Carbon-Based Electrocatalysts for Hydrogen and Oxygen Evolution

Subscriber access provided by UNIV NEW ORLEANS

Perspective

Carbon-based Electrocatalysts for Hydrogen and Oxygen Evolution Reactions Lulu Zhang, Jin Xiao, Haiyan Wang, and Minhua Shao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02718 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

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 Catalysis

Carbon-based Electrocatalysts for Hydrogen and Oxygen Evolution Reactions Lulu Zhang,† Jin Xiao,†,§ Haiyan Wang,†,‡ Minhua Shao†,#,* †

Department of Chemical and Biological Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China §

College of Electrical and Optoelectronic Engineering, Changzhou Institute of Technology, Changzhou 213032, China

‡

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

#

Energy Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China *

[email protected]

Abstract Hydrogen and oxygen evolution reactions (HER and OER) are important for many electrochemical systems. Besides traditional noble metal-based catalysts, carbon-based materials have been found effective to catalyze these reactions. Various carbon structures doped with heteroatoms (N, S, P, B and transition metals) and graphitic layer encapsulated metal and compounds particles have shown good activities toward HER and OER at universal pHs. In this perspective, recent researches on the development of carbon-based electrocatalysts for HER and OER, as well as their challenges and opportunities are discussed. Key words: Water splitting, electrolyzer, core-shell structure, non-precious metal electrocatalyst, metal organic framework

1

ACS Paragon Plus Environment

ACS Catalysis

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

1. Introduction The society has been facing growing concerns on environmental pollution and global energy crisis caused by the consumption of fossil fuels. Numerous efforts have been stimulated to search for renewable and clean energy alternatives, such as wind, tidal, solar and geothermal energy.1-2 However, the lack of suitable energy storage technologies has prevented these intermittent energy sources from wide applications. Hydrogen is a promising clean energy carrier that can be generated by splitting water with excess renewal energy in an electrolyzer. Electrocatalytic water splitting, which is a reverse process of fuel cell reactions, involves the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Both reactions require efficient catalysts to accelerate reaction kinetics in order to make the electrolyzer practically feasible. Precious metals (Pt) and noble metal oxides (IrO2) possess the best performance (activity and durability) for the HER and OER in acidic media, respectively, while large-scale industrial application of electrolyzer has been impeded by their high cost and scarcity.3 Numerous efforts have been made in developing non-precious metal based catalysts for HER and OER. Transition-metal based compounds such as carbides,4 nitrides,5 phosphides,

6-8

sulfides9 and selenides10 have been studied as alternative electrocatalysts

for these reactions. However, their catalytic activity and durability are still worse than precious metal based ones.11-12 Recently, carbon-based materials were found to possess unexpected performance in HER and OER.13 In this perspective, we briefly review the recent progress on the development of this type of electrocatalysts, including metal and 2


Page 2 of 34

Page 3 of 34

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 Catalysis

non-metal doped carbon structures, graphitic layer encapsulated metal and compound particles, and hybrid nanostructures. 2. Hydrogen evolution reaction The first step in HER is the discharge of H3O+ or H2O dependent on the pH to produce a hydrogen atom absorbed on the catalyst’s surface (Volmer reaction). The second step can be two different reactions. One is the electrochemical desorption step, which is known as the Heyrovsky reaction. The other possible pathway is the Tafel recombination reaction involving two adsorbed hydrogen atoms. The elemental reactions involved in acid and alkaline media are listed in Table 1.14 Table 1. Elemental reaction steps in HER in acid and alkaline media Acid

Alkaline

Volmer reaction

H3O+ + e- → Had + H2O

H2O + e- → OH- + Had

Heyrovsky reaction

Had + H+ + e- → H2

Had + H2O + e- → OH- + H2

Tafel reaction

Had + Had → H2

Had + Had → H2

According to the above-mentioned reaction mechanism, HER includes the adsorption and removal of adsorbed hydrogen atoms on the electrode surface. These are competitive processes and a good catalyst should have an ideal balance between binding and releasing of adsorbed reaction intermediates. Theoretical calculations concluded that the overall reaction rate of HER was highly dependent on the Gibbs free energy of hydrogen adsorption (∆GH). According to the so-called volcano plot,15 a good catalyst, Pt for instance, should have a ∆GH close to zero, i.e., the binding energy on the surface is not 3


ACS Catalysis

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

Page 4 of 34

too strong nor too weak. Due to the high price of Pt, many efforts have been made in developing precious metal free catalysts including carbon-based ones.10, 16,17 2.1 Non-metal and transition metal doped carbon Carbon itself is inert for most of the electrochemical reactions, including HER. Surface /subsurface modifications are required to tailor its structure in order to increase the binding energies of reactants and reaction intermediates on carbon. So far, most of the surface modification strategies have been adopted from those in oxygen reduction reaction (ORR).18-19 Doping heteroatoms into the carbon matrix has been found to be an effective way to modify the electronic properties of carbon and create active sites for HER.20 Due to the different electronic density from carbon, these dopants can affect the valance orbital energy levels of its adjacent carbon atoms, where hydrogen atoms can be adsorbed. The doping elements range from non-metals to transition metals. In many cases, more than one types of elements were doped in carbon to further improve the performance of carbon. Some typical doped carbon catalysts and their overpotentials at 10 mA cm-2 are summarized in Table 2. It is worth noting that the overpotentials were corrected based on the Tafel slopes and the mass loadings of the catalysts (a same mass loading of 0.285 mg cm-2 were used) reported in the same study so that their activities can be directly compared. Table 2. Summary of overpotentials in HER of selected metal and non-metal doped carbon structures Overpotential at 10 mA cm-2 (mV)a,b 185 217

Catalyts N-doped hexagonal carbon22 Graphitic-C3N4 with N-doped graphene23 4


Page 5 of 34

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 Catalysis

S-doped graphene nanosheets 26 N/P-co-doped carbon32 N/S-co-doped carbon31 Graphitic carbon nitride with S/Se-co-doped graphene21 Co/N co-doped carbon nanocomposites40 Co/N co-doped porous carbon137 CoNx on carbon39 Atomic Co in N-doped graphene42 Atomic Ni in graphitic carbon43 (0.1 M NaOH)

290 167 210 262 270 270 124 147 34

a

The overpotentials were derived from the Tafel slope by normalizing the mass loading of 0.285 mg cm-2. b The electrolyte used in activity evaluation was 0.5M H2SO4 unless stated otherwise.

Non-metal atoms such as N, B, S and P, have been employed as dopants to improve the activity of carbon for HER.21 Among them, N-doped carbon is one of the most studied materials for HER.22-23 Various carbon support can be used for doping including carbon black, hollow sphere, and even graphene-like structures. Huang et al. reported that the N-doped graphene showed an overpotential of 239 mV at 10 mA cm-2 in a 0.5 M H2SO4 solution with a catalysts mass loading of 204 µg cm-2.24 Besides the most intensively researched N dopants, other non-metal heteroatoms such as B and S were also found to be effective in enhancing the HER activity of carbon.25-26 The S-doped graphene showed an overpotential of 290 mV.26 Sathe et al. reported a facile way to dope B into graphene by using borane tetrahydrofuran.27 Electrochemical testing showed that the overpotential of B-doped graphene at 10 mA cm-2 was about 430 mV, which was 100 mV smaller than its parent material (defective graphene without borane). This result implies that B-doping is somewhat less effective than N and S. The discovery that single heteroatom doping can improve the catalytic activity of carbon-based materials toward HER has recently given way to research on multiple heteroatoms doped carbon materials, including dual and treble dopants. Most commonly 5


ACS Catalysis

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

used dopants can be classified into two categories according to their charge populations in carbon matrix: electron acceptor (N, O) and donor (F, S, P and B). Different heteroatom combinations, such as N /P, N /S and N /B, have been chosen as co-dopants to obtain more active carbon for HER.28-31 Zhang et al. prepared N and P co-doped porous carbon networks by pyrolysis of a mixture of melamine, phytic acid, and graphene oxide.32 The overpotential of this material was only 170 mV at 10 mA cm-2 in a 0.5 M H2SO4 solution. The high catalytic activity was mainly attributed to the synergistic coupling effect between N and P. Jiao et al. investigated additional doping of S, P and B in a N-doped carbon and found that activity followed the trend of N /S > N /P > N > N /B (Figure 1c).25 In other words, by introducing another non-metal element like S and P the activity of N-C could be further improved. On the contrast, B has a negative impact on the activity, which is also consistent with the result on the single-atom doping discussed above. From a theoretical perspective, the activity of those materials correlated well with the ∆GH on different surfaces (Figure 1b) with that of N /S being the closest to zero. For these dual-doped carbon surface, the active site is expected to be the carbon near the dopants (Figure 1a).

Figure 1. (a) Atomic configurations of three dual-doped models with the lowest computed |∆GH|. Green, pink, blue, red, gold, purple and white represent: C, B, N, O, S, P and H atoms, respectively. (b) The three-state free energy diagram for the pure, single- and dualdoped graphene models. (c) Electrochemical measurements on various graphene-based 6


Page 6 of 34

Page 7 of 34

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 Catalysis

materials in 0.5 M H2SO4 showing polarization curves with the benchmark MoS2 under the same conditions for direct comparison.25 Transition metal and nitrogen co-doped carbon-based materials prepared via pyrolysis have been widely recognized as excellent catalysts for ORR in both acidic and alkaline solutions.18, 33-34 The metal center coordinated with nitrogen atoms is believed to be the active site for ORR.35-36 This structure motivated a new approach to prepare efficient carbon-based materials for HER. Wang et al. synthesized a three dimensional Co-N-C structure by carbonization of cobalt salts and polyaniline on carbon fiber (Figure 2a).37 It was found that heat treatment temperature played a crucial role in the activity of the CoN-C with 750 ºC being the optimized temperature (η =138 mV at 10 mA cm-2), as shown in Figure 2b. The much lower overpotential of Co-N-C than that of N-C indicates the important role of Co in this composite. Density functional theory (DFT) calculation results showed that the ∆GH value on Co coordinated with 3 C atoms and 1 N atom was close to zero. Similar Co-N-C composites synthesized by different methods also demonstrated reasonable good activity toward HER in both acidic and alkaline solutions. 8, 38-40

All these results confirmed that the metal-centered and nitrogen-coordinated matrix

(Me-N-C, Me = transition metal) attributes to the enhanced activity for HER. However, the optimal coordination configuration of metal, N and C has not been agreed. Another interesting observation on Me-N-C is that pH does not have a significant effect on the overpotential as was observed on many precious metals.40 The overpotential of HER on Co-N-C was only increased by 35 mV at 10 mA cm-2 when the electrolyte was changed from 0.5 H2SO4 to 1 M KOH. The difference is one of the smallest among reported values. Not surprisingly, the electrocatalytic activity is also dependent on the types of transition metals in the active center. As shown in Figure 3,38 the overpotential of Me-N7


ACS Catalysis

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

C prepared by heating the mixture of transition metal precursors and Zeolitic Imidazolate Framework (ZIF)-8 at 1 mA cm-2 in a H2SO4 solution (pH=1) followed the order of Co < Cr < Zn < W < Ni < Cu, Mo, Mn, Fe. This trend is somewhat different from that in ORR, where Fe-N-C showed the highest activity.35 This discrepancy so far has not been investigated and requires further study.

Figure 2. (a) Schematic illustration of the synthetic process; (b) Polarization curves of PPANI750, PANICo550-950A, and Pt/C in 0.5 M H2SO4; (c) Calculated free-energy diagram of HER at the equilibrium potential for the four catalysts, and the inset is the model structure of the Co-3C1N catalyst.37

8


Page 8 of 34

Page 9 of 34

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 Catalysis

Figure 3. Overpotentials at 1 mA cm-2 for Me-N-C catalysts at pH=1 prepared by heating the mixture of transition metal precursors and ZIF-8.38 The synthesis of Me-N-C materials generally involves high-temperature pyrolysis, in which most transition metals form metal particles, metal carbides and sulfides, instead of Me-Nx coordinated sites. It will be ideal if the transition metal can be achieved at the single-atom level during the synthesis.41 By simply heating the mixture of graphene oxide and small amounts of cobalt salts in a gaseous NH3 atmosphere, Fei et al. obtained the atomic Co dispersed in carbon.42 Figure 4a shows that Co (bright dots) are atomically dispersed in the nitrogen-doped graphene matrix with a mass loading of 0.57%. This single-atom based structure showed excellent HER activity with a low onset potential of 30 mV and an overpotential of 147 mV at 10 mA cm-2 in a 0.5 M H2SO4 solution (Figure 4b). Figure 4b again confirms the importance of synergetic effect between transition metal, N and C as the Co, N co-doped carbon showed much higher activity than that doped with Co or N. Other metal atoms can also be atomically dispersed in carbon matrix and show similar activity improvement.43 Single-atom based catalysts are a new research trend and have attracted increasing attention.44 More efforts will be taken on

9


ACS Catalysis

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

Page 10 of 34

understanding the reaction pathways on this kind of unique structure, as well as solving the issues associated with large-scale synthesis, limited activity site density and durability assessment.

Figure 4. (a) HAADF-STEM image of the Co-NG, showing many Co atoms welldispersed in the carbon matrix. Scale bar, 1 nm. (b) LSV curves in 0.5 M H2SO4 at a scan rate of 2 mV s-1.42 2.2 Graphitic layer encapsulated metal or compound particles Transition metals and many of their compounds (carbides,45-46 nitrides5, phosphides6-7 etc.) have shown considerable activity toward HER. However, their stability is a big concern especially in strong acidic electrolytes. Interestingly, recent studies revealed that graphitic layer (usually doped with N) encapsulated metal and compound particles can improve both their activity and stability.13,

47-49

With a protective carbon layer, metal-

based particles don’t directly contact with the electrolyte but through electron transfer to the carbon shell, the composites still possess high catalytic activity for HER. The activities of some carbon layer encapsulated nanostructures for HER are summarized in Table 3. Table 3. Summary of overpotentials toward HER of encapsulated carbon-based materials Catalyts

Overpotential 10


Page 11 of 34

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 Catalysis

Carbon-coated bimetallic Co-W carbide nanoparticles31 (1M KOH) Co nanoparticles embedded in N-doped carbon60 N-doped carbon-wrapped Co on N-doped graphene nanosheets

at 10 mA cm-2 (mV)a, b 199 265 190

6

FeCo alloy in N-doped carbon nanotube47 (0.1M H2SO4) CoNi alloy in N-doped graphene shells51 (0.1M H2SO4) Mo2C embedded in carbon56 MoxC-modified N-doped carbon vesicle encapsulating Ni nanoparticles52 MoS2 nanoplates embedded in N-doped carbon nanofibers57 Co in N-rich carbon nanotubes50 Co in N-doped carbon65 MoO2 with P-doped porous carbon, and reduced graphene oxide66 Au covered by Zn/Fe embedded carbon64 MoCx in N-doped graphene58 FeCo in N-doped graphene55

284 229 76 101 138 199 337 51 104 95 262

a

The overpotentials were derived from the Tafel slope by normalizing the mass loading of 0.285 mg cm-2. b The electrolyte used in activity evaluation was 0.5M H2SO4 unless stated otherwise.

Zhou et al. synthesized N-doped carbon-wrapped Co nanoparticles on N-doped graphene nanosheets by the decomposition of cyanamide and reduction of Co2+.6 It showed a much better activity than naked Co particles with a small overpotential of 190 mV at 10 mA cm-2. Besides Co,50 other metal and metal compound nanoparticles can serve as the core (Table 3).51-59 It is interesting to see that Co@NC has a much higher activity than that of Fe@NC, suggesting that the type of metals in the core has a significant impact on the activity.60 This discrepancy is also consistent with the previous observation that Co-N-C performed better than Fe-N-C in HER. Besides pure metal particles, transition metal (Co, Fe, Ni) alloys embedded in carbon had the similar activity enhancement (Figure 5).47, 51 Deng et al. found that the N doping in the carbon shell could decrease the values of ∆GH, i.e., promoting the adsorption of H on the carbon shells 11


ACS Catalysis

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

Page 12 of 34

and the nature of covered cores would also affect the values of ∆GH (Figure 6).47 Besides metal particles, transition metal compounds covered by carbon layers also showed promising catalytic activity toward HER. For instance, Zhang et al. prepared Mo2C encapsulated in N-doped carbon nanotubes (Mo2C-NCNTs) and found that it exhibited a much smaller onset potential than bulk Mo2C.61 It is worth noting that the electronic effect from the metal-based core is dependent on the thickness of the graphitic layer. It is expected that the effect will be diminished if the carbon layer is too thick. The thickness dependent activity has not been systematically studied, which is important to understand the true synergistic effect between the core and the carbon-based shell.

(a)

(b)

(c)

Figure 5. (a-b) HRTEM images of CoNi@NC, showing the graphene shells and encapsulated metal nanoparticles. (c) Schematic illustration of the CoNi@NC structure shown in (b).52

12


Page 13 of 34

1.4 1.2 1

∆G (eV)

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 Catalysis

0.8 0.6 0.4 0.2 0 -0.2

Figure 6. Adsorption free energy for hydrogen (∆G) on various model surfaces.47

Metal organic frameworks (MOFs), a kind of inorganic-organic hybrid materials composed of metal centers and organic ligands have been used as an ideal template to fabricate carbon-based catalysts recently.62-63 The tunable porous structures and components make them ideal precursors in preparing carbon encapsulated materials.64 Zhang et al. prepared Co nanoparticles embedded in a N-doped carbon matrix by heating the ZIF-67 in N2 at 600 °C.65 It exhibited an overpotential of 339 mV at 10 mA cm-2 with a loading of 0.028 mg cm-2 and good stability in acidic solutions for HER. Tang et al. found that a metal oxide core MoO2 encapsulated in a P-doped nanoporous carbon supported on a reduced graphene oxide (RGO) substrate (MoO2@PC-RGO) prepared from carbonization of polyoxometalate-based MOFs showed excellent activity.66 The overpotential was only 64 mV at 10 mA cm-2 in a 0.5 M H2SO4 solution with a loading of 0.14 mg cm-2, approaching the performance of Pt. The remarkable performance was attributed to the synergistic effect between the well dispersed MoO2 nanoparticles and P-

13


ACS Catalysis

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

doped carbon. Since the metal center, pore size, and the size of the ligands can be easily adjusted in MOFs, it opens up a new door for the design of more efficient catalysts.

2.3 Hybrid carbon structures Both pristine and doped carbon can be supports for other types of HER catalysts, such as sulfides,67-71 carbides,72 phosphides,73-75 etc. The hybrid structures have been found to perform better than any individual component. Though the exact role of carbon supports is not very clear, they may help improve the dispersion and stabilize catalysts, and increase the electric conductivity and facilitate the electron transfer.76-80 Zheng et al. grafted a dense layer of CoSe2 nanobelts on carbon fiber felt (CoSe2/CFF) and found that its overpotential at 10 mA cm-2 was much smaller (141 mV) than that of physical mixture of CoSe2 and CFF mixture (252 mV).81 Despite the factor that CFF itself had no activity toward HER, the possible charge-transfer between CFF and CoSe2 together with a densely grafted structure resulted in a superior HER activity. Shinde et al. synthesized SnS on N-reduced graphene sheets (SnS/N-rGr) through a solution process as shown in Figure 7.82 SnS/N-rGr showed good catalytic activity for HER with a small onset overpotential of 125 mV, while freestanding SnS nanoparticles were almost inert for HER. The small size and high dispersion of SnS on graphene yielded numerous available edges that could serve as active sites for HER. This research shows the important role of carbon support that further improves the HER catalytic activities. Table 4 lists more hybrid carbon structures and their performance for HER.

14


Page 14 of 34

Page 15 of 34

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 Catalysis

Figure 7. Synthesis of SnS with and without graphene sheets. (a) Schematic synthesis process of SnS/N-rGr hybrid. (b) HER polarization curves of various hybrid electrocatalysts in 0.5 M H2SO4.82

Table 4. Summary of overpotentials toward HER of hybrid carbon structures Catalyts WS2 nanolayers with P/N/O-doped graphene80 Co-doped FeS2 nanosheets with carbon nanotubes70 CoS2 nanopyramid array on carbon fiber paper71 CoSe2 nanobelts on carbon fiber felt81 MoS2 nanosheets with carbon nanotubes76 FeP nanoparticles on graphene sheets75 MoS2 nanosheets with carbon nanopapers77 MoSx clusters decorated N-doped graphene78 (0.1M H2SO4) Mo2C encapsulated by N/P co-doped carbon shells and N/P co-doped reduced graphene oxide72 WS2 anchored to hollow N-doped carbon nanofibes69 15


Overpotential at 10 mA cm-2 (mV)a, b 104 162 94 221 201 123 100 181 24 280

ACS Catalysis

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

a

SnS on N-reduced graphene sheets82 MoSx layer on vertical N-doped carbon nanotube79

Page 16 of 34

130 92

The overpotentials were derived from the Tafel slope by normalizing the mass loading of 0.285 mg cm-2. b The electrolyte used in activity evaluation was 0.5 M H2SO4 unless stated otherwise.

3. Oxygen evolution reaction The oxygen evolution involves four-electron transfer and is much more complicated than HER. The possible elemental reactions in OER in acidic and alkaline media are given in Table 5. As shown in the following equations, the OER can be divided into four steps. The first step is the formation of adsorbed OH* on the catalysts with the first electron transfer. The second step is the transformation of OH* to O*. The third step is the transformation of O* to OOH* with another H2O molecule or OH-. The last step is the release of O2. Each step occurs with the release of one electron. The overpotentials of the OER can be determined by the reaction free energies of all four steps. The sluggish fourelectron transfer kinetics for OER process can severely hamper its further practical applications. Currently, the state-of-the-art catalysts for OER in acidic media are based on expensive and stable noble metals such as Ir. There are no good alternatives due to the stability issues at high potential and low pH. In the alkaline solutions, however, more materials, such as metal oxides and carbon-based materials83 are stable even in the potential range where OER occurs. This perspective only discusses studies conducted in alkaline solutions. Table 5. Elemental reaction steps in OER in acidic and alkaline media a Acid84

Alkaline85

16


Page 17 of 34

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 Catalysis

H2O (l) + * → OH* + (H+ + e-)

OH- + * → OH* + e-

OH* → O* + (H+ + e-)

OH* + OH- → O* + H2O (l) + e-

O* + H2O (l) → OOH* + (H+ + e-)

O* + OH- → OOH* + e-

OOH* → * + O2 (g) + (H+ + e-)

OOH* + OH- → * + O2 (g) + H2O(l) + e-

a

where (l) and (g) refer to gas and liquid phases, respectively; * stands for the active site on the catalyst, and O*, OH*, and OOH* are adsorbed intermediates.

3.1 Non-metal and transition metal doped carbon Heteroatoms doped carbon materials have also been proven to be efficient electrocatalysts for OER.86-96 Although the complicated electrochemical reactions with multi- electron-transfer steps make it difficult to describe the detailed mechanism of the OER, considerable progress has been made in developing carbon-based catalysts for OER. Single element, especially N-doped carbon materials have been explored extensively as bifunctional electrocatalysts for ORR and OER.87-91 As discussed in Section 2.1, the introduction of N atoms into carbon lattice can alter the electronic property and electric conductivity of carbon.84 The active sites for N containing species are believed to locate at carbon atoms near doping elements.88 Zhao et al. fabricated the nitrogen-doped carbon, which had an overpotential of 380 mV at 10 mA cm-2 in 0.1 M KOH with a mass loading of 0.2 mg cm-2 (Figure 8a).89 The OER activity was mainly dependent on the populations of pyridinic- or/and quaternary-N related active sites (Figure 8b). The high activity was also observed on N-doped mesoporous carbon nanosheets (η = 320 mV at 10 mA cm-2 with a mass loading of 0.21 mg cm-2 ) due to N-doping together with an extremely high

17


ACS Catalysis

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

Page 18 of 34

active surface area of the nanosheet structure.90 Other types of carbon structures including mesoporous carbon,91 graphene,92, carbon nanotubes,93 and carbon fiber94 showed reasonable good activity toward OER in alkaline by heteroatom doping, some typical non-metal doped carbon structures and their performance (overpotential normalized to the mass loading of catalyst) are listed in Table 6.

Figure 8. (a) OER activities of the different catalysts. (b) Relationship between the different elemental contents and OER activities of the N/C electrocatalyst (the potentials required to achieve 10 mA cm-2). GSA: Geometric surface, EASA: Electrochemical active surface area.89

Compared with the single heteroatom doping, the multiple heteroatoms doping is likely to create more active sites, thus leading to a high catalytic activity for OER. It was also confirmed that the OER activity could be tailored by altering the doping types, sites, levels and the distance between the co-dopants.85,

93, 97

Among the dual-heteroatoms

doping systems, N/P co-doping and N/S co-doping have been most employed. For example, Li et al. synthesized N/P co-doped graphene/carbon nanosheets by pyrolysis of graphene oxide, polyaniline, and phytic acid.92 The obtained material generated a low overpotential of 340 mV at 10 mA cm-2 with a mass loading of 0.141 mg cm-2, which is

18


Page 19 of 34

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 Catalysis

lower than that of N-and P-doped graphene, indicating that the multi-heteroatoms doping is better than the single one. This observation is also in agreement with the DFT calculation results reported by Zhang et al.85 Besides, N/P, N/S co-doped carbon materials including graphite foam, carbon nanotubes, also showed good OER activity with overpotentials around 360 mV at 10 mA cm-2.93, 98 Table 6. Summary of overpotentials toward OER for non-metal and metal doped carbonbased materials Catalysts N-doped porous carbon@graphene87 Carbon nitrogen-nanotubes88 N-doped mesoporous carbon nanosheet/carbon nanotube hybrids90 N-doped ordered mesoporous carbon/graphene framework91 P-doped graphitic carbon nitride grown on carbon-fiber paper94 N /P co-doped graphene/carbon nanosheets92 N /S co-doped carbon nanotubes93 (1 M KOH) Surface-oxidized multiwall carbon nanotubes95 S-doped carbon nanotubes96 (1 M KOH) Fe-N-co-doped graphene99 Co and N co-doped graphene with inserted carbon nanospheres100 Atomic dispersion of Fe-Nx species on N and S codecorated hierarchical carbon layers104

Overpotential at 10 mA cm-2 (mV)a.b 412 455 313 341 391 319 351 489 341 402 443 397

a

The overpotentials were derived from the Tafel slope by normalizing the mass loading of 0.285 mg cm-2. b The electrolyte used in activity evaluation was 0.1 M KOH unless stated otherwise.

As discussed in Section 2.1, the electrocatalytic activity of carbon can be further improved by combing non-metal and transition metal doping. This strategy also holds

19


ACS Catalysis

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

true for OER. 99-102 Among these materials, metal and nitrogen co-doped carbon materials (Me-N-C) have demonstrated remarkable OER activity. Zhao et al. synthesized the Fe-NC and Co-N-C with low overpotentials of ~ 360 mV and 380 mV at the current density of 10 mA cm-2, respectively. Their performance was comparable with that of IrO2/C (η =370 mV at 10 mA cm-2).102 Even though the synergistic effect between metal and nitrogen has been proposed to attribute to the improved OER activities,103 it is not clear whether the transition metal directly participates in the reaction and improves the kinetics as the overpotentials of these types of materials are similar with that of non-metal doping carbon (Table 6). Single-atom based transition metal doping was also explored to synthesize more active carbon materials. Chen et al. prepared atomically dispersed Fe on N /S co-doped hierarchical carbon and evaluated their activity for OER.104 The catalyst generated a reasonable good activity with an overpotential of 370 mV at 10 mA cm-2 with a loading of 0.6 mg cm-2, which was similar with other types of doped carbon structures. Thus, further improvement of the activity can be made by increasing the density of metal centers and exploring different carbon supports.

3.2 Graphitic layer encapsulated metal or compound particles As explained in the previous section, the metal or metal compounds embedded into carbon materials have advantages of higher mechanical/chemical stability and electric conductivity.105-108 These kinds of structures not only prevent the metal based nanoparticles from leaching and aggregation during reaction, but also increase the graphitization degree of carbon materials. 20


Page 20 of 34

Page 21 of 34

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 Catalysis

Table 7. Summary of overpotentials toward OER for graphitic layer encapsulated metal or compound particles Catalysts Co-embedded in N-doped carbon nanotubes105 Co nanoparticles encapsulated in N-doped carbon nanotubes106 Co/N embedded in carbon nanotubes107 Co nanoparticles embedded in porous N-rich carbon108 Co nanoparticle embedded in carbon nanotube/porous carbon hybrid112 Ni nanoparticles encapsulated in N-doped Graphene110 (1 M KOH) FeNi alloy nanoparticles encapsulated in Ndoped carbon nanotubes113 (1 M KOH) FeNi alloy embedded in single layer graphene115(1 M NaOH) Co@Co3O4 encapsulated in carbon nanografted N-doped carbon polyhedral114 CoO nanoparticles embedded in N/S-co-doped carbon fiber networks116 Co3O4 nanoparticles embedded in N-doped carbon/ CNTs hybrids117 (1 M KOH) FeP embedded in N/P dual-doped porous carbon nanosheets118 Co2P nanoparticle encapsulated in N/P codoped CNT119 (1 M KOH) Co2P and CoxN embedded in N doped nanocarbons120 CoPx nanoparticles embedded in N-doped carbon matrices121 (1 M KOH) CoP/reduced graphene oxide122 (1 M KOH) Cobalt sulfide hollow spheres embedded in N /S co-doped graphene nanoholes123 Co0.85Se nanocrystals embedded in N-doped carbon124 (1 M KOH) Co0.85Se particles coated with carbon shell125 (1

Overpotential at 10 mA cm-2 (mV)a, b 197

21


356 370 377 355 300 340 284 402 268 313 288 392 347 354 339 387 331 340

ACS Catalysis

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

M KOH) Fe3C nanoparticles encapsulated in Fe-N-doped carbon126

Page 22 of 34

351

a

The overpotentials were derived from the Tafel slope by normalizing the mass loading of 0.285 mg cm-2. b The electrolyte used in activity evaluation was 0.1M KOH unless stated otherwise.

The major challenge of non-precious metals for OER is the low efficiency and poor stability. One of the effective approaches is to fabricate catalysts with a core-shell structure, which consists of a metal-based core and a carbon shell (usually doped with N or other non-metal elements).105, 109-114 Wang et al. utilized cobalt-phthalocyanine (Co-Pc) as the cobalt and nitrogen sources for the fabrication of cobalt-embedded nitrogen doped carbon nanotube (CNCNT).105 The preparation process is shown in Figure 9a. The resulting sample exhibited highly efficient catalytic performance and stability in alkaline electrolytes, which was even better than IrO2 (Figure 9b). The overpotential at 10 mA cm2

was 197 mV in 0.1 M KOH, which is the lowest among this type of catalysts. Cui et al.

also reported that the transition metal and metal alloy nanoparticles encapsulated in a thin graphene-like carbon layer showed good OER activities in alkaline solutions. The overpotentials followed the order of FeNi@NC < CoNi@NC < FeCo@NC < Ni@NC < Fe@NC < Co@NC with the NiFe alloy core being the most active one.115 According to the DFT calculation results, the electron transfer from metal nanoparticles and N dopants can significantly change the electronic structure of the graphene layer. As a result, it will alter the binding energies of reaction intermediates on the graphene surface and the corresponding reaction activity. More examples are given in Table 7.

22


Page 23 of 34

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 Catalysis

Figure 9. (a) Schematic illustration of the preparation process of CNCNT. (b) Comparison of OER activities of the CNCNT and IrO2 in 0.1 M KOH.105

Besides metal nanoparticles, metal compounds including metal oxides, carbides, nitrides, phosphides, sulfides and selenides have been employed as the core encapsulated in various carbons (carbon nanosheets, graphene, nanotubes, spheres) as the shell, as listed in Table 7.116-127 Various methods have been developed to synthesize this type of structure. For example, Zhang et al. prepared FeP embedded in N/P co-doped 2D porous carbon nanosheets by a simple thermal annealing of iron phytate and folic acid.118 The core-shell structure showed a low overpotential of 300 mV at 10 mA cm-2 in 0.1 M KOH with a mass loading of 0.2 mg cm-2. Recently, MOFs have been widely used as precursors to prepare catalysts for ORR.128 They were also found to provide an opportunity to obtain competent OER electrocatalysts due to their large surface area, controllable composition and pore structure. Even though some MOFs can be directly used as efficient OER catalysts without calcination process,129, 130 the inferior stability limits their practical application. You et al. prepared CoPx nanoparticles embedded in Ndoped carbon (Co-P/NC) by direct carbonization of ZIF-67 followed by phosphidation (Figure 10).121 It exhibited a reasonably low overpotential of 354 mV at 10 mA cm-2 with 23


ACS Catalysis

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

a mass loading of 0.283 mg cm-2. Some other compounds (Co3O4,117, 131 Co0.85Se,124-125 Fe3C,132 etc.) encapsulated in carbon were also derived from MOFs and evaluated for OER. Although MOF-assisted strategy could be a promising method to design and synthesize carbon based OER catalysts with good nanostructures, there are still some issues that need to be addressed.133 Firstly, it is difficult to control the species and the contents of the heteroatoms in the MOF-derived carbon materials during the synthesis process. Secondly, the structure and morphology of the MOF-derived carbon materials are also difficult to control because of the difficulty to obtain uniform distribution between the metal/metal compound particles and the carbon matrix. Thirdly, there are insufficient understandings about the bonding effect, component proportional, particle distribution of particles and carbon in the MOF-derived carbon materials. The last but not the least, mass production technique of the MOF-derived carbon materials should be developed in the future.

Figure 10. (a) Schematic illustration of the two-step synthesis of Co-P/NC nanopolyhedrons. (b) OER activities of the Co/NC, Co-P/NC, IrO2 in 1 M KOH.121

24


Page 24 of 34

Page 25 of 34

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 Catalysis

There are a lot of common features of carbon-based materials in both HER and OER. Some of them can be applied as active bifunctional catalysts for overall water splitting in electrolyzers.108,

110, 122, 134-135

For example, Li et al. synthesized ultrafine Co2P

nanoparticles encapsulated in N and P dual-doped porous carbon nanosheet/carbon nanotube hybrids.136 This material showed good catalytic activities for both HER and OER in basic media and produced 10 mA cm-2 at 1.64 V. The performance was comparable to the one using Pt/C (cathode) and IrO2 (anode). 4 Summary and outlook Based on the discussions in Section 3, carbon contributed to HER and OER electrocatalysts in different ways. Figure 11a and b summarize the ranges of overpotentials at 10 mA cm-2 derived from literature of different approaches for HER and OER, respectively. Even though carbon itself is inactive for these reactions, after being doped with certain non-metal elements (N, S, P, B), they show reasonably good activities for both HER and OER. The lowest overpotential (167 mV) for HER in acidic solutions was observed on N/P co-doped carbon.32 For OER, the N-doped mesoporous carbon nanosheet/carbon nanotube showed the lowest overpotential (313 mV) in alkaline media.90 By co-doping with transition metals, the activity of carbon can be further improved for HER. Most of the catalysts showed overpotentials around 150 mV in acidic solutions. The highest activity with an overpotential of 34 mV in 0.1 M NaOH was observed on the specially designed single-atom based catalyst with Ni atoms atomically dispersed in carbon.43 The strategy of co-doing with transition metals in carbon structures seem not work on OER reflected from the relatively high overpotentials.

25


ACS Catalysis

600 Non-metal doping

HER

Metal/non-metal doping Encapsulated material

400

Hybrid material

200

(b) η at 10 mA cm-2 (mV)

(a) η at 10 mA cm-2 (mV)

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

Page 26 of 34

Non-metal doping

600

Metal/non-metal doping Encapsulated material

OER

400

200

0

Figure 11. Activity comparison of carbon-based materials for (a) HER in acid and (b) OER in alkaline. The overpotentials have been normalized by the same mass loading (0.285 mg cm-2).

Another strategy is to encapsulate metal or metal compound particles with thin graphitic layers or nanotube structures. The electronic effect from the metal-based core is believed to make the carbon layer active for HER and OER. In general, this core-shell like materials have relatively better activity than doped carbon (except for the singleatom based one) with overpotentials from 50 mV to 280 mV for HER, and 200 mV to 430 mV for OER, respectively. The lowest overpotential was observed on MoO2 encapsulated with P-doped porous carbon66 and Co particles embedded in N-doped carbon nanotubes105 for HER and OER, respectively. Besides directly catalyzing reactions, pristine and modified carbon structures can also be a good support for nanocatalysts to improve the electric conductivity and dispersion. Carbon-based materials have shown great promise in catalyzing HER and OER, there are still some open questions to be answered. 1) What’s the real active site and the reaction mechanism on various carbon-based materials? In situ microscopic and spectroscopic techniques with high resolution combined with advanced theoretical

26


Page 27 of 34

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 Catalysis

calculations, i.e., taking electrolyte and potential into account, may help fully understand roles of doping elements and the metal/metal compound cores. 2) How to increase the population of active sites. For instance, for the single-atom based catalysts, the metal atom loading is very low. It is extremely challenging to increase the metal loading while maintaining the atomic dispersion. More advanced synthesis methods need to be developed. 3) Performance evaluation in electrolyzers must be conducted to assess the activity and stability of carbon-based electrocatalysts. Almost all the evaluation data so far were reported based on the liquid cell testing and may not represent the real performance in electrolyzers. The feasibility of carbon-based materials as both anode and cathode catalysts should be assessed in electrolyzers. The results of this kind of testing are very important to guide the future development of carbon-based electrocatalysts.

Acknowledgement The authors acknowledge the support from Research Grant Council (26206115) of the Hong Kong Special Administrative Region, Guangdong Special Fund for Science and Technology Development (Hong Kong Technology Cooperation Funding Scheme (201604030012, 201704030065) and a startup fund from the Hong Kong University of Science and Technology.

References (1) Panwar, N.; Kaushik, S.; Kothari, S., Renew. Sust. Energ. Rev. 2011, 15, 1513-1524. (2) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J., Chem. Rev. 2011, 111, 3577-3613. (3) Pavel, C. C.; Cecconi, F.; Emiliani, C.; Santiccioli, S.; Scaffidi, A.; Catanorchi, S.; Comotti, M., Angew. Chem., Int. Ed. 2014, 53, 1378-1381. 27


ACS Catalysis

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

(4) Michalsky, R.; Zhang, Y.-J.; Peterson, A. A., ACS Catal. 2014, 4, 1274-1278. (5) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G., J. Am. Chem. Soc. 2013, 135, 19186-19192. (6) Zhou, W.; Zhou, J.; Zhou, Y.; Lu, J.; Zhou, K.; Yang, L.; Tang, Z.; Li, L.; Chen, S., Chem. Mater. 2015, 27, 2026-2032. (7) Lv, C.; Yang, Q.; Huang, Q.; Huang, Z.; Xia, H.; Zhang, C., J. Mater. Chem. A 2016, 4, 13336-13343. (8) Fu, S.; Zhu, C.; Song, J.; Engelhard, M. H.; Li, X.; Du, D.; Lin, Y., ACS Energy Lett. 2016, 1, 792-796. (9) Wang, H.; Lu, Z.; Xu, S.; Kong, D.; Cha, J. J.; Zheng, G.; Hsu, P.-C.; Yan, K.; Bradshaw, D.; Prinz, F. B., Proceedings of the National Academy of Sciences 2013, 110, 19701-19706. (10) Ming, F.; Liang, H.; Shi, H.; Xu, X.; Mei, G.; Wang, Z., J. Mater. Chem. A 2016, 4, 15148-15155. (11) Roudgar, A.; Groß, A., J. Electroanal. Chem. 2003, 548, 121-130. (12) Bai, S.; Wang, C.; Deng, M.; Gong, M.; Bai, Y.; Jiang, J.; Xiong, Y., Angew. Chem., Int. Ed. 2014, 53, 12120-12124. (13) Wang, H.; Lu, Z.; Kong, D.; Sun, J.; Hymel, T. M.; Cui, Y., ACS Nano 2014, 8, 4940-4947. (14) Bockris, J. M.; Potter, E., J. Electrochem. Soc. 1952, 99, 169-186. (15) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J.; Chen, J. G.; Pandelov, S.; Stimming, U., J. Electrochem. Soc. 2005, 152, J23-J26. (16) Anxolabéhère-Mallart, E.; Costentin, C.; Fournier, M.; Nowak, S.; Robert, M.; Savéant, J.-M., J. Am. Chem. Soc. 2012, 134, 6104-6107. (17) Zhou, W.; Jia, J.; Lu, J.; Yang, L.; Hou, D.; Li, G.; Chen, S., Nano Energy 2016, 28, 29-43. (18) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R., Chem. Rev. 2016, 116, 3594-3657. (19) Shao, Y.; Zhang, S.; Engelhard, M. H.; Li, G.; Shao, G.; Wang, Y.; Liu, J.; Aksay, I. A.; Lin, Y., J. Mater. Chem. 2010, 20, 7491-7496. (20) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z., Acs Nano 2015, 9, 931-940. (21) Shinde, S.; Sami, A.; Lee, J.-H., J. Mater. Chem. A 2015, 3, 12810-12819. (22) Liu, Y.; Yu, H.; Quan, X.; Chen, S.; Zhao, H.; Zhang, Y., Sci. Rep. 2014, 4, 6843. (23) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z., Nat. Commun. 2014, 5, 3783. (24) Huang, X.; Zhao, Y.; Ao, Z.; Wang, G., Sci. Rep. 2014, 4, 7557. (25) Jiao, Y.; Zheng, Y.; Davey, K.; Qiao, S.-Z., Nat. Energy 2016, 1, 16130. (26) Shervedani, R. K.; Amini, A., Carbon 2015, 93, 762-773. (27) Sathe, B. R.; Zou, X.; Asefa, T., Catal. Sci. Technol. 2014, 4, 2023-2030. (28) Ito, Y.; Cong, W.; Fujita, T.; Tang, Z.; Chen, M., Angew. Chem., Int. Ed. 2015, 54, 2131-2136. (29) Lin, Y.; Pan, Y.; Zhang, J.; Chen, Y.; Sun, K.; Liu, Y.; Liu, C., Electrochim. Acta 2016, 222, 246-256. (30) Shinde, S. S.; Sami, A.; Lee, J. H., ChemCatChem 2015, 7, 3873-3880. (31) Zhou, Y.; Leng, Y.; Zhou, W.; Huang, J.; Zhao, M.; Zhan, J.; Feng, C.; Tang, Z.; Chen, S.; Liu, H., Nano Energy 2015, 16, 357-366.

28


Page 28 of 34

Page 29 of 34

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 Catalysis

(32) Zhang, J.; Qu, L.; Shi, G.; Liu, J.; Chen, J.; Dai, L., Angew. Chem., Int. Ed. 2016, 55, 2230-2234. (33) Jaouen, F.; Proietti, E.; Lefèvre, M.; Chenitz, R.; Dodelet, J.-P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P., Energy Environ. Sci. 2011, 4, 114-130. (34) Fu, S.; Zhu, C.; Li, H.; Du, D.; Lin, Y., J. Mater. Chem. A 2015, 3, 12718-12722. (35) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., Science 2011, 332, 443-447. (36) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P., Science 2009, 324, 71-74. (37) Wang, Z.-L.; Hao, X.-F.; Jiang, Z.; Sun, X.-P.; Xu, D.; Wang, J.; Zhong, H.-X.; Meng, F.-L.; Zhang, X.-B., J. Am. Chem. Soc. 2015, 137, 15070-15073. (38) Morozan, A.; Goellner, V.; Nedellec, Y.; Hannauer, J.; Jaouen, F., J. Electrochem. Soc. 2015, 162, H719-H726. (39) Liang, H.-W.; Brüller, S.; Dong, R.; Zhang, J.; Feng, X.; Müllen, K., Nat. Commun. 2014, 6, 7992-7992. (40) Zhang, L.; Liu, W.; Dou, Y.; Du, Z.; Shao, M., J. Phy. Chem. C 2016, 120, 2904729053. (41) Qiu, H. J.; Ito, Y.; Cong, W.; Tan, Y.; Liu, P.; Hirata, A.; Fujita, T.; Tang, Z.; Chen, M., Angew. Chem., Int. Ed. 2015, 54, 14031-14035. (42) Fei, H.; Dong, J.; Arellano-Jiménez, M. J.; Ye, G.; Kim, N. D.; Samuel, E. L.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J., Nat. Commun. 2015, 6, 8668. (43) Fan, L.; Liu, P. F.; Yan, X.; Gu, L.; Yang, Z. Z.; Yang, H. G.; Qiu, S.; Yao, X., Nat. Commun. 2016, 7, 10667. (44) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T., Accounts Chem. Res. 2013, 46, 1740-1748. (45) Zhao, Y.; Kamiya, K.; Hashimoto, K.; Nakanishi, S., Angew. Chem., Int. Ed. 2013, 52, 13638-13641. (46) Wan, C.; Regmi, Y. N.; Leonard, B. M., Angew. Chem., Int. Ed. 2014, 53, 64076410. (47) Deng, J.; Ren, P.; Deng, D.; Yu, L.; Yang, F.; Bao, X., Energy Environ. Sci. 2014, 7, 1919-1923. (48) Yang, L.; Zhou, W.; Jia, J.; Xiong, T.; Zhou, K.; Feng, C.; Zhou, J.; Tang, Z.; Chen, S., Carbon 2017, 122, 710-717. (49) Zhou, W.; Xiong, T.; Shi, C.; Zhou, J.; Zhou, K.; Zhu, N.; Li, L.; Tang, Z.; Chen, S., Angew. Chem., Int. Ed. 2016, 55, 8416-8420. (50) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T., Angew. Chem., Int. Ed. 2014, 53, 4372-4376. (51) Deng, J.; Ren, P.; Deng, D.; Bao, X., Angew. Chem., Int. Ed. 2015, 54, 2100-2104. (52) Wang, S.; Wang, J.; Zhu, M.; Bao, X.; Xiao, B.; Su, D.; Li, H.; Wang, Y., J. Am. Chem. Soc. 2015, 137, 15753-15759. (53) Wang, J.; Gao, D.; Wang, G.; Miao, S.; Wu, H.; Li, J.; Bao, X., J. Mater. Chem. A 2014, 2, 20067-20074. (54) Lu, J.; Zhou, W.; Wang, L.; Jia, J.; Ke, Y.; Yang, L.; Zhou, K.; Liu, X.; Tang, Z.; Li, L., ACS Catal. 2016, 6, 1045-1053. (55) Yang, Y.; Lun, Z.; Xia, G.; Zheng, F.; He, M.; Chen, Q., Energy Environ. Sci. 2015, 8, 3563-3571. (56) Ma, R.; Zhou, Y.; Chen, Y.; Li, P.; Liu, Q.; Wang, J., Angew. Chem., Int. Ed. 2015, 54, 14723-14727. 29


ACS Catalysis

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

(57) Zhu, H.; Du, M.; Zhang, M.; Zou, M.; Yang, T.; Wang, S.; Yao, J.; Guo, B., Chem. Commun. 2014, 50, 15435-15438. (58) Yang, X.; Feng, X.; Tan, H.; Zang, H.; Wang, X.; Wang, Y.; Wang, E.; Li, Y., J. Mater. Chem. A 2016, 4, 3947-3954. (59) Liu, Y.; Li, G.-D.; Yuan, L.; Ge, L.; Ding, H.; Wang, D.; Zou, X., Nanoscale 2015, 7, 3130-3136. (60) Fei, H.; Yang, Y.; Peng, Z.; Ruan, G.; Zhong, Q.; Li, L.; Samuel, E. L.; Tour, J. M., ACS Appl. Mater. Interfaces 2015, 7, 8083-8087. (61) Zhang, K.; Zhao, Y.; Fu, D.; Chen, Y., J. Mater. Chem. A 2015, 3, 5783-5788. (62) Zhou, W.; Lu, J.; Zhou, K.; Yang, L.; Ke, Y.; Tang, Z.; Chen, S., Nano Energy 2016, 28, 143-150. (63) Song, J.; Zhu, C.; Xu, B. Z.; Fu, S.; Engelhard, M. H.; Ye, R.; Du, D.; Beckman, S. P.; Lin, Y., Adv. Energy Mater. 2017, 7, 1601555. (64) Wang, Z.; Lu, Y.; Yan, Y.; Larissa, T. Y. P.; Zhang, X.; Wuu, D.; Zhang, H.; Yang, Y.; Wang, X., Nano Energy 2016, 30, 368-378. (65) Zheng, F.; Xia, H.; Xu, S.; Wang, R.; Zhang, Y., RSC Adv. 2016, 6, 71767-71772. (66) Tang, Y. J.; Gao, M. R.; Liu, C. H.; Li, S. L.; Jiang, H. L.; Lan, Y. Q.; Han, M.; Yu, S. H., Angew. Chem., Int. Ed. 2015, 54, 12928-12932. (67) Peng, S.; Li, L.; Han, X.; Sun, W.; Srinivasan, M.; Mhaisalkar, S. G.; Cheng, F.; Yan, Q.; Chen, J.; Ramakrishna, S., Angew. Chem., Int. Ed. 2014, 53, 12594-12599. (68) Khan, M.; Yousaf, A. B.; Chen, M.; Wei, C.; Wu, X.; Huang, N.; Qi, Z.; Li, L., Nano Res. 2016, 9, 837-848. (69) Yu, S.; Kim, J.; Yoon, K. R.; Jung, J.-W.; Oh, J.; Kim, I.-D., ACS Appl. Mater. Interfaces 2015, 7, 28116-28121. (70) Wang, D.-Y.; Gong, M.; Chou, H.-L.; Pan, C.-J.; Chen, H.-A.; Wu, Y.; Lin, M.-C.; Guan, M.; Yang, J.; Chen, C.-W., J. Am. Chem. Soc. 2015, 137, 1587-1592. (71) Zhang, H.; Li, Y.; Zhang, G.; Xu, T.; Wan, P.; Sun, X., J. Mater. Chem. A 2015, 3, 6306-6310. (72) Li, J. S.; Wang, Y.; Liu, C. H.; Li, S. L.; Wang, Y. G.; Dong, L. Z.; Dai, Z. H.; Li, Y. F.; Lan, Y. Q., Nat. Commun. 2016, 7, 11204. (73) Pan, Y.; Yang, N.; Chen, Y.; Lin, Y.; Li, Y.; Liu, Y.; Liu, C., J. Power Sources 2015, 297, 45-52. (74) Liu, Q.; Pu, Z.; Asiri, A. M.; Sun, X., Electrochim. Acta 2014, 149, 324-329. (75) Zhang, Z.; Lu, B.; Hao, J.; Yang, W.; Tang, J., Chem. Commun. 2014, 50, 1155411557. (76) Cai, Y.; Yang, X.; Liang, T.; Dai, L.; Ma, L.; Huang, G.; Chen, W.; Chen, H.; Su, H.; Xu, M., Nanotechnology 2014, 25, 465401. (77) Ye, T.-N.; Lv, L.-B.; Xu, M.; Zhang, B.; Wang, K.-X.; Su, J.; Li, X.-H.; Chen, J.-S., Nano Energy 2015, 15, 335-342. (78) Chen, S.; Duan, J.; Tang, Y.; Jin, B.; Qiao, S. Z., Nano Energy 2015, 11, 11-18. (79) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O., Nano Lett. 2014, 14, 1228-1233. (80) Duan, J.; Chen, S.; Chambers, B. A.; Andersson, G. G.; Qiao, S. Z., Adv. Mater. 2015, 27, 4234-4241. (81) Zheng, Y.-R.; Gao, M.-R.; Yu, Z.-Y.; Gao, Q.; Gao, H.-L.; Yu, S.-H., Chem. Sci. 2015, 6, 4594-4598. 30


Page 30 of 34

Page 31 of 34

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 Catalysis

(82) Shinde, S.; Sami, A.; Kim, D.-H.; Lee, J.-H., Chem. Commun. 2015, 51, 1571615719. (83) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z., ACS Nano 2014, 8, 5290-5296. (84) Li, M.; Zhang, L.; Xu, Q.; Niu, J.; Xia, Z., J. Catal. 2014, 314, 66-72. (85) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L., Nat. Nanotechnol. 2015, 10, 444-452. (86) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z., ACS Catal. 2015, 5, 5207-5234. (87) Liu, S.; Zhang, H.; Zhao, Q.; Zhang, X.; Liu, R.; Ge, X.; Wang, G.; Zhao, H.; Cai, W., Carbon 2016, 106, 74-83. (88) Yadav, R. M.; Wu, J.; Kochandra, R.; Ma, L.; Tiwary, C. S.; Ge, L.; Ye, G.; Vajtai, R.; Lou, J.; Ajayan, P. M., ACS Appl. Mater. Interfaces 2015, 7, 11991-12000. (89) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K., Nat. Commun. 2013, 4, 2390. (90) Li, X.; Fang, Y.; Zhao, S.; Wu, J.; Li, F.; Tian, M.; Long, X.; Jin, J.; Ma, J., J. Mater. Chem. A 2016, 4, 13133-13141. (91) Zhang, C.; Wang, B.; Shen, X.; Liu, J.; Kong, X.; Chuang, S. S. C.; Yang, D.; Dong, A.; Peng, Z., Nano Energy 2016, 30, 503-510. (92) Li, R.; Wei, Z.; Gou, X., ACS Catal. 2015, 5, 4133-4142. (93) Qu, K.; Zheng, Y.; Jiao, Y.; Zhang, X.; Dai, S.; Qiao, S.-Z., Adv. Energy Mater. 2016, 1602068. (94) Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z., Angew. Chem., Int. Ed. 2015, 54, 4646-4650. (95) Lu, X.; Yim, W. L.; Suryanto, B. H.; Zhao, C., J. Am. Chem. Soc. 2015, 137, 29012907. (96) El-Sawy, A. M.; Mosa, I. M.; Su, D.; Guild, C. J.; Khalid, S.; Joesten, R.; Rusling, J. F.; Suib, S. L., Adv. Energy Mater. 2016, 6, 1501966. (97) Zhao, Z.; Xia, Z., ACS Catal. 2016, 6, 1553-1558. (98) Yu, X.; Zhang, M.; Chen, J.; Li, Y.; Shi, G., Adv. Energy Mater. 2016, 6, 1501492. (99) He, D.; Xiong, Y.; Yang, J.; Chen, X.; Deng, Z.; Pan, M.; Li, Y.; Mu, S., J. Mater. Chem. A 2017, 5, 1930-1934. (100) Qiao, X.; Liao, S.; Zheng, R.; Deng, Y.; Song, H.; Du, L., ACS Sustain. Chem. Eng. 2016, 4, 4131-4136. (101) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.L.; Dai, L., Nano Energy 2016, 29, 83-110. (102) Zhao, Y.; Kamiya, K.; Hashimoto, K.; Nakanishi, S., J. Phy. Chem. C 2015, 119, 2583-2588. (103) Liu, X.; Amiinu, I. S.; Liu, S.; Cheng, K.; Mu, S., Nanoscale 2016, 8, 13311-13320. (104) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y., Angew. Chem., Int. Ed. 2017, 56, 610-614. (105) Wang, Z.; Xiao, S.; Zhu, Z.; Long, X.; Zheng, X.; Lu, X.; Yang, S., ACS Appl. Mater. Interfaces 2015, 7, 4048-4055. (106) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X., Nat. Energy 2016, 1, 15006. (107) Song, J.; Zhu, C.; Fu, S.; Song, Y.; Du, D.; Lin, Y., J. Mater. Chem. A 2016, 4, 4864-4870. (108) Li, X.; Niu, Z.; Jiang, J.; Ai, L., J. Mater. Chem. A 2016, 4, 3204-3209. 31


ACS Catalysis

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

(109) Wang, J.; Wu, H.; Gao, D.; Miao, S.; Wang, G.; Bao, X., Nano Energy 2015, 13, 387-396. (110) Xu, Y.; Tu, W.; Zhang, B.; Yin, S.; Huang, Y.; Kraft, M.; Xu, R., Adv. Mater. 2017, 29, 1605957. (111) Zhu, C.; Fu, S.; Xu, B. Z.; Song, J.; Shi, Q.; Engelhard, M. H.; Li, X.; Beckman, S. P.; Sun, J.; Du, D., Small 2017, 13, 1900796. (112) Dou, S.; Li, X.; Tao, L.; Huo, J.; Wang, S., Chem. Commun. (Camb) 2016, 52, 9727-9730. (113) Tao, Z.; Wang, T.; Wang, X.; Zheng, J.; Li, X., ACS Appl. Mater. Interfaces 2016, 8, 35390-35397. (114) Aijaz, A.; Masa, J.; Rosler, C.; Xia, W.; Weide, P.; Botz, A. J.; Fischer, R. A.; Schuhmann, W.; Muhler, M., Angew. Chem., Int. Ed. 2016, 55, 4087-4091. (115) Cui, X.; Ren, P.; Deng, D.; Deng, J.; Bao, X., Energy Environ. Sci. 2016, 9, 123129. (116) Liu, T.; Guo, Y.-F.; Yan, Y.-M.; Wang, F.; Deng, C.; Rooney, D.; Sun, K.-N., Carbon 2016, 106, 84-92. (117) Li, X.; Fang, Y.; Lin, X.; Tian, M.; An, X.; Fu, Y.; Li, R.; Jin, J.; Ma, J., J. Mater. Chem. A 2015, 3, 17392-17402. (118) Zhang, R.; Zhang, C.; Chen, W., J. Mater. Chem. A 2016, 4, 18723-18729. (119) Das, D.; Das, A.; Reghunath, M.; Nanda, K. K., Green Chem. 2017, 19, 1327-1335. (120) Zhong, X.; Jiang, Y.; Chen, X.; Wang, L.; Zhuang, G.; Li, X.; Wang, J.-g., J. Mater. Chem. A 2016, 4, 10575-10584. (121) You, B.; Jiang, N.; Sheng, M.; Gul, S.; Yano, J.; Sun, Y., Chem. Mater. 2015, 27, 7636-7642. (122) Jiao, L.; Zhou, Y.-X.; Jiang, H.-L., Chem. Sci. 2016, 7, 1690-1695. (123) Qiao, X.; Jin, J.; Fan, H.; Li, Y.; Liao, S., J. Mater. Chem. A 2017, 5, 12354-12360. (124) Meng, T.; Qin, J.; Wang, S.; Zhao, D.; Mao, B.; Cao, M., J. Mater. Chem. A 2017, 5, 7001-7014. (125) Li, S.; Peng, S.; Huang, L.; Cui, X.; Al-Enizi, A. M.; Zheng, G., ACS Appl. Mater. Interfaces 2016, 8, 20534-20539. (126) Jiang, H.; Yao, Y.; Zhu, Y.; Liu, Y.; Su, Y.; Yang, X.; Li, C., ACS Appl. Mater. Interfaces 2015, 7, 21511-21520. (127) Yuan, C.-Z.; Jiang, Y.-F.; Wang, Z.; Xie, X.; Yang, Z.-K.; Yousaf, A. B.; Xu, A.W., J. Mater. Chem. A 2016, 4, 8155-8160. (128) Fu, S.; Zhu, C.; Song, J.; Du, D.; Lin, Y., Adv. Energy Mater. 2017, 1700363. (129) Zhao, S.; Wang, Y.; Dong, J.; He, C.-T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z., Nat. Energy 2016, 1, 16184. (130) Shen, J. Q.; Liao, P. Q.; Zhou, D. D.; He, C. T.; Wu, J. X.; Zhang, W. X.; Zhang, J. P.; Chen, X. M., J. Am. Chem. Soc. 2017, 139, 1778-1781. (131) Zhang, C.; Xiao, J.; Lv, X.; Qian, L.; Yuan, S.; Wang, S.; Lei, P., J. Mater. Chem. A 2016, 4, 16516-16523. (132) Li, J. S.; Li, S. L.; Tang, Y. J.; Han, M.; Dai, Z. H.; Bao, J. C.; Lan, Y. Q., Chem. Commun. (Camb) 2015, 51, 2710-2713. (133) Zhao, Y.; Song, Z.; Li, X.; Sun, Q.; Cheng, N.; Lawes, S.; Sun, X., Energy Storage Mater. 2016, 2, 35-62. 32


Page 32 of 34

Page 33 of 34

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 Catalysis

(134) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y., J. Am. Chem. Soc. 2015, 137, 2688-2694. (135) Wang, X.; Li, W.; Xiong, D.; Petrovykh, D. Y.; Liu, L., Adv. Funct. Mater. 2016, 26, 4067-4077. (136) Li, X.; Fang, Y.; Li, F.; Tian, M.; Long, X.; Jin, J.; Ma, J., J. Mater. Chem. A 2016, 4, 15501-15510. (137) Zhang, Z.; Yang, S.; Dou, M.; Ji, J.; Wang, F., Int. J. Hydrogen Energ. 2017, 42, 4193-4201.

Graphical abstract

33


ACS Catalysis

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

34


Page 34 of 34

Carbon-Based Electrocatalysts for Hydrogen and Oxygen Evolution

Recommend Documents