A new one-pot, and green strategy for the synthesis of networks of connected Pt nanoparticles decorated on MWCNTs as an excellent catalyst for anodic electrooxidation of methanol

Highlights

• Pt/MWCNTs modified electrode exhibits excellent catalytic activity and durability for MOR.

• Simple, one-pot, and new strategy was applied for the fabrication of Networks of connected Pt NPs decorated on MWCNT.

• Application of Zn/HCl system was used for reduction of Pt as a new reducing system.

Abstract

A galvanic reduction strategy was applied on Pt nanoparticles decorated on Multi-Wall Carbon Nanotubes (MWCNTs) for preparing a modified electrode for the first time in this study. In this regards, the electrode was modified by the networks of connected platinum nanoparticles supported on MWCNTs with low Pt loading. Based on the proposed method, deposited PtCl₂ on MWCNTs was directly reduced to metallic Pt nanostructure using a zinc sheet in HCl (2%w/w) solution. The physical and electrochemical characters of resultant electrocatalyst were identified via Field Emission Scanning Electron Microscopy (FESEM), X-ray Diffraction (XRD), Energy Dispersive X-Ray Spectroscopy (EDS), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Transmission Electron Microscopy (TEM), Cyclic Voltammetry (CV), and Chronoamperometry (CHA). The as-prepared Pt/MWCNTs catalyst was utilized for oxidation of methanol in alkaline electrolyte and showed higher catalytic activity (about 10 times) and durability compared to commercial Pt/C. Additionally, the tolerance to carbonaceous intermediates accumulation was greatly improved. The proposed method is not only very simple, fast and environmentally friendly but also does not require any advanced equipment and therefore can be used for production of variously modified electrodes.

Introduction

In comparison to various kinds of Direct Liquid Fuel Cells (DLFCs), Direct Methanol Fuel Cells (DMFCs) are proper candidates for application in commercial level because of their widespread advantages (e.g. facile, security, low temperature, large theoretical power-conversion yield, etc.) [[1], [2], [3], [4], [5], [6], [7]]. Synthesis of the high performance electrocatalysts should be considered before commercialization of DMFCs [8]. In this regard, profuse studies have been dedicated to the promotion of catalytic characters of palladium (Pd) and platinum (Pt)-based electrocatalysts. Compared to Pd, Pt-based catalysts are the most effective catalysts that were used as both active anode and cathode materials in Methanol Oxidation Reaction (MOR) [[9], [10], [11], [12], [13]]. It is due to the fact that the Pt‐based catalysts showed higher catalytic activity and durability for the MOR in comparison to other noble metal catalysts such as Ru, Pd, Au, etc [14]. Recently, the price of Pt is reduced dramatically compared to Pd metal and usage of a cheaper and more active catalyst for commercialization of DLFCs plays an important role [15]. Generally, the application of platinum-based catalyst boosts the methanol-conversion properties of electrocatalysts and enhances their CO tolerance [16,17]. Due to Pt-based catalyst precious features, the Pt-based catalyst should be applied completely for DMFCs and its current-level performances to MOR need to be improved essentially for application in commercial level [18].

To enhance catalytic activity, some factors such as the type of support, morphology of catalyst, dispersity of nanoparticles, and clean surface of active sites should be considered [19].

In recent years, various types of carbon allotropes were employed as the support materials (e.g. carbon nanotubes (CNTs), active carbon, graphene (G), and graphite carbon nanofibers (GCNFs)) [[20], [21], [22]]. Specifically, CNTs are announced as an appealing support to load metal nanoparticles for the electrocatalytic applications owing to their distinctive attributes including giant surface area, high electronic conductivity, mechanical and thermal durability and stability. It has been made a lot of efforts to construct CNT-supported Pt-based electrocatalysts in various structures and shapes for MOR in application of DMFCs. For instance, Wang et al. decided to provide boron-doped carbon nanotube-supported Pt nanoparticles (Pt NPs) by increasing CO tolerance for methanol oxidation [23]. Moreover, Hsieh et al. conducted experiments to synthesize Pt-Sn nanoparticles on CNTs as the catalysts utilized in MOR [5].

The morphology (including shape and dimensionality) of Pt NPs can also efficiently influence their magnetic, electronic and catalytic properties. Consequently, the majority of high fulfillment MOR electrocatalysts are prepared by dispersing small nanoparticles of the Pt-based over the carbonaceous supports that is a critical step. For the methanol electrooxidation, catalyst's performance and durability depend not only on the size and morphology of metal nanoparticles but also on the attributes of the support material and interactions between the metal particles and support [19].

Up to now, profuse routes have been successfully developed for the production of Pt-based nanomaterials using the deposition-reduction method [21]. Unfortunately, the development and fabrication of Pt-based nanomaterials suffer from time-consuming multistep procedures. Moreover, one of the main challenges in the development of Pt-based nanomaterials refers to the utilization of reductant and surfactant materials that are highly expensive and dangerous for health. In addition, the application of surfactant materials leads to the loss of catalytic performance due to their strong interaction to the surface of the catalyst that results in the blockage of active sites on the surface of the catalyst.

Based on the mentioned points, in this paper a simple, surfactant-free and eco-friendly route with the aim of decorating small Pt nanoparticles on MWCNTs is suggested for the preparation of Pt/MWCNTs/GCE modified electrode. In this method, after covering the Glassy Carbon Electrode (GCE) surface by MWCNTs, an MWCNTs/GCE modified electrode is created. Next, PtCl₂ is deposited on the MWCNTs and then reduced to Pt NPs using Zn/HCl system. Zn/HCl, as a reductant, has some advantages such as inexpensive, facile, non-toxicity, environmentally friendly and accessible. This method can be used for the production of variously modified electrodes. The as-prepared Pt/MWCNTs/GCE modified electrode was utilized as a catalyst for MOR and reflected both electrocatalytic efficiency and durability compared to the Pt/C.