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.