Cobalt-based hydroxide nanoparticles @ N-doping carbonic ......Cobalt-based hydroxide nanoparticles @ N-doping carbonic frameworks core–shell structures as highly efficient bifunctional

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Electronic Supplementary Material

Cobalt-based hydroxide nanoparticles @ N-doping carbonic frameworks core–shell structures as highly efficient bifunctional electrocatalysts for oxygen evolution and oxygen reduction reactions

Shiqiang Feng, Cheng Liu, Zhigang Chai, Qi Li (), and Dongsheng Xu ()

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

Supporting information to https://doi.org/10.1007/s12274-017-1765-2

Reagents

Cobalt sulfate heptahydrate (CoSO4·7H2O, 99.999%) and polyvinylpyrrolidone (PVP, K30) were provided by

Aladdin. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%) was provided by J&K. Iridium(IV) oxide (IrO2, 99.99%)

and 2-amino terephthalic acid (NH2-H2BDC, 99%) were provided by Alfa Aesar. Nafion solution (5 wt%) was

brought from Sigma-Aldrich. N, N-dimethylformamide (DMF, HPLC), dodecyl sodium sulfate (SDS, 99%),

phosphoric acid (H3PO4, GR,85%) and ethnol (HPLC) were brought from Sinopharm Chemical reagent Co.,

Ltd.

Synthesis of CoPi

Cobalt phosphate (CoPi) were synthesized according to the literature with modification [S1]. In a typical

procedure, 4 mL CoSO4·7H2O (50 mM), 4 mL H3PO4 (50 mM) and 2 mL SDS (0.25 M) was mixed with 40 mL urea

(1.25M) under stirring. After that, the solution was transferred into a 100 mL Teflon-lined stainless steel

autoclave and heated to 95 oC in 40 min. Then, the oven cooled down to 80 oC in 10 min and held for 1 h. After

reaction, the product was collected by centrifugation at 5000 rpm for 10 min and washed with water and ethanol

several times in order to remove the excess of the remaining reagents. Finally, the washed particles were dried

at 60 oC in the vacuum oven.

Synthesis of CoPi@aIRMOF-3

CoPi@aIRMOF-3 were synthesized according to the literature with modification [S2]. In a typical procedure, 0.5 g

Address correspondence to Dongsheng Xu, [email protected]; Qi Li, [email protected]

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PVP was dissolved into the mixed solvent 30 mL ethnol and 27 mL DMF under ultrasonic conditions. 1 mL

DMF solvent contained 19 mg CoPi was added drop by drop into the above solvent under ultrasonic conditions.

Next, 111.55 mg Zn(NO3)2 (0.375 mmol) and 27.15 mg 2-amino terephthalic acid (0.15 mmol) dissolved into

2 mL DMF were added into the above mixed solution, and the solution was treated under ultrasonic conditions

for 20 mins. Subsequently, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and

heated at 90 oC for 240 min. After reaction, the product was collected by centrifugation at 5000 rpm for 10 min

and washed with DMF and ethanol several times in order to remove the excess of the remaining reagents.

Finally, the washed particles were dried at 60 oC in the vacuum oven.

Synthesis of aIRMOF-3

0.5 g PVP was dissolved into the mixed solvent 30 mL ethnol and 28 mL DMF under ultrasonic conditions.

Next, 111.55 mg Zn(NO3)2 (0.375 mmol) and 27.15 mg 2-amino terephthalic acid (0.15 mmol) dissolved into

2 mL DMF were added into the above mixed solution, and the solution was treated under ultrasonic conditions

for 20 mins. Subsequently, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and

heated at 100 oC for 480 min. After reaction, the product was collected by centrifugation at 5000 rpm for 10 min

and washed with DMF and ethanol several times in order to remove the excess of the remaining reagents.

Finally, the washed particles were dried at 60 oC in the vacuum oven.

Sample pyrolyzed

The powder of samples were placed in a tube furnace and heat-treated at 450 oC (or other certain temperature)

for 1 h under N2 atmosphere with the heating rate of 10 oC min-1. After cooling to room temperature, the

pyrolyzed sample was washed with water to remove unstable species and dried at 60 oC in the vacuum oven.

Characterization

Scanning electron microscopy (SEM) was performed on a field emission Hitachi S-4800 scanning electron

microscope at 10.0 kV. Transmission electron microscope (TEM) was performed using Tecnai F30 instrument

with a field emission gun operating at 300 kV. High angle annular dark field scanning transmission electron

microscopy (HAADF-STEM) imaging and energy-dispersive X-ray spectroscopy (EDX) elemental mapping

were characterized by Tecnai F30 at 300 kV. Powder X-ray diffraction (XRD) measurement was performed on a

PANalytical Powder X-ray diffractometer using Cu Kα (λ = 0.154 nm) radiation (40 kV, 40 mA). The Fourier

transform infrared (FT-IR) spectra was measured on a brulker Tensor 27 spectrometer. The thermal gravity

analysis (TGA) was measured on a Q600 SDT instrument. X-ray photoelectron spectroscopy (XPS) measurements

were performed on an Axis Ultra photoelectron spectrometer. The Inductively Coupled Plasma-Atomic Emission

Spectrometer (ICP-AES) was measured by PROFILE SPEC. Brunauer-Emmett-Teller (BET) surface area and

pore size of the different samples were performed on a Micromeritics ASAP 2010 adsorption analyser. The gas

chromatograph was performed using a Techcomp GC-7890Ⅱ.

EDX was used to verify the elemental compositions before and after electrochemical treatment in alkaline

electrolyte. The results indicated that the elements of P and Zn were missing after the samples were treated by

alkaline electrolyte, which meant Co-based phosphate in the cores had been converted to Co-based hydroxide

and zinc element in the shells had been removed (Figure S1d).

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Figure S1 SEM images for (a) CoPi, (b) CoPi@aIRMOF-3, (c) CoPiCat@MCF and (d) SEM-EDX pattern of CoPiCat@MCF and CoOHCat@NCF.

Figure S2 (a) HAADF-STEM bright field image and (b) Co, P, O, Zn element mapping of CoPi@aIRMOF-3.

The results of energy-dispersive X-ray spectroscopy (EDX, Table S1) and Inductively Coupled Plasma-Atomic

Emission Spectroscopy (ICP-AES, CoPi and CoPiCat@MCF, Table S2) indicated the molar ionic ratio of [Co]/[P]

is approximately 1.5, which corresponds the ratio of cobalt phosphate. The ICP (CoPiCat@MCF, Table S2) test

also reveals that there are less than 55% CoPi in the composite CoPiCat@MCF which is helpful for calculating

the active mass.

Table S1 EDX results of CoPi

Sample Element Weight (%) Atomic (%) Uncert. (%)

Co 83.1 61.9 0.52 CoPi

P 16.9 38.1 0.24


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Table S2 ICP-AES results of CoPi, CoPiCat@MCF and the tested electrolyte

Samples Line Mean RSD Units

Co 228.615 2.8907 0.101 ug/mL

Zn 206.200 0.0711 0.138 ug/mL CoPi

P 213.618 0.8033 0.435 ug/mL

Co 228.615 1.5046 0.425 ug/mL

Zn 206.200 0.7489 0.336 ug/mL CoPiCat@MCF

P 213.618 0.4286 0.415 ug/mL

Co 228.615 -0.0178 4.540 ug/mL

Zn 206.200 1.7123 0.856 ug/mL tested electrolyte

P 213.618 0.1852 4.294 ug/mL

The tested electrolyte is the electrolyte that have been used to test CoOHCat@NCF catalyst for one month.

The content of cobalt in solution could illustrate the loss of the active material. The results indicate that there is

no cobalt in the solution which means catalyst have a good stability.

Figure S3 FTIR spectra of (a) CoPi , aIRMOF-3, CoPi/aIRMOF-3 and CoPi@aIRMOF-3, (b) CoPi, aIRMOF-3, 1-amino terephthalic and CoPiCat@MCF, (c) CoPi@aIMOF-3 treated at 400 oC, 450 oC and 500 oC; XRD patterns, (d) CoPi anealled at 700 oC and aIRMOF-3 anealled at 550 oC and 700 oC, and (e) CoPi and CoPi@aIRMOF-3 before anealled.


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The stretching vibration peaks of amino group in the CoPi@aIRMOF-3, CoPi/aIRMOF-3 and aIRMOF-3

samples are down shifted from 3507 cm-1 and 3397 cm-1 to 3340 cm-1 and 3355 cm-1 in comparison with the pure

2-amino terephthalic (Figure S3b), owing to an electron donating oxygen from the carboxylic group to the

intra-framework hydrogen bonding of amine group. Moreover, the characteristic of OH- stretching frequency

in the carboxylic acid group of the 2-amino terephthalic at around 2900 cm-1 is disappeared, suggesting

occurrence of the coordination interaction between Zn2+ ions and carboxylic acid group of 2-amino terephthalic

to form aIRMOF-3. It is noted that the relative intensity of the PO43- stretching frequency that CoPi and

Co(PO4)2/aIRMOF-3 have more strong absorption peaks than the CoPi@aIRMOF-3, due to the fact that the

CoPi NPs core is encapsulated by the thick shell. When the CoPi@aIRMOF-3 samples were pyrolyzed at 450 oC

for 1 hour, the characteristic IR peaks of aIRMOF-3 disappeared after the anneal process (Figure S3b). The

electron diffraction pattern (Figure 1a) of the CoPi NPs shows the similar result as XRD (Figure S3e). The

amorphous CoPi particles would be crystallized during the thermal treatment at 580 oC, which can be verified

by TGA and XRD (Figure S3d, S4a).

Figure S4 (a) TGA and (b) DTA anlysis for CoPi in air and CoPi, aIRMOF-3 and CoPi@aIRMOF-3 in N2 atmosphere.

Figure S5 (a) XPS spectra for CoPiCat@MCF before and after electrochemical test; Co 2p region of XPS spectra for (b) CoPiCat@MCF before electrochemical test and c) CoOHCat@NCF after electrochemical test.

The high resolution XPS (Figure S4b) of Co 2p before electro-chemical test shows a high-energy band (Co2p1/2)

and a low energy band (Co2p2/3) at 797.6 eV and 781.9 eV with two satellites at 803.5 and 786.5, respectively,

which both to Co2+. In addition, a shoulder at 778.5 eV suggests the presence of metallic Co at the interface.

Figure S4c shows the high resolution Co 2p spectrum after test which presents two prominent peaks at 780.2

and 795.3 eV. The two fitting peaks at binding energies of 796.6 and 781.4 eV are ascribed to Co2+ with two

satellites at 804.1 and 787.7, while another two fitting peaks at 795.2 and 780.2 eV are attributed to Co3+.


Nano Res.

Electrochemical characterization

Electrochemical measurements were performed at room temperature using a rotating disk working electrode

(RDE) made of glassy carbon (5 mm diameter) connected to an electrochemical workstation (Model CHI660A,

CH instruments) with a three-electrode system. The polished glassy carbon electrode (GCE) was used as

working electrode, and 1×1 cm2 Pt foil and Hg/HgO/OH (KOH a = 1) were used as counter and reference

electrodes, respectively. All the electrochemical measurement were conducted in 1 M KOH electrolytes that

were bubbled with oxygen for at least 30 min to ensure the H2O/O2 equilibrium at 1.23 V versus the reversible

hydrogen electrode (RHE) .The potential of the Hg/HgO reference was measured to be 0.900 V vs. RHE in 1 M

KOH (Figure S5).

The preparation of working electrode is described as follows. 5 mg of catalyst powder was dispersed in diluted

Nafion alcohol solution containing 2.5 mL ethanol and 50 μL Nafion, and the mixture was then ultrasonicated

for 1 h. Next, about 0.15 mg/cm2 of catalysts (IrO2 and Co-based hydroxide) was transfered onto the GCE and

dried at the condition of the infrared lamp. It should be mention that we use same content of cobalt for CoPi

and CoPiCat@MCF according to the ICP test. Finally, the modified electrode treated by the surface air plasma

for 3 min by plasmacleaner.

The Cyclic voltammetry (CV) was performed at a sweeping rate of 50 mV s-1 for activation process (Figure S7)

at room temperature. During the measurements of polarization, the RDE was continuously rotating at 1600 rpm

to remove the generated O2 bubbles. All potentials were iR-compensated to 85% with the built-in programme.

Electrochemical impedance spectroscopy (EIS) dates was performed at 1.5 V vs. RHE over a frequency range

from 100 kHz to 100 mHz. The presented current density was normalized to the geometric surface area of the GCE.

The turnover frequency (TOF) is defined as the number of O2 molecules evolved per active site per second [S3]:

04

jTOF

F

where j is the measured current density (mA cm–2), the number 4 means 4 mole electrons per mole O2, F is

Faraday constant (96485 C mol-1), Γ0 is surface concentration of active sites(mol cm–2).

The surface concentration of redox active Co sites were extracted from the slope of the linear relationship

between the peak current of the CoⅢ/CoⅡ reduction wave and the scan rate (Figure S7) [S3]:

2 2

0

4

n F Aslope

RT

where n = 1, F is Faraday constant (96485 C mol-1), A is surface area of the GCE (0.19625 cm2), Γ0 is surface

concentration (mol cm–2), R is ideal gas constant (8.314J mol-1K-1), T is temperature(298 K).

The Faradic efficiency was tested at a constant potential (0.9 V vs. Hg/HgO) without IR compensation by

monitored volumetrically using a gas burette at room temperature and atmospheric pressure. The electrocatalyst

was loaded onto carbon paper (CP) electrode (1×3 cm2). Thus, the Faradaic yield is estimated from the observed gas

volume and the theoretical gas volume calculated by the charge passed through the electrode (Figure S9) [S4]:

exp

4

erimental

m

VFaradaic yield

Q VF

where Q is the charge passed through the electrode, F is Faraday constant (96485 C mol-1), the number 4 means


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4 mole electrons per mole O2, Vm is molar volume of gas (24.5 L mol-1, 298 K, 101 KPa).

The electron transfer number is calculated on the basis of Koutecky–Levich (K–L) equation [S5]:

0.5

2 13 6

0 0 0

0

1 1 1 1 1

0.62

K L K

K

j J J J B

B nC nFD C

J nFkC

where J is measured current density, JK is kinetic- limiting current densities and JL is diffusion-limiting current

densities, is angular velocity of the disk, n is electron transfer number, F is Faraday constant (96485 C mol-1),

C is constant relating to the concentration of O2 (C0), kinematic viscosity of the electrolyte(0

), and diffusion

coefficient of O2 (D0) in 0.1 M KOH.

Figure S6 The activation process of CoOHCat@NCF.

Figure S7 The calibration of Hg/HgO/KOH 1M reference electrode with respect to RHE. E (RHE) = E (Hg/HgO) +0.90.


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Figure S8 CVs of a) CoOHCat@NCF after 12 hours in KOH, b) CoOHCat@NCF, c) Co(OH)2 catalysts in 1M KOH at different scan rates.

Figure S9 The electrochemical impedance plots for CoOHCat@NCF and Co(OH)2.

Figure S10 Faradaic yield test of a) Co(OH)2 and c) CoOHCat@NCF gas and the responding of Gas chromatogram for the experimental produced gas (b,d).


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Figure S11 Chronopotentiometric measurements at J = 100 mA cm-2 for CoOHCat@NCF.

Figure S12 Polarization curves a) at various pH and b) for samples annealing at different temperatures.

When the annealing temperature is 350 oC, aIRMOF-3 coating layer could not be carbonized completely, 550 oC,

CoPi NPs had been partially crystallized, and 650 oC, CoPi NPs had been completely crystalized and MCF

coating layer had been partially destructed. These differences determined their lower OER performance at all

these annealing temperature.


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Figure S13 Proposed pathway for water splitting by CoOHCat@NCF.

Figure S14 Linear scanning voltammetry at different rotation rates (rpm.) and the corresponding K-L plots at different potentials for RDE curves in O2-saturated 0.1 M KOH.

Figure S15 (a) Linear scanning voltammetry for different annealing temperature. b) Chronoamperometric response at 0.4 V in O2-saturated 0.1 M KOH at 900 rpm.


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[S2] Zhao, M. T.; Deng, K.; He, L. C.; Liu, Y.; Li, G. D.; Zhao, H. J.; Tang, Z. Y. Core–shell palladium nanoparticle@metal–organic

frameworks as multifunctional catalysts for cascade reactions. J. Am. Chem. Soc. 2014, 136, 1738–1741.

[S3] Pintado, S.; Goberna-Ferrón, S.; Escudero-Adán, E. C.; Galán-Mascarós, J. R. Fast and persistent electrocatalytic water oxidation

by Co–Fe Prussian blue coordination polymers. J. Am. Chem. Soc. 2013, 135, 13270–13273.

[S4] Wang, J.; Li, K.; Zhong, H. X.; Xu, D.; Wang, Z. L.; Jiang, Z.; Wu, Z. J.; Zhang, X. B. Synergistic effect between metal–

nitrogen–carbon sheets and NiO nanoparticles for enhanced electrochemical water-oxidation performance. Angew. Chem., Int. Ed.

2015, 54, 10530–10534.

[S5] Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Co3O4 nanocrystals on graphene as a synergistic

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Cobalt-based hydroxide nanoparticles @ N-doping carbonic ......Cobalt-based hydroxide nanoparticles @ N-doping carbonic frameworks core–shell structures as highly efficient bifunctional