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