i n t e r n 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 ) 2 9 8 5e2 9 9 2

Avai lab le a t www.sc iencedi rec t .com

journa l homepage : www.e lsev ier . com/ loca te /he

Synthesis and characterization of novel high-performancecomposite electrocatalysts for the oxygen evolution in solidpolymer electrolyte (SPE) water electrolysis5

Chunbao Xu, Lirong Ma, Jinlai Li, Wei Zhao, Zhongxue Gan*

State Key Laboratory of Coal-based Low Carbon Energy, ENN Group Co., Ltd., Langfang, Hebei 065001, PR China

a r t i c l e i n f o

Article history:

Received 14 November 2010

Received in revised form

28 March 2011

Accepted 6 April 2011

Available online 2 January 2012

Keywords:

Composite electrocatalysts

Oxygen evolution

Heat treatment

Water electrolysis

Solid polymer electrolyte

5 Presented at the 11th China Hydrogen En* Corresponding author. Tel.: þ86 316 259 69E-mail address: ganzhongxue@enn.cn (Z

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.031

a b s t r a c t

Novel Ru0.3Ir0.7O2/Pt0.15 composite electrocatalysts for the oxygen evolution in solid poly-

mer electrolyte (SPE) water electrolysis were prepared by a two-step method. The inter-

mediate Pt black with or without heat treatment (the final samples marked as C1 and C2

respectively) was firstly synthesized by a conventional reduction method with ultrasonic

dispersion. The composite electrocatalysts were then prepared by impregnation-reduction

method with ultrasonic dispersion followed by fusion treatment. The influence of heat

treatment of intermediate Pt black on the properties of the composites was explored. The

as-prepared composites were characterized by XRD, BET, SEM, EDX, CV and LP. The cata-

lytic performance of the as-prepared electrocatalysts has also been investigated in a 20 cm2

SPE electrolytic cell using Nafion� 117 as an electrolyte with the loading of noble metals of

1.8 mg cm�2 at anode and 0.3 mg cm�2 at cathode. It shows that the catalytic performance

of samples C1 and C2 is obviously higher than that of commercial PtIrO2 electrocatalyst and

the catalytic activity of C1 electrocatalyst for the oxygen evolution is evidently higher than

that of C2 electrocatalyst in the whole range of cell voltage. The cell voltage was only 1.76 V

and 1.90 V when the current density is 1.0 A cm�2 and 1.5 A cm�2 respectively using sample

C1 as anode electrocatalyst.

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

reserved.

1. Introduction characteristics [10e12]. Nevertheless, hydrogen production is

Energy crisis and climate change are threatening the global

security. There is an urgent need to find alternative sustainable

energy sources, which promotes significant advances in

renewable energy technologies [1e5]. Since these renewable

energy sources are intermittent, there is clearly an ever-

increasing need for the development of new technologies for

large-scale energy storage systems, which remains one of the

greatest challenges [6e9]. Hydrogenhas beenconsideredasone

of thepromising energy carrier of the future due to its attractive

ergy Conference, 24-27 O00; fax: þ86 316 259 6907.. Gan).2011, Hydrogen Energy P

probably the biggest technical and economical hurtle before

hydrogen energy systems become a competitive energy carrier

[13]. By converting renewable energy to hydrogen, both issues

mentioned above could be readily resolved simultaneously and

a substantial synergy between hydrogen and electricity could

be achieved. Although several technologies have been under

development for production of hydrogen from renewable

energy sources, it seems that water electrolysis is easiest and

most promising option [14e16]. The key is to develop more

efficient and lower-cost water electrolysis systems.

ctober 2010, Zhejiang, China.

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 ) 2 9 8 5e2 9 9 22986

Water electrolysis systems based on solid polymer elec-

trolyte (SPE) have several attractive advantages over tradi-

tional alkaline technologies including greater energy

efficiency, higher production rates, a high degree of gas purity

[15,17,18]. More importantly, SPE water electrolysis technolo-

gies are well-suited for water splitting using intermittent

power sources [19]. Therefore, SPE water electrolysis has been

identified as a key process to transform renewable electricity

into hydrogen and oxygen by the European Commission [19].

However, considerable efforts are still needed to further

improve the electrolytic efficiency as one of the most impor-

tant routes to further bring down hydrogen production costs

to meet the market requirements. The oxygen evolution

reaction is the main source of energy loss through over-

potential in SPE water electrolysis. It is therefore of great

importance to develop and optimize oxygen evolution elec-

trocatalysts to minimize the energy loss and bring down the

material costs.

The overpotential of oxygen evolution reaction is influ-

enced by adsorbed species at the catalyst surface that blocks

the approach of oxygen-containing species to the surface

catalytic sites where they are going to be oxidized and the

oxygen electrode catalysts must also be resistant to anodic

corrosion during oxygen evolution because the newly formed

oxygen-containing species are strong oxidizing agents [20].

Noble metals, metal oxides and their combinations have been

widely used as the oxygen electrode catalyst materials for the

SPE water electrolysis [15,17,20e39]. Generally, metal oxides

showhigh electrocatalytic activities for oxygen evolution than

metal electrodes. IrO2 and RuO2 as electrocatalysts are well

established in many industrial electrochemical processes in

the form of dimensionally stable anodes [40]. Pure RuO2 has

a high electrocatalytic activity for oxygen evolution, however,

it is not stable in acidic media, which makes it unsuitable for

long-term operations [29]. IrO2 exhibits higher corrosion-

resistant properties and only slightly lower activity than

RuO2 [41]. Additions of less expensive inert components to

IrO2 or RuO2 can improve the anodic stability and electro-

catalytic activity [40e46]. It was reported that both RuO2 and

IrO2 are slightly non-stoichiometric, RuO2 is an oxygen defi-

cient, while excess oxygen has been found with IrO2 [47]. To

leverage the complementary structure of IrO2 and RuO2,

mixtures of RuO2 and IrO2 and IrxRu1�xO2 have been investi-

gated as the anodic elctrocatalyst to enhance their electro-

catalytic activities and stability and better eletrocatalytic

performance and higher stability were exhibited [17,32e36].

To further increase the conductivity, stability and electro-

catalytic activity of the oxygen electrode, noble metals, such

as Pt, are added to noble metal oxides [36e39]. Enhanced

electrocatalytic performance was obtained due to specific

interactions between Pt and the metal oxides and the

continuous conduction path of electrons along Pt [36e39].

In the present work, novel Ru0.3Ir0.7O2/Pt0.15 composite

electrocatalysts for the oxygen evolution in SPE water elec-

trolysis were synthesized by a two-step method. Ru0.3Ir0.7O2

was chosen to serves as a major electrocatalyst since Ru-Ir

oxides has been exhibited to be the most promising active

oxygen evolution electrocatalyst [21,34,43,44], while a small

amount of Pt (molecular ratio of Pt to Ru to Ir is 0.15:0.3:0.7)

was added to increase the conductivity, improve the stability,

and enhance the electrocatalytic performance of the

composites. The influence of heat treatment of intermediate

Pt black on the properties of the compositeswas explored. The

freshly prepared composites were characterized by XRD, BET,

SEM, EDX, CV and LP. Their electrocatalytic performances

were investigated in a 20 cm2 SPE electrolytic cell. The

mechanism associated with electrode electrocatalytic activity

enhancement was assumed.

2. Experimental

2.1. Preparation of composite electrocatalysts

The Ru0.3Ir0.7O2/Pt0.15 composite electrocatalysts were

prepared by a two-step method. The intermediate Pt black

was firstly synthesized by a conventional reduction method

with ultrasonic dispersion. The 5 wt% aqueous precursor

solution was prepared by dissolving known amount of

H2PtCl6$6H2O (Sino-Platinum Metals Co., Ltd.) in appropriate

amount of deionized water. All deionized water was tapped

from a Milli-Q� Ultra Pure Water Systems. The resistance of

the water was 18.2 MU cm. A certain amount of aqueous

solutionswas thenweighed out and dispersed in an ultrasonic

reactor. The solution was subsequently heated to 80 �C under

air atmosphere and stirred under ultrasonic agitation for 1 h.

Appropriate amount of reducing agent HCHO was then added

to the solution and the pH of this solution was adjusted to

neutral or slightly basic levels by the addition of 0.1 M NaOH

solution. The mixture solution was kept at 80 �C under ultra-

sonic agitation for another hour. After cooling to room

temperature, the reduced and precipitated slurry was washed

repeatedly with deionized water until no visible precipitate

appeared by adding several drops of 0.1 M AgNO3 solution to

the filtrate. After being air dried at 80 �C for an hour, a certain

amount of them was then transferred into a quartz crucible

and annealed in a tube furnace at 500 �C for 30 min under

flowing Argon before cooling down to room temperature. The

intermediate Pt powder, with or without heat treatment, was

then obtained.

The composites were then prepared by impregnation-

reduction method with ultrasonic dispersion followed by

fusion treatment. Known amounts of Pt powder, with or

without heat treatment (the final samples obtained were

marked as C1 and C2, respectively), was dipped in appropriate

amount of de-ionized water containing 10 ml isopropanol to

form a suspended phase. The slurry was dispersed in an

ultrasonic reactor for 30 min at room temperature. Certain

amounts of metal precursor solutions RuCl3$nH2O and

H2IrCl6$6H2O (Sino-Platinum Metals Co., LTD.) were added

into the above slurry. The molar ratio of Pt:Ru:Ir in the slurry

was 0.15:0.3:0.7. The aqueous mixture solution was subse-

quently heated to 80 �C under air atmosphere and stirred

under ultrasonic agitation for 1 h. Then known amount of

NaNO3 was added into the mixture, which was kept at 80 �Cunder ultrasonic agitation for another hour. After being air

dried at 80 �C for 12 h, the remainder was calcined in a tube

furnace at 500 �C for 30 min to obtain Ru0.3Ir0.7O2-deposited Pt

black composite electrocatalysts. The composites were then

cooled to room temperature and washed repeatedly with

Fig. 1 e Schematic diagram of the SPE electrolytic cell.

i n t e r n 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 ) 2 9 8 5e2 9 9 2 2987

deionized water until no visible precipitate appeared by add-

ing several drops of 0.1 M AgNO3 solution to the filtrate. The

composite electrocatalysts were obtained finally by drying the

powder at 80 �C in air for an hour.

2.2. Physico-chemical characterization

The apparent surface areas and average pore size were

obtainedusing aSurfaceAreaandPorosimetryAnalyzer (NOVA

4200e) by the Brunauer-Emmett-Teller (BET) method and

Barrett-Joyner-Halenda (BJH) method based on a desorption

model, respectively. The total pore volumewas estimated from

the volume of liquid nitrogen adsorbed at a relative pressure

P/P0 ¼ 0.98. The XRD patterns of the samples were recorded by

means of an X’Pert PRO X-Ray Diffractometer with an area

detectorusing aCuKa radiation source (l¼ 1.54056 A) operating

at 40 kV and 38mA. The scanning angle (2q) region between 20�

and 80� was explored at a scan rate of 0.02� s�1. The morpho-

logical characteristics of the samples were investigated by SEM

using a SUPRA 55 Field Emission Scanning ElectronMicroscope

operated at 5.00 kV. The chemical compositions of the samples

were determined by a LINK-860 Energy dispersive X-ray (EDX)

spectroscopy attached to the SEM.

2.3. Electrochemical measurements

A glass electrochemical system with a three-electrode config-

uration was used for the electrochemical experiments. A

saturated calomel electrode (SCE) with double salt bridges in

saturated potassium chloride (KCl) solution was used as the

reference electrode. The counter electrode used was

a 1 cm � 1 cm Pt foil and the working electrode was a clean

glassy carbon disk electrode (gcde) at an electrocatalyst

loading of 0.2mg cm�2 with a cross-sectional area of 0.196 cm2

held in a Teflon cylinder. All potentials reported in this work

were corrected with respect to the SCE.

The electrochemical performance of the electrocatalysts

were studied by cyclic voltammetry (CV) and linear polariza-

tion (LP) using a PARSTAT� 2273 Potentiostats e Electro-

chemistryWorkstation fromAMETEK. CV and LP experiments

were carried out at room temperature. All the experiments

were performed in 0.5 M H2SO4 solution prepared from purity

sulphuric acid and deionizedwater. The potential range of the

CV was between 0 and 1.0 V with a scan rate of 50 mV s�1. The

potential range of the LPwas between 1.0 and 1.8 Vwith a scan

rate of 10 mV s�1.

2.4. Preparation of MEAs

A catalyst coating membrane (CCM) method was used for MEA

preparation. Nafion� 117 membranes (Dupont) were first pre-

heated in deionized water, then treated by 3 wt% H2O2 and

5 wt% sulphuric acid solution at 80 �C to remove organic

impurities and ion exchange the membrane, respectively, fol-

lowed by boiling in deionized water and finally stored in

deionized water. To prepare electrocatalyst ink for the anode,

a mixture of freshly prepared C1 or C2 electrocatalyst or

commercial PtIrO2 electrocatalyst from Johnson Matthey (for

comparison purpose), Nafion� solution (5 wt%, Dupont), iso-

propanol, and de-ionized water (in weight proportion of

7:2:30:2)were supersonically stirred for 1 h. Commercial 40wt%

Pt/XC-72 from Johnson Matthey was served as electrocatalyst

for cathode ink. An air driven spray gun was utilized to spray

known amounts of anode and cathode inks on either side of

a treated and dried Nafion� 117 membrane at 30e40 �C. Thecircular active area of the MEAs is 20 cm2. The loading of noble

metals is 1.8 mg cm�2 at anode and 0.3 mg cm�2 at cathode.

2.5. Evaluation of WE performance

Evaluation of SPE water electrolysis performance was per-

formed in a single SPE electrolytic cell. Schematic diagram of

the cell used in our experiments is shown in Fig. 1. The cell

body wasmade of stainless steel. Two porous titanium plates,

whichwere heat-treated by differentmetal precursors to form

a very thin coating of IrO2 or Pt on their surfaces to avoid

corrosion phenomena, were used as a diffusion layer and

current collector for the anode or cathode, respectively. Two

pieces of 2-mm thick titanium plates with flow fields were

used as polar plates. During the operation of water electrol-

ysis, deionizedwaterwas supplied to the anode and circulated

to perform the thermalmanagement, as well as to provide the

process water for the electrolytic cell. H2 and O2 gases

produced along their flow fields were respectively released.

Water was separated from the anode stream out of the cell at

atmospheric pressure. The temperature of the electrolysis cell

is kept at about 60 �C and the pressure in both sides of anode

and cathode is kept at atmospheric pressure.

3. Results and discussion

3.1. XRD analysis

Fig. 2 shows the XRD patterns of samples C1 and C2, Pt black

with heat treatment, and RuO2 and IrO2 powders prepared by

20 30 40 50 60 70 80

Pt black

RuO

IrO

C

C

2θ / °

Rel

ativ

e in

tens

ity /

a.u.

Fig. 2 e XRD patterns of samples C1 and C2, Pt black with

heat treatment, RuO2 and IrO2 powders prepared by Adams

fusion method.

0 10 20 30 40 50 60 70 80 900.00

0.02

0.04

0.06

0.08

0.10

0.12

Diameter / nm

BJH

cum

ulat

ive

pore

vol

ume

/ cm

g

C desorption

C desorption

Fig. 4 e BJH cumulative pore volume curves of samples C1

and C2.

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 ) 2 9 8 5e2 9 9 22988

Adams fusion method [21,40,48], the latter three of which are

used as reference patterns. It can be seen that the values of the

2q corresponding to the characteristic intense diffraction

peaks in both XRD patterns of C1 and C2 composite electro-

catalysts are slightly different from those in the XRD patterns

of individual Pt, RuO2 or IrO2. The shift in the locations for the

characteristic intense diffraction peaks indicates that there

must exist some specific interactions between Pt and the Ru-Ir

oxides and Pt and Ru-Ir oxides in both C1 and C2 electro-

catalysts do not exist as a mixture of the individual Pt, RuO2

and IrO2, but in the form of coordinative complex. The shift of

Pt peak suggests that the incorporation of IrO2 and RuO2

changes the lattice constant of Pt crystals. Compared the XRD

patterns of both C1 and C2 electrocatalysts, it can be found that

all the characteristic intense diffraction peaks are nearly the

same except the Pt (111) and (200) characteristic intense

diffraction peaks at 2q ¼ 40.3� and 46.5�, respectively. The

0.0 0.2 0.4 0.6 0.8 1.020

40

60

80

100

120

Relative pressure / p/p

Volu

me

/ cm

g

C adsorption

C desorption

C adsorption

C desorption

Fig. 3 e Nitrogen adsorptionedesorption isotherms of

samples C1 and C2.

higher peak intensities of Pt in the pattern for sample C1

indicate that higher crystallinity of Pt were obtained as

a result of heat treatment of intermediate Pt black in Argon at

500 �C for 30 min.

3.2. Textual property analysis

Nitrogen adsorptionedesorption isotherms of samples C1 and

C2 are shown in Fig. 3. The adsorption properties of sample C1

is apparently higher than that of sample C2. The isotherms

were determined to be of type IV (IUPAC classification) typical

of mesoporous materials [49], which was confirmed by BJH

cumulative pore volume curves of the prepared samples as

shown in Fig. 4. Most pore diameters of both samples are in

the range of 2 nme50 nm. The surface areas, total pore

volumes and average pore diameters for the samples are listed

in Table 1. It can be seen that the surface area and total pore

volume of the sample C1 are higher than those of the sample

C2, yet the average pore diameter of C1 is slightly smaller than

that of C2. The hysteresis loop shown in Fig. 3 was designated

H4 as recommended by IUPAC Type H4 [49]. The lower closure

point of the hysteresis loop of sample C1 indicates that its

average pore size is smaller than that of C2, which is in good

agreement with the results presented in Table 1. All these

phenomena are related to the heat treatment of the inter-

mediate Pt black. It is possible thatmost of the intermediate Pt

powders without heat treatment are amorphous phase with

a higher degree of agglomeration. On the other hand, thermal

agitation along with heat treatment might inhibit incompact

Table 1 e Textual properties of as-prepared samples C1

and C2.

Samples BET specificsurface area

(m2 g�1)

Total porevolumes(cm3 g�1)

Average porediameter (nm)

C1 165.5 0.16 3.9

C2 141.4 0.15 4.1

i n t e r n 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 ) 2 9 8 5e2 9 9 2 2989

aggregation and combination of weak-bonds, which results in

smaller particles of a lower degree of agglomeration. Mean-

while, heat treatment under Argon leads to a higher crystal-

linity of Pt black with a well-ordered porous structure.

Therefore, the composite electrocatalyst with heat treatment

(sample C1) has a higher specific surface area and total pore

volume.

3.3. SEM and EDX analysis

Representative SEM images of the samples C1 and C2 are in

Fig. 5. Fig. 5a shows that the Ru0.3Ir0.7O2/Pt0.15 particles

synthesized with heat treatment of the intermediate Pt black

are more uniform than the particles without heat treatment

and good distributions of pore structure can be observed as

well, which provide reasonable explanations for the higher

surface area of C1 electrocatalyst. Fig. 5b shows that

Ru0.3Ir0.7O2/Pt0.15 particles of different morphological charac-

teristics were obtained in sample C2. It can be seen that the

nanoparticles are highly disordered and have a larger particle

size distribution as well, leading to a decrease in the surface

area. It can be inferred that the heat treatment of intermediate

Pt black in the protection of Argon leads to more highly

ordered crystallites with porous structure. Therefore, more

ordered particles were obtained by the deposition of Ru-Ir

oxides on the surface of Pt black.

Fig. 5 e Representative SEM images of samples C1 (a) and

C2 (b).

The EDX data (Table 2) were obtained by analyzing the

above corresponding regions of the SEM images (Fig. 5) of the

C1 and C2 composite electrocatalysts. Ru, Ir, O, Pt are themajor

elements in both composites. The detected atomic contents of

the Ru, Ir and Pt in the C1 electrocatalyst are 9.73%, 15.15%,

and 2.21%, respectively. Comparedwith the C2 electrocatalyst,

the atomic contents of Ru, Ir and Pt in the C1 electrocatalyst

are all lower than those in the C2 electrocatalyst. Nevertheless,

the atomic percent ratio of Pt to Ru and Ir (Pt/( Ru þ Ir) ¼ 0.089)

in the C1 electrocatalyst is lower than that (0.093) in C2 elec-

trocatalyst, indicating a more uniform deposition of Ru-Ir

oxides on Pt black obtained in C1 electrocatalyst as a result

of highly ordered porous crystallites caused by heat treat-

ment. On the other hand, the intermediate Pt powder used in

the C2 electrocatalyst is mostly amorphous, leading to a less

uniform deposition of Ru-Ir oxides on Pt powder, therefore,

higher atomic percent ratio of Pt to Ru and Ir was obtained in

the C2 electrocatalyst. The atomic contents of the corre-

sponding elements in the C1 and C2 electrocatalysts are all

lower than the theoretically calculated values shown in the

molecular formula Ru0.3Ir0.7O2/Pt0.15 due to the uneven depo-

sition of RuxIr1�xO2 on the particles of Pt, more or lessmaterial

loss in the preparation process and the fact that the detection

depth of EDX is limited.

3.4. Electrochemical analysis

The electrochemical performances of the samples C1 and C2

electrocatalysts were evaluated by the CV and LP measure-

ments. The loading of each electrocatalyst was kept at

0.2 mg cm�2 Fig. 6 shows CV curves for both samples, which

were used to evaluate their electrocatalytic activities. It can be

seen that the electrocatalytic activity of the C1 electrocatalyst

for the oxygen evolution at positive high-potential is evidently

higher than that of the C2 electrocatalyst. In order to examine

and compare the electrocatalytic activities of the C1 and C2

electrocatalysts, LP tests were carried out and the results are

shown in Fig. 7. It is common to take the value of the kineti-

cally controlled peak current density at the highest positive

potential to compare the electrocatalytic activities of the C1

and C2 electrocatalysts. It can be found that the peak current

densities are respectively 0.14 A cm�2 for the C1 electro-

catalyst and 0.11 A cm�2 for the C2 electrocatalyst at 1.8 V

versus SCE. The heat treatment of the intermediate Pt black in

the protection of inert gas leads to an increase of 0.03 A cm�2

in the peak current density at the highest experimental

potential. Therefore, heat treatment under inert gas of the

Table 2 e The EDX data for samples C1 and C2.

Elements C1 C2

Weight% Atom % Weight % Atom %

Ru 17.91 9.73 16.88 10.01

Ir 53.01 15.15 56.12 17.50

O 21.23 72.90 18.66 69.92

Pt 7.86 2.21 8.34 2.56

Total 100.00 100.00 100.00 100.00

0.0 0.2 0.4 0.6 0.8 1.0-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

C2

Potential / V vs SCE

Cur

rent

den

sity

/ A

cm- 2

C1

Fig. 6 e Steady-state cyclic voltammograms of samples C1

and C2.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Current Density / A cm

Volta

ge /

V

PtIrO

CC

Fig. 8 e Currentevoltage curves measured during water

electrolysis using a SPE electrolytic cell.

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 ) 2 9 8 5e2 9 9 22990

intermediate Pt black is beneficial to the electrocatalytic

activity of the prepared composite electrocatalyst.

3.5. SPE water electrolysis performance

The performance of two freshly prepared composite electro-

catalysts (samples C1 and C2) was measured in a single SPE

water electrolytic cell up to current densities of 1.5 A cm�2

with other conditions the same. Commercial PtIrO2 was also

evaluated in the same condition for comparative purpose. The

loading of noblemetals for each electrocatalyst is 1.8 mg cm�2

at anode and 0.3 mg cm�2 at cathode. Typical currentevoltage

curves recorded at 60 �C are shown in Fig. 8. It can be seen that

the electrocatalytic performance of both C1 and C2 electro-

catalysts is obviously higher than that of commercial PtIrO2

electrocatalyst. Compared with PtIrO2, the employment of Ru

probably improves the molecular structure of RuxIr1�xO2-Ptyand therefore the electrocatalytic performance is enhanced as

a result of the synergy effect of oxides of Ir and Ru.Meanwhile,

there probably exists a synergy effect between Pt and the

oxides of Ir-Ru, which may also contribute to the increase in

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

0.00

0.03

0.06

0.09

0.12

0.15

C

Potential / V vs SCE

Cur

rent

den

sity

/ A

cm-2

C

Fig. 7 e Linear polarization curves of samples C1 and C2.

the higher electrocatalytic activity of prepared electro-

catalysts than that of commercial electrocatalysts. The cell

voltages of ca. 1.76 V and 1.90 V for the C1 electrocatalyst were

obtained at a current density of 1.0 A cm�2 and 1.5 A cm�2,

respectively. Results obtained with the C1 electrocatalyst

shows that its electrocatalytic performance is evidently

higher than that of the C2 electrocatalyst. More precisely, at

the current density of 1.0 A cm�2 and 1.5 A cm�2, the operating

voltages of the cell with the C2 electrocatalyst are ca. 1.81 and

1.92 V, respectively. It can be inferred that the difference

between the electrocatalytic performances of the two

as-prepared composite electrocatalysts is as a result of the

treatment of the intermediate Pt black at 500 �C for 30 min

under flowing Argon. More specifically, the enhanced elec-

trocatalytic performance of sample C1 is probably related not

only to its higher surface area and well-ordered pore struc-

ture, but also to its higher conductivity of the composite as

a result of higher crystallinity of Pt black caused by heat

treatment.

4. Conclusions

Ru0.3Ir0.7O2/Pt0.15 composite electrocatalysts for the oxygen

evolution in solid polymer electrolyte (SPE) water electrolysis

was synthesized by impregnation-reduction method with

ultrasonic dispersion followed by fusion treatment. The heat

treatment of the intermediate Pt black has an obvious influ-

ence on the physico-chemical as well as electrochemical

properties of the prepared Ru0.3Ir0.7O2/Pt0.15 electrocatalysts.

XRD analysis shows that Pt and the oxides of Ru and Ir exist in

the form of coordinative complex in both electrocatalysts. It is

found that C1 electrocatalyst has a higher BET specific surface

area than sample C2 electrocatalyst, indicating heat treatment

of the intermediate Pt black is beneficial to the electro-

chemical properties of the prepared composites, which was

further confirmed by CV and LP analysis. SEM and EDX anal-

ysis reveal that heat treatment on the intermediate Pt black

influences not only on the morphology of the composites but

also on atomic contents. The electrocatalyst particles with

heat treatment of the intermediate Pt black are more uniform

i n t e r n 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 ) 2 9 8 5e2 9 9 2 2991

in size and have a higher surface area and total pore volume.

Ru-Ir oxides are more uniformly deposited on Pt black as well.

The SPE water electrolysis performance tests show that the

electrocatalytic performance of both Ru0.3Ir0.7O2/Pt0.15composite electrocatalysts with and without heat treatment

of the intermediate Pt black is higher than that of commercial

PtIrO2 electrocatalyst, nevertheless, the electrocatalytic

activity for the oxygen evolution was obviously enhanced

with heat treatment of the intermediate Pt black. The poten-

tial on the C1 electrocatalyst in a 20 cm2 single SPE water

electrolytic cell is only 1.76 V and 1.90 V when the current

density is 1.0 A cm�2 and 1.5 A cm�2, respectively. It is

expected that the electrocatalytic activity of RuxIr1�xO2/Ptycomposite electrocatalysts can be further improved by opti-

mizing the preparation process as well as the composite

formula.

Acknowledgements

The authors acknowledge ENN Research & Development Co.,

Ltd. and Dr. Yusuo Wang, Chairman of the Board of Directors

at ENN Group Co., Ltd. for the financial support. The authors

are also grateful to Long Zhao, Pei Zhao and Qi Zhang in the

assistance of some experiments.

r e f e r e n c e s

[1] Joselin Herbert GM, Iniyan S, Sreevalsan E, Rajapandian S.A review of wind energy technologies. Renew Sustain EnergyRev 2007;11:1117e45.

[2] Harborne P, Hendry C. Pathways to commercial wind powerin the US, Europe and Japan: the role of demonstrationprojects and field trials in the innovation process. EnergyPolicy 2009;37(9):3580e95.

[3] Joshi AS, Dincer I, Reddy BV. Performance analysis ofphotovoltaic systems: a review. Renew Sustain Energy Rev2009;13:1884e97.

[4] Celik AN, Muneer T, Clarke P. A review of installed solarphotovoltaic and thermal collector capacities in relation tosolar potential for the EU-15. Renew Energy 2009;34:849e56.

[5] Kazmerski Lawrence L. Solar photovoltaics R&D at thetipping point: a 2005 technology overview. J ElectronSpectrosc and Relat Phenom 2006;150:105e35.

[6] Wilson IAG, McGregor PG, Hall PJ. Energy storage in the UKelectrical network: estimation of the scale and review oftechnology options. Energy Policy 2010;38:4099e106.

[7] Hall PJ, Bain EJ. Energy-storage technologies and electricitygeneration. Energy Policy 2008;36:4352e5.

[8] Ibrahima H, Ilincaa A, Perron J. Energy storage systems-characteristics and comparisons. Renew Sustain Energy Rev2008;12:1221e50.

[9] Chen HH, Cong TN, Yang W, Tan CQ, Li YL, Ding YL. Progressin electrical energy storage system: a critical review. Prog NatSci 2009;19:291e312.

[10] Nejat Veziroglu T, Barbir F. Solar-hydrogen energy system:the choice of the future. Environ Conserv 1991;18:304e12.

[11] Kazim A. Strategy for a sustainable development in the UAEthrough hydrogen energy. Renew Energy 2010;35:2257e69.

[12] van Ruijven B, van Vuuren DP, de Vries B. The potential roleof hydrogen in energy systems with and without climatepolicy. Int J. Hydrogen Energy 2007;32(12):1655e72.

[13] Elam CC, Gregoire Padro CE, Sandrock G, Luzzi A, Lindblad P,Hagen EF. Realizing the hydrogen future: the internationalenergy agency’s efforts to advance hydrogen energytechnologies. Int J Hydrogen Energy 2003;28:601e7.

[14] Barbir F. PEM electrolysis for production of hydrogen fromrenewable energy sources. Sol Energy 2005;78:661e9.

[15] Grigoriev SA, Porembsky VI, Fateev VN. Pure hydrogenproduction by PEM electrolysis for hydrogen energy. IntJ Hydrogen Energy 2006;31:171e5.

[16] Kruger P. Electric power requirement in California for large-scale production of hydrogen fuel. Int J Hydrogen Energy2000;25:395e405.

[17] Marshall A, Borresen B, Hagen G, Tsypkin M, Tunold R.Hydrogenproductionbyadvancedprotonexchangemembrane(PEM) water electrolysis e reduced energy consumption byimproved electrocatalysis. Energy 2007;32:431e6.

[18] Garcıa-Valverde R, Miguel C, Martınez-Bejar R, Urbina A.Optimized photovoltaic generator-water electrolysercoupling through a controlled DC-DC converter. Int JHydrogen Energy 2008;33:5352e62.

[19] Millet P, Ngameni R, Grigoriev SA, Mbemba N, Brisset F,Ranjbari A, et al. PEM water electrolyzers: fromelectrocatalysis to stack development. Int J Hydrogen Energy2010;35:5043e52.

[20] Ma LR, Sui S, Zhai YC. Preparation and characterization ofIr/TiC catalyst for oxygen evolution. J Power Sources 2008;177:470e7.

[21] Rasten E, Hagen G, Tunold R. Electrocatalysis in waterelectrolysis with solid polymer electrolyte. Electrochim Acta2003;48:3945e52.

[22] Siracusano S, Baglio V, Di Blasi A, Briguglio N, Stassi A,Ornelas R, et al. Electrochemical characterization of singlecell and short stack PEM electrolyzers based on a nanosizedIrO2 anode electrocatalyst. Int J Hydrogen Energy 2010;35:3558e68.

[23] Sedlak J, Lawrence R, Enos J. Advances in oxygen evolutioncatalysis in solid polymer electrolyte water electrolysis. IntJ Hydrogen Energy 1981;6:159e65.

[24] Ardizzone S, Carugeti A. Properties of thermally preparediridium dioxide electrodes. Electroanal Chem 1981;126(1e3):287e92.

[25] Takenaka H, Torikai E, Kawami Y, Wakabayashi N. Solidpolymer electrolyte water electrolysis. Int J Hydrogen Energy1982;7:397e403.

[26] Millet P, Durand R, Pineri M. Preparation of new solidpolymer electrolyte composites for water electrolysis. IntJ Hydrogen Energy 1990;15:245e53.

[27] Yamaguchi M, Ohisawa K, Nakanori T. Development of highperformance solid polymer electrolyte water electrolyzer inWE-NET. IEEE; 1997:1958e65 (3e4).

[28] Ma LR, Sui S, Zhai YC. Investigations on high performanceproton exchange membrane water electrolyzer. Int JHydrogen Energy 2009;34:678e84.

[29] Andolftto F, Durand R, Michas A, Millet P, Stevens P. Solidpolymer electrolyte water electrolysis: electrocatalysisand long-term stability. Int J Hydrogen Energy 1994;19(5):421e7.

[30] Slavcheva E, Radev I, Bliznakov S, Topalov G, Andreev P,Budevski E. Sputtered iridium oxide films as electrocatalystsfor water splitting via PEM electrolysis. Electrochim Acta2007;52:3889e94.

[31] Ma HC, Liu CP, Liao JH, Su Y, Xue XZ, Xing W. Study ofruthenium oxide catalyst for electrocatalytic performance inoxygen evolution. J Mol Cat A: Chem 2006;247:7e13.

[32] Kotz R, Stucki S. Stabilization of RuO2 by IrO2 for anodicoxygen evolution in acid media. Electrochim Acta 1986;31:1311e6.

[33] Wen TC, Hu CC. Hydrogen and oxygen evolutions on Ru-Irbinary oxides. J Electrochem Soc 1992;139(8):367Ce468C.

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 ) 2 9 8 5e2 9 9 22992

[34] Mattos-Costa FI, de Lima-Neto P, Machado SAS, Avaca LA.Characterisation of surfaces modified by sol-gel derivedRuxIr1-xO2 coatings for oxygen evolution in acid medium.Electrochim Acta 1998;44:1515e23.

[35] Cheng JB, Zhang HM, Chen GB, Zhang YN. Study of IrxRu1-xO2

oxides as anodic electrocatalysts for solid polymer electrolytewater electrolysis. Electrochim Acta 2009;54(26):6250e6.

[36] Di Blasi A, D’Urso C, Baglio V, Antonucci V, Arico’ AS,Ornelas R, et al. Preparation and evaluation of RuO2-IrO2,IrO2-Pt and IrO2-Ta2O5 catalysts for the oxygen evolutionreaction in an SPE electrolyzer. J Appl Electrochem 2009;39:191e6.

[37] Ioroi T, Kitazawa N, Yasuda K, Yamamoto Y, Takenaka H.IrO2 deposited Pt electrocatalysts for unitized regenerativepolymer electrolyte fuel cells. J Appl Electrochem 2001;31:1179e83.

[38] Ye F, Li JL, Wang XD, Wang TT, Li SM, Wei HJ, et al.Electrocatalytic properties of Ti/Pt-IrO2 anode for oxygenevolution in PEM water electrolysis. Int J Hydrogen Energy;in press.

[39] Zhang YJ, Wang C, Wan NF, Mao ZQ. Deposited RuO2eIrO2/Ptelectrocatalyst for the regenerative fuel cell. Int J HydrogenEnergy 2007;32:400e4.

[40] Marshall A, Børresen B, Hagenm G, Tsypkin M, Tunold R.Preparation and characterisation of nanocrystallineIrxSn1�xO2 electrocatalytic powders. Mat Chem Phy 2005;94(2e3):226e32.

[41] Hu JM, Zhang JQ, Cao CN. Oxygen evolution reaction on IrO2-based DSA� type electrodes: kinetics analysis of Tafel linesand EIS. Int J Hydrogen Energy 2004;29:791e7.

[42] De Pauli CP, Trasatti S. Electrochemical surfacecharacterization of IrO2þSnO2 mixed oxide electrocatalysts.J Electroanal Chem 1995;396:161e8.

[43] Marshall A, Børresen B, Hagenm G, Tsypkin M, Tunold R.Electrochemical characterisation of IrxSn1�xO2 powders asoxygen evolution electrocatalysts. Electrochim Acta 2006;51(15):3161e7.

[44] Marshall AT, Sunde S, Tsypkin M, Tunold R. Performance ofa PEM water electrolysis cell using IrxRuyTazO2

electrocatalysts for the oxygen evolution electrode. Int JHydrogen Energy 2007;32:2320e4.

[45] Ardizzone S, Bianchi CL, Cappelletti G, Ionita M,Minguzzi A, Rondinini S, et al. Composite ternary SnO2-IrO2-Ta2O5 oxide electrocatalysts. J Electroanal Chem 2006;589:160e6.

[46] Santana MHP, De faria LA, Boodts JFC. Effect of preparationprocedure of IrO2-Nb2O5 anodes on surface andelectrocatalytic properties. J Appl Electrochem 2005;35:915e24.

[47] Trasatti S. Physical electrochemistry of ceramic oxides.Electrochim Acta 1991;36:225e41.

[48] Adams R, Shriner RL. Platinum oxide as a catalyst in thereduction of organic compounds. iii. Preparation andproperties of the oxide of platinum obtained by the fusion ofchloroplatinic acid with sodium nitrate. J Am Chem Soc 1923;45:2171e9.

[49] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA,Rouquerol J, et al. Reporting physisorption date for gas/solidsystems with special reference to the determination ofsurface area and porosity. Pure Appl Chem 1985;57:603e19.