The Recent Advancement of Graphene-Based Cathode Material for Rechargeable Zinc–Air Batteries

Abstract

Graphene-based materials (GBMs) are a prospective material of choice for rechargeable battery electrodes because of their unique set of qualities, which include tunable interlayer channels, high specific surface area, and strong electrical conductivity characteristics. The market for commercial rechargeable batteries is now dominated by lithium-ion batteries (LIBs). One of the primary factors impeding the development of new energy vehicles and large-scale energy storage applications is the safety of LIBs. Zinc-based rechargeable batteries have emerged as a viable substitute for rechargeable batteries due to their affordability, safety, and improved performance. This review article explores recent developments in the synthesis and advancement of GBMs for rechargeable zinc–air batteries (ZABs) and common graphene-based electrocatalyst types. An outlook on the difficulties and probable future paths of this extremely promising field of study is provided at the end.

1. Introduction

Because of their high potential energy density and ability to use atmospheric free oxygen as fuel, metal air batteries hold great promise as a power source. The positive electrode in metal–air batteries is made of carbon and is coated in metals to react with oxygen. One or more metals, such as zinc, aluminum, magnesium, or lithium, make up the negative electrode. These batteries are classified as fuel cells because air is passing through the cathode membrane’s pores inside the cell [1,2]. Zinc–air batteries are a particularly promising type of metal air battery because of their high theoretical energy density, affordability, and built-in safety [3]. Figure 1 shows the advantages of ZABs over other types of batteries [4]. Zinc–air batteries (ZABs) have attracted interest as a viable substitute because of their attractive features, which include their remarkable theoretical energy densities of 1218 Wh kg⁻¹ (gravimetric) and 6136 Wh L⁻¹ (volumetric), their environmentally friendly ability to extract power from Zn and atmospheric oxygen, their compact form factor because of the air cathode, and their remarkably low operating cost of < $10 kW⁻¹ h⁻¹. The most significant energy source for hearing aids is primary zinc–air batteries; however, the focus of current research is on the use of electrically rechargeable zinc–air batteries [5,6]. The power density, energy efficiency, and cycle life of zinc–air batteries that are rechargeable are influenced by the electrocatalysts present in the air cathode [7]. Since oxygen is one of the active materials in the positive electrode and is derived from the air, oxygen is provided from outside the battery; as a result, the negative electrode material occupies the majority of the battery’s interior [8]. Since oxygen is a potent oxidizing agent, light in weight, and readily available, it will reach the electrolyte through the air without the help of any solid substance. For an oxygen reduction reaction (ORR) to occur, the air electrode that is produced needs to contain a lot of pores in order to facilitate the easy transport of oxygen towards the electrolyte [9,10]. For the purpose of stabilizing gas movement and preventing electrolyte invasion, the material’s surface between the pores needs to be hydrophobic. “Triple-phase boundaries” (TPBs) are the locations where ORR is finished, and they are formed by the intersection of oxygen, electrolyte, and catalyst. Accordingly, in terms of oxygen evolution reaction (OER), hydroxyl ions arrive at the two-phase zone known as the electrolyte electrode interface with the purpose of producing and releasing oxygen. A hydrophilic catalytic surface that interacts with the electrolyte is necessary to optimize and balance the three-phase and two-phase interfaces [11,12]. Because of their large surface areas; strong thermal, mechanical, and chemical durability; and high electronic conductivity, graphene materials—which are made up of individual sheets of carbon atoms—are thought to hold great promise for a variety of uses, including electrocatalysis [13,14]. The graphene structure has the strongest C–C bond in-plane, while the out-of-plane π bond contributes to a delocalized network of electrons that allows for weak interaction between graphene and catalyst or between graphene layers, which is what causes graphene to conduct electrons [15]. To increase the air cathode’s catalytic performance, the electrocatalyst’s structures must be modified [16]. Because of their intriguing qualities, graphene-based nanocomposites are thought to be the most promising electrocatalyst for use in any kind of energy storage device. This article presents a prospective approach for the production of graphene, graphene oxide, and reduced graphene oxide with an emphasis on improving graphene and graphene-based electrode materials.

2. Working Principle of Rechargeable Zinc–Air Battery

Three components make up rechargeable zinc–air batteries: an aqueous alkaline solution (electrolyte), zinc metal (anode), and an air electrode (cathode). The air electrode, which controls performance, is the most crucial and intricate component of the zinc–air battery [18]. Figure 2 illustrates how oxygen from the environment is reduced during cell discharging to release hydroxyl ions (OH⁻) into the electrolyte and how hydroxide ions are oxidized during cell charging to produce oxygen. These processes are equivalent to the well-known complex and kinetically sluggish multistep reactions known as the ORR and the OER. The basic characteristics and structure of the electrocatalyst, as well as experimental parameters like solution pH and active material concentration, all influence how these reactions work [19]. The following diagram illustrates the air cathode’s reaction mechanism as shown in Equations (1) and (2).

ORR on discharge:

O₂ (g) + 2H₂O (l) + 4e⁻¹ → 4OH⁻ (aq)

(1)

OER on charge:

4OH⁻ (aq) → O₂ (g) + 2H₂O (l) + 4e⁻

(2)

A hydrophilic electrocatalyst layer and a hydrophobic gas diffusion layer often make up an air electrode bilayer. With the help of a binding agent, an active catalyst is put onto one side of the air electrode. In the case of nonconductive catalysts, conductive carbon is occasionally added to the active catalysts in order to increase the electrical conductivity [19,20]. Figure 1 illustrates a specific kind of rechargeable zinc–air battery that displays the cathode electrode’s catalytic layers.

Figure 2. Schematic representation of prismatic zinc–air battery configuration. Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license [21]. Copyright (2023), MDPI.

Figure 2. Schematic representation of prismatic zinc–air battery configuration. Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license [21]. Copyright (2023), MDPI.

The low stability of the air electrode during battery charging is one of the fundamental drawbacks of rechargeable zinc–air batteries. The zinc–air cell at the positive electrode uses up oxygen from the surrounding air during discharge. For the gas to react, it is therefore better to use an electrode with a very high effective surface area [22]. Only when the reactant gas, in this case oxygen, is in contact with both the liquid electrolyte over this large surface area and an electronic conductor acting as a current collector can the electrochemical reaction take place [23]. As a result, the air electrode is made to function with an interface that is triple phase, meaning it consists of a solid, liquid, and gas. At every triple-phase reaction point, a catalyst is also required to minimize electrochemical losses. By employing a porous electrode that allows both air and the liquid electrolyte to pass through, these triple-phase reaction interfaces are produced [24].

3. The Recent Advancement of Graphene

Because of its significant physical and chemical properties, graphene is a promising candidate for a sustainable clean energy source and is present in many innovative materials [25]. Graphene, a single-atomic layer-thick carbon allotrope, serves as the fundamental building block for carbon compounds in all other dimensions [26]. In general, graphene is more effective than other carbon-containing materials, including fullerene, graphite, and carbon nanotubes [27]. It is thought that the highly conjugated and delocalized π system shared by the sp² carbons in graphene is chemically inert [28,29,30]. There will undoubtedly be some intrinsic or extrinsic flaws introduced into the graphene lattice in order for graphene to become functional. Topological defects, dangling bonds, armchair edges, zigzag edges, and vacancies are examples of the structural flaws that can form [31]. The conjugated sp2 systems’ homogeneity and symmetry are broken by point n defects, which include vacancies in the basal plane and zigzag/armchair edges at the lattice boundary. The boundary carbon atoms with unpaired π electrons, localized spins, and high chemical potential in catalytic processes are made possible by the localized electrons in edging sites [32]. Graphene is utilized as a template because of its regularly repeated unit structure. The aggregation of nanoparticles is limited when a homogenous composite of particles is created using graphene as a template [33]. Large surface area, good mechanical and thermal stability, charge redistribution, change in electronic structure, and ease of functionalization are the main reasons for the dominance of nanostructured carbonaceous materials [29]. Consequently, graphene has a wide range of uses in the creation of ZABs with strong electrochemical activity due to its special properties. Graphene by itself, however, often exhibits negligible electrocatalytic activity toward ORR/OER. For graphene to be useful for ORR/OER reactions in rechargeable zinc–air batteries, it must be modified utilizing a variety of modification procedures [34]. Graphene by itself, however, often exhibits negligible electrocatalytic activity toward ORR/OER. For graphene to be useful for ORR/OER reactions in rechargeable zinc–air batteries, it must be modified using a variety of modification procedures [26]. Chemical doping is one of the most interesting methods for functionalizing graphene because it introduces new active sites, controls the electron/spin cultures of the carbon lattice, and significantly accelerates carbocatalytic reactions [30]. Oxidizing substances such as H₂O₂, O₂, and CO₂ and concentrated acids like H₂SO₄, HNO₃, and H ₃PO₄ can be used to potentially add oxygen-containing groups to carbon compounds. There are several different types of oxygen groups, most of which are found on graphene’s edge. Proton-donating and -accepting oxygen functions are classified into acidic, neutral, and basic species [35]. Numerous investigations have been performed on the intrinsic catalytic centers and activities of oxygen groups thus far. The species and amount of oxygen groups in graphene can have an impact on the material’s reactivity during electrochemical catalysis [36]. Heteroatom doping, which modifies graphene’s electron donor characteristics, is one of the other most intriguing graphene functionalization methods [37,38]. Metal-free elements, including boron, nitrogen, phosphorus, sulfur, and halogens, are widely employed as dopants. These elements can be added to graphene by post-treating it with organic or inorganic precursors containing the elements [39,40]. A few methods for openly synthesizing heteroatom-modified graphene have also been established [41]. In a redox reaction, the substitution of foreign atoms modifies the neighboring carbons’ charge density and catalytic activities. Their varying electronegativities dictate which way electrons flow from the dopants to the carbon [42]. In addition, the electrical configuration to establish a covalent connection with sp² carbon and the relative radius of the alien atom to carbon dictate the doping amount. In particular, doping levels should be controlled below reasonable bounds; otherwise, graphene’s structural stability will be compromised. A new class of single-atom carbocatalysts for chemical processes has been made possible recently by the use of metal-free dopants, particularly pyridinic nitrogen, as anchoring sites to coordinate with isolated transition/noble metal atoms in the graphene layer [43]. Three characteristics of the dopant element—its size, electronegativity, and number of electrons in the outer shell—are primarily linked to the effect of the dopants on the electrocatalytic activities of graphene-based materials [44]. Direct plasma [45], nonplasma (chemical vapor deposition (CVD)) [46], and hydrothermal method [47] are the methods used to produce N-doped graphene. N-doped exfoliated graphene is also prepared via the electroexfoliation technique [48]. The enhanced graphene has superior electrochemical capabilities. Consequently, one of the most promising substitutes for electrode materials in energy-related devices has been thought to be graphene and graphene-based materials [49,50,51]. Graphene can be utilized as a cathode material alone or in combination with other dopants such as non-metals, transition metals, transition metal oxides, and halides, for the reasons previously mentioned. Moreover, there are two different kinds of defects in graphene: (i) intrinsic defects, which are found naturally in graphene and include carbon adatoms, Stone–Wales defects, single-vacancy defects, multiple-vacancy defects, and line defects; and (ii) extrinsic defects, which are defects that are physically introduced and include substitutional impurities and foreign adatoms. On the surface of graphene, the dopants are utilized to form functional groups and active sites [52]. Heteroatom doping is a useful method for producing active sites that can accommodate specific functional groups that support redox catalysis. Doping is therefore anticipated to increase graphene’s catalytic activity for the activation of oxidants (O–O bonds) by altering the surface’s chemical composition and textural structure [53]. Figure 3 provides a brief overview of the roadmap for representative research on graphene in electrochemical energy storage (EES) devices.

4. Preparation of Graphene

Techniques for preparing graphene can be classified into two groups: top-down and bottom-up. Techniques such as chemical exfoliation, mechanical exfoliation, and chemical synthesis fall under the top-down approach classification scheme, while techniques such as thermal chemical vapor deposition (CVD), pyrolysis, and epitaxial growth fall under the bottom-up approach [55]. The use of pure graphene in the creation of graphene-based air catalysts is essentially limited because it is practically insoluble and cannot be dissolved in water or any other organic solvent [56]. The top-down and bottom-up approaches of graphene preparation are summarized in the diagram below. Regretfully, it is difficult to synthesize graphene, particularly on a large scale. Consequently, the discovery of graphene led to the development of its derivatives, including reduced graphene oxide (rGO) and graphene oxide (GO). Compared to graphene, these materials are comparatively simpler to produce on a wide scale [50]. Thus, this section will go into further detail regarding the preparation techniques for both reduced and graphene oxide [57].

4.1. Synthesis of Graphene Oxide (GO)

As seen in Figure 4, the top-down method is primarily used to synthesize graphite oxide (GO). This involves treating graphite with strong oxidants, such as sulfuric acid and potassium permanganate, and then using mechanical peeling techniques, like sonication and shearing stress, to achieve the exfoliation step [58,59].

Moreover, bottom-up synthesis techniques like chemical vapor deposition (CVD) can be used to produce GO [61]. The sp2 structure of graphite layers is broken during the treatment process, and various oxygen-containing functional groups, such as carboxyl, hydroxyl, or epoxy groups, are acquired [62]. The graphite structure’s interplanar gap grows as a result of graphite layer oxidation. To create a solution of uniform graphene oxide layers, the next step involves exfoliation, which divides the graphite oxide layers [63]. The final GO structure and the degree of oxidation are strongly influenced by a number of factors, including the initial oxidation conditions, energetic input, source, and lateral size of the graphite utilized as a beginning material [64]. Distinctive qualities like low electrical conductivity, high specific surface area, high mechanical strength, and high fracture strength are caused by the disruption of the sp2 bonding network [58]. Figure 5 shows the synthesis of graphene oxide using Hummer’s method, while Figure 6 shows the quick synthesis of graphene oxide from graphene [65].

4.2. Synthesis of Reduced Graphene Oxide (RGO)

One appealing way to achieve graphene-like properties is through the reduction of graphene oxide. Reduced graphene oxide structures can be obtained by thermal, chemical, or photo-thermal reduction techniques [68].

4.3. Chemical Reduction of GO to RGO

Because chemical reduction can produce stable dispersions, which are necessary for many applications, and because it can produce RGO of excellent quality and productivity, it has supplanted nonchemical reduction methods in the field [69]. Certain compounds are utilized, such as sodium borohydride, hydrohalic acids, strong alkalis [70], hydrazine, hydrazine hydrate, and ascorbic acid [71]. The structural characteristics, mechanical strength, stability, dispersibility, and reactivity of GO are all drastically altered throughout the reduction process (Figure 7) [72]. These modifications have a direct bearing on the removal of compounds that include oxygen from the GO structure and the subsequent restoration of the sp² structure following the reduction process [73]. Even with its reduced size, reduced graphene oxide is still beneficial due to its regulated functionality, high electrical and thermal conductivity, initial material availability, and inexpensive, scalable manufacturing method [74].

4.4. Thermal Reduction of GO to RGO

One of the most promising methods for producing significant amounts of reduced graphene oxide (rGO) is the thermal reduction of graphene oxide (GO) [75]. Large-scale production can benefit from the quick, easy, and nontoxic nature of thermal reduction. Three elements—temperature, time, and atmosphere—have a significant impact on the structure and characteristics of rGO during the thermal reduction process [76,77]. As seen in the figure, the reduction process will eliminate the oxygen functional group of GO. The C/O ratio in their structures is the primary distinction between GO and rGO (Figure 8). Although the carbon-to-oxygen ratio is extremely low in GO structures, it is much higher in rGO structures that are virtually entirely oxygen-free. This variation in C/O ratios accounts for the majority of the remaining variations between GO and rGO materials [43].

4.5. Photo-Thermal Reduction of GO to RGO

In the area of CO₂ conversion, photothermal reduction has garnered a lot of attention as an effective catalytic technique that combines thermochemical and photochemical processes [79]. Given that the two systems may operate concurrently, determining which pathway primarily drives the system can be extremely difficult and opaque at times [80]. Figure 9 summarizes the preparation method for graphene.

5. Electrocatalysts for ZABs

Three components make up zinc–air batteries: an air cathode (a carbon substrate with a catalytic layer), an alkaline electrolyte (often concentrated KOH), and a zinc anode [81]. One of the most costly cell components that affects the battery’s performance is the air cathode in zinc–air batteries [82]. The main element of the air cathode that is essential to determining the energy conversion efficiency is the electrocatalyst [83]. Cathode electrocatalysts can be made of a wide range of materials. Some of the cathode catalyst materials employed in zinc–air batteries are noble metals, transition metal oxides, and various carbon materials such as activated carbon, carbon nanotubes, and graphene [84,85]. Since its initial isolation in 2004 [78], graphene has gained popularity as a material for electrochemical energy storage systems [86]. With a 2D “aromatic” monolayer of carbon atoms tightly packed in a honeycomb crystal lattice, this sp²-linked carbon nanosheet offers a remarkable two-fold advantage: rapid heterogeneous electron transport at the edges and unusually high electron mobility at ambient temperature [87,88]. Furthermore, compared to one-dimensional carbon nanotubes (1315 m² g⁻¹) and three-dimensional graphite (10 m² g⁻¹), graphene has an ultrahigh surface area of 2630 m² g⁻¹. Recently, there has been a lot of interest in decorating graphene with metal nanoparticles [89]. Additionally, graphene-based materials perform better than graphene in a number of applications, including electrocatalysis, environmental remediation, energy applications, capacitors, transparent electrodes, sensors, and electrical or optical detectors [90,91]. Graphene-based devices have proven effective because they are excellent single-atom-thick substrates for the development of functional nanomaterials [92]. Graphene-based nanocomposites are primarily driven by electrochemical processes, wherein graphene substrates can function as a large electron carrier for highly efficient electronic exchange between graphene and the decorations [93]. Because active sites are selectively exposed, combining graphene substrates with the matching recognition sites of addends can prevent errors in electrical conductivity and self-construction while also clearly demonstrating their electrocatalytic activity [94]. Furthermore, complex structural addenda are rather essential to the further enhancement of graphene-based nanocomposites’ electrocatalytic activity [95]. Figure 10 shows the graphene’s usual structure.

Both experimental and theoretical research reveals that the effects of various dopants can improve the electrocatalytic activities of graphene. In these graphene-based metal-free electrocatalysts, the chemical dopants and geometric lattice defects of graphene play an important role in their superior catalytic activities. Several studies have verified that cathode catalysts featuring large active surfaces are conducive to the deposition and decomposition of solid discharge products, thereby enhancing both capacity and reversibility. Furthermore, the migration of constituents such as O₂, electrolyte ions, reaction intermediates, by-products, and primary discharge products within MABs directly influences the catalytic efficiency of the cathode [96].

Due to its exceptional qualities, which make it an excellent electrocatalyst in many applications, graphene has drawn a lot of attention. These qualities include high electron mobility at room temperature, fast heterogeneous electron transfer at the edges, superior electric conductivities, and excellent mechanical strength and elasticity [97,98,99]. Graphene is an extremely promising electrocatalyst because of all these qualities. Graphene-doped graphene, graphene/metals (metal oxides), and other graphene-based nanomaterials were all rationally constructed, and under alkaline circumstances, they all demonstrated increased activity and stability for ORR, comparable to the commercial Pt/C catalyst. In comparison to the commercial Pt/C catalyst currently utilized for the manufacture of zinc–air batteries, certain forms of electrocatalysts offer superior catalytic characteristics at a lower cost [100]. In order to adjust the surface reactivity and physical characteristics of graphene-based materials, such as band structure, carrier concentration, and magnetism, graphene can now be changed using metal nanoparticles or non-metals. There are structural flaws in graphene, which can improve its functionality even further. In the meantime, chemical or ion irradiation processes can be used to purposefully induce imperfect structures in graphene. More opportunities for supported catalysts to reside on the surface are provided by the enhanced chemical reactivity of the defect sites that are formed [90,101].

Figure 10. Graphene and its derivatives. Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license [102]. Copyright (2023), MDPI.

Figure 10. Graphene and its derivatives. Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license [102]. Copyright (2023), MDPI.

6. Graphene-Based Cathode Material for Zinc–Air Batteries

One of the crucial cell elements that may be seen as essential to managing ZAB operation is the air cathode. The materials utilized and the electrocatalyst’s structure both have a role in improving the performance of air cathodes. Because of their intriguing features, graphene-based nanocomposites are thought to be the most promising of the known electrocatalysts for use in any kind of energy storage device. This section will discuss the characteristics of transition metal single-atom-doped graphene (TMSAC), nitrogen-doped graphene (N/G), metal co-doped graphene (M/N/G), metal oxide co-doped graphene (MOx/N/G), and mixed metal oxide co-doped graphene (M₁M₂O_x/N/G) [50,103].

6.1. TMSAC

Transition-metal single-atom catalysts (M-N-C SACs, M = Fe, Co, Zn, Cu, Mn, etc.) are one of the non-platinum group metal (NGM) electrocatalysts that have emerged as an attractive option owing to their excellent atomic efficiency, tunable coordination structures, and designable geometric configurations [104]. A single-atom catalyst has many unique properties that are different from those of its nanostructured or subnanostructured counterparts. When the dispersion levels of the atoms in the catalyst reach atom scale, there is a sharp increase in surface apparent energy, quantum size effect, and unsaturated coordination environment, as well as a different interaction between the metal and the support. Due to the fact that only the coordinately unsaturated atoms on the surface of catalysts can be involved in the catalytic process, all these results endow the catalyst with superior catalytic performance compared with their bulky and nanostructured counterparts. In addition, TMSAC inherits the merits of both heterogeneous and homogeneous catalysts. In catalyzing the ORR/OER in ZABs, it is expected that single-atom catalysis exhibits the potential to make best use of the advantages while also bypassing the disadvantages of heterogeneous and homogenous catalysis [100]. The most effective approach to enhancing the performance of SACs is probably to increase the density of active sites on the support while promoting the intrinsic activity of the active sites. To increase the number of active sites, the well-established and most often employed synthetic methodologies are based on defect engineering, spatial confinement strategies, and coordination design strategies [105]. Tour’s group reported a low-cost, simple, and scalable method to prepare Co TMSAC by simply heat-treating cobalt salts and graphene oxide under an NH₃ atmosphere for the first time. The Co–NG catalysts exhibit excellent catalytic activity under both acidic and alkaline conditions. They also suggested that the catalytically active centers originate from Co metal centers coordinated to the N atoms. In addition, Fan et al. reported that a Ni–C-based material can be activated to obtain the Ni SACs on graphitic carbon after 4000 cyclic voltammogram cycles, consequently displaying high catalytic activities [106].

6.2. Nitrogen-Doped Graphene (N/G (GO,rGO))

Doping graphene with non-metal atoms such as N, B, S, and P, or their combination, improves its chemical characteristics, forms active sites, and significantly boosts its catalytic activity. Nitrogen is the most common non-metal dopant, followed by sulfur. By altering the exterior electronic structures and promoting the exterior hydrophilicity of carbon materials, N atoms placed into the graphitic lattice might enhance oxygen molecule adsorption and the ensuing electron transfer process. For graphene-based materials to have ORR catalytic activity, their shape and structure are also crucial. The electrophilic properties of pyridinic and graphitic N atoms in the graphene framework cause adjacent carbon atoms to become electropositively charged, which increases the amount of oxygen that can be absorbed on the catalyst surface and intensifies the oxygen reduction reaction. Of the four types of N atoms in the graphene framework, pyridinic and graphitic N atoms play a key role in this process [34,107,108,109]. The N-doped structures in nitrogen-doped graphene operate as catalysts, and these can be isolated from one another by primarily inactive undoped areas. Thus, distinct oxygen transport behaviors in the graphene region may be displayed by the nitrogen-doped graphene sheets [110]. There is much disagreement over the precise location of the N-doped carbon active centers, which is believed to be either at the graphene’s edge or basal plane. There is also disagreement over the centers’ chemical makeup and whether or not they contain graphitic or pyridinic N sites. Pyridinic-N has frequently been thought of as the catalytic active site because it has a lone pair of electrons, which improves O₂ adsorption. However, recent studies have shown that graphitic-N sites play a significant role in ORR employing graphene or carbon black generated by chemical vapor deposition (CVD) [111]. To increase the electron dispersion on graphene’s surface, elements with varying electronegativities can be doped into the material. Doping N, which has a higher electronegativity than carbon, may change the electronic characteristics and chemical reactivity of graphene by forming a unique electronic structure. This would result in increased electrocatalytic activity. At the same loading in an alkaline electrolyte, N-doped graphene exhibits equivalent ORR electroactivity to the commercial Pt/C catalyst due to its huge species surface area, superior conductivity, and durability. Drawing from the research of Piotr Kamedulski and Jerzy P. Lukaszewicz, the electroexfoliated method of N-doping enhances the performance of the graphene cathode. A great way to create an electrocatalyst without any metal is to use the N-doped graphene-based catalysts produced. Nitrogen’s presence enables graphene-based catalysts to carry out ORR as intensely and successfully as Pt-loaded commercial catalysts. Furthermore, nitrogen doping produces extremely stressed regions in SP² carbon networks, which results in the creation of edge defects, vacancies, and active sites [48,112].

In order to achieve optimal oxygen adsorption and suitable active sites, two heteroatoms with differing electronegativities may occasionally be added to the reduced graphene oxide structure [113]. Consider the elements boron and nitrogen. Adding nitrogen atoms with strong electronegativity produces positively charged neighboring carbon atoms, which act as O₂ molecules’ adsorption sites. In the meantime, the positively charged boron species is converted into active sites by doping boron atoms with low electronegativity [114]. In alkaline electrolytes, carbon that has been doped with N or B exhibits enhanced catalytic activity. When used as an electrocatalyst, N/B co-doped graphene performs better for ORR activity than N- or B-doped graphene, according to Chen et al. [115]. Reaction kinetics in ORR were accelerated by the more advantageous locations for O₂ adsorption created by the electron-deficient characteristic of B atoms. That being said, low-density N-doped graphene, which forms a considerably thicker catalyst layer, would restrict its volumetric species activity, which would worsen mass transfer in the Zn–air battery [116]. Because co-doping graphene with N and S helps to build non-neutral electron sites that permit effective four-electron transfer ORR activity, it is also a promising approach to producing an ORR catalyst. To create N and S co-doped graphene nanosheets, we employ thiourea as the only precursor in a reaction with graphene oxide (GO). XPS provides proof that N, B, and S coexist on the graphene surface. In addition to their ORR activity, N, B, and S’s primary role on the graphene surface is to create a conductive network that promotes effective electron transmission [117]. This article focuses on the characteristics and transmission electron microscope (TEM) structures of materials used as the cathode in zinc–air batteries, including reduced graphene oxide (RGO), graphene oxide (GO), and doped and co-doped graphene (G). The TEM pictures of nitrogen- and boron-doped graphene are displayed in Figure 11.

6.3. Metal Co-Doped Nitrogen-Doped Graphene (M/N/G (GO, rGO))

In an effort to address the issue and enhance ORR activity, transition metal nanoparticles (such as Co, Fe, Mn, and Ni) with ORR activity have recently been encapsulated in carbon composites. The process of electron transfer is directly impacted by the interface between graphene and metal nanoparticles. N/B co-doped reduced graphene oxide aerogel adorned with iron nanoparticles (Fe-NBrGO) is one of the greatest examples of two heteroatoms with distinct electronegativities encapsulated with a metal. Yuyun Irmawati has created an oxygen catalyst for use in alkaline and neutral electrolytes [114,115]. Figure 12 shows the charging–discharging curves, power density, stability, and voltaic efficiency of ZAB in alkaline and neutral media.

According to certain studies, the encapsulation of metal–metal carbide nanoparticles in the graphene layer reduces the carbon layer’s work function, activating the structure electrochemically toward ORR. It has been demonstrated that the ORR performance was directly correlated with the FeN, Fe₃C, and FeP in catalysts as modifications, which may further enable quicker electron transfer [102,103,119,120]. The TEM image presented in Figure 13 demonstrates the homogeneous dispersion of spherical Fe particles on graphene sheets.

Emilia Morallon reports that direct dispersion of graphene oxide (GO) generated by electrochemical expansion has been successfully used to incorporate iron, cobalt, and manganese phthalocyanines (FePc, CoPc, and MnPc, respectively) into graphene oxide [104]. A favorable outcome was obtained when the ORR activities of the synthesized GO-based electrocatalysts were compared with those of commercial Pt/C [121]. Figure 14 shows the TEM pictures of the produced GO-based electrocatalysts and the bare GO.

Table 1. Electrochemical data of the different composites and GO and comparison with Pt/C in an O₂-saturated 0.1 M KOH solution [121].

Catalyst	Eonset a	E ½	Electron Transfer Number b	jlim b	Tafel Slope
	(V)	(V)		(mAcm−2)	(mV dec−1)
GO/FePc	0.95	0.87	4	−6.9	−31
GO/MnPc	0.87	0.69	3.7	−4.4	−61
GO/CoPc	0.84	0.59	3.6	−4.7	−86
Pt/C	0.99	0.87	4	−6.4	−52
GO	0.80	0.66	2.7	−2.1	−81

^a Determined at −0.1 mA cm⁻². ^b Determined at 0.2 V vs. RHE.

provides an overview of the electrocatalytic activity for ORR of the various composites and the commercial Pt/C.

In some cases, graphene can be coupled with metals that do not contain phthalocyanines or other supports to act as an electrocatalyst. Fe/N/S is one of these kinds, where the addition of atoms to carbon materials raises the density of their intrinsic active sites and encourages the adsorption of oxygen. Active sites should be accessible as far into the material as feasible to maximize their usage; this can be achieved by optimizing porosity and dimensionality [122]. The morphological structures of N/S- and Fe/N/S-doped carbon are displayed in Figure 15.

Furthermore, catalysts made of non-platinum group metals (non-PGMs) have improved electron conductivity. Fe/N/Gr, Co/N/Gr, and Mn/N/Gr are the most often utilized dopants from N/G/(non-PGM) combinations, and they are investigated. Among these catalysts is Co/N/rGO(NH₃), which exhibits better ORR activity. This could be because the carbon atoms in the GO matrix may offer special carbon chemistry, which could help the ORR active site form [123,124,125]. Figure 16 shows the morphological structure of this electrocatalyst.

6.4. Metal Oxide Co-Doped Nitrogen-Doped Graphene (MOx/N/G (GO, rGO))

Because of their excellent ORR catalytic activity, low cost, and high electrochemical stability, transition metal oxides have garnered a lot of interest. Specifically, carbon electrocatalysts doped with nitrogen and transition metals exhibit good catalytic activity, are readily available, and are reasonably priced. Nonetheless, the majority of transition metal oxides are semiconductors or insulators, which prevent electrons from moving between supporting electrodes and catalysts as well as inside them. Therefore, carbon composites with high conductivity and good durability are typically used to support the transition metal oxide nanoparticles in order to prevent these problems. Furthermore, the introduction of nitrogen atoms into carbon can modify its pristine electronic states and significantly increase its active sites (pyridinic-N, graphitic-N, and so on). For instance, if we take CoO, the combination of CoO nanoparticles and materials that dope carbon with nitrogen can modify CoO’s conductivity and encourage *O, *OH, and O₂ to adsorb, transfer, and diffuse on the catalytic surface. Enhancing ORR kinetics is further aided by the interaction between CoO nanoparticles and nitrogen-doping carbon materials, wherein the ORR reaction is catalyzed by the N-doped carbon skeleton present in catalysts combined with CoO nanoparticles. It is therefore vital to investigate hybrid nanomaterials made of nitrogen-doped carbon and CoO nanoparticles that are well related to one another in order to make sense of the advantages of both types of nanoparticles. In practical applications and the commercial development of zinc–air batteries, CoO/NG catalyst as the air-cathode shows promise [126,127,128,129]. Figure 17 displays the CoO/NG electrocatalyst’s SEM images.

The other form of cobalt oxide is Co₃O₄, which is one of the transition metal-based oxide materials in the form of spinel and perovskite structures that have been studied for a long time as highly efficient catalysts. The graphene provides high electrical conductivity, and the large surface of graphene creates more electrochemically active sites, which may improve the electrochemical performance of reduced graphene-Co₃O₄ (RG-Co₃O₄) composite materials in rechargeable zinc–air batteries. As the SEM image shows in Figure 18, the rod-like Co₃O₄ is randomly dispersed on the surfaces of the graphene, and this composite structure is beneficial for the performance of the catalyst by improving the electrical conductivity of Co₃O₄ [131].

6.5. Bimetallic Metal Oxides Co-Doped Nitrogen-Doped Graphene (M₁M₂O_x/N/G (GO,rGO))

Bimetallic oxides, such as Co-Ni, Fe-Ni, and Fe-Co oxides, are also frequently used because they perform better in electrocatalysis than their single-element equivalents. Therefore, the synthesis of cost-effective bimetallic oxide/N-doped carbon composite materials is extremely desirable in order to provide bifunctional oxygen electrocatalysts with well-defined architectures that are devoid of noble metals. Fe–Co is the most economical bifunctional oxygen electrocatalyst for ORR and OER in alkaline solutions among bimetallic oxides. FeCoOx@NG composite is an N-doped graphene sheet packed with in situ generated Fe–Co bimetallic oxide particles with a self-antistacking structure [111,128]. Figure 19 displays the composite material’s SEM pictures.

The other hybrid material is made up of nitrogen-doped reduced graphene oxide and CoMn₂O₄ nanoparticles covalently connected as a highly stable and active bifunctional electrocatalyst for zinc–air batteries generated by a hydrothermal technique (Figure 20) [133].

LaNiO₃ nanorods, which have an anisotropic shape and a longitudinal axis structure that enhances dispersion, are one of the known La-based catalysts. They can also be used to improve catalytic utilization, mass, and electron transport. It has been demonstrated that this catalyst works well with rechargeable Zn–air batteries. As fully conducting conduits, the interconnected rGO sheets can also increase the pace at which electrons move from the semiconducting catalyst to the external circuit. These imply that rGO combined with the structure of the mixedvalent Ni–O molecule could result in high catalytic activity for the LNO-NR/RGO hybrid [134]. In the initial discharge-charge cycle, LaMnO₃, the other catalyst based on La, has been found to demonstrate better catalytic activity for both ORR and OER. This is particularly true for the ORR that corresponds to the zinc–air battery’s discharge. An average valence of Mn(III)–Mn(IV) is typically found in the most active catalyst for Mn-based oxides, providing a moderate binding strength between the catalyst surface and the reactant or product [135].

Because N atoms have a lower electronegativity than O atoms, nitrides often have superior conductivity than oxides. Nitrides have a tendency to create interstitial compounds, which causes the d band to decrease and the distance between metal atoms to increase. This increases the density of states close to the Fermi level, which is advantageous for electrocatalysis. But in many instances, O₂’s adsorbing capacity on nitrides is still less than that on oxides, which makes it less likely to cause ORR. Consequently, it is thought that integrating nitrides and oxides into a single integrated system is a worthwhile strategy to achieve high catalytic activity and comparatively good conductivity. Furthermore, the presence of metallic phases can enhance electronic transmission [126,127]. Co/Co₃O₄/CoN/NG is one of the greatest examples of a mixed electrocatalyst; in this case, N-doped graphene is coupled with CoN and Co₃O₄ (Figure 21). For alkaline zinc–air batteries, sulfides can also be utilized in addition to oxides and nitrides. As an illustration In an alkaline medium, cobalt sulfide nanoparticles supported on graphene have been employed as an ORR catalyst. In an alkaline medium, carbon and graphene doped with sulfur and nitrogen also show respectable ORR activity. Fuel cells employ graphene-supported cobalt sulfide (CoS₂) nanoparticles as an electrocatalyst. Because of their enhanced OER and ORR activities, cobalt sulfide nanoparticles anchored onto graphene oxide substrates display a better charge-discharge profile than the valuable catalyst Pt-Ru/C (40 wt%) [136].

7. Challenges and Prospectives

We attempt to provide an overview of the preparation process and several graphene-based composites used as cathode materials for ZABs in this review. The TEM (SEM) images and the morphological interactions between the graphene and cathode materials are highlighted in those graphene-based composites. Graphene-based cathode materials have shown significant success in ZABs because of their unique 2D structure, high electrical conductivity, huge surface area, customizable surface chemistry, and outstanding chemical and electrochemical stability. For this reason, it is important to carefully control the shape, pore size, and surface chemistry of graphene-based cathode materials in order to maximize their capacity by increasing their effective surface area. Furthermore, sufficient attention should be paid to heteroatom doping and porosity engineering in order to achieve high-capacity and high-rate capacitor-type cathodes. In addition to increasing capacity through quick redox reactions, doping can also improve electrical conductivity. Reasonable and adjustable porosity can also help with the diffusion of the electrolyte ions, which, when combined, can produce exceptional rate capability. One of the key components for large-scale applications is the affordable preparation of graphene-based cathode materials. Another significant but crucial problem is the development of materials suitable for batteries that have a high rate of stability over time. To create high-rate anodes, it is therefore always necessary to design nanostructured materials with tunable porosity and combine them with highly conductive materials. Graphene’s typical precursor is GO, which requires a number of processes and chemicals to transfer from graphite, despite graphite being a plentiful precursor on Earth. The conventional method of synthesizing graphene oxide (GO) requires a large number of chemicals, which raises costs and may harm the environment. Numerous fascinating outcomes have been obtained from the exploration of cathode materials based on graphene. It is recommended to assess the cathode performance using electrocatalysts based on graphene. More innovative materials based on graphene are anticipated to be developed in the future, with tremendous potential for use as integrated cathodes with high performance in rechargeable ZABs. The following elements need to be carefully taken into account in order to enhance the performance of rechargeable zinc–air batteries.

• Developing an extensive performance assessment of different graphene flaws in relation to OER/ORR activities is crucial. Systematic studies on the causes of altered OER/ORR performance resulting from different faults are still lacking. The explanation of each faulty site’s contribution is necessary to enhance comprehension of their respective responsibilities for graphene-based materials and enable their utilization in ZABs. Naturally, rigorous control studies on carbon materials with a single kind of defect are also required. Furthermore, it is important to understand how the location and quantity of dopants affect the degree of defect.
• In order to understand the electrocatalytic process of graphene-based catalysts for ZABs and to effectively develop and promote active sites, critical cathode electrode efficiency studies are desperately needed. Typically, catalyst recharge processes can result in structural changes to the active sites that are not detectable by standard characterization techniques.
• Research on the functions of individual components in hybrid graphene-based ZAB catalysts is still needed. Researchers typically attribute these roles to the “synergistic effects” among the several components. In addition to proving the impact exists, we also need to clarify if the doping level and component location will affect this synergistic effect. Through comprehension and utilization of individual component contributions, we may enhance the efficiency of carbon-based cathodes for ZAB design.
• Since energy storage devices require rapid charging, rate capability is a crucial technical requirement for applications involving rechargeable ZABs. Because of their enormous surface area and exceptional conductivity, graphene-based materials are commonly used in high-performance rechargeable zinc–air batteries. Nevertheless, prior investigations have focused mostly on how using graphene materials in air cathodes can impact the rate capability of rechargeable ZABs.
• The inherent redox pathways of OER/ORR in air cathodes are the reason for the restricted gains in ZAB energy efficiency. In order to lower charge potentials, it is imperative to create more effective cathode systems, which might be motivated by the development of innovative cathodes made of carbon-based materials.

8. Conclusions

Compared to lithium-ion batteries (LIBs), zinc–air batteries (ZABs) have a greater theoretical energy density (1218 Wh kg⁻¹), which increases their energy efficiency in terms of form factor and allows for a lighter and less expensive design. This points to a viable direction for significant advancements in the field of ZABs for flexible and portable forms that are simple to incorporate into wearable electronics such as smart wristbands, electronic skins, and cell phones. From an environmental perspective, ZABs are also a far more promising option because they employ airborne oxygen as a reactant, negating the requirement for heavy metal components in the battery. Since ZABs do not rely on finite metal resources, they are a more environmentally friendly choice than LIBs. However, ZABs are currently limited by issues with electrolyte management and electrode deterioration, making it difficult for them to reach their maximum theoretical energy density. Hence, in order to attain high energy storage performances for LIBs and ZBBs, this article aims to summarize the latest developments in GBM technology. Graphene and its derivatives are crucial for energy storage because they act as both a capacity storage medium and a conductive template. Graphene-based materials exhibit significant potential in ZAB air cathodes. Advanced research methods in defect engineering, synergistic effects tuning, and morphological design have been developed; however, there are still issues that need to be resolved, and new carbon material applications for ZABs need to be investigated. Using fictitious examples, graphene-based electrocatalyst materials with the general forms of (N/G), (M/N/G), (MOx/N/G), and (M₁M₂O_x/N/G) have been elaborated. Because graphene is practically insoluble and cannot be dissolved in water or any other organic solvent, its use in the creation of graphene-based air catalysts is severely limited. Here, we address these issues and implement effective techniques to fully realize the potential advantages of ZAB technology.