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Effect of partial substitution of Cr on electrocatalytic properties of CoFe2O4 towards O2 -evolution in alkaline medium
International Journal of Hydrogen Energy 31 (2006) 701 – 707 www.elsevier.com/locate/ijhydene
Effect of partial substitution of Cr on electrocatalytic properties of CoFe2 O4 towards O2 -evolution in alkaline medium夡 R.N. Singha,∗ , N.K. Singha , J.P. Singha , G. Balajib , N.S. Gajbhiyeb a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, India b Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India
Available online 22 August 2005
Abstract Some nanocrystalline ternary ferrites (∼30–40 nm) having molecular formula, CoFe2−x Cr x O4 (0 x 1.0) and face centered cubic structure were investigated by electronic, EPR, magnetization, impedance and Tafel polarization techniques. Results indicate that the combination of antiferromagnetic superexchange interactions of the spinel lattice determines the Curie temperature (Tc ) value. The AB interaction dominates over the sublattice interactions, and Fe3+ occupies A sites, and Cr 3+ and Co2+ ions occupy octahedral sites because of large crystal field stabilization energy. The saturation magnetization (s ) coercivity (Hc ), remnant field (Hr ) and electrical resistivity are observed to decrease with the progressive replacement of Fe3+ ions (d5 , five unpaired electrons) by Cr 3+ ions (d3 , three unpaired electrons). However, the electrocatalytic activity of the oxides towards the oxygen evolution reaction (OER) in 1 M KOH at 25 ◦ C increases with the increase in x; the optimum improvement in the apparent electrocatalytic activity being with 1.0 mol Cr. At low overpotentials, the OER on substituted compounds displayed a Tafel slope of b = 50 ± 5 mV decade−1 and the reaction order with respect to OH− concentration as unity, regardless of the composition of the oxide catalyst. It seems that the electrocatalytic activity and saturation magnetization/Curie temperature for the oxide are inversely related. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Spinel ferrite; Electrocatalysis; Oxygen evolution; Cation distribution; Saturated magnetization
1. Introduction Fe3 O4 and its substituted products with the spinel structure of the type, AB2 O4 , where A and B are transition metals, possess relatively high overpotential [1] compared to cobaltites [2–8], for the oxygen evolution reaction (OER) in alkaline solution, and are scantly investigated for their use as oxygen anodes [2,9,10]. However, recent studies have shown 夡 Presented at the Fourth International Symposium on Electrocatalysis—“From Theory to Industrial Applications” held in Como, Italy, 22–25 September, 2002. ∗ Corresponding author. Fax: +91 542 2369951. E-mail addresses: [email protected] (R.N. Singh), [email protected] (N.S. Gajbhiye).
[11–13] that the oxygen overpotential on ferrospinels can be reduced considerably by using low temperature preparation methods and substituting suitable metal ions for Fe in the Fe3 O4 -matrix partially. For instance, at 100 mA cm−2 in 1 M KOH at 25 ◦ C, Fe3 O4 , MnFe2 O4 , NiFe2 O4 and CoFe2 O4 recently obtained by a hydroxide precipitation method at controlled pH 11, produced the oxygen overpotentials 524, 338, 379 and 395 mV, respectively. These values of overpotentials are much lower than those obtained for similar anodes prepared by ceramic methods [9,10], but are comparable to those recently reported for Co3 O4 [14–16] and NiCo2 O4 [17–19]. Further, the overpotential data indicate that the effect of a d-electron deficient transition metal (i.e. Mn, 3d5 ) substitution in Fe3 O4 is more pronounced compared to that of a d-electron rich transition metal
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.07.003
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
(Co, 3d7 or Ni, 3d8 ) substitution. Very recently, Singh et al. [20] observed that the replacement of Fe3+ by Cr 3+ in the CoFe2 O4 matrix reduced the O2 overpotential significantly. For instance, at j = 100 mA cm−2 in 1 M KOH at 25 ◦ C, CoFe2 O4 and CoCr0.2 Fe1.8 O4 produced the O2 overpotentials, 0.395 and 0.312 V, respectively. However, the observed beneficial effect of Cr is not well understood. Also, the Ni substrate is not well protected from the electrolyte contact by the oxide overlayer and that the film electrodes in 1 M KOH indicated the formation of Ni (III)–Ni (II) redox couple in cyclic voltammetry (CV) [11]. In order to understand the role of Cr 3+ in oxygen evolution electrocatalysis visà-vis to correlate electrocatalytic and electronic properties, Cr-substituted products obtained in [20] were further investigated by electronic, EPR, conductivity and magnetization measurements. Also, to avoid the possible interference of Ni2+ in the study of O2 evolution (OE), electrochemical investigations were carried out on films of oxides obtained on Pt supports. Details of results, so obtained, are described in the present paper.
-2.4
-2.9
log (σ / Ω-1 cm-1)
702
-3.4
-3.9
-4.4
-4.9
-5.4 1.5
2.0
1 × 10 / T -3
2. Experimental Cr-substituted cobalt ferrites used in the present investigation were the same as reported in [20]. They were prepared at 70 ◦ C by adding 5 M NaOH in dropwise manner into an aqueous solution of salts of constituent metals of the oxide, dissolved in stoichiometric ratio, under stirred condition and continued oxygen bubbling; details of which are already given elsewhere [20]. For conductivity measurements, the oxide powders (sintered at 400 ◦ C) were pressed into pellets using 4–5 tons pressure by the pressure pump. No binder was used in making the pellet. To ensure good contact and uniform distribution of current, both the surfaces of the pellet were painted with silver conductive paste. The electrical conductivity was determined at different temperatures by recording impedance spectra at a DC potential of 50 mV and f = 50 kHz. Magnetization measurements were carried out using a vibrating sample magnetometer, Par-Model 150A, in conjugation with a Varian electromagnet assembly model (V7200) that provides a magnetic field up to 11 kG. The temperature variation of magnetization was studied using a furnace assembly model 151 associated with the magnetometer and it can provide a temperature up to 1000 K. The oxide powders were transformed in the form of film on a pretreated platinum support by an oxide-slurry painting method as described elsewhere [21,22]. The oxide films were heat treated in an electrical furnace at 380 ◦ C for 1 21 h and they were removed only when the furnace attained the temperature of < 80 ◦ C. The pretreatment of the support and electrical contact with the oxide film were made as described previously [21,22]. The oxide loadings were 3–5 mg cm−2 . A conventional three-electrode single-compartment Pyrex glass cell involving pure Pt-foil (∼ 8 cm2 ) and Hg/HgO/1 M
3.0
2.5
3.5
k-1
Fig. 1. Plot of log vs 1/T for CoFe2−x Cr x O4 (temperature range: 298–593 K); ◦–◦: CoFe2 O4 , –: CoFe1.6 Cr0.4 O4 , –: CoFe1.2 Cr0.8 O4 .
KOH (E 0 = 0.098 V vs SHE), respectively, as reference and auxiliary electrodes was employed for electrochemical investigations. All potential values given in the text correspond to this reference only. Impedance and Tafel polarization studies were carried out using an electrochemical impedance system (Model 273 A, EG & G, PARC, USA) provided with a lock-in-amplifier (Model 5208), a potentio-galvanostat (Model 273A) and a PS/2 (Model 35 SX) IBM computer. The softwares used in measurements of impedance and Tafel polarizations were M 388 and M 352 Corrosion analysis, respectively. As mentioned earlier [20], the electochemical impedance study of the oxide film electrodes in 1 M KOH has been carried out with the AC voltage amplitude of 5 mV in the frequency range of 0.02–105 Hz at two DC potentials, 0 and 50 mV. The double layer capacitance (Cdl ) at each DC potential was analyzed by using the “Equivalent Circuit” program written by Boukamp [23]. Procedure followed in the study of Tafel polarization was previously described in [15,22]. 3. Results and discussion 3.1. Conductivity, UV-visible, EPR and magnetic studies The bulk conductivity () of compounds was determined in the temperature range 298–593 K. Results show that the room temperature conductivity increases with increase in x. The Arrhenius plot (log vs 1/T ) shown in Fig. 1 indicates
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
703
Table 1 Activation energy for conduction
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
104 (mho m−1 ) at 298 K
5.5 5.6 8.1 18.8 34.2 26.7
two linear regions, one between temperatures 298 and 413 K (I) and the other between temperatures 413 and 593 K (II). The activation energies for conduction (Table 1) were ranged between 0.029 and 0.188 eV and 0.44 and 0.61 eV in the low and higher temperature regions, respectively. The observed linearity in log vs 1/T curves at higher temperatures may be due to the role of valence electrons. However, the lower temperature linear region might be caused due to the presence of minor donor impurities in the oxides. The increase in conductivity of the base oxide with increase in x may be explained on the basis of symmetry of orbitals occupied by electrons. Interaction between Fe3+ (d5 ) in A and B sites 3 –t 3 and e2 –e2 configurations. When are larger due to t2g g g 2g 3 –t 3 interaction Fe3+ in B sites are replaced by Cr 3+ , t2g 2g remains but interaction of eg orbitals disappears and con-
sequently, the number of unpaired electrons increases and hence the conductivity of the oxide increases. The UV-visible spectra of oxides excepting CoFe1.2 Cr0.8 O4 showed a broad peak around 500 nm. This peak may be due to presence of Co2+ /and or Fe3+ at the octahedral sites. Contrary to this observation, CoFe1.2 Cr0.8 O4 did not show any peak around 500 nm, however, it exhibited an intense peak around 300 nm which, thereby, indicates the strong charge transfer from O to Fe3+ or O to Cr 3+ . The similar intense peak was also indicated by CoFeCrO4 . The observed single broad band in the EPR spectra of catalysts in the low field region indicates the interaction mainly between Fe3+ centers. Co2+ and Fe2+ appear to be EPR silent as they do not show their respective signals. Further, estimates of the g-values for pure and Cr-substituted CoFe2 O4 indicate greater ferromagnetic interaction between Fe3+ centers in the absence of Cr 3+ . The g-values for oxides with x = 0, x = 0.4, x = 0.8 and x = 1.0 in low field (maximum) and high field (minimum) were 10.3 and 2.27, 7.94 ± 0.05 and 1.7 ± 0.01, 8.38 and 1.80 and 5.48 ± 0.11 and 1.82 ± 0.04, respectively. In general, the intrinsic magnetic properties of spinel ferrite having particle size in the nanometer range are influenced by surface, particle size and cation distribution [24]. The cation distribution in magnetic nanoparticles is com-
Activation energy of conductivity (eV) First region (298–413 K)
Second region (413–593 K)
0.029 0.188 0.11 0.12 0.13 0.12
0.61 0.58 0.44 0.47 0.45 0.59
20 Cr 0.0 0.2 0.4 0.6 0.8 1.0
16 σ (emu/g)
Oxides
12 8 4 0 300
400
500
600
700
800
T (K) Fig. 2. Temperature dependence of magnetization curves for CoFe2−x Cr x O4 particles.
pletely different from that observed in bulk. It is well known that the magnetic properties of spinel ferrite particles are influenced by constituent metal ions and the cation distribution in the spinel lattice depending on the synthetic methods employed [25]. In the present work, the aim is to study the effect and consequence of progressive substitution of Fe3+ by Cr 3+ ions in nanosize CoFe2 O4 particles. Fig. 2 shows the temperature dependence of magnetization curves for Cr 3+ substituted CoFe2 O4 particles. As the temperature increases, the magnetization value decreases due to the thermal fluctuation phenomenon. Further, at higher temperatures, the magnetization decreases steadily and falls to zero at the Curie temperature (Tc ) signifying a ferrimagnetic to paramagnetic transition. The Tc values, saturation magnetization (s ), coercivity (Hc ) values and remnant field values (Hr ) are tabulated in Table 2. It is worth noticing that the Tc value of CoFe2 O4 particles is greater than that of the bulk value (793 K) [26]. This may be due to particle size effect. Also, the magnetization
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R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
Table 2 Magnetic parameters for CoFe2−x Cr x O4 (0.0 x 1) Oxides
Curie temperature, Tc (K)
s (emu/g)
Saturation magnetization,
Remnant magnetization, Mr (emu/g)
Coercive field, Hc (T)
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
798 783 778 758 773 753
49.1 54.3 42.1 36.1 27.1 17.09
16.29 20.62 15.65 10.94 8.71 5.04
0.1013 0.0909 0.0713 0.0583 0.0825 0.0650
60 40 20 σ (emu/g)
decreases with Cr 3+ substitution. The Tc values decrease from 798 to 753 K with increasing concentration of Cr 3+ varying from 0.2 to 1.0 mol at Fe3+ sites. This trend is due to the decrease of total exchange energy in the spinel lattice because of Cr 3+ substitution. The magnetic structure of CoFe2 O4 material is ferromagnetic and can be envisaged by two magnetic sublattices A and B, which are coupled antiparallel and occupy tetrahedral and octahedral sites, respectively. Then, it is possible to have a combination of antiferromagnetic superexchange interactions (namely, AA, BB, AB) and the total net magnitude of superexchange energy of the spinel lattice determines its Tc value. It is known that AB interaction dominates over the sublattice interactions and the Fe3+ occupy A sites (inverse spinel). But both Cr 3+ (d3 ) and Co2+ (d7 ) are with three unpaired electrons, which must have large crystal field stabilization energy (CFSE) for octahedral occupancy. Thus, in our system, we predict that Cr 3+ ions possibly occupy either A and/or B sites. On progressive Cr 3+ substitution, the stronger Fe(A)–O–Fe(B) and Co(A/B)–O–Fe(B/A) interactions are replaced by weaker Co(A/B)–O–Cr(B/A), Fe(A/B)–O–Cr(B/A) and Cr(A)–O–Cr(B) interactions and hence Tc value is decreased. Tc value of CoCr2 O4 is very much less than that of CoFe2 O4 [27,28]. Fig. 3 shows the field dependence of magnetization for all the samples. It is clear that saturation magnetization (s ) decreases with increasing Cr 3+ substitution. The magnetic moment of ions in sublattice B orient parallel to the applied magnetic field and antiparallel to the sublattice A. Hence, the total magnetization of the spinel lattice is dependent on the moment of ions occupying B site (assuming these oxides are insulators). In the present study, Fe3+ (d5 ion, five unpaired electrons) is replaced by Cr 3+ (d3 ion, three unpaired electrons). Thus, when Fe3+ is substituted more, the magnetization value decreases progressively. The coercivity (Hc ) and remnant field (Hr ) are inversely dependent to s [28]. Hence, it may be expected that Hc and Hr should increase on increasing Cr 3+ substitution. But the experimental findings in the present study do not corroborate this prediction. However, both Hc and Hr are found to decrease and this observed trend is related to the dominant magnetocrystalline anisotropy (K) factor for these samples, which is being explored in further study.
0 Cr 0.0 0.2 0.4 0.6 0.8 1.0
-20 -40 -60 -1.0
-0.5
0.0 H (T)
0.5
1.0
Fig. 3. Field dependence of magnetization for CoFe2−x Cr x O4 particles.
3.2. Impedance spectra/roughness factor The feature of electrochemical impedance (EI) spectra for the CoFe2−x Cr x O4 -films in 1 M KOH was almost similar. A typical complex impedance plot for Pt/CoFe1.2 Cr 0.8 O4 in 1 M KOH (25 ◦ C) is shown in Fig. 4. In order to observe the nature of complex plot particularly at high frequencies, the high frequency part of Fig. 4 has been expanded and reproduced in Fig. 4(a). Fig. 4(a) shows a small and ill-defined semicircle at high frequencies and that it may be produced due to the contribution of interior grains of the oxide mass, but the complex feature of the EI spectrum in the latter frequency region can be ascribed to the charging and discharging of the double layer formed at the oxide/1 M KOH interface and also to the diffusion of the charge species (H+ /OH− ) into the oxide films through defects and pores [11]. As reported earlier [20], the two equivalent circuits, LR(RQ)(R(W(C))) and LR(RQ)(CW), were used to analyze the circuit parameters in the present study also. Symbols, L, R, R, R, Q, W and C, used to describe the circuit description code (CDC) were inductance (H ), solution resistance (), oxide film
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
of the oxide roughness (RF ) were estimated from the ratio of the observed Cdl of the oxide catalyst and the Cdl of a smooth oxide surface (60 F cm−2 ) [22,29,30]. The replacement of Fe by Cr (0.2–1.0 mol) in the CoFe2 O4 -matrix did not indicate any significant influence on the oxide roughness. Almost similar results were also obtained when these catalytic films were studied on a Ni-support [20].
0.80
0.02 Hz
1.5
0.40
3.3. Electrocatalytic activity
1 kHz
(a) -z img / Ω cm2
-z img x 104/Ω cm2
0.60
0.06 Hz
1.0 2.51 kHz 0.5 2.51 kHz
0.20
0.0
79.4 kHz 100 kHz
0.26 Hz 1.5 1.18 Hz
2.0 2.5 zreal / Ω cm2
3.0
1000 kHz
0.00
0.20
0.40 0.60 z real x 104/Ω cm2
Fig. 4. Complex impedance plot for (l = 4.4 mg cm−2 ) in 1 M KOH (25 ◦ C).
0.80
Pt/CoFe1.2 Cr0.8 O4
90
l Zl / Ω cm2
60 1k 30 100 0
Phase angle / deg.
Measurement Simulation
10 k
10 -30 1 100 m 1
705
100 10 1k Frequency / Hz
10 k 100k
Fig. 5. Bode plot for Pt/CoFe1.2 Cr0.8 O4 (l = 4.4 mg cm−2 ) in 1 M KOH (25 ◦ C).
resistance (), charge transfer resistance (), constant phase element (−1 s n ), Warburg impedance (−1 ) and capacitance i.e. Cdl (F), respectively. As the potentiostat introduces some inductance at high frequencies [23], the component L has been used to obtain the best fit of the experimental data. Based on proposed circuit models, EI spectra obtained agreed reasonably well with the experimental curves. A set of typical Bode plot (simulated and experimental) for (CoFe1.2 Cr0.8 O4 is shown in Fig. 5, and estimates of the circuit parameters are shown in Table 3. The relative values
The jR-corrected anodic Tafel polarization curves (E vs log j ) recorded at a scan rate of 0.2 mV s−1 for cobalt ferrite films on Pt in 1 M KOH at 25 ◦ C are shown in Fig. 6. The feature of the E–log j curves was almost similar for any x. They seemed to have two Tafel slopes, one at low and the other at higher potential. The first Tafel slope (b) was approximately the same (50 ± 5 mV decade−1 ) with each oxide. The second Tafel slope was found to be ∼ 104 and ∼ 116 mV decade−1 in the case of oxide with x = 0 and x = 0.2, respectively; however, it was not very clear in case of other oxides. The order for the OER with respect to OH− concentration was found to be approximately unity with each electrocatalyst. To determine the order, the anodic E vs log j curves were recorded on four representative oxide films with x = 0.2, x = 0.4, x = 0.6 and x = 0.8 at varying KOH concentrations. Concentrations of KOH used in this particular investigation were 1.01, 2.05, 3.11, and 4.22 m. As concentrations of the electrolyte used in the study were not low, the ionic strength of the medium was not maintained constant. With the aid of the anodic polarization curves determined at different KOH concentrations, the linear log j vs log aOH− curves at constant potentials were constructed in the first Tafel region and the reaction order was estimated by measuring the slope of these curves. The activity coefficient (f ) corresponding to each molal concentration of KOH was estimated by constructing a calibration curve, f vs [KOH]/m from the data of activity coefficients of the electrolyte at 25 ◦ C given as Appendix 8.10 in [31]. Values of the electrode kinetic parameters (b and p) (Table 4) suggest that the OER on pure and Cr-substituted cobalt ferrite follow almost similar mechanistic paths. The apparent (ja ) and the true (jt = ja /RF ) electrocatalytic activities of oxides determined at two potentials (0.65 and 0.70 V) from Fig. 6 (Table 4) indicate that Cr substitution increases both the apparent as well as the true electrocatalytic activity of the base oxide significantly. Further, results demonstrate that Cr-substitutions mainly improve electronic properties of the material in favor of the electro-formation of O2 , while geometrical properties seem to be little influenced with Cr. Results of conductivity, electronic, EPR, and magnetic studies also indicate modification of electronic properties of the oxide with Cr-substitution. Thus, the observed increase in electrocatalytic activity of the base oxide in presence of Cr can be attributed to the decrease in the electrical resistivity vis-a-vis the saturation magnetization/Curie temperature of the oxide. Based on values of jt at E = 0.7 V
706
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
Table 3 Values of the equivalent circuit parameters for oxide electrodes in 1 M KOH at 25 ◦ C Electrodes
107 L (H)
R1 ()
R2 ()
104 Q1 (−1 s n )
n1
104 W(−1 )
10−3 R()
104 C (F)
RF
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
9.99 10.14 6.70 9.82 8.00 8.08
1.95 1.74 1.85 2.63 1.91 2.34
365.7 16.83 78.99 594.5 215.4 174.5
5.84 9.52 11.71 10.88 21.89 7.07
0.74 0.77 0.73 0.70 0.68 0.72
3.23 4.29 1.51 2.02 2.85 1.17
58.4 56.9 107.9 44.0 – –
2.52 1.08 1.80 2.36 3.89 2.98
∼4 ∼2 ∼3 ∼4 ∼6 ∼5
Table 4 Electrode kinetic parameters for oxygen evolution on the Pt/CoFe2−x Cr x O4 (0 x 1.0) electrodes in 1 M KOH at 25 ◦ C Electrodes
Oxide loading (mg cm−2 )
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
3.2 ± 0.4 3.3 ± 0 4.4 ± 0.2 3.7 ± 0.4 4.2 ± 0.3 3.4 ± 0.1
Tafel slope (mV decade−1 )(b)
49 52 51 52 50 51
Order (p)
– 1.1 1.1 0.8 0.9 –
0.82 0.78
E/V
0.74 0.70
a b
0.66
c d
e
0.62 0.58 0.54 -4.0
-3.5
-3.0
-2.0 -1.5 -2.5 log ( j / A cm-1)
-1.0
-0.5
Fig. 6. j R-free Tafel plots for Pt/CoFe2−x Cr x O4 (0 x 1.0) oxide electrodes in 1 M KOH at 25 ◦ C (scan rate = 0.2 mV s−1 ); (a) x = 0, (b) x = 0.4, (c) x = 0.6, (d) x = 0.8, (e) x = 1.0.
in 1 M KOH at 25 ◦ C, the different oxide catalysts can be placed in the following catalytic order: CoFeCrO4 > CoFe1.2 Cr 0.8 O4 > CoFe1.4 Cr 0.6 O4 ∼ CoFe1.6 Cr 0.4 O4 > CoFe1.8 Cr 0.2 O4 > CoFe2 O4 It is noteworthy that, the apparent as well as the true catalytic activities of films of CoFe2−x Cr x O4 on Pt are
j (mA cm−2 ) at E(mV)
E(mV) at j (mA cm−2 ) 10
100
650 ja
jt
700 ja
jt
665 ± 4 671 ± 3 650 ± 6 643 ± 2 619 ± 1 615 ± 5
747 ± 5 753 ± 1 717 ± 5 711 ± 5 680 ± 1 675 ± 5
5±1 4±1 11 ± 3 15 ± 1 37 ± 2 45 ± 9
1.3 2.1 3.6 3.7 6.1 9.0
38 ± 5 31 ± 4 68 ± 13 91 ± 10 172 ± 3 200 ± 23
9.4 15.6 22.6 22.9 28.7 40.1
significantly lower than ones obtained on Ni under similar conditions. For instance, at E = 0.65 V in 1 M KOH at 25 ◦ C, the catalytic films of CoFeCrO4 obtained on Pt and on Ni [20] produced the true current densities (jt ) ∼ 9 mA cm−2 (ja ∼ 45 mA cm−2 , Rf ∼ 5) and ∼ 49 mA cm−2 (ja ∼ 195 mA cm−2 , Rf ∼ 4), respectively. Results indicate that the electrocatalytic activity of the oxide layer is strongly influenced by the nature of the substrate. It is thought that Ni2+ ions, produced at the Ni/oxide interface due to oxidation of the nickel support [11,12] during electrolysis, somehow influence the activity of the catalytic films. Further, there seems no controversy with regard to the order of the OER, however, the observed b values, particularly at low overpotentials are found to be low on films on Ni (b ∼ 2 × 2.303 RT/3F) [20] compared to those on Pt (b ∼ 2.303 RT/F). The observed b and p values were in fair agreement with those recently reported for similar films on Pt and ITO/ glass (b = 53–55 mV decade−1 , p = 1.1–1.2) [7], Co3 O4 films on Ni [32] and Ti, Co, Nb and Ta [33] prepared by different methods. Values of p and b obtained in the present study could be explained by considering a mechanism similar to that the peroxide path [34]. This mechanism is considered to involve the discharge of a molecule of OH− with the formation of the adsorbed OH intermediate as a fast step and subsequent electrochemical transformation, in presence of OH− , into a physisorbed oxygen atom as the rate determining step. However, the possibility of the other mechanisms
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
similar to that already given by Krstajic and Trasatti [35] could not be ruled out. [10]
4. Conclusion
[11]
The study shows that the partial substitution of Cr for Fe in the CoFe2 O4 matrix greatly enhances electrocatalytic activity of the oxide. The true catalytic activity of the highest active electrode, CoFeCrO4 , at E = 650 mV in 1 M KOH (25 ◦ C) was ∼ seven times higher than that of the base oxide. Cr-substitutions improved mainly the electronic properties (resistivity and magnetic) of the base oxide. The saturation magnetization, coercivity and remnant field decreased with increasing Cr 3+ substitutions in the oxide lattice.
[12] [13] [14] [15] [16] [17] [18]
Acknowledgments Authors thank the Council of Scientific and Industrial Research (CSIR), government of India, New Delhi and Dr. Vinay K. Singh (RA), IIT Mumbai, for financial support (Project no. P-25/225) and carrying out electronic and EPR studies, respectively.
[19] [20] [21] [22] [23]
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707
Iwakura C, Nishioka M, Tamura H, Kashi Nippon Kagaku Denki 1981;49:355. Orehotsky J, Huang H, Davidson CR, Srinivasan S. J Electroanal Chem 1979;95:233. Singh NK, Tiwari SK, Anitha KL, Singh RN. J Chem Soc Faraday Trans 1996;92(13):2397. Singh NK, Singh RN. Ind J Chem 1999;38A:491. Singh JP, Singh NK, Singh RN. Int J Hydrogen Energy 1999;24:433. Lal B, Singh NK, Samuel S, Singh RN. J New Mat Electrochem System. 1999;2:59. Singh SP, Samuel S, Tiwari SK, Singh RN. Int J Hydrogen Energy 1996;21(3):171. El Baydi M, Poillerat G, Rehspringer JL, Gautier JL, Koenig JF, Chartier P. J Solid State Chem 1994;109:281. Tiwari SK, Samuel S, Singh RN, Poillerat G, Koening JF, Chartier P. Int J Hydrogen Energy 1995;20:9. Singh JP, Singh RN. J New Mat Electrochem System 2000;2:137. Singh RN, Pandey JP, Singh NK, Lal B, Chartier P, Koenig JF. Electrochim Acta 2000;45:1911. Singh RN, Singh NK, Singh JP. Electrochim Acta 2002;47:3873. Singh RN, Tiwari SK, Singh SP, Singh NK, Poillerat G, Chartier P. J. Chem Soc Faraday Trans 1996;92(14):2593. Tiwari SK, Chartier P, Singh RN. J Electrochem Soc 1995;142:148. Boukamp BA. Solid Sate Ionics 1986;20:31; Boukamp BA. In: Copyright Boukamp BA. Equivalent circuit; user’s manual, University of Twente, Netherlands, 1988/89. p. 43. Gajbhiye NS. Metals Mater Proc 1998;10:247. Gajbhiye NS, Balaji G, Ghafari M. Phys Status Solidi A 2002;189:357. Gajbhiye NS, Prasad Seema, Balaji G. IEEE Trans MAG1999;35:2155. Chen JP, Sorensen CM, Klabunde KJ, Hadjipanayis GC, Devlin E, Kostikas A. Phys Rev B 1996;54:9288. Cullity BD. Introduction to Magnetic Materials. MA: AddisonWesley; 1972. Levine S, Smith AL. Discuss Faraday Soc 1971;52:290. Bockris JO’M, Otagawa T. J Electrochem Soc 1984;131:290. Robinson RA, Stokes RH. Electrolyte solution, 2nd ed. London: Butterworths; 1959. p. 494. Singh NK, Singh JP, Singh RN. Int J Hydrogen Energy 2002;27:895. Iwakura C, Honji A, Tanura H. Electrochim Acta 1981;26: 1319. Bockris JO’M, Otagawa T. J Phys Chem 1983;87:2960. Krstajic N, Trasatti S. Electrochem Soc Proc 1997;95–26:155.
Effect of partial substitution of Cr on electrocatalytic properties of CoFe2 O4 towards O2 -evolution in alkaline medium夡 R.N. Singha,∗ , N.K. Singha , J.P. Singha , G. Balajib , N.S. Gajbhiyeb a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, India b Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India
Available online 22 August 2005
Abstract Some nanocrystalline ternary ferrites (∼30–40 nm) having molecular formula, CoFe2−x Cr x O4 (0 x 1.0) and face centered cubic structure were investigated by electronic, EPR, magnetization, impedance and Tafel polarization techniques. Results indicate that the combination of antiferromagnetic superexchange interactions of the spinel lattice determines the Curie temperature (Tc ) value. The AB interaction dominates over the sublattice interactions, and Fe3+ occupies A sites, and Cr 3+ and Co2+ ions occupy octahedral sites because of large crystal field stabilization energy. The saturation magnetization (s ) coercivity (Hc ), remnant field (Hr ) and electrical resistivity are observed to decrease with the progressive replacement of Fe3+ ions (d5 , five unpaired electrons) by Cr 3+ ions (d3 , three unpaired electrons). However, the electrocatalytic activity of the oxides towards the oxygen evolution reaction (OER) in 1 M KOH at 25 ◦ C increases with the increase in x; the optimum improvement in the apparent electrocatalytic activity being with 1.0 mol Cr. At low overpotentials, the OER on substituted compounds displayed a Tafel slope of b = 50 ± 5 mV decade−1 and the reaction order with respect to OH− concentration as unity, regardless of the composition of the oxide catalyst. It seems that the electrocatalytic activity and saturation magnetization/Curie temperature for the oxide are inversely related. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Spinel ferrite; Electrocatalysis; Oxygen evolution; Cation distribution; Saturated magnetization
1. Introduction Fe3 O4 and its substituted products with the spinel structure of the type, AB2 O4 , where A and B are transition metals, possess relatively high overpotential [1] compared to cobaltites [2–8], for the oxygen evolution reaction (OER) in alkaline solution, and are scantly investigated for their use as oxygen anodes [2,9,10]. However, recent studies have shown 夡 Presented at the Fourth International Symposium on Electrocatalysis—“From Theory to Industrial Applications” held in Como, Italy, 22–25 September, 2002. ∗ Corresponding author. Fax: +91 542 2369951. E-mail addresses: [email protected] (R.N. Singh), [email protected] (N.S. Gajbhiye).
[11–13] that the oxygen overpotential on ferrospinels can be reduced considerably by using low temperature preparation methods and substituting suitable metal ions for Fe in the Fe3 O4 -matrix partially. For instance, at 100 mA cm−2 in 1 M KOH at 25 ◦ C, Fe3 O4 , MnFe2 O4 , NiFe2 O4 and CoFe2 O4 recently obtained by a hydroxide precipitation method at controlled pH 11, produced the oxygen overpotentials 524, 338, 379 and 395 mV, respectively. These values of overpotentials are much lower than those obtained for similar anodes prepared by ceramic methods [9,10], but are comparable to those recently reported for Co3 O4 [14–16] and NiCo2 O4 [17–19]. Further, the overpotential data indicate that the effect of a d-electron deficient transition metal (i.e. Mn, 3d5 ) substitution in Fe3 O4 is more pronounced compared to that of a d-electron rich transition metal
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.07.003
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
(Co, 3d7 or Ni, 3d8 ) substitution. Very recently, Singh et al. [20] observed that the replacement of Fe3+ by Cr 3+ in the CoFe2 O4 matrix reduced the O2 overpotential significantly. For instance, at j = 100 mA cm−2 in 1 M KOH at 25 ◦ C, CoFe2 O4 and CoCr0.2 Fe1.8 O4 produced the O2 overpotentials, 0.395 and 0.312 V, respectively. However, the observed beneficial effect of Cr is not well understood. Also, the Ni substrate is not well protected from the electrolyte contact by the oxide overlayer and that the film electrodes in 1 M KOH indicated the formation of Ni (III)–Ni (II) redox couple in cyclic voltammetry (CV) [11]. In order to understand the role of Cr 3+ in oxygen evolution electrocatalysis visà-vis to correlate electrocatalytic and electronic properties, Cr-substituted products obtained in [20] were further investigated by electronic, EPR, conductivity and magnetization measurements. Also, to avoid the possible interference of Ni2+ in the study of O2 evolution (OE), electrochemical investigations were carried out on films of oxides obtained on Pt supports. Details of results, so obtained, are described in the present paper.
-2.4
-2.9
log (σ / Ω-1 cm-1)
702
-3.4
-3.9
-4.4
-4.9
-5.4 1.5
2.0
1 × 10 / T -3
2. Experimental Cr-substituted cobalt ferrites used in the present investigation were the same as reported in [20]. They were prepared at 70 ◦ C by adding 5 M NaOH in dropwise manner into an aqueous solution of salts of constituent metals of the oxide, dissolved in stoichiometric ratio, under stirred condition and continued oxygen bubbling; details of which are already given elsewhere [20]. For conductivity measurements, the oxide powders (sintered at 400 ◦ C) were pressed into pellets using 4–5 tons pressure by the pressure pump. No binder was used in making the pellet. To ensure good contact and uniform distribution of current, both the surfaces of the pellet were painted with silver conductive paste. The electrical conductivity was determined at different temperatures by recording impedance spectra at a DC potential of 50 mV and f = 50 kHz. Magnetization measurements were carried out using a vibrating sample magnetometer, Par-Model 150A, in conjugation with a Varian electromagnet assembly model (V7200) that provides a magnetic field up to 11 kG. The temperature variation of magnetization was studied using a furnace assembly model 151 associated with the magnetometer and it can provide a temperature up to 1000 K. The oxide powders were transformed in the form of film on a pretreated platinum support by an oxide-slurry painting method as described elsewhere [21,22]. The oxide films were heat treated in an electrical furnace at 380 ◦ C for 1 21 h and they were removed only when the furnace attained the temperature of < 80 ◦ C. The pretreatment of the support and electrical contact with the oxide film were made as described previously [21,22]. The oxide loadings were 3–5 mg cm−2 . A conventional three-electrode single-compartment Pyrex glass cell involving pure Pt-foil (∼ 8 cm2 ) and Hg/HgO/1 M
3.0
2.5
3.5
k-1
Fig. 1. Plot of log vs 1/T for CoFe2−x Cr x O4 (temperature range: 298–593 K); ◦–◦: CoFe2 O4 , –: CoFe1.6 Cr0.4 O4 , –: CoFe1.2 Cr0.8 O4 .
KOH (E 0 = 0.098 V vs SHE), respectively, as reference and auxiliary electrodes was employed for electrochemical investigations. All potential values given in the text correspond to this reference only. Impedance and Tafel polarization studies were carried out using an electrochemical impedance system (Model 273 A, EG & G, PARC, USA) provided with a lock-in-amplifier (Model 5208), a potentio-galvanostat (Model 273A) and a PS/2 (Model 35 SX) IBM computer. The softwares used in measurements of impedance and Tafel polarizations were M 388 and M 352 Corrosion analysis, respectively. As mentioned earlier [20], the electochemical impedance study of the oxide film electrodes in 1 M KOH has been carried out with the AC voltage amplitude of 5 mV in the frequency range of 0.02–105 Hz at two DC potentials, 0 and 50 mV. The double layer capacitance (Cdl ) at each DC potential was analyzed by using the “Equivalent Circuit” program written by Boukamp [23]. Procedure followed in the study of Tafel polarization was previously described in [15,22]. 3. Results and discussion 3.1. Conductivity, UV-visible, EPR and magnetic studies The bulk conductivity () of compounds was determined in the temperature range 298–593 K. Results show that the room temperature conductivity increases with increase in x. The Arrhenius plot (log vs 1/T ) shown in Fig. 1 indicates
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
703
Table 1 Activation energy for conduction
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
104 (mho m−1 ) at 298 K
5.5 5.6 8.1 18.8 34.2 26.7
two linear regions, one between temperatures 298 and 413 K (I) and the other between temperatures 413 and 593 K (II). The activation energies for conduction (Table 1) were ranged between 0.029 and 0.188 eV and 0.44 and 0.61 eV in the low and higher temperature regions, respectively. The observed linearity in log vs 1/T curves at higher temperatures may be due to the role of valence electrons. However, the lower temperature linear region might be caused due to the presence of minor donor impurities in the oxides. The increase in conductivity of the base oxide with increase in x may be explained on the basis of symmetry of orbitals occupied by electrons. Interaction between Fe3+ (d5 ) in A and B sites 3 –t 3 and e2 –e2 configurations. When are larger due to t2g g g 2g 3 –t 3 interaction Fe3+ in B sites are replaced by Cr 3+ , t2g 2g remains but interaction of eg orbitals disappears and con-
sequently, the number of unpaired electrons increases and hence the conductivity of the oxide increases. The UV-visible spectra of oxides excepting CoFe1.2 Cr0.8 O4 showed a broad peak around 500 nm. This peak may be due to presence of Co2+ /and or Fe3+ at the octahedral sites. Contrary to this observation, CoFe1.2 Cr0.8 O4 did not show any peak around 500 nm, however, it exhibited an intense peak around 300 nm which, thereby, indicates the strong charge transfer from O to Fe3+ or O to Cr 3+ . The similar intense peak was also indicated by CoFeCrO4 . The observed single broad band in the EPR spectra of catalysts in the low field region indicates the interaction mainly between Fe3+ centers. Co2+ and Fe2+ appear to be EPR silent as they do not show their respective signals. Further, estimates of the g-values for pure and Cr-substituted CoFe2 O4 indicate greater ferromagnetic interaction between Fe3+ centers in the absence of Cr 3+ . The g-values for oxides with x = 0, x = 0.4, x = 0.8 and x = 1.0 in low field (maximum) and high field (minimum) were 10.3 and 2.27, 7.94 ± 0.05 and 1.7 ± 0.01, 8.38 and 1.80 and 5.48 ± 0.11 and 1.82 ± 0.04, respectively. In general, the intrinsic magnetic properties of spinel ferrite having particle size in the nanometer range are influenced by surface, particle size and cation distribution [24]. The cation distribution in magnetic nanoparticles is com-
Activation energy of conductivity (eV) First region (298–413 K)
Second region (413–593 K)
0.029 0.188 0.11 0.12 0.13 0.12
0.61 0.58 0.44 0.47 0.45 0.59
20 Cr 0.0 0.2 0.4 0.6 0.8 1.0
16 σ (emu/g)
Oxides
12 8 4 0 300
400
500
600
700
800
T (K) Fig. 2. Temperature dependence of magnetization curves for CoFe2−x Cr x O4 particles.
pletely different from that observed in bulk. It is well known that the magnetic properties of spinel ferrite particles are influenced by constituent metal ions and the cation distribution in the spinel lattice depending on the synthetic methods employed [25]. In the present work, the aim is to study the effect and consequence of progressive substitution of Fe3+ by Cr 3+ ions in nanosize CoFe2 O4 particles. Fig. 2 shows the temperature dependence of magnetization curves for Cr 3+ substituted CoFe2 O4 particles. As the temperature increases, the magnetization value decreases due to the thermal fluctuation phenomenon. Further, at higher temperatures, the magnetization decreases steadily and falls to zero at the Curie temperature (Tc ) signifying a ferrimagnetic to paramagnetic transition. The Tc values, saturation magnetization (s ), coercivity (Hc ) values and remnant field values (Hr ) are tabulated in Table 2. It is worth noticing that the Tc value of CoFe2 O4 particles is greater than that of the bulk value (793 K) [26]. This may be due to particle size effect. Also, the magnetization
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R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
Table 2 Magnetic parameters for CoFe2−x Cr x O4 (0.0 x 1) Oxides
Curie temperature, Tc (K)
s (emu/g)
Saturation magnetization,
Remnant magnetization, Mr (emu/g)
Coercive field, Hc (T)
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
798 783 778 758 773 753
49.1 54.3 42.1 36.1 27.1 17.09
16.29 20.62 15.65 10.94 8.71 5.04
0.1013 0.0909 0.0713 0.0583 0.0825 0.0650
60 40 20 σ (emu/g)
decreases with Cr 3+ substitution. The Tc values decrease from 798 to 753 K with increasing concentration of Cr 3+ varying from 0.2 to 1.0 mol at Fe3+ sites. This trend is due to the decrease of total exchange energy in the spinel lattice because of Cr 3+ substitution. The magnetic structure of CoFe2 O4 material is ferromagnetic and can be envisaged by two magnetic sublattices A and B, which are coupled antiparallel and occupy tetrahedral and octahedral sites, respectively. Then, it is possible to have a combination of antiferromagnetic superexchange interactions (namely, AA, BB, AB) and the total net magnitude of superexchange energy of the spinel lattice determines its Tc value. It is known that AB interaction dominates over the sublattice interactions and the Fe3+ occupy A sites (inverse spinel). But both Cr 3+ (d3 ) and Co2+ (d7 ) are with three unpaired electrons, which must have large crystal field stabilization energy (CFSE) for octahedral occupancy. Thus, in our system, we predict that Cr 3+ ions possibly occupy either A and/or B sites. On progressive Cr 3+ substitution, the stronger Fe(A)–O–Fe(B) and Co(A/B)–O–Fe(B/A) interactions are replaced by weaker Co(A/B)–O–Cr(B/A), Fe(A/B)–O–Cr(B/A) and Cr(A)–O–Cr(B) interactions and hence Tc value is decreased. Tc value of CoCr2 O4 is very much less than that of CoFe2 O4 [27,28]. Fig. 3 shows the field dependence of magnetization for all the samples. It is clear that saturation magnetization (s ) decreases with increasing Cr 3+ substitution. The magnetic moment of ions in sublattice B orient parallel to the applied magnetic field and antiparallel to the sublattice A. Hence, the total magnetization of the spinel lattice is dependent on the moment of ions occupying B site (assuming these oxides are insulators). In the present study, Fe3+ (d5 ion, five unpaired electrons) is replaced by Cr 3+ (d3 ion, three unpaired electrons). Thus, when Fe3+ is substituted more, the magnetization value decreases progressively. The coercivity (Hc ) and remnant field (Hr ) are inversely dependent to s [28]. Hence, it may be expected that Hc and Hr should increase on increasing Cr 3+ substitution. But the experimental findings in the present study do not corroborate this prediction. However, both Hc and Hr are found to decrease and this observed trend is related to the dominant magnetocrystalline anisotropy (K) factor for these samples, which is being explored in further study.
0 Cr 0.0 0.2 0.4 0.6 0.8 1.0
-20 -40 -60 -1.0
-0.5
0.0 H (T)
0.5
1.0
Fig. 3. Field dependence of magnetization for CoFe2−x Cr x O4 particles.
3.2. Impedance spectra/roughness factor The feature of electrochemical impedance (EI) spectra for the CoFe2−x Cr x O4 -films in 1 M KOH was almost similar. A typical complex impedance plot for Pt/CoFe1.2 Cr 0.8 O4 in 1 M KOH (25 ◦ C) is shown in Fig. 4. In order to observe the nature of complex plot particularly at high frequencies, the high frequency part of Fig. 4 has been expanded and reproduced in Fig. 4(a). Fig. 4(a) shows a small and ill-defined semicircle at high frequencies and that it may be produced due to the contribution of interior grains of the oxide mass, but the complex feature of the EI spectrum in the latter frequency region can be ascribed to the charging and discharging of the double layer formed at the oxide/1 M KOH interface and also to the diffusion of the charge species (H+ /OH− ) into the oxide films through defects and pores [11]. As reported earlier [20], the two equivalent circuits, LR(RQ)(R(W(C))) and LR(RQ)(CW), were used to analyze the circuit parameters in the present study also. Symbols, L, R, R, R, Q, W and C, used to describe the circuit description code (CDC) were inductance (H ), solution resistance (), oxide film
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
of the oxide roughness (RF ) were estimated from the ratio of the observed Cdl of the oxide catalyst and the Cdl of a smooth oxide surface (60 F cm−2 ) [22,29,30]. The replacement of Fe by Cr (0.2–1.0 mol) in the CoFe2 O4 -matrix did not indicate any significant influence on the oxide roughness. Almost similar results were also obtained when these catalytic films were studied on a Ni-support [20].
0.80
0.02 Hz
1.5
0.40
3.3. Electrocatalytic activity
1 kHz
(a) -z img / Ω cm2
-z img x 104/Ω cm2
0.60
0.06 Hz
1.0 2.51 kHz 0.5 2.51 kHz
0.20
0.0
79.4 kHz 100 kHz
0.26 Hz 1.5 1.18 Hz
2.0 2.5 zreal / Ω cm2
3.0
1000 kHz
0.00
0.20
0.40 0.60 z real x 104/Ω cm2
Fig. 4. Complex impedance plot for (l = 4.4 mg cm−2 ) in 1 M KOH (25 ◦ C).
0.80
Pt/CoFe1.2 Cr0.8 O4
90
l Zl / Ω cm2
60 1k 30 100 0
Phase angle / deg.
Measurement Simulation
10 k
10 -30 1 100 m 1
705
100 10 1k Frequency / Hz
10 k 100k
Fig. 5. Bode plot for Pt/CoFe1.2 Cr0.8 O4 (l = 4.4 mg cm−2 ) in 1 M KOH (25 ◦ C).
resistance (), charge transfer resistance (), constant phase element (−1 s n ), Warburg impedance (−1 ) and capacitance i.e. Cdl (F), respectively. As the potentiostat introduces some inductance at high frequencies [23], the component L has been used to obtain the best fit of the experimental data. Based on proposed circuit models, EI spectra obtained agreed reasonably well with the experimental curves. A set of typical Bode plot (simulated and experimental) for (CoFe1.2 Cr0.8 O4 is shown in Fig. 5, and estimates of the circuit parameters are shown in Table 3. The relative values
The jR-corrected anodic Tafel polarization curves (E vs log j ) recorded at a scan rate of 0.2 mV s−1 for cobalt ferrite films on Pt in 1 M KOH at 25 ◦ C are shown in Fig. 6. The feature of the E–log j curves was almost similar for any x. They seemed to have two Tafel slopes, one at low and the other at higher potential. The first Tafel slope (b) was approximately the same (50 ± 5 mV decade−1 ) with each oxide. The second Tafel slope was found to be ∼ 104 and ∼ 116 mV decade−1 in the case of oxide with x = 0 and x = 0.2, respectively; however, it was not very clear in case of other oxides. The order for the OER with respect to OH− concentration was found to be approximately unity with each electrocatalyst. To determine the order, the anodic E vs log j curves were recorded on four representative oxide films with x = 0.2, x = 0.4, x = 0.6 and x = 0.8 at varying KOH concentrations. Concentrations of KOH used in this particular investigation were 1.01, 2.05, 3.11, and 4.22 m. As concentrations of the electrolyte used in the study were not low, the ionic strength of the medium was not maintained constant. With the aid of the anodic polarization curves determined at different KOH concentrations, the linear log j vs log aOH− curves at constant potentials were constructed in the first Tafel region and the reaction order was estimated by measuring the slope of these curves. The activity coefficient (f ) corresponding to each molal concentration of KOH was estimated by constructing a calibration curve, f vs [KOH]/m from the data of activity coefficients of the electrolyte at 25 ◦ C given as Appendix 8.10 in [31]. Values of the electrode kinetic parameters (b and p) (Table 4) suggest that the OER on pure and Cr-substituted cobalt ferrite follow almost similar mechanistic paths. The apparent (ja ) and the true (jt = ja /RF ) electrocatalytic activities of oxides determined at two potentials (0.65 and 0.70 V) from Fig. 6 (Table 4) indicate that Cr substitution increases both the apparent as well as the true electrocatalytic activity of the base oxide significantly. Further, results demonstrate that Cr-substitutions mainly improve electronic properties of the material in favor of the electro-formation of O2 , while geometrical properties seem to be little influenced with Cr. Results of conductivity, electronic, EPR, and magnetic studies also indicate modification of electronic properties of the oxide with Cr-substitution. Thus, the observed increase in electrocatalytic activity of the base oxide in presence of Cr can be attributed to the decrease in the electrical resistivity vis-a-vis the saturation magnetization/Curie temperature of the oxide. Based on values of jt at E = 0.7 V
706
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
Table 3 Values of the equivalent circuit parameters for oxide electrodes in 1 M KOH at 25 ◦ C Electrodes
107 L (H)
R1 ()
R2 ()
104 Q1 (−1 s n )
n1
104 W(−1 )
10−3 R()
104 C (F)
RF
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
9.99 10.14 6.70 9.82 8.00 8.08
1.95 1.74 1.85 2.63 1.91 2.34
365.7 16.83 78.99 594.5 215.4 174.5
5.84 9.52 11.71 10.88 21.89 7.07
0.74 0.77 0.73 0.70 0.68 0.72
3.23 4.29 1.51 2.02 2.85 1.17
58.4 56.9 107.9 44.0 – –
2.52 1.08 1.80 2.36 3.89 2.98
∼4 ∼2 ∼3 ∼4 ∼6 ∼5
Table 4 Electrode kinetic parameters for oxygen evolution on the Pt/CoFe2−x Cr x O4 (0 x 1.0) electrodes in 1 M KOH at 25 ◦ C Electrodes
Oxide loading (mg cm−2 )
CoFe2 O4 CoFe1.8 Cr0.2 O4 CoFe1.6 Cr0.4 O4 CoFe1.4 Cr0.6 O4 CoFe1.2 Cr0.8 O4 CoFeCrO4
3.2 ± 0.4 3.3 ± 0 4.4 ± 0.2 3.7 ± 0.4 4.2 ± 0.3 3.4 ± 0.1
Tafel slope (mV decade−1 )(b)
49 52 51 52 50 51
Order (p)
– 1.1 1.1 0.8 0.9 –
0.82 0.78
E/V
0.74 0.70
a b
0.66
c d
e
0.62 0.58 0.54 -4.0
-3.5
-3.0
-2.0 -1.5 -2.5 log ( j / A cm-1)
-1.0
-0.5
Fig. 6. j R-free Tafel plots for Pt/CoFe2−x Cr x O4 (0 x 1.0) oxide electrodes in 1 M KOH at 25 ◦ C (scan rate = 0.2 mV s−1 ); (a) x = 0, (b) x = 0.4, (c) x = 0.6, (d) x = 0.8, (e) x = 1.0.
in 1 M KOH at 25 ◦ C, the different oxide catalysts can be placed in the following catalytic order: CoFeCrO4 > CoFe1.2 Cr 0.8 O4 > CoFe1.4 Cr 0.6 O4 ∼ CoFe1.6 Cr 0.4 O4 > CoFe1.8 Cr 0.2 O4 > CoFe2 O4 It is noteworthy that, the apparent as well as the true catalytic activities of films of CoFe2−x Cr x O4 on Pt are
j (mA cm−2 ) at E(mV)
E(mV) at j (mA cm−2 ) 10
100
650 ja
jt
700 ja
jt
665 ± 4 671 ± 3 650 ± 6 643 ± 2 619 ± 1 615 ± 5
747 ± 5 753 ± 1 717 ± 5 711 ± 5 680 ± 1 675 ± 5
5±1 4±1 11 ± 3 15 ± 1 37 ± 2 45 ± 9
1.3 2.1 3.6 3.7 6.1 9.0
38 ± 5 31 ± 4 68 ± 13 91 ± 10 172 ± 3 200 ± 23
9.4 15.6 22.6 22.9 28.7 40.1
significantly lower than ones obtained on Ni under similar conditions. For instance, at E = 0.65 V in 1 M KOH at 25 ◦ C, the catalytic films of CoFeCrO4 obtained on Pt and on Ni [20] produced the true current densities (jt ) ∼ 9 mA cm−2 (ja ∼ 45 mA cm−2 , Rf ∼ 5) and ∼ 49 mA cm−2 (ja ∼ 195 mA cm−2 , Rf ∼ 4), respectively. Results indicate that the electrocatalytic activity of the oxide layer is strongly influenced by the nature of the substrate. It is thought that Ni2+ ions, produced at the Ni/oxide interface due to oxidation of the nickel support [11,12] during electrolysis, somehow influence the activity of the catalytic films. Further, there seems no controversy with regard to the order of the OER, however, the observed b values, particularly at low overpotentials are found to be low on films on Ni (b ∼ 2 × 2.303 RT/3F) [20] compared to those on Pt (b ∼ 2.303 RT/F). The observed b and p values were in fair agreement with those recently reported for similar films on Pt and ITO/ glass (b = 53–55 mV decade−1 , p = 1.1–1.2) [7], Co3 O4 films on Ni [32] and Ti, Co, Nb and Ta [33] prepared by different methods. Values of p and b obtained in the present study could be explained by considering a mechanism similar to that the peroxide path [34]. This mechanism is considered to involve the discharge of a molecule of OH− with the formation of the adsorbed OH intermediate as a fast step and subsequent electrochemical transformation, in presence of OH− , into a physisorbed oxygen atom as the rate determining step. However, the possibility of the other mechanisms
R.N. Singh et al. / International Journal of Hydrogen Energy 31 (2006) 701 – 707
similar to that already given by Krstajic and Trasatti [35] could not be ruled out. [10]
4. Conclusion
[11]
The study shows that the partial substitution of Cr for Fe in the CoFe2 O4 matrix greatly enhances electrocatalytic activity of the oxide. The true catalytic activity of the highest active electrode, CoFeCrO4 , at E = 650 mV in 1 M KOH (25 ◦ C) was ∼ seven times higher than that of the base oxide. Cr-substitutions improved mainly the electronic properties (resistivity and magnetic) of the base oxide. The saturation magnetization, coercivity and remnant field decreased with increasing Cr 3+ substitutions in the oxide lattice.
[12] [13] [14] [15] [16] [17] [18]
Acknowledgments Authors thank the Council of Scientific and Industrial Research (CSIR), government of India, New Delhi and Dr. Vinay K. Singh (RA), IIT Mumbai, for financial support (Project no. P-25/225) and carrying out electronic and EPR studies, respectively.
[19] [20] [21] [22] [23]
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