Pt–Ru nanoparticles anchored on poly(brilliant cresyl blue) as a new polymeric support: Application as an efficient electrocatalyst in methanol oxidation reaction

https://doi.org/10.1016/j.ijhydene.2019.10.071Get rights and content

Highlights

  • Poly(brilliant cresyl blue) is used as an efficient support for development of Pt-based catalysts.

  • All steps of electrode modification are done in aqueous solutions.

  • The poly(brilliant cresyl blue) supported nanoparticles possess smaller size and more uniform distribution.

  • The proposed catalyst provides mass activity of 898.5 A g−1.

Abstract

The glassy carbon electrode is modified by poly(brilliant cresyl blue) (PBCB) to be applied as a new green and efficient platform for Pt and Pt–Ru alloy nanoparticles deposition. Surface composition, morphology and catalytic activity of these modified electrodes towards methanol oxidation are assessed by applying X-ray diffraction, field emission scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy techniques. The X-ray diffraction patterns reveal that the highly crystalline Pt and Pt–Ru alloy and RuO2 nanoparticles with low crystallinity are deposited on the PBCB modified glassy carbon electrodes. The microscopic images indicate smaller size and better distribution of deposited nanoparticles on the surface of PBCB modified electrodes. Cyclic voltammetry and electrochemical impedance spectroscopy results reveal that PBCB supported Pt and Pt–Ru nanoparticles have better electrocatalytic performance and durability towards methanol oxidation rather than the unsupported nanoparticles. From the obtained results it can be concluded that the presence of PBCB not only improves the stability of nanoparticles on the surface, but also leads to the formation of smaller size and more uniform distribution of nanoparticles on the surface, which, in turn, cause the nanoparticles to provide a higher accessible surface area and more active centers for the oxidation of methanol. The results will be valuable in extending the applications of this polymer in surface modification steps and in developing promising catalyst supports to be applied in direct methanol fuel cells.

Introduction

Over the past few decades, fuel cells have been and are being greatly studied as clean and renewable energy converging device for applications in a variety of future fields of technology [1]. The main challenges for the commercialization of direct methanol fuel cells (DMFCs) are the limited resources and high cost of Pt, which is currently the most active and most widely applied material as catalyst in these cells. Easy poisoning of Pt surface by the intermediate of methanol oxidation, like carbon monoxide, is another barrier in applying Pt as catalyst in DMFCs [2]. Adding a second metal like Ru, Ni, Sn, and Pd to the Pt not only can overcome these drawbacks, but also may improve the electrocatalytic performance of the catalyst [[3], [4], [5]]. Among different bimetallic catalysts, the Pt–Ru is commonly accepted as the best electrocatalyst for methanol oxidation. Presence of Ru may increase the anode activity by reducing the effects of carbon monoxide poisoning by generating oxygen-containing species at lower potentials than that of Pt, which in turn contributes in the oxidation of carbon monoxide molecules adsorbed on catalyst sites into carbon dioxide [6,7]. The performance of these catalysts for the oxidation of methanol strongly depend on the Pt and Ru alloying degree and the type of support for catalyst fabrication [8]. It is well-known that the catalytic properties of catalysts are closely related to their size and morphology [9,10]. The dispersion of Pt and Pt-based nanoparticles on a proper support can be due to the production of smaller nanoparticles with more uniform distribution on the surface [11,12]. Applying different support for nanoparticles immobilization is one of the most common practiced strategies in reducing the amount of Pt on the surface and improving its catalytic performance [13]. The significance of the supporting material structure for the catalyst activity is assessed in [14,15]. Efforts are being made for developing different conducting supports for fuel cell applications.

The polymer-assisted synthesis of nanoparticles and nanocomposits are of great concern in recent years for preparation of nanoparticles with narrow size distribution [16]. There exist many studies on applying conjugated polymers with high conductivity like polyaniline (PANI) [17], polypyrrole [18], polythiophene [19], polyvinylpyrolidone (PVP) [20], and poly(3,4-ethylenedioxythiophene) (PEDOT) [21,22] as substrates for electrodeposition of Pt or Pt–Ru nanoparticles in methanol oxidation reaction (MOR) applications. Accomplishing a uniform dispersion of nanoparticles in a narrow size range, high utilization and good stability is the common property among these polymers. Due to the electrical conductivity of these polymers, it is possible in an electrocatalytic process to shuttle the electrons through the polymer chains between the electrode and the immobilized metal nanoparticles. Some studies in this field reveal that the catalytic performance and durability of the polymer supported nanoparticles can be efficiently improved compared to the unsupported Pt-based catalysts [22]. Some of the outstanding examples of catalytic activity, stability, CO tolerance and durability consist of: carbon black supported-PtRu electrocatalyst sandwiched between (PVP) and poly(2,5-benzimidazole) (ABPBI) [23], Pt–Ru nanoparticles deposited on para-phenyldiamine-functionalized carbon nanotubes [24], Pt nanoparticles decorated on graphene oxide-PVP hybride material [20], Pt–Ru supported on PANI [25] and PANI-coated Pt–Ni alloy nanoparticles [26]. The improvement of the electrocatalytic activity, anti-poisoning ability, and mechanical properties of the commercial Pt/C and PtRu/C catalysts in presence of polymer nanofibers is reported in [27]. In most of these studies, application of water insoluble monomers for production of polymer films is of concern. Consequently, it is of interest to extend such studies on other polymers, which might be appropriate as host material for developing new nanocatalysts.

In this study, the preliminary results concerning the modification of glassy carbon electrode with polyphenoxazines, well-known as redox mediators with high conductivity and their implementation as supports for electrochemical deposition of Pt and Pt–Ru nanoparticles are assessed. For this purpose, the brilliant cresyl blue (7-amino-3-diethylamino-8-methyl-phenoxazine chloride; BCB) is applied as a monomer to provide new support for methanol oxidation studies. Applying the BCB as the redox mediator in designing sensors and biosensors is reported in [28]. To the best of the authors knowledge here, there exists no report on PBCB film application as catalyst support in MOR. Due to its ease in synthesis as a conducting film with good adhesion to the substrate, high chemical and electrochemical stability, and water solubility of BCB monomer which makes its polymerization in aqueous solutions possible, PBCB is assumed to be a proper choice to develop new polymer-based catalysts for promising applications in fuel cell technology. With respect to the available methods in preparation of Pt-bimetallic alloys, the electrochemical deposition is the most effective in their size and morphology control [29,30] therefore, this method is adopted in this study. By applying the electrochemical measurements, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), highly efficient catalytic activity and improved electron transfer properties of the Pt and Pt–Ru nanoparticles are supported on the PBCB modified glassy carbon electrode towards the oxidation of methanol in an acidic media is revealed. The major contribution of PBCB support in improving the durability and catalytic properties of the developed catalysts is assessed in detail, by comparing the performance of different modified electrodes in absence or presence of polymeric support.

Section snippets

Chemicals and reagents

The RuCl3·3H2O is purchased from Sigma. BCB, H2PtCl6·6H2O and all other chemicals are of Merck analytical grade and are consumed without any further purification. Deionized water is consumed throughout the experiments. Solutions are deoxygenated by purging with pre-purified argon gas. The 0.1 M phosphate buffer solution (PBS, pH 7.0) is prepared from Na2HPO4 and NaH2PO4 aqueous solutions.

Fabrication of modified electrodes

Initially, the GCE surface of 2 mm diameter is carefully polished with alumina slurry to gain a mirror

Characterization of modified electrodes

The XRD characterization of the PBCB-supported nanoparticles is performed to identify the crystalline structure of the nanoparticles, Fig. 1, where the characteristic peaks at 2θ = 26.38, 42.22, 44.39, and 54.54 in XRD patterns are attributed to (002), (100), (101), and (004) planes of carbon surface. All XRD patterns show three characteristic diffraction peaks at 2θ = 39.76° (with slightly shifts to higher degrees for PtRu alloys), 46.24°, and 67.45° corresponding to (111), (200) and (220)

Conclusions

Here, a fast, green, and efficient two-step electrochemical approach is introduced to develop a new polymer supported Pt–Ru catalyst to be applied in MOR. In this approach, Pt and Ru (mainly as RuO2) nanoparticles are electrochemically deposited on PBCB electropolymerized on the GCE surface. In spite of a decrease in the amount of loaded nanoparticles, deposition of nanoparticles on the PBCB-modified electrodes are found to improve the stability and catalytic performance (ECSA, mass activity,

Acknowledgements

The authors would like to extend their appreciations to University of Isfahan, for the financial support.

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