The electrocatalytic oxidation of methanol using a new modified electrode based on NiCo2O4 nanoparticles incorporated into zeolite-4A

Abstract

In this work, a new catalyst based on carbon paste (CP) modified by spinel NiCo₂O₄ nanoparticles and zeolite-4A was proposed and used for the electrochemical oxidation of methanol. NiCo₂O₄@zeolite-4A was obtained through the calcination of the physical mixture of NiCo₂O₄ and zeolite-4A. Their physicochemical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectra (SEM-EDS), and FT-IR techniques. The electrocatalytic performance of modified electrode based on NiCo₂O₄@zeolite-4A was tested using cyclic voltammetry (CV) and chronoamperomerty (CA). The result of cyclic voltammetry indicated a significant rise in the oxidation current on the surface of NiCo₂O₄@zeolite-4A/CPE modified electrode. The rate constant for the catalytic reaction (k) of methanol was calculated by means of a chronoamperometric technique. To sum up, the incorporation of NiCo₂O₄ nanoparticles into zeolite-4A provided the binary electroactive sites for catalysis of methanol oxidation as it exhibited a long-term stability for catalytic reaction, and thus a promising application.

Resumen

En este trabajo se propuso un nuevo catalizador basado en pasta de carbono (CP) modificado por espinela de nanopartículas de NiCo₂O₄ y zeolita-4ª, y se utilizó para la oxidación electroquímica del metanol. Se obtuvo NiCo₂O₄@zeolita-4A a través de la calcinación de la mezcla física de NiCo₂O₄ y zeolita-4A. Sus propiedades fisicoquímicas se caracterizaron por difracción de rayos X (XRD), espectros de rayos X de barrido de energía con microscopía electrónica de barrido (SEM-EDS) y técnicas de FT-IR. El rendimiento electrocatalítico del electrodo modificado basado en NiCo₂O₄@zeolita-4A se probó utilizando la voltametría cíclica (CV) y cronoamperometría (CA). El resultado de la voltametría cíclica indicó un aumento significativo de la corriente de oxidación en la superficie del electrodo modificado con zeolita-4A/CPE de NiCo₂O₄. La constante de velocidad para la reacción catalítica (k) del metanol se calculó mediante una técnica cronoamperométrica. En resumen, la incorporación de nanopartículas de NiCo₂O₄ en la zeolita-4A proporcionó los sitios electroactivos binarios para la catálisis de la oxidación de metanol, ya que exhibió una estabilidad a largo plazo para la reacción catalítica y, por lo tanto, una aplicación prometedora.

Introduction

Fuel cells (FCs) are among the most common technologies in energy storage. They prove to be environmentally friendly and highly efficient in solving the energy problems of the modern world [1]. Although hydrogen is currently the predominant fuel as, by way of example, fuel cell vehicles under commercial development all use hydrogen fuel, methanol could be a promising alternative owing to its simplicity of operation, transportation and storage [2]. The most commonly used electrodes in direct methanol fuel cell (DMFC), however, exhibit low kinetic and high overpotential. Likewise, Pt-based metal, bimetal and their alloys such as Pt, Pt-Ru, Pt-Fe-Ru are also known to be the best catalysts in investigating the mechanism and kinetic of electrocatalytic oxidation of methanol [3–9] as they are very active in the oxidation of methanol. But these catalysts are expensive and have low natural abundance. Therefore, there is a need to develop alternative low-cost transition metal oxides based catalysts that could overcome these drawbacks.

Transition metal oxides are utilized in heterogeneous catalysis and electrocatalysis by virtue of their intrinsic redox and tunable chemical, morphological, and textural properties, which can be rigidly bound on the substrate materials, i.e., nanoporous architectures with high surface areas and pore volumes [10–12]. Several substrate materials such as carbon nanotube, graphene oxide, and mesoporous silica have been hitherto exploited to improve electro-catalytic activity of metal species [13–17]. The ordered mesoporous materials with high surface area and porous nature known as zeolites are not only among the most commonly used nanoporous supports and substrate but also considered among the best solutions to overcome the pitfalls of catalytic oxidation of alcohol for fuel cells [18]. It should be noted that zeolite comes in different types, of which zeolite-4A is the most widely studied and employed framework, functioning as a substrate for different catalytic reactions [10–24].

Similarly, spinel crystal structure of the type AB₂O₄ has been used in a broad range of disciplines, from electrochemistry, through catalysis to electronics. In a normal spinel, the A cations occupy the tetrahedral sites while the B cations fill the octahedral sites in the cubic closed packing formed by the anions O²⁻. In an inverse spinel structure, in contrast, part of the B cations occupy the tetrahedral sites, possibly distorting the lattice by their size, while the A cations fill the octahedral sites [24–26]. One of the binary metal oxides known as NiCo₂O₄ adopts an inverse spinel structure in which nickel ions fill the octahedral sites whereas cobalt ions are scattered across the tetrahedral and octahedral sites of the structure. NiCo₂O₄ demonstrates higher electronic conductivity and electrochemical activity than those of nickel and cobalt oxides and hence higher redox chemistry than each single component [27–30]. That being so, NiCo₂O₄ has been utilized in such diverse fields as magnetic materials, electro-catalysts, optical limiters and switches, chemical sensors, and lithium ion batteries [31].

In the present research, nickel cobaltite nanoparticles (NiCo₂O₄ NPs) was synthesized via a facile hydrothermal method in the presence of urea and NiCo₂O₄ nanoparticles were incorporated into the zeolite-4A. In the end, electrocatalytic behavior of NiCo₂O₄@zeolite-4A modified electrode toward methanol oxidation was observed using the cyclic voltammetry and chronoamperomerty in a conventional three-electrode electrochemical setup. The finding of this study indicated that the electrode modified by NiCo₂O₄@zeolite-4A might be more efficient in the oxidation of methanol than other catalysts.

Experimental

Materials

All chemicals used in this study were of analytical grade and utilized as received without undergoing any further purification. Sodium aluminate, Cetyl Trimethyl Ammonium Bromide (CTAB), NaOH pellets and Tetraethylorthosilicate (TEOS) were purchased from Sigma–Aldrich; and urea, cobalt (II) chloride hexahydrate and nickel (II) chloride hexahydrate were obtained from Merck.

Synthesis of zeolite-4A, NiCo₂O₄ and NiCo₂O₄@zeolite-4A

Zeolite-4A was prepared using a formerly reported method [32]. That is to say, 4 g of NaOH pellets and approximately half of the required amount of CTAB were dissolved in deionized water, in which 8 g of sodium aluminate was added later. The resulting mixture was stirred until a clear solution was formed (solution A). 15.6 g of TEOS was added to the rest of the required amount of CTAB solution. After 2 h of continuous stirring, a homogeneous and clear solution was created (solution B). A blend of solutions A and B formed the initial clear solution for synthesis which was then crystallized hydrothermally at 100 °C for 24 h in a stainless-steel autoclave. Next, the precipitation product was filtered and washed using deionized water. The final product was eventually dried in a vacuum oven at 60 °C, and later calcined at 550 °C.

NiCo₂O₄ was synthesized in a similar way [33]. Namely, 10 mmol CoCl₂·6H₂O, 5 mmol NiCl₂·6H₂O and 50 mmol urea were dissolved in 70 mL deionized water and stirred until a bright pink colored solution was formed. The mixture was then transferred into a Teflon-lined stainless-steel autoclave, and hydrothermally treated at 120 °C for 6 h. Autoclave was already cooled at room temperature while the reaction mixture was filtered and washed with distilled water and ethanol. The material was later calcined at 300 °C for 4 h. In the end, NiCo₂O₄ and zeolite-4A with weight ratio 40 to 60, denoted as NiCo₂O₄ (40%)/zeolite-4A were grounded uniformly with ethanol using mortar and pastel. Ethanol was slowly removed during the mixing process. The mixture was then heated at 473 K in air for 45 min to remove the residual ethanol followed by calcination at 613 K for 12 h.

Results

The FT-IR spectra of samples are given in Fig. 1. The FT-IR spectra of pristine NiCo₂O₄ spectrum showed two strong peaks at 557 and 652 cm⁻¹, referring to the stretching vibrations of the Ni–O and Co–O bonds in nickel cobalt oxide, respectively [30]. Given the high degree of hydration, the FT-IR spectra of pure zeolite-4A and NiCo₂O₄@zeolite-4A exhibited a broad peak around 3400 cm⁻¹, indicating the presence of hydroxyl groups. All absorption peaks around 450–1000 cm⁻¹ alluded to metal–oxygen bonds, owing to the existence of metal complexes or metal oxide nanoparticles brought about by low concentrations of metal loaded on the zeolite-4A. The comparison between FT-IR spectrum of NiCo₂O₄@zeolite-4A and the pure zeolite-4A spectrum revealed that the original zeolite spectrum about 973 cm⁻¹ ((Si, Al)−O) appeared in lower region. This peak shifted 5 cm⁻¹ and appeared in 968 cm⁻¹ which could be caused by the interaction between zeolite network and nickel cobalt oxide nanoparticles [34–37].

Fig. 1.

FT-IR of NiCo₂O₄, zeolite-4A and NiCo₂O₄@zeolite-4A.

The X-ray diffraction (XRD) pattern of all samples are shown in Fig. 2. As depicted in XRD, all the diffraction peaks for NiCo₂O₄ and the space groups Fd-3m corresponded to the standard patterns of the spinel NiCo₂O₄ phase (JCPDS card No. 73-1702) and all the diffraction peaks of zeolite-4A with the space groups Fm-3c were, likewise, confirmed by (JCPDS card No. 39-222). Diffraction peaks of NiCo₂O₄ NPs were observed at XRD pattern of zeolite-4A after loading NiCo₂O₄ NPs on zeolite-4A, which indicated that NiCo₂O₄ NPs were well loaded into zeolite-4A.

Fig. 2.

XRD pattern of NiCo₂O₄, zeolite-4A and NiCo₂O₄@zeolite-4A.

Fig. 3 illustrates SEM images of NiCo₂O₄, zeolite-4A and NiCo₂O₄@zeolite-4A. As can be seen, the NiCo₂O₄ nanoparticles were composed of uniform flower-like structures whose average size is 1.6 μm in diameter. These 3D flower-like structures were assembled by a large number of nanosheets with an average thickness of 16 nm (Fig. 3a and b). Fig. 3c and d indicates that NiCo₂O₄@zeolite-4A had the spherical structure with particle diameter ranging from 46.2 nm to 57 nm. The comparison between Fig. 3c, d and e revealed that the cubic structure morphology of zeolite transformed into a spherical structure subsequent to the loading of NiCo₂O₄ nanoparticles.

Fig. 3.

SEM images of NiCo₂O₄ (a, b), NiCo₂O₄@zeolite-4A (c, d), zeolite-4A (e) and EDX spectra of NiCo₂O₄@zeolite-4A (f).

It can be inferred from the EDX analysis that the obtained NiCo₂O₄@zeolite-4A (Fig. 3f) had the purity of the samples containing only Si, Al, O, Na, Ni and Co elements. The loading of NiCo₂O₄ nanoparticles to the zeolite-4A had already been shown in Fig. 3c.

Preparation of the working electrodes

Electrochemical measurements were carried out by means of a Sama-500 electrochemical workstation while the electrocatalytic performance was tested using cyclic voltammetry (CV) and chronoamperometry (CA). All the measurements were performed in a conventional three-electrode system containing Ag/AgCl as the reference electrode (3 M KCl) a platinum wire (both from Azar Electrode Co., Iran) as the counter electrode and NiCo₂O₄ modified carbon paste electrode (CPE) as the working electrode. The NiCo₂O₄@zeolite-4A and zeolite-4A modified carbon paste electrode were prepared by mixing NiCo₂O₄@zeolite-4A and zeolite-4A in a ratio of 30:70% (w/w) with the graphite powder and then with diethyl ether. After solvent evaporation, paraffin was blended by hand in a mortar, the resulting paste of which was inserted in the bottom of a glass tube (with internal radius 1.5 mm). The electrical connection was implemented by a Cu wire. The unmodified CPE were produced in a similar manner, except for the addition of an appropriate amount of NiCo₂O₄ to the graphite powder. The electrodes were ultimately polished on a very fine smooth paper.

Electrochemical behavior of modified electrode

The electrochemical behaviors of different modified electrodes were examined by cyclic voltammetry recorded at a potential sweep rate of 25 mV s⁻¹ for methanol oxidation (Fig. 4). As shown in Fig. 4, NiCo₂O₄@zeolite-4A/CPE displayed large oxidation current with the oxidation peak current of 60 μA (curve a). On the surface of zeolite-4A/CPE and CPE (curve c, d), however, no redox peaks were observed under the same condition. It can be inferred from Fig. 4c and d that the zeolite-4A played no significant role in the oxidation of methanol. The oxidation peak current on the surface of NiCo₂O₄/CPE was 28 μA (curve b) which proved that the presence of NiCo₂O₄ in CPE could increase methanol oxidation peak currents. The findings revealed that the presence of synthesized NiCo₂O₄ nanoparticles and zeolite-4A on NiCo₂O₄@zeolite-4A/CPE surface were significantly improved by virtue of electrochemical oxidation response of methanol.

Fig. 4.

Cyclic voltammogram recorded on (a) NiCo₂O₄@zeolite-4A/CPE, (b) NiCo₂O₄/CPE, (c) zeolite-4A/CPE and (d) CPE in the presence of 0.5 M methanol in 0.1 M NaOH at scan rate of 25 mV s⁻¹.

Fig. 5a displays the cyclic voltammetry behavior of the NiCo₂O₄@zeolite-4A/CPE toward the methanol concentrations in the range of 0–0.6 V at the scan rate of 25 mV s⁻¹. The concentration of methanol changed from 0 to 0.5 M in 0.1 M NaOH solution. As evident in Fig. 5, the amount of electrocatalytic current increased after adding the concentration of methanol until 0.5 M which confirmed the excellent electrocatalytic behavior of NiCo₂O₄@zeolite-4A/CPE.

Fig. 5.

Cyclic voltammograms of the NiCo₂O₄@zeolite-4A/CPE at the scan rate of 25 mV s⁻¹ with different concentrations of methanol: (a) 0.0, (b) 0.2, (c) 0.25, (d) 0.3, (e) 0.4, and (f) 0.5 M, respectively in 0.1 M NaOH. Inset shows plot of I_pa vs. ν^1/2 for the oxidation of methanol on NiCo₂O₄@zeolite-4A/CPE.

The examination of scan rate can shed more light on electrochemical mechanisms and kinetic characteristics of methanol. Fig. 6a depicts the cyclic voltammograms of methanol on NiCo₂O₄@zeolite-4A/CPE in 0.1 M NaOH between 0 and 0.6 V at various scan rates. As can be seen in Fig. 6c, the anodic oxidation current (I_p) increased linearly proportional to the square roots of scan rates from 10 to 100 mV s⁻¹ indicating a probable occurrence of a diffusion-controlled process in the oxidation of methanol on NiCo₂O₄@zeolite-4A/CPE. To obtain further information on the rate determining step, a Tafel plot was developed for the methanol on the surface of NiCo₂O₄@zeolite-4A/CPE using the data derived from the raising part of the log ν–voltage curve (Fig. 6b). The slope of the Tafel plot is equal to n(1 − α)F/2.3RT and α is 0.59.

Fig. 6.

(a) Cyclic voltammograms of the NiCo₂O₄@zeolite-4A/CPE in the presence of 0.1 M methanol, at different scan rates of (a) 10, (b) 20, (c) 40, (d) 60, (e) 80 and (f) 100 mV s⁻¹ in 0.1 M NaOH. (b) and (c) Plot of E_p vs. log ν and I_pa vs. ν^1/2 for the oxidation of methanol on NiCo₂O₄@zeolite-4A/CPE.

Chronoamperometric studies

Double potential step chronoamperometry along with some other electrochemical methods were used to examine the catalytic rate constant of the methanol oxidation on the NiCo₂O₄@zeolite-4A/CPE. As depicted in Fig. 7, the current–time profiles recorded by setting the potential of modified electrode at 520 mV (in first step) and 320 mV (in second step) vs. Ag/AgCl (KCl 3 M) were determined based on peak potential of redox process at various concentrations of methanol. The plot of I vs. t^−1/2 showed a linear dependence in the absence of methanol (Fig. 7c). From exponential I–t amperograms (Fig. 7a) a diffusion controlled process occurs which can be modeled by a Cottrell equation [38]. From chronoamperograms, the rate constant for the chemical reaction between the methanol and redox sites of NiCo₂O₄@zeolite-4A/CPE modified electrode can be evaluated according to following equation [39]:

where I_C is the catalytic current of NiCo₂O₄@zeolite-4A/CPE in the presence of methanol, I_L is limiting current in the absence of methanol and γ = kC₀t (C is the argument of error function). C₀ in the argument refers to the concentration of methanol in bulk solution. When γ is more than 2, the error function is almost equal to 1 and the above equation can be summarized as follows:

where k, C₀ and t are the catalytic rate constant (cm³ mol⁻¹ s⁻¹), methanol concentration (mol cm⁻³) and time passed (s), respectively. From the slope linear plot of I_C/I_L vs. t^1/2 obtained from the plot of chronoamperogram of NiCo₂O₄@zeolite-4A/CPE, the value of k for a given concentration of substrate can be easily calculated. Fig. 7b displays one such plot, constructed from the chronoamperogram of the NiCo₂O₄@zeolite-4A/CPE in the absence and presence of 0.05 M methanol. The mean value of k was found to be approximately 1.1 × 10³ cm³ mol⁻¹ s⁻¹. From Eq. (2), the findings confirmed that NiCo₂O₄@zeolite-4A/CPE represents a good catalytic activity toward the methanol oxidation.

Fig. 7.

(a) Chronoamperograms obtained at the NiCo₂O₄@zeolite-4A/CPE in absence (a) and presence of (b) 0.05, (c) 0.1, and (d) 0.2 M of methanol, first and second potential steps were 0.52 and 0.32 V vs. Ag–AgCl, respectively, in 0.1 M NaOH solution. (b) Dependence of current on t^−1/2, derived from the data of chronoamperograms. (c) Dependence of I_C/I_L on t^1/2 derived from the data of chronoamperogams of in the main panel.

Stability of NiCo₂O₄@zeolite-4A/CPE

The electrochemical stability of the modified electrode is also a very important factor to be evaluated. In order to investigate the stability of modified electrode, cyclic voltammograms of modified electrode were recorded in presence of 0.1 M methanol (Fig. 8). As seen in Fig. 8, the CV plots are quite stable and the current density at 0.55 V (vs. Hg/HgO) exhibits 95% retention after 50 cycles at 25 mV s⁻¹.

Fig. 8.

Successive cyclic voltammograms (50th cycles) of NiCo₂O₄@zeolite-4A/CPE in 0.1 M NaOH and 0.5 M methanol solution. Scan rate 25 mV s⁻¹.

Conclusion

In this study, spinel NiCo₂O₄ nanoparticles and zeolite-4A were prepared via a facile hydrothermal route. The findings of SEM and XRD revealed that NiCo₂O₄ nanoparticles of the flower-like 3D structures were assembled by a large number of nanosheets with an average thickness of 16 nm while zeolite-4A had the spherical structure with particle diameter ranging from 46.2 nm to 57 nm. The behavior of modified electrode in electrocatalytic process of methanol oxidation was investigated using cyclic voltammetry and chronoamperometry. From cyclic voltammetry, the modified electrode delivered an anodic oxidation current about 60 μA at 0.6 V in 1 M KOH and 0.5 M methanol and displayed a good stability with 95% current retention after 50 cycles at 30 mV s⁻¹. The modified electrode could enhance the electro-catalytic oxidation and overcome the low kinetic of reaction of methanol due to its rich binary electro-active sites of Ni and Co species, high intrinsic electron conductivity and superior surface structures.