Lai
Yu†
,
Jie
Li†
,
Nazir
Ahmad
,
Xiaoyue
He
,
Guanglin
Wan
,
Rong
Liu
,
Xinyi
Ma
,
Jiacheng
Liang
,
Zixuan
Jiang
and
Genqiang
Zhang
*
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China. E-mail: gqzhangmse@ustc.edu.cn
First published on 12th March 2024
Zinc-ion hybrid capacitors (ZHCs) have gained increasing attention due to their numerous advantages such as cost-effectiveness, environmental friendliness, improved safety, high energy/power densities, and long-term cycling stability. However, despite these benefits, the development of ZHCs is still in its early stages with several challenges. Carbon materials have emerged as promising cathode candidates for application in ZHCs due to their low cost, abundance, diverse structures, and good electrical conductivity. In this review, we systematically summarize the research progress on carbon materials and the electrolytes for ZHCs, including activated carbon, graphene, porous carbon, and heteroatom-doped carbon materials. The synthetic methods, morphology characterization, electrochemical performance, and energy storage mechanisms for zinc-ion storage based on various types of carbon cathodes are comparatively discussed. Finally, the current challenges and prospects of carbon materials in zinc-ion storage systems are proposed. This review provides a comprehensive understanding of the research on carbon materials, which will be beneficial for the practical application of ZHC devices with high performance.
Fig. 1 (a) Ionic radius, standard reduction potential, theoretical specific capacity, volumetric capacity, and metal cost of different metals. (b) Energy density and power density profiles of ZHCs and other energy storage devices. Copyright 2020, Wiley-VCH.15 (c) Number of publications on ZHCs according to Web of Science (from 2016 to December 2023). |
Among the various cathode materials, carbon materials have recently garnered extraordinary focus due to their abundant resources, high electronic conductivity, super-specific surface area and tunable pore structures.34–36 Currently, carbon materials with different morphologies and structures are employed as cathode materials in ZHCs, such as AC and porous carbon, demonstrating superior electrochemical performances. The carbon cathode can store electrical energy based on reversible ion adsorption/desorption, while the physical and chemical properties of carbon materials can determine the electrochemical performance of ZHCs.15 However, despite their significant merits, there are still some challenges for carbon cathodes to acquire a high performance because of the kinetic imbalance between the carbon-based cathode and Zn anode and the lack of understanding of the charge storage mechanism. Therefore, there is still much room for improvement to get high-performance ZHCs by designing and optimizing carbon materials. Hence, in this review, we systematically summarize the application and research progress of carbon materials, including AC, graphene, porous carbon, and heteroatom-doped carbon materials in ZHC devices (Fig. 2). The synthesis, structure, electrochemical performance, and energy storage mechanisms of carbon materials are investigated systematically. Finally, based on the in-depth understanding of the achievements of carbon cathodes, the existing challenges and perspectives of carbon materials in zinc ion storage systems are proposed.
Fig. 3 Schematic of the chemical reactions in different types of energy storage devices: (a) metal ion battery, (b) supercapacitor, and (c) zinc ion hybrid capacitor. |
Zn ↔ Zn2+ + 2e− | (1) |
Regarding the capacitor-type carbon material cathode, the capacitance is mainly contributed from the EDLC storage with reversible ion adsorption/desorption and pseudocapacitance by chemical adsorption of Zn ions at/near the surface of the carbon electrode, especially those involving oxygen functional groups or doped heteroatoms to gain a superpower density, which is expressed as follows:17,47
C + X−↔ C//X−; C + Y+↔ C//Y+ (X: anions, Y: cations) | (2) |
C–OH + Zn2+ + 2e− ↔ C–O–Zn or C–OH + Zn2+ + e− ↔ C–O–Zn + H+ | (3) |
COOH + Zn2+ + 2e− ↔ COO–Zn + H+ | (4) |
However, there are some side reactions in aqueous electrolytes besides the primary reactions on the electrodes, where the OH− ions produced from the hydrogen evolution reaction can react with Zn2+ in the aqueous electrolyte (such as ZnSO4 and Zn(CF3SO3)2). The process can be presented as follows:48,49
4Zn2+ + 6OH− + SO42− + 5H2O ↔ Zn4SO4(OH)6·5H2O | (5) |
2Zn2+ + 4OH− + 2Zn(CF3SO3)2 + 3H2O ↔ 2[Zn(CF3SO3)2·Zn(OH)2]·3H2O | (6) |
The formation/decomposition of Zn4SO4(OH)6·5H2O/[Zn(CF3SO3)2·Zn(OH)2]·3H2O is reversible on the cathode electrodes during the charge/discharge process. The researchers proposed that side reactions can lead to the loss of capacity, while some workers think that the reactions may contribute slightly to the whole energy storage. Therefore, the storage mechanism still needs to be investigated in depth.
Fig. 4 (a) Schematic illustration of working mechanism and (b) electrochemical performance of the ZHCs based on AC cathode.23 Copyright 2018, Elsevier. (c) Schematic of AC//ZnSO4 (aq.)//Zn energy storage system and (d) corresponding electrochemical performance.46 Copyright 2018, Elsevier. (e) Schematic of the fabrication process of Zn-ion hybrid micro-supercapacitors, (f) rate capability, and (g) two connected Zn-ion hybrid micro-supercapacitors lighting up a red LED.61 Copyright 2019, Wiley-VCH. |
Biomass-derived AC is a potential carbon material for energy storage devices due to its unique physical/chemical properties, abundant functionality, and economic value, which is prepared by physical, chemical, physiochemical, and microwave-assisted activation.62,63 He and co-workers64 prepared AC derived from corncob (denoted as ACC), which had a high SSA of 2619 m2 g−1 with a hierarchical porous structure by tuning the parameters in a calcination–activation process (Fig. 5a–c). The assembled ZHC exhibited a high energy density of 94 W h kg−1 at 68 W kg−1 and superior cycling stability with 98.2% capacitance retention after 10000 cycles at 5 A g−1. Recently, Zhou and co-researchers65 synthesized coconut shell-activated carbon (CSAC) via a steam activation method (Fig. 5d). The SSA of CSAC was 1260 m2 g−1 with a pore volume of 1.8 cm3 g−1, and its porosity was concentrated at around 1.03 nm (Fig. 5e), which could accelerate the kinetics of the electrolyte ions and provided sufficient active sites for charge storage. Consequently, a robust, flexible quasi-solid-state ZHC device (Fig. 5f) was fabricated using CSAC as the cathode and cross-linked poly(vinyl alcohol)/montmorillonite/Zn(ClO4)2 as the gel electrolyte, which showed high energy/power densities (49.1 W h kg−1/6.5 kW kg−1 at −20 °C and 138.6 W h kg−1/18 kW kg−1 at 60 °C), and excellent cycling stability in a wide temperature range from −50 °C to 80 °C (such as 99% capacity retention at 25 °C and 98% capacity retention at −20 °C after 10000 cycles at 5 A g−1, respectively). Likewise, the electrochemical performance of the quasi-solid-state ZHC device remained virtually unchanged at different angles from 0° to 120°, demonstrating its admirable mechanical flexibility. According to the above discussion, the use of AC with high SSA and tailored porosity as the cathode electrode is necessary for ZHCs (or multifunctional zinc ion energy storage devices) to achieve high energy/power density and superior cycling stability.
Fig. 5 (a) Schematic illustration for the preparation of ACC, (b) the SEM and TEM images of ACC, (c) N2 adsorption–desorption isotherms, and pore size distribution of ACC.64 Copyright 2019, American Chemical Society. (d) Schematic illustration of the coconut cycle of coconut, coconut shell, and biochar and (e) the structural study of CSAC. (f) The electrochemical performance of quasi-solid-state ZHC device.65 Copyright 2021, Elsevier. |
Fig. 6 (a) Schematic diagram of the interlayer space evolution of graphene films during the hydrothermal reaction and (b) cycling performance of the ZHC device at a high current density of 15 A g−1.69 Copyright 2020, Elsevier. (c) Schematic of the configuration and working mechanism of ZHCs with surface oxygen functional groups and (d) DFT calculations. (e) Schematic diagram of the as-assembled quasi-solid-state ZHC, (f) electrical performance, and (g) photograph of the device-powered timer at different discharge times.72 Copyright 2020, Wiley-VCH. |
Oxygen-bonded and defective rGO was synthesized via a modified Hummers' method, which could be employed as an active material for ZHCs.71 The device exhibited a specific capacity of 200.4 F g−1 at 0.1 A g−1 and a capacity retention of 92.06% after 10000 cycles at 5 A g−1. When the system was exposed to air, a higher specific capacity (370.8 F g−1 at 0.1 A g−1) and high energy density (100.9 W h kg−1 at 70 W kg−1) could be obtained, as well as superior cycling stability with 94.5% capacitance retention after 10000 cycles. More importantly, apart from the physical adsorption/desorption of ions on the surface of graphene, oxygen-containing functional groups and defects can be generated in the oxygen reduction reaction and provide more active sites for the electrochemical adsorption/desorption of Zn2+, providing extra capacitance to enhance the performance of ZHCs. However, the influence of oxygen substituents of the carbon cathode on the energy storage behavior of ZHCs is vague. Therefore, Sun et al.72 studied the impact of the surface oxygen substituents on the pseudocapacitance contribution and chemical adsorption of Zn ions (Fig. 6c). They chemically synthesized rGO with oxygen functional groups using a series of reductants and varying experimental conditions, which could provide redox-active pseudocapacitance and strengthen the Zn ion adsorption/desorption to enhance the overall ZHCs electrochemical performance. Moreover, the researchers used density functional theory (DFT) to confirm that the carboxyl and carbonyl groups could reduce the chemical adsorption barrier of Zn2+ (Fig. 6d), boost the surface wettability, and extend the stable working potential, thus enhancing the Zn-ion chemical adsorption ability and electrochemical charge storage. Consequently, the ZHC-based rGO cathode synthesized through hydrogen peroxide-assisted hydrothermal reduction exhibited a high specific capacitance of 277 F g−1 and capacitance retention of 97.8% after 20000 cycles even at a relatively low current density of 2.5 A g−1 in 1 M ZnSO4 electrolyte. Alternatively, a quasi-solid-state ZHC with an rGO cathode and a polyacrylamide gel electrolyte was assembled to investigate its possible application (Fig. 6e). The ZHC device, with great flexibility, could deliver an outstanding areal capacitance of 1257 mF cm−2, high areal energy density of 342 μW h cm−2, and power density of 880 mW cm−2, as well as a great capacity retention of 98.9% after 1900 cycles (Fig. 6f). An electric timer could be powered for more than 12 h by two quasi-solid-state ZHCs connected in series (Fig. 6g), demonstrating that the quasi-solid-state ZHC device is a promising flexible power source. As can be seen, the rGO-based ZHCs exhibited a superior performance due to the additional pseudocapacitance provided by the oxygen-containing functional groups.
The agglomeration effect of multilayer graphene sheets causes an unsatisfactory capacitance in graphene electrodes. Thus, to solve this problem, Xu et al.73 designed a p-phenylenediamine (PPD) organic molecule-functionalized rGO film via simple hydrothermal reduction using PPD as a reducing and functional agent. Benefiting from the three-dimensional (3D) structure of the covalently grafted PPD, increasing the wettability and enhancing the electroconductivity of the RGO film, the fabricated ZHCs utilizing the optimized rGO@PPD (6:7, mass ratio) film cathode and Zn anode displayed a high areal capacitance (3012.5 mF cm−2 at 1 mA cm−2), sufficient energy density (1.1 mW h cm−2 at a power density of 0.8 mW cm−2) in aqueous 1.0 M Zn(CH3COO)2 electrolyte, and retention of the initial areal capacitance after 4000 cycles at 7 mA cm−2. Prasit Pattananuwat et al.74 combined the conducting polymer polypyrrole (PPy) with nitrogen-doped graphene (N-rGO) via in situ polymerization for ZHCs. The synthetic PPy nanoparticles embedded on N-rGO showed a 3D network structure with a high SSA of 89.82 m2 g−1 and large mesopore structure, facilitating ion/charge transportation and affording rapid transport channels for Zn ion adsorption/desorption. The constructed ZHCs using zinc foil as the anode and N-rGO/PPy as the cathode with 0.2 M ZnSO4 exhibited a high specific capacity (145.32 mA h g−1 at 0.1 A g−1 in the potential range of 0–1.6 V), remarkable energy density (232.50 W h kg−1 at 160 W kg−1), and excellent cycling stability of 85% retention up to 10000 cycles at 7.0 A g−1. In addition, the energy storage mechanism of the N-rGO/PPy cathode was the adsorption/desorption of Zn2+ ions on the cathode surface and the intercalation/deintercalated of Zn2+ ions in the PPy structure via a faradaic reaction, enhancing the storage capability of the N-rGO/PPy cathode-based ZHCs. Likewise, Jiang et al.75 fabricated a ZHC device using electrochemically exfoliated graphene/polyaniline (EG/PANI) as the cathode and porous organic polymer–tetra(4-aminophenyl)porphyrin–1,4,5,8-naphthalenetetracarboxylic dianhydride (POP–TAPP–NTCA) as the anode in 2 M ZnSO4 aqueous electrolyte, which exhibited a specific capacitance of 172 F g−1, maximum energy density of 48 W h kg−1 and capacitance retention of 90% after 1100 cycles at a low current density of 0.3 A g−1. Also, Yang et al.76 reported the fabrication of a ZHC device using a polypyrrole/electrochemical graphene oxide (PPy/EGO) composite cathode in aqueous 1 M ZnCl2 electrolyte, exhibiting a great energy density of 117.7 W h kg−1 at 0.34 kW kg−1, high power density of 12.4 kW kg−1 at 72.1 W h kg−1, and long-term cycling stability with 81% capacitance retention after 5000 cycles. Interestingly, they thought the PPy/EGO cathode charge storage mechanism in ZHCs mainly involved fast monovalent anion (e.g., Cl− and Br−) insertion/de-insertion into/from PPy instead of Zn2+ ion insertion/de-insertion. Therefore, compounding polymers with graphene is an effective way to improve the specific capacitance of ZHCs. However, some issues still need to be overcome, such as the high cost and complicated operation of the method for the synthesis of graphene-based composites. Furthermore, understanding the charge storage mechanism of graphene-based composite systems needs further research.
Fig. 7 (a) Schematic illustration of the working principle of the ZHCs and corresponding morphology of HCSs, (b) N2 adsorption–desorption isotherms and pore diameter distributions of HCSs, and (c) cycle performance of the HCS cathode at 1.0 A g−1.80 Copyright 2019, The Royal Society of Chemistry. (d) Synthesis process and application of MCHSs, (e) N2 sorption isotherms (inset is the pore size distribution), (f) rate performance, and (g) scheme of the ion transport processes in MCHSs with different mass loadings.82 Copyright 2020, Elsevier. |
Another method is the activation method, among which chemical activation is the most common using activating agents such as KOH, K2CO3, and NaOH.84 Wang et al.85 prepared a new type of sharpened pencil-like nanoporous carbon (MPC-x, where x represents the ratio of KOH) by using a metal–organic framework (MIL-47) as the precursor combined with the chemical activation method (Fig. 8a–d). MPC-2 possessed a high SSA of 2125 m2 g−1 and a large pore volume of 2.21 cm3 g−1, which facilitated the expansion of the active interface and promoted the ion transfer rate. The assembled ZHCs achieved a high energy density of 130.1 W h kg−1 at 180.3 W kg−1 and a large power density of 7.8 kW kg−1 at 59 W h kg−1 under a relatively wide operating voltage of 0–1.8 V, as well as excellent cycling stability over 10000 cycles at 10 A g−1. Likewise, Fan et al.86 obtained porous honeycomb carbon (PPC) with a high SSA of 2926.4 m2 g−1 by a pre-oxidation and KOH-activated process. Due to the interconnected structure of porous carbon, the Zn//PPC ZHCs displayed a high specific capacity of 238 mA h g−1 at 0.1 A g−1, maximum energy density of 193.6 W h kg−1 at 76.6 W kg−1 and maximum power density of 3981 kW kg−1 at 109.5 W h kg−1, together with a superior cycle life of up to 20000 cycles with a capacity retention of 83% at 2 A g−1 (Fig. 8e). As shown in Fig. 8f, Wang et al.87 used a mild KHCO3 as the activating agent to generate nanopores within carbon nanoflakes (PCNFs) and investigated the effect of KHCO3 on the activation process by adding the amount of activating reagent. It can be seen the nanoflakes with some folded morphology were interconnected with each other to form a network structure (Fig. 8g). As demonstrated in Fig. 8h, the optimized PCNF-based ZHC device delivered a high specific capacitance (177.7 mA h g−1 at 0.5 A g−1), excellent rate performance (85.5 mA h g−1 at 20 A g−1), and outstanding cycling stability of 10000 cycles (90% capacity retention at 10 A g−1). More importantly, the ZHC exhibited a high energy density of 142.2 W h kg−1 at 400.3 W kg−1 and retained 68.4 W h kg−1 even at the maximum power density of 15390 W kg−1. The activation method is easy to operate, which is suitable for preparing hierarchically porous carbon materials with high SSA and pore volume, thus obtaining a high performance in Zn ion energy storage devices.
Fig. 8 (a) Schematic illustration of the preparation process of MPC, (b) SEM and (c and d) TEM images of MPC-2, and (e) cycling stability at 10 A g−1.85 Copyright 2022, Elsevier. (f) Schematic illustration of the synthetic process of PCNFs, (g) SEM and TEM images of PCNFs, and (h) electrical performance of PCNF cathode materials.87 Copyright 2020, Elsevier. |
Considering environmental factors, biomass-derived porous carbon has attracted increasing attention due to its low cost and facile preparation.88 For instance, bagasse and coconut shells can be mixed to prepare carbon materials. Liang's group89 synthesized hierarchical porous carbon (HPC) with a 3D interconnected structure through a hydrothermal-assisted molecular-scale mixing strategy (Fig. 9a). The HPC showed a higher SSA, a large pore volume, and better electric conductivity than that of porous carbon derived from cellulose-rich bagasse (PC-B) and lignin-rich coconut shell (PC-CS). Consequently, the HPC cathode exhibited higher specific capacities than PC-CS and PC-B cathodes at different current densities. The HPC-based ZHC exhibited an ultrahigh capacity of 305 mA h g−1, high energy density of 118.0 W h kg−1, and excellent cycling stability for up to 20000 cycles with over 94.9% capacity retention at 2 A g−1, which was superior to the commercial microporous carbon (YP-50)-based ZHCs. Zhang et al.48 fabricated porous carbon derived from pencil shavings (PSC-Ax, where x corresponds to the temperature) by KOH-assisted activation at the required temperature (Fig. 9b). PSC-A600 materials can provide a large SSA (948 m2 g−1) and numerous active sites for the adsorption of ions by temperature optimization. As a result, the assembled Zn//PSC-A600 in 1 M Zn(CF3SO3)2 electrolyte exhibited a specific capacitance of 413.3 F g−1 at a mass loading of 2.0 mg cm−2 and a high areal capacity of ∼4.5 F cm−2 even at an ultrahigh loading of 24 mg cm−2. Meanwhile, it also achieved a high energy density of 147.0 W h kg−1 at 136.1 W kg−1, maximum power density of 15.7 kW kg−1 at 65.4 W h kg−1, and superb durability over 10000 cycles with 92.2% initial capacity retention at a high current density of 10 A g−1. Notably, the ex situ experiments and theoretical calculations (Fig. 9c) were performed to investigate the energy storage process in Zn(CF3SO3)2-based electrolyte, which was the typical adsorption/desorption of electrolyte ions and the reversible precipitation/dissolution of 2[Zn(CF3SO3)2Zn(OH)2]·3H2O cluster, respectively. More importantly, when applying the PSC-A600 cathode in quasi-solid-state ZHCs with a unique anti-freezing hydrogel electrolyte (Fig. 9d), the flexible device could well perform at various bending states and maintain about 63.9% of its initial capacitance (20 °C) and ∼100% coulombic efficiency after 80 cycles even at the relatively low temperature of −15 °C. As can be seen from the above-mentioned results, biomass-derived porous carbon should receive increasing attention because it is not only environmentally friendly but also has a high surface area and porosity, which facilitates ion transport to realize ZHCs with excellent electrochemical performances.
Fig. 9 (a) Schematic of the PC-B, PC-CS, and HPC synthetic processes.89 Copyright 2019, Elsevier. (b) Schematic preparation of PSC and PSC-Ax, (c) theoretical calculations on the specific composition of byproduct generated in Zn(CF3SO3)2-based electrolyte, and (d) electrochemical performance of the as-assembled quasi-solid-state Zn//PSC-A600.48 Copyright 2020, Elsevier. |
Designing oxygen-rich porous carbon is another effective strategy to realize satisfactory electrochemical performance. For example, Yin et al.90 prepared oxygen-rich porous carbon (PC) by directing the pyrolysis of pyromellitic acid tetra-potassium salt with acid etching (Fig. 10a–d). PC800 possessed a large SSA of 1094.7 m2 g−1, pore volume of 1.11 cm3 g−1, and high oxygen content of 6.7 at% (Fig. 10e and f). In 3 M Zn(ClO4)2 aqueous electrolyte, the assembled ZHC with PC cathode and Zn anode showed a high capacitance of 340.7 F g−1 in a wide voltage window of 0–1.9 V (Fig. 10g), high energy density of 104.8 W h kg−1 at 58 W kg−1, large power density of 48.8 kW kg−1 at 40.4 W h kg−1, and excellent cycling stability over 30000 cycles with a high capacity retention of 99.2%. This excellent performance was attributed to the highly reversible hydrogen and oxygen redox reactions on the porous carbon besides the typical EDLC (Fig. 10h–j). It is known that oxygen functional groups can offer additional pseudocapacitance to enhance the overall performance of ZHCs. Based on this, Wang et al.91 investigated the synergistic mechanism between the material structure and oxygen functional groups during the charge/discharge process. The oxygen-functionalized hierarchical porous carbon (HPC) materials were constructed via the pyrolysis of potassium citrate followed by acid etching. Benefiting from its high SSA (1259.7 m2 g−1), suitable micro-/mesopores structure, and high oxygen content (5.86%), the ZHCs based on HPC cathode exhibited a high capacity of 169.4 mA h g−1, maximum energy density of 125.1 W h kg−1 at 0.1 A g−1 and ultrahigh power density of 16.1 kW kg−1 at 20 A g−1. Especially, an ultralong cycle lifespan of up to 60000 cycles at a high current density of 10 A g−1 with a high capacity retention of 93.1% could be obtained. The authors demonstrated that the oxygen functional groups (hydroxyl and carboxyl groups) can undergo reversible Faraday reactions with zinc ions to form C–O–Zn and C–OO–Zn by using ex situ measurements, which could effectively boost the performance of ZHCs during charge/discharge processes. Alternatively, the by-products of Zn(CF3SO3)2 [Zn(OH)2]3·xH2O caused capacity fading during cycling due to the effect of ion adsorption/desorption on the HPC cathode. Furthermore, the corresponding quasi-solid ZHC (Zn//gelatin/Zn(CF3SO3)2//HPC) device also showed a satisfactory rate performance and cycling stability. Meanwhile, He et al.92 employed electrospinning and acid-assisted oxidation technology to synthesize an oxygen-enriched super hydrophilic, flexible porous carbon fiber (OPCNF-20) with super-hydrophilic character. As expected, the OPCNF-20 cathode-based ZHCs displayed a high energy density of 97.74 W h kg−1, power density of 9.92 kW kg−1, and ultralong-term cycling stability (retention rate of 81% after 50000 cycles at 5 A g−1).
Fig. 10 (a) Schematic of the synthetic process of PC, (b) SEM and (c and d) TEM images of PC800, (e) N2 adsorption–desorption isotherms, (f) XPS survey spectra, and (g) rate capability of PC cathodes. (h–j) Study of charge storage mechanism of PC800 cathode.90 Copyright 2020, Wiley-VCH. |
Porous carbon materials were introduced based on specific aspects including their morphologies, SSA, porous structures, synthesis approaches, and electrochemical performances. Importantly, oxygen-rich porous carbon can significantly improve the performance of ZHCs by coupling EDLC and the reversible redox reaction of oxygen functional groups. Meanwhile, facile and green preparation processes are encouraged to be deeply studied to meet the requirements of high efficiency and environmental friendliness. Table 1 presents a summary of the electrochemical performances of porous carbon and AC cathode materials used in ZHCs, guiding the selection of suitable carbon cathode materials.
Materials | SSA (m2 g−1) | Electrolyte | Capacity | Energy density/W h kg−1 | Power density/kW kg−1 | Cycling stability | Ref. |
---|---|---|---|---|---|---|---|
AC | 3384 | 1 M Zn(CF3SO3)2 | 170 F g−1/0.1 A g−1 | 61.6 | 1.725 | 91%/20000 cycles (2 A g−1) | 23 |
AC | 1990 | 2 M ZnSO4 | 132 mA h g−1/0.1 A g−1 | 140.8 | 2.85 | 72%/20000 cycles (4 A g−1) | 33 |
AC | 1923 | 2 M ZnSO4 | 121 mA h g−1/0.1 A g−1 | 84 | 14.9 | 91%/10000 cycles (1 A g−1) | 46 |
HPAC | 3525 | 3 M Zn(CF3SO3)2 | 231 mA h g−1/0.5 A g−1 | — | 11.4 | ∼70%/18000 cycles (10 A g−1) | 57 |
MSAC | 2527 | 2 M ZnSO4 | 176 mA h g−1/0.5 A g−1 | 188 | 10.666 | 78%/40000 cycles (10 A g−1) | 58 |
ACC | 2619 | 2 M ZnSO4 | 318 F g−1/0.1 A g−1 | 94 | 7 | 98.2%/10000 cycles (5 A g−1) | 64 |
CSAC | 1260 | 3 M Zn(ClO4)2 | 423.5 F g−1/0.1 A g−1 | 190.3 | 24.5 | — | 65 |
ACC | 745 | 1 M ZnSO4/1 M Na2SO4 | 153 mA h g−1/1 mA cm−2 | 100 | 1.692 | Stable cycling up to 20000 cycles | 120 |
AC | 1386 | 2 M ZnSO4 | — | 82.9 | 10 | 98.8%/5000 cycles (1 A g−1) | 121 |
AC | — | 2 M ZnSO4 | — | 70.4 | 7.6 | ∼100%/5000 cycles (2 A g−1) | 122 |
PSC-A600 | 948 | 1 M Zn(CF3SO3)2 | 413.3 F g−1/0.2 A g−1 | 147.0 | 15.7 | 92.2%/10000 cycles (10 A g−1) | 48 |
HCSs | 819.5 | 1 M ZnSO4 | 86.8 mA h g−1/0.5 A g−1 | 59.7 | ∼9 | 98%/15000 cycles (1 A g−1) | 80 |
MCHS | 1275 | 2 M ZnSO4 | 174.7 mA h g−1/0.1 A g−1 | 129.3 | 13.7 | 96%/10000 cycles (1 A g−1) | 82 |
MPC-2 | 2125 | 3 M Zn(CF3SO3)2 | 289.2 F g−1/0.2 A g−1 | 130.1 | 7.8 | 96.7%/10000 cycles (10 A g−1) | 85 |
PCNFs | 1770 | 1 M ZnSO4 | 177.7 mA h g−1/0.5 A g−1 | 142.4 | 15.39 | 90%/10000 cycles (10 A g−1) | 87 |
3D HPC | 3401 | 2 M ZnSO4/1 M Na2SO4 | 305 mA h g−1/0.1 A g−1 | 118.0 | 3.2 | 94.9%/20000 cycles (2 A g−1) | 89 |
PC800 | 1094.7 | 3 M Zn(ClO4)2 | 179.8 mA h g−1/0.1 A g−1 | 104.8 | 48.8 | 99.2%/30000 cycles (20 A g−1) | 90 |
HPC-600 | 1259.7 | 1 M Zn(CF3SO3)2 | 169.4 mA h g−1/0.1 A g−1 | 125.1 | 16.1 | 93.1%/60000 cycles (10 A g−1) | 91 |
OPCNF-20 | 532.5 | 1 M ZnSO4 | 136.4 mA h g−1/0.1 A g−1 | 97.74 | 9.92 | 81%/50000 cycles (5 A g−1) | 92 |
Single-doped carbon materials, especially nitrogen (N) doped materials are commonly studied because they are next to the carbon atom in the periodic table and are relatively easy to bond with carbon chemically.95–97 Lu et al.97 designed N-doped hierarchically porous carbon (HNPC) using zeolite NaY as the template and furfuryl alcohol as the carbon source and subsequent thermal treatment to introduce N dopants and hierarchical pores (Fig. 11a). The SSA of HNPC was up to 2762.7 m2 g−1 and the N content was 2.21%, thus increasing the surface wettability (the contact angle decreased from 140° to 76°), the conductivity and active sites, which could produce pseudocapacitance (Fig. 11b). Importantly, not only a series of ex situ measurements applied to study the relevant working principle but also DFT calculations were performed to investigate the effect of N atoms on the adsorption/desorption process of Zn ions at the HNPC/electrolyte interface. The results showed that the overall capacitance of Zn//HNPC was conjointly contributed by the typical EDLC and the additional pseudocapacitance via the chemical adsorption of Zn ions (C–OH + Zn2+ + e− ↔ C⋯O⋯Zn + H+). At the same time, N dopants could effectively lower the energy barrier of chemical interaction between C–O and Zn2+, thus effectively boosting the chemical adsorption of Zn ions on the electrode surface (Fig. 11c). Consequently, the ZHCs based on the HNPC cathode in 1 M ZnSO4 electrolyte exhibited an exceptionally high energy density of 107.3 W h kg−1 at 4.2 A g−1, superb power density of 24.9 kW kg−1, and ultralong-term lifespan over 20000 cycles with high retention of 99.7% at a high current density of 16.7 A g−1, which is superior to state-of-the-art ZHCs. Particularly, the quasi-solid-state Zn//HNPC in poly(vinyl alcohol) gel electrolyte still exhibited a sufficient capacity of 148.2 mA h g−1, high energy density of 91.8 W h kg−1 and a remarkable power density of 27.6 kW kg−1. This is an important study illustrating the energy storage mechanism of heteroatom doping to enhance the electrochemical performance of ZHCs. Subsequently, Liu et al.98 prepared nitrogen-doped hierarchical porous carbon (N-HPC) via one-step method. Benefitting from the existence of micro–mesopores and nitrogen doping, the ZHCs using N-HPC as the cathode manifested a high capacity of 136.8 mA h g−1 at 0.1 A g−1 (corresponding to an energy density of 191 W h kg−1 at 58.5 W kg−1), large power density of 3633.4 W kg−1 and cycling stability of 5000 cycles with 90.9% capacity retention. Similar, diblock copolymer micelle-derived nitrogen-doped hierarchically porous carbon spheres (N-HPCSs) with a N content of 4.32 at% were fabricated for ZHCs,99 which delivered a high specific capacity (180.4 mA h g−1 at 0.5 A g−1), excellent rate performance (58.3 mA h g−1 even at an extremely high current density of 100 A g−1), high energy/power densities (144.3 W h kg−1/79.9 kW kg−1), and outstanding electrochemical durability with an exciting capacity retention of 98.2% after 50000 cycles at 5 A g−1, showing promising potential for practical applications. In addition, the highly reversible formation/dissolution processes of Zn4SO4(OH)6·5H2O indicated that the electrochemical active sites on the N-HPCS surface were well retained according to the ex situ XRD tests. Another unique sulfur (S) doping has emerged and arousing interest in the research of ZHCs. Wang et al.100 developed a versatile synthetic route for producing S-doped 3D porous carbon (S-3DPC-800) by employing sustainable pine needles as the carbon source and potassium thioacetate as the activation agent without any additives (Fig. 11d). The prepared S-3DPC-800 showed a unique 3D architecture with a large specific surface area (2336.9 m2 g−1), abundant ions, accessible micropores/mesopores, and a certain amount of sulfur (1.71 at%, Fig. 11e). As a result, the ZHC device with S-3DPC-800 cathode in 2 M ZnSO4 solution delivered a high specific capacity (203.3 mA h g−1 at 0.2 A g−1), superior rate performance (81.0 mA h g−1 at 20 A g−1), and outstanding cycling stability with 96.8% capacity retention after 18000 cycles at 10 A g−1 (Fig. 11f). Moreover, it displayed a maximum energy density of up to 162.6 W h kg−1 (99.0 W h L−1) at a power density of 160 W kg−1 (97.4 W L−1). It can be seen that heteroatom-doped carbon materials exhibit excellent capacitive behavior in ZHC devices.
Fig. 11 (a) Schematic illustration of the synthetic procedure of HNPC, (b) high-resolution N 1s spectra, and contact angles of HNPC, respectively. (c) Study of the working principle of HNPC cathode.97 Copyright 2019, Wiley-VCH. (d) Schematic illustration of the synthesis route of S-3DPCs, (e) XPS survey spectra of S-3DPCs obtained at different calcination temperatures, and (f) corresponding electrochemical performance.100 Copyright 2021, Wiley-VCH. |
Compared with single-heteroatom doping, dual- and triple-heteroatom doping can significantly change the electronic structure of carbon via their synergistic coupling effects. Zhao et al.47 fabricated N, O co-doped hierarchical porous carbon (HPC), integrated with flexible carbon cloth (CC) through a drop-coating and carbonization approach as the cathode material for ZHSs. Benefiting from the high SSA (197.45 m2 g−1), suitable pore size distribution, interconnected conductive network, and N/O dual-doped (N: 6.52 at%, O: 3.76%), the HPC/CC-based ZHS delivered a high specific capacity of 138.5 mA h g−1 at 0.5 A g−1 and superb rate performance of 75 mA h g−1 even at a high current density of 20 A g−1. Therefore, this device achieved an energy density of 110 W h kg−1 at 499 W kg−1, maximum power density of 20 kW kg−1, and excellent cycling stability without decay after 10000 cycles at 5 A g−1. More importantly, they first proposed the dual cation (H+ and Zn2+) chemical absorption process for extra energy storage capacity and verified it by ex situ experiments (C⋯O + Zn2+/H+ + e− ↔ C⋯O⋯Zn/C⋯O⋯H and N⋯O + Zn2+/H+ + e− ↔ N⋯O⋯Zn/C⋯O⋯H). Also, they explained the precipitation/dissolution process of zinc hydroxide sulfate hydrate, which was deemed a partial source of energy storage capacity. The O dopants can be used as the active sites for pseudocapacitance, while N atoms can facilitate the physical/chemical adsorption and desorption processes. Similarly, Lu et al.101 proposed the preparation of N/S co-doped carbon materials (NPC) derived from pitch, in which the N dopants could participate in the surface redox reaction to provide extra Zn ion storage ability and improve the electrochemical kinetics. Consequently, the NPC//Zn ZHCs delivered a high capacity of 136.2 mA h g−1, satisfactory rate performance (50.8% capacity retention from 0.3 A g−1 to 15 A g−1), and excellent anti-self-discharge ability (95.6% capacity retention after 24 h of rest). For the anti-self-discharge, Dong group102 also realized high-energy and anti-self-discharge ZHSs based on fibrous carbon cathodes with a hierarchically porous surface and O/N heteroatom functional groups.
Compared to carbon, phosphorus (P) possesses a low electronegativity and high electron-donating ability, while boron (B) has higher electronic conductivity due to the shift in its Fermi level to the conducting band.103 Thus, Xu group104 reported a green gas-steamed MOF approach to preparing open-wall carbon cages, while simultaneously offering N, P dopants functionalized in the carbon matrix (OCCs) in a one-step process, while the content of doped N, P in OCCs was as high as 2.53 and 2.96 at%, respectively. The OCCs with unique open-wall structure and hierarchical micro/meso/macropore structure could provide more channels for high-speed mass transport of reactants to the accessible active sites, while the N/P dual-doping could effectively lower the energy barrier of the chemical interaction between the carbon surface and Zn2+, further boosting the chemical adsorption of Zn ions on the carbon surface. Also, according to their report, the electrons of the adjacent C atoms tend to flow toward the N/P atoms with higher electronegativity. The C atoms function more like electron acceptors with respect to the nearby O atoms, and thus the resultant electron-withdrawing inductive effect promotes the formation of C–O–Zn bonds. Another possible pathway is that the sulfur functional groups (such as –PO3−) can directly facilitate the chemical adsorption of Zn ions. As a result, the OCC-based ZHC coin-type cells displayed a wide operating voltage of 2.0 V, high specific capacity of 225 mA h g−1 at 0.1 A g−1, and ultralong cycling life of up to 300000 cycles with a superb capacity retention of 96.5% even at a high current density of 50 A g−1. Based on the superior performance of the OCC cathode, a soft-pack ZHSC device was fabricated, which showed a high energy density of 97 W h kg−1 and superb power density of 6.5 kW kg−1. More importantly, the soft-pack device could operate in a wide temperature range of −25 to +40 °C, holding tremendous possibilities for practical applications in everyday life. Also, heteroatom doping can efficiently improve the pseudocapacitance of carbon cathodes. Zhang and co-workers105 designed carbon clusters assembled with P/N dual-doped hollow nanospheres (PN-CHoNS) via a dual-functional manganese oxide nanoflower template-induced strategy and subsequent carbonization treatment (Fig. 12a). The ZHCs using the PN-CHoNS cathode delivered a high specific capacity of 164.4 mA h g−1 at 0.2 A g−1, superior rate performance with a retained capacity of 66.7 mA h g−1 at 40 A g−1, exceptional energy density of 116.0 W h kg−1 at 141 W kg−1 and maximum power density of 21660 W kg−1 with a decent energy density of 36.1 W h kg−1, as well as ultra-long cycling stability of 12000 cycles with a capacity retention of 80.6%, which was superior to the control sample (Fig. 12b). The excellent electrochemical performance was attributed to the unique features of the 3D nanosphere with a porous structure for ion/electron transfer, hollow interior for good electrolyte/electrode contact, and shortened ion transfer path, as well as the dual-doping engineering electronic structure. Especially, the DFT calculation results (Fig. 12c) revealed that P/N dual-doping could effectively boost the chemical absorption/desorption kinetics of Zn ions, which is favorable for the high electrochemical property of ZHCs. Sun et al.107 proposed that N and P dopants are energetically favorable for promoting the chemical adsorption process of Zn2+ on the cathode interface according to theoretical simulations. Alternatively, N and B dual-doping has also been regarded as an effective strategy to enhance the energy storage performance of carbon cathodes. For instance, Lu et al.108 designed layered B/N co-doped porous carbon (LDC) guided by facile retention after 6500 cycles at 5 A g−1. Soon afterward, Pan et al.109 designed a hierarchically porous B, N dual-doped carbon by thermally annealing metal–organic frameworks and boric acid. The obtained B, N dual-doped carbon showed high heteroatom contents (3.82 at% B and 6.4 at% N), high SSA of 926.67 m2 g−1, and abundant hierarchical pore structure (micropore, mesopore, and macropore), which benefitted the electrochemical performance. Alternatively, B, N dual doping could significantly influence the electronic structure and density state of the carbon surface, thus enhancing the pseudocapacitance contribution. As a result, the B, N dual-doped carbon cathode exhibited a high energy density of 115.7 W h kg−1 at 711.6 W kg−1 in the voltage range of 0.2–1.8 V and a superior cycling stability with 99.2% capacity retention after 10000 cycles.
Fig. 12 (a) Schematic illustration of the formation process of PN-CHoNS, (b) electrochemical performance of PN-CHoNS cathode-based ZHCs, and (c) DFT calculation.105 Copyright 2021, Elsevier. (d) Preparation process of BCF, (e–f) elemental mapping images of BCF, and (g) schematic working mechanism of the aqueous BCF-based ZHC device during the charging process.106 Copyright 2021, Elsevier. |
Triple-heteroatom doping has been researched due to the success of dual-doped carbon cathode materials. For example, Fan et al.106 synthesized a B, N, and O co-doped flower-like carbon (BCF) via the spheroidizing growth of hydrothermal carbon as an assembly drive and in situ-formed melamine cyanurate nanosheets as structure-directing agent (Fig. 12d–f). The contents of N, O, and B on the surface of BCF were 7.43%, 6.59%, and 0.79%, respectively. As presented in Fig. 12g, N-doping could enhance the electronic conductivity and was prone to providing pseudo-capacitance as an electron donor, B-doping with electron deficiency was favorable to improve the chemical adsorption of SO42− on the carbon cathode surface (B + SO42− ↔ B2+//(SO4)2−), while O-doping can react with Zn ions to provide additional pseudocapacitance (C–O–Zn ↔ C–O–Zn2+ + 2e−). Consequently, the BCF-based ZHC delivered a specific capacity of 133.5 mA h g−1 at 1 A g−1 and a high preservation of 70 mA h g−1 at 20 A g−1. Especially, the device achieved a superior energy density of 119.7 W h kg−1 at 890 W kg−1, large power density of 16.5 kW kg−1 even at 63.5 W h kg−1, and outstanding cycle life of 20000 cycles with 92% retention at 10 A g−1. However, the preparation of carbon precursors usually requires tedious steps, and thus Zhu and co-workers synthesized N, F, and O-doped carbon materials via a straightforward and scalable one-step method.110 The obtained N, F, and O-doped carbons possessed a high SSA (∼1700 m2 g−1 to ∼3300 m2 g−1) and regulated pore distribution (0.87 cm−3 g−1 to 2.26 cm−3 g−1). Heteroatom doping can improve the electrolyte surface wettability and enhance the number of electrochemical pseudocapacitance active sites. Therefore, the ZHCs using N, F, O-doped carbon as the cathode and zinc foil as the anode in 2 M Zn(CF3SO3)2 electrolyte exhibited a high energy density of 131.9 W h kg−1 at 0.5 A g−1 and highest power density of 30.8 kW kg−1 at 40 A g−1. Besides, an excellent cycling performance at 10 A g−1 over 12000 cycles with almost no capacity fading was achieved. To better compare the effects of heteroatom doping on carbon cathode materials for ZHCs, we summarize the performances of heteroatom-doped carbon materials in Table 2.
Materials | SSA (m2 g−1) | Electrolyte | Capacity | Energy density/W h kg−1 | Power density/kW kg−1 | Cycling stability | Ref. |
---|---|---|---|---|---|---|---|
HNPC | 2762.7 | 1 M ZnSO4 | 177.8 mA h g−1/4.2 A g−1 | 107.3 | 24.9 | 99.7%/20000 cycles (16.7 A g−1) | 97 |
PVA gel | 148.2 mA h g−1/4.2 A g−1 | 91.8 | 27.6 | 88.3%/10000 cycles | |||
N-HPC | 879 | 2 M ZnSO4 | 136.8 mA h g−1/0.1 A g−1 | 191 | 3.633 | 90.9%/5000 cycles (1 A g−1) | 98 |
N-HPCS | 789.2 | 2 M ZnSO4 | 180.4 mA h g−1/0.5 A g−1 | 144.3 | 79.9 | 98.2%/50000 cycles (5 A g−1) | 99 |
S-3DPC-800 | 2336.9 | 2 M ZnSO4 | 203.3 mA h g−1/0.2 A g−1 | 162.6 | 16 | 96.8%/18000 cycles (10 A g−1) | 100 |
NPC | 1707 | 1 M ZnSO4 | 136.2 mA h g−1/0.2 A g−1 | 81.1 | 12.8 | 98.9%/60000 cycles (10 A g−1) | 101 |
N/O co-doped HPC/CC | 197.45 | 2 M ZnSO4 | 138.5 mA h g−1/0.5 A g−1 | 110 | 20 | 104.3%/10000 cycles (5 A g−1) | 47 |
BGC-750 | 3657.5 | 3 M Zn(CF3SO3)2 | 257 mA h g−1/0.5 A g−1 | 168 | 61.7 | — | 123 |
HPCF (O/N heteroatom) | 2000 | 2 M ZnSO4 | 141 mA h g−1/0.1 A g−1 | 112 | 14.5 | 93%/6000 cycles (20 A g−1) | 102 |
N, P-doped OCC-900 | 1733 | Saturated Zn(CF3SO3)2 | 225 mA h g−1/0.1 A g−1 | — | — | 96.5%/300000 cycles (50 A g−1) | 104 |
167 mA h g−1/0.1 A g−1 | 97 | 6.5 | — | ||||
PN-CHoNS | 29.7 | 2 M ZnSO4 | 164.4 mA h g−1/0.2 A g−1 | 116.0 | 21.66 | 80.6%/12000 cycles (5 A g−1) | 105 |
CNPK | 2038 | 1 M ZnSO4 | 103.2 mA h g−1/0.1 A g−1 | 81.1 | 13.366 | 100%/10000 cycles (5 A g−1) | 107 |
B/N co-doped LDC | 597 | 1 M ZnSO4 | 127.7 mA h g−1/0.5 A g−1 | 97.6 | 12.1 | — | 108 |
Gelatin/ZnSO4 (gel) | 116.8 mA h g−1/0.5 A g−1 | 86.6 | 12.2 | 81.3%/6500 cycles (5 A g−1) | |||
BN-ZIC-3 | 926.67 | 2 M KOH and 2 M ZnSO4 | 162.6 mA h g−1/1 A g−1 | 115.7 | — | 99.2%/10000 cycles (10 A g−1) | 109 |
B, N and O co-doped flower-like carbon | 282 | 2 M ZnSO4 | 133.5 mA h g−1/1 A g−1 | 119.7 | 16.5 | 92%/20000 cycles (10 A g−1) | 106 |
N, F, O-doped carbon (R6) | 3354.9 | 2 M Zn(CF3SO3)2 | 168.4 mA h g−1/0.5 A g−1 | 131.9 | 30.8 | ∼100%/12000 cycles (10 A g−1) | 110 |
(1) Improving the electrochemical performance of carbon cathode materials: the specific capacity of carbon cathode materials fails to match the high theoretical capacity of Zn electrodes, leading to the unsatisfactory performance of ZHCs. Therefore, effective modification strategies are urgently needed to achieve high-performance devices. These include nanostructure design, pore engineering, and heteroatom doping.
(2) Exploring the detailed energy storage mechanism of carbon cathode-based ZHCs: the storage capacitance is mainly contributed by EDLC storage with ion adsorption/desorption at/near carbon surface and pseudocapacitance by reversible faradaic surface reactions. However, the electrochemical mechanism of carbon cathode-based ZHCs is not sufficiently understood, especially heteroatom-doped carbon. Alternatively, although theoretical calculations have been applied to understand the storage mechanisms based on the theoretical model of graphene, the model is not entirely consistent with the real-world situation. Therefore, efforts should be made to investigate the electrochemical behaviors of electrodes in ZHCs during energy storage using advanced in situ technologies combined with theoretical calculations based on more reasonable models.
(3) Considering the practical application of ZHCs: in addition to coin cells of ZHCs, multifunctional zinc ion energy storage devices (such as flexible quasi-solid-state ZHCs and micro devices) should be constructed to keep up with the demands of society. For example, flexible ZHCs play a critical role as a safe power source in realizing wearable electronic products. Soft/flexible electrodes must be developed to cater to various on-demand applications for flexible ZHCs or micro devices, achieving wide commercial applications, such as good flexibility, wide working temperature, anti-freezing ability, and superior anti-self-discharging capability. Furthermore, standardizing tests and performance evaluation standard systems referring to devices should be carefully considered in practical applications.
In conclusion, developing ZHCs as appealing energy storage technology has significant opportunities and challenges. Tremendous efforts in electrode material studies are required to enhance the electrochemical performance of zinc-ion storage devices, accelerating the development of ZHCs in practical application. This review will provide valuable and constructive suggestions for carbon material studies to attract researchers to explore zinc-ion energy storage systems.
Footnote |
† Co-authors. |
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