Electrooxidation of 2-propanol and 2-butanol on the Pt–Ni alloy nanoparticles in acidic media

Electrochimica Acta 88 (2013) 157–164

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Electrooxidation of 2-propanol and 2-butanol on the Pt–Ni alloy nanoparticles in acidic media Biuck Habibi ∗ , Elaheh Dadashpour Electroanalytical Chemistry Laboratory, Department of Chemistry, Faculty of Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran

a r t i c l e

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Article history: Received 8 August 2012 Received in revised form 8 October 2012 Accepted 8 October 2012 Available online 16 October 2012 Keywords: 2-Propanol 2-Butanol Oxidation Electrocatalyst Platinum–nickel nanoparticles

a b s t r a c t The platinum–nickel nanoparticles carbon-ceramic modified electrode (Pt–Ni/CCE) was used as a potent electrocatalyst for the electrooxidation of 2-propanol and 2-butanol in a mixture of 0.15 M 2-propanol (or 0.15 M 2-butanol) and 0.1 M H2 SO4 solutions. The Pt–Ni/CCE catalyst shows excellent electrocatalytic activity for electrooxidation of these fuels in comparison with platinum nanoparticles of carbon-ceramic modified (Pt/CCE) and smooth Pt electrodes due to the presence of Ni atoms in the alloy which enhances the electrocatalytic activity of Pt toward the oxidation of 2-propanol and 2-butanol and reduces the amount of Pt in the anodic material of direct 2-propanol and 2-butanol fuel cells. Furthermore, the Pt–Ni/CCE catalyst has satisfactory stability and reproducibility for the electrooxidation of 2-propanol and 2-butanol in acidic media when stored in ambient conditions or when used in constant potential applying (chronoamperometry) and continuous potential cycling (cyclic voltammetry) which makes it more attractive for alcohol-based fuel cell applications. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, the development of fuel cells that operate on the basis of alcohols direct oxidation without any external reforming has attracted considerable interest because of its applications to alternative power sources, ranging from portable power for consumer electronics to transport applications [1–3]. The direct alcohol fuel cells (DAFCs) have several advantages, including low cost, low weight, and simple construction without a reformer [3,4]. Although, methanol has several advantages as a promising fuel for the DAFCs [4], the development of DAFCs based on methanol fuel is facing serious difficulties [5,6]: slow electro-kinetic property of methanol oxidation, high methanol crossover, high toxicity and low boiling point (65 ◦ C) of methanol. Therefore, other alcohols have been considered as alternative fuels. Recent studies [7–12] have focused on 2-propanol as an alcohol fuel because it shows a lower over-potential than methanol. 2-Propanol is the smallest secondary alcohol which is less toxic than methanol and its electrochemical oxidation is of great interest due to its particular molecular structure. The fuel cells using 2-propanol fuel show much higher performance and a much lower crossover current [5,7–18]. On the other hand, four-carbon alcohols have received particular attention because they can be produced in significant quantities from a

∗ Corresponding author. Tel.: +98 412 4327541; fax: +98 412 4327541. E-mail address: [email protected] (B. Habibi). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.10.020

variety of renewable feed stocks [19–21] and have several advantages over short-chain alcohols [22]. On the basis of general knowledge [23–25], the best performing fuel cell electrodes as an anode in the DAFCs are either Pt or Pt-based materials. These catalysts have been one of the most expensive materials in the fabrication of components for DAFCs. Therefore, it is necessary to develop active catalysts for these fuel cells to make them economically viable. The search for new and less expensive alternative materials as anodes for DAFCs has been a topic of current interest [26]. Various routes adopted to achieve this goal include the dispersion of the Pt nanoparticles in polymer matrix [27–29], deposition of Pt layers on the less expensive metal materials [30], reduction of relative loading of Pt on the catalyst surface by introducing a second promoting metal [31], and employing a substrate that not only offers enhanced surface area but also assists in the catalytic activity of the noble metal deposits during electrooxidation of the fuel [32]. It should be taken into consideration that Pt and Pt-based materials are also highly sensitive to surface-adsorbed intermediates poisoning: the catalyst’s surface is progressively poisoned by the adsorbed intermediates, which is formed as a result of the stepwise dehydrogenation of the fuel in the oxidation reaction [33]. Poisoning adsorbed intermediate species can be oxidatively removed from the Pt surface with neighboring OHad species electrosorbed from water at more positive potentials [34]. Alloying of Pt with oxophilic metals enables electrochemical dissociation of water on oxophilic metal sites at more negative potentials compared to pure Pt and, therefore, this allows electrocatalytic oxidation of surface-adsorbed intermediates at

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lower anodic level over potentials [35]. Pt–Ni alloy electrocatalysts demonstrate a superior activity toward electrooxidation of fuels and have been intensively investigated and they show electrocatalytic activity for carbon monoxide [36] and methanol [37] oxidation. Therefore, these catalysts deserve closer attention as promising candidates for possible applications as anode material in low-temperature fuel cells. In our previous work [38], we have successfully demonstrated the possibility of the fabrication of Pt–Ni nanoparticles on the carbon-ceramic electrode (CCE) which has several interesting characteristics such as high conductivity, relative chemical inertness, good mechanical properties, physical rigidity and high porosity as a substrate in the catalyst’s deposition. The obtained catalyst (Pt–Ni/CCE) was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and cyclic voltammetery. Based on the electrocatalytic behavior of Pt–Ni nanoparticles toward formic acid [38], methanol and ethanol oxidation [39], the present work is focused on the electrooxidation of 2-propanol and 2-butanol on the bimetallic Pt–Ni alloy nanoparticles. This is the continuation of our previous work on the role of metal or metal alloy nanoparticles in fuel cell applications [40–43]. At any rate, to the best of our knowledge, the Pt–Ni bimetallic catalyst for 2-propanol and 2-butanol electrooxidation in acidic media has not been reported yet. 2. Experimental procedure 2.1. Chemicals Methyltrimethoxysilane (MTMOS), 2-propanol, 2-buthanol, H2 PtCl6 ·5H2 O, NiCl2 , HCl, H2 SO4 and graphite powder of high purity were obtained from Merck or Fluka. All solutions were prepared with double distilled water. 2.2. Procedure of Pt–Ni/CCE preparation The Pt–Ni/CCE was prepared according to our previous work [38]. Briefly, the sol–gel processing method was used for fabricating CCE according to the following procedure: An amount of 0.9 ml MTMOS was mixed with 0.6 ml of methanol. After the addition of 0.6 ml HCl 0.1 M as a catalyst, the mixture was magnetically stirred (for about 15 min) until it produced a clear and homogeneous solution. Then, 0.3 g graphite powder was added and the mixture was shaken for another 5 min. Subsequently, the homogenized mixture was firmly packed into a Teflon tube (with 3 mm inner diameter and 10 mm length) and dried for at least 24 h at the room temperature. After polishing of the CCE, the Pt–Ni alloy nanoparticles were electrodeposited [potentiostatically at −0.4 V versus a saturated calomel electrode (SCE)] on CCE from an aqueous solution of 0.1 M Na2 SO4 (pH = 3.8) containing H2 PtCl6 ·5H2 O and NiCl2 (the total concentration of the two salts was equal to 1 mM with 70/30 ratio) at 25 ◦ C. Under the deposition conditions [Pt–Ni (70:30)], a stable Ni contribution (0.31) can be easily got [38]. The charge resulting from the complete reduction of the precursor salts at the given time was 1384 mC cm−2 . This value corresponds to an equivalent amount of platinum of 700 ␮g cm−2 when platinum is considered as the only metal deposited [28,42]. The actual active surface area of the Pt–Ni/CCE was equivalent to 5.10 cm2 accordingly the number of Pt sites available for hydrogen adsorption/desorption [38]. 2.3. Instrumentation The electrochemical experiments were carried out using an AUTOLAB PGSTAT-100 (potentiostat/galvanostat) equipped with a USB electrochemical interface and driven GEPS software was used for electrochemical experiments. A conventional three-electrode

Fig. 1. CVs of the Pt–Ni/CCE in 0.1 M H2 SO4 in the presence (curve a) and absence (curve b) of 0.15 M 2-propanol and also bare CCE in the same conditions (curve c and d) at a scan rate of 50 mV s−1 . Inset is the CV of 2-propanol at the smooth Pt electrode.

cell was used at the room temperature. The smooth Pt or the nanoparticles-modified electrode (Pt/CCE or Pt–Ni/CCE) was used as a working electrode. A SCE and a platinum wire were used as the reference and auxiliary electrodes, respectively. A JULABO thermostat was used to control the cell temperature at a constant value. 3. Results and discussion 3.1. Electrocatalytic properties of Pt-Ni/CCE toward the oxidation of 2-propanol and 2-buthanol Fig. 1 curve a shows the cyclic voltammogram (CV) of 0.15 M 2-propanol in the 0.1 M H2 SO4 solution on the Pt–Ni/CCE electrocatalyst at the scan rate of 50 mV s−1 and curve b shows the CV of the same electrocatalyst in supporting electrolyte (0.1 M H2 SO4 ). The inset of Fig. 1 also shows the CV of 2-propanol oxidation on the smooth Pt electrode. As can be seen in the inset, no CV features of hydrogen adsorption/desorption at a Pt electrode appeared in the potential region of hydrogen adsorption/desorption, which indicated effectively that a part of Pt surface was covered by adsorbates of 2-propanol like other alcohols [30,44]. Higher than these potentials, in the positive going potential sweep (PGPS), the oxidation of 2-propanol occurred in two anodic peaks (a1 and a2 ) in the range of potential between 0.0 and 1.5 V. While, in the negative going potential sweep (NGPS) the oxidation of 2-propanol took place in the relatively wide ranges of potential and a current peak occurred at 0.25 V (a3 ). However, a small negative current due to the reduction of the surface Pt oxides which is formed at higher potentials in the PGPS, was observed in the range of potential between 0.63 and 0.45 V (c1 ). Based on some previous reports [45–49], it is considered that the main electrochemical reaction in the oxidation of 2-propanol on the Pt and Pt-based electrocatalysts is dehydrogenation, conversion of 2-propanol to acetone, with the charge-transfer reaction of hydrogen. Besides the above main reaction, the electrochemical reaction including an intermediate,

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2-propanol → intermediate → carbon dioxide, proceeds successively in parallel. Although the exact molecular structure of the intermediate has not been determined, the intermediate is probably an acetone species or acetone itself. Considering these pathways, a simple reaction model can be assumed that involves parallel reactions for 2-propanol electrooxidation:

Pathway 1 expresses the dehydrogenation of 2-propanol with the successive charge-transfer reaction of hydrogen, the kinetic study shows that the dehydrogenation is the fastest reaction step for the overall oxidation of 2-propanol on Pt electrodes [49], and in pathway 2, conversing of intermediate to carbon dioxide is the reaction involving the rate-determining step for the electrooxidation of the adsorbed intermediate [45]. It should be noted that, based on the previous studies during the 1990s [48,49], the major oxidation product is acetone at low potentials (<0.4 V vs RHE) on the smooth Pt electrode; the mechanism of electrooxidation is proposed to occur via C–H activation at the alkoxy carbon, and this is the reason why there is some activity for this reaction at potential lower than 0.4 V on smooth Pt catalyst. While in potentials higher than about 0.4 V decomposition of water on smooth Pt surface provide the OH adsorption and therefore both pathway (1 and 2) could occur at higher potentials [50]. Electrooxidation of 2-propanol on the Pt–Ni/CCE occurs at two oxidation peaks, A1 and A2 , the first and the second anodic peaks, in the PGPS and at another oxidation peak, A3 , in the NGPS. The first anodic peak (A1 ) appears around 0.42 V with the onset potential (Eonset = −0.1 V), the second anodic peak (A2 ) appears around 1.1 V. Considering that the OH adsorption by water decomposition on the Pt surface occurs at about 0.4 V, the low onset potential is probably due to presence of Ni metal in the alloy and dehydrogenation of 2-propanol. On the other hand, in presence of Ni atoms, because the bifunctional mechanism could be operational at lower potential, both mechanisms could occur at these potentials. Peaks A1 and A2 are related to the direct oxidation of adsorbed species formed during the adsorption of 2-propanol. The appearance of the two peaks on the PGPS can be ascribed to the oxidation of the fuel by the two kinds of chemisorbed oxygen species [51]. The A3 peak appears around 0.29 V which was attributed to the oxidation of the 2-propanol. Indeed, in the NGPS, the platinum surface becomes free of adsobed species (oxides, hydroxides and adsorbed organic species) and then becomes very active for the 2-propanol electrooxidation [48] and anodic current starts immediately after the electroreduction of Pt oxide and renewal of Pt atoms. In comparison with the smooth Pt, Pt–Ni nanoparticlesmodified CCE showed an enhanced electrocatalytic activity toward the 2-propanol oxidation. The anodic current density of 2-propanol oxidation on the Pt–Ni nanoparticles in PGPS is found increased to the JA1 = 0.62 mA/cm2 (vs real electrochemical active sur-

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significantly enhanced after the introduction of Ni metal. The effects of the Ni atoms in the Pt based binary alloy on the electrooxidation reaction of 2-propanol can be explained on the basis of (a) the bifunctional mechanism [52,53], (b) the electronic interaction between the alloying element and Pt [53–55] or the synergistic

role of Pt and Ni atoms in the catalyst and (c) the increase in electrochemical active surface area of the alloying element due to the “leaching out” effect [56]. In addition, another effect of Pt nanoparticles surface modification by Ni atoms which can explain the higher activity of Pt–Ni alloy is that the dilution of Pt adsorption site could modify the balance between adsorbed organic species and OH adsorbed species and favors the oxidation and desorption of adsorbed 2-propalnol as already reported to others organic species such as ethylene glycol and alcohols more than one carbon [57,58]. Curves c and d in Fig. 1 show the CV of the bare CCE in the absence (c) and presence (d) of 2-propanol in supporting the electrolyte. As can be seen, within the applied potential range, there are no observable redox peaks on the CCE: this indicates that bare CCE has no catalysis effect in the electrooxidation of 2-propanol and even in the hydrogen adsorption/desorption processes. Fig. 2 shows the CVs of 0.15 M 2-butanol in the 0.1 M H2 SO4 solution on the Pt–Ni/CCE electrocatalyst and the Pt/CCE (inset) at 50 mV s−1 . Three oxidation peaks, 0.36 (A1 ), 0.95 (A2 ) and 0.25 V (A3 ) are observed for the electrooxidation of 2-butanol on the Pt–Ni/CCE electrocatalyst. The appearance of these two main peaks on the PGPS for the electrooxidation of 2-butanol on the Pt–Ni/CCE electrocatalyst can be ascribed to the oxidation of this fuel by two kinds of chemisorbed oxygen species [1,9,59]. In the NGPS, an anodic peak appears and this is attributed to the renewed oxidation of the fuel. This behavior is very similar to that shown by 2-propanol oxidation. A comparison of the CV of Fig. 2, its inset and the oxidation of 2-butanol on the smooth Pt (not shown here) shows that the CV profile of the Pt–Ni/CCE electrocatalyst has the usual characteristic of smooth Pt and Pt/CCE except the fact that

face area) and JA2 = 0.34 mA/cm2 , which is Ja1 = 0.21 mA/cm2

and Ja2 = 0.17 mA/cm2 for the smooth Pt in the same condition while in the NGPS, the anodic current density is found JA3 =

0.63 mA/cm2 , which is JA3 = 0.18 mA/cm2 for the smooth Pt. All of these indicate that Pt-Ni nanoparticles possess many active locations, higher electrochemical active surface area and excellent catalytic capability to 2-propanol electrooxidation. Moreover, the comparison of the anodic currents density of 2-propanol oxidation on the Pt–Ni nanoparticles with Pt nanoparticles (alone) on CCE (Pt/CCE) (CV not shown here) [JA1 = 0.35 mA/cm2 , JA2 =

0.23 mA/cm2 , JA3 = 0.38 mA/cm2 ] shows an enhanced electrocatalytic activity toward the 2-propanol oxidation. Therefore, the electrocatalytic activity toward the 2-propanol oxidation is

Fig. 2. CV of 0.15 M 2-butanol at the Pt–Ni/CCE in 0.1 M H2 SO4 at a scan rate of 50 mV s−1 . Inset is the CV of 2-butanol at the Pt/CCE in the same conditions.

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Fig. 3. Effect of scan rate on the electrooxidation of 0.15 M 2-propanol at the Pt–Ni/CCE in 0.1 M H2 SO4 . Scan rates are shown on CVs.

the oxidation current on the Pt–Ni/CCE electrocatalyst, both for the PGPS and NGPS, is much higher than on smooth Pt and Pt/CCE. Furthermore, Pt–Ni/CCE exhibits a high electrocatalysis performance in 2-butanol oxidation. Obviously, the anodic currents densities of 2-butanol oxidation in PGPS are significantly enhanced after the introduction of Ni atoms in the electrocatalyst. Additionally, it is noted that the onset potential of 2-butanol oxidation on the PtNi/CCE is even lower than that on the smooth Pt and Pt/CCE like as 2-propanol. These results indicate that the addition of Ni into Pt catalysts can significantly improve the electrode performance for the 2-butanol electrooxidation at a relatively less cost. In order to investigate the kinetic characterization of 2-propanol and 2-butanol oxidation on the Pt–Ni/CCE, we looked into the effect of the scan rate on the behavior of these fuels’ oxidation. Fig. 3 shows the CVs of the 2-propanol on the Pt–Ni/CCE at the different scan rates. The result clearly reveals that the peak current associated to the 2-propanol electrooxidation increases linearly with the scan rate. In Fig. 4A and B, the peak current densities were plotted as a function of the square root of the scan rate (1/2 ) and the scan rate (), respectively. As can be seen, the current density values of the two main anodic peaks and reduction peak in NGPS are linear vs. 1/2 . These behaviors indicate that the electrocatalytic processes under study are controlled by diffusion. Fig. 4C shows the effect of the scan rate on the current density of the anodic peak in the NGPS. It can be seen that the current of the reverse anodic oxidation process (A3 ) increases by increasing the scan rate with a hill-shape relationship. In different scan rates, due to different amounts of time (time windows) for poisoning species to adsorb on the nanoparticles’ surface, the surface is occupied by these species by different amounts. On the other hand, in different scan rates, the conversion of Pt to PtO is accelerated or decelerated; so, in NGPS, because of different clean active sites available on nanoparticles’ surface, the current of the reduction of PtO peak and the re-oxidation peak are altered. Therefore, in the low scan rates, there is not enough time for poisoning species to occupy the nanoparticles’ sites, so nanoparticles’ surface is clean to electrocatalyzing 2-propanol. Whereas, in the high scan rates, Pt converts to PtO and produces enough clean Pt surface for 2-propanol to be oxidized and this results in an increase of the current of the reverse anodic oxidation process. We can also see that the potential of different anodic and cathodic peaks vary with the increase of the scan

rate. As can be seen in Fig. 4D the potential of the reverse anodic oxidation peak shifts to low positive potentials with the increase of the scan rate. Whereas, the potentials of peak A1 (curve a1 ), A2 (curve a2 ), and C1 (curve c1 ) shift to high potentials with the increase of the scan rate (Fig. 4E and 4F). Fig. 5 shows the CVs of the 2-butanol oxidation on the Pt-Ni/CCE at different scan rates. The result clearly reveals that the two main peaks (A1 and A2 ), the reduction peak (C1 ) and the reverse oxidation peak (A3 ) currents associated to the 2-butanol electrooxidation and PtO reduction increase linearly with the scan rate like the 2propanol oxidation. In order to reveal the correlation between 2-propanol and 2butanol oxidation and Pt oxide species, we have studied the effect of the upper limit potentials (EU) in the cyclic potential scanning on the 2-propanol and 2-butanol oxidation. The reason, as reported in the literature, is the fact that the different ranges of potential over which the formation and dissolution of surface oxides occur on the smooth Pt or on the Pt-based electrode, form a striking feature of the electrochemical behaviors of these electrodes [60]. It is also reflected in a kinetic irreversibility of most electrocatalytic oxidation even in the reduction reactions that proceed on these electrodes [61]. Fig. 6 shows the CVs of 2-propanol oxidation on the Pt–Ni/CCE electrode for EU of 1.1–1.5 V. As seen in Fig. 6, by increasing the final positive potential limit, the anodic current density of 2-propanol oxidation in the PGPS remains unchanged, but the oxidation current density in the NGPS is decreased. In the lower limit potential, the Pt oxides with a high valence do not develop greatly, so the effect of the Pt oxides with a high valence on the 2-propanol oxidation in the NGPS is relatively small. It can be also seen that the potential of the 2-propanol oxidation peak remains invariable in the PGPS, while the potential of the 2-propanol oxidation peak shifts positively in the NGPS. On the other hand, the peak current density in the NGPS decreased as the EU increased. Indeed by increasing the final positive potentials, the conversion of Pt to PtO is accelerated and this causes a decrease of the oxidation current density in the NGPS, which further demonstrates that 2-propanol can only be oxidized on a clean metallic platinum nanoparticles’ surface [54]. In Table 1, we have summarized some data from the analysis of CVs in different anodic reversal potentials. From the obtained results in Table 1, it can be stated: by increasing the final positive potential (i) the E = Ea1 − Ea3 and E = Ea2 − Ea3 , (ii) the ratio of Ia1 /Ia3 , Ia2 /Ia3 and I = Ia1 − Ia3 , I = Ia2 − Ia3 and (iii) the E = Ea1 − Ec1 increases, E = Ec1 − Ea3 is almost constant and (iv) I = Ia1 − Ia3 , I = Ia2 − Ia3 , the ratio of |Ia1 /Ia2 | and |Ia1 /Ia3 | increases. These results show that there is a relationship between the anodic reversal potential and the structures and status of metallic nanoparticles and various electrochemical phenomena in the NGPS. The inset of Fig. 6 shows the CVs of 2-butanol oxidation on the Pt–Ni/CCE for EU of 1.0–1.5 V. As can be seen, by increasing the final positive potential limit, the current of both two anodic peaks (especially the second peak) in the PGPS remains unchanged, but the oxidation current in the NGPS is almost constant (to E = 1.0) and then increases (to E = 1.3) and finally decreases. The re-oxidation peak of 2-butanol is related to the oxidation of 2-butanol and/or 2-butanol residues in NGPS and on the other hand, the reaction pathway, intermediate and product of 2-butanol oxidation depend on the potential [62]. Therefore, until an anodic reversal potential (E = 1.0), the re-oxidation peak of 2-butanol is the main reaction; while after that (E > 1.0) the re-oxidation peak corresponds to 2butanol and 2-butanol residues oxidation. To evaluate the power of Pt–Ni/CCE for the electrooxidation of 2-propanol and 2-butanol, the effect of these compound concentrations on the corresponding anodic peaks’ currents in PGPS was investigated by CV. According to the experimental data, the peak currents of 2-propanol and 2-butanol were increased by these compound concentrations, and they reached a nearly constant value

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Fig. 4. Plot of Jp of two main anodic peaks (a1 and a2 ) and reduction peak (c1 ) of 2-propanol oxidation at the Pt–Ni/CCE vs square root of scan rate (curve a) (A) and vs scan rate (B), plot of Jp for reveres anodic peak (a3 ) vs square root of scan rate and vs scan rate (C), variation of the peak potential of reveres anodic peak (a3 ) vs square root of scan rate and vs scan rate (D), variation of the peak potential of two main anodic peaks (a1 and a2 ) and reduction peak (c1 ) vs scan rate (E) and vs square root of scan rate (F).

for the concentrations higher than 1.5 M for 2-propanol and 1.2 M for 2-butanol. We assume this effect is caused by the saturation of active sites at the surface of the electrode. The long-term stability of the electrocatalyst is important from the viewpoint of the practical application. In order to evaluate the stability of the electrocatalytic activity of the Pt–Ni/CCE toward 2-propanol and 2-buthanol electrooxidation and also poisoning-resistance of the electrocatalyst, chronoamperometric measurements were performed. The obtained results on Pt–Ni/CCE and smooth Pt in 0.15 M 2-propanol + 0.1 M H2 SO4 solution were shown in Fig. 7 (for 2-propanol as an example). When the potential was fixed at +0.45 V, due to the continuous oxidation of 2-propanol

on the catalyst surface, tenacious reaction intermediates would begin to accumulate if the kinetics of the removal reaction could not keep pace with that of 2-propanol oxidation. A more gradual decay of anodic current density with time implies that the catalyst has good poisoning-resistance ability [63]. As also seen from Fig. 7, the decay of the oxidation current density on Pt–Ni/CCE is very slower than that on the smooth Pt. This indicates that Pt–Ni/CCE has an acceptable stability in the electrooxidation of 2-propanol. On the other hand, the oxidation current on Pt–Ni/CCE is very larger than that on Pt electrode at the end of experiment (2000s). This indicates that Pt–Ni/CCE is a good poisoning-resistance electrocatalyst for 2-propanol oxidation. In addition, the continuous cycling

Table 1 Some data from the analysis of CVs of 0.15 M 2-propanol oxidation in different anodic reversal potentials at the Pt–Ni/CCE in 0.1 M H2 SO4 . Eanodic 1.1 1.2 1.3 1.4 1.5

reversal

Ea1

Ea2

Ec1

Ea3

Ia1

Ia2

Ia3

Ea1 − Ea3

Ea2 − Ea3

Ea1 − Ec1

Ec1 − Ea3

Ia1 /Ia3

Ia2 /Ia3

Ia1 − Ia3

Ia2 − Ia3

0.426 0.426 0.430 0.432 0.432

– 1.137 1.137 1.137 1.137

0.449 0.403 0.361 0.327 0.304

0.337 0.306 0.268 0.240 0.209

3.07 3.07 3.14 3.25 3.25

– 2.32 2.32 2.32 2.32

3.37 3.20 2.85 2.65 2.25

0.089 0.120 0.165 0.196 0.228

– 0.831 0.869 0.897 0.928

−0.02 0.023 0.069 0.105 0.128

0.111 0.097 0.092 0.086 0.094

0.91 0.96 1.10 1.23 1.44

– 0.73 0.81 0.88 1.03

−0.30 −0.13 0.30 0.60 1

– −0.88 −0.53 −0.33 0.07

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Fig. 7. Chronoamperometric curves (at +0.45 V) of 2-propanol (0.15 M) oxidation on the Pt–Ni/CCE (thick line) and smooth Pt electrode (thin line) in 0.1 M H2 SO4 solution. Inset is the plot of anodic peak current density of the electrooxidation of 0.15 M 2-propanol on the Pt–Ni/CCE in the same condition as a function of scan number in cyclic voltammetric method at a scan rate of 50 mV s−1 .

Fig. 5. Effect of scan rate on the electrooxidation of 0.15 M 2-butanol at the Pt–Ni/CCE in 0.1 M H2 SO4 . Scan rates are shown on CVs.

in the CV method can indicate that the present electrocatalyst has how stability in long-term usage. The effect of continuous cycling and long-term stability of Pt–Ni/CCE was examined in 0.1 M H2 SO4 solution containing 0.15 M 2-propanol and 0.15 M 2-butanol. It can be observed from the obtained results (inset of Fig. 7 for 2-proanol) that the anodic current density remains constant with an increase in the scan number at the initial stage and then it starts to decrease after 60 scans. The peak current density of the 200th scan is about 90% than that of the first scan. In general, the loss of the catalytic activity after successive scans may result from the consumption of 2-propanol during the CV scan. It may also be due to poisoning and the structure change of the Pt–Ni nanoparticles as a result of the perturbation of the potentials during the scanning in aqueous solutions, especially in presence of the organic compounds. Another factor involved might be the diffusion process occurring between the surface of the electrode and the bulk solution. With an increase in the scan number, 2-propanol diffuses gradually from

the bulk solution to the surface of the electrode. After the long-term CV experiments, the Pt–Ni/CCE was stored in water for a week; then 2-propanol oxidation was carried out again by CV, and excellent catalytic activity toward 2-propanol oxidation was still observed. This indicates that the Pt–Ni/CCE composite prepared in our experiment has good long-term stability and storage properties. Similar behavior was obtained for 2-butanol oxidation in stability study and continuous cycling. 4. Conclusion Electrooxidation of 2-propanol and 2-butanol was studied on the carbon-ceramic electrode modified bimetallic platinum–nickel nanoparticles as a potent electrocatalyst in acidic media. The results were compared with those at the same electrode modified by platinum nanoparticles and smooth Pt electrode alone. It was found that Pt–Ni electrocatalyst was catalytically more active than platinum nanoparticles on the same substrate and Pt electrode. The high electrocatalytic activity of the Pt–Ni electrocatalyst toward the oxidation of 2-propanol and 2-butanol may be directly related to the effect of the Ni atoms in electrocatalytic activity of the Pt and its large electroactive surface area. Moreover, it contributes to the reduction of the amount of the noble metal in the anode of DAFCs, which remains one of the challenges to make the technology of DAFCs economically viable. Also, the Pt–Ni electrocatalyst exhibits satisfactory stability and reproducibility when stored in ambient conditions and in constant potential applying or continuous cycling; which makes it attractive as the anode in direct alcohol fuel cells (DAFCs) and the related applications. Acknowledgment The authors gratefully acknowledge the research council of Azarbaijan Shahid Madani University for financial support. References

Fig. 6. Effect of upper limit of potential scanning region on the electrooxidation of 0.15 M 2-propanol at the Pt–Ni/CCE in 0.1 M H2 SO4 . (1) 0.0–1.1 V, (2) 0.0–1.2 V, (3) 0.0–1.3 V, (4) 0.3–1.4 V, and (5) 0.3–1.5 V. Scan rate 50 mV s−1 . Inset is the effect of upper limit of potential scanning region on the electrooxidation of 0.15 M 2-butanol at the Pt–Ni/CCE in 0.1 M H2 SO4 solution.

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