III. RESULTS AND DISCUSSION

Inspired by previous reports, we designed a 3D GO paper with multiscale pores including macropores and mesopores (denoted as MPGP).

The facile synthesis procedure for MPGP can be briefly described in Fig. 1. Typically, the MPGP was synthesized in three steps: fabrication of the free-standing ZnO/GO paper by vacuum filtration of ultrasonicated mixed aqueous suspension containing GO and ZnO (100–500 nm), followed by removal of ZnO to generate 3D macroporous GO paper (denoted as PGP). Finally, controllable amounts of H₂O₂ aqueous solution with PGP were sealed in a Teflon-lined autoclave and heated to 120 °C for 4 h to yield the MPGP. The vacuum filtration process is reported to be suitable for assembling GO nanosheets into paper-like electrodes with high flexibility and good electrical conductivity.^{Reference Wan, Zhu, Yu, Sun, Wong and Liao24,Reference Ferrero, Sevilla and Fuertes25} However, the GO nanosheets tend to stack into graphite during vacuum filtration, which results in significant loss of surface area. The incorporation of spacer materials (metal oxide nanomaterials,^{Reference Tang, Li, Zhang, Zhu, Wang, Shi and Wei26} polystyrene nanospheres,^{Reference Wang, Yan, Liu, Chen, Shi, Zhang, Zhong and Zhang27,Reference Liu, Chen, Policastro, Becker and Zhu28} polymethyl methacrylate spheres,^{Reference Xu, Lin, Bi and Huang29} etc.) between graphene or GO nanosheets appears to be efficient to retain the high surface area of graphene or GO, especially after subsequent removal of spacer materials. Accordingly, the as-prepared 3D porous structure facilitates the electrolyte ions to transport at two-dimensional graphene planes. For graphene-based materials under highly compressed form or with a large packing density (>1.0 g/cm³), a large dead volume without accessing to the electrolytes will be resulted in. Therefore, the ion diffusion shortcuts between different graphene layers are highly required to make the electrolytes accessible to the entire compressed film. To this end, H₂O₂ was adopted to create a highly continuous network of micropores and mesopores for efficient ion transport in three dimensions. The carbon atoms around active defective sites of GO can be partially oxidized and etched into carbon vacancies, which further lead to nanopores.^{Reference Xu, Lin, Zhong, Huang, Weiss, Huang and Duan30} In detail, H₂O₂ generates higher OH˙ radical concentrations under hydrothermal condition. Subsequently, the OH˙ radical attacks the C–C sp ² bonds of reduced GO and produces an OH adduct. Complete oxidation of carbon atoms to CO₂ is further caused by continued attack of OH˙ radicals, finally leading to nanopores on the reduced GO layer. Thus the as-created mesopores could occur across the entire nanosheets due to the uniformly distributed defective carbon sites on the basal plane of GO.

FIG. 1.

Schematic illustration of the procedure to fabricate the 3D and multiscale porous GO paper through ZnO nanodisk templates and a subsequent H₂O₂ etching process.

The template-directed approach has been reported to be an efficient way to prepare hierarchical porous carbon-based materials with a tunable pore structure. Generally, 3D graphene or GO with microstructure, mesostructure, or macrostructure is synthesized via utilizing various templates such as PS nanospheres, PMMA spheres, SiO₂ nanospheres. Yet, these templates suffer from complicate synthesis and removal process (acid or base etching, calcinations), high cost, etc. In contrast, the unique features of ZnO nanomaterials such as easy control of scale and facile dissolubility even in weak acid or basic solution make ZnO an ideal template for construction of carbon-based materials with various nanostructures.^{Reference Zhao, Jiang and Xie31} ZnO nanodisks with different sizes ranging from 100 to 500 nm were prepared via a previously reported method.^{Reference Zhai, Xie, Zhao, Sun, Lu, Yu, Xu, Xiao and Tong32} Subsequently, ZnO nanodisks and GO suspension were mixed under controlled pH and vacuum filtrated for ZnO/GO paper films. During this process, the ZnO nanodisks were sandwiched in-between GO layers as “spacer” to form a composite that effectively prevents the GO layers from serious agglomeration in the vacuum filtration process. The morphologies of the intermediate ZnO/GO composites during fabrication of MPGP are shown in Fig. 2. For simplification, the sample of ZnO nanodisks with a diameter of 100 nm sandwiched by GO layers was denoted as ZnO-100/GO and so on. Clearly, the scanning electron microscopy (SEM) images of ZnO-100/GO, ZnO-180/GO, ZnO-250/GO, and ZnO-500/GO all demonstrate that ZnO nanodisks were uniformly wrapped by GO nanosheets, which can be attributed to the electrostatic and hydrophobic interactions between each other in aqueous solution. In detail, ZnO will hydrolyze in water solution to form hydroxide layers at the surface (≡Zn–OH) and then the hydroxide surface will be positively charged through reacting with H⁺ in water solution.^{Reference Degen and Kosec33} In contrast, the GO layers in water are negatively charged because of the ionization of carboxylate groups on the surface. Consequently, the negatively charged GO nanosheets bind electrostatically to the positively charged ZnO nanodisks. The spacer materials of ZnO-100/GO possess a smaller diameter of ∼100 nm than that of as-prepared ZnO-180/GO, ZnO-250/GO, and ZnO-500/GO samples and previously reported SiO₂/GO^{Reference Bae, Park, Kim, Lee, Kim and Seo34} and PMMA/GO,^{Reference Xu, Lin, Bi and Huang29} thus leading to a porous GO with a higher surface area after removal of spacer materials. Significantly, porous GO with a tunable surface area can be easily fabricated via the ZnO nanodisks with controllable size from 100 to 500 nm, indicating the feasibility of our method.

FIG. 2.

Magnified SEM images of (a) ZnO-100/GO, (b) ZnO-180/GO, (c) ZnO-250/GO, and (d) ZnO-500/GO composites, respectively.

The selective removal of ZnO nanodisks in ZnO/GO composites leaves behind a porous GO paper (denoted as PGP). The macropore size of the PGP depends on the scale of the ZnO nanodisks as spacer materials, which controls the pore size in the range from 100 to 500 nm. The PGP with a macropore diameter of 100 nm was selected because of their potentially high-capacitive performance originated from a large surface area. Figure 3(a) presents the cross-sectional SEM image of the PGP with an average thickness of around 42.4 μm, which is substantially higher than most of the reported graphene or GO papers (<20 μm).^{Reference Ye, Zhou, Jia, Tang, Zhu and Wan35,Reference Chi, Zhang, Xi, Huang, Xiao, Wang and Liu36} The thickness of the electrode has appeared to be important for high-volumetric capacitance. More importantly, electrodes with larger thickness as well as high-volumetric capacitance are more preferred in design of energy storage devices (LIBs, SCs, etc.) due to the advantages as follows: (i) the ratio of active mass can be effectively enlarged, i.e., a high-energy density calculated based on the whole device can be achieved; (ii) less steps are needed to achieve the total energy capacity, thus decreasing the manufacturing cost. However, the ionic transport along the thickness direction in the electrodes is still problematic, especially for highly packed electrodes. Thus, electrodes with appropriate pore structures in more dimensions are highly required. Figure 3(b) shows the magnified SEM image of the PGP. The porous structure with a pore diameter of ∼100 nm retains well after the removal of the ZnO template, which can be ascribed to the interconnected feature of GO layer walls in 3D structure. Moreover, the porous structure throughout the entire thick GO paper again confirms the important role of ZnO nanodisks as spacers in preventing the restack of GO layers. The PGP was further treated by H₂O₂ (for details, see Sec. II) to generate micropores and mesopores. The SEM image of MPGP in Fig. 3(c) demonstrates that the as-assembled 3D porous GO structure did not collapse and no obvious morphology change was observed on the GO layer. Transmission electron microscopy (TEM) was performed to further investigate the microstructure and nanostructure of the MPGP. Figure 4(a) shows the TEM image of the MPGP, presenting the well-defined porous structure with interconnected and overlapped pores. Moreover, no traces of the ZnO “spacer” in the MPGP can be observed, which indicates the complete removal of ZnO “spacers”. The macropores are determined to be about 100 nm, which is in well accordance with the diameter of the ZnO nanodisks that confirmed by SEM results [Fig. 2(a)]. Furthermore, the high-resolution TEM image of the selected area (marked by yellow dashed box) in Fig. 4(b) presents abundant uniformly distributed mesopores with a diameter of 2–3 nm on the GO layer, indicating the carbon atoms of GO were etched efficiently by H₂O₂. The as-designed MPGP with hierarchical pores to facilitate efficient ionic transport would be desirable in SCs.

FIG. 3.

(a) Low-magnified and (b) high-magnified cross-sectional SEM images of the PGP. (c) High-magnified cross-sectional SEM image of the MPGP.

FIG. 4.

TEM images at different magnifications of MPGP.

X-ray diffraction was used to further confirm the complete removal of ZnO nanodisks in the MPGP and PGP. The XRD pattern of the GO paper (denoted as GP) was added for comparison. As shown in the XRD pattern of ZnO-GO in Fig. 5(a), peaks located at 31.8°, 34.4°, 36.3°, 47.5°, and 56.6° can be well indexed to the wurtzite structure [JCPDS # 36-1451; a = b = 3.249 Å, c = 5.206 Å, space group: P63mc(186)]. After the etching process, no peaks ascribed to ZnO can be observed in PGP and MPGP samples, confirming the full removal of ZnO nanodisks from the 3D composites. No characteristic peaks ascribed to GO can be observed in the four XRD patterns due to the poor crystallinity of the as-prepared GO film. We carried out the BET measurements to further characterize the detailed porous structure of the MPGP. Figure 5(b) shows the nitrogen adsorption–desorption isotherm of the MPGP. Obviously, the isotherm of the MPGP presents an increase in the amount of N₂ absorbed at both low relative pressure (P/P ₀ < 0.1) and high-relative pressure (P/P ₀ > 0.9), indicating the existence of amounts of micropores and macropores, respectively.^{Reference Zhao, Lai, Lyu, Jiang, Xie, Wang, Wu, Yang, Jin, Ma, Liu and Hu37} In addition, the presence of mesopores can be suggested by the hysteresis located between P/P ₀ = 0.4 and P/P ₀ = 1.0.^{Reference Wang, Wang, Lu, Ling, Yu, Zhai, Tong and Li38} The inset of Fig. 5(b) shows the pore size distribution of MPGP calculated based on the absorption–desorption isotherm using the Barrett–Joyner–Halenda (BJH) method. A large quantity of pores with diameter from 1.7 to 3.2 nm were available for MPGP, again indicating the occurrence of micropores and mesopores. The high micropore and mesopore volume can be attributed to the aforementioned H₂O₂ etching process. Moreover, a significant high-BET surface area is determined to be 960 m²/g, which is considerably higher than some reported carbon materials at a similar thickness level.^{Reference Shu, Wang, Li, Zhao, Yang, Liu and Wallace39}

FIG. 5.

(a) XRD pattern of GO, ZnO/GO, PGP, and MPGP, respectively. (b) Nitrogen absorption–desorption isotherms of the MPGP collected at liquid nitrogen temperature (77 K). The inset of (b) is the pore size distribution calculated based on the absorption–desorption isotherm using the Barrett–Joyner–Halenda (BJH) method (pore size ranges between 1 and 25 nm).

To evaluate the enhanced capacitive performance of the multiscale porous GO paper, the electrochemical performance was determined in a three-electrode system with a SCE as the reference electrode, a Pt electrode as the counter electrode, and 3 M KOH as electrolytes. The electrochemical performance of the GP and PGP was also characterized for comparison. Figure 6(a) compares the GCD profiles of MPGP, PGP and GP electrodes collected at a current density of 1 A/g. All the three curves exhibited a symmetrical triangular shape, indicating the good EDL behavior. Significantly, the charge/discharge curve of the MPGP is more symmetric and substantially prolonged as compared to that of the GP and PGP. Notably, the MPGP electrode possesses a slightly larger IR drop (a voltage drop that appears at electrode with resistance during a current flow) of 0.005 V than that of the PGP (0.003 V). It shows that the oxidation during H₂O₂ treatment has a minor effect on the electrical conductivity of the MPGP. Besides, the IR drop of the MPGP is smaller than the value of GP (0.01 V), revealing the good electrical conductivity of MPGP. The Cs (specific capacitance) value of the MPGP at 1 A/g is determined to be 246 F/g (307.5 F/cm³), which is substantially larger than that of the PGP (147 F/g; 183.8 F/cm³) and GP (139 F/g; 173.8 F/cm³). Additionally, the value is also larger than or comparable to that of the previously reported graphene or GO paper electrodes at the same current density.^{Reference Ferrero, Sevilla and Fuertes25,Reference Ye, Zhu, Tang, Wan and Jia40,Reference Shao, Tang, Lin, Li, Qin, Yuan and Qin41}

FIG. 6.

(a) GCD curves of MPGP, PGP, and GP at a current density of 1 A/g. (b) GCD curves of the MPGP collected at different current densities from 1 to 16 A/g.

The micropores and mesopores of the MPGP provide amounts of active sites for charge accumulation, which further boost the specific capacitance of the MPGP. Figure 6(b) shows the GCD curves of the MPGP collected at current densities ranging from 1 to 16 A/g. The charge curves at different current densities are relatively symmetric to their corresponding discharge curves, again confirming their good capacitive behavior. The specific capacitance slowly decreased from 246 to 192 F/g as the current density increased from 1 to 16 A/g. An outstanding retention of 78% was achieved due to the multiscale porous structure, which provides effective pathways for fast ion transfer, enabling the good rate capability.

Figure 7(a) displays the specific capacitance of the MPGP, PGP, and GP as a function of scan rates. The values were extracted from their corresponding CV curves. As expected, the MPGP outperforms PGP and GP in specific capacitance at all scan rates ranging from 5 to 400 mV/s. In specific, the MPGP delivers an attractive capacitance of 264 F/g (330 F/cm³; 5.33 mg/cm²) at 5 mV/s, which is much larger than 161 F/g (204.5 F/cm³; 5.4 mg/cm²) and 105 F/g (165.7 F/cm³; 3.9 mg/cm²) for PGP and GP, respectively, at the same scan rate. The enhancement in capacitance is in well accordance with the aforementioned results from charge/discharge curves and can be ascribed to the increased active sites raised from mesopores. Moreover, the values are also higher than that of previously reported graphene or GO paper electrodes, such as the graphene paper (80 F/g; 2.4 mg/cm²),^{Reference Ye, Zhu, Tang, Wan and Jia40} holey graphene paper (50 F/g; 3.0 mg/cm²),^{Reference Ferrero, Sevilla and Fuertes25} and CNT (carbon nanotube)-spaced graphene aerogels (245 F/g).^{Reference Shao, Tang, Lin, Li, Qin, Yuan and Qin41} Notably, the thickness of the paper-like electrode is a major factor that should be considered when comparing with the reported electrodes. A thicker electrode with high packing density is critical for the application of SCs. Due to the high packing density of 1.25 g/cm³, the MPGP achieves a substantially high-volumetric capacitance of 327 F/cm³ at 5 mV/s, enabling it as highly promising as SC electrodes with high-volumetric performance for applications with limited space, e.g., miniature electronic devices. In addition, the MPGP exhibits a more stable cycling life with a capacitance retention of ∼96% after 10,000 CV cycles at 400 mV/s in the 3 M KOH electrolyte, while that of the PGP and GP is around 94% and 89%, respectively. Notably, the three samples were characterized directly without any binders, which further leads to exfoliation and consequent capacitance degradation during cycling test. The multiscale porous structure of the MPGP is suitable for alleviating the capacitance degradation during the repeated charge/discharge process. The above results confirm that the enhanced capacitive behavior can be attributed to the following aspects: (i) the outstanding electric conductivity of the GO layer; (ii) the boosted surface area and highly efficient ion transport originated from the multiscale porous structure. The advantages also unambiguously proved that the MPGP is a promising scaffold for pseudocapacitive materials to achieve high-volumetric capacitance at high rates.

FIG. 7.

(a) Comparison of specific and volumetric capacitances as a function of different scan rates for MPGP, PGP and GP. (b) Cycling stability of MPGP, PGP, and GP collected at a scan rate of 200 mV/s for 10,000 cycles.

Increasing the cell voltage of SCs is proved to be more efficient to boost the energy density as compared to the enlargement of capacitance.^{Reference Zhai, Wan, Sun, Chen, Sun, Xia and Xia1} ASCs in aqueous electrolytes can achieve a voltage up to 2.6 V by making full use of potential windows of two different electrodes. In this work, we evaluated the capacitive performance of the as-prepared MPGP at the device level through constructing an ASC device. The ASC device was assembled based on MnO₂ on the MPGP (denoted as MnO₂/MPGP) as the cathode and MPGP as the anode (for details, see Sec. II). Manganese oxides (MnO₂) have attracted intensive interest as a promising pseudocapacitive material in SCs due to its large theoretical capacity (1370 F/g), abundant and environmentally friendly nature, etc.^{Reference Zhai, Xie, Yu, Fang, Liang, Lu and Tong42} We incorporated MnO₂ with the MPGP to further boost the pseudocapacitive performance of MnO₂. The MnO₂/MPGP was synthesized via an electrochemical deposition method (for details, see Sec. II). Figure S1 shows the SEM images of MnO₂ nanosheets on the MPGP, presenting that the MnO₂ nanosheet-assembled nanoparticles with a diameter of less than 300 nm are uniformly distributed on the MPGP scaffold. The X-ray photoelectron spectroscopy (XPS) spectra including Mn 3s [Fig. S2(a)] and Mn 2p [Fig. S2(b)] core-level spectra were performed to confirm the composition of MnO₂, which are consistent with previous reports.^{Reference Zhai, Xie, Yu, Fang, Liang, Lu and Tong42,Reference Ma, Shao, Fan, Wang, Song and Shen43} CV curves of the MnO₂/MPGP electrode were collected at different scan rates ranging from 10 to 100 mV/s in the 0.5 M Na₂SO₄ electrolyte. As shown in Fig. S3(a), characteristic quasirectangular shapes can be observed for the MnO₂/MPGP electrode at all the scan rates in a potential window of 0–1.0 V versus SCE, indicating the typical pseudocapacitive behavior of MnO₂. Significantly, a high-volumetric capacitance of 180.6 F/cm³ was achieved at a scan rate of 5 mV/s [Fig. S3(b)]. Moreover, it retained 91.7 F/cm³ at 100 mV/s, with 50.7% retention of initial capacitance at 5 mV/s, which is higher or comparable to previously reported MnO₂-based composites.^{Reference Ma, Shao, Fan, Wang, Song and Shen43,Reference Wang, Yang, Liu, Zeng, Li, Huang and Hu44}

For performance maximization of the as-assembled ASC device, we used the charge balance theory (for details, see Sec. II) to balance the charge between the positive and negative electrodes. Figure S4 shows the CV curves of the MPGP and MnO₂/MPGP electrode collected at a scan rate of 100 mV/s. The mass ratio (MnO₂/MPGP:MPGP) for the ASC device is calculated to be about 0.86:1. According to the CV studies in Fig. S4, the operating voltage of device is expected to be extended to 1.8 V. Figure 8(a) demonstrates the CV curves of the MnO₂/MPGP//MPGP ASC device collected at different voltage windows at 100 mV/s. As expected, the as-assembled MnO₂/MPGP//MPGP device is electrochemically stable in the voltage window of 0–1.8 V. The GCD measurement (Fig. S5) also confirms the stable voltage window for the ASC device. CV studies of the MnO₂/MPGP//MPGP ASC device collected at various scan rates are shown in Fig. 8(b) to further illustrate its capacitive performance. CV curves at all scan rates present a rectangular-like shape and remain unchanged even when the scan rate is increased to 100 mV/s, confirming the ideal capacitive behavior and fast responsive properties of the MnO₂/MPGP//MPGP ASC device. Figure 8(c) shows the volumetric capacitance of the MnO₂/MPGP//MPGP as a function of scan rates. The device achieved 26.3 F/cm³ at 5 mV/s, which is substantially higher than those recently reported values for MnO₂-based ASC devices, such as the Fe₃O₄@Fe₂O₃//Fe₃O₄@MnO₂ ASC device (1.49 F/cm³, 1.25 mA/cm²),^{Reference Tang, Jia, Zhai and Xia45} MnO₂/CNT/nylon fiber SC (3.8 F/cm³, 10 mV/s),^{Reference Choi, Kim, Sim, Lee, Choi, Kim, Lepró, Spinks, Baughman and Kim46} CNT spring fiber SC (18.12 F/cm³, 10 mV/s),^{Reference Zhang, Bai, Cheng, Ren, Weng, Chen, Fang, Zhang and Peng47} and MnO₂/Au/MnO₂ nanospike SSC (symmetric supercapacitor) device (20.3 F/cm³, 0.2 mA/cm²).^{Reference Gao, Jin, Lin, Li, Tavakoli, Leung, Tang, Zhou, Wa Chan and Fan48} Moreover, a good rate capability of 36.5% retention of initial capacitance was delivered as the scan rates increased from 5 to 100 mV/s. Notably, the capability was achieved in a device with both the thick positive electrode and thick negative electrode. Thus, the rate capability is quite competitive with previously reported ASC devices. In addition, the charge–discharge curves of the MnO₂/MPGP//MPGP device (Fig. S5) show the triangular shapes at different current densities, again confirming the outstanding electrochemical properties of the as-assembled ASC device. As one of the most important parameters for high-performance SCs, the long-term cycling stability of the MnO₂/MPGP//MPGP ASC device was evaluated at a scan rate of 50 mV/s for 5000 cycles. As shown in Fig. S6, the ASC device exhibits an outstanding electrochemical stability, with a capacitance retention of 92.4% of the initial capacitance values after 5000 cycles. The good cycling performance of the as-assembled ASC is comparable with recently reported high-performance ASC devices.^{Reference Zhang, Bai, Cheng, Ren, Weng, Chen, Fang, Zhang and Peng47,Reference Gao, Jin, Lin, Li, Tavakoli, Leung, Tang, Zhou, Wa Chan and Fan48}

FIG. 8.

(a) CV curves of the MPGP//MnO₂/MPGP ASC device collected at 50 mV/s in different voltage windows. (b) CV curves of the MPGP//MnO₂/MPGP ASC device collected at different scan rates. (c) Volumetric capacitance of MPGP//MnO₂/MPGP as a function of scan rates. (d) The Ragone plot of MPGP//MnO₂/MPGP.

The energy density (E) and average power density (P) are also key parameters to determine the electrochemical performance of SCs (for detailed calculations, see Sec. II). Figure 8(d) illustrates the Ragone plots calculated based on the CV curves in Fig. 8(b). The values reported for other MnO₂-based ASC devices have also been demonstrated for better comparison. The as-assembled MnO₂/MPGP//MPGP ASC device exhibits a volumetric energy density of 11.8 W h/L (based on the whole volume of the device) at a scan rate of 5 mV/s, and this retains 4.47 W h/L at 100 mV/s, confirming the high-volumetric performance and outstanding rate capability of the as-assembled ASC device. The volumetric energy density values are considerably higher than those of recently reported MnO₂-based ASC devices, such as Fe₂O₃@NiCo₂S₄//MnO₂ (2.3 W h/L),^{Reference Jia, Zhu, Sun, Zhai and Xia49} Co₉S₈//Co₃O₄@RuO₂ (1.2 W h/L),^{Reference Xu, Wang, Wang, Xiang, Liang, Chen and Shen50} Fe₂O₃@HP-CF//NiCo₂S₄@HP-CF (1.0 W h/L),^{Reference Chen, Xu, Zhou, Zhao and Wong51} VOS@C//MnO₂/graphene (0.88 W h/L),^{Reference Zhai, Lu, Ling, Yu, Wang, Liu, Liang, Tong and Li8} and C//MnO₂@C (0.77 W h/L).^{Reference Zhong, Shi, Liu, Huang, Cheng, Cheng, Li, Liao and Tang52} Additionally, a maximum average power density of 894.6 W/L was achieved at a current density of 100 mV/s, which is also much higher than recently reported ASCs.^{Reference Zhai, Xie, Yu, Fang, Liang, Lu and Tong42,Reference Tang, Jia, Zhai and Xia45,Reference Feng, Ye, Lu, Tong and Li53,Reference Yu, Moore, Calderon, Zhai and Thomas54} The results above unambiguously proved that the MPGP is very promising as a high-performance electrode as well as a good support for pseudocapacitive materials, opening up new opportunities for constructing advanced SCs with high-electrochemical performance per volume.