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Graphitic Carbon Nitride for Electrochemical Energy Conversion and Storage Wenhan Niu, and Yang Yang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01594 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Energy Letters

Graphitic Carbon Nitride for Electrochemical Energy Conversion and Storage Wenhan Niu†, Yang Yang†,‡,* †NanoScience

Technology Center (NSTC), ‡Department of Materials Science and Engineering,

University of Central Florida, Orlando, FL, 32826, USA *Corresponding author: [email protected] Abstract Owing to the rising pressure on the requirement of commercializing sustainable and environmental-friendly energy technologies such as proton exchange membrane fuel cells (PEMFCs), metal-air batteries and water-splitting electrolyzers, it is urgent to develop highly efficient electrocatalysts to replace costly platinum group metals. Graphitic carbon nitrides (gC3N4) have been attracted intensive focus due to their unique properties and impressive performance in challenging the widely accepted nitrogen-doped carbon materials in electrocatalytic fields. However, the relatively poor conductivity limits the further improvement of g-C3N4–based electrocatalysts. Thus, this review is primarily focused on the recent progress in the functionalization of g-C3N4 materials for oxygen reduction reaction and water splitting reaction. Especially, an innovative and in-depth understanding of g-C3N4 materials is presented through systematically summarizing the function of g-C3N4 material, such as serving as active sites, coordination complex and supporter/protective coating, in contributing the catalytic performance. Finally, the main challenges and future perspectives of g-C3N4-based nanomaterials in electrocatalytic fields are also discussed. It should be noted that, in the text, we acknowledge that "many" (in fact, most) of the g-C3N4 materials have been polymeric amorphous phases within the C-N-H system. For avoiding the debate and confusion on the naming of previously reported “graphitic carbon nitride” materials in our review, we decide to nominate all these materials as "g1 ACS Paragon Plus Environment

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C3N4" in accordance with previous reports.

TOC

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ACS Energy Letters

In recent decades, the increasingly serious energy crisis and the gradually emerging awareness in environmental protection have inspired intensive researches on the development of sustainable and environment-friendly energy technology alternatives to traditional fossil energy industry.1 H2 energy has become a feasible and easy-available energy source that can effectively alleviate current problems brought by fossil consumption.2,3 H2 energy-related technologies ranging from H2 production to H2 energy applications are of special importance to determine the efficiency and cost of H2 consumption. Of these, fuel cell technologies containing proton exchange membrane fuel cells (PEMFCs), H2-O2 fuel cells, metal-air batteries and water splitting electrolyzer have been considered to be the most promising candidates as the new generation of sustainable energy devices.4,5 H2 production by water splitting electrolyzers generally comprises two half-reactions of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), and the sluggish OER at the anode of water splitting reaction greatly limits the efficiency of H2 production at cathode, which needs the highly efficient electrocatalysts to reduce the potential barriers and thereby drive the elementary reactions more readily.3,6-8 In addition, the efficient utilization of H2 in PEMFCs is also relied on oxygen reduction reaction (ORR) at the cathode. Platinum (Pt) group metals (PGMs), such as ruthenium (Ru) and iridium (Ir) etc., appear as the state-of-the-art electrocatalysts at present, which could be valuable but not ideal catalysts for electrocatalytic reactions due to their scarcity, high cost, and poor durability.9,10 In recent decades, nitrogen (N)-doped carbon electrocatalysts, such as N-doped graphene and N-doped carbon nanotubes, have been popular metal-free catalysts for the ORR, OER and HER due to their remarkable activity.11-14 It is well-known that when a nitrogen atom is doped into carbons, three typical N configurations, including quaternary N (or graphitic N), pyridinic N, and pyrrolic N, are formed within the carbon skeleton.15 Of these, pyridinic N and quaternary N have



been widely accepted as the main active sites for ORR, OER, and HER.11,16 In addition, although with decades of efforts in the development of N-doped carbon electrocatalysts, several barriers still exist, e.g., the relatively low nitrogen content (2~5 at.%), the poor stability and catalytic performance in extreme electrochemical condition (or in the PEMFC operation environment).17 Moreover, there has been a long debate on whether the catalytic activity of N-doped carbons is derived from its unique electronic properties or metal residue from its metal involved synthesis process.18 Therefore, the studies on N-doped carbon electrocatalysts are facing an unprecedented challenge. Being that with a similar two-dimensional structure to N-doped carbons (e.g. N-doped graphene), carbon nitride (referred to as CxNy) have attracted extensive attention previously since its extraordinary performance in the application of photocatalytic HER. Graphitic carbon nitride (g-C3N4) is a polymer semiconductor comprised of replicate s-triazine units, with a bandgap around 2.7 eV, thus enabling the material to absorb UV and portion of visible light, which can be facilely synthesized from a nitrogen-containing precursor via a series of polycondensation reactions without any metal involvement.19 In addition, g-C3N4 has been previously demonstrated to show a robust stability for a variety of photocatalytic reactions.20 Owing to its high nitrogen content (theoretically up to 60%), facile synthesis procedure and easy-tailored structure, g-C3N4 has shown its potential as a feasible nitrogen-containing electrocatalyst.21 Especially, g-C3N4 is designed with a nanostructure, or combined with other highly conductive materials (graphene/carbon tubes, ITO, and other metal compounds) could further enhance its catalytic activity. On the basis of experimental observation and theoretic calculation, pyridine-like N in triazine configuration is predictably related to the active sites for electrocatalytic reactions.22 With many merits in structure and composition, g-C3N4 would be the most promising alternative to N-


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ACS Energy Letters

doped carbons and PGM catalysts. But, there remain many challenges, such as poor conductivity and limited specific surface area, which need to be well addressed before their wide application in electrocatalytic fields. Despite that many reviews summarized the recent progress in synthesis and application of g-C3N4 materials, few mechanisms and functions of g-C3N4 materials in contributing electrocatalytic activity were systematically discussed and summarized, especially when these functionalized g-C3N4 nanomaterials were used as electrocatalysts in batteries and electrolyzers.2325

Therefore, this review not only focuses on the recent progress in the design, synthesis, and

application of g-C3N4-based electrocatalysts for electrochemically sustainable energy conversion but also discusses the synergetic effect and mechanism of g-C3N4 in promoting the electrochemical performance. The role of g-C3N4 materials in g-C3N4-based electrocatalysts will be divided into three types based on their structure and composition: 1) g-C3N4 takes the major role in serving as active center for electrocatalytic reactions, and this type of catalysts is referred as metal-free gC3N4 electrocatalysts; 2) g-C3N4 acts as a supporter or protective coating to offer a synergetic effect in enhancing the catalytic activity and stability of metal-based catalysts; 3) with a lot of triazine units in g-C3N4, which can undertake the coordination complex to anchor metal atoms or clusters to form M-Nx/C active sites. In summary, the unique insight into understanding the mechanism of g-C3N4 functionalizing various electrocatalysts will provide a significant impact in pushing the development of outstanding g-C3N4-based electrocatalysts for energy conversion. Finally, some conclusive remarks and perspectives on the recent progress and future directions in the g-C3N4-related research are presented. Traditional Synthesis of Bulk g-C3N4. The carbon nitride and its family were first discovered by Berzelius and named by Liebig in 1834, close to the ideal“melon” structure that comprised of structural formula C6N7(NH)(NH2), or C2N3H. However, the initial “melon” was synthesized



through the reaction of NH4SCN and KSCN with Cl2 treatment, which follows Scheele’s studies on prussic acid (HCN) and its salts. Their works lead to the extensive studies of the thiocyanate (SCN-) anion-containing compounds, such as the study of mercurous thiocyanate conducted by Porret.26 Interestingly, the typical Hg(SCN)2 as a member of mercuric group compounds was firstly prepared by Berzelius in 1821.27,28 Strictly speaking, the most of carbon nitride materials studied recently are synthesized by the thermolytic polymerization of liquid nitrogen-containing molecular such as urea, dicyandiamide and melamine, resulting in a series of polymeric CxNyHz compounds with the C-N-H system. In fact, few reports described their materials with C3N4 stoichiometry because these C3N4 are based on linked triazine (C3N3) rings instead of heptazine units.29-33 Thus, the most of g-C3N4 previously reported are amorphous polymeric materials containing the linked triazine-based structures rather than the ribbons of condensed polyheptazine chains with perhaps some side-ways polymerization giving rising to the onset of sheet-like units (Figure 1).34-36 However, in our review, for avoiding the debate and confusion regarding the naming of g-C3N4 in electrochemical studies, we unify all the name of so-called “graphitic carbon nitride” materials as the g-C3N4, consistent with those in the pioneering reports.36 Since 2009, g-C3N4 has attracted wide interest because that Wang and his coworkers first demonstrated that g-C3N4 could serve as a visible-light absorption photocatalyst for HER.37 Generally, there are several allotropes in a C3N4 system such as α, β, cubic, pseudocubic, and graphitic structures. In comparison to other C3N4 allotropes, g-C3N4 possesses the most stable chemical structure under varied extreme environment, for instance, it can hold up a 600 oC under air atmosphere, which is considered to be a promising stable-type catalyst toward electrochemical catalysis. Of these, the one allotrope of g-C3N4 constructed by the substructure of the polyheptazine (or tris-s-triazine) unit is considered to be more stable than the same one derived


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ACS Energy Letters

from triazine. With extensive effort in the study of g-C3N4, many skillful methods have been developed for the synthesis of g-C3N4 nanomaterials. But, most of the methods are based on thermal condensation of nitrogen-containing liquid precursors (e.g. urea, thiourea, melamine, cyanamide, and dicyandiamide) under optimal condition (Figure 1). As a typical synthesis procedure reported by Wang et al.,37 g-C3N4 was synthesized by thermal polycondensation of cyanamide under temperatures between 673 and 873 K (ramp: 2.2 K min−1) for 4h. The thermal gravimetric analysis (TGA) and X-ray diffraction (XRD) techniques tracked the intermediate compounds during the polycondensation and verified that the synthesis included many polycondensation steps. For example, the cyanamide molecules were firstly polycondensed to dicyandiamide or melamine monomer at low temperatures within 203~234 °C. Then, the ammonia groups were volatilized from dicyandiamide or melamine monomer to form the low polymerized tri-s-triazine-based products around 335 °C. The ordered and linked tri-s-triazine units were built up via the reconstruction of melamine monomers at 390°C. Finally, the polymeric g-C3N4 was obtained at ca. 520 °C. When the g-C3N4 underwent a further thermal treatment beyond 600 °C, it became unstable and was then decomposed into nitrogen and cyano-containing gas molecules. Despite the synthesis of g-C3N4 by using above nitrogen-containing liquid precursors, Wolf et al. reported a strategy for the synthesis of close-to-crystalline g-C3N4 by using 2-amino-4,6dichlorotriazine as a precursor at a high pressure and temperature, in which the released HCl gas likely served as a template to fill the nitridic in-plane pores of a triazine-based pattern.38 This gC3N4 was found to show a high-quality structure due to the highly ordered and regular in-plane pattern of the triazine units. Except for the synthesis of g-C3N4 by the thermal polymerization, Xie et al. also reported a solvothermal method for the synthesis of g-C3N4 with using cyanuric chloride and sodium amide as precursors, and benzene as solvent under 200 °C for 8~12 h.39 In the



meantime, Montigaud et al. also demonstrated the solvothermal condensation strategy for the large-scale production of g-C3N4 using melamine and cyanuric chloride precursors.40 In the synthesis, the first condensation was initiated at 250 °C under air pressure of 140 MPa, which resulted in a low polymerization of melamine with many NHx groups. The second step was further heat treatment of melamine in presence of hydrazine at 800~850 °C within 2.5 GPa, which led to the product of g-C3N4 possessing a perfect structure, but a large loss of nitrogen groups and little remaining nitrogen in regular distribution. Regarding the synthesis of g-C3N4 catalysts with high-activity properties, it has been reported that the deprotonation of g-C3N4 could result in the alteration of its surface properties (for example, increasing the Lewis basicity of g-C3N4), and subsequent improvement of its photocatalytic performance. In other side, surface terminations and defects are also considered to be more important for mediating the electronic band structure and band gap, which are very significant for g-C3N4 material. Consequently, for bulk g-C3N4 materials, they are thought to perform rather poorly in some catalytic processes due to the less-defective structure or unfavorable surface terminations. In contrast, the construction of nanostructures seems to be a wise way of enhancing the catalytic performance of g-C3N4 according to recent studies ， which can be achieved by involving hard templating and soft templating strategies, viz. nano-casting and supramolecular self-assembly of melamine, dicyanamide and other liquid precursors with hard templates and soft templates, respectively, under certain condensation temperatures.41,42 These nanostructured gC3N4 catalysts were believed to provide abundant and accessible active sites to the electrolyte and reactive molecules and enable the acceleration of mass transfer system during catalytic reactions. In contrast, the bulk g-C3N4 materials are far from showing their advantages in the electrocatalysis. 1.1. Synthesis of Porous g-C3N4 Materials


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ACS Energy Letters

As mentioned above, the catalytic performance of g-C3N4 materials is related to their structure and nature. Porous g-C3N4 materials are of most interest especially when they are applied in heterogeneous catalysis due to their high density of active sites and large specific surface areas.23,24,43 Generally speaking, the direct thermal treatment of dicyanamide, melamine or other nitrogen-containing liquid precursors can only synthesize g-C3N4 with a limited specific surface area and small pore volume (less than 10 m2 g-1 and 0.1 cm3 g-1) that is not favorable for mass transfer as well as electron mobility. But, macro-/meso-porous g-C3N4 can offer a high specific surface area and large porosity (up to 830 m2 g-1 and 1.25 cm3 g-1), which is considered as a more promising metal-free catalyst that provides the larger numbers of accessible active sites, the higher selectivity of size or shape and the shorter pathway for electron transfer. Overall, the strategies for the synthesis of porous g-C3N4 are established on the basis of templating methods including hardor soft-templating methods. As for soft-templating method, it employs liquid polymers or surfactants as volatilizable templates in the synthesis under a certain temperature. For instance, Zhang et al reported soft-templating syntheses of meso-g-C3N4 via various surfactant templates i.e., Triton X-100, P123, F127, Brij30, Brij58, and Brij76.44 In the synthesis, the porous structure was formed through the self-assembly of surfactant molecules and dicyandiamide, followed by precise control of thermal polymerization leading to decomposition and volatilization of surfactants. However, the carbon residue on the surface of g-C3N4 from the pyrolysis of templating surfactants greatly decreased the density of surface nitrogen active sites, and even seriously reduced its catalytic activity. For addressing this issue, Yan et al demonstrated the soft-templating synthesis of mesoporous g-C3N4 catalysts by employing Pluronic P123 surfactant as template and melamine as a precursor. Owing to that the precursor melamine is more chemical-stable than that of dicyanamide, the cross-linking reaction between the precursor and the surfactant could be



efficiently avoided during the pyrolysis, giving rise to the g-C3N4 polymerization and surfactant decomposition following their own pathways with little carbon residue. At last, the as-prepared mesoporous g-C3N4 with a trace amount of carbon dopants showed a sufficient light absorption capability with a large absorption wavelength up to 800 nm, which was finally demonstrated to be favorable for photocatalytic H2 production.44 As compared to the soft-templating method, hard-templating method is more controllable and facile, and the resultant porous material also exhibits a higher specific surface area and a more consistent pore structure. Experimentally, the first step for preparation of porous g-C3N4 is to nanocasting of the g-C3N4 precursor into the hard template (e.g. SBA-15, KIT-6 and silicon oxide sphere),45-48 which is driven by the capillary force of channel. Subsequently, the oriented-grown g-C3N4 is formed by a polycondensation within the channel of template due to the space confinement effect. As a result, nanoporous g-C3N4 was synthesized by a removal of the template in HF or NH4HF solution. It is worth noting that the surface hydrophilicity and pH value of templates determine the nanocasting performance of precursor within hard-template. For instance, most of the g-C3N4 precursors (dicyanamide, melamine, urotropine and so forth) present with an alkalinity that is not suitable for bonding with surface –SiOH of the template, resulting in a poor filling of the precursor. For this, Wang et al demonstrated a strategy to realize the sufficient filling of the precursor into silica template by acid-alkaline effect, viz. to introduce silica template with the acidic surface by the pre-treatment of the silica template in acidic solution. In contrast to the templates without acidic treatment, the synthesis involving acid-mediated template produced the meso-g-C3N4 with a highly ordered structure.49 With respect to the template-free method for synthesizing nanoporous g-C3N4, Wang et al reported a strategy to prepare honeycomb-like g-C3N4 by adding distilled water into the thermal


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ACS Energy Letters

condensation of urea, in which the additional H2O molecules can serve as “bubbles” that generate cavities in g-C3N4 framework during the synthesis.18 Hu et al employed KOH as an active agent to etch g-C3N4 to obtain hierarchically porous g-C3N4. Specifically, the bulk g-C3N4 was first synthesized，and then etched by KOH under 350 °C to generate small pores. Meanwhile, the small pores can be enlarged with the increase of etching time, resulting in a macro-/meso-porous structure of g-C3N4. Then, the hierarchically porous g-C3N4 can be purified by dissolving the product in an acid solution for the removal of KOH active agent.50 1.2. Other Strategies Based on many attempts mentioned above in improving the conductivity of g-C3N4, it was found that integrating g-C3N4 with carbon materials (such as graphene, carbon nanotubes and porous carbons) is the most effective way. However, the route to combine g-C3N4 with conductive carbons includes mechanical mixing and in-situ growth methods. The mechanical mixing method indicates that the bulk or nanostructured g-C3N4 are mixed with conductive carbons during the preparation of catalyst ink. For low graphitized carbons, such as graphene oxides, it usually needs a post-heat or solvent-thermal treatment of g-C3N4/graphene oxide mixture in a bid to reduce graphene oxides to reduced graphene resulting in a high electron-transfer property. On basis of physical contact, the electron transfer only occurs within the interface between g-C3N4 and conductive carbons, even if when the bulk g-C3N4 is shaped into quantum dot ， nanosheet or nanoribbon with a high specific surface area.51,52 Thus, there remains a limitation making full use of conductive carbons to enhance the conductivity of g-C3N4. However, the strategy using in-situ growth of g-C3N4 on conductive carbons renders a chance to enhance the electron-transfer capability by the electron coupling of g-C3N4 and carbon substrate. For example, the thermal polycondensation of nitrogen-containing liquid precursor with graphene or carbon nanotubes can 11 ACS Paragon Plus Environment


introduce the formation of covalent -C-N-/-C=N- or π-π stacking within the interface of g-C3N4 and carbon substrate, resulting in the enhanced electron transfer through the π-electron system.5355

Except for the conductive carbon materials, the metal or metal compounds, e.g., Pt, Pd, Au, and

ITO, can be also employed as conductive “additives” to facilitate the electron transferability of gC3N4 through the injection of transportable electrons from their atomic d orbits.56-60 Without the introduction of an external substrate, doping g-C3N4 with heteroatoms, such as B, S, P and I, also affects its electronic property.61-65 Zhu et al found that the P-doped g-C3N4 showed a narrower band gap than that of pristine one ascribing that the P atoms in g-C3N4 skeleton presented as a bonding condition with adjacent C and N atoms, which results in the delocalization of electron cloud from the π-conjugated tri-s-triazine of g-C3N4.66 Obviously, the relatively poor conductivity of g-C3N4 materials could be explained by the replacement of most of the graphitelike sp2-hybridized C atoms in the hex-atomic ring by N atoms. In recent years, the researchers found that the electronic properties of g-C3N4 could be further optimized by the introduction of Ndeficiency. Chen et al synthesized a nitrogen-deficient g-C3N4 by treating pristine g-C3N4 with magnesium powder at 750 oC.67 The obtained nitrogen-deficient g-C3N4 showed an improved electron-transfer capability attributing to the re-arrangement of C atoms in triazine to form a graphene-like honeycomb structure and the vaporization of N species of g-C3N4. However, the limited performance can be achieved in such the g-C3N4 hybrid with a Van der Waals heterostructure. Our group recently demonstrated the concept that conjugating g-C3N4 with crystalline carbon domains can effectively increase the electron mobility within the planar of gC3N4 due to the construction of a huge π-electron system among tris-s-triazine units of g-C3N4.68 Overall, facilitating the charge transfer, and simultaneously providing all the N sites with sufficient electrons are still the main challenge for achieving a high-performance g-C3N4–based


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ACS Energy Letters

electrocatalysts. In the following part, we summarized and discussed most of g-C3N4–based electrocatalysts reported within the recent decade (Table 1). Besides, we carefully investigated the function of g-C3N4 in improving the catalytic performance of g-C3N4–based catalysts. g-C3N4 Serves as Major Active Species. As well known, g-C3N4 possesses similar nitrogen configurations to nitrogen doped carbons in its skeleton, which should be a potential metal-free electrocatalyst for electrochemical ORR. The typical g-C3N4 electrocatalyst was first reported by Lyth and his coworkers.69 According to their study, the bulk g-C3N4 was synthesized by the solvothermal method at 220 °C for 22 h. The synthetic process involved reactive cyanuric chloride and sodium azide within the benzene solution, which underwent the following reaction: C3N3Cl3 + 3NaN3→g-C3N4 + 3NaCl + 4N2. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) measurements verified the subunit of melam in the product, which was consistent with the regular structure of g-C3N4. The ORR performance of g-C3N4 was testified by using a rotating disk electrode (RDE) technology in O2-saturated 0.5 M H2SO4 solution (Figure 3a). The catalyst electrode containing g-C3N4 and carbon black (CB) shows the better ORR activity than those of g-C3N4 and CB electrodes, suggesting that the ORR activity of bulk g-C3N4 could be improved by physically mixing it with conductive CB probably due to the enhanced electronic conductivity. In addition, cyclic voltammograms (CVs) of g-C3N4+CB showed a higher double layer capacitance than that of g-C3N4, implying the improved ORR activity is related to the corresponding increased electrochemical surface area (Figure 3b). For confirming the ORR activity being from g-C3N4, CVs of pure g-C3N4 in O2-saturated electrolyte showed an apparent ORR current within -0.2~0.2 V vs NHE, verifying that the g-C3N4 was responsible for the ORR activity (inset of Figure 3b). Besides, they found that the active sites in g-C3N4 are primarily from pyridinic N and graphitic N in the g-C3N4 skeleton. These results are the first time to reveal that



g-C3N4 materials can be served as the electrocatalysts for ORR, but, only with the condition when they are physically combined with conductive carbon support. Considering the potential application of g-C3N4 as an ORR catalyst, g-C3N4 electrocatalysts should be a promising activity comparable to that of commercial Pt electrocatalysts. Therefore, the limited effective surface area and poor conductivity in the bulk g-C3N4 catalyst are the main challenges in the further optimization of g-C3N4 electrocatalysts. It has been well-known that two-dimensional (2D) materials can provide a much higher specific surface area with more exposed active sites on the surface as compared to the bulk counterpart. In addition, there are numerous studies demonstrating 2D g-C3N4 being of excellent photocatalytic abilities in water splitting.70 Unlike photocatalysts, the high electrical conductivity is particularly required for its application in electrocatalysis except for the high specific surface area.71 For this, the strategy for integrating 2D nanostructure into the g-C3N4 electrocatalysts is more valuable. In general, 2D g-C3N4 can be synthesized by top-down strategy, for example, separating the stacked layers of bulk g-C3N4 by thermal oxidation etching71 or liquid sonication exfoliation in polar solvents.72,73 Whereas the oxidation etching usually results in a low yield of 2D g-C3N4 nanosheets, and the liquid sonication exfoliation also requires an extremely long time to peel bulk g-C3N4 into nanosheets (normally, 10-16 hrs).74 This could be attributed to the strong hydrogen bonds between the interlayers of polymeric melon units with NH/NH2 groups, in contrast, the stacked layers are considered to be more effective by acidic/thermal exfoliations.71,75 As an idea conductive substrate, graphene nanosheet possesses a 2D carbon atoms network following a honeycomb pattern, which exhibits an excellent conductivity for electron transfer, and an excellent chemical stability and mechanical flexibility to be as an extremely stable substrate or supporter.76,77 With the similar structure to g-C3N4, it is meaningful to integrate graphene into the 14 ACS Paragon Plus Environment

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ACS Energy Letters

g-C3N4 structure to address the poorly electrocatalytic performance resulted from the low electrical conductivity of g-C3N4. Yang et al demonstrated tactics for introducing graphene substrate into gC3N4 nanosheets by using a nano-casting technology with oxide graphene supported silica nanosheets as a rigid template.78 Not surprisingly, the graphene supported g-C3N4 nanosheets presented a remarkable electrocatalytic performance with low overpotential, long stability and high selectivity with 4 e- transfer for ORR, which were superior to those of monophasic g-C3N4 material and commercial Pt. This excellent ORR performance, better than bulk g-C3N4, might be due to the following factors: 1) the individual dispersion of g-C3N4/graphene composite (Figure 4a) enables the more accessible active sites on the inner plane of g-C3N4/graphene unit and avoiding the isolation of active sites by π-π stack of graphene supporters; 2) the strong combination of nanoporous g-C3N4 with highly conductive graphene results in a facilitated electron transfer in g-C3N4 skeleton (Figure 4b), and the nitrogen groups from g-C3N4 (viz. pyridinc N and graphitic N) also furnish graphene with abundant active sites; 3) the optimal thickness (Figure 4c) of nanoporous g-C3N4 layer shortens the electron transfer pathway; 4) more importantly, a high specific surface area (up to 542 m2g-1) was achieved in this catalyst, and the nanoporous structure, which is smaller than 5 nm, offers the high electrochemically active surface area (ECSA) of gC3N4 catalyst with sufficiently exposed active sites to electrolyte and reactive molecules. In the meantime, Shi et al also reported a similar 2D graphene supported g-C3N4 catalyst, which was prepared by the thermal condensation of a mixture precursor including melamine and graphene oxides. They found that the graphene supported g-C3N4 hybrid showed a better ORR activity in comparison to the pure g-C3N4 material, and even comparable to that of graphene loaded Pt nanoparticles with mass loading of ca. 23 wt%.79 They claimed that the enhanced activity in this g-C3N4/graphene hybrid was contributed to the similar ORR mechanism occurred on the active



sites of N-doped carbon materials, for example, the nitrogen doping into carbon materials introduces an electron-withdrawing effect on adjacent carbon atoms, which results in positive charge status on the carbon atoms that are favorable for O2 adsorption during ORR. Unfortunately, much effort has been devoted into enhancing the catalytic activity of g-C3N4 by the in-situ growth of g-C3N4 on a 2D conductive substrate, but these hybrid catalysts still show a weak combination and interaction between g-C3N4 interface and graphene basal, which gives rise to a poor electron transfer from graphene to g-C3N4. In addition, it remains challenging for gC3N4 based catalyst outperforming commercial PGM catalysts because of unclear ORR mechanism on g-C3N4. Within this context, Wang et al synthesized a kind of one-dimensional (1D) g-C3N4 dots (MTCs) with monatomic diameter by oxidative exfoliation of g-C3N4 into nanodot structure, and followed by the assembly of g-C3N4 onto graphene to form a face-to-face plane contact via a hydrothermal process (Figure 5a). The final product was referred to as MTC@graphene (MTCG).80 As they expected, the MTCG was demonstrated to possess a much better ORR activity than that of the benchmark Pt/C catalyst (Figure 5b and c). They claimed that the apparent improvement in ORR activity of g-C3N4 stemmed from the sufficient exposure of active sites in g-C3N4 dots, and the strong absorption between monatomic g-C3N4 dots and graphene substrate via a π-π interaction enabling a facilitated electron migration at the interface of g-C3N4 and graphene. One another advantage of this catalyst is that the graphene support can not only offer a continuous channel for electron migration to the g-C3N4 dots but also construct a 3D graphene framework that links all the catalytic sites within microporous and macroporous channels, accelerating the diffusion of reactants and electrolyte toward the efficient ORR. Furthermore, they conducted the density functional theory (DFT) analysis for clarifying the optimized ORR activity, which confirmed that the synergistic effect through the coupling interaction between g-C3N4 and 16 ACS Paragon Plus Environment

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ACS Energy Letters

graphene should be responsible for the obvious enhancement in catalyzing ORR. Unlike other gC3N4 catalysts, the 1D g-C3N4 dots were precisely controlled within a small diameter comprising monatomic C and N skeleton that provides an accurate active site for building a model in DFT simulation. In contrast to 1D and 2D g-C3N4 catalysts, a type of g-C3N4 catalysts constructed by threedimensional (3D) nanostructures show much better practicality and catalytic activity in fuel cells and water splitting devices，owing to their individual dispersion status and strong mechanical flexibility on electrode preventing the isolation of in-plane active sites through the stack driven by physical and chemical forces. Recently, Qiao’s group proved the excellent electrochemical performance of 3D mesoporous g-C3N4 catalyst by the rational design of nanostructure and composition.81 For example, g-C3N4 was integrated with highly ordered macroporous carbon (Figure 6a), which exhibited a superior ORR performance to N-doped carbons in a KOH solution.81,82 They found that the remarkable ORR activity was contributed to the highly dispersed g-C3N4 in the ordered porous carbons (CMK-3) framework that facilitates the mass transport, and further increases the density of reactive sites within the carbon surface of CMK-3. Whereas these nanopores in previously investigated g-C3N4/carbon materials, which are mainly derived from the pyrolysis of g-C3N4 precursors with a polymer,69,81,83 stacked graphene nanosheets,78,79 and porous carbon,82,84 are pretty small. Besides, the pore diameters of these porous g-C3N4 materials are considerably difficult to be controlled and even too narrow for catalyzing the electrochemical reactions with smooth mass transport and high accessibility. On the basis of the above consideration, they further presented the design and preparation of a 3D macroporous g-C3N4/C with ordered porous structures by using silica microsphere templates (Figure 6b).82 Interestingly, the ORR activity of this macroporous g-C3N4/C (pore size= 150 nm) was found to show a better 17 ACS Paragon Plus Environment


ORR activity than those of mesoporous g-C3N4/C (pore size= 12 nm) and commercial Pt/C. Furthermore, this macroporous g-C3N4/C catalyst also presented a higher stability than other reported g-C3N4/C catalysts and Pt/C in alkaline media, which confirms that the smooth mass transfer in macroporous g-C3N4/C enable a low overpotential for electrocatalysis ORR, and thus prevent the electrochemical corrosion of active sites at a high operating potential. In addition, the enhanced ORR stability on the macroporous g-C3N4/C can be also attributed to their uniquely porous structure constituted of interconnected pores, which efficiently avoid the pore collapse and the detachment of active sites that usually takes place in electrochemically catalytic reactions on individual carbon supported g-C3N4 catalysts. As mentioned above, graphene-based nanohybrids such as graphene supported 1D and 2D gC3N4 materials, named “van der Waals heterostructures”, render a great potential in serving as ORR catalysts, which could be derived from the sufficient exposure of active centers located at the interface of g-C3N4 plane and conductive graphene basal, and the intimate interaction between these two components. Duan et al presented a 3D g-C3N4/N-graphene hybrid film as an electrocatalyst, which was prepared by the integration of 2D porous g-C3N4 nanolayers with 3D N-doped graphene sheets through one step of vacuum filtration. However, this catalyst still maintains a similar microarchitecture as the van der Waals heterostructures, but with a 3D macroscopic architecture and hierarchical porous structure (Figure 6c), which was verified to exhibit a remarkable HER activity with a low overpotential in an acidic electrolyte.85 In the meanwhile, they also reported a 3D g-C3N4 nanocomposite prepared by the assembly of g-C3N4 nanosheets and carbon nanotubes (CNTs) (Figure 6d).84 The spontaneous assembly between gC3N4 NSs and CNTs is attributed to the electrostatic effect and p–p stacking of the negatively charged CNTs and the positively charged g-C3N4 nanosheets. Finally, this hybrid electrocatalyst 18 ACS Paragon Plus Environment

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ACS Energy Letters

displayed an impressive activity and strong stability in OER, which was better than the commercial IrO2 catalyst, owing to the robust 3D interconnected network with a robustly porous carbon framework, a large specific surface area (149 m2 g-1) and a high concentration of N atoms (23.7 wt%). Like N-doped carbons, it also has shown that doping g-C3N4 with other heteroatoms (such as B, P, S and so on) could mediate its electronic and textural properties, and further enhance its photocatalytic activity in HER.61-64,86,87 Qiu et al reported a P-doped g-C3N4 electrocatalyst comprising P-doped g-C3N4 nanosheets and NH2-modified carbon black, which showed an impressive activity toward ORR (Figure 6e).88 The excellent ORR performance of P-doped g-C3N4 composite may ascribe to the P doping into carbon skeleton to offer lone pair electrons to the adjacent π-conjugated tri-s-triazine, facilitating the local delocalization of the hole excitons and the charge transfer for increasing the conductivity of P-doped g-C3N4 materials. Ma et al synthesized a flower-like P-doped g-C3N4 on carbon fiber paper (CFP) (Figure 6f), which is capable of offering a good bifunctional activity and durability toward both ORR and OER, outperforming that of P-free g-C3N4.89 It is worth noting that integrating P-doped g-C3N4 with CFP to form a freestanding ORR/OER electrode could effectively avoid the side effect of binder polymer (i.e. Nafion) and other additives involved in electrode fabrication. This freestanding Pdoped g-C3N4 catalytic electrode finally showed an impressive bifunctional property in zinc-air batteries, with reduced charging overpotential and enhanced cyclic stability. As demonstrated above, it cannot be ignored that the synergetic effect from conductive supporters (graphene, CNT, CFP, and CB) indeed promoted the catalytic performance of g-C3N4 catalysts. Therefore, there also raises an increased concern on the optimization of conductive supporter to enhance the contribution from a conductive supporter. Shinde et al synthesized an S and Se-codoped 19 ACS Paragon Plus Environment


nanoporous graphene supported g-C3N4 composite, which exhibited a unique chemical and physical properties for efficient catalyzing HER. The double-doping with S and Se into graphene leads to the formation of additional active sites on the edge of nanopores in graphene. However, the most important advantage of these g-C3N4 hybrids is the strong synergistic effect derived from S, Se, N-relevant active sites. More specifically, the N-doped active sites in g-C3N4 provide reactive centers for hydrogen adsorption, meantime, the S and Se co-doped active sites in porous graphene boost the mobility of electrons for proton reduction.90 Although much progress has been made in improving g-C3N4-based electrocatalysts via various modifications in physical and chemical properties, such as building the multidimensional nanostructures, enlarging the specific surface area, or doping them with different heteroatoms and molecules, most of the above modifications are only related to g-C3N4 materials in its powder form. Besides, owing to the large particle morphology and the hydrophobic and lipophobic nature of g-C3N4 materials, the common deposition techniques in the preparation of catalytic electrode, such as spin-coating and screen-printing, give rise to a poor coverage of g-C3N4 powder on electrode, and a low stability and conductivity for electrocatalytic reactions stemming from the detachment of separated catalyst individuals.91,92 On the basis of nano-casting techniques, g-C3N4 could be grown in ordered porous carbons,81 or into graphene-supported mesoporous silica nanosheets78 for improving the conductivity. Whereas, for the practical application of g-C3N4 materials in fuel cells and water splitting devices, it is essential to conceive a novel and facile methodology to grow g-C3N4 materials on various conductive “platform” for a wider application. At this point, Shalom et al designed a general approach to deposit highly ordered g-C3N4 on different substrates (glass, FTO, and TiO2) for catalyzing HER (Figure 7).93 The synthetic protocols, which were carried out on the glass and F-doped tin oxide (FTO) by using cyanuric acid 20 ACS Paragon Plus Environment

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ACS Energy Letters

and melamine complex as a precursor, provided the ordered g-C3N4 rods with a higher specific surface area in comparison to the bulk g-C3N4. Taking the g-C3N4/FTO electrode as an example (Figure 6), they found that this electrode could offer a low overpotential for HER, which were located at 0.25 and 0.1 V for the electrolyte with pH 6.9 and 13.1, respectively. These values are close to those of outstanding non-PGM catalysts for HER, such as Ni electrode. Besides, they found that the FTO substrate was almost completely covered by g-C3N4 layer, due to that the reduction peak of FTO (the reduction peak at 0.1 V corresponds to SnO2 to SnO) was evidently disappeared when the g-C3N4 rods were grown on FTO, suggesting that FTO substrate did not participate in the electrochemical process, and was isolated by g-C3N4 rods layer. However, the pristine FTO substrate could be electronic oxidation and reduction in the electrolyte, which means that there is an electronic interaction between the g-C3N4 layer and FTO substrate, and the g-C3N4 should be responsible for the HER. Moreover, the stability of the g-C3N4/FTO electrode was verified by testing the sample within the CV scan of 50 cycles at a rate of 25 mV s-1. For further proving the deposition of a g-C3N4 layer on various substrates, the glass and TiO2 substrates supported g-C3N4 layers were also prepared using the same method. Interestingly, the g-C3N4/TiO2 electrode also showed a satisfactory HER activity in various electrolytes, which outperforming those observed on the pristine TiO2 electrode. They declared that the efficient HER catalysis on the g-C3N4/TiO2 electrode was ascribed to the electronic interaction between the g-C3N4 layer and the TiO2 substrate, and the facilitated electron transfer but not the electron accumulation within the in TiO2 substrate for HER. g-C3N4 Serves as a Supporter or Protective Coating. Graphene and carbon nanotube have already become extensive conductive supports or carbon shells in many transition metal electrocatalysts due to their unique chemical stabilities and conductive properties. But, g-C3N4 has 21 ACS Paragon Plus Environment


a similar carbon skeleton to those of N-doped carbon materials, which could provide rich nitrogen sites for anchoring many kinds of metals, and make a strong electronic interaction between metal particles and nitrogen sites from g-C3N4 that should be favorable for the activity improvement of PGM and transition metal. Noto et al reported a carbon core–g-C3N4 shell supported Pt alloy materials as ORR catalysts for application in PEMFC. The “core” was referred to the graphitic carbon supporter, which acted as electron-conductive “footstone” for infusing electrons into Pt alloy nanoparticles. Whereas the “shell” indicates g-C3N4 layer, which was used as “trap” for anchoring PtFe94 and PtNi.95 As a result, these core-shell catalysts showed better activities and stability than that of Pt/C. This unique structure enables this g-C3N4 to stabilize the metal nanoparticles through a chemical absorption between the carbon supporter and the nitrogen species. Interestingly, the g-C3N4 shell with an extremely low N content (less than 2.0 at.%) rendered the “shell” with a smooth electron transferability. Considering high-price, scarcity and poor stability of Pt metal, later, they tried to synthesize a series of tri-metallic Pt alloy electrocatalysts based on the g-C3N4 shell as the supporter and coordinator (such as “core-shell”-like g-C3N4 supported PdCoNi, PdCoFe, and PdCoAu).96,97 The Pt nanoparticles were alloyed with transition metals effectively reducing the cost of Pt, and increasing the activity and stability of Pt in fuel cells. Likewise, Kundu et al reported a thin carbon nitride sheets supported gold aerogel for its application as ORR and HER bifunctional catalyst. The ORR performance of this carbon nitridebased Au aerogel catalyst outperformed that of state-of-the-art Pt/C catalyst, highlighted with a high selectivity toward ORR following a 4 e- pathway. In addition, this catalyst also presented with an excellent activity for HER. All these prominent behaviors of this carbon nitride-based Au aerogel in HER and ORR are based on the unique structure, where carbon nitride or g-C3N4 not only acts as the nitrogen active sites but also serve in anchoring Au nanoparticles. Due to the close


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ACS Energy Letters

contact between Au nanoparticles and nitrogen-containing moieties, the density of electron cloud in the surface Au atoms might be changed by electronic interaction or electron transfer, which has a significant impact on the binding energy or absorption energy of Au atoms to O2 molecules or protons. With a great breakthrough in the exploitation of promising alternatives to PGM catalysts, transition metals compounds (such as oxides, sulfides, and phosphides) have shown more effectively catalytic performance toward ORR, OER and HER than those of commercial PGMs. Accordingly, the rational design of the nanostructure and component of transition metals-based catalysts, especially the effective combination of these transition metals with nitrogen-containing substrates, is extremely important for their application in electrochemical conversion devices. The g-C3N4 materials can offer a possibility to mediate the activity of transition metals by their abundant nitrogen sites and controllable structure.98 In addition, it has been already known that the smaller size of transition metal nanoparticles could provide the larger electrochemically active area with more reactive sites, thus leading to the higher activity toward electrocatalytic reactions. On basis of the two aspects mentioned above, recently, our group presented a g-C3N4-based electrocatalyst including g-C3N4 nanomeshes and amorphous CoSx nanoparticles, which exhibited pronounced ORR and OER performance.99 The nanomesh-like g-C3N4 exhibited a highly dispersed pore structure resulting in a facilitated mass diffusion system in the composite catalyst (Figure 8a). Interestingly, the g-C3N4 nanomeshes were prepared by a top-down approach without template involvement, which successfully prevented the active site loss from the post removal of the template in harsh etching condition. Obviously, the effect of amorphous CoSx nanoparticles (Figure 8b) in the contribution to the whole activity of this composite catalyst could not be neglected, because that the CoSx nanoparticles on g-C3N4 surface could not only serve as active 23 ACS Paragon Plus Environment


sites for bonding with reactive intermediates but also establish a conductive channel for freely transferring electrons to nitrogen active sites in skeleton of g-C3N4 nanomeshes. As a consequence, the graphene supported CoSx nanoparticles modified g-C3N4 performed a prominent effect in achieving a bifunctional electrocatalysis for both ORR and OER in alkaline (Figure 8c), which was contributed by the following aspects: 1) the g-C3N4 nanomeshes offered a stable scaffold for the CoSx nanoparticles, as well as a high specific surface area with abundant nitrogen active sites; 2) the amorphous CoSx nanoparticles provided an additional active sites for ORR and OER; 3) the graphene substrate facilitated the electron mobility between g-C3N4 matrix and CoSx nanoparticles. With all these advantages in this structure and competent, the g-C3N4-based composite catalyst showed a better electrochemical performance than that of graphene supported g-C3N4 nanosheet. For a potential application, this g-C3N4-based composite was assembled as the bifunctional cathode for ORR and OER in the rechargeable zinc-air batteries, which showed a favorable rechargeability and practicality within the severe environment (Figure 8d), e.g., the strong alkaline with 6 M KOH solution and the high voltage during the charging. Low price ， high activity and high stability are the three essential characteristics for nonprecious electrocatalysts in their efficient applications of electrochemical conversions. However, in addition to the use of carbon nitride or g-C3N4 in regulating the electrocatalytic activity of PGM catalysts, there is a requirement for using them to promote the stability of these metal catalysts. Nevertheless, g-C3N4 can stabilize precious metal or transition metal nanoparticles on its surface via the effect of electronic interaction, the side effect from the Ostwald ripening and the dissolution of these metal nanoparticles could not be inevitable in the harsh condition of fuel cells.100 At this point, there appears a new concept in fabrication of long-life ORR catalysts that preventing the direct reaction of active metal nanoparticles catalysts with reactants, such as O2, OH- or H+ in harsh 24 ACS Paragon Plus Environment

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ACS Energy Letters

environments (such as strong acid and alkaline electrolyte, CO and sulfur contaminations) by the growth of carbon-protective layer surrounding metal nanoparticles, which could further improve catalytic stability of these metal nanoparticles.101,102 Interestingly, the metal nanoparticles encapsulated within carbon shell still maintain a comparable ORR activity and a superior stability to those metal nanoparticles catalysts without the carbon-protective layer. Recent DFT studies gave explanations that the metal nanoparticles encapsulated within carbon layers could result in the electron migration from metal nanoparticles to the surface carbon atoms thus leading to the reduction of local work function on the surface carbons, and finally improving the catalytic activity.101,103 Within this context, g-C3N4 should be also a promising “coating” for enhancing the overall electrochemical performance of metal-based nanoparticles catalysts. In response, Jin reported a graphene-based CoO core-g-C3N4 shell composite electrocatalyst (Figure 9a and b), which rendered a further improved electrocatalytic performance toward ORR as compared to the counterpart CoO composite without g-C3N4 shell (Figure 9c), confirming the critical role of gC3N4 shell in optimizing electrochemical activity and stability of this “core-shell” type catalyst.104 However, unlike carbon-encapsulated transition metal catalysts in previous researches, they found that g-C3N4 layer in the “core-shell” structure functioned as either the “protective coating” to protect the core Co nanoparticles or the coordination sites to capture Co atoms to form other active species. They assumed that the excellent stability of this composite catalyst might be derived from the two reasons: 1) the g-C3N4 shell took the primary responsibility for serving as active sites for ORR, and also acted as the “protective coating” to prevent the CoO nanoparticles from the corrosion by electrolyte; 2) the Co ions were coordinated with the nitrogen sites in the g-C3N4 shell to form CoN2 or CoN6 species that might be active for ORR (Figure 8d), in the meantime, the core Co nanoparticles could be released and migrated to the g-C3N4 shell to recover the disabled active



sites like CoN2 or CoN6. Despite the active sites remaining unclear, the synergetic effect from the g-C3N4 shell in optimizing the catalytic activity and stability of metal nanoparticles is obvious. It should be noted that the graphene supporter in this composite catalyst also plays a vital role in facilitating the electron transfer within the g-C3N4 shell for ORR because of the poor conductivity of g-C3N4 materials. g-C3N4 Acts as Coordination Complex. Although many findings confirm the role of g-C3N4 materials in modifying the structure and catalytic performance of PGM and non-PGM electrocatalysts, the advanced function of g-C3N4 materials is not well investigated and exploited. Furthermore, the g-C3N4-based electrocatalysts are far from the efficient application in fuel cells or water splitting devices. More importantly, their electrocatalytic performance is still inferior to that of commercial PGM in an acid electrolyte. In contrast, the transition metal−macrocyclic complexes and transition metal coordinated nitrogen species in carbon skeleton (e.g., M−Nx/C, M = Co, Fe and Mn) has attracted intensive attention because of their highly efficient activities for ORR.98,105,106 For instance, the direct pyrolysis of nitrogen-containing polymer (such as polyaniline and polypyrrole) with transition metal salts to obtain M-Nx/C nanocomposites, which displayed excellent activity toward ORR due to the formation of M−Nx/C sites.18 There has been a long debate on whether the metal-assisted nitrogen sites are responsible for ORR or not. But, integrating transition metal with nitrogen sites indeed apparently enhances the activity of N-doped carbons. The g-C3N4 has a similar carbon framework to that N-doped graphene, but with more abundant pyridine-like nitrogen atoms, which is considered to be a potential “main body” to trap abundant the “guest” of transitional metals to form active M-Nx/C sites. To this end, Liu et al first reported the g-C3N4 used as “coordination complex” to capture transition metal ion to form a graphenebased Co-g-C3N4 nanosheet electrocatalyst (Co-g-C3N4@rGO) for ORR.107 For such a 2D g-C3N4 26 ACS Paragon Plus Environment

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ACS Energy Letters

composite, the g-C3N4 framework and the Co element are highly distributed within the graphene substrate (Figure 10a). Surprisingly, no Co particles and bulk g-C3N4 tissue can be observed on the graphene surface. The reason is probably attributed to that the graphene with a huge π-electron system can absorb the g-C3N4 precursor (viz. melamine) on its surface, and subsequently control the growth of g-C3N4 with a highly dispersed pattern on the graphene by the π-π bonding. In the meanwhile, through the coordination between the nitrogen species from g-C3N4 and the Co atoms, the nucleation and crystallization of Co clusters can be controlled and even prevented. Apparently, the ORR activity of Co-g-C3N4@rGO is better than those of g-C3N4@rGO and rGO (Figure 10b), confirming the effect of graphene substrate in assisting the electron transfer for catalyzing ORR. Besides that, the obvious improvement in the ORR activity is also related to the formation of CoNx active sites within the g-C3N4 frameworks (Figure 10c), which is a novel active species with respect to g-C3N4-based catalysts. Simultaneously, the graphene bonded with g-C3N4 through a strong electronic coupling can further enhance the electron transferability for ORR, which is much different from the 3D-structured g-C3N4 electrocatalysts in previous studies because of most of the 3D-structured g-C3N4 electrocatalysts becoming almost inactive for ORR after removing 3D conductive scaffolds. It is obvious that the electrocatalytic performance of g-C3N4 composite is strongly related to its conductivity. There have been many well-developed strategies for the enhancement of electron transferability, for example, the construction of 1D or 2D nanostructures to enlarge the electron transfer area between conductive carbon scaffold and g-C3N4 skeleton, thereby shorten the electron-transfer pathway and increase the number of fluent electrons from the conductive substrate to active nitrogen species in g-C3N4. Based on this heterogeneous structure, this kind of g-C3N4-based electrocatalysts is generally limited by the van der Waals contact, which results in a 27 ACS Paragon Plus Environment


tremendous energy loss during the electrocatalytic process.85,108 Except for the previous example illustrated above, for instance, the growth of highly dispersed g-C3N4 on the graphene through the π-π bonding could obviously promote the conductivity of g-C3N4. But, the bottleneck of van der Waals contact could not be fully addressed. There is a more acceptable concept for solving it as follows: firstly, as we all know, there is a large amount of pyridine-like nitrogen atoms within the triazine configuration of g-C3N4; secondly, this kind of pyridine-like nitrogen has an additional lone-electron pair offering to adjacent carbon atoms; finally, it is assumed that the introduction of a π-electron system adjacent to the pyridine-like nitrogen atoms will doubtless give rise to the delocalization of lone-pair-electrons on pyridine-like nitrogen atoms, and thereby make the electrons transportable for electrocatalytic reactions. Although many types of conductive substrates with π-electron systems, such as graphene and carbon nanotubes, have been already used as scaffolds for the deposition of g-C3N4, they are particularly difficult to be grown in the gC3N4 framework. Therefore, the enhancement of electron transfer in the plane of the g-C3N4 framework is more important and effective than that in van der Waals-structured g-C3N4. In addition, the transition metals coordinated triazine units in g-C3N4 can also be further improved and electro-activated by this new concept. As a proof, our recent work developed a novel strategy to implant π-electron system species into g-C3N4 framework without the addition of foreign conductive scaffolds. We found that the introduction of lactose and Co ion into the thermal polycondensation of g-C3N4 could achieve the conjugation of g-C3N4 with crystalline carbon, and improve the in-plane electron transport of g-C3N4 framework (Figure 11a).68 Of these, the Co metal not only acted as a catalyst for the growth of crystalline carbon during the pyrolysis but also played the role as the active center for ORR and OER when it was bonded with pyridine-like nitrogen within triazine units. On the other side, the introduction of lactose can provide more carbon sources


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ACS Energy Letters

for the synthesis of crystalline carbons because of the low concentration of carbon atoms in g-C3N4 at an elevated temperature.68 As a result, the crystalline carbon domains can be clearly observed within the g-C3N4 framework (Figure 11 b-d). As expected, this crystalline carbon-conjugated gC3N4 with atomic Co species (Co-C3N4/C) showed a much-improved activity toward both ORR and OER as compared to that of the g-C3N4 catalyst with a van der Waals heterostructure. For example, the onset potential was tested at 0.95 V for ORR, and the overpotential of 10 mA cm-2 was verified at 1.65 V for OER, both of which were superior to those of commercial Pt/C, RuO2/C, and other kinds of g-C3N4-based electrocatalysts. In the meanwhile, this catalyst also showed a potential application as a bifunctional cathode in rechargeable aluminum-air batteries with a longlife rechargeability and a high power density. It is obvious that the pyridine-like nitrogen in triazine unit of g-C3N4 can provide lone-pairelectrons to capture transition metal atoms to form M-Nx/C species by ligand coordination. Despite that the implantation of crystalline carbons into g-C3N4 can obviously promote the conductivity and catalytic activity of g-C3N4, the M-Nx/C species (e.g., Fe or Co-Nx/C) seems to be more favorable for the significant improvement in the catalytic activity of g-C3N4 materials due to its unique property for ORR.109 In contrast, traditional M-Nx/C materials are difficult to control and investigate because that these materials always include complicated nitrogen isoforms (such as pyridine N, graphitic N and pyrrolic N) but with a low content. At this point, g-C3N4 is a perfect alternative to traditional M-Nx/C materials that can provide more uniform nitrogen coordination sites to transition metals but in high concentration. As mentioned above, previous studies have shown that transition metal-coordinated g-C3N4 materials possess a high activity toward ORR, even though g-C3N4 is almost inactive for electrocatalytic reactions.110 However, before full exploitation of g-C3N4-based M-Nx/C catalyst, there remain many issues unsolved: 1) the precise 29 ACS Paragon Plus Environment


configuration of metal-coordinated site in g-C3N4 is unclear, and thus the activity source of MC3N4 catalysts is hard to be investigated; 2) the practicability of g-C3N4 as a multifunctional platform to capture metals are under exploitation; 3) the application of M-C3N4 catalysts are only limited for their use as ORR catalysts.111 To this end, Zheng et al synthesized and investigated a series of g-C3N4 based M-C3N4 electrocatalysts for both ORR and OER. They found that all the Co metals in the catalyst are in atomic condition (Figure 12a and b). In addition, they found that the CNT supported Co-C3N4 exhibited better ORR and OER performances than those of CoO, CoN/C and g-C3N4 counterparts (Figure 12c and d). Then, the experimental observations and theoretic calculations further confirmed that the active sites are comprised of uniform Co-N2 coordination species within the g-C3N4 matrix. More specially, Co atom is coordinated to two neighboring pyridinic N atoms from two separated triazine units. In addition, they demonstrated that g-C3N4 can act as a versatile coordination ligand to coordinate various transition metals to replace the traditional M-Nx/C electrocatalysts. The g-C3N4 in Symmetrical and Asymmetrical Supercapacitors. In recent years, g-C3N4 also shows a great potential in being as the electrode material for supercapacitors mainly due to its high density of nitrogen, strong corrosion resistance, and easily tailored structure. Chen et al developed a 3D graphene supported g-C3N4 electrode (3D g-C3N4@G ) for symmetrical supercapacitor (Figure 13a).112 The synthesis of hydrothermal process enables the high mass loading of g-C3N4 onto the graphene basal with a 3D framework (the mass density is 13.2 mg cm−3) leading to the sufficiently accessible sites for double layer capacitance. Furthermore, the elemental mappings confirm that the uniform dispersion of g-C3N4 units on graphene due to the strong adsorption by πtr electron interaction (Figure 13b). With many advantages in the structure and mass loading of g-C3N4, the g-C3N4@G shows a higher capacitance of 264 F g−1 at 0.4 A g−1 30 ACS Paragon Plus Environment

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than 152 F g−1 of the pristine graphene hydrogels (Figure 13c). The excellent capacitive performance of 3D g-C3N4@G is found to be attributed to the introduction of g-C3N4 into graphene leading to the formation of 3D structure that facilitates the mass and charge transfer during the charge-discharge process. In contrast with symmetrical supercapacitors, asymmetrical supercapacitors have been demonstrated to show a more promising prospect in electrochemical energy storage due to their larger capacitances, higher discharge current and wider voltage window. For example, Shi et al developed a g-C3N4 supported Ni(OH)2 electrode with a specific capacitance of 505.6 F g−1 at 0.5 A g−1, and yet, the poor cycling stability is found in the asymmetrical supercapacitor with remaining 71.5% capacity over 1000 cycles.113 Xu et al. synthesized a gC3N4/α-Fe2O3 electrode for the asymmetrical supercapacitor that achieves a low specific capacitance of 167 F g−1 at 1 A g−1.114 Zhu et al prepared a supercapacitor electrode comprised of g-C3N4 supported Co3O4, which shows a high specific capacitance of 780 F g−1 at 1.25 A g−1 and a high stability with remaining 80% capacity over 1000 charge-discharge cycling at 3 A g−1. Interestingly, the capacity of the Co3O4/g-C3N4 composite is much larger than 419 F g−1 of pure Co3O4, suggesting that the g-C3N4 in providing more electron carrier and double layer capacitance.115 Despite the examples mentioned above, the combination of g-C3N4 materials with more conductive materials, such as CoS,116 NiCo2S4 117 and polyaniline118, also show high specific capacitances and excellent cycling stabilities in supercapacitors, implies the great potential of gC3N4 materials in improving the electrode materials for high-performance supercapacitor. The g-C3N4-Based Anodes in Lithium-Ion Batteries (LIBs). The previous theoretic simulations and experimental observations have demonstrated the possible product of Li4C3N4 through the reaction of g-C3N4 and Li ions.119,120 However, Veith et al. found that Li ions were only reacted with the graphitic nitrogen of g-C3N4 with an irreversible process. In addition, the irreversible 31 ACS Paragon Plus Environment


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reaction could damage the structure of graphitic nitrogen leading to the irreversible capacity loss of g-C3N4.121 For example, Yang et al. found that g-C3N4 in lithium-ion batteries delivered a low discharge capacity at 134.9 mAh g-1 with the almost irreversible capacity loss up to 90% after 7 cycles of charge-discharge. The huge capacity loss probably results from the decomposition of gC3N4 during the delithiation process.119 Furthermore, Hankel et al. claimed that the pyridinic nitrogen should be responsible for the stable uptake of Li ions in g-C3N4.122 Thus, the reduction of the amount of graphitic nitrogen and the optimization of conductivity of g-C3N4 will be the two objectives that lead to the high performance of g-C3N4 toward lithium storage capacity and cycling stability. Based on this point, many nano-engineering strategies have been devoted to improving the

g-C3N4 materials.

For

instance,

Hou

et

al.

reported

a

three-phase

g-

C3N4/NRGO/MoS2 electrode for lithium-ion battery anode, which achieved a high cycling stability over 1000 cycles and a large specific capacity of 855 mAh g-1 under 100 mA g-1 discharging. In fact, the g-C3N4 with graphene substrate only acts an electron-donating component and framework structure for facilitating the mass transfer of electron, lithium ions and electrolyte.123 In other words, the nitrogen content and configuration determine the function of g-C3N4 in contributing the capacity and stability of g-C3N4 anode-based LIBs. Chen et al synthesized one kind of nitrogendeficient g-C3N4 (ND-g-C3N4) by the magnesiothermic denitriding method, which exhibits a low nitrogen content and a high electron conductivity leading to the improved lithium storage capacity and cycling stability (Figure 13d). The excellent performance is attributed to the porous morphology of ND-g-C3N4 with a high degree of porosity (Figure 13d and e) that provides more accessible reactive sites for the lithiation/delithiation process. This conclusion can be further evidenced by the ND-g-C3N4 anode delivering a smaller interfacial electron-transfer and lithiumion diffusion resistances than those of pristine g-C3N4 (Figure 13g). Besides, the low content of


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graphitic nitrogen species in g-C3N4 provides the LIBs with a high stability and lithium storage capacity at 2753 mAh g-1 over the 300 cycles (Figure 13h), which is in accordance with the previous assumption that the higher content of pyridinic nitrogen could further stabilize the stability of LIBs with g-C3N4 anodes.67 The g-C3N4-Based Cathodes in Lithium-Sulfur Batteries (LSBs). Although some studies have demonstrated the successful application of g-C3N4 in energy storage devices of supercapacitors and LIBs, the research regarding the development of g-C3N4 cathode into LSBs is rarely reported. This is due to the complicated reactions and problems involved in LSBs. For example, LSBs are facing the following challenges: (i) the low utilization efficiency of electro-insulating sulfurcontaining cathode leads to the sluggish kinetics of the reaction between lithium and sulfur; (ii) the dissolution and diffusion of lithium polysulfide (LiPS) to lithium anode, and the unfavorable endproduct of Li2S/Li2S2 give rise to the irreversible capacity loss and lithium anode damage; (iii) the dramatic volume change of the S-cathode destroys the pristine structure and specific surface area of the S-cathode resulting the rapid decay of lithium capacity. Recently, Pan et al prepared a Ndoped graphene/CNT supported g-C3N4 compound with a high loading of sulfur up to 14.9 mg cm−2 by using carboxymethyl cellulose as the binder in the S-cathode fabrication .124 Owing to the space confinement and electrical adsorption, the g-C3N4 hybrid electrode exhibits a higher capacity of 1340 mA h g−1 than 1150 mA h g−1 of pristine g‐C3N4 electrode at a high discharge rate. The g-C3N4 in the hybrid electrode as a stabilizer enables the high cycling stability of LSBs over 100 cycles. Zhang et al reported a 3D porous sulfur/graphene@g-C3N4 (S/GCN) hybrid sponge for being directly applied as a free-standing cathode for LSBs (Figure 13i). More especially, the high loading of sulfur (82 wt.%) in S-cathode is achieved by employing the assembly of microemulsion; besides, the g-C3N4 within the macroporous S/GCN (Figure 13j) acts as the N-adsorption sites for 33 ACS Paragon Plus Environment


anchoring polysulfides and LiPS limiting the volume expansion and diffusion of LiPS at Scathode.125 As a result, the 3D S/GCN electrode delivers the larger capability (Figure 13k) than those of 2D S/GCN electrode and GCN-Li2Sn, highlighting the 3D structure in offering the higher specific surface area and the more reactive sites for lithium ions. In addition, the S/GCN exhibits the high discharge capacities and cycling stability at the different rates, verifying that the 3D structure combined with the enriched nitrogen site from g-C3N4 facilitates kinetics of lithiumsulfur reactions and limits these reactions within the porous S/GCN electrode. Despite the complicated and multiple reactions differ from those in a supercapacitor, extensive studies have demonstrated the enormous potential of g-C3N4 to be as the auxiliary electrode material for improving the electrochemical performances of LIBs and LSBs. Generally speaking, the g-C3N4 materials within LIBs and LSBs primarily take part of offering the enriched N-reactive sites for anchoring or reacting with lithium ion, and yet, when it comes to supercapacitors, the gC3N4, in most case, serves as the “coordinator” and “electron donator” for stabilizing the major capacitance-contributors (e.g., metal compounds, carbon materials and polymers) and providing them with more electron carrier and double layer capacitance leading to the high stability and capacitance of supercapacitors. However, with the similarity to the g-C3N4-based electrocatalysts, the poor conductivity and low utilization efficiency remain the main obstacles for wide application of the g-C3N4-based electrodes into energy storage devices even though many nano-engineering technologies have been tried to modify the g-C3N4 with conductive materials. Therefore, to optimize the inherent physical and chemical (e,g, band gap and nitrogen content) of g-C3N4 will be the most effective way to greatly promote the electrochemical performance of g-C3N4 in those energy storage applications. Future Perspectives and Concluding Remarks. As a metal-free semiconductor, the g-C3N4 material 34 ACS Paragon Plus Environment

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has been already demonstrated to exhibit a very promising application in energy conversion and storage technologies. This review is targeted to discuss the recent progress of g-C3N4 synthesis and performance in electrocatalysis, supercapacitors, lithium-ion batteries, and lithium-sulfur batteries. Progress in synthesis of nanostructured g-C3N4 such as meso-g-C3N4 and 2D or 3D gC3N4 materials paved a way for its utilization as an efficient electrode material in fuel cells, water splitting, and energy storage devices. There are several factors that can, in our opinion, lead to a breakthrough of g-C3N4 based materials in electrochemical reactions. The first one lies on the fabrication of continuous, atomic-thickness and homogeneous g-C3N4 layers. This kind of g-C3N4 can be synthesized by optimizing the conditions of chemical and physical deposition methods such as a controllable growth of atomic g-C3N4 through the in-situ vapor deposition, or through the alteration of the monomers, deposition temperature and reaction condition in gas-phase. Despite the great effort has been made, the control of thickness, uniformity and surface properties of gC3N4 based materials is still pursued. In addition, the post treatments of the g-C3N4 material (e.g. exfoliation and etching processes) should be a promising way for improving the catalytic performance by providing extender defects on the surface. The second factor is related to the electron-transfer ability of the g-C3N4 material. Currently, the low conductivity greatly limits the performance of g-C3N4 in devices. To overcome it, the adjustment of the structure of g-C3N4 materials (such as nanorods, spheres, 2D layers, etc.) offers a myriad of opportunities to facilitate the mass transfer during electrocatalysis. The electronic and catalytic properties can also be adjusted by the insertion of heteroatoms as boron, sulfur, and phosphorus into the g-C3N4 network, thus leading to new materials with tunable electronic properties. We strongly believe that by overcoming these challenging issues g-C3N4 can be implemented in the near future in a broad range of electrochemical energy conversion and storage due to its price, stability, environmentally



friendly nature, and tunable catalytic and electronic properties. Corresponding Authors *E-mail:

[email protected] (Y. Yang).

Notes The authors declare no competing financial interests. Biographies Wenhan Niu received his Ph. D under the supervisor of Professor Shaowei Chen and Ligui Li in Environment and Energy from the South China University of Technology in 2016. He is currently a postdoctoral associate in Professor Yang Yang’s group, University of Central Florida. His current research interests include electrochemical energy conversion, lithium-sulfur batteries, and metalair batteries. Yang Yang obtained his Ph.D. from Tsinghua University in 2010. From 2010 to 2012 he was supported by the Alexander von Humboldt Postdoctoral Fellowship and worked with Prof. Dr. Patrik Schmuki at the University of Erlangen-Nuremberg. From 2012 to 2015 he was supported by the Peter M. & Ruth L. Nicholas Postdoctoral Fellowship and worked with Prof. Dr. James M. Tour at Rice University. Since 2015 he has been an assistant professor at the University of Central Florida. His current research interests cover nanostructured thin-film materials, electrochemical energy generation and storage, and plasmonic photocatalysis. http://www.yangyanglab.com/. Acknowledgments This work was financially supported by the University of Central Florida through a startup grant (No. 20080741). References (1) Shao, M. H.; Chang, Q. W.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts


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ACS Energy Letters

Table 1. Summary of g-C3N4 based electrocatalysts for ORR, OER, and HER, mostly reported in the last decade. Electrocatalysts Morphology ORR:

Mass

Onset potential (E0) or half-wave potential

Electrolyte Ref.

loading (mg cm-2)

(E1/2) OER: Potential at 10 mA cm2 (E ) 10

HER: Onset potential (ƞ0) ( E vs. RHE) g-C3N4/carbon

bulk

ORR: E0 = 0.76 V

black

N/A

0.5M H2SO4


69


graphene-based mesoporous

nanosheet

g-

ORR： E0 = 0.87 V, E1/2

Page 54 of 70

0.071

78

0.1 M KOH

= 0.67 V

C3N4 g-C3N4@CMK-

nanoporous

ORR: E0 = 0.86 V

0.085

3 composites Carbon

M

81

M

89

M

82

M

51

M

110

M

126

KOH

fiber nanoflower-

ORR: E0 = 0.90 V, E1/2 =

paper supported like

0.63 V

P-doped g-C3N4

OER: E10 = 1.59 V

3D

0.1

N/A

0.1 KOH

Ordered macroporous ORR: E0 = 0.84 V

0.085

Macroporous g-

0.1 KOH

C3N4/Carbon Composite Graphene supported

nanoribbon-

HER: ƞ0 = -0.08 V

0.143

g- like

0.5 H2SO4

C3N4 nanoribbons Ordered mesoporous

mesoporous

ORR: E0 = 0.75 V

0.124

g-

0.1 HClO4

C3N4 Graphene/

g- bulk

ORR: E0 = 0.86 V

C3N4 composite

0.283

0.1 KOH


Page 55 of 70 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Ti3C2 supported nanosheetg-C3N4

OER: E10 = 1.65V

M

127

~0.283

0.1M KOH

107

1.4

like

0.1 KOH

nanosheets Graphene supported

nanosheet Co-

ORR: E0 = 0.91 V, E1/2 = 0.81 V

doped g-C3N4 nanosheet

OER: E10 = 1.77 V

~0.50

0.1M KOH

128

Carbon

molecular

ORR: E0 = 0.90 V, E1/2 =

0.408

0.1

M

111

nanotube

level

0.85 V

M

99

M

68

gC3N4/graphene composites

KOH

OER: E10 = 1.60 V

supported Co−gC3N4 Graphene based nanomesh

ORR: E0 = 0.89 V

CoSx

OER: E10 = 1.57 V

modified

0.408

0.1 KOH

g-C3N4 composites Crystalline

nanosheet

carbonconjugated

ORR: E0 = 0.95 V OER: E10 = 1.65 V

g-

C3N4


0.61

0.1 KOH


Porous

g-C3N4 porous

nanolayers@N-

Page 56 of 70

HER: ƞ0 = -0.008 V

N/A

0.5

nanolayer

M

85

M

129

H2SO4

Graphene Films g-C3N4

nanosheet

HER: ƞ0 = -0.18 V

0.196

0.5

supported MoS2

H2SO4

dots

N

(cyanamide)

NH2

N

N

H2 N

N

N

NH2

(dicyanamide)

N

N

N

N

H2 N

N

N

N

(g-C3N4)

oC

N

NH2 NH2

NH2

N

N

HN

NH2

(melamine)

H2 N

N

N

N

N

N

N

N

(g-C3N4)

N

N

N

N

NH2

390 H2 N

N

N

N

N

NH2

N N

N

H2 N

240

N

(tri-s-triazine/heptazine)

N

N

N

N

NH2

(triazine) N

N

N

N N

N

N

N

NH2

b

NH2

NH

HN

150 oC

a

N

N

N

N

N H

(melam)

oC

N

520

N

oC

N

N

N

N

N

N

N

NH2

N

-NH3

N

NH

H2 N

N

-NH3 NH2

N

N

N

N

N

N

N

N

N

N

N

N

(melem)

Figure 1. Schematic illustration of the synthetic routes of g-C3N4 through thermal condensation of cyanamide; g-C3N4 includes two types of allotropes: (a) Triazine and (b) tri-s-triazine (heptazine). 56 ACS Paragon Plus Environment

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ACS Energy Letters

Figure 2. The strategies for improving the conductivity of g-C3N4 materials.




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ACS Energy Letters

Figure 3. (a) Cyclic voltammograms (CVs) of ORR on carbon black (CB), Pt/C, g-C3N4, and gC3N4+CB electrodes (The inset refers to the amplified CVs curves at the onset potential). (b) CVs for g-C3N4 and g-C3N4+CB. The inset demonstrated the ORR activity of g-C3N4 electrode in O2saturated KOH; reprinted with permission from ref. 69, Copyright 2009 American Chemical Society.



Figure 4. (a, b)Transmission electron microscope (TEM) and (c) Atomic Force Microscope (AFM) images of graphene-based g-C3N4 nanosheets (G-CN). (d) Nitrogen adsorption/desorption isotherm of G-CN (inset: pore-size distribution), confirming the mesoporous structure with a Brunauer-Emmett-Teller (BET) surface area of 542 m2g-1; reprinted with permission from ref. 78, Copyright 2011 Wiley-VCH .


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ACS Energy Letters

Figure 5. (a) AFM image and (c1–c2) the corresponding diameter of the g-C3N4 dots graphene. (b) RDE polarization curves of the series electrodes in O2-saturated 0.1 M KOH solution. (c) Chronoamperometric responses of the MTCG; the inset highlights the excellent stability of MTCG in the O2-saturated KOH solution; reprinted with permission from ref. 80, Copyright 2015 Royal Society of Chemistry.



Figure 6. (a) TEM image of g-C3N4@CMK-3, reprinted with permission from ref. 81, Copyright 2012 American Chemical Society; (b) scanning electron microscope (SEM) image of macroporous g-C3N4/C and the corresponding silica spheres template (inset), reprinted with permission from ref. 82, Copyright 2012 Wiley-VCH; (c) SEM image of porous C3N4 nanolayers (PCN) @Ngraphene hybrid film and TEM images of PCN, reprinted with permission from ref. 85 2015 American Chemical Society; (d) SEM and TEM inset of g-C3N4 nanosheets and carbon nanotubes composite, reprinted with permission from ref. 84, Copyright 2014 Wiley-VCH; (e) SEM image of P-doped g-C3N4 nanosheets and NH2-functionalized carbon black composite, reprinted with permission from ref. 88, Copyright 2016 American Chemical Society. (f) SEM and TEM of P-gC3N4 nanosheets, reprinted with permission from ref. 89, Copyright 2015 Wiley-VCH.


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ACS Energy Letters

Figure 7. SEM images of carbon nitride deposited on glass, FTO, and TiO2; reprinted with permission from ref. 93, Copyright 2014 Wiley-VCH.



Figure 8. (a) TEM and (b) High-Resolution TEM (HRTEM) of graphene supported CoSx nanoparticles modified g-C3N4; (c) Linear Sweep Voltammograms (LSVs) of CoSx@PCN/rGO and other reference samples; (d) charge-discharge curves of zinc-air battery assembled with CoSx@PCN/rGO; reprinted with permission from ref. 99, Copyright 2017 Wiley-VCH.


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ACS Energy Letters

Figure 9. (a)TEM image of graphene supported CoO core-g-C3N4 shell (Co-N-GS), and (b) the crystal embedded CoO core; (c) LSV curves of a series of Co-N-GS catalysts; (d) schematic illustration of the Co configuration in g-C3N4; reprinted with permission from ref. 104, Copyright 2013 Royal Society of Chemistry.



Figure 10. (a) TEM of Co-g-C3N4@rGO; (b) LSVs curves of these catalysts in alkaline; (c) schematic illustration of ORR on Co-g-C3N4@rGO electrode; reprinted with permission from ref. 107, Copyright 2013 American Chemical Society.


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ACS Energy Letters

Figure 11. (a) Schematic illustration of the synthetic process of Co-C3N4/C; (b-d) HRTEM images of Co-C3N4/C and the corresponding lattice spaces for crystalline carbon domains; reprinted with permission from ref. 68, Copyright 2018 American Chemical Society.



Figure 12. (a) HRTEM and (b) high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images of Co-C3N4/CNT; the circles and arrows in panel b indicate single Co atoms and Co clusters respectively; LSV curves for (c) ORR and (d) OER on various Co-based catalysts electrodes; reprinted with permission from ref. 111, Copyright 2018 American Chemical Society.


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ACS Energy Letters

Figure 13. (a) SEM and the inset of the photograph, (b) TEM and elemental mapping, (c) capacitance at the different discharge rates of 3D graphene supported g-C3N4 electrode (3D gC3N4@G) in a supercapacitor, reprinted with permission from ref.112, Copyright 2015 Royal Society of Chemistry; (d) TEM, (e) HR-TEM, (f) CVs, (g) EIS and (h) specific capacities and coulombic efficiency of nitrogen-deficient g-C3N4 (ND-g-C3N4) in a lithium-ion battery, reprinted with permission from ref.67, Copyright 2017 American Chemical Society; (i) TEM, (j) HR-TEM and (k) specific capacity and coulombic efficiency of 3D porous sulfur/graphene@gC3N4 (S/GCN) hybrid sponge in the lithium-sulfur battery, reprinted with permission from ref.125, Copyright 2017 Wiley-VCH. 69 ACS Paragon Plus Environment


Quote 1. The g-C3N4 materials are challenging the status of nitrogen-doped carbon materials in many electrocatalytic reactions due to their abundant and uniform nitrogen species, and strong corrosion resistance in acid and basic solution. Quote 2. Although the poor conductivity has been the main obstacle to the further application of g-C3N4 materials in energy conversion and energy storage devices, many novel strategies, such as the integration of them with conductive carbon and metal materials by nano-engineering technologies, have been successfully demonstrated to addressing the problems at some extent.

Quote 3. The rational optimization of nitrogen content and configuration and other inherent properties of gC3N4 will be the most effective way to enhance the electrochemical performance of g-C3N4-based electrode materials.


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