Self-assembly formation of high density but porous carbons starting from graphene oxide

Typically, the hydrothermal treatment of a graphene oxide (GO) suspension results in the formation of a 3D hydrogel-like assembly through an effective interlinkage of graphene nanosheets and freeze drying is normally used to fix the 3D network constructed of interlinked nanosheets and results in a spongy assembly (normally obtained porous graphene macroform, denoted as PGM). The upper part of Fig. 1a schematically illustrates the formation of PGM, that after freeze drying, retains the morphology of the parent hydrogel without visible shrinkage. PGM is thus characterized by abundant macropores and mesopores and of course, some micropores, resulting in a very low apparent density (~0.02 g cm−3). By comparison, the present study reports a unique graphene assembly, HPGM, with a highly compact but porous microstructure. As demonstrated in the lower part of Fig. 1a, evaporation-induced drying (herein vacuum drying at room temperature) is used for the removal of water from the hydrothermal hydrogel, which results in the formation of a very stiff rod-like material with an apparent volume shrinkage of about one eightieth the volume of the parent hydrogel. The novel graphene-derived macroform material, HPGM, is characterized by an apparent density as high as 1.58 g cm−3 (see details of the density measurement and determination in Methods Section and Supplementary Table S2), which is ~70% of the theoretical density of graphite (2.2 g cm−3). Of interest to us, HPGM is still characterized by a porous microstructure, indicated by a very simple experiment shown in the left panel of Fig. 1b, in which such a high density macroform generates bubbles in water due to the desorption of the absorbed air. The adsorption measurement gives a SSA of 367 m2 g−1 and a pore volume of 0.16 cm3 g−1. Interestingly, this value is very close to that of PGM (370 m2 g−1) prepared from the same hydrogel, although both samples have entirely different volumes and apparent densities. Thus, the evaporation-induced drying of the graphene hydrogel produces a unique carbon not only with a high density but also a porous structure.

Figure 1 Self-assembly formation of highly dense but porous graphene-based monolithic carbon (HPGM). (a) Schematic of the formation of graphene-based 3D porous macroforms with different drying process and the SEM images of the resultant PGM and HPGM. (b) PGM and HPGM in water, an enlarged view shows a rod-like HPGM with bubbling due to desorption of the adsorbed air. Note that material shown here is PGM with adsorbed water due to the hydrophilicity. (c) Rod-like HPGM (lower) for writing with a soft pencil (upper) as the reference. (d) Photograph of HPGM monoliths with different shapes but with similar SSA (300 ~ 500 m2 g−1). Full size image

The self-assembly technique used allows HPGM to be moldable in two ways. On the one hand, we can obtain porous monoliths with desired shapes according to the mold used in the hydrothermal process. On the other hand, the hydrothermal product can be sliced or cut into any required shapes before further drying. Some typical examples for porous monolithic HPGMs with different shapes but similar SSA are presented in Fig. 1d and Supplementary Fig. S1.

Identification of highly compact but porous microstructure

Due to its highly compact structure, a rod of HPGM can be used for writing (the lower panel of Fig. 1c). In the X-ray diffraction (XRD) patterns (Fig. 2a), graphite and the soft pencil lead, which are similar in density to HPGM, are characterized by sharp (002) peaks (~26.5°) due to the layered structure, while such peaks are much broader and weaker for HPGM and annealed HPGM (denoted HPGM-800, annealed at 800°C). The results show that the arrangement of the graphene nanosheets in the graphene-derived porous carbons here is totally different from those in graphite or graphite products. It is likely that in HPGM highly wrinkled nanosheets are interlinked with each other to form a porous structure in a disordered but highly compact way.

Figure 2 Structure and morphological characterization of HPGM. (a) XRD patterns of graphite, soft pencil lead, HPGM and HPGM-800. (b) Cross sectional SEM images of HPGM, with the inset showing the cross-section of HPGM at low magnification. (c, d) TEM images of HPGM-800. Full size image

Scanning electron microscopy (SEM) indicates the dense structure of HPGM (Fig. 1a and Fig. 2b). The typical cross-sectional SEM image shows a compact microstructure and no pore openings can be identified, indicating the totally different microstructure from spongy PGM in which large pores are clearly observed (Fig. 1a). In other words, graphene nanosheets in HPGM are closely and neatly packed to form a very compact structure. High-resolution transmission electron microscopy (HRTEM) shows structural details at the nanoscale and intertwined nanosized pores constructed of curved graphene layers can be confirmed in HPGM under a microscope (Supplementary Fig. S2). As shown in Fig. 2c and 2d, with a post-annealing treatment at 800°C, HPGM-800 presents more distinct fringes for the curved nanosheets indicative of the contour of interconnected nanopores. Fig. 2c shows an overview for a region of such a novel carbon where totally interconnected pores, several nanometers in size (micropores and small mesopores), are observed and an unimpeded channel for ion transport is identified. This is naturally formed during the direct evaporation of the water trapped in the 3D pore network. In other words, nanopores in HPGM are formed after the 3D continuous porous structure of parent hydrogel. The shrinkage would not change the 3D interconnected porous character but only with a much smaller pore size, thereby the nanopores in HPGM being mutually connected. In Fig. 2d, many curved graphene layers and resulting cylindrical pores can be identified and the pore walls normally consist of 2 ~ 4 layered nanosheets.

Since the present carbon is constructed of 3D interlinked nanosheets to produce intertwined nanopores, it is difficult to have an overall view in a single microscope image of a pore and accurate information about the pore size. More accurate information about the porous structure of HPGM is obtained by adsorption measurements35. The nitrogen cryoadsorption isotherms are shown in Fig. 3a and HPGM exhibits a Type I isotherm together with some characteristics for Type IV. That is, obvious micropore filling occurs at very low relative pressure and the adsorption process quickly reaches a well-defined plateau. A very small but identifiable hysteresis loop indicates the existence of a very limited amount of mesopores. Pore size distribution (PSD) curves (Fig. 3b) give more detailed information on the pore structure13. Except for a small peak around 2 nm, a single peak (centered at 1.1 nm) is observed in the micropore range (<2 nm in size) for HPGM, indicating that this carbon with cylindrical pores is a microporous carbon with a very limited amount of small size mesopores (slightly larger than 2 nm). That is, HPGM is a microporous carbon (mostly with pores 1.1 nm in diameter) and free of large mesopores and macropores when used as a monolithic carbon. After annealing at 800°C (HPGM-800), the isotherm totally transforms into a composite isotherm combining Type I and Type IV. An increased adsorption amount at low pressure indicates an increase of the micropore volume while the appearance of a wide and more pronounced hysteresis loop is associated with more developed mesopores36. The analysis of the isotherm gives a SSA up to 720 m2 g−1 and a pore volume of 0.46 cm3 g−1 for HPGM-800. The more developed pores result in a density decrease to 1.07 g cm−3, which is still higher than for most commercial activated carbons and reported novel porous carbons (0.3 ~ 0.8 g cm−3)9,13,23,24,25,26,27,28,29,37 due to its monolithic form. The PSD curve (Fig. 3b) shows that HPGM-800 possesses pores that are mostly in the micropore (<2 nm) and mesopore ranges (2.0 ~ 3.7 nm). Compared with HPGM, HPGM-800 has a slightly smaller pore size in the micropore range possibly due to thermal shrinkage and the appearance of pores from 1.3 to 3.7 nm may be due to the evolution of pores resulting from the removal of trapped water and bound oxygen. Microporosity in HPGM is further confirmed by iodine adsorption which is normally used for probing micropores38 (Fig. 3c). HPGM-800 with more micropores, as expected, shows higher iodine adsorption than HPGM. In comparison, powdered graphene (GNS), which is totally free of micropores, shows a very small iodine adsorption although it possesses a larger SSA (450 m2 g−1) than does HPGM. Moreover, a relatively high adsorption rate for iodine indicates unimpeded channels in HPGM for adsorbates.

Figure 3 Adsorption behaviors and surface nature of HPGM. (a) Nitrogen adsorption isotherms and (b) pore size distributions (DFT) of HPGM and HPGM-800. (c) Iodine adsorption isotherms of HPGM and the reference materials. (d) TPD spectra of HPGM and HPGM-800, including H 2 , H 2 O, CO, CO 2 evolution. Full size image

According to the X-ray photoelectron spectroscopy (XPS) analyses (Supplementary Fig. S3a), graphitic C (284.8 eV) is dominant in HPGM and the material contains far less oxygen (~16 at.%) than the parent graphene oxide (30 ~ 40 at.%), indicating a partial reduction for HPGM in the hydrothermal process. Similar to the reduction process reported by Loh et al., the supercritical water produced in the hydrothermal condition plays the role of reducing agent and offers an effective reduction approach for graphene oxide39. The rest of oxygen element in HPGM is mainly in the forms of C-O (286.2 eV) and C = O (287.6 eV) groups. After annealing at 800°C, the surface oxygen content decreases to ~4 at.% (Supplementary Fig. S3b and Table S3). Such a big decrease in oxygen content indicates that most oxygen-containing functional groups are removed and the resulting materials are reduced to a large extent. Temperature-programmed desorption (TPD) measurements indicate the thermal chemical desorption behavior of bound species on graphene nanosheets. In the case of HPGM, obvious CO 2 and CO peaks, which may respectively originate from carboxyl and lactone groups and phenol and carbonyl groups40, can be identified from the TPD spectra, revealing that there is still a considerable amount of surface oxygen groups left after the hydrothermal reduction. Significantly, an H 2 O peak of HPGM centered at ~210°C suggests the existence of bound water trapped in the 3D porous network and results show that the trapped water is totally removed as the annealing temperature increases to 550°C (Fig. 4c). In contrast, after the annealing, for HPGM-800, there is only a small CO peak remaining at ~1000°C in addition to the total disappearance of the CO 2 peak. The results indicate that almost all surface groups and trapped water are removed except for a small amount of carbonyl groups. For HPGM-800, a very large hydrogen peak is observed and indicates the existence of a large amount of edge carbon atoms (saturated with hydrogen to form an open network). Such a hydrogen peak totally disappears above ~1550°C and therefore, for HPGM-1600 (annealed at 1600°C), no visible hydrogen peak is observed (Supplementary Fig. S4a), which suggests a decrease in the number of edge carbons, indicative of a closed network. Such a difference in the hydrogen evolution peak for HPGM-800 and HPGM-1600 is consistent with the measured SSA values for the two carbons (the former with many edge carbons indicating an open network: 720 m2 g−1; the latter with fewer edge carbons indicating a closed network: 10 m2 g−1 (Supplementary Fig. S4b)). Moreover, TEM images of HPGM-1600 (Supplementary Fig. S5) further demonstrate the changes of graphene network after annealing treatment.

Figure 4 Electrochemical performance of HPGM. (a) Room temperature I-V curves of HPGM and HPGM-800: the insets show the measurement apparatus for the conductivity test. (b) CV results of HPGM measured at scan rates of 5, 10, 20, 50, 100 mV s−1. (c) Cycle performance of HPGM, GNS and AC at a current density of 0.5 A g−1: the inset is a diagram of the two-electrode measurement device. (d) Rate performance of HPGM. (e) Volumetric Ragone plots comparing HPGM with AC, GNS and carbon materials reported in the reference number indicated. Full size image

Together with high density, shrinking during the vacuum drying increases the interlinking of graphene nanosheets, contributing to an acceptable conductivity of ~16 S m−1 for HPGM, which was directly measured as schematically shown in Fig. 4a. For comparison, spongy PGM has a lower conductivity of ~0.4 S m−1, which is agreement with values reported elsewhere22. The annealing exerts a substantial effect on the microstructure of HPGM and a treatment temperature up to 1000°C results in both increases in SSA and conductivity. With a higher temperature treatment (>1000°C), the sample shows an apparent increase in conductivity but a substantial loss of SSA. HPGM-800 with higher SSA shows a good conductivity of 115 S m−1, while HPGM-1600 shows a conductivity as high as 500 S m−1 but a very low SSA (~10 m2 g−1).

Compact structure and monolithic form needed to achieve an ultrahigh volumetric capacitive performance

Two-electrode supercapacitor cells (Supplementary Fig. S6) were constructed to assess the electrochemical performance in an aqueous system (6 M KOH). Note that due to its monolithic form and acceptable conductivity, sliced HPGM is directly used as an electrode in the assembly of supercapacitor devices without adding binders and conducting additives. PGM and HPGM show similar gravimetric capacitances in the aqueous system, the former with 235 F g−1 and the latter with 238 F g−1 at a current density of 0.1 A g−1. In contrast, because of its low apparent density, PGM shows a very limited volumetric capacitance, even lower than 10 F cm−3. In sharp contrast, with a very high density (much reduced empty space), HPGM shows an ultrahigh volumetric capacitance, up to 376 F cm−3, which, to the best of our knowledge (see the references in Supplementary Table S1 for comparison), is the highest value reported for carbon-based supercapacitor materials in an aqueous system. Even for a fabricated supercapacitor device, the energy density is very high due to the absence of any additives. Note that although thermal treatment results in an increase in both SSA (more developed porous structure) and conductivity, HPGM-800 possesses a much lower capacitance (about 80 F g−1 and 86 F cm−3, Supplementary Fig. S7) than HPGM. Considering above results, it is likely that surface chemistry and trapped water other than specific surface area are the determining factors for the high capacitive performance of HPGM. In the case of HPGM-800, the substantial removal of oxygen-containing groups results in a big loss of pseudocapacitance as well as charged surface area which largely related to the wettability of graphene surfaces accessible to electrolyte ions41,42. The loss of trapped water hinders the formation of interconnected water passages, which reduces the full utilization of the graphene surface and brings the increased ion transfer resistance (Supplementary Fig. S8)43. Further investigations are ongoing to give more quantitative evidence for how various factors have influence on the capacitive performance of such a novel material.

For reference, the gravimetric and volumetric capacitances of powdered GNS and activated carbon (AC) as two typical commercial electrode materials for EDLCs are measured under the same conditions as for HPGM. Fig. 4b shows cyclic voltammetry (CV) profiles of HPGM at scan rates from 10 to 100 mV s−1 and all curves show quasi-rectangular shapes indicating an ideal capacitive behavior. The gravimetric and volumetric capacitances were calculated based on galvanostatic charge/discharge curves (Supplementary Fig. S9) and the maximum volumetric capacitance of HPGM can reach 376 F cm−3 at 0.1 A g−1. HPGM shows an excellent cyclability and, as shown in Fig. 4c, retains a specific capacitance up to 203 F g−1 and 321 F cm−3 at 0.5 A g−1 over 4000 cycles (the retention rate is over 96%). Remarkably, although totally free of conducting additives, HPGM shows a very good rate capability while the capacitance was able to retain around 69% of the maximum at a high current of 15 A g−1 (Fig. 4d and Supplementary Fig. S10). The excellent rate performance can be attributed to the 3D porous network of interlinked graphene nanosheets which guarantee fast electron transfer and ion transport. On the one hand, with an acceptable conductivity, the 3D network of the HPGM provides an easily accessible conducting network. On the other hand, although HPGM only contains very small pores (mainly micropores together with very small mesopores), its interlinked 3D network structure together with trapped water in the pore network provides an unimpeded channel for fast ion transport. This is consistent with the previously reported results for fast ion transport in both interlinked micropores by Nishihara et al13. and water-trapped nanochannels by Li et al43. The Nyquist plot (Supplementary Fig. S11) shows very small equivalent series resistance (ESR) values corresponding to high conductivity and low internal resistance. As an important index for the ion diffusion process from electrolyte to the surface of the electrode material, the small Warburg resistance implies fast ion diffusion in the 3D network of HPGM. To better reveal the performance of HPGM, the Ragone plot of a HPGM based supercapacitor device is shown in Fig. 4e and Supplementary Fig. S12, with a comparison with AC, GNS and other carbon materials44,45,46. We can see that in the aqueous system, the maximum energy density of such a supercapacitor device is up to 13.1 Wh L−1 at a power density of 39.5 W L−1 and the power density is up to 5.9 kW L−1 at an energy density of 9.1 Wh L−1. A much higher energy density can be achieved for a HPGM-based supercapacitor when an organic electrolyte is used (37.1 Wh L−1 at a power density of 98.8 W L−1, as shown in Supplementary Table S4 and Fig. S13). These values are much higher than those of devices based on conventional carbon material, AC and the newly emerging nanomaterials including GNS and carbon nanotubes for the same measurement conditions.