Constructing High Performance Hybrid Battery and Electrocatalyst by

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Constructing high performance hybrid battery and electrocatalyst by heterostructured NiCo2O4@NiWS nanosheets Depeng Zhao, Meizhen Dai, Hengqi Liu, Li Xiao, Xiang Wu, and Hui Xia Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01904 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Crystal Growth & Design

Constructing high performance hybrid battery and electrocatalyst by heterostructured NiCo2O4@NiWS nanosheets Depeng Zhao, †1 Meizhen Dai, †1 Hengqi Liu, † Li Xiao, † Xiang Wu†,* Hui Xia ‡,* †School

of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, P.R. China

‡School

of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, P.R. China

Abstract It is very important to design hybrid core-shell structured electrode materials to obtain superior electrochemical and electrocatalytic performance. In this work, we reported hybrid NiCo2O4@NiWS nanostructures through a facile solution approach. The as-synthesized product can be used directly as the electrode materials for the hybrid battery and the electrocatalysts. The as-synthesized hybrid NiCo2O4@NiWS structure exhibits a high specific capacitance 580 C g-1 at current density of 1 A g-1. The as-assembled supercapacitor device shows high energy density of 41.1 Wh kg-1 at a power density of 1032.2 W kg-1. Moreover, NiCo2O4@NiWS structure possesses excellent oxygen evolution reaction with small overplant of 290 mV and excellent cycle stability.

Keywords: Hybrid structure, NiCo2O4@NiWS, hybrid battery, Oxygen evolution reaction *Correspondences 1 These

should be addressed: [email protected]; [email protected]

authors contributed equally to this work 1

ACS Paragon Plus Environment

Crystal Growth & Design 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

1. Introduction Nowadays, with the increasingly serious energy crisis and environmental pollution, the global energy demand has increased dramatically and traditional fossil fuel energy sources are limited vastly.1-3 Thus, exploring the renewable and sustainable energy conversion and storage systems has attached widespread attention to solve the energy problems.4-6 Among the various energy systems, hybrid battery and electrocatalysts are considered promising candidates due to excellent characteristics, which could use different mechanisms to store and convert energy. Hybrid battery has attracted much attention due to high power density, fast charge-discharge and long cycle stability.7-10 However, low energy density of hybrid battery limits their large-scale commercial use in the future electronics industry. A feasible solution is to improve the energy density by increasing the specific capacitance and broadening the operating voltage. In addition, electrochemical water splitting is considered as an efficient way to produce oxygen and hydrogen, which can be divided into two kinds of reactions: hydrogen evolution reaction (HER) in the cathode and oxygen evolution reaction (OER) in the anode.11 The efficient, cheap and stable electrocatalysts for OER play an important role in the full electrochemical water splitting because of the intrinsic kinetics.12,13 Although H2 production from electrocatalytic water splitting is relatively easy, O2 production is more complicated and requires more overpotential than the theoretical value of 1.23 V due to its sluggish kinetics process.14,15 Transition metal oxide or sulfide have attracted much interest as the electrode 2


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materials for hybrid battery and OER applications.16-18 Thus, the electrode materials with low cost, variable valence states, high reversible capacitance and environmental friendliness have been extensively investigated.19 However, the spinel metal oxide with limited diffusion length limits the capacitance contribution near the surface of the electrode material.20 Wolframite-type nickel tungstate (NiWO4) structure delivers high electrical conductivity (10-7-10-3 S cm-1) and can be considered as a potential candidate for hybrid battery and electrocatalysis.21 Moreover, metal sulfide presents an electrical conductivity about 100 times higher than the corresponding oxides, which more convenient to the transmission of the electrons.22 Therefore, to design hybrid oxide @ sulfide structured electrode materials is a prior alternative due to the synergistic effects between two individual materials. At the same time, the hybrid structured electrode can also improve the whole active area and promote the ion transmissions from the electrolyte. In previous reports, Niu et al. reported hybrid NiCo2S4@Ni3V2O8 structure as a binder-free electrode through a facile hydrothermal process combined through a simple co-precipitation method, through a facile hydrothermal process combined with a simple co-precipitation method it present a high specific capacity of 512 C g-1 at a current density of 1 A g-1 and excellent cycle stability.23 Zhou et al. fabricated hierarchical NiCo2S4@Ni(1-x)Cox(OH)2 core-shell structures with the specific capacitance of 3.54 C cm-2 at 1 mA cm-2 and excellent rate capability.24 However, to accelerate reaction kinetics and decrease the overpotential of OER, many efforts have been made to develop an efficient non-noble metal based electrocatalysts through morphological and structural tailoring. Lu’s groups prepared 3



NiO/NiCo2O4 nanocrystals through a simple hydrothermal method, which exhibits a low overpotential of 264 mV at the current density of 10 mA cm-2.25 Zhang et al. reported NiCo/NiCo2S4@NiCo/Ni foam electrocatalysts, which presents a low overpotential of 294 mV at a high current density of 100 mA/cm2 for the OER.26 Herein, we have prepared several hybrid NiCo2O4 based core-shell structure as the supercapacitor electrode and the electrocatalyst for oxygen evolution reaction. Meanwhile, the as-synthesized hybrid NiCo2O4@NiWS structure delivers areal capacitance of 3.49 C cm-2 at 4 mA cm-2. As electrocatalyst, the hybrid structured NiCo2O4@NiWS demonstrates the low overpotential of 290 mV at 10 mA cm-2, respectively. The binder free hybrid structured electrodes can improve the electrical conductivity between the core and the shell and effectively eliminates the agglomeration among adjacent particles.

2. Experimental Section 2.1 Material Preparation Preparation of NiCo2O4 nanowires on Ni foam All reagents were of analytically grade and used without further purification. Before a typical synthesis, a piece of Ni foam was cleaned by immersed in 0.5 M HCl, absolute ethanol and deionized water under sonication for 20 min, respectively to remove to the grease in the surface of nickel oxide. The Ni foam (4 x 4 cm2) was dried at room temperature. NiCo2O4 nanowires were synthesized though a simple hydrothermal process. 0.5820 g cobalt nitrate (Co(NO3)2.6H2O), 0.2910 g nickel nitrate (Ni(NO3)2.6H2O), 0.0926 g ammonium fluoride (NH4F) and 0.3 g urea were dissolved in 60 ml deionized water 4


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with constant magnetic stirring for 10 min. then a piece of Ni foam was transferred into 100 ml of Teflon-lined autoclave and kept at 120 oC for 6 h. After cooling down to room temperature, Ni foam covered with the precursor was taken out and washed with absolute ethanol and deionized water for several times. Finally, the as-synthesized products were calcined in 350 °C for 2 h in air and the average mass loading was 1.2 mg cm-2. Preparation of NiCo2O4@NiWO4 hybrid structures 0.659 g sodium tungstate dihydrate (Na2WO4·2H2O), 0.474 g nickel nitrate (Ni(NO3)2.6H2O) were dissolved in 40 ml deionized water and stirred for 10 min for form a homogeneous solution. Then, the as-synthesized NiCo2O4 nanowires was immersed into 80 ml Teflon-Lined stainless steel autoclave and heated at 120 oC for 4 h. After cooling to room temperature, the as-synthesized products were repeated wash three times with deionized water and ethanol, and dried with at 60 oC for overnight. Finally, the as-synthesized products were calcined at 300 °C for 2 h in air. The average mass loading was 2.5 mg cm-2. Preparation core-shell hybrid structures of NiCo2O4@NiWS In a typical process, 0.35 g Na2S was dissolved in 40 ml deionized water. Then, the as-synthesized core-shell structures of NiCo2O4@NiWO4 on supporting Ni foam were placed in 80 ml Teflon-Lined stainless steel autoclave, which was kept at 120 oC for 4 h. After cooling to room temperature, the as-synthesized products were repeatedly washed for three times and dried for 12 h at 60 °C. The average mass loading of NiCo2O4@NiWS was 2.8 mg cm-2. 5



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3.2 Materials characterization Crystallographic structure and phase purify of the as-synthesized products were tested though X-ray diffraction analyzer (XRD, 7000, Shimadzu) with Cu Kα radiation (λ= 1.5406 Å). The morphology and structure of the samples were characterized by using scanning electron microscope (SEM, Gemini 300-71-31) and TEM analysis was carried out on a FEI Tecnai F20 transmission electron microscope (TEM) with a voltage of 200 kV. X-ray photoelectron spectra (XPS) measurements were conducted to investigate the element composition through ESCALAB250 with using an Al Kα sources. Long cycle stability measurements were conducted though Landan tests system. 3.3 Electrochemical measurement The as-synthesized products (d = 1 cm) were directly act as working electrode for electrochemical measurement though a CHI660E electrochemical workstation. The cyclic voltammetry (CV) curves, galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were conducted in a three-electrode system at room temperature with 3 M KOH aqueous electrolyte. Pt foil was used as the reference electrode and Hg/HgO served as the counter electrode. Electrochemical impedance spectroscopy (EIS) was measured in a frequency range from 0.01 to 100 kHz with an open circuit potential of 5 mV. The specific capacitance and areal specific capacitance of the electrode was calculated from the GCD curves by following equations; Ca = I t / s

(1) 6


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Cs = I t / m

(2)

Where Ca, Cs, I, ∆t, s and m represents the areal specific capacitance, the specific capacitance, discharge current density (mF cm-2), the discharging time (s), the active area of the electrode, the mass of electroactive materials on Ni foam, respectively. 3.4 Fabrication of flexible asymmetric hybrid battery To assemble asymmetric hybrid battery (ACS), the as-synthesized hybrid structure NiCo2O4@NiWS nanosheets were used as positive materials, AC as the negative electrode. The device was fabricated though a traditional slurry coating method. Active carbon, acetylene black and polytetrafluoroethylene (PTFE) were mixed with a mass ratio of 7: 2: 1 and the mixture slurry were casted on clean Ni foam (2×2 cm) and dried at 60 °C for 24 h. Then, an asymmetric supercapacitor was assembled with above electrode materials and PVA-KOH gel as the electrolyte. 3.5 Electrocatalytic measurement The electrocatalytic performance of the as-synthesized products were investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and i-t curves on a CHI660E electrochemical workstation with a three electrode standard system in 1 M KOH (pH=13.7) aqueous solution. Pt plate as counter electrode and saturated calomel electrode (SCE)

was used as reference electrode, All used

potentials were converted with respect to reversible hydrogen electrode (RHE) though the Nernst equation ERHE = ESCE + 0.197 + 0.059×pH, where ESCE is experimentally measured potential against the SCE reference electrode. The overpotential (η) was 7



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calculated using the equation: η = E RHE -1.23.

4. Results and discussion The schematic illustration for the as-synthesized products is showed in Fig. 1. 3D highly conductive porous Ni foam was directly used to grow NiCo2O4 nanowires. First, NiCo2O4 nanowires are obtained in the nickel substrate via the hydrothermal process. Second, the hybrid NiCo2O4@Ni-W-O core-shell structure precursor is obtained

through

a

hydrothermal

approach.

After

annealing,

hybrid

NiCo2O4@NiWO4 core-shell structures are formed. Lastly the precursors react with Na2S and form core-shell NiCo2O4@NiWS structures though an anion-exchange process. The crystal structures of the as-synthesized products are characterized through XRD analysis, as illustrated in Fig. 2a. The diffraction peaks at 2 values of 44.4, 51.6 and 76.1 degree are well in accordance with Ni foam substrate (JCPDS no. 04-0850). The major diffraction peaks at 2 values of 31.2, 36.7, 59.1 and 65.1 degree can be well identified as the (220), (311), (440) and (511) crystal planes of spinal NiCo2O4 phase (JCPDS no. 20-0781). It is clearly found that the diffraction peaks of hybrid structures contain the NiCo2O4 peaks, which confirm the successful preparation of NiCo2O4 and NiCo2O4 hybrid structure. Other diffraction peaks can be attributed to the NiWO4 structure (JCPDS no. 15-0755). Especially, for NiCo2O4@NiWS hybrid structure, other peaks could be considered NiWS phase although there is no standard PDF card. Thus, a complete phase transfer is proved. Sharp diffraction peaks indicate the as-synthesized hybrid structure presents excellent 8


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crystallinity. X-ray photoelectron spectroscopy (XPS) is applied to further characterize the element composition and chemical valence state of the hybrid structure, as shown in Fig.2b-2g.

Using

the

Gaussian

fitting

method,

Ni

2p

emission

spectra

(NiCo2O4@NiWO4 and NiCo2O4@NiWS) are well fitted with two spin-orbit doublets and two shakeup satellites (noted as Sat.,). Fig. 2c show the Co 2p spectra (NiCo2O4@NiWO4 and NiCo2O4@NiWS) are fitted into two spin-orbit doublets and two satellite peaks. The binding energies at 853.4 eV and 856.5 eV correspond to the Ni 2p3/2, 874.5 and 871.6 eV for Ni 2p1/2, respectively. The binding energy difference between Ni 2p3/2 and Ni 2p1/2 is 17.7 and 18 eV, respectively, indicating the existence of Ni2+ and Ni3+. The satellite peak presents most of Ni existing in Ni2+ ion.27 Fig. 2c show the Co 2p spectrum (NiCo2O4@NiWO4 and NiCo2O4@NiWS) is fitted into two spin-orbit doublets and two satellite peaks, containing two low energy bands (Co 2p3/2) and two high energy bands (Co 2p1/2), which further confirm the existence of Co2+ and Co3+ in the as-synthesized products. The weak satellite peak reveals that the majority of Co exists in the form of Co3+ state.28,29 After vulcanization, it is easy to find that the Ni 2p and Co 2p peaks shift to high binding energy, revealing that Ni and Co elements mainly exist in high valence state. Fig. 2d shows the S 2p spectrum, the binding energy at 161.3 eV and 162.5 eV correspond to S 2p1/2 and S 2p3/2, respectively. The binding energy at 162.5 eV is a typical of metal-sulfur bond, which is benefit to electrochemical activity.30,31 W (NiCo2O4@NiWO4)spectrum could be divided into three peaks at binding energies of 35.8, 37.9 and 40.1 eV, respectively, 9



which correspond to W 4f7/2 and W 4f5/2.32 The W 4f (NiCo2O4@NiWS) can be divided into three peaks, the banding energy of 33.2 eV and 34.8 eV are ascribed to W4+,33,34 It can be found that W 4f peak position shift to low binding energy after vulcanization, suggesting that the valence of W element decreases after vulcanization (From +6 to +4). Which confirm the formation of NiWS after vulcanization. It is consistent with the XRD results. The O 1s spectrum is fitted with three components, the peaks at 529.5 eV is ascribed to O2- forming oxides with relation to the metal elements (Ni, Co) and the biding energy at the 530.9 eV corresponds to the OH-.35 The morphology of the as-synthesized hybrid structure are characterized first by SEM. Fig. 3a1 shows the signal of NiCo2O4 nanowires, it is clearly found that Ni foam was totally covered with a large of NiCo2O4 nanowires. The surface of NiCo2O4 nanowires is smooth and the average diameter is 80 nm. The morphology of NiCo2O4@NiWO4 core-shell hybrid structure is shown in Fig. 3a2, the average diameter is 100 nm, and the surface become rough, revealing that NiWO4 structure is coated on the surface of NiCo2O4 nanowires. In order to obtain more structural information, TEM characterization is carried out. The low magnification TEM image is depicted in Fig. 3a3. It is obviously that the hybrid structures are consists of NiCo2O4 core and a thin NiWO4 shell. HRTEM image of NiCo2O4@NiWO4 structure (Fig. 3a4) shows the interplanar spacings of 0.141 nm and 0.123 nm, which correspond to the (440) plane of NiCo2O4 and the (004) plane of NiWO4, respectively. Fig. 3b1 shows the low magnification of NiCo2O4@NiWS core-shell hybrid structure, NiCo2O4 nanowires are completely decorated with the NiWS structures. From high 10


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magnification SEM images, NiWS product is sheet-like shape, as shown in Fig. 3b2. Low magnification TEM image of hybrid NiCo2O4@NiWS structure (Fig. 3b3), it is obviously that the average thickness of NiWS nanosheets is 20 nm. The lattice fringes in Fig. 3b4 can be readily indexed to the (222) and (311) crystal planes of NiCo2O4 phase. To evaluate the electrochemical performance of the as-synthesized hybrid structures, cycle voltammetry (CV) and galvanostatic charge-discharge curves are tested in 3 M KOH aqueous electrolyte under three-electrode system. Fig. 4a depicts the comparison of CV curves of NiCo2O4, NiCo2O4@NiWO4 and NiCo2O4@NiWS electrodes at same scan rate of 5 mV s-1, respectively. It can be seen that all CV curves possess obviously redox peaks, revealing that the capacitance is mainly based on the faradaic redox reaction, which could be ascribed to M-O/M-O-OH and OH- in the electrolyte (M stands for Ni or Co).36,37 According to previous work, the capacitance contribution from the Ni foam is negligible.38 CV curves of NiCo2O4, NiCo2O4@NiWO4 and NiCo2O4@NiWS electrodes are presented in Fig. S1. It is obviously found that the CV integrated area of core-shell structure is bigger than that of NiCo2O4 nanowires, indicating the increase in electron mobility and excellent pseudocapacitive performances. Compared with the GCD curves of NiCo2O4, NiCo2O4@NiWO4 and NiCo2O4@NiWS electrodes at same current density of 4 mA cm-2 are shown in Fig. 4b. It can be noticed from the GCD curves that the hybrid structure delivers much longer discharge time than NiCo2O4 nanowires electrodes, which is in consistent with results from CV curves. The areal capacitances of 11



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NiCo2O4, NiCo2O4@NiWO4 and NiCo2O4@NiWS are 0.42, 1.38 and 3.49 C cm-2, respectively. The specific GCD curve for each electrode is shown in Fig. S2, respectively. It was further found that NiCo2O4@NiWS exhibit longer discharge times, indicating that vulcanization could improve the electrochemical performance of the electrode materials. Fig. 4c shows the GCD curves of different electrode materials at same current density of 1 A g-1. The specific capacitances of NiCo2O4, NiCo2O4@NiWO4 and NiCo2O4@NiWS are 264, 517 and 580 C g-1, respectively. The specific GCD curves for the above electrode materials are shown in Fig. S3. It is obviously that the hybrid structure possesses much high specific capacitance, which might be attributed the unique architecture of electrode materials. CV measurement is also a powerful strategy to understand the electrochemical kinetics of electrode materials and further obtain insight into the reaction kinetics. The full CV curves of NiCo2O4@NiWO4 and NiCo2O4@NiWS electrodes at different scan rates from 5 to 50 mV s

-1

are shown in Fig. S1(a-d), which are found that the

integrated areas increase as the sweep speed increasing, and redox peaks in anodic and cathodic sweeps present the pseudo-capacitive characteristics. Peak position shifts with the scan rate increasing, revealing that the as-synthesized hybrid structures possess low resistance and fast ion and electron transport rate. Simultaneously, capacitive effect can be qualitatively calculated according to following equation: 39-40 i = avb

(3)

Where a and b represent the constant. Meanwhile b value is between 0.5 and 1.0, the larger b values the faster reaction speed. The calculated b value for oxidation 12


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peaks of NiCo2O4, NiCo2O4@NiWO4 and NiCo2O4@NiWS electrodes is 0.88, 0.62 and 0.73, respectively (Fig. S4). Those of corresponding reduction peaks are 0.71, 0.57 and 0.71 respectively. The capacitive contribution ratio can be further quantified by the following equation: i = k1v+k2v1/2

(4)

Where i, ν, k1 and k2 stands for the measured current, sweep rate and the constant, respectively. The calculated capacitance and diffusion-controlled redox values at different scan rates are shown in Fig. 4d. The pseudocapacitance controlled capacitance proportions of NiCo2O4@NiWO4 and NiCo2O4@NiWS structure are 43.4% and 26.1% at the scan rate of 30 mV s-1, respectively. It is obviously that diffusion-controlled reactions are predominant in the total capacitance for four hybrid electrodes, demonstrating that the hybrid structures benefit the permeation of OH- fast. Fig. 4h exhibits the EIS measurement of the as-synthesized hybrid structures in the frequency range from 100 kHz to 0.01 Hz with the amplitude of 5 mV. The straight line in low frequency zone shows the diffusive resistance of the electrolyte ions (Rw). In high frequency region, the intersection with the real axis represents bulk resistance (Rs) and the diameter of semicircle shows the charge transfer resistance (Rct).41 The equivalent

series

resistance

(ESR)

of

the

hybrid

NiCo2O4@NiWO4

and

NiCo2O4@NiWS are 0.81 and 0.66 Ω, respectively. Long life cycling performance of the as-prepared electrodes is evaluated by repeatedly charge/discharge at a current density of 20 mA cm-2 and the results are shown in Fig. 4i. For all the curves, the capacity retention slowly decreases with the cycle number increasing. The hybrid 13



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electrodes show excellent capacitance retention of 80.6 and 83.7%, respectively. The capacitances increase during the first 200 cycles, which could be attributed to the complete activation process of electrode materials. SEM images of the hybrid electrodes after 10000 cycles are shown in Fig. S5, it is observed that the morphology of the electrode materials can still be maintained, which further confirm that the heterostructured materials present good cycle stability. To investigate practical application of the as-synthesized electrode materials, an asymmetric supercapacitor is fabricated by employing NiCo2O4@NiWS as the positive material and AC as the negative material, as shown in Fig. 5a. According to the charge balance, the mass of the electrodes are matched through following equation: m+ / m- =C- × V- / C+× V+

(5)

where C+ and C- represent specific capacitance of the positive material and negative material at a current density of 1 A g-1, respectively. V+ and V- is the potential window of the electrodes. Loading mass ratio is closed to be 0.32. The total mass loading of NiCo2O4@NiWS and AC are 6.2 mg. CV curves of the hybrid NiCo2O4@NiWS and AC electrode are shown in Fig. 5b, It is seen that hybrid NiCo2O4@NiWS and AC electrode possess the potential window from 0 to 0.6 V and -1 to 0 V, respectively. Fig. 5c shows CV curves at different scan rates. With the sweep rate increasing, the shape of the curve remains unchanged, revealing that the device possesses ideal capacitance performance. The electrochemical performance of the device is further verified by GCD measurement at different current density with a 14


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voltage window of 1.5 V, as depicted in Fig. 5d, the voltage window of CV and GCD curves is inconsistent, which might be ascribed to the existence of coulombic efficiency. In addition, the triangular-shaped GCD curves verify capacitive characteristics of fast current response and small resistance. The energy densities and power densities of the device are calculated and compared with the previous reports, as shown in Fig. 5e. The device exhibits an excellent energy density of 41.1 Wh kg-1 at 1353.2 W kg-1 and a high power density of 10800 W kg-1 at 30.1 Wh kg-1, respectively. It is much better than previous reports.42-47 The long cycle stability measurement of NiCo2O4@NiWS//AC device is conducted via GCD tests at a current density of 10 mA cm-2 (Fig. 5f). The device possesses excellent capacitance retention of 84.2% after 10000 cycles. In order to evaluate the electrocatalytic performance for OER, different electrode materials are measured though the linear sweep voltammetry (LSV) with a applied-voltage range from 1.0 to 1.7 V (vs. RHE) at a scan rate of 2 mV s−1, and corresponding results are shown in Fig. 6a. It can be seen that the as-synthesized NiCo2O4@NiWO4 and NiCo2O4@NiWS structures present a small overpotential, which is lower than that of NiCo2O4 and Ni foam at current density of 10 mA cm-2. Especially, the hybrid NiCo2O4@NiWS structures show low overpotential and high current densities, revealing a superior electrocatalytic activity of the hybrid structure. Tafel slope is a key parameter to verify the kinetic of electron transfer in electrocatalytic process. Fig. 6b exhibits the Tafel slope of different electrodes. It is seen that the hybrid structures present smaller Tafel slope than NiCo2O4 nanowires 15



electrode, and the bare Ni foam possesses a Tafel slope of 160.1 mV dec-1, revealing that OER reaction activity presents small polarization with anodic current increasing. The electrochemical active surface area (ECSA) can be used to explain catalytic activity and the effect of electrocatalysts on the ability of water oxidation, which can be determined though the double layer capacitance (Cdl). Scan rate can be calculated from the slope of the curve, the slope is a function of current density.48,49 Electrochemical double layer capacitance (Cdl) can be obtained by CV tests in a non-Faradic potential window at various scan rates from 1 to 5 mV s-1, which could be attributed to Cdl. The CV curves of the different electrode materials are shown in Fig. S6, which are found that the area of the electrode materials increase with the scan rate increasing. Fig. 6c shows that the Cdl value of NiCo2O4@NiWO4 and NiCo2O4@NiWS is 0.17703 and 0.12577 F cm-2,respectively, which are higher than that of NiCo2O4 (0.0353 F cm-2). It demonstrates that the hybrid structures possess enhanced ECSA than NiCo2O4 nanowires and easily expose much more active sites, which might be attributed to enhanced specific capacitance and superior OER activity. Table S1 shows the comparison of the electrocatalytic properties of heterostructures with those reported in the literature. It is clearly found that hybrid structured NiCo2O4@NiWS exhibits excellent electrocatalytic performance. Long cycle stability of the electrocatalyst is another parameter of OER catalysts. Fig. 6d exhibits the chronoamperometric curve of the hybrid structures at a potential of 1.65 V, which is

16


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found that the hybrid NiCo2O4@NiWO4 and NiCo2O4@NiWS decrease by 3.6 and 2.9% after 13 h, respectively, revealing the excellent cycle stability.

5. Conclusion In summary, the hybrid core-shell structured electrodes are grown directly on Ni foam substrate through facile the hydrothermal route. The hybrid structures exhibit excellent specific capacitance and cycle stability. The as-assembled device delivers the energy density of 41.1 Wh kg-1 at a power density of 1353.2 W kg-1. Moreover, the hybrid NiCo2O4@NiWS presents the overpotential of 290 mV at 10 mA cm-2. It could be concluded that vulcanization can increase reaction kinetics and drop the polarization voltage of the hybrid structure. The as-synthesized hybrid structure might be a promising candidate for the fabrication of high performance energy storage devices and excellent OER electrocatalysts.

Supporting information: Supporting Information is available from the Internet at https://pubs.acs.org/ or from the author.

Acknowledgment: This project is supported by State Key Laboratory of New Ceramic and Fine Processing Tsinghua University（No. KF201807）

Conﬂict of Interest The authors declare no conﬂict of interest

17



References: (1) Schwietzke, S.; Sherwood, O. A.; Bruhwiler, L. M.; Miller, J. B.; Etiope, G.; Dlugokencky, E. J.; Michel, S. E.; Arling, V. A.; Vaughn, B. H.; White, J. W. Upward revision of global fossil fuel methane emissions based on isotope database, Nature 2016, 538, 88-91. (2) Zhong, Y.; Xia, X. H.; Deng, S. J.; Zhan, J. Y.; Fang, R. Y.; Xia, Y.; Wang, X. L.; Zhang, Q.; Tu, J. P. Popcorn inspired porous macrocellular carbon: rapid puffing fabrication from rice and its applications in lithium-sulfur batteries, Adv. Energy Mater. 2018, 8, 1701110. (3) Chu, S.; Majumdar, A. Opportunities and challenges for a sustainable energy future, Nature 2012, 488, 294-303. (4) Zhang, Q.; Uchaker, E.; Candelaria, S. L.; Cao, G. Nanomaterials for energy conversion and storage, Chem. Soc. Rev. 2013, 42, 3127-3171. (5) Zhao, D. P.; Liu, H. Q.; Xiang, W. Bi-interface induced multi-active MCo2O4@MCo2S4@PPy (M=Ni, Zn) sandwich structure for energy storage and electrocatalysis, Nano Energy 2019, 57, 363-370 (6) Li, Y.; Xu, J.; Feng, T.; Yao, Q. F.; Xie, J. P.; Xia, H. Fe2O3 Nanoneedles on Ultrafine Nickel Nanotube Arrays as Efficient Anode for High-Performance Asymmetric Supercapacitors, Adv. Funct. Mater. 2017, 27, 1606728. (7) Wu, X.; Yao, S.Y. Flexible electrode materials based on WO3 nanotube bundles for high performance energy storage devices, Nano Energy 2017, 42, 143-150.

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(20) He, G.; Li, J.; Li, W.; Li, B.; Noor, N.; Xu, K.; Hu, J. Parkin, I. P. One pot synthesis of nickel foam supported self-assembly of NiWO4 and CoWO4 nanostructures that act as high performance electrochemical capacitor electrodes, J. Mater. Chem. A, 2015, 3, 14272-14278. (21) Ma, F. X.; Yu, L.; Xu, C.Y.; Lou, X.W. Self-supported formation of hierarchical NiCo2O4 tetragonal microtubes with enhanced electrochemical properties, Energy Environ. Sci. 2015, 9, 862-866. (22) Wei, T. Y.; Chen, C. H.; Chien, H. C.; Lu, S. Y. Surface-core-functionalized polyphenylene dendrimers for organic electronics, Adv. Mater. 2010, 22, 347-349. (23) Niu, L.Y.; Wang, Y. D.; Ruan, F. P.; Shen, C.; Shan, S.; Xu, M.; Sun, Z. K.; Li, C.; Liu, X. J.; Gong, Y. Y. In situ growth of NiCo2S4@Ni3V2O8 on Ni foam as a binder-free electrode for asymmetric supercapacitors, J. Mater. Chem. A, 2016, 4, 5669-5677. (24) Zhou, W. W.; Yu, K.; Wang, D.; Chu, J.; Li, J. Y.; Zhao, L. M.; Ding, C. Y.; Du, Y.; Jia, X. T.; Wang, H. T. Hierarchically constructed NiCo2S4@Ni(1-x)Cox(OH)2 core/shell nanoarrays and their application in energy storage, Nanotechnology, 2016, 27, 235402. (25) Chang, C.; Zhang,

L.; Hsu, C.W.; Chuaha, X. F.; Lu, S. Y.; Mixed

NiO/NiCo2O4 Nanocrystals grown from skeleton of 3D porous nickel network as efficient electrocatalysts for oxygen evolution reaction, ACS Appl. Mater. Interfaces, 2018, 10, 417-426. (26) Ning, Y. Y.; Ma, D. D.; Shen, Y.; Wang, F. M.; Zhang, X. B. Constructing 21



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hexagonal WO3 thin films for high performance supercapacitor application. Electrochim. Acta, 2017, 224, 397-404. (33) Jaegermann, W.; Ohuchi, F.; Parkinson, B.; Interaction of Cu, Ag and Au with van der Waals faces of WS, and SnS2. Surf. Sci. 1988, 201, 211-227. (34) Xie, X.; McCarley, R. E. Synthesis, structure, and characterization of N-ligated tungsten selenide cluster complexes W6Se8L6. Inorg. Chem. 1995, 34, 6124-6129. (35) Chen, F. S.; Ji, S.; Liu, Q. B.; Wang, H.; Liu, H.; Brett, J. L.; Wang, G. X. Wang, R.F.

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Structured

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Interface-modulated

fabrication

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yolk-shell

Co3O4/C


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25



Figures caption Fig. 1 Synthesis schematic illustration of the hybrid structures Fig. 2 (a) XRD patterns of the as-synthesized hybrid structure products (b-f) XPS spectra of NiCo2O4@NiWO4 and NiCo2O4@NiWS (b) Ni 2p (c) Co 2p (d) W 4f (e) S 2p (f) O 1s Fig. 3 (a1) SEM images of NiCo2O4 nanowires grown on Ni foam (a2) NiCo2O4@NiWO4 nanosheets high magnification (a3) TEM image of hybrid NiCo2O4@NiWO4 nanosheets (a4) HRTEM image (b1-b2) different magnification SEM images of hybrid NiCo2O4@NiWS sheets (b3) TEM image of hybrid NiCo2O4@NiWS nanosheets (b4) HRTEM image of hybrid NiCo2O4@NiWS nanosheets Fig. 4 (a) Comparison of CV curves of the as-synthesized hybrid structure and NiCo2O4 electrodes at a scan rate of 5 mV s-1 (b) Charge-discharge curves of hybrid structure and NiCo2O4 nanowires at a current density 4 mA cm-2 (c) Comparison of GCD curves of the as-synthesized hybrid structure and NiCo2O4 electrodes at current density of 1 A g-1 (d) Comparison of contribution ratio between capacitive capacities and diffusion-limited capacities for different peak currents of NiCo2O4@NiWS and NiCo2O4@NiWO4 nanosheets (e) Nyquist plots of hybrid structures and NiCo2O4 nanowires (f) cycle stability of hybrid structure at current density of 20 mA cm-2 26


Page 26 of 34

Page 27 of 34 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


Fig. 5(a) device structure diagram (b) CV comparison of the NiCo2O4@NiWS nanosheets and the AC at a scan rate of 50 mV s-1 (c) CV curves of the asymmetric device at various scan rates from 10 to 100 mV s-1 (d) galvanostatic charge-discharge curves of asymmetric device at different current densities (e) Ragone plots of asymmetric device (f) cycling performance at a current density of 10 mA cm-2 Fig.6 (a) Polarization curves of hybrid structure, NiCo2O4 nanowires and Ni foam in 1 M KOH at scan rate of 2 mV s-1 (b) Tafel plots (c) double-layer capacitance (Cdl) obtained from the cyclic-voltammetry (CV) curves at different scan rate (d) Chronoamperometric stability tests of hybrid structure and NiCo2O4 nanowires in 1 M KOH at 1.65 V (vs. RHE)

27



Co2+ NH4F Ni2+

Page 28 of 34

Urea Step II

Step I

NiCo2O4@NiWO4 nanosheets Step III

Ni foam

NiCo2O4 nanowires

NiCo2O4@NiWS nanosheets

Fig. 1 Depeng Zhao et al.

28


Intensity (a.u.)

(a) NiCo2O4@NiWS

(b)

 Ni foam

(002) (200)

(220) (013)

NiCo2O4(222)

(400)

(220)

 10

20

30

40

Intensity(a.u.)

NiCo2O4@NiWO4

(440) (422)  50

2 Theta / degree W 4f7/2 W 4f 5/2

Ni 2p3/2

NiCo2O4@NiWO4 Ni 2p1/2

Sat.

Ni 2p3/2

Ni 2p1/2

30

70

80

850

Sat.

36

860

870

NiCo2O4@NiWS Co 2p 2p1/2

Sat.

775

880

Sat.

780

785

(e)

42

45

155

160

795

800

(f)

S2p

Sat.

2p1/2

790

805

Binding energy (eV)

O 1s

Intensity (a.u.)

Intensity (a.u.)

NiCo2O4@NiWS

39

Sat.

2p3/2

Binding energy (eV)

NiCo2O4@NiWO4

Binding energy (eV)

NiCo2O4@NiWO4 2p1/2 Co 2p



60

W 4f

33

2p3/2 Sat.

2p3/2

W4f7/2 W4f5/2

(c)

Sat.

NiCo2O4@NiWS Sat.

W 4f

Intensity (a.u.)

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


Intensity (a.u.)

Page 29 of 34

165

Binding energy (eV)

170

175

528

531

534

537

540

Binding energy (eV)


29


543

810


(a1)

200 nm (b1)

(a2)

Ni

N i O O

Co

(a3)

C o

(a4)

W

d=0.123 nm (004)

200 nm (b2)

Page 30 of 34

100 nm Ni

O

(b3)

Co

(b4)

W

S

1 um

d=0.141 nm (440)

2 nm

d=0.243 nm (311)

d=0.234 nm (222)

100 nm

50 nm


30


2 nm

Page 31 of 34

0.05

0.00 -0.01

NiCo2O4

-0.02

NiCo2O4@NiWS

-0.03 0.0

0.1

0

0.5

NiCo2O4

0.1

NiCo2O4@NiWO4 NiCo2O4@NiWS

0.0

0.6

0

10

5

10

20

30

-1

Scan rate (mV s )

63.7

83.2

74.5

80.6

90.3

91.4

97.0

90.2

diffusion

NiCo2O4@NiWS

50

pesudocapacitance

20

pesudocapacitance diffusion

40

0.4

NiCo2O4@NiWO4

(d)

100

60

0.3

0.2

400

800

1200

Times / s

Potential / V vs Hg/HgO

120

80

0.2

90.2

-0.04

NiCo2O4@NiWO4

0.3

NiCo2O4@NiWO4

0.3 0.2 0.1

1 A g-1

0.0

1600

2000

0

200

400

600

800

1000 1200 1400

Times / s

(e)

100%

8 6 4 Ni NiCo2O4

2

NiCo2O4@NiWS

0.4

Capacitance retention (%)

0.01

0.4

NiCo2O4

(c)

0.5



0.02

4 mA cm-2

(b)

0.5

-Z'' / Ohm

0.03

5 mV s-1

(a)

88.7

Current density / A

0.04

Contribution (%)

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


(f) 80.6%

100% 83.7% NiCo2O4@NiWS

NiCo2O4@NiWO4

0

NiCo2O4@NiWO4

NiCo2O4@NiWS

0

2

4

Z' / Ohm

6

0

2000

4000

6000

Cycle number / times


31


8000

10000


(a)

0.12

Negative

0.10

(b)

Active carbon NiCo2O4@NiWS

0.06

(c)

0.04

0.06

Current / A

Current / A

0.08

0.04 0.02 0.00

0.02 0.00

10 mV s-1 30 mV s-1 50 mV s-1 100 mV s-1

-0.02

-0.02

-0.04 -0.06

Potential / V

1.2 1.0 0.8 0.6 0.4 0.2

40

-0.9

-0.6

-0.3

0.0


0.3

0.6

Ref 42 30 ZnCo2O4//NGN/CNTs Ref 43 Ni(OH)2-CNTs//AC 20

Ref 44 MnO2@ppy//AC

Ref 45 NiWO4//AC Ref 46 NiCo2S4//AC

Ref 47 NiCo2S4//RGO

50

100

150

200

Time / s

250

300

350

0.9

80

1.5

84.2 % 1.5

60 40

1.5

1.2

1.2

0.9

0.6

0.9 0.6 0.3

0.3

20

0.0

200

400

600

67800

Power density (W kg-1)

10000

0

2000

4000

68000

68200

6000

8000

Cycle number (times)


32


68400

68600

Number (times)

Number (times)

0

1000

1.2

0.0

10 0

0.6

(f)

100

0

0.0

0.3

Potential / V

Our works

(e)

0.0

Potential(V)

1 A g-1 2 A g-1 4 A g-1 6 A g-1 8 A g-1

1.4

Energy density (Wh kg-1)

(d)

1.6

-0.04

Potential (V)

-0.08

Capacitance retation (%)

Positive



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

Page 32 of 34

10000

Page 33 of 34

0.28

(a)

Overpotential ( V vs. RHE)

Current density (mA cm-2)

120

Ni foam NiCo2O4

90

NiCo2O4@NiWS NiCo2O4@NiWO4

60

30

0

0.24

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

(c)

-2

NiCo2O4

0.9

slo

NiCo2O4@NiWO4

0.1 p e=

7F 257

mV 160.1

-1

dec

mV 150.4

-1

dec

0.16 0.12 0.08

106.7 mV

Ni foam NiCo2O4

-1

dec

95.2 mV

NiCo2O4@NiWS

-1

dec

NiCo2O4@NiWO4

0.2

Potential (V vs.RHE) 1.2

(b)

0.20

0.04

Current density (mA cm-2)

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


0.3

0.4

0.5

Log (j/mA cm-2)

0.6

0.7

(d)

cm

NiCo2O4@NiWS -2

0.6

0.1 p e= o l s

0.3

0.0

0

1

77

F 03

2

cm

-2

53 F cm slope=0.03

3

Scan rate (V s-1)

4

5


33



Page 34 of 34

For Table of Contents Use Only Constructing high performance hybrid battery and electrocatalyst by heterostructured NiCo2O4@NiWS nanosheets Depeng Zhao, †1 Meizhen Dai, †1 Hengqi Liu, † Li Xiao, † Xiang Wu†,* Hui Xia ‡,*

Co2+ NH4F Ni2+

Urea Step II

Step I

NiCo2O4@NiWO4 nanosheets Step III

Ni foam

NiCo2O4 nanowires

NiCo2O4@NiWS nanosheets

In this work, heterostructured NiCo2O4@NiWS nanosheets are prepared through a facile solution route. The as-synthesized products can be used directly as the electrode materials for high performance hybrid battery and electrocatalyst.

34


Constructing High Performance Hybrid Battery and Electrocatalyst by

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