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Reversible thermal-responsive electrochemical energy storage

based on smart LDH@P(NIPAM-co-SPMA) films

Yibo Dou, Ting Pan, Awu Zhou, Simin Xu, Xiaoxi Liu, Jingbin Han, Min Wei,*

David G. Evans and Xue Duan

State Key Laboratory of Chemical Resource Engineering, Beijing University of

Chemical Technology, Beijing 100029, P. R. China

CORRESPONDING AUTHOR FOOTNOTE

Corresponding author. Phone: +86-10-64412131. Fax: +86-10-64425385. E-mail:

weimin@mail.buct.edu.cn.

Experimental Section

Synthesis of LDH nanoplatelets array on Ni foil substrate: The LDH

nanoplatelets array was synthesized by a homogeneous hydrothermal method.[1]

Typically, Ni(NO3)2·6H2O (15 mmol), Al(NO3)2·9H2O (5 mmol), NH4F (20 mmol)

and Ni(NH2)2 (50 mmol) were dissolved in 100 ml of water, and was transferred into

a Teflon-lined stainless steel autoclave. A piece of clean Ni foil (2 cm×6 cm,

thickness: 60 µm) was then immersed into the solution. The autoclave was sealed

and maintained at 110 C for 8 h. After the reaction, the LDH nanoplatelets array was

observed on both sides of Ni foil. Subsequently, the Ni foil was washed thoroughly to

remove the surface deposited material, followed by coating with a thin layer of

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electrochemically inert and nonconductive poly(methyl methacrylate) (PMMA) via a

cast-coating technique. Finally, the substrate was dried at 100 C for 5 min.

Synthesis of P(NIPAM-co-SPMA) polymer: The P(NIPAM-co-SPMA) polymer

was synthesized via free radical dispersion polymerization according to the reported

method.[2,3]

Typically, N-isopropylacrylamide (NIPAM; 4.343 g),

2-acrylamido-2-methyl propane sulfonicacid (SPMA; 2.011 g), N,N-methylenebis

(acrylamide) (0.082 g) and sodium dodecyl sulfate (0.050 g) were dissolved in 250 ml

of deionized water with vigorous stirring at 72 °C in nitrogen atmosphere. A

potassium persulfate solution (0.218 g, 50 ml) was added and stirred for 5 h under

nitrogen. The reaction mixture was centrifuged and washed three times with deionized

water, and then filtrated using a membrane filter (1.2 μm, Millipore) to remove any

unreacted monomer and other impurities. The obtained P(NIPAM-co-SPMA) polymer

was then exhaustively dialyzed against deionized water (changing the dialysate twice

daily for a week).

Fabrication of highly-arrayed LDH@P(NIPAM-co-SPMA) and

LDH@P(SPMA) electrode: A thin coating of P(NIPAM-co-SPMA) polymer or

P(SPMA) polymer was deposited onto the surface of LDH nanoplatelets array by dip

coating method.[4]

The prepared LDH film on Ni foil was immersed into the

P(NIPAM-co-SPMA) or P(SPMA) solution by the use of the Deposition Robots

(Riegler & Kirstein GmbH). The Ni foil was placed vertically in the solution for 5

min, followed by withdrawing the substrate out of the solution with a ascent velocity

of 0.05 cm/min. The resulting film were dried in air for 15 min. The whole process

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(immersion, withdrawing, drying) was repeated 10 times. In order to enable the

P(NIPAM-co-SPMA) or P(SPMA) polymer to stick to the LDH surface tightly, the

obtained LDH@P(NIPAM-co-SPMA) film was thermally cross-linked by the

following process:[5,6]

the film was placed in a sealed container purged with N2 for 15

min, which was then slowly heated to 85 °C (approximately 1 h ramping time) and

sustained for 2 h.

Fabrication of pristine P(NIPAM-co-SPMA) planar film: The planar film of

P(NIPAM-co-SPMA) was fabricated by applying the spin-coating procedure.[7]

The

P(NIPAM-co-SPMA) solution was spin-coated onto a Ni substrate (60 s per cycle,

1500 rpm, 5 cycles). The films were dried at room temperature after each spin-coating

process. Finally, the substrates coated with pristine P(NIPAM-co-SPMA) planar film

were obtained for control experiment.

Fabrication of randomly-stacked LDH/P(NIPAM-co-SPMA) electrode: the

LDH@P(NIPAM-co-SPMA) film was scratched from the Ni foil substrate to give a

comparison study with their well-aligned array counterpart. The working electrode

was prepared as follows: 2 mg of the scratched material was first mixed with

polytetrafluoroethylene (PTFT) (LDH@P(NIPAM-co-SPMA):PTFT=50:1, w/w) and

then was dispersed in ethanol; the suspension was drop-dried onto a Ni foam (1.6

cm2) at 80 °C overnight. The foam was pressed at 5 MPa before measurement.

Material characterizations: X-ray diffraction (XRD) patterns were recorded by a

Rigaku XRD-6000 diffractometer, using Cu Kα radiation (λ = 0.15418 nm) at 40 kV,

30 mA. The morphology of films was investigated using a scanning electron

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microscope (SEM; Zeiss SUPRA 55) with the accelerating voltage of 20 KV, a

NanoScope IIIa atomic force microscope (AFM) from Veeco Instruments and a

transmission electron microscopy (TEM; JEOL JEM-2100). X-ray photoelectron

spectroscopy (XPS) measurements were performed using an ESCALAB 250

instrument (Thermo Electron) with Al Kα radiation. The UV-vis absorption spectra

were collected in the range 200700 nm on a Shimadzu U-3000 spectrophotometer,

with the slit width of 1.0 nm. Water contact angle of films was measured using a

sessile drop at three different sites of each film sample using a commercial drop shape

analysis system (DSA100, KRüSS GmbH, Germany). The volume of water droplets

used for measurement is 2 μl. The water contact angle was determined by the average

of at least five measurements. Cyclic voltammetry (CV), electrochemical impedance

spectroscopy (EIS) and galvanostatic (GV) charge-discharge measurements were

performed on a CHI 660C electrochemistry workstation using a three-electrode mode

in 1 M KOH aqueous solution. The highly-arrayed LDH@P(NIPAM-co-SPMA) on

Ni sbustrate or randomly-stacked LDH@P(NIPAM-co-SPMA) on Ni foam was

directly used as the working electrode. The reference and counter electrode were

Hg/HgO and a platinum wire, respectively.

Average specific capacitance value was calculated from the CV curves using the

following equation:

H

L)(

1

LH

V

V

idVVVsm

C (1)

where i is the oxidation or reduction current; s is scanning rate; m denotes the mass of

the active material; VH and VL represent high and low potential limit of the CV tests.

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Specific capacitance was also calculated from the galvanostatic charge-discharge

curves, by the equation:

Vm

tiC

Δ

Δ (2)

where i is the discharge current; Δt is the discharge time; m denotes the mass of the

active material, and ΔV corresponds to the voltage change after a full charge or

discharge process.

Figure 1. XRD patterns of as-prepared NiAl-LDH film on the Ni foil substrate and

the corresponding powdered sample scraped from the substrate (two reflections

denoted with blue triangles originate from the substrate).

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Figure 2. The water contact angle as a function of temperature for the

LDH@P(NIPAM-co-SPMA) film and pristine P(NIPAM-co-SPMA) planar film,

respectively.

Figure 3. The transmittance of P(NIPAM-co-SPMA) solution at 600 nm upon

increasing the temperature from 20 °C to 40 °C (inset: the photographs of

P(NIPAM-co-SPMA) solution at 20 and 40 °C, respectively).

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Figure 4. The reversible variation in water contact angle between 20 and 40 °C for

five consecutive cycles towards the LDH@P(NIPAM-co-SPMA) film, in comparison

with the pristine P(NIPAM-co-SPMA) planar film.

Figure 5. The specific capacitance corresponding to the highly-arrayed

LDH@P(NIPAM-co-SPMA) electrode in the temperature range 2040 °C.

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Figure 6. Temperature-dependent CVs curves in the range 2040 °C for (A) LDH

electrode and (B) LDH@P(SPMA) electrode, respectively.

Figure 7. Galvanostatic discharge curves at a current density of 1 A/g for the

randomly-stacked LDH/P(NIPAM-co-SPMA) electrode in the presence of 1 M KOH

solution in the temperature range 2040 °C.

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Figure 8. Galvanostatic charge-discharge curves at a current density of 1 A/g for the

LDH@P(NIPAM-co-SPMA) switch with normal and bending test in the presence of 1

M KOH solution at 20 to 40 °C, respectively.

Figure 9. The galvanostatic charge-discharge curves at a current density of 1 A/g for

(A) the as-prepared LDH@P(NIPAM-co-SPMA) switch, (B) the switch after storage

for 50 days in the presence of 1 M KOH solution at 20 to 40 °C, respectively.

References:

[1] J. Han, Y. Dou, J. Zhao, M. Wei, D. G. Evans and X. Duan, Small, 2013, 9, 98.

[2] J. Wong, A. Gaharwar, D. Muller-Schulte, D. Bahadur and W. Richtering, J.

Electronic Supplementary Material (ESI) for Chemical CommunicationsThis journal is © The Royal Society of Chemistry 2013

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Colloid Interface Sci., 2008, 324, 47.

[3] H. Senff and W. Richtering, Colloid Polym. Sci., 2000, 278, 830.

[4] Y. Zhao, S. He, M. Wei, D. G. Evans and X. Duan, Chem. Commun., 2010, 46,

3031.

[5] J. L. Stair, J. J. Harris and M. L. Bruening, Chem. Mater., 2001, 13, 2641.

[6] L. Mariniello, C. Giosafatto, P. Pierro, A. Sorrentino and R. Porta,

Biomacromolecules, 2010, 11, 2394.

[7] Y. Dou, J. Han, T. Wang, M. Wei, D. G. Evans and X. Duan, J. Mater. Chem.,

2012, 22, 14001.

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