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 6 ( 2 0 1 1 ) 1 2 6 9 8e1 2 7 0 5

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New ternary mixed oxides of Fe, Ni and Mo for enhancedoxygen evolution

M. Kumar a, R. Awasthi a, A.K. Pramanick b, R.N. Singh a,*aDepartment of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi 221005, IndiabNational Metallurgical Laboratory, Jamshedpur 831007, India

a r t i c l e i n f o

Article history:

Received 19 April 2011

Received in revised form

1 July 2011

Accepted 10 July 2011

Available online 9 August 2011

Keywords:

Ternary mixed oxides

Oxygen evolution reaction

Electrocatalytic properties

Specific activity

* Corresponding author. Tel.: þ91 542 670159E-mail address: rnsbhu@rediffmail.com (

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.07.029

a b s t r a c t

Ternary mixed oxides of Fe, Ni and Mo with molecular formulas FexNi1�xMoO4 (x ¼ 0.25,

0.50 and 0.75) have been prepared by a co-precipitation method and investigated for their

structural and electrocatalytic properties by XRD, AFM, electrochemical impedance spec-

troscopy and anodic Tafel polarization. Results indicate that the apparent oxygen evolution

activity of the base (NiMoO4) electrode significantly increases with introduction of Fe from

0.25 to 0.75 mol. The Tafel slope for the oxygen evolution reaction at low overpotentials is

found to be only w35 mV on Fe-substituted oxides, while it was w75 mV on the base oxide.

The reaction follows the first order kinetics with respect to OH� concentration, regardless

of Fe content in the oxide.

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

reserved.

1. Introduction binary and ternarymetal ferrites, MxFe3�xO4 (M¼Co, Ni orMn;

The splitting of water either electrochemically or photo-

electrochemically has the potential to provide a sustainable

source of hydrogen for powering fuel cells, reducing CO2 to

fuels (e.g., CH4, CH3OH), and removing oxygen from biomass

[1e3]. One of the main drawbacks to make these processes

viable on an industrial scale is the high overpotential of

available electrocatalysts for the oxygen evolution reaction

(OER). For this reason, concerted efforts have been made to

obtain catalysts with low overpotentials [4e11]. The oxides of

Ru and Ir are considered as the best OER catalysts for use in

acid and base [12e15] respectively, but these metals are

amongst the rarest elements on the earth and, hence, are not

economical for large-scale applications. Cobalt, on the other

hand, is earth-abundant and both Co3O4 and its substituted

products, MxCo3�xO4 (M ¼ Ni, Fe or Cu), exhibit good OER

activity and have been studied extensively [4,6,7,16e18]. Also,

6; fax: þ91 542 2368127.R.N. Singh).2011, Hydrogen Energy P

0 � x � 1) [19e24] and MFe2�xMx0O4 (M ¼ Cu, Co, Ni or Mn;

0 � x � 1 and M0 ¼ Cr) [25e28] have been reported to display

good OER activity.

Recently, Singh and coworkers [29e32] reported a new type

of transition metal mixed oxides with general formula,

MMoO4 (where, M ¼ Co, Ni or Fe) for the OER in alkaline

solutions. The oxides were obtained by thermal decomposi-

tion of mixed metal nitrates at 1123 K [29] and also by a co-

precipitation method under controlled pH [30e32]. The

oxides, obtained by both the methods, followed the mono-

clinic crystal geometry. The oxygen evolution activities of

these newmixed oxide catalysts were found to be comparable

to those of active Co-based spinel type oxides [33,34]. It seems,

therefore, desirable to improve the electrocatalytic perfor-

mance of these new oxides further. Very recently [35], we have

introduced Fe for Co in the CoMoO4 matrix partially and

investigated the structural and electrocatalytic properties of

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

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 6 ( 2 0 1 1 ) 1 2 6 9 8e1 2 7 0 5 12699

new ternary products toward oxygen evolution in alkaline

solutions. It was noted that 0.25 mol Fe substitution enhanced

the apparent electrocatalytic activity of the oxide more than

10 times in 1MKOH. This result encouraged us to substitute Fe

for Ni in NiMoO4 also and carry out the similar study. Results

showed that 0.75 mol Fe substitution improved the apparent

(in ‘mA mg�1’) and specific (in ‘A m�2’) electrocatalytic activ-

ities of the oxide w22 and 90e120 times, respectively. Details

of results of the investigation are described in this paper.

Fig. 1 e XRD powder patterns of FexNi1LxMoO4 (x [ 0.0,

0.25, 0.50 and 0.75), sintered at 648 K for 10 h.

2. Experimental

2.1. Mixed oxide preparation

Mixed oxides of Fe, Ni and Mo with molecular formulas Fex-Ni1�xMoO4 (x ¼ 0, 0.25, 0.50, 0.75) were prepared by a copreci-

pitation method as described elsewhere [32]. In brief, for the

preparation of a particular ternary oxide, the stoichiometric

amount of (NH4)6Mo7O24$4H2O (Merck) was first dissolved in

100 ml of redistilled water and adjusted pH of the solution

approximately 2 by using concentrated HNO3. This solution

was then kept at 353 K and to this added the mixed metals

nitrate solution (prepared by dissolving stoichiometric

amounts of Fe(NO3)3$9H2O (Merck) and Ni(NO3)2$6H2O (Merck)

in redistilled water) slowly with vigorous stirring. After the

completion of the mixed metals nitrate solution addition, the

whole content of the solution was kept at 373 K for 1.5 h under

stirred condition. The solution was then filtered and the

precipitate, so obtained, was repeatedly washed with hot

distilled water, dried overnight at 393 K and finally calcined at

648 K for 10 h. Similarly, the base oxide (NiMoO4) was

prepared.

2.2. Characterization of oxides

The structural characterization wasmade by the study of XRD

(X-ray diffraction), BET surface area and AFM morphology of

the oxide catalysts. XRD powder pattern of the catalyst was

recorded on an X-ray diffractometer (Rigaku DMAX III) using

Cu-Ka as radiation source (l ¼ 1.542 �A). The BET surface area

was determined by a surface area analyzer (Micrometrics,

USA, ASAP 2020 Model). The AFM study was made for three

catalysts, Fe0.25Ni0.75MoO4, Fe0.5Ni0.5MoO4 and Fe0.75Ni0.25-MoO4. Prior to the study, themoisture content of the sample, if

any, was removed by keeping it under incandescent lamp for

2 h. The dried powders of the catalysts were then dispersed on

freshly cleaved mica surface. The excess powders were

removed by hand blower and again kept under lamp for 1 h.

The samples were investigated in non-contact force mode of

atomic force microscope (Model SPA 400, SEICO, Japan).

2.3. Electrode preparation

The catalyst electrodes were prepared by coating a homoge-

neous slurry of the oxide on pretreated Ni supports. Prior to

use, Ni-plates (1.5 � 1 cm) were etched for 5 min in hot aqua

regia, washed thoroughly with double distilled water,

degreased in acetone, cleaned ultrasonically in double

distilled water and then dried in air. A homogeneous slurry

was made by grinding 80 mg of oxide powders with 2e3 drops

of Triton (X-100) in agate pastel Mortar. The oxide coated Ni

supports were dried in air and then heated in an electrical

furnace at 673 K for 1.5 h. Electrical contacts with the oxide

films were made as described previously [36]. The oxide

loading was w2 mg cm�2. Electrical connection to the oxide

film was obtained by using copper wire, silver adhesive paint

and araldite Epoxy.

2.4. Electrochemical studies

Electrochemical investigations were carried out in a three-

electrode single compartment Pyrex glass cell. The reference

and counter electrodes were respectively an Hg/HgO/1 M KOH

(E� ¼ 0.098 V vs. SHE) and pure Pt-foil (w8 cm2). All potential

values mentioned in the text are given against this reference

only. The electrochemical impedance spectroscopy (EIS) study

of the oxide film electrodes in 1M KOHhas been carried out by

an electrochemical impedance system (EG&G, PAR Model

273A) employing an ac voltage amplitude of 10 mV. The

frequency range used in the study was 0.02e20 � 103 Hz &

softwares employed were ‘Power Sine’ & ‘ZsimpWin’ version

3.00.‘M 352 Corrosion Analysis’ software was used to perform

the anodic Tafel polarisation study. The electrocatalytic

activities of the oxide electrodes given in the text are average

ones and have been obtained by the study of triplicate elec-

trodes of each catalyst.

3. Results and discussion

3.1. XRD

The XRD powder patterns for FexNi1�xMoO4 with x ¼ 0,

x ¼ 0.25, x ¼ 0.50 and x ¼ 0.75 recorded between 2q ¼ 20� and2q ¼ 80� are shown in Fig. 1. It is observed that 2q and the

corresponding d values of all the diffraction lines for the base

oxide shown in Fig. 1 shows the best match with JCPDS ASTM

Fig. 2 e AFM image of (a) Fe0.25Ni0.75MoO4 (b) Fe0.5Ni0.5MoO4 and (c) Fe0.75Ni0.25MoO4.

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files 32-0692 for the compound concerned (i.e. NiMoO4). From

comparison of XRD data of catalysts (Fig. 1) it is noted that the

most intense diffraction peak (220) and the first, a relatively

less intense, diffraction peak (021) of the base oxide slightly

shift toward the lower angle with introduction of Fe, the

magnitude of shift in 2q being 1.7e1.6� and 0.9�, respectively,on contrary, the second (202) and fourth (022) prominent

diffraction peaks slightly displaced toward the higher angle

(2q) by 0.4� and 0.8�, respectively. This result indicates the

formation of single (Fe, Ni) MoO4 phase with introduction of

Fig. 3 e Nyquist plot of Fe0.75Ni0.25MoO4 at varying

potentials (E [ 0.55 V, 0.56 V, 0.57 V, 0.58 V, 0.59 V and

0.60 V).

Fig. 4 e Nyquist plots (experimental and simulated) for the

Fe0.25Ni0.75MoO4 electrode at E [ 0.55 V (a) and for the

Fe0.75Ni0.25MoO4 electrode at E [ 0.59 V (b) in 1 M KOH

(298 K).

Table 1 e Estimates of the equivalent circuit parameters for Fe0.25Ni0.75MoO4 electrode at 298 K.

E/V Rs/U cm2 R1/U cm2 102 Q1/S cm�2 sn n1 R2/U cm2 103 Q2/S cm�2 sn n2

0.50 2.2 4.3 � 108 41.19 0.30 2987 5.32 0.94

0.52 2.2 8.2 � 1016 35.37 0.30 937.6 6.11 0.93

0.53 2.1 2.0 � 108 59.0 0 .20 478.6 6.50 0.92

0.54 2.2 0.8 8.20 0 .40 228.8 7.05 0.92

0.55 2.2 0.9 8.40 0.40 112.3 7.50 0.92

0.57 2.2 0.8 4.92 0.47 29.2 8.0 0.90

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Fe (0.25e0.75 mol) in NiMoO4. Estimates of the crystallite size,

based on the Scherrer formula [17], were found to be 11.8, 18.5,

18.0, and 18.4 nm respectively for the oxide with x ¼ 0, 0.25,

0.50, and 0.75. Values of the crystallite size suggest that

addition of Fe somewhat increases the crystallite size, the

magnitude of increase, however, being nearly same, regard-

less of the Fe content in the oxide. The most intense diffrac-

tion peak observed in the diffractogram of a particular oxide

was employed for determination of the crystallite size of that

oxide.

3.2. AFM

The AFM images for all the three catalysts, Fe0.25Ni0.75MoO4,

Fe0.5Ni0.5MoO4 and Fe0.75Ni0.25MoO4 are taken for 20 mm� 20 mm

and 5 mm� 5 mmareas. But, only 5 mm� 5 mm images are being

reported in the present study. All the samples are triangular

shaped particles. Most of the particles are appeared as scat-

tered individually on the sample surface. But, few triangular

shaped clusters are also noticed in the AFM images.While AFM

imaging, all the particles and clusters had shown a tendency to

align in a particular direction. These features are predominant

for all the compositions. The characteristic feature for each

particle is noticed that particles have a sharp boundary at their

base. In the middle of the particles they create plateau regions

and toward the apex of the triangles the plateau region is

slowly converging toward the base of sample surface. These

features are clearly visible in the three dimensional images in

Fig. 2(aec) and hence one can infer that the particle shapes are

triangular plates. However, in Fig. 2c, for composition

Fe0.75Ni0.25MoO4, one can observe minutely that particle

surfaces are not totally flat; rather it is protruding in the z-

direction, giving a sense of tetrahedral shape of the particle. It

is considered that the triangular structure of the particles ari-

ses, particularly in Fe0.75Ni0.25MoO4 (Fig. 2c), due to tetrahedral

structure of (MoO4)4� present in the selected compositions of

this study [37].

Table 2 e Estimates of the equivalent circuit parameters for Fe

E/V 107L/H Rs/U cm2 R1/U cm2 102Q1/S cm�

0.52 e 2.4 0.2 42.31

0.53 e 2.5 1.0 33.35

0.54 e 2.4 0.9 21.80

0.55 e 1.9 1.0 22.91

0.56 e 2.0 0.9 14.11

0.57 e 1.9 0.5 5.32

0.58 8.3 1.9 0.6 8.04

0.59 9.1 1.9 0.5 6.48

The particles for compositions Fe0.25Ni0.75MoO4, Fe0.5Ni0.5-MoO4 and Fe0.75Ni0.25MoO4 are found to vary in the range of

(223.50e409.76) nm, (223.06e396.23) nm and (227.75e380.04)

nm with corresponding particles height in the range of

(397.16e430.07) nm, (290.12e459.70) nm and (227.75e380.04)

nm. The particle size for Fig. 2(aec), was analyzed on the basis

of ten data sets where average width (a) and height (c) of the

particles for compositions Fe0.25Ni0.75MoO4, Fe0.5Ni0.5MoO4 and

Fe0.75Ni0.25MoO4 in pairwise are found as (307.60e399.78) nm,

(315.41e383.60) nm and (292.30e319.99) nm. In the same

sequence of compositions the average (c/a) ratios are found as

1.32, 1.23 and 1.11 where range of variation in (c/a) ratios are

observed as (1.04e1.77), (1.30e1.16) and (0.81e1.18). At this

point it can be remembered that the least (c/a) value for ideally

close packed tetrahedral structure with similar atoms is 0.806

and the <MoeO> distance for the MoO4 tetrahedron is 1.76 �A

in uranyl molybdates containing MoO4 tetrahedra [37].

Therefore, a similarity of (c/a) ratios in the present experi-

mental results c/a ¼ 0.8 and c/a ¼ 1.77 may be visualized with

the least tetrahedral distance and distance between hMoeOibonds.

The average roughness data for particles for compositions

Fe0.25Ni0.75MoO4, Fe0.5Ni0.5MoO4 and Fe0.75Ni0.25MoO4 are found

to be 6.080, 5.805 and 3.372 nm, where range of variations are

noticed as (4.27e10.84), (2.92e9.75) and (0.002e6.224) nm.

3.3. BET surface area

Estimates of the BET surface area were 12.8, 1.6, 2.8 and

2.5 m2 g�1 for the oxide with x ¼ 0, x ¼ 0.25, x ¼ 0.50 and

x ¼ 0.75, respectively. Thus, the BET surface area is consider-

ably reduced in the presence of Fe in the oxide, the magnitude

of the reduction, however, being practically the same in case

of each ternary oxide. Values of the crystallite size determined

from the XRD data were also approximately same

(18.3 � 0.2 nm) for ternary oxides. The decrease in the specific

surface area and increase in the crystallite size may be caused

0.5Ni0.5MoO4 electrode at 298 K.2 sn n1 R2/U cm2 103 Q2/S cm�2 sn n2

0.43 328.4 7.0 0.94

0.41 165.4 9.50 0.96

0.43 79.6 10.70 0.97

0.40 37.1 11.40 0.98

0.43 18.8 11.70 0.98

0.55 9.0 12.60 0.97

0.46 5.5 12.60 0.97

0.50 3.7 12.80 0.96

Table 3 e Estimates of the equivalent circuit parameters for Fe0.75Ni0.25MoO4 electrode at 298 K.

E/V 106 L/H Rs/U cm2 R1/U cm2 102 Q1/S cm�2 sn n1 R2/U cm2 103 Q2/S cm�2 sn n2

0.55 1.1 1.3 0.4 7.05 0.58 22.6 9.17 0.94

0.56 1.0 1.4 0.4 4.66 0.53 14.7 9.40 0.93

0.57 1.0 1.4 0.3 10.5 0.69 9.3 9.60 0.91

0.58 1.1 1.4 0.5 6.62 0.46 6.0 10.0 0.93

0.59 1.3 1.2 0.5 8.00 0.43 3.9 9.90 0.94

0.60 1.3 1.2 0.6 8.20 0.44 2.8 10.10 0.94

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due to lowering of the oxide sintering temperature in presence

of Fe in the base oxide [35].

Fig. 5 e Anodic Tafel polarization curves for FexNi1LxMoO4

(x [ 0.0, 0.25, 0.50 and 0.75) in 1 M KOH at 298 K.

3.4. Electrochemical impedance spectroscopy (EIS)

The EIS study of FexNi1�xMoO4 electrodes has been carried out

at different constant dc potentials in 1M KOH at 298 K. Prior to

the start of EIS measurements, the oxide electrodes were first

equilibrated in 1 M KOH at each applied dc potential for 300 s.

The EIS spectra, so obtained, are shown in Fig. 3.

Features of EIS spectra for all the electrodes shown in Fig. 3

appear to be similar. EachNyquist curve seems to have a small

arc at high frequencies and a large semicircle at intermediate

and low frequencies. High frequency arc is found to be prac-

tically independent of the nature of the electrode material as

well as the applied potential, however, the latter strongly

influence the diameter of the semicircle produced at lower

frequencies. The diameter of the semicircle decreases with

the increase in the applied potential. This indicates that the

semicircle is produced due to the OER taking place at the

electrodeeelectrolyte interface and that the diameter repre-

sents the charge transfer resistance [23,38e40]. As the charge

transfer resistance is inversely related to the rate of the

reaction, a decrease in the charge transfer resistance means

an increase in the rate of the reaction.

The impedance data were simulated using the equivalent

circuit, Rs (R1Q1) (R2Q2). Symbols Rs, R1, and R2 are used to

represent the solution resistance, oxide film resistance and

the charge transfer resistance and Q1 and Q2 are the constant

phase elements (CPE) for the oxide bulk and the oxide/solution

interface, respectively. Using this circuit model, Nyquist

curves determined tally exactly to the corresponding experi-

mental curve (Fig. 4). Estimates of the circuit parameters are

listed in Tables 1e3.

As n2 values shown in Tables 1e3 are close to 1, Q2 can be

considered as the capacitance of the double layer (Cdl). The

observation of Tables 1e3 shows that the Cdl values of the

oxide/1 M KOH interface, regardless of potential, are nearly

constant in the oxygen evolution region. This shows that the

electrode surface does not change practically with the appli-

cation of the potential in the oxygen evolution region.

However, as the potential increases from E ¼ 0.500 V to

0.600 V, the R2 reduces due to the enhancement in the rate of

the OER. Based on values of R2, the oxide with x ¼ 0.25 is low

active for the OER compared to that of the oxide with x ¼ 0.50

or x ¼ 0.75. The BET surface area data also indicate a higher

activity for 0.50 and 0.75 mol Fe-substituted oxides. However,

this point would be clearer in the study of the anodic Tafel

polarization.

3.5. Tafel polarisation study

The oxygen evolution activities of oxide electrodes were

determined by recording the IRs (where Rs is the solution

resistance in ohm and I is the current in mA) compensated

Tafel polarization (E vs log j ) curves at a slow scan rate of

0.2 mV s�1 in 1 M KOH at 298 K and curves, so obtained, are

shown in Fig. 5. The IRs was automatically compensated at an

interval of 10 s using the current interrupt technique provided

in electrochemical impedance system. Each curve, as shown

in Fig. 5, displays two Tafel slopes, one at low and the other

one at high overpotentials (h). Values of these Tafel slopes are

given in Table 4. Nearly the same Tafel slope values for the

OER on FexNi1�xMoO4 indicate that the OER follows similar

mechanisms, regardless of Fe content in the oxide catalyst. To

compare the catalytic activities of electrocatalysts, the

apparent current densities for the OER at a constant potential

(E ¼ 0.650 V) were noted from Fig. 5 and are given in Table 4.

The reaction order in OH� concentration has also been

determined. For the purpose, E vs. log j curves at 0.2 mV s�1

was recorded at varying KOH concentrations, maintaining

the ionic strength of the medium (m ¼ 2.0) constant. KNO3

was used as an inert electrolyte. With the help of these

curves the linear log j vs log [OH�] plots were constructed at

a constant potential (Fig. 6). The order was then determined

by measuring the slope of these straight lines (Table 4). For

the construction of log j vs. log [OH�] plot, the first linear

Table 4 e Electrode kinetic parameters for O2 evolution on FexNi1LxMoO4 in 1 M KOH at 298 K.

Electrode Loading (mg cm�2) Tafel slope (mV) Order At E ¼ 0.650 V

b1 b2 jap/mA cm�2 jap0/mA mg�1 SA (A m�2)

NiMoO4 1.90 � 0.40 75 � 1 e 1.6 7.9 � 1 w4 w0.3

Fe0.25Ni0.75MoO4 2.11 � 0.02 34 � 1 78 � 5 1.0 123 � 2 w58 w36

Fe0.5Ni0.5MoO4 1.95 � 0.2 34 � 1 65 � 5 1.1 148 � 23 w76 w27

Fe0.75Ni0.25MoO4 1.69 � 0.07 33 � 1 57 � 4 1.2 154 � 21 w90 w36

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region of E vs. log j curves, determined at varying KOH

concentrations, were considered.

Results shown in Table 4 show that partial replacement of

Ni by Fe in the NiMoO4 matrix increases the apparent current

density ( jap/mA cm�2) and also the apparent current permg of

the oxide loading, jap0 (¼ jap,mA cm�2/oxide loading,mg cm�2).

The increase in jap0 is found to be 14e22 times with substitu-

tion of Fe from 0.25 to 0.75 mol. The increase in the apparent

catalytic activity can be ascribed to the increase in number of

unpaired d electrons in the oxides with Fe introduction. Esti-

mates of the specific catalytic activity (SA ¼ jap0, A g�1/BET

surface area, m2 g�1) of the Fe-substituted products shown in

Table 4 indicate that all the three Fe-substituted products

seem to have nearly the same values of the SA, however, these

values are greatly higher (more than 90 times) than one found

for the base oxide (i.e. NiMoO4) under similar experimental

condition. This shows that introduction of Fe modifies the

electronic properties of the material, which are highly favor-

able for electrocatalysis of oxygen evolution.

Values of the Tafel slope (b) found on Fe-substituted oxides

are in fair agreement with those recently reported for similar

oxides, namely MMoO4 (Co, Fe or Ni) (b1 z 40 mV) [29],

Fe2(MoO4)3 (b1 z 35 mV) [32] and for ternary ferrites, namely

CoFe2�xCrxO4 (b1z 40e51mV) [27], MnFe2�xCrxO4 (b1z 40mV)

[28] and NiFe2�xCrxO4 (b1 z 40 mV) [26] and for electro-

deposited CoeNi cobaltites (b1 z 40e48 mV) [40]. However,

there seems controversy regarding the reaction order with

Fig. 6 e Log j versus log [OHL] plot for O2 evolution of

FexNi1LxMoO4 (x [ 0.0, 0.25, 0.50 and 0.75) electrodes at

a constant potential (0.590 V).

respect to OH� ion. Both fractional and second order reactions

have been reported in literature [19e23,29,32,35,40,41].

The apparent electrocatalytic activity of new ternary

oxides, FexNi1�xMoO4 (at jap ¼ 100 mA cm�2; E ¼ 0.645 V for

x ¼ 0.25; E ¼ 0.640 V for x ¼ 0.50 and E ¼ 0.641 V for x ¼ 0.75)

electrodes is greatly higher than those of binary oxides,

NiMoO4 (at jap ¼ 100 mA cm�2, E ¼ 0.790 V) [30], CoMoO4 (at

jap ¼ 100 mA cm�2, E ¼ 0.731 V) [31] and Fe2(MoO4)3 (at

jap ¼ 100 mA cm�2, E ¼ 0.674e0.685 V) [32] electrodes, recently

reported in literature. Thus, there is a reduction of 34e150 mV

of the overpotential at jap ¼ 100 mA cm�2 with introduction of

0.25e0.75 mol Fe.

Also, the apparent electrocatalytic activities of the new

ternary electrodes are found to be somewhat better than those

of many active Co- and Fe-based spinel oxide [18,23] elec-

trodes recently reported in literature under similar electrol-

ysis conditions. For instance, Chi et al. [42] observed

jap ¼ 100mA cm�2 at E ¼ 0.620 V vs. SCE (z0.764 V vs. Hg/HgO)

at the NiCo2O4/Ni electrode, prepared through hydroxide

precipitation method. Hamdani et al. [41] found

jap¼ 16mA cm�2 at E¼ 0.80 V vs. SCE (z0.944 V vs. Hg/HgO) on

Li-doped Co3O4 on glass, obtained by spray pyrolysis. Svegl

et al. [43] found jap ¼ 100 mA cm�2 at Ez 0.640 V (z0.740 V vs.

Hg/HgO) and 0.715 V vs. Ag/AgCl (z0.815 V vs. Hg/HgO) for

solegel derived Co3O4 and Li-doped Co3O4 films on Pt,

respectively. Tavares et al. [44] prepared catalytic films of

NiCo2O4 on Ni by thermal decomposition of aqueous nitrate

solutions and found E ¼ 0.650 V vs. Hg/HgO corresponding to

jap z 68 mA cm�2 (5 M KOH). However, the electrocatalytic

activity of active ternary FeeNieMoeO oxides were lower

compared to those of NiFeCrO4 ( jap¼ 100mA cm�2, E¼ 0.586 V

vs. Hg/HgO) prepared by precipitation [26], La-doped Co3O4

( jap¼ 100mA cm�2, E¼ 0.527e0.539 V vs. Hg/HgO) obtained by

microwave assisted thermal decomposition [33], ZnCo2O4

( jap ¼ 100 mA cm�2, h ¼ 0.256e0.203 V) obtained by electro-

phoretic deposition [45], and electrodeposited Co þ Ni mixed

oxide catalyst ( jap ¼ 100 mA cm�2, E ¼ 0.60 V vs. Hg/HgO) [40].

4. Summary

The study has shown that partial replacement of Ni by Fe in

the NiMoO4matrix decreases the Tafel slope and increases the

electrocatalytic activity of the catalyst toward the OER. It is

observed that with 0.25e0.75 mol Fe substitution, the Tafel

slope for the OER on the base oxide gets reduced to nearly one

half and the specific activity gets increased by more than 80

times. Thus, the presence of Fe in the NiMoO4 matrix has

a very strong influence on the electrocatalytic activity as well

as on the reaction mechanism.

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 6 ( 2 0 1 1 ) 1 2 6 9 8e1 2 7 0 512704

Acknowledgment

One of the authors (MK) thanks Banaras Hindu University,

Varanasi-221005 (India) for providing UGC Fellowship to carry

out the investigation.

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