MOF-derived Hierarchical Hollow NiRu-C Nanohybrid for Efficient Hydrogen Evolution Reaction①

WU Yu-Lin XIE Ning LI Xiao-Fang② FU Zhao-Ming WU Xin-Tao,c,d ZHU Qi-Long,c,d②

a (College of Chemistry,Fuzhou Uniνersity,Fuzhou 350108,China)

b (State Key Laboratory of Structural Chemistry,Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences,Fuzhou 350002,China)

c (Fujian Science &Technology Innoνation Laboratory for Optoelectronic Information of China,Fuzhou 350108,China)

d (Uniνersity of Chinese Academy of Sciences,Beijing 100049,China)

e (Physics and Electronic Information College,Yunnan Normal Uniνersity,Kunming 650500,China)

ABSTRACT Designing efficient electrocatalysts for efficient hydrogen evolution is extremely desired but challenging.Herein,we report a facile MOF-assisted strategy to synthesize the hierarchical hollow spherical NiRu-C nanohybrid with closely packed rod-like bulges on the surface.Benefited from the more exposed active sites of NiRu-C nanohybrid and the efficient electron/mass transport in its unique hierarchical hollow spherical nanostructure,the optimized nanohybrid showed excellent performance for alkaline hydrogen evolution with ultralow overpotentials,which are much superior to those of Pt/C and the overwhelming majority of reported electrocatalysts.The interpretation of the reaction mechanism was further discussed with DFT calculations.Our research may provide a guidance for the development of advanced electrocatalysts with controlled morphology and excellent performance for future energy applications.

Keywords:hydrogen evolution reaction,NiRu,hollow spherical nanostructure,electrocatalysis;DOI:10.14102/j.cnki.0254-5861.2011-3153

1 INTRODUCTION

Transition-metal-based catalysts,particularly the Ni-based catalysts with good durability and low cost,have proven to be universal and competitive electrocatalysts for various electrochemical reactions,which sheds light on the developing efficient catalysts for HER[19].Nevertheless,the pure Ni-based catalysts are usually unable to meet the requirement in terms of activity to replace the Pt-based catalysts,especially for HER due to the weak binding interactions between Ni and hydrogen.The past endeavor has shown that alloying noble metals with Ni,even with the amount of noble metals lower by an order of magnitude,is an effective route to modify the chemical properties of Ni-based catalysts with good balance between cost and efficiency[20].Among noble metals,Ru shows a more economic advantage with much lower cost than Pt.Although Ru exhibits the stronger bonding strength with hydrogen,the energy barrier of water dissociation is much less than that over Pt,which has aroused widespread attention over Ru-based catalysts towards alkaline HER[21].Therefore,the alloying of Ni and Ru can not only result in a relatively moderate metal-hydrogen bonding energy through a balance effect and accelerated water dissociation for efficient alkaline HER,but also cut down the dosage of noble metals for large-scale applications.Further-more,the activity of NiRu alloy is also highly relevant to their nanostructures,such as the morphology and dispersion state.Often,severe agglomeration of NiRu alloy leads to the burying of surface active sites and a deterioration in its catalytic performance.Carbon supports with good stability,high conductivity and confinement effect are widely used to construct unique nanostructures,which can not only make the NiRu alloy evenly dispersed but also improve the catalytic performance[22].

As a kind of crystalline porous materials with periodic network structures formed by self-assembly of transition metal ions and organic ligands,metal-organic frameworks (MOFs) have been extremely widespread as the precursors for the controllable preparation of carbon supported metal-based nanomaterial.As a kind of crystalline porous materials with periodic network structures formed by self-assembly of transition metal ions and organic ligands,metal-organic frameworks (MOFs) have been extremely widespread as the precursors for the controllable preparation of carbon supported metal-based nanomaterials[23-25].Furthermore,benefited from the tunability of MOFs,the postsynthetic modification of MOFs is capable of resulting into novel multicompositional MOF-based analogues,which can be used to construct unique nanostructures with designable composi-tion,exhibiting the superiority in facilitating the structural engineering[26,27].Herein,inspired by the above advantages,a nanohybrid with NiRu alloy nanoparticles (NPs) supported in the hierarchical hollow spherical carbon (NiRu0.06-C) was designed and synthesized by using a hollow spherical Ru3+-doped nickel-based MOF as the starting materials,followed by the pyrolysis under inert atmosphere.The NiRu0.06-C nanohybrid maintained the pristine hollow spherical structure with lots of rod-like bulges on the surface,where the NiRu alloy NPs with a low Ru content are homogeneously dispersed.By virtue of the unique hierar-chical hollow spherical nanostructure,the NiRu0.06-C nanohybrid showed an excellent performance towards alkaline HER with ultralow overpotentials,ranking top of the majority of the state-of-the-art catalysts.DFT calculations were further conducted to unveil the reaction mechanism.Our research may provide a guidance for the development of advanced electrocatalysts with controlled morphology and excellent performance for future energy applications.

2 EXPERIMENTAL

2.1 Materials and characterization

The PXRD measurements were performed on a Miniflex 600 Benchtop X-ray diffractometer at 40 kV with CuKαradiation to analyze crystal phases of the as-synthesized products.The SEM images were obtained at the accelerating voltage of 5 kV by a JEOL JSM6700-F instrument.The TEM and HRTEM images were recorded at the accelerating voltage of 200 kV by the FEI Tenia F20 instrument.The XPS data with monochromatic AlKαradiation were tested by a Thermo Fischer ESCALAB 250Xi X-ray photoelectron spectrometer (hν=1486.2 eV).Raman spectra were recorded on a Labram HR800 Renishaw in Via system (Horiba) by using a 532 nm laser.

2.2 Electrocatalytic HER measurements

The catalyst ink was prepared by mixing H2O (0.7 mL),EtOH (0.2 mL),nafion solution (0.1 mL,5 wt% in isopropanol) and catalyst (5.0 mg) together,followed by ultrasonication for 2 h.Then,the ink (6μL) was uniformly loaded onto a freshly polished glassy-carbon electrode (diameter=3.0 mm) as the working electrode with a loading of 0.4 mg·cm-2catalyst.The electrochemical measurements were performed at a CHI 660E electrochemical station (CH Instruments,USA) with a three-electrode electro-chemical cell,using a carbon rod and saturated calomel electrode (SCE) as the counter and reference electrodes,respectively.1 M KOH solution (pH=14) or 1 M KOH was used as electrolyte.All potentials of the electrochemical tests were performed as follows:

ERHE=ESCE+0.245+0.059 × pH (V).

Before going on the test,high-purity Ar gas was bubbled through the solution for at least 30 min.The LSV curves,unless otherwise specified,were recorded at a scan rate of 5 mV·s-1until the CV curves were stable without iR-compensa-tion in different electrolytes.TheCdlmeasured with CV curves,at a potential window between 0.07 and 0.12 V versus RHE,was performed with different scan rates from 15 to 40 mV·s-1.Based on the LSV curves,the Tafel plots were obtained by potential against log(current density).The EIS was recorded in the frequency ranging from 105to 0.01 Hz at a 5 mV AC amplitude.

2.3 DFT calculation

First-principle calculations were performed with using the Viennaab initiosimulation package (VASP),by which the geometric structures of all systems were relaxed and the energies were obtained correspondingly[28,29].The interaction between the electrons and ions was characterized by the method of projector augmented wave (PAW)[30].Electronic exchange and correlation interactions were described by the functional of generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE)[31].We expanded the Kohn-Sham orbitals by plane waves with a kinetic energy cutoff of 550 eV.The Monkhorst-Pack meshes of Brillouin zone is 4×4×1.The force and energy criteria on each atom are -0.05 eV/? and 10-5eV,respectively.The Ru occupied in Ni(200) and Ni(111) surfaces were modeled by supercells.A vacuum layer was added to eliminate the interactions of two neighboring supercells along thez-axis.

2.4 Synthesis of the complexes

Polyvinylpyrrolidone (PVP,Mw=40000) and ruthenium trichloride (RuCl3·xH2O,99%,37.5～41 wt% Ru) were purchased from Adamas Reagent Co.Ltd.China.Nickel nitrate hexahydrate (Ni(NO3)2·6H2O,≥ 98.0%),trimesic acid (H3BTC,A.R.),N,N-dimethylformamide (DMF,A.R.) and ethanol (C2H5OH,A.R.) were bought from Sinopharm Chemical Reagent Co.Ltd.China.All chemicals were used without further purification.

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2.4.1 Hollow spherical Ni-MOFs

The hollow spherical Ni-MOF was synthesized under solvothermal conditions.Typically,216 mg of Ni(NO3)2·6H2O,750 mg of PVP and 75 mg of H3BTC were dissolved in 15 mL of a solution containing water,ethanol and DMF (The volume ratio is equal to 1/1/1).After vigorous stirring for 40 min,the uniform light green solution was transferred into a 30 mL Teflon-lined stainless-steel autoclave and heated to 150 ℃ for 16 h.The green products were collected by centrifugation at 4000 rpm for 10 min,washed with ethanol for 3 times and dried at 60 ℃ for 8 h,giving hollow spherical Ni-MOF.

2.4.2 Solid spherical Ni-MOFs

The solid spherical Ni-MOF was synthesized by using similar procedures with hollow spherical Ni-MOF,except for reducing the solvothermal reaction time to 3 h.

2.4.3 Hollow spherical NiRu-C nanohybrid

The hollow spherical NiRu-C nanohybrid was synthesized by using Ru3+-doped Ni-MOF as the precursor.Firstly,the as-synthesized hollow spherical Ni-MOF was immersed into the ethanol solution containing different amounts of RuCl3·xH2O for 12 h.Then the impregnated Ni-MOF was dried at 40 ℃ for several hours,obtaining Ru3+-doped Ni-MOF.Finally,the hollow spherical NiRu-C nanohybrid was prepared by pyrolyzing the Ru3+-doped Ni-MOF at 600 ℃ for 1 h at a heating rate of 2 ℃·min-1under Ar atmosphere.For simplicity,the obtained samples prepared with 0.04,0.06 and 0.08 g RuCl3·xH2O were labeled as NiRu0.04-C,NiRu0.06-C and NiRu0.08-C,respectively.

3 RESULTS AND DISCUSSION

Fig.1 shows the schematic illustration for the preparation of hierarchical hollow spherical NiRu0.06-C nanohybrid.Firstly,the hollow spherical Ni-MOF was solvothermally synthesized,during which the initially solid spherical Ni-MOF particles with smooth surface were gradually transformed into the hollow ones with uniform rod-like bulges on the surface[32].The growth mechanism can be explained by Ostwald ripening,where the inner substance of the solid spheres dissolved and diffused to the surface,forming the hollow interior and generating rod-like bulges on the exterior with the prolonged reaction time.Then,the hollow spherical Ni-MOF was impregnated in an aqueous solution of RuCl3·xH2O to obtain the hollow spherical Ru3+-doped Ni-MOF.The powder X-ray diffraction (PXRD) patterns of both Ni-MOF and Ru3+-doped Ni-MOF show similar characteristic peaks,which suggests that the incorporation of Ru3+has no influence on the crystalline structure of the Ni-MOF (Fig.2).The typical scanning electron microscopy (SEM) image of Ru3+-doped Ni-MOFs in Fig.3a also reveals its similar hollow spherical nanostructure to that of parent Ni-MOF.Finally,the Ru3+-doped Ni-MOF was pyrolyzed in an Ar atmosphere toin situsynthesize the hierarchical hollow NiRu0.06-C nanohybrid,during which the Ni and Ru atoms joined to form NiRu alloy NPs that homogeneously dispersed onto the hierarchical hollow carbon shells.

Fig.1.Schematic illustration for the preparation of hierarchical hollow NiRu0.06-C nanohybrid

Fig.2.PXRD patterns of hollow spherical Ni-MOF and Ru3+-doped Ni-MOF

Fig.3.SEM images of (a) hollow spherical Ru3+-doped Ni-MOF;(b,c) SEM,(d,e) TEM and (f) HRTEM images of hollow spherical NiRu0.06-C nanohybrid;(g) HAADF-STEM and corresponding element mapping images of NiRu0.06-C for Ni,Ru,C and O elements

The SEM images of the as-prepared NiRu0.06-C nanohybrid are shown in Figs.3b～3c.As it can be seen,the NiRu0.06-C nanohybrid derived from the Ru3+-doped Ni-MOF can well maintain the hollow spherical nanostructure,where there are closely packed rod-like bulges on the surface.The transmission electron microscopy (TEM) images further attested the hollow spherical nanostructure of the NiRu0.06-C nanohybrid with rod-like bulges on the exterior (Fig.3d).The average length of the bulges was about 300 nm.The magnified TEM image of the hollow shell in Fig.3e unambiguously revealed the distribution of crystalline RuNi alloy NPs over the carbon layer,as evidenced by the energy-dispersive X-ray (EDX) analysis (Fig.4).

Fig.4.EDX analysis of the NiRu0.06-C nanohybrid

The crystalline structure of the NiRu alloy NPs was clearly identified in high-resolution TEM (HRTEM) image (Fig.3f),in which,besides the looming lattice fringes of graphitic carbon with the interlayer distance of 0.34 nm,the lattice fringes with spacing of 0.199 and 0.226 nm near the edge are closely to the (101) and (100) planes of NiRu alloy.In addition,the lattice fringes with spacing of 0.210 nm attributed to the (111) plane of the cubic Ni can also be clearly observed.The contents of Ni and Ru in the NiRu0.06-C nanohybrid were determined to be 66.41 and 2.67 wt% by the inductively coupled plasma atomic emission spectrometer (ICP-AES),which is consistent with the EDX result.Moreover,the high-angle annular dark-field scanning TEM (HAADF-STEM) and the corresponding element mapping are shown in Fig.3g,confirming the homogeneous distribution of Ni and Ru elements throughout the hollow spherical nanostructure of the NiRu0.06-C nanohybrid.

Fig.5.(a) N2 sorption isotherms and corresponding pore size distribution;(b) PXRD and (c) Raman spectra of NiRu0.06-C and Ni-C;High-resolution (d) C 1s+Ru 3d,(e) Ru 3p and (f) Ni 2p spectra of NiRu0.06-C

The specific surface and porosity of the hierarchical hollow spherical NiRu0.06-C nanohybrid were investigated by the Brunauer-Emmett-Teller (BET) method.As shown in Fig.5a,similar I/IV-type isotherms with high N2absorption and characteristic hysteresis loop at higher pressure verified the hierarchically porous structures of both hollow spherical NiRu0.06-C and Ni-C.The obvious mesopores and micropores with a sharp peak in aperture distribution at about 1.1 and 3.8 nm can be observed over the NiRu0.06-C nanohybrid (in the inset).Benefited from the distributed abundant micropores and mesopores within the shells,as well as the macropores between the closely packed bulges,the NiRu0.06-C nanohybrid exhibits a BET surface area of 136.71 m2·g-1and a pore volume of 0.42 cm3·g-1,which are similar with those of the Ni-C sample (142.38 m2·g-1and 0.45 cm3·g-1).Furthermore,the crystalline structure of the NiRu0.06-C nanohybrid was characterized by the PXRD analysis.As shown in Fig.5b,besides the diffraction peak of graphite carbon at about 21.0°,all the peaks of NiRu0.06-C can be attributed to the cubic Ni,which suggests that the NiRu alloy keeps the crystalline structure of metallic Ni,as evidenced by the TEM analysis.While a slight shift to lower diffraction angle was observed for NiRu0.06-C when compared with those of the Ni-C sample,implying the slight lattice expansion caused by the Ru atoms entered into the Ni lattices.In addition,the Raman spectrum of NiRu0.06-C nanohybrid shows obvious D band (1327 cm-1) and G band (1594 cm-1),which are corresponding to thesp3-type disordered carbon with edge plane defects and the defect-freesp2-type graphitized carbon,respectively (Fig.5c).The same intensity ratio of D and G bands (ID/IG) for NiRu0.06-C and Ni-C suggests that the formation of NiRu alloy does not distinctly disturb the structure of the graphitic carbon.Moreover,X-ray photoelectron spectroscopy (XPS) was performed to investigate the electronic structure and chemical state of the NiRu0.06-C nanohybrid.The survey spectrum in Fig.6a confirms the existence of Ni,Ru,C and O elements in the NiRu0.06-C nanohybrid.The high-resolution C 1sspectrum mixed with the Ru 3dspectrum in Fig.5d can be accorded with C-C,C-O and C=O of graphitic carbon.The further high-resolution Ru 3pspectrum of NiRu0.06-C with the binding energies at 485.6 and 463.0 eV are attributed to the Ru 3p1/2and Ru 3p3/2of Ru0(Fig.5e).The high-resolution Ni 2pspectrum of NiRu0.06-C in Fig.5f exhibits the characteristic peaks of Ni0with the binding energies at 856.2 and 874.1 eV for Ni 2p1/2and Ni 2p3/2,respectively.No metal-O bonding signal can be observed in the high-resolution O 1sspectrum (Fig.6b).These results verify the successful formation of bimetallic NiRu alloy and graphitic carbon support through doping Ru3+into Ni-MOF followed by pyrolysis treatment.

Fig.6.(a) Survey spectrum of the NiRu0.06-C nanohybrid;(b) High-resolution O 1s spectrum of the NiRu0.06-C nanohybrid

Fig.7.HER performance of the NiRu0.06-C nanohybrid and compared samples:(a) LSV plots,(b) Corresponding overpotentials at 10 mA·cm-2,(c) Tafel plots,(d) Current density differences vs. scan rates and corresponding yielded Cdl,(e) EIS plots and (f) Chronoamperometric measurement

Inspired by the unique hierarchical hollow spherical nanostructure with rod-like bulges on the surface,the electrocatalytic performance of the NiRu0.06-C nanohybrid for HER was evaluated in 1.0 M KOH in a typical three-electrode system,where the carbon rod and saturated calomel electrode (SCE) were used as the counter and reference electrodes,respectively.For comparison,the commercial 20 wt% Pt/C,the hollow spherical Ni-C and the NiRux-C nanohybrids with different Ru contents were also tested under the same conditions.All the electrochemical potentials were calibrated versus reversible hydrogen electrode (νs.RHE).As shown in Fig.7a,the current density of NiRu0.06-C was more negatively shifted compared with other compared samples,showing the excellent electrocatalytic activity for HER.The overpotential at 10 mA·cm-2for NiRu0.06-C is only 22 mV,which is far better than those of Pt/C (30 mV),Ni-C (74 mV),NiRu0.04-C (30 mV) and NiRu0.08-C (46 mV),respectively (Fig.7b).Even when compared with the recently reported electrocatalysts for alkaline HER,the fairly lower overpotential of NiRu0.06-C ranks it among the top few (Table 1).Furthermore,the outstanding HER performance of NiRu0.06-C was confirmed by its smaller Tafel slope (42.81 mV·dec-1) than those of Pt/C (45.18 mV·dec-1),Ni-C (76.54 mV·dec-1),NiRu0.04-C (53.53 mV·dec-1) and NiRu0.08-C (59.82 mV·dec-1),elucidating its favorable HER kinetics (Fig.7c).

Table 1.Comparison of Selected Recently Reported Representative HER Electrocatalysts

To probe the origin of the superior HER activity over the hierarchical hollow spherical NiRu0.06-C nanohybrid,the electrochemically active surface areas (ECSA) that are in direct proportion to the double layer capacitance (Cdl) were evaluated by the test of cyclic voltammetry (CV) in the non-faradaic potential region (Figs.7d and 8).The calculatedCdlvalue of NiRu0.06-C is 62.88 mF·cm-2,much larger than those of Pt/C (57.10 mF·cm-2) and Ni-C (21.86 mF·cm-2),implying that the NiRu alloy can provide more accessible active sites for electrocatalysis owing to the formation of hierarchical hollow nanostructure and high distribution of the active species.Furthermore,the catalytic kinetic of NiRu0.06-C was also estimated by electrochemical impedance spec-troscopy (EIS).As shown in Fig.7e,the smallest charge transfer resistance (Rct) of NiRu0.06-C indicates the rapid electron/proton transfer at the interfaces of the catalyst and electrolyte,which can facilitate the catalytic process and thus bring an excellent HER activity.Moreover,the macropores between the closely packed bulges and the hollow structure can serve as the buffering reservoirs where the electrolyte can shorten the diffusion distances and easily diffuse into the mesopores and micropores of NiRu0.06-C,thus greatly accelerating the mass transport during HER.Therefore,it can be logically deduced that the excellent HER activity of NiRu0.06-C is attributed to the more exposed active sites in the nanohybrids and the efficient electron/mass transport in the unique hierarchical hollow spherical nanostructure with rod-like bulges on the surface.

Fig.8.CV curves of (a) NiRu0.06-C,(b) Pt/C and (c) Ni-C between 0.07 and 0.12 V vs.RHE with an increasing scan rate of 5 mV·s-1 in 1.0 M KOH.The CVs of all samples exhibit the typical rectangular shapes of electrical double layer capacitors and the currents originate solely from electrical double layer charging and discharging

Besides,the stability of NiRu0.06-C nanohybrid plays an important role in the evaluation of its catalytic performance.As shown in Fig.7f,after the long-term stability tests for 80,000 s at an overpotential of 22 mV,NiRu0.06-C maintained much better activity as compared to Pt/C.In addition,the hierarchical hollow spherical nanostructure of NiRu0.06-C with rod-like bulges was well preserved after the chronoampero-metric measurement (Fig.9a).The PXRD analysis showed no significant crystalline change for NiRu0.06-C before and after the chronoamperometric measurement (Fig.9b).These results further confirm the excellent electrocatalytic stability of the NiRu0.06-C nanohybrid.

Fig.9.(a) SEM images of the NiRu0.06-C nanohybrid after the chronoamperometric measurement;(b) PXRD patterns of the NiRu0.06-C nanohybrid before and after chronoamperometric measurement

Fig.10.Atomic configurations of the intermediates on the surface of (a) NiRu-type I (200) and (b) NiRu-type II (111).Light gray,dark purple,red and white represents the Ni,Ru,O and H atoms,respectively.The free energy diagrams over (c) NiRu-type I (200) and Ni (200),(d) NiRu-type II (111) and Ni (111) during alkaline HER reaction

To reveal the underlying mechanism for the outstanding HER activity of NiRu0.06-C,DFT calculations were peformed.From the PXRD analysis of NiRu0.06-C,the NiRu alloy should present the Ni (200) and Ni (111) planes as the dominantly observed surface.Thus,the NiRu-type I (200) with Ru doping in Ni (200) surface and NiRu-type II (111) with Ru doping in Ni (111) surface were modeled to represent the NiRu alloy in NiRu0.06-C.Based on the experimentally measured Tafel slope values,the alkaline HER reaction over NiRu0.06-C follows more likely the Volmer-Heyrovsky step,which includes the dissociation of H2O to form the surface-bonded H*and OH*,the desorption of OH*to release the surface sites and finally the desorption of H*to generate gaseous H2.The atomic configurations of the intermediates during HER on the modeled surfaces are shown in Figs.10a and b.The corresponding free energy diagrams over the Ni surfaces with and without the Ru doping were also calculated.As shown in Fig.10c,the energy barrier for the desorption of H*can be obviously reduced over NiRu-type I (200) with Ru doping compared to the prinstine Ni (200).Similar enhancement effect derived from the Ru doping can also be observed over NiRu-type II (111) (Fig.10d).Although the further comparison in Fig.11 indicates that the prinstine Ni (200) and NiRu-type I (200) surfaces are more active than the prinstine Ni (111) and NiRu-type II (111) surfaces for the alkaline HER reaction,the desorption of H*over both Ru-doped surfaces are thermodynamically favourable due to the decreased energy barrier,which can lead to the superior HER activity of NiRu0.06-C than that of Ni-C,well consistent with the experimental demonstration.

Fig.11.Free energy diagrams over (a) Ni (200) and Ni (111),(b) NiRu-type I (200) and NiRu-type II (111) during alkaline HER reaction

4 CONCLUSION

To sum up,we report a scalable MOF-assisted strategy to synthesize the hierarchical hollow spherical NiRu-C nanohybrid with closely packed rod-like bulges on the surface.Benefited from the exposed active sites of NiRu alloy with high intrinsic activity and the efficient electron/mass transport in the unique hierarchical hollow spherical nanostructure,the optimized NiRu0.06-C nanohybrid showed excellent perfor-mance for alkaline HER with ultralow overpotentials,which are superior to those of Pt/C and the majority of reported electrocatalysts.Our research may provide a guidance for the development of advanced electrocatalysts with controlled morphology and excellent performance for future energy applications.