Electrospun NiCu Nanoalloy Decorated on Carbon Nanofibers as Chemical Stable Electrocatalyst for Methanol Oxidation

Ayman Yousef,a,b,c,z Robert. M. Brooks,dMohammad A. Abdelkareem,b,eJabril A. Khamaj,c M.M. El-Halwany,b,f Nasser. A. M. Barakat,b,e,zMohamed H. EL-Newehy,gand Hak Yong Kimb,z

aMathematics and Physics Engineering Department, Faculty of Engineering in Matteria, Helwan University, Cairo, Egypt

bOrganic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, South Korea

cFaculty of Engineering, Jazan University, Jazan, Saudi Arabia

dCivil and Environmental Engineering Department, Temple University, Philadelphia, Pennsylvania 19122, USA

eChemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt

fEngineering Mathematics and Physics Department, Faculty of Engineering, Mansoura University, El-Mansoura, Egypt

gPetrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

zE-mail:; ;

Abstract

NiCu-carbon nanofibers (CNFs) compositewasintroduced as electrocatalyst for methanol oxidation. Nanofibers (NFs)were synthesized bycalcination of electrospun nanofiber mats composed of nickel (II) acetate tetrahydrate, Copper (II) acetate monohydrate, and polyvinyl alcohol (PVA) in the argon/hydrogen atmosphere at 700oC for 2hr.The introduced NFs showed a very good ectrocatalytic activity for methanol oxidation as compared tothat of Ni nanoparticles (NPs) and NiCNFs. As the current densities ~ 110 mA/Cm2, 85 mA/Cm2, and 60 mA/Cm2 for NiCuCNFs nanocomposite, NiCNFs, and Ni NPs, respectively, were obtained.

Manuscript submitted May 4, 2015; revised manuscript received July 6, 2015. Published xx xx, xxxx.

Introduction

Energy, like water is one of the most important daily needs for the human beings to survive. New energy source draws immediate attention as the conventional energy sources deplete. Direct alcohol fuel cells(DAFCs) are believed a promised candidates for energy producing devices[1]. Among of them, direct methanol fuel cells (DMFCs) are very attractive due to their low cost, performance at the room temperature, and use liquid methanol as fuel. Moreover, methanol has a high energy density enabling it tobe easily stored and distributed [2, 3]. InDMFCs, aqueous methanol is electrooxidized to carbon dioxide and wateron the surface of highly active anode consisting of precious platinum–based materials[4]. However, theyface a major challenge in commercialization of the fuel cells due to their highly cost[5]. Moreover, theyare poisoned by the adsorption of CO or CHO species that form during the methanol oxidation [2]. Thus, huge effort has been expended to replace Pt-based materials. Recently, transition metals due to their low cost and ease of preparation are used as alternatives for precious metals in DMFCs. Especially, the use of bimetallic nanoalloyfor catalytic chemical reaction has been thought to be useful owing to the formation of new properties dissimilar from monometallic [6, 7]. Generally, alloy nanomaterials have featured binding properties with reactants in contrary to those of monometallic metal catalysts[7]. Among alloy nanomaterials, nickel copper nanoalloy has been used in electrooxidation of alcohols and other organics [8][9]. However, nude nanoalloy suffers from corrosion during methanol electrooxidation.

Electrooxidtion of methanol is a combination of electrochemical reaction and adsorption of reactants on the anode surface. Carbon nanostructures with ordered graphite-like structure have been incorporated in many recentlyreported as electrocatalytic materials in fuel cells Due to their high adsorption capacity, electrical properties, chemical stability and corrosion resistance[2, 4, 10]. Accordingly, covering NiCu nanoalloy withnano-carbon might be enhancing corrosion resistance and methanol electrooxidation. Among nanostructures, NFshave been extensively applied in a wide variety of applications ranging from catalysts and photocatalysts to biological and medical application. As compared to nanoparticles, nanofibers have a large axial ratio that faciles electrons transfer, an important factor in electrocatalytic process[6, 11-15].

In this study, synthesized NiCunanoalloy-decorated CNFs were introduced as non-precious catalysts for methanol electrooxidation. NFswere synthesized by a cheap and facile electrospinning technique. The introduced nanofibers demonstratedvery good performance for methanol electrooxidation.

2. Experimental

Nickel (II) acetate tetrahydrate (NiAc, 98%)aqueous solutionwas prepared and then mixed withPolyvinylalcohol (PVA, molecular weight (MW) =85000-124000 g/mol). The materials werepurchased from DC chemical Co., South Korea.The same procedure was used to prepare NiCu-CNFs. Typically, 0.2 gm copper (II) acetate monohydrate (CuAc, 98%) and 0.8 gm NiAc was dissolved in 5 gm water and 15gm PVA solution (10 wt%). The mixture wascontinuous stirred for 5 hr at 50 oC. The formed solutions were electrospun at 20kV using a DC power supply. The produced nanofibers mat was dried under vacuum for 24 h at 60oC and then calcined at 700oC for 1.5h in argon/hydrogen atmosphere.

The electrochemical activity of the prepared materials was evaluated by acyclic voltammetry measurement using three electrode cell structure in 1M KOH solution in the potential range from 0 to 0.8V [vs. Ag/AgCl]. The cell consisted of a platinum wire as counter electrode, silver/silver chloride electrode as reference electrode and glassy carbon electrode as the working electrode. The potential was controlled using a VersaStat4 potentiostat device. The catalyst ink was prepared by dispersing 2mg of the prepared catalyst with 20μL Nafion solution (5wt%) in 400μL isopropanol in an ultrasonic bath for 30 min at room temperature. Three successive layers of the catalyst ink (5μL each) was deposited on the glassy carbon electrode. Finally, the prepared electrode was dried at 80 oCfor 20min.

Results and discussion

3.1Characterization

Fig. 1A and Bshow the FE-SEM images of the sinteredelectrospun nanofiber mats in Ar/H2 atmosphere. The obtained powder after calcinationshowed a good nanofibrous morphology as nanofibers structure waspreserved under the proposed calcination condition. Fig. 2indicates XRD analysis of the produced powder. As shown in the fiurethe existence of the strong diffraction peaksat 2θ~ 43.9o, 51.0o, and 75.5ocorrespondingwith the crystal planes (111), (200), and (220) respectively indicated the formation of Ni-Cu nanalloy. Moreover, it can be seen that a broad peak at 2θ~26oagree to the formation of graphite like-carbon (d (002), JCPDS; 41-1487). It is noteworthy that NiCu was formed during the calcinationof electrospun NF mats under inert gas promoting the catalytic activity of carbonization process of the usedPVA [16].To confirm the chemistry of obtained carbon, Raman spectroscopy was used. As shown in panel B, two peaks are centered at around 1330 (D band) and 1580 cm-1(G band).The first peak is present in all graphite-like carbons,while, the second one is corresponding to the palanar vibrations of carbon atoms in the graphite-like materials [17]. This result supported the XRD data indicating that the produced NFs contained carbon.

Fig. 3A displaysthe normal TEM images of the obtained powder.As shown in figure,the bimetallic NiCunanoalloy was covered by a thin layerof carbon as can see nanofibers made core/shell- likestructure.To investigate this hypothesis, Elemental analysis was widely usedto investigate the element distribution within a specified portion. As shown in panels B and C, nickel and copper have the same distribution within the selected part in panel B; this verifies the aforementioned hypothesis that NiCu nanoalloywas incorporated inside CNFs.Panel D contains the HR-TEM image, showing, the sheathing of bimetallic nanoalloy in a shell fromgraphite-like carbon. The inset in Panel D shows the characteristic rings in the electron diffraction pattern (SAED); it is clear from SAED image that a good crystalline structure of the synthesized nanofiber was formed. Interestingly, the graphite-like carbon wasable to protect the metal nanofibers during the harsh electrooxidation reaction of methanol. In other words, it canincrease the chemical stability of the metallic nanofibers. Furthermore, it has a high adsorption capacity that can be enhance methanol electrooxidation.

3.2 Electrochemistry study

It is well known that Ni-based materials have to be activated by forming NiOOH layer on the catalyst surface. This layer is used to initiate the electrochemical activity[4, 18]. Fig.4displays CV behavior of NiCuCNFs in 1M KOH. The scanning ranged from 800 mV to -200 mV in the cathodic direction and then wasreversed in the opposed direction (anodic direction) from -200 mV to 800 mV. The inset showed a large scale for the marked area. Two redox peaks emerged at 400 and 317were shown in Fig. 4the peaks were assigned to the Ni(II)/Ni(III) redox couple according to the following reactions[19]:

Ni + 2OH- Ni(OH)2+ 2e- (1)

Ni(OH)2 + OH- NiOOH + H2O + e- (2)

The NiOOH layer thickness was not preferable to be increased in catalytic oxidation of methanol because it increases the resistance of electrode[20]. The electrocatalytic oxidation ofadsorbed methanol molecules occur at slightly higher potential more than the oxidation of Ni (II) to Ni (III) species. Accordingly, methanol oxidation takes places on the electrocatalyst surface of Ni (III)[8].

Fig.5 shows the effect of methanol concentration on the corresponding current density for the prepared nanocatalysts at the scan rate of 50 mV/s,at potentials between 800 mV to -200 mV, (see the panels), The increasing of methanol concentration leads to an increase in the current density which showing the oxidation of methanol on the surface of the nanocatalysts.Howevere, the NiCNFs were showed a higher current density than that of the Ni NPs. This high performance may be due to the excellent electrical conductivity and adsorbtion capacity of CNFs which improve the methanol electrooxidation.Furthermore, NFs structure has an excellent impact in electrooxidation as the electrons are facile to transportion better than in other structures. Copper is used to enhance electrocatalytic activity of nickel in methanol oxidation. However, it does not participate in the redox processes in the potential range of nickel electroactivity[19].Thus, the formation of nanocomposite from Ni, Cu, and CNFs can enhance the methanol electrooxidation. As shown in panel C, higher current density was obtained as compared that of Ni NPs and NiCNFs. Furthermore, oxidation current of the backward scan (anodic condition) was intersected by that of the forward scan. Therefore,the intermediate product resulted from the methanol oxidation was electrochemically oxidized along with methanol during the reverse scan under the anodic conditions[21, 22].It is worth mentioning that the increase in the methanol concentration more than the introduced values led to the decrease in the current densities.

Fig. 6 showsinfluence of the scan rate on the electrocataytic activity of prepared nanofibers at the various scan rates (10, 50, 100, 200, 300 mV/s) in 2 M methanol.As shown in these figures, the current density in methanoloxidation was not affected by the scan rate. In other words, the current density did not increase with the increase in the scan rate. This emerged the fact that the electrocatalytic of methanol oxidationtook a fast electrosorption mechanism withthe CH3OH moleculeand then fragmented into intermediates on the surface sites making progress towards the final oxidation products[23].

Chronoamperatory (CA) test was done at the constant voltage~ 0.6V for 900 Sec (Fig. 7). As shown in the figure, the initial current corresponded to the value in the CV curves (Fig. 5). Also, there is a little drop on the initial current. Interstingly, in all electrochemical experiments only one working electrode was used.Additionally, the performance of electrode was not affected due to the multiple usage of introduced catalyst supportingthe better stability of the synthesized NFs. The better stability might be attribuited to the coverage of the bimetallic nanoalloy by a thin layer of carbon (Fig. 3A and D).

Conclusion

NiCu nanoalloy enveloped in graphite-like CNFswere prepared by the electrospinning process. Typically, calcination of electrospinning of a sol-gel consistedof copper acetate, nickel acetate and PVA in Ar/H2 atmosphere at 700 oC.the calcination revealed a good morphology as NiCu nanoalloywas sheathed by thin layer of graphite. The introducedNiCuCNFsshoweda very good electrocatalytic activity toward methanol oxidationat the current density ~ 110 mA/Cm2 and onset potential ~ 570 vs. NHE. Overall, the present study introduced a low cost and an effective electrocatalyst for methanol oxidation.

Acknowledgement

This work was supported by the IT R&D Program of MKE/KEIT(10041957, Design and Development of Fiber-based Flexible Display)(No. 10041947). The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this Research group no. (RGP#.021).

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Fig. 1. FE-SEM imagesobtained powder after the sintering process.

Fig. 2. XRD (A) and Raman patterns (B) of the obtained powder.

Fig. 3. Panel A normal TEM image, B displays STEM image for one nanofiber along with the line EDX analysis, C demonstrates line analysis TEM EDX results for the line in B, and D represents the high resolution TEM image (HR-TEM). The inset in B demonstrates the selected area electron diffraction pattern (SAED).

Fig. 4. Cyclic voltammograms for activation the introduced NFs.

Fig. 5. Cyclic voltammograms in the presence of different concentration from Methanol.

Fig. 6. Cyclic voltammograms of the prepared NFs at different scan rates.

Fig. 7.Chronoamperatorytest at a cell potential of 0.6V and 2.0 Mmethanol.

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