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Enhancement of Hydrogen Evolution Reaction Performance of Graphitic Carbon Nitride with Nickel Boride Incorporated Meng Cao, Xinyu Zhang, Jiaqian Qin, and Riping Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02994 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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ACS Sustainable Chemistry & Engineering

Enhancement of Hydrogen Evolution Reaction Performance of Graphitic Carbon Nitride with Nickel Boride Incorporated

Meng Cao†, Xinyu Zhang*,†, Jiaqian Qin *,‡,§, and Riping Liu†

†

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, P. R. China

‡

Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330, Thailand

§

Research Unit of Advanced Materials for Energy Storage, Chulalongkorn University, Bangkok, Thailand

*Corresponding Author. Fax: +66 2611 7586 E-mail: [email protected] (J. Qin), [email protected] (X. Zhang)

ABSTRACT: Graphitic carbon nitride (g-C3N4) working as a semiconductor catalyst sustainable and clean energy carrier to substitute fossil energy is drawing plenty researches. Hydrogen is regarded as the right energy carrier to meet the future conditions, hence developing hydrogen economy are intensively investigated for its great electrocatalytic property. However it’s blocked as an electrocatalyst due to the poor conductivity and electrochemical activity. Therefore to conquer such shortcomings by changing the nanostructure and increasing active site are studied.

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Here, we report a facile method by incorporation of nickel boride (Ni2B) nanoparticles into bulk g-C3N4 to enlarge the specific surface area, exfoliate the layers, and improve the electronic conductivity. The obtained catalysts exhibit better hydrogen evolution reaction (HER) performance with a low onset overpotential of 300 mV, a Tafel slope of 221 mV decade-1 and overpotential of 707 mV at the current density of 10 mA cm-2, superior to original g-C3N4 which demonstrates that Ni2B-decorated g-C3N4 is an illuminating method to enhance the HER performance. This work also provides a reference for g-C3N4 based catalyst working in broader field. KEYWORDS: Electrocatalysts; g-C3N4; Ni2B; HER

INTRODUCTION Environmental issues are attracting intensive attention in recent days. The researchers are finding more and more alleviative to fossil energy in industrial production to ease the concerns of environmental problems such as air pollution and fossil energy depletion1-4. Electrolysis and photo-electrolysis are two significant methods to produce fuel-cell grade H2. Efficient electrolysis for hydrogen evolution reaction needs stable and reliable electrocatalysts which can efficiently oxidize H2O molecule by reducing the energy barrier and enhancing the reaction kinetic. Recently Pt-based catalysts exhibit the best performance in HER from water splitting and gain numerous researches. However, the sparse amount and luxurious price limit its application in Large-scale commercialization5-7. So it’s really significant to study on some


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earth-abundant non-noble transition metals and its composites to replace Pt-based catalysts, such as transition metals Fe, Ni, Co, Mo, Cu, phosphide and sulfide8-13. On the other hand, carbon related catalysts have also been experimentally and theoretically proved to efficiently electrocatalysts14-17 and photocatalysts18-21. Graphitic carbon nitride (g-C3N4) with layered graphite-like structure is intensively noticed in catalyst field such as water splitting，visible light photocatalysis for hydrogen evolution and CO2 reduction owing to its unique electronic structure, excellent chemical and thermal stability22-27. Traditional g-C3N4 catalyst is restricted in electrocatalytic reaction because of its low conductivity and low specific surface area28-31. Several improved methods have been reported to solve these shortcomings, such as hydrothermal carbonization17, quantum dots engineering16 , co-monomer control19, nanoparticles encapsulation14 and organic-metal hybrid32. Nevertheless incorporation is an facile and efficient way to conquer such shortcomings by modifying the surface morphologies and electronic structures of materials ， there are several selected elements has been studied, showing better performance than the intrinsic g-C3N433. The boron atom that introduced in tri-s-triazine can enhanced the ionic conductivity since hexagonal boron nitride (BN) and ternary B-C-N systems were formed and electronic structure were tuned34,with an appropriate structure makes the material highly active in electrocatalysis21. As known, Ni and its composites drew increasing attentions in electrochemistry because it’s steady for HER in alkaline solutions35-36. Meanwhile higher activity and better selectivity was obtained among Ni-B amorphous



catalysts, showing the promoting effect of the alloying B on the hydrogenating reactions, the presence of Ni causes B enrichment on the surface and also provides excess electrons to active sites. Therefore, modifying the microstructure to study the correlations of the structure engineering and the chemical properties is greatly important and extremely challengeable to explore a robust C3N4 based electrocatalysts. Furthermore, the modifying g-C3N4 with nickel boride not only increases the conductivity of g-C3N4 but also improves the stability of electrocatalyst in alkaline solutions. Herein, we report a facile method for preparation of Ni2B modified g-C3N4 electrocatalysts. The Ni2B nanoparticles can serve as a spacer and conductive material to enhance the surface area, reduce the thickness of layers, and further improve their electronic conductivity. Meanwhile, the synergic interaction between active catalyst and Ni2B helps to reduce the adsorption energies of some catalytic species and decreases the required overpotential for the electrocatalysis, enhancing their catalytic activity. As expected, Ni2B modified g-C3N4 exhibits excellent HER activity in alkaline solution. The superior HER performance with an onset potential of 396 mV and a Tafel slope of 221 mV dec-1 is better than that of pure g-C3N4, indicating a low-cost and high-performance electrocatalyst for water splitting.

EXPERIMENTAL SECTION Preparation of g-C3N4. g-C3N4 was prepared by directly heating the nitrogen rich urea in a semi-closed system. In a typical preparation, 10 g urea was heated


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(heating speed 10 ºC min-1) in the alumina crucible with covering lid at 500 ºC for 2 h under air atmosphere. Preparation of Ni2B and Ni2B decorated g-C3N4. Ni2B nanoparticles was synthesized by a redox reaction, that NiCl2 acted as oxidant and Ni source while NaBH4 acted as reducing agent and boron source. Specifically NiCl2 powder was dissolved evenly in deionized water, three times in molar ratio of NaBH4 was added in the NiCl2 solution and an intensely redox reaction took place that turned the transparent aqueous solution into black with severe effervescence. Collecting the remnant black powder precipitation after the effervescence faded. Ultrasonic treatment of the black powder in ethanol and deionized water was utilized for cleaning. The black Ni2B powder and g-C3N4 was mixed in the agate mortar then stirred with ethanol for 2h. The mixture was annealed at 500 ºC for 2h with a heating rate of 10 ºC/min in a muffle furnace under atmosphere. Five different Ni2B contents (0%, 0.03wt%, 0.05wt%,0.07wt%, 0.1wt%) of catalysts were prepared,

and named as

CN-0, NC-3, NC-5, NC-7, and NC-10, respectively. Characterization. Structural characterization of all catalysts were detected by X-ray diffraction (XRD) conducted on a Rigaku D/MAX-Rb operated at 40 kV and 100 mA with Cu Kα radiation, with a scanning rate of 1°/min and scanning rage from 10° to 80°. X-ray photoelectron spectroscopy (XPS) was operated at 150 W with monochromatic Al Kα (1486.6 eV) radiation with an ESCALAB 250. C 1s peak at 284.6eV was the reference to analysis the binding energies of all elements. Scanning



electron microscopy (SEM, Hitachi, S4800) was performed to study the surface morphologies. Transmission electron microscopy (TEM) was used with operating voltage of 120 kV on a JEOL-ARM200F to determine the size of the catalyst layers. Surface areas and pore distribution of samples were determined by ASAP 2020 using the Brunauer−Emmett−Teller (BET) method. Atomic force microscope (AFM) was engaged to detected the thickness of graphitic nanosheets by multimode 8 at a scan rage of 10µm×10µm×2.5µm. FT-IR was recorded on E55+FRA106 with a wavelength range of 500-4000cm-1 and the samples were prepared with KBr powder. Electrochemical measurement. All the HER electrochemical tests were assessed on a typical three-electrode cell employing an electrochemical station (CHI 660E, Shanghai Chenhua Instruments). A saturated silver/silver chloride electrode (Ag/AgCl, saturated KCl) and Pt foil (2 × 2cm2) were used as the reference electrode and counter electrode respectively. Working electrode was prepared on a polished 3mm-diameter glassy carbon electrode. Specifically, 2 mg of catalyst together with 20 µl of 5wt% nafion were ultrasonic dispersed in 0.4 ml ethanol for 30 min till the homogeneous yellow suspension was formed. 2 µl of the suspension was deposited onto the 3 mm-diameter glassy carbon electrode which was polished with particle size of 0.3 µm and 0.05 µm alumina powder in advance and left it dried in air. In this work, all potentials were calibrated to the reversible hydrogen electrode (RHE). The potentials were corrected with iR compensation. All the tests are measured in 1M KOH. Linear sweep voltammetry (LSV) curves were recorded at a scan rate of 2 mV/s, and Electrochemical impedance spectroscopy (EIS) was performed with a


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frequency from 100 kHz to 1 Hz under overpotential=200mV with an amplitude of 5 mV. Reusability behavior of the catalyst was tested by conducting cyclic voltammetry sweep for 1000 cycles in the range between −0.3 V and 0.1 V (vs Ag/AgCl, saturated KCl) with a scan rate of 100 mV/s. 20 cycles of cyclic voltammetry scans with a range from -0.3 V to 0.1 V(vs Ag/AgCl, saturated KCl) and scan rate at 0.1 V/s was taken place before every catalyst’s electrochemical measurements to ensure the electrocatalyst activity.

RESULTS AND DISCUSSION In this work, a facile way was developed to synthesis the g-C3N4 (Fig. 1a). Five different Ni2B contents (0%, 0.03wt%, 0.05wt%,0.07wt%, 0.1wt%) modified g-C3N4 catalysts were prepared by thermal treatment at 500 ºC for 2 h, and named as CN-0, NC-3, NC-5, NC-7 and NC-10, respectively. SEM and TEM images (Fig. S1 and Fig. S2) exhibit the granular morphology of Ni2B precursor, the grain size of Ni2B particles is about 10-20 nm. The obtained results are in conformity with the previous reports10, 37. The elemental mappings (Fig. S3) revealed the coexistent and homogenerous distribution of Ni and B elements on g-C3N4 sheets. TEM imageines in Fig. 1 show the nanoplate structures of CN-0 and NC-5, CN-0(Fig. 1b and 1c) displayed a board and flat graphitic carbon nitride sheets and some porous structure meanwhile NC-5(Fig. 1d and 1e) exhibit a lamellar structure stacked by winkle and curved smaller layers with pores. The SEM imagines of CN-0, NC-3, NC-5 and NC-7 samples exhibit the general assembling of nanoplates.



The Fig. 2a and 2b are corresponding to CN-0, the g-C3N4 exhibits the bulk morphology, there are smooth and flat g-C3N4 layers stacked on the surface. When the Ni2B content is 0.03 wt.%, the addition of Ni2B can strip the sheets of bulk g-C3N4 and the separate g-C3N4 sheets are curved with relatively larger surface area (Fig, 2c and 2d). Fig. 2e and 2f show the morphology of NC-5, the increasing of Ni2B can further peel off and separate the g-C3N4 sheets, and the area of the sheets are also reduced significantly. The SEM images of NC-7 are shown in Fig. 2g and 2h, the g-C3N4 sheets are cracked and winkled like NC-5 but thicker. The BET results (Fig. 3a) indicate that the NC-5 exhibits the larger specific area (154 m2 g-1) and pore volume (0.7 cm3 g-1) than those of CN-0 (42 m2 g-1 and 0.14 cm3 g-1), which could be caused by the smaller, porous and wrinkled morphology of NC-5 samples due to the incorporation of Ni2B and thermal exfoliation. AFM measurements (Fig. S4) show that the average thickness of NC-5 is much thinner (2.68 nm) than that of CN-0 samples (5.35 nm). The SEM, TEM images and AFM measurements reveal that the incorporation of Ni2B and thermal exfoliation treatment can decrease the size and thickness of the graphitic carbon nitride sheets. Moreover, the cracks on the g-C3N4 sheet might be induced by the boron atoms doped into aromatic CN rings, resulting in the structure changed of tri-s-triazine. The porous and thin lamellar structures could provide more active sites for reactant and area to contact electrolyte meanwhile adsorb more H* and OH-, thereby the electrical conductivity would be improved. The crystal structure of the Ni2B was detected by XRD and shown in Fig. S5a. A peak around 2θ=45º indicating the amorphous character of Ni2B powders. The XRD


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patterns of Ni2B modified g-C3N4 composites were also measured (Fig. S5b). All samples represent the typical diffraction peak at 27.4º and a weaker peak around 13º, which means the incorporation of Ni2B doesn’t change the graphitic in-plane structure. The intensive peaks at 27.4º corresponding to the (002) planes and attribute to the reflection of conjugated aromatic systems stacking between interlayers and the weaker peak at 13.1º corresponding to (100) planes reflects repeat packing of continuous heptazine network38. There is no obvious peak related to Ni2B because of the little amount for detected. The surface chemistry and elemental constituent characters were analyzed by XPS measurement. The Ni2B can be detected by XPS (Fig. S6) and indicated that the electron enriched metallic Ni is a good active site for HER. Fig. 3b shows the survey scan of NC-5 composite. The results identify the existence of certain elements such as C, N, O, B and Ni. Furthermore, Fig. 3c-f show the high-resolution spectra of C 1s, N 1s, B 1s and Ni 2p. The strong peak at 284.6 eV in Fig. 3c is attributed to C-C coordination of standard reference carbon, the peaks at 285.1 eV and 288.3 eV correspond to C-(N)3 group and C-N-C, respectively39-40. The strong peak appeared in high-resolution imagine of N 1s as shown in Fig. 3d presents the N-replaced aromatic rings (C-N=C), and peaks ascribed to tertiary nitrogen groups (N-(C)3) and charging effects can be found at 401.2 and 404.3 eV, respectively41-42. The peak at 399.9 eV (C-BN2 groups) demonstrates that some boron atoms are brought into g-C3N4 frame43-44. As shown in Fig. 3e, B1s can be deconvoluted into three peaks with binding energy at 190.1 eV, 191.8 eV and 192.8 eV. The binding energy at 190.1 eV is



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corresponded to h-BN groups45, The binding energy at 191.8 eV peak could be assigned to the formation of C-BN groups which is consistent with a reported BCN compounds with a binding energy at 192 eV46. The B 1s peak at 192.8 eV presents the oxidized form of the B chemical state47. As shown in Fig. 3f, the Ni 2p spectrum contains Ni 2p3/2 peak at binding energy of 856.3 eV and Ni 2p1/2 peak at binding energy at 874 eV and their corresponding shake-up peaks. The peaks with binding energy at 861.6 eV and 867.1 eV are attributed to the oxidized composite and metallic nickel, respectively9, 12. Nickel boron compound and nickel oxide was formed and incorporated into the layers of g-C3N4. All the samples were further characterized by FT-IR spectroscopy (Fig. 4). The results display the typical vibration bonds futures of g-C3N4 indicating their comparable arrays formed regardless of different incorporation of Ni2B. The strong peaks between 1200-1700 cm-1 are assigned to the characteristic stretching mode of aromatic CN heterocycles, the peak at 808 cm-1 is related to characteristic breathing of triazine units, the broad peaks range from 3290-3500 cm-1 can be assigned to the stretching

modes

of

amines

and

their

inter-molecular

hydrogen-bonding

interactions47-49. The HER activities of Ni2B decorated g-C3N4 samples were investigated in 1 M KOH solution as electrolyte using a three-electrode setup with a certain amount of sample loading on glassy carbon electrode. All polarization curves were recorded with iR compensation to eliminate the effect of ohmic resistance. Prior to all electrochemical measurements, the catalysts were activated by 20 cyclic voltammetry


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scans. Polarization curves with a scan rate of 2 mV s-1 were record. Samples loaded on glassy carbon electrode, Ag/AgCl, and Pt electrode were applied as working electrode, reference electrode, and counter electrode, respectively. The raw glassy carbon electrode and Pt/C commercial electrocatalyst were also performed for comparison. As shown in Fig. 5a, the glassy carbon electrode exhibites weak electrocatalytic activity for HER while the Pt/C commercial electrocatalyst shows the best HER activity in alkaline solution as expected. The onset overpotential of Pt/C working electrode is 4 mV and overpotential at cathodic current densities of 10 mA cm-2 is 56 mV. The CN-0 sample was also detected as comparison and poor HER activity can be obtained, the onset overpotential is 686 mV and overpotential at cathodic current densities of 10 mA cm-2 is 1056 mV. However, the Ni2B modified g-C3N4 samples show the better HER activity, NC-5 emerged the best HER activity, which the overpotential decreases to 707 mV and 773 mV at cathodic current densities of 10 mA cm-2 and 20 mA cm-2 respectively while the onset overpotential decreases to 396 mV, about 30% decreasing on HER overpotential at 10 mA cm-2 current density can be achieved compare to that of pure g-C3N4 although the bad conductivity. The electrochemical properties of Ni2B particles and the NC-5 etched by acid (NC-5-AE) were also examined (Fig. S7(a)). The onset overpotential of Ni2B is 335 mV and the overpotential to drive cathodic current density of 10 mA cm-2 reaches 459 mV, meanwhile the onset overpotential of NC-5-AE working electrode is 640 mV and overpotential at cathodic current density of 10 mA cm-2 is 875 mV. The Tafel slope (Fig. S7(b)) of NC-5-AE (~237 mV dec-1, and 136 mV dec-1 for the Ni2B) is



much lower than pure g-C3N4 samples (372 mV dec-1), it reveals that the incorporation of Ni2B particles on g-C3N4 layers can really enhance the electrocatalyst performance. The slope of Tafel plot (Fig. 5b) can also imply the excellent improvement of NC-5 catalyst. The Tafel slope of NC-5 (~221 mV dec-1, and 36 mV dec-1 for the Pt/C working electrode) is much lower than pure g-C3N4 samples (372 mV dec-1). Tafel slope can give additional insight into HER and reveal multi-step electrochemical process occurring on the surface of an electrode. Other Ni2B incorporation g-C3N4 samples also display the improved HER activity and show in Table S1. Moreover, the obtained Ni2B modified g-C3N4 catalysts also exhibit better HER oerpotential than those of transition metals-modified g-C3N4 and Ni-NiO-modified g-C3N4 (Table S2), but it is still lower than some reported results (Table S2), the development of good g-C3N4-based electrocatalysts are still big challenge. A long-term stability over 1000 cycles was also examined. As shown in Fig. 5c, the potential has slightly increased indicates that the durability is good. Electrochemical impedance spectroscopy at overpotential of 200 mV (Fig. 5d) was also examined. The lower Faradaic resistances for NC-5 sample demonstrate that the enhanced electrocatalytic performance due to the better electrical conductivity and the chemical interaction between g-C3N4 and Ni2B. The proposed electrocatalystic process for HER on the Ni2B modified g-C3N4 layers is illustrated in Fig. 6. The Ni2B attached on g-C3N4 layers, the electron enriched metallic Ni can be the good active site to break the H-O-H bond by


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electrochemical reduction of H2O into adsorbed OH– and H*. The slits and pores in g-C3N4 layers provide the place for adsorbing OH- and H*. The generated H* combines with H2O then receive electrons from the active sites on the catalysts surface to form H2. The adsorbed H* can also form H2 on the g-C3N4 surface.

CONCLUSIONS In summary, the modified structure samples with thinner layers and larger specific surface areas were synthesized in a facile method by incorporation of Ni2B into g-C3N4. Great improvements could be achieved for HER in alkaline solution with less catalyst loading on electrode compared to the other kinds of electrocatalysts, the onset overpotential decreases obviously compared to the unmodified g-C3N4 electrode. This current work demonstrates that the potential of g-C3N4 industrialized and large scalable can be applied as electrocatalyst and other field such as supercapacitor, photocatalysts, batteries and fuel cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures S1-S7, and Table S1-S2



AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research is financially supported by the Thailand Research Fund (RSA6080017). We also gratefully acknowledge the NSFC (grant 51421091), and National Science Foundation for Distinguished Young Scholars for Hebei Province of China (grant E2016203376).

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Figure Legends Fig. 1. (a) Flow chart of the Ni2B doped g-C3N4 electrocatalyst synthesis process, (b, c) TEM imagines for CN-0, (d, e) TEM imagines for CN-5, Fig. 2. SEM imagines for nickel boride incorporated graphitic carbon nitride samples, (a, b) for CN-0, (c, d) for CN-3, (e, f) for CN-5, (g, h) for CN-7. Fig. 3. (a) N2 adsorption–desorption isotherms of CN-0 and NC-5, (b) XPS survey scan spectrum of NC-5, (c-f) XPS high-resolution spectra of C 1s, N 1s, B 1s and Ni2p respectively. Fig. 4. FT-IR spectra of g-C3N4 (CN-0), and the Ni2B modified g-C3N4 (CN-3, CN-5,



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CN-7, and CN-10). Fig. 5. (a) LSV curves of the samples in 1M KOH, (b) Corresponding Tafel plots fitting by the polarization curves in 4a, (c) LSV curves for the NC-5 before and after 1000 CV sweeps under 1M KOH solution, (d) Electrochemical impedance spectroscopy

data

for

CN-0

and

NC-5

which

were

collected

under

overpotential=200mV. Fig. 6. The possible HER process take place on the surface of Ni2B modified g-C3N4 electrocatalyst.

For Table of Contents Use Only Ultrasmall Ni2B nanoparticles modified g-C3N4 catalysts were synthesized and exhibit good catalytic performances and long-term stabilities toward HER in alkaline solution.


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Fig. 1. (a) Flow chart of the Ni2B doped g-C3N4 electrocatalyst synthesis process, (b, c) TEM imagines for CN-0, (d, e) TEM imagines for CN-5. 199x260mm (300 x 300 DPI)



Fig. 2. SEM imagines for nickel boride incorporated graphitic carbon nitride samples, (a, b) for CN-0, (c, d) for CN-3, (e, f) for CN-5, (g, h) for CN-7. 200x69mm (256 x 256 DPI)


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Fig. 3. (a) N2 adsorption–desorption isotherms of CN-0 and NC-5, (b) XPS survey scan spectrum of NC-5, (c-f) XPS high-resolution spectra of C 1s, N 1s, B 1s and Ni2p respectively. 199x228mm (300 x 300 DPI)



Fig. 4. FT-IR spectra of g-C3N4 (CN-0), and the Ni2B modified g-C3N4 (CN-3, CN-5, CN-7, and CN-10). 199x154mm (300 x 300 DPI)


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Fig. 5. (a) LSV curves of the samples in 1M KOH, (b) Corresponding Tafel plots fitting by the polarization curves in 4a, (c) LSV curves for the NC-5 before and after 1000 CV sweeps under 1M KOH solution, (d) Electrochemical impedance spectroscopy data for CN-0 and NC-5 which were collected under overpotential=200mV. 199x140mm (300 x 300 DPI)



Fig. 6. The possible HER process take place on the surface of Ni2B modified g-C3N4 electrocatalyst. 199x99mm (300 x 300 DPI)


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Enhancement of Hydrogen Evolution Reaction Performance of

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