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

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, PbO₂-PANi and PbO₂ 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(NO₃)₂ + 0.05 M Cu(NO₃)₂ + 0.1 M HNO₃ + 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.

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).

The SEM image of PbO₂ (figure 1(a)) demonstrated that lead dioxide existed in tetragonal β-modification. However, after it was immersed into acidic aniline solution to form composite of PbO₂-PANi (figure 1(c)) we could observe only spongy surface owing to knitted nano PANi fibres formed from the following oxidation reaction [9]

$$\begin{eqnarray}&&\text{P}{{\text{b}}^{4+}}+2{{\text{C}}_{6}}{{\text{H}}_{5}}\text{N}{{\text{H}}_{2}}\to \text{P}{{\text{b}}^{2+}}+{{\text{C}}_{6}}{{\text{H}}_{5}}\text{N}{{\text{H}}_{2}}^{+}.\end{eqnarray} \tag{ 1 }$$

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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 Pb²⁺ in the PANi lattice can be solved becauce of using 0.1 M HNO₃ as an electrolyte [8]. Figure 1(b) represents PbO₂-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 PbO₂-PANi into acidic aniline medium.

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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 PbO₂. Both of them had size in nano range. The gained results from SEM and TEM analyses explained that nanostructural PbO₂-PANi composites were succesfully prepared not only by cyclic voltammetry but also by combining chemical and cyclic voltammetric methods.

XRD pattern for determining structure of regarded materials is shown in figure 3. In the spectrum of CV-deposited PbO₂ (a) three small peaks at 2θ degree of near 30°, 32, 49° and one strong peak at over 62° indicated β-PbO₂ 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-PbO₂ 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-PbO₂ modification. It explains the existence of β-PbO₂ in our prepared composites. This is evidence to prove that only a part of the surface of PbO₂ layer reduced by aniline to Pb²⁺ which might be moved into electrolyte and the rest of it remains in composite matrix.

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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.

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As reported in [7, 8], the electro-oxidation of methanol on the surface of anodic PbO₂-PANi composites can occur following the reaction:

$$\begin{eqnarray}&&\text{C}{{\text{H}}_{3}}\text{OH}+{{\text{H}}_{2}}\text{O}\to 6{{\text{H}}^{+}}+6{{\text{e}}^{-}}+\text{C}{{\text{O}}_{2}}.\end{eqnarray} \tag{ 2 }$$

The difference of current Δi representing an electro-oxidation current of methanol at those composites in figure 5 can be calculated by the formula

$$\begin{eqnarray}&&\Delta i=i-{{i}_{\text{base}\,\text{line}}},\end{eqnarray} \tag{ 3 }$$

where i is corresponding current measured in acidic methanol solution and i_{base line} is that measured in acidic solution without methanol (base line).

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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.

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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-PbO₂ composites prepared by combining chemical and cyclic voltammetric methods can more positively catalyse for methanol oxidation than those by chemical and pulsed current methods.

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