Accepted Manuscript Title: Synthesis and enhanced electrochemical performance of Pt-Ag/porous polyaniline composites for glycerol oxidation Authors: Aijuan Xie, Feng Tao, Lina Hu, Yanfang Li, Wenliang Sun, Chao Jiang, Feifan Cheng, Shiping Luo, Chao Yao PII: DOI: Reference:
S0013-4686(17)30362-6 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.086 EA 28951
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
5-8-2016 15-2-2017 15-2-2017
Please cite this article as: Aijuan Xie, Feng Tao, Lina Hu, Yanfang Li, Wenliang Sun, Chao Jiang, Feifan Cheng, Shiping Luo, Chao Yao, Synthesis and enhanced electrochemical performance of Pt-Ag/porous polyaniline composites for glycerol oxidation, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and enhanced electrochemical performance of Pt-Ag/ porous polyaniline composites for glycerol oxidation
Aijuan Xie, Feng Tao, Lina Hu,Yanfang Li, Wenliang Sun, Chao Jiang, Feifan Cheng, Shiping Luo *, Chao Yao
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China
Pt-Ag/porous PANI were fabricated successfully via electrodeposition method
Pt-Ag/porous PANI (300/300) exhibited the best catalytic activity
Porous PANI and synergic effect between Pt and Ag can enhance utilization of Pt
Abstract: Porous polyaniline (PANI) with enhanced surface area was obtained using attapulgite (ATP) as a template and then etched with HF acid. Pt-Ag/porous PANI nanocomposites were fabricated successfully via step-by-step electrodeposition method, and then characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), attenuated total reflection fourier transform 2
infrared spectroscopy (ATR-FTIR), and UV-visible spectrophotometer (UV-visible). The influence of deposition order and deposition laps of Pt, Ag towards glycerol oxidation was also studied. The results showed that when Pt was deposited first then Ag, and the deposition laps of both Pt and Ag were 300 laps, Pt-Ag/porous PANI (300/300) exhibited the best effect. The catalytic activity of Pt-Ag/porous PANI (300/300) was 3.14 times higher than that of the commercial 20 wt.% Pt/C. In addition, the Electrochemical Impedance Spectroscopy (EIS), chronoamperometry and stability testing proved that the Pt-Ag/porous PANI (300/300) modified electrode showed not only excellent electrocatalytic activity but also better poison resistance and good stability towards glycerol oxidation in contrast to other as-prepared composites as well as commercial 20 wt.% Pt/C catalysts, which may be ascribed to high utilization of Pt resulted from porous structure of support and the synergic effect between Pt and Ag.
1. Introduction Environmental pollution caused by fossil fuels and their rapid depletion have initiated the increasing demand for efficient and green energy sources [1,2]. Direct alcohol fuel cells (DAFCs) technology is one of such alternative energy sources and has been recognized as an attractive green power sources, for the reason that it can convert the chemical energy into electricity via chemical reactions [3,4]. The simplest alcohol (methanol) exhibits a relatively good reactivity, however its 3
high toxicity and high solubility in water can result in environmental hazards [5]. In this case, the use of glycerol as an alternative fuel for the DAFCs has obtained increasing attention in the recent years on the account of its high power density, renewability, low toxicity and cost, non-flammability, non-volatility, convenient storage with a theoretical energy density of 6.4 KWh L -1 and so on [6–8]. Up to now, platinum is still the essential ingredient of catalysts in DAFCs. Nevertheless, the higher cost and lower activity of Pt-based catalysts are the major obstacles to hinder the commercialization of DAFCs [9–11]. Therefore, much research has been devoted to reduce the content of Pt while enhancing the activity of the Pt-based catalysts [12]. As compared to conventional monometallic Pt nanoparticles, bimetallic nanoparticles have recently attracted widespread attention due to their tunable surface plasmon resonance and high catalytic activity [13]. Bimetallic nanoparticles are distinct not only from the corresponding mono-metal, but also from the bulk metals in optical, magnetic, electronic and catalytic properties [14]. To date, a number of transition metals and rare earth metals such as Pd, Ru, Au, Mn, and Ag and so forth have been used to make the different structures so as to develop highly efficient catalysts with low Pt-loadings [15–17]. Although PtAg, PdAg, Pt–Pd/Ru and AuAg/C systems have been reported to display excellent catalytic properties towards the redox reaction [18–22], there has few report using an Ag nanoparticle with Pt/support system as a redox reaction catalyst. A good support system can also reduce the Pt loading, polyaniline (PANI) is one of the most potential conducting polymers as the electrode materials for DAFCs due to its environmental stability, easy synthesis, exciting electrochemical and optical properties [2314]. However, the tight block structure of PANI with a lower specific surface area formed by the conventional chemical polymerization easily leads to the difficulties of metal particle loading [2423]. Therefore, the modification to of PANI has 4
become necessary. Generally, PANI is modified by the methods of self-assembly and template [25–2624,25]. In our recent study [2726], we have demonstrated attapulgite (ATP) as a sacrificial template on the electrocatalytic activity towards glycerol oxidation exhibited a noticeable effect. Aniline is liable to be adsorbed over ATP surface through polymerization without further chemical modification owing to the high surface area, complicated pore structure and a few hydroxyl groups of ATP. In this study, porous PANI was obtained using ATP as a sacrificial template and then etched with HF acid as our previous study [2726], and then Pt-Ag/porous PANI composite was developed via step-by-step electrodeposition approach and characterized by various physicochemical analyses such as scanning electron microscopy (SEM), transmission electron microscope (TEM), energy spectrum (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), attenuated total
reflection
fourier
transform
infrared
spectroscopy
(ATR-FTIR),
and
UV-visible
spectrophotometer (UV-visible). And the application of Pt-Ag/porous PANI composite was focused on exploring electrochemical performance towards glycerol oxidation. 2. Experimental 2.1. Reagents and apparatus H2PtCl6, AgNO3 were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Pt/C (20 wt.%) catalyst was obtained from Sigma-Aldrich (Shanghai Trading Co., Ltd) for comparison to the new catalysts. All other reagents not mentioned were of analytical grade and purchased from Linfeng Chemical Reagent Co., Ltd. (Shanghai, China) or Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). And deionized water (18.2 MΩ) was used in all runs. All electrochemical measurements were carried out on a CHI 660D electrochemical workstation 5
(Huake 101 Putian Instrumental Co., Beijing, China). The TEM were collected using a transmission electron microscope working at 200kV (JEM-2100, JEOL, Japan). The SEM and EDS images were recorded using a scanning electron microscope (JSM-6360LA, JEOL, Japan). The XRD analyses of the powered samples were performed using an X-ray diffractometer with Cu anode (D/Max 2500 PC, Rigaku Corporation, Japan). The X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250, USA) by Al Kα radiation were used to evaluate surface composition and oxidation state of metal species on the surface of the catalysts. The binding energy was corrected using the C 1s spectrum at 284.8 eV. The ATR-FTIR spectra were acquired using a Nicolet iS50 FT-IR spectrometer (Thermo Fisher, USA). The UV spectra were obtained via UV-visible spectrophotometer (UV-2450, Shimadzu, Japan). The deposition amount of Pt was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Varian, USA). 2.2. Preparation of Pt-Ag/porous PANI Preparation of porous PANI was described in details in our previous study [2726]. A 10 μL dispersion solution of porous PANI (1.0 mg mL-1) was dropped on the glassy carbon electrode (GCE) that was then allowed to dry in air. A series of Pt/porous PANI composites was acquired by depositing Pt on the porous PANI in 1.0 mg·mL-1 mg mL-1 H2PtCl6 solution containing 0.5 mol L-1 H2SO4 with different deposition laps via cyclic voltammetry (CV). And then 1.0 mg mL-1 of Ag was deposited over the Pt/porous PANI electrode (marked Pt-Ag/porous PANI). For comparison, the other composites such as Ag-Pt /porous PANI, Ag/porous PANI, and Ag, Pt modified GCE and so on were prepared by the identical procedures only in different deposition order or in the absence of porous PANI. The whole fabrication process was depicted in Scheme 1. 3. Results and discussion 6
3.1. Optimization of composites 3.1.1. Influence of deposition order of Pt, Ag for glycerol oxidation Fig. 1 displays the CV curves of different composites towards glycerol (0.5 mol L-1 of glycerol was used in all the latter experiment) oxidation in 0.5 mol L-1 H2SO4 at the scan rate of 100 m Vs-1. Where Pt-Ag represented that Pt was deposited first then Ag on the surface of GCE, conversely Ag-Pt represented Ag was deposited first then Pt. It can be observed from Fig.1, the typical CV profiles of electro-oxidation reactions of glycerol display two anodic current peaks (ca. 0.62 V and 0.32 V vs SCE) in the positive and negative scans, respectively. The rising and falling of the current density represents the formation and consumption of intermediates in an electroactive interface and/or competition among them [2827]. More specifically, these are related to the oxidation reactions of hydrocarbons in the positive scan and incomplete oxidization of carbonaceous residues on the catalyst surface during the negative scan. The latter intermediates are strongly adsorbed on the Pt surface, blocking the effective and active catalyst sites from the next turnover, and thus making the anodic reactions more sluggish [2928]. The reactivation of platinum electrode is through the formation of free platinum sites due to the reduction of platinum oxides in the negative scan, which are then available for reacting with glycerol molecules [2726]. In comparison with other modified electrodes, Pt-Ag alloy displayed the best electrochemical activity, the reason may be ascribed to electrochemical surface dealloying phenomenon, that is, the element with more active electrochemical properties in the alloy is selectively dissolved into the electrolyte by potential difference between group of alloy, consequently the surface of alloy become porous or rough, and thus leads to significant activity enhancements for the oxygen reduction redox reaction [4]. In addition, the CV profile of Ag is very close to that of bare GCE, indicating that the catalytic activity 7
of pure Ag towards the glycerol electro-oxidation in acidic medium is very low. Also compared with pure Pt, the activity of Pt-Ag improved 2–3 folds after Ag dealloying from Pt-Ag alloy surface region [3029]. In view of the above results, that Pt was deposited first then Ag was chosen in the next study. 3.1.2. Influence of different composites The GCE modified by the different composites was investigated via the CVs in 0.5 mol L-1 H2SO4 at scan rate of 100 mV·s-1 mV s-1 (Fig. 2) with 100 deposition laps. Generally, the performances of all electrocatalysts examined were evaluated in terms of the following three aspects: (1) onset potential (Eonset), indicating the catalytic activity over alcohols oxidations, (2) forward anodic peak current density (If), showing the catalyst maximum performance, and (3) the ratio of the forward peak current density to the reverse peak current density (If/Ir), demonstrating the catalyst toleration over the adsorbed carbonaceous intermediate species [2928]. It can be noted from Fig.1 and Fig. 2, under the same preparation condition, the current density of Pt-Ag/porous PANI modified electrode is higher than that of the corresponding mono-metal or alloy. The values of forward anodic peak current density of Pt-Ag, Pt /porous PANI, Pt-Ag/porous PANI are 383.4, 407.7, and 425.5 µA cm-2 peak current is about 1.2 and 1.8 folds compared with Pt-Ag and Pt/porous PANI, respectively. This indicates that the electrocatalytic activity of Pt-Ag/porous PANI is superior to the other composites. The reason may be that an abundant of channels in porous PANI prepared with ATP as a sacrificial template and then etched with HF acid can provide space for the platinum deposition and more active site during the glycerol electrocatalytic oxidation. Meanwhile the electrochemical surface dealloying effect and synergic effect between Pt and Ag can also 8
improve the electrocatalytic activity of Pt [3029]. 3.1.3. Influence of deposition laps In order to seek the optimal electrochemical activity of Pt-Ag/porous PANI, the deposition laps of both Pt and Ag was investigated through CV method. The CV curves of different deposition laps of both Pt and Ag are displayed in Fig. 3A and B. As noted in Fig. 3A, when Pt is deposited over the surface of porous PANI for 300 laps, Ag 100 laps (marked Pt-Ag/porous PANI (300/100), the rest were marked in the same manner), the electrocatalytic activity of as-prepared hybrids towards glycerol oxidation is best. However, it is not monotonic increasing with increasing content of Pt. The electrocatalytic activity of Pt-Ag/porous PANI (100/100) is obviously better than that of Pt-Ag /porous PANI (200/100). The reason may be that the synergic effect and proper proportions between metals plays a critical role in the Pt-Ag/porous PANI (100/100). In Fig. 3B, the catalytic effect is superior to the other composite electrodes when both Pt and Ag deposited for 300 laps. The reason may be that Pt and Ag have not completely deposited on the electrode surface in Pt-Ag/porous PANI (200/100) and Pt-Ag/porous PANI (100/100). The peak current increases significantly with the increase of deposition laps and almost reaches a steady state when deposition laps is 300, then decreases after 300 laps. This is probable because too much deposit leads to particle reunion and decline in specific surface area, and thus influences catalytic properties [2726]. Two relatively good composite electrodes were compared with commercial 20 wt.% Pt/C in Fig. 3C. It is found that the oxidation peak current of Pt-Ag/porous PANI (300/300) is higher as compare to the other composites, which can be also evaluated from the ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib). The higher If/Ib value indicates higher 9
tolerance to intermediate carbon species accumulated on electrode surface, which means glycerol can be oxidized to carbon dioxide much more efficiently [31, 3230,31]. The If/Ib value of Pt-Ag/porous PANI (300/300), Pt-Ag/porous PANI (300/100) and commercial 20 wt.% Pt/C are 1.5, 0.64 and 0.87, respectively. Thus there is less carbonaceous accumulation in Pt-Ag/porous PANI (300/300) modified electrode and hence it has much more tolerance to CO poisoning during the electrooxidation process of glycerol [3332]. Furthermore, the electrochemical surface areas (ECSA) of Pt are calculated based on the hydrogen adsorption–desorption in the potential region of -0.2 to 0.15 V vs SCE (Fig. 3D) [3433]. Two resolved peaks in Fig. 3D are associated with weakly and strongly bonded hydrogen species on different crystal faces of Pt [3534]. And the deposition amount of Pt was obtained from detecting the concentration of metal ions in the solution before and after the deposition using ICP. The ECSA of Pt-Ag/porous PANI (300/300), Pt-Ag/porous PANI (300/100) and commercial 20 wt.% Pt/C are 40.07 m2 g-1, 38.89 m2 g-1 and 37.64 m2 g-1, respectively. The value of positive peak current density is about 2.10 and 3.14 folds compared with Pt-Ag/porous PANI (300/100) and commercial Pt/C (20%), further evidencing that Pt-Ag/porous PANI (300/300) employs excellent electrocatalytic performance. 3.2. Characterization 3.2.1. SEM and TEM imaging SEM and TEM images of different composites are showed in Fig. 4. It is very evident that the ATP nanorods cover on the surface of bulk PANI by comparing Fig. 4A and C. And an abundant of channels can be discerned on the surface of porous PANI in Fig. 4E, indicating ATP has been successfully introduced into PANI and removed as a template to form uniform porous PANI. These 10
points can be also demonstrated by TEM images (Fig. 4B, D and F), ultra-thin sheet-like PANI (Fig. 4B) is very different from rod-like ATP (Fig. 4D). Clearly visible channels can be observed in Fig. 4F by brightened and dimmed shadow on the surface of porous PANI. The diameters of channels are mainly in the range of 5–20 nm, these channels play a significant role in improving the conductivity of PANI composites [3635]. Fig. 5A is the typical SEM image of as-synthesized Pt/porous PANI. Pt nanoparticles are uniformly decorated without agglomeration on the porous PANI support to form relatively regular nanoflower feature. In comparison with Fig. 5A, the SEM image of Pt-Ag/porous PANI shows clear nanoparticles with several hundred nanometers in Fig. 5C, implying that many tiny Ag nanoparticles have covered uniformly on Pt nanoparticles. and sheltered Pt nanoflower feature to form bulk structure. Whereas no obvious Ag particles can be observed after glycerol oxidation (Fig.5E), indicating that most of Ag have dissolved from the surface of Pt. The EDS spectra of Pt/porous PANI and Pt-Ag/porous PANI before and after glycerol oxidation are displayed in Fig. 5B, D, and F. As can be seen, the Pt/porous PANI is mainly composed of Pt except a small amount of C and O. Pt and Ag is are main elements in Pt-Ag/porous PANI composite, and the content of Ag decreases from 7.65% to 1.93% after glycerol oxidation, further demonstrates Ag has dissolved to form the dealloyed caves, which can increase the reactive sites and help to improve electrochemical activity. In addition Therefore, it can also be deduced from Fig. 5 that Pt and Ag hashave been successfully deposited on the porous PANI composite. 3.2.2. X-ray diffraction Fig. 6 illustrates the XRD patterns of PANI, porous PANI and Pt-Ag/ porous PANI composites. 11
The broader and weak peak of PANI implies an amorphous nature. The broad band appearing at 2θ = 27.24° with (200) crystal plane is attributed to the periodicity parallel to the polymer chains (Fig.6a) [3714]. The weak peaks at 2θ = 20.22° and 15.82° are ascribed to (020) and (011) crystal planes of PANI (Fig. 6a) [3836]. In Fig. 6b, the characteristic diffraction peaks of ATP, such as intense peak of 2θ = 8.34°, 19.76° and 26.62° almost disappears. On the contrary, the distinct peaks of PANI (20.22°, 27.24°) can be clearly observed, indicating that the removal of ATP do not destroy the structure of PANI. The additional intense and sharp peaks appeared in Fig.6c are ascribed to the natural crystalline of Pt-Ag. Pt-Ag/porous PANI nanocomposite is more crystalline in nature than PANI. The peaks of platinum (2θ = 39.8°, 46.18° and 67.4°) and silver (2θ = 38.10°, 44.21° and 64.42°) can be ascribed to the characteristic (111), (200) and (220) diffraction, respectively, which is corresponding to the face-centered cubic (fcc) crystal structure. The peaks of the bimetallic nanoparticles occupy in between those of Pt and Ag nanoparticles [3714], which fully demonstrated the formation of Pt-Ag intermetallic nanocomposite due to their similarity in lattice constant. The average particle grain size of the Pt-Ag/porous PANI nanocomposite based on the (220) diffraction and calculated from the Debye-Scherrer equation is 60 6 nm. 3.2.3. XPS analysis The chemical states of different elements of Pt-Ag /porous PANI nanocomposites were investigated by XPS characterization. The wide survey spectrum of Pt-Ag/porous PANI composite, which reveals the presence of C, O, Pt, and Ag elements, is displayed in Fig. 7A. The high-resolution Pt 4f spectra (Fig. 7B) display that Pt 4f spectrum is composed of the doublets of Pt 4f7/2 and 4f5/2. The deconvolution of the Pt 4f5/2 peak yields two peaks at 69.5 and 70.4 eV while the 12
Pt 4f7/2 peak yields two peaks at 73.1 and 73.8 eV, which could be assigned to Pt2+ and Pt0 oxidation states, respectively. The presence of Pt2+ could probably be due to the surface oxidation during the experimental processes [3937]. Furthermore, the doublet (Ag 3d5/2 and Ag 3d3/2) corresponding to metallic Ag can be observed clearly, which is shown in Fig. 7C. The binding energies at 367 eV are ascribed to Ag 3d3/2, and the binding energies at around 373 eV are attributed to Ag 3d5/2 of the metallic silver [4038], respectively. A significant negative shift of the Pt binding energy is observed in Pt-Ag /porous PANI as compared with single Pt (71.0 eV), which may be caused by different electro-negativity between Pt and Ag and the slight strain effect imposed by Ag covered on the Pt surface. It may also be attributed to synergistic effect between Pt and conducting polymers (PANI) as a support [3534]. In addition, the content of Pt4f and Ag3d for the Pt-Ag/porous PANI before and after glycerol oxidation are calculated to be 45.59%, 54.41% and 67.37%, 32.63% on the basis of XPS results, respectively, which are in accordance with the results of EDS patterns. 3.2.34. ATR-FTIR and UV-vis spectral analysis ATR-FTIR spectra of porous PANI and Pt-Ag/porous PANI are almost similar as shown in Fig. 78A. The stretching vibrations of C–C, C–N and N–H occur at 1118, 1383 and 3400 cm-1, respectively. An intense peak appeared at 1593 cm-1 areis stretching vibrations of C=C for the quinone ring. After Pt-Ag is incorporated into the porous PANI, a shift from 3397 cm-1 to 3410 cm-1 in N–H stretching can be observed, illustrating that the Pt-Ag has interacted with the nitrogen atom of the porous PANI matrix [3714]. UV-vis absorption spectra of porous PANI and Pt-Ag/porous PANI are displayed in Fig. 78B. Two characteristics absorption bands appeared at 330 and 650 nm are corresponded to PANI. The 13
former band is assigned to π→π* transition and the latter to n→π*, which is associated with the transition of benzenoid into quinoid ring [37]. After Pt and Ag were introduced into porous PANI, the absorption bands generated blue shift (from 330 to 270 nm, 650 to 550 nm, respectively), which implies that the bimetallic Pt-Ag nanoparticle has interacted with the heteroatom of the porous PANI and hence leads to blue shift [3714]. 3.3. Electrocatalytic analysis 3.3.1. Electrochemical impedance characteristics Electrochemical Impedance Spectroscopy (EIS) can be used to reveal a complex phenomenon of electron interception and diffusion at the electrode–electrolyte interface [39]. Therefore, EIS of the Pt-Ag/porous PANI (300/300), Pt /porous PANI, porous PANI and commercial 20 wt.% Pt/C were investigated within a frequency range from 0.1 to 105 Hz. And the Nyquist plots are presented in Fig. 89A. The high frequency intercept of the semicircle with the real axis gives the internal resistance, including the resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance at the interface active material/current collector [40]. As can be observed, the internal resistance of the Pt-Ag/porous PANI (300/300) is smallest among all composites, which means that the microchannels in the as-fabricated Pt-Ag/porous PANI (300/300) is conducive to the ion transfer between the solution and the electrode active center. Furthermore, the semicircle in the high frequency region manifests that there is a surface charge transfer resistance (Rct). Meanwhile, the high slope means the faradic pseudocapacitance of the electrode. The semicircle radius in Pt-Ag/porous PANI (300/300) is smallest and can hardly be observed as compare to the other composites, illustrating that Pt-Ag/porous PANI (300/300) has the smallest Rct and best 14
electrochemical performance. Also, a slope of a nearly vertical line arising in the low frequency region on the curve testifies Pt-Ag/porous PANI (300/300) possesses the high pseudocapacitive performance. This result is in perfect accordance with CV results [40]. 3.3.2. Chronoamperograms The tolerant ability to CO intermediates of the Pt-Ag/porous PANI (300/300) were evaluated by the chronoamperometry (CA) in 0.5 mol L-1 H2SO4 containing 0.5 mol L-1 glycerol under a constant potential of 0.62 V, and the results are shown in Fig. 89B. During the successive CV experiments of the Pt-Ag/porous PANI (300/300) modified electrode, a remarkable current decay is found for all of the catalysts in the first several minutes, which could arise from the accumulation of poisonous intermediates such as species containing CO. The higher pseudo-current density shows the stronger capacity to decrease the poisoning from CO intermediaries [41]. Among the all composites, Pt-Ag/porous PANI (300/300) has the highest pseudo-current density and thus possesses the strongest capacity to decrease the poisoning from the incomplete oxidation reaction of glycerol. The order of initial activity in the CA tests was in good agreement with the results both in the above CV measurements and the value of If/Ib, which suggests its superior performance for glycerol electroxidation. 3.3.3. Electrochemical behavior and electrocatalytic stability The transport characteristics and concentration dependence of glycerol oxidation on the Pt-Ag/porous PANI (300/300) modified electrode was explored. The CVs of Pt-Ag/porous PANI (300/300) towards glycerol oxidation in the 0.5 mol L-1 H2SO4 containing 0.5 mol L-1 glycerol solution at different scan rates are shown in Fig. 89C. A good linear relationship (R2=0.9923) between current density and square root of scan rates (v1/2) come from the fitted plot (Insert of Fig. 15
89C) illutrates that glycerol oxidation reaction on the Pt-Ag/porous PANI (300/300) electrode is diffusion-controlled process [42]. Further investigation was performed to examine the stability of the Pt-Ag/porous PANI (300/300), and the results are shown in Fig. 89D. After repetition of 200 times for the Pt-Ag/porous PANI (300/300) modified electrode in 0.5 mol L-1 H2SO4 containing 0.5 mol L-1 glycerol during the successive CV experiments, the decay in the current intensity of oxidation peak of glycerol (at 0.62 V) is very small and only declines 15.3% of its initial value, indicating that the as-fabricated Pt-Ag/porous PANI (300/300) has a relatively good electrocatalytic stability. 4. Conclusions A series of Pt-Ag/porous PANI catalysts waswere fabricated successfully via electrochemical approach and used as electrocatalysts for glycerol electro-oxidation. The influence of deposition order, different materials and deposition laps of Pt, Ag were investigated. Among all catalysts, on account of bimetallic nanoparticles of Pt and Ag and synergistic effect between Pt and conducting polymers (PANI) as a support, the Pt-Ag/porous PANI (300/300) catalyst exhibited excellent electrocatalytic activity with 3.14-fold of commercial 20 wt.% Pt/C towards glycerol oxidation. The values of ECSA and If/Ib indicated that Pt-Ag/porous PANI (300/300) modified electrode displayed not only excellent electrocatalytic activity but also better poison resistance towards glycerol oxidation in contrast to other as-prepared composites as well as commercial 20 wt.% Pt/C catalysts, which can be further proved by EIS, CA tests. Therefore, Pt-Ag/porous PANI can be considered as a good candidate for electro-catalytic oxidation of glycerol. Acknowledgements 16
This work was supported by Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University; Production, Practical Experience and Research of Prospective Joint Research Projects of Jiangsu Province (BY2014037–23); Jiangsu International Cooperation Project (BZ2015040); Important research and development program (BE2015103); Social development Fund of Jiangsu Province (BE2016654); Advance Catalysis and Green Manufacturing
Collaborative
Innovation
Center,
PPZY2015B145)
17
Changzhou
University
(TAPP
NO:
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Captions of Figure and Scheme Fig.1. CVs of Pt-Ag, Ag-Pt, Pt, Ag and bare GCE towards glycerol oxidation in the 0.5 mol L-1 H2SO4 + 0.5 mol L-1 glycerol solution at 100 mV /s mV s. Fig.2. CVs of Pt-Ag, Pt-Ag/porous PANI, Pt/porous PANI and Ag/porous PANI towards glycerol oxidation in the 0.5 mol L-1 H2SO4 + 0.5 mol L-1 glycerol solution at 100 mV /s mV s with 100 deposition laps. Fig.3. CVs of different nanocomposites in 0.5 mol L-1 H2SO4 + 0.5 mol L-1 glycerol solution (A, B and C) and 0.5 mol L-1 H2SO4 solution (D) at 100 mV /s mV s, respectively. Fig.4. SEM and TEM images of PANI (A, B), PANI/ATP (C, D), and porous PANI (E, F). Fig.5. SEM images of Pt/porous PANI (A), Pt-Ag/porous PANI before glycerol oxidation (C), and after glycerol oxidation (E); EDS images of Pt/porous PANI (B), Pt-Ag/porous PANI before glycerol oxidation (D), and after glycerol oxidation (F). Fig.6. XRD spectra of PANI (a), porous PANI (b) and Pt-Ag/porous PANI (c). Fig.7. XPS survey spectra of Pt-Ag/porous PANI sample (A), high-resolution Pt 4f spectra (B) and Ag 3d (C) for Pt-Ag/porous PANI before and after glycerol oxidation. Fig.8. ATR-FTIR spectra of (a) PANI and (b) Pt-Ag/porous PANI (A), UV-vis spectra of PANI and Pt-Ag/porous PANI (B). Fig.9. EIS curves of differentcomposites in 0.1 mol L-1 KCl + 5 mmol L-1 [Fe(CN6)]3−/4− (A). Chronoamperograms of different composites in 0.5 mol L-1 H2SO4 + 0.5 mol L-1 glycerol solution at a constant potential of 0.62 V (B). CVs of Pt-Ag/porous PANI (300/300) towards glycerol oxidation in the 0.5 mol L-1 H2SO4 + 0.5 mol L-1 glycerol solution at different scan rates, Inset: Plot of peak current at 0.62 V versus square root of scan rate (C). Variations in peak current with successive CVs of the Pt-Ag/porous PANI (300/300) in 0.5 mol L-1 H2SO4 + 0.5 mol L-1 glycerol solution at scan rate of 200 mV s-1 (D). Scheme 1. Schematic illustration of the fabrication process of the Pt-Ag/porous PANI.
