Electrochemical Reversible Formation of Alane · 2020. 9. 23. · Dendrites Formation in Cell 30 40...
Transcript of Electrochemical Reversible Formation of Alane · 2020. 9. 23. · Dendrites Formation in Cell 30 40...
Electrochemical Reversible Formation of Alane
Ragaiy Zidan (PI) Brenda Garcia, Andrew Harter, Ashley Stowe, Josh Gray
Savannah River National Laboratory June 13, 2008
Project ID STP19This presentation does not contain any proprietary, confidential, or otherwise restricted information
Program Overview
• Start: 10/01/2006
• End: In Progress
• Percent Complete: 20%
• Store hydrogen required for conventional driving range (greater than 300)
• The weight, volume and cost of these systems
Technical Targets• System Gravimetric Capacity > 6 %• Storage System Cost < 30 % of hydrogen cost
• Brookhaven National Laboratory
• University of Hawaii
• 250K Funding received in FY07• 400 K Funding for FY08
Budget Partners
Timeline Barriers
ObjectivesDevelop a low-cost rechargeable hydrogen storage material
with cyclic stability and favorable thermodynamics and kinetics fulfilling the DOE onboard hydrogen transportation goals
Aluminum hydride (Alane- AlH3), having a gravimetric capacity of 10wt% and volumetric capacity of 149 g/L H2and desorption temperature: ~60oC to 175oC (depending on particle size) meets the 2010 DOE targets for desorption
Specific Objectives– Avoid the impractical high pressure needed to form AlH3 by
utilizing electrolytic potential to increase hydrogen activity and/ or drive chemical reactions to recharge AlH3
– The process used is based on Gibbs free energy and Faraday’s equation
MilestonesKey MilestoneQuantify formation reaction yield, determine energy requirements and demonstrate feasibility of electrolyticallycharging/forming alane
Specific Milestones:• Design/build Ambient Pressure Cell (APC) Test Matrix • Complete APC 2nd Test Matrix• Complete Elevated Pressure Cell (EPC) 1st Test Matrix
Go/NoGo Decision on SRNL Charging Process
Following FY07 activities a GO decision was made and determined that the potential of this material and process to meet > 8 wt% hydrogen capacity under reasonable operating conditions should be furtherinvestigated. Additional work in FY08-09 will include optimization of both the process and the material purity as well as examinations into coatings to improve material handling and safety.
Approach
Utilize electrolytic potential, E, to increase hydrogen activity to hydrogenate Al. Based on Gibbs free energy and Faraday equation:
( )fnF
ln RT- E =nFEG −=Δ
Ambient Pressure Non-Aqueous Electrochemical CellElevated Pressure Non-Aqueous Electrochemical Cell
Approach:
Motivation: Electrochemical recharging represents a very different, promising and complementary approach to AlH3 recharging.
Al + 3/2 H2 → AlH3
Concern: Because Al and AlH3 will be oxidized in an aqueous environment, protection of the Al surface from water/oxygen must be achieved by either using coating such as Pd, or using non-aqueous solvents:
AlH3 Electrochemical Recharging
Electrochemical synthesis is analogous to direct chemical means…
ΔG = ΔH - TΔS(Gibb’s Equation)
RTlnP = ΔH - TΔS(van’t hoff Equation)
ΔG = ΔH - TΔS = - nFERTlnP = - nFE
(Electrochemical Analogue)
We know there are at least 3 phases of AlH3with different enthalpies of formation. We want the α phase, the most usable phase.
To select for the enthalpy of the α phase, we need to control T (mostly), and E.
Direct link betweenE (electrochemical potential), T (temperature) and the ΔH of the formation.
ΔH = ΔG + T ΔS = -nFE + nFT (∂E/∂T)P
ApproachThermodynamic Bases for Regeneration
(Avoiding Thermodynamic Sink)
• Load Pd/Ag tube with dehydridedAlH3
• 5 volts (Max)• 50 mA (Max)• 5 hr
Approach
H2 bubbles were Observed at cathode
glass frit
Electrolyte0.1M KOH
O2H2 - +
current
Thermocouple
Al Pt
Pd/Ag
Ambient Pressure Aqueous Cell
AlH3 was NOT detected
Approach
Utilizing Polar Conductive Solution (LiAlH4, NaAlH4 ,KAlH4) in THF or Ether
Example:NaAlH4/THF Na+/AlH4
-/THF
3 AlH4- + Al+ (Cathode) 4AlH3 (Combined with Ether)
Electrode is expected to dissolve
Na+ + H-/ Pt (Anode) NaH
Non-Aqueous Ionic Solution system 1
Bubbled hydrogenH2 2H
Approach
NaAlH4
NaH 4AlH3
Na+ + H- 3AlH4- + Al+
Al metal
H2 used
Possible recharging facility
Ti cat.
H2
~ Electricity + H2
electrochemical cell
(LiAlH4, NaAlH4 ,KAlH4)/THF or EtherNon-Aqueous Solution system 1
H2
AlH3
A C
Al3+
Al3+Al3+
Al3+
Al3+
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Non-Aqueous Ionic Solution system 2( )
33 / 3Solution THF AlAlCl Cl+ −←⎯⎯⎯⎯→
2
33
3 3 3 1.85
Cath
2
d
3
o e
oc
A
H e HV
lE
H AlH
− −
−+
⎫+ ↔ ⎪ = −⎬⎪+ ↔ ⎭
33 Anode
3 1.69 oaAl e AlCl E VCl − −+ − → = −
( )2 33 0.16 46.52
o ocell cellAl H AlH E V G kJ
− − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
+ ↔ = − Δ =
Approach
Cl-
Cl-
Summary of Technical Accomplishments3/2007 → Present
Status at 3/2007:Built facilityEstablished electrochemical approach (Non Aqueous Electrolyte, Elevated Pressure Cell)Seen initial AlH3 in early experiments
Since 3/2007:•Modified electrochemical cell design and materials to achieve larger quantities of AlH3
•Demonstrated feasibility of electrolytically forming AlH3 (larger quantities)•Devise and conduct experiments to understand the mechanism of any reactions taking place and show theoretical production capability
•Characterized reaction products with XRD and other methods•Quantified yield and efficiencies of recharging reaction
Go Decision on SRNL Charging Process was made at the end of FY 2007,based on charging efficiencies, product yield, process conditions
00 cellcell
GEnFΔ
= −
0 0 0cell cathode anodeE E E= −
4 4
4 3
04 3 2
02
0 04 3
( )/SolutionCell P
( ) 1 1.732
1 2.37
otential
261.2 0.64
an
ca
cell cell
NaAlH AlH Na
NaAlH AlH H Na
NaAlH AlH H e Na E V
H Na e NaH E V
NaAlH AlH NaH G kJ E V
− +
− +
− +
+ −
↔ +
↔ +
↔ + + + = −
+ + ↔ = −
≈ + Δ = = −
0 04 2 3
33 4 3 154 0.802
+ + ↔ + Δ = = −cell cellNaAlH Al H AlH NaH G kJ E V
0 02 3
3 46.5 0.162
+ ↔ Δ = = −cell cellAl H AlH G kJ E V
Cell Thermodynamics
Accomplishments/Progress/Results
Translating Chemical Energy into Electrical energy
Alane Production Cyclic Voltammetry (CV)
-15.00
-10.00
-5.00
0.00
5.00
10.00
15.00
-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0
Voltage vs. NHE (V)
Cur
rent
(mA
)
Al Cyclic Voltammagram
Constant Voltage Experiment @ -1.5 V vs. NHE
Limiting Current ~ 9.7 mA
(normal H2 electrode)
AlH3
Na
Accomplishments/Progress/Results
Bulk Electrolysis for AlH3 Production
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0 100 200 300 400 500 600 700Time (s)
Cur
rent
(mA
)
Al Potential = -1.5 V vs. NHE
Limiting Current Increases with Time
~0.37 mg of AlH3 formed over 10 minutes (assuming 100% electrical efficiency)
~450 hours needed to generate 1 gram of AlH3 (too slow)
∴ Need more electrode area
Accomplishments/Progress/Results
Accomplishments/Progress/ Results
nF = 96500 C / mol AlH3
Results
03 2 4
1 1.732
AlH H Na e NaAlH E V+ −+ + + ↔ = −
For 59.8 C of charge passed during reaction
Original Solution85 mL · 0.5 M = 42.5 mmol NaAlH4
359.8 0.62 mmol AlH
96,500=
0.62 1.4% conversion42.5
=20 30 40 50 60 70 80
Two-Theta (deg)
0
100
200
300
400
500
600
700
Inte
nsity
(Cou
nts)
Film
α-AlH3 Aluminum Hydridefrom Aluminum formed electrochemically
Alane Conversion−−− ++== eHAlHHAlHAlH 2334 2
1)(AlH3 filmDendrites
Al- Aluminum
1 0 2.3722H Na e NaH E Vca
+ −+ + ↔ = −
Desired Reaction
Dendrites Formation in Cell
30 40 50 60 70 80Two-Theta (deg)
0
250
500
750
1000
1250
Inte
nsity
(Cou
nts)
NaAlH4 -Sodium Aluminum Hydride
Na3AlH6 -Sodium Aluminum Hydride
Al- Aluminum
Film
Film
-Na
Al Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
-Na
Al Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
NaAl Na
Na
H
H H
HH
HNa
Al Na
Na
H
H H
HH
H
3 3 3 14 3 62 2 2 2
3 14 3 62 2
NaAlH Na e Na AlH Al
NaAlH Na e Na AlH Al
+ −+ + → +
+ −+ + → +
Accomplishments/Progress/ Results
Fraction Charge/Molar and Formation of Dendrites
Cathode
Dendrites composition is consistent with our theory based on x-ray measurements
Electrochemical Alane Formation Energy-1
Basis
Work = Q · V
5.66 kWhe / kg H2
nF = 289,000 C / mol AlH3
nF · E°cell = 61.2 kJ / mol AlH3
1 kg H2 = 10 kg AlH3 or333 mol AlH3
Comparison
*H2 Compression:(7000 psig)
*H2 Liquefaction: 12.5 – 15 kWhe / kg H2
2.6 – 3.6 kWhe / kg H2
(ideal)
*Source: PraxAir 2003
@ $0.05 / kWh = $0.28 / kg H2 (ideal case)
4 4
02
04 3 2
0 04 3
1 2.372
1 1.732
___________________________61.2 0.64
ca
an
cell cell
NaAlH AlH Na
H Na e NaH E V
NaAlH AlH H e Na E V
NaAlH AlH NaH G kJ E V
− +
+ −
− +
↔ +
+ + ↔ = −
↔ + + + = −
≈ + Δ = = −
Accomplishments/Progress/ Results
02
04 3 2
0 04 3
1 2.372
1 1.732
___________________________61.2 0.64
ca
an
cell cell
H Na e NaH E V
NaAlH AlH H Na e E V
NaAlH AlH NaH G kJ E V
+ −
+ −
+ + ↔ = −
↔ + + + = −
≈ + Δ = = −
3
C289,000mol AlH
ocell cell
Work Q V
nF
E E η
= ⋅
=
= +
( )
( )
3 3
3 3
3
2
3
2 2
:
33.3 mol AlH 10 kg AlH61.2 kJ 1 kWh $0.05@mol AlH kg AlH kg H 3,600
kJ61.2mol AlH
kWh $0.28 5.66kg H
kJ0.3 V 90.7mol
kJ
AlH
kg HkWh
:
oIdeal cell
Real cell
W
Real
nF EI
Cos
deal
Ideal
W n
C
F
t
E
t
os
η
= =
=
= = ⇒ =
= =
=2
3
3 3 2 2
333.3 mol AlH 10 kg AlH90.7 kJ 1 kWh $0.05@mol AlH kg AlH kg H 3,60
kWh $0.428.40k0 k g HJ kWh kg H
= ⇒ =
61.2 67%90.7R
I l
ea
a
l
deAlane Formation Efficiency WW
= = =
Electrochemical Alane Formation Energy-2
@ $0.05 / kWh = $0.42 / kg H2
Accomplishments/Progress/ Results
Electrochemical Cell
Alane Film +NaAlH4
0.25 inchGas being H2 was verified, using RGA
Aluminum Electrode
Accomplishments/Progress/ Results
Hydrogen desorption (TGA), showing almost 9% by weight hydrogen capacity, consistent with alane capacity of 10wt%
Summary
• A novel electrochemical methods were developed to reversibly form Alane (AlH3)
• Bulk AlH3 formed in electrochemical regeneration
• Aluminum electrode dissolved as expected
• The needed energy was found not to exceed 30% of the cost of H2 stored
• 0.37 mg of AlH3 were formed in 10 minutes
• At this reaction rate and using the bench size cell, ~ 450 hours of reaction time would be needed to form 1 g of alane
Counter Electrode
Working Electrode
Reference Electrode
Electrochemical Cell Glasswared=
3.35
17
d=3.
1165
d=2.
1153
d=1.
7370
d=1.
4572
30 40 50 60 700
500
1000
1500
2000
2500
Inte
nsity
(Cou
nts)
[ ] g ; p00-004-0787> Aluminum - Al
00-023-0761> AlH 3 - Aluminum Hydride
Alane formation
Comparison of H2 Storage Methods
Summary
Storage Method(kJ) (V)
61.2 -0.64
-0.16
---
H2 Liquefaction --- --- 70 12.5 - 15 0.63 – 0.75
---
46.5
---
---
Storage Density(kg H2/m3)
StorageEnergy Cost(kWhe/kg H2)
Storage Cost@ $0.05/kWh
($/kg H2)
Ideal: 5.66 0.28
η = 0.3 V: 8.40 0.42
Ideal: 4.28 0.21
η = 0.3 V: 12.33 0.62
Wet Synthesis 150 Ideal: 107 5.3
H2 Compression(7000 psig)
79.8 2.6 - 3.6 0.13 – 0.18
150
150
2 332
Al H AlH+ ↔
4 3NaAlH AlH NaH≈ +
ocellGΔ o
cellE
3 / 3 solution+ −Al Cl
FY08• Upgrade the Electrochemical Cell with larger surface area working (Al) and
counter electrodes and focus effort on using alanate solutions• Examine the use of catalysts (Ti) in accelerating the electrochemical
regeneration• Use other techniques (e.g. NMR and Raman) to quantify and characterize
AlH3• Work closely with other AlH3 partners BNL and Hawaii
FY09• Optimize all parameters needed for producing several grams of AlH3 efficiently
• Design and construct a lager Electrochemical Cell (EC) capable of producing
larger quantities of AlH3
• Develop efficient methods of extracting AlH3 from (EC)
• Work closely with other AlH3 partners BNL and Hawaii
Future Work