N and V co-incorporated Ni nanosheets for enhanced hydrogen

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N and V co-incorporated Ni nanosheets for enhanced hydrogen evolution reaction Rui Tong, Zhi Sun, Fang Zhang, Xina Wang, Jincheng Xu, Xing-Qiang Shi, Shuangpeng Wang, and Hui Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03600 • Publication Date (Web): 04 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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ACS Sustainable Chemistry & Engineering

N and V co-incorporated Ni nanosheets for enhanced hydrogen evolution reaction Rui Tong a, Zhi Sun b, Fang Zhang a, c, Xina Wang d, Jincheng Xu a, Xingqiang Shi c, Shuangpeng Wang a,* and Hui Pan a,* aJoint

Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Macau, China. bState

Key Discipline Laboratory of Wide-Bandgap Semiconductor Technologies, School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China. cDepartment

of Physics, Southern University of Science and Technology, Shenzhen, Guangdong Province, 518055, China. dHubei

Key Laboratory of Ferro & piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, People's Republic of China. * E-mail: [email protected] (S. P. Wang); [email protected] (H. Pan); Fax: +853 88222426; Tel: +853 88224427;

Abstract: Searching for earth abundant and efficient electrocatalysts for large-scale hydrogen generation from the electrolysis of water is of paramount importance. Incorporating non-metal or metal element into host catalysts is a possible way to tune the catalytic capability of electrocatalyst on the hydrogen evolution reaction (HER). Here, a simple and facile way was presented to synthesize N and V co-incorporated Ni nanosheets on self-supported conductive carbon paper (NV-Ni/CP) as noble metal-free catalysts for HER. Compared with Ni/CP, the NV-Ni/CP shows better HER property with low overpotential of 95 mV (10 mA cm-2), small Tafel slope (140 mV dec-1), and superior long-term stability. We further show that the outstanding HER performance of NV-Ni/CP is ascribed to the increased active sites and enhanced conductivity. It is expected that the co-incorporation of non-metal and metal elements will provide more chances to enhance the HER catalytic ability and extend the scope of cost-effective electrocatalysts. Keywords: NV-Ni/CP; Nanosheets; Co-incorporation; HER

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Introduction Hydrogen has drawn the attention of researchers because of its cleanness and renewability. It is well known that the electrolysis of water is a simple and hopeful approach for hydrogen evolution via HER.1-4 The production of hydrogen via electrocatalytic water splitting needs highly active and abundant electrocatalysts.5 Until now, many Pt-based materials are known as the most effective catalysts for HER.6-9 But, high price and limited abundance impede their large-scale applications in HER. As a result, exploiting low-cost, earth-abundant and highly efficient catalytic materials is significantly important. It is about 100 years ago that nickel was found to possess HER activity.10 And nickel has been used as industrialized water reduction catalyst.11,12 However, the overpotential (η10) of pure Ni metal is high (~210 mV), and the stability is poor.13,14 Although Ni-based compounds, such as Ni nanoparticles embedded into CFC (NiΦCFC),15 NiO/Ni heterostructures on CNT (NiO/Ni-CNT),16 ultrathin Ni nanosheet array,17 Ni-C-N nanosheets,18 and Ni/ceria-rGo,19 have been extensively studied and present enhanced HER activity, further improvement of its catalytic performance is still desired. Recently, it has been reported that incorporating non-metal nitrogen element into some host materials could modified the electronic properties and exposed more active sites, resulting in optimized adsorption energy and boosting intrinsic conductivity.20-22 For example, nitrogen-doped Ni3S2/VS2 showed excellent electrocatalytic HER property with a low overpotential (151 mV at 10mA cm-2) and long stability.23 Similarly, another effective method such as incorporating metal elements (such as Fe, Co, Zn, Al and V) into the host material has been reported to enhance the HER ability of electrocatalyst.24-29 More recently, vanadium-based materials, such as VS2 nanoflowers,30 V-doped Ni3S2 nanowire arrays,31

V doped CoP,25

V doped NiFe LDH,32 and NiVS/NF,33 had shown

superior electrocatalytic properties for water splitting, indicating the incorporation of V could be promising to improve the electrochemical ability. In this work, we present an active catalyst (NV-Ni/CP) for HER by introducting both N and V into two-dimensional (2D) nickel nanosheet arrays on the carbon paper (CP). We find that the 2 ACS Paragon Plus Environment

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co-incorporation dramatically enhances the electrocatalytic behavior of Ni nanosheets, including a η10 of 95 mV and a superior long-term stability achieved in 1 M KOH. We show that the co-incorporation of nitrogen and vanadium into Ni can increase active sites, improve electrical conductivity of the Ni, and further boost the HER activity of Ni. Meanwhile, our results shows that the co-incorporation of non-metal and transition metal atoms can be an efficient method to achieve other novel and highly active electrocatalysts.

Experimental section Preparations of N-Ni/CP and NV-Ni/CP. V doped Ni-OH nanosheets (NSs)/CP (V-Ni-OH/CP) were prepared as the following hydrothermal method.34 Typically, 4.5 mmol Ni(NO3)2·6H2O (1.308 g), 0.5 mmol VCl3 (0.079 g), 20 mmol urea (1.201 g), 8 mmol NH4F (0.296 g) and 60 mL deionized water were added into a 100 mL beaker. After continuous stirring fo 30 min, the light green solution was formed. The carbon paper (CP) was washed in 3 M HCl solution, water and ehanol for 30 minutes, then dried in air before used as substrate. Then the solution and cleaned CP were put into an autoclave. The reaction temperature was hold at 120 ºC for 6 h. After the hydrothermal reaction is over, V doped Ni-OH nanosheets on the CP (V-Ni-OH/CP) were washed with water and ethanol for 3 times. Then, the V-Ni-OH/CP electrode was put into a vacuum oven and dried at 60 ºC overnight. To prepare NV-Ni/CP, the precursor V-Ni-OH/CP was annealed in a NH3 atmosphere at 425 ºC for 3 h. And the heating rate is 5 ºC/min. The NVx-Ni1-x/CP (x is the mole ratio of V3+ and Ni2+ plus V3+, the total molar of Ni2+ and V3+ is 5 mmol, x equals 0.05 or 0.15) electrodes were also synthesized with the same way. To obtain V-Ni/CP, V-Ni-OH/CP was calcined in Ar with 10% H2 at 425 ºC for 3 h at the same heating rate. Similarity, the Ni/CP was synthesized only without VCl3. The mass density of the material grown on CP substrate is 1.1 mg cm-2. Synthesis of Pt/C electrode. Firstly, 5 mg commercial Pt/C (20 wt%) was sonicated in the solution including 750 μL deionized water, 250 μL isopropanol and 85 μL of Nafion to form a dark ink. Secondly, a glassy carbon electrode (GC) was polished and 5 μL Pt/C ink was droped onto it. Finally, the GC was dried at 60ºC in a vacuum oven.35 3 ACS Paragon Plus Environment


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Characterizations. X-ray diffraction (XRD, Rigaka, Smartlab) was used to determine the crystal structure. We used a Zeiss field-emission scanning electron microscopy (SEM) to detect the morphologies of samples. The transmission electron microscopy (TEM) images and element mappings of samples were obtained by a TEM equipment (Talos, F200S). X-ray photoelectron spectroscopy (XPS) tests were done on an Escalab 250Xi with Al Kα radiation. Elemental contents were investigated by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 730 spectrometer). The N2 adsorption-desorption isotherms (Micromeritics, 3Flex instrument) was used to calculate the BET surface area. Electrochemical measurements. We used a Solartron electrochemical analyzer to measure all the electrochemical performances. And we adopted a typical three-electrode system. A carbon rod was as the counter electrode, the as-prepared samples (1 cm × 1 cm) were as the working electrode, and a saturated Ag/AgCl electrode was as the reference electrode. All the tests were done at ambient temperature. And all the data were not IR-corrected. Electrochemical impedance spectroscopy (EIS) was tested from 105 Hz to 10-1 Hz at -0.08 VRHE. We used the Z-View software (Scribner Associates Inc.) to analyze the EIS results. The scan rate of Linear Sweep Voltammetry (LSV) was 5 mV s-1. And the sweep rate of long-term Cyclic Voltammetry (CV) was 100 mV s-1. The following equation was used to calculate the potentials vs reversible hydrogen electrode (RHE): E(vs RHE) = E(vs Ag/AgCl) + 0.1976 + 0.0591 × pH

(1)

At a bias of 100 mVRHE, the amount of H2 production in 45 min was tested by Gas chromatography (Agilent GC 7890B). Comparing with the actual and theoretical quantity of H2, we calculated the Faradaic efficiency.

Results and discussion The surfaces of CP are covered evenly with (V)-Ni-OH nanosheet arrays (Figure S1a and b). We found that the nanosheet array still maintains the initial morphology with rough surface after heat-treatment under 10% H2/Ar and NH3 atmosphere (Figure 1a-d). Meanwhile, the nanosheets


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become small and thin after V introduced, suggesting that incorporating vandium can effectively change the morphology. The surface of Ni-V bimetallic nanosheet with nitrogen incorporated becomes even rough. The specific surface areas (Figure S2a) of Ni/CP, V-Ni/CP, N-Ni/CP and NV-Ni/CP are 3.44, 4.51, 3.81 and 7.14 m2/g, respectively, indicating that incorporating N and V can enhance the specific surface area. We used the XRD measurements to determine the crystal structures of the samples. The diffraction signals for the (V)-Ni-OH/CP (Figure S3a) are indexed to (Ni(OH)2(NiOOH)0.167)0.857 (JCPDS No. 89-7111) and Ni(OH)2 (JCPDS No. 73-1520). From the XRD patterns of the four samples (Figure 2), the diffraction peaks at 26.4° and 54.5° come from the CP substrate, the remaining peaks at 44.7°, 51.8°, 76.2° consistent with (111), (200) and (220) facets are indexed to Ni (JCPDS No. 04-0850). After the NV co-incorporation, no other significant signals are detected except a small peak at around 40°, this peak may be attributed to vanadium oxide, which is induced by surface oxidization. The intensities of diffraction peaks in the nitrogen and vanadium incorporated systems are lower than those for pure Ni, verifying that they could have poor crystallinity. In addition, the diffraction peaks at 51.8° for NV-Ni/CP and V-Ni/CP shift to small angle (Figure S3b), suggesting that NV-Ni and V-Ni have extended lattice constants than pure Ni and V should be introduced into the crystal lattice of Ni. TEM measurements were done to determine the microstructure of NV-Ni. The low TEM images (Figure 3a) show that many Ni nanoparticles (~10 nm) form the NV-Ni nanosheets. In the high resolution TEM image (Figure 3b), the lattice fringes with distance of 0.20 nm is ascribed to the (111) planes of Ni. And no lattice fringes are found for other compounds. The energy-dispersive X-ray spectra (Figure S4) exhibit the existences of Ni, V and N elements, and the elemental mappings (Figure 3c-e) imply the uniform distribution of Ni, V and N elements within NV-Ni. All these results confirm the successful preparation of the NV-Ni nanosheet arrays on CP. The surface compositions of Ni/CP and NV-Ni/CP were analyzed by XPS measurements. Only the peaks of Ni, O and C are presented in the survey spectra (Figure 4a), while after the NV co-incorporation, the expected V and N are examined. To ensure more accurate, we used the peak at



285.0 eV for C 1s as a reference. In the XPS spectrum of Ni 2p (Figure 4b), the peaks appeared at 852.7 eV and 855.3 eV in Ni/CP are assigned to metallic Ni and Ni2+ oxidation states,36-39 respectively. The peak at 861.0 eV is the satellite of Ni 2p3/2. After the NV co-incorporation, the peaks for Ni shift towards higher binding energy, confirming the Ni-V coordination formation.40 For V 2p (Figure 4c), the peaks at 514.0 and 521.4 eV are attributed to V3+, and the peaks at 516.7 and 524.0 eV are assigned to V4+. The oxidized V species are formed due to the air contact.23,30,41,42 In the XPS spectrum of N 1s (Figure 4d), the obvious peak at 397.7 eV is assigned to Ni-N coordination in NV-Ni/CP.22,43 Additionally, V 2p and N 1s signals are not detected (Figure 4c-d) in the high resolution XPS results from Ni/CP. Our XPS results show that both N and V are successfully incorporated into Ni nanosheets. The molar content of vandium in NV-Ni/CP is obtained to be 7.2% by the ICP-AES results. In addition, the relative atomic concentrations of C, N, Ni, V, and O in NV-Ni/CP are 35.84%, 4.52%, 22.53%, 1.82%, and 35.59% as estimated from the XPS analysis. Additionally, the theoretical calculation results show that the formation energy for the co-incorporation of N and V as a pair is lower than that for them to be far away (Table S1), indicating the strong interaction among Ni, N and V due to electrostatic attraction.44,45 The electrocatalytic properties of the NVx-Ni1-x/CP (x=0, 0.05, 0.1, 0.15) for HER were measured in 1 M KOH (Figure S5). Among these NVx-Ni1-x/CP catalysts, NV0.1-Ni0.9/CP (NV-Ni/CP) exhibits the best HER performance. Therefore, we focus on NV0.1-Ni0.9/CP in the later analysis and label it as NV-Ni/CP. In contrast, the HER performances of Pt/C, pure NF (nickel from) and CP electrodes were evaluated (Figure 5a). The LSV curves shows that the activities of the catalysts follow the trend Pt/C > NV-Ni/CP > N-Ni/CP > V-Ni/CP > Ni/CP > NF in the electrolyte. For convenience, we used η10 and η20 to represent the overpotentials at the current densities of 10 and 20 mA cm-2. The η10 and η20 for Ni/CP are 210 and 251 mV, respectively (Figure 5c,). After V incorporated into Ni/CP, the values decease to 178 (η10) and 222 (η20) mV, respectively. With the incorporation of N, they are 148 (η10) and 210 (η20) mV, respectively. Interestingly, the overpotentials are dramatically reduced to 95 (η10) and 139 (η20) mV after N and V co-incorporated into Ni/CP, respectively, demonstrating signicantly enhanced HER activity. The performance of NV-Ni/CP compares favorably to most Ni-based 6 ACS Paragon Plus Environment

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catalysts (Table S2) and other noble-metal-free catalysts (Table S3). In addition, Tafel slope is one of most important index for the catalysts, which can be obtained by the following equation46: η = c+ blog j

(2)

where b is the Tafel slop, η is the overpotential corresponding to the current density (j), and c is the intercept. The Tafel slopes (Figure 5b) for Ni/CP, V-Ni/CP, N-Ni/CP and NV-Ni/CP are 191, 163, 183 and 140 mV dec-1, respectively. In basic solution, HER has two basic steps. One step is the Volmer process (H2O + e- → Hads + OH-), and another step is the Tafel process (Had + Had → H2) or Heyrovsky process (Had + H2O + e- → H2 +OH-). The theoretical Tafel slopes corresponding to Volmer, Tafel and Heyrovsky step are 120, 30, and 40 mV dec-1, respectovely.47,48 According to our results, the rate determining step (RDS) for our catalysts is the hydrogen adsorption (Volmer process).49,50 The different Tafel slops of the as-prepared catalysts may associate with the hydrogen adsorption, indicating the introduction of N and V can modality the Ni surface. However, the RDS (reaction mechanism) does not change. The cycling stability is an important index to assess the electrocatalytic performance. To estimate the stability in 1 M KOH, 5000 cycles of LSV tests were performed from 0.1 to -0.3 VRHE (vs. RHE) at 100 mV s-1. After 5000 cycles tests, the LSV curve (Figure 5d) of NV-Ni/CP remains almost unchanged. Meanwhile, the XRD and SEM show that there have no significant changes on the composition and morphology of NV-Ni/CP (Figure S6). In addition, the overpotential of NV-Ni/CP at 10 mA cm-2 (insets in Figure 5d) exhibits a small fluctuation after working for 12 hours, suggesting its excellent stability. After 12h stability test, the SEM of NV-Ni/CP have no significant change expect the surface becomes more rough (Figure S7a). From the high resolution XPS spectra (Figure S7b-d), compared with the as-prepared NV-Ni/CP, the peaks of Ni 2p and N 1s have no obvious change, whereas the peaks of V3+at 514.0 and 521.4 eV disappear and the intensity of V4+ at 516.7 and 524.0 eV reduces, suggesting that V at the surface is partially leached after the long term experiment. In addition, the NV-Ni/CP electrode has a 97% faradaic efficiency at an overpotential of 100 mVRHE (Figure 5e), suggesting an efficient electron transfer in the electrolysis reaction. We use the



EIS test to investigate the influence of NV co-incorporation on the HER kinetics. The inset is fitted equivalent circuit corresponding to the Nyquist plots (Figure 5f). The semicircle represents the charge transfer resistance of H+ between the electrolyte and electrode. The charge transfer resistance is labeled as Rct. The calculated Rct value for the NV-Ni/CP electrode is 4.7 Ω, considerably smaller than that of Ni/CP (23.5 Ω), V-Ni/CP (16.9 Ω) and N-Ni/CP (15.2 Ω) electrodes. Furthermore, the solution resistances (Rs) of Ni/CP, V-Ni/CP, N-Ni/CP and NV-Ni/CP electrodes are 2.2, 2.0, 1.9, and 1.8 Ω, respectively. These findings verify that the NV co-incorporation can significantly enhance the electrical conductivity and charge transfer.51-53 The catalytic properties of the as-prepared catalysts were also investigated in acidic solutions (Figure S8). Similarly, NV-Ni/CP performs the best HER catalytic performance. The η10 and Tafel slop are 152 mV and 115 mV dec-1, which are superior to the Ni/CP (278 (η10) mV, 196 mV dec-1), V-Ni/CP (217 (η10) mV, 168 mV dec-1), and N-Ni/CP (182 (η10) mV, 196 mV dec-1). Meanwhile, the catalyst stability was also measured in acidic solution. Compared with the as-prepared NV-Ni/CP, the overpotential (η10) of 100 cycled NV-Ni/CP electrode (100 cycles of LSV tests from 0.1 to -0.3 VRHE at 100mV s-1) decreases sharply to 190 mV. The poor stability in in acidic solution is attributed to the reaction between the catalyst and acid. After 100 cycles test, the catalyst on the CP surface is corroded (Figure S8d). We calculated the electrochemical double-layer capacitances (Cdl) through the CV measurments in a non-Faradic potential region at different scan rates (Figure S9). The Cdl of Ni/CP, V-Ni/CP, N-Ni/CP and NV-Ni/CP are 0.30 mF cm-2, 1.60 mF cm-2, 1.47 mF cm-2 and 6.34 mF cm-2, respectively. Obviously, NV-Ni/CP has the largest Cdl value, which is about 21-fold higher than that of Ni/CP, indicating that the incorporation of V and N into Ni generates more active sites for HER. We normalized the LSV curves in 1 M KOH by the specific surface area (Figure S2b). To achieve the same specific activity (0.1 mA cm-2BET), the overpotential of NV-Ni/CP (160 mV) was much lower than that of Ni/CP (83 mV), indicating the impressive intrinsic activity of NV-Ni/CP for HER. Hence, it is concluded that the enhanced property is due to the synergic effect of high electrochemical surface area and intrinsic catalytic activity of NV-Ni/CP.


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Conclusions In conclusion, we present the preparation of NV co-incorporated Ni nanosheets as efficient electrocatalysts for HER via a simple and effective method. The NV co-incorporated sample exhibits a reduced overpotential and improved high durability, which are much better than pure and single-element incorporated Ni samples. The excellent HER porperty of NV-Ni/CP is mainly attributed to highly exposed active sites and enhanced diffusion kinetics. Our strategy on metal and non-metal co-incorporation to enhance the HER activity of eletrocatalyst may generally apply to design effective catalysts for hydrogen production.

Acknowledgements This work was supported by Science and Technology Development Fund from Macau SAR (FDCT-132/2014/A3

and

FDCT

(MYRG2018-00003-IAPME,

084/2016/A2)

and

MYRG2017-00027-FST,

Multi-Year

Research

Grants

MYRG2017-00149-FST,

and

SRG2016-00085-FST) from the Research & Development Office at the University of Macau.

Supporting Information Top-view SEM images of Ni-OH/CP and V-Ni-OH/CP; Nitrogen adsorption/desorption isotherms and the LSV curves in 1 M KOH normalized by the BET surface area; XRD patterns of (V)-Ni-OH/CP; The enlarged XRD patterns; EDX spectrum; HER polarization curves of NVx-Ni1-x/CP; XRD pattern and SEM images of NV-Ni/CP after 5000 cycles measurement; SEM images and XPS spectra of NV-Ni/CP after 12h stability test; The catalytic property of the as-synthesized samples in acidic solutions; Cdl and CV curves; The calculated formation energy of VN-codoping; Summary of HER performance of recently reported Ni-based catalysts and other noble-metal-free catalysts; Calculation Details.



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Figure Captions

Figure 1. Low- and high (inset)-magnification SEM images of (a) Ni/CP, (b) V-Ni/CP, (c) N-Ni/CP, and (d) NV-Ni/CP. Figure 2. XRD patterns of Ni/CP, V-Ni/CP, N-Ni/CP and NV-Ni/CP. Figure 3. (a) TEM and (b) HRTEM images of NV-Ni nanosheets. (c-e) Corresponding EDS mappings of elemental Ni, V and N from the purple rectangular area in the TEM image (a). Figure 4. (a) XPS survey scans for the NV-Ni/CP and Ni/CP electrodes. (b-d) High-resolution XPS spectra of Ni 2p, V 2p and N 1s peaks from NV-Ni/CP and Ni/CP. Figure 5. (a) LSV curves of Ni/CP, V-Ni/CP, N-Ni/CP, NV-Ni/CP, NF, CP and Pt/C in 1 M KOH. (b) The corresponding Tafel of the catalysts in 1 M KOH. (c) The comparisons of the overpotential (η) obtained at current densities of 10 and 20 mA cm-2. (d) Polarization curves of NV-Ni/CP at the first cycle and after 5000 cycles. The inset in (d) is the chronopotentiometry curve of NV-Ni/CP conducted 14 ACS Paragon Plus Environment

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at 10 mA cm-2. (e) Faradaic efficiency (right y axis) and H2 production (left y axis) over a period of 40 min electrolysis under an overpotential of 100 mVRHE. (f) Nyquist plots of the three electrodes in 1 M KOH at -0.1 VRHE.

Figures Figure 1




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Figure 2



Figure 3


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Figure 4



Figure 5


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Graphical Abstract

N and V co-incorporated Ni nanosheets on carbon paper (NV-Ni/CP) were obtained as noble metal-free catalysts for high efficient HER.


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N and V co-incorporated Ni nanosheets for enhanced hydrogen

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