Electro catalytic performance of pt-supported poly (o-phenylenediamine) microrods for methanol oxidation reaction
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Electro catalytic performance of pt-supported poly (o-phenylenediamine) microrods for methanol oxidation reaction

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Poly (o-phenylenediamine) (PoPD) microrods were obtained by interfacial...

Poly (o-phenylenediamine) (PoPD) microrods were obtained by interfacial
polymerization using ferric chloride as oxidant and without any template or
functional dopant. Pt/PoPD nanocatalysts were prepared by the reduction of chloroplatinic
acid with sodium borohydride, and the composite catalysts formed were
characterized by X-ray diffraction and electrochemical methods. The nanocomposite
of Pt/PoPD microrods has been explored for their electro-catalytic performance
towards oxidation of methanol. The electro-catalytic activity of Pt/PoPD was
found to be much higher (current density 1.96 mA/cm2 at 0.70 V) in comparison to
Pt/Vulcan electrodes (the current density values of 1.56 mA/cm2 at 0.71 V) which
may be attributed to the microrod morphology of PoPD that facilitate the effective
dispersion of Pt particles and easier access of methanol towards the catalytic sites.

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  • 1. Res Chem Intermed (2012) 38:383–391DOI 10.1007/s11164-011-0354-3Electro-catalytic performance of Pt-supported poly(o-phenylenediamine) microrods for methanoloxidation reactionT. Maiyalagan • C. Mahendiran • K. Chaitanya •Richa Tyagi • F. Nawaz KhanReceived: 16 May 2011 / Accepted: 22 July 2011 / Published online: 4 August 2011Ó Springer Science+Business Media B.V. 2011Abstract Poly (o-phenylenediamine) (PoPD) microrods were obtained by inter-facial polymerization using ferric chloride as oxidant and without any template orfunctional dopant. Pt/PoPD nanocatalysts were prepared by the reduction of chlo-roplatinic acid with sodium borohydride, and the composite catalysts formed werecharacterized by X-ray diffraction and electrochemical methods. The nanocom-posite of Pt/PoPD microrods has been explored for their electro-catalytic perfor-mance towards oxidation of methanol. The electro-catalytic activity of Pt/PoPD wasfound to be much higher (current density 1.96 mA/cm2 at 0.70 V) in comparison toPt/Vulcan electrodes (the current density values of 1.56 mA/cm2 at 0.71 V) whichmay be attributed to the microrod morphology of PoPD that facilitate the effectivedispersion of Pt particles and easier access of methanol towards the catalytic sites.Keywords Methanol oxidation Á Pt supported poly (o-phenylenediamine) ÁNanostructured materials Á Electro-catalystT. Maiyalagan (&)School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 NanyangDrive, Singapore 639798, Singaporee-mail: maiyalagan@gmail.comC. MahendiranDepartment of Chemistry, Anna University of Technology Tirunelveli, Nagercoil 629004, IndiaK. Chaitanya Á F. Nawaz KhanChemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, Indiae-mail: nawaz_f@yahoo.co.inR. TyagiDepartment of Chemistry, Deshbandhu College, Delhi University, Delhi 110007, India 123
  • 2. 384 T. Maiyalagan et al.IntroductionThough fuel cells are considered as potent energy conversion devices, they have notbeen evolved as an economically viable, socially acceptable, easily manipulativetool for energy conversion [1–8]. A fuel cell essentially consists of threecomponents: the two electrodes and an electrolyte. Pt and Pt-based noble metalshave been employed as electro-catalysts in both the anode and cathode, andnowadays there are attempts to reduce the amount of noble metal loading in theelectrodes by dispersing the noble metals on suitable electronically conductingsupports with low metal loadings. To decrease the platinum loading as well as toimprove the oxidation rate and electrode stability, considerable efforts have beenapplied to study the electrode materials for the direct electrochemical oxidation ofmethanol [9–17]. Carbon is the most common catalyst support material thatconducts only electrons in the electro-oxidation reaction. An alternative is todevelop a catalyst support that conducts both protons and electrons efficiently [18–20] by employing conducting polymers possessing both protonic and electronicconductivity. Recently, conducting polymer matrices are being employed as catalystsupport materials for the oxidation, and metal nanoparticles dispersed on conductingpolymer support provide access to a large number of catalytic sites with thepossibilities of the recovery of spent catalyst. Catalytic particles dispersed on conducting polymers, mainly polypyrrole (PPY)and polyaniline (PANI), have received considerable attention as the electrodematerial for methanol oxidation [21–23]. The conducting and electroactivepolymers, such as poly(o-phenylenediamine) (PoPD), have a greater potential invarious fields of technology due to their interesting properties, different from thoseof the usual conducting polymers, like PANI or PPY, which make them promisingfor applications in electro- and bioelectro-chemical sensors. One of these propertiesrelates to an unusual dependence of the electric conductivity on the redox state ofthe PoPD polymer. Different from PANI or PPY, PoPD shows high conductivity inits reduced state, whereas the oxidized state is insulating. This determines theelectrochemical properties of PoPD, since many electrode redox processes havebeen shown to take place within a relatively narrow potential window, correspond-ing to the reduced (conducting) form of this polymer. Within this potential window,electro-catalytic oxidation of some species takes place, making it possible to usePoPD for electro-catalytic applications, such as the electro-oxidation of coenzymeNADH, electro-oxidation of methanol [24–27] and oxygen reduction [28–30]. PoPD is usually obtained through electrochemical polymerization. In thispolymerization method, the PoPD obtained usually exhibits an irregular morphol-ogy. 1D nano-structured PoPD is obtained by mixing aqueous solutions of HAuCl4and OPD without any surfactants or templates [30]. However, the materials obtainedwere not pure. Hence in this article, we describe an efficient method for thepreparation of PoPD microrods with different lengths using ferric chloride asoxidant, which takes advantage of the easy removal of FeCl2 by simple washingwith water; the PoPDs so obtained were of high purity [31]. In the present investigation, the composite material based on Pt-supported PoPDmicrorods has been compared with that of the Pt/C electrodes for electro-oxidation123
  • 3. Electro-catalytic performance of Pt 385of methanol. These materials were characterized using X-ray diffraction (XRD) andcyclic voltammetry (CV). The Pt-supported PoPD microrods electrode exhibitedbetter catalytic activity and stability compared to the 20 wt% Pt supported on theVulcan carbon electrode.ExperimentalMaterialsThe present study was carried out in aqueous solutions. Purified water obtained bypassing distilled water through a milli Q (Millipore) water purification system wasused. The reagents o-Phenylenediamine (oPD), ferric chloride (FeCl3), chloroplatinicacid (H2PtCl6), ruthenium chloride (RuCl3), sodium borohydride (NaBH4), sodiumhydroxide (NaOH) were purchased from Aldrich (India) and used as received.Methanol and sulphuric acid were obtained from Fischer Chemicals (India). Nafion 5wt% solution was obtained from Dupont (USA) and was used as received.Synthesis of POPD microrodsThe PoPD microrods were obtained by chemical oxidation of oPD using ferricchloride as an oxidant. In a typical procedure, different contents of oPD monomerwere dissolved in 30 mL distilled water at room temperature. Then, 10 mL aqueoussolutions of ferric chloride (the molar ratio of ferric chloride to oPD is 1:1) wereadded to the above mixtures under vigorous stirring at room temperature for 5 h.The resulting precipitates were washed with water twice and filtered. Finally, theproducts were dried in vacuum at 50 °C for 24 h.Synthesis of Pt supported PoPD microrodsAmounts of 0.1 gm of PoPD and 2 mL of 50 mM H2PtCl6 were mixed with 100 mLof distilled water. Then, 2 mL NaBH4 solution (50 mg mL-1) was added drop-wiseto the above solution with vigorous stirring at room temperature. Stirring wascontinued overnight before the solid phase was recovered by filtration and thenwashed copiously with water. The recovered solid was dried overnight undervacuum at room temperature. An aqueous solution of H2PtCl6 was prepared, and the pH of the solution wasadjusted to 6.8 by adding NaOH. A freshly prepared aqueous solution of NaBH4was added to this solution under stirring at room temperature. After the addition ofNaBH4, the color of the solution changed to brown, indicating the formation ofnanoparticles. The solution was stirred overnight.CharacterizationThe morphologies of the microrods were observed by an optical microscope(Olympus BX-100).The phases and lattice parameters of the catalyst were 123
  • 4. 386 T. Maiyalagan et al.characterized by X-ray diffraction (XRD) patterns employing a Shimadzu XD-D1 ˚diffractometer using Cu Ka radiation (k = 1.5418 A) operating at 40 kV and48 mA. XRD samples were obtained by depositing composite-supported nanopar-ticles on a glass slide and drying the latter in a vacuum overnight.Electrochemical measurementsAll electrochemical measurements were performed in a conventional three-electrode cell at room temperature. A Pt wire was used as a counter electrode.All electrochemical potentials in the present study are given versus an Ag/AgCl(saturated KCl) reference electrode. Glassy Carbon (GC) (electrodearea = 0.196 cm2) was polished to a mirror finish with 0.05-lm aluminasuspensions before each experiment which served as an underlying substrate ofthe working electrode. In order to prepare the composite electrode, the catalystswere dispersed ultrasonically in water at a concentration of 1.0 mg mL-1 and a20-lL aliquot was transferred on to a polished glassy carbon substrate. After theevaporation of the water, the resulting thin catalyst film was covered with 5 wt%Nafion solution. Then, the electrode was dried at 353 K and used as the workingelectrode. A solution of 1.0 M CH3OH in 0.5 M H2SO4 was used to study themethanol oxidation activity.Results and discussionThe PoPD microrods were obtained by interfacial polymerization and Fig. 1 showsthe optical micrograph of uniform PoPD microrods of several micrometers in lengthand with diameters of approximately 1–3 lm, which suggested that they werefabricated successfully by the interfacial polymerization method. The XRD patternof the catalyst (Pt/PoPD) is shown in Fig. 2. The peaks consistent with face-centered cubic (fcc) as expected for Pt were clearly observed for the Pt/PoPDcatalyst. The XRD patterns were used to estimate the average particle size using theScherrer equation: ˚ dðAÞ ¼ kk=b coshwhere k is a coefficient (0.9), k the wavelength of X-ray used (1.54056 A°), b thefull-width half maximum and h is the angle at position of peak maximum. The meanparticle size obtained from the XRD patterns were 4.48 nm for Pt/PoPD and 2.8 nmfor Pt/C. To further identify the composition of these Pt/PoPD materials, the EDXanalysis (Fig. 2b) was used to analyze the content of Pt in the Pt/PoPD compositewhich revealed a satisfactorily result close to the theoretical value (i.e., 20%)calculated based on the fact that Pt precursors being reduced completely and theentire reduced Pt particles are incorporated into the Pt/PoPD composite. The cyclic voltammograms of Pt nanoparticles-supported PopD microrodsexhibiting hydrogen adsorption–desorption peaks corresponding to platinumoxidation–reduction in 0.5 M H2SO4 at a scan rate of 50 mV/s are shown in123
  • 5. Electro-catalytic performance of Pt 387Fig. 1 Optical micrographs of poly (o-phenylenediamine) microrodsFig. 3. The appropriate cyclic voltammogram pattern of Pt nanoparticles-supportedPopD microrods was attained within five cycles in the background electrolyte. Theelectrochemical surface areas (ECSA) calculated using the hydrogen desorptionpeak and used to evaluate the surface area of the Pt-based catalysts are given inTable 1. The surface area of Pt-supported PopD microrods was found to be higherthan the surface area of Pt/C [14]. The electrochemical surface area of variouspolymer electrodes area is reported and the electrochemical surface area of Pt/Celectrode consistent with the previously reported electrodes is given in Table 1. The electro-catalytic activity of the particulate composite electrodes for methanoloxidation was studied by cyclic voltammetry. Figure 4 shows the cyclic voltam-mograms (CVs) of the electrodes in 1.0 M MeOH ? 0.5 M H2SO4 at 25 °C with a 123
  • 6. 388 T. Maiyalagan et al.(a) (b) Pt (111) (a) 20% Pt/C (b) 20% Pt/PoPD C (002) Pt (200) Intensity (a.u) (a) Pt (220) (b) 20 30 40 50 60 70 80 2θ (degrees)Fig. 2 a X-ray diffraction patterns and b EDX patterns of 20% Pt Pt/PoPd electro-catalysts prepared bysodium borohydride reduction 1.0 (a) (mA/cm Pt ) 2 0.5 (b) 0.0 Current density -0.5 -1.0 -1.5 -200 0 200 400 600 800 1000 1200 Potential (V) vs Ag/AgClFig. 3 Cyclic voltammograms of a 20% Pt/PoPD and b 20% Pt/C in nitrogen-saturated 0.5 M H2SO4 ata scan rate of 50 mV/sscan rate of 50 mV s-1. The specific activity of the catalysts normalized to Ptsurface area is shown in Table 1. The oxidation current of Pt/PoPD electrode(If = 1.96 mA/cm2) was also higher than that of Pt/C (1.56 mA/cm2). During thereverse scan, the oxidation peak at 0.50 V was obtained with a peak current ofIb = 1.45 mA/cm2. This peak is attributed to the release of adsorbed CO or CO-likespecies, which can be generated via incomplete oxidation of methanol in theforward scan [33]. These carbonaceous species are mostly in the form of linearlybonded Pt=C=O, which are oxidized in the reaction of the backward scan peak [33]. Thus, the ratio of the forward anodic peak current density (If) to the reverseanodic peak current density (Ib), i.e., If/Ib, suggests a tolerance to carbonaceousspecies accumulation on catalysts during methanol electro-oxidation. The low If/Ib123
  • 7. Electro-catalytic performance of Pt 389Table 1 Electrochemical properties of PoPD microrods-supported Pt and carbon-supported PtElectrode Electrochemical Current density surface area (m2 g-1) (mA/cm2)Pt/C 78.4 1.56Pt/C [38] 84 1.39Pt/PoPD microrods 96 1.96Pt/poly(N-acetylaniline) nanorods [39] – 2.60Pt/poly(o-anisidine) nanofiber [40] – 3.56Pt/polypyrrole [41] 117 14.1Pt/CNx [41] 114 7 2.0 (a) Current density (mA/cm ) 2 1.5 (b) 1.0 0.5 0.0 -0.5 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential (V) vs Ag/AgClFig. 4 Cyclic voltammograms of a 20% Pt/PoPD and b 20% Pt/C electrode in 0.5 M H2SO4/1 MCH3OH at 50 mV/sindicates poor oxidation of methanol to carbon dioxide during the forward anodicscan and excessive accumulation of carbonaceous residues on the catalyst surface.On the other hand, the high If/Ib indicates excellent oxidation of methanol during thereverse anodic scan and less accumulation of residues on the catalyst. The reportedIf/Ib value for the commercial E-TEK catalyst is *1 [34, 35]. The ratio observed forPt–Ru after vigorous heat treatment is *1.30 [36]. In the present study, the ratio isobserved to be 1.34 for Pt/PoPD catalysts, larger than the ratio of 1.1 [37] whichwas calculated for the Pt/C (E-TEK) sample. Therefore, the higher tolerance of thecatalysts to incompletely oxidized species is another important reason for the higherefficiency of Pt/PoPD catalysts. The stability of Pt/PoPD and Pt/C was tested by chronoamperometric curves formethanol oxidation as shown in Fig. 5.The current density has been normalized toPt surface area for the evaluation of the catalytic activity of the electrodes. In thecurves of all composite catalysts there was a sharp initial current drop, followed by a 123
  • 8. 390 T. Maiyalagan et al. 1.6 1.4 Current density (mA/cm ) 2 1.2 1.0 0.8 0.6 0.4 (a) (b) 0.2 0.0 0 500 1000 1500 2000 2500 3000 3500 Time (seconds)Fig. 5 Chronoamperometric curves for the catalysts a 20% Pt/PoPD and b 20% Pt/C recorded at the0.7 V versus Ag/AgCl for 3,600 s in nitrogen-saturated 0.5 M H2SO4 and 1 M methanol at 25 °Cslow decay. A possible reason for the slow decay of current density is the poisoningeffect by COads intermediates [32]. As observed from Fig. 5, methanol oxidation onPt/PoPD gives a higher oxidation current than that on Pt/C during the process. Thisindicated that the direct oxidation of methanol on Pt/PoPD was enhanced.Therefore, Pt/PoPD had better poisoning-tolerance ability than Pt/C. At the sametime, the Pt/PoPD maintained the highest current density compared to Pt/Celectrodes. This was mainly due to the more facilitative methanol oxidation on Pt/PoPD, which was in agreement with the aforementioned CV results. The more facilediffusion of both liquid fuel and products of the microrods-supported catalyststructure interpenetrated with the electrolyte network. Therefore, the utilizationefficiency of catalysts becomes higher. The microrods arrays may have a greatpotential in the application of direct methanol fuel cells.ConclusionsSynthesis and characterization of conducting PoPD microrods-incorporated Ptnanoparticles are reported here. Good catalytic activity was observed for the electro-oxidation of methanol at the metal–polymer microrods composite electrode. The Pt-loaded PoPD microrods not only increase the electronic-ionic contact but alsoprovide an easier electronic pathway between the electrode and the electrolyte,which increases the reactant accessibility to the catalytic sites. The electro-catalyticactivity of the microrods-based electrode was compared with those of 20 wt% Ptsupported on the Vulcan carbon electrode, using cyclic voltammetry. The Pt-loadedPoPD microrods electrode exhibited better catalytic activity and stability than the20 wt% Pt supported on the Vulcan carbon electrode.123
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