Synthesis of a carbon-coated NiO/MgO core/shell nanocomposite as a Pd electro-catalyst support for ethanol oxidation

Abstract

Carbon coated on NiO/MgO in a core/shell nanostructure was synthesized by the single-step RAPET (reaction under autogenic pressure at elevated temperatures) technique, and the obtained formation mechanism of the core/shell nanocomposite was presented. The carbon-coated NiO/MgO and its supported Pd catalyst, Pd/(NiO/MgO@C), were characterized by SEM, HR-TEM, XRD and cyclic voltammetry. The X-ray diffraction patterns confirmed the face-centered cubic crystal structure of NiO/MgO. Raman spectroscopy measurements provided structural evidence for the formation of a NiO/MgO composite and the nature of the coated carbon shell. The high-resolution transmission electron microscopy images showed the core and shell morphologies individually. The electrocatalytic properties of the Pd/(NiO/MgO@C) catalyst for ethanol oxidation were investigated in an alkaline solution. The results indicated that the prepared Pd–NiO/MgO@C catalyst has excellent electrocatalytic activity and stability.

Introduction

Recently, the focus on the chemical synthesis of nanoparticles has shifted from single component nanoparticles to hybrid nanoparticles. Hybrid nanoparticles usually contain two or more different nanoscale domains, which lead to synergistic properties due to their interfacial interactions. Therefore, hybrid nanoparticles (non-symmetrical dimmers, symmetric core–shell and any other hetero-structures) are promising due to their multi-function and designable properties [1], [2], [3]. The design of core/shell structured composites has received much attention as a means to improve the stability and surface chemistry of the core materials, and as a way of obtaining unique structures, properties, and applications via a combination of the different characteristics of the components that are not available with their single-component counterparts [4], [5], [6], [7]. The synthesis and characterization of hybrid nanostructure material is aimed at designing and arranging nanomaterials in complex functional structures, resulting in unique physical properties [8], [9], [10], [11].

Carbon-coated nanomaterials are of great interest due to their stability towards oxidation and degradation [12], [13], [14], [15]. The creation of core/shell nanostructures containing bi-metal oxides has greatly enhanced the efficiency of these structures over pure single metal oxide particles as destructive adsorbents for environmental pollutants, such as SO₂ and H₂S [16], [17]. Magnesium oxide (MgO) is a typical wideband-gap insulator. It has found many important applications for use in catalysis, refractory materials, toxic waste remediation, paint, superconductors, and substrates for thin film growth [18], [19], [20]. NiO is a very successful material used in various fields, such as catalysis [21], battery cathodes [22] and fuel cell electrodes [23], and it is also widely studied as an ant ferromagnetic as it is a chemically-stable ionic insulator with high thermal stability and good resistance to corrosion. Many researchers have prepared NiO and MgO nanomaterials by various methods, e.g., the solvo-thermal technique [24], the laser technique [25], the sol–gel method [26] and thermal decomposition [27]. The preparation of core–shell nano-wire structures of ZnO/MgO [28] obtained by hydrothermal, atomic layer deposition [29] and chemical vapor deposition (CVD) [30] methods has already been reported. NiO/MgO thin films were prepared by an ion beam-assisted deposition technique and were used for chemical transformations and catalysis applications [31].

Direct ethanol fuel cells (DEFs) have attracted much attention because ethanol has a low toxicity, as compared to the methanol used in direct methanol fuel cells (DMFs). Ethanol can be easily produced in great quantities by the fermentation of sugar-containing raw materials [32]. Pt-based catalysts are extensively studied for electrocatalytic fuel cell applications. Indeed, platinum is very efficient for the different reactions involved in fuel cells, such as methanol oxidation and oxygen reduction [33]. However, it is well known that the anode activity of Pt is adversely affected by poisoning from CO species. Several studies have investigated the addition of Ru or other metals to Pt, in order to improve the activity of the catalysts [34], [35], [36]. Nevertheless, it has been shown recently that palladium is more active than platinum for ethanol oxidation in basic media [37], [38], [39]. As Pd is 50 times more abundant in the earth and cheaper than Pt, Pd-based nanomaterials are potential electrocatalysts for replacing Pt in DAFCs. Palladium also plays a vital role in catalysis and is involved in various reactions, such as Heck, Suzuki, and Stille coupling [40], [41], [42], [43]. In order to reduce the amount of noble-metal loading on the electrodes, improvements to the dispersion of the noble metal on the support have been investigated. Metal nanoparticles have been dispersed on a wide variety of substrates, such as carbon nanomaterials [44], [45], polymer matrices [46], and oxide matrices [47], [48]. Recently, metal oxides such as CeO₂ [49], NiO [50], MgO [51], TiO₂ [52] and In₂O₃ [53] have been used as electrocatalysts for the direct oxidation of alcohol, and which resulted in a significantly improved electrode performance in terms of the enhanced reaction activity. The noble-metal shell in the core/shell nanoparticles has two roles. First, it protects the non-noble core from contact with the alkaline electrolyte. Second, the coated Pd on the shell should improve the catalytic properties of the substrate. We therefore report herein on a one-stage, reproducible, solvent free, competent, and straightforward approach for the synthesis of NiO/MgO@C core shell nanostructures, followed by its decoration by Pd using simple impregnation. The prepared material was evaluated for the electrochemical ethanol oxidation reaction for fuel cell applications in alkaline media.