VAPOUR HYDROLYSIS OF COMPLEX HYDRIDES

Elizabeth Ashton

Loughborough University

Hydrogen Storage Materials For Mobile Applications

1 2 3 4

High Energy Densities DOE Targets Toxicity Recyclability

• High Energy Densities: Gravimetric and volumetric energy densities allow comparison between hydrogen storage materials

• DOE Targets: The U.S Department of Energy (DOE) has set certain targets for onboard hydrogen storage and delivery.

• Toxicity: Hydrogen storage materials must have low toxicity if they are to become part of a commercially available product.

• Recyclability: The ability to recycle spent materials is also an important factor to consider.

5kg of hydrogen required to achieve 300miles (~500km)2x700bar tanks

https://www.audi-mediacenter.com/en/the-audi-h-tron-quattro-concept-5333/, (Accessed 9 May 2019)

Hydrogen Storage Materials

STORAGE

PARAMETERUNITS 2020 2025 ULTIMATE

System Gravimetric Capacity

Usable, specific-

energy from H2kWh/kg 1.5 1.8 2.2

System Volumetric Capacity

Usable energy

density from H2kWh/L 1.0 1.3 1.7

Durability/Operability

Min -Max delivery

pressure from

storage systembar (abs) 5-12 5-12 5-12

DOE Onboard Targets1

Targets for gravimetric and volumetric density have been set for 2020, 2025 and ultimate.

Targets look at achieving higher energy densities at more amiable temperatures and pressures.

Safety is the main consideration for system design of hydrogen storage and delivery systems.

Reference: 1) DOE Technical Targets for Onboard Hydrogen Storage for Light-Duty Vehicles | Department of Energy, https://www.energy.gov/eere/fuelcells/doe-technical-targets-onboard-hydrogen-storage-light-duty-vehicles, (accessed 27 November 2018).

Complex Metal Hydrides

Complex metal hydrides have the highest percentage weight hydrogen.

The general formula is M(M’H4)n ; where M is a light metal element found in group one or two of the periodic table.

Hydrogen can be released via thermolysis or hydrolysis.

CompressedH2 Gas

cylinder

LaNi5H6 MgH2 LiAlH4 NaBH4 LiBH4

Percentage Weight Hydrogen (%) DOE Targets

Energy Densities

Complex hydrides have the highest energy densities.

Small volume and light weight is desired for mobile applications.

System design must also be considered in overall weight and volume to meet DOE targets.

0.2

2

0.6

33.5

4.1

6

0.5

1.5

3.54

3.5

4.4

5

Gravimetric kWh/kg

Volumetric kWh/L

Best Battery

H2 Gas LaNi5H6 MgH2 LiAlH4 NaBH4 LiBH4

DOE Targets

BORO-HYDRIDES

NaBH4, LiBH4, Mg(BH4)2 etc.

Issues: Catalyst costCatalyst durability Exothermic

Issues: CostAqueous or Solid (pellets)

Issues: Additional waterLow energy density

Issues: Vapour or LiquidTemperature

Issues: InsolubleRecycling Hydroscopic

LiAlH4, NaAlH4, NaH, LiH, CaH2

Pros• The hydrolysis of these reactions occurs at RTP.

• No catalysts or acidic conditions are required.

• No insoluble metaborate by-products

• LiAlH4 is cheaper than NaBH4

Cons• The hydrolysis reactions are very exothermic.

• Less literature available.

• Some products formed are insoluble, e.g. (Al(OH)3)

OTHERHYDRIDES

Hydrogen Storage Materials For Mobile Applications

• Solid sample used.

• No extra water required to keep reactants/products in solution.

• No catalysts or acidic conditions needed.

• Yields of 90% hydrogen production at 110oc achieved for hydrolysis of NaBH4.3

Reference: 3) H. Liu, C. M. Boyd, A. M. Beaird and M. A. Matthews, Int. J. Hydrogen Energy, 2011, 36, 6472–6477.

𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐻𝐻4 + 4𝐻𝐻2𝑂𝑂 → 𝑳𝑳𝑳𝑳𝑳𝑳𝑳𝑳 + 𝑨𝑨𝑨𝑨(𝑳𝑳𝑳𝑳)𝟑𝟑 + 4𝐻𝐻2

Vapour Hydrolysis of LiAlH4

• Vapour hydrolysis cell design by Paul Brack and engineered by Intelligent Energy Ltd.

• Reaction left to run for 24h.

• Products then characterised by Powder X-Ray Diffraction (XRD), Infrared Spectrometry (IR) and Thermogravimetric Analysis (TGA)/ Differential Thermal Analysis (DTA).

• Products compared to mixture 1:2 LiOH:Al(OH)3

A

BC

D

E

F

Different Humidity

• 96% KNO3

• 86% KCl

• 76% NaCl

• 56% Mg(NO3)2

• 46% K2CO3

(a) 1:1 molar ratio LiOH:Al(OH)3

(b) LiOH

(c) Al(OH)3

(d) Vapour Hydrolysis Product [LiAl2(OH)6)]2CO3 ·𝒙𝒙H2O

• <100oC Physisorbed water. • ~150oC interlayer water • ~250-300 Dehydroxylation

and interlayer anions

Presence of CO2 Exclusion of CO2

→ CO32-

→ OH-

4) I. C. Chisem and W. Jones, J. Mater. Chem., 1994, 4, 1737–1744.

5) J. Qu, X. He, B. Wang, L. Zhong, L. Wan, X. Li, S. Song and Q. Zhang, Appl. Clay Sci., 2016, 120, 24–27.

Synthesis of [LiAl2(OH)6)]2CO3 ·𝒙𝒙H2Oand [LiAl2(OH)6]OH·𝒙𝒙H2O

• Synthesis of [LiAl2(OH)6]2CO3·𝑥𝑥H2O using published method.

(AlCl3·6H2O + LiOH·H2O + Na2CO3)4

• Synthesis of [LiAl2(OH)6]OH·𝑥𝑥H2O using published method.

(LiOH·H2O + Al(OH)3)5

• Products then compared to the products obtained from the vapour hydrolysis reactions. 4000 3500 3000 2500 2000 1500 1000 500

Tran

smis

sion

%

GB VHC Li-Al-OH Synthesised Li-Al-OH Synthesised Li-Al-CO3 VHC Li-Al-CO3

Wavenumber cm-1

Experiment performed in a glovebox box to exclude CO2

• Three characteristic endothermic peaks of a Layered Double Hydroxide.

0 200 400 600 800 1000

40

50

60

70

80

90

100

Temperature oC

% w

eigh

t

∆T/

o C

• IR match to that of [LiAl2(OH)6]2OH]·𝑥𝑥H2O

• XRD match to that of [LiAl2(OH)6]2OH]·𝑥𝑥H2O using EVA software.

Hydrolysis of Complex Hydrides in Flow • Solid sample used.

• No extra water required to keep reactants/products in solution.

• No catalysts or acidic conditions needed.

• Reduced weight and volume compared to the vapour hydrolysis cell.

𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐻𝐻4 + 4𝐻𝐻2𝑂𝑂 → 𝑳𝑳𝑳𝑳𝑳𝑳𝑳𝑳 + 𝑨𝑨𝑨𝑨(𝑳𝑳𝑳𝑳)𝟑𝟑 + 4𝐻𝐻2

Packed Bed Reactor

The hydride hydrolysis reactions were converted to flow.

A packed bed reactor maximizes gravimetric and volumetric energy densities.

Silicon glass beads were used to pack the reactor.

H2OVapour inlet

Paper frit Hydride Silica bead

H2O/H2gasoutlet

To control the rate of reaction:

• Change the length of the column

• Increase the mass of hydride

• Change the water flow rate

Hydride Reaction equation Predicted Yield of H2cm3

% Yield

LiH LiH+H2O→LiOH+H2 302 91LiAlH4 LiAlH4+4H2O→Al(OH)3+LiOH+4H2 252 61NaAlH4 NaAlH4+4H2O→Al(OH)3+NaOH+4H2 178 80CaH2 CaH2+2H2O→Ca(OH)2+2H2 114 92NaH NaH +H2O→NaOH+H2 100 530 250 500 750 1000 1250 1500

0

50

100

150

200

250

300

350 LiAlH4 LiH NaAlH4 CaH2 NaH

Am

ount

of h

ydro

gen

(cm

3 )

Time (s)

Hydrolysis of Hydrides in Flow The highest yield of hydrogen was achieved using CaH2 and LiH.

The LiH and NaH products could not be characterized as they are soluble in water.

NaAlH4 did not form a layered double hydroxide.

Conclusions and Future Work1 2 3 4

Conclusions and Future Work

Hydrogen Production Structure Recyclability

H2

Anion exchange

Cl-

• Hydrogen Production: A method of controlled hydrogen generation via vapour hydrolysis of LiAlH4 was achieved.

• Structure: The product produced was a Li-Al Layered Double Hydroxide.

• Recyclability: The LDH retains water at high temperatures which is problematic for recyclability.

• Anion Exchange: the addition of additives will be investigated, to see if structures with different anions in the interlayer spaces reduces water retention.

Acknowledgements

Supervisor: Professor Sandie Dann2nd Supervisor: Professor Upul Wijayantha

William Oakley Dr Paul BrackDr Simon Foster Professor Paul Adcock