Electrostatic Interaction‐directed Construction of Hierarchical Nanostructured Carbon Composite with Dual Electrical Conductive Networks for Zinc‐ion Hybrid Capacitors with Ultrastability

Metal–organic framework (MOF)‐derived carbon composites have been considered as the promising materials for energy storage. However, the construction of MOF‐based composites with highly controllable mode via the liquid–liquid synthesis method has a great challenge because of the simultaneous heterogeneous nucleation on substrates and the self‐nucleation of individual MOF nanocrystals in the liquid phase. Herein, we report a bidirectional electrostatic generated self‐assembly strategy to achieve the precisely controlled coatings of single‐layer nanoscale MOFs on a range of substrates, including carbon nanotubes (CNTs), graphene oxide (GO), MXene, layered double hydroxides (LDHs), MOFs, and SiO2. The obtained MOF‐based nanostructured carbon composite exhibits the hierarchical porosity (Vmeso/Vmicro: 2.4), ultrahigh N content of 12.4 at.% and “dual electrical conductive networks.” The assembled aqueous zinc‐ion hybrid capacitor (ZIC) with the prepared nanocarbon composite as a cathode shows a high specific capacitance of 236 F g−1 at 0.5 A g−1, great rate performance of 98 F g−1 at 100 A g−1, and especially, an ultralong cycling stability up to 230 000 cycles with the capacitance retention of 90.1%. This work develops a repeatable and general method for the controlled construction of MOF coatings on various functional substrates and further fabricates carbon composites for ZICs with ultrastability.


Introduction
Aqueous zinc-ion hybrid capacitors (ZICs) have attracted tremendous attention due to the unique properties of Zn metal, such as its high theoretical capacity (820 mA h g −1 ; 5855 mA h cm −3 ), low redox potential (−0.76 V vs standard hydrogen electrode, SHE), and high stability with water.[3][4][5] To date, activated carbon, [6,7] porous carbon, [8,9] hollow carbon spheres, [10] heteroatom-doped carbon, [11,12] metal-organic framework (MOF)-derived carbon, [13,14] and graphene [15,16] have been investigated as the cathode materials for ZICs due to their high specific surface area (SSA), abundant pores, and great chemical stability.However, the specific capacitance, rate capability and energy density of ZICs are mainly limited by the carbon-based cathode materials rather than the Zn metal anode due to the "short plate effect."The poor compatibility between hydrated ions of the electrolyte and cathodes dominated by micropores leads to slow ion diffusion and aggregation between the interface of cathode and electrolyte.Besides, the inferior electrical conductivity of cathodes will result in the slow charge transfer.19][20][21][22][23] To date, the liquid-liquid synthesis is the main method for obtaining MOF coatings, in which functional substrates are introduced into the precursor solution of MOFs with metal ions and organic ligands. [24]Various combinations of MOFs and functional substrates, such as oxidized carbon nanotubes (o-CNTs), [25] graphene oxide (GO), [26,27] MXene, [28,29] SiO 2 [30,31] and so on [32,33] are investigated.Shao et al. [34] fabricated the oxidized carbon fibers/UiO-66-NH 2 composite and Hoseini et al. [35] reported a direct synthesis of MWCNTs@MIL-101 composite by mixing the MIL-101 and acidtreated MWCNTs.Similarly, due to the presence of vast surface end groups on MXene sheets, Pang et al. [36] prepared the MOF/MXene composites via the electrostatic interaction combined with metal ions to prevent the aggregation of 2D MXene sheets.However, the uneven Metal-organic framework (MOF)-derived carbon composites have been considered as the promising materials for energy storage.However, the construction of MOF-based composites with highly controllable mode via the liquid-liquid synthesis method has a great challenge because of the simultaneous heterogeneous nucleation on substrates and the self-nucleation of individual MOF nanocrystals in the liquid phase.Herein, we report a bidirectional electrostatic generated self-assembly strategy to achieve the precisely controlled coatings of single-layer nanoscale MOFs on a range of substrates, including carbon nanotubes (CNTs), graphene oxide (GO), MXene, layered double hydroxides (LDHs), MOFs, and SiO 2 .The obtained MOF-based nanostructured carbon composite exhibits the hierarchical porosity (V meso / V micro : 2.4), ultrahigh N content of 12.4 at.% and "dual electrical conductive networks."The assembled aqueous zinc-ion hybrid capacitor (ZIC) with the prepared nanocarbon composite as a cathode shows a high specific capacitance of 236 F g −1 at 0.5 A g −1 , great rate performance of 98 F g −1 at 100 A g −1 , and especially, an ultralong cycling stability up to 230 000 cycles with the capacitance retention of 90.1%.This work develops a repeatable and general method for the controlled construction of MOF coatings on various functional substrates and further fabricates carbon composites for ZICs with ultrastability.
distribution of surface functional groups on substrates will result in the scattered coating of MOFs on them.To solve this problem, Zhong et al. [37] proposed a strategy by preparing polyvinylpyrrolidone (PVP) modified GO to ensure the uniform coating of ZIF-8 on GO sheets.Likewise, the MXene@multi-layers Co/Zn-ZIF@PVP composites were fabricated by the hydrogen bonds between PVP and MXene and the coordinate bonds between Co (Zn) ions and PVP. [38]Besides, carboxyl grafted SiO 2 spheres/ZIF-8 composite was prepared by Yan et al., [39] which requires an additional pre-treatment of SiO 2 spheres.Furthermore, the oxidized ZnO/Zn foil coated by ZIF-8 layers is constructed by Ruoff's groups [40] to achieved the coating of MOFs with the same type of metal species on the modified metal foils.However, in order to construct a range of MOF-based composites with highly controllable mode, a facile and general liquid-liquid synthesis method for oriented and uniform coatings of MOFs on various substrates is still urgently needed.
Electrostatic interaction-induced self-assembly has been widely used for fabricating nanomaterials and facilitating the efficient nanomaterialbased systems. [41]Herein, for the first time, the bidirectional electrostatic interaction provided by the zwitterionic dodecyl dimethyl betaine (BS-12) was studied for the precisely controlled coating of MOFs on various substrates in the liquid-liquid system.Due to the unique hydrophilic end of BS-12 with covalently bonded anionic and cationic groups, the large dipole moment between opposite charges will induce the interface of bilayers to interact with both metal ions and surface-charged substrates, [42] leading to the oriented and uniform nucleation of MOFs on a range of substrates, such as CNTs, GO, MXene, LDHs, MOFs, and SiO 2 .Furthermore, the obtained MOFbased composite was annealed to fabricate the ultrahigh N-doped (12.4 at.%) carbon-based composite with hierarchical porosity (V meso / V micro : 2.4) and "dual electrical conductive networks."As a cathode for a ZIC, it shows a high specific capacitance of 236 F g −1 at 0.5 A g −1 , a great rate performance of 98 F g −1 at 100 A g −1 , and an ultralong cycling stability of 230 000 cycles with the capacitance retention of 90.1%.

Results and Discussion
In order to reveal the importance of bidirectional electrostatic interaction provided by BS-12 bilayers for the precisely controlled coating of MOFs on the surface-charged substrates, the coating of representative MOF ZIF-8 on the typical negatively charged GO sheets was firstly studied.Scanning electron microscope (SEM) images show the structures of GO with 2D crumpled sheets and ZIF-8 with the rhombic dodecahedral shape (Figure S1, Supporting information).Figure 1a presents the surface morphology of ZIF-8/GO produced by mixing GO, zinc source, and 2-methylimidazole (2-MIM) after hydrothermal (HT) process, in which the irregular and stacked distribution of ZIF-8 on and beyond GO sheets can be clearly observed.The mixing of zinc source and GO cannot inhibit the self-nucleation of ZIF-8 to form individual ZIF-8 nanocrystals in the solution, leading to the stacked and isolated growth of MOFs.As shown in Figure 1b, after adding BS-12 into the above solution, the oriented and uniform coating of nanoscale single-layer ZIF-8 on GO sheets of ZIF-8/BS-12/GO can be found and the ZIF-8 coating can effectively inhibit the selfinduced π-π stacking effect and enhance the interlayer space of GO sheets.It can be attributed to the homogeneous and strong electrostatic interaction caused by BS-12 bilayers between the interface of GO sheets and zinc ions (Figure 1c), resulting in the completely heterogeneous nucleation of ZIF-8 nanocrystals on GO sheets and the increased nucleation rate of zinc nuclei to regulate the size of ZIF-8 nanoparticles.
The formation mechanism of ZIF-8/BS-12/GO via the bidirectional electrostatic self-assembly was also investigated by the macroscopic states in sequence of adding BS-12, Zn(NO 3 ) 2 and 2-MIM into GO solution as shown in Figure 1d (Figure S2a-d, Supporting Information).It can be seen that, after adding BS-12, the GO solution can form GO/BS-12 hydrogel immediately due to the bridge conformation, in which N + in the zwitterionic hydrophilic groups of BS-12 bilayers is binding to oxygen functional groups located in the adjacent GO sheets. [27]After adding zinc source, the negative potential of GO/ BS-12 is beneficial for forming ion pairs with positive zinc ions and thus destroying the structure of BS-12/GO hydrogel, which presents as a flocculent state above the solution.After adding 2-MIM, the solid settles down to the bottom due to the formation of ZIF-8 nanocrystals on GO sheets.Meanwhile, the macroscopic states of the mixing of GO, zinc ions, and 2-MIM without adding BS-12 are delivered (Figure S2e, Supporting Information).The gray color of ZIF-8/GO and black color of ZIF-8/BS-12/GO reflect the essential difference of the ZIF-8/GO mixture and the ZIF-8/BS-12/GO composite (Figures S3  and S4, Supporting Information).Moreover, the size, shape, and composition of ZIF-8 can be fully preserved and well defined during the bidirectional electrostatic induced self-assembly process (Figure S5, Supporting Information).SEM images in Figure 1e-j show the precise control of the size of ZIF-8 from 200 to 40 nm by regulating the amount of BS-12 from 0.00 to 0.18 mM.The diameter deviation of ZIF-8 increases from AE 10 to AE 30 nm with the size of ZIF-8 increases from 40 to 160 nm (Figure S6, Supporting Information).Although the nanoscale ZIF-8 particles are more imperfect with the decreased sizes, they still remain rhombic dodecahedral shapes.Meanwhile, the distributed degree of single-layer ZIF-8 with the size of 40 AE 10 nm on GO sheets can be regulated as well (Figure S7, Supporting Information).In the synthesis of MOFs, the particle size is decided by the nucleation rates.The formation of nanoscale MOFs is only realized when the synthesis conditions favor nucleation rather than growth, [43] demonstrating the increased nucleation rate of zinc nuclei caused by BS-12.Thanks to the above-mentioned multiple functions of BS-12, the controllable coating of nanoscale ZIF-8 with adjustable sizes on GO is achieved.
To further investigate the importance of the bidirectional electrostatic interaction for the construction of composites with highly controllable mode, BS-12 was replaced by non-ionic polyvinylpyrrolidone (PVP), cationic cetyltrimethyl ammonium bromide (CTAB), anionic sodium lauryl sulfate (SDS), other zwitterionic dodecyl sulfobetaine (LHSB), and dodecyl dimethyl tertiary amine oxide (DDAO), respectively (Figure S8, Supporting Information).Herein, ZIF-8/PVP/GO was prepared by mixing GO, PVP, zinc source, and 2-MIM during a one-step HT process, which is the same as fabricating other ZIF-8/amphiphiles/ GO composites.It can be found that the non-ionic PVP cannot avoid the formation of zinc nuclei in the solution and on GO sheets due to the weak interaction between PVP and GO (metal ions), resulting in stacked and isolated growth of ZIF-8 on/beyond GO sheets (Figure S8a, Supporting Information).Likewise, the ZIF-8/cationic CTAB (anionic SDS)/GO composites exhibit the similar surface morphologies (Figure S8b,c, Supporting Information).It is attributed to the electrostatic repulsion of like charges between the cationic (anionic) sites in the hydrophilic ends of cationic CTAB (anionic SDS) and zinc ions (negative sites of GO), inhibiting the heterogeneous nucleation of ZIF-8 on GO sheets.As expected, the betaine-type LHSB and amine oxide-type DDAO, which are all belong to the zwitterionic amphiphiles but have different hydrophilic groups, can achieve the oriented and uniform coating of single-layer nanoscale ZIF-8 on GO sheets (Figure S8d,e, Supporting Information).Because the zwitterionic amphiphiles with the covalently bonded anionic and cationic groups in the hydrophilic ends are electrically neutral but extremely high polarity.Compared with cationic and anionic amphiphiles, the charge repulsion of zwitterionic amphiphiles will be weaker, facilitating the tighter interfacial dispersion, and thus providing better interfacial properties.Moreover, since the zwitterionic amphiphiles present as intramolecular salts, they can adapt to any strong acidic and alkaline liquid-liquid synthesis systems.According to the above results, the bidirectional electrostatic interaction is the crucial factor to bind both zinc ions and GO sheets for this self-assembly method, leading to the oriented and uniform coating of ZIF-8 on GO sheets.
On the basic of above considerations, a general mechanism of fabricating the MOF/BS-12/substrate composites is proposed, as shown in Figure 2a.N + or O − in hydrophilic groups of the BS-12 bilayers binds with negatively or positively charged sites of the substrates while O − in the opposite hydrophilic groups binds with positive metal ions in the solution driven by electrostatic interactions.During the self-assembly process, metal ions further react with the ligands for the in-situ growth of MOFs on substrates.In order to demonstrate the above mechanism and the versatility of this bidirectional electrostatic interaction-induced self-assembly strategy, GO sheets were replaced by negatively charged 1D oxidized MWCNTs (o-MWCNTs), 2D Ti 3 C 2 T x MXene sheets and 3D SiO 2 spheres as well as positively charged 2D MgAl layered double hydroxide (LDH) sheets and 3D ZIF-67 particles, respectively (Figure 2b-f).The surface-charged types of above substrates are confirmed by zeta potential measurements (Figure S9, Supporting Information).
For comparison, it can be observed that ZIF-8 particles grow independently on/beyond o-MWCNTs, MXene, SiO 2 , MgAl LDH, and ZIF-67 without adding BS-12 (Figure S10, Supporting Information).Although the absolute value of zeta potentials for most surface charged substrates is relatively lower than that of GO, the uniform and oriented coatings of single-layer nanoscale ZIF-8 on these substrates can be found in Figure 2g-k, indicating the robust capability of the bidirectional electrostatic interaction.Based on this design principle, the controllable coatings of MOF ZIF-67 and inorganic iron hexacyanoferrate (FeHCF) on GO sheets (Figure S11, Supporting Information) can be also achieved via the bidirectional electrostatic interaction induced self-assembly strategy.The above results indicate the universality and repeatability of this method for the controllable coatings of MOFs on various functional substrates to construct a range of MOF-based composites with highly controllable mode.
To explore the structural and compositional characteristics of the ZIF-8/BS-12/GO derived carbon composites (ZIF-8 derived carbon/BS-12 derived carbon/reduced GO, ZDC/BDC/ rGO), the surface morphologies of all materials were observed as shown in Figure 3a,b (Figure S12, Supporting information).SEM images in Figure 3a presents that, BS-12, as links that connect GO and ZIF-8, shrinks to form continuous carbon-based networks to connect and anchor uniformly distributed ZDC particles on rGO sheets after annealing, which is verified by TEM images (Figure S13, Supporting Information).For comparison, ZDC/rGO derived from ZIF-8/GO with uneven and stacked ZDC on and beyond rGO sheets can be seen in Figure 3b.As shown in Figure 3c, the continuous BS-12 derived carbon networks and rGO sheets of ZDC/BDC/rGO play the role of "double electrical conductive networks" to provide the best electrical conductivity compared with rGO, ZDC, and ZDC/rGO, which is confirmed by the I-V curves in Figure 3d.The pyrolysis behavior of ZIF-8/BS-12/GO was studied based on the curves of thermogravimetric analysis as shown in Figure 3e, which consists of the pyrolysis of ZIF-8, reduction of GO, and greater retention of BS-12 compared with the complete decomposition of BS-12 (Figure S14, Supporting Information).The structural properties of materials were studied by measurements of N 2 adsorption/desorption isotherms.In Figure 3f, ZDC/rGO and ZDC/BDC/rGO both exhibit the type IV adsorption/desorption isotherm with an obvious hysteresis loop at the relative pressure from 0.4 to 1.0 in the desorption branch, demonstrating the hierarchically porous nanostructures.According to the Brunauer-Emmett-Teller (BET) method, the specific surface area (SSA) of rGO is only 38.2 m 2 g −1 while the SSAs of ZDC, ZDC/rGO, and ZDC/BDC/rGO is 1413.3,487.7, and 633.0 m 2 g −1 , respectively (Table S1, Supporting Information).The isotherm curve of ZDC/BDC/ rGO shows the largest hysteresis loop, which identifies with the highest V meso /V micro ratio of 2.4 compared with that of other materials (rGO: 0.5, ZDC: 0.4, and ZDC/rGO: 1.5).Pore size distribution in Figure 3g also indicates micropores in ZDC and hierarchical nanostructures of ZDC/BDC/rGO.Besides, two broad peaks at 22-24°and 42-44°in the XRD pattern of all materials are assigned to (002) and (100) graphitic planes, respectively, corresponding to the amorphous structures (Figure S15a, Supporting Information).Due to the selfstacking rGO sheets, both rGO and ZDC/rGO present a slight narrow peak at about 26°, which disappears in the XRD profile of ZDC/BDC/ rGO owing to the enhanced interlayer space between adjacent rGO sheets.Meanwhile, two typical Raman peaks can be found at 1350 cm −1 (D band) and 1580 cm −1 (G band) of all materials.Compared with the I D /I G ratio of rGO (1.08), ZDC (1.10), and ZDC/rGO (1.12), the highest I D /I G ratio (1.14) of ZDC/BDC/rGO indicates its most topological defects (Figure S15b, Supporting Information).Above all, the hierarchical porosity of ZDC/BDC/rGO can facilitate the rapid ion diffusion while the "dual electrical conductive networks" contributed by continuous carbon networks and rGO sheets can improve the electrical conductivity to enable the fast charge transfer, making ZDC/BDC/rGO an ideal electrode material for energy storage.
X-ray photoelectron spectroscopy (XPS) was performed to investigate the surface chemical composition of the above materials.XPS survey spectra of ZDC, ZDC/rGO, and ZDC/BDC/rGO show the presence of C, O, and N (Figure S16, Supporting Information).In the N 1s spectra of the above materials, three peaks located at 398.1, 399.8, and 400.8 eV are assigned to the pyridinic N, pyrrolic N, and graphitic N, respectively (Figure 4a).It is found in Figure 4b that the N content of rGO, ZDC, ZDC/rGO, and ZDC/BDC/rGO is 0.0, 8.6, 11.2, and 12.4 at.%, respectively (Table S2, Supporting Information).Compared with ZDC prepared from ZIF-8, ZDC/rGO can retain more N content due to the binding force between GO sheets and ZIF-8 coated on them.The highest N content of ZDC/BDC/ rGO is contributed by BS-12 derived N-doped carbon networks and N-doped ZDC particles.As shown in Figure 4c, the graphitic N content of ZDC, ZDC/rGO, and ZDC/BDC/rGO decreases from 26.0% to 22.1% and 19.8% while the pyrrolic N content raises from 19.2% to 26.7% and 27.8%, respectively (Table S3, Supporting Information).The pyridinic N content of ZDC/BDC/rGO (6.5 at.%) is higher than that of ZDC/rGO (5.7 at.%) and ZDC (4.7 at.%), which can be attributed to the BS-12 derived N-doped carbon networks.
Density functional theory (DFT) calculations are used to study the adsorption of Zn atom on the pure and N-doped graphene models and Zn adsorption ability is related to the partial density of states calculated for central carbon and nitrogen atoms in the graphene models (Figure 4d).Results present that graphene modified with pyridinic N (pdN-G) is more likely to adsorb Zn (adsorption energy, E a is 0.36 eV) than pristine graphene (E a for G is 0.16 eV), pyrrolic N-doped graphene (E a for prN-G is 0.15 eV), and graphitic N-doped graphene (E a for gN-G is 0.22 eV).And the shortest distance between Zn and N atoms of 3.1 Å is found in pdN-G.Meanwhile, pdN-G exhibits a higher partial density of states (DOS) at the valence band edge than that of pristine G, prN-G, and gN-G (Figure S17, Supporting Information), indicating the best affinity of pdN-G to Zn atoms.Thus, the highest content of pyridinic N (6.5 at.%) in ZDC/BDC/rGO can enhance its chemical adsorption capability of Zn ions.Thus, the unique structure with continuous Ndoped carbon networks that interconnect individual ZIF-8 derived nanostructured carbon on rGO sheets of ZDC/BDC/rGO is beneficial for the rapid Zn-ion diffusion, fast charge transfer, and robust chemical adsorption of zinc ions, making it a promising cathode material for ZICs.
The property-performance correlation of the obtained carbon composites is studied by constructing modified ZICs (m-ZICs) reported in our previous work. [44]As shown in Figure 5a, the galvanostatic charge/discharge curves of ZDC/BDC/rGO at current densities ranging from 0.5 to 100 A g −1 are provided.Figure 5b presents the cyclic voltammetry (CV) curves of ZDC/BDC/rGO from 5 to 500 mV s −1 .Compared with the highly distorted CV curves of ZDC and ZDC/rGO, CV curve of ZDC/BDC/rGO at 500 mV s −1 remains the quasirectangular shape, as shown in Figure 5c (Figure S18, Supporting Information), indicating the fast ion transport capability and great electrical conductivity of ZDC/BDC/rGO.The calculated rate performance of cathode materials shows in Figure 5d that the ZDC/BDC/rGO exhibits a superior specific capacitance of 236 F g −1 at 0.5 A g −1 and still remains 98 F g −1 at 100 A g −1 , compared to rGO (16 F g −1 @0.5 A g −1 and 3 F g −1 @100 A g −1 ), ZDC (243 F g −1 @0.5 A g −1 and and ZDC/rGO (238 F g −1 @0.5 A g −1 and 71 F g −1 @20-A g −1 ).The highest liner dependent (R 2 = 0.9982) of current density on scan rate from 5 mV s −1 up to even 1000 mV s −1 proves the ultrafast Zn-ion diffusion and charge transfer of ZDC/BDC/rGO as shown in Figure 5e.Moreover, the ZDC/BDC/rGO-based m-ZIC delivers a high energy density of 83.8 Wh kg −1 at 0.5 kW kg −1 and remains an energy density of 26.9 Wh kg −1 at an ultrahigh power density of 80.7 kW kg −1 , which is higher than other m-ZICs as shown in Figure 5f (Table S4, Supporting Information).Cycling stability was examined at a current density of 10 A g −1 , compared with ZDC/rGO-based m-ZIC (89%@20 000 cycles) and ZDC-based m-ZIC (74%@20 000 cycles), the ZDC/BDC/rGO-based m-ZIC presents the highest initial capacitance retention of 102% after 20 000 cycles (Figure S19, Supporting Information), indicating its superior reversibility for the stable zinc-ion storage.
Kinetic analysis of the ZDC/BDC/rGO cathode was further examined to understand its electrochemical behavior and evaluate the rate performance in a ZIC.CV curves of ZDC/BDC/rGO cathode at various scan rates from 5 to 100 mV s −1 all show the nearly rectangular shape (Figure 6a), indicating the fast electron and ion transport.To quantify the capacitive contribution, the peak current (i) and scan rate (v) were analyzed via the power-law equation i ¼ kv b , [23] where i is the current density, v is the scan rate, and the b value is calculated from the slope of log i-log v (log i ¼ blog v þ k).b of 1 represents an ideal capacitive behavior and b of 0.5 suggests a diffusion-controlled process.The b value of ZDC/BDC/rGO cathode is 0.88 (Figure 6b), approaching b of 1 for an ideal capacitive behavior.They can remain stable from 5 to 1000 mV s −1 , revealing the rapid charge storage for the ZIC.
We further quantify the capacitance contributions in the ZDC/BDC/ rGO cathode using Dunn's method [48,55] by the following equations: Equation ( 1) can be converted to equation (2): where k 1 and k 2 are constants, the values of k 1 and k 2 can be obtained by plotting the relationship curves of iv −1/2 against v 1/2 , thus the capacitive and diffusion-controlled contributions are evaluated.As shown in Figure 6c, the capacitive contribution from fast kinetics (the red region) dominates that of slow kinetics (the blue region) across the whole voltage window, yielding a total contribution of 67.1%.With the increased scan rates, the capacitive contribution significantly increases and reaches 98.3% at 1000 mV s −1 (Figure 6d).And among all scan rates (5-1000 mV s −1 ), the predominant fast-kinetic capacitance of ZDC/ BDC/rGO cathode ensures the outstanding rate performance, promising its application of high-power scenes.

Conclusion
We develop a bidirectional electrostatic interaction-induced self-assembly method for the construction of MOF-based composites with highly controllable mode.Due to the completely heterogeneous nucleation of MOF nanocrystals on substrates, the oriented and uniform coating of MOFs on a range of substrates, including CNTs, GO, MXene, LDHs, MOFs, and SiO 2 , can be achieved.Moreover, the obtained ultrahigh N-doped (12.4 at.%) carbon composite with the hierarchical nanostructures (V meso /V micro : 2.4) and "dual electrical conductive networks" is used as a cathode for a ZIC, which shows a high specific capacitance of 236 F g −1 at 0.5 A g −1 , a great rate capability of 98 F g −1 at 100 A g −1 , and an ultralong cycling lifespan of 230 000 cycles with the capacitance retention of 90.1%.This work provides a repeatable and universal design principle for the precisely controlled coating of MOFs on various functional substrates and further constructs MOF-derived carbon composites for ZICs with ultrastability.

Experimental Section
Preparation of ZIF-8/BS-12/GO: Graphene oxide (GO) was prepared through a modified Hummers method. [18]One hundred milligram GO powder was dispersed into 20 mL deionized water by ultrasonication for 2 h to achieve a homogeneous state.After that, 0.12 mM dodecyl dimethyl betaine (BS-12) was adding into the above GO solution for 10 min stirring.1.46 mM Zn(NO 3 ) 2 Á6H 2 O (20 mL water) and 50 mM 2-methylimidazole (2-MIM, 40 mL water) were adding into the GO/BS-12 solution in sequence.After 10 min stirring, the above mixture was transferred to a 100 mL Teflon-lined autoclave and heated in an oven at 120 °C for 4 h.The product was collected by centrifugation process (14 800 × g, 10 min), washed by deionized water for several times, and then dried at 60 °C in a vacuum oven, which was named as ZIF-8/BS-12/GO.Various reaction conditions to regulate the size of ZIF-8 in ZIF-8/BS-12/GO were performed through same processes except for varying the amount of BS-12 from 0.00 to 0.18 mM, the amount of Zn(NO 3 ) 2 Á6H 2 O from 0.73 to 2.92 mM, and the amount of 2-MIM from 25 to 100 mM, respectively.ZIF-8/GO was prepared in the same way without adding BS-12.Preparation of ZDC/BDC/rGO: ZIF-8/BS-12/GO was annealed from room temperature to 800 °C at a rate of 2 °C min −1 , and kept at 800 °C for 2 h under continuous Ar gas flow.The black products were dissolved in 3 M HCl overnight at 80 °C, and then washed by deionized water for several times until pH = 7.Finally, the as-obtained black powders were dried in oven at 120 °C for 12 h and noted as ZDC/BDC/rGO.
Characterizations: Field-emission scanning electron micrograph investigations were carried out with a Hitachi FESEM SU8220 instrument.TEM images were obtained with an FEI Tecnai G220STwin instrument.Zeta potential measurements were carried out at 25 °C on a Malvern Zeta sizer Nano ZS90 Instrument.Thermogravimetric (TG) analysis was performed on a thermal analyzer (TA-Q50).X-ray diffraction (XRD) was carried out on the Rigaku D/Max 2400 diffractometer with Cu Kα radiation (λ = 1.5406Å).XPS analysis was carried out on Thermo ESCALAB 250 to analyze the content of C, O, N of cathode materials.Nitrogen adsorption isotherms were analyzed by Micrometrics ASAP 2020 Surface Area and Porosity Analyzer at 77 K.The surface areas (S BET ) were calculated from the N 2 isotherms with p/p 0 in the range 0.1-0.25 by applying BET method and the pore size distribution was calculated by the DFT equation.
Assembly of aqueous ZICs: All cathodes were fabricated by coating the slurry consisting of active materials, carbon black, and polyvinylidene difluoride (PVDF) with a mass ratio of 8:1:1 in N-methylpyrrolidone solvent onto carbon cloth (12 mm in diameter) and subsequent drying process at 100 °C for 12 h.The loading mass of each cathode was about 2 mg cm −2 .Commercial Zn foil was applied as Zn anode after immersing in 0.1 M HCl for 5 min and washing process.CC@ZIF-8 was prepared by the method as described in our previous work. [25]ZICs were assembled in 2026 coin-type cells with CC@ZIF-8 modified Zn anode, carbon cathode, 2 M ZnSO 4 electrolyte, and glass microfiber filter as separator.
Electrochemical characterizations: Electrochemical performances of rGO, ZDC, ZDC/rGO, and ZDC/BDC/ rGO were investigated by galvanostatic charge/discharge curve (GCD) and cyclic voltammetry (CV), which were tested on the electrochemical workstation (Bio-Logic, VMP3, France).The potential window for CV and GCD tests was fixed in 0.2-1.8V.The CV scan rate was in the range of 5-1000 mV s −1 .The GCD curves were measured under the current densities ranging from 0.5 to 100 A g −1 .The cycle stability was measured at 10 A g −1 and 500 mV s −1 , respectively.The energy and power densities (Ragone plots) were evaluated from the galvanostatic discharge curves by taking account the mass of active materials.The gravimetric capacitance of ZICs was calculated from GCD and CV curves, according to the equations ( 3) and ( 4), respectively: where Δt, ΔV, I, i, v, u, and m are the discharging time (s), voltage window after removing ohmic drop (V), discharge current (A), voltammetric current (A), scan rate (mV s −1 ), the potential window between negative and positive electrodes, and the mass of active materials in carbon cathode (g), respectively.
The energy density was calculated according to the equation ( 5): The power density was calculated according to the equation ( 6): where C is the specific capacitance of samples in the two-electrode cell, V is the set voltage window (V), and t is the discharge time (s).

Computational Details
[58][59][60] Atomic orbitals of the elements were described by the LACVP basis set with effective core potential.A graphene fragment of C 121 H 27 composition was used to model the graphene.The geometry of the models without Zn was optimized by an analytical method to the energy change of 2.5 × 10 −4 atomic units for taking into account the shift of atomic position.
During the optimization of the models after adsorption of Zn atoms, the central part of graphene models was not relaxed.The adsorption energy of Zn was calculated as: E ad = E mod + E Zn − E Zn-mod , where the terms correspond to the total energies of the model (E mod ), insolated Zn atom (E Zn ), and the model with Zn Energy Environ.Mater.2024, 7, e12484 atom (E Zn-mod ).Partial density of states was calculated for central carbon and nitrogen atoms in the models without Zn.For all models, total number of atoms taken for DOS calculations was 13.

Figure 1 .
Figure 1.a-d) SEM images of a) ZIF-8/GO (inset: π-π stacked GO sheets) and b) ZIF-8/BS-12/GO (inset: GO sheets with enhanced interlayer space).c) Schematic illustration of the bidirectional electrostatic interaction-induced coating of single-layer ZIF-8 on GO sheets.d) Digital photos of macroscopic states in the order of adding BS-12, Zn(NO 3 ) 2 and 2-MIM into the GO solution, and the molecular structure of the hydrophilic group in BS-12 binding with GO and zinc ions.e-j) SEM images of GO sheets coated by ZIF-8 with a range of sizes e) >200 nm, f) 160 AE 30 nm, g) 130 AE 30 nm, h) 100 AE 20 nm, i) 50 AE 15 nm, and j) 40 AE 10 nm.

Figure 4 .
Figure 4. a) XPS N 1s spectra of ZDC, ZDC/rGO, and ZDC/BDC/rGO.b) Atomic concentration of nitrogen of all materials and c) relative content of N species of ZDC, ZDC/rGO, and ZDC/BDC/rGO.d) The comparison of zinc adsorption energy (E a ) for pristine and various N-doped graphene structures with Zn atom and C and N partial DOS at the valence band edge calculated for various graphene structures (the models contain C, N, Zn, and H atoms colored in gray, blue, red, and white, respectively).

Figure 6 .
Figure 6.Charge-storage kinetics of ZDC/BDC/rGO cathodes.a) CV curves at various scan rates from 5 to 100 mV s −1 .b) The plots of corresponding log i vs log v curves of raw data and fitted line.(Dash line is fitted by i = kv b ).c) Capacitive and diffusion-controlled contributions at the scan rate of 100 mV s −1 .d) Normalized capacitance contributions at different scan rates from 5 to 1000 mV s −1 .