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Self-assembled Coral-like Hierarchical Architecture Constructed by NiSe Nanocrystals with Comparable HydrogenEvolution Performance of Precious Platinum Catalyst 2

Bo Yu, Xinqiang Wang, Fei Qi, Binjie Zheng, Jiarui He, Jie Lin, Wanli Zhang, Yanrong Li, and Yuanfu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15719 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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ACS Applied Materials & Interfaces

Self-assembled Coral-like Hierarchical Architecture Constructed by NiSe2Nanocrystals with Comparable Hydrogen-Evolution

Performance

of

Precious

Platinum Catalyst Bo Yu‡, Xinqiang Wang‡, Fei Qi, BinjieZheng, Jiarui He, Jie Lin, Wanli Zhang, Yanrong Li, andYuanfu Chen*‡ State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China *Corresponding author: E-mail [email protected] ‡These authors have contributed equally. KEYWORDS:NiSe2nanocrystals, solvothermal reaction, hydrogen evolution reaction, ultrasmall Tafel slope, transition metal dichalcogenide, nickel dichalcogenide, elctrocatalytic activity

ACS Paragon Plus Environment

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ABSTRACT

For the first time, self-assembled coral-like hierarchical architecture constructed by NiSe2nanocrystals has been synthesized via a facile one-pot DMF-solvothermal method. Compared with hydrothermally synthesized NiSe2 (H-NiSe2), the DMF-solvothermally synthesized nanocrystallineNiSe2 (DNC-NiSe2) exhibits superior performance of hydrogen evolution reaction (HER): it has a very low onset overpotential of ~136 mV, a very high cathode current density of 40 mA/cm2 at ~200 mV, and an excellent long-term stability; most importantly, it delivers an ultra-small Tafel slope of 29.4 mV dec-1, which is the lowest ever reported for NiSe2-based catalysts, and even lower than that of precious platinum (Pt) catalyst (30.8 mV mV dec-1). The superior HER performance of DNC-NiSe2 is attributed to the unique self-assembled coral-like network, which is benefit to form abundant active sites and facilitates the charge transportation due to the inherent high conductivity of NiSe2nanocrystals. The DNCNiSe2 is promising to be a viable alternative to precious metal catalysts for hydrogen evolution.


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INTRODUCTION

Hydrogen is considered as a promising renewable clean energy in the future,1-4 with the shortage of energy and serious environmental problems become more and more severe. The hydrogen can be produced by hydrogen evolution reaction (HER); however, the high cost of noble metals (e.g. Pt) seriously hindered the large-scale applications of precious metal catalysts. 4-6

Hence, developing highly efficient, stable, low-cost and non-precious electrocatalysts through

facile preparation method using earth-abundant elements is urgent.7, 8

Recently, some transition-metal dichalcogenides (TMDs), have been reported to be used as HER catalysts. However, previous experimental data and theoretical calculation have confirmed that most of two-dimensional (2D) TMDs, such as MoS2,9,10 WS2,5,11,12 MoSe2,13,14 WSe2,15ReS2,19ReSe2,20PtSe221are still limited to the low inherent HER performance and relatively bad

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stability compared with platinum-based materials.22-24 Fortunately, among the TMDs, NiSe2 is stable in both acidic and alkaline environments and can be an effective catalyst for highperformance HER.3,25,26 Moreover, NiSe2 electrocatalyst has inherently high conductivity, which is benefit to obtain high electrocatalytic efficiency.3,27In addition, nickel is an earth-abundant element with low cost. Therefore, NiSe2 is appealing to be used as a non-noble-metal-based electrocatalysts with high HER performance and low cost.

In the last few years, some remarkable progresses have been made for the synthesis and HER performance of NiSe2-based catalysts. For example, Pu et al. synthesized NiSe2 nanoparticlesbased film on conductive Ti plate (NiSe2/Ti) with a Tafel slope of 82.0 mV dec-1;28 Zhou et al. reported that NiSe2/Ni hybrid foam had a small Tafel slope of 49.0 mV dec-1;3Kwak et al. synthesized NiSe2nanocrystals with a Tafel slope of 44.0 mV dec-1.26 Liang et al. synthesized


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NiSe2 nanoparticles via hydrothermal method, which had an overpotential of 170 mV @ 10 mA/cm2 and a small Tafel slope of 31.1 mV dec-18 However, compared with Pt catalyst with ultra-small Tafel slope, ultralow overpotential and super long-term stability, it is still urgent and significant to further improve the HER performance of NiSe2-based catalyst by developing novel synthesis method and/or optimizing its microstructure.

Herein, for the first time, a facile one-pot solvothermal method is presented to synthesize selfassembled coral-like hierarchical architecture constructed by pyrite-type NiSe2nanocrystals. Compared to hydrothermally synthesized NiSe2 (H-NiSe2), the DMF-solvothermally prepared nanocrystalline NiSe2 (DNC-NiSe2) delivers superior HER performance: it has a low onset overpotential of ~136 mV and very high cathode current density of 40 mA/cm2 at -200 mV and excellent long-term stability even after 2000 cycles; its Tafel slope is as low as 29.4 mV dec-1, which is the lowest ever reported for NiSe2-based catalysts and even lower than that of precious platinum (Pt) catalyst (30.8 mV dec-1). The superior HER performance is attributed to its unique coral-like network.

Experimental section

Synthesis of DNC-NiSe2 and H-NiSe2.

All of the chemicals were analytical grade and purchased from Aladdin Industrial Corporation and used without further purification. The DNC-NiSe2 was prepared by solvothermal method. The prepared process is schematically demonstrated in Figure 1. Firstly, 0.632 g of Se powder and 0.378 g sodium borohydride (NaBH4) were dissolved in 65 ml N, N-dimethylformamide (DMF) and stirred for 1 h; then, 0.952 g of NiCl2•6H2O was added to the solution and stirred for


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1 h until a homogeneously dispersed black-color solution was prepared; and then the as-prepared precursor solution was put into a 100 mL Teflon-lined autoclave that is maintained in an oven at 160 oC for 24 h. After cooling to room temperature naturally, the black powder was collected and washed with ethanol and DI water for several times and then treated in vacuum at 60 oC for 12 h. For comparison, the H-NiSe2 was synthesized by hydrothermal method. The synthesis procedure of H-NiSe2 is same as that of DNC-NiSe2 except for D.I. H2O is used as solvent instead of DMF.

Characterization

The crystal structures of DNC-NiSe2 and H-NiSe2 were conducted by X-ray diffraction (XRD, Rigaku D/MAX-rAdiffractometer). The Raman spectrum of NiSe2 was performed (532 nm, Renishaw). The chemical composition of the sample was examined by X-ray photoelectron spectroscopy (Kratos XSAM 800, Al Ka radiation (144 W, 12 mA, 12 kV). The distribution and morphology of the as-prepared NiSe2 was observed using scanning electron microscope (SEM, JSM-7000F) and transmission electron microscope (TEM, Tecnai F20) with an accelerating voltage of 200 kV.The size dispersity of DNC-NiSe2 and H-NiSe2was performed by a laser light scattering instrument (Malvern ZEN3690, Malvern Instruments).The specific surface area and porosity

analyseswere

conducted

using

the

Brunauer-Emmett-Teller

(BET)

method(Quantachrome, Nova 2000E).

Measurements of Electrocatalytic Properties

The electrocatalytic properties were carried out in a typical three-electrode system at an electrochemical station (CHI 660 D). To prepare the working samples, 4 mg of as-prepared NiSe2 powder was sonicated 10 min to form a homogeneous suspension in 1 mL of


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ethanol/water (1:3), then add 50 µL Nafion (5% w/w in water and 1-propanol) to the suspension with another 30 min sonication. After that, 10 µL of the above dispersion was dropped onto glassy carbon electrode (3 mm in diameter) using micropipette, each GCE was loaded with the same amount of catalyst. All the potentials were calibrated to a reversible hydrogen electrode. The polarization curves were obtained with a step interval of 1mV and scan rate of 5 mV/s was performed in 0.5 M H2SO4 using a Platinum (Pt) wire as the counter electrode and saturated calomel electrode (SCE) as the reference electrodes. Before the measurements, the 0.5 M H2SO4 was saturated with purity N2. To evaluate the stability of the electrocatalyst, a cyclic voltammetry method was performed at a scan rate of 100 mV/s for 2000 cycles.Time dependence of current density under static potential of -0.2 V vs RHE was also performed.

Results and discussion

The crystalline structures of DNC-NiSe2 and H-NiSe2 were performed by X-ray diffraction (XRD). As shown in Figure2a, the XRD diffraction patterns indicate that DNC-NiSe2 and HNiSe2 are pure pyrite-type phases with cubic structure (JCPDS card No. 88-1711). Confocal micro-Raman spectra was confirmed by excitation with an unpolarized 532 nm laser. As shown in Figure2b, there are four peaks at 149, 169, 210, and 238 cm-1 for both DNC-NiSe2 and HNiSe2. The two peaks at high energy (Ag and Tg) correspond to the stretching modes of the Se-Se pairs, while the two peaks at low energy can be assigned to the vibration modes (Tg and Eg) of dumbbell-shaped Se-Se pairs.27,30

The X-ray photoelectron spectroscopy (XPS) spectrum of DNC-NiSe2 was performed. Figure3a and 3b shows the high-resolution XPS spectra of Ni 2p and Se 3d peaks, respectively. As shown in Figure 3a, the Ni 2p3/2, and Ni 2p1/2 peaks appear at 856.1, and 873.8 eV


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respectively. The 2p3/2 peak was resolved into three bands. The XPS data of two peaks at 853.8 eV (S1) and 870.9 eV (S1’) are originated from Ni2+ ions. The peaks at 856.2 eV (S2) and 874.1 eV (S2’) are most likely originated from Ni3+ ions in the surface oxide phase. Two shakeup satellites at 860.7 eV (S3) and 880.1 eV (S3’) were observed, indicating Ni2+ oxidation state. The XPS spectrum of H-NiSe2was measured, as shown in Figure S1.These results are consistent with previous reports.26,28,31As shown in Figure 3b,it is apparent that the binding energies of signals of Se 3d (S1) is located at about 55 eV, which is also consistent with NiSe2.3,24,28,32The broad peak at 59.5 eV (S2) arises from surface oxidation of Se species.28,32

The SEM images show the morphologies of DNC-NiSe2 and H-NiSe2. As shown in Figure 4a, it is obvious that DNC-NiSe2nanocrystals are self-assembled to be a coral-like structure, while it seems that it is easier to be aggregated in H-NiSe2 (Figure 4b). It is noted that compared with HNiSe2, DNC-NiSe2 has smaller particle size with much more homogenous distribution/stacking. From Figure. 4a, one can clearly observe that NiSe2nanocrystals are homogenously distributed and stacked, and each nanocrystal tightly connects with neighboring nanocrystals, which constructs a three-dimensional coral-like conductive network. The inset is the optical microscopy of coral.The unique coral-like structure of DNC-NiSe2 can not only maintain the robust contact between the nanocrystals which will facilitate the charge transportation, but also provide abundant active sites. The different morphology between DNC-NiSe2 and H-NiSe2 might strongly influence their HER performances.

TEM and high resolution HRTEM studies were also used to identify the microstructure of DNC-NiSe2. For TEM characterization, Figure 5a shows the morphology of DNC-NiSe2 nanoparticles, which is consistent with the SEM image. As shown in Figure5b, the joint between


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DNC-NiSe2 nanoparticles was selected, we can clearly see that each nanocrystal is tightly connected with neighboring nanocrystals. Figure5c and 5d show the HRTEM images of DNCNiSe2. The distance between lattice fringes of 0.27 nm and 0.24 nm are assigned to the (210) and (211) plane of the NiSe2, respectively, which agrees with the result obtained from XRD pattern.3,8,26,29,33The elemental mapping images (Figure S2) also confirm that the formation of DNC-NiSe2 by this solvothermal method.

The HER activity of DNC-NiSe2 was studied at a scan rate of 5 mV s−1 in 0.5 M H2SO4 solution by a standard three-electrode electrochemical cell setup. Figure 6a shows the corresponding linear sweep voltammetry (LSV) curves with current density normalized by geometric surface area. For comparison, H-NiSe2, bare glassy carbon electrode, and Pt catalyst were also studied.We can find that the Pt catalyst shows excellent activity, butbare glassy carbon electrode has hardly any HER activity. We also observed that DNC-NiSe2 exhibits outstanding HER performance with a low onset potential of ~136 mV and high current density of 40 mA cm2

(at -200 mV vs RHE). We can clearly find that the onset potential of H-NiSe2 (~170mV) is

higher than DNC-NiSe2.

The Tafel plots demonstrate the linear fitted into the Tafel equation (η = b log (j) + a, where b is the Tafel slope and j is the current density), which is normally used as a reference to evaluate the efficiency of the catalytic reaction.8,34As shown in Figure 6b, the Tafel slopes of H-NiSe2, DNC-NiSe2, and Pt catalyst are 39.6, 29.4, and 30.8 mV dec-1, respectively. We can find that the Tafel slope of DNC-NiSe2 is much smaller than H-NiSe2. We think replacing water by DMF as solvent play an important role in the fabrication of DNC-NiSe2. In DMF solvent environment, the formation and dispersion of NiSe2 can be mediated.35Such ultra-small size and even spread of


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DNC-NiSe2 can improve the HER activity. Surprisingly, the Tafel slope of DNC-NiSe2 is comparable to that of Pt catalyst. Compared with other reported catalysts like MoS2,9,10 MoSe2,14, 36

WSe2,15-17 NiS2,22or the same materials NiSe2,3,23,28 the DNC-NiSe2 is better than most recently

reported catalysts, showing the high HER performance. The electrochemical performance of somepreviously reported catalysts and our work is summarized in Table S1.

In 0.5 M H2SO4 solution, there are three main steps, corresponding to the Volmer reaction (the hydrogen adsorption of electrochemistry: H3O+ + e-→Hads + H2O), the Heyrovsky reaction (the desorption of electrochemistry: H3O+ + Hads + e-→ H2↑ + H2O), or the Tafel reaction (the desorption of chemistry: Hads + Hads→ H2↑).4 Elementary steps involved in the HER can be investigated by Tafel slope. Under a specific set of conditions, Tafel slope of 30 mV dec-1 is attributed to the rate-limiting step of Tafel reaction. In this work, as shown in Figure 6b, the Tafel slope of NiSe2 is 29.4 mV dec-1, which indicates that the Tafel reaction is the rate-limiting step.

For the electrocatalysts, stability is important for practical application. As shown in Figure 6c, the polarization curve of DNC-NiSe2 after 2000 CV cycles at 100 mV/snearly overlaps to the initial curve, suggesting excellent long-term cycling stability. To further demonstrate the good stability of the DNC-NiSe2, time-dependent current density curve for DNC-NiSe2 under constant overpotential of -200 mV (vs. RHE) in 0.5 M H2SO4 was conducted for 12 h. As shown in Figure 6d, the current density of the NiSe2 modified GCE remains stable and nearly unchanged. The inset in Figure 6d shows a serrate shape of current curve from 0.6 h to 0.8 h, which is relative to the alternate processes of bubble accumulation and release.


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The cyclic voltammograms of the DNC-NiSe2 (Figure S3 a) and H-NiSe2 (Figure S3 b) were obtained in the potential range from 0.3 to 0.4 V (vs RHE) undervarious scan rates (20 – 200 mV/s). Current density differences (∆j = (Janodic − Jcathodic) at 0.35 V (vs RHE) are then plotted against the various scan rates. The electrochemical double-layer capacitance (Cdl) values are equivalent to half of the linear slopes in Figure7a and the Cdl of the DNC-NiSe2, and H-NiSe2 are 2.1, and 0.66 mF/cm2, respectively.In order to further demonstrate the advantage of DNC-NiSe2, thesize dispersity andBrunauer-Emmett-Teller (BET) measurements were performed. As shown in Figure S4, the particle sizes of DNC-NiSe2, and H-NiSe2 are concentrated around 78 nm and 148 nm, respectively. Compared with H-NiSe2, DNC-NiSe2 has smaller particle size with much more homogenous distribution. As shown in Figure S5, the calculated specific surface area of DNC-NiSe2 (13.5m2 g-1) is larger than that of H-NiSe2 (11.1m2 g-1); the total volumes of pores for DNC-NiSe2, and H-NiSe2 are0.067, and 0.034 cm3 g-1, respectively. It suggests that the DNC-NiSe2has larger specific surface area and porosity than those of H-NiSe2. As mentioned above,compared to H-NiSe2, DNC-NiSe2 has unique coral-like network with smaller particle size, more homogenous dispersity, larger specific surface areaand porosity, resulting in better HER performance.

Furthermore, electrochemical impedance spectroscopy (EIS) wasperformed to investigate the electrode kinetics under HERprocess.Figure7b displays the Nyquist plots of DNC-NiSe2 and HNiSe2. The Nyquist plots data were fitted to equivalent circuitshown in the insetof Figure 7b. Corresponding fitted charge-transfer resistances (Rct) of DNC-NiSe2, and H-NiSe2are 85.6, and 184.4Ω, respectively.Obviously, DNC-NiSe2has much lower Rct value than that of HNiSe2,indicating a faster electron transfer rate in the HER process. The lower Rct could afford much faster HER kinetics with the DNC-NiSe2 catalyst due to its unique morphology of network


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frame structure, which highlights the important role of DMF as a solvent for mediating the formation and dispersion of NiSe2nanocrystals.

As mentioned above, compared with H-NiSe2, DNC-NiSe2 exhibits superior HER performance. This can be simply interpreted as follows. Since DMF is a polar aprotic solvent with high boiling point, a small molecular solvent with relatively short carbon chain length,37,38 it is commonly used in metals,39 metal oxides,40 metal-organic frameworks,41,42 particularly metal chalcogenides17,18,43 synthesis with controlled shape and structure via solvothermal method. For example, Yanet al.demonstrated that when D.I. H2O was replaced by DMF solvent, much better dispersion (less aggregation) and more idea morphology of MoS2/CNT can be obtained.8 This is more or less similar in our case. In our work, when the D.I. H2O solvent was replaced by the polar organic DMF solvent, due to the better dispersion and stronger reactive activity of DMF, smaller crystal size, and less aggregation was observed in DNC-NiSe2, and each nanocrystal is tightly connected with neighboring nanocrystals, which constructs a three-dimensional coral-like conductive network. We attribute the superior HER performance of DNC-NiSe2 to its unique coral-like hierarchical architecture: such structure can not only guarantee the robust contact between the nanocrystals which will facilitate the charge transportation, but also provide abundant active sites. Therefore, it is reasonable that DNC-NiSe2 has much better HER activity than that of H-NiSe2.

Conclusions

In summary, we have developed a facile, low-cost one-pot solvothermal strategy to design and accomplish DNC-NiSe2 electrocatalyst and the method can be used to fabricate other diselenides. Compared with H-NiSe2, the DMF-solvothermallysynthesizednanocrystalline NiSe2 (DNC-


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NiSe2)can form a three-dimensional conductive network and exhibits excellent catalytic activity for HER. It has a low onset overpotential of ~136 mV and very high cathode current density of 40 mA/cm2 at -200 mV and excellent long-term stability even after 2000 cycles.Particularly,it has an ultra-smallTafel slope of 29.4 mV dec-1, which is better than most catalysts that have been reported, and comparable to Pt catalyst. The remarkable catalytic performance and stability, together with the facile and low-cost synthesis method, makes DNC-NiSe2 promising to be a viable alternative to precious metal catalysts for hydrogen evolution.

ASSOCIATED CONTENT Supporting Information.XPS spectra of Ni2p of H-NiSe2;Elemental mapping images of DNCNiSe2;Cyclic voltammograms; Size and dispersity; Nitrogen adsorption-desorption isotherms and pore size distribution; Summary of electrochemical performance of some catalysts.

AUTHOR INFORMATION Corresponding Author [email protected]

ACKNOWLEDGMENT


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The research was supported by the National Natural Science Foundation of China (Grant Nos. 51372033, 51202022, and 61378028), National High Technology Research and Development Program of China (Grant No. 2015AA034202), and the 111 Project (Grant No. B13042).

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FIGURES

Figure 1.The illustrations of the synthesis process of cubic DNC-NiSe2.

(421) (332)

(400)

(311) (220) (221)

(023) (321)

(211) (200)

DNC-NiSe2

H-NiSe2 PDF#88-1711

20

30

40 50 60 2θ / degree

70

80

Raman Intensity (a.u.)

(b)

(210)

(a) 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


Ag T g Eg

Tg

DNC-NiSe2

H-NiSe2

160

240 320 400 480 -1 Raman shift (cm )

560

Figure 2.(a)The XRD patterns ofDNC-NiSe2and H-NiSe2and (b) the Raman spectra of DNC-NiSe2and H-NiSe2.


17


(b)

(a)

848

Ni 2p1/2 S2' S3' S1'

856 864 872 880 Binding energy (eV)

S1 S2

Intensity (a.u.)

Ni 2p3/2 S2 S1 S3

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

888

Page 18 of 22

50

52

54 56 58 60 Binding energy (eV)

62

64

Figure 3.The high-resolution XPS spectra of (a) Ni2p and (b) Se 3d of DNC-NiSe2.

Figure

4.SEM

images

of

(a)DNC-NiSe2in

pyrite-type

phase.The

inset

is

the

opticalmicroscopyof coral. (b) H-NiSe2in pyrite-type phase.


18

Page 19 of 22

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


Figure 5. TEM images of DNC-NiSe2 (a-b) and HRTEM images of cubic pyrite-type DNCNiSe2 (c-d).


19


(b) 0.4

-10 -20 Glassy carbon H-NiSe2

-30

0.3

39.6 mV dec

-1

0.2 0.1

29.4 mV dec

-1

0.0 30.8 mV dec

H-NiSe2

-1

-0.1

DNC-NiSe2

DNC-NiSe2

Pt

-0.3

-0.2 -0.1 0.0 0.1 Potential (V vs RHE)

Pt

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 2 Log |j(mA/cm )|

0.2

(d)

-20 -30 -40 -50 -0.4

Initial After 2000 cycles -0.3 -0.2 -0.1 0.0 Potential (V vs RHE)

0.1

-25 -26

-50

-27

2

-10

j (mA/cm )

2

Current density (mA/cm )

-40 -0.4

(c) 0 2

Potential (V vs RHE)

2

Current density (mA/cm )

(a) 0

Curent density (mA/cm )

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 20 of 22

-75 -100

Bubble accumulation Bubble release

-28 -29 -30 0.60

-125 0

2

0.65

4

0.70

Time (h)

0.75

6 8 Time (h)

0.80

10

12

Figure 6.The electrochemical performance of DNC-NiSe2 and H-NiSe2. (a) Polarization curves and (b) corresponding Tafel plots of NC-NiSe2 and H-NiSe2. (c)Durability test for cubic pyritetype NiSe2 after 2000 CV cycles under air atmosphere. (d) Time dependence of current density under static potential of -0.2 V vsRHE. Inset is the enlargement of the area from0.6 h to 0.8 h.


20

Page 21 of 22

(a) 0.5

(b) 180 DNC-NiSe2 → 2.1mF

0.4

-Z'' (Ω)

0.3 0.2

H-NiSe2

120 90 60

0.1 0.0

DNC-NiSe2

150

H-NiSe2 → 0.66mF

2

∆j/2 (mA/cm )

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


30 0

40

80 120 160 Scan rate (mV/s)

200

0

0

40

80

120 160 z'(Ω)

200

240

Figure 7.(a) Estimated Cdl and relative electrochemically active surface area for the DNCNiSe2and H-NiSe2electrocatalyst. (b) Nyquist plots (100 kHz–10 mHz) of DNC-NiSe2and HNiSe2 at -0.145 Vvs RHE. The data were fitted to the equivalent circuit shown in the inset.


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Table of Contents Graphic


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Self-Assembled Coral-like Hierarchical ... - ACS Publications

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