SLIDE 1
Applied Nano Bioscience Center at ASU
Properties of aqueous alkaline sodium borohydride solutions and by-products formed during hydrolysis
presented at the HYATT REGENCY SAN FRANCISCO AT EMBARCADERO CENTER BOARD ROOM B, ATRIUM LEVEL 5 EMBARCADERO CENTER SAN FRANCISCO, CA 94111 Session 1:00 - 6:00 PM by Don Gervasio, Michael Xu and Evan Thomas Arizona State University Tempe, AZ AUGUST 17, 2005
SLIDE 2 Applied Nano Bioscience Center at ASU
Outline
- Review of ASU tasks
- Identity of product of borohydride hydrolysis, NaB(OH)4
- Solubility of hydrolysis product in aqueous solutions
- Future Research
- Characterizing hydrolysis by- products
- Identity
- Solution properties
- Chemical vs electrochemical hydrolysis of borohydride
- Seeking Polybenzimidazole (PBI) from PEMEAS for fuel cell membrane
- Conclusions
SLIDE 3 Applied Nano Bioscience Center at ASU
Task 1: ASU will support the Univ. of Illinois (UIUC) in reporting and participate in technical meetings.
Task 4.3: Optimize NaBH4 performance Task 4.3.1: ASU will develop an energy dense (>2100 Wh/liter or Wh/Kg) hydrogen storage solution component consisting of >30% sodium borohydride in aqueous >1M NaOH solution.
Task 4.3.2: ASU will develop a means of separating the hydrogen gas from the liquid hydrogen storage solution, so that only the hydrogen gas is supplied to the fuel cell anode. _____________________________________
Review of ASU tasks
Today’s talk relates to Tasks 4.3,
- maximizing hydrogen storage & optimizing fluidics,
- and is about boron-oxide identity and solution stability.
Future work will try to determine
- if the boron oxides that form during direct oxidation of borohydride on Pt is different from
the oxides that form from heterogeneous catalysis (Ru) of borohydride hydrolysis.
- Setting up an electrolysis cell and in situ electrochemical NMR cell for this purpose
SLIDE 4 Applied Nano Bioscience Center at ASU
Current Research
OVERALL GOAL: Maximize hydrogen storage capacity of aqueous alkaline sodium borohydride solution and solubility of hydrolysis by-product solution.
Hydrolysis of Borohydride:
NaBH4 + 2 H2O → 4 H2 + NaBO2
SUB-GOALS: * Characterize hydrolysis of sodium borohydride (NaBH4 ) using:
- 11B Nuclear Magnetic Resonance (NMR) Spectroscopy
- X-ray diffraction (XRD)
- Fourier Transform Infrared (FTIR) spectroscopy.
*Ascertain the chemical structure of the by-product(s) formed during hydrolysis. *Determine the mechanism of the catalytic hydrolysis of NaBH4 in alkaline solution *Evaluate the effect of additives on:
- the hydrolysis of sodium borohydride and the
- solubility of the resulting boron-oxide by-products.
SLIDE 5 Applied Nano Bioscience Center at ASU
Ru on various metal supports
Ti best support:
Alumina-borate solid lump that forms in hydrogen generation reactor from alumina dissolution 30wt% sodium borohydride in aqueous 1M NaOH solution.
Precipitates during catalytic hydrolysis
Ru on alumina Not Stable in aqueous alkaline sodium borohydride hydrogen-storage solution ! Ru on metal Ti support is stable in aqueous alkaline sodium borohydride solution!
Question…Do sodium borohydride and hydrolysis by-products stay in solution? Or Precipitate?
SLIDE 6 Applied Nano Bioscience Center at ASU
The reaction of hydrogen generation from sodium borohydride is nominally written as:
NaBH4 + 2 H2O → 4 H2 + NaBO2
Actually many boron oxides can form, dictating the amount of water needed as seen below. Boron Oxide Mole Oxygen H2O Needed Volume H2O (per NaBH4) (30% sol’n) (milliliter)
*NaB(OH)4
4 moles 32 moles 576 NaBO2
. x H2O
2+x 16 288 Na2B4O7 7/2 28 504 Na2B4O7
. 10 H2O
17/2 68 1224 Na2B4O6(OH)2
. 3 H2O
11/2 44 792 Na2B4O7
. 5 H2O
12/2 48 864 Na2B4O5 (OH)4
. 3 H2O
12/2 48 864 Na2 B4O5 (OH)4
. 8 H2O
17/2 68 1224
__________________
* ASU X-ray diffraction data indicate that NaB(OH)4 is the by-product of hydrolysis reaction.
Possible By-Products formed during NaBH4 Hydrolysis
Answer…it depends on hydrolysis by-product(s)
SLIDE 7
Applied Nano Bioscience Center at ASU
XRD of Hydrolysis Product
Same as NaB(OH)4 Hydrolysis product
SLIDE 8
Applied Nano Bioscience Center at ASU
B(OH)3 + NaOH → NaB(OH)4
Stoichiometric reaction done in water or water–tetrahydrofuran (THF) mixtures Precipitate Filter XRD of the product of this synthesis is same as NaB(OH)4 standard which is same as borohydride hydrolysis product last slide
Synthesis of NaB(OH)4 from boric acid
SLIDE 9 Applied Nano Bioscience Center at ASU
ppm (f1)
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
1.00 0.91
NMR Figure 1: B11 NMR, presumably NaB(OH)4 with BF3 diethyl etherate in external capillary
NMR of NaB(OH)4
Standard
NaB(OH)4
SLIDE 10 Applied Nano Bioscience Center at ASU
ppm (f1)
500 1000 1500 2000 2500 3000
- 17.374
- 59.083
- 59.712
- 60.341
- 60.970
- 61.600
1.00 0.08
B11 NMR, 30% NaBH4 w/ Ru catalyst in situ after 3 weeks
NMR of Borohydride Hydrolysis Product
SLIDE 11 Applied Nano Bioscience Center at ASU
ppm (f1)
100 200 300 400 500 600 700
1.00 0.06
11B NMR, neat BF3 diethyl etherate in external capillary and 0.05M
NaBO2·4H2O, 5% D2O in sample tube. Boric acid: 0 ppm.
NMR Spectrum of Sodium Metaborate, Na BO2
BF3
Not NaBO2
- 16.5 is not close to -17.2
SLIDE 12
Applied Nano Bioscience Center at ASU
NMR like XRD indicates hydrolysis product if NaB(OH)4 Will do thermal analysis next to confirm match Conclusion on Identification of Borohydride Hydrolysis Product
SLIDE 13 Applied Nano Bioscience Center at ASU
Since solution NMR is in agreement with X-ray scattering of solids, that both techniques indicate that NaB(OH)4 is the principal oxide from the hydrolysis of 10 to 30% NaBH4 in aqueous 1 M NaOH, we accelerate studies of solubility by direct synthesis of NaB(OH)4 and testing of its solubility.
Continuing solubility tests of NaB(OH)4
- air dry & weigh solid NaB(OH)4
- place in “solvent”
- warm, stir
- filter
- air dry and reweigh
- solubility is weight loss/volume of solvent
Oxide Identity and Solubility Tests
SLIDE 14
Applied Nano Bioscience Center at ASU
B(OH)3 + NaOH → NaB(OH)4
Stoichiometric reaction done in water or water–tetrahydrofuran (THF) mixtures Precipitate Filter XRD of the product of this synthesis is same as NaB(OH)4 standard which is same as borohydride hydrolysis product last slide
Synthesis of NaB(OH)4 from boric acid
SLIDE 15 Applied Nano Bioscience Center at ASU
Gravimetric storage density: % H2 by weight Solid NaBH4 10.6 % NaBH4-20 solution (20 wt% NaBH4, 3 wt% NaOH, 77 wt% H 2O) 4.3 % NaBH4-25 solution (25 wt% NaBH 4, 3 wt% NaOH, 72 wt% H2O) 5.3 % NaBH4-30 solution (30 wt% NaBH 4, 3 wt% NaOH, 67 wt% H2O) 6.4 % NaBH4-35 solution (35 wt% NaBH4, 3 wt% NaOH, 62 wt% H2O) 7.5 % Volumetric storage density: 1 Liter NaBH4-20 solution 44 grams or 526 Standard Liters 1 Liter NaBH 4-25 solution 55 grams or 658 Standard Liters 1 Liter NaBH4-30 solution 66 grams or 789 Standard Liters 1 Liter NaBH4-35 solution 77 grams or 921 Standard Liters
Hydrogen Content of alkaline sodium borohydride solutions
- Goal: use as high a concentration of borohydride in solution as possible
From Millenium Cell website, www.milleniumcell.com
SLIDE 16
Applied Nano Bioscience Center at ASU
Solubility of Sodium Borohydride in Various Solvents
From Rohm and Haas Borohydride digest www.Rohm&Haas.com
For all liquid reactor, solubility of NaB(OH)4 not NaBH4 may be critical!
SLIDE 17 Applied Nano Bioscience Center at ASU
Solubility Testing of NaB(OH)4
At Room Temp. Sample Solvent Amt. (mL) Solid Wt. (g) Undissolved Solid Collected (g) Solubility Limit (g/50mL) Solubility Limit (g/L) H2O 50 10.017 7.2637 2.7533 55.066 0.1M NH4OH 50 9.9978 7.1196 2.8782 57.564 29.2wt% NH4OH 50 9.9968 7.6224 2.3744 47.488 0.1M NaOH 50 9.9982 7.5556 2.4426 48.852 At Elevated Temp. (40ºC) Sample Solvent Amt. (mL) Solid Wt. (g) Undissolved Solid Collected (g) Solubility Limit (g/10mL) Solubility Limit (g/L) H2O 10 2.0056 0.6650 1.3406 134.060 0.1M NH4OH 10 2.0606 0.7379 1.3227 132.270 29.2wt% NH4OH 10 2.0150 1.5155 0.4995 49.950 0.1M NaOH 10 2.0200 1.2629 0.7571 75.710 H2O w/ 2% 90k CMC polymer 10 2.0229 0.8689 1.1540 115.400 0.1M NaOH w/ 2% 90k CMC polymer 10 2.0054 1.3401 0.6653 66.530 1M NH4OH 20wt% NH4OH Solubility of NaB(OH)4 in Various Solvents
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SLIDE 18 Applied Nano Bioscience Center at ASU
Recall: in 1000 g of fuel (NaBH4-30), there are:
- 2.8 g NaOH or 0.07 moles of NaOH per kilo of solution
- 700 g H2O or 39 moles of free water per kilogram of solution.
- 300 g NaBH4 or 8 moles of NaBH4 per kilo of solution.
NaBH4 + 4 H2O → 4 H2 + NaB(OH)4 Status: Hydrolysis of 30% NaBH4 to NaB(OH)4
After the above hydrolysis reaction of 30wt% borohydride solution, there are:
- 32 moles or 571 g of water consumed
- 8 moles or 807.5 g of NaB(OH)4 formed
- 0.07 or 2.8 g of NaOH remain
- 125 ml of free water remains.
Questions for Future Work Q1: Is NaB(OH)4 the only hydrolysis by-product? A: Yes Q2: Can 800 g of NaB(OH)4 dissolve in 125 ml of pure water? With 0.07 moles of NaOH? A: Water or base does not seem to matter but this is work in progress Q3: Does the hydrolysis process influence identity of by-product(s)?
- heterogeneous catalytic hydrolysis over Ru? (NMR ex-situ and in-situ)
- electrolysis on Pt electrode? (NMR ex-situ and in-situ)
A: work in progress
SLIDE 19
Applied Nano Bioscience Center at ASU
Conclusions
! A key problem with the hydrogen generation reactor is keeping the boron oxide (borohydride hydrolysis by-product) from precipitating and obstructing the micro-fluidics in system. ! Boron oxide is definitively established through NMR spectroscopy and XRD to be Na B(OH)4 ! Additives being tested to keep the boron oxide in solution.
