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Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol
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Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol M.U. Anu Prathap, Rajendra Srivastavan Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, India Received 28 January 2013; received in revised form 10 April 2013; accepted 10 April 2013
KEYWORDS
Abstract
Spinel; Cyclic voltammetry; Electrocatalytic oxidation; Methanol oxidation
NiCo2O4 is synthesized by the hydrothermal route in the presence of urea. Material is characterized by a complementary combination of X-ray diffraction, nitrogen sorption, and Scanning electron microscopy. The electrochemical oxidation of methanol is investigated at NiCo2O4 modified electrode in the alkaline medium using cyclic voltammetry and chronoamperometry methods. Electrocatalytic activity of NiCo2O4 is compared with the NiO and Co3O4 modified electrodes. A detailed investigation is made for the electrocatalytic oxidation of methanol by varying several reaction parameters such as potential scan rate, methanol concentration, etc. Mechanism of methanol oxidation is proposed based on the cyclic voltammetry study. Double steps chronoamperometry study shows that the methanol electrooxidation is an irreversible reaction. Electrocatalytic activity of the methanol oxidation at NiCo2O4 modified electrode is found to be significantly higher than that of NiO and Co3O4 modified electrodes. & 2013 Elsevier Ltd. All rights reserved.
Introduction Transition metal oxides find application in heterogeneous catalysis and electrocatalysis due to their intrinsic redox properties and tunable chemical, morphological, and textural properties [1–6]. Transition metal based composite materials exhibit synergistic enhancement in activities that are much better than a simple combination of individual components, for example: bimetallic and multi-metallic
n
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395. E-mail address: [email protected] (R. Srivastava).
catalysts have shown promising properties with respect to stability, chemical activity, or resistance to poisoning, once combined [7]. A wide variety of composite materials such as dioxides, perovskites, pyrochlores, and spinel-type transition metal oxides have been investigated. Economical, low toxicity and high natural abundance attracted significant attention of the researcher to develop nickel and cobaltbased materials, which have shown tunable textural properties and exhibited superior electrochemical capacitive properties [8]. The advantages of using these oxides as electrode materials are associated with their activity, availability, thermodynamic stability, and low electrical resistance [9]. High electrical conductivity of these materials is due to the ease of electron transfer taking place with
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.04.003 Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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relatively low activation energy between cations of different valencies by hopping processes. Since the electrocatalysis depends on the electronic and geometrical factors [10], therefore, these compounds have demonstrated promising activity and stability in the alkaline medium as oxygen anode and cathode [11]. Methanol is an economical and readily available raw material for fuel cell and transportation applications [12]. Direct methanol fuel cell (DMFC) is considered as a promising candidate for future energy demand. Nevertheless, its practical application is hindered by high over-potential associated with the direct electro-oxidation of methanol. A great deal of interest has recently been shown with respect to the choice of electrode materials, cheaper than platinum [13]. Cobalt-containing spinel oxides (MCo2O4, where M = Cu, Mn, Ni, Zn, Mg, etc.) have drawn considerable attention due to their superior physicochemical properties and exhibited many technological applications, ranging from catalysts and sensors to electrode materials and electrochemical devices [14]. NiCo2O4 materials have shown excellent electrocatalytic activities for many electrode reactions, for example: O2 [15] and Cl2 evolution [16] and O2 reduction [17], as well its application as a negative electrode for lithium-ion batteries [18,19]. Spinel-oxides are low cost, easily available and have outstanding corrosion stability in alkaline solutions [20]. Therefore, in this study, we attempted to demonstrate the applicability of NiCo2O4 as an electrode material towards methanol oxidation in the alkaline medium. The electrocatalytic activity of NiCo2O4 modified electrode was compared with the single component oxides such as NiO and Co3O4. Furthermore, cyclic voltammetry and chronoamperometry methods were used to ascertain the mechanism of methanol oxidation.
Synthesis of Co3O4 CoCl2 6H2O (1.79 g) was dissolved in ethylene glycol (60 mL) to form a transparent solution. Sodium acetate (5.4 g) was added into the above solution. The mixed solution was magnetically stirred until it became transparent. Reaction mixture was transferred in a Teflon-lined stainless-steel autoclave and hydrothermally treated at 453 K for 12 h. After the reaction, autoclave was allowed to cool to room temperature. The products were filtered, rinsed with ethanol, and dried at 353 K for 24 h. The obtained material was calcined using programmable Furness (by maintaining the heating rate of 5 K min−1) at 773 K for 0.5 h to obtain Co3O4. It may be noted that when material was calcined at 573 K a mixed phase consists of Co(OH)2 and Co3O4 was obtained, therefore to obtain pure Co3O4, material was calcined at 773 K.
Synthesis of NiCo2O4 NiCo2O4 was prepared by following the reported procedure [19]. In a particular synthesis, 2.37 g CoCl2 6H2O, 1.19 g NiCl2 6H2O, and 2.7 g urea were dissolved in 75 mL distilled water and stirred for about 10 min to obtain a transparent pink colored solution. The reaction mixture was then transferred into a Teflon-lined stainless steel autoclave, and hydrothermally treated at 393 K for 6 h. Autoclave was cooled to room temperature and the reaction mixture was filtered, washed with distilled water and ethanol, followed by drying at 353 K for 24 h. Finally, the material was calcined at 573 K for 4 h using programmable Furness by maintaining the heating rate of 5 K min−1.
Preparation of the working electrodes
Experimental Materials All chemicals were analytical grade and used without further purification. NiNO3 6H2O, NiCl2 6H2O, CoCl2 6H2O, sodium acetate, and urea were obtained from Spectrochem Pvt. Ltd., India. Deionized water from Millipore Milli-Q system (resistivity 18.2 MΩ cm) was used in the electrochemical studies.
Synthesis of NiO 1.45 g NiNO3 6H2O was dissolved in distilled water (250 mL) and the solution was stirred for 15 min at room temperature. 0.6 g of urea was added to the above solution and magnetically stirred at room temperature for 3 h. Then the reaction mixture was transferred to Teflon lined steel autoclave and hydrothermally treated at 373 K for 10 h. The green precipitate of Ni(OH)2 was collected, washed with distilled water and dried in an oven at 353 K for 24 h. The obtained Ni(OH)2 was calcined using programmable Furness (by maintaining the heating rate of 5 K min−1) at 573 K for 4 h to form black powder NiO.
Cyclic voltammetry (CV) and chronoamperometric studies were performed using Potentiostat-Galvanostat BASi EPSILON, USA. A three electrode electrochemical cell was employed with Ag/AgCl as the reference electrode (3 M KCl), NiCo2O4 mounted glassy carbon electrode (GCE) (3 mm diameter) as the working electrode, and Pt foil as the counter electrode. Before modification of GCE, the polished electrode was ultrasonicated in ethanol and deionized water for 5 min, respectively. 10 mL aliquot of NiCo2O4 suspension (a homogenous sonicated solution of 10 mg of NiCo2O4 and a mixture of 0.1 mL of Nafion and 0.9 mL of water) was placed onto the electrode surface, the electrode was dried in air leaving the material mounted onto the GCE surface.
Materials characterizations X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5–901 with a scan speed of 2°/min on a PANalytical X'PERT PRO diffractometer using Cu Kα radiation (λ = 0.1542 nm, 40 kV, 20 mA). Scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM6610LV, to investigate the morphology. During the SEM investigation, energy-dispersive X-ray spectroscopy (EDS) was utilized in the sample characterization, and the EDS elemental maps were obtained. Nitrogen adsorption
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol measurement at 77 K was performed by Autosorb-IQ Quantachrome Instruments volumetric adsorption analyzer. Samples were out-gassed at 423 K for 4 h in the degas port of the adsorption apparatus. The specific surface area was determined by Brunauer–Emmett–Teller (BET) method using the data points of P/P0 in the range of about 0.05–0.3.
Results and discussion Materials prepared in this study were subjected to XRD investigation (Fig. 1a). The XRD patterns of NiO exhibited peaks at 2θ = 37.381, 43.381, 62.681, 75.481, and 79.481, which can be indexed as cubic NiO phase (JCPDS 02-1216). No reflection peaks corresponding to precursor Ni(OH)2 was detected in the XRD pattern of NiO (Fig. 1a), indicating that the sample is having high phase purity. XRD pattern of Co3O4, matches well with XRD pattern of the standard cubic spinel Co3O4 (JCPDS Card no. 43-1003), and the observed reflections are assigned to (111), (220), (311), (222), (400), (422), (511), and (440) (Fig. 1a). XRD pattern of NiCo2O4 shows the formation of a homogeneous spinel phase
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0
ð1Þ
where α is the absorption coefficient, Ephoton is the discrete photon energy, K is a constant relative to the material [23]. The plot of (αEphoton)2 vs. Ephoton based on the direct transition is shown in Fig. S2 (inset). The extrapolated value (the straight lines to the x axis) of Ephoton at α= 0 gives an absorption edge energy corresponding to Eg. Using DRUV– visible, Eg of the NiCo2O4 was found to be 2.06 and 2.63 eV. The internal electron transitions are found to play an important role in the photo excitation, as in the case of BaCr2O4 [24].
0.0 40
(Fig. 1a). Diffraction peaks observed for NiCo2O4 can be easily indexed as a face-centered cubic NiCo2O4 (JCPDS Card no. 73-1702). No peaks from other crystallized phases were observed, indicating the formation of pure NiCo2O4 product. It may be noted that the XRD pattern of Co3O4 and NiCo2O4 shown here seems to be similar, but they belong to different materials, which is well reported in the literature [21]. The cationic distribution in the Co3O4 lattices can be classified as normal spinels, whereas as inverse spinels in the NiCo2O4 [21]. The textural properties of NiCo2O4 were investigated by nitrogen adsorption–desorption measurements (Fig. 1b). Material exhibits type IV isotherm with H3 hysteresis loop, which is the characteristics of a mesoporous material. BJH pore size distribution (inset of Fig. 1b) shows a somewhat narrow pore size distribution centered at ∼3.8 nm along with a wide pore size distribution centered at ∼20.4 nm, respectively. Surface area of NiO, Co3O4, and NiCo2O4 were found to be 141, 25, and 68 m2 g−1. The SEM photomicrographs provide the surface morphology of the NiO, Co3O4, and NiCo2O4 (Fig. 2). SEM results show nanoflake aggregated micron size ball like morphology for NiO (Fig. 2a and b). Co3O4 exhibited irregular spheroidal morphologies (Fig. 2c), which are built with nanometer sized small crystallites (Fig. 2d). NiCo2O4 exhibited urchin-like morphology (Fig. 2e and f), which are composed of nanowires radially grown from the center, and forming a hierarchical micro/nanostructure. EDS mapping confirms the finely dispersed metal ions in the NiCo2O4 matrices (Fig. S1). Diffused reflectance UV–visible (DRUV–visible) investigation was used to determine the electronic structure and optical properties of NiCo2O4. It is generally known that in the spinel structure of NiCo2O4, the divalent cations Co2+ (one-half of the Co cations) occupy the tetrahedral sites, whereas the trivalent Ni3+ and Co3+ cations (the other half of Co cations) reside in the octahedral sites [22]. The NiCo2O4 exhibits two broad absorptions in the range of 350–500 and 600–800 nm (Fig. S2), which are due to the transition from O 2p to Co 3d-eg (or Ni 3d-eg) or from O 2p to Co 3d-t2g (or Ni 3d-t2g), and the internal electron transition from Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg). The absorption band gap energy (Eg) can be calculated by the following equation: ðαE photon Þ2 ¼ KðE photon −E g Þ
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Fig. 1 (a) XRD patterns of NiO, Co3O4, and NiCo2O4. (b) N2 adsorption–desorption isotherm of NiCo2O4. Inset shows pore size distribution.
Electrocatalytic oxidation of methanol at NiCo2O4 modified electrode Prior to the implementation of NiCo2O4 modified electrode for the oxidation of methanol, the electrochemical behavior
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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0.5 µm Fig. 2
SEM images of (a, b) NiO, (c, d) Co3O4, and (e, f) NiCo2O4.
of NiCo2O4, NiO, and Co3O4 modified electrodes were investigated using CV in 0.1 M NaOH solution. NiCo2O4 modified electrode exhibits a pair of well-defined redox peaks (Fig. S3(a)) corresponding to the oxidation of Co(II) and the reduction of Ni(III) [25,26]. These anodic peaks can be correlated with the oxidation of Co(II) to Co(III) to Co (IV). The cathodic peaks correspond to the reduction of Co (IV) to Co(III) to Co(II) and Ni(III) to Ni(II). It may be noted that the redox potential of Ni(II)/Ni(III) is very close to Co (III)/Co(IV) [25,26]. NiO modified electrode exhibits a pair of well-defined redox peaks corresponding to Ni(II)/Ni(III) redox couple (Fig. S3(a)) [27]. These peaks represent the oxidation of the Ni(OH)2 to the nickel oxy-hydroxide (NiOOH) in the anodic run and the reduction of the produced oxy-hydroxide to Ni(OH)2 in the cathodic half cycle, respectively [27]. The presence of cobalt in the NiCo2O4 structure increased the charge-acceptance of Ni in the form of the Ni(II)/Ni(III), therefore, the redox peaks shift to more negative potential values than NiO [28,29]. It may be noted that the enclosed area of CV loop of the NiCo2O4 is larger than the NiO; therefore, the current densities of the NiCo2O4 are higher than those of the NiO, implying its better electrochemical reactivity. CV of Co3O4 in 0.1 M NaOH consists of two pairs of redox peaks (I/IV and II/III), resulting from the reversible transition between Co3O4 and CoOOH (I/IV) and the transition between CoOOH and CoO2 (II/III) (Fig. S3(b)) [30]. This section describes the preliminary investigation of methanol oxidation at NiCo2O4, NiO, and Co3O4 modified electrodes in the presence of 1 M methanol. CV shows a strong oxidation current in the presence of 1 M methanol
when it was compared with the CV of NiCo2O4 modified electrode in the absence of methanol (Fig. S4). Similar behavior was obtained for NiO modified electrode, but with a low current response compared to NiCo2O4 modified electrode, indicating that the presence of Nickel is required for methanol oxidation (Fig. S5). However, when the similar study was performed at Co3O4 modified electrode in the presence of 1 M methanol, it did not show an increment in the oxidation current (Fig. S6), suggesting no electrocatalytic activity for methanol oxidation. Based on these observations, one can easily conclude that among the three materials investigated in this study, NiCo2O4 modified electrode exhibited the highest activity towards methanol oxidation. Fig. 3a shows a comparison of the CVs obtained at NiCo2O4 and NiO modified electrodes in the presence of 1 M methanol. Based on the CV, one can conclude that the current response at NiCo2O4 modified is much higher than that of NiO modified electrodes. Furthermore, NiCo2O4 modified electrode exhibited a decrease in the over-potential compared to NiO electrode. NiCo2O4 showed higher electrochemical activity, which are originated from the synergistic contributions from both nickel and cobalt ions, than those of the monometallic NiO and Co3O4. The addition of cobalt promotes a redox processes to less positive potentials, which mean that cobalt acted as a chemical modifier and caused a structural change or electronic effect [31,32]. Secondly, cobalt allows the nickel to reach a higher oxidation state during the oxidation process and promotes the electron transfer for methanol oxidation [33,34]. This section describes the mechanistic investigation for the electrochemical oxidation of methanol at NiCo2O4
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol
consumption of NiOOH species for oxidation of methanol with the formation of nickel hydroxide (Eq. (3)) (Fig. S7(a)) [37,38]. NiO modified electrode exhibited similar CV, but with a low current response compared to NiCo2O4 modified electrode (Fig. S7(b)).
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Fig. 3 (a) CV of NiCo2O4 and NiO modified electrodes in the presence of 1 M methanol in 0.1 M NaOH solution at a scan rate of 50 mV s−1. (b) CV of NiCo2O4 modified electrode in 0.1 M NaOH solution at a scan rate of 50 mV s−1 in the presence of (i) 0.1 M, (ii) 0.4 M, and (iii) 1 M methanol.
modified electrode. The electrochemical behavior of NiCo2O4 modified electrode in the presence of 0.1 M methanol shows an increment in the anodic peak current for peak (a1) followed by the appearance of a new oxidation peak (a2) at more positive potential and a peak current (c) during the reverse scan rate of potential (Fig. S7(a)), which is indicative of the methanol oxidation at NiCo2O4 modified electrode. The anodic peaks (a1) and (a2) demonstrate the existence of two different crystallographic structures of Ni (OH)2, (α-Ni(OH)2 and β-Ni(OH)2), which are responsible for methanol oxidation (Eqs. (2) and (3)) [35,36]. NiOOH species are formed from α-Ni(OH)2 (anodic peak a1), which can be formed even in the absence of methanol (Eq. (2)), further chemical reduction of NiOOH with methanol takes place to form β-Ni(OH)2 (Eq. (3)). Subsequently, β-Ni(OH)2) is converted to NiOOH at higher potentials, leading to the appearance of a new anodic peak (a2) (Eq. (4)). A cathodic peak (c), as a result of the oxy-hydroxide reduction, with very low current value compared to anodic peak is also observed. The relative decrease of the cathodic peak height in the presence of methanol is attributed to the partial
α-Ni(OH)2+OH−↔NiOOH+H2O+e−
(2)
NiOOH+methanol↔β-Ni(OH)2+product
(3)
β-Ni(OH)2+OH−↔NiOOH+H2O+e−
(4)
In order to understand the mechanism of the electrochemical oxidation of methanol, first the effect of methanol concentrations on the CV responses of NiCo2O4 modified electrode was investigated (Fig. 3b). At low concentration of methanol, two distinguished anodic peaks (a1 and a2) were observed (Fig. 3b (i, ii)). With the increase in the methanol concentration, first anodic peak, a1 current density decreases and further disappeared (methanol concentration40.4 M), whereas the current density of anodic peak, a2 became significant. With the increase in the methanol concentration, more β-Ni(OH)2 was formed due to the consumption of more NiOOH (Eq. (3)). Further, produced β-Ni(OH)2 could be oxidized to form more NiOOH with higher current densities (Eq. (4)), [35,36], which is responsible for significantly high-current density of a2. Based on the results obtained (Fig. 3b), one can conclude that the rate limiting step for the methanol oxidation (methanol concentration40.4 M) at the NiCo2O4 modified electrode is the reaction between methanol and NiOOH [39]. Furthermore, it was also observed that with the increase in the methanol concentration, the reduction current in the negative sweep decreases (c). This observation demonstrates that the methanol reacts with NiOOH formed on the electrode surface. The amount of NiOOH formed on the electrode surface decreases due to its involvement in the chemical reaction with methanol, which causes the decrease in the charge of Ni3+ (NiOOH) in the reverse sweep direction. The effect of scan rate on the methanol oxidation over NiCo2O4 modified electrode was investigated in the presence of 0.03 M methanol using CV (Fig. 4). It was observed that the current ratio of peak Pa1 and Pa2 (Pa1:Pa2) increases with the increase in the scan rate. At a scan rate of 200 mV s−1, only peak (a1) was observed, while the peak (a2) had almost disappeared. On the other hand, only peak (a2) was found in the voltammogram recorded at a slow scan rate (10 mV s−1). Therefore, it can be said that the reaction shown in Eq. (3) is not so fast that it does not match with reaction shown in Eq. (2). Therefore, the anodic current is mainly contributed by reaction shown by Eq. (2). As a result, the peak a2 reduces with the increase of the scan rates and finally disappeared at a scan rate of 200 mV s−1. Double potential step chronoamperometry, as well as other electrochemical methods, was employed for the investigation of the electrode processes at NiCo2O4 modified electrode. The double-step chronoamperograms for the NiCo2O4 modified electrode by setting the working electrode potential at 700 mV (first step) and 350 mV (second step) vs. Ag|AgCl|KCl (3 M) for different concentrations of methanol has been shown (Fig. 5). It was found that the current observed from chronoamperograms was in good
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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Fig. 4 CV of NiCo2O4 modified electrode recorded in 0.1 M NaOH+0.03 M methanol at a scan rate of (i) 600, (ii) 200, (iii) 100, (iv) 50, and (v) 10 mV s−1.
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of the modified electrode in the blank solution showed an almost symmetrical chronoamperogram with almost equal charges consumed for the oxidation and reduction of surface-confined Ni(II)/Ni(III) sites (Fig. 5A (a′)). However, in the presence of methanol, the charge value associated with the forward chronoamperometry is greater than that observed for the backward chronoamperometry (Fig. 5A (c′)). The current is negligible when potential is stepped down to 350 mV, indicating that the electrocatalytic oxidation of methanol is irreversible. The rate constant for the chemical reaction between the methanol and redox sites of NiCo2O4 can be evaluated by chronoamperometry according to the method described in the literature [37]. IC =IL ¼ γ 1=2 ½π 1=2 erfðγ 1=2 Þ þ expð−γÞγ −1=2
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Fig. 5 Chronoamperograms obtained at the NiCo2O4 modified electrode in the absence (a) and in the presence of (b) 0.03 M, and (c) 0.05 M of MeOH in 0.1 M NaOH solution. First and second potential steps were 700 and 350 mV vs. Ag|AgCl|KCl (3 M), respectively. Inset (A): dependence of charge (mC) vs. time, (a′) and (c′) are derived from the data of chronoamperograms of (a) and (c), respectively in the main panel. Inset (B): dependence of IC/IL on time1/2 is derived from the data of chronoamperograms of (a) and (c) in the main panel. Inset (C): dependence of current (mA) on time−1/2 is derived from the data of chronoamperogram of (a) in the main panel.
agreement with the current observed from CV. Furthermore, the current increases as the methanol concentration increases. This result supports our conclusion about the catalytic role of NiOOH in the methanol oxidation. The forward and backward potential step chronoamperometry
ð5Þ
where IC is the catalytic current of the NiCo2O4 in the presence of methanol, IL is the limiting current in the absence of methanol and γ =kC0t (C0 is the bulk concentration of methanol) is the argument of the error function. In cases, where γ exceed 2, the error function is almost equal to 1, and the above equation can be reduced to IC =IL ¼ π 1=2 γ 1=2 ¼ π 1=2 ðkC0 tÞ1=2
ð6Þ
where k, C0 and t are the catalytic rate constant (cm3 mol−1 s−1), methanol concentration (mol cm−3), and time elapsed (s), respectively. From the slope of the IC/IL vs. time1/2 plot, the value of k for a given concentration of methanol can be calculated. Inset B of Fig. 5 shows one such plot, constructed from the chronoamperogram of the NiCo2O4 in the absence and in the presence of 0.05 M methanol. The mean value for k was found to be 5.5 103 cm3 mol−1 s−1. Plotting of net current of NiCo2O4 in the absence of methanol with respect to time−1/2, presented a linear dependence (see the inset C of Fig. 5), which shows that the process is dominated by diffusion. The mean values of the sensitivity were
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol 2500
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Authors thank Department of Science and Technology, New Delhi for financial assistance (DST Grant SB/S1/PC-91/ 2012). AP is grateful to CSIR, New Delhi for SRF fellowship. We acknowledge Director, IIT Ropar for constant encouragements.
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Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.04.003.
1.0
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Fig. 6 CV of NiCo2O4 modified electrode in the presence of different concentrations of methanol in 0.1 M NaOH solutions at a scan rate of 50 mV s−1. Inset shows the calibration plot.
found to be 80 and 700 μA M−1 for NiO and NiCo2O4 modified electrodes, respectively. An excellent linear relationship (based on CV responses) between oxidation peak currents and methanol concentrations was observed in the range of 0.01–1.8 M for NiCo2O4 with high-current (see the inset of Fig. 6). Linear relationship between oxidation peak currents and methanol concentrations was observed in the range of 0.01–1.6 M (see the inset of Fig. S8) for NiO but with low current compared to NiCo2O4. High electrocatalytic activity of NiCo2O4 compared with the NiO may be correlated with the facile internal electron transitions between Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg). Based on the experimental evidence, one can conclude that NiCo2O4 exhibited superior current sensitivity compared to NiO modified electrodes investigated in this study.
Conclusions The electrocatalytic oxidation of methanol on NiCo2O4, NiO, and Co3O4 modified electrodes was studied in the alkaline solution. The electrocatalytic activity of NiCo2O4 modified electrode in the methanol oxidation is significantly higher than that of NiO modified electrode. Co3O4 modified electrode was found to be inactive for the methanol oxidation. Double steps choronoamperograms confirm that the oxidation of methanol at NiCo2O4 modified electrode is an irreversible and diffusion controlled process. A significant reduction in the over-potential was observed for the NiCo2O4 modified electrode when compared to the NiO modified electrode. The linear proportionality of catalytic peak current for methanol oxidation and also relatively low over-potential toward methanol oxidation lead us to conclude that NiCo2O4 modified electrode provides a promising new catalyst for the methanol oxidation.
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[39] A. Nozad Golikand, S. Shahrokhian, M. Asgari, M. Ghannadi Maragheh, L. Irannejad, A. Khanchi, Journal of Power Sources 144 (2005) 21–27. M.U. Anu Prathap received his Master in Technology in Polymer Science and Rubber Technology from Cochin University of Science and Technology, India. He is currently a graduate student working towards PhD degree in Department of Chemistry from Indian Institute of Technology Ropar, Punjab, India. His research interest includes the development of nanomaterials for electrochemical biosensors and energy conversion device applications. Rajendra Srivastava is an Assistant Professor of Chemistry at IIT Ropar. He obtained his doctoral degree from NCL Pune, India. He worked as a post-doctoral research fellow at KAIST, South Korea and as a JSPS fellowship recipient at Hokkaido University, Japan. He has published more than 50 research papers and has, to his credit, seven international patents. His doctoral work was adjudged the best thesis for the year 2006 by Catalysis Society of India. His research interests are in the areas of development of nanostructure materials and find their applications in catalysis (homogeneous and heterogeneous catalysis/electrocatalysis) and bio-sensing.
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol M.U. Anu Prathap, Rajendra Srivastavan Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, India Received 28 January 2013; received in revised form 10 April 2013; accepted 10 April 2013
KEYWORDS
Abstract
Spinel; Cyclic voltammetry; Electrocatalytic oxidation; Methanol oxidation
NiCo2O4 is synthesized by the hydrothermal route in the presence of urea. Material is characterized by a complementary combination of X-ray diffraction, nitrogen sorption, and Scanning electron microscopy. The electrochemical oxidation of methanol is investigated at NiCo2O4 modified electrode in the alkaline medium using cyclic voltammetry and chronoamperometry methods. Electrocatalytic activity of NiCo2O4 is compared with the NiO and Co3O4 modified electrodes. A detailed investigation is made for the electrocatalytic oxidation of methanol by varying several reaction parameters such as potential scan rate, methanol concentration, etc. Mechanism of methanol oxidation is proposed based on the cyclic voltammetry study. Double steps chronoamperometry study shows that the methanol electrooxidation is an irreversible reaction. Electrocatalytic activity of the methanol oxidation at NiCo2O4 modified electrode is found to be significantly higher than that of NiO and Co3O4 modified electrodes. & 2013 Elsevier Ltd. All rights reserved.
Introduction Transition metal oxides find application in heterogeneous catalysis and electrocatalysis due to their intrinsic redox properties and tunable chemical, morphological, and textural properties [1–6]. Transition metal based composite materials exhibit synergistic enhancement in activities that are much better than a simple combination of individual components, for example: bimetallic and multi-metallic
n
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395. E-mail address: [email protected] (R. Srivastava).
catalysts have shown promising properties with respect to stability, chemical activity, or resistance to poisoning, once combined [7]. A wide variety of composite materials such as dioxides, perovskites, pyrochlores, and spinel-type transition metal oxides have been investigated. Economical, low toxicity and high natural abundance attracted significant attention of the researcher to develop nickel and cobaltbased materials, which have shown tunable textural properties and exhibited superior electrochemical capacitive properties [8]. The advantages of using these oxides as electrode materials are associated with their activity, availability, thermodynamic stability, and low electrical resistance [9]. High electrical conductivity of these materials is due to the ease of electron transfer taking place with
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.04.003 Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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relatively low activation energy between cations of different valencies by hopping processes. Since the electrocatalysis depends on the electronic and geometrical factors [10], therefore, these compounds have demonstrated promising activity and stability in the alkaline medium as oxygen anode and cathode [11]. Methanol is an economical and readily available raw material for fuel cell and transportation applications [12]. Direct methanol fuel cell (DMFC) is considered as a promising candidate for future energy demand. Nevertheless, its practical application is hindered by high over-potential associated with the direct electro-oxidation of methanol. A great deal of interest has recently been shown with respect to the choice of electrode materials, cheaper than platinum [13]. Cobalt-containing spinel oxides (MCo2O4, where M = Cu, Mn, Ni, Zn, Mg, etc.) have drawn considerable attention due to their superior physicochemical properties and exhibited many technological applications, ranging from catalysts and sensors to electrode materials and electrochemical devices [14]. NiCo2O4 materials have shown excellent electrocatalytic activities for many electrode reactions, for example: O2 [15] and Cl2 evolution [16] and O2 reduction [17], as well its application as a negative electrode for lithium-ion batteries [18,19]. Spinel-oxides are low cost, easily available and have outstanding corrosion stability in alkaline solutions [20]. Therefore, in this study, we attempted to demonstrate the applicability of NiCo2O4 as an electrode material towards methanol oxidation in the alkaline medium. The electrocatalytic activity of NiCo2O4 modified electrode was compared with the single component oxides such as NiO and Co3O4. Furthermore, cyclic voltammetry and chronoamperometry methods were used to ascertain the mechanism of methanol oxidation.
Synthesis of Co3O4 CoCl2 6H2O (1.79 g) was dissolved in ethylene glycol (60 mL) to form a transparent solution. Sodium acetate (5.4 g) was added into the above solution. The mixed solution was magnetically stirred until it became transparent. Reaction mixture was transferred in a Teflon-lined stainless-steel autoclave and hydrothermally treated at 453 K for 12 h. After the reaction, autoclave was allowed to cool to room temperature. The products were filtered, rinsed with ethanol, and dried at 353 K for 24 h. The obtained material was calcined using programmable Furness (by maintaining the heating rate of 5 K min−1) at 773 K for 0.5 h to obtain Co3O4. It may be noted that when material was calcined at 573 K a mixed phase consists of Co(OH)2 and Co3O4 was obtained, therefore to obtain pure Co3O4, material was calcined at 773 K.
Synthesis of NiCo2O4 NiCo2O4 was prepared by following the reported procedure [19]. In a particular synthesis, 2.37 g CoCl2 6H2O, 1.19 g NiCl2 6H2O, and 2.7 g urea were dissolved in 75 mL distilled water and stirred for about 10 min to obtain a transparent pink colored solution. The reaction mixture was then transferred into a Teflon-lined stainless steel autoclave, and hydrothermally treated at 393 K for 6 h. Autoclave was cooled to room temperature and the reaction mixture was filtered, washed with distilled water and ethanol, followed by drying at 353 K for 24 h. Finally, the material was calcined at 573 K for 4 h using programmable Furness by maintaining the heating rate of 5 K min−1.
Preparation of the working electrodes
Experimental Materials All chemicals were analytical grade and used without further purification. NiNO3 6H2O, NiCl2 6H2O, CoCl2 6H2O, sodium acetate, and urea were obtained from Spectrochem Pvt. Ltd., India. Deionized water from Millipore Milli-Q system (resistivity 18.2 MΩ cm) was used in the electrochemical studies.
Synthesis of NiO 1.45 g NiNO3 6H2O was dissolved in distilled water (250 mL) and the solution was stirred for 15 min at room temperature. 0.6 g of urea was added to the above solution and magnetically stirred at room temperature for 3 h. Then the reaction mixture was transferred to Teflon lined steel autoclave and hydrothermally treated at 373 K for 10 h. The green precipitate of Ni(OH)2 was collected, washed with distilled water and dried in an oven at 353 K for 24 h. The obtained Ni(OH)2 was calcined using programmable Furness (by maintaining the heating rate of 5 K min−1) at 573 K for 4 h to form black powder NiO.
Cyclic voltammetry (CV) and chronoamperometric studies were performed using Potentiostat-Galvanostat BASi EPSILON, USA. A three electrode electrochemical cell was employed with Ag/AgCl as the reference electrode (3 M KCl), NiCo2O4 mounted glassy carbon electrode (GCE) (3 mm diameter) as the working electrode, and Pt foil as the counter electrode. Before modification of GCE, the polished electrode was ultrasonicated in ethanol and deionized water for 5 min, respectively. 10 mL aliquot of NiCo2O4 suspension (a homogenous sonicated solution of 10 mg of NiCo2O4 and a mixture of 0.1 mL of Nafion and 0.9 mL of water) was placed onto the electrode surface, the electrode was dried in air leaving the material mounted onto the GCE surface.
Materials characterizations X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5–901 with a scan speed of 2°/min on a PANalytical X'PERT PRO diffractometer using Cu Kα radiation (λ = 0.1542 nm, 40 kV, 20 mA). Scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM6610LV, to investigate the morphology. During the SEM investigation, energy-dispersive X-ray spectroscopy (EDS) was utilized in the sample characterization, and the EDS elemental maps were obtained. Nitrogen adsorption
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol measurement at 77 K was performed by Autosorb-IQ Quantachrome Instruments volumetric adsorption analyzer. Samples were out-gassed at 423 K for 4 h in the degas port of the adsorption apparatus. The specific surface area was determined by Brunauer–Emmett–Teller (BET) method using the data points of P/P0 in the range of about 0.05–0.3.
Results and discussion Materials prepared in this study were subjected to XRD investigation (Fig. 1a). The XRD patterns of NiO exhibited peaks at 2θ = 37.381, 43.381, 62.681, 75.481, and 79.481, which can be indexed as cubic NiO phase (JCPDS 02-1216). No reflection peaks corresponding to precursor Ni(OH)2 was detected in the XRD pattern of NiO (Fig. 1a), indicating that the sample is having high phase purity. XRD pattern of Co3O4, matches well with XRD pattern of the standard cubic spinel Co3O4 (JCPDS Card no. 43-1003), and the observed reflections are assigned to (111), (220), (311), (222), (400), (422), (511), and (440) (Fig. 1a). XRD pattern of NiCo2O4 shows the formation of a homogeneous spinel phase
(311)
Intensity (a.u.)
(222)
NiO Co3O4 NiCo2O4
(220)
(111)
(200)
(440)
(422) (511)
(400)
(222)
(111)
(220)
(311)
20
30
40
50
(440)
(422) (511)
(400)
(222)
(111)
(220)
(311)
60
70
80
90
2θ 160
Dv (log d) (cm3/g)
Adsorbed amount (mL/g)
120
80
3.8 nm
0.3
20.4 nm
0.1
0
ð1Þ
where α is the absorption coefficient, Ephoton is the discrete photon energy, K is a constant relative to the material [23]. The plot of (αEphoton)2 vs. Ephoton based on the direct transition is shown in Fig. S2 (inset). The extrapolated value (the straight lines to the x axis) of Ephoton at α= 0 gives an absorption edge energy corresponding to Eg. Using DRUV– visible, Eg of the NiCo2O4 was found to be 2.06 and 2.63 eV. The internal electron transitions are found to play an important role in the photo excitation, as in the case of BaCr2O4 [24].
0.0 40
(Fig. 1a). Diffraction peaks observed for NiCo2O4 can be easily indexed as a face-centered cubic NiCo2O4 (JCPDS Card no. 73-1702). No peaks from other crystallized phases were observed, indicating the formation of pure NiCo2O4 product. It may be noted that the XRD pattern of Co3O4 and NiCo2O4 shown here seems to be similar, but they belong to different materials, which is well reported in the literature [21]. The cationic distribution in the Co3O4 lattices can be classified as normal spinels, whereas as inverse spinels in the NiCo2O4 [21]. The textural properties of NiCo2O4 were investigated by nitrogen adsorption–desorption measurements (Fig. 1b). Material exhibits type IV isotherm with H3 hysteresis loop, which is the characteristics of a mesoporous material. BJH pore size distribution (inset of Fig. 1b) shows a somewhat narrow pore size distribution centered at ∼3.8 nm along with a wide pore size distribution centered at ∼20.4 nm, respectively. Surface area of NiO, Co3O4, and NiCo2O4 were found to be 141, 25, and 68 m2 g−1. The SEM photomicrographs provide the surface morphology of the NiO, Co3O4, and NiCo2O4 (Fig. 2). SEM results show nanoflake aggregated micron size ball like morphology for NiO (Fig. 2a and b). Co3O4 exhibited irregular spheroidal morphologies (Fig. 2c), which are built with nanometer sized small crystallites (Fig. 2d). NiCo2O4 exhibited urchin-like morphology (Fig. 2e and f), which are composed of nanowires radially grown from the center, and forming a hierarchical micro/nanostructure. EDS mapping confirms the finely dispersed metal ions in the NiCo2O4 matrices (Fig. S1). Diffused reflectance UV–visible (DRUV–visible) investigation was used to determine the electronic structure and optical properties of NiCo2O4. It is generally known that in the spinel structure of NiCo2O4, the divalent cations Co2+ (one-half of the Co cations) occupy the tetrahedral sites, whereas the trivalent Ni3+ and Co3+ cations (the other half of Co cations) reside in the octahedral sites [22]. The NiCo2O4 exhibits two broad absorptions in the range of 350–500 and 600–800 nm (Fig. S2), which are due to the transition from O 2p to Co 3d-eg (or Ni 3d-eg) or from O 2p to Co 3d-t2g (or Ni 3d-t2g), and the internal electron transition from Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg). The absorption band gap energy (Eg) can be calculated by the following equation: ðαE photon Þ2 ¼ KðE photon −E g Þ
0.2
3
20 40 60 80 100 Pore diameter (nm)
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 1 (a) XRD patterns of NiO, Co3O4, and NiCo2O4. (b) N2 adsorption–desorption isotherm of NiCo2O4. Inset shows pore size distribution.
Electrocatalytic oxidation of methanol at NiCo2O4 modified electrode Prior to the implementation of NiCo2O4 modified electrode for the oxidation of methanol, the electrochemical behavior
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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M.U. Anu Prathap, R. Srivastava
0.5 µm Fig. 2
SEM images of (a, b) NiO, (c, d) Co3O4, and (e, f) NiCo2O4.
of NiCo2O4, NiO, and Co3O4 modified electrodes were investigated using CV in 0.1 M NaOH solution. NiCo2O4 modified electrode exhibits a pair of well-defined redox peaks (Fig. S3(a)) corresponding to the oxidation of Co(II) and the reduction of Ni(III) [25,26]. These anodic peaks can be correlated with the oxidation of Co(II) to Co(III) to Co (IV). The cathodic peaks correspond to the reduction of Co (IV) to Co(III) to Co(II) and Ni(III) to Ni(II). It may be noted that the redox potential of Ni(II)/Ni(III) is very close to Co (III)/Co(IV) [25,26]. NiO modified electrode exhibits a pair of well-defined redox peaks corresponding to Ni(II)/Ni(III) redox couple (Fig. S3(a)) [27]. These peaks represent the oxidation of the Ni(OH)2 to the nickel oxy-hydroxide (NiOOH) in the anodic run and the reduction of the produced oxy-hydroxide to Ni(OH)2 in the cathodic half cycle, respectively [27]. The presence of cobalt in the NiCo2O4 structure increased the charge-acceptance of Ni in the form of the Ni(II)/Ni(III), therefore, the redox peaks shift to more negative potential values than NiO [28,29]. It may be noted that the enclosed area of CV loop of the NiCo2O4 is larger than the NiO; therefore, the current densities of the NiCo2O4 are higher than those of the NiO, implying its better electrochemical reactivity. CV of Co3O4 in 0.1 M NaOH consists of two pairs of redox peaks (I/IV and II/III), resulting from the reversible transition between Co3O4 and CoOOH (I/IV) and the transition between CoOOH and CoO2 (II/III) (Fig. S3(b)) [30]. This section describes the preliminary investigation of methanol oxidation at NiCo2O4, NiO, and Co3O4 modified electrodes in the presence of 1 M methanol. CV shows a strong oxidation current in the presence of 1 M methanol
when it was compared with the CV of NiCo2O4 modified electrode in the absence of methanol (Fig. S4). Similar behavior was obtained for NiO modified electrode, but with a low current response compared to NiCo2O4 modified electrode, indicating that the presence of Nickel is required for methanol oxidation (Fig. S5). However, when the similar study was performed at Co3O4 modified electrode in the presence of 1 M methanol, it did not show an increment in the oxidation current (Fig. S6), suggesting no electrocatalytic activity for methanol oxidation. Based on these observations, one can easily conclude that among the three materials investigated in this study, NiCo2O4 modified electrode exhibited the highest activity towards methanol oxidation. Fig. 3a shows a comparison of the CVs obtained at NiCo2O4 and NiO modified electrodes in the presence of 1 M methanol. Based on the CV, one can conclude that the current response at NiCo2O4 modified is much higher than that of NiO modified electrodes. Furthermore, NiCo2O4 modified electrode exhibited a decrease in the over-potential compared to NiO electrode. NiCo2O4 showed higher electrochemical activity, which are originated from the synergistic contributions from both nickel and cobalt ions, than those of the monometallic NiO and Co3O4. The addition of cobalt promotes a redox processes to less positive potentials, which mean that cobalt acted as a chemical modifier and caused a structural change or electronic effect [31,32]. Secondly, cobalt allows the nickel to reach a higher oxidation state during the oxidation process and promotes the electron transfer for methanol oxidation [33,34]. This section describes the mechanistic investigation for the electrochemical oxidation of methanol at NiCo2O4
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol
consumption of NiOOH species for oxidation of methanol with the formation of nickel hydroxide (Eq. (3)) (Fig. S7(a)) [37,38]. NiO modified electrode exhibited similar CV, but with a low current response compared to NiCo2O4 modified electrode (Fig. S7(b)).
2000 NiCo2O4 NiO
Current (μA)
1500
1000
500
0 0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
2500 (ii)
(i)
(iii)
2000 a2
Current (μA)
1500 a2 1000
500
a2
a1
a1 0
-500 0.0
0.5
1.0
5
0.0 0.5 1.0 Potential (V)
0.0
0.5
1.0
Fig. 3 (a) CV of NiCo2O4 and NiO modified electrodes in the presence of 1 M methanol in 0.1 M NaOH solution at a scan rate of 50 mV s−1. (b) CV of NiCo2O4 modified electrode in 0.1 M NaOH solution at a scan rate of 50 mV s−1 in the presence of (i) 0.1 M, (ii) 0.4 M, and (iii) 1 M methanol.
modified electrode. The electrochemical behavior of NiCo2O4 modified electrode in the presence of 0.1 M methanol shows an increment in the anodic peak current for peak (a1) followed by the appearance of a new oxidation peak (a2) at more positive potential and a peak current (c) during the reverse scan rate of potential (Fig. S7(a)), which is indicative of the methanol oxidation at NiCo2O4 modified electrode. The anodic peaks (a1) and (a2) demonstrate the existence of two different crystallographic structures of Ni (OH)2, (α-Ni(OH)2 and β-Ni(OH)2), which are responsible for methanol oxidation (Eqs. (2) and (3)) [35,36]. NiOOH species are formed from α-Ni(OH)2 (anodic peak a1), which can be formed even in the absence of methanol (Eq. (2)), further chemical reduction of NiOOH with methanol takes place to form β-Ni(OH)2 (Eq. (3)). Subsequently, β-Ni(OH)2) is converted to NiOOH at higher potentials, leading to the appearance of a new anodic peak (a2) (Eq. (4)). A cathodic peak (c), as a result of the oxy-hydroxide reduction, with very low current value compared to anodic peak is also observed. The relative decrease of the cathodic peak height in the presence of methanol is attributed to the partial
α-Ni(OH)2+OH−↔NiOOH+H2O+e−
(2)
NiOOH+methanol↔β-Ni(OH)2+product
(3)
β-Ni(OH)2+OH−↔NiOOH+H2O+e−
(4)
In order to understand the mechanism of the electrochemical oxidation of methanol, first the effect of methanol concentrations on the CV responses of NiCo2O4 modified electrode was investigated (Fig. 3b). At low concentration of methanol, two distinguished anodic peaks (a1 and a2) were observed (Fig. 3b (i, ii)). With the increase in the methanol concentration, first anodic peak, a1 current density decreases and further disappeared (methanol concentration40.4 M), whereas the current density of anodic peak, a2 became significant. With the increase in the methanol concentration, more β-Ni(OH)2 was formed due to the consumption of more NiOOH (Eq. (3)). Further, produced β-Ni(OH)2 could be oxidized to form more NiOOH with higher current densities (Eq. (4)), [35,36], which is responsible for significantly high-current density of a2. Based on the results obtained (Fig. 3b), one can conclude that the rate limiting step for the methanol oxidation (methanol concentration40.4 M) at the NiCo2O4 modified electrode is the reaction between methanol and NiOOH [39]. Furthermore, it was also observed that with the increase in the methanol concentration, the reduction current in the negative sweep decreases (c). This observation demonstrates that the methanol reacts with NiOOH formed on the electrode surface. The amount of NiOOH formed on the electrode surface decreases due to its involvement in the chemical reaction with methanol, which causes the decrease in the charge of Ni3+ (NiOOH) in the reverse sweep direction. The effect of scan rate on the methanol oxidation over NiCo2O4 modified electrode was investigated in the presence of 0.03 M methanol using CV (Fig. 4). It was observed that the current ratio of peak Pa1 and Pa2 (Pa1:Pa2) increases with the increase in the scan rate. At a scan rate of 200 mV s−1, only peak (a1) was observed, while the peak (a2) had almost disappeared. On the other hand, only peak (a2) was found in the voltammogram recorded at a slow scan rate (10 mV s−1). Therefore, it can be said that the reaction shown in Eq. (3) is not so fast that it does not match with reaction shown in Eq. (2). Therefore, the anodic current is mainly contributed by reaction shown by Eq. (2). As a result, the peak a2 reduces with the increase of the scan rates and finally disappeared at a scan rate of 200 mV s−1. Double potential step chronoamperometry, as well as other electrochemical methods, was employed for the investigation of the electrode processes at NiCo2O4 modified electrode. The double-step chronoamperograms for the NiCo2O4 modified electrode by setting the working electrode potential at 700 mV (first step) and 350 mV (second step) vs. Ag|AgCl|KCl (3 M) for different concentrations of methanol has been shown (Fig. 5). It was found that the current observed from chronoamperograms was in good
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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of the modified electrode in the blank solution showed an almost symmetrical chronoamperogram with almost equal charges consumed for the oxidation and reduction of surface-confined Ni(II)/Ni(III) sites (Fig. 5A (a′)). However, in the presence of methanol, the charge value associated with the forward chronoamperometry is greater than that observed for the backward chronoamperometry (Fig. 5A (c′)). The current is negligible when potential is stepped down to 350 mV, indicating that the electrocatalytic oxidation of methanol is irreversible. The rate constant for the chemical reaction between the methanol and redox sites of NiCo2O4 can be evaluated by chronoamperometry according to the method described in the literature [37]. IC =IL ¼ γ 1=2 ½π 1=2 erfðγ 1=2 Þ þ expð−γÞγ −1=2
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Fig. 5 Chronoamperograms obtained at the NiCo2O4 modified electrode in the absence (a) and in the presence of (b) 0.03 M, and (c) 0.05 M of MeOH in 0.1 M NaOH solution. First and second potential steps were 700 and 350 mV vs. Ag|AgCl|KCl (3 M), respectively. Inset (A): dependence of charge (mC) vs. time, (a′) and (c′) are derived from the data of chronoamperograms of (a) and (c), respectively in the main panel. Inset (B): dependence of IC/IL on time1/2 is derived from the data of chronoamperograms of (a) and (c) in the main panel. Inset (C): dependence of current (mA) on time−1/2 is derived from the data of chronoamperogram of (a) in the main panel.
agreement with the current observed from CV. Furthermore, the current increases as the methanol concentration increases. This result supports our conclusion about the catalytic role of NiOOH in the methanol oxidation. The forward and backward potential step chronoamperometry
ð5Þ
where IC is the catalytic current of the NiCo2O4 in the presence of methanol, IL is the limiting current in the absence of methanol and γ =kC0t (C0 is the bulk concentration of methanol) is the argument of the error function. In cases, where γ exceed 2, the error function is almost equal to 1, and the above equation can be reduced to IC =IL ¼ π 1=2 γ 1=2 ¼ π 1=2 ðkC0 tÞ1=2
ð6Þ
where k, C0 and t are the catalytic rate constant (cm3 mol−1 s−1), methanol concentration (mol cm−3), and time elapsed (s), respectively. From the slope of the IC/IL vs. time1/2 plot, the value of k for a given concentration of methanol can be calculated. Inset B of Fig. 5 shows one such plot, constructed from the chronoamperogram of the NiCo2O4 in the absence and in the presence of 0.05 M methanol. The mean value for k was found to be 5.5 103 cm3 mol−1 s−1. Plotting of net current of NiCo2O4 in the absence of methanol with respect to time−1/2, presented a linear dependence (see the inset C of Fig. 5), which shows that the process is dominated by diffusion. The mean values of the sensitivity were
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003
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Authors thank Department of Science and Technology, New Delhi for financial assistance (DST Grant SB/S1/PC-91/ 2012). AP is grateful to CSIR, New Delhi for SRF fellowship. We acknowledge Director, IIT Ropar for constant encouragements.
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Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.04.003.
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Fig. 6 CV of NiCo2O4 modified electrode in the presence of different concentrations of methanol in 0.1 M NaOH solutions at a scan rate of 50 mV s−1. Inset shows the calibration plot.
found to be 80 and 700 μA M−1 for NiO and NiCo2O4 modified electrodes, respectively. An excellent linear relationship (based on CV responses) between oxidation peak currents and methanol concentrations was observed in the range of 0.01–1.8 M for NiCo2O4 with high-current (see the inset of Fig. 6). Linear relationship between oxidation peak currents and methanol concentrations was observed in the range of 0.01–1.6 M (see the inset of Fig. S8) for NiO but with low current compared to NiCo2O4. High electrocatalytic activity of NiCo2O4 compared with the NiO may be correlated with the facile internal electron transitions between Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg). Based on the experimental evidence, one can conclude that NiCo2O4 exhibited superior current sensitivity compared to NiO modified electrodes investigated in this study.
Conclusions The electrocatalytic oxidation of methanol on NiCo2O4, NiO, and Co3O4 modified electrodes was studied in the alkaline solution. The electrocatalytic activity of NiCo2O4 modified electrode in the methanol oxidation is significantly higher than that of NiO modified electrode. Co3O4 modified electrode was found to be inactive for the methanol oxidation. Double steps choronoamperograms confirm that the oxidation of methanol at NiCo2O4 modified electrode is an irreversible and diffusion controlled process. A significant reduction in the over-potential was observed for the NiCo2O4 modified electrode when compared to the NiO modified electrode. The linear proportionality of catalytic peak current for methanol oxidation and also relatively low over-potential toward methanol oxidation lead us to conclude that NiCo2O4 modified electrode provides a promising new catalyst for the methanol oxidation.
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[39] A. Nozad Golikand, S. Shahrokhian, M. Asgari, M. Ghannadi Maragheh, L. Irannejad, A. Khanchi, Journal of Power Sources 144 (2005) 21–27. M.U. Anu Prathap received his Master in Technology in Polymer Science and Rubber Technology from Cochin University of Science and Technology, India. He is currently a graduate student working towards PhD degree in Department of Chemistry from Indian Institute of Technology Ropar, Punjab, India. His research interest includes the development of nanomaterials for electrochemical biosensors and energy conversion device applications. Rajendra Srivastava is an Assistant Professor of Chemistry at IIT Ropar. He obtained his doctoral degree from NCL Pune, India. He worked as a post-doctoral research fellow at KAIST, South Korea and as a JSPS fellowship recipient at Hokkaido University, Japan. He has published more than 50 research papers and has, to his credit, seven international patents. His doctoral work was adjudged the best thesis for the year 2006 by Catalysis Society of India. His research interests are in the areas of development of nanostructure materials and find their applications in catalysis (homogeneous and heterogeneous catalysis/electrocatalysis) and bio-sensing.
Please cite this article as: M.U. Anu Prathap, R. Srivastava, Synthesis of NiCo2O4 and its application in the electrocatalytic oxidation of methanol, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.04.003