Redox Chemistry of Mn2+ on N‐Doped Porous Carbon Fibers for High‐Performance Electrochemical Energy Storage

Earth‐abundant manganese (Mn) compounds have multifarious valence states that make Mn advantageous for electrochemical energy storage applications. Benefiting from the unity of opposites in redox chemistry, a novel aqueous all‐Mn energy storage device (AMESD) based on the redox chemistry of Mn2+ is proposed, which is a simple‐structured battery that can be initially assembled by two bare carbon fiber cloths (CFCs) and the electrolyte. Moreover, the effects of electrolyte optimization and the modification of CFCs on the energy storage performance of the novel battery are discussed. The device based on N‐doped porous CFCs achieves the highest areal capacity of 1.46 mAh cm−2 (≈152.1 mAh g−1), along with an energy density of 1.10 mWh cm−2 and a power density of 9.66 mW cm−2. More impressively, the charge–discharge cycles at a capacity of 0.33 mWh cm−2 are performed 40 000 times and it can maintain a high‐capacity retention rate of 93.5%. Even at a low current of 1 mA cm−2, the capacity retention rate after 100 cycles is maintained to 86%. Herein, a new option for low‐cost, ultrastable aqueous battery design is provided and the possibility of Mn‐based anodes for energy storage applications is explored.


Introduction
Large-scale electrical energy conversion and storage technologies play a pivotal role in addressing global environmental issues such as the energy crisis and climate warming in today's society. [1]fter extensive research, conventional ion (Li, Na, K) batteries still suffer from poor cycle stability, low power density, and noticeable safety hazards. [2]To meet the increasing demand for mobile portability, mechanical electrification, and artificial intelligence of various devices around the world, research on aqueous batteries with attributes such as high safety attributes, high-power characteristics, durability, and low cost of scale has gradually increased. [3]nderstanding the chemical mechanism in the energy storage process has important implications for improving battery performance.The insertion/extraction and chemical conversion mechanisms are regarded as two types of typical charge storage mechanisms. [4]There are still differences in the energy storage mechanisms of various aqueous battery systems.For instance, aqueous sodium-ion batteries rely on the Na þ insertion/extraction mechanism in the cathode/anode host for charge storage. [5]Zn-ion batteries are mainly dominated by the insertion/extraction mechanism of cathodic materials and the chemical conversion mechanism based on the deposition/stripping of metal anodes. [6]However, research on batteries with the dual chemical conversion (DCC) mechanism of cathode and anode is still in its infancy.The ideal batteries have outstanding characteristics such as high capacity, high power output, and high stability. [7]Hence, the DCC batteries open up a new way for novel energy storage devices with excellent electrochemical performance.
The typical reaction based on the chemical conversion mechanism in the battery cathode is the deposition/dissolution process of two-electron Mn 2þ /MnO 2 . [8]Mn 2þ /MnO 2 batteries by matching different anodes have been reported successively.Specifically, Cui et al. assembled an H 2 -MnO 2 battery earlier through the conversion reaction between H 2 and H 2 O, which achieved no capacity fading within 10 000 cycles and provided Earth-abundant manganese (Mn) compounds have multifarious valence states that make Mn advantageous for electrochemical energy storage applications.Benefiting from the unity of opposites in redox chemistry, a novel aqueous all-Mn energy storage device (AMESD) based on the redox chemistry of Mn 2þ is proposed, which is a simple-structured battery that can be initially assembled by two bare carbon fiber cloths (CFCs) and the electrolyte.Moreover, the effects of electrolyte optimization and the modification of CFCs on the energy storage performance of the novel battery are discussed.The device based on N-doped porous CFCs achieves the highest areal capacity of 1.46 mAh cm À2 (%152.1 mAh g À1 ), along with an energy density of 1.10 mWh cm À2 and a power density of 9.66 mW cm À2 .More impressively, the charge-discharge cycles at a capacity of 0.33 mWh cm À2 are performed 40 000 times and it can maintain a high-capacity retention rate of 93.5%.Even at a low current of 1 mA cm À2 , the capacity retention rate after 100 cycles is maintained to 86%.Herein, a new option for low-cost, ultrastable aqueous battery design is provided and the possibility of Mn-based anodes for energy storage applications is explored.
an energy density of 139 Wh kg À1 . [9]Qiao et al. designed a lowcost electrolytic Zn-MnO 2 battery with high output potential (1.95 V), delivering an energy density of 409 Wh Kg À1 . [10]Zhi et al. also proposed to use three common metals (copper, zinc, and bismuth) as anodes to assemble M-MnO 2 batteries, thus opening the investigative prologue in M-MnO 2 battery research. [4]Immediately afterward, Xia et al. also successively developed Cu-MnO 2 and Pb-MnO 2 batteries based on acidic electrolyte systems, both of which showed considerable energy density and excellent stability with no capacity decay after 10 000 cycles. [11,12]Chen et al. also proposed Mn 2þ /MnO 2 batteries based on other metal anodes, such as Cd-MnO 2 , [13] Cr-MnO 2 , [14] and Al-MnO 2 [15] batteries, all of which exhibited excellent rate capability (up to 200 C) and superior cycle stability (over thousands of cycles).The design of the above batteries is based on the acidic electrolyte, and the acid environment helps to improve the reversibility of the Mn 2þ /MnO 2 conversion reaction, but on the negative side, most metal anodes have selfcorrosion behavior in a strong acid environment, which is not conducive to the long-term stability of the battery.18] However, the introduction of the membrane, while increasing its manufacturing cost, further complicates the structural design of the energy storage device.Meanwhile, considering a series of environmental and safety problems that may be caused by strong corrosive solutions such as acid and alkali, the DCC batteries based on mild weakly acidic electrolytes have application prospects.At present, researchers have chosen to design Mn 2þ /MnO 2 batteries in mild electrolytes, and this mildly acidic electrolyte (pH around 4) can achieve reversible deposition/dissolution reactions of MnO 2 , [19][20][21] in which the negligible anodic corrosion reaction further improves the stability of the assembled device, especially in Zn-MnO 2 batteries.This opens up a new horizon for later research on aqueous batteries.
The anodes with different properties have practical effects on the overall performance of Mn 2þ /MnO 2 batteries.Most of their previous studies assembled Mn 2þ /MnO 2 batteries by matching different metal anodes, but this demands the introduction of the corresponding common metals and their salts into the system, which undoubtedly increases the cost of the battery system.The main components of the electrolyte of these batteries are all Mn 2þ salts, and Mn is the third most abundant transition metal reserve on the Earth's crust, making it more suitable for largescale commercial applications.Furthermore, Mn 2þ oxidation to MnO 2 is expected, with the corresponding reduction process of Mn 2þ occurring on the counter electrode via electrodeposition technology.Due to its oxygen sensitivity, metal Mn will form a layer of Mn 3 O 4 with a specific thickness on its surface as the reduction product of Mn 2þ . [22]In theory, both Mn metal and Mn 3 O 4 can be used as anode materials in energy storage devices. [23,24]As a result, designing an all-Mn energy storage device (AMESD) based on the Mn 2þ source is feasible.
The assembly of such devices requires only two pieces of current collectors and electrolytes, which can be free from the traditional complicated process of manufacturing electrode materials and the reliance on various precision manufacturing equipment, further reducing the expected cost for large-scale production of high-performance devices.Although the structure of the device is relatively simple, the current collector as a key component will determine the overall energy storage performance of the device.Carbon fiber clothes (CFCs) are flexible conductive substrates with high electrochemical stability that can be easily chemically processed and modified, making them ubiquitous in the design of DCC batteries. [25]The effect of chemical modification of CFCs on device performance is rarely discussed in previous research on DCC batteries.
In this work, we proceeded from the modified CFCs with high specific surface area (SSA), excellent electrical conductivity, and abundant metal-philic sites by KOH activation strategy at high temperature and hydrothermal nitrogen doping treatment.Moreover, for designing a Mn-based DCC battery, we were inspired by the formulation of electrolytic manganese to introduce (NH 4 ) 2 SO 4 into the MnSO 4 electrolyte and discussed its effect on the uniform deposition of products.The weak acidity of (NH 4 ) 2 SO 4 and MnSO 4 further provides a positive electrolyte environment for the reversible deposition/dissolution of MnO 2 , and the optimal electrolyte formulation was determined.Based on the assembly of nitrogen-doped porous CFC (NPCFC) and the optimal electrolyte, we successfully activated the Mn-based anode and cathode of a DCC battery by a one-step galvanostatic electrodeposition method.Impressively, benefitting from a chemical conversion mechanism, the novel aqueous battery can deliver excellent charge storage performance and superior cycle stability.The CFCs were washed with acetone, ethanol, and deionized water.The hydrophilic CFCs were obtained by immersing them in a mixed solution of concentrated H 2 SO 4 and concentrated HNO 3 with a volume ratio of 3:1 at 60 °C for 12 h and the residual acid of CFCs with deionized water was washed until the washing solution was neutral.The desiccated CFCs samples were marked as HCFCs.Also, KOH was used for the etching medium of CFC.The cut HCFC (4 cmÂ6 cm) was dipped into aqueous KOH solution (30% wt) and kept at 60 °C for 12 h to fully achieve uniform distribution of KOH on the CFC.The water in CFC was removed in the vacuum oven at 80 °C, and the desiccated KOH-attached CFC was annealed at 700 °C for 2 h under the protection of N 2 atmosphere.After returning to room temperature, the annealed CFC was quickly immersed in 0.1 M dilute H 2 SO 4 solution, then washed repeatedly with deionized water until the cleaning solution became neutral, and the desiccant CFC sample was labeled as PCFCs.The N doping of CFC was carried out by a hydrothermal reaction system using NaN 3 as the nitrogen source.The clean PCFC was saturated in 60 mL NaN 3 solution (0.05 M), transferred into a Teflon-lined autoclave, and kept at 180 °C for 12 h.The washed sample was labeled as NPCFCs.

Assembly of AMESD
A predeposition process was achieved by galvanostatic electrodeposition.Specifically, several electrolytes SO 4 , and 3 M MnSO 4 þ 0.7 M (NH 4 ) 2 SO 4 ) were prepared to discuss the key to realizing the design of AMESD.We adopted a three-electrode system to conduct the electrodeposition process, the same CFC was used as the working and counter electrodes, respectively, and a saturated calomel electrode (SCE) was used as the reference electrode.The electroplating process was carried out at a current density of 100 mA cm À2 for 20 min.Long deposition time ensured the adequate loading of products on the surface of porous CFCs.The CFC substrates of the working electrode and counter electrode were washed in deionized water and dried in a vacuum oven at 50 °C for 24 h.The weight of the samples was determined using an electronic balance with a 0.01 mg accuracy (Sartorius BT-25S).The mass loading of CFCs per unit area before and after predeposition in the desiccated state was used to calculate the loading mass of the cathode and anode, which was 5.2 and 4.4 mg cm À2 , respectively.Two desiccated electrodes were assembled directly in a fresh plating solution for electrochemical measurement.

Preparation of Gel-Based Electrolyte
To demonstrate the flexibility of AMESD based on CFC substrates, the corresponding gel-based electrolytes were formulated.Sodium carboxymethylcellulose (CMC-Na) was chosen as the host.Specifically, 3 g CMC-Na was swollen in 100 mL deionized water for 12 h, stirred at 95 °C for 3-4 h until it was completely dissolved, and then 16.9 g of MnSO 4 •H 2 O and 6.6 g of (NH 4 ) 2 SO 4 were dissolved in the CMC-Na aqueous solution.It can be used after cooling to room temperature.

Characterization and Electrochemical Measurements
Details about the material characterization and electrochemical measurements are shown in the Supporting Information.

Characterizations of Current Collectors (HCFC, PCFC, and NPCFC)
Chemical activation of the carbon substrate by KOH at high temperature and facile N doping reaction under hydrothermal conditions can prepare a flexible substrate with a multilevel pore structure, large SSA, and excellent electrical conductivity. [26]s shown by the scanning electron microscope (SEM) images in Figure 1a,b and S1, Supporting Information, the chemical activation roughens the smooth surface of the initial CFC.The elemental mapping of SEM in Figure 1c shows the uniform distribution of N elements on the surface of carbon fibers, which preliminarily verifies the successful introduction of N elements and facilitates the uniform distribution of active sites.The edge of the carbon fiber shows obvious voids and etching in the transmission electron microscope (TEM) profile (Figure 1d).Apparently, there is high porosity in the graphite microcrystallite region under the high-resolution TEM (HRTEM) of Figure 1e, which confirms the obtainment of porous carbon substrate with large SSA.The N 2 isotherm adsorption-desorption isotherms in Figure 1f show that the adsorption capacity of both PCFC and NPCFC increases rapidly at lower relative pressure, which reflects the adsorption characteristics of micropores, and the isotherm hysteresis loops appeared at higher relative pressure prove the existence of the mesoporous structure. [27]Moreover, the Horvath-Kawazoe and Barrett-Joyner-Halenda methods were used to evaluate the distribution of microporous and mesoporous structures, respectively.The pore size distribution results in Figure S2, Supporting Information, also confirm the coexistence of the microporous and mesoporous structures in PCFC and NPCFC.By the Brunauer Emmett Teller multipoint analysis method, the SSAs of the HCFC, PCFC, and NPCFC were calculated to be 7.1, 156.9, and 162.2 m 2 g À1 , respectively.The detailed analysis data is shown in Table S1, Supporting Information.KOH high-temperature etching and hydrothermal nitrogen doping introduced abundant micropores and mesopores into the CFC substrates, which made their SSA increase rapidly.
As illustrated in Figure 1g, the peaks located at %1343 cm À1 in the Raman spectra of HCFC, PCFC, and NPCFC correspond to the D band, which is a disordered vibrational peak of graphene that can characterize structural defects in carbon materials, [28] and the peaks at %1575 cm À1 are attributed to G band caused by the in-plane vibrations of sp 2 -hybridized carbon atoms, which can reflect the degree of graphitization. [29]The intensity ratio of the D band and G band (I D /I G ) can reveal a higher number of defects in carbon materials.It can be seen that the I D /I G of PCFC decreases compared with HCFC, which can be ascribed to the graphitization of CFC at high temperatures.The I D /I G of NPCFC is higher than that of PCFC and HCFC, indicating that the N doping treatment introduces more structural defects into the porous substrate. [30]X-ray photoelectron spectroscopy (XPS) was used to characterize the surface elemental composition of NPCFC.The N 1s spectrum in Figure 1h shows the existence of pyridinic N (398.4eV), pyrrolic N (400.4eV), and graphitic N (402 eV) in NPCFC, [31] which indicates the successful introduction of N element in PCFC, and the fitting peak at 286.4 eV in the C 1s spectrum corresponds to C-N (Figure S3, Supporting Information), [32] which further confirms N doping.The above results fully demonstrate the successful preparation of nitrogen-doped surface-modified CFC substrates with large SSAs and abundant micro-/mesopores.

The Optimization of Electrolytes for AMESD
(NH 4 ) 2 SO 4 is the common additive in the synthesis of industrial electrolytic Mn. [33] It is crucial to discuss the introduction of (NH 4 ) 2 SO 4 in the plating solution for the optimal design of AMESD.The predeposition of the corresponding active substance on the NPCFC can be achieved by galvanostatic electrodeposition in the aqueous solutions with different ammonia concentrations, and the conversion of Mn 2þ to MnO 2 can be achieved based on the electrochemical oxidation mechanism, while conversely, the main cathodic reaction is the reduction process of Mn 2þ .Considering the extreme operating condition of the later assembled energy storage devices, the current density of galvanostatic electrodeposition is controlled at 100 mA cm À2 .Correspondingly, the morphologies of the cathodic products obtained by predeposition in the four electrolytes are shown in Figure 2a-d.Using field-emission scanning electron microscopy (FESEM), it can be observed that all the cathodic products are uniformly attached to the surface of NPCFC, and the morphology of the cathodic products gradually evolves from flakes to the particulate structure by increasing the concentration of NH 4 þ .The formation of nanosheet structures can be attributed to the generation of manganese hydroxides, which precipitate out from the aqueous solution and deposit onto the carbon fiber. [33]hen 0.3 M (NH 4 ) 2 SO 4 was introduced, the size of the nanosheets is significantly reduced (Figure 2b).As the ammonia concentration was further increased, the regular overlapping particle morphology is observed, and this microstructural mutation confirms the initial formation of metallic Mn (Figure 2c,d).The above results show the differences after the introduction of different concentrations of (NH 4 ) 2 SO 4 .In addition to improving the ionic conductivity of the solution, (NH 4 ) 2 SO 4 can form a soluble [Mn(NH 3 ) x=1-6 ] 2þ ion group by complexing the NH 3 with Mn 2þ . [34]Moreover, the buffering effect of (NH 4 ) 2 SO 4 keeps the pH of the electrolyte stable at (%3).All the above factors help to suppress the formation of manganese hydroxide in the electrolyte solution. [22]To further verify the possibility of using a specific mixture of MnSO 4 and (NH 4 ) 2 SO 4 as the electrolyte for the novel aqueous battery, we used the corresponding plating solution directly as the electrolyte for the electrochemical performance measurement of the devices.The resistance behavior of devices composed of the resulting active material after predeposition in electrolytes with different ammonia concentrations can help to further determine the intrinsic properties of the resulting material.The Nyquist plots in Figure 2e clearly show that the charge transfer resistance (R ct ) value of the predeposited devices tends to decrease with increasing (NH 4 ) 2 SO 4 addition, which can be correlated with the production of conductive Mn metal in the cathodic reduction products.The strong bonding between the generated Mn and the conducting substrate ensures fast electron transport.
The electrolyte system with 0.5 M (NH 4 ) 2 SO 4 and 3 M MnSO 4 providing the optimizing capacitive performance can be evaluated from the comparison of cyclic voltammetry (CV) curves at 5 mV s À1 (Figure 2f ).It demonstrates that the energy storage mechanism at a suitable ammonia ion concentration tends to be a battery-type Faraday reaction process, and the capacity difference is closely related to the electrode structure's electron transporting capacity and the energy storage mechanism. [35]Figure 2g depicts the rate performance of Mn-based batteries with varying ammonia concentrations.The system with the addition of 0.5 M (NH 4 ) 2 SO 4 outperformed all other conditions in terms of capacity.The device, in particular, can achieve a maximum areal capacity of 1.17 mAh cm À2 at 1 mA cm À2 while still maintaining 39.6% of the maximum areal capacity at 10 mA cm À2 , demonstrating its outstanding rate capability.Figure S4a-4f, Supporting Information, shows the detailed galvanostatic charge-discharge (GCD) profiles for these systems with various ammonium introductions (þ0, 0.3, 0.5, 0.7, 0.9 M (NH 4 ) 2 SO 4 ).The introduction of (NH 4 ) 2 SO 4 changes the linearity of GCD profiles, implying that the generation of Mn in the anodic active substance affects this system.Furthermore, because the electric double-layer capacitance (EDLC) generated by the high surface area of the NPCFC contributes significantly to the system capacity, the electrochemical performance of NPCFC-based symmetrical devices in 1 M (NH 4 ) 2 SO 4 solution is measured (Figure S5, Supporting Information).Furthermore, the device has an areal capacity of up to 0.48 mAh cm À2 at 1 mA cm À2 , which is significantly lower than the capacity level of the designed optimal electrolyte.Based on the above results, we can conclude that (NH 4 ) 2 SO 4 acts as a buffer to prevent the formation of manganese hydroxide as pH increases during deposition, as well as improving ionic conductivity and promoting the reduction reaction of Mn 2þ .The optimum NH 4 þ addition can be determined as 0.5 M. The composition of the electrolyte formulation was determined to optimize and complete the system of the novel battery, and the preliminary results also showed excellent electrochemical performance, proving its promising application for a new generation of energy storage devices.

The Optimization of Current Collectors for AMESD
NPCFC is chosen as the substrate for the novel battery and further discusses the effect of CFC modification on optimizing the charge storage performance of AMESD.At first, we performed the corresponding structural characterization for the products derived from predeposition on the NPCFC surface.Figure 3 presents the material characterization results of the cathodic predeposition products.The SEM image (Figure 3a) shows irregular particles uniformly adhering to the surface of NPCFC, with the particle size ranging from 600 to 800 nm, which is consistent with the TEM image in Figure 3b.In high-resolution transmission electron microscopy (HRTEM) images, the presence of the graphite microcrystalline structure proves that the product is deposited on the NPCFC.The apparent lattice fringes are analyzed, and the interplanar spacing is 0.210 nm, corresponding to the (330) main crystal plane of α-Mn, [36] and the detailed analytical results are presented in Figure S6, Supporting Information.The X-rays diffractions (XRD) data in Figure 3d also provides accurate evidence for the generation of α-Mn (JCPDS #89-2105) and Mn 3 O 4 (JCPDS #80-0832), with three sharp peaks located at 43.03°, 47.84°, and 78.87°corresponding to (330), (332), and (721) crystalline planes of α-Mn. [37]The autoxidation reaction on the electrolytic Mn surface is responsible for the spinel structure of Mn 3 O 4 .The α-Mn phase appears with increasing deposition time, according to the XRD results of anodes at different deposition times (Figure S7, Supporting Information), which is attributed to the formation of a thin Mn layer on the surface of NPCFC that is easily oxidized to Mn 3 O 4 at a shorter deposition time.The amount of manganese deposited increases as the deposition time increases, forming a Mn 3 O 4 layer with a specific thickness on the surface of the inherent Mn metal to prevent further oxidation, as confirmed by the XPS result based on surface analysis without Ar þ sputtering.Figure 3e shows a pair of spin-orbit peaks at binding energies of 641.2 and 652.8 eV attributed to Mn 2þ , as well as two spin-orbit doublets at 642.6 and 654.2 eV corresponding to Mn 2p 3/2 and Mn 2p 1/2 of trivalent Mn. [38] The ratio of trivalent Mn to divalent Mn is close to 2:1.From the surface analysis of the anode after Ar þ sputtering depth of about 120 nm, it can be concluded that the 0-valent Mn appears, which corresponds to a pair of spin-orbit peaks at binding energies of 639 and 650 eV in Figure 3f. [22]The energy-dispersive spectrometer (EDS) mapping of the Mn anode on the NPCFC substrate shows the distribution of elements dominated by Mn, C, and N (Figure 3g), which is consistent with the XPS survey spectra in Figure S8, Supporting Information.Taken together, this is sufficient evidence that the cathodic product obtained by predeposition is Mn@Mn 3 O 4 .
In addition, the characterization results of the anodic oxidation products on NPCFC are presented in Figure 4.The corresponding electron microscopy profiles demonstrate that MnO 2 particles of 10-20 nm in size are closely packed on the surface of NPCFC (Figure 4a-c).The abundant hierarchical pore structure in NPCFC enhances the adsorption capacity for Mn 2þ , thereby contributing to the final capacity output.The HRTEM image in Figure 4d shows the selected-area electron diffraction (SAED) pattern of diffuse spots representing the amorphous phase.Small-sized nanoparticles tend to form low crystalline products due to the limitation of nucleation space. [39]Similar graphitic microcrystalline regions are also found in the HRTEM image, which is ascribed to the strong binding effect between the stripped carbon fiber and the deposited product.The SEM mapping images in Figure 4e-f show regional distributions of Mn, O, C, and N, proving the deposition of anodic products on NPCFC.
The predominantly diffuse peaks of the XRD pattern in Figure 4g further demonstrate the low crystallinity of the resulting product and the phase can be determined with diffraction peaks located at 37.12°, 42.4°, 56.03°, and 66.76°, which correspond to the (100), ( 101), (102), and (110) crystal planes respectively of ε-MnO 2 . [40]The XPS survey spectrum in Figure S9, Supporting Information, also proves the existence of Mn, O, C, and N elements.The high-resolution spectrum of Mn 2p in Figure 4h contains two spin-orbit doublets at 642.5 and 654.2 eV, assigned to Mn 2p 3/2 and Mn 2p 1/2 of tetravalent Mn, [41] and Figure 4i depicts two Gaussian peaks of O 1s located at 529.7 and 531.4 eV, corresponding to the Mn-O band and oxygen vacancies. [42]In summary, the predeposited anodic product is amorphous ε-MnO 2 and the simultaneous deposition of Mn@Mn 3 O 4 and MnO 2 on the NPCFC in a 3 M MnSO 4 þ 0.5 M (NH 4 ) 2 SO 4 solution can be achieved and confirmed using only one electrodeposition process.
To visually compare the advantage of the NPCFC substrate in enhancing the performance of the novel batteries, we predeposited MnO 2 cathodes and Mn@Mn 3 O 4 anodes on three substrates (HCFC, PCFC, and NPCFC) and tested their charge storage performance of the corresponding devices.The differences in the type of CV curves can reflect the specific electrochemical reaction type of the electrode material.Figure 5a shows the CV curves of the batteries based on three substrates at 5 mV s À1 .In the potential interval of 0-1.8 V, obvious differences in the integrated area of the curves are presented, and the curve of NPCFC exhibits superior capacity performance.[45] Figure 5b highlights the variation of CV curves for the NPCFC substrates at different scan rates.The CV curves at high scan rates exhibit characteristic curves similar to supercapacitors, presumably with excellent power output characteristics for the AMESD, and additional CV data for the three substrates are presented in Figure S11, Supporting Information, for comparative illustration.Furthermore, the GCD profiles at different current densities are provided to illustrate the concrete capacity level of the battery (Figure 5c).The AMESD can deliver a maximum capacity of 1.46 mAh cm À2 at 0.5 mA cm À2 and a reversible capacity of 0.25 mAh cm À2 that is still available at 20 mA cm À2 .Figure S12, Supporting Information, shows the GCD data of the AMESD based on PCFC and HCFC substrates for comparison.
Figure 5d shows the rate capabilities of full batteries based on HCFC, PCFC, and NPCFC substrates.None of these batteries show a sharp capacity decay with continuous current changes.Moreover, it can be seen that the battery based on NPCFC exhibits capacity advantages at all current densities, and it can still maintain the capacity of 1.15 mAh cm À2 when reverting to 1 mA cm À2 from 20 mA cm À2 , which is indistinguishable from the capacity (1.17 mAh cm À2 ) determined at the initial current density of 1 mAcm À2 .The Nyquist plots in Figure 5e demonstrate the improved conductivity and hydrophilicity of the NPCFC.The series resistance (R s ) of NPCFC is significantly smaller than that of PCFC and HCFC, which is attributed to the good wettability of the NPCFC substrate in the electrolyte system.Meanwhile, the smaller charge-transfer resistance (R ct ) also reflects the superior conductivity of NPCFC compared to that of PCFC.[48][49][50][51][52][53][54][55][56][57][58][59] Also at the capacity level of 1 mA cm À2 , this battery still maintained 86% of the initial capacity after 100 charge-discharge cycles (Figure 5g) and the long-cycle test results at 15 mA cm À2 are shown in Figure 5h; the charge-discharge cycle test at the capacity level of 0.33 mAh cm À2 was conducted 40 000 times and finally a capacity retention rate of 93.5% was achieved, which is far beyond the stability of most aqueous batteries reported so far (Table S2, Supporting Information).The above results fully demonstrate the excellent charge storage performance of AMESD and confirm that the NPCFC is the eligible collector in the energy storage system.The large surface area of the CFC substrate can reduce the local current density of the electrode and provide a volume space for the microregion deposition/dissolution of active materials, which is beneficial in suppressing the generation of corresponding byproducts during charging and discharging and improving the cycling stability of the battery. [60,61][64] The presence of five available valence electrons in nitrogen enables nitrogen-doped carbon to form strong covalent bonds, resulting in a stable conductive framework. [65]o further explore the reasons for the superior stability and the charge storage mechanism of the battery, we characterized the structure of the cathode and anode materials after 40 000 charge-discharge cycles.Figure 6a-c shows the electron microscope profiles of the tested MnO 2 cathode.It can be seen that the morphology and microstructures have not changed significantly after the cycle test, and the nanoparticles are firmly wrapped on the surface of NPCFC.The XRD analysis in Figure 6d shows that the phase composition of the cathode material has not changed, and it is still MnO 2 with low crystallinity.The XPS resolution spectrum of Mn 2p in Figure 6e shows that the valence state of Mn is tetravalent after the test, which is consistent with the results before the test (Figure 4h). [41]The appearance of C-O peaks in the O1s spectrum in Figure S13, Supporting Information, can be attributed to the thickness variation of the deposits on the NFCFC after long-cycle testing.The material structure remains unchanged after the ultralong cycle, attributed to the continuous progress of the reversible chemical conversion reaction of the MnO 2 cathode in the GCD test.To further clarify the mechanism, ex situ XRD measurement, and ex situ SEM analysis were performed on the blank NPCFC.As shown in Figure 6f, when charging to 1.2 V, a diffused broad peak appears at 37.1°on the pattern, corresponding to the main peak of ε-MnO 2 on the (100) plane. [40]As the potential increased to 1.5 and 1.8 V, the intensity of the peak increased.During discharge, with the decrease of the potential, the broad peak intensity of the (100) crystal plane gradually decreases, and when the discharge is to 0 V, there is no obvious characteristic peak of ε-MnO 2 .Moreover, ex situ SEM images at different chargedischarge potentials are provided in Figure S14, Supporting Information, to further confirm the deposition/dissolution process of MnO 2 .During the initial charging process (0-1.2V), the surface of NPCFC first formed the microstructure dominated by a sheet-like structure (Figure S14a-c, Supporting Information), which can be attributed to the formation of MnOOH from the hydrolysis of Mn 3þ obtained by the electrochemical oxidation of Mn 2þ . [66]When the potential reached 1.5 V, it could be observed that MnO 2 nanoparticles began to be generated, which indicated that the generated flakes of MnOOH converted to MnO 2 by electrochemical oxidation at high potentials.When the potential increased to 1.8 V, the nanoparticles continue to be formed.The MnO 2 coating layer gradually roughened from relatively flat during the discharge, and disordered flake morphology begins to appear on the surface of NPCFC at specific potentials (Figure S14g, Supporting Information), which is related to the reduction process of tetravalent manganese to trivalent manganese.When the discharge potential reached 0 V, there were no visible particles attached to the carbon fiber, and there was no significant difference in weighing mass before and after the test.The results show that the ultimate transformation state of trivalent manganese exists during MnO 2 deposition and dissolution, which is consistent with previous related literature. [67,68]e MnO 2 cathode was performed the CV test in a threeelectrode system and the results are shown in Figure S15, Supporting Information.At low sweep speeds, redox peaks are present in the high potential interval, corresponding to the deposition/dissolution of the Mn 2þ /MnO 2 , and the absence of the peak at higher scan rates is attributed to the rapid Faraday reaction process on the surface.The cathode material-free device was also assembled and tested (Figure S16, Supporting Information), and it exhibited EDLC characteristics similar to those in (NH 4 ) 2 SO 4 (Figure S5, Supporting Information), which was attributed to the hybrid ion supercapacitor's capacitive mechanism without Faraday processes occurring at the cathode. [69]The above results confirm that the cathode's high capacity is primarily derived from NPCFC EDLC and the subsequent redox reaction of Mn 2þ /Mn(III)/MnO 2 .Furthermore, previous research concluded that it is accompanied by the insertion of cations such as H þ in the weakly acidic electrolyte system. [70]imilarly, the mechanism of the charge-discharge process of the anode is explored.Figure 6g-i shows the change in the morphology of the anode on the NPCFC after 40 000 chargedischarge cycles.The SEM and TEM images at different magnifications show that the particle sizes are reduced to 200-400 nm.The HRTEM profile in Figure 6j shows two interplanar spacings of 0.21 and 0.248 nm corresponding to the (330) plane of Mn metal and the (211) plane of Mn 3 O 4 , respectively. [37,66]The detailed analysis results are presented in Figure S17, Supporting Information.The XRD pattern of the Mn anode after testing in Figure 6k also confirms the existence of Mn 3 O 4 and MnOOH phases in addition to the α-Mn phase. [68,71]From the Mn 2p spectrum in Figure 6l, a pair of spin-orbit peaks at binding energies of 641.2 and 652.8 eV are attributed to Mn 2þ , and a satellite peak located at 644.2 eV also accompanies divalent Mn.Similarly, two spin-orbit doublets located at 642.6 and 654.2 eV correspond to Mn 2p 3/2 and Mn 2p 1/2 of trivalent Mn. [38] Furthermore, based on the three-electrode electrochemical test of the Mn@Mn 3 O 4 anode, we found that the anode exhibits pseudocapacitance characteristics under a wide negative potential window and has a significant increase in energy storage capacity when compared to the CV curve of the NPCFC (S18).And, at a low scan rate, Mn@Mn 3 O 4 appears to have a milder polarization tendency than the pure NPCFC substrate, indicating that the surface inorganic Mn 3 O 4 layer provides capacity while also passivating the hydrogen evolution activity. [72]Combining the above findings, the anode's energy storage mechanism is primarily dominated by the electrochemical conversion of solid-phase divalent and trivalent manganese in Mn 3 O 4 , which is consistent with previous related research findings. [45,73]The schematic diagram in Figure 7a demonstrates the energy storage mechanism of the AMESD as confirmed by the above results.Besides, to demonstrate the practical application of AMESD based on flexible NPCFC, we used sodium carboxymethylcellulose (CMC-Na) as the host material to prepare the CMC-Na/ MnSO 4 gel electrolyte, which is the electrolyte and separator for the flexible device (Figure 7b).The device assembled after effective packaging is directly used to power the electronic watch with the rated voltage of 1.5 V.As shown in Figure 7c, this single device successfully started the electronic watch and still displayed time after 15 min, which reflects its practicality as a flexible power supply equipment.

Conclusion
In summary, we realized the design of an AMESD assembled by NPCFC and aqueous electrolyte based on the redox chemistry of Mn 2þ and confirmed that (NH 4 ) 2 SO 4 in the manganese plating solution can improve the ionic conductivity to promote the reduction reaction of Mn 2þ , which acts as a buffer to prevent the formation of manganese hydroxide with the increase of pH during the deposition process.The device based on 3 M MnSO 4 þ 0.5 M (NH 4 ) 2 SO 4 electrolyte demonstrated attractive charge storage performance, proving the advantages of AMESD in terms of capacity performance and cycle stability.Furthermore, porous structure and nitrogen doping modification in the carbon substrates further optimize the overall performance of AMESD.The assembled device achieves the highest areal capacity of 1.46 mAh cm À2 (%152.1 mAh g À1 ), and the rate performance at the current level of 20 mA cm À2 , which enables it to output a power density of 9.66 mW cm À2 .Also, the outstanding capacity retention rate (93.5%) after 40 000 charge-discharge cycle tests fully demonstrated the superiority of the AMESD based on the DCC mechanism.This work proposed a new insight into the design of low-cost, high-energy, and highstability aqueous batteries.The research on Mn metal as battery anodes is still in the preliminary exploration stage, and there are still some specific problems to be solved, such as the side reaction of hydrogen evolution.Future work can further focus on specific optimization strategies to achieve highly reversible plating/stripping of Mn metal anodes.To perform the experiments on human subjects, rules or permissions from the relevant national or local authorities are not in place in the country where the experiments were performed.Informed written consent was acquired from the human participants for the said experiments.

Figure 3 .
Figure 3. a) FESEM images of Mn anode, b,c) TEM image of Mn anode d) XRD pattern of Mn anode; e) XPS spectra of Mn 2p (no Ar þ sputtering); f ) XPS spectra of Mn 2p (Ar þ sputtering for 600 s); and g) EDS elemental mappings of Mn anode.

Figure 4 .
Figure 4. a, b) FESEM images of MnO 2 cathode; c,d) TEM image of MnO 2 cathode; e,f ) EDS elemental mappings of MnO 2 cathode; g) XRD pattern of MnO 2 cathode; h) XPS spectra of Mn 2p; and i) XPS spectra of O 1s.

Figure 5 .
Figure 5. Electrochemical performance comparison of AMESD based HCFC, PCFC, and NPCFC: a) CV curves at 5 mV s À1 ; b) CV curves at the different scan rates; c) GCD curves at various current densities; d) rate capability; e) Nyquist plots; f ) the Ragone plot; g) the cycle performance at 1 mA cm À2 ; and h) the cycle performance at 15 mA cm À2 .

Figure 6 .
Figure 6.Characterization results of MnO 2 cathode and Mn anode after 40 000 cycle test: a,b) FESEM images of MnO 2 cathode; c) TEM image of MnO 2 cathode; d) XRD pattern of MnO 2 cathode; e) XPS spectra of Mn 2p for MnO 2 cathode; f ) ex situ XRD patterns of MnO 2 cathode; g,h) FESEM images of Mn anode; i,j) TEM image of Mn anode; k) XRD pattern of Mn anode; and l) XPS spectra of Mn 2p for Mn anode.

Figure 7 .
Figure 7. a) Schematic diagram of energy storage mechanism of AMESD.b) Schematic diagram of the assembly of the flexible AMESD.c) Optical photo of flexible AMESD as an electronic watchband.