11.

Task１
Task １ １

Development of Hydrogen Storage Materials

11.1

R&D Goals

In fiscal year 2001, the following three activities were performed :(1) Research and development for promising alloys and technologies with WE-NET
target performance- Effective hydrogen storage capacity:

more than 3 mass%- Temperature for hydrogen desorption :- Durability

100 ℃ or below: hydrogen storage capacity more than 90 % of the initial capacity
after 5,000-cycle use(2) Investigation and inspection of new carbon materials and others
11.2

Achievement of research and development in fiscal year 2001

In the fiscal 2001, following three items were conducted: ①improvement of capacityand endurance of V-base and Ti-Cr base alloys developed in the previous year, ②
research for next generation alloys with more than 5 mass% capacity, ③ start ofresearch for chemicals and carbonaceous materials
11.2.111.2.1.1

Metal Hydride
Improvement of capacity and endurance of developed alloys(1) V base alloys
Practical properties of the novel alloys of V-10 at%Ti-12.5 at%Cr-3 at% Mn alloy andV-10 at%Ti-11 at%Cr-2 at%Mn-1.5 at% Ni with a very large effective hydrogen capacity
were investigated.

These alloys showed neither pyrophoricity nor ignitability whileconventional practical hydrogen storage alloys of AB 5 and AB 2 types are dangerous
substance, and a good durability for 1,000 charge/discharge cycling.

Although thehydrogen capacity was decreased by oxidizing gas such as O 2 , CO and CO 2 , it wasrecovered by flashing treatment with pure hydrogen gas.

The hydrogen reservoirs

using these both alloys were demonstrated to have larger hydrogen volume- and masscapacity than using conventional best alloy.
(2) TiCrMo alloys
In this year, the hydrogenation property of Ti-Cr alloys was improved by changing
producing condition and added element.

Floating zone (FZ) method is greatly

effective on improving the capacity of developed Ti-Cr alloys and we can expect toreach the goal of 3 mass% or more effective hydrogen storage amount(3) Durability Tests for Hydrogen Storage Alloys
In this work, some hydrogen storage alloys developed in WE-NET have been
investigated on durability thereof.

All tests were carried out at temperatures-106-

between 293 and 353 K and at H 2 pressures between vacuum (less than 1 hPa) and 4.5
MPa.

Every alloy is examined its degradation of characteristics during 1,000 cycles ofabsorption and desorption.
11.2.1.2

Research for next generation alloys with 5 mass% (provisional) capacity(1) Enlargement of plateau of V base alloys
In FY2000, new alloys of V-Cu-Ni and V-Cu that have a monohydride and dissociatehydrogen at 140 ℃ around 0.1 MPa were found in WE-NET.

In FY2001, we searched

for alloys that formed monohydride and dihydride and can dissociate hydrogen atlower than 140 ℃ around 0.1 MPa from both hydrides.
found.

However such alloys were not

The alloys which formed dihydride were found not to have monohydride which

dissociated hydrogen at lower than 140 ℃.
We succeeded in producing nano order thin foil laminates by cold rolling method.
The hydrogen storage properties of this super-laminate of V-15 at%Pd were similar tothat of casted V-15 at%Pd instead of vanadium.

However, the super laminates of V-25

at%Pd had a two-step plateau on the PCT curves and the monohydride dissociated at
140 ℃ around 0.1 MPa.
(2) CaMg 2 alloys with nano size composite structure
New alloy with fine composite structure of CaMgNi 4 (1.0 mass%, hydrogen desorption
temperature: 70 ℃) and CaMg 2 (5.4 mass%, 350 ℃ ) was produced by ball-milling
method.

This composite material showed hydrogen desorption, which was notobserved in single phase of CaMgNi 4 or CaMg 2 and this composite method is expected
to be a new method for improving the hydrogen desorption rate of Ca-Mg system.
(3) New alloys of Ca-Mg system
An alloy, which contains CaMg 2 phase that is one of Mg-Ca-Ni system, has been
investigated to improve the absorption/desorption property with some additives.

In

this research, the (Ca 1-x A x )Mg 2 C z alloy that can absorb 4 to 5 mass% at 25 ℃ was
found.

This alloy did not disproportionate after hydrogen absorption.

(4) Ti base alloy designed with the space factor principle
We utilize a space factor as a guiding principle for searching new hydrogen storage
materials.

The space factor is defined as an average space size per atom in acompound.

We found that the average space size of several hydrogen storage alloys

ranges from 0.013 nm 3 to 0.015 nm 3 , and has a correlation with the hydrogen pressure
and capacity.

We can succeed in increasing hydrogen pressure of a Ti-V binary alloy

by

the

reducing

space(Ti 0.44 V 0.39 Al 0.07 Ni 0.10 ).

factorto0.01552

nm 3

by

Al

and

Ni

addition

Consequently, we ascertained that the adjustment of the spacefactor corresponding to the above criterion is effective to obtain a new hydrogen alloy-107-with the capacity beyond 5 mass%.
(5) New Mg alloys
Mg-Ti system alloys were investigated in order to create new alloys that have a
possibility of achievement of 5 mass% capacity.
alloy does not exist.

Until now, the BCC structure Mg-Ti

However Mg-Ti alloy with BCC structure and α -Ti solid

solution consisted of Mg and Ti were successfully synthesized by ball milling of Mg andTi powders for 7.2×10 5 s under argon atmosphere.

The composition of the BCC phaseand α-Ti solid solution were Mg 1-x Ti x (x=0.34-0.62) and Ti 1-y Mg y (y<0.2), respectively.
This BCC alloy is expected to absorb hydrogen in large amount because the alloy
consisted of light-weight hydrogen absorbing elements.
(6) Ternary R-Mg-Ni alloy
Recently, it has been reported that pseudobinary MgNi 2 -YNi 2 and NgNi 2 -CaNi 2
alloys take the C15b structure and can absorb-desorb hydrogen reversibly.

In thepresent work, MgRNi 4 (R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb) alloys
were prepared and structures of these alloys before and after hydrogenation wereidentified.

It was found that these ternary C15 Laves phases with the atomic radiusratio larger than 1.37 can keep their crystal structure and less than 1.37 amorphize byhydrogenation.

The hydrogen content increased with increasing the lattice constantof MgRNi 4 alloys.

Therefore the atomic radius ratio plays the important role andalloys of R=Y or Tb with relatively small lattice constant is expectable for absorbinghydrogen.
(7) Improvement of hydrogen desorbing kinetics of Mg alloys
The Mg 85 Ni 10 Nd 5 alloy that can readily absorb about 5 mass% hydrogen at 373-573 K
and can fully desorb it with a moderate speed at 453-573 K in vacuum has beendeveloped.

This alloy has a composite-phase structure in which Mg 2 Ni and Nd-

hydride particles, 〜 1 nm and 〜 30 nm in diameter, respectively, are precipitateduniformly in the Mg matrix phase with nano-grains of 〜 5 nm.

This refined

microstructure, coupled with a catalytic action of Nd-hydrides, strongly enhances theabsorption/desorption kinetics.

However application of this alloy to hydrogen

reservoir is difficult because of it low adsorbing pressure.
(8) New magnesium intermetallic compounds including Ge
New intermetallic compounds including group 14 elements which bond withhydrogen covalently, such as Mg 2 Ge, were synthesized by mechanical alloying.

Themechanically alloyed Mg 2 Ge absorbs and desorbs hydrogen reversibly at moderatetemperature, though the amount of hydrogen was small.

The amount of hydrogen

released from the compound was enhanced by addition of nickel.

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The tendency wasremarkable at the composition of Mg 2 Ge 0.8 Ni 0.2 (the composite of Mg 2 Ge and Mg 2 Ni).
(9) Formation of unstable hydrides by electrochemical charging
Previous works on preparing non-stable hydrides (hydrides with high dissociationpressures) by electrochemical charging were reviewed and extensive studies werecarried out on some iron alloys.

Some iron alloys that were strongly distorted bycoherent nitride particles were electrochemically charged in a similar way employing a
KOH electrolyte and the formation of their hydrides were tested.
dispersed iron absorbed hydrogen, however the amount was small.

The particle-

It was suggested

that the characteristics of electrolyte is very important for the formation of unstable
hydrides (FeH).
(10) High-pressure Synthesis Novel Hydrides in Mg-M Systems
New hydrides in Mg-M (M = Ca, Y, Mn) systems have been prepared at GPa orderhigh- pressure by using a cubic-anvil-type apparatus.
① In MgH 2- x%CaH 2 system, Mg 1-x Ca x H 2 (x=0.4-0.6) was synthesized at 1,223 K, 5 GPa.
The decomposition temperature of the new hydride was 646 K.
② In MgH 2- x%YH 3 system, new hydride of a FCC-type structure was synthesized
nearby x = 67 (MgY 2 H x ) at 3 GPa or higher.
③ The Mg 3 MnH x phase exhibited a different crystal structure from Mg 3 MnH 7 that has
been synthesized at 1,073 K for 2 h under 2 GPa.
the hydride was 620 K.

The decomposition temperature of

And its hydrogen content was found to be 3.69 mass%,corresponding to Mg 3 MnH 4.86 .
(11) Hydriding properties induced by the interaction between inter- and intrananostructured grains
Nanostructured vanadium-hydrides were prepared by mechanical milling underhydrogen atmosphere.

The hydrogen concentration in the grains x G decreases withdecreasing grain size from x G = 0.82 in 80 nm grains to 0.72 in 10 nm grains.

Thehydrogen concentration in the intergrains, x IG 0.5-0.6, is smaller than in the grains,and nearly independent of the grain size.

The hydrogen diffusivity in the intergrain

regions measured by NMR is substantially higher than inside the grains.

The

exchange of hydrogen atoms between the grains and the intergrain regions occurs very
slowly.
(12) Nano-composite Mg thin film
The nano-composite Pd-Mg multi-layer film that absorbed 4.4 mass% of hydrogen for
Mg and all the hydrogen desorbed at temperature lower than 120 ℃ have beenreported.

Using an in-situ system, the Pd(50nm)/Mg(200nm)/Pd(50nm), Pd(10nm)/

Mg(200nm)/Pd(10nm) films were prepared and hydriding properties of them were-109-

examined.

Hydrogen can be desorbed at temperature lower than 150 ℃ even after

10 times hydriding-dehydriding reaction.
(13) Be 2 Ti alloy① Be is categorized as a specified chemical and regulated legally.
stable metal chemically and really generates toxic substances.

However Be is veryIn the case of vehicle

part materials, some risks might be assumed but we could take measures to meetthem.
② Be 2 Ti alloy can be produced by melting-heat treating-pulverizing process.

In thematerials prepared this time, metallic Ti exists and it spoils the hydrogen absorbing
properties of the alloy.
next trial.

Some improving of production conditions may be required in

In hydrogenation at 200 ℃ , 10 MPa (these are not the best conditions),

these samples absorbed 1.4-1.5 mass% hydrogen, which is less than the amount of 4.4
mass% reported in the paper.
11.2.2

Chemicals for hydrogen storage

In WE-NET two kinds of chemicals, hydrogen complex compound and naphtenes, arestudied as 5-8 mass% hydrogen storage materials that can be used repeatedly.
11.2.2.1
11. 2.2.1

Hydrogen complex compound(1) NaAlH4 for hydrogen storage material
The kinetics of the reverse hydrogenation reaction have been determined over arange of temperatures and H 2 pressures and acceptable charging capacities rates attemperatures as low as 100 ℃ under 10 MPa of H 2 have been observed.

From thethermdynamical analysis it becomes clear that the kinetics of hydriding of doped
mixtures Na 3 AlH 6 and Al to NaAlH 4 are limited by the micro-reverse, of thefundamental, Al-H bond breaking process.
might be developed from this viewpoint.

Some useful technology for rehydriding

A long duration hydrogen cycling study ofthe hydrogenation reaction and its reverse are also conducted.

To date 3.0 mass%hydrogen has been discharged and re-charged in 24 cycles with no loss of capacity.
(2) Novel complex-type hydrogen storage material
A vibrating-milling technique for 1 hour was used to prepare titanium (III) chloride(TiCl 3 1/3AlCl 3 )-doped LiAlH 4 powder nanocrystallites.

This mixture started torelease hydrogen from 100 ℃ on the first heating process in hydrogen atmosphere ata pressure of 4 MPa.

On the following cooling process, an exothermic thermal effectthat indicates hydrogenation of the decomposed material on the heating process, LiH,
to Li 3 AlH 6 was appeared.

Further study on reversible dehydrogenation of LiAlH 4

through optimization of preparation process and investigation of reaction mechanism
would contribute to development of a lighter hydrogen storage system.

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(3) Safety dealing with NaAlH 4 and disposal condition
The objectives of this study are clarifying the properties of NaAlH 4 and collection of
the data to design hydrogen storage tank.

NaAlH 4 is highly flammable solid andreacts violently with water or other oxidizer.

The results of reactivity test with someliquid material, i-propanol showed relatively mild reaction and silicone oil showed noreaction with Ti-doped NaAlH 4 .

Safety manual dealing with NaAlH 4 and disposal

condition were established.
(4) NaAlH 4 System for Hydrogen Storage
Design and fabrication of the equipment, which has 10 g reactor and 100 g reactor,
for investigation of dehydriding/hydriding properties were performed.

The result of

the investigation with 10g reactor in first stage of hydrogen release shows very low
reaction rate for ordinary Ti-doped NaAlH 4 at 100 ℃ .

Hydrogen transfer remains

only 63% of theoretical value, however deterioration with cycle was not observed.
These results show the possibility of NaAlH 4 application for hydrogen storage tank.
11.2.2.2

H ydrogen transport and storage using naphtenes

Naphtenes like cyclohexane are very promising as chemical carriers for transportingand storing hydrogen because their hydrogen contents are exceeding 7 mass% and they
are in liquid state at room temperature like gasoline.
(1) Research for hydrogen transport and storage system of cyclohexane
The major technical problem involving this system is how the hydrogen caneffectively be taken out of naphtens in the dehydrogenation process, which is limitedby thermodynamic equilibrium.

First, the type of membrane reactor is discussed fromthe viewpoint of obtaining an equilibrium shift as well as pure hydrogen.

For such a

purpose, it is found that a membrane reactor using palladium membrane is the mostsuitable.

Next, selection of a better operation manner of the palladium membranereactor is made.

The reaction should be carried out under an elevated pressure so as

to keep the partial pressure of hydrogen higher than the pressure on the perm-side.
Then experiments with cyclohexane dehydrogenation at 0.1-0.3 MPa are carried out toestablish the rate of reaction, which is needed for the reactor modeling.
(2) Phase behavior for solid-liquid equilibrium of naphtene system
At first, some physical properties of the pure compounds were cited.

However,

there is no report for other two systems at all except the data for benzene +cyclohexane system.
equilibria.

Thermodynamic model was applied to predict the solid-liquid

In case of assuming the ideal solution, the predicted eutectic temperature

is lower than the reported one for benzene + cyclohexane system.

In case of assuming

non- ideal solution the solid-liquid equilibria of benzene + cyclohexane system was well-111-

predicted.
(3) Quantum chemical molecular dynamics study on the permeation process ofhydrogen in palladium membrane
The permeation process of hydrogen in palladium membrane was studied by thetheoretical approach, in order to perform a screening of new non-palladium materials
for hydrogen separation.

The simulation on the hydrogen adsorption on thepalladium membrane was performed using our accelerated quantum chemicalmolecular dynamics program “Colors”.

From this simulation, it was found that theelectron transfer from the palladium atoms to the hydrogen molecule promotes thedissociation of the hydrogen molecule.

In order to investigate the equilibrium state of

the hydrogen absorption in Pd-Ag alloys, we succeeded in the development of newMonte-Carlo simulation program “MONTA”, which enables us to simulate thedissociative absorption of the hydrogen molecules in transition metals and their alloys.
11.2.3

Carbonaceous materials for hydrogen storage

Following in the wake of last year ’s accomplishment of this working group, in which
we had established accurate measurement techniques (balance method and Sievert’s
method), we investigated hydrogen storage capacity in several carbon materials.
11.2.3.1

Hydrogen uptake of singlesingle - walled carbon nanotubes(SWNT) synthesized inChina

At first, a new setup used for synthesis of SWNTs by the hydrogen arc discharge
method with a designed yield of one kilogram of SWNTs per year was established.
Secondly,amulti-steppurification

andpretreatment processwasdeveloped.

Ultrasonic sonication, hydrothermal treatment, acid washing, bromination, airoxidation, and high temperature vacuum treatment were involved in these processes.
Thirdly,themeasurements.

pretreated

SWNTs

were

used

for

volumetrichydrogenstorageIn order to calibrate the measurement apparatus, the standard

sample among WE-NET Group, Maxsorb, was used.

The H 2 storage capacities weobtained for different SWNT samples were between 0.9 to 4.3 mass%, depending upon
the sample states.
11.2.3.2

StructureStructure - controlled carbon nanotubes

The structure-controlled carbon nanotubes have special wall structure that consistsof ultrafine graphite plane fragment with very large specific surface area.

The

property of the hydrogen storage of this material was 3.1 mass% at 10 MPa.

The

Temperature Programmed Desorption (TPD) measurement showed that this material
had more than two types of adsorption site and the particular structure.

It was alsosuggested that this new carbon material differed from the other carbon materials on-112-

the structure and the adsorption mechanism.
11.2.3.3
11.2.3. 3

Carbon nanohorn

Carbon nanohorn was synthesized by laser vaporization method without metal
catalyst.

Synthesized material consists a single wall structure.

The mean length ofa horn is 30-50 nm and the mean diameter of born-like tube is 2-3 nm.
Hydrogen absorbing amount of carbon nanohorn at 303K is 0.23 mass% at 10 MPa.
This amount is very low considering the expected value as hydrogen storage.
Pretreatment of oxidization at various temperatures and washing in nitric acid or
alkali were tried.

The caps at the both end of a horn were removed by oxidization atelevated temperature and the amount of hydrogen absorbed at 10 MPa increased tomore than twice, even though it was only 0.52 mass%.
element was not clear.

The effect of noble metal

Therefore it was suggested that a carbon nanohorn does notpossess enough absorbing ability as hydrogen storage material.

The results of thisresearch demonstrate that increasing the amount of specific surface area andmicropore volume is very important to develop high storage capacity carbonaceous
materials.
11.2.3.4

Effect of local electronic structure on desorption from nanostructured
graphite

Nanostructured hexagonal (h)-BN was prepared by mechanical milling underhydrogen atmosphere.

The hydrogen concentration reaches up to 2.6 mass% after

milling for 80 h, and this value corresponds to ca. 35% of that of nanostructured
graphite as was previously reported.

In addition to the hydrogen desorption startingat about 570 K, nitrogen desorption was also detected at about 700 K.
recrystallization phenomenon at least below 1,173 K.

There was no

The dissimilarities on the (de-)

hydriding properties between nanostructured h-BN and graphite might be due to thedifferent local electronic structure near the specific defects.
11.2.3.5

Study of adsorbed hydrogen properties by NMR measurement

The mechanism of hydrogen storage in carbonaceous adsorbents has been evaluatedby means of

1 H-NMRrelaxation measurement.

These results indicate that thecharacteristics of surface structure of carbon strongly affect the hydrogen adsorption
properties.

It has become apparent that the amount of hydrogen adsorbed on theactivate carbon is independent of conventional pore characters such as specific surface
area and total pore volume.

Thus, in-situ measurement of the change in theproperties of adsorbed hydrogen, which is difficult to evaluate using conventional
techniques such as gravimetric analysis, has became possible using
relaxation technique.

1 H-NMRThe results obtained in this study clearly demonstrate the-113-advantage of 1H-NMR relaxation measurement for the evaluation of hydrogen
adsorbed on carbonaceous materials.

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