Perspective Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9568-9585
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Ball-Milled Carbon Nanomaterials for Energy and Environmental Applications Honghong Lyu,†,‡,§ Bin Gao,*,§ Feng He,∥ Cheng Ding,⊥ Jingchun Tang,† and John C. Crittenden‡ †
Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), Tianjin Engineering Center of Environmental Diagnosis and Contamination Remediation, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China ‡ Brook Byer Institute for Sustainable Systems and School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Department of Agricultural and Biological Engineering, University of Florida, Gainesville, Florida 32611, United States ∥ College of Environment, Zhejiang University of Technology, Hangzhou 310014, China ⊥ School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, China ABSTRACT: With exceptional physicochemical properties, carbon nanomaterials (CNMs) have been widely applied in various energy and environmental applications including energy conversion, energy storage, and environmental remediation. Recent efforts have been made to prepare modified CNMs with improved electrical and chemical properties, hence broadening their potential applications. It is highly desirable to produce high-quality CNMs at a low cost and large scale, which remains a challenging task. Ball-milled CNMs (BMCNMs) are a novel class of engineered materials that may provide new opportunities to satisfy the need. To promote the research on BM-CNMs, this work provides a comprehensive review on recent research development in (1) synthesis of various types of BM-CNMs, (2) effects of ball milling on physicochemical properties of BM-CNMs, and (3) energy and environmental applications of BM-CNMs. Different ball-milling processes for preparing BMCNMs and modified BM-CNMs with desired particle size, structure, and surface properties are summarized and discussed. The physicochemical properties of pristine CNMs and BM-CNMs are compared. Because of BM-CNM’s unique properties, such as excellent catalytic, electrochemical, and sorptive properties, their potential applications in energy conversion, energy storage, and environmental remediation are also discussed. Key challenges and further research needs are proposed at the end. KEYWORDS: Carbon nanoparticles, Ball-milled nanoparticles, Catalyst, Fuel cell, Battery, Sorbent
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INTRODUCTION Carbon nanomaterials (CNMs) such as graphene, carbon nanotubes (CNTs), and nanofibers possess extraordinary physical, chemical, and mechanical properties for applications in various fields to improve life quality.1,2 CNMs have attracted considerable research interest to optimize and expand their applications, especially with respect to energy and environmental applications such as energy conversion (i.e., solar cells and fuel cells),3−9 energy storage (i.e., supercapacitors and batteries),10−12 and environmental remediation (i.e., removal of heavy metals and organic contaminates from water and soils).13−15 When applied to energy conversion and storage, CNMs have shown excellent electrochemical stability and charge transfer/storage properties.16−20 In addition, CNMs are excellent adsorbents for various organic and inorganic contaminants, including antibiotics,21 PAHs,22−24 organic pesticides,25−27 and heavy metals.28−32 It has also been reported that modification of CNMs can introduce new structures to further enhance their physicochemical properties. Both CNMs © 2017 American Chemical Society
and modified CNMs are promising high-performance structural and multifunctional materials for energy and environmental applications. The properties and functions of CNMs are highly dependent upon their production methods. Several technologies have been developed for the syntheses of CNMs, including mechanical exfoliation,33 laser ablation, arc discharge, hydrocarbon combustion,34 chemical exfoliation,35 chemical vapor deposition (CVD),36−38 and ball milling.19 Although facile and valuable, some physical methods, such as mechanical exfoliation and laser ablation, possess some disadvantages regarding material purity, uniformity, batch-to-batch consistency, and industrial applicability.39 On the other hand, chemical syntheses of CNMs often involve expensive procedures and toxic reagents that may impose economic and environmental burdens.40 For example, chemical Received: July 1, 2017 Revised: August 25, 2017 Published: September 17, 2017 9568
DOI: 10.1021/acssuschemeng.7b02170 ACS Sustainable Chem. Eng. 2017, 5, 9568−9585
S/CNT and NiCl2·6H2O
Sulfur and MWCNTs S/CNT and Zn(CH3COO)2· 2H2O
Multiwalled CNTs (MWCNTs)
Graphite nanoplatelets and epoxy
GS, ZrO2, and N-methylpyrrolidone (NMP)
GO and melamine
Graphite
Graphite and OA mixed in a stainless-steel jar; Ball milling; Washed with HCl and distilled water; Heating to 600 °C under an argon atmosphere Graphite added in a hardened steel jar; Ball milling; Cooled down and unloaded in air GO and melamine particles mixed in the deionized water; Heated to 60 °C to dissolve melamine; Ultrasonic dispersion; Highenergy wet ball milling GS, ZrO2, and NMP mixed in a stainless-steel grinding bowl; Ball milling; Centrifugation; Washed with deionized water; Freezedried Graphite nanoplatelets dissolved in acetone, epoxy added; Sonication; Ball milling; Heating to remove acetone; Curing Dry ball milling: MWCNTs and zirconia balls mixed into a cylindrical stainless pot; Wet ball milling: MWCNTs, zirconia balls, and distilled water mixed into a cylindrical stainless pot Sulfur and MWCNTs mixed in an agate tank; Ball milling Zn(CH3COO)2·2H2O, ethanol and S/CNT-60% mixed in an agate tank; String; Concentrated ammonia added; Wet ball milling; Washed with water and ethanol; Dried NiCl2·6H2O, ethanol, and S/CNT-60% mixed in an agate tank; String; Concentrated ammonia added; Wet ball milling; Washed with water and ethanol; Dried
Graphite and oxalic acid (OA)
Process
Graphite and dry ice mixed in a stainless-steel jar; Ball milling; Sonicated in HCl solution; Freeze-dried
Graphite and dry ice
Feedstock
Table 1. Summary of Synthetic BM-CNMs Condition
NiCl2·6H2O = 0.477 g, ethanol = 5 mL, S/CNT-60% = 0.6 g, and concentrated ammonia (30 wt %) = 1 mL; Milling at 500 rpm for 3 h
Dry ball milling: MWCNTs = 0.5 g and zirconia balls = 105.0 g; Wet ball milling: MWCNTs = 0.5 g and zirconia balls = 105.0 g, and 20 mL distilled water; Milling at 200 rpm, 300 rpm, 400 and 500 rpm for 1 h, respectively Sulfur:MWCNTs = 3:2 or 1:1; Milling at 300 rpm for 3 h Zn(CH3COO)2·2H2O = 0.4 g, ethanol = 5 mL, S/CNT-60% = 0.6 g, and concentrated ammonia (30 wt %) = 1 mL; Milling at 500 rpm for 3 h
Milling at 300 rpm for 30 h
GS:ZrO2 = 1:40; Milling at 300 rpm for 50 h
Graphite-to-ball charge ratio = 1:7; Harden steel balls diameter = 5 mm and 7 mm; Milling at 345 and 152 rpm, respectively; Milling for 2−24 h GO = 5 mg, GO:melamine= 1:6, zirconia grinding balls (diameter = 1.4−1.7 mm, volume = 6 mL) Milling at 500 rpm for 8, 16, 24, and 32 h.
Graphite = 25 g and dry ice = 20 g; Stainless steel balls (300 of 6 mm in diameter and 200 of 10 mm in diameter); Revolution speed = 150 rpm and autorotation speed = 300 rpm; Milling for 24 h Graphite = 50 g and OA = 80 g; Stainless steel balls diameter = 10 mm; Ball mill speed = 40%; Milling for 20 h and rest for 1 h per cycle
Product
Ni(OH)2@S-CNT
Sulfur/CNT(S/CNT) ZnO@S/CNT
MWCNTs
Epoxy/mechanically exfoliated GR (MEG)
Ball-milled GO (BGO)
147
147 147
146
145
131
74
66
Ball-milled graphene oxide (GO) nitrogen-doped GO (N-G) catalyst
119
144
Refs
Ball-milled GR
graphene nanosheets (GNs)
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DOI: 10.1021/acssuschemeng.7b02170 ACS Sustainable Chem. Eng. 2017, 5, 9568−9585
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ACS Sustainable Chemistry & Engineering
Figure 1. (a) Principles of ball-milling method.74 (b) Schematic illustration of the mechanochemical reaction between graphite and C60 in a sealed ballmill crusher. The synthesis routes with LiOH affording the directly bonded GR-C60 hybrid or without LiOH leading to the formation of oxygenated graphene nanoplatelets (GnPs) and unreacted C60 are both shown.152
physiochemical properties of resulting BM-CNMs, (3) lay out the main applications of BM-CNMs in energy conversion, energy storage, and environmental remediation, and (4) identify critical knowledge gaps and future research needs.
exfoliation relies on acid oxidation of the carbon-rich precursors, resulting in potential contamination risks as well as significant damage of the CNM planes.41 Large-scale production of CNMs and modified CNMs by CVD can be very expensive because of the high cost associated with complex manufacturing equipment and processes.42 In comparison with the traditional CNM production methods, ball milling, a simple but efficient approach which mechanically reduces the grain size of solids to nanoscale particles, has attracted much research interest recently.19,43−45 It has been widely used in large-scale production processes due to its low cost, flexibility, and simplicity.46 A number of studies have also been conducted recently to evaluate the potential applications of ball-milled CNMs (BM-CNMs) in various fields including energy, environment, and biomedicine.44,47−53 Nevertheless, BM-CNM is still a relatively new concept that may introduce promising and innovative technological breakthroughs to benefit society, especially with respect to improving energy and environmental sustainability. The overarching goal of this work is to promote the research on BM-CNMs by providing a critical review of recent research development of the production and characterization of BMCNMs and their energy and environmental applications. The specific objectives are as follows: (1) summarize the fundamental approaches and principles developed in the past few decades to prepare BM-CNMs and their derivatives (i.e., modified BMCNMs), (2) discuss the effect of ball milling on the
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PREPARATION OF BM-CNMS
As an emerging technology, ball milling was first developed by John Benjamin in 1970 for the synthesis of oxide dispersionstrengthened alloys capable of withstanding high temperature and pressure.54 Ball milling is a powerful nonequilibrium processing method, which refines the grain size of solids into nanoscale after a balance between the defect introduced by deformation of milling and its recovery by thermal treatment.55,56 Table 1 summarizes the synthesis of various types of BM-CNMs. In ball milling, a suitable powder charge is placed in a high energy mill, along with a milling medium.57 The moving balls apply their kinetic energy to the powder charge, break chemical bonding, reduce the particle size, change the particle shape, produce fresh surfaces, and synthesize BM-CNMs by fracturing feedstock particles (Figure 1a).1 For modified BMCNMs, carbon powders are milled in the presence of modification chemicals, i.e., hexane,17 yttrium,58 melamine,59 ammonium bicarbonate,60 magnesium,61 aluminum,62 iron,63 dry ice,64 hydrogen, carbon dioxide, sulfur trioxide, and carbon dioxide/sulfur trioxide mixture;20 thus, various heteroatoms, 9570
DOI: 10.1021/acssuschemeng.7b02170 ACS Sustainable Chem. Eng. 2017, 5, 9568−9585
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Figure 2. Schematic representation of (a) preparing of edge-nitrogenated graphene nanoplatelets (NGnPs) by dry ball milling,73 (b) the nanoscale highenergy wet ball-milling experimental process,74 and (c) the synthesis of the silicon@carbon/graphene sheets (Si@C/G) composites through combining high-energy wet ball milling and pyrolysis treatment.79
ability to produce finer particles. Therefore, planetary ball mills have often been used for the preparation of BM-CNMs recently.67 To reduce chemical degradations (e.g., oxidation) and physical fragmentation/destruction of CNMs during the ball-milling process, an inert gas such as argon (Ar), NH3, or C2H4 is often used to fill in the milling chambers for BM-CNM preparation.68−70 Chen et al.71 have developed an approach to prepare curved or closed-shell carbon nanostructures by ball milling of graphite in a dry pure Ar atmosphere. Ball milling was carried out in a conventional planetary ball mill at a mass ratio of steel balls:graphite powder = 40:1 for 150 and 250 h, respectively. Under this condition, closed-shell carbon nanostructures, nearly carbon onions, were obtained by bending of the flat sp2 sheets under the heavy mechanical deformation for 150 h. Liu et al.49 reported the formation of shorter and open-ended multiwall carbon nanotubes (MWNTs) by ball milling of MWNTs. During the synthesis, ball milling of MWNTs was conducted in a planetary ball mill in an Ar gas environment to avoid oxidation of the MWNTs. Milled MWNTs were obtained at a rotational speed of 32 rpm for 10 h. Compared to MWNTs without milling (0.11 wt %), milled MWNTs (0.66 wt %) have six times higher hydrogen adsorption capacity, which is ascribed to ball milling increasing the defects and surface area of short MWNTs. Chang et al.72 reported an approach to prepare Co-carbon nanofibers and Co-CNT composites using ball milling in an Ar atmosphere.
such as nitrogen, hydrogen, carboxylic acid, Al, or Mg, can be introduced to the surface and/or edge of BM-CNMs (Figure 1b). The kinetic energy transferred to the carbon powder from the balls is governed by the type of mill (e.g., tumbler ball mills, attrition mill, vibratory tube mills, planetary ball mills, and so on), dry or wet milling, the powder supplied to drive the milling chamber, milling speed, size distribution of the balls, temperature of milling, and the reaction time of milling.57 The tumbler ball mill is a cylindrical container rotated about its axis in which balls impact upon the powder through rolling down the surface of the powders. The attrition mill process provide balls and powders a much higher degree of surface contact than that of the tumbler ball mill due to the vertical rotating shaft with horizontal arms (impellers). For the vibratory tube mill, whose cylindrical container is vibrated, the high milling forces are obtained at high vibrational frequencies and small amplitudes of vibration.65 The planetary ball mill is designed to generate both impact and shear forces to the particles.66 Compared to the other ball mills, planetary ball mills are smaller and mainly used in laboratories. A planetary ball mill consists of at least one grinding jar which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars. For a planetary ball mill, the high dynamic energies, which are generated by the difference in speeds between the balls and grinding jars produces an interaction between frictional and impact forces, resulting in its higher grinding efficiency and 9571
DOI: 10.1021/acssuschemeng.7b02170 ACS Sustainable Chem. Eng. 2017, 5, 9568−9585
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energy ball milling could be energy intensive if long operation time is required to prepare BM-CNMs with desired properties. A feasibility test and cost analysis thus might be necessary for largescale practical applications of the ball-milling technology to CNMs.
Using ammonia or hydrocarbon (C2H4 or CH4) as a protection gas, Xing et al.68 successfully prepared graphene nanosheets by ball milling of its bulk crystal. They found that the obtained graphene remained flat and maintained its single-crystalline structure with a low defect density even after a long period of milling time. When modifiers or reactants such as modification chemicals are added to the milling chambers with the carbon powder, one can produce modified BM-CNMs. Jeon et al.73 reported an approach to introduce nitrogen (N) onto the edges of graphene nanoplatelets (GnPs) to produce edge-nitrogenated GnPs (NGnPs) through dry ball milling in the presence of N2. During the synthesis, 5 g of pristine graphite was placed into a 500 mL stainless steel ball-mill capsule at a graphite:balls mass ratio of 1:100. After charging with N2, the sealed capsule was agitated on a planetary ball-mill machine at 500 rpm for 48 h and subsequent exposure to air moisture to produce NGnPs. The resultant 5.67 g of dark black powder (NGnPs) contained at least 0.67 g of N, indicating the fixation of N2 into the GnPs by ball milling (Figure 2a). In addition to gaseous reactants, liquid solutions with modification chemicals are also used in wet ball milling to prepare the modified BM-CNMs. With wet ball milling of graphene oxide (GO) in the presence of melamine, Zhuang et al.74 have successfully developed a nitrogen-doped GO (N-G) catalyst for electrochemical systems (Figure 2b). During the synthesis, 5 mg of GO and 30 mg of melamine were added into 8 mL of deionized water and heated at 60 °C to dissolve melamine. The mixture was then milled in a steel grinding jar with 6 mL of zirconia grinding balls at 500 rpm for 8, 16, 24, and 32 h. After the decomposition of melamine by thermal treatment, the resultant N-G has a nitrogen content up to 32.7%, which is higher than that of the NGnPs prepared by dry ball milling (11.8%),73 CVD (16%),75 heat treating (10.1%),76 gas annealing (5%),77 and plasma treatment (8.5%).78 The X-ray photoelectron spectroscopy (XPS) confirmed the existence of pyrrolic N−H, pyridinicN, graphitic-N, and pyridinic +N−O− functional groups, indicating that the N-G catalysts have been successfully synthesized by the wet ball-milling approach. In another study, silicon@carbon/graphene sheets (Si@C/G) nanocomposites were synthesized by a novel method combining the advantages of wet ball milling and pyrolysis treatment (Figure 2c).79 Expanded graphite, sucrose, and Si nanoparticles were first mixed in a water:ethanol (1:1, v/v) solution. The suspension was added into a zirconia vial at a zirconia balls:raw material mass ratio of 20:1 and milled for 2 h at 2000 rpm. The Si@C/G composite was then obtained by drying and in situ carbonization of the sucrose coated on the surface of the Si nanoparticles by pyrolysis in a tube furnace for 2 h in an Ar atmosphere with a heating rate of 10 °C/ min to a final temperature of 600 °C. Previous studies have also pointed out some drawbacks of the ball-milling technology to material synthesis, including heterogeneity of the particle size80 and contamination from balls and jars.57 For example, when hard minerals (e.g., quartz sand) are added into the milling chambers with the carbon powder to synthesize modified BM-CNMs, it is difficult to obtain homogeneous grain size due to the difference between the hardness of the minerals and CNMs.80 To reduce particle size heterogeneity, feedstocks of similar hardness should be used to prepare the modified BM-CNMs. When steel balls and containers are used in the milling process, the samples may be contaminated with iron.57 Balls and the container lining of the same or similar alloys as the composition of the samples are often used to minimize the sample contamination. Furthermore, high
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PROPERTIES OF BM-CNMS Ball-milling treatments can bring new properties to the matter depending on the grain size and feedstock compositions.81 In
Figure 3. TEM images of (a) pristine CNTs and (b) ball-milled CNTs (10 h).49
general, BM-CNMs possess a smaller grain size (i.e.,
