Int. J. Electrochem. Sci., 6 (2011) 3196 - 3209
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org
Facile Synthesis of Composite Electrodes Containing Platinum
Particles Distributed in Nanowires of Polyaniline-Poly(Acrylic
Acid) for Methanol Oxidation
Chung-Wen Kuo1,*, Chang-Cian Yang1, and Tzi-Yi Wu 2,3,*
1
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan
2
Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan
3
Department of Materials Engineering, Kun Shan University, Tainan 71003, Taiwan
*
E-mail: [email protected], [email protected]
Received: 14 June 2011 / Accepted: 7 July 2011 / Published: 1 August 2011
This work demonstrates a novel and simple route for preparing a composite that comprises platinum (Pt) nanoparticles and polyaniline (PANI) doped with poly(acrylic acid) (PAA) and hydrochloric acid (HCl) via “simultaneous doping- deposition” to obtain PANI-(PAA+HCl)-Pt composite electrodes. PANI-(PAA+HCl) is characterized using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS results indicate that PANI-(PAA+HCl) has more positively charged nitrogen atoms compared to PANI doped with HCl (HCl). SEM images reveal that (PAA+HCl) is composed of highly porous nanowires. The morphology and structure of the PANI-(PAA+HCl)-Pt composite are further characterized by transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). TEM images and AES results indicate that Pt particles are dispersed uniformly into the spatial regions of PANI-(PAA+HCl). Cyclic voltammetry results and chronoamperometric response measurements show that PANI-(PAA+HCl)-Pt electrodes have good electrocatalytic activity of methanol oxidation with low CO poisoning.
Keywords: Polyaniline, poly(acrylic acid), nanowires, cyclic voltammetry, TEM, methanol oxidation
1. INTRODUCTION
Direct methanol fuel cells (DMFC) are highly attractive power sources for a variety of applications due to their high energy efficiency, low emissions, low noise, and environmental friendliness [1-8]. DMFCs are based on methanol electro-oxidation at the anode. Among the numerous materials used for the anode of DMFCs, Pt has been established as a powerful electrocatalyst for the oxidation of methanol [9,10]. However, the use of Pt in the form of smooth foils for the direct
oxidation of methanol has been found to be inefficient due to (i) high cost and (ii) the formation of strongly adsorbed intermediates such as CO (referred to as COads) as a result of the dissociative
adsorption of methanol [11].
In order to mitigate COads-like poisoning, various strategies have been developed to improve
the electrocatalytic activity for methanol electro-oxidation and oxygen reduction reactions, including the addition or incorporation of a second element into Pt electrocatalysts, such as catalyst supporters [12,13]. It has been shown that the use of conducting polymers (CPs) as a catalyst supporter is a simple and useful way of reducing catalyst poisoning. CPs in their various oxidation states interconvert each other, which allows a redox cycle to form for catalytic reactions. Thus, the electrochemical deposition of metals on electrodes modified with CP films is a convenient and inexpensive route for developing anode materials. Studies on CPs as host materials for Pt nanoparticles have focused on polyaniline (PANI) [14,15], polypyrrole (PPy) [16,17], and polythiophene (PT) [18] for methanol oxidation. The advantage of using CPs over other materials is that they are permeable to electroactive species, sufficiently conductive for current flow between the solution and substrate, easily modified using various techniques, and easy to coat onto various substrates. CPs also act as electrocatalyst and current collectors.
Poly(styrenesulfonic acid) (PSS), a polymer acid, has been shown to be easily incorporated in a CP matrix as a dopant as a support for Pt particles. Huang et al. [19] reported that PANI-PSS acts as a matrix that leads to the uniform distribution of Pt particles. As a result, the electrocatalytic activity for methanol oxidation of PANI-PSS-Pt is much higher than that of PANI-Pt. Kuo et al. [20] reported that highly dispersed Pt particles became homogeneously distributed in a poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) matrix. However, these investigations mainly focused on the manufacture of PSS as a dopant in a CP matrix; less attention has been given to the processing of a CP modified with a polymer acid containing carboxylic acid groups for use as a supporting material in DMFCs.
In the present study, a simple simultaneous dopingdeposition method is used to introduce -CO2H groups (poly(acrylic acid), PAA) and HCl with Pt4+ ions into a PANI matrix. PANI doped
simultaneously with PAA and HCl forms a spatial network structure, which behaves as a 3D-random matrix for the deposition of Pt particles. We believe that PANI-(PAA+HCl) may act as a stabilizer for Pt particles, preventing their aggregation. The PANI-(PAA+HCl)-Pt composite is expected to enhance electroactivity for methanol oxidation.
2. EXPERIMENTAL
2.1. Preparation of PANI-(PAA+HCl)
A mixture solution of 50 mM ANI (Merck) and 0.5 M H2SO4 (Merck) aqueous solution was
prepared. Electrochemical polymerization of the solution was carried out using a potentiostatic method with stainless steel 316 (SS) as the working electrode for a total charge of 0.1 C cm-2, details of the procedure are described elsewhere [21]. Before each experiment, SS was cleaned in an ultrasonic bath
using detergent, deionized water, and isopropanol, and then dried with a dry nitrogen flow. The electrochemically deposited PANI film was rinsed with double distilled water for 5 min and then dried at 120 °Cfor 3 min. The Emeraldine base form (EB) of PANI was obtained by treating the PANI film in 0.1 M ammonium hydroxide (Aldrich) for 3 min. The EB film was then simultaneously redoped with poly(acrylic acid) (PAA) (Mw = 450,000, Aldrich) and HCl in 0.01 M PAA +0.01 M HCl solution
containing 5 mM H2PtCl66H2O. EB film redoped with PAA and HCl is denoted as PANI-(PAA+HCl).
For comparison, EB film was redoped with HCl in 0.01 M HCl solution containing 5 mM H2PtCl66H2O (denoted as PANI-HCl).
2.2. Deposition of Pt into PANI-(PAA+HCl) matrix
Pt particles were incorporated into PANI-(PAA+HCl) film via electrochemical deposition from 0.01 M PAA + 0.01 M HCl + 0.1 M KCl solution containing 5 mM H2PtCl6． 6H2O
(PANI-(PAA+HCl)-Pt) with a constant deposition charge of 0.15 C at a constant potential of -0.2 V (vs. Ag/AgCl). For comparison, Ptparticles were also deposited into a PANI-HCl matrix (PANI-HCl-Pt) under deposition conditions similar to those used for PANI-(PAA+HCl). After Pt particle incorporation, the electrodes were rinsed with double distilled water for 5 min and then dried at 120 °C for 3 min. The amount of Pt loaded into PANI-(PAA+HCl) or deposited onto PANI-HCl was calculated using:
where M is the atomic weight of Pt, F is the Faradic constant, and Z is the number of electrons transferred (taken as four for the formation of Pt). The amount (m) was calculated using the charge (Qdep) utilized for the deposition of Pt particles.
2.3. Characterization of PANI-(PAA+HCl)-Pt composite electrode
A X-ray photoelectron spectroscopy (XPS) study was performed using an ESCA 210 spectrometer with Mg Kα(hν= 1253.6 eV) irradiation as the photon source. The primary tension was 12 kV and the pressure during the scans was approximately 10-10 mbar. The surface morphologies of PANI-(PAA+HCl)-Pt and PANI-HCl-Pt films were compared using a scanning electron microscope (SEM) (JEOL JSM-6700F) equipped with an energy dispersive spectroscopy (EDS). The morphology was characterized by Transmission electron microscopy (TEM, JEOL 1200 EX) at a 100 kV accelerating voltage. Specimens for TEM were prepared by spreading a small drop of one of the sample solutions onto a 400-mesh copper grid. The drop was dried in air at room temperature for nearly 4 days. Auger electron spectroscopy (AES) depth profiles were obtained with a Microlab 310 D (VG Scientific Ltd.) spectrometer at emission currents of 0.1 and 8 mA with gun tensions of 10 (electron) and 3 kV (ion), respectively.
Qdep M
m =
Electrochemical characterizations of PANI-(PAA+HCl)-Pt and PANI-HCl-Pt composite electrodes were carried out using an CHI627D electrochemical analyzer (U.S.A.). All experiments were performed in a three-component cell.
An Ag/AgCl electrode (in 3 M KCl), Pt wire, and SS (1-cm2 area) were used as the reference, counter, and working electrodes, respectively. A Luggin capillary, whose tip was set at a distance of 1-2 mm from the surface of the working electrode, was used to minimize errors due to iR drop in the electrolytes.
2.4. Methanol Electro-oxidation and Stability of PANI-(PAA+HCl)-Pt composite electrode
The catalytic activities of PANI-(PAA+HCl)-Pt and PANI-HCl-Pt composite electrodes were examined by cyclic voltammetry (CV) at 10 mV sec-1 ranging from -0.2 to 1.0 V. Chronoamperometric response curves were obtained at 0.6 V in 0.1 M CH3OH + 0.5 M H2SO4
solution. All the electrochemical experiments were carried out at room temperature.
3. RESULTS AND DISCUSSION
3.1. Characterizations of PANI-(PAA+HCl)
An investigation was made into the charge transport mechanism in the PANI-(PAA+HCl)-Pt and PANI-HCl-Pt composite electrodes at various scan rates (v) by linear sweep voltammograms (LSVs). The double logarithmic plots of peak current versus v (Fig. 1) for the two types of electrode are linear with nearly identical slopes.
It is known that the linearity of a plot of peak current versus v corresponds to the surface-bound transport process and a different type of linear for the plot of peak current versus v1/2 signifies a diffusion control process [22]. The slope values of the double logarithmic plots are 0.95 and 1.03 for PANI-HCl and PANI-(PAA+HCl), respectively. Thus, the PANI-HCl and PANI-(PAA+HCl) composite electrodes have surface-bound transport processes.
XPS analyses of the surfaces of PANI-HCl and PANI-(PAA+HCl) were conducted to investigate variations at nitrogen binding sites. Fig. 2 shows the XPS spectra of core-level N1s for
PANI-HCl and PANI-(PAA+HCl).
The signals of N1s were fitted with peaks at 398.8, 399.6, 400.7, and 401.8 eV, which
correspond to quinonoid imine (=N-), benzenoid amine (-NH-), protonated amine (-N+), and protonated imine (=N+), respectively [23]. Kumar et al. [24,25] attributed the last two peaks to the presence of polarons (radical cations) and bipolarons (dications). The ratio of these two N1s
components at 400.7 and 401.8 eV (positively charged nitrogen atoms) can be considered as a direct estimation of the doping level of PANI [26]. The area ratios of the four nitrogen constituents in PANI were calculated; their results are listed in Table 1. The doping level for PANI-(PAA+HCl) (27 %) is higher than that observed for PANI-HCl (23 %) due to the existence of PAA.
1.0 1.2 1.4 1.6 1.8 2.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 lo g ( I pa ) ( m A c m -2 )
log (scan rate) (mV sec-1)
(a) (b) 398 399 400 401 402 403 C o u n ts
Binding energy (eV)
Figure 1. Dependence of the peak current on the scan rate by linear sweep voltammograms (LSVs): (a) PANI-HCl and (b) PANI-(PAA+HCl).
Table 1. XPS results of PANI-HCl and PANI-(PAA+HCl)
Electrode =N-(%) -NH-(%) -N+(%) =N+(%) [-N+ + =N+/N] (%)
PANI-HCl 19 58 19 4 23
PANI-(PAA+HCl)
a higher current density and lower onset potential toward methanol oxidation than PANI-HCl-Pt. The PANI-(PAA+HCl)-Pt composite electrode is a promising material for catalysts for methanol oxidation. The enhanced electrocatalytic activity of Pt in PANI-(PAA+HCl) opens up the possibility to using smaller amounts of Pt in DMFC applications.
ACKNOWLEDGEMENTS
The financial support of this work by the National Science Council of Taiwan under NSC 99-2218-E-151-003 is gratefully acknowledged.
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