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Stabilizing and Activating Metastable Nickel Nanocrystals for Highly Efficient Hydrogen Evolution Electrocatalysis Qi Shao, Yu Wang, Shize Yang, Kunyan Lu, Ying Zhang, Chongyang Tang, Jia Song, Yonggang Feng, Likun Xiong, Yang Peng, Yafei Li, Huolin L. Xin, and Xiaoqing Huang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06896 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Stabilizing and Activating Metastable Nickel Nanocrystals for Highly Efficient Hydrogen Evolution Electrocatalysis Qi Shao†#, Yu Wang‡#, Shize Yang§‖#, Kunyan Lu†, Ying Zhang†, Chongyang Tang†, Jia Song†, Yonggang Feng†, Likun Xiong┴, Yang Peng┴, Yafei Li‡*, Huolin L. Xin‖, and Xiaoqing Huang†,* †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China.

‡

Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China. §

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States. ‖

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States.

┴

Soochow Institute for Energy and Materials Innovation, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou, Jiangsu 215006, China. #

These authors contributed equally.

*

Address correspondence to: [email protected]; [email protected].

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ABSTRACT: Exploring high-performance and cost-efficient electrocatalysts with unusual metastable phase offers opportunities for improving the electrochemical hydrogen generation, while it remains a great challenge to achieve them with desirable activity and stability. Herein, we report that the doping engineering in a metastable, hexagonal-close-packed nickel (hcp Ni) electrocatalyst is a largely unrevealed yet important factor in achieving the extremely active and stable electrocatalyst towards alkaline hydrogen evolution reaction (HER). Theoretical predications and experimental results suggest that, while the stability of metastable hcp Ni electrocatalyst can be largely improved via the manganese (Mn) doping due to the lower formation energy and lattice stabilization, the MnO/hcp Ni surface promotes the HER via intrinsic favorable H2O adsorption and fast water dissociation kinetics. Consequently, the Mn doped hcp Ni electrocatalyst shows a small overpotential of 80 mV at 10 mA/cm2 and a low Tafel slope of 68 mV/dec. The result is even approaching to that of the commercial Pt/C, being one of the best reported non-noble metal HER electrocatalysts in alkaline media. Under long-term chronopotentiometry measurement, such electrocatalyst can endure at least 10 h with negligible activity decay and structure change. The present work demonstrates the dimension in boosting the electrocatalysis by doping engineering of metastable electrocatalyst.

KEYWORDS: metastable, nickel, hexagonal-close-packed, doping, hydrogen evolution reaction


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Hydrogen (H2) is considered as one of the most promising energy carriers for the future sustainable energy conversion owing to its clear and renewable features.1-3 Pursuing highly efficient electrocatalysts for electrochemical water splitting to generate H2 has thus paid much attention. Although platinum (Pt) has been shown the best hydrogen evolution reaction (HER) performance,4,5 its high cost and low abundance largely restrict its practical applications.6,7 It is highly desirable to design cost-efficiency and high-performance candidates to replace the precious electrocatalysts. As the fourth most abundant transitional metal on earth,8 particular emphasis has been placed on nickel (Ni)-based electrocatalysts.9-15 Unfortunately, the current Ni-based catalysts still require large overpotentials to achieve favorable activity since the unsuitable absorption energy on Ni metal surface, leading to the slow desorption rate.16,17 Therefore, pursuing an optimal absorption of the adsorbates on Ni for HER becomes a subject of interest to be studied. In principle, achieving metastability in a solid may endows many possibilities for catalytic reaction due to the unequilibrated metal surface caused by the high reactivity and fast mobility of the surface atoms.18 Ni owns a metastable phase, hexagonalclose-packed (hcp) phase, while the synthetic methods reported so far are limited to the high-temperature chemical reactions or thin film technology, heavily hindering their applications.19,20 In addition, since the electrocatalysis is a surface-sensitive reaction, the surface-active metastable phase provides an important platform for generating a complete different surface environment, which may overcome the limitation of Ni in HER.21,22 Although the metastable catalyst owns the attractive catalytic activities, it endows a native drawback of inherent low stability because of the quick elimination of the active metastable phase under the electrochemical reaction.23 How to improve the stability yet enhance the activity of metastable electrocatalysts becomes an open challenge.


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Herein, we report the creation of a class of metastable hcp Ni nanoparticles (NPs) via the doping engineering as an extremely active and stable electrocatalyst towards alkaline HER. While the stability of the metastable hcp Ni electrocatalyst in electrochemical reaction can be largely improved via Mn doping due to the lower formation energy and lattice stabilization, the MnO/hcp Ni surface promotes the HER via favorable H2O adsorption and fast water dissociation kinetics, as understood by both the theoretical predications and experimental results. As a result, it showed an extremely low overpotential of 80 mV at 10 mA/cm2, a Tafel slope of 68 mV/dec, as well as a high TOF value of 0.53/s at the overpotential of 100 mV, which is regarded as one of the best non-precious electrocatalysts so far. The Mn doped hcp Ni electrocatalyst is also very stable with negligible activity decay and structure change after long-term chronopotentiometry measurement of at least 10 h. This work highlights the importance of doping engineering of electrocatalyst for practical electrocatalysis and beyond. RESULTS AND DISCUSSION The crystal structures of hcp Ni and face centered cubic (fcc) are exhibited in Figure 1a, where the lattice constants a and c of hcp Ni are 2.651 Å and 4.343 Å and the lattice constant a of face centered cubic (fcc) Ni is 3.523 Å. The packing densities of hcp Ni and fcc Ni are calculated to be 60.4% and 73.1% (Figure S1 and Note S1), in which the packing density of fcc Ni is 21.0% larger than that of hcp Ni. Therefore the hcp phase is loosely packed, which is detrimental for maintaining the crystal structure under the severe corrosion condition. In order to stabilize this metastable phase, doping the secondary element with the radius larger than Ni may provide an effective way to manipulate the lattice stability. To better reveal the doping effect on hcp Ni stabilization, we computed the formation energy (Ef) of hcp Ni with different transition metal dopants (including: cobalt (Co), iron (Fe), Mn, copper (Cu), and calcium (Ca)) by using density functional theory (DFT) simulations (Figure 1b and Note S2).


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Encouragingly, it was found that the Mn dopant manifests a considerable low Ef, indicating that Mn is a desirable candidate on stabilizing the metastable hcp Ni.

Figure 1. (a) Schematic structures of hcp Ni and fcc Ni. (b) The calculation of formation energy (Ef) and phases of hcp Ni with metal-dopants. (c) XRD patterns of Ni with different metal dopants.

To verify the theoretical predictions, we attempted to prepare metal (including Co, Fe, Mn, Cu, and Ca) doped hcp Ni by using a nonaqueous synthetic methodology, where glucose acted as the reducing agent and oleylamine was the solvent. The atom radii of these elements are listed as follows: RCo = 1.25 Å, RFe = 1.26 Å, RMn = 1.27 Å, RCu = 1.28 Å, and RCa = 1.97 Å. The atomic radius of Ni is 1.24 Å.24 As shown in Figure 1c, (1) when RNi (1.24 Å) < Rdopant < RMn (1.27 Å), both the fcc and hcp phases were


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existed in the products. The radii of these metal dopants are not large enough and can only partly stabilize the metastable phase; (2) When Rdopant = RMn (1.27 Å), only the hcp phase was formed in the product, indicating that the radius of Mn dopant can fully stabilize the hexagonal phase; (3) When Rdopant > RMn (1.27 Å), both the fcc and hcp phases were observed in the products, indicating that the larger radius of this metal dopant is not suitable to stabilize the metastable phase; (4) When Rdopant (1.97 Å) >> RMn (1.27 Å), only the fcc phase was generated in the product. The radius of the dopant is too large to stabilize the metastable phase in the hcp Ni. For structural and electrocatalytic comparisons, hcp Ni NPs and fcc Ni electrocatalyst (fcc Ni/C) were also created, which were synthesized by keeping the same parameters with Mn-hcp Ni NPs but without metal dopant and by annealing the hcp Ni/C, respectively (see Experimental Procedures for details).20 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscope (TEM) observations reveal the highly dispersed Mn-hcp Ni NPs and hcp Ni NPs have similar morphologies with the average diameters of 32.1 nm and 28.9 nm, respectively (Figure 2a,d and Figures S2-7). The TEM and high resolution TEM (HRTEM) images show the stacking faults in the hcp Ni particles, which might be due to the fast growth kinetics (Figure S8). No obvious morphological changes were observed in fcc Ni/C (Figure 2c and Figures S9c,7). The crystal structures were identified by the HRTEM image and the selected-area electron diffraction (SAED) (Figure 2d-f). The related diffraction patterns confirmed the hcp phase of Mn-hcp Ni NPs and hcp Ni NPs and the fcc phase of fcc Ni NPs (Figure 2d-f insets). X-ray diffraction (XRD) shows that only the sharp diffraction peaks belong to hcp Ni (JCPDS 89-7129, P63/mmc) were detected in Mn-hcp Ni NPs (Figure 2g). No detectable traces of Mn and MnO2 were observed. If we have a close view on the XRD patterns of Mn-hcp Ni NPs and hcp Ni NPs, a peak shift towards the smaller angle is noticed, which is likely attributed to the larger atom radius of Mn (1.27 Å) than that of Ni (1.24 Å). The XRD pattern also


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confirms the formation of fcc Ni (JCPDS 88-2326, Fm3m). The molar ratio of Ni to Mn in Mn-hcp Ni NPs is ~ 98.5 : 1.5 (Figure 2h).

Figure 2. (a-c) STEM images and (d-f) HRTEM images and corresponding SAED (insets) of (a, d) Mn-hcp Ni NPs, (b, e) hcp Ni NPs and (c, f) fcc Ni/C. (g) XRD patterns of Mn-hcp Ni NPs, hcp Ni NPs and fcc Ni/C. (h) TEM-EDX analyses of Mn-hcp Ni NPs and hcp Ni NPs. Scale bars: (a, b) 50 nm, (c) 100 nm and (d, e, f) 1 nm.

The detailed elemental distribution of Ni and Mn in Mn-hcp Ni NPs was initially characterized using the HAADF-STEM energy-dispersive X-ray spectroscopy (EDS) element mappings (Figure 3a), where the Ni, Mn, and combined images confirm the homogeneous doping of Mn into the Mn-hcp Ni NPs. Since electrocatalysis is a surface-sensitive reaction, the abreaction-corrected HAADF-STEM technique was further carried out to reveal the surface structure of Mn-hcp Ni NPs. HAADF-STEM image shows that the particle edge has random regions with low contrast compared to that of the interior,


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indicating a different phase partially covers on the particle (Figure 3b). The surface element distribution was further checked by electron energy loss spectroscopy (EELS) elemental mapping, where Ni (blue), Mn (pink) and O (yellow) display the coverage of Mn and O on the Ni surface (Figure 3c). EELS analysis indicates that Mn at the surface is in the form of MnO (Figure 3d). To identify the phase distribution of MnO and hcp Ni, the enlarged abreaction-corrected HRTEM image was deeply studied. The tiny cubic MnO cluster were observed on the surface of Mn-hcp Ni NPs, where (11-1) and (2-20) planes of MnO and (0002) and (11-20) planes of hcp Ni were observed, suggesting the epitaxial growth of MnO on hcp structure (Figure 3e). All the above results collectively confirm the successful doping of Mn into Mn-hcp Ni and the island epitaxial growth of cubic MnO clusters on Mn-hcp Ni (Figure 3f).

Figure 3. (a) STEM-EDS elemental mapping of Mn-hcp Ni NPs. (b) High-resolution TEM image, (c) STEM-EELS elemental mapping and (d) EELS analysis of surface Mn-hcp Ni NPs. (e) Atomic-resolution HRTEM image and simulated HAADF image of surface Mn-hcp Ni NPs. The atomic models are superimposed on the experimental images. (f) Schematic diagram of Mn-hcp Ni NPs. Scale bars: (a) 10 nm, (b) 2 nm and (e) 0.5 nm.


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Considering that Ni is a promising candidate for HER, the HER performance of the Mn-hcp Ni with simultaneous phase and doping control deserves evaluation. As shown in Figure 4a, the overpotentials of hcp Ni/C and fcc Ni/C are 105 mV and 117 mV at the current density of 10 mA/cm2 in 1.0 M KOH, respectively, indicating that the hcp phase owns higher HER activity than the fcc phase. More interesting, with doping Mn into hcp Ni, only overpotentials of 80 and 111 mV at current densities of 10 and 20 mA/cm2 are achieved, respectively, approaching to those of the commercial Pt/C (62 mV and 107 mV at 10 and 20 mA/cm2, respectively). The current density of Mn-hcp Ni/C is even larger than that of Pt/C when the overpotential is larger than 121 mV. The Mn-hcp Ni/C also exhibits the lowest Tafel slope of 68 mV/dec (Figure 4b), which is comparable to that of the Pt/C (51 mV/dec). Encouragingly, both the overpotential and Tafel slope realized by Mn-hcp Ni/C are lower than most previous reported HER catalysts in alkaline electrolyte (Table S1). We further explored the turnover frequency (TOF) values of different electrocatalysts for evaluating the intrinsic electrocatalytic activity (Notes S3-5). As exhibited in Figure 4c, the Mn-hcp Ni/C achieved TOF values of 0.14 H2/s, 0.53 H2/s and 1.45 H2/s at the overpotentials of 50 mV, 100 mV and 200 mV, much higher than the TOF values of hcp Ni and fcc Ni at the overpotentials of 50 mV, 100 mV and 200 mV (0.08 H2/s, 0.35 H2/s and 1.01 H2/s and 0.03 H2/s, 0.19 H2/s and 0.44 H2/s). The TOFs achieved by Mn-hcp Ni/C are also higher than many reported catalysts, such as α-Mo2C,25 γ-Mo2N,25 NiMo,26 Ni5P4,27 and S-CoO NRs,28 confirming the excellent HER performance of the Mn-hcp Ni/C.


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Figure 4. (a, b) HER polarization plots and the Tafel slopes of Mn-hcp Ni/C, hcp Ni/C, fcc Ni/C and Pt/C. (c) TOF plots of Mn-hcp Ni/C, hcp Ni/C and fcc Ni/C against overpotential and TOF values of the commercial Pt/C and the reported electrocatalysts at specific overpotentials (α-Mo2C,25 γ-Mo2N,25 NiMo,26 Ni5P4,27 S-CoO NRs28 and MoNi411). a

The electrolyte used in Ref. 26 is 2 M KOH. (d-h) The polarization curves and the comparison of overpotentials at -10

mA/cm2 before and after 10 h chronopotentiometry tests of Mn-hcp Ni/C, hcp Ni/C, fcc Ni/C and Pt/C.

Significantly, the Mn-hcp Ni/C also exhibits excellent electrochemical stability. As shown in Figure 4d-h and Figure S14, after long-term chronopotentiometry (CP) measurement at a constant current density of 10 mA/cm2 for 10 h, Mn-hcp Ni/C shows an overpotential increase of only 7 mV. On the sharp contrast, the hcp Ni/C, fcc Ni/C and Pt/C undergo obvious overpotential increases of 27 mV, 24 mV and 21 mV, respectively, clearly showing the enhanced electrochemical stability of Mn-hcp Ni/C. In addition, although the serious aggregation was observed for the commercial Pt/C, Mn-hcp Ni/C can largely keep the original structures (Figures S15-17). The excellent performance achieved by Mn-hcp Ni/C demonstrates that doping Mn not only improves the electrocatalytic activity, but also enhances the electrocatalytic stability.


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Figure 5. (a) Surface valence band photoemission spectra of Mn-hcp Ni, hcp Ni and fcc Ni. The black lines point to the locations of d-band center. (b) The comparison of d-band centers of Mn-hcp Ni, hcp Ni and fcc Ni. (c) Calculated energy diagram of water dissociation on the three metal slabs. (d-f) Atomic configurations of (d) initial state, (e) transition state, and (f) final state of water dissociation on Mn-hcp Ni. Sky-blue, pink, dark yellow and green balls represent Ni, Mn, O, and H atoms, respectively, and the outermost Ni atoms are highlighted by light-cyan.

It is known that the electronic effect plays an important role on manipulating the HER performance by changing the chemisorption energies of different adsorbates, and the position of d-band center is intrinsically associated with the binding intensity of adsorbate, where a moderately strong adsorption to H2O is indispensable to launch alkaline HER process (2H2O + 2e-  H2 + 2OH-) and a temperately higher d-band center is required for a good HER catalyst. To this end, we firstly computed the d-band centers of fcc Ni (111), hcp Ni (0001), and Mn-hcp Ni (0001) planes by DFT to understand the origin of the excellent HER activity of Mn-hcp Ni/C (Computational details and Figure S18). Figure 5a,b exhibit an up-shift of the location of d-band center from fcc Ni (-1.32 eV) to hcp Ni (-1.31 eV) and Mnhcp Ni (-0.83 eV), indicating that metastable-Ni and especially Mn-doping may strengthen H2O adsorption. As expected, Mn-hcp Ni has the strongest binding energy to H2O (EB[H2O]) with a value of -


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0.53 eV, followed by hcp Ni (-0.10 eV) and fcc Ni (-0.09 eV). Intriguingly, the correlation between the d-band center and EB[H2O] is identified in three metal surfaces (Figure S19). The above results indicated that the transformation of Ni phase from fcc to hcp would lead to a tiny upshift of d-band center and result in the slightly improved binding strength between H2O and Ni surface. Nevertheless, the addition of Mn would further promote an upshift of 0.48 eV for the d-band center, leading to a significantly enhanced H2O adsorption. Therefore, H2O molecule can be well activated on the surface of Mn-hcp Ni, which accelerates the Volmer reaction (H2O + e-  H* + OH-) thermodynamically. As the activity of HER catalyst in the alkaline media is also mainly dominated by the kinetics of water dissociation to produce adsorbed H, we further theoretically investigated the energy barrier of water dissociation step (Ebar). The energy diagram of the dissociation process is shown in Figure 5c, and the related key reaction states of Mn-hcp Ni are listed in Figure 5d-f. From the energy diagram, it reveals that the Ebar of hcp Ni (0.99 eV) is slightly lower than that of fcc Ni (1.00 eV), implying that the adsorbed H formation on metastable hcp Ni in alkaline media is more kinetically favorable. Remarkably, the Ebar of hcp Ni can be significantly reduced to be 0.63 eV with Mn-modification, which is a big advantage for alkaline HER. Note that the relatively small Ebar in Mn-hcp Ni also demonstrate that its thermodynamic EB[H2O] is not too robust to hinder the subsequent step, although Mn-hcp Ni exhibits the strongest H2O binding among these three surfaces.29 Overall, Mn-hcp Ni can exhibit superior HER activity in alkaline solution due to the intrinsic favorable H2O adsorption and fast water dissociation kinetics, followed by pristine hcp Ni and fcc Ni, achieving good agreements with experimental results. CONCLUSIONS To summarize, we report a doping engineering approach for making a class of metastable hcp Ni NPs as a highly efficient and stable electrocatalyst towards alkaline HER. The native weak stability of metastable electrocatalysts can be greatly alleviated via introducing the metal (Mn) due to the lower


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formation energy and lattice stabilization. DFT calculation shows that the MnO/hcp Ni surface plays a critical role on boosting the HER performance by the intrinsic favorable H2O adsorption and fast water dissociation kinetics. Consequently, the Mn-hcp Ni achieves a low overpotential of 80 mV at 10 mA/cm2, Tafel slope of 68 mV/dec as well as a high TOF value of 0.53 s-1 at the overpotential of 100 mV, being regarded as one of the best non-precious electrocatalysts so far. Under long-term chronopotentiometry measurement, the Mn doped hcp Ni electrocatalyst can endure at least 10 h with negligible activity decay and structure change. This work highlights the design of the highly active and stable metastable electrocatalysts for future energy applications and beyond.

EXPERIMENTAL SECTION Chemicals. Nickel acetate tetrahydrate (Ni(Ac)2∙4H2O, C4H14NiO8, 98%) was purchased from STREN. Copper acetylacetonate (Cu(acac)2, C10H14CuO4, 97%), cobalt(II) acetylacetonateb (Co(acac)2, C10H14CoO4, 98%), ferrous acetylacetonate (C4H14FeO8, 98%), manganese acetylacetonate (Mn(acac)2), 97%),

nickel

acetylacetonate

(Ni(acac)2,

C10H14NiO4,

97%)

and

oleylamine

(CH3(CH2)7CH=CH(CH2)7CH2NH2, C18H35NH2, OAm, 68-70%) were purchased from J&K. Nickel formate dehydrate (Ni(HCO2)2∙2H2O, 96%) was purchased from Alfa Aesar. Nafion 117 solution (~5% in a mixture of lower aliphatic alcohols and water) was purchased from Sigma-Aldrich. Calcium acetylacetonate (Ca(acac)2, C10H14CaO4, 98.5%), glucose (C6H12O6, 99%), potassium hydroxide (KOH, 96%),

nickel

chloride

hexahydrate

(NiCl2∙6H2O),

cyclohexane

(C6H12,

>99.5%),

ethanol

(C2H6O, >99.7%) and isopropanol (C3H8O, >99.7%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The commercial Pt/C (20 wt% loading, Pt size of 2-5 nm) was purchased from Johnson Matthey (JM). Carbon powder (XC72R) was purchased from Vulcan. The water (18 MΩ/cm) used in all experiments was prepared by passing through an ultra-pure purification system. Electrocatalyst preparation. In the typical synthesis of hcp Ni nanoparticles (NPs), 12.8 mg Ni(Ac)2∙4H2O, 60 mg glucose, and 5.0 mL OAm were put into a 35 mL glass bottle. The mixture was sonicated for 2 h before heating at 190 °C for 5 h in an oil bath. The resulting black product was washed with a mixture solution (Vcyclohexane : Vethanol = 9 : 1) by three times, then naturally dried at room


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temperature. For Mn-hcp Ni NPs, 1.2 mg Mn(acac)2 was added with keeping the other parameters the same as those of hcp Ni NPs. To prepare the electrocatalyst, one glass bottle product was mixed with 10 mg carbon powder (Vulcan XC72R carbon) in 5 mL cyclohexane and 15 mL ethanol. The mixture was sonicated for 2 h before collected by centrifugation and dried naturally. The resulted products were then annealed at 200 °C in the 5%H2/95%N2 mixed gas for 1 h. The fcc Ni electrocatalyst was obtained by annealing hcp Ni electrocatalyst at 300 °C under the 5%H2/95%N2 mixed gas for 1 h. Characterizations. Crystal structure was analyzed by using an X`Pert-Pro X-ray powder diffractometer equipped with a Cu radiation source (λ = 0.15406 nm). TEM images were collected by transmission electron microscope (TEM, Hitachi, HT7700) with the accelerating rate of 120 kV. The HRTEM, STEM, HAADF-STEM, EDS and EDS elemental mapping were obtained by FEI Tecnai F20 TEM at an accelerating voltage of 200 kV. XPS analysis was performed on a Thermo Scientific ESCALAB 250 XI X-ray photoelectron spectrometer. All data were calibrated by C 1s peak at 284.8 eV. The annular darkfield images (ADF) were collected using a Nion UltraSTEM200 microscope operated at 200 kV. Some of the images were filtered using a Gaussian function (full width half maximum = 0.12 nm) to remove high-frequency noise. The convergence half angle of the electron beam was set to 30 mrad and the collection inner half angle of the ADF detector was 80 mrad. The inner collection angle for EELS spectrum was 48 mrad. The samples were baked in vacuum at 150 °C overnight before STEM observation. Electrochemical measurements. The electrochemical measurements were performed by using a CHI 660E potentiostat with a three-electrode configuration. All the experiments were carried out at room temperature. Saturated calomel electrode and carbon rod were used as the reference and counter electrodes, respectively. The inks of different electrocatalysts were prepared by sonicating 5 mg catalyst with 0.5 mL isopropanol and 20 μL Nafion for 10 min. The working electrode was then fabricated by dropping 40 μL ink on a glass carbon electrode (GCE) with a geometric area of 0.192 cm2. The commercial Pt/C ink was prepared by sonicating 5 mg Pt/C (JM) with 0.5 mL isopropanol and 20 μL Nafion for 10 min. 10 μL ink was dropped on the GCE. Linear-sweep voltammograms and chronopotentiometry measurements were carried out to study the catalytic activity and stability, respectively. The potential at the current density of 10 mA/cm2 was chosen for the stability measurement. The electrolyte used was 1.0 M KOH and all polarization curves were iR corrected.


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Computational details. First-principles DFT computations were performed using VASP,[1,2] with ionelectron interactions described by the projector-augmented plane wave approach.[3,4] The exchangecorrelation interactions were modeled by RPBE within generalized gradient approximation functional. [5] In all computations, a cutoff energy of 400 eV was used with spin-polarization and a Methfessel-Paxton smearing of 0.2 eV. Convergence thresholds for energy and force were set as 3×10-5 eV and 0.03 eV/Å, respectively. The active surfaces of fcc Ni and hcp Ni were constructed as the 3×3×4 fcc Ni (111) and hcp Ni (0001) plane slabs, respectively, where only the top layer plus adsorbates were allowed to relax with (4×4×1) k-space sampling. For Mn-hcp Ni, following the Cs-STEM analysis, we focused on the interface between MnO and hcp Ni, which was represented by the 4×4×3 hcp Ni (0001) plane slab with a MnO cluster. Vacuum space was kept to be larger than 15 Å. Denser k-space sampling of 8×8×1 was used in electronic property calculations. Specially, the dipole corrections and aspherical contributions were included. The climbing-image nudged elastic band (CI-NEB)[6] technique was employed to search the transition state of water dissocation. The lowest-energy initial and final states have been checked carefully, and all the transition states have confirmed by frequency analyses (only one imaginary frequency).

ASSOCIATED CONTENT Supporting Information. Experimental details and data. Figures S1-16&Notes S1-5&Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected]; [email protected]. ACKNOWLEDGMENT This work was financially supported by Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135, 21522305), Young Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20150045, BK20170003) and Natural Science Foundation of Jiangsu Higher Education Institutions


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(17KJB150032), the project of scientific and technologic infrastructure of Suzhou (SZS201708), the start-up supports from Soochow University and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The electron microscopy at ORNL (S.Y.) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The work at BNL was supported by the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DESC0012704. Q. Shao, Y. Wang and S. Yang contributed equally.

REFERENCES 1. Chu, S.; Cui, Y.; Liu, N. The Path Towards Sustainable Energy. Nat. Mater. 2017, 16, 16-22. 2. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, Ib.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. 3. Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. 4. Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.-C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2Pt Interfaces. Science 2011, 334, 1256-1260. 5. Wang, P.; Zhang, X.; Zhang, J.; Wan, S.; Guo, S.; Lu, G.; Yao, J.; Huang, X. Precise Tuning in PlatinumNickel/Nickel Sulfide Interface Nanowires for Synergistic Hydrogen Evolution Catalysis. Nat. Commun. 2017, 8, 14580. 6. Gray, H. B. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, 7. 7. McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.; Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Evaluation of Pt, Ni, and Ni-Mo Electrocatalysts for Hydrogen Evolution on Crystalline Si Electrodes. Energy Environ. Sci. 2011, 4, 3573-3583.


16

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8. De, S.; Zhang, J.; Luque, R.; Yan, N. Ni-Based Bimetallic Heterogeneous Catalysts for Energy and Environmental Applications. Energy Environ. Sci. 2016, 9, 3314-3347. 9. Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. A Synthetic Nickel Electrocatalyst with a Turnover Frequency above 100,000 s-1 for H2 Production. Science 2011, 333, 863-866. 10. Jiang, J.; Gao, M.; Sheng, W.; Yan, Y. Hollow Chevrel-Phase NiMo3S4 for Hydrogen Evolution in Alkaline Electrolytes. Angew. Chem. Int. Ed. 2016, 55, 15240-15245. 11. Zhang, J.; Wang, T.; Liu, P.; Liao, Z.; Liu, S.; Zhuang, X.; Chen, M.; Zschech, E.; Feng, X. Efficient Hydrogen Production on MoNi4 Electrocatalysts with Fast Water Dissociation Kinetics. Nat. Commun. 2017, 8, 15437. 12. Li, H.; Chen, S.; Jia, X.; Xu, B.; Lin, H.; Yang, H.; Song, L.; Wang, X. Amorphous Nickel-Cobalt Complexes Hybridized with 1T-phase Molybdenum Disulfide via Hydrazine-Induced Phase Transformation for Water Splitting. Nat. Commun. 2017, 8, 15377. 13. Gong, M.; Zhou, W.; Tsai, M.-C.; Zhou, J.; Guan, M.; Lin, M.-C.; Zhang, B.; Hu, Y.; Wang, D.-Y.; Yang, J.; Pennycook, S. J.; Hwang, B.-J.; Dai, H. Nanoscale Nickel Oxide/Nickel Heterostructures for Active Hydrogen Evolution Electrocatalysis. Nat. Commun. 2014, 5, 4695. 14. Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270. 15. Duan, J.; Chen, S.; Zhao, C. Ultrathin Metal-Organic Framework Array for Efficient Electrocatalytic Water Splitting. Nat. Commun. 2017, 8, 15341. 16. Hammer, B.; Nrskov, J. K. Why Gold is the Noblest of All the Metals. Nature 1995, 376, 238-240. 17. Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and HydrogenInvolving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. 18. Spencer, M. S. Stable and Metastable Metal Surfaces in Heterogeneous Catalysis. Nature 1986, 323, 685-687.


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19. Papaconstantopoulos, D. A.; Fry, J. L.; Brener, N. E. Ferromagnetism in Hexagonal-Close-Packed Elements. Phys. Rev. B 1989, 39, 2526-2528. 20. Jeon, Y. T.; Moon, J. Y.; Lee, G. H.; Park, J.; Chang, Y. Comparison of the Magnetic Properties of Metastable Hexagonal Close-Packed Ni Nanoparticles with Those of the Stable Face-Centered Cubic Ni Nanoparticles. J. Phys. Chem. B 2006, 110, 1187-1191. 21.

Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Norskov, J. K. Computational High-

Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2016, 5, 909-913. 22. Wu, B.; Zheng, N. Surface and Interface Control of Noble Metal Nanocrystals for Catalytic and Electrocatalytic Applications. Nano Today 2013, 8, 168-197. 23. Orikasa, Y.; Maeda, T.; Koyama, Y.; Murayama, H.; Fukuda, K.; Tanida, H.; Arai, H.; Matsubara, E.; Uchimoto, Y.; Ogumi, Z. Direct Observation of a Metastable Crystal Phase of LixFePO4 under Electrochemical Phase Transition. J. Am. Chem. Soc. 2013, 135, 5497-5500. 24. Speight, J. G. Lange's Handbook of Chemistry, Sixteenth Edition, McGraw-Hill Education, 2005. 25. Ma, L.; Ting, L. R. L.; Molinari, V.; Giordano, C.; Yeo, B. S. Efficient Hydrogen Evolution Reaction Catalyzed by Molybdenum Carbide and Molybdenum Nitride Nanocatalysts Synthesized via the Urea Glass Route. J. Mater. Chem. A 2015, 3, 8361-8368. 26. McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. Ni-Mo Nanopowders for Efficient Electrochemical Hydrogen Evolution. ACS Catal. 2013, 3, 166-169. 27. Laursen, A. B.; Patraju, K. R.; Whitaker, M. J.; Retuerto, M.; Sarkar, T.; Yao, N.; Ramanujachary, K. V.; Greenblatt, M.; Dismukes, G. C. Nanocrystalline Ni5P4: a Hydrogen Evolution Electrocatalyst of Exceptional Efficiency in Both Alkaline and Acidic Media. Energy Environ. Sci. 2015, 8, 1027-1034. 28. Ling, T.; Yan, D.-Y.; Wang, H.; Jiao, Y.; Hu, Z.; Zheng, Y.; Zheng, L.; Mao, J.; Liu, H.; Du, X.-W.; Jaroniec, M.; Qiao, S.-Z. Activating Cobalt(II) Oxide Nanorods for Efficient Electrocatalysis by Strain Engineering. Nat. Commun. 2017, 8, 1509. 29. Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L.; Han, Y.; Chen, Y.; Jaroniec, M.; Qiao, S.-Z. High Electrocatalytic


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Hydrogen Evolution Activity of an Anomalous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 1617416181.


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Stabilizing and Activating Metastable Nickel ... - ACS Publications

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