Cobalt-containing ZIF-derived catalysts for Zn–air batteries

Abstract

Zinc–air batteries (ZABs) are safe, environmentally friendly and stable energy storage devices. However, the two important electrochemical reactions (ORR and OER) in ZABs have high reaction barriers, and thus require the help of a catalyst. At present, the efficient commercial bifunctional catalyst (Pt + RuO₂) has some drawbacks such as high price and poor stability. Thus, it is extremely important to find efficient and inexpensive bifunctional electrocatalysts for ZABs. In this case, zeolitic imidazolate frameworks (ZIFs) are emerging functional materials with highly ordered pore structures and extremely strong catalytic activity, which have attracted much attention due to their high specific surface area, tunable pore size, and structural diversity. Especially, cobalt-containing catalysts derived from ZIFs containing cobalt have excellent bifunctional electrocatalytic properties. Cobalt-containing ZIFs with different amounts of metal possess different synergistic effects and active sites. This review summarizes the preparation and modification methods and active sources of ZIF-derived catalysts containing different amounts of cobalt. Furthermore, we summarized the application of ZIF-derived catalysts containing different amounts of cobalt in ORR and OER. Finally, we briefly discussed the development prospects of ZIF-derived catalysts containing different amounts of cobalt in the field of oxygen electrocatalysis and ZABs.

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Yansheng Fan

Yansheng Fan received his Undergraduate Degree from the Applied Technology College of Soochow University in 2021. Currently, he is a Master's student at Hubei University of Automotive Technology. His current research direction is zinc–air battery electrocatalysts.

Wenhui Wang

Wenhui Wang is currently an undergraduate student at Hubei University of Automotive Technology. Her current undergraduate major is New Energy Materials and Devices, and her research interest focuses on zinc–air battery materials and their properties.

Yixin Chen

Yixin Chen received her Undergraduate degree from Henan Institute of Science and Technology in 2021. She is currently a Master's student at Hubei University of Automotive Technology. Her current research direction is electrocatalysis.

Zhenyi Xu

Xu Zhenyi received his Undergraduate degree from Hubei University of Automotive Technology in 2023. He is currently a Master's student at Hubei University of Automotive Technology. His current research direction is optoelectronic energy materials and devices.

Miao Xu

Miao Xu received her Master's Degree from Hubei University in 2016, and now works as a Lecturer at the College of Mathematics, Physics and Optic-electronic Engineering, Hubei University of Automotive Technology. Her research interests are focused on micro- and nano-scale energy materials.

Rui Tong

Rui Tong obtained his doctorate from the Institute of Applied Physics and Materials Engineering (IAPME) at University of Macau in 2020. He is currently working as an Associate Professor at the College of Mathematics, Physics and Optic-electronic Engineering, Hubei University of Automotive Technology. His research interests are focused on the synthesis of nanomaterials as well as their applications in catalysis, energy storage and conversion.

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1. Introduction

In the 21st century of global modernization, fossil fuels (including non-renewable fuel resources such as coal, oil, and natural gas) are still widely used by countries worldwide.¹ However, the continuous use of fossil fuels has resulted in a series of problems such as energy shortage, global warming, environmental pollution, and resource scarcity.² Thus, presently, to reduce the consumption of non-renewable fossil fuels, the effective utilization of renewable energy sources, especially solar, wind, and hydro energy, is a top priority for society.^3,4

In current energy storage technology, various types of batteries have become an important medium for energy acquisition and utilization.⁵ However, although Li-ion batteries, which are well known to the general public, are widely used in electric vehicles, aerospace, and other applications, alternatives have emerged due to their shortcomings such as high cost, insufficient energy density, poor safety and low recycling rate.^6,7 Particularly, ZABs have emerged as vital energy storage candidates because they have many advantages in practical applications. Firstly, ZABs have low-cost electrode materials because their zinc negative and oxygen positive electrodes have richer reserves worldwide.⁸ Secondly, ZABs have a theoretical energy density of 1086 W h kg⁻¹, which is significantly higher than that of conventional lithium-ion batteries.⁹ Thirdly, the electrolyte used in ZABs is a safe and environmentally friendly aqueous electrolyte.¹⁰ However, ZABs have some drawbacks (such as growth of zinc dendrites and low power) in practical applications.¹¹ The key material affecting the performances of ZABs is the cathode oxygen catalyst.¹² Noble metal-based materials (such as Pt/C,¹³ RuO₂,¹⁴ and IrO₂¹⁵) are considered superior oxygen electrocatalysts; however, their scarcity and high cost limit their practical large-scale application. Thus, recently, researchers have devoted their efforts to studying non-precious metal electrocatalysts (transition metal oxides,^16–18 sulfides,^19–21 phosphides,^22–24 nitrides,^25–27 carbides,^28–30 alloys,^31–33 and carbon materials^34–36).

Metal–organic frameworks (MOFs), a new class of porous materials that emerged at the end of the 20th century, have³⁷ become one of the most promising candidates as functional and novel carbon materials due to their highly controllable structural and surface properties.³⁸ MOFs are organic–inorganic hybrid materials with intramolecular pores formed by the self-assembly of organic ligands and metal ions or clusters through coordination bonds.³⁹ By changing their different coordination modes and adjusting the size of their organic ligands, materials with different functions can be obtained, which have been widely used in many fields such as gas storage, gas separation, catalysis, medicine, sensors, and energy storage devices.⁴⁰ MOFs have advantages in electrocatalysis, such as high specific surface area, porous structure, higher proportion of metal sites, catalytically active atoms, and easy structural characterization.⁴¹ Thus, based on these advantages, MOFs are compatible with electrochemical applications, especially batteries.⁴²

One of the prominent types of MOFs is zeolitic imidazolate frameworks (ZIFs), which are preferentially used in electrochemical applications.⁴³ ZIFs are emerging functional materials with highly ordered pore structures and extremely strong catalytic activity.⁴⁴ ZIFs are organic–inorganic hybrid systems of zeolites and MOFs, which consist of an inorganic porous skeletal system tightly coupled with imidazolium-type organic linkers, resulting in a highly ordered and spatially regular pore structure.⁴⁵ ZIF derivatives are often used as ideal catalysts for battery electrodes because they not only retain the original advantages of ZIFs, such as high specific surface area, adjustable pore size, and structural diversity, but also the unique advantages of zeolites, such as adsorption, catalytic, and ion exchange properties and thermal stability.⁴⁶ In recent years, simple transition metal cobalt materials have shown high ORR and OER activity due to their excellent stability as non-precious metals with heat resistance, abrasion resistance, corrosion resistance, ferromagnetism, flexibility, and good electrochemical properties. Also, cobalt can be more readily bonded to form ZIFs than other metals.⁴⁷

Due to their excellent bifunctional electrocatalytic performance in OER and ORR, there are many reports on the application of cobalt-ZIFs and their derivatives in the field of electrocatalysis.⁴⁸ This review summarizes the cobalt-containing ZIF-derived catalysts applied in Zn–air batteries.⁴⁹ Currently, most reviews only focused on ZIF derivatives with a fixed quantity of metal.⁵⁰ Therefore, in this review, we systematically summarize the methods for the preparation and modification of cobalt-based ZIF derivatives, cobalt–M-based ZIF derivatives, and cobalt-based trimetallic ZIF derivatives as ORR electrocatalysts.⁵¹ Regarding the strategies for the preparation and modification of cobalt-based monometal derivatives, we mainly summarize them based on five aspects including pyrolysis, oxidation, sulfuration, doping and hybridization.⁵² Among them, hybridization is an important strategy to improve the properties of cobalt-based ZIF derivatives, including compounding with conductive carbon (carbon nanotubes, carbon fibers, etc.) to improve their conductivity and binding with highly active ORR electrocatalysts (Co₃O₄, NiCo-layered double hydroxides (LDH), etc.) to increase their number of active sites, and hybridizing with organics (supramolecular gels (SMG), benzimidazole (BIm), etc.) to enhance their structural stability.⁵³ Similarly, in this review, we summarize methods for the preparation and modification of cobalt-based trimetallic ZIF derivatives (mainly ZIFs formed by cobalt metal and transition metals such as Fe, Ni, and Zn).⁵⁴ Finally, the challenges and prospects of developing cobalt-containing ZIF-derived electrocatalysts with bifunctional ORR and OER activity are outlined to provide a comprehensive understanding.⁵⁵

2. Fundamentals of Zn–air batteries

ZABs mainly include two types, i.e., primary ZABs and rechargeable/secondary ZABs, and mainly comprised of three parts including a zinc plate or zinc powder electrode (anode), air electrode (cathode), and alkaline electrolyte. Primary zinc–air batteries convert chemical energy into electrical energy for external circuits and are powered only by internal chemical reactions.⁵⁶ Alternatively, secondary ZABs are discharged and charged by atmospheric oxygen through ORR and OER at the air electrodes, respectively.

2.1 Zn electrodes

Zinc is abundant in the Earth's crust, and simultaneously zinc is fully recoverable. Furthermore, it is cheaper and more environmentally friendly than other metal materials, making it an excellent anode material for ZABs. However, zinc reacts violently in acidic electrolyte, leading to corrosion, and thus alkaline electrolyte is generally employed in ZABs. During the discharge process, Zn loses electrons and is converted to Zn²⁺. The O₂ at the cathode receives electrons from the anode to undergo ORR to obtain OH⁻, subsequently forming Zn(OH)₄²⁻ with the Zn²⁺ produced at the anode, and when the Zn(OH)₄²⁻ deposition exceeds the saturation limit, it is further decomposed into insoluble ZnO and deposited on the surface of the anode.⁵⁷ The continuous accumulation of ZnO in the insoluble insulating layer on the zinc electrode produces a passivation layer, which blocks the pores and active sites, leading to an increase in internal resistance and a gradual decrease in conductivity, discharge capacity, and power capacity. Ultimately, this blocks the entry of ions, further affecting the utilization and reversibility of zinc and adversely affecting the overall reaction of the cell.⁵⁸ Thus, to address the problem of passivation, many reports employed different approaches, such as increasing the concentration of zinc oxide, causing it to be greater than a critical value, accelerating the dissolution of the metal to avoid passivation;⁵⁹ and adding surfactants to the electrolyte to reduce the surface tension and the surface free energy, eliminating the interface between the two phases and loosening the passivation layer.⁶⁰ In addition, during the charging process, ZnO gains electrons and undergoes a reduction reaction to be converted into Zn, but the deposition process of Zn²⁺ into Zn is not uniform due to the uneven flow of current in the cell during this process, with further problems such as the growth of dendrites (dendrites: the part of the needle protrusion that the deposits gather together and the deposits gather together to form a small, hard tree structure). Clade crystals and dendrites can penetrate the diaphragm inside the battery and cause a short circuit. There are many reports on alleviating dendrites, such as by optimizing the electric field distribution on the electrode surface, resulting in the uniform deposition of zinc.⁵⁹ Also, the addition of additives to the electrolyte reduces the rate of dendrite formation.⁶⁰ In addition, in the electrolyte on the anode surface, the OH⁻ generated in the electrolyte through ionization of water will compete with ZnO for electrons, and then H₂ is generated through a reduction reaction, and this process is known as the hydrogen evolution reaction (HER), i.e., Zn + H₂O → ZnO + H₂.⁶¹ Given that it competes with the anode for electrons, it can impact the charging performance of the battery, greatly reducing the charging efficiency. At the same time, it can also cause corrosion of the zinc anode, and hydrogen generation increases the risk of the expansion or even explosion in ZABs. In this case, the concentration of hydrogen ions is adjusted, namely changing the electrode potential of hydrogen ions;⁵⁹ reducing their ability to grab electrons, namely appropriately increasing the pH;⁶⁰ and adding corrosion inhibitors and other methods to inhibit or slow down the occurrence of HER.⁶² The above-mentioned four issues (passivation of zinc during discharge, formation of dendrites during charging, and hydrogen evolution during charging) limit the development of ZABs.

2.2 Air electrodes

The air electrode, as the cathode of the secondary ZABs, is accompanied by OER and ORR during the charging and discharging operation of secondary ZABs, respectively, where their reaction rate directly determines the performance of the battery.

The ORR is divided into two-electron and four-electron paths. The two-electron path is represented by the following equation:

O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻ (1)

HO₂⁻ + H₂O + 2e⁻ → 3OH⁻ (2)

According to these equations, the two-electron path involves the conversion of O₂ into intermediate HO₂⁻ firstly, which later produces OH⁻. Due to the corrosive nature of peroxides, they can harm the catalyst, leading to a decrease in the performance of the oxygen electrocatalyst and ZABs.⁶³ Therefore, the four-electron path is the ideal ORR path. The equations for the four-electron path are as follows:

* + O₂ + H₂O + e⁻ → OOH* + OH⁻ (3)

OOH* + e⁻ → O* + OH⁻ (4)

O* + H₂O + e⁻ → OH* + OH⁻ (5)

OH* + e⁻ → * + OH⁻ (6)

Overall reaction:

O₂ + H₂O + 4e⁻ → 4OH⁻ (7)

where * denotes the catalyst surface and O*, OH*, and OOH* denote the oxygen intermediates produced when oxygen binds to the active site. The adsorption and dissociation of these intermediates are effective for the ORR and OER performances. Given that the reduction potential of the four-electron path (1.23 V) is higher than that of the two-electron path (0.695 V), ORR is more prone to undergo the two-electron path when the catalytic efficiency of the cell is low. In the case of ZABs, the most desirable reaction is still the four-electron path, given that it avoids cell corrosion while improving the cell performance with efficient use of the ORR reaction.

During charging, OER is the opposite reaction ORR and is the key reaction for rechargeable ZABs. The reaction equation for OER is expressed as follows:

* + OH⁻ → OH* + e⁻ (8)

OH* + OH⁻ → O* + H₂O + e⁻ (9)

O* + OH⁻ → OOO* + e⁻ (10)

OOH* + OH⁻ → * + O₂ + H₂O + e⁻ (11)

Overall reaction:

4OH⁻ → O₂ + 2H₂O + 4e⁻ (12)

According to these equations, OER is a reaction in which OH⁻ is converted to oxygen and water by transferring four electrons in sequence through four steps. In the standard case, the thermodynamic equilibrium potential of OER is 1.23 V. However, in practice, due to the hindrance of some kinetic processes, the working potential of the actual reaction often needs to be higher than the value of the equilibrium potential, and the voltage exceeding the theoretical value is called overpotential. Therefore, efficient redox electrocatalysts with optimal binding energy are needed to reduce the overpotential.

2.3 Overall reactions

ZABs are chemical power sources with oxygen in the air as the positive active substance and zinc metal as the negative active substance, with the following reaction formula when discharged at the cathode:

O₂ + 4e⁻ + 2H₂O → 4OH⁻ (13)

The reactions at the anode are as follows:

Zn + 4OH⁻ → Zn(OH)₄²⁻ + 2e⁻ (14)

Zn(OH)₄²⁻ → ZnO + H₂O + 2OH⁻ (15)

The overall reaction is:

2Zn + O₂ → 2ZnO (16)

ZABs work on the principle that Zn on the anode is oxidized by oxygen to ZnO and releases electrons, and the oxygen on the cathode gains the electrons and is reduced to OH⁻. This process produces a potential difference between the cathode and anode, generating a current to realize charging and discharging.

2.4 OER/ORR evaluation parameters

In OER and ORR tests, a three-electrode system is employed, which is comprised of a working electrode, reference electrode, and counter electrode.⁶⁴ Firstly, we introduce the working electrode, which is usually a glass carbon electrode because of its good electrical conductivity, easy cleaning, strong durability, and good stability. The second is the reference electrode, which is usually Ag/AgCl, calomel, and mercury oxide electrodes under acidic, neutral, and alkaline conditions, respectively. Finally, as the counter electrode, a graphite rod is usually preferred, and Pt filament or Pt sheet can be selected. However, because Pt will dissolve in the electrolytic cell for a long time, the dissolved Pt ions will be deposited on the working electrode, which will have a certain impact on the test results. Therefore, graphite rods are usually used as the counter electrode.

To reasonably evaluate the activity of electrocatalysts and obtain information about the reaction mechanism, it is necessary to determine the evaluation indexes of the electrocatalytic kinetics. The main kinetic indicators currently in use include are presented below.

2.4.1 OER overpotential. The overpotential can directly reflect the catalytic activity of OER catalysts, which is the difference between the actual potential and the theoretical potential when a certain current density is reached during the catalytic reaction.⁶⁵ The overpotential can be calculated using the following equation:

E_j = E_i (actual potential) − E_t (theoretical potential) (17)

According to this equation, it can be seen that the overpotential cannot be zero, but theoretically, the smaller the overpotential, the better the performance of the catalyst, the lower the actual voltage required to reach the related current density, the relatively smaller the energy consumption, and the higher the catalytic activity. However, it has to be compared at the same current density or the same potential, otherwise the comparison will be meaningless. The performance of catalysts is generally compared using a current density of 10 mA cm⁻² as the benchmark, and the corresponding overpotential is noted as E_j=10.

2.4.2 Onset potential. The onset potential is a key parameter for evaluating the catalytic activity of ORR. The starting potential is the equilibrium potential reached by an electrode in contact with a solution in the absence of any external interference. The potential at a current density of 0.1 mA cm⁻² or the potential corresponding to 5% of the limiting current is usually determined as the starting potential.⁶⁶ The onset potential is affected by various factors, such as pH, temperature, and ion concentration. Among them, one of the most important factors is pH. When the pH changes, the nature of the oxide or reductant film on the metal surface also changes, resulting in a shift in the onset potential.

2.4.3 Half-wave potential. The half-wave potential is another key parameter reflecting the catalytic activity of ORR, which is expressed as E_1/2.⁶⁷E_1/2 refers to the value of the potential corresponding to more than half of the limiting current in ORR. The factors that affect the half-wave potential are the type of electrolyte and its concentration, temperature, etc. When the electrolyte concentration and testing temperature remain constant, the value of the half-wave potential is also unchanged.

2.4.4 Limiting current density. The limiting current density is the maximum current density of the electrode reaction,⁶⁸ which is mainly affected by the pH value, gas concentration, and temperature, and all are proportional to it. Additionally, is an essential parameter to characterize the reaction speed of the electrode.

2.4.5 Potential gap. The potential gap is a key parameter reflecting the catalytic activity of a catalyst and is expressed as ΔE.⁶⁹ It is the potential difference between the OER potential at a current density of 10 mA cm⁻² and the half-wave potential of ORR. The smaller the potential gap, the better the bifunctional catalytic performance.

2.4.6 Tafel equation and Tafel slope. The Tafel equation is an essential equation in electrochemical kinetics that relates the electrochemical current density to the overpotential. Tafel analysis is a powerful tool for evaluating or discussing the steps involved in the determination of the electrocatalytic rate. Tafel analysis enables the analysis of the Tafel slope, from which the relationship between current density and overpotential can be known further to understand the rate and mechanism of the reaction. The relationship between the Tafel slope and overpotential can be expressed mathematically by the Tafel equation as follows:

η = a + b log j (18)

where η and j are absolute values, η is the overpotential, and j is the current density. Also, a represents the overpotential value at a unit value of current density (1A cm⁻²) and b is called the Tafel slope.⁷⁰ Depending on the magnitude of the a-value, it is possible to compare the ease of performing the electron transfer step in different electrode systems. The other essential parameters that can be obtained based on the magnitude of b include the electron mobility number, charge mobility coefficient, and reaction rate. When the Tafel slope (b) is positive, it indicates that the electrochemical reaction controls the electrode reaction rate. When the Tafel slope (b) is negative, it indicates that the electrochemical reaction controls the diffusion rate of the electrolyte. The smaller the Tafel slope, the better the kinetic performance of the response, the smaller the overpotential in the catalytic process, and the more desirable the electrocatalytic activity.

2.4.7 Real surface area. The real surface area is the surface area with catalytic sites. There are two methods to calculate the catalyst surface area, namely the BET test and calculation of the electrochemical active surface area (ECSA).⁷¹ The BET test is a gas adsorption method that uses the adsorption of gas on the material surface to evaluate its specific surface area and pore structure. However, not all the electrochemical active sites measured by this method are catalytically active. In practical applications, BET testing is usually used with other characterization methods to allow a more comprehensive evaluation of the material properties. In addition, the ECSA refers to the electrochemically active area, which is the effective activated surface area involved in the electrochemical reaction. The electrochemical active area of the catalyst is calculated based on the following equation:

ECSA = C_dl/C_s (19)

where C_dl is the bilayer capacitance measured electrochemically and C_s is the capacitance in the ideal plane of the catalyst. The measurement of the electrochemical surface area can give information and signals, namely, the changing trend of the catalyst activity, the reaction rate of the electrode surface, and the capacity of the electrochemical reaction, which are the main parameters affecting the electrochemical reaction.

2.4.8 Electron transfer number and HO₂⁻ percentage. The electron transfer number of ORR can be expressed by the Koutecky–Levich equation (K–L equation for short) as follows:

1/j = 1/j_L + 1/j_K = 1/Bω^1/2 + 1/j_K (20)

where j is the experimentally measured electrode current; j_L and j_K are the limiting and kinetic current densities, respectively; B is regarded as a constant and written as 0.620 nFC₀D₀^2/3ν^−1/6; ω is the disk electrode rotation rate; n is the electron transfer number in ORR; F is the Faraday constant (96 485 C mol⁻¹); C₀ is the concentration of O₂ in solution; D₀ is the diffusivity of oxygen molecules; and ν is the kinetic viscosity of the solution.⁷² Therefore, based on the experimental data, at the same voltage, different rotational speeds and their corresponding current densities are plotted, yielding the scatterplot of 1/j and 1/ω^1/2. These points are all on a straight line with a slope of 1/K, and the electron transfer number, n, can be determined. The ORR test set is generally RRED. The yield of HO₂⁻ can be determined using the following equation:

%(HO₂⁻) = 200I_R/N/(I_D + I_R/N) (21)

where I_D is the disk current, I_R is the ring current, and N is the collection efficiency of RRED.

2.4.9 Turnover frequency. The turnover frequency (TOF) is used to characterize the intrinsic activity of the active sites of different electrocatalysts,⁷³ which is the number of molecules of H₂ or O₂ generated per unit of time at one catalyst active site, with units of 1/h. In practice, it is impossible to calculate the number of active sites involved in the reaction because the active sites cannot come into contact with the reactants when the catalyst is covered. Thus, practical methods for accurately determining the total active sites on the surface of electrocatalysts still need to be discovered. Most research calculations assume that all the active sites can contact the reactants and the TOF calculated accordingly. The calculated values will be smaller than the actual values, but they can still be compared.

2.5 ZAB evaluation parameters

Many studies have applied catalysts to ZABs to test their performance in practical applications. Here, we present a few of the key performance parameters of ZABs.

2.5.1 Open circuit voltage. The open circuit voltage (OCV) is a characteristic parameter of battery systems. The terminal voltage of the battery is in the open circuit state, namely the difference between its positive and negative electrode potentials when disconnected. The OCV is calculated using the following equation:

OCV = E_c − E_P (22)

where E_c and E_p are the positive and negative electrode potentials of the battery, respectively. The main factors affecting the OCV are the nature of the system constituting the poles of the battery, the material nature, the electrolyte composition and concentration, and the temperature.

2.5.2 Charge–discharge performance. The charging and discharging performance of a battery is a capacity limit based on comprehensive physical and chemical considerations of safety performance. This depends on the type of material, composition ratio, structural design, the battery itself, etc. The charge–discharge curve represents the relationship between the charge/discharge current and voltage of the battery. Representing the battery material charging and discharging behavior, the analysis of the battery charging and discharging curves is of great significance in understanding the properties and electrochemical behavior of materials, especially for the analysis of half-cell charging and discharging curves, which can be targeted to analyze the behavior of the characteristics of a specific type of material.

2.5.3 Peak power density. The power of the battery is obtained from the product of voltage and current. In the I–V curve of the battery, the voltage value at each point is multiplied by the corresponding current density value to obtain the power density curve. The maximum value in the peak power density curve is the fitted power density, where a higher peak power density indicates more efficient battery operation and less energy consumption.

2.5.4 Specific capacity. The specific capacity is one of the leading performance indicators of a battery. There are two types of specific capacity, i.e., weight-specific capacity and volume-specific capacity. Zinc–air batteries are measured using weight-specific capacity, which is the amount of electricity that can be discharged per unit weight of battery or active substance. The formula for calculating the weight-specific capacity of a battery is as follows:

Specific capacity = current density × service hours/weight of consumed zinc (23)

Generally, the higher the specific capacity, and the more stable the voltage and the better the discharge performance of the battery. Conversely, the lower the particular capacity and the greater the voltage fluctuation, the worse the discharge performance of the battery. The specific capacity of a battery is usually affected by its structure, electrolyte, electrodes, temperature, and other factors. Thus, to improve the specific capacity of the battery, these factors need to be fully considered, and the battery structure, electrolyte, and electrode materials need to be rationally selected to achieve a higher specific capacity.

2.5.5 Charge–discharge cycling performance. The charge–discharge cycling performance testing of batteries is an essential test method. Charge–discharge cycling simulates the actual use of the battery by repeatedly charging and discharging it to test its performance, life, and stability to optimize its performance.

The cycling performance of a battery is measured by three leading indicators, i.e., number of cycles, first discharge capacity, and retained capacity. The cycle number is the number of times the battery has cycled through multiple charge–discharge cycles; the discharge capacity is the discharge capacity of the battery during the first charge–discharge test; and the retention capacity is the discharge capacity maintained by the battery after it has completed a certain number of cycles of charging and discharging. With the same number of cycles, the greater the ratio of retained capacity to first discharge capacity, the better the cycling performance of the battery.

3. Recent research progress in cobalt-containing ZIF-derived catalysts

3.1 Co-Based ZIF derivatives

Cobalt-ZIFs can be directly used as ORR and OER electrocatalysts. However, their catalytic performance is relatively poor, which is possibly due to their organic linkers blocking their active site and the poor interactions between their metal center and the ligand. Thus, to improve the catalytic performance of cobalt-ZIFs as ORR and OER electrocatalysts, many reports have studied various strategies, including pyrolysis, oxidation, sulfuration, doping, and hybridization.

(1) Pyrolysis. Cobalt-based ZIF derivatives usually require pyrolysis to fully expose their active sites. Liu et al. designed highly dispersed cobalt quantum dot-embedded (diameter of 4.57 nm) ultra-thin nitrogen-doped carbon layer (2 nm), denoted as Co@NCL.⁷⁴ It showed excellent catalytic activity against ORR and OER due to the unique structural features of the obtained cobalt with NCL catalysts, including highly dispersed cobalt quantum dots, a carbon substrate with high nitrogen doping content, and a large specific surface area. Co@NCL had an ORR half-wave potential of 0.84 V_RHE (V vs. RHE) and OER overpotential of 1.63 V_RHE at 10 mA cm⁻². Guan et al. reported the synthesis of unique cobalt/cobalt nitride nanoparticles (NPs) with a two-dimensional nanoarray-based catalyst (NC–Co/CoN_x).⁷⁵ The ORR half-wave potential and OER overpotential of NC–Co/CoN_x were determined to be 0.87 V_RHE and 289 mV_RHE, respectively. Furthermore, the covered NC–Co/CoN_x nanoarrays were used as the cathode, carbon fiber as the anode, and gel as the electrolyte to assemble coaxial fiber solid-state ZABs. These coaxial fiber solid-state ZABs exhibited a significantly enhanced volume power density and good flexibility, with broad application prospects in flexible energy storage devices. The solid-state ZABs with the NC–Co/CoN_x cathode had a peak power density of 41.5 mW cm⁻³, and the cells containing the NC–Co/CoN_x cathode operated longer than Pt/C in both flat and curved states, showing better structural deformation tolerance.

Many reports suggest that cobalt-ZIFs may produce carbon nanotubes (CNTs) during pyrolysis. Chen et al. investigated the effects of the structure of cobalt-ZIFs and the subsequent carbonization atmosphere on the production of CNTs and catalyst performance.⁷⁶ The surface of the two-dimensional nanosheet ZIF-L was covered with large amounts of CNTs, and the tip of CNTs was coated with highly dispersed cobalt nanoparticles. The abundant Co–N–C active sites and large specific surface area endowed Co–NCS@CNTs with a half-wave potential of 0.86 V_RHE. Furthermore, the ZABs assembled by the catalyst had a peak power density of 90 mW cm⁻² and a specific capacity of 798 mA h g⁻¹.

Traditional heat treatment methods are used to convert cobalt-ZIFs into carbon materials, which require high temperatures, long preparation time, and a unique gas environment. Zou et al. used a laser-induced carbonization strategy (LIC) to rapidly convert cobalt-ZIFs into a conductive carbon grid structure in milliseconds (Fig. 1a–c).⁷⁷ More interestingly, LIC-ZIF-67-M10 exhibited a better bifunctional electrocatalytic performance than the carbon materials obtained by the conventional carbonization process. Specifically, LIC-ZIF-67-M10 had an ORR half-wave potential of 0.77 V_RHE and OER overpotential of 390 mV_RHE. The ZAB assembled by this catalyst exhibited a high specific capacity of 780 mA h g⁻¹ and a long cycle stability of 220 h (Fig. 1d). These excellent properties most likely originated from their high specific surface area and unique honeycomb open channel structure, which facilitated electrolyte storage and mass transfer processes.

Fig. 1 (a) Illustration of the fabrication process of LIC-ZIF-67-M10. (b) and (c) SEM images of LIC-ZIF-67-M10. (d) Galvanostat charge–discharge of LIC-ZIF-67-M10 and Pt/C + RuO₂. Copyright 2022, Elsevier. (e) Schematic demonstration of the process for the fabrication of NP-Co₃O₄/CC. (f) SEM images of NP-Co₃O₄/CC. (g) ORR and OER potential gaps of various samples. (h) Aqueous zinc–air battery voltage and power density with NP-Co₃O₄/CC and Pt/C + Ir/C as the air electrode. (i) Galvanostatic discharge–charge cycling curves at 5 mA cm⁻² for aqueous rechargeable Zn–air batteries with NP-Co₃O₄/CC and Pt/C + Ir/C catalysts as the air electrode, respectively. Copyright 2020, Elsevier. (j) Schematic representation of the formation of Co/Co₉S₈-NCL. (k) SEM images of Co/Co₉S₈-NCL. (l) LSV curves of Co/Co₉S₈-NCL, Co–N–C, and Pt/C. Copyright 2022, Elsevier. (m) Synthesis route of Mn/Co–N–C-0.02–800. (n) Photograph of an LED lit by a series connection of three all-solid-state ZABs. (o) Galvanostatic discharge–charge cycling profiles of Mn/Co–N–C-0.02–800 all-solid-state ZAB at 2 mA cm⁻² with a per cycle period of 10 min. Copyright 2019, the American Chemical Society.

The use of cobalt ZIFs as precursors for pyrolysis may produce single atoms of cobalt. Sun et al. reported the use of cobalt-ZIFs to form nitrogen-doped carbon nano/microtubes confined to metallic cobalt nanoparticles and single atoms (Co@NCNT/Co-SA@NCMT).⁷⁸ The NCMT was only 1.6 μm, while ordinary carbon nanotubes were 200 nm. These microtubules facilitated the better diffusion of oxygen molecules to the active site, improved the local concentration, and promoted the catalytic performance at a high current density. In addition to microtubule production, cobalt single atoms were found in the final product of carbonization. Therefore, the final catalyst named Co@NCNT/Co-SA@NCMT exhibited an excellent ORR catalytic performance (E_1/2 = 0.87 V_RHE) and OER catalytic performance (E_j=10 = 1.6 V_RHE). The ZABs assembled by this catalyst had a peak power density of 155 mW cm⁻² and a specific capacity of 846 mA h g⁻¹. Furthermore, they showed a stable voltage gap during charging and discharging for 300 h at a current density of 5 mA cm⁻².

(2) Oxidation. In recent years, an increasing number of reports has studied the role of cobalt oxides in ORR and OER. Zhang et al. designed a high-performance ORR/OER electrocatalyst with a pore structure combining Co–N_x and cobalt oxide using an in situ sodium borohydride reduction strategy (Co₃O₄–Co@NC-2).⁷⁹ The in situ sodium borohydride reduction strategy employed in this study solved the problem of metal agglomeration in ZIF-67 during pyrolysis at high temperatures. In addition, the XPS and BET characterization results showed that Co₃O₄–Co@NC-2 had the highest proportion of pyridine-N and the largest specific surface area. This finding suggests that the in situ sodium borohydride reduction strategy can effectively increase the active site and specific surface area of ZIF-67-derived catalysts. Thus, Co₃O₄–Co@NC-2 had a half-wave potential of 0.86 V_RHE, the potential difference between the OER and ORR was 0.72 V_RHE, and the liquid ZABs based on Co₃O₄–Co@NC-2 had a peak power density of 158 mW cm⁻², a high specific capacity of 758 mA h g⁻¹. In addition, constant current charging and discharge were performed for 200 h at a current density of 10 mA cm⁻². Wang et al. reported the synthesis of ultrafine nitrogen-doped cobalt oxide nanoparticles on carbon cloth (NP-Co₃O₄/CC) via a mild oxidation method (Fig. 1e and f).⁸⁰ Owing to its abundant accessible surface active sites and pores as well as optimized oxygen adsorption, NP-Co₃O₄/CC delivered a particularly low potential gap of 0.66 V_RHE (Fig. 1g). As a difunctional oxygen electrode for rechargeable liquid zinc–air batteries and all-solid-state flexible ZABs, the ultra-high power density of the NP-Co₃O₄/CC-based batteries reached up to 200 mW cm⁻² and 99.8 mW cm⁻³, respectively (Fig. 1h). In addition, the liquid ZABs had a long cycle life of 400 h, showing excellent battery stability (Fig. 1i).

(3) Sulfuration. In addition to using cobalt-ZIFs as a precursor, many reports convert cobalt-ZIFs into sulfide as efficient electrocatalysts. For instance, Huang et al. reported the simple sulfation of dithioxamide (DTO) with two-dimensional leaf-shaped ZIF-L, preparing Co/Co₉S₈ nanoparticle-decorated nitrogen and sulfur double-doped carbon materials (Co/Co₉S₈-NCL) with excellent ORR performance (Fig. 1j and k).⁸¹ The core–shell structure of this catalyst could shorten the three-phase interface reaction pathway, expose more active sites, and solve the problem of possible structure collapse of ZIF-L during carbonization, thus improving the catalytic performance of ORR. The Co/Co₉S₈-NCL had an ORR half-wave potential of 0.852 V_RHE, and the ZAB assembled by this catalyst has a high power density of 112 mW cm⁻² and high specific capacity of 799 mA h g⁻¹ (Fig. 1l).

(4) Doping. Pyrolysis of cobalt-ZIFs usually causes problems such as the uneven decomposition and severe accumulation of metals, resulting in insufficient electrocatalytic activity of the final compound. Many reports have addressed this issue by tuning the composition and microstructure of cobalt-ZIF precursors. For example, doping cobalt-ZIFs with other metal ions can provide a practical approach for synthesizing high-performance electrocatalysts. Pendashteh et al. reported the preparation of an efficient bifunctional ZIF-9_Fe3_Pyrol electrocatalyst by adjusting the iron doping and calcination temperature with iron-doped cobalt-ZIF as the precursor.⁸² In this catalyst, high-speed doping could be achieved without changing its crystal structure, and iron doping could significantly affect its electrocatalytic activity by increasing the number of transferred electrons. ZIF-9_Fe3_Pyrol exhibited an ORR start potential of 1.55V at 10 mA cm⁻² and ORR half-wave potential of 0.81 V_RHE. In addition, the ZAB assembled by the catalyst showed a specific capacity of 815 mA h g⁻¹ and good stability. Similarly, Wei et al. reported a simple dipping solution method to prepare Mn/Co–NC-0.02–800 with a stable dodecahedral morphology using manganese-doped cobalt-ZIF as the pyrolysis precursor (Fig. 1m).⁸³ Due to the enhanced conductivity and activity of the catalyst after Mn doping, it exhibited good electrocatalytic activity for ORR/OER, with an ORR half-wave potential of 0.80 V_RHE and OER overpotential of 1.66 V_RHE. The battery assembled with this catalyst was cycled for 250 h at the current density of 5 mA cm⁻². The peak power density was 136 mW cm⁻², which also could power a light-emitting diode (LED) viewing screen with three batteries in series (Fig. 1n). After charging and discharging for 60 cycles at the current density of 2 mA cm⁻², the increase in the charge and discharge potential difference was negligible (Fig. 1o).

(5) Hybridization. In addition to separate pyrolysis, oxidation, sulfation, and doping, many reports use hybridization to design catalysts. Cobalt-based ZIF catalysts with multiple material advantages can be obtained through the hybridization of different substances with cobalt ZIF derivatives.

Many reports have hybridized cobalt ZIFs with carbon materials and carbon sources (CNTs,⁸⁴ supramolecular gels (SMG),⁸⁵ benzimidazole (BIm),⁸⁶ α-amylase,⁸⁷ polyaniline (PANI),⁸⁸ Pluronic F127,⁸⁹ glucose,⁹⁰etc.) to improve their conductivity, structural stability, and adsorption capacity for oxygen and water. The carbonization of cobalt-ZIF materials often results in poor electrical conductivity, while carbon nanotubes are carbon materials with good electrical conductivity. Thus, many reports have structured ZIF materials with carbon nanotubes to improve their electrical conductivity and electrocatalytic properties. The traditional method usually involves simply mixing cobalt-ZIFs with CNTs, resulting in their uneven and insufficient combination. Lv et al. used a simple solvent thermal method to synthesize cobalt-ZIFs and CNT suspensions into flexible Co–N–C/CNT films (Fig. 2a and b).⁸⁴ This method of film preparation avoided the use of adhesives and prevented the stripping of the catalyst particles. The ORR half-wave potential of the catalyst was 0.87 V_RHE (Fig. 2c). The flexible ZAB assembled with this flexible Co–N–C/CNT film showed a stable platform discharge at 1.15V and charge at 1.97 V under a current density of 1 mA cm⁻², respectively.

Fig. 2 (a) Schematic showing the steps for the preparation of Co–N–C/CNT films. (b) Photograph of Co–N–C/CNT film. (c) ORR LSV curves of Co–N–C/CNT films. Copyright 2019, Elsevier. (d) Schematic illustration of the formation mechanism for the decussation-shaped D-Co@NC nanostructure. (e) SEM image of D-Co@NC. (f) ORR LSV curves of D-Co@NC. Copyright 2022, Elsevier. (g) Schematic diagram of Co–O–ZIF/PANI synthesis path. (h) SEM image of Co–O–ZIF/PANI. (i) Charging and discharging curves. (j) Photograph of a red LED powered by two liquid ZABs in series. Copyright 2022, Elsevier. (k) Scheme illustration for synthesizing CoS₂@MoS₂@NiS₂ double-shelled polyhedron. TEM image (l) and HRTEM image (m) of CoS₂@MoS₂@NiS₂. (n) OER LSV curves. Copyright 2022 Elsevier.

Zhang et al. designed a simple enzyme-assisted synthesis strategy to convert Co-ZIF precursors into structurally stable crossover shapes, obtaining Co nanoislands grafted on decussate N-doped carbon nanoleaves (D-Co@NC) (Fig. 2d and e).⁸⁷ The structural mediator of α-amylase plays the key role in forming reciprocal crossover frames by coordinating competition with organic ligands and regulating the nucleation and growth process of the ZIF precursors. Due to its powerful interpenetration structure and unique composition, D-Co@NC showed stronger catalytic activity in ORR and OER compared to catalysts derived from ZIFs and conventional three-dimensional ZIFs, and its ORR half-wave potential was E_1/2 = 0.852 V_RHE (Fig. 2f). In the ZAB, the peak power density was 115.4 mW cm⁻². The energy density was 879.6 W h kg⁻¹ and the long cycle stability exceeded 200 h, which was better than that of the benchmark catalyst of Pt/C + RuO₂. Lei et al. reported the combination of PANI with a cobalt-ZIF as a bifunctional electrocatalyst (Co–O–ZIF/PANI) (Fig. 2g and h).⁸⁸ This Co–O–ZIF/PANI exhibited excellent surface adsorption capacity and a suitable Co³⁺/Co²⁺ ratio, which facilitated the progress of the electrocatalytic reaction. In situ Raman spectroscopy showed that the surface chemisorption properties of O₂ and H₂O brought about by the Co–O bond and PANI also enhanced the catalytic properties. The half-wave potential of Co–O–ZIF/PANI was 0.7 V_RHE. The voltage gap of ZABs based on Co–O–ZIF/PANI had no significant change after 300 cycles (100 h) at a current density of 5 mA cm⁻² (Fig. 2i). As a demonstration of practical applications, two ZABs connected in series could illuminate a red LED with a stable working status, confirming the favorable application prospects of Co–O–ZIF/PANI-based ZABs (Fig. 2j).

Many reports have also hybridized cobalt ZIFs with metal alloys (NiFe alloy,⁹¹ CoFe alloy,^92,93 CuCo alloy,⁹⁴etc.). Metal alloys are one of the recent research hotspots of catalytic materials. Compared with single metal–NC, metal alloys have better bifunctional electrochemical properties due to the synergistic interaction of different metal sites. Therefore, many reports used doped metal alloys to obtain catalysts with polymetallic–NC active sites. For instance, Zhu et al. reported the successful synthesis a bifunctional electrocatalyst (NiFe–Co@NC-450) composed of NiFe alloy by simple solution immersion and pyrolysis using ZIF-67 as the template.⁹¹ This study explored the activity of the catalyst by changing the metal composition and content, and concluded that the samples with polymetallic and higher metal content had better electrocatalytic properties. NiFe–Co@NC-450 exhibited an ORR half-wave potential of 0.833 V_RHE, and the potential difference between OER and ORR was 0.857 V_RHE. The liquid ZABs had a high specific capacity of 798 mA h g⁻¹ and a high power density of 100 mW cm⁻² over 200 h at a current density of 10 mA cm⁻². Du et al. developed a nitrogen-doped carbon material (CoFe/NC) of composed CoFe alloy using a simple pyrolysis method.⁹² CoFe/NC had a high surface area, nitrogen content, and well-dispersed CoFe alloy nanoparticles, also exhibiting an ORR half-wave potential of 0.82 V_RHE. The liquid ZAB had a peak power density of 173 mW cm⁻² and a stable cycle of 230 h (about 690 cycles) at a current density of 10 mA cm⁻².

In addition, there have been many reports on the hybridization of cobalt sulfide derived from cobalt ZIFs and other metal sulfides. Liu et al. reported the preparation of CoS₂@MoS₂@NiS₂ nanopolyhedra with a double-shell structure using cobalt-ZIFs as the precursor (Fig. 2k–m).⁹⁵ The catalytic performance of CoS₂@MoS₂@NiS₂ mainly originated from its double shell hollow structure, which exposed more active sites and provided a smooth diffusion path for the rapid transport of reactants and products during the reaction. The double shell structure could also limit the electrolyte in the shell, thus providing a high driving force for the electrochemical reaction. The OER overpotential of CoS₂@MoS₂@NiS₂ was determined to be 200 mV_RHE (Fig. 2n).

In conclusion, this chapter addressed electrocatalysts derived from cobalt-based ZIFs. Among them, the NP-Co₃O₄/CC electrocatalyst exhibited the best difunctional oxygen electrocatalytic performance. NP-Co₃O₄/CC was composed of ultrafine nitrogen-doped cobalt oxide nanoparticles on carbon cloth. The relevant electrochemical properties of Co-based ZIF derivatives are shown in Table 1.

Table 1 A comparison of cobalt-based ZIF derivatives for oxygen electrocatalysis and ZABs

Electrocatalysts	ORR	OER	ΔE (VRHE)	ZABs					Ref.
	E 1/2 (VRHE)	E j=10 (VRHE)		OCV (V)	Peak power density (mW cm^−2)	Specific capacity (mA h g^−1)	Cycling stability	Electrolyte
Co@NCL	0.84	1.63	0.79	1.47	170	766.35	200 h (10 mA cm^−2)	6 M KOH	74
								−0.2 M Zn(Ac)2
NC–Co/CoNx	0.87	1.52	0.65	1.40	41.5	—	1500 min (10 mA cm^−2)	KOH–ZnO gel	75
Co-NCS@CNT	0.86	2.07	1.21	1.42	90.6	789	80 h (5 mA cm^−2)	6 M KOH	76
								−0.2 M Zn(Ac)2
LIC-ZIF-67-M10	0.77	1.49	0.72	1.36	80	780	220 h (10 mA cm^−2)	6 M KOH	77
								−0.2 M Zn(Ac)2
Co@NCNT/Co-SA@NCMT	0.87	1.29	0.65	1.4	120	846	300 h (5 mA cm^−2)	6 M KOH	78
								−0.2 M Zn(Ac)2
Co3O4–Co@NC-2	0.86	1.58	0.72	1.46	158	758	200 cycles (10 mA cm^−2)	6 M KOH	79
								−0.2 M Zn(Ac)2
NP–Co3O4/CC	0.9	1.56	0.66	1.57	200	—	400 h (5 mA cm^−2)	6 M KOH	80
								−0.2 M Zn(Ac)2
Co/Co9S8-NCL	0.852	—	—	1.48	112	799	—	6 M KOH	81
								−0.2 M Zn(Ac)2
ZIF-9_Fe3_Pyrol	0.89	1.7	0.81	1.25	32	815	15 h (2 mA cm^−2)	6 M KOH	82
								−0.02 M ZnSO4
Mn/Co–N–C-0.02-800	0.80	1.66	0.86	1.39	136	—	250 h (5 mA cm^−2)	6 M KOH	83
								−0.2 M Zn(OAc)2
Co–N–C/CNT	0.87	1.58	0.71	—	—	—	4 h (1 mA cm^−2)	polyvinyl alcohol–KOH–Zn(OAc)2 gel	84
Co@N–PCP/NB-CNF-2–800	0.85	2.22	1.37	1.47	143.8	700	110 h (10 mA cm^−2)	6 M KOH	85
								−0.2 M Zn(Ac)2
Co–N–PCD	0.886	—	—	1.41	259.4	773.53	25 h (20 mA cm^−2)	6 M KOH	86
								−0.2 M Zn(Ac)2
Co–O–ZIF/PANI	0.7	351 mV (50 mA cm^−2)	—	1.40	123.1	748.2	100 h (5 mA cm^−2)	6 M KOH	87
								−0.2 M Zn(Ac)2
D-Co@NC	0.852	1.718	0.866	1.41	115.4	745.5	200 h (10 mA cm^−2)	6 M KOH	88
								−0.2 M Zn(Ac)2
Co4N@NC-2	0.84	1.519	0.679	1.48	74.3	769.4	750 h (5 mA cm^−2)	6 M KOH	89
								−0.2 M Zn(Ac)2
Co/NGC-3	0.85	1.63	0.78	1.40	134.4	716	120 h (5 mA cm^−2)	6 M KOH	90
								−0.2 M Zn(Ac)2
NiFe–Co@NC-450	0.833	1.69	0.857	1.44	100	798	200 h (10 mA cm^−2)	6 M KOH	91
								−0.2 M Zn(Ac)2
CoFe/NC	0.82	—	—	—	173	—	230 h (10 mA cm^−2)	6 M KOH	92
								−0.2 M Zn(Ac)2
CoFe–Co@PNC	0.887	1.55	0.663	1.46	152.8	786	200 h (10 mA cm^−2)	6 M KOH	93
								−0.2 M Zn(Ac)2
CuCo@NC	0.866	1.568 (50 mA cm^−2)	—	1.23	303.7	751.4	100 h (2 mA cm^−2)	6 M KOH	94
								−0.2 M ZnCl2
CoS2@MoS2@NiS2	0.80	2.00	1.20	1.39	80.28	708	24 h (10 mA cm^−2)	6 M KOH	95
								−0.2 M Zn(Ac)2

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3.2 Co–M (M = Zn, Ni, Fe, etc.)-based ZIF derivatives

Many studies investigated the addition of other metals for the preparation of cobalt-ZIFs to improve their electrocatalytic performance. In contrast to monometallic catalysts, bimetallic catalysts usually exhibit a synergistic effect, which improves the catalytic activity by adjusting the electronic structure to optimize the adsorption and desorption of the reactants. There are many similar reports, and next, we will summarize and analyze the recent cobalt–M (M = Zn, Ni, Fe, etc.) ZIF derivatives.

3.2.1 C–Zn-based ZIF derivatives. N-Doped amorphous carbon frameworks derived from ZIF-8 have a large surface area and porosity but poor conductivity. The graphite-carbon frame encapsulated by cobalt nanoparticles (Co-NPs) of ZIF-67 has higher electrical conductivity but lower surface area. Therefore, ZIF-67 and ZIF-8 were combined as catalyst precursors. The CoZn-ZIF derivatives have abundant Co–N_x, high conductivity, and high surface area. These advantages can significantly improve the ORR and OER activity of the catalyst. However, although CoZn-ZIFs combine the advantages of both cobalt-ZIFs and zinc-ZIFs, some of their aspects can be improved. For example, their structure collapses after high temperature treatment and they possess a low-density of Co–NC active sites and poor electrocatalytic stability. Consequently, these problems severely limit the development of CoZn-ZIFs in the field of catalysis, and many reports propose novel approaches to address them.

At present, many metal sulfides derived from CoZn-ZIFs are used as oxygen electrocatalysts, but the oxygen electrocatalytic performance of zinc sulfide derived from CoZn-ZIFs still needs to be enhanced. Huang et al. reported the synthesis of cobalt nanoparticle and ZnS-decorated N,S co-doped carbon nanotubes (ZnS/Co-NSCNTs) (Fig. 3a and b).⁹⁶ According to the XPS analysis, ZnS/Co-NSCNTs contained a C–S–C bond, indicating that the production of CNTs was associated with the C–S–C bond formed under the catalysis of cobalt. In this novel composite, the N-doped porous carbon substrate of ZIF-8 derivatives stimulated the diffusion dynamics of O₂, which enabled the C–S–C bond and zinc sulfide to absorb O₂. The samples were tested for their ORR performance, and it was found that ZnS/Co-NSCNTs had the highest half-wave potential of 0.871 V_RHE, indicating its high ORR catalytic performance (Fig. 3c). The ZnS/Co-NSCNT-based ZABs had a peak power density of 182.6 mW cm⁻² and specific capacity of 755.0 mA h g⁻¹ (Fig. 3d).

Fig. 3 (a) Schematic illustration describing the synthesis of ZnS/Co-NSCNTs. (b) HRTEM image of ZnS/Co-NSCNTs. (c) ORR LSV curves of ZnS/Co-NSCNTs. (d) Typical galvanostatic discharge curves (5 mA cm⁻²) of ZnS/Co-NSCNTs. Copyright 2021, Elsevier. (e) Schematic illustration of the synthesis of Co–N–PHCNTs. (f) SEM image of Co–N–PHCNTs. (g) ORR LSV curves of Co–N–PHCNTs. Copyright 2019, Elsevier. (h) Schematic illustration of the preparation of NGPC@CoO_x. (i) SEM image of NGPC@CoO_x. (j) ORR LSV curves of NGPC@CoO_x. Copyright 2022, Elsevier. (k) Schematic illustration of the synthesis of LDH@N–CoO_x@C. ORR (l) and OER (m) LSV curves of LDH@N–CoO_x@C. Copyright 2022, Elsevier.

CoZn-ZIFs have the problem of structural collapse after high temperatures. In this case, many reports use carbon materials (carbon fiber,⁹⁷ bacterial cellulose (BC),⁹⁸ and PVP⁹⁹) to protect the structure of CoZn-ZIFs from destruction and improve their electrical conductivity and catalytic properties. Pan et al. reported the synthesis of a self-supported N-doped Co/Zn carbon nanofiber membrane (Co/Zn@NCF) with superior ORR activity and durability.⁹⁷ The cycle life of the liquid ZABs based on the Co/Zn@NCF catalyst exceeded 666 h, with a charge–discharge voltage difference of 0.64 V_RHE. The prepared flexible solid-state ZABs enabled it to charge and discharge stably at different bending angles. Due to the three-dimensional porous structure of BC and the adsorption of many hydroxyl groups on the fiber surface, it could be used as an ideal supporting material. Zhang et al. prepared a new and three-dimensional porous carbon nanofiber network rich in CoN_xC active sites, which was denoted as CoNC@N-CNF.⁹⁸ Furthermore, Guan et al. used PVP as a surfactant to synthesize cobalt and N codoped carbon nanotubes embedded in a nitrogen-doped hexagonal carbon nanosheet, denoted as Co–N–PHCNTs (Fig. 3e and f).⁹⁹ PVP could inhibit the vertical growth of CoZn-ZIFs to form a favorable two-dimensional hexahedral structure, which remained unchanged during the subsequent high-temperature pyrolysis. Co–N–PHCNTs showed good ORR activity with a high starting potential (E_onset = 0.98 V_RHE) and a better half-wave potential of Co–N–PHCNTs (E_1/2 = 0.89 V_RHE) than that of Pt/C (E_1/2 = 0.87 V_RHE) (Fig. 3g).

Many reports have hybridized CoZn-ZIFs with carbon materials (CNTs,¹⁰⁰ GO,¹⁰¹etc.) For example, Wang et al. reported the synthesis of Co₃O₄ nanoparticle-embedded carbon nanotubes (CNT–Co₃O₄/NC) using CoZn-ZIF as the precursor.¹⁰⁰ The cobalt and Co₃O₄ nanoparticles played an essential role in the ORR performance. In addition, the graphite carbon on the catalyst provided good conductivity and improved the internal charge conductivity, while the carbon nanotubes grafted on the carbon frame improved the external conductivity. Therefore, CNT–Co₃O₄/NC had an excellent ORR catalytic performance with an ORR half-wave potential of −0.124 V (vs. Ag/AgCl). Zhao et al. reported the preparation of CoO/Co₃O₄ nanoparticles by low-temperature oxidation evenly distributed in three-dimensional layered nitrogen-doped graphene mesh (NGPC@CoO_x) (Fig. 3h and i).¹⁰¹ The results demonstrated that CoZn-ZIFs could serve as a protective layer to prevent the severe accumulation of reduced graphene oxide (rGO) and as a precursor to construct a functional nanoporous carbon layer modified with highly distributed nitrogen and CoO_x dopants. The ORR half-wave potential and OER overpotential of NGPC@CoO_x were 0.86 and 1.61 V_RHE, respectively, and the ZABs based on NGPC@CoO_x had a peak power density of 184.4 mW cm⁻² and a high specific capacity of 805.3 mA h g⁻¹ (Fig. 3j). In addition, during the charge–discharge performance test at 2 mA cm⁻², the initial charge–discharge voltage gap of the ZABs based on NGPC@CoO_x was 0.76 V, which was low after 256 charge–discharge cycles and kept at 1 V.

Some reports hybridized CoZn-ZIFs with carbon materials to prepare single-atom catalysts. Li et al. reported a simple spatial isolation strategy for fabricating atomic cobalt catalytic sites anchored on layered porous N-doped carbon (Co-SAs/N–C/rGO) as catalysts for ORR in a wide pH range.¹⁰² The resultant Co-SAs/N–C/rGO catalysts exhibited pH-universal ORR activity with half-wave potentials (E_1/2) of 0.84, 0.77, and 0.65 V_RHE in basic, acidic, and neutral media, while the half-wave potentials for commercial Pt/C were 0.85,0.79 and 0.65 V_RHE, respectively. These superior ORR catalytic activities were attributed to the combined action of the atomic active sites, larger specific surface area, and hierarchical porous structures. The OCV of liquid ZABs with Co-SAs/N–C/rGO as the cathode catalyst was 1.52 V, and the discharge capacity was 671.94 mA h g⁻¹, which were better than that of the commercial Pt/C + RuO₂ (1.49 V and 657.32 mA h g⁻¹, respectively).

As a substance with high-efficiency catalytic properties, NiFe-LDH was hybridized with CoZn-ZIF-derived oxide to prepare an efficient catalyst. For instance, Hao et al. used a simple chemical method to combine NiFe-LDH with N–CoO_x@C derived from CoZn-ZIFs to synthesize a bifunctional catalyst with a core–shell structure (LDH@N–CoO_x@C) (Fig. 3k).¹⁰³ The core–shell structures of CoO_x and NiFe-LDH increased the number of defects in the catalyst and exposed more active sites, as confirmed by the I_D/I_G value of 1.03 for LDH@N–CoO_x@C. The OER and ORR characterization of LDH@N–CoO_x@C showed that its OER starting potential and ORR half-wave potential were 273 mV_RHE and 0.838 V_RHE (Fig. 3l and m), respectively. The LDH@N–CoO_x@C-based ZAB exhibited a peak power density of 155.5 mW cm⁻² and specific capacity of 685.6 mA h g⁻¹ at 5 mA cm⁻².

3.2.1.1 ZIF-67@ZIF-8 (core–shell structure) derivatives. ZIF-67@ZIF-8 functions as a CoZn-ZIF with a particular core–shell structure and has been attracting significant attention. Precisely, the preparation of ZIF-67@ZIF-8 mainly involves two steps, where the first step is to mix Zn²⁺ and 2-MeIm in water or methanol solution to form ZIF-8, and the second step involves wrapping ZIF-67 formed by Co²⁺ and 2-MeIm on the surface of ZIF-8. Compared to traditional CoZn-ZIF derivatives, ZIF-8@ZIF-67 derivatives have a particular core–shell structure. The core–shell structure has the following advantages: (1) the hollow structure of the ZIF-8 core can provide channels for the material transport required for the catalytic reaction, and its outer layer is ZIF-67-derived Co@NC, which can give a rich active site for the catalytic reaction. (2) Because Zn is inside the material, Zn evaporation from inside to outside in the pyrolysis process is more conducive to the formation of vacancies and defects, better exposing the active site of the material and improving its electrocatalytic performance. (3) The interacting forces between the core and shell components allow modifications to the chemical/electronic configurations of the catalytic sites, which are critical for optimizing the reaction intermediate bindings and improving the intrinsic activity. (4) Core–shell nanostructures with unique active sites are expected to synergistically address multiple fundamental steps in water oxidation reactions.

Currently, there are many reports on ZIF-67@ZIF-8-derived catalysts, but there needs to be more research on the impact of the incorporation of Co²⁺ on the overall electrochemical performance. Zhu et al. explored the effect of the Co²⁺ content on the overall catalytic performance by adjusting the amount of Co²⁺ incorporated in the catalyst.¹⁰⁴ This study showed that the incorporation of an appropriate content of Co²⁺ allowed the catalyst to obtain more carbon defects and cobalt–N_x, which could achieve an enhanced ORR catalytic performance. Among the samples, Co–N/PCNs-2 had the best ORR activity, with the highest half-wave potential of 0.85 V_RHE. The open-circuit voltage of the ZAB based on Co–N/PCNs-2 was 1.49 V_RHE and its maximum power density was 135 mW cm⁻², while there was almost no voltage loss during 7 h discharge at the current density of 20 mA cm⁻².

Simple pyrolysis is usually required to expose the active sites in the ZIF-67@ZIF-8 material. For instance, Li et al. reported the preparation of N-CNT hollow polyhedra using simple epitaxial growth and pyrolysis methods.¹⁰⁵ The activity of Co@N-CNT-HC mainly originated from the efficient integration of 0D cobalt nanoparticles, 2D carbon nanotubes, and 3D hollow nanocarbons, which synergistically enhanced the interfacial reaction dynamics of oxygen and accelerated the charge transfer. The ORR half-wave potential of Co@N-CNT-HC was 0.84 V_RHE. Similarly, Zhang et al. reported the simple one-step pyrolysis of ZIF-8@ZIF-67 to prepare a porous carbon material codoped with cobalt and nitrogen (Co–N@CNT-C₈₀₀) (Fig. 4a and b).¹⁰⁶ It was comprised of abundant cobalt and N codoped carbon nanotubes, with a large surface area (428 m² g⁻¹). The unique three-dimensional shape of Co–N@CNT-C₈₀₀ guaranteed a particular surface area and higher porosity, facilitating mass and electron transfer for ORR. Consequently, Co–N@CNT-C₈₀₀ exhibited a high ORR half-wave potential (0.841 V_RHE) (Fig. 4c).

Fig. 4 (a) Schematic illustration of the preparation of Co–N@CNT-C₈₀₀. (b) SEM image of Co–N@CNT-C₈₀₀. (c) ORR LSV curves. Copyright 2023, Elsevier. (d) Schematic representation of the formation of Co, N-PCL. (e) and (f) SEM images of Co, N-PCL. (g) ORR LSV curves. Copyright 2019, Elsevier. (h) Schematic illustration of the three-step synthesis of ZnCoFe–N–C. (i) XPS spectrum of Zn 2p for ZnCoFe–N–C. (j) ORR LSV curves. Copyright 2021, the American Chemical Society. (k) Schematic Illustration of the synthesis process for Co/NHCNT. (l) TEM image of Co/NHCNT-160. (m) Chronoamperometric measurement of Co/NHCNT-160 and Pt/C. Copyright 2023, Elsevier.

In addition to the traditional three-dimensional structure of ZIF-8@ZIF-67 as the precursor, there are many recent reports on the preparation of catalysts using two-dimensional ZIF-L@ZIF-67 as the precursor. For instance, Park et al. reported the preparation of a highly active ORR catalyst (Co, N-PCL) using ZIF-L@ZIF-67 as the precursor (Fig. 4d and e).¹⁰⁷ Co, N-PCL possessed many features that favor ORR, such as many carbon nanotubes, large surface area, and large pore volume (Fig. 4f). The ORR half-wave potential of Co, N-PCL was 0.846 V_RHE (Fig. 4g).

Furthermore, several studies reported the use doping to improve the catalytic performance of ZIF-8@ZIF-67 derivatives. For example, Li et al. used iron-doped CoZn-ZIFs as precursors, preparing Zn, Co, and Fe triple-doped nitrogen–carbon nanocages (ZnCoFe–N–C) by annealing at high temperature (Fig. 4h).¹⁰⁸ The XPS spectrum of ZnCoFe–N–C showed the presence of Zn in the sample, possibly because some Zn elements had formed stable Zn–N_x species before gasification. Co–N_x and Fe–N_x also existed in the sample (Fig. 4i). Due to the synergistic action of the three metals, the catalyst showed enhanced ORR catalytic activity with a half-wave potential of 0.878 V_RHE (Fig. 4j). Subsequently, ZnCoFe–N–C was assembled into liquid ZABs, which had a peak power density and specific capacity of 350.2 mW cm⁻² and 794.7 mA h g⁻¹, respectively. The ZnCoFe–N–C-based ZABs were stabilized at 1.36 V for 40 h at a current density of 2 mA cm⁻², and these results are better than that of Pt/C-based ZABs.

Transition metal sulfides exhibit excellent oxygen electrocatalytic performances. Accordingly, many reports use ZIF-8@ZIF-67 precursors to prepare metal sulfides. Peng et al. reported Co₉S₈ nanoparticles embedded in nitrogen-sulfur co-doped hollow carbon nanosheets (Co₉S₈/NSC-3) as a high-efficiency oxygen electrocatalyst, where 3 represents the mass ratio of TAA to ZIF-8@ZIF-67 precursor.¹⁰⁹ Four coexisting XPS nitrogen types were observed for Co₉S₈/NSC-3, namely pyridine N, pyrrole N, graphite N, and oxidized N, in which pyridine N commonly serves as the active catalytic site of ORR. Co₉S₈/NSC-3 had a half-wave potential of 0.82 V_RHE. Furthermore, the ZAB based on Co₉S₈/NSC-3 had a peak power density of 85 mW cm⁻² and a high specific capacity of 804 mA h g⁻¹. The ZABs based on Co₉S₈/NSC-3 also had a stable charge–discharge performance, with the voltage gap of around 0.962 V_RHE, which was more durable than the ZAB based on Pt/C + RuO₂.

To improve the electrical conductivity and electrocatalytic properties of ZIF-67@ZIF-8 derivatives, many reports hybridized them with better conductive carbon materials and carbon sources (rGO,¹¹⁰ GO,¹¹¹ PAN,¹¹² PS/PVP,¹¹³etc.) For instance, Qi et al. reported the synthesis of nitrogen-doped carbon nanotubes containing Co/Co₄N nanoparticles (Co/Co₄N@N-CNTs/rGO).¹¹⁰ The TEM images of Co/Co₄N@N-CNTs/rGO showed that its surface was very rough, and the cobalt nanoparticles were encapsulated on top of N-CNTs. N-CNTs could effectively inhibit the aggregation or shedding of cobalt nanoparticles during long-term cycling. In addition, the BET characterization indicated the presence of mesopores and micropores in the sample, which could fully expose the active sites. The ORR half-wave potential and OER overpotential of Co/Co₄N@N-CNTs/rGO were 0.85 V_RHE and 372.1 mV_RHE, respectively. The liquid ZABs based on Co/Co₄N@N-CNTs/rGO had a peak power density of 200 mW cm⁻² and specific capacity of 783 mA h g⁻¹. The ZABs based on Co/Co₄N@N-CNTs/rGO completed 144 h of stability tests and showed good stability. He et al. prepared a catalyst (Co/NHCNT-160) by using polystyrene (PS) spheres as the sacrificial template and ZIF-8@ZIF-67 and PVP as cobalt–NC sources and adhesives, (Fig. 4k and l), respectively.¹¹³ Co/NHCNT-160 possessed a unique hollow core–shell structure and abundant carbon nanotubes with excellent stability, abundant electron conduction channels, high specific surface area and three-dimensional interconnection. Consequently, Co/NHCNT-160 had a high starting potential (0.955 V_RHE), half-wave potential (0.88 V_RHE), limited current density (6.3 mA cm⁻²), and current retention rate of 99.7% after 7 h (Fig. 4m).

3.2.2 Co–Ni-based ZIF derivatives. As an efficient active site, Ni-NC has attracted significant attention. Moreover, the cooperation of cobalt and nickel nanoparticles enriches the active center and enhances the intrinsic catalytic activity, thus improving the catalytic performance. Therefore, increasing reports use CoNi-ZIFs as precursors to prepare efficient electrocatalysts.

CoNi-ZIFs usually require high-temperature pyrolysis to expose their active sites. For example, Ahmed et al. reported the preparation of an Ni-modified bimetallic cobalt nickel catalyst rich in carbon nanotubes (Ni/CNT/CoNi) via a simple pyrolysis process.¹¹⁴ The carbon nanotubes were observed in the SEM images of Ni/CNT/CoNi. In addition, the TEM characterization of the samples showed that the cobalt distribution in CNT/CoNi was uniform, and all Ni almost wholly overlapped with Co, indicating the presence of bimetallic CoNi. Ning et al. reported the preparation of porous N-doped carbon-encapsulated CoNi alloy nanoparticles (Co₁Ni₁@N–C) derived from CoNi-ZIF as efficient bifunctional oxygen electrocatalysts.¹¹⁵ This report explored the molar ratio of cobalt and nickel elements and the pyrolysis temperature. The results indicated that the optimal molar ratio of cobalt and nickel elements was 1 : 1, and the optimal pyrolysis temperature was 800 °C. The ORR half-wave potential of Co₁Ni₁@N–C was 0.82 V_RHE.

The wrapping layer of SiO₂ could effectively prevent the aggregation of metal nanoparticles and the collapse of the carbon matrix structure under high-temperature calcination conditions. Li et al. reported the use of an SiO₂-wrapped CoNi-ZIF as the precursor to obtain CoNi@NPC with a core–shell structure (Fig. 5a).¹¹⁶ According to the TEM images, they observed that CoNi alloy nanoparticles were evenly distributed in the central region of NPC. NPC built a good conductive layered shell, which is conducive to accelerating electricity/mass transfer and exposing the active sites. The binding of CoNi alloy nanoparticles with NPC could improve the catalytic activity and stability. Jiang et al. reported that the CoNi nanoalloy in N-doped porous carbon frameworks (CoNi-NCF) was obtained by the pyrolysis of SiO₂-modified CoNi-ZIF.¹¹⁷

Fig. 5 (a) Schematic illustration of the preparation of CoNi@NPC. Copyright 2022, Elsevier. (b) Illustration of the synthesis of CoFe/N-HCSs. Copyright 2021, Elsevier. (c) Schematic illustration of the preparation process of Co₃O₄/Mn₃O₄/CN_x@CNFs. ORR (d) and OER (e) LSV curves of Co₃O₄/Mn₃O₄/CN_x@CNFs. Copyright 2020, Elsevier. (f) Schematic illustration of the synthesis procedure of Ru–Cl-N SAC. (g) AC-HAADFSTEM image of Ru–Cl-N SAC. (h) ORR LSV curves of Ru–Cl-N SAC. Copyright 2021, Elsevier. (i) Specific capacity curves of Ru–Cl-N SAC. Copyright 2022, Elsevier. (j) Synthesis route of Co₉S₈/CeO₂/Co–NC. (k) SEM image of Co₉S₈/CeO₂/Co–NC. ORR (l) and OER (m) LSV curves of Co₉S₈/CeO₂/Co–NC. Copyright 2021, The Royal Society of Chemistry.

3.2.3 Co–Fe-based ZIF derivatives. As a common element with great catalytic potential, Fe is often used in various forms in many fields, where the Fe element is also employed in the field of oxygen electrocatalysis. Fe almost overlaps with Ni and Co in the volcano map, indicating its high electrocatalytic potential. Therefore, many reports combine Co and Fe to prepare CoFe-ZIFs and use novel methods to explore and improve their electrocatalytic performance.

Many reports used CoFe-ZIFs as a precursor to prepare oxides. For example, Chong et al. produced a catalyst with one- and two-dimensional structures grown on flexible carbon cloth (Co_1−xFe_xO@NC).¹¹⁸ The TEM image of Co_1−xFe_xO@NC showed the presence of two-dimensional nanosheets on one-dimensional nanowires, and this structure could increase the surface area of the catalyst and improve its catalytic performance. The unique structure of Co_1−xFe_xO@NC also resulted in excellent catalytic performances with an ORR half-wave potential and OER overpotential of 0.81 V_RHE and 260 mV_RHE, respectively.

Some reports hybridized CoFe ZIFs with carbon materials and carbon sources (rGO,¹¹⁹ hollow polypyrrole spheres (h-PPy),¹²⁰etc.) to prepare metal alloy catalysts. Liu et al. designed a reduced graphene oxide-wrapped Co_9–xFe_xS₈/Co,Fe–N–C composite (S-Co_9–xFe_xS₈@rGO) as a bifunctional oxygen electrocatalyst.¹¹⁹ This report explored the effects of one-step carbonization, simple semi-vulcanization followed by carbonization, and deep-vulcanization followed by carbonization on the catalytic performance. Li et al. designed a template-assisted method to reduce the crystal size of CoFe-ZIF via the in situ growth of hollow polypyrrole spheres (h-PPy) (Fig. 5b).¹²⁰ As a result, well-dispersed ultrafine CoFe nanoalloy was obtained with N-doped hollow carbon microspheres (CoFe/N-HCSs) as the support. Owing to its unique alloy structure with ultrafine size and the uniform dispersion of the CoFe nanoalloy on the highly conductive N-HCS support via strong interactions, the proposed CoFe/N-HCSs exhibited high catalytic activity and stability toward ORR.

3.2.4 Others. In addition to Zn, Ni, and Fe, many other metals, such as Mn,¹²¹ La,¹²² Ce,¹²³ and Ru,¹²⁴ can be combined with cobalt to prepare ZIF precursors. This chapter summarizes the CoM-ZIFs formed by co-binding with these metals and the methods employed to improve their electrocatalytic properties.

Li et al. prepared highly efficient electrocatalysts (Co₃O₄/Mn₃O₄/CN_x@CNFs) using electrospinning, pyrolysis and oxidation processes (Fig. 5c).¹²¹ The XPS characterization results of Co₃O₄/Mn₃O₄/CN_x@CNFs proved the presence of Co₃O₄, Mn₃O₄, Co–N_x and Mn–N_x. Many reports demonstrated the excellent electrocatalytic performance of Co₃O₄, Mn₃O₄, Co–N_x, and Mn–N_x. This study tested their ORR and OER, showing a high ORR half-wave potential of 0.85 V_RHE and low OER overpotential of 1.63 V_RHE (Fig. 5d and e), respectively. The potential difference between OER and ORR was only 0.78 V_RHE, demonstrating that Co₃O₄/Mn₃O₄/CN_x@CNFs had efficient ORR and OER bifunctional catalytic activity.

Zhou et al. designed a high-efficiency bifunctional oxygen electrocatalyst (La₂O₃–Co/NC) with CoLa-ZIFs as the precursor.¹²² The SEM images showed no significant change in the La₂O₃–Co/NC structure after CoLa-ZIFs/AB through pyrolysis at high temperatures. Meanwhile, the CoLa-ZIFs not attached to AB underwent significant structural changes after pyrolysis, indicating that AB benefited the structural maintenance of CoLa-ZIFs after exposure to high temperatures. La₂O₃–Co/NC had a high ORR half-wave potential of 0.86 V_RHE.

Sun et al. designed a novel two-dimensional “senbei”-like nitrogen-doped carbon nanosheet (Co₉S₈/CeO₂/Co–NC) (Fig. 5j and k).¹²³ A phase transition from three-dimensional spherical cobalt-ZIFs to two-dimensional Co/Ce-ZIFs was achieved by introducing Ce ions. Co₉S₈/CeO₂/Co–NC with a highly open two-dimensional structure and numerous distributed carbon nanotubes increased the specific surface area of the catalyst, thus providing more active sites. In addition, Co₉S₈/CeO₂/Co–NC, with a unique Co₉S₈/CeO₂ heterostructure and the synergy of the two components, showed excellent electrocatalytic performances in ORR and OER. The ORR half-wave potential and OER overpotential of Co₉S₈/CeO₂/Co–NC were 0.875 and 1.60 V_RHE (Fig. 5l and m), respectively. Furthermore, when used as a bifunctional air electrode, Co₉S₈/CeO₂/Co–NC reached a peak power density of 164.24 mW cm⁻², showing an outstanding cyclic charge–discharge stability (over 668 h) at a current density of 5 mA cm⁻².

Chen et al. designed a Cl, N-coordinated Ru single-atom multifunctional electrocatalyst (Ru–Cl-NSAC) with CoRu-ZIFs as the precursor.¹²⁴ The ORR half-wave potential of Ru–Cl-NSAC was 0.90 V_RHE. The ZABs based on Ru–Cl-NSAC exhibited a specific capacity of 804.26 mA h g⁻¹ at a current density of 10 mA cm⁻². The excellent performance of the catalyst was mainly due to the following reasons: (1) the ZIF-67-derived three-dimensional porous carbon frame facilitated the charge and mass transport between the electrocatalyst and the electrolyte. (2) The doping of different types of N atoms could regulate the electronic structure of the carbon substrate. (3) The introduction of Cl and N coordinating with Ru atoms could adjust the electronic environment of the Ru single atom and reduce the adsorption free energy of oxygen-containing intermediates, thus significantly improving the OER and ORR performances.

Briefly, this chapter summarized the synthesis and modification methods of Co–M (M = Zn, Ni, Fe, Mo, etc.) ZIF derivatives. Among them, the electrocatalysts of Ru–Cl-NSAC showed the best difunctional oxygen electrocatalytic performance. The introduction of Cl and N coordinating with Ru atoms reduced the adsorption free energy of oxygen-containing intermediates. Therefore, the Ru–Cl-NSAC has excellent oxygen electrocatalytic performance. The relevant electrochemical properties of cobalt–M-based ZIF derivatives are shown in Table 2.

Table 2 A comparison of cobalt-based bimetallic ZIF derivatives for oxygen electrocatalysis and ZABs

Electrocatalysts	ORR	OER	ΔE (VRHE)	ZABs					Ref.
	E 1/2 (VRHE)	E j=10 (VRHE)		OCV (V)	Peak power density (mW cm^−2)	Specific capacity (mA h g^−1)	Cycling stability	Electrolyte
ZnS/Co–NSCNTs	0.871	—	—	1.52	182.6	750.1	10 h (5 mA cm^−2)	6 M KOH	96
								−0.2 M Zn(OAc)2
Co/Zn@NCF	0.84	1.69	0.85	1.42	202	760	666 h (5 mA cm^−2)	Polyacrylate-KOH–ZnCl2	97
CoNC@N-CNF	0.8	1.61	0.81	1.43	308	—	63 h (5 mA cm^−2)	6 M KOH	98
Co–N–PHCNTs	0.89	1.62	0.73	1.4	125.14	—	673 h (5 mA cm^−2)	6 M KOH	99
								−0.2 M Zn(Ac)2
CNT–Co3O4/NC	−0.124 (AgCl)	0.819 (AgCl)	0.943	1.38	267	814	300 cycles (15 mA cm^−2)	6 M KOH	100
NGPC@CoOx	0.86	1.61	0.75	1.41	184.4	805.3	32 h (2 mA cm^−2)	6 M KOH	101
								−0.2 M Zn(Ac)2
Co-SAs/N–C/rGO	0.84	—	—	1.52	104.91	671.94	26.31 h (1 mA cm^−2)	6 M KOH	102
								−0.2 M Zn(OAc)2
LDH@N–CoOx@C	0.838	1.503	0.665	1.43	155.5	685.6	150 h (5 mA cm^−2)	6 M KOH	103
								−0.2 M Zn(Ac)2
Co–N/PCNs-2	0.88	—	—	1.49	135	—	7 h (20 mA cm^−2)	6 M KOH	104
								−0.2 M Zn(Ac)2
Co@N-CNT-HC	0.84								105
Co–N@CNT-C800	0.84								106
Co, N-PCL	0.83								107
ZnCoFe–N–C	0.878	—	—	1.52	350.2	794.7	10 h (5 mA cm^−2)	6 M KOH	108
Co9S8/NSC-3	0.82	1.79	0.97	1.42	85	804	140 h (10 mA cm^−2)	6 M KOH	109
								−0.2 M Zn(Ac)2
Co/Co4N@N-CNTs/rGO	—	1.6	—	1.33	200	783	440 h (5 mA cm^−2)	6 M KOH	110
Co@N-HPCF-800	0.831	—	—	1.40	136.2	151	50 h (5 mA cm^−2)	6 M KOH	111
								−0.2 M Zn(Ac)2
Co–N–C@GNP	0.86	1.64	0.78	1.60	236.2	—	91 h (5 mA cm^−2)	gel	112
Co/NHCNT-160	0.88								113
Ni/CNT/CoNi							520 h (5 mA cm^−2)	6 M KOH	114
								−0.2 M Zn(Ac)2
Co1Ni1@N–C	0.82								115
CoNi@NPC	0.77	1.72	0.95	1.54	224	—	40 h (5 mA cm^−2)	6 M KOH	116
Co1−xFexO@NC	0.81	1.49	0.68	1.30	52	673	150 h (5 mA cm^−2)	6 M KOH	118
								−0.2 M Zn(Ac)2
S-Co9−xFexS8@rGO	0.84	1.52	0.68						119
CoFe/N-HCSs	0.791	1.52	0.729	1.387	96.5	777.4	160 h (5 mA cm^−2)	6 M KOH	120
								−0.2 M Zn(Ac)2
Co3O4/Mn3O4/CNx@CNFs	0.78	1.56	0.78	1.51	265	—	50 h (5 mA cm^−2)	6 M KOH	121
La2O3–Co/NC	0.86	1.53	0.67	1.48	80.12	—	170 h (2 mA cm^−2)	6 M KOH	122
								−0.2 M Zn(Ac)2
Co9S8/CeO2/Co–NC	0.875	1.60	0.725	1.4	164.24	—	668 h (5 mA cm^−2)	6 M KOH	123
								−0.2 M Zn(Ac)2
Ru–Cl-NSAC	0.90	1.46	0.56	1.45	205	804.26	360 h (2 mA cm^−2)	6 M KOH	124
								−0.2 M Zn(Ac)2

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3.3 Co-based trimetallic ZIF derivatives

The construction of bifunctional electrocatalysts with two or more different components is essential to provide abundant OER/ORR active sites, which show better OER and ORR performances than single-component catalysts. Through the interaction of trimetallic carbon and nitrogen compounds, they can reduce the binding energy of the reactants, intermediates, and active substances. Therefore, many reports employ polymetallic ZIF derivatives as bifunctional oxygen electrocatalysts.

Many reports have demonstrated that Co–NC and Fe–NC have excellent ORR and OER catalytic properties, and thus an increasing number of studies focus on compounds containing CoFe, among which CoZnFe-ZIFs have attracted much attention as single-atom catalyst precursors. For example, Zhong et al. used hydrothermal and pyrolysis processes to prepare cobalt-iron single atoms (CoFe–NC-1) anchored to a nitrogen-doped carbon substrate (Fig. 6a and b).¹²⁵ The high I_D/I_G values in the Raman spectrum of FeCo–NC-1 indicated that this catalyst has many defects, favoring the exposure of the active sites. The BET characterization results for FeCo–NC-1 demonstrated that it possessed microporous and medium pores. The unique structure of FeCo–NC-1 also resulted in excellent catalytic properties, with an ORR half-wave potential of 0.84 V_RHE (Fig. 6c). The FeCo–NC-1-based ZABs had a peak power density of 122.0 mV cm⁻², specific capacity of 726.2 mA h g⁻¹, and good stability, which exceeded that of Pt/C-based ZABs. Similar reports on a single atoms have also been reported. Chen et al. utilized a one-step pyrolysis method, where the precursor CoZnFe-ZIF was converted to a nitrogen-doped carbon material containing Fe and Co double single-atom electrocatalysts (FeCo-IA-NC) (Fig. 6d and e).¹²⁶ The synergy between the atomic-dispersed Fe and Co in FeCo-IA-NC provided many active sites for the catalyst. Therefore, FeCo-IA-NC showed an excellent ORR performance with an ORR half-wave potential of 0.88 V_RHE, reaction electron transfer number between 3.66 and 3.95, and reactive hydrogen peroxide yield of less than 8% (Fig. 6f and g).

Fig. 6 (a) Schematic illustration of the synthetic process of FeCo–NC. (b) HAADF-STEM image of FeCo–NC. (c) ORR LSV curves of FeCo–NC. Copyright 2019, Elsevier. (d) Synthesis route of FeCo-IA/NC. (e) AC-STEM image of FeCo-IA/NC. (f) ORR LSV curves FeCo-IA/NC. (g) K–L plots at different potentials FeCo-IA/NC. Copyright 2020, The Royal Society of Chemistry. (h) Synthesis route of Co₉S₈/MnS-USNC. (i) SEM image of Co₉S₈/MnS-USNC. (j) XRD patterns of Co₉S₈/MnS-USNC. (k) Cycling test of the all-solid-state zinc–air batteries based on Co₉S₈/MnS-USNC. Copyright 2021, The Royal Society of Chemistry. (l) Schematic illustration of the synthetic strategy of CoNi/N-CNN. (m) Typical Raman spectra image of CoNi/N-CNN. (n) ORR LSV curves of CoNi/N-CNN. (o) Polarization and power density plot of CoNi/N-CNN-based zinc–air batteries. Copyright 2022, Elsevier.

In addition to preparing single-atom catalysts through one-step pyrolysis of CoZnFe-ZIFs, there are also some reports that first hybridize CoZnFe-ZIFs with carbon materials, and then perform pyrolysis to prepare single-atom catalysts. Nguyen et al. used polystyrene ball (PS)-coated polypyrrole (PPy) as the core and CoZnFe-ZIFs as the shell to obtain a hollow structure catalyst (H–FeCo–NC).¹²⁷ Given that the spherical surface of PS-PPy obtained after sodium sulfite treatment had a negative charge, it could attract Co and Zn ions, facilitating the growth of CoZn-ZIFs on its surface and avoiding the possible structural collapse of CoZn-ZIFs after pyrolysis at high temperature. The unique hollow structure of H–FeCo–NC and the synergistic interaction of the Fe single-atom and cobalt nanoparticles inferred its efficient catalytic performance. The ORR half-wave potential of H–FeCo–NC was 0.888 V_RHE.

Some reports have also hybridized CoZnFe-ZIFs with carbon materials to prepare metal oxide catalysts. Sun et al. prepared self-supporting electrodes composed of two-dimensional nanosheets (Co₃O₄/Fe₂O₃NAs@CNFs) embedded with Co₃O₄ and Fe₂O₃ composed of carbon nanofibers by electrospinning and pyrolysis.¹²⁸ The XPS characterization of Co₃O₄/Fe₂O₃NAs@CNFs illustrated the presence of Fe–N_x and Co–N_x, which were the active substances of ORR and provided the ORR catalytic performance of the catalyst. The ORR half-wave potential of Co₃O₄/Fe₂O₃NAs@CNFs was 0.80 V_RHE, and its ZABs had a peak power density of 246 mV cm⁻², exceeding that of 175 mV cm⁻² for Pt/C + RuO₂, and it also showed a better performance than Pt/C + RuO₂ in the stability test.

In addition to compounds of CoZnFe-ZIFs, many reports used CoZnMn-ZIF derivatives as bifunctional electrocatalysts. Ye et al. designed catalysts (Co/Co₂Mn₃O₈@NCFs) with a unique three-dimensional structure.¹²⁹ The Co/Co₂Mn₃O₈@NCFs had a unique three-dimensional structure, coupled with the rich active sites brought by Co₂Mn₃O₈ and Co–N_x, which would have good bifunctional catalytic activity. The study tested their ORR and OER performances, and the test results showed that their ORR half-wave potential was 0.826 V_RHE, and their OER catalytic performance was also better than that of RuO₂. Li et al. used CoZnMn-ZIFs as the precursor to prepare a two-dimensional petal-thin Co₉S₈/MnS/S/N codoped carbon nanosheet (Co₉S₈/MnS-USNC) (Fig. 6h).¹³⁰ The SEM image of Co₉S₈/MnS-USNC showed the presence of a petal-like structure and a large number of metal particles on its surface (Fig. 6i). The XPS and XRD characterization demonstrated the successful doping of S and N in the carbon substrate and the presence of two active species (Co₉S₈ and MnS) (Fig. 6j). The characterization of the ORR and OER performances of Co₉S₈/MnS-USNC showed an ORR half-wave potential of 0.90 V_RHE and OER overpotential of 360 mV_RHE. The ZABs based on Co₉S₈/MnS-USNC performed better than that based on Pt/C + Ir/C in the stability tests (Fig. 6k).

Ni–NC has also attracted significant attention as a substance with high-efficiency bifunctional electrocatalytic properties similar to that of Fe–NC and Mn–NC. Many reports used CoZnNi-ZIFs as precursors to prepare CoNi compounds as bifunctional electrocatalysts. For example, Li et al. preformed simple pyrolysis to produce a sea-like structural catalyst (CoNi/N-CNN) composed of carbon nanotubes embedded with CoNi nanoparticles (Fig. 6l).¹³¹ According to the Raman spectrum of CoNi/N-CNN, its I_D/I_G value was about 1.1, indicating a good balance between the degree of graphitization and the number of carbon defects, which could provide a lot of active sites and ensure good conductivity (Fig. 6m). The ORR and OER performance tests on CoNi/N-CNN showed that its ORR half-wave potential was 0.813 V_RHE and its OER overpotential was 1.718 V_RHE. The ZABs based on CoNi/N-CNN had a peak power density of 209 mV cm⁻² and specific capacity of 810 mA h g⁻¹ (Fig. 6n and o), respectively.

In short, this chapter summarized the methods for the synthesis and modification of cobalt-based trimetallic ZIF derivatives. The electrocatalysts of Co₉S₈/MnS-USNC have the best difunctional oxygen electrocatalytic performances. The synergistic effect of Co₉S₈ and MnS provides a good oxygen electrocatalytic performance. The relevant electrochemical properties of cobalt-based monometallic ZIF derivatives are shown in Table 3.

Table 3 A comparison of cobalt-based trimetallic ZIF derivatives for oxygen electrocatalysis and ZABs

Electrocatalysts	ORR	OER	ΔE (VRHE)	ZABs					Ref.
	E 1/2 (VRHE)	E j=10 (VRHE)		OCV (V)	Peak power density (mW cm^−2)	Specific capacity (mAh g^−1)	Cycling stability	Electrolyte
CoFe–NC-1	0.84	—	—	1.50	122.0	726.2	—	6 M KOH	125
								−0.2 M Zn(Ac)2
FeCo-IA-NC	0.88	—	—	1.472	115.6	635.3	100 cycles (10 mA cm^−2)	6 M KOH	126
								−0.2 M ZnCl2
H–FeCo–NC	0.888								127
Co3O4/Fe2O3NAs@CNFs	0.80	1.6	0.8	1.533	246	—	—	6 M KOH	128
								−0.2 M Zn(Ac)2
Co/Co2Mn3O8@NCFs	0.82	—	—	1.45	118.19	841.83	120 h (10 mA cm^−2)	6 M KOH	129
								−0.2 M Zn(Ac)2
Co9S8/MnS-USNC	0.90	1.594	0.694	1.45	146	802	600 h (5 mA cm^−2)	18 M KOH	130
								−0.02 M Zn(Ac)2
CoNi/N-CNN	0.819	1.718	0.899	1.42	209	810	90 h (2 mA cm^−2)	6 M KOH	131

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4. Summary and future perspective

In summary, the main problem associated with the current ZABs is the design of dual-functional electrocatalysts with high ORR and OER activity and cheap and safe materials that can replace the precious metal-based catalysts. ZIFs have received significant attention as a new type of crystalline porous materials with large surface area, high porosity, large pores, large volume, easy functionalization, and the production of highly conductive carbon-based electrocatalysts. In contrast to other non-noble metal ZIF derivatives, most of the cobalt-containing ZIF derivatives showed high ORR and OER activity, as described earlier herein. This review demonstrated that cobalt-containing ZIF derivatives can be used as various electrocatalytic materials to enhance ORR and OER and applied in ZABs. With the combination of different metal species, these materials were classified into cobalt-based ZIF derivatives, cobalt–M based ZIF derivatives, and cobalt-based trimetallic ZIF derivatives. This review highlighted the advantages of these cobalt-containing ZIF materials, their synthetic and modification parameters, various derivative compounds, and synergistic effects of metals. These cobalt-containing ZIF derivatives showed better ORR and OER activity and ZAB performances due to the synergistic generation of more active sites and tunable metal composition.

Despite the massive surge in the number of cobalt-containing ZIF derivatives in this area, various deficiencies still need attention.

(1) The main drawbacks in their electrochemical performance are well known: the original cobalt-ZIFs and cobalt-ZIF composites have poor electrical conductivity. A possible solution to this problem is synthesizing cobalt-containing ZIF derivatives with better electrical conductivity. In recent years, many researchers have attempted to design cobalt-containing ZIF derivatives with better electrical conductivity using the following methods: (i) combining better conductive materials (carbon nanotubes, graphene, PVP, etc.) with cobalt-containing ZIF derivatives and (ii) using the dual-ligand strategy to synthesize cobalt-containing ZIF derivatives. The typical organic ligands with good electrical conductivity are BIm, SMG, and GNP. However, most of these studies will affect the morphology and performance of cobalt-containing ZIF derivatives. Therefore, more studies are needed to improve the conductivity of cobalt-containing ZIF derivatives and make them ideal for electrocatalysis.

(2) To expose more active sites in cobalt-containing ZIF derivatives, pyrolysis is usually required to prepare cobalt-containing ZIF derivatives. This may cause problems such as the collapse of the structure of cobalt-containing ZIF derivatives and the agglomeration of metal particles. Thus, in recent years, many reports have adopted novel approaches to address these problems, as follows: (i) using a surfactant as a structure mediator (α-amylase, PVP, Pluronic F127, etc.) and organic ligand competitive coordination and regulating the nucleation and growth process of cobalt-containing ZIF precursors to construct more stable cobalt-containing ZIF derivatives. (ii) Using template-assisted strategies to synthetize, common template materials such as CNTs, PS and PAN, where these materials are wrapped on the surface of cobalt-containing ZIF derivatives to avoid their structural collapse during high-temperature pyrolysis. However, these methods usually require one-step or multi-step experimental steps, increasing the operation and time of material synthesis. In addition, these methods are followed by pyrolysis, which requires an additional energy-consuming step. Therefore, discovering other methods to expose more active sites is required.

(3) Cobalt-containing ZIF derivatives have achieved more significant development as the air cathode of ZABs. For example, cobalt-based ZIF derivatives, cobalt–M-based ZIF derivatives, and cobalt-based trimetallic ZIF derivatives with a sodalite (SOD) topology are widely used in the synthesis of cobalt-containing ZIF derivative-based air electrodes. However, the remaining topologies, such as rhodesite (RHO), Linde-type A (LTA), and gismondite (GIS), are rarely utilized. ZIFs with any topology structure have permanent porosity and high thermal and chemical stability, which is one reason why they have become attractive candidate materials for many applications, such as gas separation and storage. However, the practical application and development of ZIFs with different topologies still need to be improved. For example, LTA ZIFs with good ion exchange, adsorption, and catalytic performance are only used as water softeners in detergents and catalysts for petroleum refining. Therefore, the synthesis of cobalt-containing ZIF derivative-based air electrodes has not been fully developed, and researchers need to develop cobalt-containing ZIF derivatives with different topologies and link them to their electrocatalytic properties, design novel and efficient bifunctional electrocatalysts and apply them in ZABs.

(4) In the process of ORR and OER, it is difficult to determine the main catalytic active substances and the reactions/products during the reaction affecting these active substances. Therefore, advanced operation or in situ detection methods are required to determine the active substances, which will help researchers better understand the natural reaction mechanism of these materials and improve the catalytic performance and stability of catalysts.

Although many challenges remain, investigating the electrocatalytic applications of cobalt-containing ZIF derivatives, as well as a deeper understanding of their stability and reaction mechanism, will provide more information to guide researchers to design better cobalt-containing ZIF derivative-related electrocatalytic materials, which is expected to enable large-scale applications in industrial applications. The common methods for the industrial preparation of catalysts include solid-state method, hydrothermal method, and ball milling method. Thus, the application of laboratory-prepared cobalt-containing ZIF derivative catalysts in industry still has a long way to go.

Abbreviations

ZABs	Zinc air batteries
ZIFs	Zeolitic imidazolate frameworks
ORR	Oxygen reduction reaction
OER	Oxygen evolution reaction
MOFs	Metal–organic frameworks
SMG	Supramolecular gels
BIm	Benzimidazole
HER	Hydrogen evolution reaction
LDH	Layered double hydroxides
ECSA	Electrochemical active surface area
TOF	Turnover frequency
3D	Three-dimensional
2D	Two-dimensional
NPs	Nitride nanoparticles
CNT	Carbon nano-tube
LIC	Laser-induced carbonization
DTO	Dithiooxamide
LED	Light-emitting diode
PANI	Polyaniline
NGC	Nitrogen-doped graphite carbon
O2-TPD	Temperature-programmed desorption of oxygen
h-PPy	Hollow polypyrrole
SEM	Scanning electron microscopy
BC	Bacterial cellulose
rGO	Reduced graphene oxide
OCV	Open circuit voltage
ALD	Atomic layer deposition
EPR	Electron paramagnetic resonance
PS	Polystyrene
PPy	Polypyrrole
PVP	Polyvinyl pyrrolidone

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the PhD Research Startup Fund (BK202013) from Hubei University of Automotive Technology (HUAT), the Open Fund (QCCLSZK2021A02) from Hubei Key Laboratory of Critical Materials of New Energy Vehicles (HUAT), the Open Fund (ZDK1202201) from Hubei Key Laboratory of Automotive Power Train and Electronics (HUAT) and the Open Fund (ZDK22023A05) from Hubei Key Laboratory of Energy Storage and Power Battery (HUAT).

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