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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12457−12463

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Watermelon-like Metallic Co/Graphene-like Nanohybrids from Electrochemical Exfoliation of Anthracite Coal as Superior Oxygen Reduction Reaction Electrocatalyst Jianmei Wang† and Congwei Wang*,‡

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Key Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, 18 Xin-kuang-yuan Road, Taiyuan 030024, P. R. China ‡ CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, P. R. China S Supporting Information *

ABSTRACT: Coal has been employed as both a significant energy supply and resource for the production of modern chemicals and novel carbon materials. Here, we present the synthesis of watermelon-like metallic Co/graphene hybrids with coal as the starting material by approaches of catalytic graphitization, electrochemical exfoliation, and in situ functionalization. The ([BMIm]2[CoCl4]) ion liquid acted as both an electrochemical exfoliation electrolyte and active dopant for the catalyst preparation. It is exhibited that the CoCl4−-based intercalator could be effectively expanded into the gallery region of graphitized coal and resulted in exfoliation of graphene-like nanocarbons. Interestingly, the absorbed Co ligands could be converted into watermelon-like metallic Co encapsulated by 1-butyl-3-methylimidzolium ligand-derived carbons. The electrochemical performance of the Co@CEG catalyst exhibited superior ORR activity to benchmark Pt-C with excellent halfwave potential, higher current density, a direct four-electron pathway, superior methanol tolerance, and stability. This work provides an alternative methodology for the production of graphene-like functional nanocarbons from coal with promising potential in energy conversion applications. KEYWORDS: Graphene, Nanocarbon, Metallic Co, Oxygen reduction reaction, Electrochemical exfoliation

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carbon as the evaporation anode;8 carbon nanotubes were fabricated based on thermal plasma, 9 chemical vapor deposition (CVD),10 and arc-discharge methods11 from coal. As a novel two-dimensional carbon material, graphene has attracted significant attention during the past decade from both academia and the industrial market because of its outstanding properties.12,13 Fabricating graphene-like nanocarbons from abundant coal resources has been developed and received great attention, exhibiting significant potentials in supercapacitors, lithium ion batteries, and photoelectro-catalysis.7 Indeed, coal holds abundant polyaromatic structures that are relatively

INTRODUCTION

As an abundant and cost-effective fossil fuel, coal provides most of the energy supply in China contributing to the rapid economic development in last 40 years. Coal could also be exploited as a significant resource for the production of modern chemicals, such as benzene, methanol, glycol, phenol, etc., via the modern coal chemical industry.1,2 Moreover, coalderived carbon materials, such as activated carbon,3 carbon dots,4 carbon molecular sieve,5 and synthetic graphite,6 have been fabricated widely for applications in metallurgy, chemical synthesis, and civil engineering.7 With the discovery and advances of nanocarbons, the development of carbon nanomaterials from this cheapest natural carbon source has triggered vast interests in the past decade. For example, fullerene was prepared via arc discharge with coal-derived © 2019 American Chemical Society

Received: April 11, 2019 Revised: May 19, 2019 Published: June 18, 2019 12457

DOI: 10.1021/acssuschemeng.9b02029 ACS Sustainable Chem. Eng. 2019, 7, 12457−12463

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic graph showing the preparation of coal-derived watermelon-like metallic Co/graphene-like nanocarbon hybrids.

Figure 2. (a−c) TEM images of Co@CEG at different magnifications. Inset in panel (b) shows the particle size distribution of Co species. Inset in panel (c) highlights the clear fringe pattern of metallic Co clusters. (d) SEM image of Co@CEG.

similar to sp2 bonding configurations of graphene; therefore, it is reasonable to expect that coal can be employed as a starting resource for the fabrication of graphene-like nanocarbons. Currently, the most widely used approach is the modified Hummers method.14 Coal would be first graphitized then undergo a severe oxidation process to intercalate hydroxyl, carboxyl, and water molecules, weakening the strong interlayer van der Waals forces, resulting in exfoliated coal-derived graphene-like nanocarbons. However, the usage of strong acid, alkali, and tons of leaching water would inevitably bring rigorous environmental concerns. Therefore, a green and facile method to extract high-quality graphene-like material from earth-abundant coal with less pollution is highly desired. Electrochemical exfoliation has been established as an industrial approach for making graphene, featuring no strong oxidants, recyclable electrolytes, and a scalable setup.15−17 However, to the best of our knowledge, coal has not been

investigated as a starting material for graphene production via electrochemical exfoliation up to now, not to mention further functionalization and energy conversion applications. Herein, we synthesized a watermelon-like metallic cobalt encapsulated in amorphous carbon@graphene-like nanocarbons using coal as the carbon source via an electrochemical exfoliation methodology. The selected ([BMIm]2[CoCl4]) ion liquid acted as not only the electrolyte and intercalator to weaken the interlayer interactions but also as a dopant to in situ provide transition metals and nitrogen precursors simultaneously. The electrochemical exfoliation process was carefully optimized to fabricate high-quality few-layer graphene-like nanocarbons while decorating cobalt species. After thermal annealing, the cobalt precursor would be reduced to metallic Co clusters encapsulated by carbon phases converted from the imidazole content. The obtained catalyst exhibited excellent half-wave potential, higher current density, an efficient four12458

DOI: 10.1021/acssuschemeng.9b02029 ACS Sustainable Chem. Eng. 2019, 7, 12457−12463

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Figure 3. (a) XRD patterns of raw coal, GC, Co@CEG, and standard metallic Co (JCPDS No. 15-0806), (b) Raman spectra, (c) N2 adsorption/ desorption isotherm curves of raw coal and GC Co@CEG (inset shows the BET surface area comparison), (d) pore distribution.

electron pathway, excellent methanol tolerance, and long-term stability. The unique watermelon-like microstructure and composition were probed as the key active sites contributing to its outstanding electrocatalytic performance.

exfoliated graphene-like nanocarbon. It is noted that the electrochemical charging/discharging time was carefully adjusted to optimize the exfoliation process, as shown in Figure S2. The insufficient charging/discharging time would result in inadequate exfoliation of GC, whereas the excessive time would also be a waste of energy and overly adsorb cobalt ions, resulting in superabundant Co particle aggregations in the following pyrolysis, leading to compromised catalytic performance. The high magnification of exfoliated graphene-like nanocarbons and the resultant layer number distribution are exhibited in Figure S3, demonstrating that most of the exfoliated nanosheets were below 10 graphitic layers. Moreover, nanoparticles ranging around 50 nm were also dispersed on this 2D sheet after pyrolysis. Interestingly, even smaller watermelon-like nanocrystals/clusters of 3−5 nm could be observed within these “large” nanoparticles, as highlighted in Figure 2(b, c). These “watermelon seeds” were further identified as Co-based compounds derived from Co species in electrolytes; the “watermelon flesh” was converted from the graphitization of the imidazole content in the electrolyte. Specifically, the CoCl4− ions in the electrolyte would interact closely as intercalators to embed into the gallery region of GC. The electrostatic repulsion between CoCl4− ions and the viscosity of the ionic liquid would prevent Co ion aggregation, resulting in fine Co clusters as “watermelon seeds” after thermal annealing. Meanwhile, the absorbed organic cations would be transferred to the carbon content after graphitization. The particle size distribution of Co clusters is shown in the inset of Figure 2b, where most of the Co species were around 2−3 nm. The lattice fringe d-spacing of these “watermelon seeds” nanocrystals is about 0.180 nm, corresponding to the (200) plane of metallic Co. This unique watermelon-like structure could hold the key for understanding the following superior catalytic enhancement as metallic cobalt has rarely been reported for catalytic activity and stability and is still short of expectations for practical applications.19 However, this unique hierarchical watermelon-like microstructure of Co@ CEG could potentially protect the active cobalt cluster from

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RESULTS AND DISCUSSION The synthesis of coal-derived watermelon-like metallic Co@ graphene-like nanohybrids was carried out by an electrochemical exfoliation and in situ functionalization approach, as schematically shown in Figure 1. Specifically, the grinded fine anthracite powder, whose compositional analysis is shown in Table S1, was first graphitized with the assistance of iron chloride and boric acid as reported before.18 After eliminating the iron compounds by acid leaching, the graphitized coal (GC) powder was mildly exfoliated in an electrolyte ([BMIm]2[CoCl4]) under the cyclic charging/discharging processes (±4 V) and in situ functionalized with Co species simultaneously. After centrifuging to separate the unexfoliated “heavy” GC particle sediment with excessive electrolyte, the collected supernatant containing exfoliated functionalized graphene sheets was then dried and annealed at 550 °C for 2 h to give watermelon-like metallic cobalt encapsulated in amorphous carbon@coal-derived electrochemically exfoliated graphene, denoted as Co@CEG. We used electron microscopy to record each stage of Co@ CEG preparation. Scanning electron microscopy presented that the raw coal powder exhibited a three-dimensional nanoparticle morphology with tens of micrometers, as shown in Figure S1(a). During the graphitization, GC powder retained its particle morphology, but the emerging microsteps at the edges, representing the transformed graphitic layers of GC, could be clearly identified (Figure S1(c)). After the electrochemical exfoliation and purification, the GC nanoparticles were exfoliated into two-dimensional nanosheets, as shown in Figure S1(d). Figure 2 exhibits the typical transmission electron microscopy (TEM) images of obtained Co@CEG at different magnifications, demonstrating the resultant flat surface morphology with random wrinkles of 12459

DOI: 10.1021/acssuschemeng.9b02029 ACS Sustainable Chem. Eng. 2019, 7, 12457−12463

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Figure 4. (a) Long range XPS spectrum of Co@CEG and deconvoluted spectra of (b) N 1s, (c) Co 2p, and (d) C 1s.

catalyst showed distinct porosity of nanopores around 3 nm and mesopore ranges of 10−100 nm, whereas limited porosity was observed in both raw coal and GC, as demonstrated in Figure 3(d). This hierarchical characteristic could provide crucial and favorable accessibility for the transport of reactive species, benefiting the electrocatalytic activity.23−25 X-ray photoelectron spectroscopy (XPS) was examined to illustrate the surface elemental composition and bonding configuration of catalysts. The long-range spectrum of the Co@CEG catalyst portrays the presence of C, Cl, N, O, and Co, as listed in Figure 4(a). Both of the observed Co and N peaks, indicating successful Co and N codoping on the exfoliated 2D nanosheets, derived from the pyrolysis of the [BMIm]2[CoCl4] electrolyte that adsorbed on the graphene surface during the electrochemical exfoliation process. The contents of the above elements are 75.6 at. % (C), 6.1 at. % (Cl), 7.4 at. % (O), 6.2 at. % (Co), and 4.7 at. % (N). It has been widely studied that heteroatoms could contribute to a materials’ electrocatalytic performance significantly;26−28 therefore, a comprehensive investigation of bonding configurations of N and Co is crucial for understanding their electrocatalytic performances and probing associated catalytic mechanisms. The spectral deconvolutions of N 1s peaks were primarily used to probe the configurations of N in Co@CEG, as shown in Figure 4(b). The deconvolution peaks at about 398.7, 399.7, and 400.5 eV correspond to pyridinic N, pyrrolic N, and graphitic N, respectively. Significantly, the ORRfavored graphitic and pyridine N occupied above 90% of the doped N, indicating that the doped N could be potential active sites for enhanced electrocatalytic performances. The bonding configurations of the Co content is shown in Figure 4(c), in which both of the metallic Co0 and Co2+ could be identified. It is noted that most of Co existed as metallic Co as highlighted in both TEM observations and XRD patterns, whereas a Co2+ signal could be attributed to the spot of cobalt oxidized status because of the high reactivity of Co0 and XPS surface sensitivity.

harsh electrolytes and reactive environments while maintaining and realizing high activity.20,21 X-ray diffraction (XRD) reveals the characteristic peaks of high crystallinity of metallic cobalt (JCPDS No. 15-0806) in as-prepared Co@CEG in Figure 3(a), verified in the above TEM observation. The raw coal exhibited relatively low peak intensity, indicating its low crystalline degree. The broad peak ranging from 24.5° to 27° could be assigned to the (002) band of aromatic layers stacking. However, this graphite/carbon present in coal is not in pure mineralogical form, and it is not closely arranged as it contains an inorganic component and also an organic part with volatile substances. After graphitization (>2500 °C), only the graphite phase could be observed with much stronger intensity, indicating a much improved crystalline degree of GC powders. The as-prepared Co@CEG catalyst exhibited a typical (002) peak at 2θ 26.6° that matches well with a multilayer graphene layer and metallic Co peaks around 44.17°, indicating successful decoration with metallic Co clusters. Raman spectra in Figure 3(b) provided the direct structure and defect evolution of graphitic materials. The D band at 1343 cm−1 originated from the structural defects, and the G band at 1560 cm−1 was an index of graphitization. The extent of defects in the catalyst is quantified by the relative intensities of these two bands (ID/IG).22 The raw coal, containing lots of aromatic layers and defects, has the largest ID/IG. After high temperature treatment, the graphitization degree of GC and Co@CEG was restored significantly, exhibiting a much decreased ID/IG. It is noted that Co@CEG was slightly higher than that of GC becaue of the decoration of Co and nitrogen heteroatoms, both of which would be beneficial for the electrocatalytic reactions as potential active sites. Brunauer− Emmett−Teller (BET) surface area and pore size distribution were measured by N2 adsorption−desorption analysis as shown in Figure 3(c). The exfoliated two-dimensional Co@ CEG exhibited the largest BET surface area of 97.2 m2g−1 with a type IV behavior hysteresis loop, which is about seven times higher than that of unexfoliated GC powder. The Co@CEG 12460

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Figure 5. (a) CV curves of Co@CEG in O2 and N2 saturated electrolyte. (b) Polarization curves of raw coal, GC, GC+Co, EG+Co, Co@CEG, and commercial Pt-C. (c) LSV curves of Co@CEG at different rotation speeds, ranging from 400 to 2500 rpm (inset shows the K-L plot of Co@CEG). (d) Tafel plots of various catalysts. (e) Durability of electrodes. (f) Current density loss-time CA responses of Co@CEG and Pt-C electrodes at −0.3 V in oxygen saturated electrolyte, rotating speed 1200 rpm. The arrow indicates the addition of 3 M methanol into the electrochemical cell.

sample, the introduction of metallic Co nanoparticles could act as potential active sites, exhibiting much improved halfwave potential about −0.25 V, but its limited surface area and large Co particle were not preferred for the electrocatalytic process. Though EG+Co exhibited better activity than GC +Co, indicating that the exfoliated graphene could facilitate the ORR process, this two-step approach faces the relatively complicated exfoliation process plus an additional functionalization process. For the Co@CEG catalyst, the unique watermelon-like microstructural metallic Co clusters and nitrogen-doped graphene hybrids demonstrated surprisingly enhanced ORR performance with comparable half-wave potential with a commercial Pt-C (20 wt %) catalyst. Moreover, the abundant active sites in Co@CEG resulted in an expressively boosted current density, which is 770 times higher than raw coal and nearly 3 times that of GC. This excellent activity could be attributed the proper modification of electronic structures of [email protected],32 Specifically, electron transfer would be expected among metallic Co, carbon “watermelon flesh”, and graphene-like nanocarbons via affecting the position of the Fermi energy and the surface work function of the catalyst, resulting in the facilitation of adsorption and reactions on the surface of the whole “watermelon-like” structure.33 This superior activity illustrates that our strategy for electrochemically exfoliating graphitized coal with in situ functionalization could significantly benefit the

Electrocatalytic performances of the exfoliated catalyst were investigated comparatively using an Autolab electrochemical analyzer (PGSTAT204). To adequately understand the influences of both watermelon-like metallic Co encapsulated in amorphous carbon@N-doped graphene, as active sites toward ORR, CV curves were first recorded. The measured CV curves of the Co@CEG catalyst is exhibited in Figure 5(a). It is illustrated that a distinct cathodic peak centered at −0.23 V (vs Ag/AgCl) could be observed in O2 saturated 0.1 M KOH, indicating its outstanding ORR activity. In order to comparatively probe and explore the kinetics of the ORR process, an investigation with linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) was employed. A detailed LSV investigation of Co@CEG along with controlling samples and commercial Pt-C (20 wt %) is presented in Figure 5(b), in which Co@CEG exhibited the best ORR activity. Raw coal shows rare ORR activity because of the lack of active sites, limited surface area, and poor conductivity. Though graphitization could significantly enhance the material’s conductivity as shown in Figure S4, the greatly restored sp2 hybridized hexagonal lattice could only provide limited active sites, displaying an unfavorable negative onset potential of −0.47 V (vs Ag/AgCl). It is noted that the unexpected slightly larger semicircle of Co@CEG impedance arcs reflects the complex interfacial charge-transfer resistance brought by the unique hierarchical microstructure.29,30 For the GC+Co control 12461

DOI: 10.1021/acssuschemeng.9b02029 ACS Sustainable Chem. Eng. 2019, 7, 12457−12463

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electrocatalyst for the oxygen reduction reaction. Our methodology pursues a relatively complete utilization of the raw materials, especially that the electrolyte could be recycled, meeting the standard of green chemical synthesis. In situ decoration of watermelon-like metallic cobalt and nitrogen heteroatoms results in the excellent combination of the conductive graphene-like network, with a much enhanced surface area with uniformly dispersed encapsulated watermelon-like metallic cobalt. The exfoliated Co@CEG catalyst exhibited superior ORR electrocatalytic activity to benchmark Pt-C with outstanding onset/half-wave potential, higher catalytic current density, an efficient four-electron pathway, excellent methanol crossover tolerance, and long-term stability. The key active sites for boosting the ORR activity lie in the unique hierarchical watermelon-like microstructure, in which the active Co clusters could not only promote the electron transfer at the metal/carbon interfaces but could also protect the surrounding carbon from corrosion, showing great promise for efficient electrochemical energy conversion. Therefore, our strategies could lead to a new green approach for fabricating a high-end electrocatalyst using cheap and abundant coal as the carbon source with novel structures.

material’s ORR performance via constructing a preferred microstructure and compositions, which is among the top level of metallic Co/graphene hybrids reported for ORR, as highlighted in Table S2. Additionally, the preparation process has also been optimized to improve the catalyst’s performance. Samples obtained at different electrochemical exfoliation times were tested to evaluate their performances, as shown in Figure S5(a). GC exfoliated at 48 h (Co@CEG) exhibited the most desirable positive onset potential, whereas insufficient or excessive exfoliation time could compromise their performances due to the incomplete exfoliation or superabundant Co particle aggregations, as highlighted in Figure S2. Various annealing temperatures were also applied to the exfoliated GC, and the sample obtained at 550 °C (Co@CEG) exhibited the most favorable activity compared with those annealed at 450/ 650 °C, as shown in Figure 5(b). This temperature dependency could be the balance between the fine dispersion and size of Co species after annealing (relatively low temperature preferred) and the enhancement of conductivity and heteroatom doping levels (relatively high temperature preferred). A Koutecky−Levich (K-L) plot was further calculated to reveal the catalytic mechanism and the number of electrons transferred (n) during ORR in the alkaline medium. As shown in Figure 5(c), a series of LSV curves for Co@CEG was recorded from 400 to 2500 rpm, and it is shown that with the increment of rotation speed, the current density gradually increased correspondingly because of the shortening of the diffusion distance at higher speed. The K-L plot for Co@CEG is shown in the inset, wherein a good linearity and parallelism of the plot suggest first-order reaction kinetics toward the electrochemical reduction of oxygen. The resultant n number is 3.97 at potentials from −0.25 to −0.40 V, indicating a more preferred four-electron reaction pathway and a direct formation of water. Tafel plots of various catalysts are presented in Figure 5(d), in which Co@CEG exhibited one of the most preferred performances in terms of Tafel slope with the closeness of benchmark Pt-C. Both the stability and tolerance to methanol crossover poisoning are important considerations for practical applications. The stability of Co@ CEG with respect to Pt-C was evaluated via chronoamperometric (CA) measurements. As demonstrated in Figure 5(e), the CA response for Co@CEG exhibited slow attenuation with considerable current retention of 82.88% after 15,000 s, whereas Pt-C showed a much faster degradation with 72.05% retention, indicating the much better stability of the watermelon-like encapsulated metallic Co microstructure from harsh electrolyte corrosion. Furthermore, the methanol crossover effect was also assessed by methanol injection. It is shown that no obvious performance decay was identified for Co@CEG after methanol injection at about 150 s, whereas a sharp jump in the current retention appeared for the Pt-C catalyst, indicating an excellent ability to avoid crossover poisoning.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02029.

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Experimental section: chemicals; preparation of watermelon-like metallic Co/graphene-like nanocarbon hybrids (Co@CEG) and control sample; instrumental characterization; electrochemical measurements; analysis data of raw coal, TEM images of sample obtained at different exfoliation times; EIS Nyquist plots of samples; comparison of recently reported metallic Co/graphene based ORR catalysts; ORR LSV curves of catalysts obtained at different exfoliation times and annealing temperatures. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Congwei Wang: 0000-0001-7316-870X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51502320) and Shanxi Scholarship Council of China (2016-142).

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CONCLUSIONS In summary, a fully green strategy has been reported to synthesize a coal-derived watermelon-like metallic cobalt encapsulated in amorphous carbon@graphene-like nanocarbon using graphitized coal as the carbon sources based on electrochemical exfoliation with in situ functionalization. During this process, the raw coal and electrolyte([BMIm]2[CoCl4]) have been converted into two-dimensional nanosheets with metallic Co, which was employed as the

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