Amorphous cellulose nanofiber supercapacitors

Despite the intense interest in cellulose nanofibers (CNFs) for biomedical and engineering applications, no research findings about the electrical energy storage of CNF have been reported yet. Here, we present the first electroadsorption effects of an amorphous cellulose nanofiber (ACF) supercapacitor, which can store a large amount of electricity (221 mJm−2, 13.1 Wkg−1). The electric storage can be attributed to the entirely enhanced electroadsorption owing to a quantum-size effect by convexity of 17.9 nm, an offset effect caused by positive polar C6=O6 radicles, and an electrostatic effect by appearance of the localised electrons near the Na ions. The supercapacitor also captures both positive and negative electricity from the atmosphere and in vacuum. The supercapacitor could illuminate a red LED for 1 s after charging it with 2 mA at 10 V. Further gains might be attained by integrating CNF specimens with a nano-electromechanical system (NEMS).

www.nature.com/scientificreports/ corresponds to 221 mJm −2 (13.1 Wkg −1 , and 1.6 Whkg −1 ). We illuminated a red LED to prove the electric energy storage of ACF. The device with a surface area of 2400 mm 2 , lit the LED for 1 s (Fig. 1d) after it was charged with 2 mA at 10 V. Further gains might be attained by integrating CNF specimens with a nano-electromechanical system (NEMS) (see Fig. S9 in SI).

Complex evaluation of electric storage and I-V characteristics.
To analyse non-destructively the electrostatic contribution of the specimen, we measured the AC impedance from 1 to 1 MHz. A Nyquist plot of the impedance data is shown in Fig. 2a. The impedance variation with frequency data of ACF follows the combined pattern of a quadrant, straight horizontal line (inset of Fig. 2a), line with a slope of π/4 rad, and nearvertical line, attributing to a series-RC circuit 9,11,12 . The π/4 rad region (Warburg region) is a consequence of the distributed resistance/capacitance in the porous electrode 19 . The imaginary and real impedances rapidly increase up to 4 and 2 MΩ in the lower-frequency region of the Bode diagram, respectively (Fig. 2b). Moreover, the decrement in phase angle to -90°with decreasing frequency is another evidence of DC charging (Fig. 2c). This means that each capacitor on the ACF specimen is connected to a series circuit, C = n k=1 Ck = nC . Therefore, the ACF offers an approximately ideal electric distributed constant (EDC) structure for enhancing electrical power storage. The experimental curve of series capacitance Cs can be expressed as Cs = 1.85f −0.494 (r 2 = 0.9984), where modulating the frequency f allows a considerable increase in DC capacitance. Figure 2d represents double I-V and R-V characteristics between − 200 and + 200 V in air. The curves are asymmetric relative to zero bias (inset of Fig. 2d), which is similar to the Coulomb blockade behaviour 15 . The current I reached zero at − 6.5 and + 6.5 V upon increasing and decreasing the applied voltage V, respectively. The zero current at − 6.5 and + 6.5 V correspond to the emission of electrons from the negative to the positive electrode and from the concave to the convex portions, respectively. This proves the electricity switching effect in rechargeable dry solid AAO supercapacitors 12,15,17 . Structural morphologies and surface characteristics by TEM and AFM. We investigated the structural morphologies and surface characteristics of the ACF specimen. The wide-field X-ray diffraction pattern (Fig. 3a) shows that the specimen consists of an amorphous cellulose phase, characterised by two broad peaks at  Figure 3b shows the changes in atomic pair distribution functions under a strong irradiation of 100-200 keV (SI Fig. S3). The intensity peaks of the C 1 -O 5 , C 1 -C 3 , and C 3 -C 6 bonds are at approximately 0.14, 0.26, and 0.39 nm, respectively 20 , and the corresponding change rates of distance as functions of the applied voltage are presented in Fig. 3c. Electroadsorption 21 causes volume shrinkage of C 3 -C 6 bonds to 2% due to a -1.98 GPa Maxwell compressive stress (electric field stress) (SI S6). This could be caused by the offset effect of positive polar C 6 =O 6 radicles of carbonyl groups 11,22 , with a rotation of C 6 -sodium carboxylate side chain about the C 5 -C 6 bond 23 . Amorphous structure prefers to rotate. Figure 3d shows an atomic force microscopy (AFM) image of the surface structure of specimen, with a convex diameter of 17.9 nm and a concave diameter of 13.1 nm. The fibrous appearance of the outer-surface resembles the uneven surface of ATO 9,10 , APP 11 , and AAO 12,13,15 (SI Fig. S6). When the applied voltage changed from − 20 V to + 40 V in 600 s in air (SI Fig. S7), the electrostatic potential distribution histogram shifted to negative values (Fig. 3e). The negative shift (inset of Fig. 3e) can be attributed to the electron emission from the negative cantilever and an electrostatic induction from the positive cantilever in noncontact AFM.
Molecular structure and quantum-size effect for electric storage. The ACF structure, consisting of sodium (1→4)-β-D-poly-glucuronate containing C=O radicles 24 with permanent dipoles (Fig. 4a) 22,25 , is similar to the APP structure 11 . Therefore, we inferred that the superior electric storage on the uneven ACF specimen is derived from the same quantum-size effect 26,27 described in Fig. 4b, which induces a relative increase in the combined electrons. This leads to fewer free outer electrons due to the screening effect. Figure 4c presents the convex diameter dependencies of the calculated electrostatic potential and the induced outer electron pressure of carbon atoms surrounding the ACF structure, by help of the Thomas-Fermi screening effect (discussed in the SI S9). The decrease in diameter make increase the negative potential and the positive pressure. The calculated potential is − 22.5 eV at 17.9 nm convex diameter. Therefore, the work function of ACF is negatively higher than those (− 5.5, − 10.1, and − 20.7 eV) of ATO 10 , APP 11 , and AAO 15 , respectively (SI Fig. S9). Figure 4d shows a morphological schematic proposing a possible mechanism for large electrical charges. The uneven surface serves as the EDC circuit ( Fig. 4e) with an insulating layer containing tiny capacitors throughout the bulk. Similar to how www.nature.com/scientificreports/ plant-produced cellulose absorbs atmospheric CO 2 via photosynthesis, ACF is also expected to capture both positive and negative electricity from the atmosphere, preventing the greenhouse effect. Thus, the ACF supercapacitor would be suitable for applications of light electricity such as handheld electronic devices, transportation, and renewable energy storage for power grids. However, this paper is the first report that presents the high-performance electric storage of "dry" ACF supercapacitor. We must next investigate electrical characteristics such as specific capacitance, energy efficiency of the charge-discharge process, cyclic stability for practical use.
Optimised structure of ACF and its electronic role. Finally, we investigate the reason why the CNF possesses superior electric adsorption. We optimized the local structures around CH 2 OH and COONa radicals in TEMPO-oxidised native cellulose (C 12 H 20 O 10 ) and NaOH-extracts of TEMPO-oxidised native cellulose (C 12 H 17 O 11 Na), respectively. The local structures of these celluloses are depicted in Fig. 5a,b, respectively. We then simulated the density of sates (DOS) for C 12 H 20 O 10 and C 12 H 17 O 11 Na, respectively. In sharp contrast to DOS of C 12 H 20 O 10 with OH radical, an isolated electronic state appears at − 4.5 eV in the band gap for C 12 H 17 O 11 Na with COONa radical (Fig. 5d). This localized state corresponds to the empty 3 s orbital of Na cation shown in Fig. 5b. Here, it should be noted that the localized electrons present near the two-atomic vacancies in the AlO 6 cluster of AAO induce positive charges (electrostatic effect) on the inside of the insulating oxide surface, resulting in the adsorption of many electrons under electron-beam irradiation 15 . Hence, we infer by analogy that the occurrence of the localized electron in the vicinity of COONa radical induces positive charges (electrostatic effect) on the inside of the insulating ACF surface, leading to high adsorption of many electrons from the atmosphere and in vacuum. Thus, the COONa radical in flexible ACF plays an important role for superior electric adsorption.

Conclusions
We demonstrated the high-performance electric energy storage (221 mJm −2 , 13.1 Wkg −1 ) of "dry" amorphous nanocellulose fiber supercapacitor with nanometre-sized cavities and high work function, based on the quantumsize and enhanced electroabsorption effects, respectively. From appearance of localized electrons, C 6 -sodium www.nature.com/scientificreports/ carboxylate (COONa) radicals in ACF play crucial role (electrostatic effect) for high-performance electric storage. The integration of the film with NEMS is likely to provide potential applications in light electricity such as handheld electronic devices, transportation, and renewable energy storage for power grids.

Methods
TEMPO-oxidized CNFs (COONa content: 1.48 mmolg −1 , TC-01A) with 3-nm diameters were prepared by Nippon Paper Industries. The 10-μm-thick ACF specimen was fabricated on the Al substrate via slip casting. Sample structure was examined by X-ray diffraction (XRD) in reflection mode with monochromatic Cu Kα radiation. To avoid destroying of nanofibrils under strong and long-term electron-beam irradiation, we conducted selected-area electron diffraction (SAED) analyses at 100-200 keV with the electron density of 3 nA/ m 2 . Electron-refraction and -irradiation were performed using transmission electron microscopy (JEM-2100, JEOL). The noncontact-scanning Kelvin probe-atomic probe microscopy (NC-AFM, JSPM-5200, JEOL), based on the measurement of the electrostatic force gradient was applied to measure the absolute electrical potential between the Pt-coated cantilever tip from − 100 to + 100 V and the ACF surface as the work-function difference. The AC impedance and DC charging/discharging behaviours of each RC combination and illumination test of a red LED light (2 V-100 μA) were analysed using the galvanostatic charge/discharge measured using a potentiostat/galvanostat (SP-150, BioLogic Science Instruments) with DC's of 10 V, 10 pA ~ 100 mA for ~ 900 s and charging currents of 10 mA at 293 K. A 2 × 10 -4 W red LED lump was used to verify the electric energy storage. The optimised local atomic configurations of the C 12 H 20 O 10 and C 12 H 17 O 11 Na were determined through a plane-wave-based first-principles density functional calculation (VASP 5.3) 28 . www.nature.com/scientificreports/