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Ultrathin Graphene Layers Encapsulating Nickel Nanoparticles Derived Metal−Organic Frameworks for Highly Efficient Electrocatalytic Hydrogen and Oxygen Evolution Reactions Lunhong Ai,* Tian Tian, and Jing Jiang* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, 1# Shida Road, Nanchong 637002, People’s Republic of China

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ABSTRACT: The development of cheap and efficient electrocatalysts for promoting full water splitting is still challenging. Here, we report a composite architecture that consists of onion-like ultrathin graphene shells encapsulating uniform metallic nickel nanoparticles (Ni@graphene) derived by a straightforward thermal treatment of a Ni-based metal−organic framework in an inert atmosphere. The resulting Ni@graphene is highly catalytically active for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in 1.0 M KOH solutions. It only requires relatively low overpotentials (OER ∼ 370 mV; HER ∼ 240 mV) to yield a catalytic current of 10 mA/cm2, which compares favorably to most previously reported Ni-based elecrocatalysts for water splitting. The excellent performance would be attributed to the catalytic sites of metallic Ni and the intact metal protection effect of the outer graphene layers. KEYWORDS: Water splitting, Oxygen evolution, Hydrogen evolution, Nickel, Electrocatalyst

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carbon materials.6,7 Bao et al. found that encapsulation of CoNi alloy into graphene layers can efficiently prevent the corrosion of metals and simultaneously promote catalytic HER on the carbon surface owing to electron penetration from the encapsulated metals.8 Ye et al. reported the enhancement of HER activity by implanting metal nanoparticles in carbon cages and demonstrated that the outer carbon layers not only effectively avoided the corrosion of the catalyst under a harsh environment but also can be modified at the electronic Fermi level by the encapsulated cobalt cluster resulting from the charge transfer from cobalt to carbon atoms nearby.9 Furthermore, some carbon-encapsulated transition metal nanoparticles exhibit both good electrocatalytic activity and durability, including metal@graphdiyne, 10,11 Ni@C, 12,13 NiFe@C,14,15 NiMo@C,16 and [email protected]−19 Despite these advances, the outer carbon layers around the embedded metal nanoparticles in these structures are relatively thick, limiting the efficient exposure of the open metal-based active sites. With the benefit of the unique structure of metal−organic framework (MOF), we herein employ a Ni-based MOF as a pyrolytic precursor to create confined reactors and in situ implant metallic Ni nanoparticles in onion-like graphene shells for achieving a robust and durable electrocatalyst for water splitting. The resulting Ni@graphene exhibits excellent bifunc-

INTRODUCTION The search for clean and sustainable energy carriers to replace traditional fossil fuels has attracted extensive research attention in the past few decades, which is expected for a solution to the worldwide energy crisis and environmental pollution.1,2 Hydrogen with zero emission is generally regarded as a potential energy carrier for the future energy infrastructure. Water electrolysis provides an environmentally friendly manner for producing sustainable hydrogen. However, its realization has been a mirage so far because both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) of water splitting suffer from intrinsically sluggish kinetics.3,4 The solution of this challenging issue requires effective electrocatalysts to promote these reactions. Presently, the state-of-theart OER and HER electrocatalysts are Ru/Ir-based oxides and Pt-based metals, respectively, but widespread applications are severely limited due to their scarcity and high cost. Toward this end, the exploration of cost-effective and highly efficient electrocatalysts is urgently needed. Recently, 3d transition metals (TMs) and their derivatives, typically those based on Fe, Co, and Ni, have received attention for the design of effective electrocatalysts to potentially replace Pt or Ru/Ir based materials.5 However, most of the catalysts generally suffer from corrosion and passivation under extreme environments. It is well documented that coupling metal nanoparticles with carbon materials can efficiently promote electrocatalytic activities due to the effective surface corrosion retardation and excellent electron transport properties from © 2017 American Chemical Society

Received: January 16, 2017 Revised: March 28, 2017 Published: April 12, 2017 4771

DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) TGA curves of the Ni-MOF precursor under a N2 atmosphere. (b) FTIR spectra and (c) XRD patterns of the Ni-MOF precursor and Ni@graphene. (d) Raman spectrum of the Ni@graphene. Pt wire as the auxiliary electrode, and catalyst-modified GCE as the working electrode. All potentials measured were reported as a form of reversible hydrogen electrode (RHE) according to the Nernst equation:

tional electrocatalytic activities for both the OER and HER in 1.0 M KOH solutions and demands relatively low overpotentials (OER ∼ 370 mV, HER ∼ 240 mV) to yield a catalytic current of 10 mA/cm2, which compares favorably to most previously reported Ni-based elecrocatalysts for water splitting.

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E RHE = EAg/AgCl + 0.197 + 0.059 pH

(1)

OER and HER activities of the catalysts were evaluated with linear sweep voltammetry at a scan rate of 5 mV/s. The current density was calculated based on the geometric area of GCE (0.07 cm−2).

EXPERIMENTAL SECTION

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Preparation of Ni@graphene from Ni-MOF. For Ni-MOF synthesis, nickel nitrate (Ni(NO3)3·6H2O, 4.4 mmol) and trimesic acid (BTC, 2.4 mmol) were dissolved in methanol (70 mL) under magnetic stirring. The mixture solution was pored into a Teflon-lined 100 mL autoclave, which was then sealed and heated to 150 °C and maintained at this temperature for 1 day. After that, the resulting precipitates were collected from the supernatant solution by centrifugation, rinsed with methanol several times, and dried overnight at 60 °C in a vacuum. By directly annealing Ni-MOF at 600 °C for 6 h under an Ar gas flow, the Ni@graphene was finally produced. Characterization. The phase structures of products were detected by a Rigaku Dmax/Ultima IV X-ray diffractometer with monochromatized Cu Kα radiation. Surface groups of products were determined with a Nicolet 6700 FTIR spectrometer using the KBr pellet method. The morphology observations of products were performed on a FEI Tecnai G20 transmission electron microscope (TEM) and a JEOL JSM-6510LV scanning electron microscope (SEM) combined with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) was recorded on a PerkinElmer PHI 5000C spectrometer with an Al Kα excitation. Electrochemical Measurements. Ni@graphene powder (5 mg) and Nafion (5 wt %, 10 μL) were added in a 3:1(v/v) water/ethanol solution (1 mL), which underwent sonication treatment for half of an hour to get homogeneous inks. Catalyst inks (5 μL) were then deposited on the glassy carbon electrode (GCE, diameter: 3 mm) prior to electrochemical tests. Electrocatalytic water splitting experiments were conducted in a typical three-electrode setup controlled by a CHI 660E electrochemical workstation in 1.0 M KOH solutions, using a KCl-saturated Ag/AgCl electrode as the reference electrode, a

RESULTS AND DISCUSSION It has been demonstrated that ultrathin-graphene-layerwrapped metal nanoparticles can be fabricated by annealing a suitable precursor in a controlled atmosphere.8,12,20,21 For the synthesis of Ni@graphene, we here employ Ni-MOF as a reactive template and then converted them into carbon layers encapsulated metal nanoparticles by annealing in an argon atmosphere. The carbonization process can be determined by TGA measurement. Figure 1a shows the TGA curves of the NiMOF precursor under a N2 atmosphere. Two stages of weight loss are obviously observed. The first stage presents a mass loss of ∼8%, corresponding to the evaporation of absorbed water and methanol molecules. A significant weight loss appears at the temperature range of 300−550 °C, which can be attributed to the carbonization of organic ligand and the transformation of metal ions to metal nanoparticles. Notably, the total weight loss at the second weight loss is ∼49.8%, which is relatively lower compared with theoretical value in the phase conversion process of Ni-MOF to Ni(0), implying the appearance of a carbonization phenomenon under such conditions. In terms of TGA results, the Ni-MOF would be converted to Ni@graphene efficiently by choosing an annealing temperature above 550 °C under an inert atmosphere. FTIR spectra shown in Figure 1b confirm a remarkable transformation for surface groups of NiMOF annealed under an Ar atmosphere. As for pristine Ni4772

DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777

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ACS Sustainable Chemistry & Engineering

Figure 2. SEM images of Ni-MOF precursor (a) and Ni@graphene (b,c). SEM-EDS mapping images of Ni@graphene (d).

Figure 3. TEM (a,b), HRTEM (c,d) images, SAED pattern (e), and line profiles (f) of the Ni@graphene.

MOF, the intense absorbance at 1200−1700 cm−1 represents characteristic peaks of carboxyl groups and the aromatic ring in the BTC ligand.22−24 It is clear that these absorbance peaks vanish in the annealed product and are replaced by three new peaks at 1583, 1400, and 1098 cm−1 relative to stretching vibrations of CO, C−O, and C−C groups, respectively,25 implying that the oxygen-containing groups are left in the resultant Ni@graphene. XRD patterns shown in Figure 1c further confirm the phase conversion of the Ni-MOF. The NiMOF precursor exhibits the amorphous crystalline characteristic, matching with previously reported results.26−28 After pyrolysis treatment, the distinct diffraction peaks located at 44.4° (111), 51.6° (200), and 76.3° (220) appear in carbonized products and are indexed to characteristic metallic Ni in the cubic structure (JCPDS file no. 65-0380). Additionally, a weak peak at 25.6° (002) would be associated with graphitic carbon.29 These results suggest the successful chemical transformation of Ni-MOF to Ni@graphene. The Raman spectrum further confirms the carbon species in Ni@graphene (Figure 1d). Typical D and G bands of carbon species in Ni@ graphene are observed at about 1340 and 1590 cm−1, corresponding to the characteristic features of disordered and crystalline graphitic structures, respectively, which agrees well with the XRD results. The morphology change of Ni-MOF after the pyrolysis treatment along with the above structure transformation can be further determined by SEM and TEM observations. A typical SEM image of Ni-MOF is shown in Figure 2a. The precursor is composed of numerous regular spherical microparticles with a diameter of 4−8 μm. After annealing, the carbonized product perfectly retains the sphere-like shape of the original Ni-MOF

precursor (Figure 2b). However, their sizes appear to shrink (Figure 2c) owing to a great loss of organic ligands in Ni-MOF after thermal treatment.17,30 The EDS analysis (Figure S1, Supporting Information) confirms that Ni@graphene consists of nickel, carbon, and oxygen elements with a mass fraction of 74.10%, 21.26%, and 4.74%, respectively. The detected oxygen elements originated from the nickel oxidation and some oxygen-containing functional groups left in the resulting Ni@ graphene. In addition, the detailed elemental mapping images (Figure 2d) also indicate that these elements are homogeneously distributed among Ni@graphene. TEM observations were further performed to gain insight into the microstructures of Ni@graphene. Figure 3a displays a representative TEM image of the Ni@graphene spheres, which are composed of numerous fine particles uniformly inlaid along the carbon matrix. The magnified TEM image reveals that the metallic Ni nanoparticles with diameters less than 15 nm are actually wrapped by graphene shells in individual microspheres (Figure 3b). The detailed microstructures of the Ni@graphene were thoroughly examined by HRTEM. Clearly, the uneven onion-like graphene shells with a thickness of about 2−6 nm coat on the surface of crystalline Ni nanoparticles (Figure 3c,d and Figure S2 in the Supporting Information). The formation of the graphene outer layer may be relevant to the catalytic effect of Ni nanoparticles under annealing conditions in an inert atomsphere.8,20,31 Such self-catalysis-formed graphene shells would efficiently enhance the electronic conductivity and corrosion resistance and thus improve the electrocatalytic efficiency.12 The measured surrounding lattice fringe spacing is 0.34 nm, attributing to a (002) plane of graphene, while an interplanar distance of 0.20 nm corresponds to a (111) plane of 4773

DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777

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ACS Sustainable Chemistry & Engineering

Figure 4. XPS scans of Ni@graphene: (a) survey, (b) C 1s, (c) Ni 2p.

Figure 5. (a) OER polarization curves and (b) corresponding Tafel curves of the Ni@graphene, Ni-MOF, and commercial RuO2 catalysts. (c) Chronopotentiometric curves of Ni@graphene (∼10 mA/cm2). (d) OER polarization curves of Ni@graphene prepared at different annealing temperatures.

scan in Figure 4b is well fitted into two peaks for a large quantity of sp2-hybridized carbon and few oxygen−carbon containing moieties, respectively. The high resolution Ni 2p XPS deconvolution (Figure 4b) yields three peaks at binding energies of 852.8, 855.0, and 860.2 eV, revealing the coexistence of metallic Ni0 and surface oxidized Ni2+ in the Ni@graphene, consistent with reported results of Ni@ graphene12 and [email protected]

metallic Ni (Figure 3f). Similarly, diffraction rings in the SAED pattern of Ni@graphene (Figure 3e) are indexed to the (002) planes of graphene and (111), (200), (220), and (311) planes in the fcc structure of metallic Ni. The surface elemental composition and electronic states of Ni@graphene were probed by XPS analysis. The survey spectrum (Figure 4a) indicates Ni@graphene is constituted of nickel, carbon, and oxygen elements. The high-resolution XPS 4774

DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777

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ACS Sustainable Chemistry & Engineering

Figure 6. (a) HER polarization curves of Ni@graphene, Ni-MOF, and Pt/C. (b) HER polarization curves of Ni@graphene prepared at different annealing temperatures. (c) Tafel plots of Ni@graphene, Ni-MOF, and Pt/C. (d) Chronoamperometric curve of Ni@graphene at an overpotential of ∼200 mV.

Of note, the Ni@graphene experiences an initial electrochemical activation process, as the required potential gradually increases during the first 3 h of the electrolysis, attributed to the oxidation of Ni nanoparticles in Ni@graphene for in situ activation, which is confirmed by XPS analysis on the post-OER catalyst (Figure S3, Supporting Information) and coincident with the phenomenon occurring in OER-catalytic Ni2P/Ni/NF and [email protected],40 Considering the heat treatment effects remarkably on the formation of Ni@graphene, the OER activities of different products obtained by varying annealing temperatures were further studied (see detailed characterizations in Figure S4 and Figure S5, Supporting Information). As shown in Figure 5d, the annealing temperature could significantly affect the OER activities of Ni@graphene. The Ni@graphene catalyst prepared at an annealing temperature of 600 °C displays the highest OER activity, which may be attributed to its optimized active sites. Next, we evaluated the HER activity of Ni@graphene in alkaline solutions. Figure 6a shows polarization curves of Ni@ graphene at a scan rate of 5 mV/s in 1.0 M KOH, along with Ni-MOF and commercial Pt/C for comparison. Under this test condition, the Pt/C reference displays the best activity. NiMOF presents poor activity with an overpotential (at 10 mA/ cm2) of above 500 mV for the electrocatalytic HER. As expected, Ni@graphene displays remarkable HER performance with earlier onset potential (∼120 mV). More specifically, to drive a HER current density of 10 mA/cm2, the Ni@graphene requires only 240 mV. Such a value compares to that of Nibased HER electrocatalysts previously reported, such as Ni nanoparticles/carbon nanotube (∼350 mV),41 MnNi/C (∼360 mV),39 Ni0.9Fe0.1/NC (∼231 mV),14 commercial Ni wires (∼430 mV),42 Ni nanopowders (∼270 mV),43 and Ni2P nanoparticles (∼205 mV).44 Similar to OER activity, the Ni@ graphene catalyst prepared at an annealing temperature of 600

The electrocatalytic OER performance of the Ni@graphene was first tested using linear sweep voltammetry in an alkaline solution. Figure 5a presents polarization curves of Ni@ graphene in 1.0 M KOH with a scan rate of 5 mV/s, along with Ni-MOF and commercial RuO2 for comparison. The bare GCE exhibits negligible catalytic activity, while the RuO2 presents the expected superior OER activity. Ni-MOF is able to electrocatalyze OER, but its activity is not high. In contrast, Ni@graphene exhibits superior activity to Ni-MOF, as evidenced by its earlier onset potential (∼1.53 V) and larger catalytic current. Ni@graphene only requires an overpotential of ∼370 mV to produce a catalytic current of 10 mA cm−2, which is merely 70 mV behind RuO2 (∼300 mV). This value compares favorably to most of the reported nickel-based electrocatalysts, such as MWCNTs/Ni(OH)2 (∼470 mV, 0.1 M KOH),32 NiO/Ni foam (∼390 mV),33 MOF-derived Fe0.5Ni0.5Ox (∼584 mV, 0.1 M KOH),34 NiCo-LDH/carbon paper (∼367 mV),35 NiCo2O4 nanowires/Ti (∼370 mV),36 and Ni3N nanosheets (∼430 mV)37 and is even superior to the previously reported metallic nickel-based OER electrocatalysts, including Ni@NC (∼420 mV),14 Ni@NC (∼371 mV),14 Ni3Fe/C (∼420 mV),38 and Mn0.1Ni (∼430 mV).39 A Tafel slope obtained from the linear region of Tafel plots was used to estimate the electrocatalytic OER kinetics of Ni@ graphene. As given in Figure 5b, the RuO2 yields a Tafel slope of about 65 mV/dec agreeing with the reported one. Ni@ graphene presents a Tafel slope of 66 mV/dec, which is similar to that of RuO2 and much smaller than that of Ni-MOF, implying favorable reaction kinetics for Ni@graphene. The durability of Ni@graphene for electrocatalytic OER under alkaline conditions was also evaluated with a chronopotentiometry test (Figure 5c). The Ni@graphene holds original activity after the chronopotentiometric test for more than 10 h, confirming good long-term durability in OER electrocatalysis. 4775

DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777

ACS Sustainable Chemistry & Engineering °C presents the best HER activity (Figure 6b). To understand the HER activity of Ni@graphene, the catalyst was treated with HCl to leach metallic Ni (HCl-Ni@graphene). The resulting HCl-Ni@graphene exhibits poor activity toward HER (Figure S6, Supporting Information), confirming that the metallic Ni would be catalytically active sites for HER. Generally, actual active sites in metal-based electrocatalysts can be deactivated with thiocyanate ions in alkaline solutions.17,45,46 In our case, once 5 mM of SCN− was added to a KOH solution, the HER activity of the Ni@graphene was significantly inhibited (Figure S7, Supporting Information), indicating that SCN− ions poison a certain number of metal sites, which further supports active centers of metallic Ni in Ni@graphene. In light of previous observations on TM@graphene structures, the interaction between the graphitic shell and the metallic core would change the local work function of the outer shell, thus affording it with surprisingly high chemical activities.8,47 We thereby attribute the excellent electrocatalytic performance of the Ni@graphene to the cooperation between the catalytic sites of metallic Ni, the intact metal protection effect, and enhanced electron penetration effect of the outer graphene layers. The HER kinetics of Ni@graphene are also examined with Tafel plots (Figure 6c). The Pt/C reference gives a Tafel slope of 54 mV/ dec in alkaline solution, consistent with the previously reported values. Ni@graphene (120 mV/dec) and Ni-MOF (127 mV/ dec) exhibit close Tafel slopes, reflecting their similar HER kinetics. The durability of Ni@graphene during HER catalysis was also evaluated with chronoamperometry. As shown in Figure 6d, a stable HER current density can be retained for 10 h, demonstrating its good electrocatalytic durability.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS In summary, the ultrathin graphene-encapsulated metallic nickel nanoparticles were successfully synthesized from a Nibased MOF using a simple pyrolysis approach. The resultant Ni@graphene exhibits excellent HER and OER performance in an alkaline solution. To afford a current density of 10 mA/cm2, it only requires low overpotentials (OER ∼ 370 mV and HER ∼ 240 mV), which compares favorably to most Ni-based elecrocatalysts previously reported for water splitting. The excellent performance would be attributed to the catalytic sites of metallic Ni and the intact metal protection effect of the outer graphene layers. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00153. EDS spectrum, SEM, TEM images, Ni 2p XPS spectra, and OER activity of Ni@graphene (PDF)

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This work was supported by the National Natural Science Foundation of China (51572227), the Sichuan Youth Science and Technology Foundation (2013JQ0012), and the Major Cultivating Foundation of Education, Department of Sichuan Province (17CZ0036).

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86-817-2568081. E-mail: [email protected]. *Tel./Fax: +86-817-2568081. E-mail: [email protected]. ORCID

Jing Jiang: 0000-0001-8592-5130 Notes

The authors declare no competing financial interest. 4776

DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777

Research Article

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DOI: 10.1021/acssuschemeng.7b00153 ACS Sustainable Chem. Eng. 2017, 5, 4771−4777