air-stable magnesium nano composites provide rapid and high-capacity hydrogen storage without heavy...

8/7/2019 Air-Stable Magnesium Nano Composites Provide Rapid and High-capacity Hydrogen Storage Without Heavy Metal C

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A-ab ag acp pv

ap a hgh-capacy hyg ag wh

g havy-a caay

Figure S1. TEM analysis of reaction mixture a.) before addition of reductant, b.)

immediately thereafter, c.) 20 minutes after addition of reductant (davg = 3.56 0.59 nm),

and d.) after a standard 20h growth period (davg = 4.59 1.04 nm). Histograms of the

magnesium nanocrystal sizes are also shown for the e.) 20 minute and f.) 20 hour reaction

times. These data support the burst-nucleation model hypothesized in the manuscript.

SUPPLEMENTARY INFORMATIONdoi: 10.1038/nmAt2978

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Figure S2. Histogram of the diameters of the magnesium nanocrystals present in the

nanocomposite materials as measured by HRTEM. Sizes were recorded on over a dozen

samples from independent syntheses.

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10

NumberofParticles

Diameter (nm)

2 nAture mAteriAls | www.nature.com/naturematerials

SUPPLEMENTARY INFORMATION doi: 10.1038/nmAt2978
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Figure S3. Additional TEM images of Mg NCs/PMMA composite samples from

independent syntheses.




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Figure S4. X-ray diffraction pattern (top) of as-synthesized Mg NCs/hexadecylamine

composite with references (bottom) of hexagonal Mg (solid black line, JCPDS 04-0770),

cubic MgO (long dashed grey line, JCPDS 89-7746) and hexagonal Mg(OH)2 (short

dashed light grey line, JCPDS 07-0239). Mg NCs composites formed with

hexadecylamine encapsulation showed evidence of immediate oxidation of the Mg NCs

to Mg(OH)2 and trace MgO.

30 35 40 45 50 55 60 65

Mg

MgO

Mg(OH)2

2! (degree)

(100)

(002)

(101)

(102)

(110)

(103)




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Figure S5. Typical thermogravimetric analysis (TGA) and differential scanning

calorimetry (DSC) traces of Mg NCs/PMMA composites, red solid line and dashed line,

respectively. TGA trace of pure PMMA is shown as a control (black solid line). The Mg

NCs/PMMA composite TGA trace displayed two distinct slopes: the first weight loss

from room temperature to 250oC is attributed to the evaporation of residual solvent and

the removal of low molecular weight polymers, while the second weight loss which

plateaus around 500 oC is attributed to the degradation of the PMMA polymer matrix. In

the pure PMMA TGA trace, weight loss was complete at ~440 oC when all of the

polymer has been burned off, which corresponds closely with the TGA data obtained on

the Mg-PMMA composites. The remaining weight in TGA of the Mg NCs/PMMA

composites is thereby attributed to the pure Mg metal. Based upon this value, it is

concluded that the weight of the hydrogen storage active material (pure Mg) in the Mg

NCs/PMMA compositesis 61% of the total nanocomposite weight.

0

20

40

60

80

100

120

-2

0

2

4

6

8

10

12

14

0 100 200 300 400 500 600 700

Mg-PMMA

PMMA

M

assLoss(wt.%,

TGA)

DSC(V)

Temperature (o

C)




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Figure S6. Low loss electron energy loss spectrum (EELS) of a 50 nm MgO powder at

80kV under TEAM 0.5 (at 80 kV). MgO was stable during the 10 minutes of beam

exposure, with the largest MgO plasmon energy loss occurring at 22.3 eV. The overall

spectrum shape is consistent with Ref S3.

0 5 10 15 20 25 30 35

Energy Loss (eV)

Coun

ts(a.u.)




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Figure S7. Determination of activation energy for absorption and desorption of

hydrogen in Mg NC/PMMA nanocomposites. Hydrogen absorption and desorption was

measured at three different temperatures (T = 473, 523, and 573 K) and the activation

energies were determined by plotting the log of the rate constant versus 1/T.




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Figure S8. Kinetic models of hydride formation in the Mg NC/PMMA nanocomposite: a,

chemisorption, b, 2-dimensional growth, c, 3-dimensional growth, and d, core-shell

growth. The large black circles represent the resulting curves of different kinetic

equations applied to the experimental hydrogen uptake data of Mg NCs/PMMA

composites (initial 6 minutes); the linear fit R2

value is listed below. The small black

circles represent a linear fit to the data. Insets: MgH2 growth schematics.




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Table S1. Determination of X-ray diffraction Mg NC diameter using the Debye-Scherrer

equation: Diameter = 0.9/*cos, where is the wavelength of the X-ray (0.154 nm)

and is the full width at half maximum of the diffraction peak. Four samples were

examined at 3 separate indices, as listed below. The average diameter determined by

XRD was 15 2 nm (1 standard deviation).

Sample # Index 2 (degree) (degree) Diameter (nm)

1 (100) 32.20 0.4861 17.0

(002) 34.50 0.4538 18.3

(101) 36.74 0.4213 19.9

2 (100) 32.19 0.5000 16.5

(002) 34.44 0.6389 13.0

(101) 36.64 0.6111 13.73 (100) 32.22 0.5000 16.5

(002) 34.42 0.5833 14.3

(101) 36.64 0.5834 14.3

4 (100) 32.39 0.5834 14.2(002) 34.53 0.6611 12.6

(101) 36.79 0.6222 13.5

Table S2. Johnson-Mehl-Avrami models with description (S4, S5).

Model equation Description

(2) Surface controlled (chemisorption)

(3) 1 1( )1/ n

= ktContracting volume, n-dimensional growth with

constant interface velocity

(4) 12

3

1( )

2 / 3

= ktContracting volume, 3-dimensional growth diffusion

controlled with decreasing interface velocity



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