Supporting Information

Supporting Information

for Adv. Funct. Mater., DOI: 10.1002/adfm.201901510

Two Birds with One Stone: Metal–Organic Framework

Derived Micro-/Nanostructured Ni

P/Ni Hybrids Embedded in

Porous Carbon for Electrocatalysis

and Energy Storage

Xiaobin Liu, Wenxin Li, Xudong Zhao, Yongchang Liu,

Ce-Wen Nan, and Li-Zhen Fan*

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016.

Supporting Information

Two Birds with One Stone: Metal-Organic Frameworks Derived Micro/Nano-Structured Ni2P/Ni Hybrids Embedded in Porous Carbon for Electrocatalysis and Energy Storage

Xiaobin Liu, Wenxin Li, Xudong Zhao, Yongchang Liu, Ce-Wen Nan and Li-Zhen Fan*

Prof. L.-Z. Fan, X.B. Liu, X.D. Zhao, Prof. Y.C. Liu

Beijing Advanced Innovation Center for Materials Genome Engineering

Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China

E-mail: [email protected]

Prof. C.-W. Nan, W.X. Li

School of Materials Science and Engineering Tsinghua University

Beijing 100084, China

I. Materials and Methods

1.1 Chemicals: All chemicals were purchased from commercial suppliers and used as received

without further purification. Nickel nitrate hexahydrate (Ni(NO3)26H2O), glycol,

dimethylformamide (DMF), N-methyl pyrrolidinone (NMP) and ethanol were purchased from

Sinopharm Chemical Reagent Co.,Ltd (Beijing, China). 1,3,5-benzenetricarboxylic acid

(H3BTC) was purchased from Aladdin (Shanghai, China). Sodium hypophosphite (NaH2PO2)

was purchased from Macklin (Shanghai, China). Commercial platinum on graphitized carbon

(20 wt% Pt/C) and Nafion (5 wt%) were purchased from Sigma-Aldrich (Beijing, China). The

water for experiments was deionized (DI) water (18 MΩ).

1.2 Characterizations:

X-ray diffraction (XRD, Rigaku D/max-RB) was performed with Cu Kα radiation at 40 kV and

30 mA. The IR spectra were recorded on a Nexus FTIR Spectrometer within the 2200-400 cm

-1 _{region. The morphologies, element distributions, and microstructures were characterized by}

field emission scanning electron microscopy (SEM, JEOL JSM-6330), and high-resolution

transmission electron microscopy (HRTEM, JEM-2010F). The element analysis and oxidation

state study of the samples were carried out by X-ray photoelectron spectroscopy (XPS, EscaLab

250Xi Instrument). Raman spectrum was tested by LabRAM HR Evolution. Nitrogen

were carried out on an inductively coupled plasma atomic emission spectrometry (ICP-AES)

with Perkin-Elmer ICP Optima 2000DV instrument.

II. Experimental Section

2.1 Synthesis of Ni-MOFs.

Typically, a mixture of 0.28 g Ni(NO3)26H2O and 0.14 g H3BTC was dissolved in 30 mL

mixture of DMF, glycol and water (1:1:1, ratios of the volume) under magnetic stirring for 10

min. The mixture was sealed in 100 mL Teflon-lined autoclave and heated at 145 °C for 24 h.

After cooling down to room temperature naturally, green precipitate was separated by

centrifugation, washed with DMF and ethanol for several times and finally dried in a vacuum

oven at 60 °C overnight.

2.2 Synthesis of Ni@C.

The as-prepared Ni-MOFs were placed in a tube furnace and heated to 300 °C for 1 h. Then the

temperature was increased to 600 °C and held for 1 h (heating rate 2 oC min-1) under an argon

atmosphere (70 sccm). After naturally cooled to ambient temperature, carbon-wrapped Ni

nanoparticles (Ni@C) were obtained.

2.3 Synthesis of Ni2P/Ni@C hybrids.

(Ni2P/Ni@C) were obtained when cooled down to room temperature. A series of samples

including Ni2P/Ni-H@C, Ni2P/Ni-L@C and Ni2P@C were achieved by changing the weight of

NaH2PO2 to 300 mg, 700 mg and 900 mg, respectively. In order to remove the insecure and

exposed metal particles, all the samples were treated in 0.5 M H2SO4 aqueous solution for 2 h

followed by a repeatedly filtered and washing process in deionized water.

For comparison, the Ni2P/Ni/C were also prepared by sol-gel route. The detail description

please see the "Synthesis of Ni2P/Ni/C (Sol-gel route)" part below.

2.4 Hydrogen evolution reaction characterizations and performance tests.

A standard three-electrode electrochemical cell on a CHI660C electrochemical workstation was

used to evaluate the electrochemical activities of Ni2P/Ni@C. Typically, 5 mg of sample and

60 µL Nafion solution were dispersed in solution containing 0.5 mL DI water and 0.44 mL

ethanol followed by ultrasonication for 1 hour to form an uniform suspension. To prepare the

working electrode, 5 μL of the above dispersion was loaded onto a glassy carbon electrode (3

mm in diameter) and allowed to dry under air flow at ambient temperature. A Ag/AgCl (KCl

saturated) electrode was used as the reference electrode and a graphite rod electrode was used

as the counter electrode. All potentials reported in this paper were referenced to a reversible

hydrogen electrode (RHE): E(RHE) = E(Ag/AgCl) + 0.197 + 0.059 pH. Electrolyte was

through the EIS technique. EIS was performed at a given potential with frequency from 0.1 to

100,000 Hz. Cyclic voltammograms (CVs) taken with various scan rates (20, 40, 60, 80, 100

mV s-1) were collected in the 0-0.1 V (vs. RHE) region and were used to estimate the

double-layer capacitance. Long-term stablility tests were conducted by potential cycling between 0.1

and −0.15 V (vs. RHE) at a scan rate of 100 mV s-1, and time dependent current density curves

tested by chronopotentiometry method at –150 mV vs. RHE.

2.5 DFT Calculations.

In the computational part, first-principles investigations based on density functional theory

(DFT) were performed via Vienna ab initio simulation package (VASP)[1] using a plane-wave

basis set. The projector augmented wave (PAW) method was adopted to describe the

interactions between ions and electrons.[2] RPBE functional[3] is a revision of the PBE functional

(Perdew, Burke, and Ernzerhof functional)[4] and was adopted in this work to describe the

electron exchange and correlation. The energy cutoff was set to 500eV. The (10×10×10) and

(7×7×10) k-point meshes were used for Ni and Ni2P bulk, respectively. The surfaces were built

by slab model, and (111) surface and (001) surface were considered for Ni and Ni2P in this

work, respectively. During relaxation, the convergence tolerance for the force and energy were

set to 0.01 eV/Å and 10-5 eV, respectively.

or

+ + ∗→

where the * stand an active site of the catalyst, and H* stand the adsorbed H on the surface. Our

first-principles study achieves the free energy of hydrogen adsorption of eq.1, which is

generally regarded as one of the important indicators to evaluate the performance of HER

catalyst.[5]

The computed free energy change was calculated by:

Δ = Δ + Δ − Δ

where Δ stand the adsorption energy of H, Δ is the energy difference of the zero-point energy (ZPE) between the adsorbed H and the gas-phase H2, T is 298.15 K and Δ is the

entropy change between the adsorbed H and the gas-phase H2.

The adsorption energy ( ) of hydrogen atom was computed by eq.1:

= −1

2 −

where , , and stand the total energy of the surface with H adsorbed, the

pure surface, and the gas-phase H2, respectively.

Vibrational frequency of H obtained by calculation could be used to evaluate the ZPE and

entropy change, and the latter could be calculated by the following equation:[6]

where , , ℎ, and stand the universal gas constant, the Boltzmann constant, the Plank’s

constant, and the Avogadro’s number, respectively. is the frequency of the normal mode, and

is the number of adsorbed H. The entropy of gas-phase H2 is taken from the NIST database.

In general, the closer Δ is to zero, the better the HER catalyst is.

2.6 Lithium-ion batteries characterizations and performance tests.

The LIB tests were carried out using CR2032 coin cells in a glove box filled with argon

atmosphere. The working electrode consists of active material, acetylene black, and binder of

polyvinylidenfluorid (PVDF) in a weight ratio of 8:1:1. After carefully grinding with

appropriate amount of N-methyl pyrrolidinone (NMP), the slurry was roller coated on a copper

substrate film. The average loading of composites is about 1.5 mg cm-2, and the active material

is based on Ni2P for electrochemical testing. The Metal lithium and glass fiber were selected as

counter electrode and separator, respectively. The electrolyte used is 1.0 M LiPF6 in a 1:1:1 (by

volume) mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate

(DMC). Galvanostatic charge and discharge profiles were recorded by a LAND battery-test

instrument (CT2001A) ranging from 0.01 to 3.0 V (vs Li+/Li). Cyclic voltammetry testing was

performed on a CHI 660C electrochemical workstation between 0.01 to 3.0 V with different

scan rates of 0.2, 0.4, 0.6, 0.8, 1 and 1.2 mV s-1, respectively. Electrochemical impedance

III. Supplementary Figures and Tables

Figure S1. SEM image of the Ni-MOF.

Figure S2. XRD pattern of the Ni-MOF.

10 20 30 40 50 60 70 80

Ni-MOF

Figure S3. FT-IR spectra of H3BTC and Ni-MOF.

Figure S5. Crystal structure of (a) Ni and (b) Ni2P.

In Figure S5a, each nickel metal atom possesses a coordination number of twelve, showing

a face-centered cubic stack geometry. As to the asymmetric unit of Ni2P (Figure S5b), there are

two crystallographically independent NiII ions. Ni1 is coordinated by four phosphorus atoms in

a tetrahedral coordination geometry, whereas Ni2 is coordinated by five phosphorus atoms in a

Table S1. Mole ratio of Ni2P/Ni calculated by ICP and the carbon content determined by

elemental analysis for Ni@C, Ni2P/Ni-H@C, Ni2P/Ni@C, Ni2P/Ni-L@C and Ni2P@C.

Sample Ni2P/Ni [molar ratio] Carbon content [wt%] Ni@C / 17.8 Ni2P/Ni-H@C 0.66 15.6 Ni2P/Ni@C 1.67 14.3 Ni2P/Ni-L@C 3.15 13.1 Ni2P@C / 10.2

Figure S7. Raman spectrum of Ni2P/Ni@C.

Table S2. Comparison of the HER activity for several recently reported TMPs-based

electrocatalysts in acid solution (0.5 M H2SO4).

Sample η10/mV

vs. RHE

Tafel slope

/mV dec-1 Reference

Ni2P/Ni@C 149 61 This work

Ni2P nanorods/Ti 180 76 7 Ni2P Ps-3 158 73 8 Ni2P hollow NPs 137 49 9 Ni12P5 hollow NPs 208 75 Ni12P5 nanoparticle 137 63 10 Ni5P4/Ni foil 140 40 11 Co/CoP-5 178 59.1 12 WP2 SMPs 161 57 13 Cu3P NW/CF 143 67 14

Figure S8. Cyclic voltammograms (0-0.1 V vs. RHE) of (a) Ni@C, (b) Ni2P/Ni-H@C, (c)

Ni2P/Ni@C, (d) Ni2P/Ni-L@C and (e) Ni2P@C recorded in 0.5 M H2SO4.

Table S3. Simulated impendence parameters (Rs, Rct) of the Ni2P/Ni@C samples.

Sample Rs (Ω) Rct (Ω) Ni@C 4.62 158.68 Ni2P/Ni-H@C 4.28 128.32 Ni2P/Ni@C 4.79 30.87 Ni2P/Ni-L@C 4.82 53.15 Ni2P@C 4.71 92.15

Figure S9. TOFs at different overpotentials from 146 to 160 mV by assuming that every Ni

atoms are catalytically active.

The TOF values were calculated using the equation (1) as follow:

= ⁄2 (1)

where j is the current density at a given overpotential ranging from 146 to 160 mV, A is the

area of the glassy carbon electrode, n is the number of moles of metal ions on the electrode. F

Synthesis of Ni2P/Ni/C (Sol-gel route):

Figure S10. The digital images of (a-b) gel combustion and (c) the collected NiO/Ni/C (sol-gel

route) precursor powder.

Citric acid monohydrate (20 mmol) was dissolved in 50 mL deionized water to form

solution A, and nickel nitrate hexahydrate (10 mmol) was dispersed in 50 mL deionized water

to form solution B. Then solution B was added into solution A, and the mixed solution was

stirred and heated to 80 °C for 0.5 h. With the temperature further increased, the gel reaches to

ignition point and combust (Figure S10a-b), obtaining the loose gray powder Figure S10c) of

NiO/Ni/C (sol-gel route).

The as-prepared NiO/Ni/C was placed in a tube furnace and heated to 300°C for 1 h. Then

the temperature was increased to 600 °C and held for 1 h (heating rate 2 oC min-1) under an

argon and hygrogen atmosphere (90:10 sccm). After naturally cooled to ambient temperature,

Figure S11. (a-b) SEM images and (c) XRD pattern of NiO/Ni/C (sol-gel route). (d-e) SEM

images and (f) XRD pattern of Ni/C (sol-gel route).

During the formation of the sol, the complexing agent of citric acid connect with the nickel

ions in the solution to form a complex crosslinked network, which will be retained in the gel

and eventually decomposed and oxidized in the self-combustion process. Figure S11a-b exhibit

SEM images of NiO/Ni/C (sol -gel route) by the sol-gel self-combustion method, and the

successful synthesis of NiO/Ni/C (sol-gel route) is supported by the XRD pattern in Figure

S11c. Upon the further reduction process, Ni/C (sol-gel route) can be obtained and the

Figure S12. (a-b) SEM images of Ni2P/Ni/C (sol-gel route) and (c) XRD pattern of Ni2P/Ni/C

(sol-gel route). (d) EDX mapping images of Ni and P for Ni2P/Ni/C (sol-gel route).

Table S4. Mole ratios of NiO/Ni and Ni2P/Ni for NiO/Ni/C and Ni2P/Ni/C (sol-gel route),

respectively, and their corresponding carbon content.

Sample NiO/Ni [molar ratio] Ni2P/Ni [molar ratio] Carbon content [wt%] NiO/Ni/C 1.78 / 0.36 Ni2P/Ni/C / 1.44 0.11

Upon the further phosphating process for the Ni/C, Ni2P/Ni/C (sol-gel route) was obtained

and the SEM images were shown in Figure S12a-b. The presence of the Ni2P and Ni was proven

by the XRD profiles (Figure S12c). Figure S12d exhibit EDX mapping of the Ni2P/Ni/C

(sol-gel route), which demonstrates that Ni and P components are homogeneously distributed in the

frameworks. The chemical compositions of NiO/Ni/C and Ni2P/Ni/C are characterized by

ICP-AES and element analysis in Table S4 (Supporting Information). According to the

Brunauer-Emmett-Teller (BET) method, the specific surface area of NiO/Ni/C is 32.5 m2 g-1 and the

average pore-size distribution is centered at 6.1 nm. After the phosphating process, the specific

surface area of Ni2P/Ni/C decreased to 14.1 m2 g-1 (Figure S13).

Figure S13. (a) N2 adsorption–desorption isotherm and (b) pore size distribution curve of

Figure S14. (a) Linear sweep voltammetry curves and (b) corresponding Tafel plots of

Ni2P/Ni@C (MOF-based route) and Ni2P/Ni/C (sol-gel route) in 0.5 M H2SO4 at a sweep rate

of 10 mV s-1.

Figure S14a shows the LSV curves of the HER in 0.5 M H2SO4 catalyzed by the

Ni2P/Ni@C (MOF-based route) and Ni2P/Ni/C (sol-gel route). The overpotential at 10 mA cm -2 _{for Ni}

2P/Ni/C (sol-gel route) is 274 mV, which is higher than that observed on the Ni2P/Ni@C

(MOF-based route). Besides, Tafel plots depicited in Figure S14b exhibited a similar trend with

a value of 109.3 mV dec-1.

Table S5. Lattice parameters of Ni2P and Ni before and after optimal geometrization.

Compound Space group Lattice parameter[Å] Experiment Theoretical a b c a b c Ni2P P-62m 5.862 5.862 3.372 5.90 5.90 3.37

Figure S15. Schematic models of (a) Ni2P and (b) Ni.

Figure S17. Top-view schematic models showing (a) Ni and (b) Ni with H.

Figure S18. CV curves of the Ni2P/Ni@C electrode for the initial three cycles at a scan rate of

0.0 0.5 1.0 1.5 2.0 2.5 3.0 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10

C

u

rr

e

n

t (

m

A

)

Potential vs. Li

/Li

Figure S19. Cycling performances of (a) Ni2P/Ni-H@C, Ni2P/Ni@C, Ni2P/Ni-L@C and (b)

Ni@C electrodes at a current density of 100 mA g-1.

As shown in Figure S19b (Supporting Information), the cycling performance of the

precursor Ni@C was tested at 100 mA g-1 with a reversible capacity of ~25 mA h g-1 after 200

cycles (the active material is based on Ni@C). The low lithium capacity primarily comes from

carbon coating since no alloy can be formed between nickel and lithium.

Table S6. Comparison of the results in this study with reported performance of the TMPs as

LIB anodes.

Sample Rate Capability Cyclic Stability Reference

Ni2P/Ni@C 617 mA h g-1 at 100 mA g-1 611 mA h g-1 at 200 mA g-1 583 mA h g-1 at 300 mA g-1 559 mA h g-1 at 500 mA g-1 521 mA h g-1 at 1000 mA g-1 483 mA h g-1 at 1500 mA g-1 449 mA h g-1 at 2000 mA g-1 419 mA h g-1 at 3000 mA g-1 597 mA h g-1 at 1000 mA g-1

(1000 cycles) This work

Ni2P/NF 612 mA h g-1 at 50 mA g-1 416 mA h g-1 at 100 mA g-1 342 mA h g-1 at 200 mA g-1 507 mA h g -1 at 50 mA g-1 ₁₅

Ni2P/C Nanotubes 533.4 mA h g -1 at 0.1 C 396.2 mA h g-1 at 1C 310 mA h g-1 at 5 C (2.17 A g-1₎ (100 cycles) 16 Ni2P 625 mA h g-1 at 0.2 C 539 mA h g-1 at 0.5 C 509 mA h g-1 at 1 C 436 mA h g-1 at 5 C 410 mA h g-1 at 10 C 625 mA h g-1 at 0.2 C (1 C=524 mA g-1₎ (200 cycles) 17 3D Yolk-Shell-Like Ni2P/G 520 mA h g-1 at 100 mA g-1 449 mA h g-1 at 200 mA g-1 397 mA h g-1 at 500 mA g-1 325 mA h g-1 at 800 mA g-1 291 mA h g-1 at 1000 mA g-1 283 mA h g-1 at 2000 mA g-1 246 mA h g-1 at 5000 mA g-1 511 mA h g-1 at 100 mA g-1 (250 cycles) 457 mA h g-1 at 300 mA g-1 (500 cycles) 18 Ni2P@NPC 1631 mA h g-1 at 50 mA g-1 1293 mA h g-1 at 100 mA g-1 1145 mA h g-1 at 200 mA g-1 972 mA h g-1 at 300 mA g-1 863 mA h g-1 at 500 mA g-1 734 mA h g-1 at 1000 mA g-1 631 mA h g-1 at 2000 mA g-1 584 mA h g-1 at 3000 mA g-1 489 mA h g-1 at 5000 mA g-1 1555 mA h g-1 at 100 mA g-1 (130 cycles) 603 mA h g-1 at 1000 mA g-1 (800 cycles) 19 (Fe) Ni2P/graphene composite 0.2 C to 20 C 350 mA h g-1 at 20 C 642 mA h g-1 at 0.2 C (200 cycles) 1 C = 542 mA g-1 20 FexNi2-xP/P-C 680 mA h g-1 at 100 mA g-1 595 mA h g-1 at 200 mA g-1 525 mA h g-1 at 500 mA g-1 430 mA h g-1 at 1000 mA g-1 360 mA h g-1 at 2000 mA g-1 775 mA h g-1 at 100 mA g-1 (400 cycles) 21 CoP 525 mA h g-1 at 0.3 C 440 mA h g-1 at 0.5 C 352 mA h g-1 at 1 C 314 mA h g-1 at 3 C 256 mA h g-1 at 5 C 630 mA h g-1 at 0.2 C (100 cycles) 1 C = 890 mA g-1 22

Figure S20. (a) Nyquist dots of the Ni2P/Ni@C and Ni2P@C electrodes, as well as the

corresponding simulated solid curves based on the insert equivalent circuit. (b) Real parts of

the impedance (Z') versus the reciprocal square root of angular frequency (ω-1/2) in the low

frequency region of the above Ni2P/Ni@C and Ni2P@C samples.

Rs is related to the solution resistance, and constant-phase element (CPE) refers to the

double-layer capacitance, taking into account the roughness of the particle surface. We can evaluate

the diffusion coefficient of Li+ ions (DLi) within the electrode by EIS measurement:

= (2)

′ = + + / ₍₃₎

In eq 2, R is the gas constant (R = 8.314 J K-1 mol-1), T is the absolute temperature (T = 293.15

K), A is the surface area of electrode (A = 1.54 cm-2), n is the number of electrons per molecule

during oxidization (n = 3), F is the Faraday constant (F = 96,500 C mol-1), C is the concentration

the impedance) through eq 3 and its value can be obtained from the slope of the line between

Z' and ω-1/2 (the reciprocal square root of the angular frequency) as shown in Figure S20b.

Table S7. Simulated impendence parameters (Rs, Rct) and calculated Warburg factor (σ) of

the Ni2P/Ni@C samples.

Sample Rs (Ω) Rct (Ω) σ

Ni2P/Ni@C 2.3 105.7 24.1

Ni2P@C 3.1 170.4 135.1

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