Study of Ammonia Borane – Polyvinylpyrrolidone Hydrogen Storage Composite MaterialsSahithya Pati and Ozge Gunaydin-Sen

Department of Chemistry and BiochemistryLamar University, Beaumont, TX 77710

Ammonia borane (NH3BH3), a potential hydrogen storage system, reveals a structural phase transition around ̃223 K. The transition mechanism was studied by heat capacity measurements, clearly indicating a first-order transition.NH3BH3 (Ammonia Borane, AB) crystallizes in

the tetragonal, I4mm, space group at room temperature and it becomes orthorhombic below the phase transition temperature.

Due to poor thermal kinetics practical application of AB is still hindered to overcome this we are using polymers to improve thermal properties and suppress the byproducts formation.

Introduction

Experimental

AmmoniaBorane (AB) and Polyvinylpyrrolidone (PVP, 40,000) were purchased from Sigma Aldrich.

Composites of different ratios were prepared with AB, PVP and were studied.

AB-based composite of 1:1 mas ratio was obtained mixing 0.10 g of AB and with an addition of 1 mL of deionized water. Different mass ratios were prepared as well.

Heat Capacity measurements were made with Differential Scanning Calorimeter (TA Instrument DSC Q20) covered the range of 300 K down to 180 K. Decomposition experiments were performed between 298 K to 573 K.

Transmittance measurements were made with Nicolet IS-10 FT-IR at room temperature.

Conclusions

Synthesis

Table 1. ΔH, ΔS and Tp values of AB, PVP and AB-based composites.

References

Figure 2. Cp and Enthalpy of AB:PVP(1:1) vs. Temperature with ramp of 1 K/min (Heating).

Figure 5. FT-IR graph of AB, PVP and AB-based polymer composites.

1-Gunaydin-Sen, O.; Achey, R.; Dalal, N. S.; Stowe, A. and Autrey T. J. Phys. Chem. 2012, 112, 1544-1549. 2- Klooster, W. T.; Koetzle, T. F.; Sieghbahn, E. M.; Richardson, T. B.; Crabtree, R. H. J. Am. Chem. Soc. 1999, 121, 6337-6343.3- Tang, Z.; Li, S.; Yang,Z.; and Yu,X.; J. Mater. Chem., 2011, 21, 14616

Figure 4. Heat Capacity (Cp) vs Temperature of AB, AB:PVP(1:1), (1:2) with ramp of 1 K/min (Heating).

Cp and Enthalpy

Due to interaction between the PVP and ammonia borane a decrease in ΔH and ΔS values is observed when the composites are studied in DSC.

The interaction could possibly be disturbing the dihydrogen bonding network.

Decomposition studies revealed that there is a decrease in the melting and hydrogen release temperatures with the increase of polymer proportion in the composite which is an evidence for the kinetic enhancement.

Results and Discussion IR Analysis

FT-IR spectra of pure AB, PVP and the polymer composites showed changes in their functional groups

Future StudiesVarious compositions of AB and PVP will be prepared

and subjected to DSC and TGA to compare with bulk AB.

VT-IR studies will be carried for change in chemical interactions and effect of PVP on dihydrogen bond present in AB.

Kinetic analysis will be done, activation energies will be calculated.

Electrospun fibers with AB:PVP will be investigated.

Acknowledgement Lamar University and Welch Foundation

Cp/T and Entropy

Figure 3. CP/T and Entropy of AB:PVP(1:1) vs. Temperature with ramp of 1 K/min (Cooling).

Ramp(1 K/min)

ΔH (J/g) ∆S(J/gK) Tp (K)

AB 31.81 0.1056 222.81 (±0.5)

AB:PVP(1:1)

9.01 0.06538 222.89(±0.5)

AB:PVP(1:2)

9.465 0.0339 222.76(±0.5)

Figure 7. The proposed thermolysis mechanism of AB in polymeric system [3].

300 350 400 450 500 550-8

-6

-4

-2

0

2

4

6

8

Hea

tflo

w (W

/g)

Temperature (K)

AB

AB:PVP(1:1)

PVP

AB:PVP(1:2)

Figure 5. DSC decomposition graphs 298 K-573 K

Kinetic Studies

Figure 6. Kinetic studies of AB

Figure 1. a) Conformation of the closest N-H…H-B contact from the neutron diffraction structure of NH3BH3 [1, 2], b) polyvinylpyrrolidone.

a) b)

ΔH =

ΔS =

Ozawa method Kissinger's method ln β = -Ea /RTd+ C ln (β/Tp

2) = -Ea /RTd+ C

Where, β is the heating rate, Td is the peak temperature of the thermal decomposition and R is the Universal gas constant .

50 100 150 200 250-30

-20

-10

0

10

20

30

Hea

t Flo

w

Temperature (oC)

5 OC/min 10 OC/min 20 OC/min

1900 1850 1800 1750 1700 1650 1600 1550 1500

30

40

50

60

70

80

90

100

Tran

smitt

ance

(%)

Wavenumber (cm-1)

Bulk AB PVP (1:1) AB:PVP (1:2) AB:PVP

1650 cm-1

1597 cm-1

1600 cm-1

1645 cm-1

1647 cm-1

C=O stretch

N-H deformation

200 220 240 260 2801

2

3

4

5

6

7

8

Cp

(J/g

K)

Temperature (K)

AB:PVP (1:1) AB:PVP (1:2) AB

4000 3500 3000 2500 2000 1500 1000

Tran

smitt

ance

Wavenumber (cm-1)

2206 cm-1

2273 cm-12313 cm-1

3186 cm-1

3239 cm-1

3305 cm-1

1280 cm-11417 cm-11651 cm-1

AB:PVP(1:2)

AB:PVP(1:1)

H-N Bond

B-H2 Bond

N-H bond

B-H and B-H2 strechN-H,N-H2 and N-H3 strech AB

PVP

725 cm-1

180 200 220 240 260 280 300

2

4

6

8

10

Cp

(J/g

K)

Temperature (K)

0

40

80

120

160

200

Enth

alpy

(J/

g)

180 200 220 240 260 280 300

0.01

0.02

0.03

0.04

0.05

0.06

Temperature (K)

Cp/T

(J/g

K2 )

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Entr

opy

(J/g

K)