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
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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
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
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 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
Applied Nano Bioscience Center at ASU
Current ResearchOVERALL GOAL:
Maximize hydrogen storage capacity of aqueous alkaline sodium borohydridesolution and solubility of hydrolysis by-product solution.
* 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)
Applied Nano Bioscience Center at ASU
XRD of Hydrolysis Product
Same as NaB(OH)4
Hydrolysis product
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
Applied Nano Bioscience Center at ASU
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
Applied Nano Bioscience Center at ASU
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
Applied Nano Bioscience Center at ASU
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
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
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
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
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
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.