1. Hydrogen generation and storage from sodium borohydride
Heat & Mass Transfer Institute NAS, Minsk, Belarus
Hydrogen generation and storage from sodium borohydride
Valentina Minkina, S. Shabunya, V. Kalinin

2. Why sodium borohydride? Which form of sodium borohydride storage
Why sodium borohydride?
Which form of sodium borohydride storage
(before its submitting to reactor) is preferable?
Efficiency, durability and shape of catalyst
Accumulation, hydration and recycling of
metaborate (byproduct) into borohydride

3. NaBH4  H2  Fuel cell  Power
Low toxicity product
Stable to 350 deg.
Reactive:
pyrolysis >300 deg.
hydrolysis
Nonflammable
Storage safety
Good handling properties
Availability:
Aviabor (Russia)
Sigma-Aldrich
Borosciences
NaBH4
NaBH H2O 4H NaB(OH)  210kJ/mol
Catalyst
High content of H2 (30% solution contains 6.7% wt H2)
Rate control of hydrogen generation
recycling NaBO2NaBH4
Not required additional heat
Humidified pure hydrogen is supplied directly to the fuel cell

4. Product (H2) Pressure Experiment
Measurement of pressure in system as a function of time.
The change of pressure in reactor is proportional to the amount of generated hydrogen.
Perform reaction at constant temperature
A: Water thermostat
B: Temperature sensor
C: Pressure sensor
D: Pressure release valve
E: Stainless steel pressure reactor
F: Reacting composition

5. Hydrolysis rate as a function of H2O/NaBH4 molar ratio and temperature
NaBH4-H2O-NaOH
Given hydrolysis rates fixed at 10% decomposition of NaBH4 in 1 N NaOH

6. Solubility of NaBH4 and NaBO2
NaBH4 + (2+x) H2O  NaBO2.xH2O + 4H2
The stability of various borates
NaBO2·4H2O stable up to ~55°C
NaBO2·2H2O stable up to ~120°C
The amount of NaBH4 is limited
to 16g /100g H2O to keep NaBO2 in solution.
Excess of water - x≥4
Thus, molar ratio H2O/NaBH4=6. This choice is determined by the maximum possible concentration of NaBH4 in working solution.

7. Hydrolysis rate of alkali NaBH4 solution

8. Hydrolysis rate as a function of NaOH concentration and temperature
Given hydrolysis rates fixed at 10% decomposition of NaBH4 in solution having molar ratio H2O/NaBH4=6.

9. Stability of NaBH4 in solid form
At room temperature in open air
NaBH4
crystalline hydrate NaBH4.2H2O

10. Stability of NaBH4 in slurry at 40oC
H2O/NaBH4=2.5
H2O/NaBH4=3.0
0.25 N NaOH
H2O/NaBH4=4.0
H2O/NaBH4=3.5
H2O/NaBH4=3.0
1.0 N NaOH
H2O/NaBH4=2.5
0.125 N NaOH

11. Comparison of NaBH4 stability in slurry and solution
H2O/NaBH4=3.0
H2O/NaBH4=6.0
1.0 N NaOH

12. Catalysts supported on Al2O3
Content , wt. %
Shape
Dimensions, mm
Surface area, m2/g Pt
0.5
cylinder
 3.2, h=3.6
82.3 Pd
1.0
 3.2, h=3.7
86.1 Rh
0.1
sphere
 5
5.5 Ni 20
 2 , h=3
89.0 Pt Pd Rh Ni
These catalysts demonstrated significant acceleration of the hydrolysis reaction and catalytic activity that was suitable for the study of the hydrogen generator capabilities.

13. The effect of solvent on NaBH4 stability
25.9 wt.% wt.%
20.8 wt.% wt.%
14.9 wt.%
NaBH4-H2O-NaOH
NaBO2-H2O-NaOH
NaBH4-H2O-NaBO2
NaBO2-H2O

14. Hydrogen Generator
The pump is used for circulation of working solution through catalytic unit. Circulation of solution in reactor occurs up to the end of NaBH4 hydrolysis.
At start-up working solution is preheated by heating device.
After completion of hydrolysis in the reactor part of the solution is discharged into the dosing unit and fed a new portion of NaBH4. Step-by-step removal of solution from reactor allows to control the solution temperature in the reactor.
The amount of hydrogen supplied to the fuel cell is controlled by flow controller.

15. The results of the corresponding temperature changes for NaBO2 solution formed as a result of complete hydrolysis of the working solution at different concentration of NaBH4 in adiabatic conditions
It is easier to cool down the circulating solution than the whole reactor itself.

16. The results of test with Pt catalyst
H2O/NaBH4=12 (15 wt. % NaBH4)
Evolution of hydrogen production
Stable operation of generator with 0.4 Nm3/h of hydrogen has been demonstrated.
Pressure evolution in reactor (magenta) and receiver (green)

17. The results of test with Pd catalyst
H2O/NaBH4=7 (23 wt. % NaBH4)
Evolution of hydrogen production
Increasing the hydrogen flow rate by flow controller from 0.3 to 1.0 Nm3/h demonstrates stability of system operation.
Pressure evolution in reactor (magenta) and receiver (green)

18. The results of test with Ni catalyst
H2O/NaBH4=9 (19 wt. % NaBH4)
Temperature evolution in reactor
Evolution of hydrogen production
Generating 1.5 Nm3/h of hydrogen is equivalent to 2.5 kW.
Trace of solvens: ~1.6·10-4 vol. % C6H5OH and ~1.8·10-4 vol. % CH3OH
Technical specifications of the generator allows achieving hydrogen performance up to 3 Nm3/h and higher.

19. Conclusions
Stability of reactor work raises with increase of working pressure and temperature in reactor. The lower level of temperature is equal to 120 °С, and the lower level of pressure is equal to 7-8 bars.
The working solution having molar ratio H2O/NaBH4=10 (~20 wt. % NaBH4) is accepted as the upper limit.
Instead of water-alkaline solution it is proposed to use sodium metaborate solution saturated at room temperature as a solvent and stabilizer to prepare working solutions.
NaBH4 should be stored in anhydrous dry form.

20. The advantages of circulating scheme
It is easier to achieve complete hydrolysis of NaBH4.
The volume of catalytic unit can be smaller.
Less efficient catalyst can be used.
Preparation of working solution at room temperature before
hydrolysis is easier implemented.
Temperature regime of catalyst is more homogeneous.
Final degree of hydrolysis can be easier controlled.
There is no strict requirement on concentration of inlet solution.
Conducted tests have demonstrated stable operation of hydrogen generator in steady mode.

21. European Patent Application EP1787952
Recycling NaBO2 into NaBH4
1 kg of NaHB4 ~ kJ
Scheme
kJ/kg NaBH4
3NaBO2 +12CH3OH = 3NaOCH3 + 3B(OCH3)3 + 6H2O
1246
4B(OCH3)3 + 12H2 = 2B2H6 + 12CH3OH
12368
3NaOCH3 + 2B2H6 = 3NaBH4 + B(OCH3)3
3NaBO2 + 12H2 = 3NaBH4 + 6H2O
13614
Low energy costs
The minimum number of reactions (only three)
All these reactions can be realized in industrial conditions.
European Patent Application EP

22. Thank you for your attention
Thank you for your attention

23. Thank you for your attention
Thank you for your attention

24. The advantages of circulating scheme
It is easier to achieve complete hydrolysis of NaBH4.
The volume of catalytic unit can be smaller.
Less efficient catalyst can be used.
Preparation of working solution at room temperature before
hydrolysis is easier implemented.
Temperature regime of catalyst is more homogeneous.
Final degree of hydrolysis can be easier controlled.
There is no strict requirement on concentration of inlet solution.
Conducted tests have demonstrated stable operation of hydrogen generator in steady mode.
Technical specifications of the generator allows achieving hydrogen performance up to 3 Nm3/h and higher.

25. Hydrogen Generator
The pump is used for circulation of working solution through catalytic unit. Circulation of solution in reactor occurs up to the end of NaBH4 hydrolysis.
At start-up working solution is preheated by heating device.
After completion of hydrolysis in the reactor part of the solution is discharged into the dosing unit and fed a new portion of NaBH4. Step-by-step removal of solution from reactor allows to control the solution temperature in the reactor.
The amount of hydrogen supplied to the fuel cell is controlled by flow controller.