An Ideal Electrode Material, 3D Surface-Microporous Graphene for

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An Ideal Electrode Material, 3D Surface-Microporous Graphene for Supercapacitors with Ultrahigh Areal Capacitance Liang Chang,† Dario J. Stacchiola,‡ and Yun Hang Hu*,† †

Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931-1295, United States ‡ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: The efficient charge accumulation of an ideal supercapacitor electrode requires abundant micropores and its fast electrolyte-ions transport prefers meso/macropores. However, current electrode materials cannot meet both requirements, resulting in poor performance. Herein, we creatively constructed three-dimensional cabbage-coral-like graphene as an ideal electrode material, in which meso/macro channels are formed by graphene walls and rich micropores are incorporated in the surface layer of the graphene walls. The unique 3D graphene material can achieve a high gravimetric capacitance of 200 F/g with aqueous electrolyte, 3 times larger than that of commercially used activated carbon (70.8 F/g). Furthermore, it can reach an ultrahigh areal capacitance of 1.28 F/cm2 and excellent rate capability (83.5% from 0.5 to 10 A/g) as well as high cycling stability (86.2% retention after 5000 cycles). The excellent electric double-layer performance of the 3D graphene electrode can be attributed to the fast electrolyte ion transport in the meso/macro channels and the rapid and reversible charge adsorption with negligible transport distance in the surface micropores. KEYWORDS: cabbage-coral-like graphene, surface micropores, electric double-layer capacitors, ultrahigh areal capacitance, scalable electrodes

1. INTRODUCTION Electric double-layer capacitors (EDLCs) store energy through electrostatic charge accumulations on the surface of porous electrodes under applied voltage.1−4 EDLCs possess higher energy density than conventional capacitors and larger power density than batteries with excellent reversibility and long-life expectation as well as wide range operation.5−8 Those unique features make EDLCs an indispensable part of renewable energy systems to collect and utilize intermittent wind power, hydro power, and solar energy.9−12 EDLCs are usually composed of two symmetrically porous carbon electrodes with a separator and electrolyte between them. The performance of EDLCs is strongly dependent on the surface area and pore structure of carbon electrodes.13−15 It is generally recognized that micropores are beneficial for charge accumulation to form electric double layers due to enhanced surface area and enriched active sites.16,17 However, microporous electrode materials often suffer the inhibited transport of electrolyte ions, leading to inaccessible surface areas and thus poor electrochemical behavior. Activated carbon (AC), which is a representative microporous material with surface areas up to thousands of m2/g, is widely used in commercialized EDLCs. However, the capacitance of AC electrodes possesses a limited capacitance of 40−70 F/g. Great efforts were made to improve its performance by expanding the micropores to meso/ macropores, but the enhancement is far below expectation.18−21 © XXXX American Chemical Society

To achieve better EDLC performance, an ideal electrode material should possess a large proportion of micropores for efficient electric double-layer formation without the transport limitation issue of electrolyte ions in micropores.22−24 Graphene has attracted much attention for supercapacitors due to its unique mechanical and electrical properties as well as large surface area. A single layer graphene sheet possesses a large theoretical surface area of 2630 m2/g with an ultrahigh electrical conductivity, predicting a high theoretical capacitance of 526 F/g.25 However, so far, the obtained capacitance of 2Dgraphene-based supercapacitors is still far below the theoretical value (especially for a large mass loading), because the aggregation of graphene sheets inhibits electrolyte ions to contact all surface area.26,27 To solve the issue, post-treatment strategies for 2D graphene were exploited for the enhancement of its electrochemical performance, including chemical surface modification,28 KOH-activation,29 3D graphene self-assembly,30 novel laser reduction,31 and electrolyte-mediated chemical conversion.32 Different from those technologies, we recently synthesized a new type of graphene sheets with a 3D honeycomb-like structure (HSG) via an invented reaction between Li2O and CO.33 Furthermore, we combined our Received: May 24, 2017 Accepted: July 3, 2017 Published: July 3, 2017 A

DOI: 10.1021/acsami.7b07381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Characterization of 3D SMG: (A) FESEM image, (B) TEM image, (C) XRD pattern, and (D) N2 adsorption/desorption curves at 77 K.

strengths in the synthesis of 3D graphene sheets and the conversion of CO2 greenhouse gas, leading to the successful synthesis of 3D cauliflower-fungus-like graphene (CFG) directly from CO2 via a one-step reaction.34 The 3D CFG was explored for supercapacitors, resulting in a gravimetric capacitance of 112.4 F/g (with total surface area of 462 m2/g). In addition, we synthesized porous Na-embedded carbon nanowalls (Na@C) and employed it for supercapacitors, leading to a gravimetric capacitance of 143.2 F/g (with total surface area of 555 m2/g).35 Although those two carbon nanomaterials are mesoporous materials with fully accessible surface areas, their total surface areas are not high enough to obtain very high capacitance. This has stimulated us to design and synthesize new types of graphene materials for better electrochemical performance. Herein, we demonstrate a new strategy to create a novel 3D surface-microporous graphene (SMG), which can meet two requirements for an ideal supercapacitor electrode: its fully accessible surface micropores ensure efficient charge accumulation, and its meso/macro channels provide fast paths for electrolyte-ion transport. As a result, a high gravimetric capacitance of 200 F/g was obtained with 83.5% retention associated with 20 times increase in current density. Furthermore, the SMG electrode exhibited an ultrahigh areal capacitance of 1.28 F/cm2.

Figure 2. Illustration of 3D SMG structure: Meso/macro channels are formed by graphene walls, and micropores are created in the surface layer of the graphene walls.

simple and novel approach to synthesize a novel material, 3D surface-microporous graphene (SMG): (1) CO2 reacts with Na liquid to generate graphene sheets and Na2CO3 nanoparticles, (2) the produced Na2CO3 plays a role to prevent the aggregation of graphene sheets and determine their shapes, (3) CO2 can in situ oxidize graphene walls to create surface micropores, and (4) the obtained solid products can be treated by an aqueous solution of HCl to remove Na2CO3 nanoparticles, generating meso/macro channels in the 3D graphene. The feasibility of this strategy was confirmed by the following experiments. Na cubes (1 g) were loaded into a ceramic tube reactor, and 50 psi CO2 was then introduced into the reactor, followed by increasing the temperature to 520 °C and maintaining the temperature for 12 h, leading to 90% conversion of Na. The

2. RESULTS AND DISCUSSION We invented the reaction between Na liquid and CO2 gas for graphene synthesis:36 4Na(l) + 3CO2 (g) = C(graphene)(s) + 2Na 2CO3(s)

The reaction is thermodynamically favorable with a negative Gibbs free energy change (−627 kJ/mol) and enthalpy change (−1063 kJ/mol). Herein, based on this reaction, we propose a B

DOI: 10.1021/acsami.7b07381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Electrochemical performance of AC and 3D SMG electrodes: (A) CV curves at a scan rate of 100 mV/s, (B) galvanostatic charge/ discharge profiles at a current density of 1 A/g, (C) current densities vs specific capacitances, and (D) Nyquist plot with 0.01−105 frequencies.

90% of the surface area of the graphene originates from micropores. The average diameter of micropores, which was obtained from N2 adsorption measurement, is 1.6 nm. To estimate the average deepness of micropores, we treat the micropores as typical cylinder pores and thus express their wall surface area (Swall) (excluding their bottom area) as

mechanism of this reaction is described in Figure S1. The solid products were treated with 37 wt % HCl overnight, followed by deionized water washing for several times until PH = 7 and drying at 80 °C for overnight. The obtained black carbon product was subjected to characterizations. The field emission scanning electron microscopy (FESEM) image shows cabbagecoral-like graphene with meso/macro channels (Figures 1A and S2A). The widths of the channels (which were built by coiled and bent graphene sheets) ranged from 200 nm to 1 um. The 3D cabbage-coral-like structure of the graphene sheets was further confirmed by the transmission electron microscopy (TEM) image in Figure 1B, and the 2.2 nm thickness for each of the graphene sheets is shown in Figure S2B. Furthermore, the lattice spacing of graphene layers, which was calculated from the XRD pattern (Figure 1C), is 3.55 Å. Therefore, each of graphene sheet consists of 6 graphene layers. The defects of the 3D graphene were evaluated by Raman spectra. As shown in Figure S3A, one can see a D-band at 1350 cm −1 (corresponding to sp3 carbon atoms caused by disordering graphite) and a G-band at 1580 cm−1 (corresponding to sp2 carbon atoms due to ordered graphite). The intensity ratio of the D-band and G-band (ID/IG) is 0.94, indicating rich defects in the 3D graphene sheets, which would be due to the formation of micropores, as demonstrated in the following section. The surface area and pore structure of the 3D graphene were evaluated by N2 adsorption at 77 K (Figure 1D). Its total surface area is 890 m2/g, in which micropores contribute 789 m2/g as microporous surface area. This indicates that almost

Swall = nπDh

(1)

where n is the number of micropores per gram of graphene, D the average diameter of micropores, and h the average deepness of the micropores. The number of micropores per gram of graphene and the wall surface area are 1.68 × 1020 (1/g) and 452 (m2/g), respectively (see the Supporting Information). Using eq 1, we obtained a very small average deepness (h) of 0.54 nm, indicating that the micropores are only in the surface graphene layer. We can illuminate this novel 3D SMG material in Figure 2. The bent and coiled graphene sheets constitute interconnected meso/macro channels with a cabbage-coral-like shape. This well-defined morphology can act as a reservoir to provide a buffer for electrolyte, leading to a very limited electrolyte diffusion distance. Furthermore, the surface micropores of the graphene walls can directly touch with the buffered electrolyte ions in the pore channels, which makes ions can rapidly diffuse with negligible transport distance in the meso/ macro channels and form efficient electric double layers on the micropore surface at a high rate. Thus, the 3D SMG electrode can be expected to achieve high capacitance and excellent rate performance for EDLCs. C

DOI: 10.1021/acsami.7b07381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Electrochemical performance of the 3D SMG electrode: (A) CV curves at scan rates from 30 to 300 mV/s, (B) galvanostatic charge/ discharge profiles at current densities of 0.5−10 A/g, (C) current densities vs specific capacitances, and (D) cycling performance at 10 A/g.

into account, theoretical specific gravimetric capacitances of AC and 3D SMG electrodes are 170.1 and 178.0 F/g, respectively. The theoretical capacitance of the AC electrode is much larger than its experimental value (70.8 F/g), indicating that most of the AC surface area is not accessible for electrolyte ions. In contrast, all of the surface area of SMG is fully accessible for electrolyte ions, reflected by the larger specific gravimetric capacitance (200 F/g) of 3D SMG than its theoretical value. Those indicate that, totally different from conventional micropores, SMG surface micropores can be fully utilized for charge accumulation to form efficient electric double layers. This happened because the transport distance of electrolyte ions is negligible in the surface micropores due to their very small deepness (0.54 nm). The excellent electric double-layer behavior of the 3D SMG electrodes was further demonstrated by the following observations: only slight polarization in CV curves and invisible IR drop appeared in galvanostatic charge/ discharge profiles, even when the scan rate was increased from 30 to 300 mV/s or current density from 0.5 to 10 A/g (Figure 4A,B). It should be noted that the 3D SMG cell was first charged at a current density of 0.5 A/g, leading to the apparent polarization (Figure 4B). This happened because the 3D SMG cell was being activated during the first charge, which was confirmed by the disappearance of the polarization in the second charge (Figure S5). Furthermore, the 3D SMG electrode exhibited excellent rate capability; namely, when the current density was increased by 20 times, the capacitance decreased from 200 to 167 F/g, indicating 83.5% capacitance retention (Figure 4C). In addition, the 3D SMG electrode is

To experimentally demonstrate its excellent electrochemical performance, 3D SMG was used to fabricate two identical electrodes for a symmetrically two-electrode supercapacitor cell with a glassy fiber separator and 2 M KOH electrolyte. As shown in Figure 3A, one can see that the cyclic voltammetry (CV) curve of 3D SMG electrodes possesses an ideal rectangular shape, indicating a reversible charge adsorption/ desorption process.37 The excellent electric double-layer behavior is further confirmed by the symmetrical triangle in the galvanostatic charge/discharge profile (Figure 3B). As a surface-microporous material, 3D SMG (200 F/g) reached 3 times larger gravimetric capacitance than conventional-microporous AC (70.8 F/g) at a current density of 0.5 A/g, although both materials have comparable surface areas. This indicates that other factors, such as pore morphology and pore distribution, can also affect the formation of electric double layers.38,39 The excellent EDLC performance of 3D SMG can be maintained even when the current density increases to 10 A/ g (Figure 3C). The huge difference in EDLC performance between 3D SMG and AC electrodes is also reflected by CV areas (Figure 3A) and charge/discharge times (Figure 3B). Furthermore, Nyquist plots were employed to evaluate ion diffusions in the SMG electrodes, because the greater slope in the low frequency region would reflect a faster ion diffusion.40 As shown in Figure 3D, one can see that the slope in the low frequency region is much larger for the SMG electrode than for the AC electrode, indicating that the SMG electrode possesses a faster ion-diffusion rate than the AC electrode. If total surface areas of AC (853 m2/g) and 3D SMG (890 m2/g) are taken D

DOI: 10.1021/acsami.7b07381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Electrochemical performance of 3D SMG electrodes with different mass loadings (T1 for 2.8 mg/cm2 and T2 for 6.4 mg/cm2): (A) CV curves at a scan rate of 20 mV/s, (B) galvanostatic charge/discharge profiles at a current density of 2 A/g, (C) current densities vs specific capacitances, and (D) areal capacitances at different current densities.

very stable. After 5000 galvanostatic charge/discharge cycles at a current density of 10 A/g, capacitance retention is as large as 86.2% (Figure 4D). However, the retention is slightly smaller than those reported in recent published works.4,23 The 3D SMG electrode also showed excellent scalability for supercapacitors. When the mass loading of the 3D SMG electrode increased from T1 (2.8 mg/cm2) to T2 (6.4 mg/ cm2), the rectangular CV curve at 20 mV/s and symmetrical charge/discharge triangle curve at 2 A/g remained unchanged (Figure 5A,B), leading to almost constant capacitance (namely, from 200 to 199.7 F/g). When the current density was increased by 10 times, the electrode still showed excellent electrochemical performance (Figure 5C). This indicates that the 3D SMG electrode is suitable for fast charging/discharging even with high mass loading. Furthermore, the scalable feature of the SMG electrode creates its ultrahigh areal capacitance of 1.28 F/cm2 (Figure 5D). As shown above, the gravimetric capacitance (200 F/g) of the 3D SMG electrode is even larger than its theoretical value (178.0 F/g). The exceeding capacitance over the theoretical value might be attributed to a redox reaction of functional groups. This was supported by energy dispersive spectroscopy (EDS) measurement, which shows the oxygen content of 4.88% in the SMG. Furthermore, the oxygen functional groups were further evaluated with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). As shown in Figure S6A, the C 1s XPS curve can be deconvoluated into 4 peaks. The two peaks at 286.2 and 288.1 eV can be assigned to C−O and CO groups, respectively. The other

peaks at 284.2 and 290.1 eV are associated with CC and π−π*, respectively.41 The results can be further verified by Fourier transform infrared spectroscopy (FTIR); namely, the C−O and CC groups were further confirmed by their stretching IR bands at 1064 and 1537 cm−1 (Figure S6B).42 Those oxygen functional groups in the 3D SMG not only provide redox capacitance but also improve wettability and thus benefit electrochemical performance.

3. CONCLUSION In summary, a novel strategy was demonstrated to synthesize an ideal supercapacitor electrode material, 3D surface-microporous graphene (3D SMG) with a cabbage-coral-like structure. In this unique material, the meso/macro channels (formed by graphene walls) provide excellent paths for fast electrolyte ion transport, and the rich micropores in the graphene surface layer are efficient for charge accumulation to form efficient electric double layers. Consequently, the 3D SMG electrode can achieve a high gravimetric capacitance of 200 F/g with aqueous electrolyte, 3 times larger than that of commercially used activated carbon (70.8 F/g). Furthermore, it exhibited an ultrahigh areal capacitance of 1.28 F/cm2 and excellent rate capability (83.5% from 0.5 to 10 A/g) as well as impressive cycling stability (86.2% retention after 5000 cycles).

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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/acsami.7b07381. E

DOI: 10.1021/acsami.7b07381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Experimental section, characterization of SMG, characterization of activated carbon, calculation of average micropore deepness of SMG, different reaction condition influence on materials characterization and its corresponding electrochemical performance, and electrochemical performance of SMG in three-electrode configuration (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yun Hang Hu: 0000-0002-5358-8667 Author Contributions

Y.H.H. supervised the project. L.C. synthesized materials, fabricated devices, tested device performances, and conducted material characterizations. D.J.S. conducted the XPS measurements with deep analysis. All authors were involved in analysis and discussion of results. Y.H.H. and L.C. wrote the manuscript with input from all other authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (CBET-0931587 and CMMI-1661699). L.C. and Y.H.H. also thank Charles and Carroll McArthur for their great support.

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DOI: 10.1021/acsami.7b07381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX