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Construction of Polarized Carbon−Nickel Catalytic Surfaces for Potent, Durable, and Economic Hydrogen Evolution Reactions Min Zhou,†,‡ Qunhong Weng,*,‡ Zakhar I. Popov,§ Yijun Yang,∥ Liubov Yu. Antipina,§,⊥ Pavel B. Sorokin,§,⊥ Xi Wang,*,∥,# Yoshio Bando,‡,¶ and Dmitri Golberg*,‡,∇ †
College of Physical Science and Technology, Institute of Optoelectronic Technology, Yangzhou University, Yangzhou 225002, China ‡ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan § National University of Science and Technology MISIS, Leninskiy prospekt 4, 119049 Moscow, Russia ∥ School of Science, Beijing Jiaotong University, Beijing 100044, P. R. China ⊥ Technological Institute for Superhard and Novel Carbon Materials, Centralnaya st. 7a, Troitsk, 108840 Moscow, Russian Federation # Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, Tianjin University and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China ∇ School of Chemisty, Physics, and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, 2 George St., Brisbane, Queensland 4000, Australia ¶ Australian Institute for Innovative Materials, University of Wollongong, Squires Way, North Wollongong, NSW, Australia S Supporting Information *
ABSTRACT: Electrocatalytic hydrogen evolution reaction (HER) in alkaline solution is hindered by its sluggish kinetics toward water dissociation. Nickel-based catalysts, as low-cost and effective candidates, show great potentials to replace platinum (Pt)-based materials in the alkaline media. The main challenge regarding this type of catalysts is their relatively poor durability. In this work, we conceive and construct a charge-polarized carbon layer derived from carbon quantum dots (CQDs) on Ni3N nanostructure (Ni3N@CQDs) surfaces, which simultaneously exhibit durable and enhanced catalytic activity. The Ni3N@CQDs shows an overpotential of 69 mV at a current density of 10 mA cm−2 in a 1 M KOH aqueous solution, lower than that of Pt electrode (116 mV) at the same conditions. Density functional theory (DFT) simulations reveal that Ni3N and interfacial oxygen polarize charge distributions between originally equal C−C bonds in CQDs. The partially negatively charged C sites become effective catalytic centers for the key water dissociation step via the formation of new C−H bond (Volmer step) and thus boost the HER activity. Furthermore, the coated carbon is also found to protect interior Ni3N from oxidization/hydroxylation and therefore guarantees its durability. This work provides a practical design of robust and durable HER electrocatalysts based on nonprecious metals. KEYWORDS: electrochemical catalysis, hydrogen evolution reaction, carbon, nickel, polar
H
economically prompt such reaction in regard to both hydrogen evolution reaction (HER) on a cathode and oxygen evolution reaction (OER) on an anode. To date, the cathodic H2 is
ydrogen is a clean and high-energy-density medium for energy storage and conversion. Production of hydrogen through simple electrolysis of water using sustainable energies, such as solar irradiation, wind/water power, etc., is a very attractive route.1−5 Although electrochemical water splitting has been studied for a long time, it is still a big challenge to design proper catalysts to efficiently and © XXXX American Chemical Society
Received: December 10, 2017 Accepted: March 15, 2018
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DOI: 10.1021/acsnano.7b08724 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Structural characterization of Ni3N@CQDs. (a) XRD patterns of Ni3N@CQDs and Ni3N powders in comparison with the standard hexagonal Ni3N (JCPDS 895144). (b) O K-edge XANES spectra of Ni3N@CQDs, commercial NiO, and the CQDs treated in NH3 at 370 °C. (c) TEM image of Ni3N@CQDs and (d) HRTEM image. The inset is the simulated charge distributions in the proposed Ni3N@CQDs structural model. The loss and the gain of the charge are denoted by yellowish and bluish colors, respectively, with the isosurface values of Δρ = ± 2 × 10−3 e/Å3.
for O−H bond cleavage during water splitting reactions. Gao et al. obtained an HER overpotential of only 59 mV at the current density of 10 mA cm−2 from atomically thin Ni3N nanosheets in an acid solution. However, this value climbed to 305 mV when the experiment had been performed in alkaline conditions.26 Assembling Ni3N nanosheets over the inner and outer walls of hollow carbon fibers can reduce the overpotential to 115 mV due to excellent electrical conductivity and highly dense utilizable active sites of the catalyst.29 A cutting-edge HER performance of Ni3 N-based catalysts in alkaline conditions was reported by Zheng et al. for Pt-decorated Ni3N nanosheets. At the overpotential of 160 mV, the Pt/Ni3N hybrid exhibits an HER areal current density of 200 mA cm−2, which is 2 times of the current density of commercial 20% Pt/C catalyst at the same overpotential.30 These results indicate that design of the state-of-the-art HER catalysts based on the Ni3N compound is a promising route. Meanwhile, nickel-based catalysts usually suffer from corrosion and passivation under alkaline environments, where they are quickly oxidized under the HER operation potentials.21 Therefore, conceiving catalytic surfaces toward low-cost, highly active, and durable HER processes within alkaline media is highly imperative. Herein, we report on the catalyst surface design and the HER studies of an ultrathin carbon layer-coated Ni3N nanocomposite (denoted as Ni3N@CQDs to clarify the source of the carbon layer) in which the polarized carbon surfaces not only enhance the catalytic activity but also simultaneously
mainly produced from a chlor-alkali or water electrolysis process.6 The most effective HER catalysts are precious platinum (Pt)-based materials. However, the latter cannot be extensively used due to their low abundance and high cost.7−11 Thus, design of high-performance and economic catalysts based on abundant elements becomes an interesting and timely warrant topic. Over the past few years, transition-metal-based catalysts, including Co, Ni, Mo, and their compounds, emerged as promising HER electrocatalysts to rival Pt.12−20 However, in alkaline solutions, the HER activity of these nonprecious catalysts is significantly lower than those of the Pt-based catalysts.21,22 The sluggish HER kinetics in alkaline solutions is caused by the slow water dissociation process (Volmer step). The HER catalytic activity of a catalyst working in acid media is generally dominated by its Gibbs free energy for adsorption and desorption of the reactive hydrogen intermediates (ΔGH). Under alkaline solutions, however, the activity depends not only on the ΔGH but also on the Gibbs free energy of water dissociation to form adsorbed hydrogen during the initial Volmer step.23 Accordingly, substantial efforts have been made to improve the HER activity of transition-metal-based catalysts through speeding up the Volmer step in an alkaline electrolyte. Ni3N is a type of metallic material whose surface shows special affinity to O2 and water molecules and thus has been explored for catalytic applications.24 Several Ni3N nanostructures,25,26 and heterostructures,27−30 are found to be effective B
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Figure 2. Chemical composition and elemental distribution maps of Ni3N@CQDs. (a) HAADF image. The inset is the accumulated EDX spectrum from the sample. (b−e) Ni, N, O, and C K-edge spatially resolved EDX maps. (f) An overlay of Ni, N, O, and C K-edge maps.
Ni3N phase were also prepared. The as-synthesized CQDsfunctionalized Ni3N shows a porous structure (Figure S2, Supporting Information). From recorded XRD patterns of the product, we can index the main diffraction peaks to a hexagonal Ni3N phase (JCPDS 895144). The difference between Ni3N@ CQDs and pristine Ni3N is the appearance of a broad carbon (002) peak at 2θ of 20−30° in the Ni3N@CQDs sample, as shown in Figure 1a. Crystalline Ni3N nanoparticles are joined together (Figures 1c and S3) and form a porous nanosheet-like structure. High resolution TEM image (Figure 1d) reveals a thin, only a-few-atom-thick amorphous carbon layer formed on the Ni3N surfaces. From X-ray absorption near edge structures (XANES, Figure 1b), the sharp peaks at ∼533 eV can be attributed to the X-ray photoexcitation of O 1s electrons to the Ni 3d-O 2p hybridization state, which can be seen in both Ni3N@CQDs and referred NiO samples. The peaks located at 536 eV in both Ni3N@CQDs and CQDs can be attributed to the excitation of O 1s to π* (C−O) orbitals, while the broad peak at ∼541 eV can be assigned to the O 1s → σ* (C−O).35−38 These results demonstrate that there are oxygen atoms covalently bridging carbon and nickel atoms and forming Ni−O−C structures. This is because hydroxyl groups on the CQDs and Ni(OH)2
strengthens the durability of the catalyst through physical separation of Ni3N from the alkaline electrolyte. Density functional theory (DFT) calculations reveal that the binding energy for the initial water dissociation step is significantly lowered on the carbon-coated Ni3N surfaces compared to the pristine Ni3N surfaces. The resultant Ni3N@CQDs exhibits an outstanding electrochemical HER activity with an overpotential of 69 mV at the current density of 10 mA cm−2; this is maintained for at least 20 h without any decay. These results demonstrate enhanced catalytic activity and durability of HER catalysts using the material surface design.
RESULTS AND DISCUSSION The Ni3N@CQDs was synthesized from a microsphere-like Ni(OH)2 precursor having numerous few-nanometer-thick nanosheets and carbon quantum dots (CQDs).31,32 After dipping Ni(OH)2 in the CQD solution and recovering the solids after drying, the material was heated to 370 °C in ammonia atmosphere to convert the Ni(OH)2 to Ni3N. It is noted that the negatively charged CQDs would be adsorbed on the surfaces of Ni(OH)2 in water, which are positively charged in neutral aqueous solutions.33,34 As a control, Ni3N nanoparticles loaded on graphene oxide (Ni3N-GO) and pristine C
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Figure 3. Electrochemical HER performances of Ni3N@CQDs. (a) LSV polarization curves of Ni3N@CQDs in comparison with platinum (Pt) electrode, pristine Ni3N, CQDs, and glassy carbon (GC) electrode in a 1 M KOH aqueous solution. (b) Tafel slopes. (c) Normalized HER amperometric I−t curves of Ni3N@CQDs and Ni3N at a constant overpotential of 77 mV (−1.1 V vs Ag/AgCl). (d,e) Comparison of HER Volmer reaction step and the resultant binding energies on carbon-coated Ni3N(110) (d) and pristine Ni3N(110) (e) surfaces, N, Ni, C, O, and H atoms are marked in blue, gray, brown, red, and cyan colors, respectively.
Ni3N@CQDs sample are 2.0 and 4.7 wt %, respectively (Table S1). We then performed linear sweep voltammetry (LSV) to obtain the HER polarization curves for the prepared materials. As shown in Figure 3a and Table S2, the HER overpotentials on Ni3N@CQDs surfaces were measured to be 69 mV at a current density of 10 mA cm−2, which are 47 mV and 89 mV lower than those of the commercial Pt electrode and pristine Ni3N reference sample, respectively (at the same testing conditions). Note that these performances are given without considerations of the catalyst particle surface area. We can see the HER activity for the Ni3N@CQDs and Ni3N is inferior compared with that of Pt electrode after normalizing their surfaces, but the HER activity of the Ni-based catalysts shows the same trend (Figure S7). To the best of our knowledge, this performance surpasses those of recently reported Ni3N catalysts working in alkaline solutions.26,29 To make clear the exact roles of CQDs and Ni3N during the HER electrochemical catalysis, we also treated the CQDs at the same temperature of 370 °C in ammonia and tested their HER activities. The result confirms a weak HER performance for the CQDs and indicates that the high HER activity of Ni3N@CQDs structure is arisen from the synergetic effects of CQDs and Ni3N components. We also investigated the effect of CQDs mass loading on catalytic activity and found no significant difference when
dehydrate during the nitridation process at the elevated temperatures and form the Ni−O−C bonds between CQDs and Ni3N interfaces. Furthermore, the O 1s → π* (CO) excitation present in CQDs should arise from the residual C O groups of the glucose precursor, which could be further eliminated by treating the sample at 370 °C and in reductive NH3. Comparative Ni 2p XPS of Ni3N@CQDs and pristine Ni3N and Fourier transform infrared spectroscopy (FTIR) analysis (Figures S4 and S6) further support this conclusion. In addition to the main Ni−N bonds, there are Ni−O bonds shown in the Ni3N@CQDs Ni 2p XPS spectrum. The FTIR spectrum of Ni3N@CQDs shows vibration modes of CC, C−O, and O−H bonds inherited from the CQDs. Notably, the relative intensity of νC−O/νO−H in Ni3N@CQDs is larger than that in CQDs. This should be caused by the elimination of −OH groups in both CQDs and Ni(OH)2 during the dehydration reaction, whereas considerable C−O bonds are expected to remain by forming the Ni−O−C bonds. Highresolution spatially resolved EDX elemental mapping directly reveals the elemental distributions. As shown in Figure 2, oxygen signals are seen to be closer to the surfaces of Ni3N than the carbon signals, indicating that the oxygen-rich components should mainly locate between the carbon layer and Ni3N particles. The quantitatively measured C and O contents in the D
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Figure 4. Elemental mapping of Ni3N@CQDs after amperometric I−t tests for 20 h. (a) HRTEM image. (b) HAADF image, Ni, N, O, and C K-edge spatially resolved EDX maps and their overlay.
composite, revealing that the Ni3N nanocrystals are attached to the graphene sheet. This indicates that not all the Ni3N surfaces are wrapped with graphene. The measured overpotential of Ni3N-GO was 117 mV at 10 mA cm−2, thus 47 mV lower than that of the pristine Ni3N and comparable with the Pt electrode. In Ni3N-GO, a large proportion of the material surfaces are pristine Ni3N or graphene sheets, besides the graphene-covered Ni3N. Based on the polarization curves of the samples taken in a Faradaic potential region (Figure S12), the estimated exposed Ni3N surfaces in pristine Ni3N and in Ni3N-GO are comparable and much higher than that of the Ni3N@CQDs. With the presence of graphene-covered Ni3N, a higher HER activity is expected for the Ni3N-GO than the pristine Ni3N. Moreover, we observed that the HER activity of Ni3N-GO is not as potent as that of Ni3N@CQDs at the same testing conditions. This is because only a small portion of Ni3N nanocrystal surfaces were covered by graphene in Ni3N-GO. Given the same amount of catalysts loaded onto electrodes, the most potent catalytic surfaces in Ni3N-GO may be still smaller than those in Ni3N@ CQDs. Stability of the HER catalysts was studied by amperometric I−t tests (Figure 3c). We observed that the HER stability for the Ni3N@CQDs and Ni3N was significantly different. Under an overpotential of 77 mV, the current density of Ni3N quickly decayed, and only ∼43% was maintained after a 20 h test; in contrast, the current density for the Ni3N@CQDs catalyst even slightly increased to ∼115% at the same conditions. The enhancement of the catalytic activity of Ni3N@CQDs during the stability measurement might be caused by the activation effect that increased utilizable active sites of the catalyst. This means that CQDs coating not only enhances the HER activity for the Ni3N surfaces but also stabilizes the catalysts in alkaline electrolytes during the reactions. To uncover the mechanisms behind the remarkable stability difference between the Ni3N surfaces with and without CQD functionalization, the catalyst particles after the 20 h amperometric tests were collected for high-resolution TEM (HRTEM) and elemental distribution analyses. An amorphous Ni hydroxide layer was formed on the pristine Ni3N nanostructure surfaces after the HER I−t reaction (Figure S13). A similar oxidization process was also previously observed for Ni-based HER catalysts, suggesting the instability of Ni-based nonoxide catalysts in the alkaline solutions.21,41 As a comparison, Figure 4 displays HRTEM and EDX elemental maps of Ni3N@CQDs after the I−t test. The main structural feature of the catalyst, that is, the position of an atomically thin carbon layer on the crystalline Ni3N surface, was confirmed to
varying the feeding amount of CQDs (Figure S8). The calculated Tafel slopes (Figure 3b) derived from their polarization curves are around 100 ± 10 mV dec−1, confirming that the Volmer step is the rate-determining step.23 The HER reaction pathway on the Ni3N@CQDs surface is considered to consist of two steps: (i) Volmer step, that is, a hydrogen atom is attached to the catalyst surface by the dissociation of a H2O molecule; and Heyrovsky (ii) or Tafel (iii) step, during which a hydrogen molecule is detached via the interaction of a H atom and water molecule or combination of two H atoms.23 C + H 2O + e → C−H + OH− C−H + H 2O + e → C + H 2 + OH
(i) −
(ii)
or 2C−H → 2C + H 2
(iii)
where C represents carbon atoms in the CQD layer attached to Ni3N. In the case of pristine Ni3N surface, the reaction follows the same pathways, except for active sites, which become nitrogen atoms. As shown in the simulated results, the thin carbon layer anchored to ionic Ni3N surfaces was efficiently polarized (inset of Figure 1d and Figure S9); and the electron density distribution in the originally homogeneous carbon structure is not uniform anymore. The partially negatively charged C atoms interact with H2O molecules with the formation of C−H bonds and splitting of O−H bonds. Indeed, previous reports have suggested that in graphene-encapsulated transition-metal composites, electrons can transfer between core metals and external graphene, thus modifying the surface electronic structures and charge distributions.39,40 DFT simulations were further performed to get in-depth understanding of the HER catalytic mechanisms. According to the reaction pathways, we simulate the enthalpies of ratedetermining Volmer step on both carbon-coated and pristine Ni3N (110) surfaces. As shown in Figure 3d, the binding energy of H2O for dissociation on a carbon-coated Ni3N (110) surface is significantly reduced, by 0.67 eV, compared with the pristine Ni3N (110) surface (Figure 3e). This result is in accordance with the experimentally observed enhancement of HER activities in Ni3N@CQDs and suggests that the enhanced catalytic activity is largely governed by the more energy-favored water dissociation process. To check whether other carbon structures on Ni3N surfaces show a similar activity, we also prepared the Ni3N on graphene (Ni3N-GO). Figure S11 shows the TEM images of a Ni3N-GO E
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as N- and O-containing standard substances, respectively. The used sample quantity for each measurement was 5−15 mg. Electrochemical Characterization of HER Catalytic Activity. A standard three-electrode electrochemical measurement was applied for material electrochemical HER activity evaluations on a Solartron potentiostat in a 1 M KOH aqueous solution. An Ag/AgCl (saturated KCl) electrode and a Pt wire electrode were adopted as the reference and counter electrodes, respectively. Active materials were loaded onto a glassy carbon electrode (i. d. = 3 mm) for the preparation of working electrodes, see ref 31 for details. The loaded active material was ∼0.18 mg cm−2 on the glassy carbon electrode surfaces. A Pt electrode with the same diameter (i. d. = 3 mm) was used to test the Pt HER activity at the same measurement conditions for reference. Linear scan voltammetry (LSV) measurements were performed to obtain HER polarization curves for the samples at a scan rate of 10 mV s−1. I−t curves were recorded at a constant potential of −1.1 V vs Ag/ AgCl (77 mV vs RHE) for 20 h to evaluate the stability performances of the samples. The potential values vs RHE were calculated using an equation: ERHE = EAg/AgCl + 0.197 V + 0.059 V × pH. Computational Method. We consider a sp2-hybridized graphene layer placed on a Ni3N(110) facet as a simplified structural model for calculations. The natural defectiveness of the sp2 network was modeled as vacancy defects. Interfacial oxygen atoms link with the graphene vacancy and Ni atoms on the Ni3N(110) facets based on the experimental results, see Figure S9. All ab initio calculations of the atomic structures and stability of the graphene/Ni3N and Ni3N systems were performed using DFT within the Perdew−Burke−Ernzerhof functional.44 We used the projector augmented wave method45 approximation with the periodic boundary conditions and a Vienna ab initio Simulation Package.46−49 The planewave energy cutoff was set to 400 eV. To calculate the equilibrium atomic structures, the Brillouin zone was sampled according to the Monkhorst−Pack scheme50 with a 4 × 4 × 1 grid in the k-space. The structural relaxation was performed until the forces acting on each atom became
