Recent advances in functional materials and devices for Zn-Ion hybrid supercapacitors

Abstract

Zinc-ion hybrid supercapacitors (ZHSCs) are attracting significant attention due to their high energies/power densities, safety, and low cost. In this review, recent advances in the development of ZHSCs are summarized. Particular emphasis is placed on state-of-the-art cathodes (including carbon, metal oxides, MXenes, and redox-active polymers), anodes (including Zn-based composites and Zn-free materials) and electrolytes for ZHSCs. Furthermore, the latest research on functional ZHSC devices with miniaturized ZHSCs, fiber-shaped ZHSCs, self-chargeable ZHSCs and self-healing devices is reported. Finally, further developments with ZHSCs are envisaged for future research in this thriving field.

Introduction

With the popularity and use of smart grids and electric vehicles, the demand for energy storage devices with high energy and power densities, excellent safety, and long cycling lives is increasing^1,2. Rechargeable batteries and supercapacitors are widely researched and have been used for commercial applications^3,4,5. Rechargeable batteries consist of two battery-type electrodes that store energy through redox reactions, such as ion intercalation/deintercalation, phase transformations, or stripping/plating at the anode and cathode; these batteries typically have very high energy densities and are mainly used in portable electronics and electric vehicles. However, the redox reactions in rechargeable batteries usually lead to short cycling lives and low power densities owing to volume variations and sluggish kinetics. Supercapacitors consist of two capacitive electrodes that store and convert energy through electrochemical double-layer capacitive or pseudocapacitive behavior; therefore, supercapacitors have high power densities and are used mainly in consumer electronics, backup power sources, renewable energy generation systems, rail transportation, military equipment, aerospace, and other fields, etc. However, the energy densities of supercapacitors are insufficient and need to be increased. For this reason, the concept of hybrid supercapacitors was developed. Hybrid supercapacitors combine the advantages of both batteries and supercapacitors by using capacitive and battery-type materials as electrodes. During charging and discharging, the capacitive electrodes serve as electrochemical double-layer capacitors or pseudocapacitors, and the battery-type electrodes carry out redox reactions (Fig. 1)^5,6. When constructing hybrid supercapacitors, it is necessary to ensure that the supercapacitors have capacitive electrodes and battery-type electrodes. That is, hybrid supercapacitors can be classified into two types: one has a capacitor-type cathode and a battery-type anode; the other, on the contrary, has a battery-type cathode and a capacitor-type anode.

Fig. 1: Electrochemical properties of the Zn-ion hybrid supercapacitors.

Battery-type electrodes and capacitor-type electrodes make up zinc-ion hybrid supercapacitors.

Among the various hybrid supercapacitors, nonaqueous lithium-ion hybrid capacitors were demonstrated early. However, considering the scarcity and safety issues associated with lithium resources and organic electrolytes, researchers have started to explore the prospects of other ion hybrid capacitors because of their lower costs and safety. Zinc (Zn) is very promising due to its high theoretical capacity (820 mAh g⁻¹), low redox potential (−0.76 V vs. the standard hydrogen electrode (SHE)) and high reliability. Therefore, zinc-ion hybrid capacitors (ZHSCs), which combine the advantages of Zn-ion batteries, such as low cost, environmental friendliness, and low redox potentials of the Zn anodes, and the advantages of supercapacitors, including fast charge‒discharge rates, high power densities and long cycling lives, show attractive application prospects. Conventional ZHSCs use conventional carbon materials as the cathodes, commercial Zn foils as the anodes, and aqueous Zn salts as the electrolytes. Conventional carbon materials, such as activated carbon (AC), exhibit capacitive properties, but their limited specific capacitance cannot be matched with those of Zn anodes with high capacities, resulting in a waste of resources^7,8. On the anode side, the need to polish the surface oxide layer of commercial Zn foil may cause fine traces to form on the surface of the Zn foil, leading to the formation of dendrites during cycling, which can affect the cycling life^9,10,11. In addition, the interactions between water molecules and the anions in water-soluble Zn salts (ZnCl₂, ZnSO₄, Zn(CF₃SO₃)₂, and Zn(CH₃COO)₂) can affect ion adsorption/desorption or deposition/exfoliation, which may cause a range of issues, such as byproduct formation and the hydrogen evolution reaction^12,13. Here, we present the latest cathode and anode materials, compare the effects of advanced electrolytes on the performance of ZHSCs, and summarize the prospects for application of new ZHSCs. Moreover, novel design strategies for functional ZHSC devices are highlighted. In addition, we provide our perspectives on the remaining challenges and future research opportunities for the development of electrode materials, electrolytes, and devices for ZHSCs.

Functional cathode materials

In ZHSCs, the cathode is a crucial component. The development of new cathode materials with high electrical conductivities and large specific capacities is the most effective approach to increasing the energy densities and power densities of ZHSCs. Currently, potential candidate materials for ZHSCs include three-dimensional (3D) carbon, metal oxides, MXenes, redox-active polymers, and other cathode materials. All of the performance metrics are benchmarked in Table 1.

Table 1 Comparison of the properties of various cathode materials.

3D carbon

Carbon materials are among the most common electrode materials used in supercapacitors due to their large specific surface areas, high electrical conductivities, and tunable porosities. Early aqueous ZHSCs were prepared with AC as the cathode, Zn as the anode, and a ZnSO₄ aqueous solution as the electrolyte⁷. The assembled ZHSCs exhibited high specific capacities (121 mAh g⁻¹ at 0.1 A g⁻¹) and excellent cycling lives. It is well known that electrochemical double-layer capacitive behavior depends on the specific surface area; therefore, carbonaceous materials with various morphologies have been explored to optimize their electrochemical properties. Porous carbon has been widely studied as a cathode material in ZHSCs due to its mature synthetic methods and tunable nature. For example, porous carbon was developed as a high-performance cathode for ZHSCs by the molten salt method using bitumen as the carbon source and was denoted as a porous carbon assisted by molten salt carbonization (MSPC) (Fig. 2a)¹⁴. The prepared MSPCs had high specific surface areas (499.3 m² g⁻¹) and abundant mesopores, which provided highly active sites and facilitated ion transport. In addition to porous carbon, flower-like carbons with increased specific surface areas and hydrogen-bonded frameworks (HOFs) were rapidly synthesized by recrystallization at room temperature¹⁵. Heavy fractions of bio-oil were encapsulated in HOFs to obtain bio-oil@HOF (BH) composites, which were carbonized to prepare carbon materials (Fig. 2b). Due to the high utilization of active sites, the ZHSCs exhibited energy densities of up to 117.5 Wh kg⁻¹ at 890 W kg⁻¹ and maintained 60.7 Wh kg⁻¹ even at 16.2 kW kg⁻¹. Another example is hollow carbon nanospheres. The hollow structures of these materials expand the contact area between the electrolyte and the electrode, increasing the energy density. To obtain mesoporous carbon hollow spheres (MCHSs), synthetic phenolic resins and pore-forming particles were coated onto tetrapropylorthosilicate templates to form SiO₂@SiO₂/RF core-shell structures, which were subsequently carbonized, after which the template was removed (Fig. 2c)¹⁶. As a cathode material, mesoporous hollow carbon spheres have been demonstrated to have excellent electrochemical performance.

Fig. 2: Morphologies of typical carbon materials used in ZHSCs.

a MSPC¹⁴ (porous carbon). Reproduced with permission from ref. ¹⁴. Copyright © 2020 Elsevier. b BH¹⁵ (flower-like carbon). Reproduced with permission from ref. ¹⁵. Copyright © 2021 Elsevier. c MCHS¹⁶ (hollow nanospheres carbon). Reproduced with permission from ref. ¹⁶. Copyright © 2019 Elsevier. d MPC²¹ (pencil-like carbon). Reproduced with permission from ref. ²¹. Copyright © 2021 Elsevier.

The properties of carbon materials can also be enhanced by doping with other elements. N,S-codoped porous carbons (NSPCs) with well-developed pores and large surface areas were synthesized via coactivation¹⁷. Their unique interconnected lamellar structures with cavities provided many pathways for electron transport and abundant channels for rapid ion transport. More importantly, the introduction of N and S heteroatoms provided additional active sites for ion adsorption. In addition, MnO₂ nanoflowers were used as bifunctional templates to form P/N dual-doped hollow nanospheres (PN-CHoNS) after induced carbonization¹⁸. The 3D porous structures shortened the ion transport paths, the hollow structures enabled fast kinetics and smaller volume changes, and P/N dual doping enhanced the adsorption of Zn ions. Thus, ZHSCs based on the PN-CHoNS cathode had high energy densities of 116.0 Wh kg⁻¹ and high power densities of 21,660 W kg⁻¹. Similarly, N, P, and O codoped carbon materials (RNP-1-2-Air) were prepared by combining air plasma treatment with N, P, and O codoping¹⁹. The plasma treatment increased the hydrophilicities of the prepared materials. Moreover, N, P, and O codoping was used to adjust the functional and electronic properties of carbon microspheres. In a similar vein, cottonwood-derived quasi-two-dimensional tile-shaped carbon sheets (denoted as carbon tile, CT) were developed²⁰. The large specific surface areas, proper curvature, thin walls, and multifunctional doping (O, P, N) provided short ion penetration paths, ensured fast ion diffusion, and improved electron transport efficiency. In general, doping carbon cathodes with heteroatoms such as N and P increased the pseudocapacitance by enhancing the chemisorption of Zn ions.

Electronegative oxygenated groups can reduce the Zn-ion adsorption barrier and form an electrostatic electrochemical double-layer capacitance, thereby improving the electrochemical double-layer capacitance. Moreover, similar to N and S dopants, oxygen functional groups provided additional pseudocapacitance by introducing a rapid redox reaction on/near the carbon surface and making the electrodes more hydrophilic. Therefore, the introduction of surface redox groups increased the number of adsorption sites and provided additional redox pseudocapacitance. To obtain sharpened pencil-like nanoporous carbon (MPC) with a high specific surface area, hierarchical porous structure, and numerous oxygen functional groups, a metal-organic framework (MOF) was used as a precursor in combination with chemical activation methods (Fig. 2d)²¹. Assembled ZHSC devices achieved high energy densities of up to 130.1 W h kg⁻¹ at a power output of 180.3 W kg⁻¹. At a maximum power density of 7.8 kW kg⁻¹, a high energy density of 56 Wh kg ⁻¹ was still provided. However, these oxygen functional groups with structural defects significantly reduced the electrical conductivity of the carbon material, which also seriously affected the specific capacitance. Accordingly, it is extremely important to optimize the rGO reasonably²². It was found that rGO produced by modulating the oxygen functional groups exhibited excellent electrochemical properties, and the assembled quasisolid flexible ZHSCs increased the areal energy density to 342 µW h cm⁻². The carboxyl and carbonyl groups also significantly improved the aqueous electrolyte wettability, Zn-ion chemisorption, and pseudocapacitive redox activities.

Generally, carbon materials offer great promise in the development of ZHSCs due to their diverse structures, large specific surface areas, and mature synthetic methods. Increases in the specific surface areas, while increasing the capacitance, also affect charge storage and the cycling life. Future research should be focused on modulating the pore sizes and synthesizing ordered structures to regulate the specific surface area, as well as improving the electrochemical properties through doping with heteroatoms and introducing functional groups. Based on an understanding of the pore sizes and pore shapes of carbon materials in supercapacitors, the optimal pore size would be twice the radius of the solvated ions, which would increase the capacitance.

Transitional metal oxides

Transition metal oxides (TMOs) have high specific capacitances (100–2000 F g⁻¹) and excellent chemical stabilities and are therefore widely used as new cathode materials in ZHSCs.

RuO₂ has a high theoretical capacitance (2000 F g⁻¹) and fast faradaic redox reactions, good electrical conductivity, high chemical and thermal stability, and a large voltage window. RuO₂ is therefore a most ideal cathode material for pseudocapacitive charge storage. Amorphous RuO₂·H₂O was first used in the cathodes for ZHSCs and assembled ZHSCs based on a pseudocapacitive storage mechanism to achieve fast charging/discharging rates (36 s), high energy densities (82 Wh kg⁻¹), and ultralong lifetimes (10,000 cycles). Kinetic analyses indicated that ultrafast Zn²⁺ storage in the RuO₂·H₂O cathode originated from a redox reaction²³. Furthermore, the structural water in RuO₂·H₂O played a crucial role in the storage of Zn²⁺. By comparing anhydrous RuO₂ and RuO₂·H₂O, it was found that although anhydrous RuO₂ typically has a higher electrical conductivity than aqueous RuO₂·H₂O, the redox pseudocapacitance reaction was significantly suppressed in anhydrous RuO₂.

Materials such as manganese oxides, vanadium oxides, and Prussian blue analogs were used as cathodes in Zn-ion batteries^9,10,24. These unique materials provided inspiration for the design of battery-type cathodes in which Zn ions were inserted/extracted for ZHSCs. Among these materials, manganese-based oxides offered unique advantages, such as abundant resources, low toxicities, and low costs, endowing them with the potential for use as battery-type cathode materials for ZHSCs. MnO₂, AC, and ZnSO₄ were used as the cathodes, anodes, and electrolytes, respectively, of ZHSCs²⁵. The ion storage mechanism, i.e., reversible insertion/extraction of Zn²⁺ in MnO₂ nanorods, ionic adsorption/desorption on the surface of AC particles, and partially reversible formation/dissolution of the byproduct Zn₄(OH)₆SO₄·nH₂O, was investigated. The electrolyte was then optimized to suppress the formation of byproducts and the dissolution of manganese. Interestingly, the addition of Mn²⁺ gave an increase in the energy density, while the replacement of ZnSO₄ with Zn(CF₃SO₃)₂ alleviated both problems. However, the relatively low electrical conductivity and structural collapse during cycling severely limited the reaction kinetics and reduced the structural stability of MnO₂. MnO₂ and CNTs were combined to obtain MnO₂-CNT with a higher electrical conductivity²⁶. In contrast, a core-shell heterostructured Co₉S₈@MnO₂ cathode with tunable oxygen vacancies was constructed²⁷. Due to the Co₉S₈ nanotube arrays and the presence of oxygen vacancies, electron transport through the interface was facilitated; this provided a larger surface area, increased the intrinsic electrical conductivity of MnO₂, and increased the electrochemically active area. As a result, the assembled ZHSCs exhibited excellent electrochemical properties. XRD also revealed that protons were reversibly intercalated and extracted in Co₉S₈@MnO₂ during cycling. In addition, Zn_xMnO₂ was grown on carbon cloth by Zn²⁺ preembedding²⁸. Zn-ion preintercalation improved the stabilities of the manganese-based cathodes, and the assembled ZHSCs maintained 83.1% of the initial capacity after 5000 cycles at 15 mA cm⁻², far exceeding the efficiencies of MnO₂-based ZHSCs (45.3% capacitance retention after 5000 cycles at 15 mA cm⁻²).

Due to its high theoretical capacity based on multiple electron transfers (589 mAh g⁻¹), V₂O₅ shows great potential as a cathode material for ZHSCs. Thus, new ZHSCs with V₂O₅ as the battery-type cathodes were designed²⁹. As expected, the ZHSCs exhibited good electrochemical performance with high area capacitances (129 mF cm⁻²) and high energy densities (48.9 μWh cm⁻²). However, the areal power densities of ZHSCs with V₂O₅ cathodes were still inferior to those of ZHSCs with capacitive or pseudocapacitive cathodes owing to the sluggish kinetics of battery-type redox reactions.

MXenes

MXene materials constitute another class of cathode materials for ZHSCs because of their graphene-like two-dimensional (2D) layer structures, excellent metallic conductivities, high hydrophilicities, superior mechanical strengths, and large numbers of surface pseudocapacitance charge storage sites. Ti₃C₂ MXene was obtained by HF removal of Al from the layered Ti₃AlC₂ precursor followed by centrifugation and vacuum filtration. Ti₃C₂ MXene and Zn nanosheets were assembled to form degradable rechargeable ZHSCs with excellent resistance to self-discharge, and Zn nanosheets were obtained by the deposition of Zn on Ti₃C₂³⁰. The assembled ZHSCs stored/converted electrical energy via ion adsorption/desorption on the Ti₃C₂ MXene cathodes and deposition/exfoliation of Zn²⁺ on the Zn anodes. Over 82% of the capacitance was maintained after 1000 cycles. Notably, the assembled capacitors exhibited minimum self-discharge rates of 6.4 mV h⁻¹ (Fig. 3a) and were completely degraded in phosphate-buffered saline within 7.5 days. In addition, Ti₃C₂T_x/nanofibrillated cellulose (NFC) was obtained by LiF/HCl etching and alkylation of Ti₃C₂T_x complexed with NFC³¹. Due to the presence of NFCs, which suppressed aggregation problems and increased electrolyte contact, the assembled ZHSCs exhibited large specific capacities (92.1 mAh g⁻¹, 0.5 mA cm⁻²) and excellent cycling stabilities (94.31% capacity retention after 10,000 cycles at 8 mA cm⁻²).

Fig. 3: Exploration of MXenes for use in ZHSCs.

a Self-discharge of the Zn//Ti₃C₂ MXene. The inset shows continuous monitoring of the OCV over 72 h³⁰. Copyright © 2019 American Chemical Society. Schematic illustration of the synthesis of b 2D Ti₃C₂T_x³², Copyright © 2022 American Chemical Society, c Ti₃C₂³³, Copyright © 2021 American Chemical Society, and d Sn⁴⁺-Ti₂CT_X/C sphere³⁶, Copyright © 2020 Wiley-VCH.

The abovementioned synthetic methods for MXene materials required F-based reagents (HF, LiF/HCl, etc.). These methods are time-consuming and present safety hazards. Moreover, F-based etching methods can create F-terminal functional groups on the MXene surface that impede the transport of electrolyte ions and reduce the number of electroactive sites, thus decreasing the electrochemical performance. Hence, Ti₃C₂T_x was etched with a simple, time-saving fluorine-free electrochemical method (Fig. 3b)³². Additionally, layered Ti₃C₂T_x was obtained directly after sonication and without the use of any hazardous organic embedding agents. Flexible quasisolid ZHSCs were constructed that provided large area energy densities of 249.9 and 199.1 μWh cm⁻² at power densities of 312.4 and 6471 μW cm⁻², respectively. Moreover, additive-free Ti₃C₂ MXene inks were prepared via a divalent cation-assisted gelation strategy, and the 3D-printed Ti₃C₂ MXene was used in ZHSCs³³ (Fig. 3c). The Ti₃C₂ MXene electrodes were systematically investigated with in situ/ex situ methods to reveal a dual ion storage mechanism, i.e., pseudocapacitive behavior for H⁺ and bilayer capacitive behavior for Zn²⁺. Impressively, excellent rate performance of the 3D-printed MXene cathode (184.4 F g⁻¹ at 10 A g⁻¹) was achieved.

However, restacking and agglomeration of the MXene layers can lead to a loss of electrochemically active sites and slow penetration of the electrolyte. Interestingly, the insertion of other substances on the surfaces or between the layers of MXene prevented restacking. After obtaining the Ti₃AlC₂ phase by chemical etching, N-Ti₃C₂ was also synthesized by a one-step hydrothermal method using urea as the nitrogen source³⁴. The introduced nitrogen heteroatoms increased the interlayer repulsion, suppressed self-aggregation of the Ti₃C₂ sheets, and improved the structural stability of Ti₃C₂. A 3D porous H-MXene film (3D-PHMF) was constructed via a reduced rejection and freeze-casting assembly method³⁵. Compared to the MXene film and 3D-porous MXene film, the as-prepared 3D-PHMF had a larger specific surface area. The abundant pores provided larger surface areas with abundant active sites for Zn²⁺ storage, and together with the introduction of H⁺, electrostatic repulsion between the weakened layers was attenuated, and the assembled ZHSC exhibited remarkable cycling stability for up to 20,000 cycles. Similarly, Sn-Ti₂CT_x/C spherical cathodes were obtained by inserting Sn⁴⁺ into Ti₂CT_x after distributing MXene sheets vertically on the outer surfaces of the carbon spheres in a highly ordered manner (Fig. 3d)³⁶. Due to the presence of Sn⁴⁺, the layer spacing was expanded, and the ion diffusion kinetics were optimized. The assembled ZHSCs exhibited extremely long cycle lives (12,500 cycles at 0.5 A g⁻¹). In addition, a new pizza-like ternary heterostructure, Ti₃C₂T_x/Bi₂S₃@NC, was recently reported³⁷. The synergistic interactions among the three components (Ti₃C₂T_x, Bi₂S₃, and NC) increased the number of active sites and stabilized the overall structure. Consequently, Zn//Ti₃C₂T_x/Bi₂S₃@NC exhibited a high specific capacitance (653 F g⁻¹).

The high electrical conductivity and layered structure of MXenes provided ZHSCs with excellent rate performance. Increasing the contact between the electrolyte and electrode by constructing pores on the surface of the MXenes and improving the stability of the structure by embedding other substances between the layers are effective ways to extend the lifetimes of ZHSCs.

Redox-active polymers

Cost-effective, safe, and environmentally friendly, redox-active polymers with fast and reversible redox reactions are promising materials for various energy storage devices. Polydopamine (PDA) has several redox-active sites and can be used as a redox-active agent to increase the Zn-ion storage capacitance. PDA-coated carbon cloths (PDA@PCC) were fabricated as cathodes for aqueous ZHSC through air calcination and hydrothermal treatment³⁸. The abundant oxygen-containing groups and large specific surface areas facilitated the adsorption of Zn ions. Ex situ XPS and EDS demonstrated that Zn²⁺ underwent quinone coordination reactions, electrical double-layer absorption/desorption at the cathode, and deposition/exfoliation at the anode (Fig. 4a). Inspired by the modular nature of the covalent organic framework (COF), a phenanthroline covalent organic framework (PA-COF) was developed with an electron-rich backbone and nitrogen-rich active sites for use as cathodes in ZHSCs (Fig. 4b)³⁹. The Zn²⁺/H⁺ coinsertion mechanism was confirmed with electrochemical analyses and solid-state NMR, and the phenanthroline functional group was demonstrated to be the active site for Zn-ion storage. The capacitance decay of the assembled ZHSCs was only 0.38% after more than 10,000 cycles. Poly(4,4′-thiodiphenol, TDP)-modified nanoporous AC was used with Zn foil to assemble the ZHSCs (Fig. 4c), and the redox-active carbonyl groups responsible for charge storage in the poly(4,4′-TDP) molecular chains increased the capacitance and expanded the voltage window (0.1–1.9 V)⁴⁰.

Fig. 4: Illustration of redox-active polymer materials for use in ZHSCs.

a Working mechanism for the Zn//PDA@PCC ZHSCs³⁸. Copyright © 2020 American Chemical Society. Synthetic routes for b PA-COF³⁹, Copyright © 2020 American Chemical Society, and c poly(4,4′-TDP)⁴⁰, Copyright © 2019 The Royal Society of Chemistry.

Other cathode materials

In addition to the cathode materials discussed above, there are several other emerging cathodes for ZHSCs, such as siloxanes, phosphorene, and TiN. Although silicon is nontoxic and abundant, there have been few reports on the use of silicon-based materials as electrodes for ZHSCs. In 2021, the use of few-layered siloxanes as cathodes for ZHSCs was reported for the first time⁴¹. Theoretical calculations suggested that the Zn ions preferred to be adsorbed on the Si–Si rings of the siloxene. The diffusion energies for Zn ions on monolayer or trilayer siloxenes were calculated to be much lower than that for the bulk silicon suboxide. The assembled ZHSCs based on few-layer siloxanes exhibited excellent rate performance (71.4% at 5 mA cm⁻²) and excellent capacitance retention (94.3%) after 16,000 cycles. Like few-layer siloxanes, few-layer phosphorene (FL-P) has a large specific surface area, abundant adsorption sites, good mechanical strength, and high carrier mobility and thus is a potential material for supercapacitors. Recently, electrochemical exfoliation was applied to obtain 2D FL-P cathodes⁴². The operating voltages of the assembled ZHSCs with high salt concentrations were increased to 2.2 V, and a capacitance of 145.9 F g⁻¹ was maintained even at 6.4 A g⁻¹. When Et₄NBF₄/PC was used as the electrolyte, the voltage increased to 2.5 V. TiN was creatively proposed as a cathode for ZHSCs, and its electrochemical properties in different electrolytes were compared⁴³. Studies based on the energy difference and adsorption sites of ${{\rm{SO}}}_{4}^{2-}$, Ac⁻ and Cl⁻ on TiN materials demonstrated that the ${{\rm{SO}}}_{4}^{2-}$ anion had the lowest adsorption energy and the best electrochemical properties.

Functional anode materials

3D Zn composites

With a high theoretical specific capacity and a low redox potential, Zn is one of the most promising anode materials for aqueous energy storage systems⁴⁴. When Zn is used as an anode material for ZHSCs, problems such as dendrite growth, corrosion, and hydrogen evolution often occur. Moreover, excessive use of the Zn in ZHSCs can lead to a waste of resources. To solve these problems, various strategies, such as optimizing the Zn structure, alloying, and modifying the coating surface, have been proposed.

3D Zn structures or porous Zn sponges inhibited the growth of Zn dendrites on surfaces in contact with the septum and limited dendrite formation within the internal porous structure^45,46,47. Different carbon substrates for Zn anodes were designed and constructed by controlled self-assembly of the corresponding polymer precursors⁴⁸. A comparison of carbon flowers (C_flower), carbon disks (C_disk), and carbon spheres (C_sphere) revealed that C_flower, composed of outwardly oriented nanoribbons, had a 3D nanostructure with abundant pores and zincophilic oxygen/nitrogen doping (Fig. 5a–d). Among them, ether, carboxyl, and pyrrole nitrogen groups guided nucleation and growth of the Zn in a heterogeneous epitaxial mode. As a result, the assembled ZHSCs exhibited longer lifetimes without dendrite overgrowth.

Fig. 5: Exploration and modification of Zn anodes.

SEM images of a C_flower, b C_disk, and c C_sphere⁴⁸. d Illustration of ideal transitions from monomers to polyimide precursors, polyimide, and O-/N-codoped carbons⁴⁸. Copyright © 2022 American Chemical Society. e Schematic illustration of 2D-Zn⁵⁰. Copyright © 2019 WELY-VCH. f ZIF-8⁵⁸. Copyright © 2022 Elsevier.

With an electrodeposition strategy, a Zn anode with a large specific surface area was obtained, which prevented the formation of Zn dendrites. Zeolite imidazole framework-8 (ZIF-8), a traditional MOF, was synthesized, and the Zn²⁺ in the framework was thermally reduced to form a trace amount of ZnO after heat treatment at 500 °C. The thus-prepared Zn@ZIF-8-500 was used as the anode for a ZHSC⁴⁹. The inherent porous structure and trace amounts of ZnO in the framework increased the number of nucleation sites, provided uniform deposition, and blocked side reactions. The assembled Zn@ZIF-8-500//AC supercapacitors exhibited excellent stabilities and retained 72% of the capacitance and 100% of the coulombic efficiency after 20,000 cycles. In addition, a 2D layered nanostructured Zn (2D-Zn) anode was constructed by electroplating (Fig. 5e)⁵⁰. Due to the 2D structure, the ion diffusion rate increased, and the assembled supercapacitor had a high energy density and a long cycling life.

Alloying provided Zn deposition sites induced uniform deposition and stripping of Zn. A deep eutectic solvent (DES) was developed for Cu–Zn alloying reactions, which produced a Cu–Zn alloy with a Zn-rich interlayer. The resulting alloy had a high Zn utilization ratio with uniform Zn plating/stripping reactions⁵¹. DES-Zn//AC capacitors showed 70.9% capacitance retention after 7000 cycles at a current density of 4 A g⁻¹. Alternatively, due to the low melting point of Hg, zinc amalgams (Zn₃Hg) can be synthesized at room temperature⁵². Instantaneous alloying of the formed Zn₃Hg prevented dendrite growth and corrosion. Thus, the Zn₃Hg anode exhibited a cycling life of 2000 h. Moreover, compared with Zn//AC (64 mAh g⁻¹, 0.1 A g⁻¹ and 1827 h, 2 A g⁻¹), Zn₃Hg//AC exhibited a higher capacity (80 mAh g⁻¹, 0.1 A g⁻¹) and longer lifetime (5000 h, 2 A g⁻¹).

Protecting Zn anodes by modifying the functional interphase layers has been effective in reducing polarization, inhibiting dendrite formation, and enhancing interfacial stability^{53,54,55,56,57}. Mesoporous hollow carbon sphere interlayers on Zn foils were used to regulate dendrite/protrusion growth on Zn anodes¹⁶. Moreover, MOFs are interesting coating and modification interlayers for Zn anodes due to their large surface areas and adjustable pore sizes. A simpler approach was taken to synthesize ZIF-8 (Fig. 5f)⁵⁸. ZIF-8, which grew in situ on the surface of Zn, is hydrophobic and enables homogeneous deposition of Zn during charge‒discharge processes and inhibits side reactions. Thus, the prepared dendrite-free Zn electrodes exhibited low polarization (89.0 mV at 10 mA cm⁻²) and high cycling stabilities (more than 800 h at 10 mA cm⁻²), and the developed ZHSC exhibited a high capacitance retention rate of 96% after 13,000 cycles at 5 A g⁻¹. Konjac glucomannan (KGM) is composed mainly of β-d-glucose and β-d-mannose. The numerous oxygen functional groups served as active sites to redistribute the Zn²⁺ concentration field. Therefore, Zn anodes with KGM coatings (Zn@KGM) were carefully designed⁵⁹. Due to the strong coordination ability triggered by the –OH and =CO groups, Zn²⁺ flux was coordinated, and the tip effect was suppressed. Zn@KGM//AC exhibited a capacitance retention rate of 98.8% after 5000 cycles. Our group also investigated the effects of different interlayers on the modification of Zn anodes. By comparing the electrochemical performance and deposition principles of Zn anodes modified with pure electronic, ionic, and hybrid conductor coatings, the hybrid coating-modified Zn anodes (Alg-Zn+AB@Zn) combined the advantages of both electronic and ionic conductors and exhibited longer cycle lives. The assembled Alg-Zn+AB@Zn//AC exhibited an excellent cycling life of up to 16,000 cycles⁶⁰.

Zn-free anodes

Typically, excess Zn is used as the anodes in ZHSCs, which leads to a low energy density for the whole device. Moreover, the electrochemical properties of ZHSCs with excessive Zn anodes were not reliable enough for practical application⁶¹. Therefore, further research into new anode materials with lower redox potentials than Zn for ZHSCs is needed to enable commercial applications of ZHSCs. Here, zinc-free anodes refer to Zn host materials without Zn. AC is a typical Zn-free anode and was an early alternative to the Zn anodes in ZHSCs^25,28. However, when AC was used as a capacitive electrode, it exhibited limited capacitance due to the energy storage mechanism of the electrochemical double-layer capacitor.

Due to their high electrical conductivities, hydrophilic surface areas, and pseudocapacitive mechanisms, MXenes are potential research objects. New ZHSCs were fabricated with a binder-free MXene (Ti₃C₂T_x) anode. The absence of binders and conductive additives reduced the weight of the electrodes and increased the energy density of the ZHSC device²⁶. Therefore, a high energy density (98.6 Wh kg⁻¹) and high power density (2481 W kg⁻¹) were achieved. Moreover, cotton cloth was placed in a solution containing MXene (Ti₃C₂T_x) to obtain MXene@COC with different MXene mass loadings, which were subsequently used as anodes for ZHSCs⁶². The assembled ZHSCs exhibited excellent electrochemical properties, such as high energy densities (90 Wh kg⁻¹) and high power densities (3838 W kg⁻¹). They also showed excellent flexibility. Even if the temperature was increased to 150°, the CV and GCD curves basically remained unchanged (Fig. 6a, b). Similarly, Ti₃C₂T_x MXene anodes were fabricated by electrodeposition on graphite paper substrates, and the assembled ZHSCs exhibited good electrochemical stabilities, maintained 77% of the initial capacitance, and showed ~98% coulombic efficiency after 10,000 cycles²⁹.

Fig. 6: Other anode materials for ZHSCs.

a CV curves generated at a scan rate of 10 mV s⁻¹ and b GCD curves generated at a current density of 1.01 A g⁻¹ for the ZHSCs, which were bent from 0° to 150°⁶². Copyright © 2019 Elsevier. c Structures and storage mechanism for TiSe₂//AC ZHSCs⁶³. Copyright © 2021 Elsevier. d The Zn²⁺-storage mechanism of the PI-COF⁶⁴. Copyright © 2020 American Chemical Society. e Schematic illustration of the synthesis of POP-TAPP-NTCA⁶⁵. Copyright © 2020 Elsevier.

In addition, TiSe₂ was used in a Zn-ion energy storage system for the first time, and the feasibility of using TiSe₂ for (de)intercalation of Zn ions was demonstrated through experiments and theoretical calculations (Fig. 6c)⁶³. The TiSe₂ electrode works over a low voltage range of 0.05–0.60 V vs. Zn/Zn²⁺. Over a wider voltage range of 0.05 to 1.80 V, the fabricated ZHSC with the TiSe₂ anode and activated carbon (AC) cathode exhibited a specific capacitance of 60.6 F g⁻¹ at 0.20 A g⁻¹. The Zn²⁺ diffusion coefficient in the TiSe₂ anode was calculated to be in the range of 10⁻⁹–10⁻¹⁰ cm² s⁻¹, which was greater than those of many metal oxide electrodes.

Most organic materials reported for Zn²⁺ storage have high redox potentials (>1 V vs. Zn²⁺/Zn), making them more suitable for use as cathodes than anodes. However, a 2D polyarylimide covalent organic framework (PI-COF) anode with rapid Zn²⁺ storage kinetics was designed⁶⁴. The sloped charge/discharge curves of the PI-COF electrode without a clear voltage plateau indicated charge storage behavior similar to that of a supercapacitor. Additionally, PI-COF electrodes stored Zn²⁺ through a two-step redox process, which involved enolate formation through progressive coordination of carbonyl oxygens with Zn²⁺ in a reversible manner (Fig. 6d). In addition, our group⁶⁵ constructed a polymeric tetra(4-aminophenyl)porphyrin-1,4,5,8-naphthalenetetracarboxylic dianhydride (POP-TAPP-NTCA) from PI and porphyrin, and it served as a Zn storage anode formed via a scalable bottom-up strategy (Fig. 6e). The storage of Zn ions in microspherical POP-TAPP-NTCA involved both Faradaic and non-Faradaic processes. Density functional theory (DFT) calculations revealed that the carbonyl and porphyrin nitrogen sites of POP-TAPP-NTCA enabled the storage of Zn ions, but storage of the Zn ions at the porphyrin nitrogen sites was irreversible.

For Zn-free anode hybrid supercapacitors, the problem is poor stability (<85%). Therefore, in the future, it is necessary to explore new anode and cathode materials and optimize the electrolyte for Zn-free anode hybrid supercapacitors.

Functional electrolyte materials

Organic/ionic liquid electrolytes

The widely used aqueous electrolytes in ZHSCs have narrow electrochemical stability windows. To increase the operating voltages of ZHSCs, researchers have developed a series of organic and ionic liquid electrolytes.

Recently, an ionic liquid (IL) electrolyte was used to increase the voltage to 2.4 V, thus greatly increasing the volumetric energy density to 4.3 Wh L^−1 66. Furthermore, after the ILs were replaced with acetonitrile (AN) and ZnCF₃SO₃, the power density of the ZHSC increased from 17.7 kW L⁻¹ to 18.8 kW L⁻¹, and no capacitance degradation was observed after 60,000 cycles. Then, a new hybrid electrolyte (Zn(TFSI)₂/[[Pyr₁₄TFSI]₃]₁₆/[AN]₄) was designed by combining a Zn salt (Zn(TFSI)₂), an organic solvent (AN), and an IL (Pyr₁₄TFSI)⁶⁷. Due to the synergistic effect of Pyr₁₄TFSI and AN, the electrochemical window was widened to 3.3 V vs. Zn/Zn²⁺. As a result, the ZHSCs had high operating voltages (2.1 V) and very good cycling stabilities. In addition to increasing the voltage window, the ionic electrolytes (such as Zn(BF₄)₂/[EMIM]BF₄) optimized the deposition of the Zn anode⁶⁸. Due to the high reduction/oxidation stability of Zn(BF₄)₂/[EMIM]BF₄, the formation of Zn dendrites and the occurrence of side reactions during the charging and discharging processes were avoided. Therefore, after cycling for 1200 h, the coulombic efficiency was still as high as 99%. Similarly, a ZHSC was fabricated with organic N,N-dimethylformamide (DMF) containing Zn(TFSI)₂, which, owing to the presence of Zn²⁺-DMF complexes, allowed uniform deposition of Zn ions while avoiding irreversible hydrogen evolution reactions⁶⁹. Impressively, the assembled hybrid supercapacitor worked between −65 °C and 100 °C because of its low freezing point and high boiling point.

Water in salt electrolytes

The water in salt electrolytes (WiSs) provides extended potential windows, suppression of water splitting reactions, and improved safety and thus shows potential for use in ZHSCs.

ZHSCs with aqueous ZnCl₂ electrolytes exhibited superior capacitances and rates compared to ZHSCs with aqueous ZnSO₄, Zn(CF3SO₃)₂, Zn(Ac)₂ and Zn(NO₃)₂ electrolytes⁷⁰. Interestingly, with increasing concentrations, chloride ions (Cl⁻) caused the formation of [ZnCl]⁺(H₂O)_n−1 (n = 1–6). The solvation and desolvation energies of [ZnCl]⁺(H₂O)_n-1 (n = 1–6) were significantly lower than those of [Zn(H₂O)_n]²⁺, which favored desolvation. Accordingly, a salt-in-water hydrogel electrolyte filled with ZnCl₂ was constructed for use in ZHSCs. In contrast, the effects of different aqueous ZnCl₂ concentrations on the adsorption/desorption of Zn on MXene electrodes were explored⁷¹. It is clear that the electrode performance was improved due to the acidity of the electrolyte at high concentrations, leading to both Faradaic (redox by proton insertion) and non-Faradaic (proton and Zn²⁺ adsorption) reactions at the MXene electrodes. Generally, ZnCl₂ is the most common salt for high-voltage ZHSCs in WiSs. In addition, acetates are also used in WiSs. The effects of different potassium acetate (KAc) and zinc acetate (ZnAc₂) ratios on Zn plating/stripping were systematically examined⁷². The 30 m KAc + 1 m ZnAc₂ electrolyte showed the highest reversibility and stability for Zn plating/stripping (m = mol kg⁻¹). The coulombic efficiency of the assembled ZHSCs approached 100% after 10,000 cycles.

Redox electrolytes

Great effort has been expended to optimize electrolytes and increase the energy densities and cycling stabilities of ZHSCs. The introduction of redox additives into the electrolyte usually induces additional redox reactions (additional redox couples into the electrolyte) on the electrode surfaces, through which the capacitance and energy density of the ZHSC are significantly increased.

Inspired by Zn-iodine batteries, ZHSCs were designed and fabricated with B,N dual-doped porous carbon microtubes (BN-CMT) and Zn as the cathode and anode, respectively, and the ultrahigh capacity (416.6 mAh g ⁻¹) and energy density (472.6 W h kg ⁻¹) were increased by introducing redox-active iodide ions into the electrolyte⁷³. The capacity retention rate of the ZHSC was 99.1% after 10,000 cycles. Additionally, based on the above research, a hydrogel electrolyte was designed with ZnBr₂⁷⁴. Owing to the additional Faradaic contribution provided by the redox reaction (3Br⁻/Br³⁻) occurring at the electrode/electrolyte interface, a high energy density of 605 W h kg⁻¹ was maintained at a power density of 1848 W kg⁻¹. In addition, a Zn-bromine “supercapattery” system consisting of an S/P codoped carbon-based cathode, a carbon cloth (CC) anode, and an electrolyte with a soluble double redox additive (3Br⁻/Br³⁻) was constructed⁷⁵. The anode exhibited electrochemical double-layer capacitive charge storage from the CC, pseudocapacitive charge storage from the underpotential deposition of monolayer Zn²⁺ on the surface of the CC, and battery-type Zn²⁺ deposition on the Zn//CC surface. Similarly, the charge storage contributions of the S/P codoped carbon cathodes resulted from capacitive/pseudocapacitive processes below potentials of 1.3 V vs. Zn/Zn²⁺ and from battery-type processes in the voltage range 1.3 − 1.8 V vs. Zn/Zn²⁺. Hence, after integrating the triple functions of capacitive/pseudocapacitive and battery-type charge storage in both the cathode and anode, the assembled ZHSC achieved a battery-level energy density and a capacitor-level power density.

Polymer electrolytes

To address the inevitable leakage of liquid electrolytes from flexible ZHSCs, there is a strong need to develop electrolytes that combine high ionic conductivity, good mechanical and electrochemical stability, excellent wettability, and a high safety profile. Through rational design, polymer or quasisolid electrolytes may be useful candidates. Polymeric or quasisolid electrolytes consist mainly of Zn salts, polymeric backbones, and small amounts of water. Common polymeric backbones include PVA^76,77, PAA⁷⁸, PAM^36,79, gelatin^80,81, etc. For example, polymer electrolytes were used in ZHSCs by adding copolymer-derived hollow carbon spheres, Zn deposited on carbon cloth, and a polyacrylamide hydrogel as the cathode, anode, and electrolyte, respectively⁷⁹. The assembled ZHSCs exhibited excellent cycling stability (98% capacitance retention after 15,000 cycles at a current density of 1 A g⁻¹). In addition, supramolecular gel polymer electrolytes (SGPEs) formed dynamic reversible physical cross-linking with hydrogen bonds, ionic bonds and π-π interactions exhibiting the advantages of simple fabrication methods and environmental friendliness⁸². The assembled Zn@SGPE//AC exhibited a long cycling life with 84% capacity retention after 110,000 cycles at 5 A g ⁻¹.

To meet the operating requirements of ZHSCs, composite polymers or quasisolid electrolytes that combine the advantages of individual components have been explored extensively. For example, electrolytes consisting of sodium alginate (SA)/polymethacrylic combinations displayed excellent regeneration capabilities⁸³. In addition, a frost-resistant hydrogel electrolyte (AF-PVC-CMC/Zn(CF₃SO)₂) was prepared by adding ethylene glycol (EG) to a network of antifreeze polyvinyl alcohol carboxymethyl cellulose (AF-PVA-CMC) containing aqueous Zn(CF₃SO₃)₂⁸⁴. An AF-PVA-CMC/Zn(CF₃SO)₂ electrolyte with enhanced mechanical properties enabled uniform Zn deposition and hindered dendrite overgrowth. Additionally, due to the high boiling point and high dielectric constant of ethylene glycol (EG), the interaction between polyvinyl alcohol and ethylene glycol inhibited the formation of ice crystals and prevented freezing at low temperatures. Therefore, EG is often used as an antifreeze in ZHSCs^85,86. In general, gel electrolytes tend to fail at high temperatures. As a response, heat-resistant and nonflammable polymer electrolyte (HRNPE) films with the pyrrolidinium-based IL (1-methyl-1-propylpyrrolidine bis(trifluoromethyl sulfonyl)imide) as an additive were prepared⁸⁷. ZHSCs with HRNPE separators had safe operating temperatures of up to 100 °C. At room temperature, the developed ZHSCs showed a capacitance retention rate of 88% after 10,000 cycles and cycled 30,000 times even at a high temperature of 80 °C.

Functional devices

Miniaturized devices

Compared with traditional supercapacitors, the small sizes and footprints of Zn-ion hybrid microsupercapacitors (ZMSCs) enable their use with multifunctional devices in nanorobots, wireless sensor networks, microelectromechanical systems, and wearable flexible electronics⁸⁸. This all-in-one compact device increases work efficiency and reduces maintenance costs^89,90,91.

ZMSCs were assembled with AC and electroplated with Zn⁹². The as-prepared ZMSCs exhibited large areal capacitances (1297 mF cm⁻² at 0.16 mA cm⁻²) and high energy densities (115.4 µWh cm⁻² at 0.16 mW cm⁻²). Subsequently, a simpler laser writing method was proposed to construct a ZMSC with Ti₃C₂T_x as the cathode (Fig. 7a)⁹¹. After proper annealing, the ZMSC exhibited excellent capacitance retention (80%, 50,000 cycles) and rate stability (2.02 mF cm ⁻², 10 mV s ⁻¹). Furthermore, a single ZMSC drove a digital timer even when the ZMSC was bent. As shown in Fig. 7b, a ZMSC with a wide voltage window (1.2 V) based on MXene/bacterial cellulose fibers (BCFs) and Zn and Zn(CF₃SO₃)₂/PAM hydrogel electrolytes was manufactured⁹³. The areal energy densities of the ZMSCs were significantly increased to 34.0 μWh cm⁻², in part due to the suppressed hydrogen/oxygen evolution reactions within the MXene/BCF main electrode. A stretchable ZMSC array (ZMSCA) consisting of a screen-printed liquid metal bridge (LMBI) and ZMSCs was constructed, and it exhibited considerable variability and stable electrochemical properties. The CV curves recorded in real-time did not change significantly even when the sample was stretched by 100%, bent 180°, or twisted 360°, and the curves maintained the same shape as the original curves (Fig. 7c). In addition, ZMSCs that combine ionic intercalation and solid‒solid redox reactions have been described⁹⁴. The ZMSCs were composed of MXene/Ag nanowires (AgNWs), a bacterial cellulose (BC) cathode, a Zn anode and a PAM/ZnCl₂ + NH₄Cl hydrogel electrolyte (as shown in Fig. 7d). The presence of AgNWs in MXene/AgNWs&BC extended the interlayer space and enhanced ion intercalation and reversibility while providing an additional phase transition (Ag + Cl⁻ ⇌ AgCl) to create a flat discharge plateau, which increased the charge storage capacitance and output stability. Therefore, energy densities of up to 227 μWh cm⁻² were achieved. Notably, ZMSCs can also be fabricated via laser engraving and electroplating techniques. Figure 7e shows the preparation processes of ZMSCs with Zn microanodes and CNT microcathodes⁹⁵. The energy densities of the as-prepared ZMSCs reached 29.6 μWh cm⁻². More importantly, an in situ Zn-supplemented electroplating method was developed. This method avoided irreversible consumption of the Zn anode and decreased the capacitance and cycling life without destroying the ZMSCs.

Fig. 7: Exploration of ZMSCs.

Schematic illustration of the fabrication processes for ZMSCs with a Ti₃C₂T_x⁹¹, Copyright © 2021 Springer Nature b MXene/BCF⁹³, Copyright © 2021 Elsevier. and e CNT microcathode)⁹⁵, Copyright © 2018 The Royal Society of Chemistry. c Real-time recorded CV curves of ZMSCs with MXene/BCF⁹³. Copyright © 2021 Elsevier. d Schematic of the assembled ZMSC with MXene/AgNWs&BC⁹⁴. Copyright © 2022 Wiley-VCH.

Fiber-shaped devices

The development of portable, bendable, foldable, and wearable energy storage devices is crucial for flexible electronic devices, such as smart textiles, robots, and biosensors^96,97. As emerging energy storage devices, Zn-ion fiber hybrid supercapacitors (ZFSCs) are gradually attracting the attention of researchers due to their attractive features, such as long cycling lives, low costs, safety, and environmental friendliness^80,98.

The first ZFSC was fabricated with rGO/CNTs, Zn-coated graphite fibers, and a polyacrylic acid (PAA) hydrogel containing ZnSO₄ as the cathode, anode and quasisolid electrolyte, respectively⁷⁸. Impressively, the assembled ZFSCs exhibited 98% capacitance retention after 10,000 charge/discharge cycles, demonstrating a long cycling life and excellent stability. The ZFSCs showed excellent mechanical flexibilities as a result of the PAA hydrogel electrolytes and fiber-shaped electrodes. Researchers subsequently explored ZFSCs, particularly for use as cathode materials. Because of the fast electron transport kinetics and high specific surface areas, a 3D N-doped carbon nanotube architecture@carbon nanotube (N-CNT@CNT) was constructed for use in ZFSCs (Fig. 8a)⁹⁹. The operating voltages of the assembled quasisolid ZFSCs were extended to 1.8 V. At different scan rates, all CV curves exhibited nearly rectangular shapes with fast current responses, demonstrating typical capacitive behavior, excellent reversibility, and ultrafast charge/discharge capability. Moreover, the Zn storage performance of onion-like carbon (OLC) was enhanced by the introduction of nitrogen and phosphorus¹⁰⁰. The structural stability of the OLC and the synergistic effect of N and P endowed the assembled aqueous supercapacitor with excellent electrochemical stability (77.8% capacitance retention after 50,000 cycles at 10 mA cm⁻²). Furthermore, compared with the carbon cathodes formed by coating, the N, P-OLC@ carbon cloth (N, P-OLC@CC), which was formed via in situ deposition of the OLC on CC and annealing, exhibited tight bonding with the substrate. Therefore, the risk of N, P-OLC loss during bending was avoided. The ZFSCs assembled in combination with Zn anodes exhibited high energy densities (32.1 mW h cm⁻²) and high power densities (48.3 W cm⁻²). Moreover, after wrapping the AC@CF with cellulose paper, 2D-Zn@CF was wound on the cellulose paper⁵⁰. Thus, the fabricated ZFSCs in a 1 M ZnSO₄/PVA gel electrolyte exhibited excellent electrochemical performance (25 μWh cm⁻², 50 μW cm⁻²) and high mechanical flexibility. Moreover, good working conditions were maintained even under water.

Fig. 8: Designs of ZHSC devices.

a Schematic illustration of the quasisolid N-CNT@CNT//Zn NSs@CNT⁹⁹. Copyright © 2021 The Royal Society of Chemistry. b Cross-sectional view of Zn//N,O-MOFC¹⁰¹. Copyright © 2021 Elsevier. Schematic illustrations of c photorechargeable ZHSC¹⁰³ Copyright © 2020 American Chemical Society, and d self-chargeable ZHSCs¹⁰⁶. Copyright © 2021 Springer Nature.

The morphologies and electrochemical activities of Zn anodes have also restricted the practical application of ZFSCs. As a solution to this challenge, highly reversible fibrous Zn anodes with controlled deposition morphologies were developed by electroplating and depositing Zn nanosheets on a carbon-derived 3D MOF containing N and OH functional groups (N,O-MOFCs) (Fig. 8b)¹⁰¹. Due to the layered structure and fast ion diffusion, the specific volumetric capacitances of the ZFSCs reached 128.06 F cm⁻³, and the volume energy density was as high as 57.63 mWh cm⁻³. More importantly, the highly efficient rechargeability of the fibrous Zn anode also ensured that the ZFSC device was sufficiently stable, with a capacitance retention rate of 99.20% after 10,000 charge‒discharge cycles.

Self-chargeable devices

Energy can be directly obtained from the surrounding environment by integrating energy conversion technology and energy storage units into all-in-one energy storage devices. Solar energy is a common source of energy and is now widely used in energy conversion devices. Graphitic carbon nitride electrodes serve as both capacitive electrodes and light-harvesting materials with the help of a conductive additive allowing electrons to pass through an external circuit. The first photorechargeable ZHSC was proposed in 2015¹⁰². However, the photocharged energy density of this system based on graphitic carbon nitride electrodes was still unsatisfactory (only 0.7 Wh kg⁻¹) and must be increased. An optimized scheme with vanadium oxide nanofibers used as optically and electrochemically active electrodes and silver nanowires as conductive additives in a photorechargeable ZHSC was subsequently proposed by the same research group (Fig. 8c)¹⁰³. Compared to carbon-based conductive additives, silver nanowires supported photoexcited hole transport and provided light scattering centers that enhanced visible light absorption. In comparison with that of the previous system, the photocharge energy density was increased to ∼4.8 Wh kg⁻¹.

Although solar energy is an easily accessible source of energy, the problem of intermittency still exists. As opposed to solar energy, air is more readily available and, therefore, more reliable. An ‘air-charged’ ZHSC was designed based on a U-shaped electrode, two electrolytes (a PAM hydrogel containing 2 M ZnSO₄ and a sodium polyacrylate hydrogel containing 6 M KOH + 0.2 M Zn(CH₃COO)₂) and a Zn anode¹⁰⁴. The “U-shaped” electrode in contact with the PAM was used as a capacitor electrode, and the other part was used as an air electrode. By simply peeling off the tape and allowing contact with air, the capacitor was charged to 88% in 10 min. Unfortunately, the use of an alkaline electrolyte inevitably corroded the Zn. Therefore, a new energy storage device was designed and constructed by combining a Zn-ion supercapacitor and a Zn–air battery in a mild electrolyte¹⁰⁵. The cathode and anode of this new hybrid energy storage device were reduced to graphene oxide and Zn, respectively. Benefiting from the interactions of Zn with the oxygen-containing functional groups on the surface of graphene, the capacity of this hybrid device was increased. Additionally, the presence of defects in the graphene provided abundant active sites for the ORR, which resulted in a significant increase in the capacitances of the assembled capacitors when exposed to air. Nevertheless, since the assembled capacitors were open systems, i.e., the electrolyzers were used as reaction vessels, further improvements are necessary for practical application. ZHSCs were constructed with high concentrations of ZnCl₂ cellulose hydrogel as the electrolytes (Fig. 8d)¹⁰⁶. The capacitor showed sustainable self-charging capability, but the self-charging capability of the ZHSCs decreased after repeated cycling, which proved that the capacitor was not reused. Therefore, it is also necessary to restore the usability of this capacitor by charging it with an external power source.

Self-healing devices

Owing to the flexibility and mechanical stabilities of hydrogel electrolytes, ZHSCs based on hydrogel electrolytes show excellent electrochemical performance, but there are still several problems, such as irreversible mechanical damage and failure caused by drying. The development of self-healing devices is essential for solving the above problems.

In an earlier self-healing study of ZHSCs, multiwalled carbon nanotube (MWCNT)-RGO, MWCNT-RGO-Zn, and polyvinyl alcohol (PVA)/Zn(CF₃SO₃)₂ hydrogels were utilized as cathodes, anodes and electrolytes, respectively⁹⁸. Due to the great flexibility and self-healing ability of the PVA/Zn(CF₃SO₃)₂ hydrogel electrolyte, the capacitance retention rates for the ZHSCs after bending and self-healing were 87.8% and 70.5%, respectively. Further studies were conducted by adding borax and nanofibrous cellulose (NFC) to PVA to prepare B-PVA/NFC composite hydrogels, which exhibited excellent flexibilities, high strengths, and toughness¹⁰⁷. Benefiting from the dynamic reversible cross-linking between B(OH)₄ and hydroxyl groups, the self-healing efficiency of B-PVA/NFC was as high as 98%.

Sodium alginate (SA) is also a common material used in the preparation of gels, and the addition of SA may enhance the mechanical strengths of hydrogel electrolytes. On this basis, a new ion-conducting natural polymer hydrogel (SPMA-Zn) composed of SA-poly(methacrylic acid) was constructed⁸³. SPMA-Zn healed rapidly when the two hydrogels remained in close contact along the cutting line without external interference. More importantly, due to dynamic and reversible interactions, even after drying and grinding into a powder, the hydrogel regained its gel form after the addition of water, demonstrating a surprising self-healing ability. The ZHSCs assembled with SPMA-Zn as the electrolyte also exhibited wide electrochemical windows (0–2.2 V) and high energy densities (164.1 Wh kg⁻¹). Moreover, since poly[2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) (PMS) has a negatively charged sulfonic acid group and a positively charged quaternary ammonium group, the charged groups interacted with water via electrostatic forces formed ion migration channels and inhibited hydrolysis. Therefore, a self-healing double network hydrogel electrolyte (PMAZn) consisting of PMS and SA was prepared, which exhibited a high operating voltage (2.4 V) and a fast healing response¹⁰⁸.

In addition, a new self-healing electrolyte was designed with a multifunctional Zn ionic hydrogel and Fe³⁺ ionic cross-linked anionic copolymers containing 2-acrylamido-2-methyl-1-propanesulfonic acid Zn/zinc acrylate/ZnCl₂¹⁰⁹. The ZHSCs assembled with this hydrogel electrolyte healed quickly and recovered 89.1% of their capacitance after being completely cut.

Summary and outlook

In conclusion, significant achievements have been made in the past few years in the designs of new electrode/electrolyte materials and novel functional devices for ZHSCs. This review outlined the latest advances in ZHSCs, with a particular focus on exploration and optimization of the cathode materials (3D carbon materials, transitional metal oxides, MXenes, redox-active polymers, etc.), modification and development of the anode materials (3D Zn composites, Zn-free anodes), regulation and development of the electrolytes (organic/ionic liquid electrolytes, WiSs, redox electrolytes, polymers or solid electrolytes) and designs of novel devices. Despite these significant advances, research on ZHSCs is still in its preliminary stage, and further exploration and innovation are still needed before practical application.

Optimization of cathode materials

Carbon materials have been extensively studied due to their capacitive properties. In this respect, optimized structures such as porous, flower-like, and hollow spheres increase the specific surface area; doped heteroatoms (e.g., P and N) increase the number of surface adsorption sites; and the addition of oxygen functional groups increases the specific capacitance. However, the effects of various factors, such as the specific surface area (pore size, porosity, structure, etc.), on the capacitance and lifetime, the interaction of heteroatoms and oxygen functional groups with Zn ions, etc., still need to be investigated. In contrast to carbon materials, several pseudocapacitive materials, such as MXenes and redox-active polymers, have exhibited good rate performance due to their high electrical conductivities but suffer from limited cycling stabilities because of chemical/structural instability. Thus, optimization strategies such as doping, coating, and embedding are needed to improve the electrochemical stabilities of pseudocapacitive materials. These strategies also work with metal oxide cathode materials that undergo structural collapse and dissolution.

Development of anode materials

The anode is affected by Zn dendrites, hydrogen evolution reactions, and byproduct generation, which lead to a limited cycle life. The design of 3D Zn anodes may improve the contact with the electrolyte to reduce the formation of dendrites. Optimization of the interfacial contact between the Zn anode surface and the electrolyte would reduce the occurrence of side reactions. However, current studies do not indicate whether the 3D framework networks still have the ability to induce homogeneous Zn deposition or whether the coating would maintain interfacial stability as cycling progresses. Moreover, when coupled with cathodes, the utilization efficiencies of Zn anodes are still low, which reduces the overall energy densities of the ZHSCs. Furthermore, since the use of Zn anodes inevitably leads to the formation of dendrites, the search for new anode materials for pseudocapacitive storage of Zn ions is also a future research direction.

Exploitation of the electrolyte

Organic electrolytes are effective at broadening electrochemical windows but are limited by safety issues. The design of an “organic in water” electrolyte should be considered in subsequent studies to address safety issues. WiSs with wide electrochemical windows reduces the binding of Zn ions and water molecules through a solvation effect, which may lead to the development of dendrite-free Zn anodes. However, the high salt concentrations also pose the problems of high cost and increased viscosity of the electrolyte, which limit the rates of ZHSCs. Gel electrolytes or solid electrolytes are cost-effective and environmentally friendly alternatives that provide ion movement channels and offer different functions depending on the composition, such as freezing resistance and self-healing effects. However, it should be noted that new functions should not be designed at the expense of the ionic conductivities, mechanical strengths, etc., of gel electrolytes or solid electrolytes.