Nanostructured PbO2-PANi composite materials for electrocatalytic oxidation of methanol in acidic sulfuric medium

Hybrid materials based on PbO2 and PANi were prepared by cyclic voltammetry combined with chemical method. Firstly, PbO2 and PbO2-PANi were deposited on stainless steel by cyclic voltammetry (CV) at a scan rate of 100 mV s−1. Next, they were immersed in acidic aniline solution (0.1 M) to form new fresh PbO2-PANi composites. The properties of materials were characterized by x-ray diffraction, IR- spectroscopy, scanning electron microscopy and transmission electron microscopy. The electrocatalytic oxidation for methanol of all PbO2-PANi layers was investigated in acidic medium by potentiodynamic measure at a scan rate of 100 mV s−1 in the range of 1.4 V to 2.2 V versus Ag/AgCl/saturated KCl electrode. The obtained results indicated that the composites prepared by above combined method could significantly enhance the electrocatalysis for oxidation of methanol.


Introduction
Lead dioxide is known as a material which has excellent chemical stability, high conductivity and chemical inertness for electrolysis in an acidic medium. Therefore, lead dioxide is an excellent electrocatalyst and catalyst carrier [1,2]. Polyaniline (PANi) as a conducting polymer has been widely used because of its interesting mechanical and electrical properties as well as high environmental stability without any toxicity. Additionally, PANi is easily synthesized by chemical and electrochemical methods with low price, so it is probably the most important conducting polymer today [3,4].
Recently, the preparation of new organic/inorganic composites has rapidly developed due to a wide range of their potential use. The hybrid composites from polyaniline (PANi) and different metal oxides like as TiO 2 , SnO 2 , MnO 2 were investigated for the applications to sensor, electrocatalysis [5][6][7]. In the previous paper [8] we have prepared PbO 2 -PANi composite by chemical und pulsed current method, however, methanol oxidation current density was limited only until 30 mA cm −2 . To improve the electrocatalytic ability of this composite for methanol, another combining method based on chemistry and cyclic voltammetry must be used. In this paper, we report the characterization of composites obtained by this combined method and their electrocatalytic ability for methanol oxidation.

Materials and methods
All chemicals used in this study were provided by Merck (Germany). Aniline was fresh distilled under vacuum before use. The stainless steel electrode was polished by sandpaper with 2000 grit. Firstly, PbO 2 -PANi and PbO 2 as prelayers were deposited on stainless steel by CV at a scan rate of 100 mV s −1 from solution of 0.5 M Pb(NO 3 ) 2 + 0.05 M Cu(NO 3 ) 2 + 0.1 M HNO 3 + 0.1 M ethylene glycol with and without aniline, respectively. Then they were immersed five times into acidic aniline solution (0.1 M) during 60 s for each time.

Detection method
The structure study of materials was carried out by infrared spectra on IMPACT 410-Nicolet unit. The surface morphology of coatings was examined by scanning electron microscopy (SEM) on an FE-SEM Hitachi S-4800 (Japan) and transmission electron microscopy (TEM) on a Jeol 200CX (Japan). The x-ray diffraction (XRD) of samples was obtained by x-ray diffractometer D5000-Siemens (Germany). The electrocatalytic oxidation of methanol was measured by potentiodynamic method on the electrochemical workstation unit IM6 (Zahner-Elecktrik, Germany).  was immersed into acidic aniline solution to form composite of PbO 2 -PANi (figure 1(c)) we could observe only spongy surface owing to knitted nano PANi fibres formed from the following oxidation reaction [9]  It can be explained that aniline has converted to anilinume cation radical which can begin polymerization reaction leading to PANi product on the surface, while Pb 2+ in the PANi lattice can be solved becauce of using 0.1 M HNO 3 as an electrolyte [8]. Figure 1(b) represents PbO 2 -PANi prepared by cyclic voltammetry showed a mix clearly of both PANi and lead dioxide in closed fine texture of uniform structure which is evidenced by TEM images in figure 2(c). Compared with image (c), image (d) showed a less spongy surface of PANi lattice due to immersion of the prelayer PbO 2 -PANi into acidic aniline medium.

TEM-images
The TEM images on figure 2 convincingly evidenced that among two clearly different colours, the light one belongs to PANi enclosing the dark one belonging to PbO 2 . Both of them had size in nano range. The gained results from SEM and TEM analyses explained that nanostructural PbO 2 -PANi composites were succesfully prepared not only by cyclic voltammetry but also by combining chemical and cyclic voltammetric methods.

X-ray diffraction
XRD pattern for determining structure of regarded materials is shown in figure 3. In the spectrum of CV-deposited PbO 2 (a) three small peaks at 2θ degree of near 30°, 32, 49°and one strong peak at over 62°indicated β-PbO 2 were observed. We found the first peak located at 2θ of 30°and the second strong peak at 2θ of over 62°on spectra b and c from CV-deposited composite and composite prepared by combined method, respectively, indicated β-PbO 2 as reported in [8,10]. In contrast, they did not appear in the case of spectrum d on which we can see a peak at 2θ of over 32°, and another strong one of over 49°illustrated β-PbO 2 modification. It explains the existence of β-PbO 2 in our prepared composites. This is evidence to prove that only a part of the surface of PbO 2 layer Adv. Nat. Sci.: Nanosci. Nanotechnol.  reduced by aniline to Pb 2+ which might be moved into electrolyte and the rest of it remains in composite matrix.

Infrared analysis
The data given on spectra from figure 4 and table 1 showed that all regarded composites contain PANi owing to vibration signals of benzoid and quinoid ring as well as some main groups similar to those reported in literature [8,12,13]. It explained that PANi in emeraldine salt form co-existed in composite lattice.

Electrocatalytic oxidation of methanol
As reported in [7,8], the electro-oxidation of methanol on the surface of anodic PbO 2 -PANi composites can occur following the reaction: The difference of current Δi representing an electro-oxidation current of methanol at those composites in figure 5 can be calculated by the formula  The data in figure 6 show that only one oxidation peak (Δi p ) of methanol appeared in the potential range of 2.05 V to 2.15 V (versus Ag/AgCl/saturated KCl electrode), however, the peak position slightly shifted on the right side when concentration of methanol in solution increased.
Additionally, in all cases the height of Δi p increased linearly with methanol concentration in solution, among them the composite prepared only by cyclic voltammetry had the best electrocatalytic ability for oxidation of methanol, because its Δi p line lay on the top of all (blue line on figure 7). The obtained oxidation current Δi for methanol in this research was until 85 mA cm −2 (in the case only by cyclic voltammetry), twice to three times as high as that one in our previous report [8] owing to combining chemical and pulsed current methods (only until 30 mA cm −2 ). This explains how PANi-PbO 2 composites prepared by combining chemical and cyclic voltammetric methods can more positively catalyse for methanol oxidation than those by chemical and pulsed current methods.

Conclusion
From the above results we conclude that nanostructured PbO 2 -PANi composite prepared by combining chemical and cyclic voltammetric methods improved its electrocatalytic ability of methanol oxidation in comparison with that prepared by combining chemical and pulsed current method.
CV-deposited PbO 2 -PANi composite had the best electrocatalytic for methanol oxidation in acidic sulfuric medium because its morphology existed in uniform structure with closed fine texture.