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Electrochemical studies of Pt/IreIrO2 electrocatalyst asa bifunctional oxygen electrode

Fan-Dong Kong, Sheng Zhang, Ge-Ping Yin*, Zhen-Bo Wang, Chun-Yu Du,Guang-Yu Chen, Na Zhang

School of Chemical Engineering and Technology, Harbin Institute of Technology, No. 92, West Da-Zhi Street, 150001 Harbin, China

a r t i c l e i n f o

Article history:

Received 20 July 2011

Received in revised form

1 September 2011

Accepted 17 September 2011

Available online 11 October 2011

Keywords:

Iridium oxide supported platinum

catalyst

Surface modification

Bifunctional oxygen catalyst

Unitized regenerative fuel cell

* Corresponding author. Tel.: þ86 451 864137E-mail address: yingeping@hit.edu.cn (G.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.09.087

a b s t r a c t

In the present study, Ir nanoparticles are deposited on the surface of IrO2 nanoparticles

with microwave-assisted polyol process. Then the obtained IreIrO2 nanocomposite is used

as a support to prepare Pt/IreIrO2 nanocomposite, which has been demonstrated as an

excellent bifunctional oxygen catalyst for unitized regenerative fuel cell. X-ray diffraction

(XRD), X-ray photoelectron spectra (XPS), and transmission electron microscopy (TEM)

characterizations indicate that ultrafine Ir nanoparticles (NPs) are uniformly dispersed on

the surface of IrO2, which provides a conductive network for Pt NPs. Electrocatalytic

activity and stability of Pt/IreIrO2 are investigated using cyclic voltammetry (CV), linear

sweep voltammetry (LSV), and potential cycling techniques on rotating disc electrode

(RDE). Results show that the electrocatalytic activity of Pt/IreIrO2 towards oxygen reduc-

tion reaction (ORR) is much higher than that of Pt/IrO2, and the electrocatalytic activity of

Pt/IreIrO2 towards oxygen evolution reaction (OER) can be comparable to that of Pt/IrO2.

Life tests reveal that Pt/IreIrO2 exhibits excellent stability. The enhanced ORR activity of Pt/

IreIrO2 catalyst can be attributed to the improvement in electronic conductivity, and its

higher stability can be assigned to the special structure, in which the interaction between

Pt NPs and Ir NPs prevents Pt from agglomerating.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction with a solar array. In terrestrial applications, it provides

Regenerative fuel cell (RFC) is an energy storage and conver-

sion system, which mainly consists of two separated subsys-

tems: a fuel cell and a water electrolyzer. When the two

subsystems are integrated into one unit, RFC becomes a unit-

ized regenerative fuel cell (URFC) with a more compact

configuration [1e3]. URFC possesses many advantages over

conventional secondary batteries, such as high specific energy

density (above 400 Whkg�1), long-term energy storage prop-

erty and no self-discharge [4e7]. In space applications, it can

better serve space vehicles or space stations by cooperating

07.-P. Yin).2011, Hydrogen Energy P

opportunities to establish off-grid power supply systems or to

level power peaks if coupled with the state grids [5,8,9].

Thekey technology inURFC is the fabricationof bifunctional

oxygen catalysts (BOC) which promote both oxygen reduction

reaction (ORR) and oxygen evolution reaction (OER). The

present challenge to fabricate BOCs is to endow themwith such

properties as considerably high catalytic activity, long-term

durability, reasonable electronic conductivity, more impor-

tantly, being anticorrosive to both the acidic medium and the

high anodic potential [10e12]. Currently, it has been reported

that the promising bifunctional oxygen catalysts are limited to

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 760

several noble metals and their oxides, i.e., Pt, Ir, Ru, IrO2 and

RuO2 [13e16]. Ptgenerally functionsasanactivecatalyst inORR,

while the others serve as active catalysts in OER [17e20].

Researches also show that Ru and its oxides are unstable in the

operating conditions although they display a high catalytic

activity to OER [21]. Therefore, Pt/IrO2 is commonly considered

the excellent bifunctional oxygen catalyst [22,23].

In design of Pt/IrO2 catalyst, attempts to date have been

made to increase the specific surface area and to modify the

electronic conductivity. Recently, numerous investigations

[5,10,24] focus on improving the specific surface area of Pt/

IrO2, but fewer studies on modifying the electronic conduc-

tivity of Pt/IrO2 catalyst are reported. It has been confirmed

that the conductivity of Pt/IrO2 catalysts can significantly

influence their ORR activity, even the overall catalytic

performance. The reason is that IrO2 agglomerates with

higher ohmic resistance trend to hinder the electronic paths

between Pt particles and lead to Pt insufficient performance

[25,26]. To solve this problem, exploring an effective strategy

for IrO2 surface modification is of significance. Considering

both catalytic activity and stability under the operating

conditions, metallic Pt and Ir are the appropriate material

used for IrO2 surface modification because they possess

reasonable conductivity, catalytic activity as well as resis-

tance to acidic condition and high potential. By comparison, Ir

is commercially practical.

In the present study, IrO2 surface modification has been

conducted by depositing metallic Ir nanoparticles (NPs) on its

surfacewithmicrowave-assisted polyol process. The resulting

IreIrO2 is used as a support to obtain Pt/IreIrO2 catalyst via

impregnation method. We focused our attention on exam-

ining the structure of the surface-modified IreIrO2 support

(serves as OER catalyst as well) and its effect on Pt/IreIrO2

catalyst. It was expected that Ir NPs on IrO2 surface could

improve the electronic conductivity and the overall catalytic

performance (Fig. 1). XRD and TEM were employed to deter-

mine the crystal structure and morphology. Cyclic voltam-

metry, linear sweep voltammetry, and accelerated potential

cycling tests (APCT) [27] were utilized to investigate the cata-

lytic activity and stability. To our knowledge, this kind of work

was first accomplished in our group. We aimed at opening

a new concept to fabricate bifunctional catalyst for URFC.

2. Experimental

2.1. Preparation of catalysts

The surface modification for IrO2 NPs was carried out by

deposition of ultrafine Ir NPs on the surface of IrO2 NPs (molar

Fig. 1 e Schematic diagram of the structure of Pt/IreIrO2

catalyst.

ratio, Ir:IrO2¼ 3:7) throughmicrowave-assisted polyol process

(MAPP) [28]. Typically, 52 mg of IrCl3$nH2O (58 wt.% Ir) was

dissolved in a solution containing 120 ml glycol (both as

solvent and reducing reagent) and 10 ml isopropanol. Ninety

milligrams of IrO2 (Johnson Matthey) was suspended in the

solution and was ultrasonicated for 30 min. The pH of the

solution was adjusted to 12 by dropwise addition of 0.5 M

NaOH (in glycol) and then themixturewas kept stirring for 3 h.

After that, it was bubbled with argon for 15 min to remove O2

in the solution and immediately heated inmicrowave oven for

50 s (ca. 130 �C). After cooling down to room temperature, the

solution was adjusted with 0.5 M HNO3 to a pH range of 3e4.

The mixture was continuously stirred for 10 h. Subsequently,

the resulting mixture was centrifuged and rinsed repeatedly

until no Cl� ions in the residue solution were detected, then

dried in a vacuum oven to yield IreIrO2 catalyst.

The preparation of Pt/IreIrO2 catalyst was carried out as

follows. Considering the fact that deposited Pt on IrO2 is

proved to be adverse to ORR [25], here an incipient wetness

procedure [17] was employed. First, desired amount of nano-

sized Pt powder (Johnson Matthey) was suspended in the

ethanol aqueous solution (volume ratio, 1:1) with the help of

ultrasonic to obtain Pt suspension of 1 mgml�1. Then, the

impregnation of IreIrO2 support into Pt suspension was con-

ducted and the resulting composite, Pt/IreIrO2, was dried at

120 �C before use. The nominal concentrations in Pt/IreIrO2

catalyst were 50 mol% Pt and 50 mol% Ir (Ir element content).

As comparison, Pt/IrO2 catalyst was also prepared by the

impregnation method using commercial Pt and IrO2 nano-

sized powders, both of which were obtained from Johnson

Matthey Corporation. The preparation procedure follows that

of Pt/IreIrO2 catalyst except that IreIrO2 was replaced with

IrO2. The nominal concentrations in Pt/IrO2 catalyst were kept

identical with Pt/IreIrO2 catalyst (50 mol.% Pt, 50 mol.% Ir).

2.2. Materials characterization

The X-ray diffraction (XRD) patterns of the catalysts were

recorded using a D/max-rB X-ray diffractometer (made in

Japan) with a Cu Ka radiation source operating at 45 kV and

100 mA. The tests were carried out in the angle (2q) range from

10� to 90� at a scanning rate of 4� min�1 with an angular

resolution of 0.05� of the 2q scan.

Transmission electron microscopy (TEM) images of the

catalysts were taken to characterize their morphology using

a JEOL TEM-1200EX (Made in Netherlands) system with

a spatial resolution of 1 nm. The samples of the catalysts were

finely ground and ultrasonically dispersed in ethanol, and

a drop of the resultant dispersion was deposited and dried on

a standard carbon membrane substrates. The operating

voltage on the microscope was 120 keV for low-resolution

tests and 300 keV for high-resolution tests.

X-ray photoelectron spectroscopy (XPS) measurements

were carried out for surface analysis using a Physical Elec-

tronics PHI model 5700 instrument. Photoelectrons were

generated by Al (ka) X-ray radiation of 1486.6 eV. Survey

spectra were collected at pass energy (PE) of 187.85 eV over

a binding energy range from 0 to 1300 eV. High binding energy

resolutionMultiplex data for individual element was collected

at a PE of 29.55 eV. The pressure inside the vacuum system

(c) Ir-IrO2

y/a.

u.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 7 61

was maintained at 1� 10�9 Pa during the experiments. All the

samples were dried under vacuum at 80 �C for 8 h before tests.

2.3. Electrochemical measurements

2.3.1. Preparation of working electrodesIn a typical preparation of working electrode, glassy carbon

electrode (GCE) of 3 mm in diameter (0.07065 cm2) was pol-

ished with 0.05 mm alumina to a mirror-finish before each

experiment. The ink with the total metals concentration of

2 mgml�1 was prepared by mixing the catalyst with ethanol

for 30 min in an ultrasonic bath. Twelve microliters of the ink

was loaded on GCE and dried at room temperature. After

completely drying, the deposited catalyst was covered with

5 ml of dilute Nafion solution (5 wt.%, DuPont Co. Ltd.) to form

a thin protective film [29,30].

2.3.2. Electrochemical measurementsThe electrochemical measurements were conducted on

a rotating disc electrode (RDE) with a three-electrode electro-

chemical cell. RDE covered with catalysts serves as the

working electrode and Pt foil of 1 cm2 as the counter electrode.

The standard Hg/Hg2SO4 electrode was used as the reference

electrode with its solution connected to the working electrode

by a Luggin capillary whose tip was placed close to the

working electrode. All experiments were performed in 0.5 M

H2SO4 solution at 25 �C using CHI650C electrochemical anal-

ysis instrument (Shanghai, China). All potentials reported in

this study are corrected with respect to RHE. Chemicals

applied were of analytical grade and the solutions were

prepared with ultrapure water (MiliQ Milipore, 18.2 MU cm).

Before testing, the catalyst was activated through cyclic

voltammetry (CV) scanning in the range of 0.05e1.2 V at a scan

rate of 50 mV s�1 until steady CV curves were obtained. To

determine the electrochemical surface area (ESA), the CVs

were recorded in an argon-purged 0.5 M H2SO4 solution in the

range of 0.05e1.2 V at a scan rate of 10 mV s�1. To evaluate the

polarization of OER, the linear sweep voltammograms (LSVs)

were recorded in the range of 1.2e1.6 V at a scan rate of

5 mV s�1. Similarly, the LSVs for ORR were also recorded on

RDE (1000 rpm) within the range of 1.0e0.6 V at a scan rate of

5 mV s�1 in an oxygen-saturated 0.5 M H2SO4 solution [31,32].

The accelerated potential cycling test (APCT) is a fast way

to evaluate the stability of a catalyst [27]. In this work, two

different potential ranges, 0.05e1.2 V and 1.2e1.6 V, were

selected to evaluate the stability of the catalysts for ORR and

OER, respectively. The tests were conducted in above elec-

trochemical system at a scan rate of 50 mV s�1.

0 20 40 60 80 100

(b) J& M corp. IrO2

(a) H om e-m ade Ir black

Inte

nsit

2 /degree

Fig. 2 e X-ray diffraction patterns of the catalysts, (a) Ir

black, (b) IrO2, and (c) IreIrO2.

3. Results and discussion

3.1. Materials characterization

In order to investigate the structure of surface-modified

IreIrO2 catalyst, XRD analyses were conducted and the

results have been presented in Fig. 2. Fig. 2a shows diffraction

peaks of Ir with a feature of broad peaks and wide line, indi-

cating that ultrafine Ir NPs are produced. The peaks in Fig. 2a

at 40.66�, 69.14�, and 83.44� correspond to the crystal planes,

(111), (220), and (311) (JCPDS: 06-0598), respectively. With the

intensest peak at 40.66� (Ir(111), d¼ 0.22 nm), the crystallite

size of Ir is estimated to be 1.6 nm by Scherrer equation [33].

Fig. 2b shows the diffraction characteristic of IrO2 (Johnson

Matthey), inwhich the broad peaks at 34.71�, 40.06�, and 57.94�

are assigned to the crystal planes, (101), (200), and (220) (JCPDS:

15-0807), respectively. The mean particle size of 6.4 nm in

diameter is also obtained by Scherrer equation. Fig. 2c shows

the pattern of the composite IreIrO2 catalyst. It can be easily

recognized that the peaks of Ir and IrO2 coexist in Fig. 2c, and

the broadening of Ir peaks are also observed, showing the

occurrence of ultrafine Ir NPs on the surface of IrO2. Moreover,

the peak of IrO2 (101) at 34.71� in Fig. 2c is significantly

weakened compared to that in Fig. 2b, while the peak of Ir (111)

at 40.66� in Fig. 2c is slightly weakened compared to that in

Fig. 2a. These results reveal that Ir NPs are uniformly

dispersed on the surface of IrO2 NPs and covered considerable

portion of IrO2 surface area.

Fig. 3 presents TEM images of IreIrO2 and Pt/IreIrO2 cata-

lysts. As shown in Fig. 3a, the well-dispersed Ir NPs on the

surface of IrO2 NPs (usually in agglomerated state, as shown in

insert) are observed. The electronic conductive network of Ir

NPs, as expected, has been established along IrO2 surface. The

states of deposited Ir NPs can be distinguished by the high-

resolution TEM image (Fig. 3b) where a microcrystalline

structure of Ir is readily recognized. The interplanar distance

(d ) of Ir (111), as marked in Fig. 3b, is calculated to be 0.22 nm,

which is consistent with the d value from XRD measurement.

Fig. 3c shows the image of Pt/IreIrO2 catalyst. It is clearly

found that Pt is uniformly supported on IreIrO2 support,

which may be interpreted in terms of the surface micro-

structure created by IrO2 surface modification and the affinity

of Ir to Pt. Meanwhile, Ir NPs play an important role in con-

necting Pt NPs to allow excellent electrons transfer (Fig. 3d).

To confirm the composition of Pt/IreIrO2 catalyst, surface

analysis was conducted using X-ray photoelectron spectros-

copy and the spectra are presented in Fig. 4. From Fig. 4amuch

higher Pt 4f5/2, 7/2 peaks and relative lower Ir 4f5/2, 7/2 peaks are

observed, and the corresponding relative-contents of Pt, Ir

elements are 67.6 mol%, 32.4 mol%, respectively. The

Fig. 3 e TEM images of the catalysts, (a) IreIrO2, (b) IreIrO2 with high-resolution, (c) Pt/IreIrO2, (d) Pt/IreIrO2 with high-

resolution.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 762

deviation from the designed contents (50 mol% Pt, 50 mol% Ir)

is due to the fact that XPS technique can only probe thin

surface layer (3e5 nm) where Pt are mainly enriched for Pt/

IreIrO2 catalyst. This result has been demonstrated by XRD

analysis described above. Fig. 4b shows the asymmetrical

spectrum of Ir 4f levels, which has been proved an intrinsic

property of Ir element in previous study [34]. The spectrum

displays two different valences of Ir element: Ir at 62.0 eV/4f7/2,

65.0 eV/4f5/2, and IrO2 at 61.1 eV/4f7/2, 64.1 eV/4f5/2. Their

relative-contents are calculated to be 33.5 mol% Ir and

66.5 mol% IrO2, repectively. Compared to the designed values

(30 mol% Ir, 70 mol% IrO2), a slightly high Ir content is also

attributed to the limited analytic ability of XPS and the

enrichment of Ir over IrO2 layer.

3.2. Electrochemical tests

In order to identify the individual contribution to electro-

chemical surface area (ESA), the CV curves of Pt, Ir, and IrO2

are specially shown in Fig. 5a. From the three CV curves, it can

be found that Pt, Ir, and IrO2 undergo different surface redox

processes. Pt displays a CV profile similar to the results re-

ported in other literature [35], and it mainly contributes to the

ESA. But no apparent peaks are observed for Ir and IrO2 in the

hydrogen region. Thus, the contribution created by Ir or IrO2 to

ESA can be neglected in discussing the ESA of a composite

catalyst, i.e., Pt/IrO2 and Pt/IreIrO2. Besides, the CV curve of

IrO2 shows higher voltammetric charge in oxygen region,

which is an indication of the electrochemical surface area

[17,36]. The peaks at about 0.9 V and 1.2 V are attributed to the

surface redox transitions of Ir (III)/Ir(IV) and Ir(IV)/Ir(VI)

surface oxyiridium groups, respectively. Metallic Ir, however,

undergoes a more complex process associated with contin-

uous transitions of Ir oxidation states with potential [36].

Fig. 5b presents the CV curves of Pt/IrO2 and Pt/IreIrO2

catalysts. In the hydrogen region, the hydrogen adsorption-

desorption peaks of Pt/IrO2 and Pt/IreIrO2 are well defined,

and the corresponding ESA values calculated from the

hydrogen desorption integrals are shown in Table 1. Appar-

ently, Pt/IreIrO2 possesses remarkable ESA value. This can be

ascribed to the contribution of Ir NPs to Pt, which effectively

modifies the IrO2 surface and provides available electronic

90 80 70 60 50

Ir 4f7 / 2

Ir 4f5 / 2

Pt 4f5 / 2

Pt 4f7 / 2

Inte

nsit

y / a

.u.

Binding Energy / eV

68 66 64 62 60 58

IrO2 4f

7 / 2

IrO2 4F

5 / 2

Ir 4f7 / 2

Ir 4f5 / 2

Inte

nsity

/ a.

u.

Binding Energy / eV

a

b

Fig. 4 e XPS spectra of Pt/IreIrO2 catalyst, (a) wide-range

spectrum, and (b) Ir 4f5/2, 7/2 peaks.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-10

-5

0

5

10

Pt Ir IrO

2

E / V vs. NHE

I / m

A m

g-1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-10

-5

0

5

10

I / m

A m

g-1

E / V vs. NHE

Pt/IrO2

Pt/Ir-IrO2

a

b

Fig. 5 e Cyclic voltammograms of the catalysts, (a) Pt, Ir,

and IrO2; (b) Pt/IrO2 and Pt/Ir at IrO2 in Ar-saturated 0.5 M

H2SO4 solution (10 mVsL1).

Table 1 e The initial electrochemical surface area (ESA)and oxygen reduction reaction (ORR) activity of Pt/IrO2

and Pt/IreIrO2 catalysts.

Catalysts ESA/m2 g�1 Pt ORR at 0.80V/A g�1 Pt

ORR at 0.85V/A g�1 Pt

Pt/IrO2 9.98 18.94 7.62

Pt/IreIrO2 22.4 30.55 10.67

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 7 63

paths. On the contrary, Pt/IrO2 has much lower ESA value

because IrO2 agglomerates have poor electronic conductivity

and trend to block the electronic paths between Pt particles.

As a result, the electrocatalytic active sites of Pt particles are

restrained, leading to an inadequate performance. This result

is supported by the literature reported by Ioroi [25]. However,

in the case of Pt/Ir catalyst (not shown), it indeed has high

conductivity, but metallic Ir can not act as excellent support

for Pt because the close-packed lattices ofmetallic Ir with high

mass density can not provide large specific surface area. That

results in poor dispersion and low performance for Pt catalyst

[37,38]. In Pt/IreIrO2 catalyst, in contrast, IreIrO2 possesses

both larger specific surface area (provided by IrO2) and excel-

lent conductivity (provided by metallic Ir), which results in

better Pt dispersion and higher catalytic performance. This

might be the reason why there have been limited studies on

Pt/Ir catalyst.

Fig. 6a shows the linear sweep voltammograms of Pt, IrO2,

and Ir catalysts for ORR. Within the applied potential window,

nearly no catalytic activity towards ORR is observed for indi-

vidual Ir or IrO2, indicating that they have negligible contri-

bution to ORR in Pt/IrO2 and Pt/IreIrO2 catalysts. Fig. 6b

presents the linear sweep voltammograms of Pt/IrO2 and Pt/

IreIrO2 catalysts. It can be clearly found that Pt/IreIrO2

exhibits an enhanced ORR activity as compared with that on

Pt/IrO2. As the ORR is under mixed kinetic diffusion control in

the potential range between 0.9 and 0.6 V, the oxygen-

reduction kinetic currents can be obtained with the well-

known mass-transport correction for RDE (KouteckyeLevich

equation) [39]:

Ik ¼ IdIId � I

0.6 0.7 0.8 0.9 1.0 1.1-20

-15

-10

-5

0

5I

/ mA

mg-1

E / V vs. NHE

Pt IrO

2

Ir

0.6 0.7 0.8 0.9 1.0-14

-12

-10

-8

-6

-4

-2

0

I / m

A m

g-1 P

t

E / V vs. NHE

Pt/IrO2

Pt/Ir-IrO2

a

b

Fig. 6 e Linear sweep voltammograms of the catalysts for

ORR, (a) Pt, IrO2, and Ir; (b) Pt/IrO2 and Pt/IreIrO2 in oxygen-

saturated 0.5 M H2SO4 solution (5 mV sL1, 1000 rpm).

1.2 1.3 1.4 1.5 1.6 1.7-100

0

100

200

300

400

I / m

A m

g-1

E / V vs. NHE

Pt IrO

2

Ir

1.2 1.3 1.4 1.5 1.6

-20

0

20

40

60

80

100

120

Pt/IrO2

Pt/Ir-IrO2

1.0 1.5

1.50

1.53

E /

V

Log I / mA mg-1

0.5 1.0 1.5 2.0

1.50

1.55

E /

V

54mV/dec

51mV/decI

/ mA

mg-1

E / V vs. NHE

a

b

Fig. 7 e Linear sweep voltammograms of the catalysts for

OER, (a) Pt, IrO2, and Ir; (b) Pt/IrO2 and Pt/IreIrO2 in 0.5 M

H2SO4 solution (5 mV sL1).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 764

where Ik is the mass-transfer-free kinetically controlled ORR

current, Id is the measured diffusion-limited current, and I is

the experimentally obtained current (with background

subtraction). The ORR data are shown in Table 1. It can be seen

that the ORR activity on Pt/IreIrO2 ismuch higher than that on

Pt/IrO2. This enhanced catalytic activity might also be

explained by the improved electronic conductivity as dis-

cussed above, which facilitates the ORR process [31].

Fig. 7a shows the linear sweep voltammograms of Pt, IrO2,

and Ir for OER. Pt indeed has somewhat catalytic activity to

OER. But significantly higher onset potential compared to that

of Ir or IrO2 is found. Considering that Pt content in Pt/IrO2 and

Pt/IreIrO2 catalysts were kept identical, it is offset in deter-

mining the observed difference in OER activity for the two

composite catalysts.

Fig. 7b shows the linear sweep voltammograms of Pt/IrO2

and Pt/IreIrO2 catalysts for OER. The onset potentials of them

are close, approximately 1.47 V. The current densities of Pt-

IrO2 and Pt/IreIrO2 measured at 1.52 V are 31.44 mAmg�1 and

29.30 mAmg�1, respectively. It can be seen that Pt/IreIrO2

shows extremely close OER activity to that of Pt/IrO2, which is

an indication that no substantial loss of OER activity after IrO2

surface modification with metallic Ir. In addition, we also

realize that the ohmic drop corrected Tafel plots (see Fig. 7b

inserts) measured at the same experimental condition nearly

parallel. The Tafel slopes of Pt/IrO2 and Pt/IreIrO2 are 51 and

54 mVdec�1, respectively. It further verifies the result ob-

tained from OER polarization measurement. The rather close

catalytic activity towards OER is related to several factors,

such as the individual OER activities of Ir and IrO2, their

contents in the composite catalysts, and their dispersion

levels.

Yim et al. [4] demonstrated that pure Ir and pure IrO2

exhibit close OER activity. Based on this result, the experi-

ments on IrO2 surface modification with metallic Ir were

performed keeping identical Ir content. We expected to

examine the single effect of the modification on Pt catalyst

(ORR) without apparent alteration for OER activity. On the

other hand, the active site density on catalyst’s surface is the

key factor that determines the catalytic activity. In OER, Some

0.0 0.2 0.4 0.6 0.8 1.0 1.2-100

-50

0

50

100 Pt/IrO2— 0 cycle— 400 cycle— 800 cycle— 1200 cycle— 1600 cycle— 2000 cycle

I / m

A m

g-1 P

t

E / V vs. NHE

0.0 0.2 0.4 0.6 0.8 1.0 1.2-40

-30

-20

-10

0

10

20

30

Pt/Ir-IrO2

— 0 cycle— 400 cycle— 800 cycle— 1200 cycle— 1600 cycle— 2000 cycle

I / m

A m

g-1 P

t

E / V vs. NHE

0 500 1000 1500 20000

5

10

15

20

25

ESA

/ m

2 g-1 P

t

Cyce number

Pt/IrO2

Pt/Ir-IrO2

a b

c

Fig. 8 e Cyclic voltammograms of Pt/IrO2 (a) and Pt/Ir at IrO2 (b), and the relationship of ESAs and cycle numbers (c) during

the APCT (50 mVsL1, 25 �C).

1.2 1.3 1.4 1.5 1.6-50

0

50

100

150

200

Pt/IrO2

— 0 cycle— 400 cycle— 800 cycle— 1200 cycle— 1600 cycle— 2000 cycle

I / m

A m

g-1

E / V vs. NHE

1.2 1.3 1.4 1.5 1.6

0

50

100

150Pt/Ir-IrO

2

— 0 cycle— 400 cycle— 800 cycle— 1200 cycle— 1600 cycle— 2000 cycle

I / m

A m

g-1

E / V vs. NHE

0 500 1000 1500 2000100

120

140

160

I / m

A m

g-1

Cycle number

Pt/IrO2

Pt/Ir-IrO2

a b

c

Fig. 9 e Cyclic voltammograms of Pt/IrO2 (a), and Pt/Ir at IrO2 (b), and the relationship of peak current densities at 1.6 V and

cycle numbers (c) during the APCT (50 mVsL1, 25 �C).

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 7 65

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 5 9e6 766

evidences show that watermolecules are first adsorbed on the

active sites of catalysts (Ir, IrO2, etc.), and then oxidized to

produce Oads and O2 [10]. In this study, the resulting Ir NPs

were uniformly dispersed on IrO2 surface, which state allows

Ir to remain high active site density. Otherwise, OER activity

might be lower if bulk Ir were substituted for it due to reduced

surface active sites density.

Fig. 8 shows the accelerated potential cycling test (APCT)

on Pt/IrO2 and Pt/IreIrO2 catalysts for ESAs. The polarization

curveswere recorded at the beginning and after 400, 800, 1200,

1600, and 2000 cycles, respectively. Fig. 8a and b show that the

ESAs for both catalysts decrease gradually during the APCT.

ESA values calculated from the hydrogen desorption integrals

with cycle number are shown in Fig. 8c, inwhich excellent ESA

retention of Pt/IreIrO2 is observed. After APCT, the ESA

reduces from 24.14 to 18.72 m2 g�1 for Pt/IreIrO2 (by 22.45%),

and from 13.90 to 8.97 m2 g�1 for Pt/IrO2 (by 35.47%). This

demonstrates that the degradation rate of Pt/IrO2 for ORR is

larger than that of Pt/IreIrO2.

One reason for the high stability of Pt/IreIrO2 is related to

the high dispersion of Ir NPs in gear-like state on the surface of

IrO2, which facilitates to disperse and stabilize Pt NPs. Another

reason may be, in essence, the difference in microstructure

between Pt/IreIrO2 and Pt/IrO2. As shown in Fig. 1, in the case

of Pt NPs deposited on Ir NPs, interaction between Pt and Ir

occurs due to affinity of Pt to Ir. In particular case, the most

probable positions for Pt to deposit might be the boundary

formed by Ir NPs and IrO2 surface, where it may have the

lowest potential energy [40]. Both cases described above

contribute to the high dispersion of Pt and can effectively

prevent Pt NPs from transferring and agglomerating.

In order to make a fast evaluation on catalysts for OER,

potential cycling in the range of 1.2e1.6 V at 50mVs�1 with

certain number of cycles was carried out. The peak current

densities measured at 1.6 V were used to evaluate the variation

in OER activities of the catalysts. Fig. 9 conveys that Pt/IrO2 and

Pt/IreIrO2 exhibit different degradation trends. For Pt/IrO2 cata-

lyst, continuous degradation (from168.86 to 143.28 mAmg�1, by

15.14%) during APCT is clearly observed, and for Pt/IreIrO2, slow

process of degradation (from 157.55 to 136.41mAmg�1, by

13.41%) is maintained after a sharp decrease in activity during

the first 400 cycles. The activity loss of Pt/IreIrO2 during the first

400 cycles may be associated with the oxidation of Ir on the

surface and the formation of iridium hydroxide hydrous (IrOx(-

OH)y(H2O)z), as well as the O2 bubble effect [41].

4. Conclusions

Pt/IreIrO2 catalyst has been fabricated by depositing Pt on

IreIrO2 that was obtained through IrO2 surface modification

with metallic Ir. Ultrafine Ir NPs are uniformly dispersed on

the surface of IrO2 NPs, which provides an electronic

conductive network for Pt NPs. The electrocatalytic activity of

Pt/IreIrO2 towards ORR is higher than that of Pt/IrO2, and the

electrocatalytic activity of Pt/IreIrO2 towards OER can be

comparable to that of Pt/IrO2. This kind of the structure of Pt/

IreIrO2 catalyst is beneficial for the stability of Pt because of

the interaction between Pt NPs and Ir NPs, which can effec-

tively prevent Pt from agglomerating. This study may open

a new concept to design highly active yet stable Pt-based

bifunctional oxygen catalyst for URFC.

Acknowledgements

This work is financially supported by National Natural Science

Foundation of China (grant no. 50872027, 21106024, and

21173062), Ministry of Science and Technology of China (863

program Grant No. 2009AA05Z111), Fundamental Research

Funds for the Central Universities (HIT.ICRST.2010006), and

Natural Scientific Research Innovation Foundation in Harbin

Institute of Technology (XWQQ5750012411).

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