Emmanuel Wirth , Rémi André , Andre Levchenko , Karl Gross

Coupling of manometric and calorimetric measurements to probe
unique characterization of solid hydrogen storage systems.
Emmanuel
1
Wirth ,
Rémi
1
André ,
Andre
2
Levchenko ,
2
Gross ,
Karl
3
Milanese ,
Chiara
Pierre Le
1
Parlouër
1: Setaram Instrumentation 7, Rue de l'Oratoire -69300Caluire – France
2: Setaram Inc./Hy-Energy LLC 8340 Central Ave, Newark, CA 94560, U.S.A.
3: Pavia H2 Lab, C.S.G.I. – Physical Chemistry Department, University of Pavia, Italy
ABSTRACT
RESULTS: NH3BH3 thermal decomposition pathway (cont.)
In the recent years, the solid hydrogen storage research has experienced a wide
development and promising new materials candidates such as complex hydrides, high surface
areas materials, and hybrid systems are now focusing a lot of interest. Concerning the fuel
cells, a lot of research is carried out to replace the expensive Pd catalyst to foster a large
development of the technology.
In parallel, the characterization for both evaluating the hydrogen sorption properties and
understanding of the mechanism of hydrogen-solid interaction requires accurate and reliable
tools. Simultaneous analyses of different properties give invaluable information for the material
scientist, especially when the repetition of experiments is challenging (small quantity of
synthesized material, lack of reversibility, slow kinetics).
The thermodynamic and kinetics of the different candidate storage systems are key
parameters for the practical application and among all the knowledge of the enthalpy of
formation/dissociation of the hydrides is fundamental.
Thermolysis of ammonia-borane was studied by heat flow calorimetry, and a simultaneous
calorimetric-volumetric-mass spectrometric technique to better characterize the thermal effects
associated with the reaction pathway. The influence of the hydrogen pressure on the
decomposition reactions was also studied with two experiments (1 bar and 20 bar).
Left: Heat flow and pressure (bold)
signals recorded during decomposition
of ammonia-borane with a heating rate
of 0.5 °C min -1. Pressure drifts at the
beginning and the end of the
experiment are linked with the thermal
expansion of the gases present in the
calorimetric vessel.
8.5
35
8
Heat : -66102 (J/mol)
HeatFlow (W/mol)
30
Exo
7
25
6.5
20
6
15
5.5
5
10
Pressure (bars)
7.5
4.5
5
4
0
3.5
3
-5
2.5
70
80
90
100
110
120
130
140
150
160
170
180
190
PCTPro
-260°C / 500°C
Vacuum to 200 bar
Large choices of sample holders
Heat : -44891 (J/mol)
Heat : -26688 (J/mol)
3
1
Exo
Heat : 13038 (J/mol)
80
Polymer decomposition
Nucleation : Formation of DADB
HeatFlow (W/mol)
0.1
0.25
0.5
1
0.002
0.5
Exo
0.001
0.25
0.1
Pressure Rate (bars/s)
1
0.003
90
100
110
120
Temperature (°C)
0
DADB
formation
Polymerization
Polymer
decomposition
160
120
80
40
0
Right: Heat flow (top) and
hydrogen loss rate (bottom)
signals recorded during the
-5
temperature scanning step
-10
carried
out
in
coupled
-15
calorimetry-volumetry-mass
2.5
5
7.5
10
12.5
15
17.5
20
22.5
25
27.5
spectrometry set-up at the
Up: Experimental separation of the exothermic hydrogen pressures of 1 and
events: time dependence of the heat flow, 20 (bold) bars.
temperature and hydrogen loss.
∆ : -5.72 (wt. %)
HeatFlow (W/mol)
HeatFlow (W/mol)
Temperature (°C)
-2.5
140
150
160
Enthalpy
(kJ·mol-1)
Step
30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
0
130
Up: Peak deconvolution procedure applied to
the heat flow experimental signal (exp, top)
obtained by heating ammonia-borane at
0.25°C/min. Deconvoluted peaks in the form
of asymmetric Gaussian type signals are
assigned to three consecutive events.
5
Right: Heat flow (top) and pressure
release rate (bottom) profiles
recorded at scanning rates (marked
in °C/min on the curves) between
0.1 C/min and 1°C/min.
Sum
2
200
Exo
Calvet Calorimeters
C80
from RT to 300 °C
BT2.15
-196 °C to 200 °C
Exp
Temperature (°C)
∆ : -7.73 (wt. %)
Calorimetric - Manometric coupled systems
20
HeatFlow (W/mol)
9
40
1
0.75
0.5
0.25
0
-0.25
-0.5
-0.75
-1
-1.25
13.7±1.7
49.8±8.0
25.5±4.0
Tm : 128.6 (°C)
-0.00025
Time (h)
-0.0005
-0.00075
Exo
-0.001
100
110
120
130
140
Temperature (°C)
150
160
170
Hydrogen Loss Rate (wt%/min) (-)
9.5
Hydrogen loss (wt. %)
Practically there are two ways to determine this enthalpy. The first one is an indirect method
widely used in the literature, i.e. the van’t Hoff technique, where the
hydrogenation/dehydrogenation enthalpy is derived from the mid-plateau pressure and the
temperature of absorption/desorption isotherms. The second one is a direct method, where the
enthalpy is measured via calorimetric technique. The biggest disadvantage of this technique is
that it gives a result per mole of solid sample and not per mole of H2. Recently the combination
of manometric technique (to quantify the amount of hydrogen absorbed/released) and
calorimetry was successful to overcome this issue and the direct measurement of enthalpy of
formation per mole of gas was reported [1].
45
Sample Temperature (°C)
EXPERIMENTAL
Growth : Polymerization of AB
RESULTS: PCT-Sensys Coupling
Destabilization of the Mg/MgH2 system by transition metals, transition metal oxides and C-based materials.
Activation of the samples is made directly in the HP-cell of Sensys by cycling at 350 °C and 15 bar/1 bar H2 pressure.
Subsequently, a TPD measurement is performed by heating at 2 °C/min from room temperature up to 450 °C @ 1 bar H2.
Sensys TG-DSC
From -120 °C to 830 °C
Up to 400 bar/600°C
with the HP DSC
Mg 79.5 wt % - Ni 14% - C 5% - TiO2 1.5% BM 1.5 h
Mg 94 wt% - C 1wt% - Nb2O5 5 wt % ball milled (BM) 1 h [6].
1
∆H 1 + 2 =
69 kJ/mol
1. Mg2NiH4
Micro DSC VII
from -45°C to 120°C
Up to 1000 bar
2. Mg H2
Very good kinetic behaviour:
1.5 min/1.7 min to absorb/release 6 wt % H2.
Starting desorption T = 292 °C
2
Mg2Ni + 2H2
Mg + H2
Good kinetic performance:
2 min/5 min to charge/release 6 wt %.
Destabilization of both the hydrides!!
The Reactive Hydride Composite (RHC) strategy: NaBH4 – MgH2 system.
First experimental calorimetric data!!
The coupled system at the same time enables the
quantification of the heat of reaction and of the desorbed gas
and the determination of the nature of the desorbed gas.
Triple-coupled system
(PCTPro, Calorimeter, RGA)
Ammonia borane: NH3BH3
It has a very high gravimetric uptake.
However, the reactions are not reversible,
and off-board regeneration is required in
case of use in a mobile application.
NH4BH4 → NH3BH3 + H2
NH3BH3 → NH2BH2 + H2
NH2BH2 → NHBH + H2
NHBH → BN + H2
wt.% H2
Desorption T, °C
6.1
6.5
6.9
7.3
< 25
< 120
> 120
> 500
NH3BH3
molelcule
2NaBH4 - MgH2 system
Reaction pathway after Stow et al. [4]
NaBH4 - 2MgH2 system
a.
Depending on the temperature range, the sample quantity
and the type of measurement, different types of thermal
analysis coupling can be carried out.
RESULTS: NH3BH3 thermal decomposition pathway
Reactions
TPD measurement by heating at 2 °C/min from room temperature up to 580 °C @ 0.2 bar H2.
a.
a. MgH2 → Mg + H2
b.
b. 2NaBH4 + Mg → 2Na + MgB2 + 4H2
Mechanism under examination…
2 NaBH 4 + MgH 2→
← 2 Na + MgB2 + 5 H 2
Full desorption of the two hydrides…
but with a sluggish kinetics (20 h taken).
Strong destabilization of both MgH2
and NaBH4 (71 kJ/mol vs 185 kJ/mol).
b.
3
1
2 MgH 2 + NaBH 4 →
← Na + 2 MgB2 + 2 Mg + 4 H 2
Full desorption of the two hydrides…
and better kinetics (6 h for full dehydrogenation).
Strong destabilization of NaBH4
(73 kJ/mol vs 185 kJ/mol).
CONCUSION-PERSPECTIVES
[ NH3 - (BH2NH2)n - BH2NH3]+ [BH4]-
The decomposition of ammonia-borane consists in two
consecutive exothermic steps [2]. Especially for the higher
temperature step, it seems that several reactions take place
simultaneously, forming different solid and gaseous
decomposition products [3]. The recent work of Stowe et al [4]
contains the description of a convincing reaction pathway of
the first step. It involves a nucleation, the formation of
diamoniate of diborane (DADB), and its polymerization with
remaining molecules of ammonia-borane.
For further information please contact: [email protected],
[email protected]
The accurate characterisation of hydrogen sorption materials is very important for
practical applications. It can be done with a volumetric Sievert’s technique and
thermal analysis technique to access to a full range of data. The characterisations
methods presented here are not limited to the presented examples and can be
extended to all type of solid or liquid media.
References:
1. C. Milanese et al., IJHE, 35 ( 2010 ) 1285 – 1295.
2. F.P. Hoffmann, G. Wolf, L.D. Hansen, Advances in Boron Chemistry, Royal Society of Chemistry, (1997) 514.
3. G. Wolf, J. Baumann, F. Baitalow, F.P. Hoffmann, Thermochim. Acta 343 (2000) 19.
4. A. C. Stowe, W. J. Shaw, J. C. Linehan, B. Schmid and T. Autrey, Phys. Chem. Chem. Phys. (2007) 9, 1831
5. R. André, E. Wirth, P. le Parlouër, Proceedings of the 7th Heat Flow Calorimetry Conference. (2010).
6. C. Milanese et al., IJHE, accepted for publication.