Boosting ion dynamics through superwettable leaf-like film based on porous g-C3N4 nanosheets for ionogel supercapacitors

Abstract

Ionic liquid (IL) electrolytes have enormous potential for the development of high energy density supercapacitors (SCs) owing to their wide potential windows, but ILs are plagued by sluggish ionic diffusion due to their high viscosity and large ion size. Exploiting superwettable electrodes possessing high compatibility with IL electrolytes remains challenging. Inspired by the biological characteristics observed in nature, a unique film electrode with a Monstera leaf-like nanostructure is synthesized and used to overcome the aforementioned bottleneck. Similar to the pores in Monstera leaves that allow the permeation of air and water vapor, the film electrode is based on porous g-C3N4 nanosheets (~1 nm thick) as ion-accessible “highway” channels, allowing ultrafast diffusion of IL ions. The film exhibits a high diffusion coefficient (3.68 × 10−10 m2 s−1), low activation energy (0.078 mJ mol−1) and extraordinary wettability in the IL electrolyte, indicating its superior IL ion dynamics. As a proof of concept, flexible ionogel SCs (FISCs) with tailorability and editability are fabricated, which exhibit a high energy density (10.5 mWh cm−3), high-power density, remarkable rate capability, and long-term durability, outperforming previously reported FISCs. Importantly, these FISCs can be effectively charged by harvesting sustainable power sources, particularly the rarely studied wind power, for practical applications.

Introduction

With the rapid development of portable and wearable electronics, developing lightweight, reliable, and flexible energy-storage devices is a critical challenge1,2. Supercapacitors (SCs), owing to their fast charge/discharge capability and almost unlimited lifetime, have been considered promising energy-storage devices to supply power for various electronic devices3,4,5. Because the energy density of SCs is proportional to the square of the working voltage, the amount of energy stored in SCs mainly depends on the stability of the applied electrolyte over the voltage range6,7,8. Owing to their wide potential window (>2.5 V), nonvolatility, nonflammability, and high thermal/chemical stability, flexible ionogel SCs (FISCs) based on ionic liquid (IL) electrolytes not only have great promise for achieving a high energy density but are also safe and have a wide usage temperature range in portable and wearable electronics9. Nevertheless, the large ion size and high viscosity of IL electrolytes always lead to poor compatibility with the electrode, resulting in sluggish ionic diffusion, which affects the electrochemical kinetics of FISCs10,11. Accordingly, it is important to design a novel electrode with a suitable structure for the development of high-performance FISCs.

In nature, many plants have numerous small pores (also known as “stomata”) in their leaves, e.g., Monstera leaves. These pores are important to the physiology of plants because they provide accessible channels for air and water vapor during photosynthesis and cellular respiration. Inspired by this biological characteristic, an effective approach is to fabricate electrodes with a well-developed porous structure to provide numerous accessible channels and a short diffusion distance for electrolyte ions12. As a novel porous material, graphitic carbon nitride nanosheets (g-C3N4 NSs) can be facilely synthesized on a large scale with low cost and exhibit an abundant, continuous, and adjustable porous structure13,14,15. In contrast to most porous materials, g-C3N4 NSs with ultrathin walls do not suffer from the inner-pore ion-transfer kinetics problem12,16. Additionally, the natural structure of g-C3N4 NSs includes a large number of pyrrolic N “hole” defects in the lattice and doubly bonded N at the edges of the vacancy, which are beneficial for the adsorption and motion of electrolyte ions13,17,18.

Extensive research has focused on directly growing or depositing pseudocapacitive materials on g-C3N4 NSs as SC electrodes19,20. For example, Zhang et al. constructed a lamellar Ni–Al double hydroxide/g-C3N4 NS hybrid electrode via an in situ self-assembly approach21 that delivered a specific capacitance as high as 714 F g−1 at 0.5 A g−1. Zhao et al. prepared a TiO2/g-C3N4 NS hybrid electrode with improved specific capacitance and no loss after 1000 cycles in SCs22. However, it is challenging to obtain a flexible SC electrode based on g-C3N4 NS hybrids owing to their inadequate mechanical properties and electrical conductivity23. It has been demonstrated that the incorporation of carbon nanotubes (CNTs) provides long continuous conductive paths and mechanical stability of the electrode for flexible energy-storage devices24,25. Conductive CNTs, which are characterized by a high aspect ratio and efficient assembly with other nanomaterials, particularly various nanosheets, contribute to the formation of robust three-dimensional (3D) interconnected architectures26,27,28. Nevertheless, neither the integration of g-C3N4 NSs with CNTs for flexible electrodes and their application in FISCs nor the investigation of g-C3N4 NSs as accessible channels for IL electrolyte ions has been reported.

In this work, a film electrode (MCNN/CNT) with a bioinspired Monstera leaf-like nanostructure is proposed and was fabricated by growing MnO2 nanoparticles on porous g-C3N4 NSs embedded in a CNT network. Owing to the porous g-C3N4 NSs as ion-accessible channels, the film electrode exhibited favorable ion dynamics and superior wettability in the IL electrolyte. Furthermore, the 3D architecture supported by porous g-C3N4 NSs and conductive CNTs ensured mechanical stability and charge transport. As a proof of concept, a prototype of these tailorable FISCs was fabricated and exhibited a high energy/power density and remarkable rate and cycling performance, outperforming previously reported FISCs. FISCs can be charged by harvesting solar/wind energy and can effectively power various portable electronics.

Materials and methods

Material detailed information

Multiwalled CNTs with high purity were purchased from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. The diameter and length of the CNTs are 10 nm and 20 μm, respectively. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) IL was obtained from the Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. EMIMBF4 IL was vacuum-dried at 100 °C overnight before use. All chemical reagents used in this study are of analytical grade and used without further purification.

Synthesis of g-C3N4 NSs

Bulk g-C3N4 was prepared via the direct thermal polycondensation of melamine as follows: 5 g of melamine powder was placed into an alumina crucible with a cover and heated to 520 °C for 2 h at a heating rate of 5 °C min−1. The yellow bulk g-C3N4 was vigorously milled into a powder using a mortar. Next, 1 g of bulk g-C3N4 was uniformly spread into an alumina ark with dimensions of 60 × 30 × 20 mm to ensure good contact between bulk g-C3N4 and air and was then heated to and maintained at 550 °C in an open system for 2 h at a heating rate of 10 °C min−1. Subsequently, the light-yellow powder was heated at 520 °C for 1 h in air at a heating rate of 20 °C min−1. Finally, porous g-C3N4 NSs (yield ~50 mg) were obtained as the resulting white powder (Fig. S1a).

Preparation of MCNNs/CNTs film

First, 12 mg of g-C3N4 NSs powder was added to 50 mL of deionized water with sonication for 5 min. Then, 20 mL of a 0.02-M KMnO4 solution and 20 mL of a 0.03-M MnSO4·4H2O solution were slowly added with stirring for 3 h. The resultant MCNN composite was collected via centrifugation, washed, and dispersed into deionized water to form a homogeneous MCNN solution. Then, the treated CNT aqueous dispersion (additional details are presented in Fig. S4) was mixed with the MCNN solution with continuous stirring for 20 h. The mass ratio of MCNNs to CNTs was approximately 60:40. Finally, the mixture solution was vacuum filtered through a cellulose ester filter membrane (pore size of 0.22 μm) and then peeled from the membrane to obtain the MCNN/CNT film (~1.65 g cm−3). For comparison, a similar process was employed for the preparation of an MnO2/CNT film without g-C3N4 NSs. Moreover, further experiments were conducted on growing MnO2 on a mixture of CNTs and g-C3N4 NSs (Fig. S18).

Assembly of FISCs

The resultant MCNN/CNT and CCNN/CNT films (additional details are presented in Fig. S8) were used as positive and negative electrodes, respectively. First, each film electrode (2 cm × 3 cm) was glued to a Ni slice using conducting Ag paste. Then, the ionogel electrolyte was slowly sandwiched between two film electrodes, which were pressed together under a pressure of ~1 MPa for 30 min. After the assembly, the FISCs were dried under vacuum at room temperature and stored in an Ar-filled glovebox for testing. The ionogel electrolyte was prepared as follows. First, 0.6 g of poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) was dissolved in an acetone solution. Then, 1.8 g of EMIMBF4 IL was added to the resulting solution under stirring until the solution became clear. EMIMBF4 was the IL electrolyte used in our research. EMIMBF4 is a relatively cheap and readily available IL electrolyte that is widely used as an electrolyte in energy-storage devices. The resulting viscous solution was cast onto a glass Petri dish to evaporate the acetone for 1 h to obtain the ionogel electrolyte.

Characterization

Field-emission scanning electron microscopy (SEM, FEI Sirion 200) and transmission electron microscopy (TEM, JEM-2100F) were carried out to characterize the morphology of the samples. Atomic force microscopy (AFM) images were recorded on a MultiMode 8 (Bruker) in ScanAsyst mode. X-ray diffraction (XRD) patterns were characterized on a powder XRD system with Cu Ka radiation, and X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD spectrometer with an Al Ka X-ray source. Nitrogen absorption and desorption measurements were performed with an Autosorb IQ instrument at 77 K. The specific surface area was calculated using the Brunauer–Emmett–Teller method, and the pore-size distribution was determined from the adsorption branch of the isotherm according to density functional theory. The surface tension of the EMIMBF4 IL was tested using a surface tensiometer (K100, Germany). The wettability of the films in the IL was measured using a contact angle measuring instrument (DCA300, Germany). The surface free energy of the films was determined using the two-liquid-phase method.

Electrochemical measurements

All electrochemical tests were performed using a VMP3 multifunctional electrochemical analysis instrument (Bio-Logic, France) via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) methods. The CV and GCD tests were performed at various scan rates and current densities. The EIS measurements were performed in the frequency range of 0.01 Hz to 100 kHz. The volumetric specific capacitance, energy density, and power density of the FISCs were calculated using the galvanostatic discharge curve, according to the two-electrode systematic calculation method:

$$C = \left( {I\times \Delta t} \right)/\left( {v\times \Delta U} \right),$$
(1)
$$E = C\times \left( {\Delta U} \right)^2/7.2,$$
(2)
$$P = 3600\times E/\Delta t,$$
(3)

where C (F cm3) is the volumetric specific capacitance, E (mWh cm−3) is the volumetric energy density, P (mW cm−3) is the volumetric power density, I is the discharge current, Δt is the discharge time, ΔU is the voltage variation during the discharge process after the “IR drop”, and v (cm3) is the total volume of the FISC (including the two film electrodes and the ionogel electrolyte). Additionally, the electrochemical kinetic behaviors of the MCNN/CNT and MnO2/CNT film electrodes in the EMIMBF4 IL electrolyte were compared using the three-electrode systematic method.

Results and discussion

As schematically shown in Fig. 1a, MCNN/CNT film electrodes were prepared through vacuum filtration via the solution-phase assembly of MCNNs and CNTs, yielding MnO2 nanoparticles grown on g-C3N4 NSs to form MCNNs and then embedded in a highly purified CNT network. The TEM image (Fig. 1b) showed a two-dimensional lamellar morphology with abundant pores on the layers of the g-C3N4 NSs. These pores were open and continuous, and an ultrathin pore wall was observed (Fig. 1c). The ultrathin wall could reduce the inner-pore migration distance of the electrolyte ions. According to the AFM image, the thickness of the g-C3N4 NSs was ~1 nm (Fig. 1d), further indicating the efficient exfoliation process of g-C3N4 NSs. Figure 1e shows the TEM image of the MCNN/CNT film electrode; spherical MnO2 nanoparticles ranging from 50 to 100 nm in size were tightly anchored on the g-C3N4 NSs, and both were entrapped in the CNT matrix. The interconnective porous g-C3N4 NSs and conductive CNTs in the film electrode jointly served as the backbone, mimicking the Monstera leaf with abundant pores and veins (inset in Fig. 1e). A cross-sectional SEM image (Fig. 1f) revealed that the average thickness of the MCNN/CNT film electrode was ~40 μm. A magnified image (Fig. 1g) indicated that the well-distributed MCNNs were homogeneously embedded in the CNT network, forming a 3D architecture. The robust skeleton ensured the high mechanical stability of the MCNN/CNT film electrode (inset in Fig. 1f). More information regarding the morphology and structure analyses of g-C3N4 NSs, CNTs, and MCNNs is provided in the Supporting Information (Fig. S1S4).

Fig. 1: Synthesis process and morphology characterizations.
figure1

a Schematic showing the fabrication process and architectural features of the MCNN/CNT film. b TEM, c high-resolution TEM, and d AFM images of g-C3N4 NSs. e TEM image of the MCNN/CNT film. The inset in e shows the structure of the MCNN/CNT film, which was inspired by the Monstera leaf. f Low- and g high-magnification cross-sectional SEM images of the MCNN/CNT film. The inset in f shows a digital photo of a flexible MCNN/CNT film electrode

Figure 2a shows the XRD patterns of CNTs, g-C3N4 NSs, and MCNN/CNT film. Apart from the peak at 27.6° corresponding to the (002) diffraction plane of the g-C3N4 NSs and 25.6° corresponding to the (002) diffraction plane of the CNTs, there were two weak diffraction peaks at ~37.8° and 67.1° in the XRD pattern of the MCNN/CNT film (Fig. 2a), which were indexed to α-MnO2·H2O (JCPDS No. 44–0141) with low crystallinity. More information about MnO2 in MCNNs is provided in Fig. S3. As shown in Fig. 2b, the XPS high-resolution N 1s spectrum indicated three N species. The dominant peaks at 399.8 and 398.3 eV could be assigned to the bridging N atoms in N(C)3 and the sp2-bonded N atoms in the triazine rings (C–N = C) of the g-C3N4 NSs, and the shoulder peak at 397.6 eV was due to the strong interaction between the CNTs and the N atoms in the g-C3N4 NSs29,30, confirming the robust self-assembly of the CNTs and MCNNs in the MCNN/CNT film electrode. Wettability is one of the most crucial properties of a film electrode10,31,32,33. For a droplet of liquid on a flat solid surface, the wetting behavior is determined by Young’s equation34,35:

$$\gamma _{sl} = \gamma _{sg} - \gamma _{lg}\times {\mathrm {cos}}\,\theta .$$
(4)
Fig. 2: Structural measurements and interfacial wettability.
figure2

a XRD pattern and b N 1s high-resolution XPS spectrum of the MCNN/CNT film. c Wetting model and contact angle of EMIMBF4 IL droplets on the MnO2/CNT and MCNN/CNT film electrodes. d Interfacial contact angle and solid surface free energy of the MnO2/CNT and MCNN/CNT film electrodes. e Schematic of the IL drop in contact with the superwettable MCNN/CNT film electrode

There is a relationship among the contact angle (θ), the surface tension (γlg) of the liquid, the surface free energy (γsg) of the solid, and the interfacial free energy (γsl) between the solid and liquid. Figure 2c shows the wetting process of the EMIMBF4 IL drop in contact with the MnO2/CNT and MCNN/CNT film electrodes. The MCNN/CNT film electrode was almost completely wetted by the IL electrolyte, exhibiting significantly better wettability than the MnO2/CNT film electrode. The small θ (18.4°) and large γsg (50.2 mJ m−2) of the MCNN/CNT film electrode in the IL electrolyte (Fig. 2d) confirmed its excellent wettability. The γlg of the EMIMBF4 IL electrolyte was measured to be ~52.4 mJ m−2, and the γsl of the MCNN/CNT film electrode in the IL electrolyte was calculated to be ~0.48 mJ m−2, which was almost four times lower than that of the MnO2/CNT film electrode (1.86 mJ m−2). The low γsl value indicated the strong adhesion between the MCNN/CNT film electrode and the IL electrolyte, which was due to the abundant pore channels and active sites of the g-C3N4 NSs in the MCNN/CNT film electrode (Figs. 2e and S5).

Figure 3a shows the CV curves of the MCNN/CNT and MnO2/CNT film electrodes in the EMIMBF4 IL electrolyte, which were obtained using a three-electrode setup. The capacitive performance of the MCNN/CNT film electrode was significantly higher than that of the MnO2/CNT film electrode, mainly owing to the favorable ionic diffusion of the MCNN/CNT film electrode in the IL electrolyte. To investigate the ionic diffusion of the film electrode in detail, the apparent activation energy, i.e., the energy barrier between the electrode and electrolyte, was calculated using the classical Arrhenius formula36,37:

$$lnC = lnC_0 - E_{\mathrm {a}}/RT,$$
(5)

where C is the amount of accumulated charge at the electrode/electrolyte interface, which is the specific capacitance of electrode; C0 is the pre-exponential constant; Ea is the activation energy; R is the universal gas constant; and T is the absolute temperature. By plotting the values of lnC with respect to 1/T, a straight line with a slope of K was obtained, allowing a straightforward calculation of −Ea (eV; 1 eV = 0.096 mJ mol−1). Figure 3b shows the corresponding Arrhenius curves of lnC (C: volumetric specific capacitance at 100 mA cm−3) vs. 1/T (T: temperatures ranging from 25 to 65 °C) for the MnO2/CNT and MCNN/CNT film electrodes in the IL electrolyte. The activation energy of the MCNN/CNT film electrode was ~0.078 mJ mol−1 (Ea = −K = 0.81 eV), which was significantly lower than that of the MnO2/CNT film electrode (~0.108 mJ mol−1 (Ea = –K = 1.12 eV)). The lower activation energy of the film electrode in the IL electrolyte represents a significantly smaller obstacle for the IL ions as they migrate from the electrolyte to the interior of the film electrode, leading to the high motion of IL ions in the MCNN/CNT film electrode. As shown in the Nyquist plots (Fig. 3c), the MCNN/CNT film electrode showed superior electrochemical kinetics compared with the MnO2/CNT film electrode. In addition, the diffusion coefficient of the IL ions in the MCNN/CNT film electrode was calculated to be 3.68 × 10−10 m2 s−1 (Fig. 3d), which was higher than that in the MnO2/CNT film electrode (2.85 × 10−10 m2 s−1), confirming the enhanced IL ion dynamics of the MCNN/CNT film electrode. More calculation details about the diffusion coefficient are shown in Fig. S6. Additional information regarding the electrochemical test results of the MCNN/CNT film electrode is provided in Figs. S7 and S8 and shows its high specific capacitance with good rate and cycle performance in the IL electrolyte.

Fig. 3: Electrochemical behaviors in IL electrolyte.
figure3

a CV curves, b Arrhenius curves, c Nyquist plots, and d diffusion coefficient curves of the MCNN/CNT and MnO2/CNT film electrodes in the IL electrolyte. e Schematic of the electrochemical process of the MCNN/CNT film electrode in the IL electrolyte

The excellent electrochemical behavior of the MCNN/CNT film electrode in the IL electrolyte was mainly due to the Monstera leaf-like structure with porous g-C3N4 NSs providing ion-accessible “highway” channels (Fig. 3e). The g-C3N4 NSs with abundant pore channels and reaction sites enhanced the interfacial wettability between the film electrode and the IL electrolyte. The relatively large mesopores of the g-C3N4 NSs offered the IL ions easy access into the film electrode, and the ultrathin wall of the g-C3N4 NSs reduced the inner-pore migration distance of the IL ions. This mechanism was similar to that of the pores in Monstera leaves for the permeation of air and water vapor. Large amounts of IL ions could rapidly diffuse into the interior of the film electrode (blue arrow) to participate in the electrochemical reaction with electroactive MnO2 nanoparticles. Furthermore, the highly conductive CNTs in the MCNN/CNT film electrode enhanced the charge-transport efficiency (white arrows), mimicking the transfer of organic nutrients through the veins of a leaf. Supplementary Movie 1 shows the ionic diffusion and charge transport in the MCNN/CNT film electrode. According to the foregoing discussion, the superwettable MCNN/CNT film electrode with a unique configuration has high compatibility with the IL electrolyte and remarkably enhances the accommodation of IL ions.

Similar to the IL electrolyte, an ionogel—a solid-state electrolyte—exhibits nonvolatility, nonflammability, high thermal stability, and a wide electrochemical potential window. Hence, FISCs were fabricated with an ionogel electrolyte using a P(VDF-HFP) copolymer as a gelator and hydrophobic EMIMBF4 IL as a solvent (Fig. 4a). Differentiated by the stable potential ranges in the IL electrolyte (Fig. 4b), the MCNN/CNT film and CCNN/CNT film (Fig. S9) were used as the positive and negative electrodes, respectively, in the device. The asymmetric design ensured that the fabricated FISCs had optimal capacitive performance. The CV curves indicated a high working voltage of 3 V (Fig. 4c), yielding electrochemical behavior significantly better than that of symmetric devices (Fig. S10). Figure 4d shows the galvanostatic charge/discharge (GCD) curves of the devices in the range of 100–800 mA cm−3. According to the SEM image, the total volume of the fabricated devices was approximately 0.06 cm3 (Fig. S11). The devices delivered a high volumetric capacitance of 8.6 F cm−3 at 100 mA cm−3, achieving a maximum volumetric energy density of 10.5 mWh cm−3 (Fig. 4e). This energy storage performance is superior to that of previously reported flexible solid-state SCs (mostly <2.5 mWh cm−3, Table S1) and several times higher than that of commercially available SCs (2.75 V/44 mF and 5.5 V/100 mF)38. The maximum volumetric power density of the SCs was 1804 mW cm−3 at a volumetric energy density of 4.2 mWh cm−3, which is comparable to that of commercially available SCs and significantly higher than that of thin-film lithium-ion batteries. As a result, the FISCs were able to power a motor fan and light a colorful LED (Fig. 4f).

Fig. 4: Fabrication procedure and electrochemical performance of FISCs.
figure4

a Fabrication procedure and digital photograph of FISCs. b CV curves of individual MCNN/CNT and CCNN/CNT film electrodes in the IL electrolyte obtained at a scan rate of 20 mV s−1. c CV curves, d GCD curves, and e Ragone plot of the FISCs. f FISCs successfully powered a motor fan and lighted the colorful display

To investigate the electrochemical mechanism of the fabricated FISCs in detail, the surface-reaction contribution and diffusion-controlled contribution of the total capacitance were examined. The linear relationship between the response current and scan rate corresponded to the surface-reaction and diffusion-controlled processes, which are defined by Eqs. (6) and (7)36.

$$i = a\times v^b,$$
(6)
$$ln\left( i \right) = ln\left( a \right) + b\times ln\left( v \right).$$
(7)

As shown in Fig. 5a, b values were in the range of 0.6–1.0, indicating a coupled diffusion-controlled and capacitive process for storage in the FISCs. The capacitive contribution to the total current was quantified using Eqs. (8) and (9)39:

$$i = k_1v + k_2v^{1/2},$$
(8)
$$i/v^{1/2} = k_1v^{1/2} + k_2.$$
(9)
Fig. 5: Electrochemical mechanism of the fabricated FISCs.
figure5

a b Values for the FISCs (inset: current response plotted with respect to the scan rate at different voltages). b Capacitive contribution to the charge storage at scan rates of 20 and 50 mV s−1

As shown in Fig. 5b, the capacitive contribution to the total current increased from 50.8% at 20 mV s−1 to 65.6% at 50 mV s−1. The results showed that the contribution of the diffusion-limited process to the total capacity was noticeable at scan rates below 20 mV s−1. At scan rates of ≥ 50 mV s−1, the response comprised surface capacitive effects rather than being diffusion controlled, which was beneficial to achieving a high power density at high current densities with the fabricated FISCs.

In addition to their satisfactory electrochemical performance, the FISCs exhibited high strength (Fig. 6a and S12) and flexibility (inset in Fig. 6b). The CV curves of the devices exhibited negligible changes under various conditions (Fig. 6b). The devices maintained nearly 100% of their initial capacitance upon bending at different angles. Under both straight and bending conditions, the FISCs showed strong durability (Fig. 6c), indicating their high electrochemical reliability under harsh mechanical conditions (Fig. S13). The FISCs exhibited a high capacitance retention ratio of 88.5% after 10,000 cycles in the straight state, outperforming previously reported SCs based on an IL or ionogel electrolyte36,40. Owing to their soft and robust structure, the FISCs can be tailored by cutting them into a particular size and shape (Fig. S14). The versatility in the shape of the FISCs gives them potential for a wide range of applications in stylish energy-storage devices.

Fig. 6: Stability evaluations under given conditions.
figure6

a Digital photos of FISCs with high strength. b CV curves of FISCs under different mechanical deformations. The inset in b shows FISCs with flexibility. c Cycle life of the devices in the straight and bending states at a current density of 200 mA cm−3 after 10,000 cycles

Energy sustainability is crucial for modern society. For instance, harvesting solar and wind energy can resolve all the present energy concerns and reduce the reliance on fossil fuels. To overcome the intermittency of solar/wind energy, an attempt to store this type of energy in FISCs was made. A commercial miniature solar-cell panel (SCP) and a wind-driven generator (WDG, Fig. S15) were assembled using the fabricated FISCs as a sustainable energy system. Four FISCs connected in a serial and parallel arrangement (Fig. S16) served as an energy-storage module, collecting solar energy generated by the SCP on sunny days and wind energy generated by the WDG in windy weather (Fig. S17). After charging, the FISCs could effectively power various portable electronics. The corresponding videos are shown in Supplementary Movies 2 and 3. For a real-world application, the sustainable energy system was directly attached to a commercial bicycle. The SCP and WDG were placed inside and on the front of the basket of the bicycle, respectively. During riding, the panel of the SCP was completely exposed to the sun, and the blades of the WDG could turn at a high speed under the wind produced by the motion of the bicycle (Fig. 7a); hence, the FISCs on the bicycle were charged by harvesting solar and wind energy (Fig. 7b, c). More importantly, the FISCs functioned as efficient, wearable energy-storage devices after charging, powering portable electronics in various complex motion states (Fig. 7d–f). The corresponding video is shown in Supplementary Movie 4. To the best of our knowledge, the strategy of developing a sustainable energy system for portable electronics has hardly been exploited, particularly systems utilizing wind power, which is rarely studied. Self-discharging performance is a crucial parameter for the practical application of SCs. As shown in Fig. S19, the fabricated FISCs underwent a rapid self-discharge process in the first few minutes and gradually slowed down after several hours. Finally, the open-circuit voltage of the devices was maintained at ~2.5 V beyond 10 h, indicating the low self-discharge rate of the fabricated FISCs after harvesting sustainable energy. Therefore, this study introduces a new avenue for sustainable energy harvesting, as well as wearable and portable energy storage, which are of great significance for practical use of renewable energy resources.

Fig. 7: Proof of concept for a variety of applications.
figure7

a Digital photograph of the FISCs on the bicycle, which was charged using solar and wind energy during cycling. Real-time image of the charging process of the FISCs through the harvesting of solar (b) and wind (c) energy during riding. df FISCs act as efficient, wearable energy-storage devices after charging and can power portable electronics in various complex motion states

Conclusions

We successfully developed and demonstrated an innovative methodology for preparing a bioinspired Monstera leaf-like MCNN/CNT film electrode. Similar to the pores in Monstera leaves for the permeation of air and water vapor, the film electrode was based on porous g-C3N4 NSs as ion-accessible channels, leading to the ultrafast spreading of IL ions. According to detailed investigations and accurate calculations, the film electrode exhibited extraordinary ion dynamics and superior wettability in an IL electrolyte. Using the CCNN/CNT film electrode, tailorable FISCs were fabricated that exhibited a high energy/power density and excellent rate and cycle capability. FISCs can be charged by harvesting solar/wind energy and can effectively power various portable electronics. The versatility in the shape of the tailorable FISCs makes them suitable for a wide range of applications in stylish energy-storage devices. Considering the facile fabrication and outstanding performance of the developed electrode, this study paved the way for the integration of sustainable energy harvesting and flexible energy storage.

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Acknowledgements

Minjie Shi and Cheng Yang contributed equally to this work. We appreciate the fruitful discussions with Prof. François Béguin at Poznan University of Technology and Dr. Qiang Gao at Oak Ridge National Laboratory. We greatly acknowledge the funding for this project through the National Natural Science Foundations of China (No. 51873083), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (No. sklpme2018-4-27), the Six Talent Peaks Project in Jiangsu Province (No. 2015-XCL-028), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_1831, SJCX18_0759), the Key University Science Research Project of Jiangsu Province (18KJA130001), and the Beijing Natural Science Foundation (No. 2192039). J.-H.A. acknowledges support from the National Research Foundation of Korea (NRF-2015R1A3A2066337). Z.S. acknowledges support from National Natural Science Foundation of China (NSFC, No. 21972010) and Beijing Natural Science Foundation (No. 2192039).

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Shi, M., Yang, C., Yan, C. et al. Boosting ion dynamics through superwettable leaf-like film based on porous g-C3N4 nanosheets for ionogel supercapacitors. NPG Asia Mater 11, 61 (2019). https://doi.org/10.1038/s41427-019-0161-7

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