4.1. NG as a Metal-Free Catalyst for ORR
Compared with NCNTs, NG has a large surface area and outstanding electrical conductivity; moreover, it also has the unique graphitic basal plane structure that could further facilitate electron transport and supply more active sites.
In 2010, Qu
et al. first reported the application of NG as catalysts for the ORR [
90]. As shown in
Figure 6, a free-standing NG film of 4 cm
2 in size consisting of only a few layer sheets was obtained by the CVD method, using gas mixtures of NH
3, CH
4, H
2 and Ar on the Ni catalyst surface. The N content in the as-synthesized NG was
ca. 4 at.%. The RRDE voltammograms measurements were conducted, in alkaline electrolyte, to investigate the catalytic properties of NG, graphene and Pt/C for ORR. From
Figure 6b, it can be seen that the graphene electrode showed a 2 e
− process for ORR with an onset potential of around −0.45 V. After doping with N, the NG electrode exhibited a one-step, 4 e
− pathway for ORR.
Figure 6.
(
a) An optical photograph of NG film floating on water; (
b) LV curves in 0.1 M KOH saturated with air of different samples. Reprinted with permission from [
90]. Copyright © 2010, American Chemical Society.
Figure 6.
(
a) An optical photograph of NG film floating on water; (
b) LV curves in 0.1 M KOH saturated with air of different samples. Reprinted with permission from [
90]. Copyright © 2010, American Chemical Society.
Calculated by the Koutecky-Levich equation, the transferred electron number per O
2 molecule of the NG was 3.6–4. It was found that the steady-state catalytic current density of the NG electrode was three times higher than the commercial Pt/C electrode. Similar to NCNTs, NG has excellent durability and good selectivity for ORR. The accelerated degradation test (ADT), which was carried out by CV in O
2-saturated electrolyte, is used to estimate the stability of the catalyst. In previous work, the graphene showed obviously more stable catalytic performance than Pt/C. Almost no significant loss in the voltammetric charge was observed after even a 100,000-cycle stability test [
91]. Another advantage of NG compared to Pt for ORR is that ORR on NG is not greatly affected by methanol [
59,
90] and CO [
90,
92]. For instance, a 40% decrease was observed at the Pt/C electrode on the introduction of 2% (
w/
w) methanol [
90], whereas the NG electrode remained unaffected under the identical condition. The high selectivity of NG toward ORR makes it very attractive for implementation in different kinds of fuel cells.
Based on these results, numerous research studies have been conducted on NG for ORR. Some of the typical works are summarized in
Table 1. It is notable that the half-wave potential and onset potential for ORR are important criteria for evaluating the activity of an electrocatalyst, and the number of the electron transfer is determined from RRDE measurements to show that whether the electron transfer mechanism is a 2e
− dominated process or 4e
− dominated process.
Table 1.
Summary of some typical work dedicated to NG as a metal-free catalyst for ORR.
Table 1.
Summary of some typical work dedicated to NG as a metal-free catalyst for ORR.
Synthesis Method and Reactants | N-Content (at.%) | Electrocatalytic Performance | Electron Transfer Number | Ref. |
---|
Thermal treatment of glucose and urea | 33 | NG (25 at.%) shows competitive ORR activities with Pt/C and much better crossover resistance and excellent stability | 3.2–3.7 | [19] |
CVD (C source, ethylene; N source, ammonia; Cu) | up to 16 | Higher onset potential as compared to Pt/C | close to 2 | [49] |
Thermal treatment of GO using melamine | 10.1 | Much higher ORR activity than grapheme | 3.4–3.6 | [57] |
N plasma treatment on graphene | 8.5 | Higher ORR activity than graphene, and higher durability and selectivity than Pt/C | - | [59] |
CVD (C source, methane; N source, ammonia, Cu) | 4 | Higher activity, better stability and tolerance to crossover than Pt | 3.6–4 | [90] |
Detonation technique with cyanuric chloride and trinitrophenol | 12.5 | Comparable to that of Pt, more stable and less expensive | 3.69 | [91] |
A resin-based methodology with N-containing resin and metal ions | 1.8 | The onset potential on the NG electrode is close to that of Pt/C. The current is almost the same for both the Pt/C and NG | 2.1–3.9 | [92] |
Hydrothermal reaction of GO with urea | 6.05–7.65 | The performance of these NG materials towards ORR is still not as good as that of Pt/C in terms of the half-wave potential and current density | ~3 | [93] |
Covalent functionalize GO using organic molecules and thermal treatment | 0.72–4.3 | The NG nanosheet exhibited a good electrocatalytic activity through an efficient one-step, 4e− pathway | 3.63 | [94] |
CVD of N-containing aromatic precursor molecules | 2.0–2.7 | The N dopants in the graphene reduce the ORR overpotential, thereby enhancing the catalytic activity | 3.5–4.0 | [95] |
GO treatment by ammonia hydroxide, heating under ammonia gas, and reaction with melamine | 6.0–6.8 | Pyridinic N plays a vital role in ORR | 3.2–3.7 | [96] |
Annealing of GO with ammonia and N-containing polymers | 2.91–7.56 | The higher limiting current density compared to Pt | 2.85–3.65 | [97] |
Thermal reaction between GO and NH3 | 2.4–4.6 | The onset potential is close to that of Pt/C | ~3.8 | [98] |
Hydrothermal reaction with GO and melamine | 26.08 | It shows lower ORR activity than Pt/C 40 wt.% | 3.2–4.0 | [99] |
Hydrothermal process using urea and holey GO | 8.6 | Superb ORR with 4e− pathway and excellent durability | 3.85 | [100] |
Thermally annealing GO with melamine | 8.05 | The nG-900 exhibits lower activity and onset potential than Pt/C, albeit higher than graphene; excellent stability | 3.3–3.7 | [101] |
Pyrolyzing GO with urea | 7.86 | The NG showed a much-higher activity than glassy carbon (GC) and graphene | 3.6–4.0 | [102] |
Redox GO with pyrrole then thermal treatment | 6 | Shows comparable onset potentials with 40 wt.% Pt/C | 3.3 | [103] |
GO and dicyandiamide under hydrothermal condition | 7.78 | The onset potentials at rGO-N was lower than that at Pt/C | 2.6 | [104] |
Pyrolysis of graphene oxide and polyaniline | 2.4 | High activity toward ORR with a superior long-term stability and tolerance to methanol crossover | 3.8–3.9 | [105] |
Thermally annealing GO 5-aminotetrazole monohydrate | 10.6 | Higher current density than Pt/C. Lower onset potential of ORR than that of the commercial Pt/C | 3.7 | [106] |
Pyrolysis of sugar in the presence of urea | 3.02–11.2 | The NG1000 has comparable ORR half-wave potential to 20 wt.% Pt/C | 3.2–3.8 | [107] |
Hydrothermal reaction of GO with urea | 5.8–6.2 | NG has higher ORR activity than grapheme, but is not yet comparable to the Pt | 3.0–4.0 | [108] |
Pyrolysis of GO and polydopamine | 2.78–3.79 | Much more enhanced ORR activities with positive onset potential and larger current density than graphene | 3.89 | [109] |
Pyrolyzing GO with Melamine, urea and dicyandiamide | 5 | Compared to Pt/C, the half-wave potential of ORR on this NG catalyst was close, wheras the n values are slightly lower | 3.5–4 | [110] |
PANI acting as a N source were deposited on the surface of GNRs via a layer-by-layer approach | 4.1–8.3 | Very good electrocatalytic activity and stability | 3.91 | [111] |
NG is synthesized by pyrolyzing ion exchange with resin and glycine | 0.98–1.65 | Doping N in graphene is good to improve the activity for ORR, but still lower than Pt/C catalyst | - | [112] |
Microwave heating of graphene under NH3 flow | 4.05–5.47 | The doping of graphite N enhanced the activity of the catalysts in the ORR in alkaline solution | 3.03–3.3 | [113] |
Facile hydrothermal method | 2.8 | Competitive with the commercial Pt/C catalysts in alkaline medium | 3.66–3.92 | [114] |
Gas-phase oxidation strategy using a nitric acid vapor | 0.52 | The onset potential is (0.755 V vs. RHE), comparable to the value of chemically synthesized NG, and the current densities are higher than those demonstrated for NG. | 3.2–3.9 | [115] |
CVD growth of graphene and post-doping with a solid N precursor of graphitic C3N4 | 6.5 | Excellent activity, high stability, and very good crossover resistance for ORR in alkaline medium. | 3.96-4.05 | [116] |
A hard templating approach | 5.07 | Outstanding ORR performance in both acidic and alkaline solutions. | 3.9 | [117] |
In spite of extensive studies, the explanations on the exact catalytic mechanisms of NG (e.g., wherein the N configuration (pyridinic N or graphitic N) is more important for the ORR activity) or even the active sites are still controversial [
94,
118]. In Sun
et al.’s research [
55], they found that NG containing 0.3892% quaternary N (the highest N content in three samples) showed the best ORR activity and the relationship between ORR activity and graphitic N contents matched very well. It revealed that graphitic type N plays the vital role for ORR activity. Luo
et al. [
49] synthesized the graphene layers doped with nearly 100% pyridinic N through the pyrolysis of methane (CH
4) and NH
3 on Cu substrate, and the as-synthesized pyridinic N-doped graphene mainly exhibited a 2e
− transfer process for ORR, indicating that pyridinic N may not, as previously expected, effectively promote the 4e
− ORR performance of carbon materials.
On the contrary, in the work of Sheng [
57], the NG mainly containing pyridine-like N atoms was obtained by the heat-treatment of GO in the presence of melamine. Since the electrocatalytic activity of the NGs toward ORR is independent of N-doping level, it may indicate that the pyridine-like N in NGs determines its ORR activity. Pyridinic N, which has a lone electron pair in the plane of the carbon matrix, could donate the electron to the π-bond, attract electrons, and therefore be catalytically active. Some results were shown in many previous works [
94,
95,
96].
In the research of Ruoff’s group [
97], NG with different N-doping formats was prepared by annealing GO together with different N-containing precursors, such as ammonia and N-containing polymers. It was prone to generate graphitic N and pyridinic N when annealing GO with ammonia, while it tended to form pyridinic and pyrrolic N species when annealing GO with polyaniline or polypyrrole. They found that the total atomic content of N rarely affects the ORR activity under alkaline conditions. Actually, the graphitic N-dominated catalysts exhibit higher catalytic activity and larger limiting current density than that of pyrrolic or pyridinic N-dominated catalysts. However, the pyridinic N could enhance the ORR onset potential and gradually convert the 2e
− dominated pass-way to the 4e
− dominated process. Also, some researchers [
119,
120,
121,
122] used the periodic DFT to simulate the ORR at the edge of NG. For example, by taking into account the experimental conditions,
i.e., the surface coverage, the water effect, the bias effect and pH, Yu
et al. [
119] presented a systematic theoretical study on the full reaction path of ORR on NG. They concluded that the rate-determining step is the O(ads) removal from the NG surface. From another perspective, by calculating energy variations during each reaction step using DFT, Zhang and Xia [
120] demonstrated that the electrocatalytic activity of NG is related to the atomic charge density distribution and electron spin density The reasons for why NG has catalytic capability (while pristine graphene does not) have also been discussed. From Kim
et al.’s results, [
121] doping of N in graphene could promote the oxygen adsorption, the first electron transfer, and the selectivity toward the 4e
− reduction pathway. More specifically, they suggested that the outermost graphitic N sites are the main active sites. Meanwhile, they also proposed that the graphitic N site which involves a ring-opening of the cyclic C-N bond at the edge of graphene could result in the pyridinic N, thus, the inter-converts conversion mechanism between pyridinic and graphitic types during the catalytic cycle may reconcile the experimental controversy about what types of N are the ORR active sites for N-doped carbon materials [
121].
Besides the doped N species, the morphology of NG also plays a significant role for the ORR properties. During the doping process of graphene, the stacking of graphene sheets is inclined to increase the diffusion resistance of reactants/electrolytes, reduce the specific area, and the exposed active sites. It is thus worth controlling the structure of NG to get more ORR activity. In this regard, there is a great deal of work on the production of N-doped holey graphene [
99,
100]. For instance, a 3D porous nanostructure which has N-doped holes on individual graphene sheets was synthesized through a hydrothermal process using urea and holey GO by Yu
et al. [
100]. Benefiting from the 3D porous nanostructure, abundant exposed sites, and high-level N doping, the as-prepared material exhibited excellent ORR performance, such as the high limiting current, strong resistance to the methanol crossover, which are competitive with the commercial 20 wt.% Pt/C catalysts.
4.2. NG as Support Material for ORR
The incorporation of N atoms within graphene sheets could contribute more functional groups, higher electron-mobility, and more active sites for catalytic reactions. Also, it is beneficial for facilitating the distribution and uniformity of metal nanoparticles. Moreover, when NG acts as the support, it could enhance the catalytic properties due to the interaction between graphene and metal nanoparticles. Consequently, NG materials have been regarded as one very promising metal catalyst support [
123,
124,
125,
126].
Typically, NG is proposed to be able to stabilize the noble metal nanoparticles, and improve the durability of the catalysts. Moreover, nitrogen doping could introduce active sites for catalytic reactions and also act as anchoring sites for metal nanoparticle deposition. Yang
et al. fabricated a composite of Pt-Au alloy nanoparticles on NG sheets by a wet-chemistry method [
127]. As shown in
Figure 7, the NG was synthesized by thermal treatment of GO powder and melamine. Then the solutions of H
2PtCl
6, HAuCl
4, NG in DMF and water underwent the microwave irradiation. The as-prepared Pt
3Au-NPs were found to be well dispersed on the NG sheets (
Figure 7b) and the HRTEM image in
Figure 7c revealed the lattice fringes of the NPs have an interplanar spacing of 0.232 nm. The fast Fourier transforms (FFTs) shown in
Figure 7d indicated the single crystallite nature of the Pt
3Au/NG on (111) plane.
Figure 7e,f showed that the corresponding potential for Pt
3Au/NG was much lower than the other two samples at a given oxidation current density. Improved electrocatalytict activity was observed due to the small size, uniform dispersion and a high electrochemical active surface area of the nanocomposites. Recently, more studies on NG- or N-rGO-supported Pt electrocatalysts have also been reported; all these results demonstrate the significant function of N doping in producing highly efficient ORR electrocatalysts [
128,
129,
130].
Additionally, it was predicted that non-precious-metal-NG hybrid materials would also lead to enhanced catalytic properties. For instance, Chen
et al. reported a strategy to synthesize ZnSe/NG nanocomposites (NG-ZnSe) [
131]. As shown in
Figure 8, [ZnSe](DETA)
0.5 nanobelts were gradually put into the GO solution, and then the sediments were processed by hydrothermal treatment. As shown in
Figure 8b, ZnSe nanorods, which were composed of ZnSe nanoparticles, were grown on a graphene surface. It can be seen from
Figure 8c that the NG-ZnSe electrode exhibited higher positive onset potential and larger current for ORR. The improved performance can be attributed to the synergetic effects between NG and alloy nanostructures. There are also a number of similar reports using non-precious metal to produce metal/NG composites, showing potential applications [
132,
133,
134,
135,
136].
Figure 7.
(
a) Fabrication of the Pt-Au alloy NPs on the NG sheets; (
b) TEM of Pt
3Au/N-G; (
c) HRTEM and (
d) FFTs of a single Pt
3Au NP on NG; (
e) CVs and (
f) LSV of Pt/C (a, black), Pt
3Au/G(b, red) and Pt
3Au/N-G catalysts (c, green). Reprinted with permission from [
127]. Copyright © 2012, Royal Society of Chemistry.
Figure 7.
(
a) Fabrication of the Pt-Au alloy NPs on the NG sheets; (
b) TEM of Pt
3Au/N-G; (
c) HRTEM and (
d) FFTs of a single Pt
3Au NP on NG; (
e) CVs and (
f) LSV of Pt/C (a, black), Pt
3Au/G(b, red) and Pt
3Au/N-G catalysts (c, green). Reprinted with permission from [
127]. Copyright © 2012, Royal Society of Chemistry.
Figure 8.
(
a) Schematic preparation of NG-ZnSe nanocomposites (blue rods-[ZnSe](DETA)
0.5 nanobelts; orange rods-ZnSe nanorods; purple balls-N; gray balls-C); (
b) SEM photograph of ZnSe/NG; (
c) LV curves in 1.0 M KOH solution with saturated O
2 of different electrodes. Reprinted with permission from Ref. [
131]. Copyright © 2012, American Chemical Society. Note: in the original paper, the authors refer to “nitrogen-doped graphene” as “GN”; here in this review, for consistency, we named it “NG.”
Figure 8.
(
a) Schematic preparation of NG-ZnSe nanocomposites (blue rods-[ZnSe](DETA)
0.5 nanobelts; orange rods-ZnSe nanorods; purple balls-N; gray balls-C); (
b) SEM photograph of ZnSe/NG; (
c) LV curves in 1.0 M KOH solution with saturated O
2 of different electrodes. Reprinted with permission from Ref. [
131]. Copyright © 2012, American Chemical Society. Note: in the original paper, the authors refer to “nitrogen-doped graphene” as “GN”; here in this review, for consistency, we named it “NG.”