Letter www.acsaem.org
Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Engineering Microsized Materials through Enhanced Colloidal Interactions of Graphene for Ultrahigh-Mass-Loading and Flexible Electrodes Yi Zhou, Runjing Zhang, Jiahe Wang, Xiaojun Yan, Congcong Liu, and Xiaowei Yang* Interdisciplinary Materials Research Center, Key Laboratory of Advanced Civil Engineering Materials (Ministry of Education), School of Materials Science and Engineering, Tongji University, Shanghai 201804, China
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S Supporting Information *
ABSTRACT: High-capacity, high-mass-loading yet low-cost electrodes are highly desirable for flexible energy-storage devices. However, previous research on flexible electrodes was mainly limited in low mass loadings of nanostructured materials, and the promising properties will be seriously degraded in practical levels of mass loadings. Taking advantages of the fantastic colloidal interactions of graphene, microsized commercial particles were processed into high-mass-loading yet flexible electrodes by using graphene as the dispersant and flexible conductive adhesives. Involved liquids can not only ensure the easily accessed pathway for electrolytes but also help to relieve the stress and strain under deformation. This electrode exhibits a superb areal capacitance of 8.6 F cm−2 at the mass loading as high as 40 mg cm−2, and ultrahigh power/energy density (50000 μW cm−2/377 μWh cm−2). This work provides a universal approach to fabricate flexible electrodes for various energy storage devices and highlight irreplaceable roles of colloidal interactions in this flexible electrode. KEYWORDS: colloidal interaction, microsized material, graphene, flexible, high areal capacitance, mass loading, supercapacitor n recent years, flexible energy storage devices have received tremendous attention for their potential applications in modern wearable and portable electronic equipment.1−3 A vital component of flexible energy-storage devices is mechanically strong, low-cost, stable, and high-specific-capacity electrodes.4−7 In previous research, the most flexible electrodes were achieved with nanostructured active materials, because their small sizes and large surface energy are favorable for assembling into integral structures evenly.8−13 However, such flexible electrodes always have low mass loading (less than 1 mg cm−2) and can hardly be scaled up to practical levels (∼10 mg cm−2) because of the increasing limitation for ion diffusion and electron conduction as well as the strain and stress in bending deformation.14−16 The promising properties achieved with ultrathin electrodes will be seriously degraded in practical devices because other components such as substrates, separators, and current collectors (totally more than 10 mg cm−2) should be included.17,18 Thus, to attain high-performance flexible energy-storage devices, the main challenge is to sustain the satisfactory electrochemical and mechanical properties at high mass loading.19−24 In today’s energy storage devices, commercial active materials, such as activated carbon (AC) or commercial LiFePO4 powder, can exhibit reasonable electrochemical performances in practical-level areal mass loading (>10 mg cm2). And, in view of the cost, mass production, and stability, these are currently the most desirable and widely used active
materials.25,26 However, strategies for flexible electrodes reported before were designed for nanostructured materials,14 which are hardly compatible with commercial active materials because of their micrometer size and granular shaped morphology. These inorganic microsized particles also have weak interactions and have to rely on polymer adhesives to fabricate integral electrodes. Therefore, in order to meet the actual application requirement of flexible energy storage devices, it is still an urgency to find an effective way to fabricate commercial microsized active materials into flexible and high-performances electrodes. As known, the solution processability could allow simple and effective ways for preparing flexible films, which mainly depend on the colloidal chemistry properties of materials. Previous research has proved that chemically converted graphene (CCG) has numerous oxygen-containing functional groups that result in hydrophilic domains.27,28 Thus, we have obtained layered graphene hydrogel film through simple vacuum filtration of CCG dispersion and revealed that liquid can act as an effective spacer to ensure the excellent pore connectivity for ionic transport.29−31 What’s more, the amphiphilic property and high aspect ratio make CCG a kind of dispersant,
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© XXXX American Chemical Society
Received: April 8, 2018 Accepted: June 18, 2018 Published: June 18, 2018 A
DOI: 10.1021/acsaem.8b00564 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 1. (a) Schematic illustration of activated carbon dispersed in chemical converted graphene dispersion and self-assembled into a freestanding hybrid hydrogel through vacuum filtration process. (b) Side view cross-section of vacuum-dried AC/CCG film. Digital photographs of AC/CCG hydrogel film with the areal mass loadings of (c) 1 and (d) 10 mg cm−2.
Figure 2. SEM images of cross-section of AC/CCG a) hydrogel film, (b) vacuum-dried film, and (c) freeze-dried film. Maximum bending angle of AC/CCG (d) hydrogel film, (e) vacuum-dried film, and (f) freeze-dried film with the same length. Schematic illustrations about stretching and folding mechanism and structure of AC/CCG (g) hydrogel film, (h) vacuum-dried film, and (i) freeze-dried film.
Herein, we demonstrate that commercial microsized active materials can be easily dispersed with chemically converted graphene and self-assemble into a freestanding and flexible hybrid hydrogel film in a vacuum filtration process, which provides a general approach to fabricate flexible electrodes. The integral hydrogel film based on activated carbon and chemically converted graphene can exhibit excellent capacitance (206 F g−1 at 1 A g−1) and rate performance (about 70% retention at 100 A g−1). Even though the mass ratio of AC is more than 80%, this simply fabricated all-carbon hydrogel possesses excellent flexibility and stability. The highly open ionic pathway and continuous three-dimensional conductive network of this all-carbon hydrogel can also lead to an ultrahigh areal capacitance. After the mass loading is increased by 40 times from 1 to 40 mg cm−2, the capacitance retention is
indicating that it has strong contacts with hydrophobic particles in the presence of water.32−35 Shi et al. have demonstarted that CCG can assemble into a 3D framework for encapsulating AC particles.36 These results implied that the large molecular size of CCG and the subtle equilibrium between colloidal interactions, including electrostatic force, hydrophilic, hydrophobic, van der Waals attraction, and solvation forces, can make this conducting carbon material analogous to soft matters such as ink, surfactant, or glue.37−39 Hence, we believe that liquid and CCG together can work as the dispersant and flexible conductive adhesive for a wide range of active materials. The liquid-rich structure can not only ensure the easily accessed pathway for electrolyte but also help to relieve the stress and strain under deformation, which is highly desirable for high-mass-loaded flexible electrodes. B
DOI: 10.1021/acsaem.8b00564 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 3. (a) Specific capacitance of AC/CCG hydrogel film and dried films at different charge−discharge rates. (b) Nyquist plots of AC/CCG hydrogel film and dried films. (c) Cyclic life of AC/CCG hydrogel film and dried films. (d) Flexible supercapacitor based on AC/CCG hydrogel film under different bending states at 5 mV s−1. Digital photograph shows the flexible supercapacitor when bended.
still more than 82.5% (about 170 F g−1) at 1 A g−1, resulting in an ultrahigh areal capacitance of more than 6.8 F cm−2. Figure 1a shows the preparation process of the AC/CCG hydrogel film. Commercial granular activated carbon can be easily dispersed in CCG dispersion due to the π−π interaction and hydrophobic interaction between them, resulting in a solution-processable suspension.32−35 Then the vacuum filtration process can be applied to prepare the AC/CCG hydrogel film (Supporting Information Figure S1). In this slow deposition process, CCG layers and AC particles can selfassemble together at the liquid−solid interface to form an integral structure.29−31,40 The AC/CCG hydrogel film was peeled off from the filter membrane once the liquid disappeared and immersed in DI water before use to prevent getting dry. SEM images (Figure 1b) of vacuum-dried sample confirmed that the structure of the AC/CCG hydrogel is uniform. The continuous conducting network also makes this hybrid hydrogel highly conductive (about 88 S m−1). The XPS and Raman spectra show typical curves of AC and CCG (Figure S4). And although the CCG and AC were mixed physically without covalent binding, the hybrid hydrogel film still has reasonable mechanical strength and flexibility even when the mass loading is up to 10 mg cm−2 (Figure 1d). To ascertain the importance of liquid to this hybrid hydrogel electrode, we prepared dried samples with different processes. After vacuum drying (VD), the composite film shrunk by about a quarter in both thickness and lateral size, and its density increased from 0.21 to 0.51 g cm−3. The freeze-drying
(FD) process can reduce the capillary effect, the sample only shrinks in thickness to about 75% with little shrinkage in the lateral dimension, and its density was about 0.28 g cm−3 (Figures S5, S6, and S7). Although 80% of solid component in this composite film is granular activated carbon, the AC/CCG hydrogel film can still display an ultimate tensile stress of 80 kPa (Figure S8), comparable to that of conventional organic hydrogel,41,42 together with an ultimate tensile strain of about 0.8%. However, both the FD sample and the VD sample are too fragile to apply mechanical tests. As shown in Figure 2, after the vacuum-drying process, the capillarity forced the CCG to wrap AC particles very closely and made the structure disordered and inhomogeneous. Because of the brittle nature of carbon materials, this composite film was too fragile to bend more than 45°. The CCG network in FD samples show an oriented layered structure with sandwiched AC particles, similar to that of oriented graphene/nanomaterials composites, except for the more than 5 μm particle size of AC,15,34,35 implying that even mixed with other components, CCG can self-assemble into an ordered structure in this filtration process. Because of the relatively ordered and crumpled structure of CCG, it may partially unfold and deform along the deformation direction to disperse strain.43 However, the large proportion of granular material seriously damaged the mechanical properties of the CCG network; the FD sample can still hardly bend over 90°. Although the profile of the hydrogel film is blurred by liquid, we can speculate that the AC/CCG hydrogel film has a relatively oriented structure C
DOI: 10.1021/acsaem.8b00564 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 4. (a) Specific capacitance (F g−1) vs current density for electrodes with different areal mass loadings. (b) Areal capacitance vs areal mass loading at different charge−discharge rates. (c) Nyquist plots of AC/CCG hydrogel film with different mass loadings. (d) Ragone plots of supercapacitors based on AC/CCG hydrogel film with different mass loadings, compared with several reported supercapacitors.
operated at a charge/discharge rate of 100 A g−1, which confirmed the highly open pore structure of this hybrid hydrogel (Figure S10). A flexible supercapacitor can also be assembled by replacing the substrates with PDMS and using graphite paper current collector. As shown in Figure 3d, at the scan rate of 5 mV s−1, the CV curve of the flexible supercapacitor based on AC/CCG hydrogel can still overlap with the flat one and even bend 180°, implying the tensile and compress strain of bending deformation has little effect on the electrochemical performances of AC/CCG hydrogel film.15 And after 150 bending cycles, the electrochemical properties still have negligible changes, confirming the feasibility of this hydrogel electrode used in flexible supercapacitors. What’s more, the self-discharge property of the supercapacitor based on AC/CCG hydrogel was even better than that of traditional AC electrode (Figure S11). The electrochemical performance of dried films shows little difference with the hydrogel film under a low scan rate, but the capacitance of both vacuum-dried and freeze-dried samples drops significantly when the operation current is increased (Figure 3a). Although the VD film is more compact than the FD film, their electrochemical performances show very little difference. So removing the inner water may cause inevitable aggregation and restack of AC and CCG, no matter what drying processes, and the impedance spectrum reveals that this will seriously damage the ionic transport (Figure 3b). It confirmed that the preinfiltrated liquid ensured the continuous and highly open porous structure in the hydrogel film and results in an excellent rate performance. Additionally, the AC/ CCG hybrid hydrogel film exhibits excellent cycle stability. It can retain over 99% of the capacitance after 50,000 cycles
similar to the FD sample, but with a larger thickness. As we reported before, liquid can be viewed as an effective “spacer” between the graphene layers in CCG hydrogel.29−31 With the very “soft” liquid “spacer” distributing between relatively rigid components, the stress and strain can be dispersed evenly. And the hydrophobic interaction force and surface tension also make liquid a kind of binder, which can glue AC and CCG together and increase the mechanical strength of the hybrid hydrogel. So the excellent flexibility of this hybrid hydrogel film can only be ascribed to the presence of liquid. To investigate electrochemical properties of the AC/CCG hydrogel film and dried films, we employed cycle voltammetry (CV) and galvanostatic charge/discharge tests in a twoelectrode configuration (Figure 3). Although the main component of this electrode is commercial granular activated carbon, the AC/CCG hydrogel film-based supercapacitor displays superior electrochemical performances comparable to many low mass-loaded nanomaterial-based supercapacitors reported before. The AC/CCG hydrogel film gives a specific capacitance of about 206 F g−1 at 1 A g−1 in 1 M H2SO4 electrolyte (Figure S9), higher than that of both the CCG hydrogel film (166 F g−1) and traditional AC electrode (152 F g−1) at this areal mass loading (use the same CCG and AC). This may be ascribed to the synergistic effect between CCG and AC that CCG provides a three-dimensional conductive network and AC acts as a kind of spacer to prevent excessive restacking of CCG. Furthermore, the absence of polymer binder may also ensure the low intrinsic resistance and fully exposed active surface area. The CV profile still retains a rectangular shape at the scan rate of 3.0 V s−1, and a capacitance of 146 F g−1 can be obtained even when it is D
DOI: 10.1021/acsaem.8b00564 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 5. (a) Side view cross-section of LiFePO4/CCG vacuum-dried film. Digital photograph shows the gel film with a mass loading of 12.5 mg cm−2. (b) Discharge rate capability of LiFePO4/CCG gel film and vacuum-dried film. (c) Charge−discharge curve of LiFePO4/CCG gel film at different rates.
under a high operation current density of 100 A g−1 (Figure 3c), confirming that the AC/CCG hydrogel film has a stable structure and the noncovalent binding between AC and CCG is very strong. Although its structure is similar to the hydrogel sample, the capacitance of the FD film retained only about 93% after 50,000 cycles. A benefit from the compact structure, the VD film retained about 96% of the capacitance after 50,000 cycles, but still much worse than the hydrogel film. It further confirmed that the hydrophobic force and surface tension between AC and CCG can help the film maintain an integral stable structure. This result also indicated that the hybrid hydrogel film is a metastable material similar to conventional gels. Once dried, the structure and properties are not recoverable. The areal mass loading of the films can be increased by simply filtrating more suspension. Figure 4 shows the relationship between specific capacitance and areal mass loading within a large range of scan rates. At low scan rate (≤1 A g−1), the specific mass capacitance of samples with different thicknesses show very little difference, so the specific areal capacitance increased linearly with the mass loading. A capacitance of about 214 F g−1 at 0.1 A g−1 can be obtained even when the mass loading reaches about 40 mg cm−2, resulting in a superb areal capacitance of about 8.6 F cm−2, and the areal capacitance can still retain 6.8 F cm−2 when the scan rate is 1 A g−1 (about 40 mA cm−2), much higher than that reported before (Table S1). Ragone plots of supercapacitors based on AC/CCG hydrogel film (Figure 4d) show that the power density up to 50,000 μW cm−2 can be obtained while still keeping the superb energy storage capability of 377 μWh cm−2. As shown in Figure 4c, the Nyquist plot consists of a negligible arc in the high-frequency region and a steep straight line in the low-frequency region for all samples. The intercept at the x-axis relates to the intrinsic resistance of the system, and the short Warburg region indicates the fast ion diffusion. With the thickness increased, both the intercept at the x-axis and the Warburg region increase slightly.20 Even with a very high areal mass loading, the AC/CCG hydrogel film can still maintain a low ion transmit impedance and intrinsic resistance. So the high areal capacitance should be ascribed to the highly open pathway for both ion diffusion and electron transfer in this hybrid hydrogel structure. In order to explore the universality of this method, we have synthesized the hybrid gel with commercial LiFePO4 and chemically converted graphene as the electrode for lithium ion battery (LIB). The weight ratio of LiFePO4 was 80%, and the granular size was about 1 μm. The inside water of this gel film
was first exchanged with ionic liquid, and the ionic liquid was then exchanged with LIB electrolyte before assembling coin cells. Figure 5a indicates that this LiFePO4/CCG gel film has an ordered and uniform structure, and exhibits good mechanical strength and flexibility even when the mass loading of the whole electrode is about 12.5 mg cm−2 (the mass loading of active material is about 10 mg cm−2). As shown in Figure 5b,c, this hybrid gel has a typical charge−discharge curve of LiFePO4 with a long platform around 3.45 V.44−48 The LiFePO4 in this gel electrode showed excellent capacity of about 165 mAh g−1 at 0.2 C and can still retain 130 mAh g−1 (about 79%) when it charges and discharges at 5 C, similar to that of traditional LiFePO4 electrode (Figure S13), while the dried film only has a capacity less than 20 mAh g−1 at this rate. We also synthesized δ-MnO2 nanorods with the particle size around 100 nm (Figures S3d and S14b),49 and a stable hydrogel film with excellent flexibility was obtained by the similar method.50 In spite of the low conductivity and pseudocapacitive natures of MnO2, the MnO2/CCG hydrogel film can exhibit a capacitance of more than 200 F g−1 even when the scan rate is up to 50 mV s−1, and the CV curve can still maintain a rectangular shape when the scan rate is up to 200 mV s−1.51,52 The inner water of AC/CCG hydrogel film can even be exchanged with high-viscosity ionic liquid, and the operation voltage can be increased to 4 V, while still maintaining a high capacitance of about 190 F g−1 (Figure S14). In summary, we have developed a unique kind of flexible hybrid hydrogel based on various microsized active materials by taking advantage of the colloidal interactions of chemically converted graphene. The all-carbon flexible hydrogel electrode based on activated carbon and chemically converted graphene possesses superior areal capacitance, excellent rate capability, and cycle stability. These indicate the easily accessible pathway for both electrons and ions as well as the strong colloidal interactions, which benefit from the fantastic gel-like structure. This provided an amazingly simple and universal strategy to use graphene as the conductive adhesive for flexible electrodes of various energy storage devices and also sheds light on the important roles of liquid in chemically converted graphene based composites. We also expect these CCG-based hybrid hydrogel materials can play important roles in many other applications, and further understanding the colloidal chemistry behavior of 2D materials will accelerate their commercialization process and lead to more exciting materials. E
DOI: 10.1021/acsaem.8b00564 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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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/acsaem.8b00564. Experimental methods; materials characterizations; digital photographs; additional electrochemical characterizations; SEM images; mechanical test and comparison of electrochemical performances (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaowei Yang: 0000-0002-4862-7422 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the National Basic Research Program of China (2014CB239700 and 2015CB965000), the National Natural Science Foundation of China (21336003 and 21303251), and Innovation Program of Shanghai Municipal Education Commission (16SG17).
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DOI: 10.1021/acsaem.8b00564 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
