O, N‐Codoped, Self‐Activated, Holey Carbon Sheets for Low‐Cost And High‐Loading Zinc‐Ion Supercapacitors

Low‐cost and high‐loading cathodes are crucial for practical application of zinc‐ion supercapacitors (ZICs) but achieving optimal performance in high‐loading electrodes faces challenges due to sluggish ion transport, increased resistance, and unstable structure. Guided by theoretical calculations, high‐loading carbon cathodes based on holey activated carbon sheets (HACS) are fabricated from a carefully chosen molecule. A simple pyrolysis‐leaching treatment transformed the molecule into HACS with large surface area, hierarchical porous structure, and electroactive oxygen/nitrogen dopants. When combined with an aqueous binder, the optimized HACS‐based high‐loading electrode (16.1 mg cm−2) exhibits high‐capacitance (454 F g−1 ) and fast‐rate (1 A g−1) characteristics under lean electrolyte (6.2 µL mg−1). More impressively, HACS is dry‐pressed into free‐standing thick electrodes up to 35.4 mg cm−2 and corresponding practical ZIC under limited Zn and low N/P ratio demonstrates ultrahigh areal capacitance (9 F cm−2) and energy density (3.47 mWh cm−2). The outstanding performance can be attributed to fast ion transport enabled by through‐plane pores of HACS, as well as abundant double‐layer and redox‐active surfaces from favorable heteroatom‐doped porous nanosheets. With its cost‐effectiveness, elemental abundance, and structural tunability, this molecular carbon strategy offers a platform for making self‐activated carbon electrodes at the molecular level towards practical supercapacitors.


Introduction
3] These include its high theoretical capacity, earth abundance, low cost, superior safety, and compatibility with water and air.[6][7][8] However, these cathodes typically suffer from poor kinetics and short lifespans due to the high diffusion barriers and active materials dissolution. For example, a large-surface-area carbon with hierarchical pores was shown to have a high capacitance, [16] while a nitrogen and oxygen-enriched carbon (N, 6.52 at%, O, 3.76 at%) with a low surface area was also capable of delivering high capacitance in corresponding ZICs. [17]espite numerous reports, a significant disparity persists between research advancements and their practical implementation, particularly concerning material and processing costs, as well as practical testing conditions.On one hand, delicate engineering pore, dopant, and micro-/nanostructure in carbon cathodes typically demand intricate material design and preparation, rendering it financially burdensome for large-scale applications.To address this, reducing cell costs can be accomplished through the selection of cost-effective raw materials for carbon fabrication, streamlining the preparation process for carbon cathodes, maximizing the utilization of Zn anodes, and opting for lowcost electrolytes and separators during device assembly. However, achieving high performance at high-loading electrodes remains a challenge due to sluggish ion/electron transport and unstable mechanical structure, leading to poor rate and short life. [19]Moreover, most of the reported carbon electrodes are dominated by macropores, resulting in low volumetric energy density.Developing cheap, porous yet dense electrodes is crucial for practical application. [24][27] Nonetheless, the high price, low surface area, and insufficient capacitive performance of holey graphene prevent it from being widely applied.
Inspired by holey graphene and guided by density functional theory (DFT) calculations, we here develop a high-loading carbon electrode based on holey activated carbon sheets (HACS) derived from a cheap molecular salt.This molecular salt was selected based on rational criteria and serves as an excellent precursor for making heteroatom-enriched HACS due to its abundance, low cost, and atom economy that includes key components such as potassium for activation, oxygen/nitrogen for doping, and benzene rings for carbon sheet formation.Through carbonization, self-activation, and subsequent acidic washing, the molecular salt can be easily converted into activated carbons with large surface area (1004 m 2 g −1 ), hierarchically porous structure (micropores and mesopores), enriched O/N dopants (O, 10.2 at%; N, 6.64 at%), and holey sheet morphology.The resulting HACS, combined with an aqueous binder, can be easily made into high-loading electrodes (16.1 mg cm −2 ) while demonstrating a high-capacity (208 mAh g −1 , capacitacance: 454 F g −1 ) and fast-rate performance (1 A g −1 ) for ZICs.Moreover, practical ZICs based on thick HACS electrodes (up to 35.4 mg cm −2 ), low N/P ratios (1-4), and lean electrolyte (2.23-6.09μL mg −1 ) was fabricated, delivering highest areal capacitance of 9.0 F cm −2 , maximal energy density of 3.47 mWh cm −2 and low cost of $327 kWh −1 .The mechanism was revealed to follow the Zn 2+ /proton co-storage mechanism involving reversible physical/chemical sorption and precipitation/dissolution in the HACS electrode during charge/discharge.

DFT Calculation and Guided Design of HACSs
To investigate the zinc-ion storage capacity of different oxygen and nitrogen dopants, DFT calculations were employed to determine binding energies between a Zn atom and an O-/N-doped graphene modeled with all possible doping sites (Figure 1a-d; Figures S1 and S2, Supporting Information), including ketone oxygen (Ok), hydroxyl oxygen (Oh), ether1 oxygen (Oe 1 ), ether2 oxygen (Oe 2 ), and carboxylic oxygen (Oc) as well as graphitic nitrogen in the bulk (Ngb), graphitic nitrogen on the edge (Nge), pyridinic nitrogen (Np), pyrrolic nitrogen (Npy), and oxidized nitrogen (No). [28]After analyzing results, it was discovered that the highest binding capability of Zn occurred on O e1 , O c , and N py sites with strong adsorption energies of −3.42, −0.20, and −0.19 eV, respectively, which were greater than other combinations (Figure 1e).However, as the Sabatier principle for catalyst design states that the optimal catalyst is achieved when the catalyst-substrate interaction is "just right," neither too weak nor too strong, [29] it is reasoned that the strongest adsorption energy in O e1 may lead to irreversible ion storage.As a result, our attention is turned to O c and N py due to their moderate energies.
Based on the above findings, we selected a molecular salt, potassium phthalimide (KP), which is composed of an aromatic benzene ring, a cyclic imide group, and an ionic potassium salt, for making carbon electrode (Figure 1f). To make activated carbon from KP, a simple pyrolysis-leaching treatment was employed.KP powder was thermally treated (carbonized and selfactivated) under a flowing nitrogen atmosphere at a high temperature (700, 800, or 900 °C).After acid washing, the resultant activated carbon denoted as KP-700, KP-800, and KP-900, respectively, was collected for fabricating the carbon electrode.To study the influence of molecular components on carbon fabrication, two other molecules were also selected, that is, phthalimide (without potassium) and phthalic anhydride (without nitrogen and potassium).However, the high-temperature treatment of these two molecules produced little carbon products due to

Formation, Mechanism, and Characterization of HACSs
To gain deeper insights into the conversion process from molecules to carbon, several experiments were conducted, including thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and electron microscopy.The TGA results revealed that the KP molecule displayed good thermal stability and high carbon yield, with a weight retention of 45% at 700 °C (Figure S5, Supporting Information).The XRD patterns indicated that the high-crystalline KP molecule transformed into a mixture of carbon and salts, specifically potassium cyanide (KCN) and potassium carbonate (K 2 CO 3 ), which acted as self-activation agents for high-temperature HACS formation.After acid washing, only two broad peaks at 24°and 42°were observed, corresponding to typical activated carbons (Figure S6b, Supporting Information). [32]Infrared spectroscopy (Figure S7, Supporting Information), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) mapping images confirmed the change from the molecule to the carbon-salt mixture and to carbon.The pyrolysis of micro-rod-shaped molecular aggregates (Figure S8, Supporting Information) resulted in a mixture of nonconductive salt particles and conductive carbon framework (Figure S9, Supporting Information), with the salt particles identified as potassium salts, and elemental potassium coating layers found on the surface of carbon (Figure S10, Supporting Information).The thermal treatment of the KP molecule generated both homogeneously and heterogeneously nucleated potassium salt particles and layers on carbon, both of which could activate carbon at high temperatures. [33]After acid leaching, only carbon remained, with KP-700 displaying a 3D bulky structure (Figure S11, Supporting Information), while KP-800 and KP-900 exhibited a 2D nanosheet morphology (Figure 2a; Figures S12 and S13, Supporting Information).These carbon sheets had a flat and smooth surface, thickness in the range of 15-106 nm (Figure S14-S15, Supporting Information), large lateral sizes up to hundreds of micrometers, and high aspect ratios up to 1000.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping images demonstrated the uniform distribution of C, N, and O elements in carbon sheets (Figure 2b), while transmission electron microscope (TEM) images showed numerous pores on the surface of carbon nanosheets (Figure 2c and Figure S15, Supporting Information), indicating the formation of O/N-doped HACS.
Based on above results, the formation mechanism for O/Ndoped HACS from the KP molecule was proposed.At temperatures above 450 °C, the KP molecule decomposes into O/N-doped carbon, KCN, and K 2 CO 3 (Figure S16, Supporting Information).At higher temperatures, KCN and K 2 CO 3 further decompose into KOH, K 2 O, and CO 2 , which activate carbon through chemical reactions, leaving behind holey carbon with numerous pores.At relatively low temperatures (e.g., 700 °C), benzene rings fuse with each other, forming carbon sheets that stack to form bulky structures.35] To investigate the porous structure, chemical composition, and heteroatom dopants in KP-700, KP-800, and KP-900, gas sorption isotherms, Raman spectra, and X-ray photoelectron spectra (XPS) were collected.N 2 adsorption and desorption measurements showed that all three activated carbons exhibited a hysteresis loop (Figure 2d), indicating the formation of micropores and mesopores.Micropores and mesopores were found to be mainly at 1.6-1.7 and 2.7-3.6 nm, respectively, with micropores dominating (Figure 2e).Micropores offer a substantial surface area for ion storage, while mesopores facilitate rapid ion transport within a carbon framework.The Brunauer-Emmett-Teller (BET) surface areas of KP-700, KP-800, and KP-900 were calculated to be 788, 1004, and 1150 m 2 g −1 , respectively, which are higher than that of holey graphene. [26]The Raman spectra (Figure 2f) reveal that all three activated carbons -KP-700, KP-800, and KP-900 exhibit two sharp peaks at 1350 and 1590 cm −1 and one broad peaks ranging from 2400-3400 cm −1 which correspond to the typical D, G, and 2D bands, respectively, of carbon materials. [36]The G band is associated with graphitic structure, while the D band relates to disorder-induced phonon. [37]The peak intensity ratio of G to D (I G /I D ) is calculated as 1.25, 1.14, and 1.12 for KP-700, KP-800, and KP-900, respectively, indicating that higher pyrolysis temperatures result in more defected carbon structures.These defected carbon structures could facilitate the exposure of surfaces and the transport of ions for ZICs.

Optimizing HACS Electrode for Zinc-Ion Capacitors
The electrochemical performance of KP-700, KP-800, and KP-900 was investigated systematically via the galvanostatic chargedischarge (GCD) technique.Figure 3a,b presents the capacity values of three samples at different current densities from 0.05 to 1.5 A g −1 .Obviously, KP-800 shows the highest capacities at all current rates among these samples, indicating the highest performance can be obtained in KP-800.As shown in the capacityvoltage curves (Figure 3c), KP-800 displays a more apparent pseudocapacitive behavior at a low rate of 0.05 A g −1 , resulting in the highest capacity of 166.6 mAh g −1 , which is larger than KP-700 and KP-900 with values of 135.7 and 159.8 mAh g −1 .At a high rate of 1 A g −1 , KP-800 experiences less internal resistance, giving rise to a capacity of 79.7 mAh g −1 , higher than that of KP-700 (43.3 mAh g −1 ) and KP-900 (67.0 mAh g −1 ). Figure 3d displays the cycling performance of KP-800.After 27 000 cycles at 2 A g −1 , KP-800 still maintains a high capacity retention of 68% and ≈100% Coulombic efficiency (CE) values, suggesting its good cycling stability.The large capacity, high rates, and long cycles achieved in KP-800 are comparable and even superior to previously reported carbon electrodes (Table S1, Supporting Information).
To comprehend the best performance of KP-800, the kinetics of different samples were analyzed via the cyclic voltammetry (CV) technique.As shown in Figure 3e and Figure S19 (Supporting Information), all samples present two couples of redox peaks, denoted as O1/R1 and O2/R2, respectively, indicating the presence of Faradic surface reactions, which should be ascribed to the respective oxygen and nitrogen heteroatom dopants.The i = av b equation was used to calculate the logarithmic relationship between peak currents and scan rates, [40] yielding the b value.A b value of 0.5 indicates a diffusion-controlled redox reaction, while a b value of 1.0 is associated with capacitive reactions.As shown in Figure 3f, the b values for the O1/R1 and O2/R2 peaks of KP-700, KP-800, and KP-900 were calculated to be 0.71/0.86,0.69/0.86;0.77/0.89,0.74/0.86;and 0.71/0.90,0.67/0.87,respectively.These values suggest that pseudocapacitive reactions dominate the reduction reaction, while diffusion controls the oxidation reaction.Using the Dunn method (i = k 1 v + k 2 v 0.5 ), [40,41] the quantitative distribution of capacitive and diffusion-controlled processes in the three electrodes were further calculated.For example, in KP-800, as the scan rate increased from 0.2 to 4.0 mV s −1 , the capacitive contribution increased from 54.1% to 62.4%, 67.1%, 70.2%, 78.8%, 82%, and 84.0%(Figure 3f).Similar results were also found in KP-700 and KP-900 (Figures S20-S23, Supporting Information), suggestive of the fast kinetics mainly from rapid surface Faradic reactions in three carbon electrodes.
The superior performance of KP-800 among the three samples can be attributed to several factors.First, it has significantly higher contents of carboxylic O and pyrrolic N, which are almost twice as much as those found in KP-900.Additionally, KP-800 has a large surface area with hierarchical pores, as well as a holey sheet morphology that differs from the bulky structure in KP-700.These features combine to create a surface area that is most conducive to ion storage and provides low-tortuosity pathways for fast mass and charge transport.It is also believed that KP-800 electrodes with enriched nitrogen/oxygen dopants and holey lamellar structures can be easily made into highloading electrodes without compromising the electrochemical performance.

Mass Loading Influence and High-Loading Performance
Mass loadings effect on ZIC performance was investigated using three levels: low (2.4 mg cm −2 ), median (5.52 mg cm −2 ), and high (10.8,16.1 mg cm −2 ).The electrolyte volume was fixed at 100 μL, resulting in a decreasing electrolyte-to-carbon (E/C) ratio as the loading increases.Despite this, specific capacities of HACS electrodes remained relatively constant across mass loading levels (2.40 to 16.1 mg cm −2 ) at current densities from 0.1 to 1.0 A g −1 (Figure 4a).The HACS electrode with a high loading of 16.1 mg cm −2 still retained a high capacity of 94.0 mAh g −1 at 0.5 A g −1 , a mere 11.5% decrease from the 2.40 mg cm −2 electrode, meaning that the high-loading HACS electrode can be fastcharged in 11.5 min while maintaining a high capacity.After replenishing the Zn anode (due to Zn dendrites and corrosion), the 16.1 mg cm −2 electrode demonstrated high cycling stability with 99% capacity retention and ≈100% Coulombic efficiency after 100 cycles at 0.5 A g −1 (Figure 4b; Figure S24, Supporting Information).Charge-discharge profiles (Figure 4c) of this 16.1 mg cm −2 electrode shows pseudocapacitive behavior at low rates (0.05 and 0.1 A g −1 ) with certain voltage plateaus.With the increase of charging rates, apparent voltage drops can be detected due to increased internal resistance from impeded mass/charge transport at high-loadings electrodes.The 16.1 mg cm −2 electrode manifests stable cycles at the high rate of 0.5 A g −1 , suggestive of the high rate and reversibility of the HACS electrode at high loadings.Areal capacity serves as an essential metric for evaluating energy storage systems at the cell level.To assess the relationship between areal capacity and mass loading at various rates, graphs were created.Figure 4d illustrates a linear increase in areal capacity with mass loading up to 16.1 mg cm −2 at rates ranging from 0.05 to 1 A g −1 .This suggests that no limitation has been reached in the mass loading at rates less than 1 A g −1 (equivalent to a current density of 16.1 mA cm −2 for the 16.1 mg cm −2 electrode).Figure 4e depicts the impact of mass loading and current rate on the areal capacity of the HACS electrode.In general, as areal current density increases, areal capacity demonstrates the same declining trend with comparable curve slopes, implying fast-rate capability at high loadings.The 16.1 mg cm −2 electrode can achieve the highest areal capacity of 3.35 mAh cm −2 (capacitance of 7.31 F cm −2 ) at 0.80 mA cm −2 .At areal current densities of 8.04, 16.1, and 17.7 mA cm −2 , the same electrode can attain high areal capacities of 1.51, 1.06, and 0.98 mAh cm −2 (areal capacitances of 3.29, 2.31, and 2.13 F cm −2 ), respectively.Areal capacities for other loading electrodes can be found in Figures S25  and S26, Supporting Information.
Energy density versus power density curves, viz., Ragone plots, were also generated, and a comparable decreasing trend was observed in the area-based Ragone plots (Figure 4f), while the curves in the mass-based Ragone plots almost overlapped for different mass loadings of HACS electrodes (Figures S27-S28, Supporting Information), further proving the excellent performance of high-loading electrodes.As shown in Figure 4f, the 16.1 mg cm −2 electrode offers the highest areal energy density of 2.40 mWh cm −2 and maximal power density of 10.7 mW cm −2 .Overall, the high-loading HACS electrodes' large areal capacity (energy density) at high rates (power density) are among the best ZICs (Table S2, Supporting Information).The highest specific energy density is 149.6 Wh kg −1 at 38.8 W kg −1 based on only the cathode but the cell-level energy/power densities are low due to the excessive use of Zn and heavy packing materials.The above results demonstrate the high-loading HACS electrodes' rapid kinetics and good stability, which are promising for developing high-energy and high-power ZICs.

Free-Standing Thick Electrodes, Energy and Cost Analysis
Supercapacitors should meet practical testing conditions for applications.Accordingly, we further assembled ZICs composed of limited Zn anode (9.9 or 19.8 mg cm −2 ), thick HACS cathode (10.2-35.4mg cm −2 ), low N/P ratio (1.5-4) and lean electrolyte (2.23-6.09μL mg −1 ).By simply pressing the mixture of KP-800 (80%) and CNT (20%) into free-standing disks, thick HACS electrodes could be obtained without the need of solvent, binder, and current collector (Figure 5a; Figure S30, Supporting information).With the increase of mass loading, HACS-based cells deliver a linear increase in areal capacitance from 2.50 F cm −2 in 9.1 mg cm −2 to 9.00 F cm −2 in 35.4 mg cm −2 , indicating no limitation has been reached in the mass loading at the rate of 0.05 Ag −1 (Figure 5b,c; Figure S31, Supporting information).The 9.1 mg cm −2 HACS-based cell delivers high areal capacitance of 2.50, 2.00, 1.55, 1.34, 1.16, and 1.06 F cm −2 at the rate of 0.45, 0.91, 1.82, 2.72, 3.63, and 4.54 mA cm −2 , separately (Figure 5d).The cell is also stable for more than 350 cycles with near-unity Coulombic efficiency (Figure 5e; Figure S32, Supporting information).Of note, it is still challenging to achieve long cycling performance in ultrahigh-loading HACS-based ZICs under limited Zn, low N/P ratio, and lean electrolyte due to the low reversibility of zinc metal.
Ragone plots were evaluated based on carbon electrode only, both electrodes, and electrodes+electrolyte.The corresponding highest energy densities are 94.7, 55.5, and 19.3 Wh kg −1 , respectively (Figure 5f).The energy density based on zinc and carbon electrodes were compared with previous reports in Figure 5g.[44][45][46][47][48] Although the cell-level performance remains low due to heavy coin cell cases (Figure S33, Supporting Information), it could be potentially enhanced by proper cell design.The overall cost of the above ZICs was also analyzed.Due to our rational selection of low-cost KP and its derived HACS with high carbon yield (28%, KP-800) as well as the use of cheap electrolyte based on ZnSO 4 and zinc metal anode, low material costs ranging from $327 kWh −1 to $630 k Wh −1 could be obtained in the free-standing HACS-based cells (Figure 5h).These values are much lower than commercial supercapacitors (900-6000 kWh −1 ) and comparable to lithium-ion batteries (150-500 kWh −1 ).The lowest cost of $327 kWh −1 could be obtained in the cell with the highest HACS loading (35.4 mg cm −2 ), in which zinc, HACS, CNT, separator, and electrolyte account for 6.54%, 40.6%, 31.2%,17.4%, and 4.2%, respectively (Figure S34, Supporting Information).Above results confirm that higher mass loadings of HACS result in higher cell-level energy density and lower overall costs.

Storage Mechanism Exploration
The storage mechanism of HACS electrodes was explored by ex-situ XRD, SEM, and EDX analysis of electrodes under different discharge/charge states (Figure 6a).The XRD patterns of discharged HACS electrodes in Figure 6b  (JCPDS card #39-0689) and Zn 4 SO 4 (OH) 6 •4H 2 O (JCPDS card #44-0673).The peak intensity of these peaks increased as the discharge potential decreased, indicating the gradual formation of zinc sulfate hydroxides during the discharge process. After charging the HACS electrode back to 1.8 V, peaks of zinc sulfate hydroxides disappeared, indicating the reversible storage of protons during the electrochemical processes.SEM images further show the gradual appearance and disappearance of zinc sulfate hydroxide nanoflakes on carbon sheets during the discharge and charge process, verifying the reversible proton sorption process.The EDX mappings in Figure 6c confirmed that nanoflakes formed during the discharged state were composed of Zn, S, and C elements, matching well with XRD patterns.The results suggest that protons play a crucial role in the energy storage during ZIC charging and discharging.
EDX quantitative analysis disclosed changes in the atomic ratio of Zn to C (Zn/C) and Zn to S (Zn/S) during discharge and charge (Figure 6d).The results indicate that the Zn/C ratio increased from 2.2 at.% to 6 at.% during discharge and then decreased back to 2.0 at.% during charging, suggesting the reversible storage of Zn atoms.This is partially from the reversible precipitation/dissolution of zinc sulfate hydroxide (with a Zn/S ratio of 4).The ratio of Zn/S ranged from 7-9 during discharge/charge, indicating Zn 2+ ion storage also occurred on the HACS electrode, probably through physical and chemical adsorption/desorption. [51]The electrode's large surface area and exposed micropores/mesopores allowed for electrolyte ions (Zn 2+ and SO 4 2− ) to be physically adsorbed onto the HACS electrode with negative or positive charges at different charge/discharge status.Chemical adsorption/desorption also occurred due to the presence of enriched oxygen and nitrogen dopants, particularly carboxylic O and pyrrolic N in the HACS electrode, as evidenced by two couples of redox peaks in CV results (Figure 3e).Based on these results, ion storage mechanisms of the HACS-based ZIC are proposed in Figure 6e, involving reversible physical adsorption of electrolyte ions, chemical sorption of cations (protons/zinc ions), and anions (sulfate), and reversible precipitation/dissolution of zinc sulfate hydroxide.

Conclusion
In summary, we have developed low-cost and high-loading cathodes for ZICs using HACS electrodes derived from a molecular salt.Our approach involved DFT calculations, molecule selection, and carbon fabrication.KP was chosen as the key precursor because it is cheap and contains the necessary components of potassium ions, oxygen/nitrogen groups, and benzene rings, which act as self-activation agents, electroactive heteroatom dopants, and carbon sheet fragments, respectively.The resulting HACSs have large surface areas, hierarchically porous structures, electroactive O/N dopants, and holey sheet microstructures, which provide double-layer and redox-active surfaces for ion storage and low-tortuosity pathways for rapid ion transport.Our HACS-based high-loading cathodes (16.1 mg cm −2 ) deliver a large capacity of 208 mAh g −1 and a fast-charging rate of 1 A g −1 , resulting in the highest energy density and power density of 2.40 mWh cm −2 and 10.72 mW cm −2 , respectively.Furthermore, dry-compressed free-standing thick electrodes enable ultrahigh mass loading up to 35.4 mg cm −2 and the resulting supercapacitor shows one of the highest areal capacitance, energy densities, and lowest cell cost.Our molecule-to-carbon strategy is promising for making self-activated carbon toward practical supercapacitors because it is simple, cost-effective, atom economical, and allows for structural tunability.

Figure 1 .
Figure 1.DFT calculation and carbon fabrication.a) Simulated structure of graphene with different O and N dopants.Model structures for the most stable adsorption status of Zn atoms on b) Oe1 c) Oc, and d) Npy groups of the graphene sheet, respectively.e) Calculated binding energies between Zn atoms and different configurations of pristine, O-and N-doped graphene.f) Schematic selection of molecule and synthesis of HACS.Molecular salt, KP was selected as the precursor for making carbon through pyrolysis and acidic washing.

Figure 2 .
Figure 2. Structural characterization of KP-700, KP-800, and KP-900.a) SEM, b) HAADF-STEM mapping, and c) TEM images of KP-800.d) N 2 sorption isotherms, and corresponding e) pore size distributions, and f) Raman spectra of different carbon materials.High-resolution g) O1s, and h) N1s XPS spectra of different carbons.i) Atomic contents of carboxylic O and pyrrolic N groups.

Figure 3 .
Figure 3. ZICs performance of KP-derived self-activated carbons.a) 3D rate profiles, b) rate performance, and c) charge/discharge curves at differed current densities (0.05 and 1 A g −1 ) of KP-700, KP-800, and KP-900.d) Cycling performance and CE profiles of KP-800.e) CV curves of KP-800, f) b values of O/R peaks calculated from CV curves of different carbons, g) diffusion and capacitive contribution in KP-800 at different scan rates.

Figure 4 .
Figure 4. High-loading performance of optimized HACS electrode (KP-800).a) Capacity versus mass loading at different rates from 0.05 to 1.5 A g −1 , b) rate and cycling, and c) charge-discharge plot of the high-loading electrode (16.1 mg cm −2 ), d) plots of areal capacity against mass loadings, e) plot of areal capacity versus areal current density, f) Ragone plots of KP-800 with different loadings.

Figure 5 .
Figure 5. Energy and cost analysis of thick HACS-based ZICs under practical testing conditions.a) Digital photos of free-standing HACS electrodes and a Canadian coin (10 cents).b) Charge-discharge curves of HACS cells with different mass loadings, c) plot of areal capacitance against mass loading, d) Charge-discharge curves of HACS cell (carbon loading, 9.1 mg cm −2 , N/P = 4) at different rates and its e) cycling performance.f) Ragone plots of ZICs based on carbon electrode, both electrodes, and electrodes+electrolyte. g) Comparison of specific energy and areal energy density with previous ZICs.h) Materials cost breakdown of ZICs with different mass loadings.

Figure 6 .
Figure 6.Mechanism exploration.a) Charge-discharge curve of KP-800 and points of different samples collected at different charge/discharge (C/DC) status (DC0.8V, DC0.5 V, DC0.15 V, C1.0 V, and C1.8 V). b) XRD patterns, and c) SEM images of different points and corresponding mapping images of C, Zn, and S elements at DC0.15 V. Scale bars are 5 μm.d) atomic ratio of Zn to C (Zn/C) and S to C (S/C) at different points.e) proposed reversible reactions at the HACS electrode during the discharge/charge process.