Nano-composite of PtRu alloy electrocatalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell

doi:10.1016/S0013-4686(03)00412-2

Electrochimica Acta

Volume 48, Issue 19, 15 August 2003, Pages 2781-2789

https://doi.org/10.1016/S0013-4686(03)00412-2 Get rights and content

Abstract

Nano-composites comprised of PtRu alloy nanoparticles and an electronically conducting polymer for the anode electrode in direct methanol fuel cell (DMFC) were prepared. Two conducting polymers of poly(N-vinyl carbazole) and poly(9-(4-vinyl-phenyl)carbazole) were used for the nano-composite electrodes. Structural analyses were carried out using Fourier transform nuclear magnetic resonance spectroscopy, AC impedance spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Electrocatalytic activities were investigated by voltammetry and chronoamperometry in a 2 M CH₃OH/0.5 M H₂SO₄ solution and the data compared with a carbon-supported PtRu electrode. XRD patterns indicated good alloy formation and nano-composite formation was confirmed by TEM. Electrochemical measurements and DMFC unit-cell tests indicate that the nano-composites could be useful in a DMFC, but its performance would be slightly lower than that of a carbon-supported electrode. The interfacial property between the PtRu-polymer nano-composite anode and the polymer electrolyte was good, as evidenced by scanning electron microscopy. For better performance in a DMFC, a higher electric conductivity of the polymer and a lower catalyst loss are needed in nano-composite electrodes.

Introduction

Recently, research efforts have concentrated on the use of proton exchange polymers such as Nafion and its modification as a route to increase proton conductivity and decrease methanol crossover and electronic conducing polymers in a direct methanol fuel cell (DMFC) [1], [2], [3], [4], [5], [13], [14], [15], [16], [17]. Interest has focused on the development of a supporting material, one of key factors in increasing the utilization of noble metal catalysts. A widely used supporting material is a carbon, on which catalysts are dispersed to minimize metal loading [6], [7], [8], [9], [10]. The role of carbon is to serve as an electrical connection between the dispersed catalysts and the porous backing materials. In addition, metal nanoparticles are physically separated by carbon, which decrease the rate of their degradation due to agglomeration. In spite of these advantages, carbon has some limitations in terms of performance in that it is not very permeable to gas and does not conduct protons. To overcome its poor proton conductivity, the catalyst layer typically contains an added proton-conducting polymer, usually Nafion ionomer [11], [12].

The introduction of a conducting polymer to electrode materials could help to increase the interfacial properties between the electrode and electrolyte. The electrode consists of metal alloys in the form of a catalysts layer and carbon as a supporting material or diffusion layer, while the electrolyte is the polymer membrane. The fabrication of the membrane electrode assembly (MEA) requires hot-pressing at a high temperature and high pressure in order to overcome poor adhesion. Although polymer electrolyte membrane might be damaged by this process, hot-pressing is the most widely used MEA preparation method in which its conditions were optimized for minimizing membrane degradation. Nevertheless, the use of a polymer as an electrode material enables to improve the interface properties and weaken hot-pressing condition.

A number of polymeric films have been tested as active electrode materials such as polypyrrole, poly(N-methyl-pyrrole), polyaniline (PANi), and poly(3-methyl-thiophene), which were prepared by different synthetic methods [13], [14], [15], [16]. Ficicioglu and Kadirgan [17] reported on the possible use of PANi films as an electrode. After the polymerization of aniline on a Pt foil, platinum was deposited on PANi films by changing the deposition potential. The electrical and catalytic properties of the surface would be altered because the electrochemical deposition potential has an effect on the crystalline structure and the morphology of the platinum surface. Hepel [18] proposed the new method for the formation of composite polypyrrole films containing a highly dispersed three-dimensional array of platinum catalyst particles. PtCl₄²⁻ anions, which were trapped in polypyrrole matrix during the electropolymerization of pyrrole, were reduced to elemental platinum particles and incorporated in the electrically conducting polypyrrole films. This method permitted the synthesis of films with uniform distribution of small platinum particles with an average size of 10 nm and showed better electrocatalytic activity than two-dimensional films because of its high surface area. In addition, Pickup and coworkers [19] reported that the electrically conducting proton exchange polymers were effective support materials for Pt and PtRu catalyst as a substitute of carbon using different types of polymer composites.

Even if other group showed the possibilities of electronically conducting polymers in fuel cell electrodes, most applications were limited to half-cell electrochemical test, especially in the form of film electrodes. Little fuel cell application has been reported yet. Here, we report the application to the DMFC using the electronically conducting polymers such as poly(N-vinyl carbazole) (PVK) and poly(9-(4-vinyl-phenyl)carbazole) (P4VPCz). We had already reported the modified synthetic method of highly stable and dispersed carbon-supported or unsupported Pt-based alloys in anhydrous solvent [20]. Also, other groups have tried to synthesize smaller Pt-based electrocatalysts in nonaqueous solutions. Based upon previous works, water-soluble PANi and polypyrrole, which are good examples of electronically conducting polymers, have the limitation of application to nonaqueous synthesis. The reason why PVK and P4VPCz were selected for electronically conducting polymers was that both polymers are soluble in THF. P4VPCz could be polymerized anionically having similar structure of PVK and its electronic conductivity would be enhanced by introduction of phenyl group. The molecular structures of these polymers are shown in Fig. 1. In this study, PVK and P4VPCz of electronically conducing polymers were used for catalyst supports and their composites with PtRu catalysts were examined to evaluate the possibility of their use as a DMFC anode.

Section snippets

Synthesis and characterization of conducting polymers

Poly(N-vinyl carbazole) was purchased from Aldrich. 9-(4-Vinyl-phenyl)carbazole (4VPCz), styrene derivatives containing carbazole moieties, was synthesized. 4VPCz was synthesized in two steps in which carbazole and dibromobenzene were coupled in the first step and a vinyl group was introduced in the second step. The homopolymerization of 4VPCz was performed using an initiator, K-Naph or s-BuLi in THF. 4VPCz including Bu₂Mg was added to the initiator in a THF solution, polymerized for 5 min, and

Homopolymerization of 4VPCz

The monomer and polymer were characterized by ¹H-NMR as shown in Fig. 3, ¹³C-NMR and FT-IR spectroscopy (not shown here). In the case of the proton peaks of 4VPCz which are shown in Fig. 3(a), the β-carbons, which correspond to CH₂, appeared as doublets at 5.61 and 5.18 ppm indicating that cis–trans isomerization occurred. The NH proton peak (11.25 ppm), which was attached to carbazole, was removed during the purification process. For the ¹³C-NMR spectrum, the β-carbon peak of 4VPCz, located

Conclusions

Composites of PtRu alloy nanoparticles and an electronically conducting polymer were synthesized and evaluated for use as a DMFC anode. Although conducting polymers such as PVK and P4VPCz have a lower conductivity than carbon, they have the potential for use as the supporting materials in DMFC. TEM image and XRD patterns confirm that PtRu nanoparticles were formed with good alloy properties and dispersed in polymer without extensive agglomeration. Voltammograms showed that the composites had

Acknowledgements

This work was supported by KOSEF Grant No. R01-2001-00424 and the Brain Korea 21 project from the Ministry of Education.

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Nano-composite of PtRu alloy electrocatalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell

Abstract

Introduction

Section snippets

Synthesis and characterization of conducting polymers

Homopolymerization of 4VPCz

Conclusions

Acknowledgements

J. Mem. Sci.

J. Electroanal. Chem.

Electrochim. Acta

Electrochim. Acta

J. Power Sources

J. Power Sources

Electrochim. Acta

Electrochim. Acta

J. Electroanal. Chem.

J. Power Sources

Polym. Degrad. Stab.

Analyst

Electrochem. Solid State Lett.