Two-Dimensional Materials on the Rocks: Positive and Negative Role

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Two-Dimensional Materials on the Rocks: Positive and Negative Role of Dopants and Impurities in Electrochemistry Shu Min Tan† and Martin Pumera*,‡,§

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†

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 ‡ Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology, Technicka 5, Praha 6 166 28, Czech Republic § Future Energy and Innovation Lab, Central European Institute of Technology, Brno University of Technology, Purkyňova 656/123, Brno CZ-616 00, Czech Republic

ABSTRACT: Two-dimensional (2D) materials, such as graphene and transition-metal chalcogenides, were shown in many works as very potent catalysts for industrially important electrochemical reactions, such as oxygen reduction, hydrogen and oxygen evolution, and carbon dioxide reduction. We critically discuss here the development in the field, showing that not only dopants but also impurities can have dramatic effects on catalysis. Note here that the difference between dopant and impurity is merely semantic−dopant is an impurity deliberately added to the material. We contest the general belief that all doping has a positive effect on electrocatalysis. We show that in many cases, dopants actually inhibit the electrochemistry of 2D materials. This review provides a balanced view of the field of 2D materials electrocatalysis. KEYWORDS: electrocatalysis, electrochemistry, layered materials, doping, catalysis, inhibition, impurity, hydrogen evolution, carbon dioxide, oxygen reduction, graphene, transition-metal chalcogenides he ever-progressing field of material science pushes the boundaries of technology and offers endless possibilities in a myriad of applications. The earliest testament of the exploitation of materials dates to the Stone Age when relics that were fashioned out of stones aided in the survival of humans against other physically superior species.1 In the modern society, harnessing the advantages of functional materials and technology has brought about tremendous economic gains. In this regard, one of the most prominent class of functional materials is that of two-dimensional (2D) layered materials. In his acclaimed lecture “There’s Plenty of Room at the Bottom” in 1959, Richard P. Feynman postulated that layered materials with “just the right layers” could be synthesized and that individual atoms could be engineered to attain advantageous properties.2 Nearly half a century later, the isolation and characterization of 2D monolayer graphene was

achieved by Geim and Novoselov,3 whose success led to the metaphorical graphene gold rush.4 As the provenance of 2D systems, layered materials have been extensively investigated with regards to their unique structures, high specific surface areas, and striking electronic properties and have been widely applied in the fields of catalysis, sensing, and energy storage purposes. By definition, layered materials possess strong intralayer chemical bonding but weak interlayer van der Waals bonding. Due to the distinctive van der Waals forces of attraction between the layers, the delamination of layered materials perpendicular to

T

© XXXX American Chemical Society

Received: October 12, 2018 Accepted: February 11, 2019

A

DOI: 10.1021/acsnano.8b07795 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Timeline representing the milestones of the development of significant graphene- and TMD-based electrocatalysts toward HER, ORR, OER, and CO2RR. Generally, for “X-Y-Z” material, X and Y denote the dopants, while Y represents TMD or graphene/carbon materials.

the c-axis can be performed with ease while retaining the integrity of each layer.5 Typically, layered materials are known for their anisotropic properties such as electrical and thermal conductivities as well as electrocatalytic performances which differ between the edge and basal planes.4,6−8 Each class of 2D layered material has its own unique characteristics; for example, graphene exhibits exceptional thermal, optical, and mechanical properties, while transition metal dichalcogenides (TMDs) display varying stacking sequences that heavily influence the coordination environment of the transition metal and hence its electronic properties.9,10 These diverse properties have been exploited toward numerous applications, ranging from biosensing,11,12 electrochemical sensing,13,14 and electroluminescence15 to batteries,16−18 supercapacitors,19,20 and data storage devices.21−23 The successful employment of 2D layered materials toward important electrocatalytic applications, such as water splitting, has largely been attributed to the exotic 2D structure which engendered high exposure of active sites for the various reactions. However, these nanomaterials often contained impurities, which were incidentally or deliberately included through the synthesis process or originated from the parent layered material.24−28 These impurities may significantly enhance or impair the electrochemical properties of 2D layered materials.29 In the pursuit of cost-efficient and highperformance electrocatalysts, studies into the effects of deliberate inclusion of impurities or heteroatoms, commonly termed as doping, have resulted in a plethora of doped 2D materials synthesized via a multitude of doping strategies. Specifically, graphene and related carbon materials, for

example, carbon nanotubes (CNTs) as well as TMDs stood out as highly active electrocatalysts toward electrochemical hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and carbon dioxide reduction reaction (CO2RR), with great potential for commercialization; with the help of these materials, these reactions have demonstrated high reaction efficiencies and are important for the twin objectives of energy storage and conversion. Commonly employed dopants include metal-based dopants, for example, iron nanoparticles, nonmetal-based dopants, for example, nitrogen or sulfur, or a combination of both, and the outstanding electrocatalytic performances of the doped materials stimulated further investigations into the doping mechanism and DFT calculations of active sites. Though most dopants were observed to have positive bearings on the performance efficiency, selectivity, durability, and susceptibility of the 2D materials to poisoning by contaminants, the optimization of doping parameters, such as doping percent, type of doping (e.g., substitutional doping or physical adsorption), and nature of dopant (e.g., metallic or nonmetallic, transition metal ,or main group metal), plays an imperative role in determining the merit or demerit of doping. It was observed that in many cases, indiscriminate addition of dopant led to inhibition of the reaction. Unfortunately, with the current technology, the perfect engineering of doped materials remains a far-fetched dream of Feynman, especially in the field of energy storage and conversion. To consolidate the immense amount of literature available in this regard, this review will be highlighting the industrial and environmental importance of HER, ORR, OER, B


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and CO2RR, the different strategies for contamination or doping of graphene and related materials as well as TMDs, and the impacts of these unintentional and deliberate inclusions of heteroatoms on their electrocatalytic properties toward HER, ORR, OER, and CO2RR. For ease of orientation for the vast amount of work presented in this review, a graphical overview of the milestones achieved in the development of HER, ORR, OER, and CO2RR electrocatalysts, with particular focus on graphene, and related carbon materials (for CO2RR) as well as TMDs, is featured in Figure 1.

(2)

HO2− + H 2O + 2e− → 3OH−

(2‐electron pathway) (3)

Between these two routes, the direct 4-electron reduction has been established as the most efficient pathway for fuel cell applications.36,37 As a consequent of the convoluted nature of electrochemical ORR involving multiple steps and intermediates with the employment of assorted electrocatalysts, catalyst support, and electrolytes, the ORR mechanism remains ambiguous despite the comprehensive research endeavors conducted toward its elucidation. Nevertheless, despite the limited mechanistic information concerning ORR, it is apparent that enthalpy inputs necessary for O2 adsorption, the ensuing bond activation, and, subsequently, OO double bond dissociation (497 kJ mol−1) led to the sluggish kinetics accorded to ORR. These factors compel the exploitation of electrocatalysts to ease the overpotential needed to drive ORR. Presently, Ptbased materials remained the most efficient electrocatalysts for oxygen reduction.34,38−40 According to Nørskov et al., the underlying principle behind the perceived ORR electrocatalytic activities of metals lies in their relative strength of oxygen bonding with respect to Pt.41 Metals with stronger bonding gave rise to slow desorption of products, while those with weak bonding were confronted with high enthalpy barriers to proton and electron transfer to oxygen and thus the dissociation of oxygen, leading to poor ORR performances. Despite its excellent performance, the premium cost of Pt is untenable in relation to industrial-scale application toward ORR. Moreover, the Pt ORR active sites are vulnerable to deactivation due to impurities crossover in fuel cells.42,43 Thus, a multitude of studies on substitutes for Pt have manifested, of which the metal-/nonmetal-doped graphene-based materials44−46 have been lauded as the most versatile category of nanomaterials with excellent ORR performances. Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). The water splitting reaction comprises of both HER and OER, occurring at the cathode and anode, respectively. To convert one molecule of water to H2 and 1/2O2 gas molecules, the requisite Gibbs free energy, ΔG, is 237.2 kJ mol−1, which corresponds to a minimum voltage contribution of 1.229 V, in accordance with the Nernst equation.47,48 In an acidic environment, water splitting is conducted in an electrolyzer with proton exchange membrane (PEM), in which the electrochemical HER occurs via the reduction of two protons, liberating a hydrogen molecule, and the electrochemical OER manifested in protons and oxygen molecule evolving from the oxidation of water molecules, as described below:

ELECTROCHEMICAL CATALYTIC REACTIONS The two-fold factors of rising population and economic development drove the surge in demand for nonrenewable fossil fuels, accelerating the depletion of finite energy sources.30,31 Moreover, the release of greenhouse gases such as carbon dioxide (CO2), nitrous oxide, and methane associated with indiscriminate consumption of fossil fuels has also led to escalating global temperature and, consequently, rising sea levels from shrinking glaciers. With the rocketing energy demand and dwindling fossil fuel reserves, coupled with aggravated climate change resulting from energy generation from these fuels, the exigency of exploring potential alternative supplies of renewable energy, which can sustain our needs and reduce the anthropogenic costs on the environment, takes center stage. To store intermittent renewable energy sources such as solar and wind energy for uninterrupted usage, hydrogen fuel produced from water splitting (HER and OER) and hydrocarbons such as methane and ethylene from CO2RR have been extensively scrutinized. Electrical energy can then be extracted from the hydrogen fuel through the application of alkaline fuel cells in which ORR is indispensable. Due to the high energy barriers associated with heterogeneous reactions involving gaseous species in an aqueous environment, economical yet highly active electrocatalysts that greatly lower these barriers are critical to the sustainable development of these renewable energy resources. Hence, prior to the dissertation on the electrocatalytic performances of doped graphene and TMDs, it is important to examine the thermodynamics and kinetics of the aforementioned reactions to facilitate the assessment of the advantages and disadvantages of impurities. Oxygen Reduction Reaction (ORR). In energy generation systems, for example, lithium-air batteries and fuel cells, that employ electrochemical methods, oxygen reduction reaction (ORR) is a crucial cathodic reaction, whereby the ease with which it occurs greatly affects the overall operating efficiency of said systems.32,33 Thus far, three different types of solvents have been investigated for ORR: acidic and alkaline aqueous solvents and nonaqueous aprotic solvents. Of the three solvent systems, the alkaline aqueous medium is favored among researchers, as it confers a milder and less corrosive reaction condition in comparison to its acidic equivalent as well as more rapid ORR kinetics.34,35 As such, greater emphasis shall be allotted to alkaline ORR in the succeeding discussion. In aqueous alkaline media, ORR proceeds via either a 2electron or a 4-electron process, in which molecular oxygen is reduced to either hydroperoxide ion or hydroxide ion, respectively, as described below: O2 + 2H 2O + 4e− → 4OH−

(2‐electron pathway)

H 2O → 1/2O2 + 2H+ + 2e− 2H+ + 2e− → H 2

(acidic OER)

(acidic HER)

H 2O → H 2 + 1/2O2

(4) (5)

(overall equation)

(6)

Despite the advantages of PEM water electrolysis system such as high energy competency and hydrogen production rate, the exorbitant costs of noble metal catalysts, which is suitable in the acidic environment, and PEM, which prevents mixing of the gaseous products, limit the industrial utility of the system.49 As such, alkaline water electrolysis has been

(4‐electron pathway) (1) C


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Figure 2. Proposed OER mechanism and phase structure on Pt electrocatalyst under alkaline conditions. Reprinted with permission from ref 55. Copyright 2017 The Royal Society of Chemistry.

procedure and may be rate-limited by one of the three steps below:52 Electrochemical discharge step (Volmer step):

proposed due to its innate cost-effectiveness resulting from the use of nonprecious metal catalysts. Under alkaline conditions, H2O substitutes H+ as the proton source, while OH− acts as the conjugate base:47 −

−

(alkaline OER)

2H 2O + 2e− → H 2 + 2OH−

(alkaline HER)

4OH → 2H 2O + O2 + 4e

H3O+ + e− → Hads + H 2O

(7)

b=

2.3RT ≈ 120mV αF (9)

(8)

Electrochemical desorption step (Heyrovský step):

A major downfall of the alkaline system is the difficulty in initiating and terminating its operation because of the need to equalize the gas pressure on both cathode and anode sides to prevent gas crossover. This restricts its functionality to employ fluctuating renewable sources of energy as energy inputs. In general, studies of HER and OER electrocatalysts focus predominantly on acidic and alkaline systems, respectively.50,51 As a result of the large activation energy of HER and OER, electrocatalysts that couple improved reaction efficiencies with excellent stability are highly sought after. Typically, electrocatalysts with greater catalytic performances minimize the reaction overpotential which lowers the energy required to drive both HER and OER. Another parameter for the monitoring of electrocatalytic performance is the Tafel slope which is characteristic of the electrocatalyst and is governed by the rate-determining step of HER or OER. From the Tafel plot, the linear portions were fit to the Tafel equation (η = b log j + a, where j = current density, and b = Tafel slope). Both establishing and interpreting the Tafel slope are essential for revelation of the elementary steps that constitute the reaction mechanism. The mechanism of electrochemical HER in acidic media involving HER electrocatalysts can be resolved as a two-step

Hads + H3O+ + e− → H 2 + H 2O

b=

2.3RT ≈ 40mV (1 + α)F (10a)

Recombination step (Tafel step): Hads + Hads → H 2

b=

2.3RT ≈ 30mV 2F

(10b)

Generally, HER commences with the initial discharge step (eq 9), subsequently, either the electrochemical desorption step (eq 10a) or the recombination step (eq 10b) ensues, progressing via the Volmer−Tafel or the Volmer−Heyrovský mechanism.52 Endowed with outstandingly high Hads coverage (θH ≈ 1), platinum (Pt) establishes itself as the bestperforming HER electrocatalyst with HER proceeding via the Volmer−Tafel mechanism, whereby the recombination reaction is the rate-determining step at low overpotentials, as exemplified by the measured Tafel slope of 30 mV dec−1.52,53 Generally, the Gibbs free energy of hydrogen adsorption (ΔGH) makes a good indicator of HER electrocatalytic activity, whereby a thermoneutral value of ΔGH ( i.e., ΔGH≈ 0) is desirable. At this value, hydrogen is adsorbed not too strongly, which will lead to higher energy barrier for desorption to form D


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alternative sources of energy generation has led to a surge in interest in the search for earth-abundant and cost-effective electrocatalysts such as transition-metal dichalcogenides (TMDs), sulfides, oxides, and doped graphene as prospective substitutes with potential for exceptional reaction activities and durability.63−66 Carbon Dioxide Reduction Reaction (CO2RR). A strategy to ameliorate the impacts of carbon emissions is to “recycle” CO2, i.e., to capture and convert anthropogenic CO2 emissions to various chemicals and synthetic fuels with greater economic value, such as carbon monoxide (CO) and formic acid, through chemical reactions driven by temperature, light, biological action, or electricity. Of these, the electrochemical CO2RR presents numerous advantages such as ease of catalyst screening without the need to couple CO2RR to a concurrent oxidation event through a donor species, greater control over products formed, as well as high Faradaic efficiency (FE). Furthermore, CO2RR can be performed under ambient environment, and the required electricity can be acquired from renewable energy sources such as wind and solar energies.67,68 Despite these advantages, it remains a formidable challenge to search for an outstanding CO2RR electrocatalyst with low operating overpotential, long catalyst lifetime, as well as good product selectivity. The electrochemical CO2RR has been performed in a variety of electrolytes, classified into aqueous-, organic-, or ionic liquid (IL)-based electrolytes, as well as acidic or alkaline medium. Generally, the rate-determining step is the initial reduction of CO2 to form the corresponding anion radical (CO2•−), with a corresponding Tafel slope of 118 mV dec−1.69 However, in an aprotic solvent, the straightforward electron-transfer event is significantly impeded by the standard reduction potential of CO2 to generate CO2•− at −1.9 V (vs NHE).70 On the other hand, the employment of a protic medium resulted in a series of proton-coupled electron-transfer pathways, whereby the enthalpy barrier of CO2RR is lowered.71 While a variety of products have been produced, including an array of hydrocarbons and alcohols,72−74 the major products of CO2RR are generally carbon monoxide (CO) and formate, with the corresponding half reactions shown below:75

molecular hydrogen, nor too weakly, which results in hindrance to proton transfer and, in turn, low surface Hads coverage, limiting the HER kinetics. Besides its role in water splitting, OER is also a critical counter reaction to CO2RR.54 However, despite the significance of OER, its mechanism is still poorly understood and varies with the pH of the medium and the electrocatalyst employed.55,56 To engender information regarding surface structural and chemical changes during OER, the Pt electrocatalyst has been established as an archetypal system. Through operando ambient pressure X-ray photoelectron spectroscopy (APXPS), atomic force microscopy (AFM), and numerical simulations, Crumlin et al. proposed a phase structure for alkaline OER occurring on Pt, as illustrated in Figure 2.55 The alkaline OER mechanism was described as follows, with PtOHads as the rate-limiting species: Chemisorption of OH−: Pt + OH− → Pt‐OHads + e−

b ≈ 120mV

(11)

−

Surface coupling and deprotonation of OH : Pt‐OHads + OH− → H 2O + e− + Pt‐Oads

b ≈ 40mV (12)

Desorption of oxygen molecule: 2Pt‐Oads → 2Pt + O2

b ≈ 30mV

(13)

It was proposed that the overpotential for OER served the twin purposes of lowering the energy barrier to the formation of Pt-OHads through positive polarization of the Pt surface and driving OH− ions electrostatically through the surface porous oxidized Pt layer. For other heterogeneous catalysts, though the actual mechanism may differ, with the formation of surface adsorbed intermediates, for example, M-OOH, M-O−, it is generally concurred that OER initiates via the adsorption of OH− followed by electron transfer to form the M-OHads species.57 Besides the Pt catalyst, oxides of other precious metals, for example, IrO2 and RuO2, have been established as the leading electrocatalysts toward oxygen evolution.58 Tafel slope analyses of the OER performances of these materials have engendered slope values close to 60 mV dec−1 at low current density while yielding values of approximately 120 mV dec−1 at high current density. Comparing the experimental value at low current density with the above-mentioned mechanism, the Tafel slope of 60 mV dec−1 is not accessible. In their study of an IrO2-based electrocatalyst, Cao et al. reconciled the discrepancy through modification of the chemisorption step whereby M-OHads*, an absorbed species with higher energy state, was generated; the more energetic species subsequently relaxed to M-OHads, as described below:59 M‐OHads* → M‐OHads

b ≈ 60mV

CO2 + H 2O + 2e− → CO + 2OH−

E 0 = − 0.10V vsRHE (15)

CO2 + H 2O + 2e− → HCOO− + OH−

E 0 = − 0.03V vsRHE (16)

Aqueous-based electrolytes afford ease of control of products through varying the solution pH; a lower pH leads to more hydrogenated products and vice versa.76 During the course of the reduction reaction (eq 15 or 16), the production of OH− ions engenders a high local pH value near the electrode surface; in such case, the employment of a buffered solution is essential to maintain the pH at the desired level for generation of the preferred products.77 The major drawback of aqueous electrolytes is the manifestation of the competing HER (0 V vs RHE) which directly impacts the FE for the various CO2RR products as well as the catalyst efficiency. To curb this prevalent dilemma plaguing aqueous electrolytes, Zhang et al. supplemented the 0.5 M NaHCO3 electrolytes with HER suppressants such as alkyltrimethylammonium bromides, successfully enhancing the FE for CO from 50% to 95% for bulk Ag electrode,78 which is on par with the Ag electrocatalyst, that is, triangular Ag nanoplates.79 Besides the

(14)

A similar mechanism was proposed separately by another group working on RuO2-based OER catalyst.60 Alternatively, it was postulated that the Tafel slope of 60 mV dec−1 was garnered from the dissociation of M-OHads to M-O− and H+ ions,61 but this hypothesis held little credence due to the unlikelihood of formation of negatively charged species under extremely oxidizing conditions. Currently, the commercialization of the precious metalbased catalyst for HER and OER faces several setbacks including its exorbitant cost, low abundance, and susceptibility to poisoning.62 The importance of both reactions toward E


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Figure 3. HER performance and stability of Mo2C nanoparticles-decorated N,P-doped rGO. (a) LSV curves of Mo2C nanoparticles with N,Pdoped rGO (Mo2C@NPC/NPRGO) and without rGO (Mo2C@NPC) as well as that of Pt/C. (b) Tafel slopes corresponding to (a). (c) CV curves of Mo2C@NPC/NPRGO at various scan rates. (Inset) The capacitive current at potential 0.32 V plotted against scan rate for Mo2C@ NPC/NPRGO. (d) Stability studies of Mo2C@NPC/NPRGO under potential cycling and (inset) at constant overpotential of 48 mV over 10 h. Reprinted with permission from ref 86. Copyright 2016 Springer Nature.

engendered from examinations of various electrocatalysts anchored on graphene heralded the onset of such catalysts as prospective alternatives to the state-of-the-art Pt. For this review, besides focusing mainly on graphene, some of its related carbon materials, for example, CNTs and graphene foams, which exhibit similar properties of large surface area for doping and catalysis, as well as excellent mechanical stability and electrical conductivity, were also considered. Nonmetal-Doped Graphene as Catalyst Support. Coupled with excellent electrical conductivity of ca. 104 Ω−1 cm−1 and high specific surface area of 2630 m2 g−1, graphene is the top material of choice for HER, especially as a catalyst support material where it enhances the activities of various electrocatalysts. In particular, N-doped graphene is especially valuable due to a myriad of reasons, for example, its role as anchors for metal atom doping, increased interaction with H+ ions due to presence of lone pair of electrons, as well as tailoring the electronic properties, that is, ΔGH of neighboring C atoms and the N dopants to achieve additional active sites.86−89 Lan et al. fabricated a composite of molybdenum carbide and N,P codoped rGO using a ternary hybrid as a precursor.86 Due to its structure, whereby the Mo2C nanoparticles are encapsulated within carbon shells, the nanoparticles were homogeneously embedded without aggregation; furthermore, the carbon shells imparted the Mo2C nanoparticles with rapid electron-transfer capability. Exceptional HER performance parameters of 0 mV (vs RHE) onset potential with an overpotential of 34 mV (vs RHE) at −10 mA cm−2 and a Tafel slope of 33.6 mV dec−1 were achieved with the Mo2C/N,Pdoped rGO hybrid, putting it almost on par with the bestperforming Pt electrocatalyst (Figure 3a−b); Mo2C without rGO displayed comparatively slower HER kinetics. The electrochemical double-layer capacitance (EDLC), from

aforementioned inherent impediment of aqueous electrolytes, the poor solubility of CO2 in water (0.03 M at 25 °C) has further divested the benefits of using an aqueous system. In comparison, organic and IL-based solvents proffer much greater CO2 solubility than water, while suppressing unwanted hydrogen evolution.80,81 The major product of CO2RR in these solvents is greatly affected by the availability of protons in the solvent,82 and its selectivity may be tuned by the addition of supporting electrolytes.83 In the case of IL-based electrolytes, their “green” properties, that is, negligible vapor pressure and low or nonflammibility,81 coupled with complexation with CO2 which radically lowered the enthalpy barrier and overpotential,84 further accentuate the significance of IL systems for CO2RR. To minimize the impacts of the high cost and high viscosity of ILs, ILs dissolved in water have been exploited as electrolytes whereby the pH, and inversely the proton concentration, may be decreased through lowering the IL-to-water ratio.85 In light of the myriad of products that can be yielded from CO2RR, electrocatalysts with high selectivity, FE and stability are highly sought after. In this regard, graphene and related materials that are doped with nitrogen atoms are at the forefront of the field. With the extensive literature available, it is timely and prudent to examine and compare the performances of these electrocatalysts.

GRAPHENE AND RELATED MATERIALS FOR HER A thorough survey of current literature has portrayed graphene as a promising HER catalyst support and, in some cases, a potential HER electrocatalyst itself. Both reduced graphene oxide (rGO) and nonmetal-doped graphene manifest properties such as electrical conductivity, large surface area, and robustness to acidic media which are indispensable toward their application as catalyst support. Favorable results F


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Figure 4. Structure, HER behavior, and electronic properties of Co@NC/NG. (a) Schematic illustration of HER at the exterior of Co@NC/ NG, with C (gray), O (red), H (blue), Co (bluish violet), and N (green) atoms. (b) LSV curves of Co@NC/NG compared with its equivalents and Pt/C. (c) CV stability test of Co@NC/NG over 1000 cycles in 0.5 M H2SO4. (d) DOS of a Co@carbon cage (red) and a pure carbon cage (black). Fermi level is set at 0 eV. (Inset) The optimized structure and HOMO of the Co@carbon cage. Reprinted with permission from ref 92. Copyright 2015 American Chemical Society.

carbon catalytic activity provided additional HER active sites, resulting in a low onset overpotential of 49 mV (vs RHE), Tafel slope of 79.3 mV dec−1 as well as excellent durability to potential cycling (Figure 4b−c). From DFT studies, approximately 0.7 electron was transferred from the Co atom to neighboring C atoms which raised the Fermi level upon insertion of the Co atom (Figure 4d); this reduced the local work function and enhanced the chemical reactivity, resulting in improved HER activity. Independently, Luo et al. fabricated dicobalt phosphide (Co2P) embedded in N,P codoped graphene which demonstrated an overpotential of 103 mV at −10 mA cm−2 and Tafel slope of 58 mV dec−1.93 The uniformly small diameter of Co2P nanoparticles and coupling of P and N dopants with numerous structural defects on graphene were reported to contribute to the exceptional HER electrocatalytic performance of the hybrid. A similarly structured CoP catalyst encased in N-doped carbon and deposited on N-doped graphene achieved a comparatively higher overpotential of 135 mV at −10 mA cm−2 but a comparable Tafel slope of 59.3 mV dec−1.94 TMDs is a class of 2D materials that have shown exceptional activity for HER electrocatalysis. Furthermore, the poor conductivity of the semiconducting TMDs, for example, MoS2, MoSe2, WS2, and WSe2, can be circumvented through the use of highly conducting catalyst support which are commonly graphene-based.95−98 In particular, much of the attention in this regard has been focused on molybdenum sulfide nanoparticles electrocatalyst on N-doped graphene as a feasible substitute for the archetypal Pt electrode. In an insightful study by Kim et al., the electrostatic attraction between the N-doped sites of N-doped carbon nanotube (NCNT) and precursor thiomolybdate anion (Figure 5a), as well as the enhanced wettability of the graphitic surface upon N-

which the electrochemically active surface area may be inferred, was 195 times larger for the Mo2C/N,P-doped rGO hybrid (17.9 mF cm−2) than the counterpart without rGO (0.092 mF cm−2) (Figure 3c). The excellent durability of the Mo2C/N,P-doped rGO hybrid over 10 h of continuous measurement at overpotential of 48 mV and 1000 cycles of potential cycling was illustrated in Figure 3d. Via DFT calculations, N-doping of rGO was found to confer the catalyst support with quaternary N and pyridinic N which demonstrated ΔGH of 0.89 and −2.04 eV correspondingly; upon the anchoring of Mo2C nanoparticles, the ΔGH values improved significantly to 0.69 and −0.22 eV, respectively. The favorable ΔGH value of Mo2C@C-pyridinic N as well as the dominant N-dopant species being pyridinic N jointly verified the high efficiency of Mo2C/N,P-doped rGO hybrid as HER electrocatalyst. Moreover, Co is often incorporated onto N-doped graphene in various forms. Single-atom catalysis was realized by Tour’s group with the dispersion of Co atoms onto N-doped graphene (denoted as Co-NG), whereby the N dopants provided coordination sites for Co incorporation; at very low Co metal content of 0.57 at. %, overpotential of 147 mV (vs RHE) at −10 mA cm−2 and Tafel slope of 82 mV dec−1 were engendered in acidic media.87 Likewise, when examined in alkaline media, the Co-NG catalyst demonstrated improved HER performance compared to its counterparts (without Co or N-doping); the Co-NG catalyst stood out with its versatility to be used over a wide pH range, whereas MoS2 and Ni2P catalysts displayed instability in basic solvents.90,91 In a separate investigation, instead of incorporating Co directly onto graphene, the Co nanoparticles were encapsulated in Ndoped carbon which was supported on N-doped graphene (represented as Co@NC/NG) (Figure 4a).92 The Co-induced G


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Figure 5. Formation processes and experimental and DFT-calculated HER performances of molybdenum sulfide-N-doped carbon nanostructure hybrids. (a) Proposed two-step MoSx layer formation mechanism on N-CNT surface. (b) LSV curves and (c) Tafel slopes of the different catalysts. Reprinted with permission from ref 99. Copyright 2014 American Chemical Society. (d) Scheme illustrating the synthesis process of MoS2/N-rGO electrocatalyst. (e) DFT-calculated free energy diagram indicating the ΔGH of MoS2 of different interlayer spacing. Reprinted with permission from ref 102. Copyright 2016 John Wiley and Sons.

favorable ΔGH of −0.052 eV than that of MoS2 nanosheets with interlayer spacing of 6.2 Å, which enabled rapid adsorption of protons and release of hydrogen molecules. Separately, 3D melamine foams were utilized by Piao et al. as template for the loading of GO which was, in turn, the catalyst support for MoS2.103 Upon heating at 700 °C, the melamine foams were converted to carbon foams, while the GO was doped to form N-doped graphene; MoS2 was grown on the surface via an annealing procedure. An increase in the N-doped graphene content was correlated to higher HER activity due to the presence of more regions for the growth of MoS2. The best performing electrocatalyst generated an overpotential of 170 mV (vs RHE) at −10 mA cm−2 with a Tafel slope of 53 mV dec−1. Metal-free HER electrocatalysts are highly sought after due to their lower costs and milder environmental impact arising from unlikelihood of metal ions release.104 The recent progress in architectural possibilities of carbon-based nanomaterials has spearheaded the development of such electrocatalysts. Qiao et al. synthesized a graphitic-carbon nitride (g-C3N4) coupled with N-doped graphene (C3N4@NG) which yielded unexpected electrocatalytic activity toward HER that was comparable to metallic catalysts; an overpotential of ca. 240 mV (vs RHE) was achieved at −10 mA cm−2, and a Tafel slope of 51.5 mV dec−1 was recorded.105 This was ascribed to

doping, was exploited for the deposition of ∼2 nm thick amorphous MoSx layers under a wet chemical procedure.99 The HER performance of the resulting hybrid electrocatalyst exceeded those of its individual components in terms of overpotential at −10 mA cm−2 (Figure 5b) and Tafel slope (Figure 5c). The N-doping of the CNT material was also found to impart cycling durability to the hybrid electrocatalyst which is dependent on the strength of catalyst−support interaction;100 the robust interaction between the d-orbital of transition metal and the p-orbital of N dopants manifested in strong bonding and hence excellent stability.100,101 Using phosphomolybdic acid, graphene oxide as well as thiourea as precursors, and at a hydrothermal temperature of 180 °C, Tang and co-workers obtained MoS2 of expanded interlayer spacing (9.5 Å compared to 6.2 Å of 2H-MoS2) deposited on N-doped reduced graphene oxide (MoS2/NrGO) (Figure 5d).102 With the employment of N-rGO as catalyst support, which increased the surface area for MoS2 deposition and lowered the overall impedance of the composite, the fabricated composite yielded outstanding HER activity in terms of overpotential at −10 mA cm−2 (56 mV vs RHE) and Tafel slope (41.3 mV dec−1), with an onset potential of −5 mV which was very close to that of Pt/C. Based on DFT calculations (Figure 5e), MoS2 nanosheets with enlarged interlayer spacing engendered an energetically more H


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Figure 6. DFT studies and HER activity of C3N4/N-doped graphene catalysts. (a) Band structure of C3N4@NG hybrid (right) and pure gC3N4 (left). (b) Electron transfer upon coupling of C3N4 with N-graphene. Cyan and yellow iso-surfaces depict electron depletion and electron accumulation. (c) Predicted DOS of C3N4@NG hybrid (bottom) and pure g-C3N4 (top). Reprinted with permission from ref 105. Copyright 2014 Springer Nature. (d) LSV curves of various porous C3N4/N-doped graphene hybrids, together with that of Pt electrocatalyst. (e) Tafel slopes of corresponding curves in (d). Reprinted with permission from ref 106. Copyright 2015 American Chemical Society.

mA cm−2 and 3.8, respectively) (Figure 7d−e), making it an excellent material for as a bifunctional HER and ORR electrocatalyst. Despite the lack of a meticulous study on the potential impact of metallic impurities in graphene on HER performance, it should be highlighted that the study by Sun demonstrated that residual trace impurities in CNTs were responsible for electrocatalytic behavior of CNTs toward HER.107 Separately, Chen et al. explored a N,S-doped graphene as HER electrocatalyst, achieving HER performance parameters which were comparable to that of MoS2.108 An overpotential of 280 mV (vs RHE) at −10 mA cm−2 and Tafel slope of 80.5 mV dec−1 were obtained, and the HER performance was attributed to the close proximity of negatively charged S dopant and positively charged N dopant at a defect site on graphene, enabling fast electron transport route for HER. Zhang and co-workers noted that approximately 50% of the active sites of N-CNT were inaccessible due to the high surface tension of the electrolyte impeding its entry into the voids between the nanotubes as well as the challenge in wetting the inner walls of N-CNT.109 As such, they successfully synthesized an N-doped graphene microtube (N-GMT) with large inner diameters of 1−2 μm, high specific surface area of 604.2 m2 g−1, and accessible macropores with great potential for HER electrocatalysis (464 mV (vs RHE) at current density of −10 mA cm−2 in 0.1 M KOH). It was observed that the superior HER performance of N-GMT compared to N-CNT and N-doped graphene (NG) (Figure 8a) improved further with increased catalyst loading (Figure 8b); the availability of the active centers on the inner walls of N-GMT and minimal overlap of active sites between the microtubes enhanced the current density remarkably in relation to the other doped carbon materials (Figure 8c).

downshift of the Dirac cone at the Γ point and redistribution of charge density from the conductive graphene N to g-C3N4, resulting in superior electron mobility in the hybrid (Figure 6a−c). Subsequently, the same group fabricated a 3D film with alternating layers of porous C3N4 and N-doped graphene which can be utilized as electrode directly without substrates.106 The exposure of large amounts of active sites in the porous and delaminated C3N4 nanosheets and highly conductive 3D graphene network garnered Tafel slope of 49.1 mV dec−1 and exceptionally low overpotential of 80 mV (vs RHE) at −10 mA cm−2 (Figure 6d−e), exemplifying the immense potential of such metal-free HER electrocatalysts. Nonmetal-Doped Graphene as HER Electrocatalyst. The exploration of graphene as a HER electrocatalyst is generally limited due to the endothermic ΔGH of 1.82 eV of pristine graphene, which indicates energetically adverse interaction with hydrogen;86 graphene was employed for HER electrocatalysis only upon modifications via the addition of nonmetal heteroatoms as dopants. Dai et al. developed a N,P codoped 3D graphitic carbon electrocatalyst through a cooperative assembly of melamine and phytic acid in an aqueous GO suspension, followed by pyrolysis at 550−1000 °C, forming a 3D porous graphitic carbon network doped with N and P (represented as MPSA/GO-X, X = pyrolysis temperature) (Figure 7a).66 The porous structure and heteroatom doping of MPSA/GO-1000 facilitated fast electrolyte and charge transfer, resulting in exceptional HER performance of overpotential of 210 mV at −30 mA cm−2 with Tafel slope of 89 mV dec−1, together with good stability over 4 h (Figure 7b−c). The same catalyst was investigated for ORR activity, and a high kinetic-limited current density (16.9 mA cm−2) and electron-transfer number (n = 3.7) were obtained which were on par with those attained by Pt/C (17.0 I


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were described by the Gibbs free adsorption energy of sulfur to the metal edge (ΔGS) and that of hydrogen to a sulfur atom (ΔGH‑S) correspondingly, that is, the doped edge with the highest θS and strongest binding to S would have the weakest H−S binding. Of the 19 transition-metal dopants, ruthenium, rhodium, cobalt, iron, manganese, and tantalum exhibited significant enhancements in ΔGH of the sulfur edge sites toward the thermoneutral value compared to the undoped Mo edge (Figure 9a). Among these dopants, the enhanced HER electrocatalytic performances of Ni, Fe, and Co-doped TMDs have been assessed experimentally, verifying the theoretical calculations.111,112 Besides the impact on ΔGH, heteroatom doping had been explored to alter the electronic properties of TMDs by Xie et al., who produced semimetallic V-doped MoS2 with intralayer V-doping via a solid-state reaction.113 With varying concentrations of V dopants, the electronic properties of MoS2, that is, conductivity and charge carrier density were tuned to achieve commendable HER performance of a low overpotential of ca. 240 mV (vs RHE) with Tafel slope of 69 mV dec−1 (Figure 9b−c). The optimal electrocatalyst, V0.09Mo0.91S2, displayed excellent durability over 1000 cycles in H2SO4, with a minimal loss in activity, as illustrated in Figure 9d. However, an intriguing study by Pumera et al. on the electrocatalytic effects of p-doping of bulk MoS2 and WS2 via niobium and tantalum dopants refuted the aforementioned theoretical improvement in ΔGH of Ta-doped MoS2.114 Through comparisons of HER and ORR activities of the Nb/Ta-doped TMDs against the undoped equivalents, the less potent performances of the doped TMDs resulting from the presence of dopants manifested as higher overpotentials at −10 mA cm−2 for HER and less positive onset potentials for ORR, contesting the current consensus that doping enhances the electrocatalytic activities of TMDs. A subsequent research conducted by the same group compared the HER electrocatalytic behaviors of an ultrapure MoS2 material synthesized under controlled conditions (MoS2-M) and a commercially obtained MoS2 (MoS2-AA) which was relatively contaminated, with impurities stemming from the fabrication process and the starting materials.115 Based on X-ray fluorescence data recorded in Table 1, the MoS 2 -M contained a low concentration of magnesium as its only impurity, while a whole range of metallic impurities were found to be present in MoS2-AA. Despite having a large concentration of iron impurities which was previously found to promote the HER electrocatalytic activity of molybdenum sulfide,112 among other impurities, MoS2-AA demonstrated a poorer HER performance than MoS2-M (Figure 10a). The observed trend implied that random doping without design or strategy detracted any possible positive enhancement of doping, and a deliberate doping strategy is required to harness the catalytic potential of doped TMDs. In an attempt to garner the superior electrocatalytic performance of Pt while reducing the associated cost, Deng and co-workers activated the inert basal surface of MoS2 via single-atom doping with Pt.116 The Pt atoms substituted inplane Mo atoms and existed as single atoms instead of nanoparticles or clusters; the resulting Pt-doped MoS2 (PtMoS2) displayed significantly enhanced HER compared to its undoped equivalent (FL-MoS2) (Figure 10b), engendering an improvement in overpotential by 60 mV at current density of −10 mA cm−2. The Tafel slope obtained for Pt-MoS2 was established to be 96 mV dec−1, which deviated from that of Pt

Figure 7. Bifunctionality of MPSA/GO-1000 toward HER and ORR. (a) Preparation process of MPSA/GO-1000. (b) HER LSV curves for various electrocatalysts in 0.1 M KOH. (c) HER stability tests of Pt/C and MPSA/GO-1000. (d) ORR LSV curves of various catalysts in O2-saturated 0.1 M KOH. (e) Number of electron transfer (n) calculated from the Koutecky−Levich (K-L) plots (not shown). Reprinted with permission from ref 66. Copyright 2015 John Wiley and Sons.

TRANSITION-METAL DICHALCOGENIDES FOR HER The pressing need for a highly efficient and economical electrocatalyst to produce hydrogen energy carrier has placed TMDs in the limelight as prime candidates to substitute the Pt electrocatalyst. Extensive research has been conducted to explore the features of TMDs that govern their HER activity and to tailor the TMD structure for improved electrocatalytic performance. Various doping strategies have been employed to further enhance the excellent HER electrocatalytic performance of TMDs, including doping with metallic elements as well as transition metal oxide and chalcogenide impurities; both advantageous and adverse impacts of such doping will be discussed shortly. Metal-Doped. The doping of TMDs with metallic heteroatoms to alter their ΔGH and accelerate the protontransfer kinetics while keeping the energy barrier to H2 desorption low has been studied both theoretically and experimentally. Through DFT calculations of 19 transition metal-doped MoS2, Tsai and co-workers identified a general correlation between reactivity of the transition metal and the consequent compensation effect on the S and H coverages at the active S-edge of MoS2 (θS and θH respectively), where higher reactivity relates to higher θS and lower θH.110 θS and θH J


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Figure 8. HER catalytic performance of N-doped graphene microtubes. (a) IR-corrected LSV curves and (b) current density at −0.5 V (vs RHE) at various catalyst loadings of N-GMT, N-CNT and NG in 0.1 M KOH. (c) Schemes with green arrows indicating the evolution of hydrogen gas at active sites of the various catalysts. Reprinted with permission from ref 109. Copyright 2015 Springer Nature.

Figure 9. DFT calculations of transition metal-doped MoS2 and experimental HER activity of V-doped MoS2 (a) ΔGH of various transition metal-doped MoS2. Reprinted with permission from ref 110. Copyright 2015 The Royal Society of Chemistry. (b) LSV curves of various doped and undoped MoS2 nanomaterials. (c) Corresponding Tafel plots of polarization curves in (b). (d) Long-term stability studies of Vdoped MoS2. Reprinted with permission from ref 113. Copyright 2014 The Royal Society of Chemistry.

(32 mV dec−1) while approaching that of the undoped MoS2 (98 mV dec−1), suggesting that the HER mechanism on Pt-

MoS2 closely resembled that of pristine MoS2, that is, adhering to the Volmer−Heyrovský mechanism. A prior study by K


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Table 1. X-ray Fluorescence Measurements of MoS2-M and MoS2-AAa (Reprinted with Permission from ref 115. Copyright 2017 John Wiley and Sons) sample MoS2AA MoS2-M

S/MO ratio

MO [wt %]

W [wt %]

S [wt %]

Cd [ppm]

Mg [ppm]

Y [ppm]

Si [ppm]

K [ppm]

Ca [ppm]

V [ppm]

Mn [ppm]

Fe [ppm]

Zn [ppm]

2.05

59.2

0.02

40.51

0

0

410

560

297

252

0

65

940

168

1.95

60.53

0

39.46

0

89

0

0

0

0

0

0

0

0

a

Concentrations of all other elements were below 0.01 wt %

Figure 10. HER activity of metal-doped MoS2. (a) LSV curves of MoS2-AA and MoS2-M. Reprinted with permission from ref 115. Copyright 2017 John Wiley and Sons. (b) LSV curves of various forms of MoS2. (c) CV stability test for Pt-MoS2 and Pt supported on MoS2 (Pt/MoS2) before and after 5000 CV cycles. (d) Chronoamperometric responses of Pt-MoS2, Pt/MoS2 and FL-MoS2 to methanol poisoning. Reprinted with permission from ref 116. Copyright 2015 The Royal Society of Chemistry.

Nørskov et al. determined the S edges of MoS2 to be the HER active sites,117 indicating that, based on the Tafel slope values, the HER activity of Pt-MoS2 stemmed from the S atoms instead of Pt atoms. In-depth DFT investigations revealed the tuning of the ΔGH of in-plane S in neighboring sites to the Pt dopants to close to 0 eV, which represented a tremendous increase in the number of HER active sites and, thus, HER activity. Owing to the Pt substitution at Mo sites, the Pt-MoS2 enjoyed superior durability over 5000 CV cycles and improved resistance against methanol poisoning compared to Pt supported on MoS2 (Pt/MoS2) (Figure 10c−d); the MoS2 trilayer structure of the former prevented contact between the reactants and the Pt dopants, while the Pt in latter material was easily accessed and poisoned. The chemical intercalation of TMDs with organolithium compounds followed by hydration in water generally leads to exfoliation of bulk to few-layer TMD nanosheets as well as a 2H → 1T phase transition; for Group VIB TMDs, the electronic transition from semiconducting to metallic nature occurs as well.118 The resulting HER performances of the exfoliated TMDs usually surpassed those of the bulk TMDs due to the electronic and structural changes. This concept of Li+ ion intercalation was emulated by Cui et al., who performed the electrochemical intercalation of Li+ ions to

tune the electronic properties and structural characteristics of pristine MoS2 nanofilms.119 Li electrochemical intercalation garnered an expansion of the interlayer spacing and a simultaneous electron donation to MoS2; as the intercalation Li content increases, the semiconducting 2H phase gradually transited to the metallic 1T phase, resulting in enhanced HER performance of the lithiated MoS2 (Figure 11). Transition-Metal Oxide and Chalcogenide Impurities. Improving the intrinsic conductivity of TMDs for rapid transport of electrons during the HER process, coupled with increasing the available HER active sites, is an unerring approach to attain exceptional HER performance which may rival that of Pt. To this end, trichalcogenides and oxides of transition metals, that is, MX3 and MO2 or MO3, are befitting as suitable dopants; MX3 disrupts the pristine 2D structure of TMDs, exposing more HER active sites, while MO2 or MO3 enhances the intrinsic conductivity of TMDs.65,120 Examinations of the impacts of these dopants on HER performances of TMDs have shown that, from straightforward physical mixtures with undoped TMDs to their incorporation within the TMD trilayer structure through varying synthesis conditions, these commonplace “impurities” are of great consequence to the resulting HER performance of TMD-based electrocatalysts.63,65,120 L


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Figure 11. Doping process and HER performance of Li-doped MoS2. (a) Galvanostatic discharge curve demonstrating the lithiation process. (b) LSV curves and (c) Tafel slopes of various lithiated and pristine MoS2. Reprinted with permission from ref 119. Copyright 2013 National Academy of Sciences.

deviations in electrocatalytic properties,65 frequently observed upon exfoliation via conventional ion intercalation means.118,121 Between the oxides and trichalcogenides, the latter displayed higher activity with regards to overpotentials at −10 mA cm−2 as well as onset potentials (Figure 13a). Subsequent study on the impacts of the trichalcogenides on the MX2 HER performances was performed through physical mixtures of MX2 with increasing amounts of MX3. Figure 13b illustrates steady decreases in overpotential at −10 mA cm−2, whereas the Tafel slope values show a drop which held steady with further increase in MX3 proportion (Figure 13c); these findings shed light on the wide range of reported overpotentials for MoS2 and WS2 synthesized through different methods and demonstrated that material composition held less sway over Tafel slope values, that is, HER mechanism of the electrocatalyst, compared to the degree of crystallinity, as was expounded in another study.122 The positive enhancement effects of MX3 were further illustrated by Pumera et al. in their investigations into the electrochemical deposition of amorphous WS3−x as a feasible synthesis method for HER electrocatalysts.123 From XPS surface elemental analysis, both S2− and S22− ligands were found to be present in the electrodeposited WS3−x material, with contributions from both WS2 and WS3. The HER performance of WS3−x outstripped those of bulk WS2 and WS3 as well as exfoliated WS2, with an overpotential of 494 mV at −10 mA cm−2 and Tafel slope of 43.7 mV dec−1. This was ascribed to the presence of two types of S ligands on the WS3−x electrocatalyst which possessed distinct metal−chalcogen binding strengths and thus different ΔGH,110 allowing for concurrent proton adsorption and hydrogen molecule desorption on separate sites of the WS3−x material. Despite the aforementioned illustrations whereby doping Group VIB TMDs with transition-metal oxides and other

In their momentous paper, Xie et al. optimized both structural and electronic properties of MoS2 through controllable disorder engineering via the incorporation of oxygen atoms into the pristine TMD.120 This was conducted through varying the synthesis temperature; at lower temperature, the sulfurization of the molybdate precursor did not proceed to completion, and Mo−O bonds imparted from the precursor were assimilated into the resulting MoS2. The doping of MoS2 with oxygen atoms led to increased disorder (Table 2) which Table 2. Degree of Disorder of MoS2 Nanosheets Doped with Oxygen (Reprinted with Permission from ref 120. Copyright 2013 American Chemical Society) atomic ratio of Mo/S S140 S160 S180 S200

degree of discorder

atom% of oxygen

By XPS

By EDS

100% 53.3−56.7% 35.0−40.3% 8.3−13.3%

4.18 3.36 2.28 1.92

1:2.10 1:2.07 1:2.04 1:2.02

1:2.10 1:2.09 1:2.05 1:2.01

generated more active sites, and the enhanced charge density triggered by the oxygen dopants amplified its innate conductivity (Figure 12). However, concurrently, the Odoping disrupted the 2D electron conjugation, resulting in the impediment of electron conductivity. With temperature modulation, the degree of disorder and O-doping were optimized to access more active sites with higher conductivity while minimizing the adverse effects of perturbed 2D structure of MoS2, achieving an onset potential of 120 mV and Tafel slope of 55 mV dec−1. Separately, the effects of MX3 and MO2/MO3 impurities were examined in detail by Latiff and co-workers, on bulk MoS2 and WS2 instead of delaminated nanosheets to diminish M


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Figure 12. Illustrations describing the structure of O-doped MoS2. (a) Scheme representing the disordered structure of O-doped MoS2, with the purple shading representing the amplification of active sites due to the disordered structure. (b) Model of an O-doped MoS2 nanodomain, with the blue shading signifying the increased charge density arising from O-doping. Reprinted with permission from ref 120. Copyright 2013 American Chemical Society.

Figure 13. Electrochemical HER performance of various Mo and W chalcogenides and oxides. (a) LSV curves of various oxides and chalcogenides of Mo and W. (b and c) Bar graphs comparing the (b) overpotentials at −10 mA cm−2 and (c) Tafel slope values for MX2 mixed with varying amounts of the corresponding MX3. Reprinted with permission from ref 65. Copyright 2016 The Royal Society of Chemistry.

The reversal of trends of increasing HER activities between bulk and exfoliated VX2 implied that surface compositions were severely altered upon exfoliation; this was attributed to the loss of chalcogen manifesting in an abrupt decrease in chalcogen-to-metal ratio upon exfoliation, together with the presence of vanadium oxides (VOx) and lithium vanadates (LixVyOz) yielding lowered conductivity and availability of HER active sites. Prior studies reported that VS2 fabricated through an alternative method such as CVD demonstrated minimal contamination with oxides and, therefore, significantly

forms of transition-metal chalcogenides improved the resulting HER upon activity, Wang and co-workers demonstrated that such benefits of doping might be not be applicable to other TMDs, for example, vanadium dichalcogenides (VX2).24 The electrocatalytic performances of VS2, VSe2, and VTe2 were examined before and after exfoliation via intercalation with nbutyllithium. As displayed in Figure 14, the VX2 materials exhibited poor HER activities which were inferior to that of bare glassy carbon (GC); the HER performance of bulk VTe2 was credited to its inherent electrochemical signals instead. N


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ORR catalysis. The improved ORR activity of rGO-IL/Mn3O4 corroborated with prior studies on the efficacy of IL in enhancing the solubility of oxygen and augmenting the electrocatalytic activity of the active species Mn3O4 nanoparticles, which engendered improved ORR performances. Apart from decorating graphene with metal oxide nanoparticles, doping graphene with nonprecious metal is a viable approach to attain ORR electrocatalysts with higher activity. Through thermal exfoliation, transition-metal (Mn, Fe, Co, Ni)-doped graphene hybrids were obtained; the ORR performances of the doped graphene hybrids produced in both inert (H2) and reductive (N2) atmospheres were much enhanced compared to the unmodified graphene (Figure 16a− b), with the Ni-doped catalysts exhibiting the largest improvements in ORR peak potentials.129 Sb doping was explored by Baek et al. and was found to occur selectively at the graphene edges; the resulting hybrid (SbGnP) displayed the ideal four-electron single-step process for ORR (n = 4 at −0.6 V vs Ag/AgCl) and excellent long-term stability, with zero loss in ORR electrocatalytic activity even after 100,000 continuous cycles (Figure 16c−d).130 The DFT calculations indicated that the bonding of a Sb atom with two C atoms at both armchair and zigzag edges resulted in good binding affinity of Sb atoms with O2, and similar free energy pathways were presented for both types of edges. It was also calculated that ORR catalyzed by the Sb-doped graphene proceeded via intermediates which exhibited alternating oxidation states (Sb3+ and Sb5+). Nonmetal-Doped. The doping of graphene and related materials with nonmetal heteroatoms is a well-explored route toward “metal-free” ORR electrocatalysts. These dopants include N, P, S, and B which constitute both n and p dopants. Regardless of the nature of the dopant, improved ORR electrocatalytic performances are always engendered, implying that the mechanism by which ORR is facilitated on the metalfree doped graphene is generally indeterminate. In this regard, a DFT study of N-graphene models by Xia et al. showed that generally, any form of doping by substitution or attachment to graphene that can result in a highly asymmetric atomic charge density and spin density on the doped graphene could greatly enhance its electrocatalytic ORR performance.131 In other investigations, quaternary and pyridinic N on N-doped graphene have been contended as main contributors to the enhancement of ORR electrocatalysis on neighboring C atoms.132−134 Müllen et al. fabricated N-doped graphene via pyrolysis to garner graphene-based carbon nitride nanosheets (G-CN).135 Compared with carbon nitride sheets without graphene (CN), G-CN exhibited consistently earlier onset potential and higher limiting current, as well as n values (Figure 17a−b), across the different pyrolysis temperatures. In another study, Dai et al. synthesized N-doped graphene (denoted as N-graphene) through chemical vapor deposition (CVD) of methane in an ammonia atmosphere.136 At a fixed mass (7.5 μg) of each electrocatalyst, N-graphene outperformed both Pt/C and undoped graphene in terms of limiting current, though the archetypal Pt/C displayed the earliest onset potential (Figure 17c). Nonetheless, N-graphene demonstrated excellent stability to both the poisoning and crossover effects of CO and methanol which was far superior to that of Pt/C (Figure 17d−e). N-graphene also exhibited no significant decrease in current density upon potential cycling for 200,000 continuous cycles (Figure 17f), indicating the

Figure 14. HER activity of VX2 materials. (a and b) LSV curves of (a) exfoliated VX2 and (b) bulk VX2 materials, with reference to GC. Reprinted with permission from ref 24. Copyright 2016 John Wiley and Sons.

higher HER performance with overpotential of 68 mV (vs RHE) at −10 mA cm−2, verifying that the presence of oxide impurities was detrimental to HER activity.124

GRAPHENE AND RELATED MATERIALS FOR ORR The search for cheap substitutes to Pt as ORR electrocatalysts has led to increased focus on graphene as potential alternatives. Graphene has been explored as an ORR electrocatalyst via two different strategies: (i) as a conductive catalyst support with high specific surface area on which metal and metal oxide electrocatalysts can reside and (ii) as a “metal-free” electrocatalyst doped with nonmetal heteroatoms. At times, both strategies were employed simultaneously with transition-metalbased nanoparticles as electrocatalyst on graphene doped with heteroatoms. Metal (Oxide)-Doped. Metal oxide nanoparticles such as manganese oxide and cobalt oxide nanoparticles are typically employed as electrocatalysts on graphene nanosheets as ORR electrocatalysts.125,126 Dai et al. grew Co3O4 nanocrystals on rGO through controlled nucleation, followed by hydrothermal reaction to form the final catalyst denoted as Co3O4/rmGO.127 Based on the rotating disk electrode (RDE) measurements (Figure 15a), the n value was calculated to be ca. 3.9 at 0.60− 0.75 V, implying a 4-electron ORR process. Further enhancement to its ORR performance was achieved through nitrogen doping of the rGO; the resulting hybrid engendered n value of ca. 4.0 over the same potential range (Figure 15b) and a lower Tafel slope of 42 mV dec−1 than that of the undoped Co3O4/ rmGO hybrid (54 mV dec−1). In the study by Kim et al., Mn3O4 nanoparticles were anchored onto graphene nanosheet via 1-(3-aminopropyl)-3-methylimidazolium bromide ionic liquid (IL) (rGO-IL/Mn3O4), which increased the limiting current and led to more positive onset potential by ca. 100 mV compared to undoped graphene (Figure 15c−d).128 Between the two doped electrocatalysts with different amounts of the nanoparticle, rGO-IL/Mn3O4 (10:1) and rGO-IL/Mn3O4 (2:1), the former displayed much higher n of 3.50 cf. n of 2.75 of the latter (Figure 15e−f), signifying more efficient O


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Figure 15. ORR performances of Co3O4 and Mn3O4-doped rGO. (a and b) RDE measurements of (a) Co3O4/rmGO and (b) Co3O4/NrmGO with scan rate of 5 mV s−1 at various rotation rates. (Insets) Corresponding K-L plots at different potentials. Reprinted with permission from ref 127. Copyright 2011 Springer Nature. (c) RDE measurements of various electrocatalysts, as labeled. (d) Onset potential and limiting current of the electrocatalysts in (c). (e and f) K-L plots of (e) rGO-IL/Mn3O4 (2:1) and (f) rGO-IL/Mn3O4 (10:1) at different potentials. Reprinted with permission from ref 128. Copyright 2011 The Royal Society of Chemistry.

immense potential of such N-doped graphene and related materials as outstanding electrocatalysts toward ORR. Separate studies by Xia et al. and Baek et al. explored the doping of graphene with boron and sulfur heteroatoms, respectively.137,138 Thermal exfoliation of graphite oxide in the presence of B2O3 yielded active defect sites for B-doping into the graphene carbon network; this resulted in improved n of the B-doped graphene compared to the undoped graphene and superior stability and tolerances to poisoning by CO and methanol in comparison to Pt electrode (Figure 18).137 The electrocatalytic activity of B-doped graphene was attributed to the doping by electron-deficient boron atoms which acted as active sites and triggered the breaking of O−O bond. On the other hand, Baek et al. performed selective sulfurization of graphene nanoplatelet edges via ball milling of graphene in a sulfur-containing atmosphere to achieve S-doped graphene.138 The enhancement in ORR electrocatalytic performance from the unmodified graphene was accredited to the strong polarization by the sulfur atoms bonded to the graphene edges, engendering strong electrocatalytic activity toward ORR. In the same study, the higher ORR activity of oxidized

S-doped graphene imparted evidence to the mechanism of improved ORR activity being associated with increased charge and spin densities.131 Dual-doped graphenes have been endeavored by Qiao et al. to harness the advantages presented by the individual dopant type. An S and N dual-doped graphene which exhibited augmented ORR performance compared to the S or N singly doped graphene was fabricated (Figure 19a−c).139 The greater activity of the S and N dual-doped graphene (n = 3.3−3.6) cf. their singly doped counterparts (n = 3.0 and 3.3 for N- or Sdoped graphene respectively) was attributed to the higher activity of C atoms with higher charge density surrounding N dopants and the increase in ORR-active C atoms resulting from the asymmetrical charge and spin densities due to the double doping of N and S. Compared to the N,S-doped graphene, the B,N-doped graphene synthesized by the same group generated an earlier onset potential and much larger limiting current than both B or N singly doped equivalents (Figure 19d); a much higher n of 3.97 was also attained by the B,N-doped graphene, which was in close proximity to that of Pt/C (n = 3.98) (Figure 19e−f).140 The enhanced performP


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Figure 16. ORR electrocatalytic performance and durability of various transition-metal- and Sb-doped graphene materials. (a and b) Peak potentials of Mn-, Fe-, Co-, and Ni-doped graphene produced via thermal exfoliation in (a) inert (H2) and (b) reductive (N2) atmospheres. Differences in peak potential compared to the undoped graphene (ΔE) are as displayed. Reprinted with permission from ref 129. Copyright 2013 John Wiley and Sons. (c) Bar graphs indicating n of graphite, Pt/C and SbGnP at −0.6 V (vs Ag/AgCl). (d) CV stability test on graphite, Pt/C and SbGnP at scan rate of 100 mV s−1 over 100,000 continuous cycles. Reprinted with permission from ref 130. Copyright 2015 Springer Nature.

20d,f) and more stable ORR performance to potential cycling (Figure 20e,g). An endeavor to fabricate a triply doped graphene material was challenged by Dai et al., who successfully developed a porous N,P,F codoped graphene as trifunctional electrocatalyst toward ORR, HER, and OER via concurrent pyrolysis of polyaniline-deposited graphene oxide and ammonium hexafluorophosphate.144 The triply doped graphene (denoted as GO-PANi31-FP) yielded efficient ORR electrocatalytic activity with a half-wave potential 40 mV lower than that of Pt/C and n of 3.85 (Figure 21a). For OER activity, GO-PANi31-FP displayed an onset potential ca. 0.09 V higher than the state-ofthe-art RuO2 catalyst, whereas the overpotentials of both catalysts at 10 mA cm−2 are similar (Figure 21b); a moderate HER performance of an overpotential of 520 mV (vs RHE) at −10 mA cm−2 was attained as well (Figure 21c). Though these nonmetal-doped graphene materials have often been touted as “metal-free” electrocatalysts,135,145,146 various studies have emerged, refuting these findings with conclusions that the active sites for ORR were essentially the metallic impurities embedded within the carbon layers.25,27,147,148 The trace metallic impurities, which persisted in the graphene, stemmed either from the parent graphite27 or contamination through synthetic routes.25 Therefore, elemental quantification methods such as inductively coupled plasma mass spectrometry (ICP-MS), together with inhibition techniques, are highly pertinent for substantiation of the absence of metallic impurities. To this end, Xu et al. introduced thiocyanate ions (SCN−), which are known to coordinate with iron species to achieve stable complexes and thus poison the iron ORR catalytic sites, to the N,P codoped hybrid catalyst

ance was ascribed to the increased activity of pyridinic N in the presence of B, as well as the highly electron-withdrawing nature of N which indirectly activated B, resulting in additional active centers on the B-dopant sites. In a separate study, Zhou and co-workers doped porous graphitized carbon with phosphorus and sulfur by sol−gel processing.141 The doping content of P is three times that of S at 1.3 and 0.4 at. %, respectively, which was ascribed to the elimination of S-containing gas carbon disulfide (CS2) upon pyrolysis. The codoping of the graphitized carbon with P and S augmented its ORR performance with an improvement in current density, earlier onset and peak potentials, as well as enhanced n; it was proposed that the synergistic effects of S,N codoping in raising the maximum spin density, which boosted the ORR activity, were applicable to P,S codoping as well. Through DFT studies, Chen et al. established a systematic method for tuning ORR performance via suitable alteration of spin density using covalent functionalization.142 A key discovery was the correlation of magnetic moments of the doped graphene with O2 adsorption energies. The fluorinefunctionalized graphene at 12.5% ratio possessed moderate magnetic moments and was found to improve O2 adsorption, with preference to the 4e− reduction pathway. An experimental embodiment of F-doped carbon was established by Sun and co-workers, whereby the F dopants inhibited the 2e− (H2O2) pathway while preferring the 4e− ORR mechanism (Figure 20a−b).143 Compared to Pt/C, the best-performing F-doped material (BP-18F) exhibited superior current density and earlier onset potential (Figure 20c) as well as significant tolerance toward methanol and CO contamination (Figure Q


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Figure 17. ORR activity and stability of G-CN, CN, and N-graphene materials. (a) RDE measurements of G-CN and CN electrocatalysts at 1600 rpm. (b) Bar graphs showing n of G-CN and CN at −0.40 V. Reprinted with permission from ref 135. Copyright 2011 John Wiley and Sons. (c) Rotating ring-disk electrode (RRDE) measurements for ORR electrocatalysis by graphene, Pt/C, and N-graphene materials with same mass (7.5 μg). (d and e) Chronoamperometric response of N-graphene and Pt/C toward (d) methanol and (e) CO. (f) CV curves of N-graphene before and after potential cycling for 200,000 continuous cycles between −1.0 and 0 V (vs Ag/AgCl). Reprinted with permission from ref 136. Copyright 2010 American Chemical Society.

that contained trace amounts of iron (ca. 0.09 wt %).149 Upon intentional inhibition of iron activity in the catalyst, negligible changes to the ORR activity of the catalyst were observed, suggesting that trace iron species have little impact on the ORR performance. Despite the authors’ efforts in addressing the possibility of ORR activity contribution by iron impurities, other metallic residues such as manganese species which are known for their significant ORR activities are likely be present, yet no further study was conducted to dispel this possibility. As substantiated by the above-mentioned studies, the doping of graphene with any heteroatom, either electron donating or withdrawing, leads to improved ORR electrocatalytic performance. This is in stark contrast with the well-established electrochemistry of semiconductors, whereby B-doped graphene demonstrates inhibiting electrocatalytic effect as opposed to the N-doped counterpart;150 the theoretical basis for the phenomenon was subsequently discussed.151

Two-Pronged Doping. Noting the success of doping graphene materials with either metal (oxide)- or nonmetalbased dopants in realizing improved ORR performances, researchers have explored a two-pronged approach to achieve doubly doped graphene as highly active ORR electrocatalysts. The doped graphene is generally attained through doping of the carbon material with a nonmetal element, together with deposition of metallic dopants. N-doped graphene is favored as a base material for metallic nanoparticles due to their high specific surface area as well as strong bonding with the metallic nanoparticles which ensures good dispersibility and improves electroconductivity.152 Furthermore, a good dispersion of highly active nanoparticles lowers production costs as less materials are necessitated. In the case of metal oxide dopants, nanoparticles of cobalt and manganese oxides demonstrate outstanding ORR performances, and various insights into their electrocatalytic activities have been garnered. As previously mentioned, N-doping of the R


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nanosheets followed by annealing at 800, 900, or 1000 °C (denoted as NG-800, NG-900, and NG-1000).157 NG-900 exhibited excellent tolerance to methanol oxidation compared to Pt/C (Figure 23a−b). Subsequent infusion of Fe nanoparticles (5.0 wt %) into NG-900 (denoted as NG/Fe5.0) led to improvements in ORR activities compared to NG-900 with its earlier onset potential and higher kinetic-limited current density as well as high n value of 3.82. (Figure 23c−d). The work by Dai et al. delved into the development of an oxidation method that partially unzipped the outer walls of CNTs, generating nanosheets of graphene appended to the inner walls (denoted as NT-G).45 The nanosheets were doped with minute amounts of iron (0.24 at. %), stemming from the seeding for CNT growth, and nitrogen impurities (5.3 at. %). The authors suggested that these impurities formed the active sites that enhanced the ORR electrocatalytic properties of NTG which was on par with the ORR activity of Pt/C (Figure 23e); the inner walls of the CNTs also facilitated charge transport with their retained electrical conductivity. Compared to the metal-free N-doped graphene (Figure 23f), NT-G displayed much higher current density as well as earlier onset potential, indicating the beneficial effect of additional Fe doping. In a systematic investigation of the utility of the graphene catalyst support material, Qiao et al. fabricated three forms of silver/rGO composites: Ag on N-doped graphene (Ag/NrGO), Ag on electrochemically reduced graphene oxide (Ag/erGO), and Ag on chemically reduced graphene oxide (Ag/crGO). Of the three electrocatalysts, Ag/N-rGO outperformed the other two through demonstration of the earliest onset potential and lowest ring current to disk current (Ir/Id) ratio, indicating the occurrence of a direct 4e− oxygen reduction. The Tafel slope of Ag/N-rGO (43 mV dec−1) was in closer proximity to N-rGO (55 mV dec−1) compared to Ag (110 mV dec−1), alluding that N-rGO, instead of Ag, possessed the active sites; Ag took up the role of a co-catalyst that suppressed peroxide production while enhancing the activity of N-rGO.

Figure 18. ORR activity and stability of B-doped graphene compared to undoped graphene and Pt. (a) Variation of n of Bdoped and undoped graphene electrocatalysts with applied potential. CV curves exemplifying the superior tolerances of (b) B-doped graphene toward methanol and CO over those of (c) Pt disk electrode. (d) CV curves indicating the good cycling stability of B-doped graphene in alkaline electrolytes. Reprinted with permission from ref 137. Copyright 2012 The Royal Society of Chemistry.

rGO catalyst support material by Dai et al. significantly boosted the ORR activity of the Co3O4/rmGO electrocatalyst, with n increasing to the ideal 4.0 and Tafel slope declining from 54 mV dec−1 to 42 mV dec−1.127 In addition, the excellent ORR electrocatalytic activity of Co3O4 affixed on N-doped rGO (Co3O4/N-rGO) was further demonstrated by Zhang and coworkers whose hybrid catalyst displayed an ORR peak potential of −0.26 V (vs Ag/AgCl) with superior stability in alkaline media compared to the archetypal Pt/C.153 It is noteworthy that the shape of metal oxide nanoparticle strongly affects its ORR behavior due to the exposed facets of the crystallized nanoparticle. Qiao et al. fabricated a series of electrocatalysts comprising of Mn3O4 sphere, cube, and ellipsoidal nanoparticles on N-doped graphene (Figure 22a− f) through varying the solvents used and the solvothermal treatment temperature and duration.152 By employing linear sweep voltammetry on RDE, it was established that the ellipsoidal Mn3O4 nanoparticles (NENG) elicited the highest ORR activity with an onset potential almost on par with that of Pt/C (−0.13 V (vs Ag/AgCl) of NENG vs − 0.09 V of Pt/C) and n of 3.81 (Figure 22g−h); this was ascribed to its highest percentage of (001) Miller-index facet which were reported to be advantageous toward ORR due to its facile adsorption of oxygen during ORR.154,155 In a separate study, Mn3O4 anchored onto N-doped rGO via a solvothermal process garnered n to be 3.85 at −0.4 V, which was higher than those of the N-doped rGO (3.18) and the physical mixture of Mn3O4 nanoparticles and N-doped rGO (3.52).156 Good stability and tolerance toward methanol oxidation reaction were obtained as well. For nonoxide metallic dopants, nonprecious transition metals are generally preferred for their economic value, long durability, as well as ease of doping into graphene nanosheets.157 Müllen et al. developed a synthesis method for Ndoped graphene with varying N content, from 4.0% to 12.0%, through immobilizing graphitic carbon nitride on graphene

GRAPHENE AND RELATED MATERIALS FOR OER In comparison to HER, OER involves a highly coordinated extraction of four electron−proton pairs per molecule of oxygen, which mandates significantly higher overpotential to surmount the OER energy barrier, making it the bottleneck reaction of the water splitting process.158,159 Various precious metal oxides, for example, IrO2 and RuO2, exhibit excellent OER catalytic performances but are unstable at high anodic potentials.160,161 To overcome the exorbitant cost of these electrocatalysts, the scientific community has shifted its attention toward potential OER electrocatalysts, with particular focus on achieving faster reaction kinetics as well as improved catalyst stability.162,163 Additionally, due to the reciprocal relationship between OER and ORR in metal-air rechargeable batteries, it is advantageous to develop a bifunctional electrocatalyst for both OER and ORR,164 as endeavored by various groups, which will be elaborated in the ensuing sections. Transition-Metal Oxide-Doped. Among the existing transition-metal-based dopants, Co-based oxides are favored due to their moderately high OER electrocatalytic activity in neutral and alkaline electrolytes.165,166 Nonetheless, the OER performance of such Co dopants has been impeded by their intrinsically mediocre conductivity as well as thermodynamic volatility.167 To improve the conductivity and stability of Co S


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Figure 19. Electrocatalytic ORR performance of dual-doped graphene. (a) RDE measurements in O2-saturated 0.1 M KOH, (b) corresponding K-L plots, and (c) bar graphs showing n of N,S dual- and singly doped graphene materials at −0.60 V. Reprinted with permission from ref 139. Copyright 2012 John Wiley and Sons. (d) RDE measurements in O2-saturated 0.1 M KOH, (e) corresponding K-L plots, and (f) bar graphs showing n of B,N dual- and singly doped graphene materials at −0.60 V. Reprinted with permission from ref 140. Copyright 2013 John Wiley and Sons.

enhanced ORR cathodic current (0.2 mA cm−2 at −0.3 V) which was 1 order of magnitude greater than that of [GR]3L (0.02 mA cm−2 at −0.3 V), empowering the composite material to be exploited as bifunctional catalyst for the oxygen reduction and evolution reactions. Improved stability was also achieved with the incorporation of graphene, with the current retention percentage of the hybrid catalyst being three times that of [Co3O4]3L. In a similar study on a sandwiched composite of graphene and Co3O4, the fabricated material outperformed the Ru/C catalyst in alkaline electrolyte in terms of both catalytic activities and stability over 10 h (Figure 24d− e).163 OER was also successfully conducted in neutral electrolyte, with an onset potential of 0.858 V (vs Ag/AgCl) and a Tafel slope value of 98 mV dec−1, outshining prior literature values (104−110 mV dec−1).167,169 Two-Pronged Doping. Similar to ORR, the strategy to dope graphene with both nonmetal and metallic components has been applied to OER. The nonmetal-doped graphene catalyst support provides high exposure of chemical groups containing heteroatoms, for example, oxygen, nitrogen, etc. for the anchoring of OER-active metallic dopants, as well as providing overall flexibility and stability to cushion any

dopants, different groups have applied the strategy frequently employed for ORR, that is, doping conductive graphene with small quantities of Co materials. To this objective, Zhao et al. fabricated a layer-by-layer composite of Co3O4 and graphene ([Co3O4/GR]nL, n = number of Co3O4-graphene bilayers) via alternating electrophoretic deposition (EPD) and chemical bath deposition (CBD) (Figure 24a−b).168 From Figure 24c, the OER performance of [Co3O4/GR]3L surpassed that of its individual components, with an earlier onset potential of 0.82 V (vs NHE) and a higher current density of 1.30 mA cm−2. This was accredited to the synergistic effects between Co3O4 and graphene, whereby the former afforded OER catalytic sites, whereas the latter presented conductive tunnels for rapid transfer of electrons produced during OER. The minor oxidative peak at 0.44 V was commonly observed for Co oxide catalysts due to the Co3+ to Co4+ oxidation, which was postulated as catalytically active sites for OER; the significantly higher peak current obtained for [Co3O4/GR]3L (Figure 24c) suggested the increased exposure of active sites with the incorporation of graphene layers into the hybrid catalyst. Conversely, the inclusion of Co3O4 into the composite led to T


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Figure 20. ORR performance and stability of F-doped carbon black compared against Pt/C. (a) The H2O2 yield and (b) n during ORR on BP-18F, carbon black (BP), and Pt/C. (c) Linear sweep voltammetry (LSV) curves of BP-18F, BP, and Pt/C in O2-saturated 0.1 M KOH. Tolerance of (d) BP-18F and (f) Pt/C toward CO and methanol addition. Long-term stability of (e) BP-18F and (g) Pt/C. Reprinted with permission from ref 143. Copyright 2013 American Chemical Society.

Figure 21. Electrocatalytic activities of N,P,F codoped graphene. (a) RRDE measurements of various catalysts for ORR, as labeled. (b and c) LSV curves of various catalysts for (b) OER and (c) HER, as indicated. Reprinted with permission from ref 144. Copyright 2016 John Wiley and Sons.

U


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Figure 22. Characterization data and ORR performance of Mn3O4 deposited on N-doped graphene. (a−f) TEM (left) and HRTEM (right) images of (a, b) NSNG, (c, d) NCNG, and (e, f) NENG. (Inset) The electron diffraction patterns of the Mn3O4 nanoparticles on N-doped graphene. (g) LSV curves recorded on RDE at 1600 rpm at scan rate of 5 mV s−1 and (h) bar graphs illustrating n of NSNG, NCNG, and NENG. Reprinted with permission from ref 152. Copyright 2013 John Wiley and Sons.

concluded to have proceeded via two different active sites: the edges of NiCo2O4127,171 as well as the N− and O−metal (Ni or Co) bonds.172,173 A systematic study by Yang and co-workers brought attention to the influence of the chemical valence of metallic dopants on the overall OER performance of the composite material. The authors investigated three different porous nickel sulfides hybridized with N,S codoped carbon nanoparticles (NixSy-NSCs), with different proportions of Ni2+ and Ni3+.174 In the same vein as the previous study by Qiao et al.,51 the porous structure of the electrocatalysts enhanced the exposed surface area and active sites, and the nonmetal-doped carbon matrix enhanced the conductivity of semiconducting NixSy, leading to augmented OER activities compared to the archetypal RuO2 catalyst (Figure 26a). The OER polarization curves of NixSy-NSCs in Figure 26a displayed an evident trend between chemical valence of Ni in nickel sulfide and the OER activity, whereby a higher proportion of Ni3+, that is, NiS1.03 > Ni9S8-NiS1.03 > Ni9S8, denoted superior OER performance; this is corroborated by the Nyquist plots obtained from electrochemical impedance measurements with NiS1.03 having the lowest charge-transfer resistance (Rct), followed by Ni9S8NiS1.03 and Ni9S8 (Figure 26b). The OER activity of NixSy was attributed to the partial oxidation of surface Ni atoms into NiOOH, engendering NiOOH/NixSy species as OER-active sites, as supported by the pair of reversible peaks observed in the CV curves (Figure 26c). Excellent stability was revealed upon 10 h of continuous measurement at 1.54 V (vs RHE), where no significant drop in current density was detected (Figure 26d), with inconsequential change in the morphology of NiS1.03-NSCs, illustrating the outstanding durability of the composite catalysts. Using an electrodeposition method, Wu et al. fabricated NiSe2 on N-doped graphene encapsulated Ni foam (NG/ NiSe2/NF), which formed pyramid-like deposits vertically grown on the substrate.170 The resulting electrocatalyst yielded

morphological and volumetric changes of the metallic dopants over long-term stability studies, enabling stability over as long as 100 h.170 To achieve a highly active OER hybrid electrocatalyst, Qiao et al. fabricated a porous N-doped graphene decorated with NiCo2O4 (PNG-NiCo) which displayed a layered structure and possessed both in-plane mesopores in the porous graphene and out-of-plane macropores due to restacking of NiCo2O4 nanoparticles.51 The in-plane mesopores of the underlying Ndoped graphene were created via etching with KMnO4 and HCl acid, which conferred higher surface area to the PNGNiCo hybrid compared to the counterpart containing nonporous graphene (NG-NiCo) (155 vs 126 m2 g−1). The hierarchical trait of the macropores and mesopores engendered highly accessible surfaces, whereby the former accelerated mass transport and electrolyte penetration, while the latter yielded high surface area and short diffusion path lengths, resulting in enhanced OER performance compared to its nonporous and non-NiCo2O4 doped equivalents (Figure 25a). Notwithstanding the relatively high value of the Tafel slope of PNG-NiCo (156 mV dec−1) compared to other electrocatalysts, it still presented more favorable kinetics than those of NG-NiCo and the hybrid with undoped graphene (PG-NiCo) (Figure 25b− c). The outstanding durability of PNG-NiCo was demonstrated in the

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