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Copper–iron–molybdenum mixed oxides as efficient oxygen evolution electrocatalysts Venkata Kali Vara Prasad Srirapu,a Chandra Shekhar Sharma,a Rahul Awasthi,a Ravindra Nath Singh*a and Akhoury Sudhir Kumar Sinhab Ternary Cu, Fe and Mo mixed oxides having a nominal compositional formula, CuxFe2 x(MoO4)3 (0 r x r 1.5), have been prepared by a co-precipitation method at pH E 2 and characterized by FT-IR, XRD, XPS, TEM and anodic polarization techniques for use as electrocatalysts for the oxygen evolution reaction (OER) in alkaline solutions. The crystallites of oxides with x r 1 have the monoclinic crystal structure. The OER study shows that replacement of Fe in the Fe2(MoO4)3 matrix by 0.25–1.0 mol Cu increases the apparent electrocatalytic activity. However, 1.5 mol Cu-addition is detrimental to the OER
Received 26th December 2013, Accepted 23rd February 2014 DOI: 10.1039/c3cp55453h www.rsc.org/pccp
activity. At E = 1.51 V (vs. RHE) in 1 M KOH, the catalytic activity of the oxide with x = 1 was approximately 50 times the activity of the base oxide (i.e. Fe2(MoO4)3). The Tafel slope of oxides with 0.25 r x r 1.5 ranged between 31 and 37 mV. The reaction order of OH concentration was nearly unity for oxides with x = 0.25 and 1.5 and it was B2 for oxides with x = 0.5, 0.75, and 1.0. Suitable reaction mechanisms consistent with the electrode kinetic parameters have also been proposed.
Introduction Water electrolysis is the simplest process to store electrical energy (surplus) in the form of non-polluting hydrogen fuel. It is, therefore, considered as a potential route to produce clean energy sources, hydrogen and oxygen. However, the oxygen evolution reaction (OER) in aqueous solutions involves high overpotentials. To minimize the latter various electrode materials such as metals and their pure as well as mixed oxides were extensively investigated and comprehensively reviewed.1–4 Among these catalysts, complex oxides with spinellic (mainly Co-based5–17) and perovskite (mainly LaNiO318–20 and LaCoO321–28) structures are considered as promising materials and have been investigated much for electrocatalysis of the OER in alkaline solutions. Recently, we have investigated a new mixed oxide system having molecular formulae, MMoO4 (where M = Mn, Fe, Co, and Ni)29–32 and M2(MoO4)3 (M = Fe and Cr).33,34 This novel oxide system is quite stable and efficiently catalyses the electroevolution of oxygen in alkaline solutions. It is observed that partial replacement of Co in CoMoO4,35 Ni in NiMoO4,36 Cr in Cr2(MoO4)3,34 and Mn in MnMoO432 by Fe increases the electrocatalytic activity of materials greatly. The OER performances of these novel ternary oxide catalysts were superior to several active
a
Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi-221005, India. E-mail: rnsbhu@rediffmail.com, [email protected]; Fax: +91-542-2368127; Tel: +91-542-6701596 b Department of Chemical Engineering, Indian Institute of Technology, Banaras Hindu University, India
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Co-based spinel oxides recently reported in the literature.6,16,37,38 These results encouraged us further to introduce Cu in place of Fe in the Fe2(MoO4)3 matrix and examine its influence on the electrocatalytic activity with the aim of developing highly efficient OER electrocatalysts. In fact, the study demonstrated that a 1.0 mol Cu-substituted oxide catalyst surpasses other previously reported binary and ternary metal molybdate electrodes in the OER activity. Results of the investigation are fully detailed in this paper.
Experimental Preparation of mixed oxide electrodes Fe2(MoO4)3 and Cu-substituted Fe2(MoO4)3 having compositional formulae CuxFe2 x(MoO4)3 (x = 0, 0.25, 0.50, 0.75, 1.0 and 1.5) were synthesized following a co-precipitation method, details of which were previously described.33 Metal salts, Fe (NO3)39H2O (Merck), Cu(NO3)24H2O (Hinweise Z 98%) and (NH4)6Mo7O244H2O (Sarabhai, Batch no. 4E740296), were used as precursors. The precipitate was thoroughly washed with hot distilled water, dried overnight at 393 K, crushed, heat treated at 773 K for 5 h, removed from the furnace and again crushed and heat treated for 3 h at 773 K to obtain the desired catalyst. As mentioned earlier,34,36 the oxide powders were transformed into the form of catalytic films on pretreated Ni supports by oxide-slurry coatings. To ascertain the adherence, the oxide films were dried in air and then heat treated at 673 K for 1.5 h. Loading of the oxide mass was maintained at B2 mg cm 2.
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Material characterization The catalyst materials were structurally characterized by FT-IR (Varian FT-IR spectrophotometer, model 3100), X-ray diffraction (XRD), BET surface area analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The XRD powder pattern of the catalyst was recorded on an X-ray diffractometer (Rigaku DMAX III) using Cu-Ka as the radiation source (l = 1.542 Å). The BET surface area was determined using a surface area analyzer (Micrometrics, USA, ASAP 2020 Model). Morphology of the catalytic films has been studied by TEM (TECNAI G2 FEI). The surface electronic state and composition of materials were investigated using an AMICUS-X-ray photoelectron spectrometer. Electrochemical characterization A conventional three-electrode single compartment Pyrex glass cell, containing pure Pt-foil (B8 cm2) and Hg/HgO/1 M KOH, respectively, as counter and reference electrodes, was employed to perform cyclic voltammetry (CV) and anodic polarization experiments. All potentials measured with respect to the Hg/HgO electrode immersed in 1 M KOH were converted to a RHE scale by adding 0.910 V to the potential. The Hg/HgO/1 M KOH electrode (Ecalibrated = 0.082 V vs. SHE) was calibrated as described elsewhere.39 Instruments and software employed in the study were the same as already mentioned in ref. 35 and 36. The apparent electrocatalytic activities (based on the geometrical area) of molybdate electrodes were determined by recording IRcompensated ‘E vs. log j’ curves at 0.2 mV s 1 in the potential region from 1.41 to 1.61 V vs. RHE in 1 M KOH at 298 K. Prior to recording ‘E versus log j’ curves, each electrode was first cycled for five runs at 50 mV s 1 between 0.91 and 1.51 V vs. RHE in 1 M KOH at 298 K. The order of the OER (p) has been determined by recording anodic ‘E vs. log j’ curves for each catalyst at different KOH concentrations (i.e., 0.5, 1.0, 1.5 and 2.0 mol dm 3). The ionic strength (m) of the cell solution was maintained constant (m = 2.0) using KNO3 as a neutral salt. j versus COH data for each catalyst at constant potentials were then obtained and produced in the form of straight line plots, log j vs. log COH ; the order was determined by measuring the slope of these straight lines. The log–log plot for each catalyst was constructed at three constant potentials; values of the order mentioned in the text are average ones. Only the first linear region of E vs. log j plots recorded at four KOH concentrations was considered for the order determination.
Fig. 1 FT-IR spectra of CuxFe2 x(MoO4)3 [x = 0.0 (a), 0.25 (b), 0.50 (c), 0.75 (d), 1.0 (e) and 1.50 (f)], sintered at 648 K for 10 h.
Features of FT-IR spectra of Cu-substituted products containing 0.5–1.0 mol Cu, however, appear to be more or less similar and they exhibit a band at 598–607 cm 1 which is found to be absent in the case of the oxide with x = 0 and x = 0.25. The appearance of a narrow band at 995–997 and a relatively broad band at 598–607 cm 1 indicates the presence of MoO3 in the catalyst.33,40,41 XRD Fig. 2 depicts the XRD powder patterns of CuxFe2 x(MoO4)3 (where x = 0, 0.25, 0.50, 1.0 and 1.5) samples recorded between 2y = 0 and 2y = 801. It is observed that both 2y and corresponding d values of all the diffraction lines of the oxide with x = 0, as shown in Fig. 2, exactly match with the data for pure Fe2(MoO4)3 with the monoclinic crystal structure given in JCPDS Card No. 35-0183. Substitution of copper from 0.25 to
Results and discussion FT-IR Fig. 1 gathers the FT-IR spectra of Cu-substituted Fe2(MoO4)3 catalysts. The FT-IR spectra of Fe2(MoO4)3 show an intense broad band at B840 cm 1 and a weak band at B961 cm 1, which are characteristic bands for the tetrahedral species of Mo in Fe2(MoO4)3 and Fe–O–Mo bond vibrations, respectively.33,40 This broad band widens with 0.25 mol Cu introduction.
7386 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
Fig. 2 XRD powder patterns of CuxFe2 x(MoO4)3 [x = 0.0 (a), 0.25 (b), 0.50 (c), 1.0 (d) and 1.50 (e)], sintered at 673 K for 10 h.
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1.0 mol for Fe into the Fe2(MoO4)3 matrix does not seem to influence the diffraction patterns of the base oxide; only the diffraction peaks slightly shift towards the higher angle. However, the higher substitution of copper (i.e., 1.5 mol) produced the product with mixed phases: MoO3, Fe2(MoO4)3 and/or CuxFe2 x(MoO4)3. With the exception of the base oxide, the diffraction peaks at 3.248 and 6.935 Å for impurity phase MoO3 are found to be present in all oxides. Moreover, the absence of MoO3 peaks does not rule out the possibility of the presence of free MoO3 in the oxide, because MoO3 is reported to be detectable by XRD only when its content is higher than 7%; below this amount the material is amorphous or microcrystalline.40,41 IR spectra of all compounds shown in Fig. 1 indicate the presence of MoO3. However, of two characteristic MoO3 bands, a narrow one at B990 and a broad one at B624 cm 1, the oxide with x = 0 and x = 0.25 exhibited only the narrow characteristic band. Thus, introduction of 0.25–1.0 mol Cu forms a solid solution with the Fe2(MoO4)3 phase. Values of the crystallite size (S), as estimated by using the Scherrer relation and the most intense diffraction peak of each XRD spectrum, were B33, B38, B53, B62, and B83 nm for oxide with x = 0, x = 0.25, x = 0.5, x = 1.0 and x = 1.5, respectively. The observed increase in S with increasing addition of Cu (atomic size = 128 pm) for Fe (atomic size = 126 pm) can be ascribed to the larger size of Cu. XPS The surface electronic state and composition of materials were investigated by XPS analysis. Mo 3d spectra of CuxFe2 x(MoO4)3 (x = 0, 0.5, 1.0 and 1.5) catalysts shown in Fig. 3A show two photoelectron peaks, 3d5/2 and 3d3/2, at binding energies (BEs):
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231.5–232.5 and 234.9–235.9 eV, respectively. Values of the BEs indicate that the surface layer contains, in addition to Mo6+, molybdenum in lower oxidation states also.42–45 The Cu 2p XPS spectra of the substituted product shown in Fig. 3B display two strong symmetrical photopeaks, 2p3/2 and 2p1/2, respectively, at 935.1–936.2 and 954.4–955 eV, thereby showing the presence of Cu essentially in +2 states.46 Some low intensity peaks are also observed between these two strong peaks, but they are not very clear. Similarly, the Fe 2p XPS spectra of samples indicated two strong photopeaks at 711.4–712.3 and 723.1–725.5 eV (Fig. 3C) and thereby indicating the presence of iron in two oxidation states, +2 and +3, in the surface layer.47 Deconvolution of Mo 3d photopeaks exhibited two peaks for the Mo 3d5/2 photopeak at 230.7–231.7 and 231.6–232.8 eV (Table 1). A typical deconvoluted Mo 3d spectrum in the case of the oxide with x = 1 is shown in Fig. 3F. Results shown in Table 1 show that molybdenum is present in lower oxidation states, Mo4+ and Mo5+, on the Fe2(MoO4)3 surface, the Mo4+/Mo5+ molar ratio being approximately unity (i.e. Mo4+/Mo5+ = 1.04). Further, introduction of Cu into the oxide matrix decreases the Mo4+/Mo5+ molar ratio in the surface layer, the decrease being from 1.04 to 0.67 with 0.5 and from 1.04 to 0.54 with 1.0 mol Cu. However, a higher addition of Cu (i.e., 1.5 mol) transforms molybdenum from lower valance states (Mo4+ and Mo5+) to the higher valance states (i.e., Mo5+ and Mo6+) in the surface layer with the Mo5+/Mo6+ molar ratio equal to 1.14. Results of deconvolution of the Fe 2p spectra shown in Table 1 and Fig. 3 indicate the presence of two shake-up satellites at 715.2–716.0 and 718–720 eV between the main peaks Fe 2p3/2 and Fe 2p1/2. The former and the latter shake-up satellite peaks
Fig. 3 XPS spectra of Mo 3d (A), Cu 2p (B) and Fe 2p (C) from CuxFe2 x(MoO4)3 [x = 0.0, 0.50, 1.0 and 1.50]. Representative deconvoluted XPS spectra of Fe 2p [x = 0 (D) and x = 1.0 (E)] and Mo 3d [x = 1.0 (F)].
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Paper Table 1
PCCP Results of deconvolution of X-ray photoelectron spectra: (A) Fe 2p, (B) Cu 2p, (C) Mo 3d
(A) Spin–orbit doublet – I
Spin–orbit doublet – II
Composite
Fe 2p3/2
Fe 2p1/2
Fe 2p3/2
Fe 2p1/2
S-I
Fe2(MoO4)3 Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
711.6 710.9 711.7 710.6
721.9 723.8 722.8 724.1
712.6 712.1 712.4 711.8
723.1 725.4 724.7 725.5
716.1 716.0 715.2 716.0
(36.9%) (27.6%) (18.0%) (9.8%)
(8.7%) (8.4%) (10.8%) (13.9%)
(29.4%) (23.4%) (20.9%) (35.9%)
(7.9%) (20.7%) (23.7%) (16.6%)
S-II
S-III
DP1 DP2 Fe2+/Fe3+
(6.4%) 717.7 (8.4%) 719.9 (2.3%) 10.4 (4.2%) 719.6 (15.8%) 12.9 (11.0%) 718.0 (15.8%) 11.1 (5.9%) 719.0 (18.0%) 13.5
10.5 13.3 12.3 13.6
1.25 1.18 0.86 0.27
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(B) Spin–orbit doublet – I
Spin–orbit doublet – II
Composite
Cu 2p3/2
Cu 2p1/2
Cu 2p3/2
Cu 2p1/2
S-I
DP1
DP2
Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
934.05 934.24 933.77
953.63 952.71 952.23
934.81 935.08 934.77
954.34 953.40 953.27
943.71 943.09 942.53
19.58 18.47 18.46
19.53 18.30 18.50
(C) Spin–orbit doublet Composite
3d5/2
Fe2(MoO4)3 Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
231.4 231.5 230.7 231.7
a
3d3/2 (28.5%) (23.8%) (27.8%) (34.9%)
232.1 232.3 231.6 232.8
(27.4%) (35.4%) (47.6%) (30.6%)
234.9 235.2 234.9 235.8
(44.2%) (40.8%) (26.6%) (34.5%)
DP
Mo4+/Mo5+/Mo6+
3.5/2.5 3.7/2.9 4.2/3.3 4.1/3.0
1.04 0.67 0.58 1.14a
Ratio of Mo5+/Mo6+.
are respectively at B6 and B8.5 eV higher binding energy than the 2p3/2 photopeak which, thereby, substantiate the presence of Fe(II) and Fe(III) species, respectively.47 The observation of Table 1 further shows that introduction of Cu in Fe2(MoO4)3 decreases the molar ratio of Fe(II)/Fe(III) at the oxide surface significantly. TEM TEM images of Cu-substituted iron molybdates with x = 0, x = 0.5, x = 1.0 and x = 1.5 are shown in Fig. 4. The observation of these images shows that all catalysts, in general, follow globular morphology with distinct grades of aggregation. BET measurements Estimation of the BET surface area, pore volume, and pore size of catalyst powders shown in Table 2 does not indicate any significant influence of Cu substitution on the surface properties of the base oxide. Electrochemical characterization Oxide roughness factor (RF). The relative roughness factors, RF (= measured double layer capacitance, Cdl/Cdl of a smooth oxide surface, 60 mF cm 2) of the oxide film electrodes were estimated in Ar-saturated 1 M KOH, as earlier,9,48 from intensities of the capacitive currents which were measured by cycling the potential at varying scan rates in a narrow range of 50 mV (i.e., 1.06–1.10 V vs. RHE) in a double layer region48 (i.e., in a region which is exempted from pseudo-capacitive and faradaic currents), near open circuit potential. The Cdl values were determined9 from half the slope of linear plot, jcap vs. scan
7388 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
rate, where jcap is the total charging current (anodic + cathodic) measured at the middle of the scan range (i.e., 25 mV). A typical plot in the case of the Cu0.75Fe1.25(MoO4)3 electrode is shown in Fig. 5. Estimates of the Cdl values were 141 0.2, 101.5 0.5, 101.9 0.7, 136 29, 166 22 and 103 13 mF cm 2 and the corresponding RF values were B2.4, B1.7, B1.7, B2.3, B2.8 and 1.7 for the oxide with x = 0, 0.25, 0.5, 0.75, 1.0 and 1.5, respectively. Electrode kinetic study. Fig. 6 gathers IR (where R is the resistance of the catalytic films which ranged between B7 and 40 O)-compensated E versus log ja ( ja = I, mA/geometrical electrode area, cm2) curves recorded for CuxFe2 x(MoO4)3 (x = 0, 0.25, 0.5, 0.75, 1.0 and 1.5) at the scan rate of 0.2 mV s 1 in the potential region from 1.41 to 1.61 V vs. RHE in 1 M KOH at 298 K. Features of curves shown in Fig. 6 appear to be similar with the initial slopes close to 31–34 mV; only the curve for the oxide with x = 1.5 indicated a slightly higher slope of 37 mV. Slopes at higher potentials are not very clear and that they seem to range between 62 and 78 mV. The observed similar Tafel slope indicates that the OER follows more or less similar mechanisms of CuxFe2 x(MoO4)3. To compare electrocatalytic activities, the oxygen evolution current densities produced on different oxide electrodes at a constant potential, E = 1.510 V vs. RHE were noted from Fig. 6 and are listed in Table 3. The order of the OER with respect to OH concentration (p) has been found to be B1 in the case of oxide with x = 0 and 0.25 and B2 in the case of oxide with x = 0.5, 0.75 and 1.0. For determination of the order, typical log j vs. log COH curves in the case of the oxide with x = 1 at constant potentials are shown in Fig. 7.
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Fig. 4 TEM images of: Fe2(MoO4)3, 200 nm (A) and 100 nm (B); Cu0.50Fe1.5(MoO4)3, 100 nm (C) and 50 nm (D); Cu1.0Fe1.0(MoO4)3, 100 nm (E) and 50 nm (F); Cu1.50Fe0.50(MoO4)3, 100 nm (G) and 50 nm (H).
Table 3 shows that partial introduction of Cu in place of Fe increases the apparent as well as the real catalytic activity ( jr = ja, mA cm 2/RF) of the base electrode (Fe2(MoO4)3) greatly,
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the activity being the highest with 1.0 mol Cu. At E = 1.510 V vs. RHE, ja and jr values were 1.26 0.2 and 0.53 mA cm 2, 9.5 0.1 and 5.59 mA cm 2, 11.5 0.3 and 6.76 mA cm 2,
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Table 2
PCCP Results of BET measurements
Composite
Surface area/ m2 g 1
103 pore volume/ cm3 g 1
Pore size/ nm
Fe2(MoO4)3 Cu0.25Fe1.75(MoO4)3 Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
10.5 9.0 8.9 10.3 9.6
6.09 5.22 5.16 5.97 5.56
2.33 2.33 2.33 2.32 2.32
Fig. 5 Typical cyclic voltammograms and linear jcap vs. scan rate plots in the potential region 1.06–1.11 V vs. RHE on the Cu0.75Fe1.25(MoO4)3 electrode (geometrical area 0.5 cm2 & oxide loading E2 mg cm 2).
Fig. 6 Anodic Tafel polarization curves for CuxFe2 x(MoO4)3 [x = 0.0, 0.25, 0.50, 0.75, 1.0 and 1.50] in 1 M KOH at 298 K.
21.5 0.3 and 9.3 mA cm 2, 78.6 0.2 and 28.1 mA cm 2, 2.2 0.4 and 1.29 mA cm 2 for oxide with x = 0, 0.25, 0.50, 0.75, 1.0 and 1.5, respectively. Thus, 1.0 mol Cu introduction improved the real (or true) catalytic activity of the base oxide by B53 times. The large increase in jr with introduction of Cu
7390 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
(3d10), in place of Fe (3d6) in the base oxide can be ascribed to the modification of the electronic properties of the material. The XPS study has also indicated a significant modification in electronic properties of the material with introduction of Cu. XPS results have shown that there exist two types of redox couples, Fe2+/Fe3+ and Mo4+/Mo5+ on the oxide (0 r x r 1) surface. The molar ratios of both the redox couples decreased significantly with Cu introduction. However, the formation of the redox couples, Fe2+/Fe3+ and Mo5+/Mo6+, are indicated on the 1.5 mol Cu-substituted oxide surface. The Mo4+/Mo5+ redox couple with a molar ratio close to 0.6 seems to be catalytically more active and the Mo5+/Mo6+ redox couple with a molar ratio of 1.14 seems to be catalytically least active toward the OER. Values of b and p found for Cu-substituted iron molybdates are similar to those recently reported for Fe-substituted manganese molybdates.32 To compare the catalytic activities of the present series of copper-substituted iron molybdate electrodes with those of other molybdate electrodes previously reported, values of the current density at a constant potential, E = 1.510 V vs. RHE and electrode kinetic parameters, b and p, obtained for the OER of all molybdate electrocatalysts studied are summarized in Table 3. As values of the electrochemically active area/RF of previously studied metal molybdates are not available, their mass activities at E = 1.510 V vs. RHE are taken for comparison, assuming the whole mass of the catalytic film as active. Table 3 shows that metal (Fe, Co, Ni) molybdates obtained by the method of mixed salts solution evaporation followed by the thermal decomposition seem to be somewhat superior in OER activities in comparison with similar compounds prepared by the precipitation method. However, iron molybdate (Fe2(MoO4)3) prepared by the precipitation method has a different molecular formula than one (FeMoO4) obtained by the thermal decomposition. Moreover, the crystallites of these binary mixed oxides follow the same monoclinic crystal geometry. Further, partial introduction of Fe for Co in CoMoO4, for Ni in NiMoO4, for Mn in MnMoO4 and for Cr in Cr2(MoO4)3 considerably reduced the oxygen overpotential (Z = E 0.303) and improved the mass activities of the respective basic oxides in 1 M KOH at 298 K (Table 3). A similar effect of Cu introduction in the Fe2(MoO4)3 matrix was also observed, the activity being the greatest with 1 mol Cu. It is worth mentioning that based on both the apparent and mass activities, this active electrode is superior to all metal molybdates previously studied by us (Table 3). Also, among the present series of oxide catalysts investigated the Cu1Fe1(MoO4)3 catalyst is found to be superior to other catalysts studied in respect of both mass activity as well as real activity under similar experimental conditions. At E = 1.510 V vs. RHE in 1 M KOH at 298 K, Cu-substituted catalysts followed the activity order Cu1Fe1(MoO4)3 4 Cu0.75Fe1.25(MoO4)3 4 Cu0.5Fe1.5(MoO4)3 4 Cu0.25Fe1.75(MoO4)3 4 Cu1.5Fe0.5(MoO4)3 4 Fe2(MoO4)3. The apparent as well as the real catalytic activity of the CuFe(MoO4)3 electrode is superior to those of active Co-based spinel oxide electrodes reported recently in the literature.6,16,37,38 For instance, Svegl et al.6 observed ja = 100 mA cm 2 at E E 1.727 and 1.658 V vs. RHE for sol–gel derived Co3O4 ( jr = 0.59 mA cm 2) and Li-doped Co3O4 ( jr = 0.35 mA cm 2) films on Pt, respectively.
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a
At 50 mA cm 2.
Fe2(MoO4)3 (Mo/Fe = 1, 1.5, 3.0)33/B1.5–1.7 FexCo1 xMoO4 (0.25 r x r 0.75)35/B1.5–2.3 FexNi1 xMoO4 (0.25 r x r 0.75)36/B1.7–2.1 FexCr2 x(MoO4)3 (0.25 r x r 0.75)34/B1.2–2.7 FexMn1 xMoO4 (0.25 r x r 0.75)32/B2 Fe2(MoO4)3/1.8 0.1 Cu0.25Fe1.75(MoO4)3/1.9 0.1 Cu0.50Fe1.50(MoO4)3/1.9 0.0 Cu0.75Fe1.25(MoO4)3/1.9 0.0 Cu1.0Fe1.0(MoO4)3/1.9 0.1 Cu1.5Fe0.5(MoO4)3/2.0 0.1
CoMoO4 /6.3 NiMoO429/4.5 NiMoO430/B2—4 CoMoO431/B1
29
FeMoO4 /6.4
29
Precipitation (pH 5, B85 1C) Precipitation (pH 5, B85 1C) + heat treated at 550 1C, 1 h Precipitation (pH 2, 100 1C 1.5 h), heat treated at 375 1C Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at375 1C Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 375 1C Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 550 1C, 5.5 h Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 500 1C, 5 h Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 550 1C, 5.5 h
Solution evaporation and subsequent thermal decomposition
Preparation method
34 31 33 34 31 37
1 2 1 1 2 1
10.5 9.0 8.9 — 10.3 9.6
B33 B38 B53 — B62 B83
Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic
2.5–12.8
1.0 1.3 2.3 2.2 2.1 1.1
B2 32–34,
B2–4
—
Monoclinic
B2
B35
1.7–9.9
27–35
Orthorhombic
B1
B34
12–19
—
1.2–2.4
10–49
— B40
—
—
1.5
p
0.8–1.6
37
b/mV (at low E)
B35
21–34
Monoclinic
1
1.2 1.9 1.2 1.2
— — 11.75 9.4
—
BET/m2 g
44 42 B70 B60
7.3 5.6 50 46
9.2
S/nm
Monoclinic Monoclinic Monoclinic Monoclinic
Monoclinic
Crystal structure
Summarized results of the OER study of metal molybdates in alkaline solutions at 298 K
Mixed oxide/loading (mg cm 2)
Table 3 2
372 350 341 338 305 379
3a 1 3 1 1 2a
336–351
332–337
337–342
355 (x = 0.25)
371–382
367 382 487 428
355
Z/mV at j = 100 mA cm
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0.7 0.1 5 0.1 6.1 0.2 11.3 0.2 41.5 2 1.1 0.1
7.8–11.5
2.9–10.2
0.9–7.0
0.7–6.3
0.87–1.2
— 0.4 0.19–0.38 1.5
1.9
I/mA mg 1 at E = 1.510 V vs. RHE
Mo/Fe = 1 is superior x = 0.25 is superior x = 0.75 is superior x = 0.5 is superior x = 0.5 is superior x = 1.0 is superior
— —
M = Fe is superior
Remarks
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The proposed mechanisms A and B are similar to Bokris’ oxide and O’Grady’s path, respectively. S is an active site on the catalyst surface and ‘z’ is the charge. Considering the bimolecular chemical interaction between adsorbed intermediates (reaction (2)), in the case of mechanism-A, as the rate determining step, the overall rate of the OER can be given as
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v = v2 = k2Y2
(1)
Considering reaction (1) under quasi-equilibrium conditions and Y (= fractional coverage by the intermediate) under Langmuir adsorption conditions, the overall current density expression can be written as j = nFv2 = nFk2[K1COH exp(FE/RT)]2 = nFk2[K12COH
Fig. 7 Plot of log j vs. log COH for Cu1Fe1(MoO4)3 at constant potentials.
Chi et al.16 observed ja E 170 and 212 mA cm 2 at E E 1.770 V vs. RHE at the NiCo2O4/Ni electrode, prepared respectively by electrophoretic deposition ( jr = 0.44 mA cm 2) and thermal decomposition ( jr = 0.25 mA cm 2), Tavares et al.38 observed ja = 100 ( jr = 0.03) mA cm 2 at E E 1.610 V vs. RHE on the NiCo2O4/Ni electrode prepared by thermal decomposition. The apparent OER activity of the CuFe(MoO4)3 electrode appears to be similar to the electrodeposited Co + Ni mixed oxide active catalyst ( ja = 100 ( jr = 0.34) mA cm 2, E E 1.526 V vs. RHE),13 while its real activity is much superior. Further, it exhibited low apparent OER activity in comparison to La-doped Co3O4 ( ja = 100 ( jr = 0.03) mA cm 2 at E = 1.437 V vs. RHE) and ZnCo2O4 ( ja = 100 ( jr = 0.42) mA cm 2 at E E 1.432 V vs. RHE) obtained by microwave assisted thermal decomposition8 and electrophoretic deposition,12 respectively, however, its true catalytic activity was much superior to these active cobaltite electrodes. It is found that b-values are practically the same for all Cu-substituted molybdates while p-values are not. Values of p for the OER of the oxide with x = 0.25 ( p = 1.3) and 1.5 ( p = 1.1) are close to unity while they are nearly two for the oxide with x = 0.5 ( p = 2.3), 0.75 ( p = 2.2) and 1.0 ( p = 2.1). Thus, values of the electrode kinetic parameters indicate that the OER follows, at least, two mechanistic paths having similar Tafel slopes and different reaction orders. Thus, we propose the following two mechanisms (A and B) which occur simultaneously on the oxide surface: Mechanism-A (1) S + OH - SOH + e (2) 2SOH - S.O + S + H2O (3) 2SO - 2S + O2 Mechanism-B (1) Sz + OH - SzOH + e (2) SzOH - Sz+1OH + e (3) 2Sz+1OH + 2OH - 2Sz + 2H2O + O2
7392 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
2
exp(2FE/RT)], symmetry factor being 0.5 (2)
Eqn (2) gives p = 2 and b = 30 mV. These values of b and p are close to those observed for the OER of Cu-substituted iron molybdates with 0.5 r x r 1. Now, assuming the charge transfer step 2 of mechanism-B as the rate determining step, the overall current density expression becomes j = nFk2K1COH exp(3FE/2RT), symmetry factor being 0.5 (3) All the terms used in eqn (2) and (3) have their usual meaning. Eqn (3) gives b E 40 mV and p = 1. Thus, the electrode kinetic parameters are similar to those observed for the oxide with x = 0.25 (b E 31 mV and p E 1.3) and 1.5 (b E 37 mV and p E 1.1) by us. It seems that when the molar ratio of Mo4+/Mo5+ (or Mo5+/ Mo6+) is near unity on the oxide surface, the OER proceeds mainly through mechanism-B and when the molar ratio of Mo4+/Mo5+ { 1 (i.e. when surface is rich in Mo5+), it proceeds essentially through mechanism-A.
Conclusions The study has demonstrated that the introduction of 0.25–1.0 mol Cu does not change the monoclinic crystal structure of Fe2(MoO4)3 practically and improves the catalytic activity towards the OER greatly. Among the present series of oxide electrodes investigated, the CuFe(MoO4)3 electrode exhibited the greatest OER activity. The OER activity of the latter electrode is superior to all metal molybdate electrodes and several Co-based active spinel-type electrodes previously reported. Thus, ternary metal molybdates are interesting and need to be investigated further. Efforts are continued to improve the activity of CuFe(MoO4)3 further by partial replacement of Mo by suitable metals.
Acknowledgements We thank the director, Institute of Medical Sciences, Banaras Hindu University and also Professor O. N. Srivastava, Physics Department BHU, for providing the TEM facilities.
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Copper–iron–molybdenum mixed oxides as efficient oxygen evolution electrocatalysts Venkata Kali Vara Prasad Srirapu,a Chandra Shekhar Sharma,a Rahul Awasthi,a Ravindra Nath Singh*a and Akhoury Sudhir Kumar Sinhab Ternary Cu, Fe and Mo mixed oxides having a nominal compositional formula, CuxFe2 x(MoO4)3 (0 r x r 1.5), have been prepared by a co-precipitation method at pH E 2 and characterized by FT-IR, XRD, XPS, TEM and anodic polarization techniques for use as electrocatalysts for the oxygen evolution reaction (OER) in alkaline solutions. The crystallites of oxides with x r 1 have the monoclinic crystal structure. The OER study shows that replacement of Fe in the Fe2(MoO4)3 matrix by 0.25–1.0 mol Cu increases the apparent electrocatalytic activity. However, 1.5 mol Cu-addition is detrimental to the OER
Received 26th December 2013, Accepted 23rd February 2014 DOI: 10.1039/c3cp55453h www.rsc.org/pccp
activity. At E = 1.51 V (vs. RHE) in 1 M KOH, the catalytic activity of the oxide with x = 1 was approximately 50 times the activity of the base oxide (i.e. Fe2(MoO4)3). The Tafel slope of oxides with 0.25 r x r 1.5 ranged between 31 and 37 mV. The reaction order of OH concentration was nearly unity for oxides with x = 0.25 and 1.5 and it was B2 for oxides with x = 0.5, 0.75, and 1.0. Suitable reaction mechanisms consistent with the electrode kinetic parameters have also been proposed.
Introduction Water electrolysis is the simplest process to store electrical energy (surplus) in the form of non-polluting hydrogen fuel. It is, therefore, considered as a potential route to produce clean energy sources, hydrogen and oxygen. However, the oxygen evolution reaction (OER) in aqueous solutions involves high overpotentials. To minimize the latter various electrode materials such as metals and their pure as well as mixed oxides were extensively investigated and comprehensively reviewed.1–4 Among these catalysts, complex oxides with spinellic (mainly Co-based5–17) and perovskite (mainly LaNiO318–20 and LaCoO321–28) structures are considered as promising materials and have been investigated much for electrocatalysis of the OER in alkaline solutions. Recently, we have investigated a new mixed oxide system having molecular formulae, MMoO4 (where M = Mn, Fe, Co, and Ni)29–32 and M2(MoO4)3 (M = Fe and Cr).33,34 This novel oxide system is quite stable and efficiently catalyses the electroevolution of oxygen in alkaline solutions. It is observed that partial replacement of Co in CoMoO4,35 Ni in NiMoO4,36 Cr in Cr2(MoO4)3,34 and Mn in MnMoO432 by Fe increases the electrocatalytic activity of materials greatly. The OER performances of these novel ternary oxide catalysts were superior to several active
a
Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi-221005, India. E-mail: rnsbhu@rediffmail.com, [email protected]; Fax: +91-542-2368127; Tel: +91-542-6701596 b Department of Chemical Engineering, Indian Institute of Technology, Banaras Hindu University, India
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Co-based spinel oxides recently reported in the literature.6,16,37,38 These results encouraged us further to introduce Cu in place of Fe in the Fe2(MoO4)3 matrix and examine its influence on the electrocatalytic activity with the aim of developing highly efficient OER electrocatalysts. In fact, the study demonstrated that a 1.0 mol Cu-substituted oxide catalyst surpasses other previously reported binary and ternary metal molybdate electrodes in the OER activity. Results of the investigation are fully detailed in this paper.
Experimental Preparation of mixed oxide electrodes Fe2(MoO4)3 and Cu-substituted Fe2(MoO4)3 having compositional formulae CuxFe2 x(MoO4)3 (x = 0, 0.25, 0.50, 0.75, 1.0 and 1.5) were synthesized following a co-precipitation method, details of which were previously described.33 Metal salts, Fe (NO3)39H2O (Merck), Cu(NO3)24H2O (Hinweise Z 98%) and (NH4)6Mo7O244H2O (Sarabhai, Batch no. 4E740296), were used as precursors. The precipitate was thoroughly washed with hot distilled water, dried overnight at 393 K, crushed, heat treated at 773 K for 5 h, removed from the furnace and again crushed and heat treated for 3 h at 773 K to obtain the desired catalyst. As mentioned earlier,34,36 the oxide powders were transformed into the form of catalytic films on pretreated Ni supports by oxide-slurry coatings. To ascertain the adherence, the oxide films were dried in air and then heat treated at 673 K for 1.5 h. Loading of the oxide mass was maintained at B2 mg cm 2.
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Material characterization The catalyst materials were structurally characterized by FT-IR (Varian FT-IR spectrophotometer, model 3100), X-ray diffraction (XRD), BET surface area analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The XRD powder pattern of the catalyst was recorded on an X-ray diffractometer (Rigaku DMAX III) using Cu-Ka as the radiation source (l = 1.542 Å). The BET surface area was determined using a surface area analyzer (Micrometrics, USA, ASAP 2020 Model). Morphology of the catalytic films has been studied by TEM (TECNAI G2 FEI). The surface electronic state and composition of materials were investigated using an AMICUS-X-ray photoelectron spectrometer. Electrochemical characterization A conventional three-electrode single compartment Pyrex glass cell, containing pure Pt-foil (B8 cm2) and Hg/HgO/1 M KOH, respectively, as counter and reference electrodes, was employed to perform cyclic voltammetry (CV) and anodic polarization experiments. All potentials measured with respect to the Hg/HgO electrode immersed in 1 M KOH were converted to a RHE scale by adding 0.910 V to the potential. The Hg/HgO/1 M KOH electrode (Ecalibrated = 0.082 V vs. SHE) was calibrated as described elsewhere.39 Instruments and software employed in the study were the same as already mentioned in ref. 35 and 36. The apparent electrocatalytic activities (based on the geometrical area) of molybdate electrodes were determined by recording IRcompensated ‘E vs. log j’ curves at 0.2 mV s 1 in the potential region from 1.41 to 1.61 V vs. RHE in 1 M KOH at 298 K. Prior to recording ‘E versus log j’ curves, each electrode was first cycled for five runs at 50 mV s 1 between 0.91 and 1.51 V vs. RHE in 1 M KOH at 298 K. The order of the OER (p) has been determined by recording anodic ‘E vs. log j’ curves for each catalyst at different KOH concentrations (i.e., 0.5, 1.0, 1.5 and 2.0 mol dm 3). The ionic strength (m) of the cell solution was maintained constant (m = 2.0) using KNO3 as a neutral salt. j versus COH data for each catalyst at constant potentials were then obtained and produced in the form of straight line plots, log j vs. log COH ; the order was determined by measuring the slope of these straight lines. The log–log plot for each catalyst was constructed at three constant potentials; values of the order mentioned in the text are average ones. Only the first linear region of E vs. log j plots recorded at four KOH concentrations was considered for the order determination.
Fig. 1 FT-IR spectra of CuxFe2 x(MoO4)3 [x = 0.0 (a), 0.25 (b), 0.50 (c), 0.75 (d), 1.0 (e) and 1.50 (f)], sintered at 648 K for 10 h.
Features of FT-IR spectra of Cu-substituted products containing 0.5–1.0 mol Cu, however, appear to be more or less similar and they exhibit a band at 598–607 cm 1 which is found to be absent in the case of the oxide with x = 0 and x = 0.25. The appearance of a narrow band at 995–997 and a relatively broad band at 598–607 cm 1 indicates the presence of MoO3 in the catalyst.33,40,41 XRD Fig. 2 depicts the XRD powder patterns of CuxFe2 x(MoO4)3 (where x = 0, 0.25, 0.50, 1.0 and 1.5) samples recorded between 2y = 0 and 2y = 801. It is observed that both 2y and corresponding d values of all the diffraction lines of the oxide with x = 0, as shown in Fig. 2, exactly match with the data for pure Fe2(MoO4)3 with the monoclinic crystal structure given in JCPDS Card No. 35-0183. Substitution of copper from 0.25 to
Results and discussion FT-IR Fig. 1 gathers the FT-IR spectra of Cu-substituted Fe2(MoO4)3 catalysts. The FT-IR spectra of Fe2(MoO4)3 show an intense broad band at B840 cm 1 and a weak band at B961 cm 1, which are characteristic bands for the tetrahedral species of Mo in Fe2(MoO4)3 and Fe–O–Mo bond vibrations, respectively.33,40 This broad band widens with 0.25 mol Cu introduction.
7386 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
Fig. 2 XRD powder patterns of CuxFe2 x(MoO4)3 [x = 0.0 (a), 0.25 (b), 0.50 (c), 1.0 (d) and 1.50 (e)], sintered at 673 K for 10 h.
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1.0 mol for Fe into the Fe2(MoO4)3 matrix does not seem to influence the diffraction patterns of the base oxide; only the diffraction peaks slightly shift towards the higher angle. However, the higher substitution of copper (i.e., 1.5 mol) produced the product with mixed phases: MoO3, Fe2(MoO4)3 and/or CuxFe2 x(MoO4)3. With the exception of the base oxide, the diffraction peaks at 3.248 and 6.935 Å for impurity phase MoO3 are found to be present in all oxides. Moreover, the absence of MoO3 peaks does not rule out the possibility of the presence of free MoO3 in the oxide, because MoO3 is reported to be detectable by XRD only when its content is higher than 7%; below this amount the material is amorphous or microcrystalline.40,41 IR spectra of all compounds shown in Fig. 1 indicate the presence of MoO3. However, of two characteristic MoO3 bands, a narrow one at B990 and a broad one at B624 cm 1, the oxide with x = 0 and x = 0.25 exhibited only the narrow characteristic band. Thus, introduction of 0.25–1.0 mol Cu forms a solid solution with the Fe2(MoO4)3 phase. Values of the crystallite size (S), as estimated by using the Scherrer relation and the most intense diffraction peak of each XRD spectrum, were B33, B38, B53, B62, and B83 nm for oxide with x = 0, x = 0.25, x = 0.5, x = 1.0 and x = 1.5, respectively. The observed increase in S with increasing addition of Cu (atomic size = 128 pm) for Fe (atomic size = 126 pm) can be ascribed to the larger size of Cu. XPS The surface electronic state and composition of materials were investigated by XPS analysis. Mo 3d spectra of CuxFe2 x(MoO4)3 (x = 0, 0.5, 1.0 and 1.5) catalysts shown in Fig. 3A show two photoelectron peaks, 3d5/2 and 3d3/2, at binding energies (BEs):
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231.5–232.5 and 234.9–235.9 eV, respectively. Values of the BEs indicate that the surface layer contains, in addition to Mo6+, molybdenum in lower oxidation states also.42–45 The Cu 2p XPS spectra of the substituted product shown in Fig. 3B display two strong symmetrical photopeaks, 2p3/2 and 2p1/2, respectively, at 935.1–936.2 and 954.4–955 eV, thereby showing the presence of Cu essentially in +2 states.46 Some low intensity peaks are also observed between these two strong peaks, but they are not very clear. Similarly, the Fe 2p XPS spectra of samples indicated two strong photopeaks at 711.4–712.3 and 723.1–725.5 eV (Fig. 3C) and thereby indicating the presence of iron in two oxidation states, +2 and +3, in the surface layer.47 Deconvolution of Mo 3d photopeaks exhibited two peaks for the Mo 3d5/2 photopeak at 230.7–231.7 and 231.6–232.8 eV (Table 1). A typical deconvoluted Mo 3d spectrum in the case of the oxide with x = 1 is shown in Fig. 3F. Results shown in Table 1 show that molybdenum is present in lower oxidation states, Mo4+ and Mo5+, on the Fe2(MoO4)3 surface, the Mo4+/Mo5+ molar ratio being approximately unity (i.e. Mo4+/Mo5+ = 1.04). Further, introduction of Cu into the oxide matrix decreases the Mo4+/Mo5+ molar ratio in the surface layer, the decrease being from 1.04 to 0.67 with 0.5 and from 1.04 to 0.54 with 1.0 mol Cu. However, a higher addition of Cu (i.e., 1.5 mol) transforms molybdenum from lower valance states (Mo4+ and Mo5+) to the higher valance states (i.e., Mo5+ and Mo6+) in the surface layer with the Mo5+/Mo6+ molar ratio equal to 1.14. Results of deconvolution of the Fe 2p spectra shown in Table 1 and Fig. 3 indicate the presence of two shake-up satellites at 715.2–716.0 and 718–720 eV between the main peaks Fe 2p3/2 and Fe 2p1/2. The former and the latter shake-up satellite peaks
Fig. 3 XPS spectra of Mo 3d (A), Cu 2p (B) and Fe 2p (C) from CuxFe2 x(MoO4)3 [x = 0.0, 0.50, 1.0 and 1.50]. Representative deconvoluted XPS spectra of Fe 2p [x = 0 (D) and x = 1.0 (E)] and Mo 3d [x = 1.0 (F)].
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Paper Table 1
PCCP Results of deconvolution of X-ray photoelectron spectra: (A) Fe 2p, (B) Cu 2p, (C) Mo 3d
(A) Spin–orbit doublet – I
Spin–orbit doublet – II
Composite
Fe 2p3/2
Fe 2p1/2
Fe 2p3/2
Fe 2p1/2
S-I
Fe2(MoO4)3 Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
711.6 710.9 711.7 710.6
721.9 723.8 722.8 724.1
712.6 712.1 712.4 711.8
723.1 725.4 724.7 725.5
716.1 716.0 715.2 716.0
(36.9%) (27.6%) (18.0%) (9.8%)
(8.7%) (8.4%) (10.8%) (13.9%)
(29.4%) (23.4%) (20.9%) (35.9%)
(7.9%) (20.7%) (23.7%) (16.6%)
S-II
S-III
DP1 DP2 Fe2+/Fe3+
(6.4%) 717.7 (8.4%) 719.9 (2.3%) 10.4 (4.2%) 719.6 (15.8%) 12.9 (11.0%) 718.0 (15.8%) 11.1 (5.9%) 719.0 (18.0%) 13.5
10.5 13.3 12.3 13.6
1.25 1.18 0.86 0.27
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(B) Spin–orbit doublet – I
Spin–orbit doublet – II
Composite
Cu 2p3/2
Cu 2p1/2
Cu 2p3/2
Cu 2p1/2
S-I
DP1
DP2
Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
934.05 934.24 933.77
953.63 952.71 952.23
934.81 935.08 934.77
954.34 953.40 953.27
943.71 943.09 942.53
19.58 18.47 18.46
19.53 18.30 18.50
(C) Spin–orbit doublet Composite
3d5/2
Fe2(MoO4)3 Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
231.4 231.5 230.7 231.7
a
3d3/2 (28.5%) (23.8%) (27.8%) (34.9%)
232.1 232.3 231.6 232.8
(27.4%) (35.4%) (47.6%) (30.6%)
234.9 235.2 234.9 235.8
(44.2%) (40.8%) (26.6%) (34.5%)
DP
Mo4+/Mo5+/Mo6+
3.5/2.5 3.7/2.9 4.2/3.3 4.1/3.0
1.04 0.67 0.58 1.14a
Ratio of Mo5+/Mo6+.
are respectively at B6 and B8.5 eV higher binding energy than the 2p3/2 photopeak which, thereby, substantiate the presence of Fe(II) and Fe(III) species, respectively.47 The observation of Table 1 further shows that introduction of Cu in Fe2(MoO4)3 decreases the molar ratio of Fe(II)/Fe(III) at the oxide surface significantly. TEM TEM images of Cu-substituted iron molybdates with x = 0, x = 0.5, x = 1.0 and x = 1.5 are shown in Fig. 4. The observation of these images shows that all catalysts, in general, follow globular morphology with distinct grades of aggregation. BET measurements Estimation of the BET surface area, pore volume, and pore size of catalyst powders shown in Table 2 does not indicate any significant influence of Cu substitution on the surface properties of the base oxide. Electrochemical characterization Oxide roughness factor (RF). The relative roughness factors, RF (= measured double layer capacitance, Cdl/Cdl of a smooth oxide surface, 60 mF cm 2) of the oxide film electrodes were estimated in Ar-saturated 1 M KOH, as earlier,9,48 from intensities of the capacitive currents which were measured by cycling the potential at varying scan rates in a narrow range of 50 mV (i.e., 1.06–1.10 V vs. RHE) in a double layer region48 (i.e., in a region which is exempted from pseudo-capacitive and faradaic currents), near open circuit potential. The Cdl values were determined9 from half the slope of linear plot, jcap vs. scan
7388 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
rate, where jcap is the total charging current (anodic + cathodic) measured at the middle of the scan range (i.e., 25 mV). A typical plot in the case of the Cu0.75Fe1.25(MoO4)3 electrode is shown in Fig. 5. Estimates of the Cdl values were 141 0.2, 101.5 0.5, 101.9 0.7, 136 29, 166 22 and 103 13 mF cm 2 and the corresponding RF values were B2.4, B1.7, B1.7, B2.3, B2.8 and 1.7 for the oxide with x = 0, 0.25, 0.5, 0.75, 1.0 and 1.5, respectively. Electrode kinetic study. Fig. 6 gathers IR (where R is the resistance of the catalytic films which ranged between B7 and 40 O)-compensated E versus log ja ( ja = I, mA/geometrical electrode area, cm2) curves recorded for CuxFe2 x(MoO4)3 (x = 0, 0.25, 0.5, 0.75, 1.0 and 1.5) at the scan rate of 0.2 mV s 1 in the potential region from 1.41 to 1.61 V vs. RHE in 1 M KOH at 298 K. Features of curves shown in Fig. 6 appear to be similar with the initial slopes close to 31–34 mV; only the curve for the oxide with x = 1.5 indicated a slightly higher slope of 37 mV. Slopes at higher potentials are not very clear and that they seem to range between 62 and 78 mV. The observed similar Tafel slope indicates that the OER follows more or less similar mechanisms of CuxFe2 x(MoO4)3. To compare electrocatalytic activities, the oxygen evolution current densities produced on different oxide electrodes at a constant potential, E = 1.510 V vs. RHE were noted from Fig. 6 and are listed in Table 3. The order of the OER with respect to OH concentration (p) has been found to be B1 in the case of oxide with x = 0 and 0.25 and B2 in the case of oxide with x = 0.5, 0.75 and 1.0. For determination of the order, typical log j vs. log COH curves in the case of the oxide with x = 1 at constant potentials are shown in Fig. 7.
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Fig. 4 TEM images of: Fe2(MoO4)3, 200 nm (A) and 100 nm (B); Cu0.50Fe1.5(MoO4)3, 100 nm (C) and 50 nm (D); Cu1.0Fe1.0(MoO4)3, 100 nm (E) and 50 nm (F); Cu1.50Fe0.50(MoO4)3, 100 nm (G) and 50 nm (H).
Table 3 shows that partial introduction of Cu in place of Fe increases the apparent as well as the real catalytic activity ( jr = ja, mA cm 2/RF) of the base electrode (Fe2(MoO4)3) greatly,
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the activity being the highest with 1.0 mol Cu. At E = 1.510 V vs. RHE, ja and jr values were 1.26 0.2 and 0.53 mA cm 2, 9.5 0.1 and 5.59 mA cm 2, 11.5 0.3 and 6.76 mA cm 2,
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Table 2
PCCP Results of BET measurements
Composite
Surface area/ m2 g 1
103 pore volume/ cm3 g 1
Pore size/ nm
Fe2(MoO4)3 Cu0.25Fe1.75(MoO4)3 Cu0.5Fe1.5(MoO4)3 Cu1.0Fe1.0(MoO4)3 Cu1.5Fe0.5(MoO4)3
10.5 9.0 8.9 10.3 9.6
6.09 5.22 5.16 5.97 5.56
2.33 2.33 2.33 2.32 2.32
Fig. 5 Typical cyclic voltammograms and linear jcap vs. scan rate plots in the potential region 1.06–1.11 V vs. RHE on the Cu0.75Fe1.25(MoO4)3 electrode (geometrical area 0.5 cm2 & oxide loading E2 mg cm 2).
Fig. 6 Anodic Tafel polarization curves for CuxFe2 x(MoO4)3 [x = 0.0, 0.25, 0.50, 0.75, 1.0 and 1.50] in 1 M KOH at 298 K.
21.5 0.3 and 9.3 mA cm 2, 78.6 0.2 and 28.1 mA cm 2, 2.2 0.4 and 1.29 mA cm 2 for oxide with x = 0, 0.25, 0.50, 0.75, 1.0 and 1.5, respectively. Thus, 1.0 mol Cu introduction improved the real (or true) catalytic activity of the base oxide by B53 times. The large increase in jr with introduction of Cu
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(3d10), in place of Fe (3d6) in the base oxide can be ascribed to the modification of the electronic properties of the material. The XPS study has also indicated a significant modification in electronic properties of the material with introduction of Cu. XPS results have shown that there exist two types of redox couples, Fe2+/Fe3+ and Mo4+/Mo5+ on the oxide (0 r x r 1) surface. The molar ratios of both the redox couples decreased significantly with Cu introduction. However, the formation of the redox couples, Fe2+/Fe3+ and Mo5+/Mo6+, are indicated on the 1.5 mol Cu-substituted oxide surface. The Mo4+/Mo5+ redox couple with a molar ratio close to 0.6 seems to be catalytically more active and the Mo5+/Mo6+ redox couple with a molar ratio of 1.14 seems to be catalytically least active toward the OER. Values of b and p found for Cu-substituted iron molybdates are similar to those recently reported for Fe-substituted manganese molybdates.32 To compare the catalytic activities of the present series of copper-substituted iron molybdate electrodes with those of other molybdate electrodes previously reported, values of the current density at a constant potential, E = 1.510 V vs. RHE and electrode kinetic parameters, b and p, obtained for the OER of all molybdate electrocatalysts studied are summarized in Table 3. As values of the electrochemically active area/RF of previously studied metal molybdates are not available, their mass activities at E = 1.510 V vs. RHE are taken for comparison, assuming the whole mass of the catalytic film as active. Table 3 shows that metal (Fe, Co, Ni) molybdates obtained by the method of mixed salts solution evaporation followed by the thermal decomposition seem to be somewhat superior in OER activities in comparison with similar compounds prepared by the precipitation method. However, iron molybdate (Fe2(MoO4)3) prepared by the precipitation method has a different molecular formula than one (FeMoO4) obtained by the thermal decomposition. Moreover, the crystallites of these binary mixed oxides follow the same monoclinic crystal geometry. Further, partial introduction of Fe for Co in CoMoO4, for Ni in NiMoO4, for Mn in MnMoO4 and for Cr in Cr2(MoO4)3 considerably reduced the oxygen overpotential (Z = E 0.303) and improved the mass activities of the respective basic oxides in 1 M KOH at 298 K (Table 3). A similar effect of Cu introduction in the Fe2(MoO4)3 matrix was also observed, the activity being the greatest with 1 mol Cu. It is worth mentioning that based on both the apparent and mass activities, this active electrode is superior to all metal molybdates previously studied by us (Table 3). Also, among the present series of oxide catalysts investigated the Cu1Fe1(MoO4)3 catalyst is found to be superior to other catalysts studied in respect of both mass activity as well as real activity under similar experimental conditions. At E = 1.510 V vs. RHE in 1 M KOH at 298 K, Cu-substituted catalysts followed the activity order Cu1Fe1(MoO4)3 4 Cu0.75Fe1.25(MoO4)3 4 Cu0.5Fe1.5(MoO4)3 4 Cu0.25Fe1.75(MoO4)3 4 Cu1.5Fe0.5(MoO4)3 4 Fe2(MoO4)3. The apparent as well as the real catalytic activity of the CuFe(MoO4)3 electrode is superior to those of active Co-based spinel oxide electrodes reported recently in the literature.6,16,37,38 For instance, Svegl et al.6 observed ja = 100 mA cm 2 at E E 1.727 and 1.658 V vs. RHE for sol–gel derived Co3O4 ( jr = 0.59 mA cm 2) and Li-doped Co3O4 ( jr = 0.35 mA cm 2) films on Pt, respectively.
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a
At 50 mA cm 2.
Fe2(MoO4)3 (Mo/Fe = 1, 1.5, 3.0)33/B1.5–1.7 FexCo1 xMoO4 (0.25 r x r 0.75)35/B1.5–2.3 FexNi1 xMoO4 (0.25 r x r 0.75)36/B1.7–2.1 FexCr2 x(MoO4)3 (0.25 r x r 0.75)34/B1.2–2.7 FexMn1 xMoO4 (0.25 r x r 0.75)32/B2 Fe2(MoO4)3/1.8 0.1 Cu0.25Fe1.75(MoO4)3/1.9 0.1 Cu0.50Fe1.50(MoO4)3/1.9 0.0 Cu0.75Fe1.25(MoO4)3/1.9 0.0 Cu1.0Fe1.0(MoO4)3/1.9 0.1 Cu1.5Fe0.5(MoO4)3/2.0 0.1
CoMoO4 /6.3 NiMoO429/4.5 NiMoO430/B2—4 CoMoO431/B1
29
FeMoO4 /6.4
29
Precipitation (pH 5, B85 1C) Precipitation (pH 5, B85 1C) + heat treated at 550 1C, 1 h Precipitation (pH 2, 100 1C 1.5 h), heat treated at 375 1C Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at375 1C Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 375 1C Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 550 1C, 5.5 h Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 500 1C, 5 h Precipitation (pH 2, 100 1C, 1.5 h) + heat treated at 550 1C, 5.5 h
Solution evaporation and subsequent thermal decomposition
Preparation method
34 31 33 34 31 37
1 2 1 1 2 1
10.5 9.0 8.9 — 10.3 9.6
B33 B38 B53 — B62 B83
Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic
2.5–12.8
1.0 1.3 2.3 2.2 2.1 1.1
B2 32–34,
B2–4
—
Monoclinic
B2
B35
1.7–9.9
27–35
Orthorhombic
B1
B34
12–19
—
1.2–2.4
10–49
— B40
—
—
1.5
p
0.8–1.6
37
b/mV (at low E)
B35
21–34
Monoclinic
1
1.2 1.9 1.2 1.2
— — 11.75 9.4
—
BET/m2 g
44 42 B70 B60
7.3 5.6 50 46
9.2
S/nm
Monoclinic Monoclinic Monoclinic Monoclinic
Monoclinic
Crystal structure
Summarized results of the OER study of metal molybdates in alkaline solutions at 298 K
Mixed oxide/loading (mg cm 2)
Table 3 2
372 350 341 338 305 379
3a 1 3 1 1 2a
336–351
332–337
337–342
355 (x = 0.25)
371–382
367 382 487 428
355
Z/mV at j = 100 mA cm
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0.7 0.1 5 0.1 6.1 0.2 11.3 0.2 41.5 2 1.1 0.1
7.8–11.5
2.9–10.2
0.9–7.0
0.7–6.3
0.87–1.2
— 0.4 0.19–0.38 1.5
1.9
I/mA mg 1 at E = 1.510 V vs. RHE
Mo/Fe = 1 is superior x = 0.25 is superior x = 0.75 is superior x = 0.5 is superior x = 0.5 is superior x = 1.0 is superior
— —
M = Fe is superior
Remarks
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The proposed mechanisms A and B are similar to Bokris’ oxide and O’Grady’s path, respectively. S is an active site on the catalyst surface and ‘z’ is the charge. Considering the bimolecular chemical interaction between adsorbed intermediates (reaction (2)), in the case of mechanism-A, as the rate determining step, the overall rate of the OER can be given as
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v = v2 = k2Y2
(1)
Considering reaction (1) under quasi-equilibrium conditions and Y (= fractional coverage by the intermediate) under Langmuir adsorption conditions, the overall current density expression can be written as j = nFv2 = nFk2[K1COH exp(FE/RT)]2 = nFk2[K12COH
Fig. 7 Plot of log j vs. log COH for Cu1Fe1(MoO4)3 at constant potentials.
Chi et al.16 observed ja E 170 and 212 mA cm 2 at E E 1.770 V vs. RHE at the NiCo2O4/Ni electrode, prepared respectively by electrophoretic deposition ( jr = 0.44 mA cm 2) and thermal decomposition ( jr = 0.25 mA cm 2), Tavares et al.38 observed ja = 100 ( jr = 0.03) mA cm 2 at E E 1.610 V vs. RHE on the NiCo2O4/Ni electrode prepared by thermal decomposition. The apparent OER activity of the CuFe(MoO4)3 electrode appears to be similar to the electrodeposited Co + Ni mixed oxide active catalyst ( ja = 100 ( jr = 0.34) mA cm 2, E E 1.526 V vs. RHE),13 while its real activity is much superior. Further, it exhibited low apparent OER activity in comparison to La-doped Co3O4 ( ja = 100 ( jr = 0.03) mA cm 2 at E = 1.437 V vs. RHE) and ZnCo2O4 ( ja = 100 ( jr = 0.42) mA cm 2 at E E 1.432 V vs. RHE) obtained by microwave assisted thermal decomposition8 and electrophoretic deposition,12 respectively, however, its true catalytic activity was much superior to these active cobaltite electrodes. It is found that b-values are practically the same for all Cu-substituted molybdates while p-values are not. Values of p for the OER of the oxide with x = 0.25 ( p = 1.3) and 1.5 ( p = 1.1) are close to unity while they are nearly two for the oxide with x = 0.5 ( p = 2.3), 0.75 ( p = 2.2) and 1.0 ( p = 2.1). Thus, values of the electrode kinetic parameters indicate that the OER follows, at least, two mechanistic paths having similar Tafel slopes and different reaction orders. Thus, we propose the following two mechanisms (A and B) which occur simultaneously on the oxide surface: Mechanism-A (1) S + OH - SOH + e (2) 2SOH - S.O + S + H2O (3) 2SO - 2S + O2 Mechanism-B (1) Sz + OH - SzOH + e (2) SzOH - Sz+1OH + e (3) 2Sz+1OH + 2OH - 2Sz + 2H2O + O2
7392 | Phys. Chem. Chem. Phys., 2014, 16, 7385--7393
2
exp(2FE/RT)], symmetry factor being 0.5 (2)
Eqn (2) gives p = 2 and b = 30 mV. These values of b and p are close to those observed for the OER of Cu-substituted iron molybdates with 0.5 r x r 1. Now, assuming the charge transfer step 2 of mechanism-B as the rate determining step, the overall current density expression becomes j = nFk2K1COH exp(3FE/2RT), symmetry factor being 0.5 (3) All the terms used in eqn (2) and (3) have their usual meaning. Eqn (3) gives b E 40 mV and p = 1. Thus, the electrode kinetic parameters are similar to those observed for the oxide with x = 0.25 (b E 31 mV and p E 1.3) and 1.5 (b E 37 mV and p E 1.1) by us. It seems that when the molar ratio of Mo4+/Mo5+ (or Mo5+/ Mo6+) is near unity on the oxide surface, the OER proceeds mainly through mechanism-B and when the molar ratio of Mo4+/Mo5+ { 1 (i.e. when surface is rich in Mo5+), it proceeds essentially through mechanism-A.
Conclusions The study has demonstrated that the introduction of 0.25–1.0 mol Cu does not change the monoclinic crystal structure of Fe2(MoO4)3 practically and improves the catalytic activity towards the OER greatly. Among the present series of oxide electrodes investigated, the CuFe(MoO4)3 electrode exhibited the greatest OER activity. The OER activity of the latter electrode is superior to all metal molybdate electrodes and several Co-based active spinel-type electrodes previously reported. Thus, ternary metal molybdates are interesting and need to be investigated further. Efforts are continued to improve the activity of CuFe(MoO4)3 further by partial replacement of Mo by suitable metals.
Acknowledgements We thank the director, Institute of Medical Sciences, Banaras Hindu University and also Professor O. N. Srivastava, Physics Department BHU, for providing the TEM facilities.
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