Technical Report
報文

Ultra-Low Platinum Loading Electrodes for PEFC
with High Durability Performance
Mariko Kohmoto* Kazuhide Totsuka* Shuji Hitomi*
Hideo Yasuda* Masanori Yamachi**

Abstract
The ultra-low platinum loading electrode (ULPLE) for PEFC has been found out to show high durability performance to be almost free from the essential failure mechanism losing the active surface area of platinum
particle judging from the fact that the value of retention of the active surface area was 90.1% and 115% forcathode and anode respectively after 600 hours operation period at the condition of H2/Air system at 80 oC and
200 mA cm-2, whereas the value of that was observed 66% and 80% for the conventional electrodes, respectively. This high durability performance of ULPLE derived from the special structure that minimum loading levelof platinum catalyst exist only for its active site where the surface of carbon particle contacts the proton-conductive passage of polymer electrolyte. This stability of platinum particles was also confirmed by the change
in the active surface area measured by the repetitious potential-sweep method covering region of potential for
dissolution–precipitation phenomenon of platinum, and its crystal structure change was recognized by cyclic
voltammograms.

1

for electrodes of fuel cells. The catalyst particles

Introduction

tend to be aggregated and to lose its active surface

A polymer electrolyte fuel cell (PEFC) is ex-area resulting in the decrease of cell performance

pected as a clean power source for automobiles as

during the continuous operation as the similar casewell as stationary applications because of its op-of phosphoric acid fuel cell (PAFC).1) Therefore, theeration at moderate temperature below 100 oC in

reduction of loading level of platinum catalyst par-contrast with other types of fuel cells. There are

ticles with its high durability has been required fortwo essential problems to be overcome for its com-a long time. A lot of efforts have been made for the

mercialization. One is high cost and the other isminimum loading level of platinum by the devel-

low durability performance. These problems are

opment of well-dispersed very small size particles

strongly concerned with the usage of large amountwith high utilization. The achieved level is not onlyof precious metal of platinum as catalyst particles

still remained around 0.1 mg cm-2, but also the useof small particle with nano-level diameter is obliged*Department-E, Corporate R & D Centerto accelerate the aggregation phenomenon by so**Corporate R & D Center

called" dissolution-precipitation mechanism or sin-

© 2004 GS Yuasa Corporation, All rights reserved.

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tering phenomenon", resulting in the low durability

ing serial constitution parts; anode gas flow plate /performance. We proposed the concept of ultra-low

gas diffusion backing / MEA / gas diffusion backing

platinum loading electrode (ULPLE) of which plati-/ cathode gas flow plate. The gas diffusion backing

num catalyst particle is loaded only on the active

was used of carbon paper of 0.2 mm thick treatedsite where the surface of carbon particle contacts

with PTFE dispersion solution followed by sintering

the proton-conductive passage of polymer electro-

at 380 oC for 15 min. under nitrogen atmospherelyte,2) though the existing catalyst is further loadedfor hydrophobic property. The existing electrode us-on the carbon powder covered by the insulator ma-

ing Vulcan XC-72 carbon with Pt (Tanaka.K.K., 30

terial of rigid hydrophobic backbones of polymer

mass% Pt/C) was also prepared for comparison. The

electrolyte. The loading of platinum catalyst for thisplatinum loading level of cathode was 0.15 mg cm-2

electrode was to be 0.01 – 0.05 mg cm-2 without theand 0.17 mg cm-2 for anode.

decrease in the cell performance.

2.2

Continuous operation test

In this paper, we will focus on the other aspect

The durability performance of single test cellsof durability performance for ULPLE by the direct

were evaluated by the continuous operation test atevaluation of change in the active surface area of

200 mA cm-2 at 80 oC for 600 hours under the condi-

platinum catalyst with its crystal structure changetion of hydrogen and air of humidified temperatureduring the continuous cell operation test and theof 80oC and 75 oC, respectively. The change in theaccelerated test by repetitious potential-sweep

electrochemically active surface area of platinum

method to clear the feature of the catalyst of thiscatalyst during the operation test was investigatedelectrode, and discuss the special feature of catalyst

by cyclic voltammograms, together with the change

activity from the viewpoint of mass activity.

in the polarization curve of cells.
2.3

Accelerated evaluation test by repetitious po-

tential-sweep method

2

Experimental

2.1

Preparation of test electrodes

The change in the electrochemically active surface area of platinum catalyst was investigated by

The ULPLE was prepared with the following

repetitious potential-sweep method for accelerated

process: the formation process of the layer com-test of failure mechanism losing its surface area

posed of carbon powders (Cabot, Vulcan XC-72)

using the wide range of potential from 0.6 V tocoated with 35 mass% proton conductive polymer of

1.0 V vs. RHE at 25 oC covering the occurrence ofNafion (DuPont, 5 mass% solution); ion-exchange

dissolution-precipitation reaction of platinum forreaction process of proton and platinum ion in the

30,000 times with sweep rate of 500 mV sec-1. Argon

proton conductive polymer by the immersion into

gas flowed into working electrode and hydrogen

50 mmol l [Pt(NH3)4]

ion solution at 60 C for 6

gas for counter electrode of the test cell during thehours; washing process with deionized water; heat-measurement. The electrochemically active surface

ing process at the lower temperature of 180 C forarea of platinum catalyst was calculated by colum-

6 hours under the hydrogen atmosphere; rinsing

bic charge of hydrogen desorption peak on cyclic

process with immersion in 0.5 mol l H2SO4 at 80

voltammogram obtained under the condition of 100

C and deionized water. The membrane electrodemV sec-1 between 0.05 - 1.2 V vs. RHE at the appro-

assembly (MEA) with 5 cm was then prepared by

priate intervals during the accelerated test.

-1

2+

o

o

-1

o

2

the hot-pressing of the cathode catalyst layer andthe anode catalyst layer on the both sides of poly-3

mer electrolyte membrane (DuPont, Nafion 115) as

Results and discussion

reported in elsewhere.2) The catalyst loading level

The change in the electrochemically active sur-for cathode was 0.055 mg cm and 0.050 mg cm forface area of ULPLE after 600 hours continuous

anode. The single cell was composed of the follow-

operation is summarized in Table 1. The retention

-2

-2

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Table 1 Change in electrochemically active surface area after 600 hours continuous operation test at 200
mA cm-2 and 80 oC.
Electrode
ULPLE

Pt loading level / mg cm-2

Active surface area / cm2 mg-1
BeforeAfter

Retention of active surface
area / %Cathode

0.055

611

551

90.1

Anode

0.050

682

782

115

0.15

616

407

66.0

0.17

594

477

80.0

Conventional elec- Cathode
trode
Anode

value of electrochemically active surface area for100

of 90.1%, though the value of conventional one is95

drastically decreased down to 66.0%. In the case90of anode for ULPLE, on the contrary, the value is85

Retention of active
surface area / %cathode of ULPLE after 600 hours is high value

increased up to 115%, whereas its value of conventional one is decreased down to 80% as the caseof cathode. This tendency of change for platinum
catalyst particles of ULPLE is totally different from

80
75
70
65

the existing one reported so far.3) In other words,

60
0.0

the failure mechanism of aggregation phenomenon
losing the electrochemically active surface area of

0.5

1.0

1.5

2.0

2.5

3.0

4

Number of cycles / X 10 Times

platinum for ULPLE is drastically suppressed forFig. 1 Change in retention ratio of electrochemically active surface area of ultra-low platinum loading electrode ( 〇 ) and conventional electrode ( △ )with repetitious potential-sweep cycles in the potential range of 0.6 - 1.0 V vs. RHE under argon gas at
25oC.

cathode and rather improved for anode.
For further confirmation of this tendency, the accelerated test of failure mechanism losing the active
surface area of platinum catalyst particles was conducted by repetitious potential-sweep method under
the condition of the wide range of potential covering
the occurrence of dissolution-precipitation reaction

100of platinum for 30,000 times. The change in the

95
Retention of active
surface area / %retention ratio of electrochemically active surface
area of platinum catalyst is shown in Fig. 1. The
retention value of active surface area of platinumfor ULPLE is gradually decreased with repetitious
cycles to show the minimum value of 80% at around
20,000 cycles and its value tends to be increased to-

90
85
80
75

wards 30,000 cycles, whereas the value for the con-70

ventional electrode continued to be decreased down

65
2
10

to 70%. For better analysis on these phenomena,

3

10

4

10

Number of cycles / Timesthe change in the retention ratio re-plotted in termsof logarithmic cycle number is shown in Fig. 2. The

Fig. 2 Logarithmic plots of change in retention
ratio of electrochemically active surface area of
ultra-low platinum loading electrode ( 〇 ) and conventional electrode ( △ ) with repetitious potentialsweep cycles in the potential range of 0.6-1.0 V vs.
RHE under argon gas at 25 oC.

retention value of active surface area of platinumfor ULPLE shows almost stable with no decrease byaround 1,000 cycles as well as the case of conventional electrode. This means the platinum particles

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maintain its size in this region with no change. The

from platinum surface appeared around 0.05 –linear decrease in the retention value is observed

0.4 V vs.RHE is decreased in both electrode after

beyond 1,000 cycles for both electrodes, and the in-

30,000 cycling. However, in the case of conventionalcrease is confirmed beyond 20,000 cycles in the caseelectrode, the shape of peak is found out to be theof ULPLE. This tendency is seemed to be crystalli-

contribution of change from (111) and (110) faces tozation from amorphous like platinum. The decreas-(100) face of platinum in conventional electrode.4)

ing rate in electrochemically active surface area ofOn the other hand, the case of ULPLE is found out

ULPLE beyond 1,000 cycles is found out to be 4/5 of

to maintain the shape indicating (111) face orienta-

conventional electrode.

tion with small platinum particle. This result clear-

In order to consider in more detail of the above-

ly shows that the platinum particle in conventional

mentioned change in the electrochemically active

electrode grows by accelerated test with potential

surface area of platinum catalyst particles, thesweep.

change in potential behaviors by cyclic voltammo-

Furthermore, the change in potential behavior ofgrams of this accelerated test after 30,000 cycles iselectrodes after the continuous operation test is also

shown in Fig. 3. The desorption peak of hydrogenshown in Fig. 4. The desorption peak of hydrogenat 0.05 V – 0.4 V vs. RHE from platinum surface
becomes to be separated for both cathode and anode

Current density / mA cm -2

of ULPLE after 600 hours operation. The separated(a)4

peak after operation at 0.1 V vs. RHE is characterized by (110) face, though the shape of cyclic

2voltamogram at the initial states shows (111) faceorientation, Especially, the peak of anode becomes0

to be sharpened resulting in the increment of electrochemically active surface area of platinum ob--2

served in Table 1. This phenomenon is derived fromthe crystal structure of platinum loaded on ULPLE-4

with the change in face orientation from indices
0.0

0.2

0.4

0.6

0.8

1.0

Current density / mA cm -2

10

1.2

(111) to (110) by continuous operation condition, in
contrast with no appearance in accelerated test. On(b)the other hand, the both cathode and anode peaksof conventional electrode become broad and small

5

after the operation test resulting in the drastic decrease in electrochemically active surface area of0platinum. Because this tendency has been remarkably observed in cathode side with clear appearance-5of shoulder peak at 0.25 V vs.RHE of which peakindicates the well known phenomenon of (100) face

-10

0.0

0.2

0.4

0.6

0.8

1.0

orientation of platinum appeared in large platinum

1.2

particle more than 3.5 nm5) with the growth of plat-

Potential / V vs. RHE

inum particle by aggregation under the operating
condition such as the case of accelerated test.

Fig. 3 Change in potential behavior of cyclic
voltammograms for cathode (a) ULPLE and（b）
conventional electrode after the accelerated test for
30,000 cycles between 0.6 ‒ 1.0 V vs. RHE under
the condition of H2/Ar at 25 oC ;initial,after.

The high durability performance of platinum
catalyst particle for ULPLE is able to be confirmedby the change in polarization curves of PEFC after
600 hours operation shown in Fig. 5. The cell per-51

GS Yuasa Technical Report

2004 年 12 月

4
2
0
-2

0.0

0.2

0.4

0.6

0.8

1.0

5
0
-5

-20

0.0
20

1.2
(b)

6
4
2
0
-2
-4
-6
-8

10

-15

-6

8

(c)

15

-10

-4

-8

Current density / mA cm-2

Current density / mA cm-2

6

第1号

20

(a)

Current density / mA cm-2

Current density / mA cm-2

8

第 1巻

0.0

0.2

0.4

0.6

0.8

1.0

0.4

0.6

0.8

1.0

1.2
(d)

15
10
5
0
-5
-10
-15
-20
0.0

1.2

0.2

0.2

0.4

0.6

0.8

1.0

1.2

Potential / V vs. RHE

Potential / V vs. RHE

Fig. 4 Change in potential behavior of cyclic voltammograms for cathode (a), anode (b) of ULPLE and cathode(c), anode (d) of conventional electrode after the continuous operation test of 600 hours under the condition of
H2/Air at 80 oC ;initial,after.

1.0

1.0

(b)

(a)
0.8

Cell voltage / V

Cell voltage / V

0.8
0.6
0.4
0.2
0.0

0.6
0.4
0.2

0

200

400

600

800

1000

0.0

0

200

400

600

800

1000

-2

Current density / mA cm

Current density / mA cm

Fig. 5 Change in polarization curves of PEFC with ULPLE (a) and conventional electrode (b) after the continuous operation test of 600 hours under the condition of H2/Air at 80 oC. ● , ▲ Initial ; 〇 , △ after.

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formance of PEFC with ULPLE after the operation

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6

Mass activity ratio of ULPLE toconventional electrodeis found out to be improved by the contribution of
the increment of active surface area of platinum
catalyst for anode and a slight decrease of active surface area for cathode. This improvement of performance seems to be the appearance of active site of(110) face of platinum particle. On the other hand,
The performance of conventional cell is also foundout to be decreased by the aggregation phenomenon
losing the active surface area of platinum catalyst
particles for both cathode and anode electrodes.

5
4
3
2
1
0
0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50

IR corrected operating voltage / V

Considering the special location of platinum catalyst particles for ULPLE, the platinum catalysts is

Fig. 6 Change in mass activity ratio of ULPLE toconventional electrode after the continuous operation test of 600 hours under the condition of H2/O2
at 80 oC. Initial 〇 , after △ .

loaded only in the proton-conductive passage in ion
cluster of polymer electrolyte, wherein the nanosize platinum contacts with the surface of carbon
powders. The platinum particles are formed by
reduction reaction of platinum complex cation byhydrogen gas at lower temperature of 180 oC by

activity ratio is around 1 at the higher voltage ofcatalytic activity of carbon. This means that the

0.70 V. That is to say, the active site of platinum

platinum catalyst particle is considered to be held

catalyst of the conventional cell at the higher volt-in the proton-conductive passage inside the ion

age of 0.70 V in the beginning is limited not only oncluster of polymer electrolyte with the low crystal-

the portion of proton-conductive passage, but also

lization. Therefore, the reason why ULPLE showson the part of rigid hydrophobic backbone of poly-

the drastic suppression of decrease in the active

mer electrolyte. However, the value is increased up

surface area of platinum catalyst particles forto 2.7 at the 0.60 V in the beginning. This suggests

cathode and rather shows the increase for anodethat the active site becomes to be almost limited toduring the cell operation is considered as described

the proton-conductive passage of polymer electro-

below. The phenomenon losing of the electrochemi-lyte, but not the portion of hydrophobic backbone,cally active surface area of platinum particles is

considering the value of ratio is around 2.7 almost

mainly based on the aggregation phenomenon byequal to the volumetric ratio of proton-conductive

dissolution-precipitation mechanism accelerated bypassage to polymer electrolyte. After 600 hours

wide range of potential change. The platinum cata-

operation, the operating voltage showing the ratio

lyst particles of ULPLE is tightly formed inside theof 1 shifts by higher voltage of 0.75 V and its value

proton-conductive passage contacts the surface ofis increased up to 3.75 at 0.60V with lowering thecarbon powders resulting in little occurrence of ag-

operating voltage. This increase of the ratio from 2.7

gregation phenomenon, though its particle is very

to 3.7 suggests that the active site of conventional

small of nano-scale size. The increase of active sur-electrode is largely decreased down after operation.

face area of platinum for anode of ULPLE seems tobe crystallized of the active site for hydrogen oxida-

4tion reaction at the potential region.

Conclusions

The change in mass activity ratio of this electrodeThe ultra-low platinum loading electrode (UL-

to conventional electrode after the continuous op-

PLE) is found out drastically suppress the aggrega-eration test is shown in Fig.6 for the explanation oftion phenomenon losing the active surface area ofhigh durability performance of ULPLE. The mass

platinum catalyst particles by holding the catalyst

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only inside of the proton-conductive passage of poly-

Therefore, ULPLE is considered to be essential

mer electrolyte in which nano-size platinum con-technology to reach low cost and high durability

tacts with the surface of carbon powders. This fea-

performance for practical application of PEFC.

ture has been clearly confirmed by change of active
surface area after accelerated condition by potential

Acknowledgement

sweep as well as continuous operation.
The active site of electrochemical reaction on

The part of this work was performed by financial

the platinum catalyst particles shifts towards the

support of New Energy and Industrial Technology

proton-conductive passage rather than the part ofDevelopment Organization (NEDO) of Japan.

hydrophobic backbone at lower voltage below 0.70 V
at the beginning and 0.75 V after 600 hours opera-

Referencestion. At the lower voltage of 0.60 V at the beginning

1) G.A.Gruver, R.F.Pascoe, and H.R.Kunz, J. Elec-and 0.55 V after 600 hours, the active site becomestrochem. Soc., 127, 1219 (1980).

to be limited to the proton conductive passage ofpolymer electrolyte, but not the portion of hydro-

2) S.Hitomi, H.Yasuda, and M.Yamachi, 40th Bat-

phobic backbone, because the value of ratio at thetery Symposium in Japan Proc ., 167 (1999).

corresponding voltage is around 2.5 equal to the3) M.S.Wilson, F.H.Garzon, K.E.Sickafus, andratio value of proton-conductive passage to polymer

S.Gottesfeld, J. Electrochem. Soc ., 140, 2872

electrolyte. The high mass activity of ULPLE was(1993).
4) N.Markovic, H.Gasteiger, and P.N.Ross, J. Elec-indicated to be 3.7 times compared with that of con-trochem. Soc ., 144, 1591 (1997).

ventional electrode after 600 hours operation. This

5) K.Kinoshita, J. Electrochem. Soc. , 137, 847

phenomenon is seemed to be depended on appearance for active site of platinum of (110) face.

(1990).

54