the cathode and anode surfaces to act as current

collectors, and the combined structure was sintered at

12503C for 4 h. The active cathode area was approxi-

mately 2.5 cm which we take as being the planar area of

the fuel cell. A "nal preparation step for selected experi-

ments was the impregnation of Pd to the anode. A dilute

Pd nitrate solution was dropped onto the anode support

side; the fuel cell was then dried and calcined at a mild

temperature to decompose the nitrate. The approximate

Pd loading was 1.8 mg/cm.The fuel cell was placed like a cap over one end of an

open AISI 446 alloy pipe (also of 25 mm diameter) with

the cathode facing outwards in order to be exposed to air

and the anode facing into the pipe in order to be exposed

to fuel contained within the pipe. Glass sealant was

applied between the cell edge and pipe rim; subsequent

heated to the melting point of glass for a few minutes

formed a gas tight seal. The cell-pipe assembly was

placed into a tubular furnace with the cathode exposed to

air in the furnace. A long thin steel feed line was inserted

through the remaining open end of the pipe, running

concentric along the inside of the pipe. This allowed

a fuel feed to be preheated to the furnace temperature

before passing through a di! user which dispersed the fuel

across the anode. After reaction at the anode, the spent

fuel #owed back in the opposite direction between the

outside of the feed line and inside of the pipe (all the

piping materials had been tested in the absence of the fuel

cell and no pre- or post-reaction was detected). The

e%uent passed to a gas chromatograph for product

analysis.

The products of reforming and other reactions of the

fuel at the anode were analysed by means of an ATIUnicam 610 series gas chromatograph. A Porapak

column and a TCD detector allowed separation and

analysis of CO, CO

, CH

, H

O and CH

OH. The H

concentration in the product was calculated from an

atomic balance, assuming that carbon mass was conser-

ved between the feed and the product and ensuring the

H : C atomic ratio across all the products was the same

as that in the feed. The O : C atomic ratio over all

products from the anode gave an indication of the

amount of oxygen transported from the cathode through

the electrolyte (see Table 1).

Various fuels for the SOFC were prepared as follows:

Externally reformed CH 3OH fuel * An equimolar

liquid mixture of CH

OH/H

O was injected by means

of a syringe pump to a tubular evaporator, which was

held at 1503C. The liquid #ow rate was controlled such

that vapour was generated at 15 ml/min (STP). The

vapour passed to the reformer, which consisted of

a "xed Cu/ZnO/Al

O

catalyst bed at 2503C, before

entering the feed line to the anode. Reaction (1)

occurred almost to completion in the (external) re-

former, producing H/CO with a molar ratio of 3 anda #ow rate of approximately 30 ml/min (STP). The

composition of the reformate measured by gas

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Table 1

Products of reaction over the anode during open-circuit operation of the fuel cell

Fuel type H

(%) CO

(%) CH

OH (%) H

O (%) CO (%) CH

(%) Ratio O : C

Composition of fuel feed to the anode

Moist H

77.5 19.5 * 3 * * 2.15

Reformed CH

OH 67.5 21.5 1 6.5 2.5 1 2.04

CHOH/HO * * 50 50 * * 2CH

OH * * 100 * * * 1

Composition of products at 6003C

Moist H

64.5 15 * 14.5 2.5 3 2.29

Reformed CH

OH 55 18.5 * 17.5 4.5 4 2.19

CH

OH/H

O 46.5 14.5 1 25 6.5 7 2.12

CH

OH 44 6 3.5 11 22 13.5 1.08

Composition of products at 5503C

Moist H

71.5 16.5 * 8.5 2.5 0.5 2.26

Reformed CH

OH 63 19 * 11 5.5 1 2.14

CH

OH/H

O 45.5 11.5 6.5 24.5 8.5 3.5 2.08

CH

OH 40.5 3 21.5 6.5 20.5 7.5 1.04

Composition of products at 5503C with Pd-promoted anode

Moist H

70.5 16.5 9.5 2.5 1 2.25

Reformed CH

OH 62 19 12.5 5.5 1.5 2.15

CH

OH/H

O 46.5 13 4 24 7.5 5 2.08

CH

OH 42.5 3.5 14 8 22.5 10 1.03

The original composition of fuel feed to the external reformer was 50% CH

OH, 50% H

O, where the ratio of O : C was 2.

At 5503C with direct CH

OH fuel, the O : C ratio increased from 1 in the feed to 1.04 in the product, indicating 4% excess of atomic oxygen in the

product as a result of electronic/ionic leakage. Given that the CH

OH vapour feed rate was 10 ml/min (STP) or 450 mol/min, the rate of ionic oxygen

permeation was 18 mol/min. The rate of permeation was similar with other fuels at this temperature, and approximately 50% greater at 6003C. These

rates of ionic permeation are signi"cant, being equivalent to an electronic current of approximately 50 mA at 5503C or 75 mA at 6003C. The O : C

ratios were greater under closed-circuit conditions as the oxygen ion #ux through the electrolyte increased in relation to the current in the external

circuit.

The product molar #ow rates can be calculated from the compositions given in the table, assuming that there was a mass balance between the feedand the product. An example calculation for the CH

OH product from the direct CH

OH fuel at 5503C with the Pd promoted anode is as follows. The

CH

OH vapour feed rate was 10 ml/min (STP) or 450 mol/min. The carbon ratio of the CH

OH product relative to other products was

14/(3.5#14#22.5#10)"0.28. Hence the CH

OH product rate was 126 mol/min.

chromatography is given in Table 1. Similar results of

external reforming have been observed previously

(Jiang, Trimm, Wainwright & Cant, 1993).

Direct CH 3OH / H 2O fuel * An equimolar liquid mix-

ture of CH

OH/H

O was injected via the syringe

pump to the evaporator. The vapour, with a #ow rate

of 15 ml/min (STP), passed directly to the anode feed

line (internal reforming at the anode by reaction (1)

would again produce a #ow rate of 30 ml/min assum-

ing complete reforming).

Experiments were also carried out with a moist H

fuel.

H

/CO

, with a molar ratio of 4, was provided from

a pre-mixed gas cylinder. A mass #ow controller de-

livered 25 ml/min (STP) through a room temperature

saturator and onto the anode feed line. The saturator

resulted in the addition of approximately 3% water

vapour (see Table 1). Finally, a dry CH

OH fuel was fed directly to the

SOFC. Here, pure CH

OH liquid was evaporated (as

above) at 10 ml/min (STP) and passed directly to the

anode feed line (the decomposition of CH

OH by

reaction (2) would evolve H

/CO with a molar ratio of

2 and a #ow rate of 30 ml/min assuming complete

reaction).

CH

OH#H

OPCO#3H

, (1)

CH

OHPCO#2H

. (2)

Electrical power generation by the fuel cell was investi-

gated at temperatures of 550, 600 and 6503C. Pt wires

connected the current collectors at the anode and cath-

ode to a Thompson Ministat precision potentiostat/

galvanostat. This instrument, operated in galvanostatic

mode, allowed the voltage across the cell to be measured

with a controlled electrical current being drawn in the

external circuit. The open-circuit voltage (OCV), i.e. thevoltage at zero current, was measured initially. After

introducing a particular fuel to the anode at the set

M. Sahibzada et al. / Chemical Engineering Science 55 (2000) 3077 }3083 3079

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Fig. 1. Cross section of LSCF/CGO (&5 m)/Ni}YSZ after 30 h op-

eration.

temperature, the OCV increased to a steady value within

a few minutes (the transient can be associated with the

reduction of NiO to the metal). Subsequently, the cur-

rent}voltage characteristic was obtained by drawing cur-

rent at incremental steps and measuring the cell voltage

at each step at steady state. The cell performance did not

deteriorate over a period of many weeks; this was

checked by carrying out a standard test with moistH

fuel in between other tests.

The microstructure of the cathode/electrolyte/anode

composite was examined by scanning electron micros-

copy. Fig. 1 shows a micrograph of a perpendicular cut

section of the fuel cell after 30 h operation at 550}6503C

under moist H

fuel. The individual structural elements

were found to exhibit good interfacial contact without

signs of delamination. Highly porous electrodes can be

seen, while the CGO electrolyte "lm was dense ('95%

of theoretical density) with a thickness of approximately

5 m.

3. Results and discussion

3.1. Product analysis

Table 1 shows the compositions of di! erent fuels sup-

plied to the SOFC. The table also shows the product

distribution after passing each fuel over the anode at 550

and 6003C under open-circuit conditions (i.e. with no

current being drawn from the fuel cell). With no current

being drawn, there should be negligible supply of oxygenthrough the electrolyte and therefore the fuel reactions

should be limited to reforming (1), decomposition (2), and

the water-gas shift (3). However, in all cases the product

contained a higher overall atomic O : C ratio than the

feed fuel. This is because Ce

Gd

O

becomes

somewhat reduced in the presence of H

and is then not

a perfect electrolyte (Sahibzada et al., 1996, 1997;

Godickemeier, Schneider & Gauckler, 1997). The resul-

tant electronic (e) leakage, or &internal short circuiting',

is accompanied by ionic (O) permeation across theelectrolyte to maintain charge balance (a sample calcu-

lation of the rate of ionic permeation is given in footnote

b in Table 1). The supply of oxygen ions leads to electro-

catalytic combustion reactions at the anode, e.g. the

oxidation of H

(4) or the oxidation of CO or CH

OH.

Finally, CH

formation also takes place, e.g. through the

hydrogenation of CO (5) or the hydrogenation of CO

or

perhaps the direct reduction of CH

OH.

CO#H

PCO#H

O, (3)

H#OPHO#2e, (4)

CO#3HPCH

#H

O. (5)

The moist H

and externally reformed CH

OH fuels,

which contained mainly H

and CO

, were involved in

reactions (3)}(5) and the extents of these reactions were

greater at the higher temperature (see Table 1). The direct

CH

OH/H

O fuel was internally reformed by reaction

(1). Given that the H

O concentration in the product was

greater than the CH

OH concentration, some of the

CH

OH may have decomposed by reaction (2) or alter-

natively water production by reactions (3)}(5) can explainthe large fraction of H

O in the product. The presence of

CH

OH in the product at 5503C indicated incomplete

reforming. The dry CH

OH fuel was mainly involved in

decomposition (2) at the anode, although the extent or

activity of this reaction was rather low, particularly at

5503C (see Table 1). Again, subsequent reactions (3)}(5)

were evident from the product distribution. Some general

trends in the product composition emerge from Table 1.

The product H

concentration decreases in the order,

moist H'reformed CH

OH'CH

OH/H

O'

CH

OH. The H

concentration derived from the last

two fuels was limited because of the incomplete extent

of internal reforming (1), some decomposition (2)

which results in lower stoichiometric H

production

than reaction (1), and greater production of CO (3) and

CH

(5) which consume H

.

CO concentration increases in the opposite order,

moist H'reformed CH

OH'CH

OH/H

O'

CH

OH, which follows from the increasing prevalence

of decomposition (2), particularly in the case of the dry

CH

OH fuel. CH

concentration also increases in this

order which suggests that reaction (5) is indeed themajor route for CH

formation (rather than CO

hydrogenation for example).

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Fig. 2. Current}voltage performance at 6503C under hydrogen and

methanol fuels.


methanol fuels.


methanol fuels.

With moist H

or reformed CH

OH, there was

a lower H

concentration at the higher temperature as

a result of a greater extent or rate of H

conversion by

reactions (3)}(5). However, with direct CH

OH/H

O

or CH

OH, there was a lower H

concentration at the

lower temperature as a result of the relatively slow

internal reforming (1) and decomposition (2).

3.2. Fuel cell operation

Figs. 2}4 show the steady-state current}voltage char-

acteristics of the SOFC under various fuels at 650, 600

and 5503C, respectively. In all cases the open-circuit

voltage (OCV) varied between 0.7 and 0.8 V. The theoret-

ical cell potential can be calculated by the Nernst equa-

tion (Sahibzada et al., 1997) and is in the region 1.0}1.1 V

for the fuels used in this investigation. The inferior experi-

mental voltages were due to internal short circuit across

the electrolyte (described above). This e! ect is more seri-

ous at higher temperature (as a result of greater reduction

of the oxide electrolyte), which explains the increasing

OCV from Fig. 2 to Fig. 4. We note that internal short

circuiting is a highly undesirable e! ect, leading to partial

combustion of the fuel and consequently reduced e$-

ciency of power generation. In the present study, the rate

of electronic leakage was enhanced by the electrolyte

thickness being only 5 m; the problem is much less acute

with self-supporting 300 m thick electrolyte plates

(Sahibzada et al., 1996). Consequently, the use of

Ce(Gd)O

as a solid electrolyte is being questioned;

newly discovered perovskite materials exhibit similarionic conductivity but substantially less electronic con-

ductivity under reducing conditions, suggesting that the-

oretical cell potentials may be approached (Ishihara,

Akbay, Furutani & Takita, 1998).

The gradient of each current}voltage pro"le shown in

Figs. 2}4 gives an indication of the overall resistance

associated with all the transport/reaction processes oc-

curring in the fuel cell. The greater resistance (steeper

gradients) at lower temperature is the result of decreased

ionic conductivity in the electrolyte, and the slower

kinetics of reactions and mass transport at the cathode

(Sahibzada et al., 1998) and the anode. At any one tem-

perature, the resistance was a! ected by the fuel supplied

to the anode, increasing in the order, moist H(re-

formed CH

OH(CH

OH/H

O(CH

OH. This may

be explained in part by the lower H

concentrations (see

Table 1) and its e! ect on the kinetics of electrocatalytic

combustion (4). The large resistance in the case of the

direct methanol fuels (at any one temperature) was the

result of the additional internal reforming/decomposition

reactions which must take place. These gradients were

particularly steep at lower temperatures and in the case

of the dry CH

OH fuel, consistent with the slow kinetics

of reactions (1) and in particular (2), and the incomplete

extents of these reactions shown in Table 1. Furthermore,

Fig. 4 provides evidence for limiting currents in the caseof the direct methanol fuels, where the gradient tends to

in"nity as the rate of combustion (4) approaches the rate


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5503C and particularly with dry methanol fuel due to the

poor kinetics of internal reforming/decomposition and

hence the restricted availability of hydrogen. The addi-

tion of Pd to the Ni-based anode promotes the internal

reforming/decomposition of methanol and reduces the

limitation imposed by the direct use of methanol fuels.

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Sr

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