Electropolymerization of aniline at the graphite electrodes was achieved by potentiodynamic method. Electrodeposition of Pd (C-PANI-Pd) and Ni (C-PANI-Ni) and codeposition of Pd-Ni (C-PANI-Pd-Ni) microparticles into the polyaniline (PANI) film coated graphite (C-PANI) were carried out under galvanostatic control. The morphology and composition of the composite electrodes were obtained using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) techniques. The electrochemical behavior and electrocatalytic activity of the electrode were characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometric (CA) methods in acidic medium. The C-PANI-Pd-Ni electrode showed an improved catalytic performance towards methanol oxidation in terms of lower onset potential, higher anodic oxidation current, greater stability, lower activation energy, and lower charge transfer resistance. The enhanced electrocatalytic activity might be due to the greater permeability of C-PANI films for methanol molecules, better dispersion of Pd-Ni microparticles into the polymer matrixes, and the synergistic effects between the dispersed metal particles and their matrixes.
Direct methanol fuel cell (DMFC) is regarded as a potential candidate as portable power sources [
The graphite block (saw cut finish grade) purchased from Graphite India Limited was used as substrate for the electrode matrix. PdCl2, H2SO4, aniline, methanol, and other chemicals were of analytical grades from Merck. All the aqueous solutions were prepared with distilled water. All electrochemical experiments were performed using a CHI680B electrochemical analyzer (CH Instruments, Inc. Austin, USA) controlled by CH instrument electrochemical software by using a double-compartment glass cell with conventional three-electrode configuration. The substrate working electrode was polished graphite (geometric area: 1.0 cm2). The counter electrode was a large area Pt-wire and the reference electrode was Ag/AgCl electrode.
Electropolymerization of aniline at the graphite electrodes was achieved by potentiodynamic method. The electrode potential was swept between −0.5 and 0.5 V versus Ag/AgCl at a scan rate of 50 mV s−1 using an aqueous solution of 0.1 M H2SO4 containing 0.3 M aniline for 40 cycles. The thickness of the deposited films (PANI film thickness: 1.2
The composite electrodes were characterized by scanning electron microscopy (SEM) and their chemical composition was analyzed by energy dispersive X-ray analysis (EDX). The SEM and EDX analysis were performed using electron microscope JSM-6390LV (JEOL Co., Japan) equipped with energy-dispersive full range X-ray microanalysis system. The methanol electrooxidation reaction at surface of the electrode was investigated in electrolyte solution containing 1.0 M methanol and 0.5 M H2SO4 by CV, Chronoamperometric, and EIS measurements. In CV measurements, the potential was scanned between −0.8 and 1.0 V at a scan rate of 50 mV s−1. In EIS measurements, the A.C. frequency range extended from 100 kHz to 1 Hz with an excitation signal of 5 mV. The impedance spectra were fitted to an equivalent circuit model using a nonlinear fitting program. A constant temperature water bath was used to keep the experiments running at a preset temperature. All current densities were calculated according to geometric areas of electrodes and were normalized to weight of the catalyst.
Electropolymerization of PANI offers the possibility of controlling the thickness and homogeneity of the conducting polymer film on the electrode surface. Figure
Cyclic voltammograms (40 cycles) during electropolymerization of aniline on graphite in 0.5 M H2SO4 solution containing 0.1 M aniline at a scan rate of 50 mV s−1 at room temperature.
The redox peaks at around 0.1 V are attributed to the transformation of PANI from the reduced leucoemaraldine (LE) state to the partly oxidized emeraldine (EM) state, and the redox peaks at around 0.4 V correspond to transition of the PANI, from LE to pernigraniline (PE) state [
The surface morphology of the catalyst was revealed through scanning electron microscopy. The SEM of C-PANI film in Figure
SEM images of (a) C-PANI, (b) C-PANI-Ni, (c) C-PANI-Pd, and (d) C-PANI-Pd-Ni electrodes.
The SEM images of C-PANI-Pd and C-PANI-Pd-Ni electrodes in Figures
EDX spectra of (a) C-PANI-Ni, (b) C-PANI-Pd, and (c) C-PANI-Pd-Ni catalysts.
The depositions of metal particles are evident from EDX spectra. The atomic percent ratio of Pd/Ni in the C-PANI-Pd-Ni catalyst is 27 : 0.1. The relative low content of Ni element in the C-PANI-Pd-Ni catalyst is ascribed to the higher reduction potential of Pd2+/Pd compared to Ni2+/Ni.
Electrocatalytic properties of the synthesized electrodes toward methanol oxidation were investigated by cyclic voltammetry in a solution of 0.5 M H2SO4 and 1.0 M CH3OH at a scan rate of 50 mV s−1 at room temperature as shown in Figure
Cyclic voltammograms for methanol oxidation on different electrodes in 0.5 M H2SO4 solution containing 1.0 M methanol at a scan rate of 50 mV s−1 at room temperature.
The current density was normalized by the geometric area of the working electrode to indicate the catalyst mass activity. The C-PANI electrode shows small methanol oxidation peak at −0.05 V which is in agreement with the reported results [
Comparison of different electrochemical parameters for methanol oxidation on different electrodes.
Electrochemical parameters | Electrodes | |||
---|---|---|---|---|
C-PANI | C-PANI-Pd | C-PANI-Ni | C-PANI-Pd-Ni | |
|
−0.297 | −0.303 | −0.203 | −0.320 |
|
−0.054 | 0.190 | 0.473 | 0.350 |
|
0.0058 | 0.031 | 0.0045 | 0.186 |
|
13.62 | 8.24 | 13.30 | 3.70 |
|
108.4 | 45.2 | 75.3 | 30.6 |
|
— | 40 | 250 | 20 |
The stability and electrocatalytic activity of different catalysts were evaluated by chronoamperometry. Figure
Chronoamperograms for methanol oxidation on different electrodes in 0.5 M H2SO4 solution containing 1.0 M methanol at 0.2 V.
The current density decreases as a function of time, at a given potential for all electrodes. This current decrease can be attributed to adsorption of poisonous intermediates, anion adsorption in the case of the H2SO4 system, and the decrease in production of CO2 as the surface oxide was consumed, leading to the formation of other soluble intermediates [
The results are consistent with those of CV measurements. The improved antipoisoning ability of the electrode may be attributed to the reaction between strongly bound CO species and OHads on neighboring of C-PANI-Pd-Ni sites [
The apparent activation energy (
Arrhenius plots for methanol oxidation on different electrodes in 0.5 M H2SO4 solution containing 1.0 M methanol at 0.2 V.
The activation energies were obtained from the slopes of the
Electrochemical impedance spectroscopy was used to investigate reaction process and kinetics of methanol oxidation on the different electrodes at a fixed potential of 0.2 V. The Nyquist plots for methanol oxidation in a solution containing 0.5 M H2SO4 and 1.0 M methanol are shown in Figure
Electrochemical impedance spectra (Nyquist) for methanol oxidation on different electrodes in 0.5 M H2SO4 solution containing 1.0 M methanol at 0.2 V.
The lower value of
The C-PANI-Pd-Ni electrode exhibited an excellent electrocatalytic activity for methanol oxidation and stability compared to C-PANI, C-PANI-Ni, and C-PANI-Pd catalysts. The metal microparticles were well distributed into the polyaniline matrix. Introduction of PANI as a conductive polymer within catalyst layer (i) improves electron transfer between catalyst particles and PANI matrix and also act as accessible mesoporous network for catalytic particles; (ii) it hinders the formation of strong adsorbed poisons and catalyzes the oxidation of strongly and weakly adsorbed poisons, so it prevents catalyst from more poisoning by intermediate products of methanol oxidation such as CO, and (iii) improves the mechanical properties of the catalyst layer by providing good and flexible connections between metal particles. Moreover, the amount of noble metal platinum at the prepared electrode was remarkably reduced, the cost of the processed catalyst was greatly decreased and the procedure was very simple. In fact, PANI acts also as a binder in catalyst layer without serious restriction in methanol diffusion to the active site. The performance of the electrode is enhanced after partial chemical displacement of dispersed Ni with Pd. Polyaniline matrix with higher electrochemical surface area and lower resistance of charge transfer to electrode manifest themselves to be highly suitable candidate for use in DMFC applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.