3D Hierarchical Nickel Cobalt Phosphide Nanoflowers as an Efficient

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3D Hierarchical Nickel Cobalt Phosphide Nanoflowers as an Efficient Electrocatalyst for Hydrogen Evolution Reaction in both Acidic and Alkaline Conditions Jianshuai Mu, Jing Li, En-Cui Yang, and Xiao-Jun Zhao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00540 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Energy Materials

3D Hierarchical Nickel Cobalt Phosphide Nanoflowers as an Efficient Electrocatalyst for Hydrogen Evolution Reaction in both Acidic and Alkaline Conditions

Jianshuai Mua, Jing Lia, En-Cui Yang*a and Xiao-Jun Zhao*ab

a

College of Chemistry, Key Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of

Education, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Tianjin Normal University, Tianjin 300387, P. R. China. E-mail: [email protected]; [email protected]; Fax: +86-22-23766556 b

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300071, China

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

ABSTRACT:

The sustainable hydrogen fuel from water electrolysis demands the development of efficient and robust non-noble electrocatalysts for hydrogen evolution reaction (HER). Tuning of morphology and chemical composition are paramount to constructing electrocatalysts with superior activity and stability. In this work, novel ternary nickel doped cobalt phosphide (Ni-Co-P) nanoflowers assembled by porous and unltrathin nanosheets were firstly prepared by a facile solvothermal reaction following a phosphidation procedure. The Ni-Co-P nanoflowers exhibited remarkable electrocatalytic HER performance, exhibiting overpotentials of as low as 83 and 92 mV at 10 mA cm−2, small Tafel slopes of 46.6 and 49.6 mV dec-1, in 1 M KOH and 0.5 M H2SO4 conditions, respectively, which was one of the most active earth-abundant electrocatalysts. Additionally, the electrocatalysts exhibited high durability for HER in both alkaline and acidic conditions. Various techniques further demonstrated that the superior activity of Ni-Co-P nanoflowers was attributed to the unique 3D hierarchical morphology and the modified electron structure due to Ni incorporation. The superior activity and stability of novel Ni-Co-P nanoflowers have promising potential for application in production of hydrogen fuel from water splitting. KEYWORDS: Nickel cobalt phosphide, Hierarchical nanoflowers, Electrocatalysis, Hydrogen evolution reaction, Non-noble metal electrocatalyst ■ INTRODUCTION With the depletion of traditional fossil fuels and concern over environment pollution, exploiting sustainable and clean energy sources has been becoming a hotspot. Hydrogen energy may be an ideal candidate in the future, because of its high energy density and zero emission.1 Most of hydrogen production at present is primarily produced by steam reforming, which suffers from the consumption of traditional fossil fuels and greenhouse gas emission.2 Water electrolysis coupled to sustainable energy sources including solar energy and wind is a promising method for large-scale hydrogen production, compared with steam reforming. Constructing active hydrogen evolution electrocatalysts is the key problem for the Water electrolysis.3 Currently, precious Pt is considered to be most efficient

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electrocatalyst for HER, but the high cost influences its widespread application.4 Consequently, the development of efficient HER electrocatalysts with low cost is desirable. Up to now, many non-noble metal catalysts, such as transition metal alloys,5 carbides,6,7 nitrides,8 oxides,9 chalcogenides10 and selenides11-15 have been explored. However, most of the above materials exhibit large overpotential over 100 mV, limiting their application for HER. Therefore, constructing non-noble metal electrocatalysts with lower overpotential still remains a challenging task. The recent several years have witnessed the development of transition metal phosphides (TMPs), because of their metalloid characteristics and good electronic conductivity for efficient HER.16-19 Among the TMPs, cobalt phosphide (CoP) has been demonstrated as the most effective HER electrocatalyst.20-21 The electrocatalytic performance not only relies on its morphology but also depends on its chemical composition. A variety of CoP with different morphologies, such as 1D nanorods,22-23 nanosheets, 1, 2427

polyhedron28 and nanoparticles,29-31 have been prepared. Different from the above morphology, the

electrocatalysts with 3D morphology exhibit superior activity, owing to their large surface area, more exposed active sites, enhanced mass transport and easy release of H2 bubbles from the surface.32-33 In addition, CoP-based ternary metal phosphide, for example, CoxFe1-xP,2 CoMnP3 and Ni1-xCoxP,34 can also enhance the electrocatalytic performance, because of the synergistic effect of different components. Therefore, fabricating ternary TMPs with novel 3D morphology may be a good strategy to further enhance their electrocatalytic performance. Herein, novel 3D ternary nickel cobalt phosphide (Ni-Co-P) hierarchical nanoflowers assembled by porous and ultrathin nanosheets were prepared and behave as an efficient HER electrocatalyst in both acid and alkaline conditions, with superior activity to CoP nanoflowers. The superior performance of Ni-Co-P nanoflowers was further investigated with various techniques including UV-Vis-NIR, XPS, Tafel slope and impedance spectroscopy. The as-prepared Ni-Co-P nanoflowers hold substantial promise for use as efficient and low-cost electrocatalyst in the electrocatalytic water-splitting technology. ■ EXPERIMENTAL

3



Preparation of Ni-Co-P nanoflowers and CoP nanoflowers. Preparation of Ni-Co-P nanoflowers: 0.4 mmol cobalt nitrate, 0.1 mmol nickel nitrate and 0.5 mmol urea were dissolved in 20 mL ethanol. The solution was transferred into a Teflon-lined autoclave (25 mL volume) for solvothermal reaction at 120 °C for 6 h. And the precipitate was obtained by centrifugation, washed with water and ethanol for three times, and dried at 60 °C for 6 h. Finally, the above precursor and 100 mg NaH2PO2 were placed in a quartz boat, where NaH2PO2 was put in the upstream side and the precursor was put in a downstream side. The furnace was heated to 300 °C with a rate of 1 °C/min for 2 h in N2 atmosphere. After the phosphidation process, the black product was obtained. Preparation of CoP nanoflowers: 0.5 mmol cobalt nitrate was dissolved in 20 mL ethanol, and the following conditions and procedures were same to that of Ni-Co-P nanoflowers. Materials characterization. X-ray powder diffraction patterns (XRD) were characterized by a X-ray diffractometer with Cu Kα radiation (D8 ADVANCE, Bruker, Germany). The morphology was performed using a scanning electron microscope (Nova Nano SEM 230, FEI, USA) and a transmission electron microscope (Tecnai G2 F20, FEI, USA). N2 adsorption-desorption isotherms were measured by a physisorption analyzer (ASAP 2020, Micromeritics, USA), and the specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. UV-Vis-NIR diffuse reflectance spectra (DRS) were carried out on a spectrophotometer (UV-2450, Shimadzu, Japan). X-ray photoelectron spectroscopy (XPS) was measured by an X-ray photoelectron spectroscopy (AXIS Ultra DLD, Shimadzu, Japan). Electrochemical measurements. 4 mg electrocatalyst was dispersed in the solution of 0.475 mL ethanol and 25 µL 5% nafion solution to obtain a homogeneous ink under ultrasonic irradiation for 20 min. Then 12 µL ink was transferred onto a glassy carbon electrode (GCE, 3 mm diameter) and dried at room temperature. For comparison, commercial Pt/C sample with the same loading amount was dropped onto the GCE. Electrochemical experiments were obtained by a three-electrode system with electrocatalysts modified GCE as the working electrode, a carbon rod as the counter electrode and a saturated Ag/AgCl as the reference electrode at 25 ºC. The saturated Ag/AgCl reference electrode was 4


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calibrated with respect to RHE according to the report.35 The measurement was performed in the high purity hydrogen saturated electrolyte with Pt mesh as the working electrode and counter electrode, a saturated Ag/AgCl as the reference electrode. CVs were run at a scan rate of 1.0 mV s-1, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions. In 1 M KOH, ERHE=EAg/AgCl + 1.019 V; in 0.5 M H2SO4, ERHE=EAg/AgCl + 0.220 V; and in 0.1 M pH 7.0 phosphate buffer, ERHE=EAg/AgCl + 0.582 V. Linear sweep voltammetry (LSV) were recorded at a scan rate of 5 mV s-1 to obtain the polarization curves. The data were corrected for background current and iR losses. Frequency (TOF) values were calculated using the equation: TOF = jSgeo /2Fn. Here, j (mA cm-2) was the current density at η = 83 mV.36 Sgeo was the surface area of working electrode (0.07065 cm2). The number 2 was the number of electrons per mole of H2. F was the Faraday’s constant (96485.3 C mol-1) and n was the moles of the metal atom on the working electrode calculated by the loading mass and molecular weight of the electrocatalysts. The electrochemically active surface areas of electrocatalysts were represented by the double-layer capacitor with the reported cyclic voltammograms (CVs) method, which were measured at different rates from 20 to 100 mV s-1 in 1 M KOH. The electrochemical impedance spectroscopy (EIS) were carried out using an Autolab potentiostat (Eco Chmie, The Netherlands) in the frequency range from 100 KHz to 0.1 Hz in 1 M KOH. The ESI data were fitted with the ZView software. The cyclic voltammetry (CV) durability was tested at a scan rate of 50 mV s-1 for 1000 and 2000 cycles, and the long-term stability was also examined by bulk electrolysis at a fixed overpotential of 83 mV, 92 mV and 216 mV (j=10 mA cm-2) respectively in alkaline, acidic and neutral conditions. ■ RESULTS AND DISCUSSION The synthesized electrocatalysts were fabricated by two steps including hydrothermal growth of the Ni−Co precursors and phosphorization. The final products were characterized by X-ray diffraction (XRD) patterns shown in Figure 1. The product of CoP nanoflowers showed diffraction peaks characteristic of orthorhombic CoP phase (JCPDS No. 29-0497).37 The diffraction peaks at 31.6º, 36.3º, 46.2º, 48.1º, 52.3º and 56.8º were attributed to (011), (111), (112), (211), (103) and (301) crystal planes of CoP, respectively. For the product of Ni-Co-P nanoflowers, other than some diffraction 5



peaks of orthorhombic CoP phase, there were several additional diffraction peaks at 41.0º, 44.9º, 54.4º and 54.7º, which could be indexed to the (111), (201), (300) and (002) crystal planes of hexagonal NiCoP phase (JCPDS No. 71-2336).38 Therefore, the crystalline phase of Ni-Co-P nanoflowers was a mixture of CoP and NiCoP.

♣

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

CoP

♦

♦ ♦ ♦

Ni-Co-P

♣♦ ♦

♣

♦

JCPDS 71-2336 JCPDS 290497

2θ (°)

Figure 1. XRD patterns of CoP and Ni-Co-P nanoflowers.

The morphologies of synthesized electrocatalysts were characterized by SEM and TEM. The CoP electrocatalyst were composed of hierarchical nanoflowers (diameter of ~1.5 µm) assembled by interconnected nanosheets (thickness of 25 nm) (Figure 2 a and b). After Ni addition, the Ni-Co-P electrocatalyst remained the similar hierarchical nanoflowers (diameter of ~1.2µm) assembled by interconnected nanosheets, while the nanosheets of Ni-Co-P (thickness of 15 nm) were much thinner than that of CoP (Figure 2 c and d). The energy-dispersive X-ray (EDX) spectrum revealed the existence of Ni, Co, and P for Ni-Co-P nanoflowers, and a 1:2.9 atomic ratio for Ni:Co (Figure S1). The TEM images showed that the nanosheets of CoP and Ni-Co-P nanoflowers were rough, and many nanoparticles and micro/mesopores were distributed in the nanosheets (Figure 3). The high-resolution TEM (HRTEM) taken from CoP nanoflowers revealed well-resolved lattice fringes with interplanar distance of 0.23 nm, which is indexed to the (201) crystal plane of CoP (Figure 3c).39 The lattice fringes of Ni-Co-P nanoflowers with an interplanar distance of 0.23 nm and 0.29 nm corresponded to the (201) crystal plane of CoP and (110) crystal plane of NiCoP, respectively. The scanning TEM (SETM) image and EDX elemental mapping images of Ni, Co, and P for Ni-Co-P nanoflowers, confirmed the uniform distribution of all elements in the whole nanoflower (Figure S2). Furthermore,

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the surface areas measured by Brunauer-Emmett-Teller (BET) analysis were comparable for the two different nanomaterials, with the CoP nanoflowers having a BET specific surface area of 14.6 m2 g-1 and the Ni-Co-P nanoflowers having a BET specific surface area of 21.7 m2 g-1 (Figure S3 a). In addition, the corresponding pore size distribution curves clearly showed their micro/mesopore size (Figure S3b). The 3-dimension hierarchical nanostructures of the ternary metal phosphides might enhance catalytic performance because of the unique morphology and the synergistic effect of highly distributed heteroatom.

Figure 2. (a, b) SEM images of CoP and (c, d) Ni-Co-P nanoflowers.

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Figure 3. (a-c) TEM and HRTEM images of CoP nanoflowers, (d-f) TEM and HRTEM images of Ni-Co-P nanoflowers.

The absorption spectra of CoP nanoflowers and Ni-Co-P nanoflowers in the range of 200-2500 nm were shown in Figure S4 a. The absorption spectrum of CoP nanoflowers showed two absorption maxima at about 691 nm in the visible band and 1526 nm in the near-infrared band. Compared with CoP nanoflowers, the absorption intensity of Ni-Co-P nanoflowers increased, indicating that introduce of Ni changed the electronic structure. Figure S4 b showed the plot between (αhν)2 and hν of CoP nanoflowers. The energy band gap (Eg) obtained from extrapolating the linear fitting was 0.95 eV, and the Eg value of Ni-Co-P nanoflowers was 0.79 eV.40 This band gap narrowing leaded to the enhanced excitation of charge carriers to the conduction band, which was beneficial for the electrical conductivity and might enhance the electrochemical HER performance of Ni-Co-P nanoflowers.41 The surface chemistry of CoP nanoflowers and Ni-Co-P nanoflowers was also studied by X-ray photoelectron spectroscopy (XPS). XPS spectrum of CoP nanoflowers suggested the existence of Co and P elements, and signals of C and O elements arose from contamination/surface oxidation of the product (Figure 4a).2 Other than the above elements, there was Ni element in the Ni-Co-P nanoflowers. In the Ni 2p spectrum (Figure 4b), the Ni 2p3/2 peaks at 853.7, 856.7 eV and 861.2 eV were assigned to the Ni components in Co-Ni-P, oxidized Ni species and the satellite, respectively, in line with 8


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previous reports on nickel phosphide.34, 42 The Co 2p3/2 region of CoP nanoflowers showed two main peaks at 779.3 and 781.2 eV and two satellites at 783.4 and 787.5 eV, respectively, while the Co 2p1/2 region exhibited one main peak at 796.6 eV accompanied with one satellite peak at 803.8 eV (Figure 4 c). The peaks at 779.3 eV and 781.2 eV were assigned to the binding energies for Co 2p3/2 in CoP and oxidized Co species, respectively.43-44 The Co 2p3/2 binding energy of Ni-Co-P nanoflowers was negatively shifted to 779.0 eV (Figure 4c). The high-resolution XPS P 2p spectrum of CoP nanoflowers showed the peak at 129.6 eV corresponding to the P-Metal bond in metal phosphides, along with one broad peak at 133.5 eV, which was assigned to oxidized phosphate species exposed to air (Figure 4d),34,

45

. Compared with CoP nanoflowers, the P 2p binding energy of Ni-Co-P

nanoflowers was negatively shifted to 127.5 eV (Figure 4d). The binding energies for Co and P of NiCo-P nanoflowers were negatively shifted compared with those of CoP nanoflowers, suggesting strong electron interactions between Ni and Co, which might have important implications in promoting the electrochemical HER catalysis. The Co 2p BE 779.0 eV of Ni-Co-P nanoflowers was positively shifted from that of Co metal (778.1−778.2 eV), and the P 2p BE 127.5 eV of Ni-Co-P nanoflowers was negatively shifted from that of elemental P (130.2 eV). It indicated that the Co had a partial positive charge (δ+) while the P had a partial negative charge (δ−), suggesting transfer of electron density from Co to P in Ni-Co-P nanoflowers. The natural hydrogenases had the corresponding pendant bases proximate to the metal centers, which were the active sites for HER. Similar to hydrogenases, the Ni-Co-P nanoflowers exhibited metal center Co (δ+), with pendant base P (δ−) close to it, and thus was expected as a potential efficient catalyst toward HER.

9


1200

Ni 2p

satellite 856.7 eV

400

0

890

880

870

860

850

Binding energy (eV)

Binding energy (eV)

(c)

853.7 eV

P 2s P 2p Co 3s Co 3p

C 1s

800

Ni 2p 3/2

Intensity (a.u.)

Ni LMM

Ni 2p 1/2

O 1s

O KLL Co 2s

Intensity (a.u.)

CoP

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(b)

Co 2p1/2 Co 2p3/2 Co LMM

(a) Ni-Co-P

(d)

Co 2p

P 2p

132.5 eV

Ni-Co-P

CoP

Intensity (a.u.)

∆E=0.30 eV

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

Ni 2p1/2 Ni 2p3/2


779.0 eV

803.8 eV 796.6 eV

781.2 eV

779.3 eV

133.6 eV 129.6 eV

Ni-Co-P CoP

783.4 eV 787.5 eV

810

800

790

780

127.5 eV

770

140

135

130

125

Binding energy (eV)

Binding energy (eV)

Figure 4. (a) Survey scan XPS spectra and high-resolution XPS spectra of (b) Ni 2p, (c) Co 2p and (d) P 2p of the CoP and Ni-Co-P nanoflowers.

The electrocatalytic HER performance of CoP nanoflowers and Ni-Co-P nanoflowers was studied by steady-state linear sweep voltammetry (LSV) using a three-electrode system in 1 M KOH and 0.5 M H2SO4 solutions. For comparison, the commercial Pt-C was also measured under the same conditions. As shown in Figure 5a, the HER polarization curve of CoP nanoflowers in 1 M KOH solution required overpotentials (η) of 116, 167, and 201 mV to reach current densities of 10, 50, and 100 mA cm-2, respectively, indicating the high electrocatalytic HER performance. In sharp contrast, after the hybrid of Ni, Ni-Co-P nanoflowers exhibited much higher HER activity, reducing the overpotentials of achieving 10, 50, and 100 mA cm-2 to 83, 120, and 165 mV, respectively (Figure 5a, Table S1). Particularly, the catalytic current density of Ni-Co-P nanoflowers even surpassed that of commercial Pt/C beyond 125 mV vs reversible hydrogen electrode (RHE); albeit, Pt/C exhibited the smallest onset potential. In addition, the intrinsic catalytic activities in 1 M KOH solution were studied on the basis of turn over frequency (TOF), in which Ni-Co-P nanoflowers showed higher TOF value

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(48.4 × 10-3 s-1), lager than that of CoP nanoflowers (6.33 × 10-3 s-1) (Table S1). The Tafel plots were calculated to elucidate the electron-transfer kinetics. The linear portions of the Tafel plots were fitted to the Tafel equation: η = b log j + a, where j is the current density and b is the Tafel slope. The Tafel slope of Ni-Co-P nanoflowers (46.6 mV dec-1) was smaller than that of Co-P nanoflowers (63.0 mV dec-1) implying its favorable HER catalytic kinetics (Figure 5b, Table S1). Compared with many reported efficient metal phosphides HER electrocatalysts (Table 1), the low overpotential and small Tafel slope of Ni-Co-P nanoflowers showed that the Ni-Co-P nanoflowers was superior to those of most reported metal phosphides HER electrocatalysts and was one of the most active earth-abundant electrocatalysts. Under the condition of 0.5 M H2SO4, the LSV curve of CoP nanoflowers presented overpotentials of 100, 155 and 191 mV at 10, 50, and 100 mA cm-2, respectively; while the Ni-Co-P nanoflowers showed the overpotentials of 92, 131 and 148 mV at the same current densities, indicating the superior electrocatalytic activity (Figure 5c, Table S2). The TOF value of Ni-Co-P nanoflowers (23.5 × 10-3 s-1) was also larger than that of CoP nanoflowers (19.1 × 10-3 s-1) in 0.5 M H2SO4 (Table S2). The Tafel slopes of CoP nanoflowers and Ni-Co-P nanoflowers were determined to be 59.1 and 49.6 mV dec-1 in 0.5 M H2SO4 solution, respectively (Table S2). Under 0.1 M pH 7.0 phosphate buffer, CoP nanoflowers exhibited overpotentials of 221 mV at 10 mA cm-2, while the NiCo-P nanoflowers owned the overpotentials of 216 mV at the same current densities, showing its slightly better electrocatalytic activity in neutral condition (Figure 5e, Table S3). The TOF value of Ni-Co-P nanoflowers (1.96 × 10-3 s-1) was also larger than that of CoP nanoflowers (0.469 × 10-3 s-1) in 0.1 M pH 7.0 phosphate buffer (Table S3). The Tafel slopes of CoP nanoflowers and Ni-Co-P nanoflowers were calculated to be 69.0 and 70.6 mV dec-1, respectively. The faradaic efficiency was estimated by measuring hydrogen amount during the electrolysis. And the faradaic efficiency was found to be all nearly 100% in 1.0 M KOH, 0.5 M H2SO4 and 0.1 M pH 7.0 phosphate buffer (Figure S6). The above result suggested that the HER of Ni-Co-P nanoflowers followed a Volmer−Heyrovsky mechanism in both acidic and alkaline conditions and the electrochemical desorption was the ratelimiting step. Thus far, only a few active electrocatalysts in both acidic and alkaline conditions have been reported, due to the catalytical incompatibility of the same electrocatalyst operating in the 11



different pH region. In this report, the excellent superior HER activity of the Ni-Co-P nanoflowers in both acidic and alkaline conditions could be potentially attributed to the synergistic effect between heteroatom Ni and Co in the nanocomposites where heteroatom Ni modified the electronic structure of CoP, and the unique morphology of hierarchical nanoflower with micro/mesopore structure which

0.2

(a)

Overpotential (V vs RHE)

0

-2

Current density (mA/cm )

could expose more electrocatalytic sites and enhance contact with the electrolyte.

1 M KOH

-50

Ni-Co-P CoP Pt/C

(b)

CoP -1 63.0 mV dec

0.1

Ni-Co-P -1 46.6 mV dec

Pt/C -1

34.9 mV dec 0.0

-100 -0.2

-0.1

0.0

0.0

0.5

0.2

(c) 0.5 M H2SO4

-50 Ni-Co-P CoP Pt/C

-100

1.5

-2

2.0

(d) -1

59.1 mV dec CoP

Ni-Co-P -1 49.6 mV dec

0.1

Pt/C -1 24.0 mV dec 0.0

-0.2

-0.1

0.0

0.0

0.5

Potential (V, vs. RHE)

1.0

1.5

-2

2.0

Log|j/(mA/cm )|

0.2

0


(e)

-2

1.0

Log|j/(mA/cm |


0

-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

Page 12 of 24

0.1 M PB

-5

Ni-Co-P CoP Pt/C

(f)

-1

mV 69.0

CoP

dec

Ni-Co-P -1

mV 70.6

0.1

dec

Pt/C -1

0.0

22.2

mV dec

-10 -0.2

-0.1

0.0

-0.4

-0.2

0.0

0.2

0.4

0.6

-2


Log|j(mA/cm )|

Figure 5. (a and b) Polarization curves of CoP, Ni-Co-P, Pt/C and corresponding Tafel plots in 1 M KOH solution. (c and d) Polarization curves of CoP, Ni-Co-P, Pt/C and corresponding Tafel plots in 0.5 M H2SO4 solution. (e and f) Polarization curves of CoP, Ni-Co-P, Pt/C and corresponding Tafel plots in 0.1 M pH 7.0 phosphate buffer.

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Table 1. Comparison of the HER performance of Ni-Co-P nanoflowers with those of metal phosphide electrocatalysts. Electrocatalysts

Mass loading -2

η10

Tafel slope

Reference

-1

mg cm

(mV)

(mV dec )

Ni-Co-P nanoflowers

1.36

83

46.6

This work

HNDCM-Co/CoP

-

135

66

4

Cu0.3Co2.7P/NC

0.4

220

122

36

FeP NPs@NPC

1.4

214

82

46

Mn−Co−P/Ti

5.61

76

52

1

(CoP)0.54–(FeP)0.46–NRs/G

0.285

97

62

47

Ni0.69Co0.31–P

3.5

96

47

33

CoP/CN

0.13

170

59

31

CoP NS/C

0.71

111

70.9

26

CoP/NCNHP

2

115

66

48

In order to investigate the reason of the Ni-Co-P nanoflowers with superior activity, the electrochemically active surface area (ECSA) as an influential factor for HER catalysts was obtained. The ESCA could be represented by the electrochemical double-layer capacitance (Cdl), which was measured by cyclic voltammetry curves at different scan rates in Faradaic silence potential range (Figure 6a and b). The Cdl value of Ni-Co-P nanoflowers was calculated to be 12.6 mF cm-2, much higher than that of CoP nanoflowers (3.7 mF cm-2) (Figure 6c). The high ESCA of the Ni-Co-P nanoflowers might be attributed to the porous and ultrathin nanosheets of 3D hierarchically morphology. The bigger ESCA leaded to expose more active sites and resulted in the enhancement of catalytic activity. In addition, the interfacial reaction and electrode kinetics in HER process was also studied by electrochemical impedance spectroscopy (EIS). Figure 6 d and e illustrated the typical Nyquist plots of CoP nanoflowers and Ni-Co-P nanoflowers at various overpotentials from 0 to 150 mV. The semicircle diameter of CoP nanoflowers and Ni-Co-P nanoflowers decreased as the overpotential increased from 0 to 150 mV (Figure 6 d and e), which indicated that the charge transfer kinetics was significantly accelerated upon increasing the overpotential. And the equivalent circuit was applied as a circuit model to fit the impedance data (insert of Figure 6f). This equivalent circuit was composed of the solution resistance (Rs), constant phase element (CPE), and charge transfer resistance (Rct) reflecting the resistance at the interface between the electrocatalysts and the electrolyte.

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At the same overpotential of 150 mV, the Rct of Ni-Co-P nanoflowers (10.4 Ω) was lower than that of CoP nanoflowers (17.2 Ω), indicating a faster electron transfer of Ni-Co-P nanoflowers, which was consistent with that of UV-Vis-NIR characterization (Figure 6f). Therefore, the Ni incorporation enhanced the electron transfer of Ni-Co-P nanoflowers electrocatalyst, thus facilitating the HER kinetics. Thus, the outstanding HER electrocatalytic activity of the Ni-Co-P nanoflowers was ascribed

CoP

-2

20-100 mV/s

Current density (mA cm )

(a)

-2


to its large electrochemically active surface area and high conductivity.

0.4

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12.6 mF cm

CoP

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∆j0.22v (mA cm )

Ni-Co-P

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-Z'' (Ω)

CoP Ni-Co-P

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3.7 mF cm

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Scan rate (mV s

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100

20

0mv 50mv 100mv 150mv

0

0

0

100

200

300

0

Z' (Ω)

20

40

Z' (Ω)

Figure 6. (a and b) CV curves of CoP nanoflowers and Ni-Co-P nanoflowers at different potential scanning rates in 1 M KOH solution. (c) Linear fitting of ∆j of CoP and Ni-Co-P nanoflowers (∆j = ja - jc) vs. scan rates at a given potential 14


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(+0.22 V vs. RHE). (d and e) Nyquist plots of electrochemical impedance spectra of CoP and Ni-Co-P nanoflowers at different overpotentials in 1 M KOH solution. (f) Comparison of Nyquist plots of CoP and Ni-Co-P nanoflowers at 150 mV overpotential. Inset of Figure 6f showed the equivalent circuit.

The stability of electrocatalysts was a key parameter in practical operation conditions. Therefore, the electrochemical stability of Ni-Co-P nanoflowers was investigated in both acidic and alkaline conditions. The cyclic voltammetry (CV) durability tests showed that negligible loss of activity after 2000 CV sweeps in alkaline condition and 1000 CV sweeps in acidic condition, indicating the high cycling stability of Ni-Co-P nanoflowers. The long-term stability test was also examined by bulk electrolysis at a fixed overpotential of 83 mV, 92 mV and 216 mV (j=10 mA cm-2) respectively in alkaline, acidic and neutral condition, which indicated that this electrocatalyst maintained high HER activity for at least 20 h (inserts of Figure 7 a and 7b, Figure S7). Besides, the morphology of Ni-Co-P nanoflowers after electrochemical tests was characterized by SEM. Figure 7c-f showed the hierarchical 3D nanostructure was preserved though the nanosheet units became swelled and rough. To further investigate the electrocatalytic process on structure, surface composition and morphology of Ni-Co-P nanoflowers, XRD, XPS, TEM, and HAADF-STEM-EDX mapping were also measured after long-time electrolysis in 1 M KOH. It was found that the structure of Ni-Co-P nanoflowers did not change after long-term HER tests, as shown in the XRD analysis (Figure S8). But XPS spectra revealed that part loss of P (Figure S9a), along with formation of high valence Ni, Co and P after test (Figure S9b-d), indicating the surface component transformation during electrocatalysis.49 TEM images showed that the nanosheet units of Ni-Co-P nanoflowers became swelled and rough after test, while the morphology and crystalline was preserved (Figure S10). And HAADF-STEM-EDX mapping also demonstrated that the Co, Ni and P elements was distributed uniformly in a Ni-Co-P nanoflower after long-time test (Figure S11). The above results proved that the high long-term reliability and durability of Ni-Co-P nanoflowers during the HER process. The integrated porous nanostructure might help endow Ni-Co-P nanoflowers with excellent stability. Furthermore, the interaction between Ni and Co might improve the electrochemical stability and resistance against oxidation by modifying the electronic structure of CoP.

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0

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(b)

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Time (h) Initial After 1000 cycles After 2000 cycles

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-0.3

-0.2

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0.5 M H2SO4 -0.1

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Figure 7. Polarization curves for Ni-Co-P nanoflowers in 1 M KOH (a) and 0.5M H2SO4 solution (b) before and after different cycles at a scan rate of 100 mV s-1. The SEM images of Ni-Co-P nanoflowers after electrocatalytic process in 1 M KOH (c and d) and 0.5M H2SO4 solution (e and f). Inserts show the time-dependent current density curves of the Ni-Co-P nanoflowers during electrolysis at 83 and 92 mV overpotential in 1 M KOH solution (a) and 0.5 M H2SO4 (b), respectively.

The outstanding HER performance of the Co-Ni-P nanoflowers could be attributed to the following factors: (1) The unique hierarchical nanoflowers assembled by porous and ultrathin nanosheets played a key role in the achievement of high HER performance. The unique 3D morphology facilitated mass transfer of electrolyte and release of the generated H2 gas bubbles. Compared with CoP nanoflowers, higher BET surface area and electrochemically active surface area of Co-Ni-P nanoflowers exposed more catalytically active sites. (2) From the results of absorption spectra and XPS, the heteroatom of

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Ni modified the electronic structure of CoP, and then enhanced the electron transfer of Co-Ni-P nanoflowers during the HER. Therefore, the synergistic effect between heteroatom Ni and Co enhanced the catalytic activity of Co-Ni-P nanoflowers. (3) It was well known that HER activity was strongly correlated with the free energy of hydrogen adsorption to the electrocatalyst surface (∆GH) quantifying the hydrogen−metal bond strength. ∆GH value of 0.0 eV can lead to optimal HER activity with intermediate binding energies due to the balance between the adsorption of proton reduction and the desorption of adsorbed hydrogen from the surface active sites. Recent density functional theory (DFT) calculations reported that CoP had the low ∆GH value (-0.14 eV) relatively far from the ideal value zero2. Upon the introduction of Ni, the negative shift of BE would weaken the H−Co binding strength of Ni-Co-P nanoflowers, and accordingly might increase the ∆GH toward zero. Compared with CoP, the bigger TOF values of Ni-Co-P nanoflowers in alkaline, acid and neutral conditions also proved that Ni-Co-P nanoflowers had more optimal ∆GH, leading to its enhanced catalytic activity. ■ CONCLUSION In summary, novel hierarchical Ni-Co-P nanoflowers assembled by porous unltrathin nanosheets were firstly fabricated through a facile hydrothermal synthesis of precursor, followed by a simple phosphorization treatment. The Ni-Co-P nanoflowers exhibited superior performance for electrochemical hydrogen evolution in both acidic and alkaline conditions. The Ni-Co-P nanoflowers only required overpotential of 83 mV and 92 mV at 10 mA cm-2 in 1 M KOH and 0.5 M H2SO4, respectively, with excellent long-term durability. The catalytic activity of Ni-Co-P nanoflowers was even superior to the commercial Pt/C electrocatalyst beyond 125 mV overpotential in 1 M KOH solution. The superior HER performance of Ni-Co-P nanoflowers was attributed to the following features: (1) unique 3D morphology, which could expose more active sites and then increase their electrochemically active surface area; (2) the heteroatom Ni incorporation, which changed the electronic structure of CoP, thus enhancing the electron transfer and leading to more optimal free energy of hydrogen adsorption. This work designed an efficient earth-abundant electrocatalyst for practical applications in the current sustainable energy systems. ■ ASSOCIATED CONTENT 17



Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: EDX spectrum, STEM and the elemental mapping images, N2 adsorption-sorption isotherm, UVVis-NIR absorption spectra, Real potential value of Ag/AgCl reference electrode with respect to RHE, Comparison of the HER performance, Faradaic efficiency, Electrocatalytic stability, XRD pattern, XPS spectra, TEM images, , STEM and the elemental mapping images after the longtime electrocatalytic process ■ AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]; Fax: +86-22-23766556 *E-mail: [email protected]

ORCID En-Cui Yang: 0000-0001-6463-166X Xiao-Jun Zhao: 0000-0002-6371-9528

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS The work was supported by National Natural Science Foundation of China (Grants 21571140, 21531005, 21671149, and 21703156), 973 Program (2014CB845601), the Program for Innovative Research Team in University of Tianjin (TD13−5074), Tianjin Universities Program for Development of Science and Technology (2017KJ122), and Doctoral Program Foundation of Tianjin Normal University (52XB1508).

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3D Hierarchical Nickel Cobalt Phosphide Nanoflowers as an Efficient

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