Novel FexCr2−x(MoO4)3 electrocatalysts for oxygen evolution reaction

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Novel FexCr2Lx(MoO4)3 electrocatalysts for oxygen evolution reaction R.N. Singh a,*, M. Kumar a, A.S.K. Sinha b a b

Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi 221005, U.P., India Department of Chemical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, U.P., India

article info

abstract

Article history:

Transition metal mixed oxides of Fe, Cr and Mo with nominal compositional formula,

Received 23 May 2012

FexCr2
x(MoO4)3 (x ¼ 0, 0.25, 0.50 and 0.75) have been obtained by a co-precipitation method

Received in revised form

and investigated for their structural and electrocatalytic properties by XRD, TEM, XPS, BET,

23 July 2012

electrochemical impedance spectroscopy and anodic Tafel polarization. Results show that

Accepted 25 July 2012

introduction of Fe for Cr from 0.25 to 0.75 mol into the Cr2(MoO4)3 matrix improved the

Available online 14 August 2012

electrocatalytic activity toward the O2 evolution reaction (OER) in 1 M KOH considerably; the magnitude of improvement being maximum with 0.5 mol Fe. Values of the Tafel slope

Keywords:

were close to 35 mV at low and 2.303RT/F at high overpotentials on Fe-substituted oxides.

Ternary mixed oxides

The OER follows nearly second order kinetics in OH
concentration at low overpotentials.

Oxygen evolution reaction

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Mixed metal molybdates Impedance spectroscopy

1.

Introduction

The electrochemical or photochemical water splitting has the potential to provide sustainable source of hydrogen for powering fuel cells. However, to make these processes viable on the industrial scale it is desired to discover a catalyst which can evolve oxygen at a reasonably reduced potential. For the purpose, numerous electrocatalysts were investigated and comprehensively reviewed [1e5]. Among these catalysts, transition metal mixed oxides with spinel (Co-based [5e23] and Fe-based [24e30]) as well as perovskite (LaNiO3 and LaCoO3) [31e37] structures have been considered most promising oxygen anodes in alkaline media. Very recently, Singh and co-workers [38e41] have synthesized and investigated a new type of transition metal mixed oxides with general formula, MMoO4 (where, M ¼ Co, Fe or Ni). To improve the electrocatalytic activities further, Fe was introduced for Co into CoMoO4 and for Ni into NiMoO4 partially. At E ¼ 0.650 V in 1 M KOH at 25  C, 0.25 Fe substitution enhanced

the apparent oxygen evolution current density of the base oxide (CoMoO4) by w11 times [42] and of NiMoO4 by w15 times [43]. It is noteworthy that the electrocatalytic performance of these oxides toward the oxygen evolution reaction (OER) was comparable and even higher than those of some active Co-based spinel type oxides, recently reported in literature [6,8,22,44]. We have now prepared Cr2(MoO4)3 and its Fe-substituted products and investigated their electrocatalytic properties toward the OER in KOH solutions. Details of results are described in the paper.

2.

Experimental

2.1.

Mixed oxide preparation

Transition metal mixed oxides of Fe, Cr and Mo with molecular formula, FexCr2
x(MoO4)3 (x ¼ 0.25, 0.50, 0.75) were prepared by a co-precipitation method as previously described [42,43]. The

* Corresponding author. E-mail address: [email protected] (R.N. Singh). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.114

15118

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

precursors used were Fe(NO3)3$9H2O (Merck), Cr(NO3)3$9H2O (Fluka) and (NH4)6Mo7O24$4H2O (Merck). For preparation of the catalyst, the required amount of (NH4)6Mo7O24$4H2O was dissolved in 100 ml distilled water, adjusted pH z 2 using concentrated HNO3 and added to this mixed metals nitrate solution slowly with vigorous stirring, and finally kept at 353 K under stirred condition for 1.5 h. 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 823 K for 5.5 h. Similarly, the base oxide Cr2(MoO4)3 was prepared.

2.2.

Characterization of oxides

The mixed oxide catalysts were characterized structurally by XRD (X-ray diffraction), BET surface area, TEM and XPS. XRD powder pattern of the catalyst was recorded on an X-ray diffractometer (Rigaku DMAX III) using Cu-Ka as the radiation source (l ¼ 1.542  A). The BET surface area was determined by a surface area analyzer (Micrometrics, USA, ASAP 2020 Model). For TEM analyses the oxide catalysts were dispersed in methanol, a drop of this suspension was placed onto a carbon coated copper grid and dried. The average particle size of metal catalysts has been determined using ‘SIS Viewer/ Measure Tool’ software available in TEM (TECNAI G2 FEI) facility.

2.3.

Electrochemical studies

A conventional three-electrode single compartment Pyrex glass cell was used to carry out electrochemical investigations. 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 working electrode was prepared by coating homogeneous slurry of the oxide onto the pretreated Ni support. The preparation of the oxide slurry and pre-treatment of the support were carried out as described previously [34]. The electrochemical impedance spectroscopy (EIS) study of the oxide film/1 M KOH interface has been carried out by an electrochemical impedance system (EG&G, PAR Model 273A) with an AC voltage amplitude of 10 mV. The frequency range used in the study was 0.02e20  103 Hz. Softwares employed were ‘Power Sine’ and ‘ZsimpWin’ version 3.00.‘M 352 Corrosion Analysis’ software was used to perform the anodic Tafel polarization study. The electrocatalytic activities of the oxide electrodes given in the text are average ones and have been obtained by the study of triplicate electrodes of each catalyst.

3.

Results and discussion

3.1.

XRD

Fig. 1 gathers the XRD powder patterns of FexCr2
x(MoO4)3 with x ¼ 0, x ¼ 0.25, x ¼ 0.50 and x ¼ 0.75 between 2q ¼ 20 and 2q ¼ 60 . It is observed that 2q and the corresponding d values of all the diffraction lines for the base oxide with x ¼ 0 (i.e. Cr2(MoO4)3) shown in Fig. 1 show the best match with JCPDS

Fig. 1 e XRD powder patterns of FexCr2Lx(MoO4)3 (x [ 0.0, 0.25, 0.50 and 0.75), sintered at 823 K for 5.5 h.

ASTM file 20-0310 which thereby substantiate that the compound, Cr2(MoO4)3, follows the orthorhombic crystal structure. Fig. 1, further, shows that Fe substitution from 0.25 to 0.75 mol does not seem to change the structure of the base oxide practically. Only, the diffraction peaks of the base oxide slightly shift toward either the higher or lower angle with introduction of Fe, the magnitude of the shift in 2q being  ranged between 0.1 and 0.4 . Further, no diffraction peaks other than the compound concerned were found in Fig. 1. Thus, results indicate the formation of a single phase compound, (Fe, Cr)2(MoO4)3 with addition of Fe in the base oxide.

3.2.

TEM

TEM pictures of ternary oxides shown in Fig. 2 demonstrate the granular morphology. The catalyst particles seem to have a tendency to exist in form of aggregates/clusters on the surface. The mean particle size was determined by ‘SIS Viewer’ software. For the purpose, 60e65 single particles of the TEM image of a particular catalyst were selected. Estimates of the mean particles size were w27, w23 and w35 nm in the oxide with x ¼ 0.25, 0.5 and 0.75, respectively.

3.3.

XPS

The electronic states of the Fe, Cr and Mo lying on the surface of the catalysts were studied and results are listed in Table 1. The Fe 2p spectra of oxide with x ¼ 0.25, x ¼ 0.5 and x ¼ 0.75 exhibited two photoelectron peaks corresponding to the Fe 2p3/2 and Fe 2p1/2 electrons at B.E. (binding energy) ¼ 709.9e711.7 and B.E. ¼ 724.8e725.9 eV, respectively. This shows that Fe in the mixed oxide system is present in þ2 and þ3 oxidation states. Similar values of B.E. for Fe core peaks 2p3/2 and 2p1/2 from mixed transition metal oxides (CoFe2O4, CoFeCrO4, NiFe1.5Cr0.5O4) were also reported by Allen et al. [45]. The Cr 2p spectra of catalysts also indicated two core peaks, 2p3/2 at B.E. ¼ 577.3e579.0 and

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

15119

Fig. 2 e TEM images of: Fe0.25Cr1.75(MoO4)3 at 200 nm (a) & 500 nm (b); Fe0.50Cr1.50(MoO4)3 at 200 nm (c) and Fe0.75Cr1.25(MoO4)3 at 200 nm (d).

2p1/2 at B.E. ¼ 587.3e588.3 eV. Almost similar binding energies of 577e579 and 586e588 eV are reported for standard Cr(III) compounds, CrCl3$6H2O and Cr(III)-acetate [46], respectively. This shows that that Cr in the mixed oxides is essentially in Cr(III) state. The Mo 3d spectra of the catalysts displayed a doublet at B.E. ¼ 232.8e233.5 and 235.7e236.6 eV for Mo 3d5/2 and Mo 3d3/2 core peaks, respectively. Thus, results show that Mo is present in a single oxidation state Moþ6 [47]. Typical XPS spectra of Fe 2p, Cr 2p and Mo 3d from Fe0.25Cr1.75(MoO4)3 and Fe0.75Cr1.25(MoO4)3 are shown in Fig. 3.

3.4.

BET surface area

BET surface area of the oxide, Cr2(MoO4)3, is considerably reduced in the presence of Fe (0.25e0.75 mol) (Table 4). Similar results were also obtained on introduction of Fe for Ni in NiMoO4 [43]. The decrease in the specific surface area of the oxide in the presence of Fe is not very clear, however, it may be caused due to lowering of the oxide sintering temperature with introduction of Fe [43].

Table 1 e Metal core photoelectron peak binding energies from FexCr2Lx(MoO4)3. x

Photoelectron peak binding energies (eV) Cr2p3/2 Cr2p1/2 Fe2p3/2 Fe2p1/2 Mo3d5/2 Mo3d3/2 O1s

0.25 0.50 0.75

577.3 578.0 579.0

587.3 588.3 587.7

709.9 710.6 711.7

724.8 725.4 725.9

232.8 233.5 233.3

235.7 236.6 236.5

531.1 531.6 531.8

3.5.

Electrochemical impedance spectroscopy (EIS)

The EIS spectra of only two electrodes (i.e. 0.25 and 0.75 mol Fe-substituted oxides) in 1 M KOH at 298 K were recorded at five different constant dc potentials between 0.50 and 0.60 V, chosen at the beginning of the OER region. Features of EIS spectra for both the electrodes were quite similar. A set of five representative Nyquist curves for the oxide with x ¼ 0.25 at E ¼ 0.565, 0.570, 0.580, 0.590 and 0.600 V are shown in Fig. 4. Each Nyquist curve shown in Fig. 4 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 practically independent of the nature of the electrode material as well as the applied potential. The small arc at high frequency shows the impedances related to the dielectric properties and the resistivity of the catalytic film, whereas the large semicircle formed at low to intermediate frequencies accounts for the impedances caused by the OER [48]. The observed decrease of the diameter of the semicircle with increase of the applied potential (Fig. 4) clearly indicates that the semicircle is produced due to the OER taking place at the electrodeeelectrolyte interface [17,30,49]. The diameter of the semicircle, in fact, represents the charge transfer resistance (Rct) which is inversely related to the rate of the electrode reaction and so, a decrease in the Rct means an increase in the rate of the OER. It is known [2] that when a metal oxide film electrode, obtained at low temperatures (w400  C), is exposed with an electrolyte, wetting of the oxide matrix takes place instantaneously. The oxide surface, thus modified, can be treated as a planar surface [39,50]. Therefore, a planar electrical

15120

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

Fig. 4 e Nyquist plot of Fe0.25Cr1.75(MoO4)3 at varying potentials; E [ 0.565 V, 0.570 V, 0.580 V, 0.590 V and 0.600 V; [KOH] [ 1 M; 298 K.

equivalent circuit with the circuit description code, LRs (RfQf) (RctQdl), has been used to simulate the experimental impedance data. Similar equivalent circuits have repeatedly been used in literature [51e53]. In this circuit the parallel (RfQf) takes into account the properties of the oxide film, while the parallel (RctQdl) combination is associated with the OER. Symbols L, Rs, Rf, and Rct represent inductance, solution resistance, oxide film resistance and charge transfer resistance and Qf and Qdl represent the constant phase elements (CPE) for the oxide film and the oxide film/1 M KOH interface (i.e. double layer ¼ dl), respectively. Constant phase elements represented by Q, instead of capacitances were used to fit the experimental data due to high degree of roughness and inhomogeneities of the electrode. The use of a CPE replacing the double layer is considered as a good approach for the study of practical electrodes with different degrees of surface roughness, physical nonuniformity or a nonuniform distribution of surface reaction sites [51]. The CPE can be defined as: Q ¼ 1/ZCPE ( ju)n, where j ¼ (
1)1/2 and n represents deviation from ideal behavior, being n ¼ 1 for perfect capacitors. Further, potentiostat introduces some inductance at high frequencies (>40 kHz) and so, the component L has been used to obtain the best fit of the experimental data [52]. It is mentioned that Nyquist curves derived from the proposed circuit model tally exactly to the corresponding experimental curve (Fig. 5). Results of impedance analyses are listed in Tables 2 and 3. Values of Cdl given in Tables 2 and 3 were estimated using the relation [39,40], h i1
n Q ¼ ðCdl Þn ðRs Þ
1 þðRct Þ
1

Fig. 3 e Typical XPS Fe 2p (a) Cr 2p (b) and Mo 3d (c) photoelectron spectra from FexCr2Lx(MoO4)3 (x [ 0.25, and 0.75).

The Cdl values shown in Tables 2 and 3 seem to be independent of the applied potential. This substantiates that the nature of the electrode surface does not change practically in the OER region, however, when the potential increases from the onset potential to 0.60 V in the OER region, the Rct decreases due to the enhanced rate of the OER. Based on the Rct values, the oxide with x ¼ 0.25 is more active for the OER compared to that of the oxide with x ¼ 0.75. However, this point would be clearer in the study of the anodic Tafel polarization. Smaller n1 values than 0.5 are not well understood. However, this may be caused due to contribution of interior gains in the oxide material.

15121

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

Fig. 6 e Anodic Tafel polarization curves for FexCr2Lx(MoO4)3 (0 £ x £ 0.75) in 1 M KOH at 298 K.

Fig. 5 e Nyquist plots (experimental and simulated) for the Fe0.25Cr1.75(MoO4)3 electrode at E [ 0.565 V (a) and E [ 0.59 V (b) in 1 M KOH and at 298 K.

3.6.

Electrode kinetic study

The kinetics of the OER and activity of oxide electrodes were determined by recording the IRs-compensated Tafel polarization (E vs log j ) curves at a slow scan rate (0.2 mV s
1) in 1 M KOH at 298 K and curves, so obtained, are shown in Fig. 6. 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. 6, displays two Tafel slopes, one at low (b1) and the other one at high

overpotentials (b2). Values of the Tafel slopes are given in Table 4. This table also contains the current density data for the OER on different catalysts of the present study at a constant potential (E ¼ 0.650 V), which were noted from Fig. 6. Nearly the same Tafel slope values found for the OER on all the three ternary electrodes indicate that the OER follows similar mechanisms, regardless of the nature the oxide catalyst. The order of the OER with respect to OH
concentration has also been determined by recording ‘E vs. log j’ curves for the catalyst at 0.2 mV s
1 at varying KOH concentrations. The ionic strength of the medium (m ¼ 2.0) was maintained constant using KNO3 as an inert electrolyte. With the help of these Tafel curves the linear log j vs log [OH
] plots were constructed at a constant potential (E ¼ 0.590 V) and the order was then determined (Table 4). For construction of the ‘log j vs. log [OH
]’ plots, only the first linear region of each ‘E vs. log j’ curve was considered.

Table 2 e Estimates of the equivalent circuit parameters for Fe0.25Cr1.75 (MoO4)3 electrode at 298 K. E/V

107 L/H

Rs/Ucm2

Rf/U cm2

103Qf/S cm
2 sn

nf

Rct/U cm2

103Qdl/S cm
2 sn

n

103Cdl/F cm
2

0.565 0.57 0.58 0.59 0.60

9.2 9.2 10.5 1.03 13.7

1.4 1.5 1.4 1.5 1.6

0.95 0.96 0.93 0.81 0.93

70.79 51.97 43.74 35.92 51.34

0.42 0.44 0.44 0.48 0.39

23.58 18.5 9.87 6.78 4.79

5.10 5.65 6.14 6.29 6.45

0.96 0.95 0.93 0.93 0.93

4.18 4.43 4.26 4.37 4.48

Table 3 e Estimates of the equivalent circuit parameters for Fe0.75Cr1.25(MoO4)3 electrode at 298 K. E/V

107 L/H

Rs/U cm2

Rf/U cm2

103Qf/S cm
2 sn

nf

Rct/U cm2

103Qdl/S cm
2 sn

n

103Cdl/F cm
2

0.54 0.56 0.575 0.585 0.60

9.9 12.2 11.6 10.3 10.7

1.3 1.2 1.4 1.4 1.5

0.29 0.41 0.27 0.25 0.12

224.60 269.40 145.4 240.2 130.34

0.49 0.21 0.33 0.35 0.77

42.04 11.65 5.49 3.83 2.34

8.91 9.93 10.14 10.18 9.99

0.96 0.93 0.93 0.93 0.93

7.45 7.07 7.25 7.42 7.02

15122

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

Table 4 e Electrode kinetic parameters for O2 evolution on FexCr2Lx(MoO4)3 (0 £ x £ 0.75) in 1 M KOH at 298 K. Electrode (x) B.E.T./m2 g
1 Loading/mg cm
2 Tafel slope Order ( p)

0 0.25 0.50 0.75

9.9 2.7 2.8 1.7

3.0  2.2  1.2  2.7 

0.2 0.2 0.2 0.1

b1

b2

w45 w35 w35 w35

w73 w69 w59 w64

Results shown in Table 4 show that partial replacement of Cr by Fe in the Cr2(MoO4)3 matrix increases the apparent current density ( jap/mA cm
2) and also the apparent current per mg of the oxide loading, j0ap (¼jap, mA cm
2/oxide loading, mg cm
2); the increment in the current density being the greatest with x ¼ 0.5. The increase in j0ap is found to be w14 to w37 times with 0.25e0.75 mol Fe introduction. However, estimates of the specific catalytic activity (SA ¼ j0ap , 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. Cr2(MoO4)3) under similar experimental conditions. The large increase in specific activity with introduction of Fe (3d6), in place of Cr (3d5) in the base oxide can be ascribed to the modification of the electronic properties of the material due to increase in the number of unpaired ‘d’ electrons and also to synergistic effect. Values of the Tafel slope (b) for OER found on Fe-substituted chromium molybdates are in fair agreement with those recently reported on oxides, namely MMoO4 (Co, Fe or Ni) (b1 z 40 mV) [38], Fe2(MoO4)3 (b1 z 35 mV) [41], MnFe2
xCrxO4 (b1 z 40 mV) [25] and NiFe2
xCrxO4 (b1 z 40 mV) [26]. However, there seems controversy regarding the reaction order with respect to OH
ion. Both fractional and second order reactions have been reported in literature [17,27e30,38,41,42,48]. The apparent electrocatalytic activities of new ternary oxides, FexCr2
x(MoO4)3 (at jap ¼ 100 mA cm
2: E ¼ 0.635e0.640 V) electrodes are greatly higher than those of binary oxides, MMoO4, where M ¼ Fe, Ni and Co (at jap ¼ 100 mA cm
2, E ¼ 0.670e0.790 V) [38e41]. The OER activities of these ternary oxides also seem to somewhat better than those of similar ternary oxides, FeeCoeMoeO (E w 0.645 V, j ¼ 100 mA cm
2) [42] and FeeNieMo (E w 0.640 V, j ¼ 100 mA cm
2) [43] recently reported in literature. Also, the apparent electrocatalytic activities of Cr2
xFex(MoO4)3 electrodes are better than those of many active Co- and Fe-based spinel oxide [9,30] electrodes recently reported in literature under similar electrolysis conditions. For instance, Chi et al. [22] observed jap ¼ 100 mA 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. [44] found jap ¼ 16 mA cm
2 at E ¼ 0.80 V vs. SCE (z0.944 V vs. Hg/HgO) on Li-doped Co3O4 on glass, obtained by spray pyrolysis. Svegl et al. [8] found jap ¼ 100 mA cm
2 at E z 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. The electrocatalytic activities of active ternary CreFeeMoeO oxides were, however, lower compared to those of NiFeCrO4

Electrocatalytic activity at E ¼ 0.650 V
2

j/mA cm w1 w2 w2 w2

12 165 186 151

   

3 15 17 5

j/mA mg
1 S (mA cm
2) ¼ j/B.E.T. surface area w4 w75 w155 w56

0.04 2.78 5.54 3.29

( jap ¼ 100 mA cm
2, E ¼ 0.586 V vs. Hg/HgO) prepared by precipitation [26], La-doped Co3O4 ( jap ¼ 100 mA cm
2, E ¼ 0.527e0.539 V vs. Hg/HgO) obtained by microwave assisted thermal decomposition [11], ZnCo2O4 ( jap ¼ 100 mA cm
2, h ¼ 0.256e0.203 V) obtained by electrophoretic deposition [21], and electrodeposited Co þ Ni mixed oxide catalyst ( jap ¼ 100 mA cm
2, E ¼ 0.60 V vs. Hg/ HgO) [17]. To account the observed kinetic parameters, a mechanism similar to one given by Bockris and Otagawa [31] is proposed for the OER. The mechanism involves the discharge of OH
ion forming the adsorbed OH intermediate (S þ OH
4 SOH þ e
, S being an active site on the oxide surface) as a fast step and subsequent electrochemical transformation, in presence of OH
ion, into the physisorbed H2O2 (SOH þ OH
/ S$H2O2 þ e
) as a slow (rate determining) step. This mechanism gives the second order reaction in OH
concentration and a Tafel slope of ca. 40 mV under the Langmuir adsorption condition.

4.

Summary

The study demonstrates that the co-precipitation method employed in-situ produces the crystalline nano-compounds with the orthorhombic crystal structure. Substitution of Cr by 0.25e0.75 mol Fe in the Cr2(MoO4)3 matrix increases the electrocatalytic activity of the catalyst toward the OER. 0.25e0.75 mol Fe substitution reduced the Tafel slope to nearly one half and increased the specific activity of the oxide by more than 80 times. Thus, the presence of Fe in the Cr2(MoO4)3 matrix greatly influences the electrocatalytic activity as well as the mechanism of the OER.

Acknowledgments One of the authors (MK) thanks Banaras Hindu University, Varanasi-221005 (India) for providing UGC Fellowship to carry out the investigation.

references

[1] O’Sullivan EJM, Calvo EJ. In: Compton RG, editor. Comprehensive chemical kinetics, vol. 27. Amsterdam: Elsevier; 1987. p. 247e360. [2] Trasatti S. In: Lipkowski J, Ross PN, editors. The electrochemistry of novel materials. New York: VCH Publishers Inc.; 1994. p. 207e95. [3] Tejuca LG, Fierro JLF, Tascon JM. Advances in catalysis, vol. 36. New York: Academic Press; 1989.

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

[4] Pena MA, Fierro JLG. Chemical structures and performance of perovskite oxides. Chem Rev 2001;101:1981e2017. [5] Hamdani M, Singh RN, Chartier P. Co3O4 and Co-based spinel oxides bifunctional oxygen electrodes. Int J Electrochemical Sci 2010;5:556e77. [6] Tavares AC, Cartaxo MAM, da Silva Pereira MI, Costa FM. Effect of the partial replacement of Ni or Co by Cu on the electrocatalytic activity of the NiCo2O4 spinel oxide. J Electroanal Chem 1999;464:187e97. [7] Rios E, Gautier J-L, Poillerat G, Chartier P. Mixed valency spinel oxides of transition metals and electrocatalysis: case of the MnxCo3
xO4 system. Electrochim Acta 1998;44:1491e7. [8] Svegl F, Orel B, Grabec-Svegl I, Kaucic V. Characterization of spinel Co3O4 and Li-doped Co3O4 thin film electrocatalysts prepared by the solegel route. Electrochim Acta 2000;45: 4359e77. [9] Fatih K, Marsan B. CuxCo3
xO4/LaPO4-bonded Ni electrodes for oxygen evolution in alkaline solution: preparation, physicochemical properties and electrochemical behavior. Can J Chem 1997;75:1597e607. [10] Fradette N, Marsan B. Surface studies of CuxCo3
xO4 electrodes for the electrocatalysis of oxygen evolution. J Electrochem Soc 1998;145:2320e7. [11] Singh RN, Mishra D, Anindita, Sinha ASK, Singh A. Novel electrocatalysts for generating hydrogen from alkaline water electrolysis. Electrochem Comm 2007;9:1369e73. [12] Singh JP, Singh RN. New active spinel-type MxCo3
xO4 films for electrocatalysis of oxygen evolution. J New Mater Electrochem Syst 2000;3:137e45. [13] Singh RN, Pandey JP, Singh NK, Lal B, Chartier P, Koenig JF. Solegel derived spinel MxCo3
xO4 (M ¼ Ni, Cu; 0  x  1) films and oxygen evolution. Electrochim Acta 2000;45:1911e9. [14] Lal B, Singh NK, Samuel S, Singh RN. Electrocatalytic properties of CuxCo3
xO4 (0  x  1) obtained by a new precipitation method for oxygen evolution. J New Mater Electrochem Syst 1999;2:59e64. [15] Tiwari SK, Samuel S, Singh RN, Poillerat G, Koenig JF, Chartier P. Active thin NiCo2O4 film prepared on nickel by spray pyrolysis for oxygen evolution. Int J Hydrogen Energy 1995;20:9e15. [16] Singh SP, Samuel S, Tiwari SK, Singh RN. Preparation of thin Co3O4 films on Ni and their electrocatalytic surface properties towards oxygen evolution. Int J Hydrogen Energy 1996;21:171e8. [17] Wu G, Li N, Zhou D-R, Mitsuo K, Xu B-Q. Anodically electrodeposited Co þ Ni mixed oxides electrode: preparation and electrocatalytic activity for oxygen evolution in alkaline media. J Solid State Chem 2004;177: 3682e92. [18] Castro EB, Real SG, PinheiroDick LF. Electrochemical characterization of porous NieCo oxide electrodes. Int J Hydrogen Energy 2004;29:255e61. [19] Castro EB, Gervasi CA. Electrodeposited NieCo-oxide electrodes: characterization and kinetic of the oxygen evolution reaction. Int J Hydrogen Energy 2000;25:1163e70. [20] Lu B, Cao D, Wang P, Wang G, Gao Y. Oxygen evolution reaction on Ni substituted Co3O4 nanowire array Electrodes. Int J Hydrogen Energy 2011;36:72e8. [21] Chi B, Li J, Yang XZ, Lin H, Wang N. Electrophoretic deposition of ZnCo2O4 spinel and its electrocatalytic properties for oxygen evolution reaction. Electrochim Acta 2005;50:2059e64. [22] Chi B, Lin H, Li J, Wang N, Yang J. Comparison of three preparation methods of NiCo2O4 electrodes. Int J Hydrogen Energy 2006;31:1210e4. [23] Li Y, Hasin P, Wu Y. NixCo3
xO4 nanowire Arrays for electrocatalytic oxygen evolution. Adv Mater 2010;22:1926e9.

15123

[24] Isabel Godinho M, Alice Catarino M, daSilva Pereira MI, Mendonca MH, Costa FM. Effect of the partial replacement of Fe by Ni and/or Mn on the electrocatalytic activity for oxygen evolution of the CoFe2O4 Spinel oxide electrode. Electrochim Acta 2002;47:4307e14. [25] Singh RN, Singh JP, Cong HN, Chartier P. Effect of partial substitution of Cr on electrocatalytic properties of MnFe2O4 towards O
2 evolution in alkaline medium. Int J Hydrogen Energy 2006;31:1372e8. [26] Singh RN, Singh JP, lal B, Thomas MJK, Bera S. New Crsubstituted nickel ferrite nano-spinels for O2 evolution in alkaline solutions. Electrochim Acta 2006;51:5515e23. [27] Singh RN, Singh NK, Singh JP. Electrocatalytic properties of new active ternary ferrite film anodes for O2 evolution in alkaline medium. Electrochim Acta 2002;47:3873e9. [28] Singh JP, Singh NK, Singh RN. Electrocatalytic activity of metal-substituted Fe3O4 obtain at low temperature for O2 evolution. Int J Hydrogen Energy 1999;24:433e9. [29] Singh NK, Tiwari SK, Anitha KL, Singh RN. Electrocatalytic properties of spinel-type MnxFe3
xO4 synthesized below 100  C for oxygen evolution in KOH solution. J Chem Soc Faraday Trans 1996;92:2397e400. [30] Anindita, Singh A, Singh RN. Effect of V substituted at B-site on the physicochemical and electrocatalytic properties of spinel-type NiFe2O4 towards O2 evolution in alkaline solutions. Int J Hydrogen Energy 2010;35:3243e8. [31] Bockris JO’M, Otagawa T. Mechanism of oxygen evolution on perovskites. J Phys Chem 1983;87:2960e71. [32] Bockris JO’M, Otagawa T. The electrocatalysis of oxygen evolution on perovskites. J Electrochem Soc 1984;131: 290e302. [33] Tiwari SK, Chartier P, Singh RN. Preparation of perovskitetype oxides of cobalt by the malic acid aided process and their electrocatalytic surface properties in relation to oxygen evolution. J Electrochem Soc 1995;142:148e53. [34] Singh RN, Lal B. High surface area lanthanum cobaltate and its A and B sites substituted derivatives for electrocatalysis of O2 evolution in alkaline solution. Int J Hydrogen Energy 2002; 27:45e55. [35] Lal B, Raghunandan MK, Gupta M, Singh RN. Electrochemical behaviour of perovskite-type La1
xSrxCoO3 (0  x  0.4) obtained by a novel stearic acid solegel method for electrocatalysis of O2 evolution in KOH solutions. Int J Hydrogen Energy 2005;30:723e9. [36] Tiwari SK, Singh SP, Singh RN. Effect of Ni, Fe, Cu and Cr substitutions for Co in La0.8Sr0.2CoO3 on electrocatalytic properties for oxygen evolution. J Electrochem Soc 1996;143: 1505e10. [37] Jain AN, Tiwari SK, Chartier P, Singh RN. Low temperature synthesis of perovskite-type oxides of La and Co and their Electrocatalytic properties for oxygen evolution in alkaline solution. J Chem Soc Faraday Trans 1995;91:1871e5. [38] Singh RN, Singh JP, Singh A. Electrocatalytic properties of new spinel-type MMoO4 (M ¼ Fe, Co & Ni) Electrodes for Oxygen evolution in alkaline solutions. Int J Hydrogen Energy 2008;33:4260e4. [39] Singh RN, Madhu, Awasthi R, Sinha ASK. Preparation and electrochemical characterization of a new NiMoO4 catalyst for electrochemical O2 evolution. J Solid State Electrochem 2009;13:1613e9. [40] Singh RN, Madhu, Awasthi R, Sinha ASK. Electrochemical characterization of a new binary oxide of Mo with Co for O2 evolution in alkaline solution. Electrochim Acta 2009;54: 3020e5. [41] Singh RN, Madhu, Awasthi R, Tiwari SK. Iron molybdates as electrocatalysts for O2 evolution reaction in alkaline solutions. Int J Hydrogen Energy 2009;34:4693e700.

15124

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 e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 1 7 e1 5 1 2 4

[42] Kumar M, Awasthi R, Sinha ASK, Singh RN. New ternary Fe, Co and Mo mixed oxide electro catalysts for oxygen evolution. Int J Hydrogen Energy 2011;36:8831e8. [43] Kumar M, Awasthi R, Pramanick AK, Singh RN. New ternary mixed oxides of Fe, Ni and Mo electrocatalysts for enhanced oxygen evolution. Int J Hydrogen Energy 2011;36: 12698e705. [44] Hamdani M, Pereira MIS, Douch J, Addi AA, Berghoute Y, Mendonca MH. Physicochemical and electrocatalytic properties of LieCo3O4 anodes prepared by chemical spray pyrolysis for application in alkaline water electrolysis. Electrochim Acta 2004;49:1555e63. [45] Allen GC, Harris SJ, Jutson JA. A study of a number of mixed transition metal oxide spinels using X-ray photoelectron spectroscopy. Appl Surf Sci 1989;37:111e34. [46] Park D, Yun Y-S, Lee HW, Park JM. Advanced kinetic model of the Cr(VI) removal by biomaterials at various pHs and temperatures. Bioresour Technol 2008;99:1141e7. [47] Abello MC, Velasco AP, Ferretti, Fierro JLG. A monte carlo approach to describe the reduction profiles of bidimensional MoOx structures grown on an alumina substrate. Latin Am Appl Res 2007;37:307e13.

[48] Chien H-C, Cheng W-Y, Wang Y-H, Wei T-Y, Lu S-Y. Ultra low overpotentials for oxygen evolution reactions achieved by nickel cobaltite aerogels. J Mater Chem 2011;21:18180e2. [49] Laouini E, Hamdani M, Pereira MIS, Berghoute Y, Douch J, Mendonca MH, et al. Impedance study of spinel type FeeCo3O4 oxide thin film electrode in alkaline medium. Int J Electrochem Sci 2009;4:1074e84. [50] Singh RN, Singh A, Mishra D, Anindita, Chartier P. Oxidation of methanol on perovskite type La2
xSrxNiO4 (0  x  1) film electrodes modified by dispersed nickel in 1M KOH. J Power Sources 2008;185:776e83. [51] Alves VA, Da Silva LA, Boodts JFC. Surface characterization of IrO2/TiO2/CeO2 oxide electrodes and faradaic impedance investigation of the oxygen evolution reaction from alkalie solution. Electrochim Acta 1998;44:1525e34. [52] Da Silva LA, Alves VA, Da Silva MAP, Trasatti S, Boodts JFC. Oxygen evolution in acid solution on IrO2 þ TiO2 ceramic films. A study by impedance, voltammetry and SEM. Electrochim Acta 1997;42:271e81. [53] Palmas S, Ferrara F, Vacca A, Mascia M, Polcaro AM. Behaviour of cobalt oxide electrodes during oxidative processes in alkaline medium. Electrochim Acta 2007;53:400e6.