Composite Aerogels of Carbon Nanocellulose Fibers and Mixed-Valent Manganese Oxides as Renewable Supercapacitor Electrodes

Bio-waste derived nanocelluloses show excellent mechanical flexibility and self-aggregated capability, which enable them to be good supporting substrates for the synthesis of electroactive materials. Herein, we present a facile route for fabricating composite aerogels consisting of carbonized nanocellulose fibers (CNF) and mixed-valent manganese oxide (MnOx), toward supercapacitor applications. Mixed solutions of nanocellulose and manganese acetate with different ratios were prepared and freeze-dried into hybrid aerogels. The hybrid aerogels were then transformed into CNF/MnOx composites by a calcination process. The CNF membranes served as porous carbon nano-reservoirs for MnOx and electrolyte. The CNF/MnOx composites also kept a 3D porous aerogel structure with hierarchical pores, which enabled stable transport of both electrolyte ions and electrons to the electrode surface, leading to low a charge-transfer impedance and good electrochemical kinetics. The CNF/MnOx-based symmetric supercapacitor showed a satisfied energy density and power density of 37.5 Wh kg−1 and 2.75 kW kg−1, respectively. All the above results demonstrate the feasibility of using sustainable nanocellulose as a nanoscale carbon substrate for the synthesis of hybrid composite electrodes toward renewable supercapacitor applications.


Introduction
Parallel to the growing need for renewable energy supply, there is an increasing demand for high-performance power sources with low-cost, lightweight, portable, and environmentally friendly features. Supercapacitors have emerged as a promising candidate for electrochemical energy storage because of their fast charge-discharge processes, providing a high energy density and a reasonable power density [1]. Both the materials and the structures are the cruxes in developing high-performance supercapacitors [2]. In terms of the materials, despite offering key performance advantages, many components are often expensive and contain fluorine, sulfur, and cyanide groups, which pose environmental hazards if discarded and disposed of using conventional landfill or incineration methods. Also, the derived supercapacitors sometimes suffer from a low energy density. Aiming at solving these problems, various categories of redox-reaction materials, including metal oxides, conductive polymers, and hybrid materials, have been employed as electrodes to dramatically improve the energy density. In regard to the structure, one key challenge is to achieve a high loading of electroactive materials per unit area with good contact of the electrodes. As the appeal of hybrid structures lies in the synergistic effects based on interfacial charge and energy transfer processes, controlling the nature of the interface and maximizing the interfacial area become critical issues.
The electrode was prepared by loading a slurry containing CNF/MnO x -m, poly(vinylidene fluoride) (PVDF) (in N-methylpyrrolidone), and acetylene black (8:1:1 wt.%) on a nickel collector. After the mixed materials were loaded, the electrode was pressed and dried in vacuum at 80 • C for 12 h. In the case of a three-electrode system, an Au-coated polyethylene terephthalate substrate was used as a working electrode, a mercurous sulfate electrode as the reference electrode, and Pt wire as the counter electrode. 1.0 M Na 2 SO 4 solution was used as an electrolyte. A symmetric supercapacitor was assembled using two pieces of 1.5 × 1.5 cm CNF/MnO x electrodes (the average weight of each electrode was 2.0 mg), with 1.0 M LiPF 6 ethylene carbonate/diethylene carbonate/ethyl-methyl carbonate (EC/DEC/EMC) (1:1:1 v/v/v) electrolyte-soaked separator in between.

Characterizations
A JEM-2100 transmission electron microscope (TEM, JEOL Ltd., Tokyo, Japan) was used for the TEM imaging. Scanning electron microscopy (SEM) images were taken on a Quanta200 microscope (FEI, Hillsboro, OR, USA), at an operating voltage of 15 kV. The surface chemical composition was determined by an energy-dispersive X-ray (EDX) spectroscope connected to the SEM. Nitrogen adsorption-desorption isotherm measurements were performed using a 3H-2000PM2 Brunauer-Emmett-Teller (BET) surface area analyzer (Micromeritics, Beishide Instrument-ST Ltd., Beijing, China). The samples were degassed at 300 • C under vacuum for 5 h before the measurements. Specific surface area was calculated using the BET method. The total pore volume was determined at a relative pressure of 0.98. Pore volume and pore-size distribution were calculated using a nonlocal density functional theory (NLDFT) model based on the nitrogen adsorption branches of the isotherms. X-ray photoelectron spectroscopy (XPS) was performed using a K-Alpha system (Thermo Fisher Scientific, West Sussex, UK) operated at 14.0 kV and all binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV. X-ray diffraction (XRD) patterns were obtained using a D/max 2200 Polymers 2019, 11, 129 4 of 14 X-ray diffractometer (Rigaku, Tokyo, Japan) with Ni-filtered CuK α (λ = 1.5406 Å) radiation at 40 kV and 30 mA. Scattered radiation was detected in the range of 2θ = 10 • −80 • at a scan rate of 4 • min −1 .

Electrochemical Measurements
Electrochemical measurements including cyclic voltammetry (CV) curves, galvanostatic charge-discharge (GCD) curves, and electrochemical impedance spectroscopy (EIS) were conducted on a CHI 660D electrochemical workstation (Chenhua Instrument Co., Shanghai, China) at room temperature. The electrochemical performance of individual electrodes was investigated using a three-electrode system prior to the fabrication of the supercapacitors. CV measurements were performed at different scan rates (2-100 mV s −1 ), within a potential window of -0.5 to 0.5 V. GCD measurements were performed with a constant current from 0.25-2 A g −1 . EIS measurements were conducted for the working electrode in a frequency range of 100 kHz to 0.01 Hz with alternating current (AC) perturbation of 5 mV. The EIS data were analyzed using Nyquist plots, which represent the imaginary part (Z ) and the real part (Z") of impedance. The performance of the supercapacitors was measured using a two-electrode method at scan rates of 5-100 mV s −1 within a potential window of 0 to 3.5 V.
The specific capacitance (C, F g −1 ) was calculated from the corresponding GCD curves at different current densities according to the equation: where I(A) is the response current, m is the average mass of the electrode, and dt/dV is the inverse of the slope of the discharge curve (V s −1 ). The maximum specific energy density (E max ) and power density (P max ) were calculated according to the equations below: E max = 0.5CV 2 max /3.6 (2)

Results and Discussion
The preparation procedure of the 3D porous CNF/MnO x composite electrodes for supercapacitors is illustrated in Figure 1. First, nanocellulose fibers were disintegrated from wood waste through biorefinery steps. The resultant nanocelluloses exhibited fibrous structures with 2-20 nm widths and lengths exceeding 1 µm (Figure 2a) [47]. Because of high aspect ratios and numerous hydroxyl groups being exposed, the nanocellulose fibers are likely to intertwine together, which facilitates the formation of the gel network, particularly for mass concentrations above 0.8 wt.% [48]. Subsequently, the nanocellulose suspension was mixed with Mn(OAc) 2 solutions containing different concentrations of solute, and freeze-dried into freestanding hybrid aerogels with tailored shapes and sizes. The obtained hybrid aerogels showed no significant differences in appearance compared with the nanocellulose aerogels. After calcination at 850 • C in an argon atmosphere, the hybrid aerogels turned into CNF/MnO x -m composite aerogels, and the colors changed into black (Figure 2b). The shape of the composites was maintained but the volume shrunk by almost 40%. Thus, the bulk density increased from 0.01 g cm −3 to 0.16 g cm −3 . Meanwhile, the composites exhibited flexible and elastic features. sizes. The obtained hybrid aerogels showed no significant differences in appearance compared with the nanocellulose aerogels. After calcination at 850 °C in an argon atmosphere, the hybrid aerogels turned into CNF/MnOx-m composite aerogels, and the colors changed into black (Figure 2b). The shape of the composites was maintained but the volume shrunk by almost 40%. Thus, the bulk density increased from 0.01 g cm −3 to 0.16 g cm −3 . Meanwhile, the composites exhibited flexible and elastic features.  The XPS survey was performed to determine the surface chemical state of the CNF/MnO x composite aerogels, and the spectra of the CNF/MnO x -4 are shown as representative examples. In the wide-scan spectrum (Figure 3a), the composition elements only contain C, O, and Mn, whereby the content of C is higher than the others, suggesting deep graphitization of CNF. This is also demonstrated by the fitting curve of the C 1s profile that is related to C-C bonds (sp 2 carbon) at a chemical shift of 284.5 eV (Figure 3c). Mn can assume multiple oxidation states (Mn 2+ , Mn 3+ , and Mn 4+ ) and diverse phases during both oxidation and reduction processes. There are two peaks of Mn 2p 1/2 and Mn 2p 3/2 , and the energy difference between these two peaks is 11.6 eV ( Figure 3b). According to the fitting curves of the O 1s profile (Figure 3d), the three deconvoluted peaks can be assigned to the chemical bonds of Mn-O-Mn, Mn-O-H, and C-O at binding energies of 529.9, 531.4, and 532.7 eV, respectively. All these indicate that two kinds of manganese oxides are generated during the calcination process. The formation of a mixed-valent compound rather than single-phase MnO x is attributed to oxygen deficiency during the calcination process.
(SAED) cyclic pattern in Figure 2j further reveal the graphite carbon structure of the CNF membranes. The MnOx nanoparticles that attached to the CNF membranes possess typical crystalline nanodomains ( Figure 2k). The CNF aerogel has a high BET surface area (SBET) value of 554.8 m 2 g −1 , whereas that of CNF/MnOx-4 drops to 219.3 m 2 g −1 . The pore sizes of CNF/MnOx-4 were calculated 1.99−305.3 nm from the pore-size distribution curves. The mesopore (2.0−58.0 nm) fraction and micropore (≤1.95 nm) fraction were calculated as 80.25% and 31.50%, respectively ( Figure 2l). The unique structural characteristics of the composite are expected to benefit the absorption of electrolytes and provide diffusion channels for electrolyte ions.  The XPS survey was performed to determine the surface chemical state of the CNF/MnOx composite aerogels, and the spectra of the CNF/MnOx-4 are shown as representative examples. In the wide-scan spectrum (Figure 3a), the composition elements only contain C, O, and Mn, whereby the content of C is higher than the others, suggesting deep graphitization of CNF. This is also demonstrated by the fitting curve of the C 1s profile that is related to C-C bonds (sp 2 carbon) at a chemical shift of 284.5 eV (Figure 3c). Mn can assume multiple oxidation states (Mn 2+ , Mn 3+ , and Mn 4+ ) and diverse phases during both oxidation and reduction processes. There are two peaks of Mn 2p1/2 and Mn 2p3/2, and the energy difference between these two peaks is 11.6 eV ( Figure 3b). According to the fitting curves of the O 1s profile (Figure 3d), the three deconvoluted peaks can be assigned to the chemical bonds of Mn-O-Mn, Mn-O-H, and C-O at binding energies of 529.9, 531.4, and 532.7 eV, respectively. All these indicate that two kinds of manganese oxides are generated during the calcination process. The formation of a mixed-valent compound rather than single-phase MnOx is attributed to oxygen deficiency during the calcination process. The phase structure of the CNF/MnOx composite aerogels was further analyzed from a crystallographic perspective. Figure 4 shows the relevant XRD patterns. The wide amorphous peak in the range of 2θ = 18-30° is ascribed to CNF. A small diffraction peak is observed at 2θ = 23.5°, which may be ascribed to the (002) lattice plane of highly graphitized carbon. The mixed crystals of   The phase structure of the CNF/MnO x composite aerogels was further analyzed from a crystallographic perspective. Figure 4 shows the relevant XRD patterns. The wide amorphous peak in the range of 2θ = 18-30 • is ascribed to CNF. A small diffraction peak is observed at 2θ = 23.5 • , which may be ascribed to the (002) lattice plane of highly graphitized carbon. The mixed crystals of Mn 3  The electrochemical performance of the CNF/MnOx electrodes was evaluated from the CV measurements. All the CNF/MnOx composites exhibit symmetric quasi-rectangular CV shapes, indicating a high reversible charge-storage capacity (Figure 5a). This is attributed to the successive multiple surface redox reactions of MnOx, behaving with a charge-storage mechanism that is different from that of most metal oxides. No obvious oxidation and deoxidization peaks are observed, but orientation polarization is displayed. The hysteresis at a relative pressure between 0.4 and 0.5 confirms the existence of mesopores. At a high scan rate of 50 mV s −1 , the CNF/MnOx electrodes also exhibit quasi-rectangular CV shapes, and the symmetry is improved (Figure 5b). As the scan rate is increased from 2 to 100 mV s −1 , the response current improves and shows certain multiplying power characteristic, and the orientation polarization subsides (Figure 5c). The amount of MnOx also has a significant influence on the specific capacitance of CNF/MnOx composites (Figure 5d). The specific capacitance of CNF/MnOx-1 is 35.9 F g −1 , which is similar to that of pure CNF (34.8 F g −1 ). The specific capacitances of CNF/MnOx-2 and CNF/MnOx-3 are twice and three times that of pure CNF. Notably, CNF/MnOx-4 shows the highest specific capacitance (138.4 F g −1 ). The reduction of specific capacitance at high scan rates is assumed to be caused by the limiting redox reaction of MnOx and hysteresis of the charge-transfer coefficient. When the mass fraction of MnOx was further increased, the specific capacitance started to decrease ( Figure S2). The electrochemical performance of the CNF/MnO x electrodes was evaluated from the CV measurements. All the CNF/MnO x composites exhibit symmetric quasi-rectangular CV shapes, indicating a high reversible charge-storage capacity (Figure 5a). This is attributed to the successive multiple surface redox reactions of MnO x , behaving with a charge-storage mechanism that is different from that of most metal oxides. No obvious oxidation and deoxidization peaks are observed, but orientation polarization is displayed. The hysteresis at a relative pressure between 0.4 and 0.5 confirms the existence of mesopores. At a high scan rate of 50 mV s −1 , the CNF/MnO x electrodes also exhibit quasi-rectangular CV shapes, and the symmetry is improved (Figure 5b). As the scan rate is increased from 2 to 100 mV s −1 , the response current improves and shows certain multiplying power characteristic, and the orientation polarization subsides (Figure 5c). The amount of MnO x also has a significant influence on the specific capacitance of CNF/MnO x composites (Figure 5d). The specific capacitance of CNF/MnO x -1 is 35.9 F g −1 , which is similar to that of pure CNF (34.8 F g −1 ). The specific capacitances of CNF/MnO x -2 and CNF/MnO x -3 are twice and three times that of pure CNF. Notably, CNF/MnO x -4 shows the highest specific capacitance (138.4 F g −1 ). The reduction of specific capacitance at high scan rates is assumed to be caused by the limiting redox reaction of MnO x and hysteresis of the charge-transfer coefficient. When the mass fraction of MnO x was further increased, the specific capacitance started to decrease ( Figure S2). The CNF/MnOx electrodes were evaluated by means of GCD measurements within −0.5 to 0.5 V. The charge-discharge curves exhibit an almost triangular shape with a small internal resistance drop (Figure 6a). The deviation of the charge-discharge curves from the linear voltage-time relationship indicates a pseudocapacitive behavior. In aqueous electrolytes, the pseudo-capacitance of MnOx comes from the intercalation and de-intercalation of protons and/or alkali ions, as well as redox reactions. The ion intercalation and de-intercalation and redox reactions take place on the surface of the MnOx under high-speed charge-discharge. The presence of the internal resistance drop at the beginning of the discharge is usually associated with the equivalent series resistance phenomenon. At the same current density, the CNF/MnOx-4 electrode shows superior performance to the others. The discharge curves of the CNF/MnOx-4 electrode display a nearly linear response at different current densities from 0.25 to 2.0 A g −1 with a corresponding discharge time of 300 s and 30 s, respectively (Figure 6b). Figure 6c shows a typical GCD curve of a CNF/MnOx-4 electrode under continuous operation for the first several cycles, which indicates ideal electrochemical reversibility and almost symmetric charge-discharge properties. The specific capacitance can be calculated from the discharge curves using Equation (1) and shows a downward tendency with the rise of current density (Figure 6d). The Nyquist plot displays a typical diffusion-controlled Warburg capacitive behavior with a diagonal line in the low-frequency region and a small depressed semicircle in the high-frequency region. From the EIS results, the Nyquist plot shows an intercept at the real impedance (Z′) of 1.1 Ω (Figure 6e). An elongated 'semicircle' can be attributed to the widely distributed pore sizes in the structure. The relatively small serial resistance suggests a compact electrode system that ensures fast charge transfer between electrolyte and electrodes. Figure 6f shows good electroconductivity of CNF/MnOx-4 electrode in a series circuit. From above all, CNF/MnOx-4 was selected as the optimal sample for constructing a symmetric supercapacitor. The CNF/MnO x electrodes were evaluated by means of GCD measurements within −0.5 to 0.5 V. The charge-discharge curves exhibit an almost triangular shape with a small internal resistance drop (Figure 6a). The deviation of the charge-discharge curves from the linear voltage-time relationship indicates a pseudocapacitive behavior. In aqueous electrolytes, the pseudo-capacitance of MnO x comes from the intercalation and de-intercalation of protons and/or alkali ions, as well as redox reactions. The ion intercalation and de-intercalation and redox reactions take place on the surface of the MnO x under high-speed charge-discharge. The presence of the internal resistance drop at the beginning of the discharge is usually associated with the equivalent series resistance phenomenon. At the same current density, the CNF/MnO x -4 electrode shows superior performance to the others. The discharge curves of the CNF/MnO x -4 electrode display a nearly linear response at different current densities from 0.25 to 2.0 A g −1 with a corresponding discharge time of 300 s and 30 s, respectively (Figure 6b). Figure 6c shows a typical GCD curve of a CNF/MnO x -4 electrode under continuous operation for the first several cycles, which indicates ideal electrochemical reversibility and almost symmetric charge-discharge properties. The specific capacitance can be calculated from the discharge curves using Equation (1) and shows a downward tendency with the rise of current density (Figure 6d). The Nyquist plot displays a typical diffusion-controlled Warburg capacitive behavior with a diagonal line in the low-frequency region and a small depressed semicircle in the high-frequency region. From the EIS results, the Nyquist plot shows an intercept at the real impedance (Z ) of 1.1 Ω (Figure 6e). An elongated 'semicircle' can be attributed to the widely distributed pore sizes in the structure. The relatively small serial resistance suggests a compact electrode system that ensures fast charge transfer between electrolyte and electrodes. Figure 6f shows good electroconductivity of CNF/MnO x -4 electrode in a series circuit. From above all, CNF/MnO x -4 was selected as the optimal sample for constructing a symmetric supercapacitor.  Figure 7a is the schematic illustration of the CNF/MnOx-4-based symmetric supercapacitor. Figure 7b shows a series of rate-dependent CV curves of the CNF/MnOx-4-based symmetric supercapacitor in the voltage range of 0-3.5 V. The CV measurements were conducted at a scan rate of 5, 10, 20, 50, and 100 mV s −1 . Similar to the results measured in a three-electrode configuration, the assembled supercapacitor exhibited quasi-rectangular CV curves. Moreover, even at a high scan rate of 100 mV s −1 , the CV curves remain symmetrical, suggesting good reversibility of the electrochemical processes. From the EIS results, the Nyquist plot shows an intercept at Z′ of 4.6 Ω (Figure 7c). Furthermore, the low-frequency region of the impedance curves shows a nearly straight sloping line, suggesting a good capacitive performance of the electrode. The calculated specific capacitances at different scan rates are present in Figure 7d. The CNF/MnOx-4-based symmetric supercapacitor shows specific capacitances from 269.7 F g −1 to 108.6 F g −1 as the scan rate rises from 5 to 100 mV s −1 . The specific capacitances represent an 80% retention of the initial capacitance after 1000 cycles of charging-discharging at 2 A g −1 (Figure 7e).
As noted from the CV and GCD studies, high energy density and power density are expected for such a supercapacitor assembly. Energy density and power density of the CNF/MnOx-based symmetric supercapacitor are calculated from the GCD profiles and are illustrated in a Ragone plot (Figure 7f). A maximum energy density of 37.5 Wh kg −1 can be stored at a power density of 2.75 kW kg −1 , under the same charge-discharge condition at a voltage of 3.5 V. From the Ragone plot, the performance of our supercapacitor can compare with or superior to those of CNT/MnO2-based supercapacitors and graphene/MnO2-based supercapacitors [49][50][51][52][53][54][55] (e.g., 3D MnO2/grapheme [49], graphene/CNT/MnO2 [51], graphene/MnO2/CNT supercapacitors [52]).   Figure 7b shows a series of rate-dependent CV curves of the CNF/MnO x -4-based symmetric supercapacitor in the voltage range of 0-3.5 V. The CV measurements were conducted at a scan rate of 5, 10, 20, 50, and 100 mV s −1 . Similar to the results measured in a three-electrode configuration, the assembled supercapacitor exhibited quasi-rectangular CV curves. Moreover, even at a high scan rate of 100 mV s −1 , the CV curves remain symmetrical, suggesting good reversibility of the electrochemical processes. From the EIS results, the Nyquist plot shows an intercept at Z of 4.6 Ω (Figure 7c). Furthermore, the low-frequency region of the impedance curves shows a nearly straight sloping line, suggesting a good capacitive performance of the electrode. The calculated specific capacitances at different scan rates are present in Figure 7d. The CNF/MnO x -4-based symmetric supercapacitor shows specific capacitances from 269.7 F g −1 to 108.6 F g −1 as the scan rate rises from 5 to 100 mV s −1 . The specific capacitances represent an 80% retention of the initial capacitance after 1000 cycles of charging-discharging at 2 A g −1 (Figure 7e). The satisfied performance of our supercapacitor is mainly contributed to the synergistic effects of material chemistry and structure on the electrochemistry performance of CNF/MnOx-based electrodes, which are summarized and ascribed in the following aspects: (1) The mixture of nanocellulose suspension and Mn(OAc)2 solution is uniform and homogeneous, which prevents the aggregation of MnOx colloids. (2) The CNF membranes are fabricated as the carbon aerogel carriers for MnOx and nanoreservoirs for electrolytes. (3) A large number of research data have concluded that the electrical properties of carbon/MnOx composite depend on the mass fraction of MnOx. When the mass fraction of MnOx is less than 70%, the electrical properties increase with the increase of the mass of MnOx. In this experiment, the optimal MnOx loading is just half of the best, indicating a high utilization efficiency of MnOx. (4) The web-like conductive percolating structure facilitates the diffusion path of electrolytes and reduces the charge-transfer impedance. Electrons can easily travel through the diffusion path and can be picked up by MnO4 − ions at other locations. (5) The high electrical conductivity and low charge-transfer resistance improve the kinetics significantly and are favorable to overcome the kinetic limitations at high-current operation. (6) The integrated structure contains the essential components (electrodes, spacer, and electrolyte) of an electrochemical device, paving the way for designing a variety of energy-storage devices [43].

Conclusions
Freestanding CNF/MnOx composite aerogels were facilely synthesized by freeze-drying an aqueous dispersion of nanocellulose and Mn(OAc)2, followed by a calcination process. Despite the straightforward and inexpensive preparation, the shaped composites showed 3D porous structures with hierarchical pores, which benefit the transportation of both electrolyte ions and electrons. The composites exhibit quasi-rectangular CV curves, pseudocapacitive behavior, and small impedance. When used in a supercapacitor, the as-prepared CNF/MnOx-based symmetric supercapacitor showed a specific capacitance of 269.7 F g −1 and possessed an energy density as high as 37.5 Wh kg −1 , as well as a power density of 2.75 kW kg −1 . Meanwhile, the supercapacitor displayed cyclic stability with capacitance retention of 80% after 1000 charge−discharge cycles at 2 A g −1 . The approach is therefore highly promising in terms of both cost efficiency and scale-up for the development of renewable energy-storage devices toward sustainability. As noted from the CV and GCD studies, high energy density and power density are expected for such a supercapacitor assembly. Energy density and power density of the CNF/MnO x -based symmetric supercapacitor are calculated from the GCD profiles and are illustrated in a Ragone plot (Figure 7f). A maximum energy density of 37.5 Wh kg −1 can be stored at a power density of 2.75 kW kg −1 , under the same charge-discharge condition at a voltage of 3.5 V. From the Ragone plot, the performance of our supercapacitor can compare with or superior to those of CNT/MnO 2 -based supercapacitors and graphene/MnO 2 -based supercapacitors [49][50][51][52][53][54][55] (e.g., 3D MnO 2 /grapheme [49], graphene/CNT/MnO 2 [51], graphene/MnO 2 /CNT supercapacitors [52]).
The satisfied performance of our supercapacitor is mainly contributed to the synergistic effects of material chemistry and structure on the electrochemistry performance of CNF/MnO x -based electrodes, which are summarized and ascribed in the following aspects: (1) The mixture of nanocellulose suspension and Mn(OAc) 2 solution is uniform and homogeneous, which prevents the aggregation of MnO x colloids. (2) The CNF membranes are fabricated as the carbon aerogel carriers for MnO x and nanoreservoirs for electrolytes. (3) A large number of research data have concluded that the electrical properties of carbon/MnO x composite depend on the mass fraction of MnO x . When the mass fraction of MnO x is less than 70%, the electrical properties increase with the increase of the mass of MnO x . In this experiment, the optimal MnO x loading is just half of the best, indicating a high utilization efficiency of MnO x . (4) The web-like conductive percolating structure facilitates the diffusion path of electrolytes and reduces the charge-transfer impedance. Electrons can easily travel through the diffusion path and can be picked up by MnO 4 − ions at other locations. (5) The high electrical conductivity and low charge-transfer resistance improve the kinetics significantly and are favorable to overcome the kinetic limitations at high-current operation. (6) The integrated structure contains the essential components (electrodes, spacer, and electrolyte) of an electrochemical device, paving the way for designing a variety of energy-storage devices [43].

Conclusions
Freestanding CNF/MnO x composite aerogels were facilely synthesized by freeze-drying an aqueous dispersion of nanocellulose and Mn(OAc) 2 , followed by a calcination process. Despite the straightforward and inexpensive preparation, the shaped composites showed 3D porous structures with hierarchical pores, which benefit the transportation of both electrolyte ions and electrons. The composites exhibit quasi-rectangular CV curves, pseudocapacitive behavior, and small impedance. When used in a supercapacitor, the as-prepared CNF/MnO x -based symmetric supercapacitor showed a specific capacitance of 269.7 F g −1 and possessed an energy density as high as 37.5 Wh kg −1 , as well as a power density of 2.75 kW kg −1 . Meanwhile, the supercapacitor displayed cyclic stability with capacitance retention of 80% after 1000 charge−discharge cycles at 2 A g −1 . The approach is therefore highly promising in terms of both cost efficiency and scale-up for the development of renewable energy-storage devices toward sustainability.

Conflicts of Interest:
The authors declare no competing interests.