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TiO2 nanocomposites for methanol electrooxidation in alkaline media
Synthetic Metals 235 (2018) 71–79
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Electrocatalytic performance of Pd/PANI/TiO2 nanocomposites for methanol electrooxidation in alkaline media
T
⁎
Mohammad Soleimani-Lashkenaria, , Sajjad Rezaeib, Jaber Fallahc, Hussein Rostamic a Fuel Cell Electrochemistry and Advanced Material Research Laboratory, Faculty of Engineering Modern Technologies, Amol University of Special Modern Technologies, Amol, 4616849767, Iran b Chemical Engineering Department, Tarbiat Modares University, Tehran, Iran c Faculty of Chemistry, University of Mazandaran, Babolsar, Iran
A R T I C L E I N F O
A B S T R A C T
Keyword: Palladium nanoparticles Polyaniline Methanol oxidation Electrocatalysis
An electrochemical method was successfully used for the Pd nanoparticles deposition on the Polyaniline/titanium dioxide (PANI/TiO2) modified glassy carbon (GC) electrode. The electrochemical activity of fabricated palladium/Polyaniline/titania (Pd/PANI/TiO2) electrocatalyst was investigated for methanol electrooxidation by cyclic voltammetry (CV) and chronoamperomtery (CA) in alkaline media. Also, the effect of different methanol concentrations and potential sweep rates were in two separate set of experiments. The prepared samples were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR) techniques. Obtained results indicated that the synthesized Pd/PANI/TiO2 catalyst not only possessed much higher electrochemically active surface area (EASA) than that of the pure Pd catalyst, but also enhanced the forward anodic peak current density (Jf) for methanol electrooxidation reaction. These observations are extracted from the combination of high charge transfer of the PANI/TiO2 nanocompsites and excellent catalytic characteristic of the Pd catalyst.
1. Introduction During the last decades, direct methanol fuel cells (DMFCs), electrochemical devices which directly convert chemical energy to electricity via electrooxidation of methanol as fuel, have become attractive due to their advantages including high efficiency, low operating temperature, and low/zero emission. But for commercialization of this technology, some barriers should be overcome. The slow reaction kinetics of methanol electrooxidation has been widely noticed as one of the most important challenges which strongly depends on the used catalysts [1]. Platinum (Pt) has been commonly implemented in DMFCs as catalyst layer on both anode and cathode side. Howevere, because of high cost and catalyst posioning towards methanol oxidation, many researchs have been done to find an alternative [2–4]. Pt alloy catalysts such as Pt/Ru, Pt/Ni, Pt/Co, Pt/Cu, Pt/Au and Pt/Ru/Ni have been demonstrated high activity and stability for application as anode catalyst in DMFCs [5–7]. Thus, Palladium-based catalysts with a lower cost than Pt and high catalytic activity in methanol oxidation reaction in alkaline media has been considered as a substitute for commonly-used Pt electrocatalysts [3,8–10].
In order to enhance the activity of pure Pd catalyst, Pd alloy catalysts such as PdSn, PdRu, and PdNi have been suggested because of their effect on oxidation of adsorbed CO intermediates to CO2 during methanol oxidation at lower potentials [11]. Another method for improving electrocatalytic activity in methanol oxidation is usage of support materials including 1) carbon-based materials such as carbon nanotube (CNT), carbon nano-fiber (CNF) and graphene, and 2) metal oxides like TiO2, SnO2, RuO2, MnO and CeO2 [12–17]. As among, Pd/ TiO2 nanocomposites have been shown high electrochemical activity and excellent stability in methanol oxidation [11,18]. In recent years, conducting polymers have been enormously used in various applications including catalysis, energy storage, electronic nanodevices bio-sensors and biomedical engineering due to their good mechanical, optical and electrical properties [19–25]. Between different conducting polymers, poly(o-phenylenediamine), polyaniline (PANI) and polypyrrole (PPy) has been broadly reported for improving the DMFCs performance [26–29]. Among them, PANI has attracted a lot of attention because of its low cost, easy synthesis methods and excellent electrical properties [30]. In addition, the modification effect of PANI on methanol oxidation reaction has been confirmed by several papers [4,31–36].
⁎ Corresponding author at: Fuel Cell Electrochemistry and Advanced Material Research Laboratory, Faculty of Engineering Modern Technologies, Amol University of Special Modern Technologies, Amol, 4616849767, Iran. E-mail address: [email protected] (M. Soleimani-Lashkenari).
https://doi.org/10.1016/j.synthmet.2017.12.001 Received 22 September 2017; Received in revised form 18 November 2017; Accepted 1 December 2017 0379-6779/ © 2017 Published by Elsevier B.V.
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followed by a color change to dark blue. After that, the prepared nanocomposites were filtered and washed with distilled water to remove oligomers and excess acid. Finally, the product was dried out for 24 h at room temperature.
In the current study, Pd nanoparticles was electrochemically deposited on the PANI/TiO2 nanocomposites modified glassy carbon electrode to form the Pd/PANI/TiO2/GC working electrode and was used as anode catalyst for methanol oxidation. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX) analyses were subsequently performed with the porpuse of morphological and elemental characterization of the fabricated Pd/ PANI/TiO2 catalyst. Moreover, functional groups of prepared catalyst were identified by means of Fourier transform infrared spectroscopy (FTIR). Then, electrochemical measurement techniques comprising cyclic voltammetry (CV) and chronoamperomtery (CA) were conducted to investigate the electroacivity of the Pd/PANI/TiO2 modified GC electrode towards methanol oxidation in an aqueous solution consisting of potassium hydroxid (KOH). Based on the experimental results, the Pd/PANI/TiO2 catalyst dramatically increased the catalytic activity of pure Pd catalyst by enlarging the elechtrochemical active surface area.
2.3. Fabrication of Pd/PANI/TiO2/GC electrode The thin-film electrode was prepared as follows: 2 mg of PANI/TiO2 nanocomposites was dispersed via an ultrasonic instrument in 1 mL of nafion + ethanol for a period of 10 min. Then, 12 microliters of the dispersed suspension were transferred onto the glassy carbon disk using a micropipette and then dried at room temperature. A potentiostatic technique was chosen to fabricate the Pd/PANI/TiO2/GC electrode, in which the Pd nanoparticles were electrodeposited from a solution containing 0.05 M PdCl2 and 0.5 M H2SO4. To achieve desirable samples, applied potential and process duration were set to −0.35 V and 300 s, respectively. Eventually, prepared Pd/PANI/TiO2/GC electrode was rinsed with double distlilled water prior to electrochemical measurements.
2. Experimental 2.1. Materials and apparatus
3. Result and discussion In the current study, aniline (C6H5NH2) was purchased from SigmaAldrich. Commercial TiO2 nanoparticles were supplied from nanosabz Co. Ammonium persulfate (APS) with the chemical formula of (NH4)2S2O8, HCl, methanol, PdCl2 and KOH were purchased from Merck. All reagents were used without any further purification. In all experimetns, double distlited water was used. All electrochemical experiments including cyclic voltammetry and chronoamperometry were conducted using a potentiostat/galvanostat instrument (Metrohm Autolab, PGSTAT204, Netherlands) at room temperature. A three-electrode cell was conducted in all electrochemical tests. A glassy carbon (GC) electrode with the diameter of 2 mm and surface area of 0.0314 cm2 and a Pt wire were used as working and counter electrodes, respectively. A saturated Ag/AgCl, (3 M KCl) system was served as the reference electrode. Hence, all potentials mentioned in this paper are against standard Ag/AgCl electrode.The structure of three-electrode system and mechanism of methanol oxidation on the surface of the Pd/PANI/TiO2 modified glassy carbon is schematically depicted in Fig. 1. In terms of electrochemical measurements, Initialy, in absence of methanol, cyclic voltammetry experiments were performed to determine the electrochemical surface area of the Pd/PANI/TiO2 and Pd catalyst loaded on GC electrode in a solution of 0.5 M KOH at a potential sweep rate of 50 mV/s between −0.8 and 0.5 V. Cyclic voltammetry measurements were carried out to evaluate the electrocatalytic activity of aforementioned catalysts towards methanol oxidation in a solution containing 0.5 M KOH and 1 M methanol at 50 mV/s scan rate. Then, the effects of methanol concentration and potential sweep rate were investigated by changing the scan rate from 10 to 1000 mV/s and varying the concentration of methanol from 0.05 M to 4.5 M in the two separate set of experiments. Chronoamperometry measurement was carried out in a solution of KOH (0.5 M) and methanol (1 M) at a constant potential of −0.1 V for 1000 s.
FESEM, EDX and FTIR techniques were successfully performed to analyze the fabricated Pd/PANI/TiO2 electrocatalyst. FESEM image and EDX spectra are presented in Fig. 2. As shown in Fig. 2A, pure PANI has smooth surface and flakes morphology. Moreover, the morphology of PANI/TiO2 nanocomposite was reformed with the presence of TiO2 nanoparticles (Fig. 2B) that indicated TiO2 is dispersed within PANI structure during the preparation of PANI/TiO2 nanocomposite. From Fig. 2C, it can be observed that a metallic layer of Pd nanoparticles is coated on the surface of the PANI/TiO2/GC electrode. As expected from chronoamperometry technique, this image also shows the agglomeration of metallic Pd particles forming a rough semi-spherical morphology on the electrode surface [37]. The EDX spectra presented in Fig. 2D prove the existence of palladium and titanium, and also oxygen and carbon species in the fabricated electrode. Fig. 3 demonstrates the FTIR spectra of three samples including pure TiO2 nanoparticles, PANI and the synthesized TiO2/PANI nanocompsites. From the TiO2 spectra, the bands related to TieO bending vibration are observed at 540 and 680 cm−1 (as expected, in the region between 400 and 800 cm−1). In this plot, moreover, the band at around 1630 cm−1 is the characteristic of OeH bending mode stemming from water molecules absorbed on the surface of TiO2 nanoparticles. From the PANI IR pattern, the bands occur at 1570 and 1490 cm−1 are attributed to C]N and C]C stretching mode, respectively. The peaks correspond to the CeN stretching mode are observed at 1294 and 1245 cm−1. Furthermore, the peaks located at 1134 and 818 cm−1 can be assigned to CeH bending vibration. Totally, from the repetition and existence of TiO2 and PANI characteristic absorption bands in the spectra of the PANI/TiO2 nanocomposite, the successful synthesis of the TiO2/PANI nanocompsites can be interpreted.
2.2. Synthesis of PANI/TiO2 nanocomposites
3.2. Electrochemical measurements
PANI/TiO2 nanocomposites were synthesized via an in-situ emulsion polymerization method. In this method, 0.1 M aniline monomer and 1 M HCl were vigorously stirred, and a certain amount (20 wt%) of commercial TiO2 nanopowder was added. Then, APS as an oxidant reagent was poured dropwise to the aniline–HCl–TiO2 mixture under constant stirring. The reaction was performed at a temperature of 5 °C. When polymerization reaction started, the color of the mixture turned light blue, proving the formation of PANI via an oxidation reaction. The stirring was maintained for 4 h to ensure complete polymerization. As a consequence, dark green PANI/TiO2 nanocomposites were formed,
3.2.1. Cyclic voltammetry studies Fig. 4 shows the cyclic voltammograms of Pd/GC and Pd/PANI/ TiO2/GC electrodes in a solution containing 0.5 M KOH by sweeping the potential between −0.8 V and 0.5 V at a scan rate of 50 mV/s. As can be seen in Fig. 4, there are two anodic peaks in the forward scan and a cathodic one in the reverse potential scan. The first anodic peak, which appears in the voltage range between −0.6 V and −0.4 V, can be attributed to the hydrogen adsorption or desorption [38]. The second anodic peak, at the potential about −0.2 V, is ascribed to Pd oxide (PdO) formation [38]. In the cathodic scan, a well-defined peak
3.1. Characterization of Pd/PANI/TiO2 catalyst
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Fig. 1. Schematic diagram of the experimental instruments used in this work.
observed at near −0.4 V which is related to the reduction of PdO [11,39]. The reduction peak corresponding to the Pd/PANI/TiO2 catalyst is larger than that of Pd catalyst indicating a significant increase in the active surface area of the Pd/PANI/TiO2 catalyst. Electrochemically active surface area (EASA) determining the catalytic activity of the prepared Pd/PANI/TiO2 and Pd catalysts can be simply evaluated using equation [40],
EASA =
Q Sl
l=
Qdep M 2F
(2)
Where Qdep, M and F represent the amount of charges passing during electrodeposition process (C), the molecular weight of Pd (gr mol−1), and Faraday constant (96,500C mol−1), respectively. Hence, the EASA values for the Pd/PANI/TiO2 and Pd were calculated to be 2.42 and 0.51 cm2/mg, respectively. It can be found that a much larger surface area for Pd is electrochemically available in the case of the Pd/PANI/TiO2 catalyst than that of the pure Pd catalyst [44]. The enhanced catalytic activity of the Pd/PANI/TiO2 sample in comparison with the pure Pd catalyst can be due to the proper conducting behavior of TiO2 and the polymeric matrix of PANI. Cyclic voltammetry analyses of the Pd/PANI/GC, Pd/TiO2/GC, PANI/TiO2/GC, Pd/GC and Pd/PANI/TiO2/GC electrodes in a solution of 0.5 M KOH and 1 M methanol, carried out to evaluate the catalytic activity towards methanol oxidation at a sweep rate of 50 mV/s and results are illustrated in Fig. 5. According to these curves, the first anodic peak in the forward potential sweep is ascribed to the oxidation of chemisorbed species from methanol adsorption on the surface of the electrocatalyst. Another anodic peak in the backward scan can be due to
(1)
Where Q is the columbic charge of the PdO reduction in Coulomb (mC) calculated by integrating the area under the reduction peak of cyclic voltammograms, l is the Pd catalyst loading in mg and S is the proportionality constant used to correlate charge with the surface area. Considering the reduction of PdO monolayer, S is assumed to be 0.405 mC/cm2 [40–42]. The following equation can be used to calculate the amount of Pd loaded on PANI/TiO2/GC electrode by assuming the complete reaction (100% yield) for Pd reduction from Pd+2 [43]:
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Fig. 2. FESEM image of PANI (A), PANI/TiO2 (B), Pd/PANI/TiO2 (C) and EDX spectra of fabricated Pd/PANI/TiO2 electrocatalyst (D).
Fig. 3. FTIR spectra of the TiO2, PANI and synthesized PANI/TiO2 nanocompsites.
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Fig. 4. Cyclic voltammograms of the Pd/GC and Pd/PANI/TiO2/GC in 0.5 M KOH at 50 mV s-1 scan rate.
Moreover, the onset potential (Es) in the forward potential scan demonstrates the electrocatalytic activity and methanol oxidation kinetics. For the Pd/PANI/TiO2/GC electrode, Es is about −0.53 V, whereas the onset potentials occur at −0.41 and −0.39 V for the pure Pd and PANI/TiO2 samples, respectively. The onset potential shift to negative values by 120 and 140 mV compared to the onset otentials of Pd and PANI/TiO2 for methanol oxidation, respectively. This improvement in methanol electrooxidation activity could be explained as follow; the hydroxyl groups of PANI/TiO2 nanocomposite enhanced the concentration of OH− groups in the electrolyte. OH− speices plays a significant role in the methanol oxidation reaction at lower potentials and Co to CO2 transformation [45,46]. The electrochemical properties of different catalysts for methanol oxidation in 0.5 M solution are
the oxidation or desorption of carbonaceous species which are not totally oxidized in the forward scan [11,45]. The forward scan peak current densities (Jf) of the Pd/PANI/TiO2/GC, Pd/TiO2/GC, Pd/PANI/ GC, Pd/GC and PANI/TiO2/GC electrodes are 80.12, 42.5, 27.3, 17.21 and 0.24 mA cm−2, respectively. As it is obvious from Fig. 5 that the Pd/PANI/TiO2 electrocatalyst represents significantly higher current density than those of the other catalysts in methanol oxidation. Electrochemical characteristics of proposed modified electrode (Pd/PANI/ TiO2/GC) which is summarized in Tablel 1 is comparable to past studies based on platinium catalyst [12–17]. The results revealed that the presence of the PANI/TiO2 nanocomposites improve the electrocatalytic activity and charge transfer of Pd catalyst towards methanol electrooxidation in alkaline media.
Fig. 5. Cyclic voltammograms of the Pd/PANI/GC, Pd/TiO2/GC, PANI/TiO2/GC, Pd/GC and Pd/PANI/TiO2/GC in KOH (0.5 M) and MeOH (1 M) at the scan rate of 50 mV s−1.
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In this mechanism, removing the absorbed intermediates (Pde(CO) ads) is somewhat slow, and, hence, the last reaction (8) is considered to be the rate-determining step. To investigate the effect of potential sweep rate on methanol electrooxidation in the presence of Pd/PANI/TiO2/GC electrode, different experiments were carried out at various scan rates ranging from 10 to 1000 mV/s in a solution containing 0.5 M KOH + 1 M methanol, and the obtained cyclic voltammograms are depicted in Fig. 6A. The methanol oxidation current density and potential peak in the forward scan grow up by increasing of scan rate. Fig. 6B represents the plot of the anodic peak current density (Jf) versus square root of the scan rate (υ1/ 2 ) acquired from Fig. 6A. From this linear plot, it can be concluded that the methanol oxidation on the Pd/PANI/TiO2/GC electrode is a diffusion-controlled process [48,49]. Fig. 7A exhibits cyclic voltammograms obtained from the survey of the impact of methanol concentration on catalytic behavior of the Pd/ PANI/TiO2 electrocatalyst for methanol oxidation in 0.5 M KOH solution. For a detailed analysis, the peak current densities in the forward sweep (Jf) were plotted with respect to methanol concentration provided in Fig. 7B. This diagram can be divided into two regions, before and after the 1.5 M methanol concentration. In other words, the upward trend turns downward once the concentration of methanol exceeds 1.5 M. Such behavior may be caused by the coverage of metal active sites by methanol molecules and hydroxyl adsorbates [11,45]. Furthermore, by elevating the methanol concentration, the peak potential moves to more positive values. This can be due to the direct correlation
Table 1 Electrochemical characteristics of MeOH oxidation on the Pd/PANI/TiO2, Pd and PANI/ TiO2 electrocatalysts in a solution of KOH (0.5 M) and MeOH (1 M) at a scan rate of 50 mV s−1. Sample
l (mg)
EASA (cm2 mg−1)
Es (V)
Jf (mA cm−2)
Jb(mA cm−2)
Pd/PANI/ TiO2 Pd PANI/TiO2
0.0055
2.42
−0.53
80.12
25.59
0.0047 –
0.51 –
−0.41 −0.39
17.21 0.24
2.81 –
summarized in Table 1. Methanol electrooxidation mechanism on Pd catalyst, which is a sixelectron process, in alkaline media is described by the following reactions [46,47]: Pd + CH3OH → Pde(CH3OH)ads Pde(CH3OH)ads + OH
−
→ Pde(CH3O)ads + H2O + e ̄
(3) (4)
Pde(CH3O)ads + OH− → Pde(CH2O)ads + H2O + e ̄
(5)
Pde(CH2O)ads + OH− → Pde(CHO)ads + H2O + e ̄
(6)
−
Pde(CHO)ads + OH → Pde(CO)ads + 4H2O + e ̄ Pde(CO)
ads
−
+ 2OH
→ Pd + CO2 + H2O + 2e ̄
(7) (8)
Fig. 6. A) Cyclic voltammograms of the Pd/PANI/TiO2/GC in KOH (0.5 M) and MeOH (1 M) at different scan rates from 10 to 1000 mV s−1. B) Anodic peak current density for methanol oxidation in the presence of the Pd/PANI/TiO2/GC obtained from (a) vs. square root of the scan rate.
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Fig. 7. A) Cyclic voltammograms of the Pd/PANI/TiO2/GC in a solution of KOH (0.5 M) in which the concentration of MeOH was varied from 0.05 to 4.5 M. B) variations of the anodic peak current density of methanol oxidation in on the Pd/PANI/TiO2/GC electrode vs. MeOH concentration.
a solution of 0.5 M KOH and 1.0 M methanol at −0.1 V during 100s. Fig. 9 shows the chronoamprometric responses of the Pd/PANI/TiO2/ GC and Pd/GC electrodes recorded at aforementioned test condition. As can be seen, although both samples experience similar decreasing exponential trend from their maximum initial values of current density, but current density dropping for Pd/GC electrode happens with a more rapidly trend compared to Pd/PANI/TiO2/GC electrode. It is found that the pure Pd catalyst shows lower stability in comparison with the Pd/ PANI/TiO2 catalysts towards methanol oxidation, indicating that Pd catalyst can be rapidly poisoned by adsorbing of CO-like or intermediate products in methanol electrooxidation reaction [52].
between overpotential of methanol oxidation reaction (IR drop) and the amount of oxidation current [50]. To further study of the Pd/PANI/TiO2 catalytic activity, stability test was conducted in which the potential of the Pd/PANI/TiO2/GC electrode was swept between −0.8 and 0.5 V for 150 cycles with the rate of 50 mV s−1 in a solution of 0.5 M KOH and 1 M methanol. The resulting cyclic voltammograms are illustrated in Fig. 8A, and the oxidation peak current densities are plotted versus the cycle number in Fig. 8B. According to Fig. 8B, the apparent decline of the anodic peak current densities with increasing cycle number can be ascribed to the low poisoning resistance of Pd/PANI/TiO2 catalyst [51]. The methanol forward anodic peak current density decays nearly 41% during 150 cycles. This confirms that the Pd/PANI/TiO2 catalyst show better stability compared to Pd towards electrooxidation of methanol in basic aqueous.
4. Conclusions The present study was designed to investigate the electrochemical performance of the Pd/PANI/TiO2, PANI/TiO2 and pure Pd electrocatalysts towards methanol oxidation by different electrochemical techniques shuch as cyclic voltammetry and chronoamperometry. According to the obtained results, as compared to Pd/GC, the Pd/PANI/ TiO2 promoted electrochemically activity of methanol oxidation
3.2.2. Chronoamperometric studies Since electrocatalysts stability in practical applications of DMFCs is importance, chronoamperometry experiments were carried out to investigation the stability of Pd/PANI/TiO2/GC and Pd/GC electrodes in 77
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Fig. 8. A) Cyclic voltammograms of the Pd/PANI/TiO2/GC electrode over 150 potential cycles in KOH (0.5 M) and MeOH (1 M) solution at the scan rate of 50 mV s−1. B) Anodic peak current density in the forward scan with respect to its cycle number.
Fig. 9. Chronoamperometric curves of the Pd/PANI/TiO2 and Pd catalysts in KOH (0.5 M) and MeOH (1 M) at −0.1 V.
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reaction it terms of current density and onset potential. For Pd/PANI/ TiO2, the onset potential of methanol oxidation shifts to more negative value compared to the PANI/TiO2 and pure Pd catalysts indicating improvement in methanol oxidation kinetics. From the stability point of view, a better stability was observed for Pd/PANI/TiO2 catalyst compare to the pure Pd catalyst. Overall, the Pd/PANI/TiO2 catalyst displays great electrocatalytic properties for methanol oxidation in alkaline media. It can be concluded that utilizing of PANI/TiO2 nanocomposite as support for pure Pd catalyst facilites the oxidation of methanol suggesting that this catalyst can be recommended for application in direct methanol fuel cells.
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Electrocatalytic performance of Pd/PANI/TiO2 nanocomposites for methanol electrooxidation in alkaline media
T
⁎
Mohammad Soleimani-Lashkenaria, , Sajjad Rezaeib, Jaber Fallahc, Hussein Rostamic a Fuel Cell Electrochemistry and Advanced Material Research Laboratory, Faculty of Engineering Modern Technologies, Amol University of Special Modern Technologies, Amol, 4616849767, Iran b Chemical Engineering Department, Tarbiat Modares University, Tehran, Iran c Faculty of Chemistry, University of Mazandaran, Babolsar, Iran
A R T I C L E I N F O
A B S T R A C T
Keyword: Palladium nanoparticles Polyaniline Methanol oxidation Electrocatalysis
An electrochemical method was successfully used for the Pd nanoparticles deposition on the Polyaniline/titanium dioxide (PANI/TiO2) modified glassy carbon (GC) electrode. The electrochemical activity of fabricated palladium/Polyaniline/titania (Pd/PANI/TiO2) electrocatalyst was investigated for methanol electrooxidation by cyclic voltammetry (CV) and chronoamperomtery (CA) in alkaline media. Also, the effect of different methanol concentrations and potential sweep rates were in two separate set of experiments. The prepared samples were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR) techniques. Obtained results indicated that the synthesized Pd/PANI/TiO2 catalyst not only possessed much higher electrochemically active surface area (EASA) than that of the pure Pd catalyst, but also enhanced the forward anodic peak current density (Jf) for methanol electrooxidation reaction. These observations are extracted from the combination of high charge transfer of the PANI/TiO2 nanocompsites and excellent catalytic characteristic of the Pd catalyst.
1. Introduction During the last decades, direct methanol fuel cells (DMFCs), electrochemical devices which directly convert chemical energy to electricity via electrooxidation of methanol as fuel, have become attractive due to their advantages including high efficiency, low operating temperature, and low/zero emission. But for commercialization of this technology, some barriers should be overcome. The slow reaction kinetics of methanol electrooxidation has been widely noticed as one of the most important challenges which strongly depends on the used catalysts [1]. Platinum (Pt) has been commonly implemented in DMFCs as catalyst layer on both anode and cathode side. Howevere, because of high cost and catalyst posioning towards methanol oxidation, many researchs have been done to find an alternative [2–4]. Pt alloy catalysts such as Pt/Ru, Pt/Ni, Pt/Co, Pt/Cu, Pt/Au and Pt/Ru/Ni have been demonstrated high activity and stability for application as anode catalyst in DMFCs [5–7]. Thus, Palladium-based catalysts with a lower cost than Pt and high catalytic activity in methanol oxidation reaction in alkaline media has been considered as a substitute for commonly-used Pt electrocatalysts [3,8–10].
In order to enhance the activity of pure Pd catalyst, Pd alloy catalysts such as PdSn, PdRu, and PdNi have been suggested because of their effect on oxidation of adsorbed CO intermediates to CO2 during methanol oxidation at lower potentials [11]. Another method for improving electrocatalytic activity in methanol oxidation is usage of support materials including 1) carbon-based materials such as carbon nanotube (CNT), carbon nano-fiber (CNF) and graphene, and 2) metal oxides like TiO2, SnO2, RuO2, MnO and CeO2 [12–17]. As among, Pd/ TiO2 nanocomposites have been shown high electrochemical activity and excellent stability in methanol oxidation [11,18]. In recent years, conducting polymers have been enormously used in various applications including catalysis, energy storage, electronic nanodevices bio-sensors and biomedical engineering due to their good mechanical, optical and electrical properties [19–25]. Between different conducting polymers, poly(o-phenylenediamine), polyaniline (PANI) and polypyrrole (PPy) has been broadly reported for improving the DMFCs performance [26–29]. Among them, PANI has attracted a lot of attention because of its low cost, easy synthesis methods and excellent electrical properties [30]. In addition, the modification effect of PANI on methanol oxidation reaction has been confirmed by several papers [4,31–36].
⁎ Corresponding author at: Fuel Cell Electrochemistry and Advanced Material Research Laboratory, Faculty of Engineering Modern Technologies, Amol University of Special Modern Technologies, Amol, 4616849767, Iran. E-mail address: [email protected] (M. Soleimani-Lashkenari).
https://doi.org/10.1016/j.synthmet.2017.12.001 Received 22 September 2017; Received in revised form 18 November 2017; Accepted 1 December 2017 0379-6779/ © 2017 Published by Elsevier B.V.
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followed by a color change to dark blue. After that, the prepared nanocomposites were filtered and washed with distilled water to remove oligomers and excess acid. Finally, the product was dried out for 24 h at room temperature.
In the current study, Pd nanoparticles was electrochemically deposited on the PANI/TiO2 nanocomposites modified glassy carbon electrode to form the Pd/PANI/TiO2/GC working electrode and was used as anode catalyst for methanol oxidation. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX) analyses were subsequently performed with the porpuse of morphological and elemental characterization of the fabricated Pd/ PANI/TiO2 catalyst. Moreover, functional groups of prepared catalyst were identified by means of Fourier transform infrared spectroscopy (FTIR). Then, electrochemical measurement techniques comprising cyclic voltammetry (CV) and chronoamperomtery (CA) were conducted to investigate the electroacivity of the Pd/PANI/TiO2 modified GC electrode towards methanol oxidation in an aqueous solution consisting of potassium hydroxid (KOH). Based on the experimental results, the Pd/PANI/TiO2 catalyst dramatically increased the catalytic activity of pure Pd catalyst by enlarging the elechtrochemical active surface area.
2.3. Fabrication of Pd/PANI/TiO2/GC electrode The thin-film electrode was prepared as follows: 2 mg of PANI/TiO2 nanocomposites was dispersed via an ultrasonic instrument in 1 mL of nafion + ethanol for a period of 10 min. Then, 12 microliters of the dispersed suspension were transferred onto the glassy carbon disk using a micropipette and then dried at room temperature. A potentiostatic technique was chosen to fabricate the Pd/PANI/TiO2/GC electrode, in which the Pd nanoparticles were electrodeposited from a solution containing 0.05 M PdCl2 and 0.5 M H2SO4. To achieve desirable samples, applied potential and process duration were set to −0.35 V and 300 s, respectively. Eventually, prepared Pd/PANI/TiO2/GC electrode was rinsed with double distlilled water prior to electrochemical measurements.
2. Experimental 2.1. Materials and apparatus
3. Result and discussion In the current study, aniline (C6H5NH2) was purchased from SigmaAldrich. Commercial TiO2 nanoparticles were supplied from nanosabz Co. Ammonium persulfate (APS) with the chemical formula of (NH4)2S2O8, HCl, methanol, PdCl2 and KOH were purchased from Merck. All reagents were used without any further purification. In all experimetns, double distlited water was used. All electrochemical experiments including cyclic voltammetry and chronoamperometry were conducted using a potentiostat/galvanostat instrument (Metrohm Autolab, PGSTAT204, Netherlands) at room temperature. A three-electrode cell was conducted in all electrochemical tests. A glassy carbon (GC) electrode with the diameter of 2 mm and surface area of 0.0314 cm2 and a Pt wire were used as working and counter electrodes, respectively. A saturated Ag/AgCl, (3 M KCl) system was served as the reference electrode. Hence, all potentials mentioned in this paper are against standard Ag/AgCl electrode.The structure of three-electrode system and mechanism of methanol oxidation on the surface of the Pd/PANI/TiO2 modified glassy carbon is schematically depicted in Fig. 1. In terms of electrochemical measurements, Initialy, in absence of methanol, cyclic voltammetry experiments were performed to determine the electrochemical surface area of the Pd/PANI/TiO2 and Pd catalyst loaded on GC electrode in a solution of 0.5 M KOH at a potential sweep rate of 50 mV/s between −0.8 and 0.5 V. Cyclic voltammetry measurements were carried out to evaluate the electrocatalytic activity of aforementioned catalysts towards methanol oxidation in a solution containing 0.5 M KOH and 1 M methanol at 50 mV/s scan rate. Then, the effects of methanol concentration and potential sweep rate were investigated by changing the scan rate from 10 to 1000 mV/s and varying the concentration of methanol from 0.05 M to 4.5 M in the two separate set of experiments. Chronoamperometry measurement was carried out in a solution of KOH (0.5 M) and methanol (1 M) at a constant potential of −0.1 V for 1000 s.
FESEM, EDX and FTIR techniques were successfully performed to analyze the fabricated Pd/PANI/TiO2 electrocatalyst. FESEM image and EDX spectra are presented in Fig. 2. As shown in Fig. 2A, pure PANI has smooth surface and flakes morphology. Moreover, the morphology of PANI/TiO2 nanocomposite was reformed with the presence of TiO2 nanoparticles (Fig. 2B) that indicated TiO2 is dispersed within PANI structure during the preparation of PANI/TiO2 nanocomposite. From Fig. 2C, it can be observed that a metallic layer of Pd nanoparticles is coated on the surface of the PANI/TiO2/GC electrode. As expected from chronoamperometry technique, this image also shows the agglomeration of metallic Pd particles forming a rough semi-spherical morphology on the electrode surface [37]. The EDX spectra presented in Fig. 2D prove the existence of palladium and titanium, and also oxygen and carbon species in the fabricated electrode. Fig. 3 demonstrates the FTIR spectra of three samples including pure TiO2 nanoparticles, PANI and the synthesized TiO2/PANI nanocompsites. From the TiO2 spectra, the bands related to TieO bending vibration are observed at 540 and 680 cm−1 (as expected, in the region between 400 and 800 cm−1). In this plot, moreover, the band at around 1630 cm−1 is the characteristic of OeH bending mode stemming from water molecules absorbed on the surface of TiO2 nanoparticles. From the PANI IR pattern, the bands occur at 1570 and 1490 cm−1 are attributed to C]N and C]C stretching mode, respectively. The peaks correspond to the CeN stretching mode are observed at 1294 and 1245 cm−1. Furthermore, the peaks located at 1134 and 818 cm−1 can be assigned to CeH bending vibration. Totally, from the repetition and existence of TiO2 and PANI characteristic absorption bands in the spectra of the PANI/TiO2 nanocomposite, the successful synthesis of the TiO2/PANI nanocompsites can be interpreted.
2.2. Synthesis of PANI/TiO2 nanocomposites
3.2. Electrochemical measurements
PANI/TiO2 nanocomposites were synthesized via an in-situ emulsion polymerization method. In this method, 0.1 M aniline monomer and 1 M HCl were vigorously stirred, and a certain amount (20 wt%) of commercial TiO2 nanopowder was added. Then, APS as an oxidant reagent was poured dropwise to the aniline–HCl–TiO2 mixture under constant stirring. The reaction was performed at a temperature of 5 °C. When polymerization reaction started, the color of the mixture turned light blue, proving the formation of PANI via an oxidation reaction. The stirring was maintained for 4 h to ensure complete polymerization. As a consequence, dark green PANI/TiO2 nanocomposites were formed,
3.2.1. Cyclic voltammetry studies Fig. 4 shows the cyclic voltammograms of Pd/GC and Pd/PANI/ TiO2/GC electrodes in a solution containing 0.5 M KOH by sweeping the potential between −0.8 V and 0.5 V at a scan rate of 50 mV/s. As can be seen in Fig. 4, there are two anodic peaks in the forward scan and a cathodic one in the reverse potential scan. The first anodic peak, which appears in the voltage range between −0.6 V and −0.4 V, can be attributed to the hydrogen adsorption or desorption [38]. The second anodic peak, at the potential about −0.2 V, is ascribed to Pd oxide (PdO) formation [38]. In the cathodic scan, a well-defined peak
3.1. Characterization of Pd/PANI/TiO2 catalyst
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Fig. 1. Schematic diagram of the experimental instruments used in this work.
observed at near −0.4 V which is related to the reduction of PdO [11,39]. The reduction peak corresponding to the Pd/PANI/TiO2 catalyst is larger than that of Pd catalyst indicating a significant increase in the active surface area of the Pd/PANI/TiO2 catalyst. Electrochemically active surface area (EASA) determining the catalytic activity of the prepared Pd/PANI/TiO2 and Pd catalysts can be simply evaluated using equation [40],
EASA =
Q Sl
l=
Qdep M 2F
(2)
Where Qdep, M and F represent the amount of charges passing during electrodeposition process (C), the molecular weight of Pd (gr mol−1), and Faraday constant (96,500C mol−1), respectively. Hence, the EASA values for the Pd/PANI/TiO2 and Pd were calculated to be 2.42 and 0.51 cm2/mg, respectively. It can be found that a much larger surface area for Pd is electrochemically available in the case of the Pd/PANI/TiO2 catalyst than that of the pure Pd catalyst [44]. The enhanced catalytic activity of the Pd/PANI/TiO2 sample in comparison with the pure Pd catalyst can be due to the proper conducting behavior of TiO2 and the polymeric matrix of PANI. Cyclic voltammetry analyses of the Pd/PANI/GC, Pd/TiO2/GC, PANI/TiO2/GC, Pd/GC and Pd/PANI/TiO2/GC electrodes in a solution of 0.5 M KOH and 1 M methanol, carried out to evaluate the catalytic activity towards methanol oxidation at a sweep rate of 50 mV/s and results are illustrated in Fig. 5. According to these curves, the first anodic peak in the forward potential sweep is ascribed to the oxidation of chemisorbed species from methanol adsorption on the surface of the electrocatalyst. Another anodic peak in the backward scan can be due to
(1)
Where Q is the columbic charge of the PdO reduction in Coulomb (mC) calculated by integrating the area under the reduction peak of cyclic voltammograms, l is the Pd catalyst loading in mg and S is the proportionality constant used to correlate charge with the surface area. Considering the reduction of PdO monolayer, S is assumed to be 0.405 mC/cm2 [40–42]. The following equation can be used to calculate the amount of Pd loaded on PANI/TiO2/GC electrode by assuming the complete reaction (100% yield) for Pd reduction from Pd+2 [43]:
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Fig. 2. FESEM image of PANI (A), PANI/TiO2 (B), Pd/PANI/TiO2 (C) and EDX spectra of fabricated Pd/PANI/TiO2 electrocatalyst (D).
Fig. 3. FTIR spectra of the TiO2, PANI and synthesized PANI/TiO2 nanocompsites.
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Fig. 4. Cyclic voltammograms of the Pd/GC and Pd/PANI/TiO2/GC in 0.5 M KOH at 50 mV s-1 scan rate.
Moreover, the onset potential (Es) in the forward potential scan demonstrates the electrocatalytic activity and methanol oxidation kinetics. For the Pd/PANI/TiO2/GC electrode, Es is about −0.53 V, whereas the onset potentials occur at −0.41 and −0.39 V for the pure Pd and PANI/TiO2 samples, respectively. The onset potential shift to negative values by 120 and 140 mV compared to the onset otentials of Pd and PANI/TiO2 for methanol oxidation, respectively. This improvement in methanol electrooxidation activity could be explained as follow; the hydroxyl groups of PANI/TiO2 nanocomposite enhanced the concentration of OH− groups in the electrolyte. OH− speices plays a significant role in the methanol oxidation reaction at lower potentials and Co to CO2 transformation [45,46]. The electrochemical properties of different catalysts for methanol oxidation in 0.5 M solution are
the oxidation or desorption of carbonaceous species which are not totally oxidized in the forward scan [11,45]. The forward scan peak current densities (Jf) of the Pd/PANI/TiO2/GC, Pd/TiO2/GC, Pd/PANI/ GC, Pd/GC and PANI/TiO2/GC electrodes are 80.12, 42.5, 27.3, 17.21 and 0.24 mA cm−2, respectively. As it is obvious from Fig. 5 that the Pd/PANI/TiO2 electrocatalyst represents significantly higher current density than those of the other catalysts in methanol oxidation. Electrochemical characteristics of proposed modified electrode (Pd/PANI/ TiO2/GC) which is summarized in Tablel 1 is comparable to past studies based on platinium catalyst [12–17]. The results revealed that the presence of the PANI/TiO2 nanocomposites improve the electrocatalytic activity and charge transfer of Pd catalyst towards methanol electrooxidation in alkaline media.
Fig. 5. Cyclic voltammograms of the Pd/PANI/GC, Pd/TiO2/GC, PANI/TiO2/GC, Pd/GC and Pd/PANI/TiO2/GC in KOH (0.5 M) and MeOH (1 M) at the scan rate of 50 mV s−1.
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In this mechanism, removing the absorbed intermediates (Pde(CO) ads) is somewhat slow, and, hence, the last reaction (8) is considered to be the rate-determining step. To investigate the effect of potential sweep rate on methanol electrooxidation in the presence of Pd/PANI/TiO2/GC electrode, different experiments were carried out at various scan rates ranging from 10 to 1000 mV/s in a solution containing 0.5 M KOH + 1 M methanol, and the obtained cyclic voltammograms are depicted in Fig. 6A. The methanol oxidation current density and potential peak in the forward scan grow up by increasing of scan rate. Fig. 6B represents the plot of the anodic peak current density (Jf) versus square root of the scan rate (υ1/ 2 ) acquired from Fig. 6A. From this linear plot, it can be concluded that the methanol oxidation on the Pd/PANI/TiO2/GC electrode is a diffusion-controlled process [48,49]. Fig. 7A exhibits cyclic voltammograms obtained from the survey of the impact of methanol concentration on catalytic behavior of the Pd/ PANI/TiO2 electrocatalyst for methanol oxidation in 0.5 M KOH solution. For a detailed analysis, the peak current densities in the forward sweep (Jf) were plotted with respect to methanol concentration provided in Fig. 7B. This diagram can be divided into two regions, before and after the 1.5 M methanol concentration. In other words, the upward trend turns downward once the concentration of methanol exceeds 1.5 M. Such behavior may be caused by the coverage of metal active sites by methanol molecules and hydroxyl adsorbates [11,45]. Furthermore, by elevating the methanol concentration, the peak potential moves to more positive values. This can be due to the direct correlation
Table 1 Electrochemical characteristics of MeOH oxidation on the Pd/PANI/TiO2, Pd and PANI/ TiO2 electrocatalysts in a solution of KOH (0.5 M) and MeOH (1 M) at a scan rate of 50 mV s−1. Sample
l (mg)
EASA (cm2 mg−1)
Es (V)
Jf (mA cm−2)
Jb(mA cm−2)
Pd/PANI/ TiO2 Pd PANI/TiO2
0.0055
2.42
−0.53
80.12
25.59
0.0047 –
0.51 –
−0.41 −0.39
17.21 0.24
2.81 –
summarized in Table 1. Methanol electrooxidation mechanism on Pd catalyst, which is a sixelectron process, in alkaline media is described by the following reactions [46,47]: Pd + CH3OH → Pde(CH3OH)ads Pde(CH3OH)ads + OH
−
→ Pde(CH3O)ads + H2O + e ̄
(3) (4)
Pde(CH3O)ads + OH− → Pde(CH2O)ads + H2O + e ̄
(5)
Pde(CH2O)ads + OH− → Pde(CHO)ads + H2O + e ̄
(6)
−
Pde(CHO)ads + OH → Pde(CO)ads + 4H2O + e ̄ Pde(CO)
ads
−
+ 2OH
→ Pd + CO2 + H2O + 2e ̄
(7) (8)
Fig. 6. A) Cyclic voltammograms of the Pd/PANI/TiO2/GC in KOH (0.5 M) and MeOH (1 M) at different scan rates from 10 to 1000 mV s−1. B) Anodic peak current density for methanol oxidation in the presence of the Pd/PANI/TiO2/GC obtained from (a) vs. square root of the scan rate.
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Fig. 7. A) Cyclic voltammograms of the Pd/PANI/TiO2/GC in a solution of KOH (0.5 M) in which the concentration of MeOH was varied from 0.05 to 4.5 M. B) variations of the anodic peak current density of methanol oxidation in on the Pd/PANI/TiO2/GC electrode vs. MeOH concentration.
a solution of 0.5 M KOH and 1.0 M methanol at −0.1 V during 100s. Fig. 9 shows the chronoamprometric responses of the Pd/PANI/TiO2/ GC and Pd/GC electrodes recorded at aforementioned test condition. As can be seen, although both samples experience similar decreasing exponential trend from their maximum initial values of current density, but current density dropping for Pd/GC electrode happens with a more rapidly trend compared to Pd/PANI/TiO2/GC electrode. It is found that the pure Pd catalyst shows lower stability in comparison with the Pd/ PANI/TiO2 catalysts towards methanol oxidation, indicating that Pd catalyst can be rapidly poisoned by adsorbing of CO-like or intermediate products in methanol electrooxidation reaction [52].
between overpotential of methanol oxidation reaction (IR drop) and the amount of oxidation current [50]. To further study of the Pd/PANI/TiO2 catalytic activity, stability test was conducted in which the potential of the Pd/PANI/TiO2/GC electrode was swept between −0.8 and 0.5 V for 150 cycles with the rate of 50 mV s−1 in a solution of 0.5 M KOH and 1 M methanol. The resulting cyclic voltammograms are illustrated in Fig. 8A, and the oxidation peak current densities are plotted versus the cycle number in Fig. 8B. According to Fig. 8B, the apparent decline of the anodic peak current densities with increasing cycle number can be ascribed to the low poisoning resistance of Pd/PANI/TiO2 catalyst [51]. The methanol forward anodic peak current density decays nearly 41% during 150 cycles. This confirms that the Pd/PANI/TiO2 catalyst show better stability compared to Pd towards electrooxidation of methanol in basic aqueous.
4. Conclusions The present study was designed to investigate the electrochemical performance of the Pd/PANI/TiO2, PANI/TiO2 and pure Pd electrocatalysts towards methanol oxidation by different electrochemical techniques shuch as cyclic voltammetry and chronoamperometry. According to the obtained results, as compared to Pd/GC, the Pd/PANI/ TiO2 promoted electrochemically activity of methanol oxidation
3.2.2. Chronoamperometric studies Since electrocatalysts stability in practical applications of DMFCs is importance, chronoamperometry experiments were carried out to investigation the stability of Pd/PANI/TiO2/GC and Pd/GC electrodes in 77
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Fig. 8. A) Cyclic voltammograms of the Pd/PANI/TiO2/GC electrode over 150 potential cycles in KOH (0.5 M) and MeOH (1 M) solution at the scan rate of 50 mV s−1. B) Anodic peak current density in the forward scan with respect to its cycle number.
Fig. 9. Chronoamperometric curves of the Pd/PANI/TiO2 and Pd catalysts in KOH (0.5 M) and MeOH (1 M) at −0.1 V.
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reaction it terms of current density and onset potential. For Pd/PANI/ TiO2, the onset potential of methanol oxidation shifts to more negative value compared to the PANI/TiO2 and pure Pd catalysts indicating improvement in methanol oxidation kinetics. From the stability point of view, a better stability was observed for Pd/PANI/TiO2 catalyst compare to the pure Pd catalyst. Overall, the Pd/PANI/TiO2 catalyst displays great electrocatalytic properties for methanol oxidation in alkaline media. It can be concluded that utilizing of PANI/TiO2 nanocomposite as support for pure Pd catalyst facilites the oxidation of methanol suggesting that this catalyst can be recommended for application in direct methanol fuel cells.
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