Technical Report
報文

Synthesis of Pt-Co Alloy by Ultra-low Catalyst
Loading Technology for PEFC
Heisuke Nishikawa*Shunsuke Mizutani*

Shuji Hitomi*

Ryoichi Okuyama*

Abstract
Platinum-cobalt (Pt-Co) alloy catalyst has been synthesized by the ultra-low catalyst loading technology basedon ion-exchange reaction between metal cations of catalyst precursor and proton of ionomer existing on carbon
surface followed by reduction reaction of their ion-exchanged cation using hydrogen gas. The alloying degree ofthe catalyst at 7 atom% of alloyed Co content is 0.6 despite the low reduction temperature of 200 oC. Its catalytic
activity is 1.5 times higher than that of pure Pt catalyst synthesized by same catalyst loading technology.

Key words : Ultra-low catalyst loading technology ; Pt-Co alloy catalyst ; Alloying degree

1 Introduction

been reports on the high activity of Pt alloyed metals with various transition metals for oxygen reduc)Polymer electrolyte fuel cell (PEFC) is expected astion reaction (ORR).5, 6 In this study, we attempteda clean power source for electric vehicles or residen-to synthesize Pt-Co alloy catalyst using this loading

tial co-generation system because of its high energy

technology for the further improvement of catalyticeﬃciency. The most important technical objective foractivity.

commercialization of PEFC is the cost reduction by
decreasing Pt amount of catalyst. For the purpose,

2

Experimental

2.1

Preparation

we have developed the ultra- low catalyst loading- )technology.1 4 Pt catalyst in this technology is loaded
mainly on the active site which is located on theCarbon- supported Pt catalyst was prepared byinterface between the carbon surface and the ion-

following procedures : (1) preparation of ionomer-

nomer cluster. The loading technology is based oncoated carbon powders by spray- drying of mixed

ion-exchange reaction of Pt complex cation and theslurry with ionomer solution and carbon powders ; (2)proton of ionomer existing on the carbon surface fol-ion-exchange of proton in the ionomer for [Pt(NH3)4]2+

lowed by chemical reduction of its cation under hy-

ion by immersing the powders into [Pt(NH3)4]Cl2 so-drogen gas atmosphere. The utilization of Pt in cata-lution at 80 oC for 6 h ; and (3) reduction of the ion-lyst is so high that the Pt amount of catalyst is able

exchanged ion under hydrogen atmosphere at 150 oC

to decrease to one third of commercially available

for 8 h.

4)

Pt/C catalyst. However, the decrease rate is not yet

Carbon- supported Pt- Co alloy catalyst was syn-enough for the ﬁnal objective. Recently there havethesized using above-mentioned Pt catalyst by following two procedures : ion-exchange of proton in* Department Ⅱ, R & D Center© 2010 GS Yuasa International Ltd., All rights reserved.

the ionomer for Co2+ ion by immersing Pt catalyst

20

GS Yuasa Technical Report2010 年 6 月

into CoCl2 solution at 80 oC for 6 h and reduction of2+第7巻第1号

by potential cycling between 0.05 and 1.0 V vs RHE-ion under hydrogen atmosphere at 200 C for 8,at 0.1 V s 1 in 0.1 M HClO4 solution saturated with24, and 48 h. The synthesized alloy catalyst was thenN2 gas. The electrochemical active surface area ofCoo2+

rinsed by 0.5 M H2SO4 to remove residual Co
2.2ion.

Pt in catalyst was calculated from the hydrogenCharacterization

desorption charge obtained by cyclic voltammetory.

The alloying degree, that is, the content ratio of al-

Hydrodynamic voltammetory with rotation rate ωloyed Co (Coal) to total Co (Cotot) is given byof 1000-2750 rpm for ORR were measured under the-potential sweep rate of 10 mV s 1 between 1.0 andCo al
Co tot＝X Co (100-Co tot )0.4 V vs RHE in 0.1 M HClO4 solution saturated with(1)(1-X Co ) Co tot

O2. Kinetically controlled current (Ik) was obtainedwith Koutecky-Levich Plots (i.e., Plots of the inverse--

where Cotot is Co atomic content in the Pt-Co alloyof observed current I 1 vs. ω 1/2) by extrapolating

catalyst obtained from inductively coupled plasma

the regression lines on the plots to the y intercept.

(ICP) analysis. Furthermore, Co atomic fraction (XCo)

Kinetically controlled current density (Jk) was calcu-

is evaluated by Vegard s law :lated by dividing Ik with the active surface area.

X Co ＝[a UL -a 0a S -a 0]3(2)XSResults and discussion
XRD patterns of carbon-supported Pt- Co alloy

where aUL is lattice parameter of synthesized alloy

catalyst and pure Pt catalyst are shown in Fig. 1.

catalyst obtained from Pt peak of X-ray diﬀraction

The diﬀraction peaks for Pt (111), (220), and (311) ofpattern, a0 and as are lattice parameters of bulk Pt

Pt-Co alloy catalyst are found out to shift to higher(0.3925 nm) and Pt3Co (0.3831 nm) respectively, andangles compared with that of pure Pt catalyst. This7)

Xs is Co atomic fraction (0.25) in Pt3Co catalyst. Cotot

result indicates that Pt and Co are alloyed under low

contains not only alloyed Co but also non-alloyed Co.

reduction temperature of 200 ℃. This phenomenon

Morphologies and compositions of the alloy catalyst

is recognized to derive from location of nanosized Pt

were evaluated by the transmission electron micro-

and Co particles in limited site of ionomer cluster.

scope (TEM) equipped with energy dispersive spec-

The eﬀects of reduction time of Co2+ ion on Co con-troscopy (EDS) analysis.
2.3Electrochemical property

The electrochemical active surface area and thePt (111)

ORR activity of alloy catalyst were measured by theIntensity / a.u.

rotating disk electrode (RDE). The measurements
were performed in 0.1 M HClO4 solution at room
temperature by using Ag/AgCl reference electrode.
The potential was corrected to that of RHE. Theelectrode catalyst ink for carbon- supported Pt- Co

(a )

*

Pt (220) Pt (311)

*

*(b )

alloy catalyst was prepared by ultrasonically mixing
it with ethanol aqueous solution. The ink was then30

placed on the glassy carbon surface of the RDE. Af-4050607080901002θ/ °ter drying, a drip of ionomer solution was applied
onto the surface of catalyst to ensure better adhesion

Fig. 1 XRD patterns of carbon-supported Pt-Co alloy catalyst reduced at 200 ℃ for 24 h prepared by
one time Co loading (a) and pure Pt catalyst (b).
＊ Refers to reﬂection from carbon.

of the catalyst on the glassy carbon surface. Priorto the ORR measurement, the working electrodesurface was cleaned and electrochemically stabilized21

GS Yuasa Technical Report2010 年 6 月

第7巻

第1号

tents and alloying degree in Pt-Co alloy catalyst areand alloyed Co contents increase with the number

shown in Fig. 2. The contents of total Co and alloyed

and reach saturated values of 10.8 and 6.7 atom% be-

Co increase with reduction time and reach constantyond four, respectively. The increase of total Co con-values of 3.2 and 2.5 atom% at 24 h, respectively. The

tent seems to be caused by newly supplying Co2+ ion

alloying degree deﬁned as ratio of alloyed Co content

at the location near Pt particle. This phenomenon isto total one slightly decreases with the time.

recognized that Pt particle contributes to reduction

The eﬀects of number of Co loadings on Co con-reaction of Co2+ ion. The alloying degree further de-tents and alloying degree were investigated. Changes

creases down to the value of 0.6. This means the in-in Co contents and alloying degree as a function ofcrease of non-alloyed Co content. To investigate thenumber of Co loadings are shown in Fig. 3. The total

cause of this increase, the morphology observationand composition distribution measurement of Pt-Co
alloy catalyst were conducted by TEM-EDS. Fig. 4 (a)1.05.0

0.8

4.0

0.6

3.0

0.4

Total Co content

0.2
0.0

2.0

Alloying degree

1.0

Alloyed Co content

01020304050

Pt-Co alloy catalyst prepared by one and six times
Co loadings, respectively. It is found out that particles are uniformly dispersed with average size of 4

Co content / atom%

Alloying degree / -and (b) show the TEM images for carbon-supported

nm for one time Co loading. On the other hand, theaggregations of large size of more than 10 nm areobserved besides small particles after six times Coloadings. Results of EDS analysis for particles and aggregations are shown in Table 1. It is found that theCo contents of the particles observed by EDS almost0.060

correspond with the Co contents by Vegard s law.

Reduction time / hIn contrast, Co content of the aggregation is remark-Fig. 2 Changes in Co contents and alloying degreeof Pt-Co alloy catalyst as a function of Co2+ ion reduction time. Reduction temperature: 200 ℃. Number of Co loadings : 1.

results seem to indicate that most of non-alloyed Co

ably high compared with that of the particles. Theselocate in the aggregation particles even after H2SO4
rinsing process.
The ORR activity of Pt - Co alloy catalyst was

1.0

12.0

0.8

10.0
8.0

0.6

6.0

0.4

Total Co content

0.2
0.0

4.0

Alloying degree
Alloyed Co content

0123

4

562.0

Co content / atom %Alloying degree / -evaluated by rotating disk electrode. Eﬀect of alloyed

(a)

0.0
7(b)

40 nm

40 nm

Number of Co loadings / Fig. 4 TEM images of carbon-supported Pt - Coalloy catalyst reduced at 200 ℃ for 24 h under hydrogen atmosphere prepared by one (a) and six (b)times Co loadings.

Fig. 3 Changes in Co contents and alloying degreeof Pt-Co alloy catalyst as a function of number of Coloadings. Reduction condition for Co2+ ion : 200 ℃for 24 h.

22GS Yuasa Technical Report2010 年 6 月

Table 1 Co contents of Pt - Co alloy catalyst in
average-sized and aggregation particles prepared by
diﬀerent number of Co loadings by EDS analysis.

16第1号

2.0

Co content / atom%
Average
Aggregation
particles
particles
‐
2.1
6.5
11.6

Jk ratio of Pt-Co to Pt / -Number of Co loadings

第7巻

Co content on Jk ratio of Pt-Co alloy catalyst to pure

1.51.00.5

Pt catalyst as a criterion for ORR activity at 0.6 Vis shown in Fig. 5. The Jk ratio turns out to increase

0.0

with the alloyed Co content above 3 atom% andreach the value of 1.5 at 7 atom%.

4Conclusions

0246Alloyed Co content / atom%

8Fig. 5 Eﬀect of alloyed Co content on Jk ratio of PtCo to Pt in 0.1 M HClO4 saturated with O2 at 25 ℃.

Carbon - supported platinum - cobalt ( Pt - Co ) alloy catalyst has been synthesized by the ultra-low

References

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