Recent progress in carbon-based materials as catalysts for electrochemical and photocatalytic water splitting

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RECENT PROGRESS IN CARBONBASED MATERIALS AS CATALYSTS FOR ELECTROCHEMICAL AND PHOTOCATALYTIC WATER SPLITTING

8

Chunyu Cui1, Jiale Xie2, Deqing Lin3, Jiakui Zhang1, Xianghong Chen1, Chunxian Guo2 and Jiantie Xu1 1

School of Environment and Energy, Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, National Engineering Laboratory for VOCs Pollution Control Technology and Equipment, South China University of Technology, Guangzhou, P.R. China 2Institute of Materials Science and Devices, Suzhou University of Science and Technology, Suzhou, P.R. China 3School of Ophthalmology and Optometry, Wenzhou Medical University, Zhejiang, P.R. China

CHAPTER OUTLINE 8.1 Introduction ................................................................................................................................. 174 8.2 Fundamental Principles ................................................................................................................ 174 8.2.1 Photocatalytic Water Splitting.................................................................................... 174 8.2.2 Oxygen Evolution Reaction ........................................................................................ 175 8.2.3 Hydrogen Evolution Reaction ..................................................................................... 175 8.3 Carbon-based Materials as Catalysts for Water Splitting................................................................. 177 8.3.1 Carbon Nanotube-based Materials.............................................................................. 177 8.3.2 Graphene-based Materials ......................................................................................... 181 8.3.3 C3N4-based Materials ............................................................................................... 185 8.4 Metal
Carbon Hybrid Materials.................................................................................................... 188 8.4.1 Noble-metal/Carbon-based Catalysts for Oxygen Evolultion Reaction ............................. 188 8.4.2 Noble-metal/Carbon-based Catalysts for Photocatalytic Water Splitting.......................... 189 8.4.3 Nonnoble-metal/Carbon-based Catalysts ..................................................................... 190 8.5 Conclusion and Perspectives ........................................................................................................ 194 Acknowledgments ............................................................................................................................... 195 References ......................................................................................................................................... 195

Carbon Based Nanomaterials for Advanced Thermal and Electrochemical Energy Storage and Conversion. DOI: https://doi.org/10.1016/B978-0-12-814083-3.00008-1 © 2019 Elsevier Inc. All rights reserved.

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8.1 INTRODUCTION To meet the increasing demands of energy and concerns regarding the energy crisis and environmental issues, clean energy harvest from sunlight provides great opportunities. Since the singlecrystal electrode of n-type TiO2 can excite the electrochemical water oxidation under photo-assist [1], the photocatalytic and photoelectrochemical water splitting to produce hydrogen as clean fuels has attracted considerable attention. Generally, primary approaches of the overall water splitting can be divided into two systems: one is the one-step water splitting approach using a single visiblelight-responsive photocatalyst to achieve overall water splitting under sufficient potential. This one-step system is still largely limited by photocatalysts, which should meet stringent essential requirements (e.g., suitable thermodynamic potential for water splitting, a sufficiently narrow band gap to harvest visible photons, and stability against photocorrosion) [2,3]. Another one is to use two different photocatalysts in one system [4]. Compared to one-step, a wider range of visible light can be used along with the spatial separation of the evolved H2 and O2 in the hybrid system. In addition, the photocatalysts with either a water reduction or oxidation potential can be adopted in one side of the two-photocatalysts system. In this system, the overall water splitting is based on a two-step photoexcitation, however, the promotion of electron transfer between two semiconductors and suppression of backward reactions often involves shuttle redox mediators [5
10]. Over the past several decades, significant advancements have been made in the synthesis and application of various nanostructured carbon-based materials in energy applications, particularly water splitting. The general advantages of carbon-based materials include the diversity of structure, good electrical and thermal conductivity, as well as a combination of mechanical strength and lightness that conventional materials cannot match. Of particular importance is the development of various metal-free catalysts for electrochemical and/or photoelectrochemical water splitting [11]. The types of carbonaceous materials for the construction of metal-free electrocatalysts or photocatalysts for water splitting mainly include carbon nanotubes (CNTs), graphene, g-C3N4, and others. These materials alone do not display significant activities for hydrogen evolution reaction (HER), oxygen evolultion reaction (OER), as well as photoelectrochemical water splitting. In order to make efficient carbon-based catalysts, doping with heteroatoms, creating defects, and/or forming composites among them are the frequently adopted strategies. Along with this, great efforts have also been made to alter their morphological properties and build mesoscopic structures to attain optimal performances. The origin of the active sites was often revealed using advanced characterization techniques, together with computational studies. In this chapter, we start with the introduction of fundamentals of photoelectrochemical water splitting (e.g., HER and OER). The reaction processes and fundamentals are also discussed. Then, various forms of carbon-based materials and their applications in water splitting are surveyed. To elicit fundamental insights, the performance enhancement mechanisms are highlighted. Moreover, the challenges and prospects of this research area are also deliberated.

8.2 FUNDAMENTAL PRINCIPLES 8.2.1 PHOTOCATALYTIC WATER SPLITTING In a photocatalytic water splitting system, there are three essential requirements that have to be considered. First of all, the system must be capable of absorbing solar illumination to generate

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sufficient excitons, namely photogenerated electrons and holes. Second, the photoinduced electron
hole pairs must be separated efficiently with low energetic loss to prevent their recombination. Third, the photoinduced electrons and holes must be able to carry out the desired chemical reactions. Under solar irradiation, the photoinduced electrons can reduce water to generate H2, while the holes can oxidize water to generate O2. The generated H2 can be collected and utilized to produce electricity in hydrogen fuel cells, or directly serve as a fuel for transportation with zero pollutants or greenhouse gas emissions. Under these regards, to find efficient photocatalysts with excellent physicochemical properties, high activity, more active sites, and optimized reaction conditions, as well as configuration of an efficient photoelectrolysis cell for water splitting become critically important. Fig. 8.1A shows the basic principle of one-step water splitting. To achieve photocatalytic water splitting using a single photocatalyst, the band gap of the semiconductor must straddle the reduction and oxidation potentials of water in the reactant solution with a pH 0, corresponding to 10 and 11.23 V versus normal hydrogen electrode (NHE), respectively [12]. Alternatively, two semiconductors connected in series with reversible redox shuttles are shown in Fig. 8.1B. In this scheme, the reduction of water to hydrogen and oxidation of reduced redox mediators occur on one photocatalyst while the reduction of oxidized redox mediators and oxidation of water to oxygen on the other photocatalyst concurrently happen. This two-step photocatalytic system is also called as the “Z-scheme”. This is due to the similarity in the excitation and transfer processes of photo-excited electrons [13]. Z-scheme water splitting proceeds even in the absence of reversible redox shuttles in some cases. This is due to the interparticle electron transfer during the physical contact between the hydrogen and oxygen evolution photocatalysts. During the water splitting, there are two fundamental reactions involved: OER and HER.

8.2.2 OXYGEN EVOLUTION REACTION OER is a reaction generating molecular oxygen through several proton/electron-coupled processes [14,15]. In acidic conditions, the reaction operates through oxidation of two water molecules (H2O) to give four protons (H1) and one oxygen molecule (O2) by losing a total of four electrons [14]. In basic conditions, oxidation of hydroxyl groups (OH2) are transformed into H2O and O2 with the same number of electrons being involved [15]. The detailed reaction formulas are described as follows: 2H2 O24H1 1 O2 1 4e2 2

2

4OH 22H2 O 1 O2 1 4e

(8.1) (8.2)

8.2.3 HYDROGEN EVOLUTION REACTION It is generally suggested that the HER involves three possible reaction steps in acidic media, though the HER mechanism in alkaline media is still ambiguous. The first one in acidic media is the Volmer step: H1 1 e2 1 Hads. The reaction of an electron and a proton produces an adsorbed hydrogen atom (Hads) on the electrode surface. After generation of Hads, the HER can proceed by the Tafel step (2Hads dse2) or the Heyrovsky step (Hads 1 H1 1 e2 1 e2) or both. Regardless of which HER routes, Hads is always involved in the HER. Thus, the free energy of

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FIGURE 8.1 Scheme of energy diagrams of photocatalytic water splitting in one-step (upper figure) and two-step (lower figure) photoexcitation systems. C.B.: Conduction band, V.B.: valence band, and Eg: band gap [12]. Reproduced with permission from K. Maeda, K. Domen, J. Phys. Chem. Lett. 1 (2010) 2655
2661. Copyright 2010, American Chemical Society.

hydrogen adsorption (ΔGH) is widely accepted to be a descriptor for a hydrogen-evolving material. For example, ΔGH is approximately zero for Pt, and correspondingly Pt is the best solidstate hydrogen evolution catalyst. If ΔGH is positive in large, the Hads is bound strongly with the electrode surface, making the initial Volmer step easy. However, the subsequent Tafel or Heyrovsky steps are difficult. If ΔGH is negative in large, Hads has a weak interaction with the electrode surface, resulting in a slow Volmer step that limits the overall turnover rate. Therefore, an optimal non-Pt HER catalyst should also provide appropriate surface properties and have a nearly zero ΔGH.

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8.3 CARBON-BASED MATERIALS AS CATALYSTS FOR WATER SPLITTING The types of carbonaceous materials for the construction of metal-free electrocatalysts and photoelectrochemical water splitting mainly include CNTs, graphene, g-C3N4, and other forms of carboncontaining materials, as summarized in Fig. 8.2. To enhance the performance, these materials have been intensively modified by doping with heteroatoms, creating defects, and/or forming composites. Additionally, to attain optimal performances, great efforts have also been made to alter their morphological properties and build mesoscopic structures.

8.3.1 CARBON NANOTUBE-BASED MATERIALS 8.3.1.1 Carbon nanotube-based materials for oxygen evolution reaction CNTs are seamless cylinders composed of one or more curved layers of graphene with either open or closed ends. Cheng et al. [16] demonstrated that pristine CNTs composed of between 2 and 7 concentric tubes and an outer diameter of 2
5 nm had a super OER activity in alkaline solution in comparison with single-walled and multiwalled CNTs (SWCNTs and MWCNTs). Fig. 8.3A shows the typical linear scan voltammetry (LSV) curves of CNTs for the OER. Fig. 8.3B and C are the plots of the average current densities measured at 1.8 V (vs RHE) and the onset potential of the OER on CNTs against the tube size or the number of walls. The electrocatalystic activity of CNTs strongly depends on the characteristics of CNTs. When the average number of walls increased to

FIGURE 8.2 Carbon-based materials for water splitting [11]. Reproduced with permission from Y. Xu, M. Kraft, R. Xu, Chem. Soc. Rev. 45 (2016) 3039
3052. Copyright 2016, Royal Society Chemistry.

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FIGURE 8.3 (A) Linear scans voltammetry curves of OER for CNTs measured in 1 M KOH at scan rate of 1 mV/s with rotating rate of 2000 rpm; plots of current density measured at 1.8 V versus RHE and onset potential as a function of (B) outer diameter and (C) number of walls of CNTs; (D) Tafel plots for the OER on CNTs and 20% Ru/C electrocatalysts, and (E) electrode polarization resistance, RP of CNTs as a function of the applied potential. The tests were conducted in 1 M KOH solution with catalyst loading of 0.025 mg/cm2. (F) The plot of the activity of CNTs for the OER in 1 M KOH solutions as a function of number of walls. The mass specific activity was measured at 1.8 V (vs RHE) at scan rate of 1 mV/s and rotating rate of 2000 rpm with CNTs loading of 0.025 mg/cm2 [16]. Reproduced with permission from Y. Cheng, C. Xu, L. Jia, J.D. Gale, L. Zhang, C. Liu, et al., Appl. Catal. B: Environ. 163 (2015) 96
104. Copyright 2014, Elsevier B.V.

three with a corresponding increase of the tube size to OD 5 3.8 nm (i.e., CNTs-3), the onset potential was shifted to a lower potential, 1.64 V and the current density significantly increased to 56 mA/cm2 at 1.8 V, almost 10 times higher than that measured on SWCNTs (CNTs-1). However, with a further increase in the number of walls (or the tube size), the activity for OER decreases again. Most significantly, the activity of CNTs for OER follows distinctive volcano-type dependence on the size or on the number of walls of CNTs. The low Tafel slope values of B60 mV dec21 observed for the OER on CNTs in Fig. 8.3D indicate the facile initial discharge of an OH2 ion on the surface of the outer walls of CNTs. And the electrode polarization resistance (Rp) of CNTs composed of between 2 and 7 concentric tubes and an outer diameter of 2
5 nm are much less sensitive to the applied dc bias, indicating the high activity of the CNTs for the OER (Fig. 8.3E). The volcano-type dependence of the activity of CNTs for the OER in alkaline solutions as a function of number of walls of CNTs is schematically shown in Fig. 8.3F. The activity of such CNTs is significantly higher than that of conventional 20% Ru/C and 50% Pt/C electrocatalysts at high polarization potentials. Such CNTs also show an excellent stability toward OER. One hypothesis is that for the OER on CNTs with specific number of walls, efficient electron transfer occurs on the inner tubes of the CNTs most likely through electron tunneling between outer wall and inner

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tubes, significantly promoting the charge-transfer reaction of OER at the surface of outer wall of the CNTs. For SWCNTs, such separation of functionality for OER is not possible, while effective electron tunneling between outer wall and inner tubes of the CNTs diminishes as the number of walls increases due to the reduced dc bias (i.e., the driving force) across the walls or layers of MWCNTs. This hypothesis is strongly supported by the observed distinctive volcano-type dependence of the electrocatalytic activity and turnover frequencies (TOF) of CNTs as a function of number of walls. Nitrogen-doped carbon nanomaterials have gained significant popularity recently. In the nanocarbon materials not only as supports for metal nanoparticles but as metal-free OER catalysts, the coexistence of oxygen-containing groups is almost inevitable. This is because the oxidation of the nanocarbon substrates is usually required to introduce oxygen-containing groups and defect sites for subsequent functionalization or doping with heteroatoms. However, the role of oxygencontaining groups on the catalytic activity remains unclear. MWCNTs themselves are effective water oxidation catalysts, which can initiate the OER at overpotentials of 0.3 V in alkaline media [17]. Oxygen-containing functional groups such as ketonic C 5 O generated on the outer wall of MWCNTs are found to play crucial roles in catalyzing OER by altering the electronic structures of the adjacent carbon atoms and facilitating the adsorption of OER intermediates. The well-preserved microscopic structures and highly conductive inner walls of MWCNTs enable efficient transport of the electrons generated during OER. Benefiting from the positive effect of heteroatoms doping, Cheng et al. [18] prepared B-MWCNTs with tunable boron content through a simple thermal annealing method. When it was used as a catalyst for the OER in alkaline media, the B-MWCNTs displays enhanced catalytic performance. CNTs have also been explored as catalyst supports to fabricate composite materials for OER [19
21]. For example, Co2O3 particles supported on oxidized MWCNTs yielded an OER current density of 10 mA/cm2 at an overpotential (η) of 0.39 V in 0.1 M KOH solution [22], significantly better than Co2O3 nanocrystals. Ultra-thin nickel-iron layered double hydroxide nanoplates supported on oxidized MWCNTs achieved a current density of 10 Ag21 at η 5 0.228 V in 1 M KOH with catalysts loading of 0.25 mg/cm2 [20]. Li et al. [23] developed a mononuclear ruthenium complex supported on MWCNTs, which show highly electrocatalytic activity and low η for water oxidation reaction.

8.3.1.2 Carbon nanotube-based materials for hydrogen evolution reaction CNT have been also considered as promising materials for HER. The electrochemical behavior of SWCNT related to the mechanism involved in the hydrogen electrode reaction applying electrochemical and spectroscopic techniques is studied by Fernandez et al. [24]. Kinetic studies have shown that hydrogen can be easily generated with low overpotential on CNTs. The high value of the exchange current density makes CNT electrodes a good candidate to replace expensive noble metals used as catalysts in acid electrolyzers. But high overpotential for hydrogen oxidation makes CNTs inadequate for use as catalysts in fuel cell applications. Cui et al. [25] developed the activated CNTs as an acid-stable metal-free HER electrocatalyst with high activity via acidic oxidation and subsequent cathodic pretreatment. Das et al. [26] reported that SWCNTs and some graphitic carbons activated by brief exposure to electrochemical potentials can induce HER in intercalating acids combined with extended soak times in such acids. Fig. 8.4A shows quasi-steady-state (5 mV/s) voltammograms of hydrogen evolution currents for a 1.5 μm thick SWCNT film working electrode with 54 μg of SWCNTs exposed

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FIGURE 8.4 (A) Quasi-steady-state voltammograms (5 mV/s) following the indicated time and electrochemical cycling in the acid; (B) Chronoamperometry at 10.3 V versus NHE immediately upon immersion in the acid (black) and after 144 h of activation (red); (C) Raman spectra of the D and G band regions for a nanotube film; (D) Proposed adjacent Tafel mechanism [26]. Reproduced with permission from R.K. Das, Y. Wang, S.V. Vasilyeva, E. Donoghue, I. Pucher, G. Kamenov, et al., ACS Nano 8 (2014) 8447
8456. Copyright 2014, American Chemical Society.

to 1 M H2SO4. The black curve exhibits the initial behavior with a high overpotential for the onset of HER current. After recording this voltammogram, the working electrode potential was scanned cyclically from 10.2 to 20.7 V versus NHE at 50 mV/s. Steady-state measurements (with 10 min settling times) recorded at a few potentials validated the quasi-steady-state curves. Fig. 8.4B shows chronoamperometry in 1 M H2SO4 of a 1.5 μm thick SWCNT film (54 μg of SWCNTs exposed to the acid) at 10.3 V upon switching the gas fed to the plenum back and forth between hydrogen and inert gas (argon) before (black curve) and after 144 h of activation in the acid (red curve). Initially, the SWCNTs show no hydrogen oxidation reaction (HOR) current with exposure to the hydrogen gas (black curve), while once activated, substantial HOR current appears (red curve).

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Raman spectra of acid-activated and non-activated SWCNT samples (Fig. 8.4C) show an upshift in the G band of 2.6 cm21. The D/G band ratio in these samples shows a slight decrease of the ratio in the activated sample as is evident from the relative intensities of the D bands, where the spectra were normalized to the same G band peak intensities, and reveal that the activation does not induce additional defects or effect chemical functionalization of the nanotube sidewalls. DFT calculations (Fig. 8.4D) show that two hydrogen atoms chemisorbed in proximity on two adjacent nanotubes have to overcome only a small activation barrier to form a hydrogen molecule.

8.3.1.3 Carbon nanotube-based materials for photocatalytic water splitting Ge et al. [27] prepared a composite of MWCNTs/g-C3N4 as a photocatalyst. After introduction of MWCNTs, the g-C3N4 photocatalysts show significant enhancement of the visible light photocatalytic activity. The optimal MWCNTs content is determined to be 2.0 wt.% with H2 evolution rate of 7.58 μmol/h, which is about 3.7-fold than that of pure g-C3N4. A possible photocatalytic mechanism is proposed based on the experimental results, as shown in Fig. 8.5. The MWCNTs have higher capture electron capability and can promote electron transfer from g-C3N4 toward their surface, leading to the improvement of photocatalytic performance.

8.3.2 GRAPHENE-BASED MATERIALS 8.3.2.1 Graphene and doped graphene for oxygen evolultion reaction Graphene is a monolayer two-dimensional sheet of carbon atoms chemically bonded in the hexagonal pattern that is characteristic of graphite. Chemically-coupled graphene is a counterpart of pristine graphene bearing some functional groups on the sheet (e.g., hydroxyl (C
OH) and carboxyl (
COOH)) and can self-assemble into an oriented hydrogel film [28
33]. Through doping or modifying graphene, the electronic structure behaviors of graphene can be tailored easily, which are

FIGURE 8.5 Schematic of photogenerated charge carrier’s separation and transfer in the MWCNTs/g-C3N4 system under visible light irradiation [27]. Reproduced with permission from L. Ge, C. Han, Appl. Catal. B: Environ. 117 (2012) 268
274. Copyright 2012, Elsevier B.V.

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important for developing active catalysts. There are many doped graphene or graphene composite materials reported as catalysts for OER and photocatalysts [34,35]. Lin et al. [36] developed a NG through the pyrolysis of graphite oxide (GO) and polyaniline (PANI). Fig. 8.6A schematically shows the bonding configurations of typical N functionalities present in NG, including N atoms doped into graphene basal plane (quaternary N), N atoms in sixmember ring (pyridinic N) and five-member ring (pyrrolic N), and N atoms bonded with O atoms (oxidized N). Fig. 8.6B shows the survey XPS spectrum of NG-1000, which represents the peaks of C, N, and O without any other impurities. The N 1 s can be deconvoluted into pyridinic N, pyrrolic N, quaternary N, and oxidized N, respectively [37]. In the oxygen saturated electrolyte, a prominent cathodic current appears with a peak centered at 20.22 V (Fig. 8.6D), indicating a high ORR catalytic activity. The enhanced catalytic activity is also observed in the extended CV potential range (Fig. 8.6E) toward OER. Fig. 8.6F reveals that OER catalytic activity of NG-1000 is higher than those of undoped graphene, the commercial Pt/C catalyst, and the glassy carbon electrode. OER on nitrogen-doped graphene nanoribbons was also analyzed by density functional theory calculations [38]. It is found that there is a linear relation between the binding energy of OOH and

FIGURE 8.6 (A) Scheme of N functionalities in NG; (B) XPS survey spectrum of NG-1000 and high-resolution N 1 s spectrum; (C) N functionalities percentage of NGs prepared at different temperatures; (D) CV curves of NG-1000 in nitrogen or oxygen saturated 0.1 M KOH with a scanning rate of 100 mV/s. (E) CV curves of NG-1000 in nitrogen saturated 0.1 M KOH with a scanning rate of 100 mV/s. (F) LSV of NG-1000, graphene, Pt/C, and glassy carbon electrode in 0.1 M KOH with a scanning rate of 10 mV/s. [36] Reproduced with permission from Z. Lin, G.H. Waller, Y. Liu, M. Liu, C.P. Wong, Carbon 53 (2013) 130
136. Copyright 2012, Elsevier B.V.

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OH for these structures. The OER active sites are identified on the armchair nanoribbons at the carbon atoms near the nitrogen atom. The armchair nanoribbons with nitrogen dopants near the edge have the minimum theoretical OER overpotential, which was estimated to be 0.4 V. Sulfur is also one of the most important dopants to tailor the electrocatalytic activities of carbons, from both experimental and theoretical perspectives [39
41]. Qu et al. [42] developed a robust, highly efficient, and an environmentally benign method to introduce S to the GO-PDA hybrids to produce N, S-codoped mesoporous carbon nanosheets. As a result, the fabricated mesoporous carbon nanosheets have exhibited much better performances than most of other benchmarked bifunctional ORR and OER catalysts. This is mainly due to their multiple doping, unique porous architecture and excellent charge-transfer ability. The nature of binding and doping induced defects is very critical to enhance the activity and stability of metal-free catalysts for OER. El-Sawy et al. [43] developed a novel strategy to tune and stabilize the active sites of metal-free graphene based catalysts. Fig. 8.7 shows a proposed mechanism for OER over the sulfur-bi-doped CNTs, and the sulfur incorporated into the carbon sp2 network of the nanotubes facilitates peroxide formation leading to oxygen evolution. The OER reaction, catalyzed by S,S0 -CNT1000 C begins by the addition of two hydroxides (OH2) to the adjacent sulfur’s carbon atoms on the thiophene ring (step 1), followed by addition of another 2OH2 to form an intermediate structure (step 2). This intermediate structure undergoes rearrangement by removal of two molecules of water and four electrons to form two oxygen radicals bonded

FIGURE 8.7 Proposed mechanism for oxygen evolution reaction of sulfur-doped CNTs [43]. Reproduced with permission from A.M. El-Sawy, I.M. Mosa, D. Su, C.J. Guild, S. Khalid, R. Joesten, et al., Adv. Energy Mater. 6 (2016) 1501966. Copyright 2015, John Wiley & Sons, Inc.

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to carbon (step 3). Step 4 involves the formation of a peroxide bond (O
O). Finally, the oxygen molecule is evolved in step 5. A new sequential bi-doping strategy was used to control the nature of sulfur active sites for sulfur-doped CNT
graphene nanolobe materials. This S doping leads to enhanced catalytic activity of bi-doped CNTs, resulting in one of the most active catalysts for the OER. This system also functions with the lowest observed overpotential, 350 mV at 10 mA/cm2, reported for metal-free catalysis until now and performs like the state of the art catalyst (20% Ir/C) but with a much higher (10 times) TOF. In addition to outstanding OER catalytic activity, the bidoped catalyst exhibited very high stability for periods up to 75 h, providing a highly active and stable OER catalyst. The bifunctionality of the bi-doped sulfur catalyst stands to currently be the most active bifunctional metal-free catalyst.

8.3.2.2 Graphene and doped graphene for HER Recent efforts have also explored the use of graphene-based materials for HER. The engineering of pristine graphene by chemical substitution of some carbon atoms with heteroatoms (e.g., N, B, P, F, and S) is an effective way to tailor its electronic structure and (electro)chemical properties [45]. Jiao et al. [44] investigated a series of nonmetal heteroatom-doped graphene for HER through the mutually corroborating electrochemical reaction rate measurements and theoretically computed adsorption energetics (Fig. 8.8). As shown in Fig. 8.8A, the Ep for the active carbon at the edge of py
N
G model is close to the Fermi level compared to that for another carbon at a non-edge site. This reveals that the higher the active center’s Ep position, the stronger the H adsorption strength. This relationship between the surface adsorption ability of a material and its electronic structure can be explained by the underlying scheme for bond formation (Fig. 8.8B). As shown in Fig. 8.8C, the N, S
G and N, P
G exhibit a lower overpotential than that on the N
G control sample, whilst N, B
G exhibits a reversed trend. This agrees well with the predicted trend represented by ΔGH values. Strikingly, the gap between the best synthesized N, S
G sample and the benchmark MoS2 HER electrocatalyst is significantly narrowed compared with that for N
G [46]. Additionally, all three samples showed similar Tafel slops B120 mV/dec (Fig. 8.8D), indicating the same reaction mechanism (Volmer
Heyrovsky) as the single-doped ones. By linking the computed value of ΔGH and the measured i0 (Fig. 8.8E), Fig. 8.8F summarizes the general relationship of electrocatalytic activity possessed by undoped, single-doped and dual-doped graphene samples. This relationship serves as compelling evidence of electronic structure engineering to achieve enhanced electrocatalyst performance on carbon-based materials. They establish a HER activity trend for graphene-based materials and explore their reactivity origin to guide the design of more efficient electrocatalysts. Following investigation of an extensive range of electronic structure descriptors, they shed light on the underlying activity origin of these materials for HER. The codoping of several elements to carbon (e.g., B, P, and/or N) could lead to a unique electron-donor property of carbon. Zheng et al. [47] designed and synthesized a nitrogen (N) and phosphorus (P) dual-doped graphene based on theoretical predictions. The N and P heteroatoms coactivated the adjacent C atom in the graphene matrix by affecting its valence orbital energy levels to induce a synergistically enhanced reactivity toward HER. As a result, the dual-doped graphene showed higher electrocatalytic HER activity than single-doped ones and a comparable performance to some of the traditional metallic catalysts. Ito et al. [48] synthesized nitrogen and sulfur codoped three-dimensional (3D) nanoporous graphene by using a nanoporous-metal-based CVD method. It was found that carbon defects alone in the graphene lattice are not catalytically active

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FIGURE 8.8 (A) The DOS for different sites of carbons in the py-N model. (B) Energy level diagram showing orbital hybridization of active sites and hydrogen adsorbate. (C
E) Electrochemical measurements on various graphenebased materials in 0.5 M H2SO4. (F) A volcano trend that includes pure (G), single-doped (A
G), and dual-doped (A, B
G) graphene samples [44]. Reproduced with permission from Y. Jiao, Y. Zheng, K. Davey, S.Z. Qiao, Nat. Energy 1 (2016) 16130. Copyright 2016, Springer Nature.

for HER, while the coupling of S and N dopants with geometric defects in the graphene lattice produces a synergistic effect in tuning the Gibbs free energy of H absorption and hence is responsible for the outstanding HER catalysis. The catalytic activity of the S and N codoped nanoporous graphene with both high concentrations of dopants and geometric defects is comparable to MoS2 nanosheets, the best Pt-free HER catalysts known in the literature.

8.3.3 C3N4-BASED MATERIALS g-C3N4 is a promising material due to its high N content, low cost, and easily tailorable structure [49,50]. Also, it is a kind of semiconductor material with a band gap of 2.7 eV [51], corresponding to a blue light absorption up to 450 nm. The absorption edge of g-C3N4 on the whole ultraviolet
visible (UV
vis) diffuse reflectance spectrum also shifts remarkably to longer wavelengths and thus covers a wide area [52], making it a proper material for applications in photochemistry and photocatalysis. Optimal use of g-C3N4 for electrochemical applications requires the improvement of its poor conductivity. Generally, two types of methods have been applied, namely, physical

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mixing of g-C3N4 with conductive carbon materials [53] and in situ immobilization of g-C3N4 onto carbon supports [54,55]. Recently, two-dimensional (2D) g-C3N4 nanosheets (g-C3N4 NSs) were synthesized by destacking the layered bulk g-C3N4 through thermal oxidation etching [56] and liquid exfoliation [57,58].

8.3.3.1 C3N4 for oxygen evolultion reaction There are few reports about the applications of C3N4 materials for OER. One example is that Ma et al. [59] synthesized 3D g-C3N4 NS
CNT porous composites with the highest activity among nonmetal OER catalysts. The porous structure enables a large number of active sites, and the composite promotes improved charge and mass transport abilities. With these unique physicochemical properties, the 3D g-C3N4 NS
CNT porous composite displays good OER activity and long-term durability [60].

8.3.3.2 C3N4 for photocatalytic water splitting Compared to commercial TiO2, g-C3N4 has a smaller band gap of 2.7 eV [51]. The absorption edge of g-C3N4 on the UV
vis diffuse reflectance spectrum also shifts remarkably to longer wavelengths and thus covers a wide area [52]. As expected, this proper band gap is especially relevant for applications in photochemistry and photocatalysis. The applications of g-C3N4 still suffer from its small specific surface area (SSA), and rapid recombination of photogenerated carriers, and low visible light utilization efficiency [61,62]. To overcome these drawbacks, various approaches have been developed, for example, modification of g-C3N4 by using other functional materials such as carbon dots, graphene [63], hydrogenase [64], semiconductors [65], aromatic compounds [66], and doping with heteroatoms such as P [67], F [68], O [69], B [70], I [62], S [71], and Fe [72]. Liang et al. [73] synthesized a 3D porous graphitic carbon nitride monolith (PCNM) using a thermal polymerization method from urea and melamine sponge (Fig. 8.9A and B). The highresolution C1 s and N1s spectra clearly show the chemical groups and bonding characteristics of the PCNM (Fig. 8.9C). The PCNM exhibits a hydrogen evolution rate of 29.0 μmol/h, which is 2.84 times higher than that of the g-CN powder (a value of 10.2 μmol/h), as shown in Fig. 8.9D. Moreover, the PCNM shows significantly improved light harvesting ability above 450 nm in comparison with the powdered g-CN (Fig. 8.9E). It is also found that no obvious peak is observed in the PL emission spectrum of PCNM, while a strong intrinsic fluorescence emission peak at 465 nm appears for the g-CN (Fig. 8.9F), which indicates that the PCNM has a larger barrier to charge recombination capability [63]. The excellent performance of the PCNM should be attributed to its unique properties including abundant porosity, good visible light capture, high SSA, as well as superior charge separation efficiency, which PCNM exhibits excellently. With its unique properties mentioned above, the PCNM is a promising candidate for applications in water splitting, CO2 reduction, pollutant degradation, as well as energy storage devices. Although possessing high chemical and thermal stability as well as proper electronic structures, pristine g-C3N4 materials still suffer from relatively low photocatalytic efficiency. To improve the performance, doping strategy is always applied to modify the electronic structures and tailor their surface properties [74,75]. Therefore, g-C3N4 can be modified by introducing suitable heteroatoms into its matrix, which could modify the molecular structure and consequently the photocatalytic activity. As an example, Guo et al. [76] reported the phosphorus-doped g-C3N4 tubes with a layered micro-nanostructure for enhanced photocatalytic water splitting. It is found that the tube morphology with the layered stack structure exhibits high SSA. Moreover, the phosphorus doping leads to a

8.3 CARBON-BASED MATERIALS AS CATALYSTS FOR WATER SPLITTING

187

FIGURE 8.9 (A) SEM image of porous graphitic carbon nitride monolith (PCNM). (B) TEM image; (C) XPS spectrum with the corresponding C 1s and N 1s spectra; (D) photocatalytic HER activity; (E) DRS spectrum; and (F) PL emission spectra of PCNM (solid line) and the CN powder (dotted line) [73]. Reproduced with permission from Q. Liang, Z. Li, X. Yu, Z.H. Huang, F. Kang, Q.H. Yang, Adv. Mater. 27 (2015) 4634
4639. Copyright 2015, John Wiley & Sons, Inc.

decreased band-gap energy and suppressed recombination of photogenerated electron
hole pairs. Eventually, the material offers greatly improved visible-light photocatalytic water splitting. The phosphorus-doped g-C3N4 tubes material could also be a good carrier material for other nanocomposites to achieve specified dimension and chemical functionality. Porous structure can provide readily access channel for reactants adsorption and more surface active sites for reaction, and also enhance light harvesting and shorten the diffusion length of photoinduced charge carriers [77
79]. Porous g-C3N4 materials have also been doped with heteroatoms to further improve the physicochemical properties. Huang et al. [80] fabricated porous network g-C3N4 with O-doping using a “one-pot” melamine
H2O2 supramolecular aggregate. The resulting material exhibits greatly enhanced light harvesting and charge separation. As a result, this material shows remarkably higher photocatalytic water splitting activity with AQE 5 7.8% at 420 nm, which is superior to bulk and 3D porous g-C3N4 without doping. This work shows that the pretreating the precursor can control the architecture and also introduce helpful foreign atoms or even monomers in the matrix for achieving highly efficient g-C3N4. It is anticipated that this doping strategy can be extended to design and fabricate other chemically-modified graphitic carbon nitride nanostructures.

8.3.3.3 C3N4/carbon composites The marginal visible light absorption and grain boundary affect the visible-light photocatalytic activity of g-C3N4 [81]. Additionally, the weak van der Waals interaction between adjacent

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conjugated planes hinders the electron coupling between the planes, which limit the electron transfer and photocatalytic activity [82]. An effective strategy is to couple g-C3N4 with other materials to form g-C3N4 composites. Recently, g-C3N4 has been coupled with graphene (GR) and graphene oxide (GO), which are promising metal-free visible-light photocatalysts [83
85]. When GO is overlaid on the surface of g-C3N4, the composite exhibits intense optical absorption in the visible light region. The g-C3N4/GO also shows more intensive separation efficiency of photogenerated charge than pristine g-C3N4. Therefore, g-C3N4/GO composite under visible-light irradiation offers remarkable enhancement of photocatalytic capability. Xu et al. [86] performed DFT calculations to explore the enhanced photocatalytic mechanism for the g-C3N4/RGO composites. The calculated energy gap of the g-C3N4 monolayer is 2.93 eV, which well agrees with the band gap of 2.97 eV. The g-C3N4/RGO composite shows seven stable adsorption configurations of O atom (i.e., five bridge sites and two top sites). The amount of charge transferred at the interfaces can be clearly reflected from the interaction of g-C3N4 with RGO with various concentration of O atom. The band structure of the g-C3N4/RGO with higher O concentration is much more dispersive in comparison with g-C3N4 monolayer, which shows that smaller effective masses of the electron and hole for rapid migration. It is also found that the band gap is greatly dependent on the concentration of O atom with a negative manner. The superior vis light photocatalytic performance of the g-C3N4/ RGO should be contributed to the combined effects. This calculation study can enrich the fundamental understanding on the interaction of g-C3N4 composites as well as rationalize the available experimental results for developing high-performance photocatalysts. Xiang et al. [87] reported graphene/g-C3N4 composite photocatalysts for high visible-light photocatalytic water splitting activity. The addition of graphene influences the textural properties, for example, SSA, and optical characteristics such as UV
vis absorption of the g-C3N4. g-C3N4 is immobilized on the surface of graphene sheets (GSs) to form a layered composite after introduction of graphene, which acts as conductive channels to efficiently separate the photogenerated charge carriers and to enhance the visible-light photocatalytic activity. The optimal graphene content in the composite is around 1.0 wt.%, and the resulting composite provides a photocatalytic H2-production rate of 451 μmol/h/g, which greatly exceeds pure g-C3N4 by more than 3.07 times. The good performance of the composite is resulted from its unique properties, particularly from the introduction of graphene for good acceptor of the photogenerated electrons, and hindered electron
hole pair recombination.

8.4 METAL
CARBON HYBRID MATERIALS 8.4.1 NOBLE-METAL/CARBON-BASED CATALYSTS FOR OXYGEN EVOLULTION REACTION Noble metals such as Pt, Pd, and Ru have been extensively used in catalytic and photocatalytic water splitting due to their high catalytic activities. To reduce the amount of noble metals, carbon materials have been widely used with noble metals to fabricate nanocomposites. Jeong et al. [88] report systematic evaluation of Pt, Pd, and Ru nanoparticles supported on rGO as composite OER electrocatalysts. All of the noble metals
rGO composites show low overpotentials, and among them Ru
rGO hybrids exhibited the lowest overpotentials and the most stable cycling performance.

8.4 METAL
CARBON HYBRID MATERIALS

189

Apparent behavior of Ru nanoparticles was found to control the nature of the catalytic products. The catalytic behavior is different from the conventional electrocatalysts, which always lower activation barrier through electron transfer. Yang et al. [89] reported the fabrication of Ag3PO4/C3N4 photocatalysts by assembling Ag1 on the surface of delaminated C3N4, followed by conversion the Ag1 to Ag3PO4 nanoparticles. By varying the solvent and the molar ratio of the starting material, the composite materials possess a sheet-like C3N4 structure with variable Ag3PO4 particles sizes. The composite materials exhibit high photocatalytic performance with regards to the OER activity along with an improved photostability. Under illumination, a Z-scheme is in situ formed, and the process involves the spontaneous formation of Ag nanoparticles in the interface of the materials. The in situ formed nanoscopic metallic Ag promotes the “addition path” of the band-gap energies, leading to efficient O2 evolution under illumination while keeping the high reduction potential of g-C3N4.

8.4.2 NOBLE-METAL/CARBON-BASED CATALYSTS FOR PHOTOCATALYTIC WATER SPLITTING Yan et al. [90] reported the fabrication of Au nanoparticles on g-C3N4 through a deposition
precipitation method. The Au/g-C3N4 heterojunctions effectively promoted the transfer of charge from light-excited g-C3N4, enabling efficient photocatalytic water splitting under visible light illumination. It is expected that surface modification of the Au/g-C3N4 with a second metal simultaneously optimize electron transfer (from the Au) and chemical catalytic activity (from the secondary metal), which could further improve photocatalytic activity. Shiraishi et al. [91] found that Pt nanoparticles (with size less than 4 nm, and a mass loading of 0.3 wt.%) loaded on g-C3N4 through thermal reduction using hydrogen as reducing agent, behave as efficient cocatalysts for photocatalytic H2 evolution under visible light. The Pt nanoparticles and g-C3N4 process a strong interaction, which benefits smooth migration of the photogenerated electrons from g-C3N4 to Pt nanoparticles for efficient electron
hole separation. Gao et al. [92] prepared a highly efficient GO
CdS
Pt nanocomposite for hydrogen evolution through a two-phase mixing method through combining the formic acid reduction process. The highest photocatalytic hydrogen production rate is achieved by the GO
CdS
Pt nanocomposite containing 0.5% of Pt is 123 mL/h/g, which is much higher than that of CdS and GO
CdS. The GO
CdS
Pt nanocomposite also displays good stability during the photocatalytic hydrogen evolution process for more than 16 h; partially reduced GO shows that improved conductivity can efficiently act as electron acceptor to reduce the recombination for efficiently producing photogenerated electron
hole pairs. Additionally, suitably positioned work functions of the components of CdS, GO, and Pt in the GO
CdS
Pt composites facilitate vectorial charge transfer. These components with synergestic effects enable high photocatalytic activity as well as inhibited photocorrosion, making the GO
CdS
Pt composite a promising candidate in photocatalytic applications. Xu et al. reported the design and fabrication of high-quality cuboid-like Pt-CNSs/RGO nanohybrids through self-assembling cuboid-like Pt nanocrystals on GO sheets, followed by chemical reduction [93]. The Pt-CNSs, having an average size of around 5.8 nm, were uniformly and firmly anchored on the RGO sheets. The Pt-CNSs/RGO nanohybrids that have excellent conductivity and high dispersibility of Pt-CNSs exhibited a much higher photocatalytic activity and better stability toward hydrogen evolution in comparison with the Pt-CNSs and RGO.

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8.4.3 NONNOBLE-METAL/CARBON-BASED CATALYSTS 8.4.3.1 Nonnoble-metal/carbon or metal oxides/C for oxygen evolultion reaction Three-dimensional (3D) carbon-based catalysts, by incorporating active species into 3D carbon scaffolds can promote high catalyst loading and good electrode contact which are helpful in improving their catalytic activity. Among various 3D carbon materials, graphene hydrogels have received great interest, although showing poor OER activity. Chen et al. [94] reports the synthesis of 3D OER electrocatalysts through incorporating ternary hydroxides into N-doped graphene hydrogels for high-performance OER catalysts. In the 3D material, the N-doped graphene hydrogel provides a multidimensional conductive network, rich macro/mesoporosity, high wettability and N-doping, and the ternary hydroxides offer the active sites. The 3D material exhibits an excellent durability because of the strong interaction between the components. The synergistic combination of 3D N-doped graphene hydrogel and active NiCo hydroxides could be used for large scale fabrication of efficient OER electrocatalysts. Long et al. [95] reported the fabrication of a composite material by alternately stacking the OER active FeNi double hydroxide cation layers and electrical conductive negatively charged GO layers for efficient OER electrocatalysts. The strong interactions of the FeNi LDH and the assembled rGO promote exposure of the catalytically active sites and improvement of the charge transport. Therefore, the composite exhibited high OER activity and good operation stability in alkaline solution. The OER overpotential of the composite is as low as 0.195 V, which is among the lowest values for nonnoble-metal electrocatalysts. The Tafel slope is 40 mV/dec, which is close to that of the Ir/C catalyst [96]. Two-dimensional layered double hydroxides (LDHs) are active for the OER catalysis [98]. In particular, single-layered LDH nanosheets that can be produced by the liquid exfoliation
delamination of bulk LDHs have shown promising OER performance, which can be comparable with the commercial IrO2 catalyst [99]. Using single-layered LDH nanosheets, Ping et al. [97] prepared a porous, highly efficient OER electrocatalyst by self-assembling exfoliated single-layer CoAl-NSs onto 3DGN via the electrostatic interaction, as shown in Fig. 8.10A. Structure and composition of the materials were confirmed by XRD and XPS characterizations in Fig. 8.10B and C. It is also found that there are oxygen-containing functional groups such as C
O and OaCO on the 3DGN (Fig. 8.10D), which should favor the electrostatic adsorption of CoAl-NSs [100]. The cobalt in the materials is in the form of Co21 (Fig. 8.10E). The material exhibits excellent OER performance, as clearly reflected from its electrochemical characterizations (Fig. 8.10F
L). A Tafel slope of 36 mV/dec is obtained for the 3DGN/CoAl-NS, and the overpotential is 252 mV when to reach a jgeometrical of 10 mA/cm2. The geometrical current densities are 45.37 and 91.74 mA/cm2 at respective η of 300 and 350 mV. The TOF value is up to 1.14 s21 for the 3DGN/CoAl-NS at η of 350 mV. The current density of 3DGN/CoAl-NS remains nearly constant under continuous electrolysis reaction for around 18 h at an applied η of 280 mV, indicating the excellent stability.

8.4.3.2 Nonnoble-metal/carbon or metal oxides/carbon for hydrogen evolution reaction MoS2 has been widely investigated as a catalyst for HER [101,102]. Nevertheless, it always suffers from low electrical conductivity. To overcome the poor electrical conductivity, Li et al. [103] used RGO sheets to support MoS2 nanoparticles via a facile solvothermal approach. The MoS2/RGO hybrid that exhibits highly exposed edges and excellent electrical coupling displayed excellent

FIGURE 8.10 (A) Scheme showing fabrication of 3D porous electrocatalyst; (B) XRD patterns of the materials; (C) XPS survey spectrum of the materials; (D) C 1s; (E) Co 2p spectra of the 3DGN/CoAl-NS; (F) Polarization curves; (G) corresponding Tafel plots; and (H
L) various comparisons of the materials [97]. Reproduced with permission from J. Ping, Y. Wang, Q. Lu, B. Chen, J. Chen, Y. Huang, et al., Adv. Mater. 28 (2016) 7640
7645. Copyright 2012, John Wiley & Sons, Inc.

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HER activity with large cathodic currents, a small overpotential of around 0.1 V as well as a Tafel slope as small as 41 mV/dec. This Tafel slope value is among the smallest one for MoS2 catalysts reported to date, suggesting that electrochemical desorption is the rate limiting step during the HER. Yan et al. [104] synthesized a networked MoS2/CNT nanocomposite with high catalytic activity toward HER by a facile solvothermal method. Both acid treatment of the CNTs and the use of DMF play critical roles in the formation of this unique structure, in which MoS2 nanosheets with poor crystalline are uniformly formed on CNTs. The MoS2 nanosheets with a high number of exposed sites are identified as the active sites for HER. The networked MoS2/CNT nanocomposite that can be fabricated feasibly holds a great potential for HER. Mo2C, Mo2N, and MoS2 nanocrystals supported on CNT
GR composites have also been investigated by Youn et al. using a modified urea
glass route [105]. The synthetic method and a schematic model of a Mo-compound loaded on CNT
GR composite are shown in Fig. 8.11A. The X-ray absorption near edge structure (XANES) spectra of Mo K-edge for the supported Mo-compounds are compared with Mo foil and unsupported Mo-compounds (Fig. 8.11B). The absorption edges of supported catalysts that denote an electric dipole transition from Mo’s core level to unoccupied states of p type are found to shift to higher energies compared to the Mo foil (Fig. 8.11C). Fig. 8.11D displaying the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra of the Mo composite catalysts shows that bare and supported catalysts possess almost the same local structures around a central Mo atom. HER performance of the materials including Mo2C/CNT, Mo2C/GR, Mo2C/C, and Mo2C/CNT
GR is shown in Fig. 8.11E and F. The onset potential of Mo2C/CNT
GR is 62 mV, which is lower than that of Mo2C/CNT (120 mV), Mo2C/GR (150 m,V) and Mo2C/C (135 mV). Furthermore, to reach a current density of 10 mA/cm2, the η10 value of Mo2C/CNT
GR (130 mV) is lower than that of Mo2C/CNT (190 mV), Mo2C/GR (242 mV), and Mo2C/C (212 mV). The Tafel slope of Mo2C/CNT
GR was also smaller than that of other control samples. The good performance is attributed to the following factors: more positive charge of the Mo atoms in Mo2C for decreased hydrogen binding energy, and the reduced aggregation between the Mo-compounds and improved electron transfer from the CNT
GR hybrid. MoSe2 nanosheets and their RGO composites have been designed and fabricated through a facile hydrothermal approach followed by low-temperature annealing by Tang et al. [106]. The composite exhibited superior HER activity with a small onset potentials of around 50 mV, which is lower than the control samples of MoS2 nanosheets and MoS2
graphene composites. The DFT calculations indicate that the Gibbs free energy for atomic hydrogen adsorption (ΔG0H) on MoSe2 edges is close to thermoneutral than that of MoS2, yet at a higher H coverage, providing a promising HER catalyst in replace with the widely studied MoS2. Liu et al. [107] has reported the fabrication of CoP/CNT nanohybrid from the low-temperature phosphidation of Co3O4/CNT. The CoP/ CNT nanohybrid shows a superior HER catalytic activity with a small onset potential of 40 mV, an exchange current density of 0.13 mA/cm2, and a Tafel slope of 54 mV/dec. Additionally, to attain current densities of 10 mA/cm2, it requires an overpotential of only 122 mV. Moreover, its catalytic activity can be well-retained for at least 18 h during the HER operation.

8.4.3.3 Nonnoble-metal/carbon or metal oxides/carbon for photocatalytic applications GSs can function as an electron transfer channel to reduce the recombination of the photogenerated electron
holes, thus leading to improved photoconversion efficiency of the photocatalytic materials [108].

FIGURE 8.11 (A) Scheme showing the fabrication processes of the material of Mo-compounds on CNT
GR hybrid support for HER. (B) Mo K-edge XANES spectra of the materials; and (C) correlation between the half step energy of the Mo K-edge with the oxidation state of various Mo-compounds. (D) Fourier-transforms of Mo K-edge EXAFS; (E) polarization curves; (F) Tafel plots; (G) stability (empty labels meaning current densities of each catalyst after 1000 cycles), and (H) Nyquist plots of the materials. (I) Polarization curves and (J) corresponding Tafel plots of the materials [105]. Reproduced with permission from D.H. Youn, S. Han, J.Y. Kim, J.Y. Kim, H. Park, S.H. Choi, et al., ACS Nano 8 (2014) 5164
5173. Copyright 2012, American Chemical Society.

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Zhang et al. [109] demonstrated the feasibility of producing H2 from water splitting by using TiO2/GSs composites as photocatalyst. A series of TiO2/GSs composites with different GSs content were synthesized by a sol
gel method, and the highest photocatalytic activity was observed for the sample with 5% GSs. This study informs that a suitable content of GS will improve the activity by reducing electron
hole recombination. CdS-based water-splitting catalysts are still suffered from low separation efficiency of electron
hole pairs and photocorrosion, greatly limiting their photocatalytic applications. Recently, a series of CdS quantum dot-coupled g-C3N4 photocatalysts with high visible light photocatalytic H2 evolution activity have been fabricated via a chemical impregnation method by Ge et al. [110]. The CdS QDs/g-C3N4 composite displayed strong absorption and a red shift in the visible light region and efficient separation of the photogenerated charge carriers. With the optimal CdS QD content (around 30 wt.%), the composite exhibited a H2 evolution rate of 17.27 μmol/h, which was about nine times higher than that of pure g-C3N4, making it a very promising candidate for the high-performance H2 evolution photocatalyst. Using N-graphene as a cocatalyst, Jia et al. [111] synthesized heterostructured N-graphene/CdS for photocatalytic water splitting under visible light irradiation. With a content of 2 wt.% N-graphene, the N-graphene/CdS composites showed the highest performance. This greatly improved performance of catalysts is attributed from the formation of graphene/CdS heterojunctions. N-graphene as a cocatalyst prevents CdS from photocorrosion under light irradiation, and can be used as a charge collector to promote separation and transfer of photogenerated carriers. This work proves the great potential of using N-graphene as a substitute for noble metals in the photocatalytic H2 production and also presents an efficient strategy to construct proper heterojunction between cocatalyst and semiconductor for high photocatalytic activity.

8.5 CONCLUSION AND PERSPECTIVES Metal-free carbon-based catalysts hold great promise as efficient HER and OER catalysts to replace precious-metal catalysts for clean energy technologies, including water splitting and metal
air batteries. General strategies for the design of superior catalysts are elemental doping, defect engineering, surface modification, and the fabrication of multidimensional architectures. Despite significant progress achieved in the area of metal-free carbon-based catalysts for water splitting, the exploration of their applications for other reactions is still in its infancy and has multiple challenges. The catalytic performance of metal-free carbon-based catalysts still needs to be further improved to meet the requirements for practical applications. Second, most studies focus on how to improve the electrocatalytic activity for the HER or OER and ignore the development of a bifunctional electrocatalyst that promotes both high HER and OER activity simultaneously. Most electrocatalysts currently under development for the HER are in acidic electrolytes. The HER performance in alkaline electrolytes is significantly worse because of the inefficient dissociation of water to initiate the Volmer reaction; all OER electrocatalysts available at present only function under alkaline or neutral conditions. This poses a potential problem when the HER is coupled with the OER in the overall water splitting. Thus, finding OER/HER catalysts that can work efficiently over a wide range of pH values has been among the holy grails of chemistry for a decade, and developing HER catalysts in alkaline media with high performance is highly desirable.

REFERENCES

195

The controllable integration of HER and OER catalysts into a nanostructured single metal-free catalyst with high electrocatalytic activities for both the HER and OER is difficult, if not impossible. Opportunities come with challenges; we believe that carbon-based catalysts will soon approach practical applications, particularly in view of the on-going worldwide interest in metal-free catalysis.

ACKNOWLEDGMENTS The authors are grateful for the financial support from the young talent fellowship program through South China University of Technology and the thousand talents plan program through Suzhou University of Science and Technology and National Natural Science Foundation of China’s general program 21605110 and 21703150.

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