Cobalt/copper-decorated carbon nanofibers as novel non-precious electrocatalyst for methanol electrooxidation
© Barakat et al.; licensee Springer. 2014
Received: 7 November 2013
Accepted: 16 December 2013
Published: 3 January 2014
In this study, Co/Cu-decorated carbon nanofibers are introduced as novel electrocatalyst for methanol oxidation. The introduced nanofibers have been prepared based on graphitization of poly(vinyl alcohol) which has high carbon content compared to many polymer precursors for carbon nanofiber synthesis. Typically, calcination in argon atmosphere of electrospun nanofibers composed of cobalt acetate tetrahydrate, copper acetate monohydrate, and poly(vinyl alcohol) leads to form carbon nanofibers decorated by CoCu nanoparticles. The graphitization of the poly(vinyl alcohol) has been enhanced due to presence of cobalt which acts as effective catalyst. The physicochemical characterization affirmed that the metallic nanoparticles are sheathed by thin crystalline graphite layer. Investigation of the electrocatalytic activity of the introduced nanofibers toward methanol oxidation indicates good performance, as the corresponding onset potential was small compared to many reported materials; 310 mV (vs. Ag/AgCl electrode) and a current density of 12 mA/cm2 was obtained. Moreover, due to the graphite shield, good stability was observed. Overall, the introduced study opens new avenue for cheap and stable transition metals-based nanostructures as non-precious catalysts for fuel cell applications.
The extensive use of fossil fuels is causing environment pollution and global warming problems. Fuel cell is a good technological option for solving energy and pollution problems. Polymer electrolyte membrane fuel cells (PEMFCs) have been investigated as high-density power sources in automobiles and in microelectronics. The efficiency of fuel cells depends on the catalytic activity of the catalysts. The use of methanol as a fuel is getting popular because it is a liquid which can be easily stored and handled. Methanol is also easier to supply to the public using our current infrastructure. In the DMFCs, methanol is directly oxidized to carbon dioxide and water, providing a new way to store and convey the energy [1–3]. The successful commercialization is quite dependent on the cost, activity, and durability of the electrocatalysts [3, 4]. At present, almost all pre-commercial low-temperature fuel cells use Pt-based electrocatalysts [5–8]. Accordingly, the manufacturing cost is relatively high which constrains their wide applications. Moreover, the catalyst poisoning by CO or CHO species is another real problem facing most of the Pt-based electrocatalysts [3, 9, 10]. Compared to the precious metals, transition metals are abundant and very cheap. Among the transition metals, cobalt has a well-known catalytic activity in many chemical reactions. It is also used as a cocatalyst to annihilate the Pt poisoning . Beside cobalt, copper-based materials also show good performance as electrocatalysts [12, 13].
In the DMFCs, methanol is oxidized to carbon dioxide at the anode according to the reaction: CH3OH + H2O = CO2 + 6H+ + 6e. The reaction is considered to be a combination of adsorption and electrochemical reaction on the anode surface [14, 15]. Accordingly, because of the adsorption capacity of carbon, it was exploited to enhance the electrocatalytic activities for many electrodes [16–20]. Addition of alloying elements to Cu has been demonstrated to provide increased electrocatalytic activity in comparison to the pure Cu electrode. For example, MnCu alloy shows a much improved electrochemical activity for the oxidation of glucose in alkaline media in comparison to that of the pure Cu electrode .
Nanofibers have gained much prominence in the recent years due to the heightened awareness of their potential applications in many fields including textiles, chemical synthesis, medicine, engineering, and defense. Among several methods for nanofiber production, electrospinning is the most widely used technique due to simplicity, high yield, effectiveness and low-cost aspects [22–24]. It is noteworthy mentioning that as compared to nanoparticles, the large axial ratio provides the nanofibrous catalysts a distinct advantage especially when utilizing in the electron transfer-based processes [25, 26]. Carbon nanofibers (CNFs) prepared by the electrospinning process gradually attracted the attention of most researchers because of the associated advantages of the synthesizing technique and the obtained product. Moreover, it is easy to control the morphology and pore structure of the obtained nanofibers . Currently, CNFs are widely used in many fields such as hydrogen energy [28, 29], electrochemical capacitors (EDLCs) , lithium-ion rechargeable batteries (LIBs) , and fuel cells .
Poly(vinyl alcohol) (PVA) is a semi-crystalline compound with comparatively high carbon content (ca. 54.5%), and easily splits into hydroxyl groups in the polymer chain which makes it favorable for use as a precursor for the production of the carbonaceous materials; however, low yield is the main constrain. The low decomposition temperature of PVA is the main reason for the low carbonization yield . Therefore, the researchers are focusing on enhancing the thermal stability of PVA. Some strategies were introduced such as dehydration of PVA from 100°C to 290°C under tension in a mixed gas atmosphere , preoxidation or subsequent dehydration , and using iodine as a stabilizer for PVA to promote dehydrogenative polymerization during the carbonization process . Recently, it was reported that cobalt strongly enhances the graphitization of PVA [22, 37].
Therefore, the main goal of this study is to introduce a new non-precious catalyst with good electrocatalytic activity. Enhancement of the activity will be based on exploiting the influence of synergetic effect of cobalt and copper bimetallic nanoparticles as well as the advantage of the nanofibrous morphology. In this study, Co/Cu-decorated carbon nanofibers are introduced as novel non-precious catalyst for methanol electrooxidation. The introduced nanofibers have been synthesized by calcination of electrospun nanofibers composed of cobalt acetate, copper acetate, and PVA in argon atmosphere at 750°C. The obtained nanofibers revealed good performance as electrocatalyst for methanol oxidation.
Solution containing copper acetate monohydrate (CuAc, 99.9 Sigma-Aldrich Corporation, St. Louis, MO, USA), cobalt acetate tetrahydrate (CoAc, 98% assay Junsei Chemical Co., Ltd, Japan), and poly(vinyl alcohol) (PVA) (10 wt.% concentration in water, molecular weight (MW) = 65,000 g/mol; DC Chemical Co, Ltd, Seoul, South Korea) was electrospun at a voltage of 20 kV using a high-voltage DC power supply. Typically, CuAc (25 wt.%) and CoAc (25 wt.% ) aqueous solutions were mixed with PVA solution to form a mixture containing 80 wt.% polymer and Cu/Co mass ratio of 1:4. The final solution was stirred at 50°C for 5 h and then subjected to the electrospinning process at 20 kV. The formed nanofiber mats were initially dried for 24 h at 80°C under vacuum and then calcined at 750°C for 5 h in argon atmosphere with a heating rate of 2.3°C/min.
The surface morphology was studied by scanning electron microscope (SEM) (JEOL JSM-5900; JEOL Ltd, Tokyo, Japan) and field-emission scanning electron microscope equipped with energy-dispersive X-ray (EDX) analysis tool (field emission scanning electron microscopy, FESEM; Hitachi S-7400, Japan). Information about the phase and crystallinity was obtained by using Rigaku x-ray diffractometer (XRD; Rigaku Corporation, Tokyo, Japan) with Cu Kα (λ = 1.540 Å) radiation over Bragg angle ranging from 10° to 90°. High-resolution image and selected area electron diffraction patterns were obtained with transmission electron microscope (TEM) (JEOL JEM-2010, JEOL Ltd, Tokyo, Japan) operated at 200 kV equipped with EDX analysis. The Raman spectra were measured using Nanofinder 30 spectrometer (Tokyo Inst. Co., Machida-shi, Tokyo, Japan) equipped with a He/Ne (λ = 633 nm laser) and the scattering peaks were calibrated with a reference peak from a Si wafer (520 cm-1). Raman spectra were recorded under a microscope with a × 40 objective in range of 0–1,600 cm-1 and 3 mW of power at the sample. The electrochemical measurements were performed in a 1 M KOH solution at room temperature. The measurements were performed on a VersaSTAT 4 (USA) electrochemical analyzer and a conventional three-electrode electrochemical cell. A Pt wire and an Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. Glassy carbon electrode was used as working electrode. Preparation of the working electrode was carried out by mixing 2 mg of the functional material, 20 μL of Nafion solution (5 wt.%) and 400 μL of isopropanol. The slurry was sonicated for 30 min at room temperature. An amount of 15 μL from the prepared slurry was poured on the active area of the glassy carbon electrode which was then subjected to drying process at 80°C for 20 min. All potentials were quoted with regard to the Ag/AgCl electrode. High-purity nitrogen gas was used before and during measurements to deaerate the electrolyte in all measurements. Normalization of the current density was achieved based on the surface area of the utilized glassy carbon electrode (0.07 cm2).
Results and discussion
Additionally, a broad peak at 2θ of 26.3° corresponding to an experimental d spacing of 3.37 Å which indicates the presence of graphite-like carbon (d (002), JCPDS card no 41–1487) can be observed in the figure.
Increasing the number of potential sweeps results in a progressive increase of the current density values of the cathodic peak because of the entry of OH– into the Cu(OH)2 surface layer, which leads to the progressive formation of a thicker CuOOH layer corresponding to the Cu(OH)2/CuOOH transition . On the other hand, it can be concluded from Figure 6 that Co has no surface activation which explains the known low electrocatalytic activity of this metal. It is noteworthy mentioning that high cobalt percentage was utilized in preparation of the introduced nanofibers to exploit the activity of Co in PVA graphitization to produce CNFs. Recently, supporting of the electrocatalysts on carbonaceous materials is carrying on to take the advantage of the adsorption capacity of the carbon and increase the active catalyst surface area [10, 47]. The bottom panel in Figure 6 displays the obtained results in case of utilizing the introduced nanofibers. As shown in the figure, the presence of carbon and the nanofibrous morphology strongly enhance the current density compared to the pristine cobalt and copper nanoparticles.
- 1.Adsorption of the methanol on the introduced catalyst (M) and partial release of the protons(10)
- 2.Further release of the protons(11)(12)
- 4.Later on, CO is oxidized by the OH group ; however, as aforementioned, CO oxidation on the surface of the introduced catalyst is not detected, so it is believed that reaction 13 does not occur. Instead, this reaction is taking place(14)
The electrospinning process can be efficiently utilized to produce carbon nanofibers decorated by metallic nanoparticles. Cobalt/copper-decorated carbon nanofibers can be produced by electrospinning of a sol–gel composed of cobalt acetate tetrahydrate, copper acetate monohydrate and poly(vinyl alcohol). Calcination of the electrospun mats in argon atmosphere produces Co/Cu-decorated CNFs. The produced decorated nanofibers can be utilized as electrocatalyst for methanol oxidation; the corresponding onset potential and current density are satisfactory. Moreover, good stability is expected because the metallic nanoparticles are sheathed inside the thin graphite layer. Overall, this study introduces new methodology to produce metal-decorated CNFs based on PVA. The proposed decorated nanofibers can be further modified to be more efficient non-precious electrocatalysts for fuel cell applications.
This research was supported by NPST program by King Saud University project number 11-ENE1721-02. We thank Mr. T. S. Bae and Mr. J. C. Lim from KBSI, Jeonju Branch and Mr. Jong-Gyun Kang from Centre for University Research Facility, for taking high-quality FESEM and TEM images, respectively.
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