Ross Hydrogen Storage 20041216

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Institute for Materials ResearchUniversity of Salford

Hydrogen adsorption on carbon

surfaces

Keith Ross, Ian Morrison, Peter Georgiev, Jian Liu,Keith Ross, Ian Morrison, Peter Georgiev, Jian Liu, XiangyuanXiangyuan

Cui, Dan Bull, Juan FernandezCui, Dan Bull, Juan Fernandez--Garcia;Garcia; Institute for Materials Research Institute for Materials Research

University of Salford, Manchester, UK.University of Salford, Manchester, UK.

Timmy RamirezTimmy Ramirez--CuestaCuesta, Mark Adams, Daniel, Mark Adams, Daniel CologneisiCologneisi , Ken, KenAnderson,Anderson,

ISIS, Rutherford Appleton Laboratory, ISIS, Rutherford Appleton Laboratory, Didcot Didcot , Oxon , Oxon



Institute for Materials Research

University of Salford

Hydrogen adsorption on carbon

surfaces

Measured using IGA

N2 BET Measurements H2 at pressures up top 20 Bars

On a Chemical Potential plot, a fixedadsorption energy gives a straightline



0 2000 4000 6000 8000 10000 12000

0.000

0.005

0.010

0.015

0.020

0.025

0.030

AP

SE

Darco

Pressure, mBar

S t o r a g e d e n s i t y ,

m g H

2 / m

2 a

d s o r b e n t

-90 -80 -70 -60 -50 -40 -30

0.000

0.005

0.010

0.015

0.020

0.025

0.030

AP

SEDarco

Chemical potential, meV

S t o r a g e c a p a c i t y ,

m g H

2 / m

2 a

d s o r b e n t

Hydrogen storage densities of the three materials as

functions of the equilibrium pressure in a, and the

chemical potential in b.

a

b



-90 -80 -70 -60 -50 -40 -30

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Data: H2 on SE @77

o

K Model: Fermi-Dirac Double

Chi^2 = 0.00002

E1

-83.88681 ±0.4791meV

C1

0.20903 ±0

E2

-41.53018 ±0.05515meV

C2 0.8 ±0T 9.43231 ±0.09288meV

Chemical potential, meV

F r a c t i o n a l c o v e r a g e , Θ

Fig. 8.6 Hydrogen adsorption isotherm of the SE grade nanotube material fitted with a two-site Fermi-Diracmodel. The site frequency was kept fixed during the fiiting.



Analysis of adsorption data The isotherms can be fitted with Fermi Dirac statistics

(Langmuir) with two different adsorption energies

Material Surfacearea,m2/g

N2sorptionat 77oK

Storagecapacity,wt% at

10Bar and77oK

Isosteric heatof adsorption,meV/H2 molecule

Effective adsorbatetemperature, oK

Carbolex

AP

CarbolexSE

HSA Darco

247

374

1325

0.70

1.05

2.43

-74.5, -32.1

-52.6, -34.9

Fitted 110 K

fixed to 77 K with twoadsorption energies



IINS Spectra from H2 on SWNT for different H2 coveragesSample is Carbolex (Selected grade) - Instrument is TOSCA at ISISSpectra for range of surface coveragesSharp peak at 14.7 meV, no recoil – broad peaks at higher energyinclude recoil

5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0

0

1

2

3 a c o v e r a g e

0 %

2 2 %

3 8 %7 7 %

1 1 0 %

E n e r g y lo s s , m e V

N e u t r o n s / m e V



Detail in the 14.7 meV peak showing the development of thedifferent peak components with increasing surface coverage.Note that the double peak structure dominates at lowercoverages

9 10 11 12 13 14 15 16 17 18 19 20

0

1

2

3 acoverage

0%

22%

38%

77%

110%

Energy loss, meV

N e u t r o n s / m

e V



Variation of peak intensities as a function of surface

coverage

2:1 peak intensity ratio suggests asymmetric site, i.e. vdW potential normal toplanar surface. The m =+/- 1 states are at a lower average potential energy thanthe m=0 level where the molecule precesses with the defined axis of spin parallel

to the surface.

-20 0 20 40 60 80 100 120 140 160

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

J=0,m=0 to J=1,m=0

transition

J=0,m=0 to J=1,m=+-1

transition of the perturbed

adsorbate Perpendicular

vibration ?

Rotationally

free H2

E=13.5meV

E=14.2meV

E=14.6meVE=15.1meV

Surface coverage, %

P e a k i n

t e g r a l i n t e n s i t y



Comparison of the rotational levels for SWNT and a highsurface area activated carbon. The activated carbon

appears to have a more strongly distorted adsorption site

10 12 14 16 18 20

SWNT

Activated carbon

Neutron energy loss, meV



Young and Koppel Expression S J-J’ (Q,ω ) =

Σ Jm,J’m F Jm,J’m (Q) δ ( ω-ωmJ,m’J’ ) * S CM (Q,ω )

F Jm,J’m (Q) is the Form Factor for the transition

S CM (Q,ω ) is the Scattering Function for the Centre of Massmotion in the potential energy surface seen by the molecule

For a bound atom, this consists of a delta function modified by the Debye-Waller factor for the molecule and an inelastic

part – which for large Q becomes like the Perfect Gas Law but with a higher temperature



Molecular form factors for different angular momentumtransitions (Y & K). NB We are measuring the 0-1

transition.

0 1 2 3 4 5

0

5

1 0

1 5

F0 -1

F1 -1

F1 -2

Molecularform-factor,Barn

Q, Å



(a) Fits to the recoil spectra for 100% and 144% surface

coverage using Y and K between 85 and 500meV(b) Plot of fitted temperature versus surface coverage

200 400 600

0

1

S(Q,ω

Neutron energy loss,meV

20 40 60 80 100 120 140 16

80

85

90

95

100

105

110

115

120

125

130

135

140

<EK>,K

Surface coverage, %of monolayer



Comparison of the Debye Waller factor derived from the

Q dependence of the rotational peak with the mean

thermal energy of the recoil peaks

80 90 100 110 120 130 140

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

<EK >, K

1 / < U 2 >



Parameters from fitting c-m elastic term and recoil together

2U

H2, content% of amonolayer

Peak position,meV

FWHH (meV)

(forward,backward)

Peak Area

(forwardBackward)

Mixingfactor

,Å2

<Ek > , K

de, Å

80, 30K

85, 25K

100, 20K

13.9±0.1

14.1±0.4

14.55±0.03

15.0±0.1

13.8±0.5

14.4±0.1

14.6±0.05

15.3±0.3

13.6±0.5

14.3±0.1

14.63±0.04

15.3±0.3

2.2±1.02.2±0.71.5±0.81.2±0.9

0.7±0.30.7±0.21.9±0.32.1±0.32.0±0.72.3±0.60.7±0.2

1.1±0.20.5±0.30.6±0.21.8±0.41.4±0.51.6±0.3

1.6±0.50.6±0.10.7±0.20.6±0.060.5±0.11.7±0.41.6±0.6

1.2±0.21.0±0.10.33±0.20.35±0.2

0.7±0.21.25±0.40.6±0.10.6±0.21.6±0.71.1±0.90.6±0.2

0.9±0.40.4±0.31.0±0.30.7±0.20.4±0.31.1±0.1

0.7±0.30.7±0.11.1±0.51.2±0.31.6±0.40.6±0.20.5±0.3

0011

1100001

111000

0111100

0.20±0.07

0.26±0.3

0.40±0.17

0.23±0.07

0.15±0.3

0.35±0.2

0.49±0.15

0.08±0.2

0.13±0.1

0.37±0.15

0.32±0.14

0.18±0.2

129±1

0.774

121±1

0.775

116±1

0.775

G h h i th t f i f th t h d



Graph showing the rate of conversion of ortho to para hydrogenwhen adsorbed on SWNT and DARCO carbon.Transition half live SWNTs 3 mins DARCO 16 mins

Magnetic SWNTs? Iron catalyst is covered with amorphouscarbon.

0 100 200 300 400

n-H2

on SWNT , 20K

n-H2

on DARCO ,20K

0 10 20 30 40 50

Time, min

EISF,arb.

units



New MARI data on SWNT sample from Heben

Monolayer coverage 140m ccs H2 at NTP

Neutron energy loss, meV

12 14 16 18 20

X Axis Title

Q=2.4-2.9, T=17K

140cc pH2

40cc

17cc

S ( Q , ω )



Q dependence of the peak intensity



Q dependence of the peak intensityfor 27 ccs (20% coverage)Inset compares 17ccs

values are <u2> for different coverages

0 1 2 3 4 5

-2

0

2

4

6

8

10

12

14

16

Y A

x i s T i t l e

27cc p-H2

T=17 K

0 1 2 3 4 5

0

2

4

6

8

10

Y

A x

i s T i t l e

X Axis Title

Q, Å-1

S

, ω

0.24±0.02Å2

0.29±0.01Å2

0.27±0.01Å2

Ross Hydrogen Storage 20041216

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