Modular Solid State Solutions for Portable H2 Applications

James M. Hanlon,* Laura Bravo Diaz, Marek Bielewski, Aleksandra Milewska, Cédric Dupuis and Duncan H. Gregory

* School of Chemistry, University of Glasgow

Outline

Introduction

- HYPER project

- Solid state H2 system

Experiment methods

Results:

- Synthesis of nanostructured Mg(OH)2and LiH/MgH2

- Mg(OH)2 + MgH2

- Mg(OH)2 + LiH

Approaches to progress forward

HYPER Project Introduction

HYPER Project

EC project launched in 2012 to develop a fully integrated fuel cell and storage

system for portable applications

Gaseous and solid state hydrogen storage to be employed

Solid state hydrogen storage (WP 3/4):

- University of Glasgow: synthesis, characterisation, testing

- JRC (EC): testing

- JEN: Theoretical calculations and cell design

- McPhy: Synthesis, testing and cell design

Introduction

Requirements for the solid state storage system

Low onset temperature of H2 release

Good wt.% of H2 released

Favourable kinetics

Recyclability of end products

Introduction- Solid State H2 storage system

Cell design

The cell will consist of a MgH2 matrix with an exothermic filler

The exothermic filler will be used to initiate dehydrogenation from the MgH2

matrix. A glow plug will be employed for initialisation

Designed from the following parameters:

- 3rd order of reaction (kinetics)

- Heat generated by reaction when T>Tonset

- Constant properties:

1) Density = 1000 kg/m3

2) Heat conductivity = 0.5 W/(mK)

3) Heat capacity = 1600 J/(kg K)

- Insulated walls (apart from initialiser)

Introduction

Filler Material

Efforts have been concentrated on two systems:

Mg(OH)2 + MgH2 2 MgO + 2 H2 (wt% H2= 4.5)

Mg(OH)2 + 2 LiH MgO + Li2O +2 H2 (wt .% H2= 5.44)

These have been extensively tested to determine their suitability as a ‘one shot’

exothermic filler material

(1) F Leardini, J R Ares, J Bodega, J F Fernandez, I J Ferrer, C Sanchez, Phys. Chem. Chem. Phys, 2010, 12,

572-577

Experimental techniques

Characterisation

Thermal analysis (TGA-DTA-MS) has been carried out using a Netszch STA409C coupled

to a Hiden HPR 20 MS inside an Ar recirculating glove box

Kinetic and stability measurements performed using a PCT Pro and Hiden volumetric

Sievert’s appartus

Bruker D8 and Panalytical X’pert Pro have been employed for PXD characterisation

For more information- please see poster by Laura Bravo Diaz!

Experimental techniques- Synthesis

Synthesis techniques

Nanostructured Mg(OH)2 has been synthesised hydrothermally in the MW

Due to air-sensitive nature of hydrides, nanostructuring options are limited

Ball milling using a Retsch PM100 has been used. Typical parameters used are 5 mins

mill followed by 5 mins rest and inverse rotation @ 450 rpm

Mg(OH)2 – MgH2 system

Preparation

H2 release

Nano(chemical) synthesis of Mg(OH)2

Dehydration of nano Mg(OH)2 (Mg(OH)2 MgO +H2O)

Dehydration commences at 290 oC with MgO as the final product

Morphology of the Mg(OH)2 is retained after heating

(2) J.M. Hanlon, L. Bravo Diaz, G. Balducci, B. A. Stobbs, M. Bielewski, P. Chung and D.H. Gregory, Submitted to

Green Chem.

Results- Mg(OH)2 + MgH2

Stability

Mixture must be stable at 65 oC- ‘rest temperature’

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H2 uptake (micromoles) - 65C

Mg(OH)2 and MgH2 system

STA of stoichometric Mg(OH)2 + MgH2 system

STA/MS have been performed using bulk and nano materials

Hand mix for 5 minutes

264 oC vs 317 oC

Mg(OH)2 and MgH2 system

Ball milling

Mixing must be improved

Ball milling has been employed @450 rpm

Effect of ball milling and reactivity?

153 oC vs 120 oC

Mg(OH)2 and MgH2 system

Effect of excess MgH2?

Excess MgH2 has been ball milled with Mg(OH)2 for 2 hours

Improvement in onset to 185 oC

Small weight loss < 3wt. % H2

Ignition simulations performed on this sample. 1100 oC needed to initiate the reaction:

density = 1000 kg/m3, k = 0.5 W/(mK), Cp=1600 J/(kg K), 3% H2

Tint= 30 oC

Hydrolysis of MgH2

How to improve hydrolysis?

Hydrolysis process could be constrained by the formation of a shell on the MgH2: (3)

(3) Tayah et al., Int. J. Hydrogen Energy , 2014, 39, 3109-3117

(4) Hiraki et al., Int. J. Hydrogen Energy , 2012, 17, 12114-12119

Could catalysts disrupt this?

Citric acid has also been used to lower the pH during hydrolysis to disrupt

this shell (4)

MgH2 + catalyst

H2 evolution

Hydrogen evolution - Thermal ramp 5 C/min

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00:21:36 00:50:24 01:19:12 01:48:00Time

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H2 MgH2 - Mg(OH)2

H2 MgH2-10wt SiC +

Mg(OH)2 sampleH2O MgH2 - Mg(OH)2

H2O MgH2 - 10 wt% SiC +

Mg(OH)2

Conclusions on Mg(OH)2 + MgH2

Conclusions From the STA/MS profiles the reaction is a 2-step process:

1) Dehydration of Mg(OH)2 and hydrolysis of some MgH2

2) Remaining hydrolysis of MgH2 due to a shell forming on the MgH2

Carbon and Silicon Carbide have been employed to try and disrupt this shell and improve

dehydrogenation of the mixture with some improvement

Kinetics for this system are too slow for its end use (please see poster by LBD)

First stage of reaction is exothermic, however total energy is too low to initialise the matrix

From calculations, this system may not produce enough heat to initiate the matrix

The end products are MgO and Mg so the starting materials can be regenerated

Results- Mg(OH)2 + LiH

Stability

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H2 uptake (micromoles) -65C

Mg(OH)2 – LiH system

STA of nano Mg(OH)2 + LiH

TG curve onset of ~92 oC with the main exothermic DTA peak is at 233 oC

Improved over bulk (onset of ~150 oC)

One step H2 release process

Kinetic testing- Mg(OH)2 – LiH system

Kinetic testing

System still needs further improvements for use as the exothermic filler

material, i.e first step of reaction, kinetics…

Improvement over MgH2 counterpart system

Conclusions on Mg(OH)2 + LiH

Conclusions

This system has improved performance to its MgH2 counterpart system in terms of kinetics

and its thermal profile

Nanostructuring improves the performance of the system

No stability issues with the system

Same issues with the ‘core shell’ forming on the hydride that limits H2 release

Calculations show the exothermic release from this system would be insufficient to

encourage MgH2 dehydrogenation from the matrix

Issues with reactivity during regeneration of the matrix?

Approaches Forward

Next Steps…

Further work needs to be undertaken on the ‘core shell’ of the hydrides

For the matrix approach to be successful, alternative exothermic filler

materials/matrix materials need to be investigated

Ideal ratio of filler to matrix composite needs to be investigated

Alternative, different solution required?

Acknowledgements

The research leading to these results has received funding from the European

Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and

Hydrogen Joint Technology Initiative under Grant Agreement number 303447

HYPER Project Introduction

Introduction- Solid State H2 storage system

Initialisation of the System

A glow plug is ideal for initialise the system

Glow plug can heat to 1000 oC in seconds time spans

Require external source to switch on the plug- increase ‘parasitic’ load

Synthesis of nanostructured Mg(OH)2

MW hydrothermal synthesis of Mg(OH)2

Kinetic experiments: Mg(OH)2 + MgH2

Very slow kinetics!

MgH2 + catalyst

TGA/MS Analysis

MgH2 + Mg(OH)2 hand mixed for 5 min of milled 5h MgH2/nano Mg(OH)2

MgH2 – 10 wt% C + Mg(OH)2 hand mixed for 5 min of milled 5h MgH2 + 10wt C/nano Mg(OH)2

MgH2 – 10 wt % SiC + Mg(OH)2 hand mixed for 5 min of milled 5h MgH2 + 10wt SiC/nano Mg(OH)2

MgH2 – 5 wt % SiC – 5 wt SiC + Mg(OH)2 hand mixed for 5 min of milled 5h MgH2 + 5wt SiC + 5 wt C/nano

Mg(OH)2

Mass change: 7.14 % - 8.19 %

Mass change: 9.80 % - 10.80 %

Mass change: 8.59 % - 9.36 %

Mass change: 9.2% - 9.93 %