A robust 2D porous carbon nanoflake cathode for high energy-power density Zn-ion hybrid supercapacitor applications

Journal Pre-proofs Full Length Article A robust 2D porous carbon nanoflake cathode for high energy-power density Zn-ion hybrid supercapacitor applications Zhongmou Pan, Zeming Lu, Lang Xu, Dewei Wang PII: DOI: Reference:

S0169-4332(20)30140-9 https://doi.org/10.1016/j.apsusc.2020.145384 APSUSC 145384

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

6 November 2019 17 December 2019 11 January 2020

Please cite this article as: Z. Pan, Z. Lu, L. Xu, D. Wang, A robust 2D porous carbon nanoflake cathode for high energy-power density Zn-ion hybrid supercapacitor applications, Applied Surface Science (2020), doi: https:// doi.org/10.1016/j.apsusc.2020.145384

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A robust 2D porous carbon nanoflake cathode for high energy-power density Zn-ion hybrid supercapacitor applications Zhongmou Pan, Zeming Lu, Lang Xu, Dewei Wang * College of Materials Science and Engineering, North Minzu University, Yinchuan 750021, People’s Republic of China. *Corresponding author. Tel: +86 951 2067378. E-mail: [email protected]

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Abstract The exploration of next-generation energy storage devices with long cycle life, superior stability, large specific capacity, ultrahigh power and energy density have attracted increased interests in recent years. However, it still remains a tremendous challenge for conventional energy storage devices to achieve the merits of both batteries and supercapacitors. Herein, we present a convenient but effective approach to synthesize porous carbon nanoflakes (PCNFs) that process high specific surface area and tunable pore size distributions. We found the amount of activating reagent has a profound influence on the morphology and textural structure of the resulting products, and a chemical etching process to transform nanocages into nanoflakes has been proposed. Importantly, Zn-ion hybrid supercapacitor with PCNFs as cathode and Zn foil as anode can overcome the disadvantages of poor rate capability and low energy density for the conventional batteries and supercapacitors. The optimized PCNFs based Zn-ion hybrid supercapacitor can deliver an ultrahigh specific capacitance, excellent rate performance, outstanding cycling stability, and impressive energy density. The facile synthetic procedure combined with its excellent electrochemical performances endow the present devices a huge possibility to be used in future electrochemical energy storage systems. Keywords: Zinc-ion; supercapacitor; carbon nanoflakes; energy density; rate capability

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1. Introduction With the explosive increase of demand for sustainable electrochemical energy storage systems and green energy in modern society, supercapacitors, featured with high power density, briefer charging and discharging time and long cycling life, have been attracted considerable research attention recently for their widely application in various

commercial

energy

storage

device

[1-6].

However,

conventional

supercapacitors have a low energy density (about 5-8 Wh/kg) as compared with secondary batteries, which seriously limits its future widespread usage as high potential next-generation electrochemical energy storage devices [7-9]. Generally, employing a variety of electrode materials in various electrolytes or construction of asymmetric supercapacitors can improve the energy storage performance of the resulting devices [10-12]. Nevertheless, it is still a challenge for them to construct next-generation energy storage devices with merits, such as high energy output, long cycling life and low costs. Recently, as a new type of energy storage devices, ionic based hybrid supercapacitors, which utilize metal foil as the anode to combine the virtues of both secondary batteries and supercapacitors such as superior energy density, outstanding power density alone with ultralong cycling life, have been regaining popularity as a powerful tool to further promote the energy storage electrochemical properties of the traditional devices [13-15]. For instance, lithium-ion based hybrid supercapacitors have achieved great interests in recent years owing to their high potential theoretical capacity and light weight [16]. Similarly, some other ionic based supercapacitors such 3

as sodium-ion and potassium-ion supercapacitors have also been reported [17]. However, these alkali metals are highly reactive and could bring security problems when directly used as the electrode materials for supercapacitors. Additionally, the storage and process of the alkali metals makes it expensive, and difficult to be used as a sustainable material for next-generation energy storage devices [15, 18-21]. Furthermore, these ionic based supercapacitors are all monovalent ions (e.g. Na+, K+, Li+), recent researches pointed out the energy storage devices with multivalent ions (e.g. Zn2+, Ca2+, Al3+) have some conspicuous merits, such as fast charge transfer dynamics and more reacted ions can be provided as compared with monovalent ion based devices. Especially, Zn with high theoretical capacity of 823 mAh/g (Zn/Zn2+), less air sensitive, more feasible during industry production and high safety has been widely used in metal-air batteries, rechargeable Zn//MnO2 batteries and Zn//Polymer hybrid supercapacitors [14, 19, 22]. For instance, the zinc-ion batteries using polymer grown on carbon nanotubes as cathode material can deliver a specific capacity of 126.2 mAh/g under a current density of 20 mA/g, and the capacity decreased to 43.2 mAh/g when the current density increased to 5000 mA/g, and the 96% of the stabilized capacity remained after 500 cycles [23]. An ultrahigh capacity of 255 mAh/g and high-capacity retention of 92% after 5000 cycles can be achieved when MnO2 nanorods were used as cathode electrode [24]. Although these cathodes can bring some improvement in specific capacitance, they also suffer from poor service life. To address these challenges, porous carbon materials would be a potential electrode material for Zn ion hybrid supercapacitors (ZHSCs) application as account 4

of its large specific surface area, low costs and favorable physicochemistry durability [20, 25, 26]. For example, Feng and coworkers have reported Zn-ion hybrid micro-supercapacitors with a specific capacity of 259.4 F/g at a current density of 0.05 A/g and stability of nearly 96% after 10000 cycles at a current density of 0.5 A/g [27]. Chen et al. have presented a zinc ion hybrid supercapacitors based on hollow carbon spheres derived from polymer can obtain a large discharge capacity of 86.8 mAh/g at a current density of 0.5 A/g, and high cycle stability of 98% capacity retention when up to 15000 cycles under a current density of 1.0 A/g [28]. Wang et. al, have developed an oxidized carbon nanotube cathode can acquire a specific capacitance of 20 mF/cm2 under a scan rate of 10 mV/s and almost stable up to 5000 cycles [29]. Obviously, replacement of polymer and MnO2 with a porous carbon cathode can improve the cycling stability significantly although they still suffered from low specific capacity and poor rate capability. Therefore, it is still a great importance and significant challenge of exploiting porous carbon based cathode to further improve the energy storage properties such as large specific capacitance, excellent rate capability and ultralong cycle life. In the current work, we have designed ZHSCs with low-cost Zn metal foil, porous carbon nanoflakes and non-corrosive ZnSO4 solution as anode, cathode and electrolyte, respectively. Compared with those of ZHSCs based other cathode materials, our device displays high specific capacity, outstanding rate capability, excellent stabilities accompanied with a low IR drop even at a high current charge-discharge density (20 A/g). The excellent electrochemical performances can be 5

attributed to the employing porous carbon nanoflakes microstructure as the cathode, which can simultaneously offer a fully ions-accessible surface area and short diffusion pathway for electrolyte ions. These findings demonstrate that the PCNFs with unique nanoflakes structure and outstanding electrochemical performances will be a promising cathode material for ZHSCs application. 2. Experimental 2.1. synthesize of PCNFs All chemicals were analytical grade (purchased from Sinopharm Chemical Regent Co., Ltd.), and were used directly without any purification. Zn metal foil has a thickness of 100 μm (purity: 99.99%, purchased from Qingyuan Metal Material Co., Ltd.). Ultrafine deionized water (18.25 MΩ•cm) was used throughout the experiment. In a typical synthetic process, 5 g of sodium polyacrylate powder (SPA) was mixed with 4 g of potassium bicarbonate powder（KHCO3, a mass ratios of 5:4) and thoroughly ground in agate mortar. Then the mixture was pyrolysis in tube furnace (OTF-1200X, Kejing, China) under a constant argon gas flow at 850 °C for an hour with a heating ramp rate of 5 °C/min to carbonize and active the precursor. Subsequently, the resulting black powder was neutralized by dilute HCl (1 M) and washed by water in order to wipe off any impurity elements and then dried in an oven at 80 °C overnight. To further understand the effect of KHCO3 on the activation process, samples were also prepared with different amount of KHCO3 (0, 3 and 5g). For simplicity, the as-obtained samples were defined as PNFC-X, where X corresponds to the mass amount of KHCO3. The carbon yields of the samples under 6

different amount of KHCO3 are about 3-7% (see Table S1). 2.2. Materials characterization Field emission scanning electron microscopy (FE-SEM JEOL-5600LV) was used to characterize the microstructure and morphologies of the samples at an accelerating voltage of 5.00 kV, transmission electron microscopy (TEM, JEM-2010 Japan) was employed to examine the ultrastructure of the samples. X-ray powder diffraction (XRD, SHIMADAZU 6000) with a Cu-Ka radiation source (λ = 1.5418 Å) at 40 kV, 100 mA to describe the phase composition and crystal structure of the samples under a scan rate of 10° min-1, while the 2θ range is from 10° to 80°. Raman spectroscopy analysis was performed on a Renishaw microRaman spectroscopy with a wave length range from 100 to 3500 cm-1. Nitrogen adsorption and desorption curves were determined by a Micromeritics APSP-2020 instrument at 77 K. The specific surface area was measured by the Brunauer-Emmett-Teller (BET) equation. The corresponding micropore and mesopores surface area were calculated by t-plot method. Total pore volume (Vt) was estimated by single point method from the amount adsorbed at P/P0=0.975. The distribution parameter of Pore size was calculated by non-local density functional theory (NLDFT) model with slit-like pores. X-ray photoelectron spectroscopy (XPS, Physical Electronics 5400 ESCA) was used to analyze the surface chemical compositions of the samples. The thermogravimetric analysis (TGA) and thermal difference analysis (TDA) data of the precursor was collected by using a STA449-F5 FAQ600 Jupiter analyzer (NETZSCH) under 850 °C for 1 hour in constant nitrogen gas under a heating rate of 10 °C/min. 7

2.3. Electrochemical measurement Firstly, the cathode electrodes were prepared by mixing active materials (PCNFs), binder (polytetrafluoroethylene, PTFE) and conductive additives (Super P) at a ratio of 8:1:1 into few drops of alcohol and coat the mixture onto a stainless current collector (14 mm in diameter), followed by pressing under a pressure of 10 MPa for one minute and finally dried at 80 °C overnight in an oven. The mass loading of the active materials on each electrode was weighted by a high-precision microbalance (Sartorius, BSA224S). The mass of PCNFs loaded on each cathode in the ZHSCs was 1.2 mg. Zn metal foil (polished by 1000 mesh sandpaper) was directly used as anode. Aqueous 1 M ZnSO4 was used as electrolyte and the cathode and anode were separated by a separator to construct a CR2025 coin cell. All the electrochemical measurements were performed on a CHI660E electrochemical workstation (Chenhua, Shanghai, China) at ambient conditions. Cyclic voltammetry (CV) measurements were carried out over the potential window of 0.1 to 1.7 V at scan rates from 10 to 100 mV/s. The galvanostatic charge/discharge (GCD) tests were carried out at the same potential range at a series of current densities. Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range between 10 mHz and 100 kHz with an AC amplitude of 5 mV. The specific capacitance (Cs, F/g), energy density (E, Wh/kg) and power density (P, W/kg) can be calculated by the following formulas [13, 26]: Cs = I·Δt/(m•ΔV) E = C•ΔV2/7.2

(1) (2) 8

P = E/Δt

(3)

In these formulas above, I represents the discharge current (mA), t represents the discharge time (s), m represents the mass loading of the cathode material (mg) and ΔV is the voltage window during discharge process (V), respectively. 3. Results and discussion Figure1 schematically illustrates the synthetic process and energy storage mechanism of the as-assembled ZHSCs. Generally, chemical activation with alkali metal hydroxides (e.g. KOH, NaOH) is a powerful strategy in forming porous carbons with high specific surface area [30]. Their strong corrosion and high reactivity, however, made it unfavorable for large-scale industrialized production. Thus, a mild KHCO3 was used as the activating agent to generate nanopores within the carbon frameworks. During the charging process, SO42- anions from the electrolyte can absorb onto the PCNFs cathode to form double electrode layers, while the Zn2+ ion deposited onto the Zn anode [20, 26]. The discharging process is the reversible reaction of charging process, of which SO42- diffused back to the electrolyte and Zn2+ formed again based on the stripping reaction from Zn into Zn2+ ions [27]. The electrochemical reactions on a Zn anode and a PCNFs cathode are all highly reversible, which ensure a long cycle life of the resulting ZHSCs. Moreover, the combination of EDLCs in the cathode and battery like energy storage mechanism in the anode can effectively improve energy storage capacity as compared with symmetric devices. The morphologies and microstructures of the PCNFs prepared with different 9

amount of KHCO3 were first characterized by FESEM. Figure 2a displays that the PCNF-0 is comprised of a great deal of interconnected carbon nanocages, and a small portion of carbon nanocages are broken. It is worth to note that the cage-liked structures are obtained by simply pyrolysis of SPA without any externally added templates, which is totally different from the previous hard templates involved methods [31, 32]. As shown in Figure S1, we believed the in-situ generated Na2CO3 (JCPDS Card File No.37-451) can serve as the in situ template to sustain a nanocage structure (Figure S2). The thermogravimetric analysis (TGA) described the mass change of the precursor while thermal difference analysis (DTA) measured the endothermic and exothermic of the SPA during the pyrolysis process. Figure 2b presents the mass change and thermal difference analysis of SPA. From the TGA pattern, the percentage of the last remaining mass is 53.4% and the pyrolysis of SPA contains three main processes. The first reaction occurs below 415 °C，while the SPA sample lose about 11.85% mass mainly due to loss of absorbed water and small decomposition of the precursor. Then between 415 °C and 507 °C, the precursor began to mass decomposition to form a semi-product along with a 25.45% weight loss. In the end, from 507 °C to 850 °C, the oxygen-containing functional group of the semi-product began to resolve and the formation of Na2CO3. After a small amount of KHCO3 was added, the nanocage structure has been etched to some extent, and these etching effects mostly occur on the edge area of carbon walls, resulting in a partially broken nanocage structure (Figure S3).

As the KHCO3 content continuous increased

to 4 g, the nanocage structure totally broken into a flake structure. As demonstrated in 10

Figure 2c and 2d, the nanoflakes have sharp edges and they are interconnected with each other to from a network structures. The microstructure of PCNF-4 was further analyzed by (HR)TEM studies. Figure 2e displays the typical TEM images of PCNF-4, it can be seen the nanoflakes are randomly aggregated with some folded morphology. The HRTEM image further revealed that PCNF-4 is an amorphous structure and numerous nanopores presented in the basal planes (Figure 2f). These nanopores can not only storage abundant electrolyte ions, but also improve the diffusion kinetics during charging/discharging process, and finally improve the electrochemical performance of the resulting device [33, 34]. On the contrary, PCNF-5 sample mainly consists irregular microparticles and only a small amount of nanoflakes can be found indicating the formation of nanocage structures are prohibited at high content KHCO3 (Figure S4). Based on the morphologies evolution with the different KHCO3 content, it can be concluded that the nanocage structures can be obtained with the in-situ formed Na2CO3 as the template, the nanocage structure are gradually collapsed into nanoflakes as a resulting of the KHCO3 induced etching process, which provide an alternative way towards 2D nanocarbons (Figure 2g). The chemical structural characteristic was measured by X-ray diffraction (XRD) analysis. From Figure 3a, it can be observed that all samples have two broad diffraction peaks at around 22° and 43°, corresponding to (002) and (101) plane reflection of amorphous carbons. The weak and broad (002) diffraction peak represents a low degree of graphitization and turbostratic carbon structure. In addition, 11

the high relative intensity at the low angel region (below 10°) indicating a large amount of nanopores in different sizes exist in these samples [35, 36], which is consistent with the HRTEM observation. The structure of the samples was further investigated by the Raman spectra, as shown in Figure 3b, all the sample two broad peaks around 1340 cm−1 and 1590 cm−1, which can be attributed to the D (carbon in disorder or defect structure) and the G-band (ordered graphitic structure) respectively [37]. It is universally accepted that the ratio of peak intensity of D band and G band (ID/IG) directly related to the extent of disorder structure in the porous carbons [38, 39]. The calculated ID/IG values for PCNF-3, PCNF-4, PCNF-5 are 0.97, 1.10 and 1.18, respectively, confirming the content of disordered or defective increased with the increased of KHCO3 amount. Results from XRD and Raman spectra verify that the PCNFs are amorphous carbons resulting from KHCO3 activation by generating porosity and defects. XPS was used to characterize the element composition, chemical states and energy bending of all the samples. As can be seen in the full scan (Figure 3c), the surface of PCNFs contains only two elements: carbon and oxygen, implying its high purity. The element composition of C and O is 91.23 at.% and 7.71 at.% in PCNF-3 while PCNF-4 has a C and O composition of 90.66 at.% and 8.06 at.% and PCNF-5 has a C and O composition of 90.64 at.% and 9.3 at.% respectively. From the C1s pattern of all samples (Figure 3d and Figure S5), the C 1s spectrum can also be divided into 4 peaks centered around 284.8 eV, 285.6 eV, 286.5 eV and 288.8 eV respectively, assigning to sp2 carbon, sp3 carbon, C-O bond, and C=O bond, respectively. The existence of oxygen-containing functional groups can improve the 12

wettability of electrode materials in aqueous electrolyte, and thus decrease the ion-inaccessible surface area [40, 41]. Because of pivotal role of the pore structure and specific surface area played in electrochemical performances, the characteristics of the pores and specific surface areas are analysis by N2 adsorption-desorption measurements. As demonstrated in Figure 4a, all the samples have a combined I/IV isotherm with hysteresis loop, indicating that as-obtained PCNFs have a combination of micropores and mesopores concluded from the IUPAC classification [42]. The sharp increase gas adsorption isotherm under low relative pressure (P/P0 < 0.1) indicates that there is a large amount of micropores in all the samples, while the distinct hysteresis loop at the relatively high pressure (0.4 < P/P0 < 0.9) also confirms the presence of mesopores. The corresponding pore size distribution curves of PCNFs are displayed in Figure 4b, with the increase of KHCO3 content, the amount of the pores centered at 0.8 nm decreases while that between 1.3 and 2.7 nm increases as a result of the enlarged and collapsed micropores into mesopores. The pore structure parameters of PCNFs are displayed in Table 1. The specific surface area of PCNF-3, PCNF-4 and PCNF-5 is calculated to be 804 m2/g, 1770 m2/g, 1781 m2/g, while the total volume of pores in PCNFs is 0.405 cm3/g, 0.64 cm3/g, 0.7797 cm3/g, respectively. Moreover, the corresponding mesoporous surface area ratio for PCNF-3 is only 37.8%, and PCNF-4 has a mesoporous surface area ratio of 42.4% while that of PCNF-5 increased to 46.7% (Table 1). These data clearly demonstrated that the specific surface area, total pore volume and proportion of mesoporous surface area increased with the content of 13

KHCO3 increased. The reason for this can be attributed to the chemical activation with low content of KHCO3 mainly generates new micropores while more mesopores are produced by a higher amount of chemical activation agent. It is worth noting that the microporous size distribution of the PCNFs centered around 0.8 nm, very close to the dimension of Zn2+ cations (0.74 nm) and SO42- anions (0.73 nm), which allow more close contact of the ion center to the pore walls, leading a large specific capacitance [43, 44]. The capacitance performances of the as-assembled ZHSCs are first evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurements. Among all the electrodes, PCNF-4 featured a larger CV area than PCNF-0, PCNF-3 and PCNF-5 at a scan rate of 30 mV, which manifests it owns the largest specific capacitance (Figure 5a). The extremely small CV area of PCNF-0 indicates it has very small specific capacitance as a result of its low BET surface area (Figure S6). In Figure 5b, it was found that all CV curves display nearly rectangular-shape with reversible redox humps at all scan rates, confirmed that the charge/discharge mechanism of ZHSCs is a combination of batteries and supercapacitors [26, 28]. Moreover, the rectangular-shape did not change significantly when the scanning rate increased from 10 mV/s to 100 mV/s, implying its fast energy storage capability in the electrochemical reaction process (Figure 5b). The GCD curves of the ZHSCs under different current densities are exhibited in Figure 5c-d and the PCNF-4 is proved have the highest specific capacitance by GCD curves of PCNFs at a current density of 0.5 A/g (Figure S7). The deviation from linearity shape also demonstrating 14

the combined energy storage mechanism [45]. By calculation, the maximum specific capacities of ZHSCs using PCNF-0, PCNF-3, PCNF-4 and PCNF-5 cathodes can reach to 1.27, 48.3, 177.7 and 108.2 mAh/g at 0.5 A/g (Figure S8, S9 and S10), respectively. Figure 5c-d shows the capacity values at a series of current densities range between 0.5 and 20 A/g. The results show that the capacity of PCNF-4 based ZHS of 85.5 mAh/g is remained at a high rate of 20 A/g, while 57 mAh/g for PCNF-5 based ZHS, 21.6 mAh/g for PCNF-3 based ZHSC. The corresponding capacities retention of 48.1%, 52.8%, and 44.7% is obtained, respectively, indicating an excellent rate capability of as-prepared PCNFs (Figure 5e-f). The relevance of IR drops and current density of the as-assembled ZHSCs are lined out in Figure 5g, they all express an obviously linear relation (R2 > 0.99). The internal resistance of PCNF-5 cathode (IR drop (V) = 0.01157 + 0.00962Im (A/g)) is the lowest when compared with PCNF-4 cathode (IR drop (V) = 0.00904 + 0.01082Im (A/g)), and PCNF-3 cathode (IR drop (V) = 0.0468 + 0.0225Im (A/g)). It can be concluding that the internal resistance is highly dependent on the mesopore surface area, that is, the high content of mesopore ratio the smaller internal resistance because of the mesopores can facilitate the fast electrolyte ions transfer and thereby increasing the active sites. However, the mesopore content should be optimized in order to realize a high specific capacity since the micropores can be served as the main active sites for ion adsorption and the mesopores have larger size (especially twice larger than that of electrolyte ions) hardly to establish a closer contact of the ion center to the pore walls than that of micropores. Another attractive character of this PCNF-4 based ZHSCs is its excellent 15

cycling stability at a high current density. From Figure 5h, the retention capacity is 90% after 10000 recharge cycles under a current density of 10 A/g. The outstanding long life cycle stability and high columbic efficiency it surpasses most of the previous reported ZHSCs based on the others cathodes. The superior electrochemical performance of PCNF-4 based ZHSCs is attributed to unique structural advantages. (1) The large BET surface area can afford sufficient electric double layers between active material and electrolytes that is the basic requirement for capacitive energy storage [2,3,5,6]; (2) Well balanced mesopore and micropore content can achieve a large specific capacitance and good rate performances simultaneously [46]; (3) The abundant ion matched micropores can allow more close contact of the ion center to the pore walls and therefore an increased specific capacitance [47]; (4) the structural features of 2D flakes with small thickness can shorten the ions diffusion distance that can guarantee fast electrolyte ions transfer during the charging and discharging process [12]. Electrochemical impedance spectroscopy (EIS) is an important electrochemical parameter of energy storage device to describe the transmission kinetics of ions and the resistance during charging/discharging process. Figure 6a presents the Nyquist plots of PCNFs at a frequency region from 10 mHz to 100 KHz. In general, the Nyquist plot mainly contains three components: the internal resistance (Rs) at a high frequency region, the charge-transfer resistance (Rct) exists on the reaction interface is presented through semicircle curve at the middle frequency region, and the Warburg resistance (Rw) is shown as inclined line vertical line at a low frequency region [48]. 16

The three samples have a similar small charge-transfer resistance attributes to their similar internal resistance. From the EIS image, it was found that the resistance of PCNFs mainly depends on the ion diffusion resistance and charge transfer resistance. Especially, PCNF-4 based ZHSCs has a much longer tail at low frequency region demonstrating the ions can enter into much deeper pores, which is helpful increasing the energy storage capability (Figure 6a). In addition, Furthermore, by means of quantitative analysis of the linear relationship between Z′ and ω

-1/2

(Figure 6b), the σ

values of the resulting ZHSCs increase in the sequence of PCNF-3 (27.28) > PCNF-4 (18.29) > PCNF-5 (12.99). Since the diffusion coefficient (D) is inversely proportional with the σ values [48], the D is increase in the sequence of PCNF-5 > PCNF-4 > PCNF-3 (Figure S11), demonstrating that the diffusion efficiency is highly dependent on the mesopore content. Therefore, the balance of specific surface area and mesopore content is crucial to simultaneously achieve large specific capacity and rapid electrolyte ion diffusion. In order to further investigate the differences of energy storage mechanisms between this hybrid supercapacitor and conventional symmetrical supercapacitors, the PCNF-4 based symmetric device also construct in 1 M ZnSO4 electrolyte. From Figure S12, it was found that PCNF-4 based symmetric device has a much lower specific capacitance than that of ZHSCs, i.e., a low specific capacitance of 110 F/g (30.6 mAh/g) at 0.5 A/g within 0-1 V. The different characters of CV and CP curves confirming again the totally different energy storage mechanism between ZHSCs and symmetric supercapacitor device. The low specific capacitance of symmetrical 17

supercapacitor own to the electrical double-layer based process where the energy storage main depends on the specific surface area as the active sites for ions to adsorption/desorption. The energy storage mechanism of ZHSCs can combine the advantages of supercapacitors and batteries where the electrochemical reaction occurs not only on the interface between electrode and electrolyte but also Zn ions deposition/stripping reactions during charge-discharge process. Benefiting from to the high capacity and wide reversible working voltage window, the PCNF-4 based ZHSCs device achieves an energy density as large as 142.2 Wh/kg at a power density of 400.3 W/kg (0.5 A/g) and remains at 68.4 Wh/kg even the high power density of 15390 W/kg (20 A/g) (Figure 7a), which reflects the excellent energy output at high current densities. For comparison, the maximum energy density of the PCNF-4 based symmetric device is only 39.1 Wh/kg at a power density of 1235 W/kg (0.5 A/g). The excellent energy-power performance compared and even outperforms most of recent reported ZHSCs devices in aqueous/no-aqueous electrolytes, such as, biomass derived porous carbon (52.7 Wh/kg at a power output of 1725 W/kg) [45], commercial activated carbon (30 Wh/kg at a power density of 14900 W/kg) [26], “rocking-chair”-type metal hybrid supercapacitor (8.7 Wh/kg at a power output of 5100 W/kg) [49], organic cathode based device (12 Wh/kg at a power output of 15300 W/kg) [50], hollow carbon spheres (59.7 Wh/kg at a power output of 447.8 W/kg) [28]. (Figure 7a). Figure 7b displays two PCNF-4 based ZHSCs devices connected in series can power two green light emitting diodes (LED, 2.2 V) at least 10 minutes after charging at 5 A/g for 60 s (Figure 7b), which demonstrate the 18

practical application of PCNF-4 based ZHSCs devices. All of these results evidently support that PCNF-4, which are derived from low cost SPA via a mild KHCO3 activation, is a potential choice for next-generation energy storage device for ZHSCs applications. Conclusions To sum up, PCNFs were produced via a facile one-step KHCO3 activation procedure. The morphologies and textural characteristics of the resulting PCNFs mainly depend on the KHCO3 etching effect. Particularly, PCNF-4 displays nanoflake-like morphology, high surface area and abundant mesopores. As the cathode for Zn ion hybrid supercapacitor, PCNF-4 based ZHSCs device has remarkable energy and power densities of 142.2 Wh/kg and 15390 W/kg, respectively, together with an excellent rate capability (48.1% capacity retained at 20 A/g), and an excellent cycling stability (~90% retention after 10000 cycles at 10 A/g), which outperforms most of recent

reported

ZHSCs

devices

and

symmetrical

supercapacitors

in

aqueous/no-aqueous electrolytes. Therefore, we believe the present work can offer a new opportunity for synthesizing 2D nanocarbons for next-generation electrochemical energy storage devices not only for ZHSCs but also for other applications including batteries, electrocatalyst, etc. Acknowledgements The authors are grateful to the financial supports from National Natural Science Foundation of China (Grant no. 51762001), CAS “Light of West China” Program (Grant no. XAB2017AW07) and Natural Science Foundation of Ningxia (Grant no. 19

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Figure captions Figure 1. Scheme illustration of the synthetic process and energy storage application of PCNFs. Figure 2. (a) FESEM image of PCNF-0, (b) TG-DTA curves of the SPA, (c-d) FESEM images of PCNF-4, (e-f) TEM and HRTEM images of PCNF-4, and (g) possible formation mechanism of PCNFs. Figure 3. (a) X-ray diffraction patterns; (b) Raman spectrums; (c) XPS survey spectrum, and (d) high-resolution C 1s spectrum of PCNF-4. Figure 4. (a) Nitrogen adsorption isotherms and (b) pore size distribution curves of the as-prepared PCNFs. Figure 5. Electrochemical performance of PCNFs. (a) CV curve of PCNFs at a scan rate of 30 mV/s, (b) CV curves of PCNF-4 at a scan rate from 10 mV/s to100 mV/s, (c and d) GCD curves of PCNF-4 under a current density from 0.5 A/g to 20 A/g, (e) specific capacitance of PCNFs at different current densities, (f) rate capability, (g) IR drops, (g) cycle stabilities and columbic efficiency under a current density of 10 A/g (insert is the GCD curves before and after cycles). Figure 6. EIS spectrum of PCNFs at open circuit voltage. (a) Nyquist plots of PCNFs and (b) quantitative analysis and simulation of the linear relationship between Z′ and ω -1/2 in the low-frequency region. Figure 7. (a) Ragone plots of the PCNFs based ZHSCs, PCNF-4 based symmetric supercapacitors and other previous reported ZHSCs. (b) Photographs of two green 27

LED lamps driven by two PCNF-4 based ZHSC after charging for 60 s at a current density of 5 A/ g.

28

29

30

31

32

33

34

Table 1. Porosity characteristics of the as-obtained PCNFs.

Sample

Vt

SBET

Smicropore

Smesopore

Smesopore/SBET

(cm3/g)

(m2/g)

(m2/g)

（m2/g）

(%)

PCNF-3

0.315

804

500

304

37.8

PCNF-4

0.721

1770

1020

750

42.4

PCNF-5

0.784

1781

950

831

46.7

35

Highlights

1. Porous carbon nanoflakes have been synthesized via mild one-step KHCO3 activation process. 2. KHCO3 induced etching process can transform nanocage into nanoflakes. 3. The obtained products displayed large specific surface area and tunable mesopore content. 4. Porous carbon nanoflakes can be used as the cathodes for Zn-ion hybrid supercapacitor. 5. The product with suitable mesopore content displays excellent electrochemical performance.

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Graphical Abstract

Author Contribution

Dewei Wang designed all the experiments. Zhongmou Pan, Zeming Lu and Lang Xu conducted the detail research. Zhongmou Pan wrote the first draft of the manuscript. Dewei Wang provided technical editing of the manuscript. All authors discussed the results and approved the final manuscript.

37

Declaration of conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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