Hierarchical MnO 2 /activated carbon cloth electrode prepared by synchronized electrochemical activation and oxidation for ﬂ exible asymmetric supercapacitors

(cid:129) A novel synthesis of hierarchical MnO 2 on activated carbon cloth was designed. (cid:129) High-voltage asymmetric supercapacitor delivered impressive volumetric energy density. (cid:129) The binder-free H-MnO 2 /ACC showed high speci ﬁ c capacitance and excellent cycling stability.


Introduction
Flexible electronic devices are indispensable parts in modern life, applied widely in mobile electrical products, smart textile and artificial electronic skin, etc. [1][2][3][4][5][6][7] In this regard, electrochemical supercapacitors (SCs) have drawn considerable attention and emerged as a promising power source of flexible and lightweight electronic devices owing to their high-power density and long lifetimes [1,3,5]. The ongoing efforts in this scenario, therefore, are intensively focused on electrode design and SC assembly configurations to increase the energy density of flexible supercapacitor (FSC) without sacrificing its intrinsic character [8,9]. Furthermore, solid-state electrolyte instead of liquid one has been used in FSC to avoid electrolyte leaking and minimize the dissolution of the electrode for reliable stability [8,10,11]. Nevertheless, it is more determinative and challenging to fabricate novel electrode materials in order to maximize the energy storage density together with excellent power density, further extending application opportunities [12,13]. A typical flexible supercapacitor electrode is generally constructed on a self-supported substrate such as plastic film, metal foil, and carbonbased material to conduct internal current to the outside supply [14][15][16]. Among them, carbon cloth (CC), endowing remarkable flexibility and mechanical stability, is a very competent candidate for the binder-free growth of electroactive materials [17][18][19]. Although CC has desirable flexible features, it still suffers from relatively hydrophobicity and low conductive property, as well as limited electroactive surface area for loading of highly active materials [20,21]. Meanwhile, cycling stability of CC-based electrodes is restricted by the detachment of active materials due to their weak deposit-substrate interaction. To address these problems, pretreatment for the surface of CC fibers is preferentially required so as to improve the adhesion between carbon fiber and pseudocapacitive materials [21][22][23]. For example, Song et al. pretreated CC by electrochemical exfoliation in a 0.5 M KNO 3 electrolyte and then deposited vanadium oxide on exfoliated CC. Amorphous VO x can, therefore, be firmly anchored on the surface of CC through C-O-V bonding [21]. Similarly, the work of Chien et al. who pretreated the surface of CC with atmospheric pressure plasma jet (APPJ) to improve its wettability is another exemplary in modifying the surface [23]. Nevertheless, it is still a substantial room in improving surface chemistry to strengthen the attachment of electroactive materials on electrodes.
With regard to reported active materials anchored on the surface of CC substrate, nanoscale layered manganese dioxide (MnO 2 ) among transition-metal oxides (TMOs) with rich valence states, stands out as one of the most eye-catching choices for high-performance SC electrodes [1,17,[24][25][26][27][28]. Nanostructured MnO 2 can offer plenty of active sites and suitable channels for ion intercalation, and CC simultaneously serves as elastic buffering layers to release the strain within MnO 2 during successive charge-discharge cycling. Unfortunately, the dense MnO 2 films formed through traditional electrodeposition method seriously restrict the utilization of inner parts of MnO 2 , resulting in low capacitance and thus hindering their large-scale applications [18,29]. As a result, a high gravimetric capacitance of MnO 2 -based hybrid materials are reported when the mass loading is less than 1 mg cm −2 [30]. Therefore, construction of electrodes with high electrochemical performance and large mass loading is an essential criteria for the designing of MnO 2 -based SC. Recently, Nagaraju et al. [31] fabricated core-shell-like MnO 2 nanostructures (NSs) decorated MnO 2 hexagonal nanoplate arrays on a CC with a high specific capacitance of 244.54 F/g at 0.5 A/g. Based on Ostwald ripening, Song et al. developed a two-step route to prepare a high mass loading MnO 2 electrode with remarkable capacitance of 618 mF cm −2 at 200 mV s −1 [18]. But all these methods involve multi-step processes which are costly, complicated and timeconsuming. Thus, a straightforward and cost-effective strategy to simultaneously improve the mass loading and potential performance of electrode is urgently desired.
Herein, we synthesized hierarchical MnO 2 (H-MnO 2 ) nanostructures by a facile one-step anodic electro-deposition (AED) process. During this AED process, the activation of CC and in-situ growth of active material can occur concurrently. Different from the reported anodic and/or cathodic electro-deposition (ED) of MnO 2 [29,31,32], usually producing the uncontrollable MnO 2 polymorphs and poor attachment on substrates, high-concentration Na 2 SO 4 electrolyte was employed in our developed anodic oxidation approach, which electrochemically activates graphic carbon layers of CC and preferentially induce the insitu firm attachment of active Na-containing MnO 2 on CC. The amount of deposited H-MnO 2 could be altered by simply changing the deposition time, ensuring the simplicity and controllability of our method. The resultant fiber-shaped flexible asymmetric supercapacitor device (FASC) based on H-MnO 2 /ACC-300 (positive electrode) and reduced carbon cloth (RCC, negative electrode) delivers a high volumetric energy density of 3.82 mWh/cm 3 at extended potential window of 4 V. Furthermore, the H-MnO 2 /ACC electrodes were systematically investigated in three-electrode cell, delivering superior electrochemical performance.

Preparation of H-MnO 2 /ACC
The carbon cloth (WOS1002) was purchased from Cetech Co., Ltd. without any further purification. Mn(CH 3 COO) 2 ·4H 2 O (0.4 M) and Na 2 SO 4 (0.6 M) were dissolved in distilled water (18.2 MΩ) as a solution for the AED process of H-MnO 2 . The AED process of H-MnO 2 was operated on a workstation (CHI660D, Chenhua, Shanghai, China) at room temperature (RT). A piece of carbon cloth (2 × 1 cm 2 , 0.32 mm in thickness) was used as working electrode in a three-electrode configuration at a constant potential of 3 V. Deposition time of 50, 100, 150, 200, 300, 400 and 500 s were applied for each of the carbon cloth. ACC-500, respectively. For comparison, MnO 2 was also deposited on CC with a commonly used potential of 1.0 V following similar procedures mentioned above. To study the change of CC after electrodeposition, the electrode was immersed in 6 M HCl to etch the surface layer away.

Assembly of fiber-shaped asymmetric supercapacitor
Flexible supercapacitors were assembled with as-prepared H-MnO 2 / ACC-300 as positive electrode and reduced carbon cloth (RCC) as the negative one, respectively. The 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 )/tetramethylammonium tetrafluoroborate (TMABF 4 ) ionogel (Lanzhou Kaite Trade Co., Ltd.) as a solid-state electrolyte was filled between two electrodes. RCC negative electrode was fabricated by electrochemical activation of CC in a mixed acid of HNO 3 and H 2 SO 4 (V/V = 1:1) using three-electrode system containing CC as work electrode and a platinum sheet as counter electrode and a saturated calomel electrode (SCE) as a reference electrode under a constant voltage of 3 V. Then, RCC was transferred into a Teflon-lined stainless-steel autoclave with 80 mL 0.5% hydrazine hydrate and maintained at 100°C for 30 min to recover its electrical conductivity. Finally, the as-prepared RCC sample was washed with distilled water and ethanol to remove any impurities. Besides, the EMIMBF 4 /TMABF 4 ionogel electrolyte was prepared as follows. 1 g PVDF-HFP (Mw = 5 × 10 5 , Sigma), 0.9 g EMIMBF 4 and 0.1 g TMABF 4 were dissolved in 6 g N-methyl pyrrolidone (NMP, Adamas) at room temperature for 12 h and a jelly-like solution was formed ionic electrolyte and separator. Prior to assembling, the H-MnO 2 /ACC electrode was soaked in the gel solution for 10 min, and then carefully entangled with the RCC electrode. After that, the sample was dried at 65°C for 24 h to allow electrolyte solidification. Finally, a thin parafilm was wrapped on FASC to ensure the integrity for further testing.

Materials characterization
Scanning electronic microscope (SEM) and transmission electron microscope (TEM) were conducted on a JEM-7001F and a JEOL-2100, respectively. The crystallographic structures were investigated by a Rigaku Smartlab X-ray diffractometer using Cu Kα radiation. Raman spectra were recorded by a confocal laser inVia Raman microscope (Renishaw, λ e = 514 nm). Chemical environment was analyzed by X-Ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermal-Fisher). The water contact angle of the samples was measured by optical contact angle measurement system (DSA100S, KRUSS GmbH, Germany). Nitrogen adsorption-desorption measurement was performed using a Quantachrome autosorb automated gas sorption system. Electrochemical properties were conducted with a electrochemical analyzer (CHI660D). Cyclic voltammetry (CV) ranging from 0 to 1 V, galvanostatic charge/discharge (GCD) as well as electrochemical impedance spectroscopy (EIS) of the individual electrode were performed in a three-electrode cell with a Pt counter electrode and a saturated calomel electrode (SCE) reference electrode.

Results and discussions
No special equipment or complicated manipulation was required in our method. Activation of CC can be accomplished during the AED process of H-MnO 2 . During oxidation process, the sulphate ions derived from Na 2 SO 4 are supposed to intercalate into the graphitic regions at the surface of CC and oxidize the nearby carbonaceous structure thereby increasing the electroactive surface area on which further MnO 2 can be deposited [33]. Moreover, water is crucial for the overall exfoliation process, as it not only serves as a solvent to decrease the viscosity of the solution, but also produce reactive oxygen-containing species (-COOH, C-OH, etc.) from anodic oxidation, which in turn facilitate the intercalation of SO 4 2− [34,35]. The electrolysis of sulfate ions and the co-intercalated water lead to the generation of continuous cavitation bubbles containing SO 2 , CO, O 2 and others ( Fig. 1a and Video S1, Supporting Information), all of which will help exfoliate the weakly bonded graphite layers from the defect sites and provide large forces to reduce MnO 2 aggregation. In a typical experiment, further applying high overvoltage (η) causes a high nuclei formation rate, as defined by electrochemical version of the Kelvin equation (Eq. (1)): where r means the critical nucleation radius, σ is the specific surface energy, V is the atomic volume in the crystal, z is the number of elementary charges (e 0 ). Accordingly, the metal ion concentration in the vicinity of CC decreases drastically and therefore the process becomes diffusion-controlled, which may restrict the continuous crystallite growth of MnO 2 . To further probe the process, elevated temperature with higher ion mobility is applied, which allows the metal ions to diffuse from the bulk electrolyte to the anode and compensate the metal ion depletion. As expected, we observed the advent of a dense layer of MnO 2 at 60°C. (Fig. S1) The cross-sectional SEM image (Fig. 1b) reveals that concentric H-MnO 2 lamellae with an average thickness of 150 nm are hierarchically deposited and strongly adhered to ACC by applying a high anodic voltage of 3 V at room temperature. Moreover, the exposed part of ACC exhibits a relatively rough surface, indicating the successful activation of carbon fiber surface (inset of Fig. S2b) [19,31]. Significantly, similar morphology can be maintained and the number of MnO 2 lamellae can be tailored over a wide range of about 1 to 8 in samples with increasing deposition time, as depicted in Fig. S2. With the increased magnification, MnO 2 lamellae with well-defined neighboring voids form an open framework offering more available active sites for electrolyte intercalating the inner surface (Fig. 1c), which is crucial for the construction of high mass loading electrodes [18,29]. Additionally, the MnO 2 nanosheets on the surface of H-MnO 2 /ACC are interconnected with each other, forming a highly porous surface (inset in Fig. 1c). The porous surface, in combination with the open voids (Fig. S2h), allows better flexibility to buffer the internal strain and offers accessible inner channels for rapid transfer of electrolyte ions [17]. According to Fig. 1d, these CC fibers are intermeshed with each other, forming a three-dimensional (3D) scaffold with a woven fibrous texture. After the AED, the H-MnO 2 /ACC can be clearly distinguished by their brownish dark color in comparison with the bare parts of the CC (inset in Fig. 1d). The TEM image in Fig. 1e indicates that the entire surface of ACC is completely covered with interconnected crumpled MnO 2 nanosheets, well consistent with SEM observations. The HRTEM image of H-MnO 2 (Fig. 1f) displays the parallel fringe with a spacing of 0.45 nm for the (1 0 1) plane and 0.72 nm for the (0 0 2) plane, corresponds to hexagonal birnessite-MnO 2 . The corresponding FFT pattern in inset of Fig. 1f can be further indexed to (1 0 1) and (0 0 6) diffraction planes of the hexagonal MnO 2 phase. Besides, EDS analysis ( Fig. S3b) unambiguously reconfirms the coaxial H-MnO 2 /ACC structure and the mapping result is consistent with the EDS spectrum in Fig. S3a, in which a trace of Na element was introduced into the interlayer of MnO 2 during the growth [17]. Fig. 2a shows the XRD patterns of H-MnO 2 /ACC-300 and pristine CC. Two broad peaks centered at 25.4°and 43.5°are attributing to the (0 0 2) and (1 0 1) diffractions from a turbostratic graphitic feature of CC [36,37]. After deposition, three additional diffraction peaks are observed at about 36.9°, 54.4°, and 66.5°, which are indexed to the (0 0 6), (3 0 1), and (1 1 9) crystal faces of hexagonal birnessite-MnO 2 without any other impurities (JCPDF 18-0802), respectively [38]. Meanwhile, the low intensity of these peaks suggests that the deposited MnO 2 has low crystallinity and fine grain size [28].
Moreover, Raman spectroscopy of the product verifies the existence of MnO 2 and successful activation of CC. Three distinct peaks are positioned at 482, 578 and 655 cm −1 (Fig. 2b). The pronounced Raman band at 655 cm −1 can be ascribed to the Mn-O symmetric stretching vibrations of MnO 6 octahedrons, while the characteristic band located at 578 cm −1 is usually attributed to the Mn-O stretching vibration in the basal plane of MnO 6 sheet [28,39,40]. For bare CC, a disorder-induced D-band peak at 1351 cm −1 and a typical sp 2 -related G-band peak at 1596 cm −1 can be observed, and the relatively low intensity ratio (ca. 0.98) between them indicates that the CC is very clean with high quality. Compared with CC in previously reported findings [41], the increasing of I D /I G reveals an intensified degree of disorder, verifying the activation of CC during the AED process.
X-ray photoelectron spectroscopy (XPS) was conducted to further study the chemical composition of H-MnO 2 /ACC-300. The full spectrum scan is shown in Fig. 3a. It reveals characteristic C, Mn and O peaks in "lamella-void-lamella" H-MnO 2 /ACC composite, in good agreement with XRD analysis results. By employing a Gaussian fitting approach, the C1s spectrum (Fig. 3b) was deconvoluted into four peaks, which are assigned to C-C (284.7 eV), -C-OH (285.8 eV), C = O (287.3 eV), and -COOH (288.5 eV) groups, confirming the dramatically increased functional groups on ACC surface after anodic oxidation process [42,43]. Based on the analysis of O 1 s spectra (Fig. 3c) [27], the oxidation state can be estimated using Eq.
where S is the peak intensity. The obtained average oxidation state for Mn is 3.6, closer to a 4 + state than a 3 + state. This result is in good accordance with the XPS results of Mn 3 s spectrum (Fig. S4). Additionally, a spin-energy separation of 11.8 eV between Mn 2p 3/2 and Mn 2p 1/2 peaks (Fig. 3d), which can be attributed to MnO 2 , allows us to further reveal the dominance of Mn 4 + in sample [26,27,44]. Abundant H-O-H groups and co-existence of tetravalent and trivalent oxidation states can dramatically facilitate the electrochemical reactions [26,44]. The nitrogen sorption measurement was carried out to explore porosity (Fig. S4b). The distinct hysteresis loop in the relative pressure range of 0.4-1.0 (inset of Fig. S4b) suggests the presence of an appreciable amount of mesopores (corresponding to a surface area of 81.37 m 2 g −1 ). The H-MnO 2 /ACC-300 reveals mesoporous nature with an average pore diameter of 3.41 nm. Such mesoporous nature benefits to the efficient charge transportation, which subsequently results in enhanced electrochemical performance. The as-synthesized H-MnO 2 /ACC-300 was assembled into solidstate flexible asymmetric supercapacitors in order to evaluate its electrochemical performance and potential application. Since the energy  . The mixed ionogel (EMIMBF 4 /TMABF 4 ) was used as a binder, separator, and electrolyte [45]. Because of large viscosity of the ionogel, the two-fiber electrodes can be closely contacted. The H-MnO 2 /ACC//RCC FASC displays ideal capacitive behavior with quasirectangular CV loops, even at a potential window of 4 V (Fig. 4a). Moreover, there is no obvious resistive behavior and the CV profiles still retain rectangular shape at a maximum cell voltage of 4 V at a scan rate as high as 1000 mV/s, suggesting an excellent rate capability without parasitic reactions (Fig. S6c) [46]. It is well-known that pseudocapacitive behavior of MnO 2 in aqueous electrolytes is induced by the intercalation/extraction of various cations (K + , Na + , Li + , and H + ) or protons (H 3 O + ) [47]. Nevertheless, as proposed in literature [48], BF 4 − anions with smaller size in ionogel electrolyte serve as working species that electro-neutralize the Mn 3+ /Mn 4+ redox transition, which reversibly enter/leave the tunnels between MnO 6 octahedral units (Eqn. (3)). The multi-cation (TMA + and EMIM + ) species can be packed more densely and adsorbed at the electrode surface, contributing to the overall capacitance.
To gain further insight into electrochemical kinetics, the total stored charge (Q) is studied by extrapolating Q versus ν -1/2 . As illustrated in Fig. 4b, two fitted linear plots can be clearly identified on both sides of 100 mV/s (in which ν -1/2 equal to 3.16). The Q decreases dramatically with respect to the region where the scan rate is higher than 100 mV/s, which arises largely from the irreversible redox transitions of MnO 2 and diffusive resistance of electrolyte ions [49,50]. Theoretically, the peak current (i) and scan rate (ν) obey a power-law relationship (Eq. (4)) [51].
where a and b are adjustable values. The value of b can be determined as the slope of log(ν)-log(i) plot, which effectively represents the kinetics of charging and discharging process. Generally, the exponent (b) is between 0.5 (diffusion-controlled storage process) and 1 (surfacecontrolled charge storage behavior). Fitting to Eq. (4) gives a b value of 0.71 for the H-MnO 2 /ACC//RCC FASC at scan rates from 10 to 100 mV/ s, exhibiting both diffusion-controlled and surface capacitive processes [52]. Furthermore, the contribution of pseudo-capacitance (PC) and electrochemical double layer capacitance (EDLC) can be separated by Trasatti method analysis (Fig. S6d). The pseudocapacitive contribution still dominates at high scan rate of 100 mV/s, evidencing high density of charge adsorption sites provided by sufficient void space of H-MnO 2 for the in-depth penetration of electrolyte. Besides exhibiting rectangular voltammograms, the nearly linear variation of GCD curves without obvious IR drop (0.03 V at 1 mA cm −2 ) is another criterion for judging the capacitive behavior (Fig. S6e). This corroborates the fact that the extended potential window exerts little impact on the efficiency of energy preservation/delivery. Based on the GCD curve (Fig. 4c), the calculated total volumetric capacitance of the H-MnO 2 / ACC//RCC FASC reaches a maximum of 1.74 F cm −3 at a current density of 1 mA cm −2 . When the current density changes from 1 to 15 mA cm −2 , the device shows a good rate capability with 74.5% of volumetric capacitance retained. The diameter of the semicircle verifies a small value (5.49 Ω) of the charge transfer resistance (R ct ) for the H-MnO 2 /ACC//RCC FASC (Fig. 4d). This can be ascribed to the fact that highly conductive paths built through intimately attached interfaces of activated CC and hierarchical MnO 2 , facilitating electrostatic attraction and ion insertion/extraction. Furthermore, a 45°slopped portion at the intermediate frequency of Nyquist plots, followed by a line almost parallel to Y-axis symbolizing minor Warburg, indicating less obstruction for ion diffusion and a prominent capacitive behavior. Compared with the representatives of the previously reported supercapacitor devices [32,[53][54][55][56][57][58][59], our device delivers superior volumetric energy density of 3.82 mWh cm −3 at a power density of 24.6 mW cm −3 and maintains an energy density of 2.2 mWh cm −3 at 285.8 mW cm −3 (Fig. 4e). Impressively, one single H-MnO 2 /ACC//RCC device can successfully drive an electronic device, as presented in the inset of Fig. 4e. After 3200 cycles, approximately 74.3% of the original capacitance is retained with a voltage window of 4 V, which indicates no significant side reactions during GCD cycling (Fig. S7). We then carried out CV measurement at 100 mV/s under different bending states to examine the flexibility and mechanical stability (Fig. 4f). The nearly identical shapes of CV profiles at different bending angles reveal electrical continuity of the FASC in the original and bended state. In addition, small deviations during bending test should be attributed to the movement of contact points and electrodes. After 1000 recycled bending, small cracks could be observed without apparent peeling off of the deposited MnO 2 layer, further exhibiting its excellent flexibility (Fig. S8). This is associated with the strengthened interfacial binding force between H-MnO 2 and ACC driven by the AED process. The device was also charged to 4 V at a current density of 1 mA cm −2 and then recorded the self-discharge with 31.4% retention after 50 h (Fig. S6f). In all, the achievement of high specific capacitance, outstanding rate capability, and wide voltage window pave a way for MnO 2 -based flexible supercapacitors with high-level energy and power performance. Usually, the specific capacitance of electrodes with more MnO 2 loading is unsatisfactory because of increased electronic resistance between the tight growth sites, prohibiting the reaction between MnO 2 and electrolyte [29,[60][61][62]. For comparison, we tested MnO 2 /CC electrode electrodeposited at common (1 V) and high voltage (3 V). The former takes longer duration (∼500 s) to reach the same mass loading of the latter. As expected, the MnO 2 /CC electrode electrodeposited at high voltage has a higher capacitance, as depicted in Fig. 5a. The GCD curve of the high-voltage sample exhibits only a much smaller IR drop of 0.008 V in comparison to the common-voltage one (0.03 V) from the discharge curve at 1 A/g, indicating its interface enhancement and thus low equivalent series resistance (Fig. 5b). From Fig. 5c, the R ct of the high-voltage sample is almost 10 times less than that of common-voltage sample, manifests the reduced internal resistance of electrodes and the mitigated ion diffusion resistance at the electrolyte/electrode interface [63,64]. As shown in Fig. S9, we measured the static contact angle of pristine CC and ACC after removing the MnO 2 layer. The pristine CC obviously presents an almost unchanged contact angle of ≈118°. After the common-voltage electrodeposition, CC is also hydrophobic with a stable contact angle of 94.6°. In contrast, the ACC after high-voltage electrodeposition reveals its hydrophilic nature with an initial contact angle of 20°and the droplet is absorbed immediately, confirming the activation of CC. These experimental results clearly indicate that conventional ED methods have limitations for improving the electrochemical performance of manganese oxide electrodes with high mass loading. Additionally, hierarchical MnO 2 , which provide more available active sites for electrolyte penetration, combined with activated carbon cloth is effective to increase capacitance and conductivity. As learnt from above characterizations, very good surface wettability together with such small resistance values also allows enhanced interactions between the electrolyte ions and a rapid ion diffusion channel, which fully contribute to charge storage.
In order to further investigate the influence of MnO 2 loading on the electrochemical performance during the anodic oxidation process, the comparative electrochemical studies were conducted for the H-MnO 2 / ACC electrodes in three-electrode cells containing 1 M Na 2 SO 4 electrolyte. From Fig. 6a, it can be clearly seen that the CV profile of H-MnO 2 /ACC-100 electrode exhibits the largest enclosed area than the other electrodes, suggestive of its highest capacitance value. Meanwhile, the slight distortion in CV profile of H-MnO 2 /ACC-50 is caused by the incomplete formation of MnO 2 film (inset of Fig. S2a). Areal capacitances of H-MnO 2 /ACC were collected at different scan rates as a function of mass loading (Fig. 6b). The capacitance portrays an increasing tendency with mass loading (up to 13 mg/cm 2 ) at scan rates increasing from 5 to 50 mV/s, as expected for a capacitor under the non-diffusion limited conditions [18]. This result reveals most of the active materials participated in charge storage. At high scan rates beyond 100 mV/s, capacitive saturation can be observed at a high mass loading over 6 mg/cm 2 (related to the H-MnO 2 /ACC-300 sample), implying that ion penetration might reach a limitation depth [65]. Negligible IR drops (less than 0.01 V) are observed in the GCD profiles of all samples at 1 mA/cm 2 (Fig. S10). The specific capacitance plots based on the GCD curves versus current densities are shown in Fig. 6c. Impressively, the capacitance of H-MnO 2 /ACC-100 can reach up to 400 F/ g at 0.5 A/g, higher than those of recently reported MnO 2 -based pseudocapacitive materials with a similar or even lower mass loading, such as core-shell-like MnO 2 @MnO 2 /carbon cloth (244.54 F/g at 0.5 A/g) [31], Na 0.5 MnO 2 /carbon cloth (287 F/g at 1 A/g) [17], MnO 2 /3D duct-like graphene (∼255 F/g at 5 mV/s) [66], MnO 2 nanoflake/CNTs/ Ni (∼340 F/g at 1 A/g) [67], MnO 2 /porous carbon (345.1 F/g at 0.5 A/ g) [68], and MnO 2 nanorod/carbon fiber paper (362.5 F/g at 0.5 A/g) [69]. Even with a 20 times increment at 10 A/g, superb capacitance retention of 79.7% of the value at 0.5 A/g can still be obtained, showing the exceptional rate capability. Although maximum specific capacitances are close for H-MnO 2 /ACC electrodes with increased MnO 2 loading, the decreased rate performance of H-MnO 2 /ACC electrode was achieved after electrodeposited for more than 300 s, indicative of restricted ion penetration into narrow voids. Typically, electrochemically deposited MnO 2 film have a dense morphology and increased ion diffusion length as the loading time increases, which might cause diffusion drag of electrolyte ions. The problem may be well-resolved just by integration of hierarchical structure with free void space between neighboring thin films for electrolyte infiltration and ion diffusion without any blocks, which is proved by the electrochemical impedance spectroscopy (EIS). Fig. 6d shows that the R ct only increases slightly when the H-MnO 2 loading increases up to about 6 mg/cm 2 . While, further increasing the mass loading of MnO 2 to 8.1 mg/cm 2 , a significant increase of R ct from 1.21 to 1.95 Ω can be obtained, suggesting the blocked ion pathways due to sheets aggregation of thicker MnO 2 film. Then, we take representative samples of H-MnO 2 /ACC-100 and H-MnO 2 /ACC-300 for further investigation. Remarkable capacitance retention is a crucial parameter for the practical application of SCs. It is found that the H-MnO 2 /ACC-100 electrode undergoes a profound loss of 19.5% after 5000 cycles, whereas the H-MnO 2 /ACC-300 electrode cycles at 5 A/g with a better capacity retention of 94.2% (Fig. 6e). The comparison of EIS spectra after cycling test is given in the inset of Fig. 6e. The R ct of H-MnO 2 / ACC-300 remains almost constant after repeated cycles, while the EIS spectrum of H-MnO 2 /ACC-100 reveals an increased R ct of 2.13 Ω, further validating the prolonged lifetime of H-MnO 2 /ACC-300 electrode. Furthermore, we measured the microstructure of H-MnO 2 /ACC-300 after 5000 GCD cycles. A 3D open network is formed by interconnected and larger MnO 2 nanosheet arrays on the surface of H-MnO 2 /ACC-300 (Fig. 6f), which can provide much more accessible area for adsorption of ions. Additionally, the narrow voids also signify its buffering effect to release the internal strain during cycling (inset in Fig. 6f). The observed changes in morphology can be attributed to the dissolution and reoxidization process of the H-MnO 2 , which can also facilitate electrolyte transport [70,71]. These results fully illustrate that the presence of hierarchical structure can dramatically enhance utilization efficiency of high mass-loading MnO 2 , which therefore holds great promise for high performance energy storage devices.
On the basis of the data presented above, we ascribe the ameliorative electrochemical performance of H-MnO 2 /ACC to the following points: (1) Compared with conventional techniques, our AED yields large amount of well-distributed active materials within exceptionally short time of reaction. Notably, the AED also allows the concentric H-MnO 2 fabrication in different precursors (Fig. S11). (2) As particular robust scaffolds, the activated CC electrode contains a high-level of surface oxygen functionalities, which serve as nucleation sites that favor the uniform growth of H-MnO 2 . (3) The strong attachment of H-MnO 2 layers onto ACC reduces the contact resistance at their interface and the hierarchical structure provides in-depth penetration of the electrolyte for available contact with active materials at high rates. A further advantage is that the voids between H-MnO 2 lamellae can act as "ion reservoir" to buffer the sharp change of ion concentration during flash charge/discharge processes, which also contributes to excellent rate performance [72].
In summary, the hierarchical concentric H-MnO 2 layers were fabricated on carbon cloth via a rapid high-voltage AED process and a perfectly hybridized ACC with tunable number of MnO 2 lamellae was obtained. This hierarchical structure ensures good mechanical stability as well as high ionic conductivity. Benefiting from highly conductive paths derived from intimately attached fiber-MnO 2 interfaces, hierarchically interpenetrated lamella MnO 2 porosity, concentric MnO 2 interlayer voids and wide potential window of the EMIMBF 4 /TMABF 4 ionogel electrolyte, the resultant FASC delivers a high volumetric energy density of 3.82 mWh/cm 3 . The optimized H-MnO 2 /ACC-100 electrode exhibits a maximum three-electrode specific capacitance of 400 F/g in 1 M Na 2 SO 4 at 0.5 A/g with excellent rate performance. In addition, the H-MnO 2 /ACC-300 electrode with high mass loading can retain 94.2% of initial capacitance upon 5000 cycles. We believe that this high-voltage AED process is a general strategy that can be extended to other materials, thus providing new opportunities for improving the performance of high mass loading SC electrodes.