Graphitic Carbon Nitride: Preparation, Properties and
Applications in Energy Storage

Graphitic carbon nitride (g-C₃N₄) has received much attention in recent years due to its unique optical and electrochemical properties. In this review, preparation, properties and some applications of g-C₃N₄ in energy storage are summarized. In order to improve the specific surface area of g-C₃N₄, hard and soft template, template-free and exfoliation methods are usually used. The properties of g-C₃N₄ including stability, optical, photoelectrochemical and electrochemical properties are introduced. Some applications in photocatalysis, electrocatalysis and Li-based batteries are also analyzed. Finally, the outlook of g-C₃N₄ is also provided.

Table of Content

In this review, preparation, properties and some applications of g-C₃N₄ in energy storage are summarized.

 

 

 

Keywords: g-C₃N₄; Synthesis; Properties; Applications.

1. Introduction

The growing population and rising consumption of fossil fuels continuously increase the people’s awareness in energy requirement and environment damage. At present, the large demand of fossil fuels has caused environmental damages. Therefore, certain solutions need to be raised to promote sustainable energy storage technologies towards urgent energy demands. Novel nanomaterials are considered as the efficient way to address the problems.^[1] Various kinds of nanomaterials with special properties have been prepared and used for energy conversion and storage. Nanostructured carbon nitrides (CNs) are promising in terms of a variety of applications. CNs can be synthesized by the pyrolysis of nitrogen rich precursors. Depending on the degree of condensation, the resulting materials are called melon (linear polymers of connected tri-s-triazines via secondary nitrogen) or graphitic carbon nitrides (g-C₃N₄, 2D sheets of tri-s-triazine connected via tertiary amines). g-C₃N₄ is considered to be the most stable allotrope of various CN structures because of its favorable intriguing properties such as high hardness, facile preparation, unique structure and chemical stability.^[1-7] Two structures have been proposed as the basic building block of g-C₃N₄.^[8] One is composed of condensed s-triazine units (ring of C₃N₃) with a periodic array of single carbon vacancies; the other is composed of condensed tri-s-triazine (triring of C₆N₇) subunits connected through planar tertiary amino groups with larger periodic vacancies in the lattice.^[8] The second one is the most stable local connection pattern.^[9,10] Therefore, tri-s-triazine is widely accepted as the basic unit for the g-C₃N₄. As a result, g-C₃N₄ has a great potential in energy conversion and storage as well as environmental applications,^[11] such as photocatalysis,^[12,13] fuel cells,^[14,15] CO₂ capture,^[16,17] and battery catalysis.^[18,19]

    g-C₃N₄ is a visible-light-response material (2.7 eV bandgap), and the energy position of conduction band (CB) and valence band (VB) is at -1.1 and 1.6 eV vs. normal hydrogen electrode (NHE), respectively. In addition, g-C₃N₄ has very high resistance to heat, strong acid, as well as strong alkaline solution. g-C₃N₄ contains carbon and nitrogen elements only and it can be synthesized from the facile pyrolysis of nitrogen-rich precursors, such as melamine,^[20,21] urea,^[22,23] thiourea^[24,25] and cyanamide.^[26,27] It has been reported that the choice of precursor and different pyrolysis temperatures have great influences on the electronic structure and bandgap of g-C₃N₄, which will further influence its potential applications in many fields.^[25-30] Recently, some significant advances have been made regarding the research of g-C₃N₄. Therefore, an article which can summarize the synthesis of g-C₃N₄-based materials and their potential applications in energy storage is necessary.

    This review article focuses on recent progress in the properties, synthesis and potential applications of g-C₃N₄ and g-C₃N₄-based nanocomposites in energy storage and conversion, such as photocatalytic hydrogen evolution, oxygen reduction reaction (ORR), and Li-based battery. Eventually, some concluding remarks and perspectives on current situations of the g-C₃N₄-related studies are presented, which are aimed to improve the understanding as well as promote the applications of g-C₃N₄-based nanocomposites. Therefore, this review gives a brief summary of synthesis, properties and applications of g- C₃N₄ and its composites.

 

2. Preparation of g-C₃N₄ materials

   g-C₃N₄ contains C and N elements only and it is a stable polymer semiconductor. At present, many nitrogen-rich organic precursors (such as urea, melamine, dicyandiamide, thiourea and cyanamide) are used for preparing g-C₃N₄. However, carbon nitride materials prepared by direct condensation of these precursors are with bulk structures, which have low specific surface areas. For practical applications as catalysts, it is necessary to introduce well-controlled porous structures in the bulk g-C₃N₄. Researchers have designed several different methods to obtain porous g-C₃N₄.

2.1 Hard and soft-template method

“Templating” essentially involves the replication of one structure into another under the structural inversion. A template, in its most general definition, is a structure- directing agent. Template method therefore is a versatile technique for the preparation of nanostructured or porous materials, as the size and shape of the resulting pore structures can be easily tuned by the appropriate template.^[28]

     The hard templates are used to fabricate porous structures and design hierarchical pore architectures in g-C₃N_4.^[4,29,30] Silica templates are used as the typical structure directing agent to control the nanostructures. Groenewolt et al.^[31] reported on synthesis of novel g-C₃N₄ nanoparticles with different diameters by employing different pore size mesoporous silica matrices. Afterwards, many researches on mesoporous g-C₃N₄ materials prepared by silica-based hard templates have been reported. Highly ordered mesoporous g-C₃N₄ with tunable pore diameters was synthesized by using aminoguanidine hydrochloride as precursor and SBA-15 as hard templates.^[32] Fukasawa et al.^[33] used uniform-sized silica nanospheres as templates to synthesize ordered porous g-C₃N₄. Xu et al.^[34] employed guanidinium chloride as precursors to synthesize mesoporous g-C₃N₄ by a nanocasting method. The as-prepared g-C₃N₄ samples possessed two types of pores and high specific surface areas. Park et al.^[35] prepared 2-dimensional (2D) and 3-dimensional (3D) mesostructured g-C₃N₄ via the incipient wetness process with mesoporous silica as a hard template. These materials exhibit open pores and large specific surface area. Notably, in the hard-template approach, the removal of the template is necessary to obtain the desired g-C₃N₄ structure. This process usually involves aqueous NH₄HF₂ or HF, which may cause damages to the environment.

Fig. 1 Schematic illustration of the synthesis of ordered porous g-C₃N₄ by using close-packed silica nanospheres (SNSs) as the primary template. Reproduced with permission^[33] (Copyright © 2010, John Wiley and Sons)

By contrast, the soft-template method is greener, since the soft template can be removed by calcination. In the soft-template method, the mesopores of g-C₃N₄ are achieved by amphiphilic surfactant molecules driven by the natural tendency of reducing interfacial energy.^[1] Therefore, the properties of organic templates are vital for the mesostructures of g-C₃N₄ materials and the templates are called structure directing agents. The soft-template process can fabricate g-C₃N₄ with different morphologies by changing the soft templates as well as simplify the preparation of g-C₃N₄. Many researchers have used different surfactants such as P123, Triton X-100, Brij30 and Brij58 or liquids as soft templates to prepare g-C₃N₄ with different morphologies and specific surface areas. For example, Yan et al.^[36] used Pluronic P123 as soft-templates to prepare g-C₃N₄ with worm-like pore, which possessed high BET surface. Yang et al.^[37] prepared mesoporous g-C₃N₄ by a soft template of Triton X-100. The hydrophobic groups of this template can form bubbles during the synthesis of mesoporous g-C₃N₄. In the preparation of 3D porous sulfur/graphene@g-C₃N₄ (S/GCN) hybrid sponge, oil emulsion droplets were used to form pores, and 3D interlinked S/GCN sponge structure through the hydrothermal process was achieved.^[40]

(Fig. 2) presents two representative synthesis routes for ordered mesoporous materials, which are suitable for the preparation of g-C₃N₄.^[38]

   The hard-template method has the advantage that the copies of the template structure can be achieved easily. A variety of inorganic nanostructures can be used as templates for the synthesis of organic replicas.^[28] The soft-template method is more environmental-friendly. However, the improper choice of template and pyrolysis can result in unsuccessful synthesis of g-C₃N₄, since the template materials will be decomposed before the formation of g-C₃N₄. Moreover, the template will leave carbon residue in the final products, which could decrease the nitrogen content and lower its catalytic activity.^[1]

Fig. 2 Scheme of two representative synthesis routes for ordered mesoporous materials: (a) soft-templating method and (b) hard- templating (nanocasting) method. Reproduced from [38] with permission from The Royal Society of Chemistry.

2.2 Template-free method

Han et al.^[38] prepared porous g-C₃N₄ by a facile thermal- treatment of dicyandiamide. The prepared porous g-C₃N₄ achieved a high BET surface area (201-209 m² g^-1) and large pore volume (0.50-0.52 m³ g^-1). Tahir et al.^[39] prepared g-C₃N₄ nanofibers (GCNNFs) by a facile template-free method. Melamine reacted with ethanol first and then was annealed at 450 °C for 2 hours to get GCNNFs, which exhibited 1D structure with a high specific surface area. Graphene modified porous g-C₃N₄ (porous g-C₃N₄/graphene) was also synthesized by the thermal calcinations.^[40] In this method, the polymerization process was carried out at different temperatures, and the porous g-C₃N₄ was obtained at high calcination temperatures.

Fig. 3 Schematic representation of chemical reaction for the synthesis of GCNNF. Reproduced with permission^[40] (Copyright © 2014, American Chemical Society)

For both the hard-template and soft-template methods, the complete removal of the template may require complicated post-treatments and be very challenging. Therefore, it is necessary to develop a template-free method. Such template-free method does have the advantage that it is one-step processes. Any template synthesis and removal thus can be avoided, and no waste is produced. The choice of precursor is vital for the template-free method, so the research focus mostly concentrates on the kinds of precursor as well as the changes of thermal treatment.

2.3 Exfoliation of bulk g-C₃N₄

2D g-C₃N₄ sheets show a unique layered structure and they can be prepared by the exfoliation of bulk g-C₃N₄. Li et al.^[20] obtained macroscopic foam-like holey ultrathin g-C₃N₄ nanosheets (CNHS) by a long-time thermal treatment of bulk g-C₃N₄ in air (Fig. 4). Holey 2D layered structure of CNHS has a large surface area and can provide more active sites for catalytic reaction (Fig.5). Besides, this unique structure of CNHS favors the electron movement. She et al.^[41] synthesized 2D graphene-like g-C₃N₄ by liquid exfoliation of bulk g-C₃N₄. The obtained g-C₃N₄ exhibits a 2D layer structure, a high specific surface area, which can promote both electron transport and photocatalytic activity.

Fig. 4 Top-down process for preparation of foam-like holey ultrathin g-C₃N₄ nanosheets. Reproduced with permission^[20^] (Copyright@ 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

 

Fig. 5 SEM images of (A) CNB, (B) CNS, and (C) CNHS samples. TEM images of (E ) CNB, (F) CNS, (G) and (H) CNHS samples, (I) photographs of (1) CNB, (2) CNS, and (3) CNHS. Reproduced with permission^[20] (Copyright@2016, WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fig. 6 TG-DSC thermograms for heating (a) melamine and (b) g-C₃N₄ obtained by heating melamine at 520 °C. Reproduced with permission^[21] (Copyright © 2009, American Chemical Society)

3.1.2 Chemical stability

Besides, g-C₃N₄ also exhibits a great chemical stability. g-C₃N₄ possesses optimized van der Waals interactions between the layers, as a result, it is insoluble in most solvents, such as water, acid and many organic solvents.^[48,49] However, treating g-C₃N₄ in molten alkali metal hydroxides or KMnO₄ results in a hydrolysis of its structure. However, treating g-C₃N₄ in concentrated acids leads to a colloidal dispersion. According to the report of Zhang et al.,^[50] acids did not chemically disintegrate g-C₃N₄. When treated in the concentrated-acid solvents at room temperature, g-C₃N₄ formed some dispersed nanosheets without destroying their layer structure.

3.2 Optical and photoelectrochemical properties

g-C₃N₄ possesses certain decisive optical properties, which have been studied by means of experimental characterizations and theoretical calculations. g-C₃N₄ has been used in various photochemistry-related areas, such as ultraviolet- visible absorption, photoluminescence and electrochem- iluminescence.

     As an inorganic semiconductor, g-C₃N₄ has a typical absorption bandgap adsorption at ~420 nm (Fig. 7).^[51] In fact, this medium bandgap can be confirmed by the yellow color of g-C₃N₄, and the results are in consistence with previous reports.^[52,53] It is also worth noting that some modification strategies can affect the absorption edge of g-C₃N₄, such as Fe, S, B doping^[54,55] and copolymerization by barbituric acid.^[56]

Fig. 7 (a) Diffuse reflectance absorption spectrum and photoluminescence (PL) spectrum (inset) under 420 nm excitation, and (b) time-resolved PL spectrum monitored at 525 nm under 420 nm excitation at 298 K for bulk g-C₃N₄ (black) and mpg-C₃N₄ (red). (c) Periodic on/off photocurrent I_ph response of mpg-C₃N₄ electrode in 0.5 M Na₂SO₄ under zero bias in a standard two electrode photoelectrochemical cell. Reproduced with permission ^[52] (Copyright © 2009, American Chemical Society)

In the report of Tahir et al.,^[39] they researched the optical properties of bulk g-C₃N₄ (GCN) and g-C₃N₄ nanofibers (GCNNFs). GCN has a bandgap at 2.67 eV, while GCNNF has a bandgap at 2.80 eV. The slight blue shift by 0.13 eV is due to the more perfect packing, electronic coupling and quantum confinement effect that shift conduction and valence band edges.^[39] Meanwhile, PL intensity of GCN is higher than that of GCNNF, which indicates the slow recombination rate of photogenerated electrons and holes. The decrease of PL intensity is caused by the defects in the crystal structure that reduces the strength of fluorescence peaks. These defects are the recombination centers for electrons and holes generated during photocatalysis. Therefore, a decrease in the number of defects in GCNNF ultimately increases its photocatalytic performance.

   g-C₃N₄ has been employed in photoelectric conversion systems (photoelectrochemical cells etc.) because of its favorable electronic band structure.^[57] Besides, in photoelectrochemical cells, g-C₃N₄’s band gap can be adjusted by changing its morphology or doping. Also, photoelectrochemical cells can operate stably in O₂ due to the excellent chemical and thermal stability of g-C₃N₄.

3.3 Electrochemical properties

g-C₃N₄ can be used as electrocatalysts for various electrocatalytic reactions, such as oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER).^[58,59] g- C₃N₄ can provide a higher electrocatalytic activity than pure carbon,^[60] which makes it become more attractive towards electrocatalysis. This is because g-C₃N₄ possesses pyridinic N atoms, which can accept electrons and serve as reaction active sites.^[49,61] However, the performance and applications of g-C₃N₄ in electrochemical reactions are restricted by its poor conductivity and electron transportation. Zheng et al.^[62] theoretically revealed that the low ORR catalytic activity of pure g-C₃N₄ is through an unfavorable 2e^- pathway. While in the g-C₃N₄@carbon composite, a nearly 100% of selectivity for 4e-ORR pathway could be achieved in alkaline aqueous solutions. Recently, it has been demonstrated that the electrocatalytic activities of g-C₃N₄ can be significantly enhanced by coupling with other conductive supports to improve electron transfer. As a result, tremendous efforts have been made to explore more strategies to achieve new-generation g-C₃N₄-based electrocatalysts with higher conductivity and electrochemical performance.

 

4. Energy storage of g-C₃N₄

4.1 Photocatalytic hydrogen production

Hydrogen has been regarded as a clean energy source without consuming fossil fuels. Photocatalytic water splitting by employing semiconductor photocatalysts and solar energy is a promising approach to generate hydrogen energy. The choice of photocatalyst is vital for achieving ideal hydrogen production performance. Various kinds of photocatalysts have been explored in hydrogen evolution reaction (HER), such as metal oxides,^[63,64] metal nitrides,^[65] metal sulfides^[66] and metal phosphides^[67] or their mixed solid solutions.^[68,69] Besides these materials, g-C₃N₄ has been considered as an efficient photocatalyst because of its structural and electronic properties.^[48] g-C₃N₄ possesses an appropriate bandgap and high specific surface area, therefore, it can promote the utilization of visible light and increase the electron transport, which will then promotes the photocatalytic hydrogen production.^[70,71]

Maeda et al.^[72] studied the performance of g-C₃N₄ as a nonmetallic photocatalyst for HER and OER under both ultraviolet (UV) and visible light. It indicated that g-C₃N₄ catalyst exhibited favorable performance for HER and oxygen evolution reaction (OER) due to a proper electron donor or acceptor. Zhang et al.^[73] used a facile one-step synthesis process to prepare g-C₃N₄ as photocatalysts. The porous structure with increased specific surface area and pore volume helped g-C₃N₄ achieve a better hydrogen production activity.

However, the photocatalytic efficiency of pristine g-C₃N₄ is still not satisfactory. As a result, many researchers have employed other strategies to promote its photocatalytic properties. For example, g-C₃N₄ could be complexed with other electron-rich nanocomposites.^[74] For example, g-C₃N₄ was complexed with nitrogen-doped graphene to catalyze HER.^[60] This hybrid catalyst exhibited desirable HER activities due to its intrinsic chemical and electronic coupling. She et al.^[75] confirmed that a small amount of α-Fe₂O₃ nanosheets formed 2D hybrid with g-C₃N₄, which exhibited an all-solid-state Z-scheme junction. The hybrid achieved a high H₂ evolution rate because the Z-scheme junction could suppress the recombination of electron-hole pairs as well as promote the overall water splitting without any sacrificial donor. Hou et al.^[76] successfully constructed MoS₂/g-C₃N₄ nanojunctions, which exhibited enhanced photocatalytic HER activities under visible light compared with bare MoS₂ and g-C₃N₄. Besides, they also found that other layered transition metal dichalcogenides (for example, WS₂) promoted HER rates after complexed with g-C₃N₄. Besides, noble metal Pt is also a good choice of cocatalyst, however the high cost greatly limits its application. Li et al.^[22] anchored isolated single Pt atoms on g-C₃N₄ with a high dispersion and stability to form a co-catalyst. It was found that Pt-CN remarkably enhanced the photocatalytic hydrogen evolution activity as well as achieved the maximum utilization of Pt atoms to reduce the cost.

On the other hand, doping selected heteroatoms is an efficient way to modify its electronic properties.^[56,77] Some non-metallic elements, such as boron (B), fluorine (F), phosphor (P) and sulfur (S) are often introduced into g-C₃N₄ to modify its electronic structure and improve its photocatalytic activity.^[1] Using P doping and thermal exfoliation, Ran et al.^[78] successfully prepared porous P-doped g-C₃N₄ nanosheets. This photocatalyst showed a high visible-light photocatalytic hydrogen production rate. After P doping, the intrinsic bandgap of g-C₃N₄ was narrowed from 2.98 to 2.66 eV, and the empty midgap states (-0.16 V vs. SHE) induced by P doping was favorable for promoting the visible light absorption of P doped g-C₃N₄ nanosheets (PCN). In another paper, P doped g-C₃N₄ was prepared by a thermally induced copolymerization method.^[79] The HER rate of P doped g-C₃N₄ reached 50.6 mmol h^-1, because P atoms changed the electronic structure of g-C₃N₄ and suppressed the recombination of charge carriers. Lan et al.^[13] synthesized Br doped g-C₃N₄ for hydrogen evolution. By doping Br, the CNU-Br_0.1 showed more than two times higher HER rates than pure CNU sample. Some metallic elements also introduced free electrons to g-C₃N₄ to modify its electronic structure as well as provide organic-metal hybrid. Thang et al.^[80] investigated cobalt (II) phosphide hydroxide co-doped g-C₃N₄ (Co-P/C₃N₄) for the hydrogen production. Results showed that the hydrogen production rate of Co-P/C₃N₄ was 14 times higher than g-C₃N₄.^[80] Combining doped g-C₃N₄ with other photocatalyst can further promote their photocatalytic activity. Chen et al.^[81] prepared a novel photocatalyst of Co-g-C₃N₄/MoS₂ by coupling Co-doped g-C₃N₄ and MoS₂ nanosheets. The photocatalytic activity of the catalyst was better than g-C₃N₄/MoS₂ and Co-g-C₃N₄, which was mainly due to the formation of a 2D heterojunction, promoted charge carrier transport, suppressed recombination of charge carriers, and increased light absorption.^[81]

Table 1. HER performance of typical g-C₃N₄-based photocatalyst.

Materials	Application	HER rate	Ref
g-C3N4 powder/Pt	water reduction	7.3 µmol h-1	[73]
porous g-C3N4	water splitting	0.25 µmol h-1	[74]
a-Fe2O3/g-C3N4	water splitting	3´104 µmol g-1 h-1	[76]
MoS2/g-CN	water splitting	20.6 mmol h-1	[77]
Pt-CN	water splitting	300 µmol h-1	[22]
P doped g-C3N4	water splitting	1596 µmol g-1 h-1	[79]
P doped g-C3N4	water reduction/ photodegradation of RhB	50.6 µmol h-1	[80]
Br-modified g-C3N4	water reduction	48 µmol h-1	[13]
Co-P/C3N4	water splitting	386.8 µmol g-1	[81]
Co-g-C3N4/MoS2	water reduction/ photodegradation of RhB	5411 µmol g-1 h-1	[82]

 

4.2 Oxygen reduction reactions in fuel cells

Fuel cells attract great interest for offering cleaner and more sustainable energy. At present, the practical applications of fuel cells are restricted by the high cost of Pt catalyst and the sluggish kinetics of ORR.^[14,82] Nitrogen-containing carbon materials, such as g-C₃N₄, are worthy investigating because they provide sufficient active sites for ORR.^[1,83,84] However, the electrocatalytic performance of g-C₃N₄ is restricted by its poor electron transfer. To solve this problem, one strategy is to use conductive carbon materials as support to improve the electron accumulation, thus to enhance the electrocatalytic performance. Lyth et al.^[58] used g-C₃N₄ as a catalyst for oxygen reduction, and they found out that although the electrocatalytic activity of g-C₃N₄ was better than pure carbon, the current densities were pretty low, which possibly was due to its low surface area. It was found that the current densities increased by mixing C₃N₄ with carbon black.

Yang et al.^[85] fabricated graphene-based C₃N₄ (G-CN) nanosheets by nanocasting. The G-CN nanosheets possessed a high nitrogen content and a high specific surface area and showed promoted electrical conductivities.^[85] As a result, the G-CN nanosheets exhibited excellent electrocatalytic activity for ORR. Macroporous g-C₃N₄/C was prepared by using silica microspheres as hard templates.^[14] This hybrid catalyst exhibited outstanding activities towards ORR in fuel cells, and was comparable with commercial Pt/C.^[14] Liu et al.^[15] doped Co into g-C₃N₄ polymer and supported it on graphene as a catalyst for ORR in alkaline fuel cells (Fig. 8). The experimental results showed that ORR catalyzed by Co-g-C₃N₄ mainly occurred in a four-electron pathway. The high ORR activity was attributed to the sufficient Co-N_x active sites and fast electron transport.^[15]

Fig. 8 (a) Schematic illustration of Co-g-C₃N₄/graphene; (b) RDE curves of four samples with a sweep rate of 5 mV s⁻¹ at 1600 rpm in O₂-saturated 0.1 M KOH. Reproduced with permission ^[15] (Copyright © 2013, American Chemical Society)

Apart from carbon-based materials, some metals are also investigated to complex with g-C₃N₄ to achieve a better ORR performance. For example, Kundu et al.^[59] reported a facile synthesis of porous gold aerogel supported on carbon nitride (Au-aerogel-CN_x). Results showed that four-electron ORR process was dominant at the catalysts in both alkaline and acidic media, and the excellent ORR performance was due to the synergistic effects between porous Au-aerogel- CN_x.^[59] Zheng et al.^[86] designed and synthesized a series of g-C₃N₄ coordinated transition metals as catalysts for oxygen electrode reactions. After theoretical calculation and experimental measurements of a Co-C₃N₄ catalyst, it was found that the high ORR activity was ascribed to the Co-N₂ coordination.^[86]

It can be seen that g-C₃N₄ shows promising activity towards photocatalytic hydrogen evolution. Especially, when combined with conductive supports (such as mesoporous carbon or graphene), g-C₃N₄ can exhibit highly efficient ORR

Table 2. Typical g-C₃N₄-based catalyst ORR performance.

Materials	Application	Onset Potential (V)	Current Density (mA cm-2)	Ref
g-C3N4/CB	fuel cells	0.76	2.21	[59]
Graphene-C3N4	fuel cells	-	7.3	[86]
g-C3N4/C	fuel cells	-0.14	4.3	[14]
Co-g-C3N4@ graphene	fuel cells	-0.03	4.9	[15]
Au-aerogel-CNx	-	0.92	10	[60]
Co-C3N4/CNT	-	0.9 V	5 mA cm-2	[87]

 

activity, which will be beneficial for the development of fuel cells. Although g-C₃N₄ has a great potential in various applications, the correlations between the g-C₃N₄ structure and its catalytic activity are still not clear. Current research basically focuses on the performance rather than the mechanism. For future studies, powerful theoretical calculation is needed to understand the physicochemical properties of carbon nitride and to clarify the relationship of nanostructure and catalytic kinetics. Besides, in current research, the combination of g-C₃N₄ and other materials has achieved promising performances, but the inherit mechanism is still needed to be found out.

4.3 Li-based battery

Because of the relatively high specific energy and good stability, some Li-based batteries, such as Li-ion battery, Li-O₂ battery and Li-S battery have attracted tremendous interests for their potential applications in electric vehicles. However, the practical applications of these batteries are restricted by the sluggish kinetics of the reactions on the electrodes. In order to solve this problem, efficient catalyst should be developed and employed. g-C₃N₄ may provide more active sites for the electrode reactions due to its high nitrogen content.^[87,88] Still, g-C₃N₄ is limited by the low electronic conductivity. Therefore, some electronic conductive materials should be used to increase electrons accumulated on g-C₃N₄ surface.^[87]

Hou et al.^[89] prepared a N-doped graphene/porous g-C₃N₄ nanosheets supporting layered-MoS₂ hybrid as Li-ion battery anodes. MoS₂ nanosheets were dispersed uniformly on the matrix to form a multilayered structure.^[89] The hybrid catalyst shows excellent cycling stability, high rate capability and large capacity due to the special structure. Yin et al.^[90] anchored SnS₂ on g-C₃N₄ nanosheets to synthesize a composite electrode for Li-ion batteries. The large surface area provided more active sites and promoted the charge transfer. Besides, the synergistic effect between g-C₃N₄ nanosheets and SnS₂ lowered the charge-transfer resistance, which was in favor of the alloying reaction between metallic Sn and Li⁺. As a result, the catalyst delivered a high discharge capacity with a high coulombic efficiency of over 99.9%, and it also achieved a high specific capacity retention rate. g-C₃N₄@carbon papers (GCN@CP) were prepared via a facile in situ method to improve the electronic conductivity of g-C₃N₄ and obtained a high nitrogen content.^[18] When employed as cathodes for Li-O₂ batteries, GCN@CP exhibited favorable electrocatalytic performance toward ORR and OER in nonaqueous electrolytes, showing good rate capability and cyclic stability. In another paper, Luo et al.^[91] synthesized a free-standing graphene@ g-C₃N₄ (G@CN) composite cathode for Li-O₂ batteries. During the reaction process, g-C₃N₄ nanosheets acted as electrocatalysts while graphene nanosheets accommodated Li₂O₂ and improved the electron transfer.^[91] As a result, the G@CN electrocatalyst showed a favorable cyclic stability with a high round-trip efficiency. Zhao et al.^[92] synthesized single-atom Pt catalyst supported on holey ultrathin g-C₃N₄ nanosheets (Pt-CNHS) to serve as cathode in Li-O₂ battery (Fig. 9). Li-O₂ batteries utilizing Pt-CNHS showed much higher discharge specific capacities than those with pure CNHS.

Fig. 9 Schematic illustration of the synthesis and reaction mechanism of Pt-CNHS. Reproduced with permission^[93] (Copyright@2019, Elsevier Ltd.)

The assembled batteries using Pt-CNHS as catalyst could deliver a 17059.5 mAh g^-1 discharge capacity, while that of pure CNHS was only 5890.1 mAh g^-1. CNHS with porous structure promoted the mass transport and electrolyte wetting, as well as accommodated Li₂O₂. Single-atom Pt was also in favor of the efficient interfacial mass transfer due to its high electrical conductivity. Meng et al.^[19] used g-C₃N₄ nanosheets as a sulfur anchoring material in Li-S batteries. The as-prepared g-C₃N₄ nanosheets possessed a high nitrogen (N) content of 56 wt% and a high specific surface area of 209.8 m² g^-1, which were in favor of the loading of lithium polysulfides. As a result, the GCN/S delivered good electrochemical performances at the initial and 750^th cycle.^[19] 3D light-weight and porous C₃N₄ nanosheets@reduced graphene oxide (PCN@rGO) network was prepared to solve the shuttle effect in Li-S batteries.^[93] The PCN@rGO network anchored polysulfides by strong chemical adsorption as well as promoted electron and mass transfer.^[93] As a result, the PCN@rGO-S cathode showed superior rate capability and ultra-long cycling stability. Zhang et al.^[94] synthesized a 3D porous sulfur/graphene@ g-C₃N₄ (S/GCN) hybrid sponge and applied it as a free-standing cathode for Li-S batteries. The N-sites in GCN provided a large amount of adhesion sites for polysulfides, achieving a “physical-chemical” dual-confinement for polysulfides. Moreover, the porous 3D graphene frameworks promoted electron/Li⁺ transport and maintained good structure integrity. By using a microemulsion encapsulation method, S/GCN cathodes achieved a sulfur loading up to 82 wt%. Therefore, the S/GCN delivered a high specific capacity and excellent rate capability as well as alleviated anode corrosion issues.

5. Summary and outlook

In summary, this review presents an overview of g-C₃N₄ and g-C₃N₄-based materials, focusing on the synthesis methods, properties and applications in energy storage. The unique properties and wide applications prove that g-C₃N₄ is promising for efficient energy storage. Therefore, the research of g-C₃N₄-based composite for energy storage and conversion will continue to accelerate in the near future.

    g-C₃N₄ exhibits great activities for photocatalytic hydrogen production in water splitting. Besides, when blending with other electrocatalyst, g-C₃N₄ can become highly efficient for ORR catalyzing. Therefore, the applications of g-C₃N₄ can be extended to fuel cells and some Li-based secondary batteries. To date, some researchers have attempted to make g-C₃N₄ based materials serve as catalysts in secondary batteries and achieved satisfying results. Still, the studies are preliminary, and more efforts should be focused on the internal reaction process during discharge and charge cycle. In general, considerable progress has been achieved regarding g-C₃N₄, but the studies are still not complete. The relationship between the g-C₃N₄ unique structure and catalytic activity is still not clear. There are still many opportunities for further research efforts. Accordingly, more studies are needed to figure out the structural and electronic properties of g-C₃N₄.

    As g-C₃N₄ related research develops, an urgent and realistic question is raised: what else can we continue to work on g-C₃N₄-based material presently? After reviewing the research works in the past, we believe that the following problems should be solved in the future studies:(1) New synthesis process for g-C₃N₄ should be proposed. As we introduced in this review, g-C₃N₄ is commonly prepared from the condensation of nitrogen rich precursors, which can be polymerized into various mediates. In order to improve the catalytic activity of g-C₃N₄, exploiting new methods to prepare the g-C₃N₄ with a higher crystalline quality is of great importance. (2) A more comprehensive understanding of the reaction mechanism is necessary for promoting the design and performance of g-C₃N₄ based catalysts. The charge carrier transfer mechanism, the thermodynamics and kinetics of surface reactions are worth studying. In order to realize the understanding of mechanism, some powerful tools, such as in-situ characterization techniques and theoretical calculation may be of great importance. (3) Apart from the research of sole g-C₃N₄, combining it with other novel catalysts may be a path towards more efficient energy storage strategies. It’s important that researchers should try to design a g-C₃N₄ based composite rather than simply find one. Therefore, the understanding of g-C₃N₄ and other novel materials is vital for the new catalyst. In a word, there are enormous opportunities regarding g-C₃N₄ and g-C₃N₄-based materials, but this may require the unremitting efforts of worldwide researchers.

 

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51672162), Shenzhen Science and Technology Plan Projects (No. JCYJ201708181053516 00).

 

Supporting information

Not applicable

 

Conflict of interest

There are no conflicts to declare.

 

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