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

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  

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 NaBH4 + 4H2O 4H2 + NaB(OH)4 +  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

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

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

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.

Hydrolysis rate of alkali NaBH4 solution

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.

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

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

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

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.

The effect of solvent on NaBH4 stability 25.9 wt.%. 23.1 wt.% 20.8 wt.%. 18.9 wt.% 14.9 wt.% NaBH4-H2O-NaOH NaBO2-H2O-NaOH NaBH4-H2O-NaBO2 NaBO2-H2O

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.

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.

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)

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)

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.

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.

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.

European Patent Application EP1787952 Recycling NaBO2 into NaBH4 1 kg of NaHB4 ~ 25400 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 EP1787952

Thank you for your attention   Thank you for your attention      

Thank you for your attention   Thank you for your attention      

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.

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.