MOF-derived Cu/nanoporous carbon composite and its application for electro-catalysis of hydrogen evolution reaction
Graphical abstract
Metal organic framework-derived Cu/nanoporous carbon composite (Cu/NPC composite) was prepared by direct carbonization of MOF-199 without addition of any carbon source at 900 °C. The Cu/NPC/GCE demonstrated an excellent electrocatalytic activity towards hydrogen evolution reaction compared with bare GCE.
Introduction
NPC (Nanoporous carbon) materials with high surface areas have been widely applied in many fields such as adsorbents [1], catalyst supports [2], and supercapacitor [3]. The advantageous characteristics of highly porous carbons, including their fast kinetics, high surface area, narrow pore size distribution, high pore volume, and high conductivity, have attracted attention for use as supports for preparation of the electrocatalyst [4], [5], [6].
Variety methods have been employed for preparation of carbon materials, including laser ablation [7], electrical arc [8], chemical vapor decomposition [9], and templating [10], [11] as well as chemical or physical activation methods [12]. Among them, the template method is an effective way for synthesis of the NPC. Two different template modes, i.e., endotemplate and exotemplate [13], can be distinguished according to function of the template. In the exotemplate case, firstly the nanoporous template is prepared. Then, the NPC is synthesized at two different ways; either with addition [14] or without any carbon precursor [15] upon the template as a porous pattern. Different nanoporous templates are used for preparation of the NPC such as nanoporous silicate (MCM-48, SBA-16 and so on) with addition of different carbon precursors such as sucrose and furfuryl alcohol [16], [17].
Another group of nanoporous templates that can be used to prepare of nanoporous carbon are MOFs (metal organic frameworks) [4], [15]. The MOF as a new class of porous crystalline materials [18] was used for catalysis, gas storage and separation [19]. The MOF can also be used as a template for synthesis of electrically conducting porous carbon materials. Liu and coworkers [20] for the first time, used porous MOFs for synthesis of the carbon nanomaterials and checked their hydrogen storage capability as well as electrochemical capacitance. Park and coworkers [15] reported fabrication of highly porous carbon adsorbents by carbonizing highly crystalline MOF without any carbon precursors and its application for H2 storage. Ali Khan et al. [4] synthesized the NPC by direct carbonization of MOF-5 and studied it as a support for preparation of electrocatalyst for ethanol oxidation.
Hydrogen is a future fuel because of its high heat of combustion, and high energy capacity per unit volume as compared with conventional fossil fuels. The combustion products of hydrogen are nearly free from pollution [21]. Therefore, there are many attempts to use different modified electrodes for the HER (hydrogen evolution reaction) [22], [23]. Although platinum has a high catalytic activity for the HER, limited reserve in earth and high cost restricts its wide application in industry which is a major challenge for commercialization of the device [24]. Therefore, many efforts were made to find the other materials for HER to replace or reduce the use of Pt [25], [26].
Herein, we have used Cu3(BTC)2[BTC = Benzene-1,3,5-tricarboxylate]also known as HKUST-1 (or MOF-199) as a template for fabrication of highly Cu species/nanoporous carbon composite (Cu/NPC composite) by direct carbonization without any carbon precursors at 900 °C. The MOF-199 is one of the most cited MOFs because it has a large surface area, high pore volume, high chemical stability, and easy synthesis. In this structure, each Cu ion is coordinated by four oxygen atoms of benzene-1,3,5-tricarboxylate ligands and by one H2O molecule that forms dimeric Cu (II) paddlewheel units [27]. The resulting framework is a cubic structure with two types of pores [28]. We have exploited the above mentioned structural characteristics of the MOF-199 and Cu/NPC composite as a catalyst in HER. The results demonstrated that the pore characteristics of the Cu/NPC composite and presence of copper metal catalyst have strongly affected the electrocatalyst activity rather than bare glassy carbon electrode. To the best of our knowledge, this is the first report on synthesis of the Cu/NPC composite and its application for use in HER in acid medium.
Section snippets
Materials
H3BTC (Benzene-1,3,5-tricarboxylic acid, 95%) was acquired from Aldrich. Cu(NO3)2.3H2O, 99.99% and ethanol as solvent were supplied by Fluka. All reagents were of analytical grade and used without further purification. Solutions were made with twice distilled water.
Preparation of the CU/NPC composite
Porous MOF-199 was prepared by using a hydrothermal method based on the reported procedure [29]. In the typical synthesis, Cu(NO3)2·3H2O (2.327 g) was dissolved into 25 mL of deionized water. The above process was followed by
Characterization of the synthesized materials
Characterization of the MOF-199 and Cu/NPC composite compounds usually involves XRD (structure identification), SEM analysis (morphology), N2 adsorption–desorption isotherm (textural properties of surface area/pore volume), and TGA (structural stability). Fig. 1a exhibits the XRD pattern for the MOF-199. The main diffraction peaks at 2θ = 9.4°, 11.62°, 17.4°, 19°, 29.3° and relative diffraction intensities of MOF-199 were found to be the same as previous report [29]. After direct carbonization
Conclusion
A novel metal organic framework-derived Cu/NPC composite was successfully synthesized via direct carbonization of the MOF-199 as template at 900 °C under N2 atmosphere without addition of carbon source. The obtained Cu/NPC composite was exhibited cauliflower-like morphology with particle size about 20 nm. It consisted of Cu metal, CuO, Cu2O according to XRD result. This material was showed a good surface area and micro-meso structure. Electrochemical measurements of the Cu/NPC/GCE indicate
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