Photo courtesy of James Courtney Effect of halide additives on the hydrogen desorption of lithium amide. Rosalind Davies

Hydrogen and Fuel Cells Doctoral Training Centre University of Birmingham, UK H2FC SUPERGEN Researcher Conference, 16th December 2014

[email protected] Outline

□ Introduction and Background

□ Amide Iodides

□ Amide Bromides

□ Amide Chlorides

□ Amide Fluorides

□ Conclusions Introduction to the LiNH2 system

This stage requires high temperatures for reversibility

Li3N + 2H2 Li2NH + LiH + H2 LiNH2 + 2LiH

This stage alone has a gravimetric storage 1 capacity of 6.5 wt.% H2

1Chen, P., et.al., Nature, 2002, 420, 6193 Reaction of lithium halides with LiNH2 forms amide halides

LiNH2

+ LiCl + LiI + LiBr

3 3 5 Li4(NH2)3Cl Li7(NH2)6Br Li3(NH2)2I 4 Li2NH2Br

3Anderson, P., A., A., et. al., Faraday Discussions, 2011, 151 4Barlage, H., and Jacobs, H., Z. Anorg. Allg. Chem., 1994, 620, 479. 5Matsuo, M., et al., Chem. Mater., 2010, 22, 2702. Adding halides lowers the desorption temperature

Lithium amide halide + LiH Temperature programmed desorption

Suppression of ammonia release

Formation of lithium imide halides, and systems can be rehydrogenated

Anderson, P., A., et. al., Faraday Discussions, 2011, 151 Gravimetric effects

Adding halides is gravimetrically unfavourable:

Li (NH ) Cl 4 2 3

Li7(NH2)6Br Li (NH ) I 3 2 2

this work investigates the lower halide doping limits. Reaction of LiI with 3 LiNH2

12 hours at 150°C

5 Li3(NH2)2I

Double-layered hexagonal structure a = 7.09109(5) Å, c= 11.50958(10) Å

5Matsuo, M., et al., Chem. Mater., 2010, 22, 2702. Amide Iodides

(3−z) LiNH2 + z LiI  Li3(NH2)(3−z)Iz

z = 1 equivalent to Li3(NH2)2I

For z between 0.5 and 1

Reaction for 12 hours at 150°C

Amide Iodides

LiNH2 is observed at all z values < 1

No change in unit cell volume of the amide iodide

Amide Iodide – formed in situ?

a LiNH2 + a LiH + LiI for a = 6,12,18

Temperature programmed desorption mass spectrometry (TPD-MS) Heated at 2°C/min to

400°C and H2 release measured

Improvements seen for all a values compared to

LiNH2, but best for a = 6

Reaction of LiBr with LiNH2

12 hours at 250°C

3 4 Li7(NH2)6Br Li2NH2Br

Hexagonal unit cell Orthorhombic unit cell a = 9.8218(3) Å, c = 8.9596(3) Å a = 12.484(2) Å, b = 7.959(1) Å Rhombohedral symmetry c = 6.385(1) Å

3Anderson, P., A., et. al., Faraday Discussions, 2011, 151 4Barlage, H., and Jacobs, H., Z. Anorg. Allg. Chem., 1994, 620, 479. Amide Bromides

(7−y) LiNH2 + y LiBr  Li7(NH2)(7−y)Bry

y = 1 equivalent to Li7(NH2)6Br

For y between 0.5 and 1

Reaction for 12 hours at 250°C

Amide Bromides

Decreasing y below 1 decreases amide bromide unit cell volume

Excess LiNH2 prevents direct determination of stoichiometry

Amide Bromide – formed in situ?

Desorption reactions of

b LiNH2 + b LiH + LiBr TPD-MS: Heated at 2°C/min to 400°C

- Improvements seen for b = 6, 12

- Not seen for b = 18 Reaction of LiCl with LiNH2

3LiNH2 + LiCl  Li4(NH2)3Cl t ≥ 6 hours t < 6 hours Body centred cubic phase Hexagonal unit cell a = 10.4303(4) Å a = 9.7020(4) Å, c = 8.9024(4) Å Rhombohedral symmetry

8000 Cubic Rhombohedral 6000

Counts 4000

2000

0

10 20 30 40 50 60 70 80 2 Degrees Amide Chlorides

(4−x) LiNH2 + x LiCl  Li4(NH2)(4−x)Clx

x = 1 equivalent to Li4(NH2)3Cl

For x between 0.5 and 1.75

Reaction for both 1 and 12 hours at 400°C

New phase observed at x = 0.57, 12 hour reaction

Structure of new phase

Structural solution for Li7(NH2)6Cl similar to Li7(NH2)6Br.

Rietveld refinement of data collected at Diamond light source synchrotron confirms the stoichiometry Gravimetric gain

Li7(NH2)6Cl, 5.28%

Li4(NH2)3Cl, 4.45% Amide Chloride – formed in situ?

6 LiNH2 + 6 LiH + LiCl

TPD-MS: Heated at 2°C/min to 400°C

No reduction in desorption temperature  Amide chloride not formed in situ Amide fluoride

LiNH2 + w LiF

Carried out for w = 0.25 to 0.5 at a range of temperatures and reaction times.

Diffraction patterns remained unchanged

No mixed amide fluoride formed due to stability of LiF 6 LiNH2 + 6 LiH + LiF TPD-MS: Heated at 2°C/min to 400°C

No reduction in desorption temperature Conclusions

Amide bromide and amide iodide formed in situ Hydrogen release temperature reduced on addition of LiBr and LiI

Amide chloride not formed in situ: improvement only seen when the amide is formed before the desorption

F− was not accommodated in an amide phase and addition of LiF did not lower the desorption temperature Further work

More detailed investigation into the phase space of the lithium magnesium amide chloride (reaction time and composition dependence)

Extension to other anions, e.g. S2−

Conductivity measurements of the new amide chloride

Investigation into the dehydrogenation process Changing chloride level: one hour reaction

Rhombohedral Li4(NH2)3Cl phase ‘a’: High Cl− content

Rhombohedral phase ‘b’ Lower Cl− content Changing chloride level: twelve hour reaction

100 R3 phase 1 I2 3 phase 90 1 80 70 60 Li4(NH2)3Cl 50 40 Cubic Phase 30 R3 phase 2 20

Phase fraction (weight %) (weight fraction Phase 10 LiCl 0

0.6 0.8 1.0 1.2 1.4 1.6 x [ (4x) LiNH + x LiCl] 2 Rhombohedral phase ‘c’ x = 0.53 − − NH2 :Cl 6:1