Shijin Zhu, Wnghn Huo, Tin Wng, Kilin Li, Xioying Liu, Junyi Ji,Honghng Yo, Fn Dong, Yuxin Zhng,＊, Lili Zhng,＊＊

a College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China

b Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China,Chengdu, 611731, China

c Engineering Research Center for Waste Oil Recovery Technology and Equipment,Ministry of Education,College of Environment and Resources,Chongqing Technology and Business University, Chongqing, 400067, China

d School of Chemical Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, PR China

e College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou City, Henan Province, 450001, PR China

Keywords:Parallel birnessite Double-exchange mechanism Supercapacitor Energy storage devices

ABSTRACT High electrochemically active birnessite is always desirable pseudocapacitive material for supercapacitor. Here,two-dimensional (2D) compulsive malposition parallel birnessite standing on β-MnO2 interconnected networks have been designed.Due to the restriction of β-MnO2,compulsive malposition,slippage of MnO6 slab,occurred in birnessite resulting in weaken binding force between birnessite slab and interlayer cations,which enhanced their electrochemical performances. Additionally, the electrical conductivity of the structure was largely promoted by the 2D charge transfer route and double-exchange mechanism in birnessite, also leading to desirable electrochemical properties. Based on the fraction of as-prepared nanostructure, the parallel birnessite exhibited good pseudocapacitance performance(660 F g-1)with high rate capability.In addition,the asymmetric supercapacitor assembled by reduced graphene oxide (RGO) and as-prepared nanostructure, which respectively served as the negative and positive electrode, delivered an energy density of 33.1 Wh kg-1 and a maximum power density of 64.0 kW kg-1 with excellent cycling stability(95.8%after 10000 cycles).Finally,the study opens new avenues for synthesizing high electrochemically active birnessite structure for high-performance energy storage devices.

1. Introduction

Due to their high-power density, fast recharge capability and long cycle life, supercapacitors have attracted intense attention to meet the increasing demand in energy storage for electron device. As well, the charge storage and capacitance of supercapacitors are greatly depending on the electrode materials [1-3]. As a mature electrode material,carbonaceous materials exhibit outstanding performances, including long cycling abilities and excellent stability owning to the simple electrical double-layer charge storage mechanism [4]. On the other hand,transition-metal oxides could deliver the promising electrochemical capacitances and thus higher energy densities,which have attracted intense interest in developing high-performance supercapacitor [5,6]. Among them, birnessite, as one of the most hopeful anode materials in supercapacitor, possess many advantages, such as large specific surface area,abundant electrochemical active sites, high theoretical capacitance,adjustable interlayer spacing and exchangeable interlayer cations[7-10]. However, the obtained capacitance for birnessite are often only 200-300 F g-1, far below the theoretical capacitance. This is always attributed to the low electrical conductivity of birnessite [5,11]. Therefore,a system composed of pseudocapacitive material-conductive matrix composite structures, is highly desirable. Hence, the highly conductive carbon materials were often mixed into electrode materials and some enhanced electrochemical properties were obtained [12,13]. Nevertheless,these obtained properties are still far away from the requirement of commercial application.

In addition,to further enhancing the electrochemical performance of birnessite-based electrode,efforts are also focused on preparing ultrathin birnessite to increase the specific surface area and promote the utilization rate of electrode materials [14-16]. Well-ordered, ultrathin, and large area layer structure is considered to be desirable for high performance energy storage materials. Thus, some works are aimed at preparing ultrathin birnessite grown on the surface of some other metal oxides,including Co3O4@birnesite [17,18], NiCo2O4@birnesite [19,20],CuCo2O4@birnesite [21,22] etc. Although some synergistic effect has been found in these birnessite-based composite structures, some other important electrochemical properties, for example cycling stability and rate capability,are highly influenced by these substrates,because of the low kinetic process. Additionally, the electrochemical feature of birnessite itself is the only key factor to improving the electrochemical properties of birnessite based electrode. Lots of works are seeking far and neglect what lies close at hand.As one of the most easily adjustable layer structure materials,the binding force between MnO6slab and interlayer cations should be brought to the forefront as it influences the kinetic process a lot because an attenuated binding force will definitely enhance the interlayer cation diffusing to the electrolyte and vice versa, which accelerate the electrochemical reactions.

Herein, we firstly developed a parallel birnessite directly grown on β-MnO2network. Because of the restriction of β-MnO2, compulsive malposition, slippage of MnO6slab, occurred in birnessite resulting in weaken binding force between birnessite slab and interlayer cations,which enhanced their electrochemical performances. Additionally, due to the unique morphology and double-exchange mechanism, this structure possesses larger specific surface area,abundant active sites and high electrical conductivity, which are favor for electrochemical reactions.Thus, a preferable specific capacitance of 318 F g-1with superior rate capability was obtained in 1.0 M Na2SO4electrolyte. When assembling the asymmetric supercapacitor consisted of this novel structure and reduced graphene oxides (RGO), we found the device exhibited outstanding electrochemical performance, could supply specific capacitance of 60 F g-1and energy density of 33.1 Wh kg-1with a maximum power density of 64.0 kW kg-1in 1 M Na2SO4electrolyte.Remarkably,almost 95.8% of the premier capacitance was remained after 10000 cycles. These promising results are owning to the artful design of the electrode structure,proper selection and combination of the negative and positive electrode, and are capable of meeting the high power and perdurable requirement as well as open a new avenue for synthesizing high electrochemically active birnessite structure for high performance energy storage devices.

2. Result and discussion

2.1. Morphology and crystalline information

The parallel birnessite/β-MnO2network structures were synthesized by a two-step solution route,free growth of MnOOH networks followed by the parallel birnessite directed growth on the networks.As illustrated in Fig. S1, after the as-prepared MnOOH networks were added into KMnO4aqueous solution, due to the unstable chemical property of MnOOH and the presence of highly oxidative KMnO4in acidic conditions,MnOOH and KMnO4would spontaneously react to form the MnO2nuclei on the surfaces of MnOOH networks. As well, the surface of MnOOH would be furtherly oxidized to beta-MnO2. It is worthy to note that the distribution of MnO2nuclei depended on the constituent of the H+in aqueous solutions is the mainly factors to form parallel birnessite nanosheets. A continuous MnO2nuclei come up on the surface of MnOOH networks after dumping KMnO4solution and then grow into parallel fin-like birnessite along with their further redox reaction and the decomposition of KMnO4in acidic solution. Finally, the substrate networks were oxidized to produce beta-MnO2.The surface of the MnOOH networks is smooth without any attachment(Fig.S2).Figs.S3a and S3b displayed the XRD patterns of the MnOOH network and as-prepared parallel birnessite/β-MnO2network structures. In Fig. S3a, all the diffraction peaks were index to the MnOOH (JCPDS no. 41-1379)without other impurity peaks, indicating the high purity of MnOOH network,which will be expected to be a desirable substrate for birnessite.While, after coupled with δ-MnO2(Fig. S3b), no peaks indexed to the MnOOH can be observed except for K-birnessite MnO2and β-MnO2(81-2261), suggesting that the samples are the mixture of δ-MnO2and β-MnO2.

In addition, the FIB/SEM was employed to investigate the structure and morphologies of as-prepared parallel birnessite/β-MnO2networks.After treated with KMnO4, the surface of the networks become rough.Fortunately,the interconnected network structure was remained without collapse(Fig.1a).The morphology of birnessite-type MnO2was affirmed to be coupled with β-MnO2network instead of mutual disinterest from Fig. 1b. As the K+between the layers can be replaced by other cations,when H+was introduced, the synergy of K+and H+and the special characteristic of the surface of MnOOH lead to the birnessite growing along the c axis of the beta-MnO2nanostructures forming brush structure with parallel flume. This structure can prominently enlarge the specific surface area.And the thickness of the layer of birnessite-type MnO2was measured to be 5 nm with the diameter increasing from 300 to around 620 nm from Fig. 1c. The open flume with several nanofilms are vertically anchored on the surface of β-MnO2,which indicates the skeleton got a section of rectangle. These findings demonstrate the morphology of birnessite can be regulated by the cations between the layers and the structure of matrix.

To further confirm the structure and morphologies of parallel birnessite and beta-MnO2in parallel birnessite/β-MnO2network, the TEM measurements were carried out. As displayed in Fig. 1d, the structure revealed its core-shell structure. The structure exhibited one single stab as core with uniform shell of parallel birnessite on the surface of β-MnO2revealing the simultaneous reactions between KMnO4and MnOOH at the initial stage.The width of the shell is about 210 nm.Abundant birnessite guaranteed the high surface area for the composites.For the skeleton,the lattice planes were marked in Fig. 1e. The lattice spacing of 0.44, 0.29 and 0.24 nm were corresponded to (100), (001) and (101) planes of β-MnO2, respectively. Thus, the β-MnO2can be affirmed to from single crystal(insert of Fig.1e)and growing along the 001 plane.The(110)and(-110)crystal planes are exposed facets along the longitudinal direction[23]. Thus, a growth mechanism about isotropy was put forward. The MnOOH network has two equal exposed facets((110)plane and(-110)plane. These two exposed facets acted as the carrying surface for birnessite-type MnO2. At the initial reaction,the MnOOH networks was oxidized to β-MnO2by the KMnO4solutions as well with continuous MnO2nucleus produced from solutions. Of course,the disintegration of KMnO4went through the whole hydrothermal process [6]. As the reaction proceeds, the MnO2nucleus turn to birnessite matching with the crystal planes of β-MnO2.After the MnOOH networks were depleted,the KMnO4continued its disintegration until the KMnO4was used up.Thus,a uniform core-shell structure was formed. However, most of the birnessite-type MnO2were flat without wrinkle. The parallel birnessite can be notarized to be single crystal from the SAED pattern and the spacing of 0.24 were corresponded to the (012) plane of birnessite(Fig.1f).The high degree of crystallinity of parallel birnessite ensures a high electrical conductivity for the composites which resulting in a good electrochemical property with high capacitance and outstanding rate capability. Furthermore, the unambiguous lattice fringes displayed in Fig.1g indicate the seamless connection between birnessite and β-MnO2.Based on the(101)planes of β-MnO2,the birnessite is growing along the[101]orientation.The interplanar spacing of 0.24 nm of birnessite is an inherent lattice distance for single birnessite layer. Hence, a single cell structural schematic diagram was given to demonstrate the juncture structure (Fig. 1h). As we can see, the β-MnO2is made of several reciprocal connected MnO6octahedrons but they are not at the same plane.While the birnessite composes of several densely linked MnO6octahedrons and they lie on the same plane.For a typical juncture structure,the MnO6octahedrons of birnessite grow along the β-MnO2. In this case,owing to the same lattice fringes of 0.24 nm,the continuous lattice plane from β-MnO2to birnessite were formed. These results show the perfect integration of the parallel birnessite and β-MnO2as a whole.As we have demonstrated in our previous work [24], a compulsive malposition occurred in adjacent birnessite slab because of the unique exposed facets of β-MnO2.The adjacent MnO6octahedra chains of β-MnO2are not lie in the same plane resulting in a smaller distance between Mn-Mn than that of birnessite. While the structure of birnessite are restrained by the exposed specific facets of β-MnO2leading to a malposed birnessite(Fig. 1i). Thus, the binding force between MnO6slab and interlayer cations in birnessite was weakened resulting in a freer interlayer cation,which means these interlayer cations can diffuse out or recombine more easily.

Fig. 1. (a-c) SEM images of parallel birnessite/β-MnO2 network; (d-f) TEM images of parallel birnessite/β-MnO2 network; (g) High resolution TEM image of the interface between parallel birnessite and β-MnO2;(h)Schematic diagram of the interface between parallel birnessite and β-MnO2;(i)Common birnessite and malposed birnessite structure.

In this composite, the abundant parallel birnessite also provides larger specific surface area (52 m2g-1). Fig. S4a demonstrates the nitrogen adsorption and desorption isotherms and the corresponding Barrett-Joyner-Halenda(BJH)pore-size distribution of three different asprepared samples.It could be easily found an obvious hysteresis loop at a relative pressure of ca. 0.45-1.0, suggesting the mesoporous nature of products [25]. The BJH plots (Fig. S4b) indicated the pore diameter of～13.2 nm, further confirming the mesoporous nature of parallel birnessite/β-MnO2network. Fourier transform infrared spectroscopy(FT-IR)was employed to characterize and analyze the functional groups of samples and the related results were shown in Fig. S4c. The several main characteristic bands can be easily observed at 447, 527, 1116,1638, and 3448 cm-1, respectively. The range of 450-650 cm-1peak could be ascribed to the Mn-O vibrations of manganese oxides.As well,the absorption bands at ～1116 cm-1were consistent with the vibrations of Mn-O-H [26]. Additionally, the weak peak at 1638 cm-1was probably attributed to the bending vibrations of the O-H groups originated from the surface adsorbed H2O, while the broad band ～3433 cm-1displayed the stretching vibrations of O-H groups from the interlayer H2O[27], suggesting the presence of plentiful water in birnessite structures,accordance with the XRD result.

2.2. Electrochemical properties

To elucidate the electrochemical reaction mechanism, a schematic diagram was shown in Fig. S5a. For surface adsorption-desorption dominated redox mechanism, high surface-active sites for the adsorption of the cations will contribute to the energy storage.For redox process dominated by the intercalation-deintercalation mechanism,the electrode ability to accommodate the cations in the electrode structure is the key point to realize the energy storage according to the following processes:

where C+is Li+, Na+, K+, etc. depending on the electrolyte. During the discharge process,one electron is injected to reduce Mn4+to Mn3+while equal amount of cation is simultaneously intercalated into the electrode structure.However,this process cannot happen if there is no room in the MnO2structure to accommodate the intercalated cation. That is to say,even there is sufficient Mn4+available in the electrode, it will not contribute to the energy storage if there is no spatial room in the structure for the intercalated cations. Therefore, fabrication of MnO2with structure that is able to accommodate more cations will enhance the charge storage ability.In bulk birnessite,the amount of the cations(Na+,K+,H+,etc)present between layers can be extracted out of and inserted into the electrode and is in direct proportion to the specific capacitance of electrode materials.Moreover,the content of Mn(III)is in direct proportion to the content of these intercalated cations(Na+,K+,H+,etc.)because of the charge balance in MnO2according to the following equation(3):

Fig. 2. (a) Cyclic voltammograms at different scan rates of parallel birenssite/β-MnO2 networks. (b) Galvanostatic charge-discharge curves of parallel birenssite/β-MnO2 networks at different current densities.(c)Variations of the capacitance of parallel birenssite/β-MnO2 networks and coulombic efficiency with current density.(d)Power law dependence of charge and discharge currents on various scan rates for parallel birenssite/β-MnO2 networks.(e)b-values for the three electrodes versus the potential for anodic and cathodic sweeps(Na+intercalation and de-intercalation).(f)Analytics of the voltammetric sweep data for the parallel birenssite/β-MnO2 networks electrode, sweep rates varied from 5 to 200 mV s-1. (g) CV curve at 50 mV s-1 of the parallel birenssite/β-MnO2 networks electrode with shadowed area representing the surface capacitive contribution. (h) Separations of diffusion-controlled intercalation and surface capacitive at different scan rates for parallel birenssite/β-MnO2 networks electrode. (i) Electrochemical reaction mechanism of parallel birenssite/β-MnO2 networks.

Therefore, birnessite that contains larger amount of intercalated cations will in term have higher content of Mn(III)and that will enhance the charge storage capability. The average oxidation state of the Mn changes during the charge-discharge cycle from 3.51 to 3.48 and 3.95 after discharge and charge respectively, indicating the low valance Mn ions in parallel birnessite/β-MnO2network was enhanced largely(Fig. S5b). The chemical formula of the parallel birnessite could be described by H0.012K0.470Mn2+0.040Mn3+0.402Mn4+0.558O2, which is calculated based on the content of Mn2+,Mn3+and Mn4+from the XPS results. However, the intercalation/de-intercalation processes are a battery feature because it is a slow kinetics process resulting in a pretty high capacitance at low current density but attenuation seriously with the increasing of current density. Note that, the reaction speed of the intercalation/de-intercalation processes are influenced a lot of factors,like tunnel size, electrical conductivity, ionic conductivity, temperature etc. Among them, the binding force between MnO6slab and electrolyte ions is very important but always be ignored as it will prevent the interlayer cations diffuse to the electrolyte, thus resulting in a low capacitance and poor rate capability.

The pseudocapacitive performance of the nanostructures was explored by cycle voltammetry(CV)and galvanostatic charge-discharge(GCD) in three-electrode systems. For comparison, pure birnessite nanoflowers were synthesized without introducing MnOOH networks as the substrate. The morphology of biressite nanoflowers was shown in Fig.S6.The capacitive performance of the prepared composite structures was investigated in three-electrode systems in 1 M Na2SO4aqueous solution. As displayed in Fig. 2a, the parallel birnessite/β-MnO2networks exhibit rectangular and symmetric CV curves, indicating the pseudocapacitive nature of this nanostructure [28]. The rectangular CV profile of parallel birnessite/β-MnO2networks can be maintained even under the scan rate of 100 mV s-1, which can be ascribed to the improved electrical conductivity of parallel birnessite and weakened binding force between MnO6slab and interlayer cations, resulting in the enhanced charge transfer and fast kinetic process. The charges originated from electrochemical reaction can been evacuated timely as the electrical conductivity of parallel birnessite has been enhanced to a great extent[29,30]. Meanwhile, the ultrathin parallel birnessite films and their arrangement also facilitated ion transfer due to the shortened migration route compared to ordinary birnessite films. Additionally, these structures also offer an unhindered electronic channel for charges originated from electrochemical reaction. More importantly, a weakened binding force between MnO6slab and interlayer cations will significantly accelerate the kinetic process, thus enhancing the electrochemical property.However, the CV curves of pure birnessite nanoflowers almost alter to shuttle-like shapes, manifesting the inferior rate capability (Fig. S7a).This can be ascribed to its bulk mass,prominently restricting the charge transfer.

The GCD plots of parallel birnessite/β-MnO2networks at different constant current densities were shown in Fig.2b.Obviously,compared to the GCD curves of pure birnessite nanoflowers(Fig.S7b),all the charge and the corresponding discharge curves of parallel birnessite/β-MnO2networks present well symmetry,suggesting a reversible and fast Faradic reaction between Na+and MnO2[31]. These plots of parallel structure displayed its line nature, suggesting a rapid charge-discharge process,which agrees with the CV tests. In addition, owning to the poor conductivity of the semiconductor,the bulky MnO2is hard to deliver all its capacitance. Thus, thick shells cannot sufficiently react with electrolyte in charge-discharge process,leading to an inferior capacitance and poor rate capability [32]. Therefore, the ultrathin birnessite will be more desirable to achieve high pseudocapacitance due to better utilization efficiency than bulk. The parallel birnessite/β-MnO2networks could provide a specific capacitance of 318 F g-1at 0.25 A g-1, with a high coulombic efficiency (Fig. 2c). Additionally, based on the weight of parallel birnessite,it could deliver an ultrahigh capacitance of 625 F g-1.About 25.1% of the capacitance at 0.25 A g-1could be retained for parallel birnessite/β-MnO2networks when the current density was increased more than 250 times from 0.25 to 64 A g-1, indicating a prominent rate capability of parallel birnessite/β-MnO2networks.This is ascribed to the content adjustment of Mn3+and Mn4+resulting in an enhanced electrical conductivity(double-exchange interaction)[24].By contrast, only 0.8% of the premier capacitance could be maintained for pure birnessite nanoflowers with the current density increased from 0.5 to 64 A g-1, which is poorer than that of parallel birnessite/β-MnO2networks. The interior performance of the birnessite nanoflower can be ascribed to the bulk mass resulting in a superficial visiting depth for alkali cations and lower electrical conductivity(Fig.S7c).Thus,the impedance analysis was carried out to investigate the otherness of the two products(Fig. S7d). Where, the Nyquist plots present a quasi-semicircle shapes over the high frequency range,whilst exhbit a line in the low frequency region,revealing a typical capacitor behavior.The Ohm resistances(Rs)of parallel birnessite/β-MnO2networks is almost the same with that of pure birnessite nanoflowers(1.07 Ω vs 1.28 Ω),indicating that these two samples possess the nearly resistance of electrolyte and the internal resistance. However, the interfacial resistance (Rct) of pure birnessite nanoflowers is almost three times of that of parallel birnessite/β-MnO2networks.Due to the Rct related to Faradic charge transfer,its difference can be used to explain the otherness of these two products in rate capability. Thus, the parallel birnessite/β-MnO2networks as electrode materials have a higher capacity and preferable rate capability.

To further explore the electrochemical behavior about charge storage of parallel birnessite/β-MnO2networks electrode, the capacitance of diffusion-controlled contribution and surface capacitive contribution were discussed by analyzing the power law equation, and the details about the equation are given in supplement materials.The linear feature shown in Fig. 2d reflects the ultrahigh electrochemical stability of parallel birnessite/β-MnO2networks electrode during the charge-discharge process. In addition, the calculated b values for parallel birnessite/β-MnO2networks electrode displayed in Fig. 2e are in the range of 0.8-1.0 and most of them are over 0.9 indicating that the current response belonged to pseudocapactive behavior. This means the stored charges in parallel birnessite/β-MnO2networks electrode are mainly come from the nonfaradaic process, contributed from the double-layer and Faradaic reaction from the charge-discharge process associated with redox reactions of surface Mn3+/Mn4+species rather than diffusioncontrolled intercalation. While, most of the calculated b values for pure birnessite nanoflowers are about the 0.8-0.9 implying the high content of intercalation/de-intercalation reaction occurred during the chargedischarge process. And the high calculated b values for parallel birnessite/β-MnO2networks electrode can be ascribed to the weakened binding forces between MnO6slab and interlayer cations, resulting in an enhanced transferring from diffusion-controlled intercalation process to Faradaic pseudocapacitance reactions.Compared to EDLCs,the pseudoreaction involves a valence converting of Mn atoms during the redox reactions, which is not chemical reaction but just charge transfer. The measured current contributed from first two process fits with a linear correlation approximately owning to their ultrafast reaction process,distinguishing with the contributed from last process. Thus, the current response under a specific potential can be ascribed to the two separate components: surface capacitive behavior and diffusion-controlled intercalation processes. And the capacitive contribution of the two different processes was distinguished quantitatively, the details of calculating analysis were demonstrated in supplement materials. As displayed in Fig. 2f, all the data points fall on the straight line with very small fluctuations indicating the high content of surface capacitive contribution in the CV curves at different scan rate. Similarly, in Fig. 2g, the shaded region in the CV curve at 50 mV s-1certified that the surface-controlled capacitance contribution is the main component.The surface capacitive contributions and diffusion-controlled intercalation contributions of the parallel birnessite/β-MnO2network electrode at different scan rates are shown in Fig.2h.The estimated surface capacitive contribution is about 214 F g-1at 5 mV s-1in CV tests, which is the main component of the total charge storage indicating effective surface charge storage for the parallel birnessite/β-MnO2network electrode. The diffusion-controlled intercalation contribution is gradually decreased with the increasing of scan rate,leading to the decrease of total capacitance at high scan rates.Based on these results, the charge storage mechanism of parallel birnessite/β-MnO2networks electrode was illustrated in Fig. S8a. The β-MnO2networks are surrounded by parallel birnessite and the exposed atoms of parallel birnessite are oxygen atoms. The electronegativity of oxygen atom results in the surface adsorbed Na+ions.During the charge process, these adsorbed Na+ions are ostracized along with disgorging interlayer ion(e.g.K+ions)of birnessite.While these adsorbed Na+ions are re-adsorbed during the discharge process with the intercalation of Na+ions. Remarkably, the swallowed and disgorged ions is identical as the original ions in birnessite is K+ions,while the cations in electrolyte is Na+ions,suggesting that the K+ions would be substituted by Na+ions unremittingly during the charge-discharge process. However, the reaction rate of Na+intercalation is quite slow compare to Na+ions adsorption, which matters most especially as the scan rate rising, and decreasing the total capacitance at high scan rates. Note that, the weakened binding force between MnO6slab and interlater cation are definitely improving the diffusion of interlayer cations, shortening the diffusion-controlled intercalation process and enhancing the Faradaic pseudocapacitance process, thus strengthen the capacitance and rate capability.

The advantages in electronic properties of parallel birnessite/β-MnO2networks is obvious compared to that of pure birnessite nanoflowers.As illuminated in Fig.2i,birnessite react with Na+ions in electrolyte during the electrochemical testing process. The charges migrate form the surfaces and birnessite layer,and then transferred to the current collector with the conductive agent (super P) assisted. For pure birnessite nanoflowers, the only migration path is the birnessite layer, which will be affected by the intrinsic electrical conductivity. However, for parallel birnessite/β-MnO2networks, the charges can be also migrated on the surface of β-MnO2, which will promote the conductivity of the active materials owning to the preferable electrical conductivity of the β-MnO2(See supplementary information). As well, due to the unique structures,the β-MnO2networks, supplied more transmission route, would own a better performance in charge transferring than single β-MnO2nanowires.Additionally, the intrinsic conductivity of parallel birnessite was also enhanced because of the increased content of Mn3+in birnessite resulting in an improved double-exchange interaction(Fig.S8b) demonstrated in our previous works [29]. Thus, based on these enhanced electrical conductivity mechanism, the electrical conductivity of parallel birnessite/β-MnO2networks will obviously been promoted.

Fig. 3. Asymmetrical supercapacitors made with the smart structures and reduced graphene oxide (RGO): (a) Cyclic voltammograms (50 mV s-1) of (a) reduced graphene oxide(RGO) and parallel birenssite/β-MnO2 networks.(b) CV curves at different cell voltages a scan rate of 50 mV s-1.(c)CV curves recorded at different scan rates with a maximum cell voltage of 2.0 V.(d)galvanostatic charge-discharge curves at different current densities.(e)Variations of the capacitance and columbic efficiency with the current densities. (f) Ragone plots of the supercapacitors. (g) Capacitance variations of the asymmetric supercapacitors with cycle number at the current densities of 2.0 A g-1.

Using the parallel birnessite/β-MnO2network structures as positive electrode (2.38 mg cm-2) and reduced graphene oxides as negative electrode (3.12 mg cm-2), an asymmetric two-electrode device was assembled with 1.0 M Na2SO4aqueous electrolyte [33]. The electrochemical behaviors of reduced graphene oxides (RGO, The detailed electrochemical performance was displayed in Fig. S9) and the parallel birnessite/β-MnO2network electrode were detected and analyzed by a three-electrode configuration (Fig. 3a). Both the positive and negative electrode exhibited rectangle shape at the scan rate of 50 mV s-1indicating the ideal capacitive properties.Fig.3b exhibited CV curves of the parallel birnessite/β-MnO2//RGO asymmetric supercapacitor cell with variation of voltage windows from 0-1.0 V to 0-2.0 V.Note that,the CV curves of the asymmetric supercapacitor displayed a nearly rectangular shape even at potential window up to 2.0 V,implying the ideal capacitive behavior. Moreover, the rectangular CV shape can keep well under a maximum cell voltage of 2.0 V at scan rate up to 100 mV s-1(Fig. 3c),suggesting an excellent rate capability of the asymmetric supercapacitor.Based on the total weight, the capacitance of the asymmetric supercapacitor was 59.5 F g-1at 0.25 A g-1(Fig. 3d) with a high coulombic efficiency over 95% (Fig. 3e). The supercapacitor delivered an energy density of 33.1 Wh kg-1at a power density of 199.0 W kg-1for a 2.0 V window voltage (Fig. 3f). The performance of energy storage are much higher than other symmetrical supercapacitors and MnO2-based asymmetric supercapacitors [33,34]. Such a promising capacitive performance can be attributed to the utilization of aqueous electrolyte,which is favor of ion migrate, and the synergetic effects of the two asymmetric electrodes.The RGO negative electrode not only accumulated charge via double-layer capacitance,but also provided fast electron transfer,whilst the ultrathin MnO2grown on β-MnO2network provided a short ions diffusion distance and more electrochemically active sites, which are favor for fast and reversible Faradic reaction. We also evaluate the rate capability and cycle stability of the asymmetric supercapacitor at a current density of 2.0 A g-1. This asymmetric device retained 95.8% of its initial specific capacitance after 10000 cycles (Fig. 3g). Finally, the assembled capacitors connected with a red LED and successfully lighted it (insert of Fig.3g).

Although the electrochemical properties of birnessite have an obviously improving via designing a compulsive malposition birnessite structure resulting in a weakened binding force between MnO6slab and interlayer cations, this design are more likely limited to the exposed crystal of the substrate (β-MnO2). It is not easy to prepare an exposed crystal structure same with β-MnO2. Additionally, β-MnO2is very poor pseudocapacitive material resulting in a large content of dead volume in electrode. Thus, exploiting new supporting substrate and reducing the content of β-MnO2will be in process.

3. Conclusion

In summary, a facile and cost-effective strategy has been proposed and developed to fabricate parallel birnessite on β-MnO2interconnected 3D network with high electrochemical performance. The perfect structural integrity between parallel birnessite and β-MnO2was demonstrated to be MnO6sharing chain model. Owing to the unparalleled paralleled morphology,weakened binding force between MnO6slab and interlayer cations and enhanced electrical conductivity (2D electron channel and double-exchange mechanism in birnessite), the portion of parallel birnessite in network composite structures exhibited an ultrahigh capacitance of 660 F g-1with a good rate capability and cycling stability. An asymmetric supercapacitor assembled by the parallel birnessite/β-MnO2network and RGO delivers an energy density of 33.1 Wh kg-1with the highest power density of 64.0 kW kg-1.Furthermore,about 95.8%of its initial capacitance was remained after 10000 cycles. These findings indicate compulsive malposition of birnessite slab is a promising design in developing high performance pseudocapacitance materials.

Declaration of competing interest

The authors have no conflict of interest to declare.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China(Grant No.51908092),Projects (No. 2020CDJXZ001, 2020CDCGJ006 and 2020CDCGCL004)supported by the Fundamental Research Funds for the Central Universities, the Joint Funds of the National Natural Science Foundation of China-Guangdong (Grant No. U1801254), the project funded by Chongqing Special Postdoctoral Science Foundation(XmT2018043),the Chongqing Research Program of Basic Research and Frontier Technology(cstc2017jcyjBX0080),Natural Science Foundation Project of Chongqing for Post-doctor (cstc2019jcyjbsh0079, cstc2019jcyjbshX0085), Technological projects of Chongqing Municipal Education Commission(KJZDK201800801), the Innovative Research Team of Chongqing(CXTDG201602014) and the Innovative technology of New materials and metallurgy(2019CDXYCL0031).The authors also thank the Electron Microscopy Center of Chongqing University for materials characterizations.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoms.2021.06.008.