Facile synthesis of petal-like NiCo/NiO-CoO/nanoporous carbon composite based on mixed-metallic MOFs and their application for electrocatalytic oxidation of methanol

https://doi.org/10.1016/j.apcatb.2018.12.013Get rights and content

Highlights

  • Petal-like NiCo/NiO-CoO/NPCC nanocomposites synthesized based on bimetallic MOF.

  • The NiCo/NiO-CoO/NPCC used as a non-precious electrocatalyst for MOR.

  • NiCo/NiO-CoO/NPCC showed excellent catalytic activity and durability.

  • Unique Petal-like structures offer large surface area and accessible active site.

  • Synergistic effect between metal oxides/metals enhanced the catalyst activity.

Abstract

Porous carbon template decorated with mixed transition metals/metal oxides with tunable architecture is becoming increasingly important and attractive as a kind of novel electrode materials. In this way, mixed-metallic metal-organic frameworks (MOFs) provide an opportunity for fabrication of homogeneous mixed metals/metal oxides distribution in the porous carbon frame without any carbon precursor additive. Also, structures, dimensions and electrochemical performance of MOFs can be readily manipulated by simply tuning the metals molar ratio. In this study, we demonstrate the design and fabrication of petal-like NiCo/NiO-CoO metal/metal oxides with a rational composition embedded in 3D ultrathin nanoporous carbon composite)NiCo/NiO-CoO/NPCC(. This nanocomposite is synthesized by a two-steps procedure involving preparation of bimetallic MOFs by partially substituting Ni2+ in the Ni-MOF structure with Co2+ (Ni-Co/BDC [BDC = 1,4-Benzene dicarboxylic acid]) and direct carbonization process in the N2 atmosphere at 900 °C. The prepared nanocomposite was used directly as a non-precious electrocatalyst for methanol oxidation reaction. The results indicated that, in comparison to the monometallic metal/metal oxides distributed in nanoporous carbon composite (Ni/NiO/NPCC and Co/CoO/NPCC), the mixed metals/metal oxides NiCo/NiO-CoO/NPCC exhibits excellent electrochemical performance toward the anodic oxidation of methanol. The unique ultrathin porous petal-like structure with free pores and the enlarged specific surface area provides fast ion/electron transfer, leading to faster kinetics, lower over-potential, and higher electro-catalytic reactivity. Besides their intriguing structural features, the excellent conductivity of carbon frame, as well as a rational composition of two constituents and synergistic effects from cobalt, nickel and their oxides provides favorable catalytic activity for the electro-oxidation of methanol. Therefore, it is believed that this novel multi-component composites demonstrates good electrocatalytic activity and suitable stability towards the methanol oxidation.

Introduction

Direct methanol fuel cell (DMFCs), as an attractive and promising power source for future energy demand, has attracted a great deal of attention because of its simplicity, potentially renewable fuel and high theoretical efficiency of the energy conversion [1]. Nevertheless, slow kinetics of the methanol oxidation reaction at the anode electrodes, which leads to large over-voltages, is one of the major challenges toward the commercialization of DMFCs [2]. Although noble metals and their alloys with effective catalytic role exhibit high electro-catalytic activities, however, great price and scarcity of the metals may restrict their commercial applications. Therefore, discovering low-cost and novel anode electrocatalysts is highly desirable and important. Lately, a great deal of effort was devoted to developing transition metals oxides as proper substitutes for the noble metals catalysts [3]. Thus, substantial researches have been centered on the use of transition metal oxides catalysts such as NiO [4], CoO [5] and Cu2O [6] for DMFCs, because of their low production cost, commercial availability and high electrocatalytic activity.

Also it worth to note that, formulation of composites is a fascinating strategy for increase the electro-catalytic performances, robustness and engagement ability of the catalysts [7]. Generally, multi-component catalysts, due to synergistic advantages, exhibit novel properties beyond each individual component through the reinforcement or modification of each other [8]. Thus, exploring novel mixed transition metal/metal oxides with the rational design of multi-component combination has become an important research direction. The obtained results indicate that creation of a link between metals rendering the mixed transition metal/metal oxides rich in redox reactions [9]. Till now, there have been some reports on the application of multi-component catalysts containing transition metal and metal oxides in the electro-oxidation reactions. For example, Liu et al. [10] fabricated a nanocomposite of CoO-NiO-NiCo supported by nitrogen-doped multiwall carbon nanotubes as a bifunctional electrocatalyst for the oxygen reduction reaction and also the oxygen evolution reaction. Wu et al. [11] prepared NiCo/NiO–CoOx ultrathin layered nanocomposites as a non-noble-metal multifunctional catalyst for catalyzing H2 generation from N2H4. Also, Peng et al. [12] prepared a homologous Ni–Co based nanowire system consisting of both nickel cobalt oxide and Ni0.33Co0.67S2 nanowires for efficient, complementary water splitting. Moreover, Zhang et al. prepared Ni-Co bimetallic MgO-based catalysts for hydrogen production via steam reforming of acetic acid from bio-oil [13]. Therefore, based on the above-mentioned work, multi-component catalysts such as NiCo/NiO-CoO can meet the requirements for designing high-performance catalysts for direct electro-oxidation of methanol.

It should be noted that, metal oxides suffer from poor electric conductivity and this problem can be compensated by synthesis the composites of them with a typical carbon-based material such as carbon fiber [14], carbon hollow particles [15], carbon nanoflakes [16] graphene [17] and carbon nanotubes [18]. Hence, it is believed that synthesis of composites containing conductive nanocarbon materials is an effective strategy to improving catalytic activity, through increasing number of available active sites and providing efficient charge transport channels. As promising candidate nanoporous carbon materials with high surface areas, narrow pore size distribution, high pore volume, and high conductivity can be considered as a suitable support for preparation of electrocatalysts. Besides the composition, the morphology, shape, and structural characteristics are other crucial approaches to efficient control of the catalytic activity of catalysts, which is attributed to the structure-activity relationships [19]. In this regard, petal-like structures assembled by interconnected two-dimensional nanosheets with porous structures are greatly desirable and can be considered as the nanostructures with efficient catalytic properties. The obtained results revealed that these materials are favorable to efficient ion and electron transport and fast reaction kinetics, due to their large surface area, high catalytic activity, high roughness, high porosity and sufficient adsorption sites for a specific electrochemical reaction [20].

The most widely used method for the preparation of nanocomposites containing metal/metal oxides and carbon nanomaterials is based on dispersing the metal components into the carbon support. As a point that should be noted, heterogeneous catalysts often suffer from particles aggregation during the reaction progress and thus resulting loss in their catalytic activity [21]. Therefore, we need to find a simple and facile route for the synthesis of the seamless nanocomposites, in order to effective immobilization of metal oxide nanoparticles with high stability and fine distribution on porous carbon supports.

As an appropriate candidate, metal-organic frameworks (MOF) a new class of porous materials assembled by metal-containing units and bridging organic ligands have attracted immense attentions [22]. MOFs present various superior properties, such as porous structure, high surface area, and diverse structural topology. Hence, MOFs can use in many applications in electrochemical energy storage, including hydrogen production and storage, fuel cells, lithium-ion batteries, supercapacitors and solar cells [23]. However, the poor conductivity and unstable nature of MOFs in aqueous solutions limited their application. Therefore, according to the conditions of usage, MOFs can be used as precursors or templates for preparation of nano-sized materials such as porous carbon, metals, metal oxides, and hydroxides [24]. Accordingly, mixed-metal MOFs are used for the preparation of highly porous carbon materials and hybrid meta/metal oxides via direct carbonization. As a result, the metal compositions can be converted into metallic or metal oxide nanoparticles while organic ligand can be transformed into porous carbon materials. Also, during the carbonization process, the primary morphology of MOFs is retained. Mixed-metal MOFs are composed of two different central metal ions into a same framework with a further degree of structural stability and conductivity [25]. Because of the similar ion radius of Co and Ni, partial substitution of nickel cations with cobalt can be occurring without serious changing in the crystalline structure of Ni-MOF. Then, mixed Ni/Co nanoparticles and NiO/CoO metal oxides distributed in the carbon frame can be prepared after the direct carbonization of bimetallic Ni-Co/MOF. In comparison to other methods, synthesis of metal/metal oxides/porous carbon nanocomposites from the corresponding MOFs has noticeable benefits. In one hand, the wide variety of MOFs makes it possible to achieve more pure metal/metal oxides with different components and homogeneous distribution. On the other hand, the obtained nanocomposites from MOFs have diverse shapes and structures with high surface area, which make them desirable as the electrocatalyst for the electro-oxidation reactions. Recently, some research groups have suggested different catalysts based on MOFs. For example, Raoof et al. [26] prepared MOF-derived Cu/nanoporous carbon composite as a non-platinum electrocatalyst for the hydrogen evolution reaction. Also, Peng, et al prepared Ni/Co bimetallic metal–organic framework nanobelts as an excellent bifunctional oxygen catalyst [27]. Pang et al. reported the fabrication of Ni/NiO/carbon frame nanocomposite from nickel-based MOFs and investigated as a nonenzymatic glucose and H2O2 sensors [28]. Also in another work synthesized ultrathin two-dimensional cobalt–organic framework nanosheets by a simple surfactant-assisted hydrothermal method for high-performance electrocatalytic oxygen evolution reaction [29].

Ali Khan et al. synthesized the nanoporous carbon by direct carbonization of MOF-5 and studied it as a support for the preparation of an electrocatalyst for the electro-oxidation of ethanol [30].

Based on the above considerations, the aim of this study is to develop noble metal-free electrocatalysts for DMFCs. Therefore, in this study we present a simple method to synthesize mixed NiCo/NiO-CoO/nanoporous carbon composite (NiCo/NiO-CoO/NPCC) with a petal-like structure. For this purpose, we have used bimetallic Ni-Co/MOF as the precursor template for the fabrication of nanocomposites containing mixed metal/metal oxide/carbon frame by direct carbonization without any carbon precursors. This process finally yielded stable composites with ultrafine distribution of metal/metal oxide nanoparticles in the carbon support. The obtained nanocomposites have large surface area, high pore volume, and high chemical stability with an easy synthesis procedure. The electro-oxidation of methanol on NiCo/NiO-CoO/NPCC has been studied by different electrochemical techniques. The obtained results reveal that, in comparison to simple Ni/NiO or Co/CoO, the mixed metal/metal oxide with unique porous structure and strong synergistic effect from Ni, Co, NiO and CoO shows a noticeable catalytic activity. To the best of our knowledge, this is the first report on the synthesis of the NiCo/NiO-CoO/NPCC nanocomposite from bimetallic MOFs and its application for use in direct methanol oxidation.

Section snippets

Materials and instrumentation

Cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O), Nickel (II) chloride (NiCl2), N,N-dimethylformamide (DMF), sodium hydroxide, ethanol, and methanol EITH analytical reagent grade were purchased from Merck. 1,4-Benzene dicarboxylic acid (H2BDC, 95%) was obtained from Aldrich. The reagents used for electrochemical impedance studies, including K3Fe(CN)6, K4Fe(CN)6 and KCl (>99%), were also prepared from Merck. All aqueous solutions were prepared using ultra-pure deionized water (18.2 MΩ, Zolalan

Characterization of the synthesized materials

XRD analysis is used to identify the crystal phase and structural information of freshly prepared Ni/MOF, Co/MOF, Ni-Co/MOF and corresponding derivatives, after the carbonization, Ni/NiO/NPCC, Co/CoO/NPCC and NiCo/NiO-CoO/NPCC (Fig. 1A and B). As depicted in Fig. 1A, XRD results for Ni/MOF and Co/MOF are in agreement with those simulated from the single-crystal data of the sheet-like structures [Ni3(OH)2(C8H4O4)2(H2O)4]·2H2O (CCDC no. 638866) and MOF-71, same as previous reports, respectively [

Conclusions

The main purpose of this study is to present a new strategy for preparation of a novel nonprecious catalyst containing mixed transition metals/metal oxides in nanoporous carbon frame by using mixed MOFs as a precursor’s material. In summary, Ni-Co/MOF/GCE were prepared through a facile hydrothermal approach and NiCo/NiO-CoO/NPCC/GCE were fabricated after carbonization process of bimetallic MOFs. In the following, for comparison, Ni-MOF/GCE and Co-MOF/GCE and related composites Ni/NiO/NPCC/GCE

Acknowledgements

The authors gratefully acknowledge the support of this work by the Research Council and the Center of Excellence for Nanostructures of the Sharif University of Technology, Tehran.

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