Properties of aqueous alkaline sodium borohydride …fsl.npre.illinois.edu/Project Presentation/fuel cell...Applied Nano Bioscience Center at ASU Properties of aqueous alkaline sodium

Applied Nano Bioscience Center at ASU

Properties of aqueous alkaline sodium borohydride solutionsand

by-products formed during hydrolysis

presented at the HYATT REGENCY SAN FRANCISCO AT EMBARCADERO CENTER

BOARD ROOM B, ATRIUM LEVEL5 EMBARCADERO CENTERSAN FRANCISCO, CA 94111

Session 1:00 - 6:00 PM

byDon Gervasio, Michael Xu and Evan Thomas

Arizona State UniversityTempe, AZ

AUGUST 17, 2005


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


Task 1: ASU will support the Univ. of Illinois (UIUC) in reporting and participate in technical meetings.

Task 4.3: Optimize NaBH4 performanceTask 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


Current ResearchOVERALL GOAL:

Maximize hydrogen storage capacity of aqueous alkaline sodium borohydridesolution 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.


Ru on various metal supportsTi best support:

- stable - Ru adheres.

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 borohydridehydrogen-storage solution !

Ru on metal Ti support is stable in aqueous alkaline sodium borohydridesolution!

Question…Do sodium borohydride and hydrolysis by-products stay in solution? Or Precipitate?


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 576NaBO2

. x H2O 2+x 16 288Na2B4O7 7/2 28 504Na2B4O7

. 10 H2O 17/2 68 1224Na2B4O6(OH)2

. 3 H2O 11/2 44 792Na2B4O7

. 5 H2O 12/2 48 864Na2B4O5 (OH)4

. 3 H2O 12/2 48 864Na2 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 HydrolysisAnswer…it depends on hydrolysis by-product(s)


XRD of Hydrolysis Product

Same as NaB(OH)4

Hydrolysis product


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


ppm (f1)-30-20-100

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

-17.

163

-19.

068

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

-17.2 ppmStandardNaB(OH)4


ppm (f1)-60-50-40-30-20-100

0

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

-17.4 ppm


ppm (f1)-25.0-20.0-15.0-10.0-5.0

0

100

200

300

400

500

600

700

-16.

578

-19.

279

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


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


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


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


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


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!


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 55.0660.1M NH4OH 50 9.9978 7.1196 2.8782 57.56429.2wt% NH4OH 50 9.9968 7.6224 2.3744 47.4880.1M 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 134.0600.1M NH4OH 10 2.0606 0.7379 1.3227 132.27029.2wt% NH4OH 10 2.0150 1.5155 0.4995 49.9500.1M NaOH 10 2.0200 1.2629 0.7571 75.710H2O w/ 2% 90k CMC polymer 10 2.0229 0.8689 1.1540 115.4000.1M NaOH w/ 2% 90k CMC polymer 10 2.0054 1.3401 0.6653 66.530

1M NH4OH20wt% NH4OH

Solubility of NaB(OH)4 in Various Solvents

NEXT


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 WorkQ1: Is NaB(OH)4 the only hydrolysis by-product? A: YesQ2: 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 progressQ3: 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


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.

Documents

Properties of aqueous alkaline sodium borohydride …fsl.npre.illinois.edu/Project Presentation/fuel cell...Applied Nano Bioscience Center at ASU Properties of aqueous alkaline sodium