11.2.1 Design and Synthesis

Nitrogen is the mostly researched doping atom for carbon materials as it has atom size similar to carbon and can bond strongly with carbon. Because N is more electronegative than carbon, it would break the charge neutrality of the pure carbon structure after doping and result in electron deficiency for the near carbon, thereby tuning the catalytic performance of the materials [27, 28]. Moreover, N can be doped in various locations in the carbon lattice, thus leading to multiple possible configurations [29, 30]. As shown in Figure 11.1, N can exist as graphitic, pyridinic, pyrrolic, amino, and oxidized type in the doped carbon materials, and they can be distinguished by X‐ray photoelectron spectroscopy (XPS) [31]. In particular, pyridinic nitrogen is obtained by doping at the edge of the graphene layer, contributing one p_π electron to the graphitic π system. Meanwhile, graphitic nitrogen is the result of in‐plane doping and contributes two p_π electrons. Among the N‐bonding structures, graphitic‐N and pyridinic‐N have received more attention as they are suggested to dominate the electrocatalytic behaviors of the N‐doped carbons [24, 32].

The enhanced electrocatalytic performance of the N‐doped carbon, abundance of active sites and defects, makes it an ideal support for metal nanocatalysts compared with traditional undoped carbon materials. Specifically, the interaction between metal and the N‐center or N‐induced defect in the carbon structure would possibly improve the electrocatalytic performance of the materials [33]. The enhanced metal–supports interaction between metal and the N‐doped carbon would improve the stability of the catalysts [34]. This interaction may also promote the charge transfer at the metal–supports interface, thus further enhancing the catalytic activity of the N‐doped, carbon‐supported metal structure [35]. In addition, the CN sites are proved to be the preferred sites for the growth of the metal nanostructures [36], and single metal atom can even be stabilized by the N‐doped carbon structures [37, 38], which offers promising advantages for the design of novel N‐doped, carbon‐supported metal electrocatalysts compared with the traditional carbon‐supported metal.

The strategies for the synthesis of N‐doped, carbon‐supported metal catalysts can be generally categorized into three ways. First is a two‐step, post‐loading method. In this method, N‐doped carbon is first fabricated and then the metal nanostructures can be loaded on the N‐doped carbon by either in situ nucleation or self‐assembly of the prepared metal nanostructures [39–42]. Second is the one‐step method in which N‐doping and nucleation of metal nanoparticles can be realized via one‐pot solvothermal synthesis [25, 43]. Usually, such a process is difficult to control the size and shape of metal nanostructures. The third way is the direct pyrolysis of N, metal, and carbon precursors to fabricate M–N–C catalysts [44–46]. In this catalyst, metal atoms are embedded into the N‐doped carbon matrix to form M–N_x‐active sites, which will be discussed in detail in the following sections.

The key factor for the two‐step, post‐loading method is the synthesis of N‐doped carbons. N‐doped carbons can be fabricated by chemical vapor deposition (CVD) [47, 48] or pyrolysis [49, 50] of C and N precursors, arc discharge [51], or plasma treatment [52, 53] of carbon materials in N precursor such as NH₃, thermal annealing [54, 55], or solvothermal treatment [56] of carbon nanostructure like graphene oxide (GO) with N precursor. Under these reacting conditions, N would bond with C and doped in the lattices of carbon planner. And the N precursor is suggested to play a key role in the bonding configuration of the N‐doped structures. For instance, Ruoff and coworkers prove that annealing of GO with polyaniline (PANI) would result in the generation of pyridinic‐N, while annealing of GO with polypyrrole (PPy) would tend to form pyrrolic‐N. If NH₃ is used as the precursor, graphitic‐N and pyridinic‐N would be obtained for the doped structure [57].

Either metal nanoparticles or metal compounds can be supported on the N‐doped carbon. Chemical reduction of metal salts in the presence of N‐doped carbon materials is the mostly used strategy to support noble metal nanoparticles on N‐doped carbon as noble metal ions such as PtCl₆²⁻ can be easily reduced to metal [1, 39]. N‐doped graphene and N‐doped carbon nanotubes (N‐CNTs) are good supports for noble metals [58, 59]. During the synthesis, the noble metal salts are first dispersed in these N‐doped carbon materials or in a solution than containing N‐doped carbon materials. Then H₂, NaBH₄, or some other reduction regents are used to reduce the metal ions to zero valence and produce N‐doped, carbon‐supported precious metal nanoparticles [60–62]. The noble metal nanoparticles are more easily anchored on the N sites of the doped structure due to the metal–N coordination, which may stabilize the metal nanoparticles and benefit the catalytic activity and stability of the hybrid materials [33, 63]. In particular, the pyridinic edge sites, which carry an extra lone‐pair of electrons, may provide stronger interaction between the metal and the support, thus inhibiting agglomeration of the nanoparticles. Additionally, such interactions may also induce the alterations of the electronic structure of the catalyst. Arrigo et al. prove that the nature of the interaction between Pd and the N species in N‐CNT is of σ‐type donation from the filled π orbital of the N atom to the empty d orbital of the Pd atom and a π back‐donation from the filled Pd atomic d orbital to the π*‐anti‐bonding orbital of the N atom (Figure 11.2) [64], which offers deeper understanding for the rational design of N‐doped, carbon‐supported metal nanocatalysts.

Different from precious nanoparticles, transition metal compounds are usually loaded on N‐doped carbon materials by solvothermal processes. Dai and coworkers developed a series of N‐doped, carbon (graphene or carbon nanotube)‐supported cobalt oxides with carbon nanomaterials, NH₄OH, and cobalt acetate as the precursors [25, 65]. Cobalt acetate was mixed with carbon nanomaterials in ethanol, followed by the addition of NH₄OH. Then, the mixture was heated to a certain temperature to obtain N‐doped, carbon‐supported cobalt oxide nanoparticles. They found that post‐annealing of N‐doped, carbon‐supported Co₃O₄ in NH₃ would result in N‐CNT‐supported CoO that showed enhanced ORR performance (Figure 11.3). N‐doped, carbon‐supported nickel sulfide was prepared by the reaction of Ni₂(CO₃)(OH)₂ and thioacetamide [66]. During the synthesis, Ni₂(CO₃)(OH)₂ was first dispersed in water with N‐doped graphene, then thioacetamide was added, and the solution was heated at 200 °C for 12 h to obtain N‐doped, graphene‐supported Ni₃S₄. In addition, N‐doped, carbon‐supported metal can also be fabricated by simply loading as‐synthesized nanoparticles on N‐doped carbon [40].

The direct pyrolysis of N, metal (Fe, Co, Ni, Mn, etc.), and carbon precursors is commonly used to prepare M–N–C type of N‐doped, carbon‐supported metal catalysts [44, 67–69]. In this process, the metal atoms would coordinate strongly with N to form M–N–C structure. Therefore, macrocycle complexes become ideal precursors for this process as they have natural metal–N₄ centers. Later study proved that pyrolyzing a mixture of metal salts and N/C sources could produce similar M–N–C structure [70, 71]. A variety of N‐containing molecules including cyanamide, 1,10‐phenanthroline, and PANI are employed as the N precursors, whereas inorganic metal salts such as chlorides and acetates are investigated as metal precursors. Zelenay and coworkers synthesized N‐doped, carbon‐supported iron catalysts with PANI as the N sources [72]. They found that the formation of FeN center would be influenced by the reaction temperature, and pyrolysis at 900 °C would lead to the largest FeN content. Moreover, they proved that the presence of sulfur‐based oxidant in the aniline polymerization could benefit the formation of FeN structure, which offered a new idea for the design of highly efficient, N‐doped, carbon‐supported metal catalysts. Recently, the metal–organic framework (MOF)‐derived carbon nanostructures formed by pyrolyzing MOFs become a new group of N‐doped, carbon‐supported metal catalysts and attract considerable attention [19, 20, 73–75]. MOFs hold large surface areas, controllable compositions, and structures and can achieve high conductivity after pyrolysis. Importantly, the metal ions can coordinate with N in MOFs and a uniform distribution of metal, N, and C could be achieved, which would contribute to their catalytic activity after pyrolysis. Zeolitic imidazolate frameworks (ZIFs) have been investigated as ideal precursors because they are rich in M–N coordinations and can be easily prepared at low cost [76, 77]. ZIFs are usually prepared by the coordination between tetrahedral metal ions like Zn, Co, and imidazolate compounds. Bimetallic zeolitic imidazolate frameworks (BMZIFs) can also be prepared using two kinds of metals. Jiang and coworkers synthesized Co/Zn BMZIFs with varied Co:Zn ratios by adding 2‐methylimidazole in methanol that contains Zn(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O [78]. Then, they heated the BMZIFs at 900 °C in N₂ atmosphere to obtain N‐doped, carbon‐supported Co (CNCo). The BMZIF‐derived CNCo demonstrated enhanced ORR performance compared with that obtained by the pyrolysis of ZIF‐8 and ZIF‐67, and its ORR activity can be further improved by P doping. Figure 11.4 displays the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of BMZIF and CNCo. The CNCo particles keep the polyhedron structure of ZIF after pyrolysis. It is of note that Co nanoparticles are also formed during pyrolysis in addition to Co–N–C structures (Figure 11.4e).

11.2.2 N‐Doped, Carbon‐Supported Metal Electrocatalysts

N‐doped, carbon‐supported metal electrocatalysts have been under intensive investigations during recent years due to the combination of advanced carbon materials with metal‐active sites. The high surface area, good electronic conductivity, and abundant defects induced by N‐doping make various N‐doped carbons, e.g. graphene and carbon nanotube, promising supporting materials for metal nanocatalysts. A large variety of metal, metal oxides, metal sulfides, and many other metal‐based nanostructures have been supported on N‐doped carbons as catalysts [79–88]. Their performance in catalyzing relevant electrochemical reactions including ORR, OER, HER, and CO₂RR will be discussed in detail in this section.

11.2.2.1 Oxygen Electrocatalysis

Noble metal nanoparticles and transition metal compounds can be supported on N‐doped carbons to catalyze the electrochemical reduction or evolution of oxygen. N‐doped carbon is suggested to be a good support for Pt and Pt alloys when used in ORR electrocatalysis. Ramaprabhu and coworkers loaded Pt and Pt₃Co nanoparticles on N‐doped, hydrogen‐exfoliated graphene (N‐HEG) by the reduction of H₂PtCl₆·6H₂O and Co(NO₃)₂·6H₂O in ethylene glycol/water mixture (Figure 11.5a–d) [79]. The N‐HEG‐supported Pt₃Co exhibits enhanced ORR activity and four times higher power density than commercial Pt/C in proton exchange membrane fuel cells (Figure 11.5e,f), which should be attributed to the excellent dispersion of Pt₃Co nanoparticles and the instinct ORR activity of the N‐doped graphene. The Pt nanowires can also be grown on N‐doped carbon in addition to Pt nanoparticles [89]. Using HCOOH as the reduction regent under room temperature, H₂PtCl₆·6H₂O can be reduced to Pt nanowire with a diameter of 2.5 nm on N‐CNTs, which would exhibit enhanced ORR activity.

In addition to precious metal‐based materials, N‐doped, carbon‐material–supported, non‐precious metal compound can also promote the reduction of oxygen. N‐doped, carbon‐supported cobalt oxides have been proved to be efficient ORR catalysts in an alkaline solution [25, 90]. Manganese oxide that supported on the mesoporous N‐doped carbon developed by the hydrothermal treatment of Mn salts and aniline followed by heat treatment can also catalyze the ORR efficiently [91]. Efficient OER electrocatalysts can also be designed with N‐doped, carbon‐supported transition metal (Fe, Co, Ni, and Mn) compounds [92]. Wang and coworkers fabricate a cobalt–cobalt oxide/N‐doped carbon hybrids (CoO_x@CN) by a one‐pot thermal treatment method [93]. CoNO₃·6H₂O was first mixed with D(+)‐glucosamine hydrochloride and melamine in water. Then, the mixture was dried and calcined at 800 °C in N₂ atmosphere for 1 h to obtain CoO_x@CN. The fabricated hybrid demonstrated high OER activity with an overpotential of 26 mV to achieve the current density of 10 mA cm⁻². N‐CNT‐supported NiCo₂S₄ (NiCo₂S₄/N‐CNT) is also synthesized to promote the reaction of oxygen reduction and oxygen evolution simultaneously [87]. The NiCo₂S₄/N‐CNT prepared by the solvothermal treatment of Ni(II), Co(II), and thiourea in the presence of CNT demonstrated comparable ORR activity to Pt/C and superior OER capability to RuO₂, thus could reduce the charge‐discharge polarization (∼0.63 V) significantly. The interaction between sulfide spinels and N‐CNT and the homogenous distribution of NiCo₂S₄ nanocrystals are suggested to contribute to the high ORR/OER activity of NiCo₂S₄/N‐CNT.

11.2.2.2 HER Electrocatalysis

Recently, transition metal compounds including chalcogenides, phosphides, carbides, and nitrides have been studied as earth‐abundant and efficient HER catalysts [94–96]. Therefore, the HER performance of the N‐doped, carbon‐supported metal nanomaterials have also been evaluated. Kim and coworkers prepared N‐CNT supported MoS_x (MoS_x/NCNT) with (NH₄)₂MoS₄ and N‐CNT as the reactants (Figure 11.6) [97]. After being treated with HCl, amorphous MoS_x would be decorated on the N‐CNT. The MoS_x/NCNT shows excellent HER catalytic performance with an onset overpotential of 75 mV. Its Tafel slope is 40 mV dec⁻¹ and can reach the current density of 10 mA cm⁻² at the overpotential of 110 mV. The N‐CNT possesses more favorable interaction with precursor molecules compared with pristine CNT. This interaction and the amorphous structure of the MoS_x are suggested to benefit the HER performance of the hybrid. After being annealed at 600 °C, MoS_x would turn to crystal MoS₂ (Figure 11.6g) and demonstrate inferior HER activity. You and coworkers synthesized MoS₂‐nanosheets‐embedded, N‐doped carbon nanofibers by electrospinning of Na₂MoO₄ and polyacrylonitrile followed by carbonization at 900 °C [98]. This hybrid also shows enhanced HER activity with a Tafel slope of 48 mV dec⁻¹ and an overpotential of 135 mV at the current density of 10 mA cm⁻². In addition, molybdenum selenide and tungsten oxynitride can also exhibit improved HER activity when supported on N‐doped carbon [99, 100].

11.2.2.3 Other Electrocatalysis

PtRu/C is a commercially available catalyst for MOR, which is the anode reaction of direct methanol fuel cells (DMFCs). Similar to those observed in oxygen reduction electrocatalysis, N‐doped, carbon‐supported metal nanoparticles catalysts draw special attention in MOR due to the improved interaction between noble metal and the N‐doped substrates [39, 70]. The noble metal nanoparticles would usually show improved catalyst dispersion, lower onset potential, and increased current density when supported on N‐doped carbon [101–103]. Moreover, some researchers suggest that the interaction between N and noble metal such as Pt will result in positive shift for its d‐band center, thereby increasing the chemisorptions of oxygen‐containing groups (e.g. OH_ads) and accelerating the CO oxidation during MOR [104]. O'Hayre and coworkers dispersed PtRu nanoparticles on N‐doped carbon via magnetron sputtering and proved that the N‐doped carbon could yield enhanced stability and performance of PtRu catalyst through metal–N interactions [105, 106]. Su et al. supported Pt nanoparticles by the ethylene glycol reduction method with H₂PtCl₆ as the precursor [61]. The obtained hybrid also showed enhanced mass activity compared with commercial Pt.

N‐doped, carbon‐supported metal nanoparticle can also be applied in the electrochemical reduction of CO₂, which is critical for relieving the over‐produced CO₂ and convert CO₂ into a reusable form of carbons (e.g. CO, CH₄, and C₂H₄). We synthesized uniformly distributed Cu nanoparticles with a diameter of 7 nm and assembled them on pyridinic‐N‐rich graphene (p‐NG) [40]. The p‐NG‐supported 7 nm Cu (p‐NG‐Cu‐7) exhibited high selectivity of 19% for the production of C₂H₄ at −0.9 V (versus reversible hydrogen electrode (RHE)) when used in the reduction of CO₂ (Figure 11.7). The pyridinic‐N is suggested to function as CO₂ and proton absorber, thus facilitating the hydrogenation and carbon–carbon coupling reactions on Cu nanoparticles.

11.2.3 Metal–Nitrogen–Carbon Catalysts for Electrocatalysis

M–N–C structures are generally investigated as ORR electrocatalysts due to their exposed M–N_x center. Different from N‐doped carbon and transition metal compounds that usually work in alkali, M–N–C structures can demonstrate high ORR activity in not only alkali but also in acid with good stability. A variety of nitrogen precursors and transition metal salts along with carbon sources can be used to prepare M–N–C catalysts [44, 107, 108]. Generally, the mixture is heated at 700–1000 °C and acid leaching and second‐heat treatment are required to remove the undesired species and improve the ORR activity of the catalysts. PANI is a commonly studied N source for M–N–C structures. With Fe salt, Co salt, and PANI as the precursors, we fabricate high active ORR catalysts (PANI–FeCo–C) that can catalyze the ORR at potentials within 60 mV of that delivered by commercial Pt/C (Figure 11.8) [109]. The aniline was polymerized in 0.5 M HCl with the addition of ammonium peroxydisulfate as well as FeCl₃ and Co(NO₃)₂·6H₂O in the presence of carbon. After the complete polymerization of PANI, the mixture was dried and heated in N₂ atmosphere. The heat‐treated sample was then pre‐leached in 0.5 M H₂SO₄ and re‐heated in N₂ to obtain the final sample. Figure 11.8a,b proves that metal‐based nanoparticle still exists in the M–N–C structure. Compared with PANI–Fe–C, PANI–FeCo–C demonstrated enhanced ORR activity (Figure 11.8c,d). The heating temperature and steps are found to have significant influence on the ORR catalytic behavior of PANI‐derived catalysts [109–111]. The PANI–Fe–C structure would display the most outstanding ORR activity after being treated at 900 °C.

MOF‐derived M–N–C materials are also fabricated as ORR electrocatalysts due to the unique properties of MOFs. These materials are generally prepared by the building of MOFs with metal ions and organic linkers, followed by pyrolysis of the MOF units. The morphology and structure of the final products are mainly dependent on the features of MOFs. Zou and coworkers prepared Co–N–C‐based polyhedron structures with ZIF‐67 as the precursor [112]. By controlling the size of ZIF‐67, the structure and the ORR catalytic performance of the Co–N–C materials can be tuned. The material derived from the smallest ZIF‐67 (300 nm) exhibited superior ORR activity with an onset potential of 0.86 V (versus RHE) and a half‐wave potential of 0.71 V in acid. Fe‐based MOFs can be synthesized through a reaction between FeCl₂ and H₃BTT·2HCl (BTT³⁻ = 1,3,5‐benzenetristetrazolate) [113]. The Fe–BTT was then pyrolyzed, acid‐leached, and re‐heated in NH₃ to obtain Fe–N–C structure with iron carbides. An onset potential of 0.91 V and a half‐wave potential of 0.81 V for ORR could be achieved for the catalysts. Recently, we fabricated M–N–C type of graphene tubes with a Co‐based, cage‐containing MOF as the template (Figure 11.9a,b) [114, 115]. Either benzene‐1,3,5‐tricarboxylic acid or 2,4,6‐tris(4‐pyridyl)‐1,3,5‐triazine was used as the ligand to prepare Co‐based MOF, and then dicyandiamide and iron(II) acetate were added. The mixture was heated at 1000 °C in N₂ atmosphere to form graphene/graphene tube nanocomposites. The N‐doped graphene/graphene tube hybrid exhibits outstanding ORR activity with excellent cathode discharge capacity (Figure 11.9c) and acceptable cyclability in LiO₂ batteries (Figure 11.9d). In addition, the M–N–C type of N‐doped graphene tubes (N‐GTs) developed using MIL‐100(Fe) MOF as a template has been used to support Pt nanoparticles (Pt/N‐GT). SEM and TEM images (Figure 11.9e,f) indicate that Pt nanoparticles are well dispersed on N‐GT supports. Compared with commercial Pt/C, the Pt/N‐GT exhibited superior activity to N‐GT and Pt/C catalysts (Figure 11.9g). In N 1s XPS spectra (Figure 11.9h), a shift in the N 1s peaks is observed when comparing Pt/N‐GT and N‐GT samples. These changes in binding energy are possibly due to an interaction and a coordination between pyridinic nitrogen and Pt particles, resulting from the richness in local electron density to facilitate the deposition of reduced Pt seeds on the support. Recently, N‐GTs as supports for FeCoNi alloy nanoparticles have been reported as efficient catalysts for OER in addition to ORR [116].

Very recently, metal nanoparticles are investigated as viable templates to fabricate structure‐defined M–N–C catalysts. Zhang and coworkers developed a facile in situ replication and polymerization strategy to fabricate FeN‐doped mesoporous carbon spheres (Fe‐NMCSs) with Fe₃O₄ microspheres as the templates (Figure 11.10) [117]. Pyrrole would penetrate into the mesoporous structure of the Fe₃O₄ particles and polymerize in the voids. The Fe³⁺ ions etched from the Fe₃O₄ could act as the oxidizer as well as the Fe sources. Thus, the materials would keep the initial structure after the in situ polymerization. After being heated at 950 °C for 1 h, the high ORR‐active Fe‐NMCSs could be obtained. From Fe 2p XPS spectrum, the Fe‐NMCSs show several kinds of Fe that originate from either Fe–N structure or Fe oxides (Figure 11.10c). In 0.1 M KOH, it could exhibit an onset potential of 1.027 V with a half‐wave potential of 0.86 V as well as four‐electron reaction route for ORR (Figure 11.10d,e).

11.3.1 Design and Synthesis

B doping is another way to tune the electronic and electrochemical properties of carbon materials [118–121]. The introduction of B will induce a charge polarization in the carbon structure, which will benefit the electrocatalysis. Compared with N, B only has three valence electrons, which is less than carbon. Thus, B doping would result in p‐type behavior for the layered carbon materials. As B is lacks electron, it can hardly coordinate with transition metal to form metal–boron–carbon structure. Therefore, the reported B‐doped, carbon‐supported metal catalysts are mainly focusing on metal nanoparticles that supported on B‐doped carbon substrates.

To synthesize B‐doped, carbon‐supported metal catalysts, B‐doped carbon should be first prepared. CVD has been widely used to prepare B‐doped carbon with precursors containing C and B. Pasupathy and coworkers chemically deposited B‐doped graphene (BG) with CH₄/B₂H₆ as the precursor and proved that B can be incorporated into the carbon lattice in graphitic form primarily and contributes ∼0.5 carriers into the graphene sheet per dopant, which was similar to the N‐doped graphene prepared by this method [122]. Cattelan et al. prepared large‐scale, B‐doped graphene with the same B and C sources [123] and demonstrated that the introduction of B would benefit the production of graphene with large area (Figure 11.11). The B‐doped carbon can also be prepared by microwave plasma and arc discharge with borane as the B sources [51, 124]. Thermal annealing of carbon with H₃BO₃ or B₂O₃ is preferred in preparing B‐doped carbon for electrocatalysis as it is more efficient to produce B‐doped carbon in large quantity [125, 126]. After obtaining B‐doped carbon, metal nanoparticles can be supported on it by either in situ nucleation or redispersion of synthesized nanoparticles.

11.3.2 B‐Doped, Carbon‐Supported Metal Nanoparticle Electrocatalysts

To date, the nanoparticles that supported on B‐doped carbon are mainly precious metal nanoparticles. Sun et al. loaded Pt nanoparticles on B‐doped graphene by a microwave‐assisted polyol process in ethylene glycol solution [127]. The Pt decorated on B‐doped graphene (Pt/BG) showed smaller size and narrower size distribution compared with that of Pt/G, thus demonstrating an enhanced catalytic behavior for MOR. The enhancement in activity is ascribed to the electronic interaction between B‐doped graphene and Pt nanoparticles [128]. Kim and coworkers also synthesized B‐doped graphene‐supported Pt with H₂PtCl₆ as the precursor (Figure 11.12) [129]. They found that carbon black (CB) is beneficial for the well dispersion of B‐doped, carbon‐supported Pt catalysts. The catalysts exhibit excellent stability in proton exchange membrane fuel cell due to the binding between Pt‐ and B‐doped graphene. Liu and coworkers embedded Au@AuPt core shell nanoparticle on B‐doped graphene by simple redisperse Au@AuPt nanoparticle on graphene substrate [126]. The hybrid show high electrochemical response to rutin and can achieve a detection limit (S/N = 3) of 2.84 × 10⁻¹⁰ M when constructed as an electrochemical sensor.