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The Role of Transition Metal and Nitrogen in Metal–N–C Composites for Hydrogen Evolution Reaction at Universal pHs Lulu Zhang, Wen Liu, Yubing Dou, Zheng Du, and Minhua Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11782 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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The Journal of Physical Chemistry

The Role of Transition Metal and Nitrogen in Metal–N–C Composites for Hydrogen Evolution Reaction at Universal pHs Lulu Zhang,+ Wen Liu,+ Yubing Dou,+ Zheng Du,# Minhua Shao+,* +

Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

#

National Supercomputing Center in Shenzhen, Shenzhen, Guangdong, 518055 P.R. China

*

[email protected]

ABSTRACT For the first time, we demonstrated that transition metal and nitrogen co-doped carbon nanocomposites synthesized by pyrolysis and heat treatment showed excellent catalytic activity toward hydrogen evolution reaction (HER) in both acidic and alkaline media. The overpotential at 10 mA cm-2 was 235 mV in a 0.5 M H2SO4 solution at a catalyst loading of 0.765 mg cm-2 for Co-N-C. In a 1 M KOH solution, the overpotential was only slightly increased by 35 mV. The high activity and excellent durability (negligible loss after 1000 cycles in both acidic and alkaline media) make this carbon-based catalyst as a promising alternative to noble metals for HER. Electrochemical and density functional theory (DFT) calculation results suggested that transition metals and nitrogen played a critical role in activity enhancement. The active sites for HER might be associated with metal/N/C moieties, which have been also proposed as reaction centers for oxygen reduction reaction.

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1. INTRODUCTION Owing to the climate change and shortage of fossil fuels, renewable energy sources such as solar and wind have become more and more attractive. However, these energies are intermittent and difficult to control.1 Water splitting provides hope to store these renewable energies in the form of hydrogen through a sustainable method.2 Up to now, the most popular cathodic electrocatalysts used in commercial solid polymer electrolyte (SPE) electrolyzers are still Pt-based materials that are highly scarce and costly.3-4 Therefore, one of the main challenges to generate hydrogen is to develop efficient and cost-effective catalysts toward hydrogen evolution reaction (HER) to replace Pt. Several classes of materials based on earth-abundant elements such as non-precious metal sulfides,5-9 selenides,10-12 carbides,13 nitrides14 and phosphides,15 have gained significant attention as promising catalysts for HER. However, their catalytic activities and durability are still inferior to Pt-based catalysts.16 Non-precious metal and nitrogen co-doped carbon-based materials (Me-N-C) synthesized via heat treatment have been extensively studied as highly active electrocatalysts for oxygen reduction reaction (ORR).17-20 Surprisingly, this class of materials has been rarely explored for HER.21 Recently, several groups reported that non-precious metal (Co, Ni, Fe, etc.) nanoparticles encapsulated by an ultrathin nitrogen-doped carbon layer showed excellent HER activity.22-26 Density functional theory (DFT) calculations suggested that the electronic property of the carbon shell was dramatically alternated by the metallic nanoparticle core.22 Even though Me/N/C moieties are generally believed to be the active sites for ORR, their role in promoting HER was not considered in these studies.22-23 Herein, we demonstrated that the Me (Me=Co, Ni)-N-C composites that have been commonly applied as ORR catalysts also showed excellent activity toward HER in both acidic and alkaline media. DFT calculation results indicated that transition metals and N atoms played a critical role in activity enhancement toward HER, and the active sites could be related to Me/N/C moieties.

2. EXPERIMENTAL SECTION 2.1 Material synthesis 2 ACS Paragon Plus Environment

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In a typical synthesis, 0.75 ml of aniline was added into 125 ml of 1.0 M HCl solution and stirred by a magnetic bar. The metal precursor Co(NO3)2·6H2O was added into the system. When the metal salt was dissolved, 1.25 g of (NH4)2S2O8 was added into the solution to polymerize aniline. The solution was vigorously stirred for 4 hrs at room temperature. 0.1 g of carbon (Ketjen Black EC-300J) was pretreated in 70% nitric acid at 80°C for 8 hrs, then was ultrasonically dispersed in 10 ml of 1.0 M HCl solution, and finally mixed with the polymerized aniline solution. The solution was stirred for 48 hrs and then dried on a hot plate at 90°C with stirring by a magnetic bar in the fume hood. The dried powder was ground by mortar and pestle. The obtained powder was heat treated at 900°C in the argon atmosphere for 1 hr. Then the powder was subsequently pre-leached in 150 ml of 0.5 M H2SO4 at 80-90°C for 8 hrs, and washed with deionized water for several times. After being dried at 100° C in a vacuum oven, the powder was heat-treated again under the same condition for 1 hr to obtain the final product.18 Co9S8 nanoparticles were also synthesized for comparison. 2.49 g of Co(CH3COO)2·4H2O and 0.76 g of thiourea were dissolved in 60 ml of ethylene glycol. The solution was placed in a Teflon-lined stainless steel autoclave and kept at 200°C in an oven for 8 hrs. Then the powder was collected by centrifugation and dried at 60 °C for 6 hrs.27

2.2 Characterization All the characterizations were done in the Materials Characterization and Preparation Facility (MCPF) at the HKUST. Crystal structures of materials were examined by a Philips PW1830 powder X-ray diffractometer equipped with a Cu Kα radiation source (λ=1.5406Å) and a graphite monochromator. The applied voltage and current were 40 kV and 20 mA, respectively. The scanning range of 2θ varied from 10 to 80° at a detector angular speed of 2°θ min-1 and the step size was 0.02°. The obtained spectra were analyzed against the patterns from the International Center for Diffraction Data (ICDD). The morphologies were recorded from scanning electron microscopy (SEM, JEOL 6390) and transmission electron microscopy (TEM, JEM 2010). The elemental composition and chemical state of the samples were measured by X-ray photoelectron spectroscopy (XPS). In this work, a Kratos Axis Ultra DLD multi-technique surface analysis system is used for XPS characterization. 3 ACS Paragon Plus Environment


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2.3 Electrochemical Tests The electrochemical measurements were performed in a three-electrode system controlled by a CH Instruments electrochemical workstation. Typically, 12.5 mg of sample and 20 µL of Nafion solution were dispersed in 5 ml water-isopropanol solution with a volume ratio of 4:1 and sonicated for 0.5 hr to form a homogeneous ink. Then 20 µl of the dispersion was loaded onto a glassy carbon electrode to form a thin catalyst film as the working electrode. An Ag/AgCl electrode was used as the reference electrode and a carbon paper as the counter electrode. All potentials were calibrated to a reversible hydrogen electrode (RHE). Two different electrolytes, 0.5 M H2SO4 and 1.0 M KOH solutions were used for electrocatalytic activity testing. Before activity measurements, the thin film electrode was firstly cycled in an Arsaturated 0.5 M H2SO4 solution between 0.05 and 1.2 V for 10 cycles at 100 mV s-1. Then linear sweep voltammetry (LSV) with a scan rate of 10 mV s-1 was conducted from 0.1 V to negative potentials. We found that the HER curves were the same in Ar- and H2-staturated solutions. To study the HOR catalytic activity, rotating disk electrode (RDE) evaluation was conducted in a H2-saturated 0.5M H2SO4 solution between -0.4 V and +0.9 V with a scan rate of 10 mV s-1 at 1600 rpm (rotation per minute).

2.4 Density Functional Theory (DFT) Calculations In this work, the electrocatalytic active sites and reaction mechanisms of HER were investigated using spin polarized DFT calculations. The CASTEP module of Materials Studio 6.1 version was used to calculate the hydrogen binding energy on different sites.28 The local density approximation (LDA) function was employed. A plane wave basis with an energy cutoff of 340 eV was used and the k-point separation was 0.035 Å-1. The convergence of energy and forces were set to be 1×10-5 eV and 0.03 eV Å-1, respectively. For Co-N and basal graphitic N sites, the supercells were in the lattice where α=β=90°, γ=60°, and a = b = 14 Å, c = 15 Å. While for sites at the edge, the supercells were in the lattice where α=β=90°, γ=60°, and a = 22 Å, b = 12 Å, c = 15 Å. All structures were fully relaxed to the ground state and spin-polarization was considered for geometry optimization. The hydrogen adsorption energy was defined by 4 ACS Paragon Plus Environment

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∆EH=E(H/Co-N-C) - E(Co-N-C) -1/2E(H2) where E(H/Co-N-C) is the total energy for Co-N-C catalysts with a hydrogen atom adsorbed on the surface, E(Co-N-C) is the total energy for Co-N-C catalysts without any hydrogen atom adsorbed on the surface and E(H2) is the energy of a gas phase hydrogen molecule. The Gibbs free energy for hydrogen adsorption was then calculated as29 ∆GH=∆EH+∆EZPE-T∆SH Where ∆EZPE is the zero-point energy difference between the adsorbed state of the system and the gas phase state and Δ SH is the entropy difference between the adsorbed state of the system and the gas phase. Here we have approximated the entropy of hydrogen adsorption as ΔSH ≈ 1/2(SH2),30 where SH2 is the entropy of gas phase H2 at standard conditions. Therefore, the overall correction is ∆GH=∆EH + 0.24eV. For the calculation of activation barrier, the initial and final structures for Volmer and Heyrovsky reactions were relaxed to the ground states and were used as reactants and products, respectively for the transition state (TS) searching. The barriers were obtained by Halgren-Lipscomb method and the value of the barrier from reactant was used as the activation barrier. The quality for TS searching was set to be fine.22, 31

3 RESULTS AND DISCUSSIONS The scanning electron microscopy (SEM) image (Figure 1a) clearly shows a porous structure consisting of carbon aggregates in Co-N-C composites. The formation of graphene sheet-like structure after heat treatment and co-doping with N and Co is demonstrated in a transmission electron microscopy (TEM) image in Figure 1b. Some metal-based particles that were identified to be CoSx by energy-dispersive x-ray spectroscopy (EDS) were also observed in TEM images (Figure 1b and Figure S1). High resolution TEM images show that CoSx particles were partially or fully encapsulated by graphitic layers (Figure S2). The existence of the cobalt sulfides was also confirmed by the x-ray diffraction (XRD) pattern of the Co-N-C composites. The



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sharp XRD peaks in Figure 1c (black line) correspond to the cubic close-packed Co9S8.27 The Co9S8 particles were formed during the first heat treatment (XRD shown in Figure S3a) and did not dissolve during acid washing. The broad peak at ~24° in Figure 1c belongs to the (002) plane of the graphite.32 The compositions of Co-N-C composites were determined to be (at.%): 2.7% Co, 2.9% N, 2.4% S, 5.4% O and 86.6% C by X-ray photoelectron spectroscopy (XPS, Figure S4). The detailed scan in the N1s region is shown in Figure 1d. It can be deconvoluted into different types of N groups. The binding energies at 398.6, 399.6, and 401.1 eV correspond to pyridinic/Co-N, pyrrolic and graphitic N, respectively.33 The binding energies at 402.5 and 404.3 eV are assigned to N oxides.34-35 Due to the small energy difference between the pyridinic and Co-N groups, the further deconvolution could not be conducted.36 Based on the peak intensity, the dominated groups are pyridinic/Co-N and graphitic N. In terms of Co XPS signal (Figure S5), the binding energies in the range of 775-790 eV, and 790-810 eV associate with the 2p3/2 and 2p1/2 levels, respectively.37 The sharp peaks at 778.4 and 793.2 eV originate from Co-S bond,38 consistent with the EDS and XRD data. The peaks at 781.5, 786.8, 797.2 and 802.9 eV mainly correspond to Co oxides and Co-N.39-40 The lack of the characteristic peak of Co3+ around 791 eV suggests that the Co oxides are mainly Co2+.


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Figure 1. (a) SEM and (b) TEM images, (c) XRD patterns (black line: Co-N-C; red line: Ni-N-C) and (d) XPS spectrum of N1s of Co-N-C composites after acid washing and the second heat treatment. The electrochemical behaviors of Co-N-C composites were investigated using a thin film glassy carbon electrode in a conventional three-electrode cell. Before electrochemical measurements, the thin film electrode was firstly cycled in an Arsaturated 0.5 M H2SO4 solution between 0.05 and 1.2 V for 10 cycles at 100 mV s-1. Figure 2a shows a typical cyclic voltammogram (CV) of Co-N-C in an Ar-saturated 0.5 M H2SO4 solution. The CV is featureless unless the potential is higher than 1.05 V where the oxidation current rises sharply. The large oxidation current at higher potential can be assigned to the oxidation of Co9S8 species.41 The HER activity of Co-N-C was evaluated using the linear sweep voltammogram (LSV) by sweeping the potential from 0.1 V to negative potentials at 10 mV s-1. The polarization curves for Co-N-C with different catalyst loadings along with that of pristine carbon black (black line) in a 0.5 M H2SO4 solution after ir-compensation and background correction are 7 ACS Paragon Plus Environment


shown in Figure 2b. As expected, the un-doped carbon black is inert for HER. On the other hand, the Co-N-C shows excellent HER activity with an onset potential of 100 mV at a catalyst loading of 0.255 mg cm-2 (blue line). The overpotential (η) to deliver a current density of 10 mA cm-2 is 275 mV. By increasing the catalyst loading to 0.765 mg cm-2, the overpotential decreases to 235 mV (red line). The amount of the reduction of η (40 mV) is consistent with the predication based on the Tafel slope of 90 mV dec-1 derived from the Tafel plots (Figure S6). The activity of Co-N-C is comparable to the most recent reported non-previous metal and metal free HER catalysts (Table S1). Compared with metal sulfides, selenides, carbides, nitrides and phosphides, one of the advantages of Me-N-C is that the metal weight percentage is much lower leading to a lower cost.

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Figure 2. (a) Cyclic voltammogram of Co-N-C in an Ar-saturated 0.5 M H2SO4 solution, scanning rate = 50 mV s-1. (b) Polarization curves of carbon black, Co9S8, PANI-C, Co-N-C and Ni-N-C in an Ar-saturated 0.5 M H2SO4 solution, scanning rate = 10 mV s-1, low loading = 0.255 mg cm-2 and high loading = 0.765 mg cm-2. (c) Durability testing in an Ar-saturated 0.5 M H2SO4 solution by cycling the electrode between 0.1 and -0.3 V. (d) Comparison of the polarization curves of Co-N-C in 0.5 M H2SO4 and 1 M KOH, catalysts loading = 0.255 mg cm-2. 8 ACS Paragon Plus Environment

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In order to elucidate the role of transition metals in the HER activity improvement, the polyaniline-carbon (PANI-C) composite, i.e., N-doped carbon black without any transition metal in it was prepared and its polarization curve (orange line) is also included in Figure 2b for comparison. Even though the PANI-C showed some improvement over pristine carbon black toward HER, its overpotential is at least 200 mV higher than that of Co-N-C. This result clearly demonstrates the essential role of transition metal in enhancing the HER activity in the Co-N-C composite. The Co9S8 particles may also act as active centers for HER since excellent HER activity have been found in some chalcogens, such as MoS2, WS2, CoS2, etc.42-45 Co9S8 nanoparticles were also synthesized and their HER activity was compared in Figure 2b (purple line). It is clear that the activity of Co9S8 nanoparticles is only better than carbon black and much worse than that of Co-N-C, indicating that they are not the main active sites. This argument was also supported by the HER activity of Ni-N-C composites, which were synthesized using the same method as making Co-N-C. The compositions of Ni-N-C composites after the second heat treatment were determined to be (at.%): 0.3% Ni, 1.7% N, 0.2% S, 0.9% O and 96.9% C by XPS spectra (Figure S7). In the first heat treatment, Ni3S4 compounds were formed according to the XRD pattern (Figure S3b). Unlike Co9S8, Ni3S4 compounds were completely dissolved during acid washing as reflected by the disappearance of associated XRD peaks (Figure 1c, red line). The much lower metal (0.3 at.% vs 2.7 %) and S (0.2 at.% vs 2.4%) loadings in Ni-N-C than those of Co-N-C also implies that Ni-S compounds in the former are negligible. Interestingly, the overpotential at 10 mA cm-2 for Ni-N-C (green line in Figure 2b) is only 250 mV, which is even 25 mV lower than that of Co-N-C with the same catalyst loading (0.255 mg cm-2). The higher HER activity of Ni-N-C with the absence of metal sulfides together with the extremely low activity of Co9S8 particles suggests that Co9S8 is not the main active species in Co-N-C. Durability is another critical measurement in the evaluation of non-precious metal electrocatalysts for HER. Accelerated degradation testing protocol by cycling the catalyst between 0.1 and -0.3 V was adopted to evaluate the durability of Co-N-C in a 0.5 M H2SO4 solution. Polarization curves were recorded after every 500 cycles. As shown in Figure 2c, the overpotential at 10 mA cm-2 increased by 18 mV after the first



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500 cycles (blue line). The additional 500 cycles (1000 cycles in total) did not cause any noticeable performance decay (red line). It has been well documented that HER activities of precious metal catalysts, such as Pt, Pd, Ir and Au are strongly dependent on pH values.46 For instance, the HER activity of Pt at pH=13 is two orders of magnitude lower than that at pH=1.47 One of the main reasons for the low activity in alkaline solutions is due to a higher energy barrier needed to generate H+ during the first step that involves the H2O dissolution at high pH values than the H3O+ dissolution at low pH values.44 For some non-precious metal based catalysts, including MoS2, Ni2P, metal organic frameworks (MOFs) derived N-C encapsulated transition metals, their HER activities in alkaline solutions are also much lower than that in acidic media.23, 48 For instance, the overpotential at 1 mA cm-2 of non-noble metal catalyst from MOFs was more than 200 mV higher at pH=13 than that at pH=1.23 The very low activity in high pH electrolytes limits the application of these materials. The polarization curves of the same Co-N-C sample in 0.5 M H2SO4 and 1 M KOH solutions are compared in Figure 2d. The HER activity of Co-N-C in the alkaline solution is only slightly lower than that in acidic solution, with an increase of overpotential by 35 mV at 10 mA cm-2. The difference in catalytic activity between acidic and alkaline solutions toward HER is one of the smallest among reported values. More impressively, the HER activity does not show any decay after 1000 potential cycles in alkaline solution (Figure S8). Despite extensive studies as promising electrocatalysts for ORR, the active sites in Me-N-C composites haven’t yet been agreed.49-50 Multiple N sites with or without coordination with transition metals have been proposed as active centers. To gain further insights into possible active sites and reaction mechanisms of HER on Me-NC composites, DFT calculations were carried out using Co-N-C as a model. We firstly calculated the hydrogen adsorption energy (∆EH) at various N sites without coordination with Co, including pyridinic, pyrrolic, authentic pyrrole, basal and terminal graphitic N (Figure 3a and Figure S9). The respective ∆EH are 1.30, 1.55, 1.77, 1.40 and -0.61 eV. These results suggest that only the terminal graphitic N could be a possible active site since the ∆EH values are too weak for other N sites. A careful study reveals that the H atom actually adsorbs on the C atom coordinated with the terminal graphitic N atom (Figure S10a). The ∆EH at the Co-N site was also examined. It was found that H was not stable on N atom, and automatically moved to the metal center (Figure S10b). The ∆EH at the Co-N was calculated to be -0.52 eV, suggesting 10 ACS Paragon Plus Environment

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that Co-N could also be a possible active center. Gibbs free energy (∆GH) has been proposed as the descriptor for HER activity in many studies.30, 51 The ∆GH should be close to zero for a good HER catalyst, for instance, Pt.6 We converted ∆EH to ∆GH by adding zero point energy and entropic corrections.52-53 The ∆GH values for HER on various sites are shown in Figure 3b. Here the HER was simply represented in a threestate diagram consisting of an initial state (H+), an intermediate state (Had), and the final product (1/2H2). Clearly, only the ∆GH for Co-N (-0.28 eV) and terminal graphitic N (-0.35 eV) are close to zero. Based on extensive studies on noble metal surfaces,4, 54 the first step of HER in an acidic solution is the Volmer reaction, in which a proton receives an electron and generates an adsorbed hydrogen atom (Had) at the active site as an intermediate: H+ + e- → Had. The second step can be the Heyrovsky reaction: Had + H+ + e- → H2 or the Tafel reaction: Had + Had → H2.55 In our Me-N-C systems, however, the Tafel reaction pathway seems unlikely since the active sites on the carbon surface are not continuous due to their low density. In other words, it is unlikely to find two nearby adsorbed Had intermediates to recombine into a H2 molecule. Thus, the possible reaction mechanism of HER on Me-N-C could be Volmer + Heyrovsky pathway. In order to further understand the reaction mechanism, the reaction barriers for both Volmer and Heyrovsky steps at Co-N and terminal graphitic N sites were calculated and shown in Figure 3c. For the Co-N site, the activation barrier for the Volmer step is negligible, suggesting that the adsorption of H+ is a spontaneous reaction at the Co-N site. While the activation barrier at the terminal graphitic N is 0.24 eV. For the Heyrovsky step, the activation barriers are 0.93 and 1.71 eV at the Co-N and terminal graphitic N sites, respectively. The much higher activation barrier for the Heyrovsky step at the graphitic N site implies that the transition metal should be involved in the true active site in Co-N-C composites. The DFT calculation results are consistent with the electrochemical data in Figure 2b, where PANI-C shows a much lower catalytic activity than Co-N-C. Based on the electrochemical and theoretical data, one may conclude that the Me-N site may be the main active center. In order to understand the intrinsic activity of the Me-N site, the turnover frequency (TOF) was calculated. Since most of the Co atoms in Co-N-C are in the form of Co9S8, we used the metal loading of Ni (0.3 at.%) to calculate the TOF of Ni-N-C assuming all Ni atoms are at the active centers. The TOF value at 10 mA cm-2 (overpotential = 250 mV) is 0.77 [H2



molecule s-1 Ni-1], which is comparable to those of non-previous metal and metal free catalysts for HER reported recently (Table S2). Another interesting observation is that, unlike noble metals, on which HER and hydrogen oxidation reaction (HOR) are highly reversible, Me-N-C catalysts do not show any HOR activity with an overpotential up to 0.9 V (Figure S11). This phenomenon deserves a further theoretical study.

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Figure 3. (a) The illustration of various N sites on a graphene sheet, (b) calculated Gibbs free energies (∆GH) in a three-state diagram, and (c) reaction barriers for both Volmer and Heyrovsky steps at Co-N and terminal graphitic N sites.

4. CONCLUSION In conclusion, for the first time, we demonstrated that the highly active ORR electrocatalysts Me-N-C composites derived from heat treatment of carbon black, nitrogen and transition metal precursors also showed excellent HER activity in both acidic and alkaline media. The active sites for HER might involve both transition metal and nitrogen atoms based on our experimental and DFT calculation results. Specifically, the active sites could be related to Me/N/C moieties, on which a 12 ACS Paragon Plus Environment

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reasonably low activation barrier (0.93 eV) for the Heyrovsky step was observed. The high activity together with excellent durability upon potential cycling makes this type of catalysts worth further study. The next step is to explore approaches to increasing the density of active sites in Me-N-C composites.

ASSOCIATED CONTENT Supporting Information Additional TEM images, XRD patterns, XPS spectra, Tafel plots, durability test result, unit cells for DFT calculations and electrochemical activity for HOR. A brief survey of HER catalytic activities and TOFs of non-precious and metal free catalysts reported in the literature. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Tel: +852-34692269. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to acknowledge the start-up fund from the Hong Kong University of Science and Technology. REFERENCES (1) Panwar, N.; Kaushik, S.; Kothari, S., Role of Renewable Energy Sources in Environmental Protection: A Review. Renew. Sustainable Energy Rev. 2011, 15, 1513-1524. (2) Zou, X.; Zhang, Y., Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180.



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TOC Graphic

2H+ + 2e- → H2


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The Role of Transition Metal and Nitrogen in Metal ... - ACS Publications

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