C3N4 Composite Coating ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Coral-like-Structured Ni/C3N4 Composite Coating: An Active Electrocatalyst for Hydrogen Evolution Reaction in Alkaline Solution Lixin Wang,†,‡ Yao Li,‡ Xucai Yin,‡ Yazhou Wang,‡ Ailing Song,‡ Zhipeng Ma,‡ Xiujuan Qin,*,†,‡ and Guangjie Shao*,†,‡ †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

‡

S Supporting Information *

ABSTRACT: Exploiting high-efficiency catalysts toward hydrogen evolution reaction (HER) is a significant assignment nowadays. We find a quick and straightforward means to produce large-scale g-C3N4, which does not use template and easily obtains uniform nanostructures. And, we fabricate onestep preparation of a non-noble-metal catalyst, consisting of carbon material and transition metal only, by coupling graphitic carbon nitride (g-C3N4) with Ni. The results show that Ni/C3N4 composite catalyst possesses coral-like structure and its unique morphology is in favor of electrochemical activity for HER. Simultaneously, the Ni/C3N4 composite catalyst presented prominent activity on HER with a high exchange current density of 1.91 × 10−4 A cm−2, a low Tafel slope of 128 mV dec−1 and small overpotentials of 356 and 222 mV to reach current densities of 100 and 10 mA cm−2, which are superior to those of the state-of-the-art HER-active Ni-based compositions, as well as majority other metal-free catalysts, and even rivaled the electrocatalytic property of commercial Pt/C catalyst. KEYWORDS: Graphitic carbon nitride, Nanostructure, Non-noble-metal catalyst, Supergravity electrodeposition, Hydrogen evolution reaction

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INTRODUCTION Environment pollution and energy shortage have become thorny problems in the modern times.1 Hydrogen is deemed as the most valuable energy replaced fossil energy because of its sustainability and environment friendly property. Electrochemical water splitting generates hydrogen by only consuming electric energy, has become the main trend for hydrogenenergy production.2−9 It is generally believed that precious metals (e.g., Pt, Pd, Rh) have good performance in terms of hydrogen evolution, however, the expensive and scarce supply largely limits their industrialized application. Therefore, it is imminent to find an efficient catalyst for hydrogen evolution. Ni-based materials (Ni-based alloys and Ni-based composites) are one of the most promising catalysts for HER, such as NiCo,10 NiW,11 NiMo,12 NiP,13 NiS,14 NiCoFe,15 NiCoSe,16 NiFeS,17 NiMnS,18 NiMoN,19 Ni-MnO2,20 Ni-CeO2,21 NiMoS2,22 etc. Recently, the studies about carbon material catalysts on electrolysis of water for hydrogen production have obtain a lot of attention, yet the pristine carbon material catalysts (biomass carbons, CNTs, graphene, carbon spheres, etc.) present poor catalytic activity.23−26 Therefore, modulating the transition metal and carbon material can surely promote the electrochemical activity, such as Ni-CNT, 27−29 Ni-graphene,30,31 and [email protected] McArthur et al.27 prepared an © 2017 American Chemical Society

effective Ni nanoparticles (NP) electrocatalyst supported on the 3D MWCNT matrix. This NP/MWCNT cathode offers a prominent increase in electrocatalytic performance in the HER relative to bulk Ni. The addition of MWCNT is conducive to charge transfer and prevent Ni particles from conglomeration. Wang et al.30 have reported a cathode catalyst with one-step approach to coelectrodeposit Ni nanoparticles and reduced graphene oxide (rGO) sheets on a three-dimensional Ni foam. The results presented that the Ni nanoparticles can be refined by introducing rGO sheets, and rGO sheets have high conductivity so that facilitating electrons transport among the composite coating. Deng et al.32 have designed a core−shell and hierarchical structure that composes of ultrathin graphene shells which encapsulate a CoNi nanoparticle to improve electrocatalytic activity on HER. The results indicate that the outstanding HER behavior due to modulating the electronic potential and the electron density distribution at the surface of graphene sheet. To date, the carbon material of graphitic carbon nitride (gC3N4) also has attracted numerous researchers33−35 to explore Received: May 18, 2017 Revised: July 23, 2017 Published: August 14, 2017 7993

DOI: 10.1021/acssuschemeng.7b01576 ACS Sustainable Chem. Eng. 2017, 5, 7993−8003

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ACS Sustainable Chemistry & Engineering

the power supply frequency of 42 Hz, and the volume ratio of water to g-C3N4 powder is 10:3. The obtained nanostructured g-C3N4 material was named as C3N4-S. Preparation of Ni/C3N4 Composite Catalysts. In this research article, the Ni/C3N4 composite cathodes were prepared by supergravity electrodeposition, which has been reported in detail in our published paper.43 First, the pure Ni was used as anode, and a Cu substrate was cleaned sequentially in C2H5OH, NaOH solution (20%), and HCl solution (10%) for 20 min to use as cathode. The Ni/C3N4 catalysts were electrodeposited under supergravity field in a green plating solution which contained 30 g L−1 NH4Cl, 10 g L−1 NiCl2· 6H2O, and 350 g L−1 Ni(NH2SO3)2·6H2O with 0, 0.05, 0.10, 0.15, and 0.20 g L−1 C3N4−S powder, respectively. The pH value of the plating solution is 3.5−3.8. The above five catalysts were respectively named as pure Ni, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/ C3N4-0.20. All composite electrodes were performed under the strength of the supergravity with G = 350 g (“g” stands for gravity acceleration) at a current density of 3 A dm−2 for 60 min at 318 K. Afterward the Cu substrate with Ni/C3N4 hybrid was washed with deionized water to remove physical adsorption residua, and then dried at 80 °C for 1 h. As a contrast, the Ni/C3N4-B electrode was prepared under the same experimental conditions with 0.10 g L−1 C3N4-B in plating solution. In addition, the pure C3N4-S electrode was constructed by dispersing 30 mg C3N4-S powder into 10 mL C2H5OH. The mixture was sonicated for 2 h and dropped on 1 cm2 Cu plate, then dried at 80 °C for 1 h. Characterization. The morphologies of as-prepared samples were investigated by transmission electron microscopy (TEM, JEM 2100F) operated at 100 kV, atomic force microscopy (AFM, Bruker Multimode 8), scanning electron microscopy (SEM, Zeiss Supra 55) with an accelerating voltage of 20 kV, and chemical component analysis of the composite coatings was carried out using an energy dispersive spectrometer (EDS) attached to the SEM. The microstructures of as-prepared samples were researched by X-ray diffraction (XRD, RigakuD/MAX 2500/PC) with Cu Kα radiation (λ = 1.5418 Å), Fourier transform infrared spectroscopy (FTIR, Is10), and Raman measurements (Horiba JY Xplora Plus) with laser excitation wavelength of 532 nm. Elemental quantitative analysis was gained using X-ray photoelectron spectroscopy (XPS) with a Kratos XSAM800 spectrometer. Electrochemical Measurements. Electrochemical testing was carried out with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai). All the experiments were measured at room temperature (∼25 °C) in a classical three-electrode cell using the as-synthesized Ni/C3N4 composite coating (10 mm × 10 mm) as the working electrode, a square Pt sheet (10 mm × 10 mm × 0.1 mm) as the counter electrode and an Hg/HgO electrode in 1 M NaOH as the reference electrode. As a contrast, the electrocatalytic performance of commercial Pt/C cathode was also measured. Pt/C ink was constructed by dispersing 30 mg of 20 wt % Pt/C into 4.95 mL of C2H5OH and 4.95 mL of DI water with 100 μL of Nafion solution. The mixture was sonicated for 2 h to obtain a uniform slurry, and the above Pt/C ink was dropped on 1 cm2 Cu plate, which was dried in room temperature. Line scan voltammetry (LSV) plots were executed at a sweep rate of 5 mV s−1 and presented with iR compensation. Tafel plots were received by the conversion of LSV. The electrochemical impedance spectroscopy (EIS) were executed at a potential of −1.05 V vs Hg/HgO (η = 125 mV) from 100 kHz to 0.01 Hz with an AC voltage amplitude of 5 mV. In addition, ZSimp software was applied to simulated the EIS spectra and obtain the related data. The stability measurement was performed for 12 h at cathodic current density of 100 mA cm−2. The Ni/C3N4 working electrodes performed at a constant current density of 30 mA cm−2 for 30 min for pretreatment before electrochemical measurements, aimed to remove the oxidation film of cathode surface and build a steady condition for HER.

its electrochemical property for water splitting. As we know, bulk g-C3N4 possesses small surface area and finite exposed edges, which provide few active sites for HER. Herein, preparing low-dimensional g-C3N4 with nanostructured morphology feature is a crucial way to enhance electrocatalytic performance. Duan et al.36 reported a method of using silica spheres as template to synthesize the porous C3N4 nanolayers, which ask for strong alkali reagent to get rid of the template. Jun et al.37 assembled low-dimensional g-C3N4, such as nanoparticles, nanotubes, and nanosheets, by an organic cooperative synthesis. Han et al.38 utilized hydrothermal pretreatment and vacuum freeze-drying process instead of directly calcining dicyandiamide. The prepared mesoporous gC3N4 with seaweed network enriches active sites for water splitting. Typically, the preparation procedure of low-dimensional g-C3N4 is relatively complex and it takes a long time to remove impurities. Therefore, finding an effective method to obtain nanostructured g-C3N4 is significant. Furthermore, for the purpose of improving electrocatalytic performance of gC3N4, a series of non-noble metal/g-C3N4 materials have been documented and their electrocatalytic properties for hydrogen evolution have been reported. Zou et al.39 synthesized Cudoped g-C3N4 materials with supramolecular structure and modest overpotential for HER. Bi et al.40 prepared Ni@g-C3N4 composites by a simple solvent thermal which is contributed to the enhanced photocatalytic H2 evolution activity. Kong et al.41 proposed a rapid light-assisted method to prepare effectively combined Ni/g-C3N4 composite photocatalysts. In this research article, we find a quick and straightforward means to produce large-scale g-C3N4, which do not use a template and easily obtain uniform nanostructures. And, we fabricate a non-noble-metal catalyst which consists of carbon material (g-C3N4) and transition metal (Ni) only by supergravity field electrodeposition. As we know from our previous work,29−31 compared with traditional electrodeposition, supergravity electrodeposition could intensify the micro mixture, mass transfer and bubble separation during the electrodeposited process, which results in that the as-prepared gC3N4 sample embeds more homogeneous in Ni-based composite coating and the specific surface area of Ni/C3N4 composite catalysts increase because of refined Ni nanoparticles. Our researches indicate that the as-prepared Ni/C3N4 composite catalyst possesses a unique coral-like architecture and its special configuration is in favor of electrochemical activity for HER. In this work, the morphology, microstructure and electrochemistry properties of Ni/C3N4 composite catalysts have been described systematically.

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EXPERIMENTAL SECTION

Preparation of Nanostructured g-C3N4 Material. g-C3N4 bulk was prepared by thermal decomposition of melamine according to the previous work.42 Melamine (6.0 g) was put into a porcelain boat in the middle of the pipe furnace, and the reaction was maintained at 500 °C for 2 h with the heating rate 2 °C min−1 in an atmosphere of N2. Then the material was calcined to 550 °C for another 2 h also with the heating rate 2 °C min−1 in an atmosphere of N2. After it was cooled down to room temperature, the yellow g-C3N4 bulk (∼3.2 g) was obtained. To change g-C3N4 bulk into nanostructured g-C3N4 material, the g-C3N4 bulk was addressed by two different ways (ballmilling and sand-milling). The first method was to mill the pristine gC3N4 bulk with zirconia (ZrO2) balls for 3 h in a ball mill equipment with 240 rpm rotation speed, and the mass ratio of ball to g-C3N4 powder is 30:1. The above g-C3N4 material was named as C3N4-B. The second method was to mill the g-C3N4 bulk with sanding machine at

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RESULTS AND DISCUSSIONS Morphology and Microstructure Analysis of C3N4 Samples. The flow diagram for preparation process of Ni/ 7994


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analysis (Figure 1b), C3N4-B characterizes two peaks at 12.88° (100) and 27.44° (002) resulting from an in-plane structure pattern and an interlayer stacking reflection of conjugated aromatic models, respectively, which conforms to the XRD pattern of bulk g-C3N4 reported in literature.38,42,48 Compared to C3N4-B, the sample of C3N4-S reflects only a broader and lower intensity diffraction peak at 27.72°, and that the peak situated at 12.88° vanishes, declaring a smaller planar size of gC3N4 after treated by sand milling which observed in TEM image as well. As revealed in Figure 2a, C3N4-S circular particles present uniform nanostructures with about 20 nm. Yet TEM image (Figure S1) of C3N4-B nanosheets exhibit uneven sheet sizes with 100−400 nm, which are larger than those of C3N4-S sample. In particular, the sample of C3N4-S can be evenly scattered in water without sediment for 1 week in the atmospheric environment as displayed in Figure S2. Thus, it is in favor of scattering in nickel plating solution in the next experimental procedure. As shown in Figure S3, the C3N4-B sample subsides soon in water. To further explore the thickness of C3N4-S sample, the C3N4-S nanoparticles are dip-coated on mica substrate after 1 h sonication treatment for AFM analysis as presented in Figure 2b. The white dashed box in Figure 2b indicates that appearing aggregation to some extent during the dip-coating process. Besides the AFM height profile in Figure 2c for the C3N4-S nanoparticles revealed that the thickness of as-prepared C3N4-S is about 1.2−2.8 nm. And the C3N4-S nanoparticles are about 20−40 nm in lateral size which is in accordance with the result of TEM image. In contrast with bulk g-C3N4 and C3N4-B samples, C3N4-S with a nanostructured high surface area can be more likely to promote catalytic behavior in water splitting. To explore the elements distribution of the samples, the C3N4-S and C3N4-B were measured by SEM and EDS analysis. Drying the wet C3N4-S sample which has treated by sand milling and took SEM picture as displayed in Figure 2d, the C3N4-S nanoparticles reunite together and become lumpish. The C/N molar ratio was calculated by the Table inserted in Figure 2e and Figure S4e. The computed results of C3N4-S and C3N4-B were 0.71 and 0.82, respectively, and both of them approximate the theoretical value of sublimate g-C3N4 (0.75). This illustrates that the unique polymerized structure of g-C3N4 successfully formed and reserved its unique structure after ball milling and sand grinding treatment. Moreover, the elemental mapping analysis was also recorded to research the homogeneity of elemental

C3N4 catalyst is shown in Scheme 1. We obtained g-C3N4 samples with nanometer size through two steps which was Scheme 1. Flow Diagram for Preparation Process of Ni/ C3N4 Catalyst

prepared by pyrolysis of melamine and then treated by sand milling. The Ni/C3N4 composite electrode was fabricated via a one-step approach of supergravity eletrodeposition which converted Ni2+ into Ni nanoparticles. The embedded carbon material of g-C3N4 plays a key role in refining Ni nanoparticles and forms the special coralline structure. In the process of HER, H 2O first gain electrons from the surface of Ni/C 3N 4 composite coating to become H*, then two H* collide into H2, which can gather and build bubbles up. The composition features of materials can be characterized by FTIR testing typically. In the cases of C3N4-B and C3N4-S samples (Figure 1a), the bands at 1560 and 1640 cm−1 are attributed to C−N stretching and −CN stretching, respectively. Bands at 1240, 1320, 1410 cm−1 owe to aromatic −C−N stretching, and in particular, one at 805 cm−1 is ascribed to the breathing mode of triazine units presented in C3N4-B and C3N4-S samples. It indicates that, after ball milling and sand grinding processing, C3N4-B and C3N 4-S samples still completely retain the composition characterizes of bulk gC3N4.44,45 It is worth noting that the slight shift to a higher wavenumber (blue shift) of C3N4-S compared with the C3N4-B in FTIR plots, which is resulted from the quantum size effect. When the grain size of g-C3N4 is reduced to a certain value, the energy level gap near the Fermi level is widened, causing the absorption peaks show a blue shift. It is proved that C3N4-S sample possesses smaller nanosize than C3N4-B. Another interpretation is that the lattice of C3N4-S becomes distorted, leading to the peaks to the higher wavenumber.46,47 For XRD

Figure 1. (a) FTIR plots and (b) XRD patterns of as-prepared C3N4-B and C3N4-S samples. 7995


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Figure 2. (a) TEM image of C3N4-S sample. (b) AFM image of C3N4-S (on mica plate) produced by sand milling (with 1 h sonication), and (c) corresponding AFM height images of as-prepared C3N4-S sample. (d) SEM image of C3N4-S sample. (e) EDS spectrum of selected area in panel d. (f) High-magnification image of selected area in panel d. (g−i) EDS elemental mapping of individual elements (C, N, O) of image (f).

Figure 3. (a) XRD patterns of pure Ni, Ni/C3N4-B, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/C3N4-0.20 catalysts. (b) Raman spetra of C3N4-S and Ni/C3N4-0.10 catalysts.

standard Ni (JCPDS 03-1051), respectively. In addition, the preferential orientation of pure Ni coating is (200) crystal plane, yet the preferential orientation of Ni/C3N4-B, Ni/C3N40.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are all the (111) crystal plane. From the perspective of previous researches, Ni (111) plane has pivotal effect on promoting the electrocatalytic performance for HER than other planes.49,50 Figure 3b shows the Raman spectra recorded for C3N4-S and Ni/C3N4-0.10 samples, respectively. To our best knowledge, the characteristic peaks at around 1355 (D-band) and 1600 cm−1 (G-band) are related to the defects

distribution as shown in Figure 2f−i and Figure S4a-d. As expected, it can be seen in all the graphs, the location of three elements (carbon, nitrogen and oxygen) distribute uniformly in as-prepared C3N4-S and C3N4-B samples. Little oxygen exists in samples because of slightly oxidized in the atmosphere. Morphology and Microstructure Analysis of Ni/C3N4 Composite Catalysts. Figure 3a displayed XRD patterns of pure Ni, Ni/C3N4-B, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/C3N40.15, and Ni/C3N4-0.20 catalysts. The diffraction peaks at 44.832°, 52.228°, 76.807°, 93.217°, and 99.397° are attributed to the (111), (200), (220), (311), and (222) lattice planes of 7996


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Figure 4. (a) Survey spectrum and high-resolution XPS spectrum (inset) of O 1s for the Ni/C3N4-0.10 catalyst. (b−d) High-resolution XPS spectra of Ni 2p, N 1s, and C 1s for Ni/C3N4-0.10 catalyst.

in the pristine pure g-C3N4.39−41 The high resolution C 1s spectrum in Figure 4d can be deconvoluted into two binding species. The main peak at 284.5 eV is from the C−N coordination in the g-C3N4, and another peak at 288.3 eV is assigned to the N−CN bonds. The O 1s at ∼531.3 eV corresponding to Ni-oxygen bonds of Ni/C3N4 catalyst as shown in inset of Figure 4a. Our XPS results coupled with Raman spectroscopy strongly support that the g-C3N4 have been embedded into the Ni-based composite electrode via electrodeposited process. The SEM images of pure Ni, Ni/C3N4−B, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/C3N4-0.20 catalysts are shown in Figure 5. As a blank sample (Figure 5a), the pure Ni coating is similar to the stalagmite morphology. The SEM image of Ni/C3N4-B sample is revealed in Figure 5b, the C3N4B flakes are embedded into the Ni matrix and the surface of C3N4-B electrode displays more rough than pure Ni. As presented in Figure 5c, the morphology of Ni/C3N4-0.05 indicates that introducing the C3N4-S nanoparticles into Ni base leads to finer Ni particles and shows the hill-like shape. The morphology of Ni/C3N4-0.10 composite catalyst makes a difference when the concentration of C3N4-S up to 1.0 g L−1 existed in nickel bath solution. As shown in Figure 5d and e, the Ni/C3N4-0.10 catalyst is similar to the coral-like morphology. To further study the distribution of C3N4 and Ni in composite catalyst, the EDS and elemental mapping analysis of Ni/C3N40.10 were also performed as revealed in Figure S5 and Figure 5f−i. The elements of C, N, and O distribute homogeneously into the Ni matrix. It benefits from introducing the supergravity field into electrodeposition, which results in intensifying micro mixture, mass transfer, and bubble separation, during the fabrication process of composite electrode material, and the distribution of C3N4 nanoparticles become more uniform

and the planar motion of sp2-hybridized carbon atoms in a carbon material structure, respectively. Different from other carbon materials, the D-band and G-band peaks of C3N4-S sample are combined into one broad envelope in the range 1242−1628 cm−1. This unique profile derives from the superposition of multiple peaks which the −C−N vibration appear at 1242 cm−1 and the −CN vibration appear at 1400 cm−1 except for D-band and G-band. Compared to as-prepared C3N4-S sample, the peak intensity of Raman spectrum for Ni/ C3N4-0.10 composite catalyst becomes shrunken stems from C3N4-S surface coated with Ni. In addition, the broad peak existed in C3N4-S sample split into D and G peaks apparently in Raman spectrum of Ni/C3N4-0.10 composite catalyst. This may be due to the −C−N stretching and −CN stretching get damaged during the electrodeposition process, leading to the intensities of these two peaks become weaker. Therefore, the Raman spectrum of Ni/C3N4-0.10 catalyst displays only D and G peaks. To uncover the surface chemical state and electron transfer of the Ni/C3N4-0.10 material, XPS measurement was further performed. The survey XPS spectrum in Figure 4a verifies the coexistence of Ni, N, C, and O on the electrode surface. As presented in Figure 4b, the high-resolution XPS spectrum for the Ni region of Ni/C3N4-0.10 shows the signals at 855.5 and 860.9 eV, corresponding to the Ni 2p3/2 levels and peaks at 873.0 and 879.6 eV belonging to Ni 2p1/2 levels. The peak intensity of Ni2+ is higher than that of Ni0 (∼852.0 eV), which can be attributed to the oxidation of Ni on the surface of the electrode. The strong signal of metallic Ni in XRD spectra (Figure 3a) only proposes that XRD is a much deeper detection means than XPS. The N element exists in three forms of bonds as displayed in Figure 4c including the C−NC at 398.4 eV, N−3C at 400.4 eV, and C−N−C at 399.3 eV, similar to those 7997


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Figure 5. SEM images of (a) pure Ni, (b) Ni/C3N4‑B, (c) Ni/C3N4-0.05, and (d) Ni/C3N4-0.10 catalysts. (e) High-magnification image, which white arrows point to image in panel d. (f−i) EDS mapping of individual elements (C, N, Ni, O) of image e. (j) SEM images of Ni/C3N4-0.15 and (f) Ni/C3N4-0.20 catalysts. The insets in panels c, j, and k are high-magnification images of the areas indicated by the white arrows.

the Ni/C3 N 4-0.20 composite coating presents random geometry as shown in Figure 5k. Electrochemical Activity and Stability of Ni/C3N4 Composite Catalysts. The electrocatalylic activity for HER in 1 M NaOH medium on Ni/C3N4 composite cathodes were carried out in a three electrode cell by a series of testing measures. Figure 6a shows the iR-corrected LSV curves of the C3N4-S, pure Ni, Ni/C3N4-B, Ni/C3N4-0.10, and Pt/C catalysts. It is apparent that both C3N4-S and pure Ni electrodes show very poor catalytic activity, which demand overpotentials of 641, 453 mV and 551, 378 mV to drive current densities of 100 and 10 mA cm−2, respectively. In addition, the Ni/C3N4-B and Ni/C3N4-0.10 catalysts prepared using different g-C3N4 samples demand overpotentials of 474, 311 mV and 356, 222 mV to drive current densities of 100 and 10 mA cm−2, respectively. And the as-prepared Pt/C cathode demands overpotentials of 200 and 45 mV to drive current

during the process of Ni grain growth. The atom ratios of C, N, O, and Ni exited in Ni/C3N4-0.10 catalyst are 33.42, 5.31, 3.82, and 57.45 at%, respectively, as displayed in Figure S5. It is noteworthy that the C/N molar ratio of Ni/C3N4-0.10 catalyst gets larger than the theoretical value of g-C3N4 (0.75). This may be due to the C−N stretching group damaged during the electrodeposition process, and N element transform into ammonium group entered into plating solution. So this is the reason why less N content existed in Ni/C3N4 composite coating, and this result is consistent with that of Raman analysis. The presence of O element is mainly because of the oxidation of Ni surface. The SEM image of Ni/C3N4-0.15 composite coating is displayed in Figure 5j, it indicates that the Ni/C3N4-0.15 catalyst still remains the coral-like morphology. However, the coral-like morphology disappears when the C3N4−S up to 0.20 g L−1 existed in nickel bath solution, and 7998


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Figure 6. (a) LSV curves, (b) Tafel plots, and (c) EIS spectra (dots represent original data and lines represent fitted data) of C3N4-S, pure Ni, Ni/ C3N4-B, Ni/C3N4-0.10, and Pt/C catalysts. The inset in panel c is equivalent electrical circuit used to model the HER kinetics process. (d) EIS spectra of Ni/C3N4-0.10 catalyst under different overpotentials. (e) Bode plots of Ni/C3N4-0.10 catalyst. (f) Tafel slope of the Ni/C3N4-0.10 fitted from EIS data.

Table 1. Electrocatalytic Parameters on HER Performed in 1 M NaOH Solution at 298 K composite electrodes C3N4-S pure Ni Ni/C3N4-B Ni/C3N4-0.05 Ni/C3N4-0.10 Ni/C3N4-0.15 Ni/C3N4-0.20

b (mV dec−1) 198 163 162 143 128 136 160

j0 (A cm−2) 1.001 4.705 1.195 4.749 1.906 1.266 1.277

× × × × × × ×

Rct (Ω cm−2)

η100 (mV)

η10 (mV)

Cdl (mF cm−2)

Rf

700.0 556.1 150.3 351.0 51.85 114.6 159.4

641 551 474 497 356 417 476

453 378 311 329 222 275 303

0.1859 0.1047 0.4048 0.2682 1.402 1.085 0.6409

9.295 5.235 20.24 13.41 70.09 54.26 32.04

−4

10 10−5 10−4 10−5 10−4 10−4 10−4

densities of 100 and 10 mA cm−2. Ni/C3N4-0.10 cathode presents an efficient HER activity because of its special corallike morphology with especially large surface area. Another a significant parameter is Tafel slope b which can provide insights into the kinetics of electrode process on HER mechanism. As shown in Figure 6b and Table 1, the Tafel slopes of C3N4-S, pure Ni, Ni/C3N4−B, Ni/C3N4-0.10, and Pt/ C catalysts are 198, 163, 162, 128, and 40 mV dec−1,

respectively. As we all know, the procedure on HER generally includes three primary reactions named the Volmer, Tafel, and Heyrovsky reactions in alkaline solution. According to the reported literature, the above four electrodes (C3N4-S, pure Ni, Ni/C3N4-B, and Ni/C3N4-0.10 catalysts) abide by Volmer− Heyrovsky reaction path and the rate-determining step of them are Volmer reaction. 7999


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Figure 7. (a) LSV curves, (b) Tafel plots, and (c) EIS spectra (dots represent original data and lines represent fitted data) of Ni/C3N4-0.05, Ni/ C3N4-0.15, and Ni/C3N4-0.20 catalysts.

In the same way, the electrochemical measurements of Ni/ C3N4-0.05, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts were also carried out. As displayed in Figure 7a, LSV curves show that the as-prepared Ni/C3N4-0.05, Ni/C3N40.15, and Ni/C3N4-0.20 cathodes demand overpotentials of 497 and 329 mV, 417 and 275 mV, and 476 and 303 mV to drive current densities of 100 and 10 mA cm−2, respectively. As shown in Figure 7b and Table 1, the Tafel slopes of Ni/C3N40.05, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are 143, 136, and 160 mV dec−1, respectively. In addition, EIS measurements are displayed in Figure 7c. The Rct values of the Ni/C3N4-0.05, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are 351.0, 114.6, and 159.4 Ω cm−2, respectively. Compared to above three cathodes, Ni/C3N4-0.10 cathode possesses lowest overpotential, Tafel slope and Rct. It obviously declares that Ni/C3N4-0.10 cathode presents superior electrochemical property for HER when the concentration of C3N4-S existed in plating solution is 0.10 g L−1. In fact, the Ni/C3N40.10 catalyst exhibits much better catalytic activity than other Ni-based catalysts on HER in alkaline solution as summarized in Table S2.51−55 Exchange current density (j0) not only can be used to describe the ability to gain electronics from an electrode, but also can reflect the complexity of an electrode reaction for HER. As listed in Table 1, the calculated values of j0 can be obtained by η = b log(j/j0), and the results of j0 values on C3N4S, pure Ni, Ni/C3N4-B, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/ C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are 1.001 × 10−4, 4.705 × 10−5, 1.195 × 10−4, 4.749 × 10−5, 1.906 × 10−4, 1.266 × 10−4, and 1.277 × 10−4 A cm−2, respectively. What is noteworthy is that the value of j0 for Ni/C3N4-0.10 catalyst can be up to 1.906 × 10−4 mA cm−2, which possesses the largest j0 value among the above catalysts. It declares that Ni/C3N4-0.10

For purpose of analyzing the electrode kinetics of composite catalysts, EIS measurements are executed at cathode overpotential of 125 mV. The electrochemical impedance spectroscopy are characterized by the Armstrong’s equivalent electric circuit pattern on account of the single-adsorbate mechanism as revealed in insert map of Figure 6c. Simulating electrochemical values from the EIS spectra which are listed in Table 1 and Table S1. Rs is the solution resistance and Rct is the charge transfer resistance. It can be observed that the Rct values of Nicarbon catalysts reduce sharply compared to the pure C3N4-S and pure Ni electrodes. Lesser value of Rct signifies more favorable kinetic process for HER. As shown in Table 1, the Rct of the Ni/C3N4-B and Ni/C3N4-0.10 catalysts are 150.3 and 51.85 Ω cm−2, respectively, which are obviously lesser than the 556.1 Ω cm−2 for pure Ni cathode and 700.0 Ω cm−2 for pure C3N4-S cathode. The results are consistent with the above analysis of LSV and Tafel, and it further illustrates that the Ni/ C3N4-0.10 catalyst prepared under g-C3N4 treated by sandmilling shows higher electrocatalytic activity compared to gC3N4 treated by ball-milling. The superior HER property of Ni/C3N4-0.10 catalyst is due to its big surface area and favorable electrical conductivity. Figure 6d shows Nyquist plots of Ni/C3N4-0.10 composite catalyst in 1 M NaOH at different overpotentiasl from 60 to 135 mV. The higher overpotential declares the smaller diameter which represents lower charge transfer resistance. Their corresponding Bode curves were presented in Figure 6e, and all these plots abide by one-time-constant behavior. Fitting log(1/Rct)-potential can also obtain Tafel slope which give an index to the charge transfer kinetic. As shown in Figure 6f, the Tafel slope derived from Nyquist plots data is about 120 mV dec−1, which is in accordance with the result of Tafel plot (b = 128 mV dec−1) as shown in Figure 6b. 8000


Research Article

ACS Sustainable Chemistry & Engineering catalyst, which prepared under supergravity field electrodeposition and existed 1.0 g L−1 C3N4 in plating bath, is an outstanding catalyst on HER. Currently, the electrochemically active specific surface area of catalysts has been regarded as the key factor on catalytic activity. The electrochemical double layer capacitance (Cdl) measurements were executed to decide the electrocatalytic active surface areas of the Ni/C3N4 catalysts using the formula proposed by Brug et al. Cdl = [Q /(R S−1 + R ct−1)(1 − n)]1/ n

that Ni/C3N4-0.10 cathode keeps almost steady during 12 h test, which confirms its high stability and durability for a long time on HER. Moreover, XRD and SEM analysis after CP testing of Ni/C3N4-0.10 cathode are displayed in Figure 8a, b. It is clear that there is no significant change on its morphology and microstructure after CP testing for 12 h.

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CONCLUSION In summary, preparing nanoscaled g-C3N4 material in two treatment approaches, which include ball-milling and sandmilling were studied. And we fabricate a non-noble-metal catalyst, which consists of carbon material (g-C3N4) and transition metal (Ni) only by supergravity field electrodeposition. First, we obtain an active Ni/C3N4 composite electrode which possesses high surface area with special corallike structure. Simultaneously, Ni/C3N4-0.10 composite catalyst displays better HER performance with low overpotential, Tafel slope, electrochemical reaction resistance and high exchange current density. Our results also state that the composition of Ni and g-C3N4 would be a high-efficiency non-noble catalyst for HER and this discovery is a pivotal part in directing scientific research on future experiments to efficient electrocatalysis on HER.

(1)

Q is the capacitance coefficient, and n is the CPE exponent as presented in Table S1. The calculated results of Cdl values on C3N4-S, pure Ni, Ni/C3N4-B, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are 0.1859, 0.1047, 0.4048, 0.2682, 1.402, 1.085, and 0.6409 mF cm−2, respectively. It is obviously seen that Ni/C3N4-0.10 cathode possesses largest electrocatalytic active surface area which benefited from its uniformly distributed Ni and C3N4 nanoparticles. Furthermore, the number of electrode surface active sites for HER can also depend on the surface roughness (Rf). The values of Rf are obtained by comparing the Cdl value with 20 μF cm−2 for surface of smooth electrode. As calculated in Table 1, the results show that the Rf values of C3N4-S, pure Ni, Ni/C3N4-B, Ni/C3N4-0.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are 9.295, 5.235, 20.24, 13.41, 70.09, 54.26, and 32.04, respectively. The larger Rf value declares that the electrode can offers more electrocatalytic active sites at per unit superficial area for HER. Furthermore, the mass loading of all electrodes are presented in Table S1. The dosages of C3N4-S, pure Ni, Ni/C3N4-B, Ni/ C3N4-0.05, Ni/C3N4-0.10, Ni/C3N4-0.15, and Ni/C3N4-0.20 composite catalysts are 30.00, 2.700, 5.590, 32.40, 32.60, 32.70, and 31.10 mg. It is easy to find that the mass loading of all Ni/ C3N4-S catalysts are about 30 mg, which are much larger than those of pure Ni and Ni/C3N4-B electrodes. It is sufficiently illustrated that Ni is more likely to form the nucleation center on the surface of C3N4-S material, leading to more active sites generated by Ni/C3N4-S catalysts. Chronopotentiometry (CP) testing is currently aimed to assess the electrochemical stability of catalyst for HER. As shown in Figure 8, Ni/C3N4-0.10 catalyst was tested at constant current density of 100 mA cm−2 for 12 h. It distinctly reveals

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01576. Additional details describing TEM image, SEM image, and EDS mapping of individual elements (C, N, O) of as-prepared C3N4−B sample, the photos of C3N4−S and C3N4−B samples after 1 week and 1 h standing, respectively, and the EDS of Ni/C3N4-0.10 catalyst. The tables include electrochemical circuit parameters obtained from the EIS measurements, mass loading of electrodes and comparison of electrocatalytic performance for other Ni-based catalysts in alkaline solution on HER (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: 0086-335-8061569. Fax: 0086-335-8059878. ORCID

Guangjie Shao: 0000-0001-6957-4828 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51674221). REFERENCES

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Figure 8. CP curve of Ni/C3N4-0.10 catalyst under 100 mA cm−2 for 12 h. Insets are (a) XRD pattern and (b) SEM image of Ni/C3N4-0.10 catalyst after 12 h CP. 8001


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