46(2002)3-14 part i the cathode challenges

8/9/2019 46(2002)3-14 Part I the Cathode Challenges

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Catalysis for

Low

Temperature

Fuel

Cells

PART

I:THE CATHODE CHALLENGES

By

T. R.

Ralph

and

M.

P. Hogarth

Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH. U.K.

Much of the pegotmance still to be gained in proton exchange membranefuel cells

PEMFCs )

in use today is available rom improvements

to

the cathode traditionally made rom unsupported

or carbon-supported platinum. The search or improved cathode electrocatalysts has resulted

in the development ofplatinum alloys which iftailored to the desired stack operating conditions

can double the activiry or oxygen reduction. Recently advances have been made in cathode

design which have raised perjormance levels in

PEMFCs.

The new electrocatalysts and cathode

designs have increased electrical eficienc.y and power densities to the

PEMFC

stack needed

for commercial use. Improvements have also been achieved at the anode by developments

in platinum-ruthenium anodes for carbon monoxide and cell reversal tolerance. In this fir st

paper; new cathode materials and designs

are

discussed; a second paper

to

be published in

the April issue will look at anode advances.

The fuel cell of choice for a wide range of appli-

cations spanning portable, stationary and

transportation markets is the proton exchange

membrane fuel cell (PEMFC)

(1).

This is princi-

pally because of the high power density and the

relatively low temperature of operation. Today the

PEMFC

typically operates at close to 80°C

although there is

a

desire to move to &her tem-

peratures close to

150°C

to mitigate the effects

of

carbon monoxide

(CO)

poisoning at the anode.

The basic unit cell in the

PEMFC

stack is shown

in

Figure 1.The membrane electrode assembly

@A

is the key component where hydrogen and

air

react

electrochemically

o

generate elecmcal power. The

MEA is typically located between a pair of flow

field plates to give a single cell. The flow field

plates are designed to distribute the reactant gases

across the face of the

MEA

and also to collect the

electrical current

from

the

MEA.

Sufficient unit

cells are connected electrically to generate the

desired power output

(1).

Depending on the appli-

cation, a

PEMFC

system may contain from tens to

a few thousand MEAs to produce from a few

watts to several hundred kilowatts of power.

The ME A is a five layer structure containing

at the centre the proton exchange membrane

Fig.

I

The bnsic unit cell of a

proton exchange membrane fuel

cell is assembled in the required

numbers tv deliver the necessay

power output. The component

lavers

of

the membrane electrode

assemblv

two

catalvsed substrates

nnd the PEM electrolyte) are

laminated together and located

between the flow field plates to

form the unit cell

Phtinrm Metah

Rm, 002,46, l),S 1 4

3


2/12

electrolyte which separates the electrode structures

to prevent reactant gas mixing and the formation

of an electrical short. Each electrode consists of a

gas diffusion substrate with the platinum-based

(Pt) electrocatalyst layers located between the

membrane and the substrate. The electrocatalyst

can be deposited either

on

he gas diffusion sub-

strate or on the proton conducting membrane

electrolyte using techniques such as screen print-

ing, flexographic printing, gravure printing,

spraying or rolling and calendering.

Electrocatalyst layers are typically from 5 to 20

p hick with the complete MEA being around

400 to 500p hick. The MEA layers are normal-

ly bonded together by hot pressing catalysed

substrates

to

the membrane or, in the case of catal-

ysed membranes, by compressing the

gas

diffusion

substrate to the membrane during stack assembly.

Electrocatalyst Layers

In the MEA the electrocatalyst layers employ

platinum group metal electrocatalysts to generate

the electrochemical power by the reduction

of oxy-

gen at the cathode and the oxidation of hydrogen

at the anode. Pt-based electrocatalysts are required

to provide stability in the corrosive environment

of the PEMFC. These are also the most active

electrocatalysts for oxygen reduction and are

among the most active for hydrogen oxidation. In

pre-commercial PEMFC systems, carbon-support-

ed Pt is employed at the cathode and carbon-

supported platinum-ruthenium at the anode.

Carbon Black Supports

To

achieve economical Pt loadings in the MEA

(<

1

mg Pt cm-2) the electrocatalysts are supported

on high surface area carbon blacks with a high

mesoporous area

(>

75 m2 g-'

C

and a degree of

graphitic character. Common supports are avail-

able from Cabot Corporation (Vulcan XC72R,

Black Pearls BP 2000 ,Ketjen Black International,

Chevron (Shawinigan) and Denka. The support

material must provide a high electrical conductivi-

ty, give good reactant

gas

access to the electro-

catalyst, have adequate water handling capability,

particularly a t the cathode where water

is

generat-

ed, and also show good corrosion resistance,

Table

I

Relationship between the P latinum Loading of

a

Ketjen Carbon Black Supported Electrocatalyst

and the Catalyst Metal Area

wt.

Pt

on carbon

40

50

60

70

XRD

Pt crystallite

size, nm

2.2

2.5

3.2

4.5

CO

chemisorption

metal area,

m2 g-' Pt

120

105

88

62

especially under the highly oxidising conditions

which occur at the cathode.

An

important design criterion is that the elec-

trocatalyst layers should be reasonably

thin.This

minimises the cell potential losses due to the rate

of proton diffusion and reactant gas permeability

in the depth of the electrocatalyst layer, which can

become limiting as the current density increases

during PEMFC operation (2). In contrast to auto-

catalysts and process catalysts which have metal

loadings of less than 5 wt. , PEMFC electrocata-

lysts typically have very high metal loadings of

2

40

wt. , to minimise the electrocatalyst layer thick-

ness. This challenges the catalyst manufacturer.

High metal loading electrocatalysts must be pro-

duced in volume with high metal dispersions.

This can be achieved by precipitating the Pt

group metals using chemical reduction and an

aqueous

slurry

of the carbon black support

(3).

The preparation provides the ease of manufacture,

high yield and reproducibility necessary for the

production of electrocatalyst at batch sizes from 1

to 25 kg and above. For example, Table

I

shows

typical Pt dispersion properties for

40

to 70

wt.

Pt on Ketjen carbon black supported electrocata-

lysts prepared at Johnson Matthey.As expected, as

the metal loading increases on the carbon support,

the Pt crystallite size increases. This is directly

reflected by a reduction in the metal area available

for

gas

phase CO chemisorption. Even at

70

wt.

Pt,

however, the Pt crystallite size is

still

less than

5 nm and the metal area is still above 60 m2 g-' Pt.

This is significantly better than the Pt crystallite

Pkzfinzm Metah Rm., 002,46, (1)

4


3/12

size of 5.5 to 6

nm

and CO metal area of 20 to 25

m2g-' Pt for an unsupported Pt black electrocata-

lyst that was used by Ballard Power Systems in the

early years of MEA development 4).

Figure 2 shows the improved Pt dispersion on

the microstructure of a 50 wt. Pt supported on

Ketjen electrocatalyst. The individual

Pt

crystal-

lites

(2-3 nm)

are visible on the primary particles

of the carbon black that fuse together to form the

carbon aggregates which are typical of fuel cell car-

bon supports.

It is

the mesoporosity within these

aggregates that gives the carbon blacks their gas

diffusion and water handling capability.

Polymer Solutions

In addition to the electrocatalyst, a second key

component in the electrocatalyst layer is the pro-

ton conducting polymer electrolyte. If the

electrocatalyst is to be utilised

in

the PEMFC elec-

trode reactions, there must be contact between

the

Pt-based electrocatalyst and the protons present in

the membrane electrolyte. It has been shown

(5)

that the utilisation is low unless the electrocatalyst

layer contains a soluble form of the proton con-

ductmg membrane electrolyte.

A number of groups employ organic perfluoro-

sulfonic acid polymer solutions

(6)

but at Johnson

Matthey the corresponding aqueous solutions are

usually employed

(7).

This

reduces the risk of

sin-

tering the electrocatalyst during the preparation

of

the electrocatalyst layer - due to the possibility of

reaction between the Pt-based electrocatalyst and

the organic solvents in the polymer solution.

There

is

also less concern that ttace levels of

organic material

wil l

be left

in

the electrocatalyst

layer after

MEA

manufacture. It is difficult

to

completely remove organic materials from the car-

bon black supports used

in

the PEMFC due to the

good adsorption properties of their highly porous

structure. Any ttace contaminant might react with

the electrocatalyst and poison the Pt or change the

water handling capability of the cathode, which

would eventually produce a water-flooded MEA.

Both effects significantly reduce the h4EA perfor-

mance, although the drop in performance may not

be seen until the PEMFC has been operating for

several thousands of hours.

Fig. 2 Trunsmission rlecrron microgruph showing flir

P t

ctytollire

distriburiori fo r

n

50

wr.8

Pr

supported on

Krfjen

cnrlmi

black

fuel cell

curulyst

Aqueous polymer solutions are also much

more friendly

in

terms of the volume manufacture

of

MEAS,

as using them avoids the risks and costs

associated with solvent h a n h hrough an

ME A

plant. Aqueous technology does, however, present

a challenge

in

terms of producing a suitable

nk

from the electrocatalyst and the aqueous polymer

solution for preparing the electrocatalyst layer.

Johnson Matthey have devoted much effort

to

ensure that the electrocatalyst is completely wetted

by the polymer solution and that an nkwith a sub-

micron particle size distribution and the correct

rheology is produced.

Cathode Electrocatalysts

A typical performance from an MEA is shown

schematically

in

Figure 3. This illustrates the dra-

matic impact that the cathode has on MEA

performance when operating the anode on pure

hydrogen. At all practical current densities, tens

of

mV are lost at the anode. At the cathode, howev-

er, even on pure Pt the most active oxygen

reduction material, in excess of 300 mV are lost

from the thermodynamic potential for oxygen

reduction at low current densities, due to the com-

parably sluggish electrochemical kinetics.

This

is

reflected

in

exchange current densities of around

PIatinwmMetuls

Rev., 2002, 46,

(1)

5


4/12

CURRENT

DENSITY.

n A ern-

lo-'' to lo-'' A cm-' for oxygen reduction

on

Pt at

ambient temperature.

The mechanism of oxygen reduction is extrem-

ely complex and a number of factors contribute

to

a lowering of the catalytic activity

on

Pt. The

requirement to break a strong

0 0

bond early

in

the ditect 4-electron reduction (Reaction (i)),

which

is

the desired pathway

to

maximise the elec-

trical efficiency

of

the PEMFC,

is

notable.

In

addition the open circuit voltage (OCV) is lowered

from the thermodynamic potential for oxygen

reduction

on

Pt due

to

the production of some

peroxide (Reaction 2)) and the formation of a

range of possible platinum oxides (Reaction

iii))

at

hlgh cell potentials.

0 2 + 4H' +

4

2H20

E 25-c

= +1.23

V VS.NHE)

i)

0 2 +

W +

2e

= H202

E02y~ +Oh8

V VS.

NHE)

ii)

Pt

+

H20 = Pt-0 + 2H'

+

2e-

Eo2yc

=

+0.88

V

VS.

NHE)

i)

Platinum

Alloys

The development

of

a more active oxygen

reduction electrocatalyst than Pt has been the sub-

ject of extensive research for a number of decades

(8). In

the 1980s UTC developed Pt-based metal

alloy electrocatalysts supported

on

carbon black

using carbothermal reduction. In the phosphoric

Fig. 3

Schematic

showing the

typical ceN

potential versus

current density out

put from a n MEA

operating onpure

hydrogen. The

major factors that

control the MEA

performance

in

the

various regions

of

the cell potential

versus current

density curve are

ident8ed along

with their relative

contribution to

the

electrical efficiency

losses

acid fuel cell (PAFC)

Pt

alloys developed by UTC

showed a higher kinetic performance than the pure

Pt analogues. The gain in performance was of the

order of 28

mV

n the linear region

of

the Tafel

plot. Since the Tafel slope was 90 mV decade-' at

the operating temperature

of

180°C, the

gain

cor-

responds to a 2-fold increase in the catalytic

activity for oxygen reduction and a

2%

increase in

the electrical efficiency of the fuel cell. The most

stable system selected for commercialisation in the

PAFC was found

to

be a ternary PtCr alloy sup-

ported on a graphitised furnace carbon black.

Kinetics

Based

on

the improved performance in the

PAFC, there has been much investigation of the

'intrinsic' kinetic performance in the PEMFC of

a

range of Pt-based metal alloy electrocatalysts

(9,

10). We prepared PtFe, PtMn, PtNi, PtCr, PtZr

and PtTi alloys, containing 20 wt. Pt, supported

on

Vulcan XC72R (9). X-ray diffraction confirmed

from a contraction (Fe,

Mn,

Ni, Cr, Ti) or expan-

sion (Zr) of the face centred cubic Pt lattice

parameter, that Pt alloys were formed, at an atom-

ic ratio of

Pt

to base metal of 75:25. The kinetic

performance in the cathode

of

MEAs, prepared by

hot pressing catalysed substrates to Dow

XUS13204.10 membrane, was evaluated in a small

single cell. The single cell operating conditions

were selected to minimise the cell potential losses at

the anode and to reduce the mass transport losses

PLafinrrm

Metah

Re .,

2002 46,

1)

6


5/12

F i g 4

7ufel

plots

.showing

tlie 'intrinsic kinetic benejit

fbr o q g e n reduction of

PtFe,

PtMn

and PtCr

alloys

conipured

to

pure

P I

supported on

Vulcan

XC72R

it 20 wr. O c PI.

The

MEAs

(<

/rig P I em- )

Lire based

on

catalysed

substrates bonded to

Dow

XUSl3203.10 mernhrane

electrolyte. A srnall single

cell 25 ni')

i s

operuting at

75°C. in hvdrogen/o.rygen,

30W377 kPa. 300/300 cin-'

niin 75 C/85 Cbottle

liumidification

950

5

E

W 900

W

LL

u

A-

5

850

I-

W

a

_I

v

B OO

750

100

1000

10000

SPECIFIC

ACTIVITY,

P A crn-'l P t )

at

the cathode. This was achieved by running the

single cell on pure hydrogen and oxygen, and by

using

high reactant gas flow rates.

This

procedure

ensured that the kinetic performance of the

cathode was controlling cell performance.

The 'intrinsic' kinetic performance of the

Pt

alloy and the pure Pt electrocatalysts was com-

pared on a specific activity basis to take account

of

any differences in the

Pt

surface area

of

the mate-

rials operating in the fuel cell. Specific activities

were calculated from the current density,

i,

and the

electrode

Pt

surface area (EPSA):

EPSA is a measure of the

maximum Pt

surface

area available for reaction in the cathode. The cath-

ode EPSA was measured by employing cyclic

voltammetry

i n - s h

n the single cell

to

measure the

protonic contact between the proton conducting

electrolyte and the Pt electrocatalyst.This nvolved

measuring the charge for the electrooxidation of

CO to COZ from the cyclic voltammogram and cal-

culating the EPSA based on the established ratio of

charge

to Pt

surface area for the catalyst

(4).

Tafel plots were constructed by obtaining the

MEA

area resistance,

R,

sing a non-linear least

squares analysis of Equation

[l]:

i/EPSA (A cm-z/cmz

Pt

cm-3

Ecd

= E,- b

logi

iR

I

where E,a is the cell potential, E, is a constant

which is dependent on the

cell

operating conditions

and the cathode electrocatalyst, b is the Tafel slope

for oxygen reduction and i is the current density.

Representative Tafel data for PtFe, PtMn and

PtCr alloys compared

to

pure Pt are shown in

Figure 4

(9).

The kinetic performance of a range of

Pt alloy cathodes was found to be of

the

order of

25

mV higher than the corresponding pure Pt

cathode in the linear region of the Tafel plot. The

Tafel slope was 60 mV decade-' at the operating

temperature of 80°C. In contrast

to

planar bulk

Pt

studies, where

a

doubling in Tafel slope corre-

sponding

to

a reduction

in

the Pt oxide surface

at

less positive cathode potentials is often reported

(for example

(ll)),

there was no indication of a

change in Tafel slope.

This

increased performance

confirmed that the improved activity found in the

PAFC also translated to the PEMFC environment

with

a

2.5-fold increase in the 'intrinsic' kinetic

activity for oxygen reduction with the Pt alloys.

Agam this

resulted in

a

2% increase in the elect r-

cal efficiency of the PEMFC.

Stability

The stability of the

Pt

alloys was also examined

by monitoring the membrane and the anode of the

MEA

for base metal content both before and after

operation in

the

single cell using electron probe

microanalysis (EPMA). This showed that only

the more stable PtCr, PtZr and PtTi base metal

doys were not leached into the acidic membrane

Phtimm Metah

b.

002,46, (1)

7


6/12

>

CURRENT DENSITY, mA c 6 ’

Fig.

5

For air operation, in Ballard Mark

5 E

hardw are, the kinetic benefit

of a

PtCr alloy cathode is masked by mass

transport losses. The comparative performance oft he PtCr alloy and a pure

Pt

cathode electrocatalyst is shown using

aic helox (21% 2 in helium ) and O2as oxidants and H, as fuel. The

MEAs

(<

I mg Pt

substrates bonded to Nafion 115 membrane electrolyte. The cell is operated at

80°C

in hydrogedaic helox, oxygen,

30W308 kPa. IS/.? 2, 10 stoichiometry,full internal membrane hu mid$cation

are based on catalvsed

electrolyte. The PtFe, PtMn and PtNi alloys

deposited Fe, nd Ni, respectively, in the mem-

brane and in the anode of the MEA. In the short

time-scale of the measurements (200 hours),

this

did not lower the performance of the

MEA,

but

with time, sufficient base metal would be leached

from the cathode to lower the kinetic benefit from

the Pt alloy electrocatalyst. In addition, unlike the

PAFC, the proton conductivity of both the mem-

brane and the proton conducting polymer present

in the anode electrocatalyst layerwill at some point

be dramatically reduced. The base metal ions occu-

py the sulfonic acid groups in the proton

conducting electrolyte normally available for pro-

ton conduction.

This

severely resmcts the type of

Pt alloys that can be used in the PEMFC.

Practical Operation

The study then moved to

evaluate

the more sta-

ble PtCr alloy in pie-commercial PEMFC stacks.

Here, the

aim

was to realise the ‘intrinsic’ kinetic

benefit of the Pt alloy under practical PEMFC

operating conditions.

A 40wt.

PtCr alloy at

a

Pt

to base metal atomic ratio of 75:25 was prepared

on

Vulcan XC72R. Catalysed substrate-based

MEAs were manufactured by hot pressing the

electrodes to Nafion 115 membrane electrolyte.

The results of tesang in

a

Ba.llard Mark 5E smgle

cell are

shown n

Figure 5 which presents the cell

potential versus current density plots for operation

with

air,

helox

(21%

0 2

n helium) and pure oxy-

gen as oxidants, and hydrogen as fuel

In

these

measurements for each oxidant the gas flow rate

was kept constant to prevent the water balance

in

the

MEA

being adjusted.

his

adjustment can lead

to changes

in

the cathode performance resulting

from modification of the proton conductivity of

the electrocatalyst layer or from

a

change in the

rate of oxygen permeability through the electrocat-

alyst layer. Figure 5 shows that when using pure

oxygen there is a clear kinetic benefit of 25 mV

with the PtCr alloy compared to the pure Pt-based

cathode. With more practical operation in

air,

however, the kinetic benefit was not realised and

the PtCr

alloy

showed comparable performance to

the pure Pt cathode.

We make use of air, helox and pure oxygen as

oxidants to distinguish between

oxygen g s

diffu-

sion and

oxygen

permeability limitations

in

the

cathode. The helox

g a i n s

(the helox-air perfor-

Phtinwn Metah

Rm, 2002,46,

1) 8


7/12

Fig . 6 For air operation

in

Ballard

Mark

5 E

hardware underfull

humidification co nditions a modified

PtCr alloy recovers the ‘intrinsic’

kinetic benefit fro m the alloy.

The MEAs (< I

mg

Pt

cm ’)

are based

on

catalysed substrates bonded

t o

Nafion

115

membrane electrolyte. The

cell is operated at

80°C

in

hydrogetdair;

30W308

kPa, I S / ?

.stoichiometp, ,full internal membrane

humidification

OPERATING LIFETIME, hours

mance) are associated with oxygen

ga s

diffusion

losses and the oxygen

gains

(02-helox perfor-

mance) with oxygen permeability losses. Figure

5

shows that the

air

and helox performance is com-

parable for the PtCr alloy and the pure Pt

electrocatalyst. The helox

gains

are therefore com-

parable. There are no additional oxygen

gas

diffusion limitations present in the cathode with

the PtCr alloy. Figure 5 also indicates that the

oxy-

gen gains are larger for the PtCr alloy.

This

confirmed poorer rates of oxygen permeability in

the electrocatalyst layer with the PtCr alloy, because

the

PtCr alloy is more hydrophilic

than

he pure Pt

catalyst.

This

resulted in the PtCr cathode retaining

more of the liquid water produced by oxygen

reduction. Consequently the oxygen permeability

in the cathode electrocatalyst layer was reduced.

Armed with this information the PtCr alloy

electrocatalyst was made more hydrophobic by the

addition of a very small quantity of a

third

metal

that was both stable in the acidic environment and

did not disrupt the PtCr alloy structure. The per-

formance of

this

modified PtCr alloy was

comparable on pure oxygen and hydrogen opera-

tion to the precursor PtCr alloy. On operation in

air,

however, the modified PtCr

alloy

showed the

25

mV improvement in activity predicted by the

kinetic performance. Further, as shown in Figure

6,

this

improved performance was retained for

500

hours of continuous operation. Examination of

the

MEA

by EPMA indicated the Cr had not been

transported into the membrane

or

into the anode

electrocatalyst layer.

Thus,

by adjusting the MEA water balance

which can probably be achieved by a number of

routes he ‘intrinsic’ kinetic benefit of the Pt

alloys can be realised under

a

variety of practical

PEMFC operating conditions. Work optimising

the benefit of the Pt alloys over the required oper-

ating lifetimes is on-going.

Why Are Pt Alloys

More

Active

for

Oxygen Reduction?

There has been much debate regarding the rea-

sons for the hgher activity of Pt alloys for oxygen

reduction dating

from

he early work on PAFCs.

Among

the reasons put forward (see, for example

(8)) are:

the measured improvement in the stability to

sintering,

0

surface roughening due to removal of some

base metal, which increases the Pt surface area,

preferential crystal orientation,

a more favourable Pt-Pt interatomic distance,

electronic effects and

oxygen adsorption differences due to modified

anion and water adsorption.

While it is not possible to be conclusive, the

improved oxygen reduction activity in the PAFC

has been linked to a reduction in the Pt particle

size effect with all Pt alloys

(12).

That larger Pt

particles are more active for oxygen reduction in

the PAFC with pure Pt electrocatalysts has been

proven by a number of groups (see, for example,

(13)).

For Pt alloys, Pt crystallites

in

the range from

2

to 4

nm,

do not show a reduction in specific

Pkztinum Metah Rcv.,

2002,46, 1)

9


8/12

50 100

1 5 0

200

250

300 350 400 450 500 550 6

L

2

u

EPSA, crn2

P t ~ m - 2

activity thereby restoring some of the lost activity

in

this

particle size range

that

is evident with pro-

gressively smaller pure Pt crystallites. It was

proposed that the Pt surface atoms restructure

during oxygen reduction resulting in a perfor-

mance loss and that the base metal within the

Pt

crystallite prevents the restructuring (12).

However, the existence of the particle size

effect in the PEMFC has not been proven,

although, Wilson

et a/. 14)

ave suggested

it

does

exist. At Johnson Matthey the Pt particle size

effect in the PEMFC was probed by measurement

of the kinetic performance of a series of cathodes

prepared from unsupported Pt black and

40 wt.

Pt

electrocatalysts on different carbon supports.

The plot

of

kinetic current for oxygen reduction

versus the EPSA for the different cathodes is

shown

in

Figure 7. The results show that at a given

EPSA there appears

to

be

a

clear trend of hlgher

oxygen reduction activity from larger Pt crystal-

lites. The Pt black (5.7

nm)

and the Shawinigan

(5.5 nm) electrocatalysts give much higher oxygen

reduction activity than the Pt electrocatalysts sup-

ported on Vulcan XC72R (3.5 nm) and the

BP

2000

(2.1 nm) carbon supports. The slopes of

the lines in Figure 7 reflect the specific activities

of

the different

Pt

electrocatalysts. The specific activ-

ities for oxygen reduction at 900 mV,

iR

free (vs.

NHE)

are 1.85

(Pt

black), 0.94 (Shawinigan),

0.48

(Vulcan XC72R) and

0.34

BP2000) mA cml’

Pt.

While these results do seem

to

support the exis-

tence of the Pt particle size effect

in the

PEMFC,

Fig.

7

The relationship

between the kinetic

performance for ow gen

reduction and the catho de

EPSA

jb r a runge of cathode

electrocatulysts with different

electrochemical ureas

ECA.0.

The

MEAs

are based

on

catal.ysed substrates bond ed

to Nafion

115

membrane

electrolyte. The Ballard Mark

5E

single cell is operated at

R O T

in hydrogen/ov gen,

30W30X kPu.

I

.5/10

stoichiometty full internal

membrane humidification

care must be exercised. In the PEMFC the size

and structure of the proton conducting electrolyte

in the electrocatalyst layer mean that the utilisation

of the Pt is

a

complicating factor that cannot be

separated from the particle size effect in Figure 7.

For example, the Pt black electrocatalyst shows a

much higher specific activity for oxygen reduction

than

40

wt.%

Pt

supported on Shawinigan,

although the Pt crystallite sizes are comparable.

It

seems that the utilisation under load is higher for

the Pt black system, due

to

a more favourable

interaction with the protons

in

the aqueous per-

fluorosulfonic acid solution present in the

electrocatalyst layer.

This

is perhaps not surprising

because of the lack of carbon in the unsupported

Pt black

to

mask the Pt surface area from the pro-

ton conducting polymer.

The effect of Pt utilisation is, however, likely

to

be less dramatic when comparing the different

carbon supported electrocatalysts. Some indica-

tion of

this

is

shown in

Figure 4 by the relative

‘intrinsic’ kinetic performance of the heat treated

Pt

and the pure

Pt

cathodes.

The effect of heat treating an electrocatalyst to

produce larger Pt crystallites should minimise the

effect of utilisation since the carbon support is

unchanged. For the 20

wt. Pt

on Vulcan XC72R

electrocatalyst, the heat treatment sintered the Pt

crystallites from 2.2 to 3.5

nm

corresponding to a

CO metal area reduction from

100

to

60

m2 g-’ Pt.

The ‘intrinsic’ kinetic performance from the heat

treated Pt cathode

is

some 10 mV higher.

It

seems

PhhnnmMetah Rev.,

2002,46,

1)

10


9/12

Table II

Kinetic Current Versus EPSA for Type

A

Cathodes

EPSA

cm2

Pt

c m 2

125

240

365

520

Cathode

Pt

loading,

0.73

0.97

Current density

at 900

rnV

(iR free),

mA

cm-*

70

120

190

255

Specific activi ty

at

900 mV

( iR free),

mA

cm-2

Pt

0.56

0.50

0.52

0.49

plausible that the Pt particle size effect is also

evident in the PEMFC.

This

would also explain

why the Pt alloys are more active

in

the PEMFC (a

reduction in the Pt particle size effect). Indeed,

that the Pt alloys are more active for oxygen reduc-

tion does itself provide support for the existence

of the particle size effect in the PEMFC. It would

also explain why no performance benefit was

observed with planar bulk Pt alloy cathodes

in

acid

electrolytes (1 1).

Alternatives to Pt-Based

Cathode Electrocatalysts

Pt alloys offer a performance

gain

of

25

mV,

which increases the electrical efficiency of the

PEMFC by

2 . This

is the

limit

of the perfor-

mance

gain

that can be expected using the

approach of modifymg the Pt face centred cubic

lamce structure by alloying with base metals. If

much more of the

300

mV available

at

the cathode

is

to

be recovered, then

a

fundamentally different

approach is required.

The search for alternative oxygen reduction

electrocatalyst materials to Pt

-

has been the

subject of extensive research over

a

number of

years. Appleby and Foulkes (8) have reviewed

much of the research. Recent literature has high-

lighted ruthenium-based chalcogenides

(15, 16),

pyrolysed Fe porphyrins (17) and metal carbides

(1

8)

as offering significant oxygen reduction activ-

ity. Indeed Co-based macrocydic compounds

have been used commercially in the cathode of

alkaline-based metal-air batteries. However, to date,

in acid, none of the alternative electrocatalysts

have been as active as Pt; their durability has to be

established and gas diffusion electrode structures

have to be prepared with the electrocatalysts pre-

sent in high surface area.

A possible area for

initial

focus might be in the

cathode of the direct methanol fuel cell

(DMFC),

since in contrast to Pt the alternative electrocata-

lysts are not deactivated for oxygen reduction by

the methanol transported

from

the anode. If the

electrocatalysts are not competitive with Pt in the

DMFC they

will

fall far short of the requirements

for the PEMFC.

The focus of the continued search for the elu-

sive electrocatalyst for oxygen reduction

in

acid

media should be on the development of materials

with the required stability, and greater activity than

Pt. Additionally, t h i s will require that the electro-

catalyst be prepared in a high surface area form to

compete effectively with carbon supported Pt

electrocatalysts. Electrocatalysts which are less

expensive than Pt, but which are less active, will

not move PEMFC technology forward. The drive

must be to raise system efficiencies for stationary

applications,

to

lower electricity costs, and to raise

power densities for transportation applications,

so that capital costs are lowered.

This

requires

more active electrocatalysts than Pt to raise the

M E A

performance.

Improved Utilisation

of the Cathode Electrocatalyst

While the development of much more active

oxygen reduction electrocatalysts is important, t

represents

a

significant challenge. As a result,

Phtinwm

Me&

h.002,46,

(1)

11

mg t

c r r

0 27

0 44


10/12

attempts

to

raise the cathode performance have

focused on improving the utilisation of Pt (the

most effective oxygen reduction electrocatalyst).

The results of our work, which produced the

much higher performance from the Pt black cath-

odes shown in Figure 7, pointed to the possibility

of significant performance

gains

available from

improving

Pt

utilisation with carbon supported

electrocatalysts. Different approaches

to

electrode

design have been adopted, depending on the target

MEA Pt loadings. Type

A

cathodes were devel-

oped to perform at higher electrode Pt loadings

(to

1.0 mg Pt cm ) and Type B cathodes were

designed for performance at lower electrode Pt

loadings

(<

0.25 mg Pt cm-'). Such low

Pt

loadings

are necessary to penetrate the large automotive

market.

Table I1 shows clearly that for a range of Type

A cathodes based on

40

wt. Pt supported on

Vulcan XC72R as the EPSA is increased (when the

Pt loading is increased) the kinetic current for oxy-

gen reduction (from pure hydrogen and oxygen

operation) increases in direct proportion.

This

is

shown by the similar specific activities calculated

from the EPSAs.

For

these cathode structures the

EPSA gives a relative measure of the active Pt sur-

face area in the electrodes. This performance

sc+

is particularly useful for some stationary

and portable applications where higher Pt loadings

can be tolerated economically to achieve an

improved kinetic performance.

09.

>

O B '

0 7 .

: 6 .

-

I

+

: 5

A

d 0 4 -

3.

2,

However, for applications requiring lower

MEA Pt loadings higher performing cathodes than

Type A can be produced. It is possible to increase

the kinetic performance over that achieved with

Type A cathodes. Maximising the interaction

between the Pt crystallites and the proton con-

ducting polymer (which has a complex structure to

accommodate the sulfonic acid groups and their

associated protons and water of hydration) can

raise the utilisation of the Pt electrocatalyst. Type

B

cathodes have been developed

a t

electrode Pt

loadings of less than 0.25 mg Pt cm-' with

improved EPSAs and improved performances

compared to Type

A

structures. With Type B

cathodes, prepared from

40

wt. Pt supported on

Vulcan XC72R, specific activities of ca.

1mA

cm-'

Pt

at

900

mV,

iR

free (vs. NHE) have been

achieved.

This

represents a doubling in the specif-

ic activity for oxygen reduction (see values for

Type A cathodes in Table II) corresponding to a

doubling in the Pt utilisation under load.

Most important, as shown in Figure 8, the

kinetic benefit translated to an improved MEA

performance under practical operating conditions

with air as oxidant and hydrogen as fuel. A kinetic

performance gain of

20

mV was evident at low

current densities as projected by a doubling in the

Pt

utilisation.

This

was reflected in an increased

EPSA (230 cm' Pt cm-' at 0.23 mg Pt cm-').

Figure 8 shows that at higher current densities

there were additional performance g a i n s (due to a

-0-

Type Acathode, 020rng P t

cni

Type Bcathode, 017 mg Pt cni'

Fig.

8

For low electrode

Pt

loadings

(<

0.25 mg

Pt

em') Type

B cathodes show the higher

performance due

to

improved

Pt

electrocatalyst utilisation, lower

electrode resistance and enhanced

mass transport performance.

The

MEAs

(<

0.5 mg Pt em ')

are

based on catalysed substrates

bonded

to Nafion

115

membrane

electrolyte. The Ballard Mark

5E

single cell

is

operated ut 80°C

in

hydrogerduir: 30W308 kPa, 1.92

stoichiomern .

ul l

internal

membrane humidification

Platinum Metuh Rev.,

2002,

46, (1)

12

2

4 0 0

6 8 1

1200 1400 1600

CURRE NT DE NS ITY

mAcm-


11/12

reduction in both the electrode resistance and

mass transport performance losses) using the Type

B

cathodes. This is reflected by a lowering in the

slope

in

the pseudo-linear region of the cell poten-

tial vs. current density graph (Figure

8).

The

improved cathode design increased the electrical

efficiency at low current densities by

2 ,

but at

higher current densities the electrical efficiency

was raised by as much as 7 . As the electrode Pt

loadmg was raised above

0.25

mg Pt cm-’, howev-

er, the increased EPSA of

the

Type

B

cathodes did

not translate to

a

higher performance.

This

can be

attributed

to

thickness limitations

in

the electrode

due to a reduction

in

the rate

of

oxygen perme-

ability through the cathode

(2).

Conclusions

Most of the performance loss from

the

ther-

modynamic potential of the PEMFC

is

due to the

cathode. Aqueous-based

inks,

prepared from hgh

Pt

loading

electrocatalysts with high Pt dispersion

and perfluorinated sulfonic acid polymer solu-

tions, have been used to prepare relatively

t in

electrocatalyst layers. Employing Pt-based metal

alloy

electrocatalysts prepared on traditional car-

bon black supports in the electrocatalyst layer

produces a

25

mV performance gain.

This

repre-

sents a

2

increase in the electrical efficiency of

the PEMFC.

Only

the more stable Pt-based metal alloys,

such as PtCr, P a r , PtTi, can be used in the

PEMFC, due to dissolution of the base metal by

the perfluorinated sulfonic acid in the electro-

catalyst layer and membrane. For improved per-

formance from the Pt-based metal alloys, it is

necessary to tailor the electrocatalyst layer

to

achieve the optimum

MEA

water balance under

the selected PEMFC operating conditions. This

can be achieved by modifjmg the

Pt

alloy electro-

catalyst with a third metal and probably also by

altering the electrocatalyst layer design.

There are strong indications that the particle

size effect is present in the PEMFC as well as in

the PAFC. The performance

gain

from the Pt-

based metal alloys is probably linked

to

a reduction

in

the particle size effect in the Pt crystallite range

from

2

to

4

nm. It is tentatively proposed that the

base metal prevents the Pt surface atoms from

restructuring during oxygen reduction.

To

develop cathode electrocatalysts with high-

er performance than the Pt-based metal alloys

requires a completely new approach. At present

there are no alternative cathode electrocatalysts to

Pt.

All

others are less active and their durability

is

unproven. The focus of future research needs

to

be on improved performance and not on lower

cost alternatives

to Pt with

comparable or lower

performance.

Different electrocatalyst layers have been

developed for high Pt loadings and for Pt loadings

below

0.25

mg Pt ern-'. Improved performance

can be achieved at the lower electrode Pt loadings

by improving the utilisation of the cathode elec-

trocatalyst. Secondary benefits are available due to

proton diffusion and mass transport performance

gains. At high electrode Pt loadings the perfor-

mance gains are negated by oxygen permeability

limitations.

Acknowledgement

This

paper is based on part of

a

keynote lecture given by

T. R Ralph a t the

17th North

American Chemical Society

Meeting

held at the Westin

Coast Harbour

Castle Hotel,

Toronto, Ontario, Canada, from the 3rd to

8th

June

2001.

References

T. R. Ralph and G.

A.

Hards, Cbem. Ind, 1998, (9),

337

T. E. Springer,

M.

S.

Wilson

and S. Gottesfeld, J.

Ekrtmcbem. Soc.,

1993,140, 3513

L.

Keck, J. S. Buchanan and

G. A.

Hards, U.S.

Patent

5,068,161; 1991

T. R. Ralph, G. A. Hards,

J.

E. Keating,

S. A.

Campbell, D. P. Willdnson, M. Davis, J. St-Pierre

and M. C. Johnson, J. Ekrtmcbem. Soc., 1997, 144,

3845

I. D . Raistrick in “Diaphragms, Separators and Ion

Exchange Membranes”, eds.

J.

W.

Van Zee,

R

E.

White,

K.

Kinoshita and H.

S.

Burney, The

Electrochemical Society Softbound Proc. Series,

Pennington, NJ, 1986, PV 86-13, p. 172

S. Gottesfeld and T. A. Zawodzinski, Ah

Ekrtmcbem.

ci Eng.,

1997, 5,231-240

J. Denton, J. M. Gascoyne and D. Thompsett,

Eumpeun

Patent 731,520; 1996

J. App leby and F. R. Fou lkes, “Fuel Cell

Handbook”, Krieger Publishing Company, Malabar,

Florida,

1993, Chapter 12

T. R. Ralph, J. E. Keating, N. J. Collis and T. I.

Hyde,

ETSU

Contract Report F/02/00038,1997

Pkatinum

Metals

Rev., 2002,46, 1)

13

7


12/12

10

11

12

13

14

15

16

17

S. Mukerjee and S. Srinivasan,].

Ekcfmand

Cbem.,

1993,357,201

S. Gottesfeld and T. A. Zawodzinski, Adv.

Ekcfmcbem. Sci Eng., 1997, 5,20>217

J.

S.

Buchanan, G. A. Hards, L. Keck and R. J .

Potter, in 1992 Fuel Cell Seminar Program

Abstracts, Tuscon,

AZ,

992,p. 536

K.

Kinoshita,

.

Ekcfmcbem. SOC.,

990,137,845

M . S. Wilson,

F.

H.

on,

K. E. Sickahs and S.

Gottesfeld,]. Ekcfmcbem. SOC .,

993,140,2872

N. Alonso-Vante and H. Tributsch,

Nature, 1986,

323,431

R W. Reeve, P. A.

Christensen,

A. Hamnett, S. A.

HaydockandS. C.Roy,].Ekxfmbz.SOL,1996,145,3463

G. 0 Sun, J. T. Wang and R F. Savinell,]. AppL

Ekcfmcbem.,

1998,28,1087

18

R Cote, G. Laande, G. Faubert, D.

Gauy,

J. P.

Dodelet and G. Denes, 1.

New

Matm Ekcfmcbem.

Systems, 1998, 1, 7

The Authors

Tom Ralph is Product Development Manager at the Johnson

Matthey Technology Centre, responsible for MEA design.

He

has

been working with PEMFCs since

1991

and with developing all

aspects

of

the MEA

-

electrocatalyst, electrocatalyst layer, gas dif -

fusion substrate and solid polymer membrane

-

and in integrating

MEAs into customer hardware.

Martin Hogarth is a Senior Scientist at the Johnson Matthey

Technology Centre and has worked in the area of DMFCs since

1992.

His main interests are in the development of new catalyst

materials and high-performance MEAs for DMFCs. More recently

his interes ts have expanded into novel high-temperature and

methanol impermeable membranes for the PEMFC and DMFC,

respectively.

The direct methanol fuel cell

(DMFC)

is a vari-

ant of the proton exchange membrane (PEM) fuel

cell and uses aqueous methanol directly without

prior reforming. In the DMFC methanol is con-

verted to carbon dioxide and hydrogen at the

anode. The hydrogen then reacts with oxygen, as

in a standard PEM fuel cell. Conventional materi-

als for DMFCs include platinum-ruthenium

(Pt-Ru) for the electrode electrocatalysts and car-

bon in various forms as the electrocatalyst support.

Electrocatalysts with h gh activity for methanol

oxidation are essential for improved performance

of DMFCs. Such catalysts are generally prepared as

unsupported metal colloids

or

nanocomposites

with the metal nanoparticles supported o n an elec-

trically conducting carbon of h g h surface area.

Mixed metal Pt-containing catalysts are presently

used for methanol oxidation.

Now, scientists from the Department of

Chemistry at Vanderbilt University,

with a

col-

league from the Corrosion Research Center,

University of Minnesota, U.S.A., have developed a

Pt-Ru/graphitic carbon nanofibre (GCNF) nano-

composite which exhibits hlgh relative performance

as a DMFC anode catalyst (E.S. Steigerwalt, G. A.

Deluga, D. E. Cliffel and C. M. Lukehart, J.

Phy. Chem. B, 2001,105,

34),

097-8101).

As

part of ongoing studies of new synthetic

strategies for preparing metal alloy/carbon com-

posites, they prepared and characterised a

Pt-Ru/GCNF composite, where the GCNF sup-

Platinum-Ruthenium Anode Catalyst for DMFC

port has the 'herringbone' atomic structure. The

source of both metals was the molecular precursor

(q-C2H4)CI)Pt(p-C1)2Ru(C1) q3:~3-2,7-dimethyloc-

tadienediyl).

Reductive decomposition of the precursor

formed widely dispersed Pt-Ru nanocrystals, and a

multistep deposition procedure ensured total metal

content of - 42 wt. at bulk Pt/Ru atomic ratio

of - 1:l.The metal alloy nanoclusters had average

particle size of 6 nm (calculated from XRD peak

widths) or 7

nm

(measured directly from T E M

images).

Small amounts of Ru metal and oxidised

Ru species were also present.

When used as an anode in a working DMFC,

the composite enhances fuel cell performance by

-

50%

relative to that recorded for an unsupported

Pt-Ru colloid anode catalyst. Further work on the

metal alloy/GCNF anode catalysts is envisaged.

Fuel Cell

Catalysts Brochure

Alfa Aesar has just published a new 4-page

brochure highlighting

a

range of noble metal,

HiSPECm

fuel

cell catalysts.JohnsonMatthey manu-

factures HiSPEC ? brand catalysts which provide

superior performance

in

proton exchange membrane

and direct methanol

fud

cells. The HiSPECm range

consists of single and bimetallic supported and

unsupported catalysts which can be used on both

anodes and cathodes.

Copies of the new Fuel

Cell

Brochure can be

obtained from Alfa Aesar: Tel:

+800-343-0660;

Fax:

+800-322-4757;

or

E-mail: [email protected].

Pkztinum Metals REX, 2002, 46, (1)

14

46(2002)3-14 part i the cathode challenges

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