Ultrafast microwave-assisted synthesis of highly nitrogen-doped ordered mesoporous carbon
Graphical abstract
Introduction
Mesoporous materials are promising for a wide range of current and emergent applications, including drug delivery [1], batteries [2], and separations [3,4]. Control of the pore size, its connectivity and details of the framework chemistry are all critical to the suitability of the mesoporous material to the application. Templated synthesis either with a hard template [5] or through cooperative self-assembly [6,7] provide a generalized route to fabricating materials with well-defined pore sizes. A wide variety of chemistries, including oxides [8], sulfides [9], metals [10], carbides [11], along with many others [12], have been demonstrated with well-defined templated mesoporous materials. In recent years, ordered mesoporous carbons [13,14] have grown in interest, largely in part from their suitability for energy applications [[15], [16], [17]]. However, high performance typically dictates doping of the carbon framework with heteroatoms for many of these applications [18].
For these energy applications, there tend to be requirements of high heteroatom doping, a high specific surface area in terms of both mass and volume, and pore accessibility along with low cost. One of the most common element to dope carbon is nitrogen [19], which can be readily incorporated into carbon framework due to their similarity in atomic size, but doping of amorphous mesoporous carbons or carbon precursors tends to lead to relatively low specific surface areas in comparison to their undoped carbon analogs [20,21]. The chemical specifics of the nitrogen incorporation into the carbon framework is critical to the properties endowed by the doping [18,22]. The three common bonding configurations for nitrogen within a carbon framework are pyridinic N, pyrrolic N, and graphitic N [22], although oxidized pyridinic N is sometimes considered a common fourth bonding environment. Pyridinic N bonds with two C through the contribution of one p electron (sp2 hybridized), while pyrrolic N contribute two p electrons (sp3 hybridized) while bonded with 2 carbon atoms. Graphitic N directly substitute for carbon in the crystal lattice of graphene with sp2 hybridization. These differences in the bonding state can dramatically influence the performance in electrochemical applications [[23], [24], [25]]. In addition to challenges with control of the nitrogen bonding details within a carbon framework, there are questions about the scaling of common doping processes. For example, chemical vapor deposition (CVD) with nitrogen-containing precursors can produce doped graphene [26], but there are challenges to obtain high productivity and low cost due to its high-vacuum requirement. Alternatively, N-doped mesoporous carbons with high specific surface area can be synthesized via the nanocasting approach [27], commonly including infiltration of nitrogen-containing polymers into the pores of the ordered mesoporous silicas, followed by in-situ carbonization and silica template removal [28], but there are concerns about the economic viability of this nanocasting approach due to cost of the sacrificial template and the etchant for removing the silica. One-pot cooperative assembly to template nitrogen-containing carbon precursors, such as melamine resins and urea-phenol-formaldehyde resin, usually result in poorly defined pore structure, low surface area and relatively low N content [[29], [30], [31]]. Block copolymers containing polyacrylonitrile offer the potential to directly form N-doped carbons, but require custom synthesis by controlled polymerization [32,33]. We have reported a cooperative assembly approach that enables high N doping while mostly maintaining the ordered nanostructure based on the triconstituent assembly of phenolic resin (resol), tetraethyl orthosilicate (TEOS), and Pluronic F127 [34]. The nitrogen dopant, melamine, is added after removal of the F127 template, but prior to carbonization of the resol. The silica in the framework derived from the TEOS was found to be critical to maintaining the porous structure for the highly doped carbon [34], but this added silica will adversely impact electrical properties [35] that are critical to many of the applications of these doped porous carbons.
Here, highly N-doped ordered mesoporous carbons are fabricated in a modified route that avoids any inorganic species, such as silica, within the framework for mechanical reinforcement. Rapid microwave heating carbonizes the framework in less than 3 min to inhibit collapse of the nanostructure. The fabrication involves cooperative assembly of resol and Pluronic F127 within a nickel foam, followed calcination to decompose the Pluronic F127 and crosslink the resol. This mesoporous crosslinked polymer is then infiltrated with melamine to provide the nitrogen source and carbonized by high power microwaves through inductive coupling with the nickel foam. The selection of nickel foam to absorb the microwaves to drive the initial carbonization reaction was motivated by the excellent microwave absorption properties of nickel and the common use of nickel foams as supports [36] for a variety of electrochemical applications, including batteries [37] and supercapacitors [38]. The microwave-assisted synthesis strategy demonstrated here yields ordered mesoporous N-doped carbons with nitrogen doping approaching 20% with large pore size (approximately 7.5 nm). The microwave power provides a facile handle to control the nitrogen incorporated into the framework. These nitrogen-doped ordered mesoporous carbons were investigated for their electrocatalytic performance for the ORR. The activity of these carbons increased with increasing microwave power, which was attributed to the higher concentration of ORR catalytically active pyridinic nitrogen and reduction in the ORR-catalytically inactive pyrrolic nitrogen in the framework. This microwave assisted method provides a simple route to highly doped ordered mesoporous carbons that could be suitable for a variety of electrochemical applications in energy generation, energy storage and sensing.
Section snippets
Materials
Pluronic F127, formaldehyde (ACS reagent, 37 wt% in H2O, contains 10–15% methanol as stabilizer), melamine (99%), sodium hydroxide (NaOH, ACS reagent, ≥97.0%, pellets), phenol (≥99%), Nafion solution (5% in aliphatic alcohol and water), and acetone (>98%) were purchased from Sigma-Aldrich. Ethanol (200 proof, anhydrous) was purchased from Decon Laboratories, Inc. Nickel foam (>99.99%, Fig. S1, Supplementary Materials) was purchased from MTI Corporation. Compressed nitrogen (UN1066, >99.95%) was
Results and discussion
The general synthetic strategy for the fabrication of the nitrogen doped mesoporous carbons is illustrated schematically in Fig. 1. The precursor solution containing resol and Pluronic F127 is infiltrated into the nickel foam. The composition was selected to obtain hexagonally packed cylinders [43]. Solvent evaporation leads to the formation of a micelle-templated coating on the foam. The resol is thermally crosslinked at 100 °C to provide a robust framework for the self-assembled
Conclusions
In conclusion, a direct and facile synthetic route to the fabrication of highly N-doped ordered mesoporous carbon has been demonstrated via a rapid microwave heating method that overcomes the need of inorganic species for mechanical reinforcement. This microwave-assisted synthesis strategy is demonstrated to fabricate N-doped ordered mesoporous carbons with large nitrogen incorporation (up to ~ 20%) and a large average pore size of ~7.5 nm. The microwave power effectively controls the doping
CRediT authorship contribution statement
Xuhui Xia: Investigation, Formal analysis, Validation, Visualization, Writing - original draft, Writing - review & editing. Chung-Fu Cheng: Investigation, Formal analysis. Yu Zhu: Resources, Writing - original draft, Writing - review & editing, Supervision. Bryan D. Vogt: Conceptualization, Project administration, Writing - original draft, Writing - review & editing, Resources, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Science Foundation under Grant No. CBET-1510612. This work used resources of the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors express thanks to Dr. Masafumi Fukuto and Ruipeng Li at NSLS-II. The contributions of Dr. Min Gao at Kent State University for TEM
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Present address: BASF Advanced Chemicals Co., Ltd., Jianxinsha Road 300, Shanghai, 200,137, China.