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Metal substrate enhanced hydrogen production of

aluminum fed liquid phase GaeIn alloy inside

aqueous solution

Xiao-Hu Yang a, Bin Yuan a, Jing Liu a,b,*

a Beijing Key Lab of Cryo-Biomedical Engineering and Key Lab of Cryogenics, Technical Institute of Physics and

Chemistry, Chinese Academy of Sciences, Beijing 100190, Chinab Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:

Received 13 November 2015

Received in revised form

2 March 2016

Accepted 3 March 2016

Available online 24 March 2016

Keywords:

Hydrogen production

Liquid metalEnhancement

Metal substrate

Surface roughness

a b s t r a c t

Hydrogen is a kind of promising clean energy with huge potential values for human being.

Developing a convenient real-time and on-demand hydrogen production and utilization

method is critical for its wide practices. Liquid metal was recently found to be able to easily

activate aluminum in aqueous solution to continuously and quickly generate hydrogen at

room temperature, and thus provides a straightforward hydrogen production way. To

further improve this technology, we are dedicated here to investigate the effects of the

container substrate on such hydrogen generation process. An interesting phenomenon

was found that metal substrate material would evidently enhance the hydrogen generation

rate. For conceptual illustration purpose, comparative experiments were conducted to

demonstrate the different hydrogen evolution modes between the present method andformer approach, and the enhancement mechanism lying behind was revealed. In addi-

tion, the influence of the surface roughness of the substrate on the hydrogen production

performance was also clarified. The present finding would help motivate an improved

extremely simple, straightforward and low cost way for the liquid metal assisted

aluminum hydrogen production.

Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Hydrogen, a clean and carbon-emission-free energy with high

gravimetric energy density, has attracted much attention over

the past few decades due to the increasingly serious issues on

fossil fuel shortage and air pollution. So far, there are already

many hydrogen production approaches ever proposed, which

mainly include: steam reforming of alcohols and coal gasifi-cation [1e3], water electrolysis and photolysis [4e6], metal

induced hydrogen evolving chemical reactions [7e9], etc.

Among them, the first approach still dominants the current

commercial market and owns advantages such as cost-

effective, technical maturity, etc. However, viewing from the

perspective of raw material, this method is essentially fossil

energy consuming. Moreover, the storage and transportation

* Corresponding author. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.

Tel.: þ86 10 82543765; fax: þ86 10 82543767.

E-mail address: [email protected] (J. Liu).

Available online at www.sciencedirect.com

ScienceDirect

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of hydrogen remains a big challenge due to its high flamma-

bility and extremely low volumetric energy density, and thus

caused technical difficulties encountered in compression and

liquefaction.

To accelerate the extensive use of hydrogen energy, it is of

great significance to develop real-time and on-demand

hydrogen production and utilization method, which can

dramatically reduce the storage and transportation cost. Suchdemand can be realized by hydrogen evolving chemical re-

actions of reactive metals andhydrogen sources such as water

and alcohol, etc. Among which, aluminumewater reaction is

identified as the most promising candidate [10,11]. Al is the

largestabundantmetalonearthcrust,anditislowcostandhas

stable storage quality. When Al is exposed in air, a denseoxide

layer (Al2O3) is formed on its surface rapidly due to its active

chemical property, which protects the Al from further oxida-

tion. However, this passivation layer also hinders the hydro-

lysis reaction of Al, which is unfortunately unfavorable for the

hydrogen production. Such problem can be solved by alloying

Al with certain low melting point elements such as Ga, Zn and

Bi, etc. which can activate Al and promote the hydrolysis re-action. By now, many important efforts have been made to

investigate the preparation and hydrogen generation perfor-

mance of such Al based alloys [9,12e17]. Gallium is considered

to be an excellent activator of Al to generate hydrogen in

normal conditions. Its activation effect is attributed to the Ga

induced destructionof theoxidelayeron theAl surface andthe

liquid metal embrittlement (LME) effect of Al caused by the

penetration ofGa into thegrainboundaryof thepolycrystalline

Al, which is well-known as the Rebinder effect [18e20].

In our latest work, a new conceptual liquid phase hydrogen

generation method was proposed [21], which is straightforward

and simple, and can work without evident time and energy

consuming alloying treatment process. In that work, a typicalroom temperature liquid metal, GaeIn alloy, was used for the

activation of the reaction. The dynamic hydrogen generation

phenomenon of aluminum fed liquid phase GaeIn alloy inside

NaOH electrolyte, which is much different from the conven-

tional hydrogen evolution process of solid Al rich alloys, was

experimentally demonstrated there. As a continuing work to

further improve the technology, this paper is dedicated to

explore the influence of different container substrates on the

hydrogen production events. We were surprised to find that,

when put on a metal substrate, such as stainless steel, the

hydrogen production process of Alewater reaction will be

significantly enhanced compared with the former process

which was carried out on a glass petri dish. More importantly,the hydrogen evolution rate in this work was promoted to 2e3

times that of the previous onewhichis of practical significance.

In addition, the liquid metal and the stainless steel substrate

used here were not consumed during the hydrogen production

process, just like catalysis. All these features are beneficial for

the future extensive applications of such a straightforward, low

cost and real-time liquid phase hydrogen production method.

Principles and methods

The Alewater reaction can occur both in deionized water and

aqueous solution, such as NaCl or NaOH solution, etc. Since

NaOH performances best, here, we choose 0.5 mol/L NaOH

solution for conceptual demonstrations, and similar results

can be obtained using deionized water or NaCl solution. It is

worthy to note that, even there is no liquid metal, the

Alewater reaction can also happen under assistance of al-

kalis, because that the alkaline solution will destroy the

passivation layer on the Al surface which hinders the reaction.

However, when in deionized water or NaCl solution, nothing can react with the oxide film layer, thus it is critical for the

reaction in this situation to deposit liquid metal for the acti-

vation of Al. A series of reactions will occur in the Alewater

reaction with assistance of alkalis, as given by:

2Al þ 6H2O þ 2NaOH/2NaAlðOHÞ4 þ 3H2 (1)

NaAlðOHÞ4/NaOH þ AlðOHÞ3 (2)

2Al þ 6H2O/2AlðOHÞ3 þ 3H2 (3)

where, reactions (1) and (2) indicate the intermediately

involved hydrogen evolution processes and Eq. (3) is an overall

expression of such processes. It can be seen that, essentially,

only Al and water are consumed in the whole reaction. The

hydrogen evolution procedure of aluminumewater reaction is

dependent on many factors such as temperature, the con-

centration of the aqueous solution, the composition, ratio and

size of the alloy, etc. Here, our focus is on exploring the metal

substrate enhanced hydrogen evolution reaction of Al fed

liquid metal and the mechanism lying behind, thus the factors

aforementioned will not be investigated here.

A stainless steel dish was chosen as the conductive sub-

strate, and was filled with 0.5 mol/L NaOH solution inside.

1 mL liquid metal (GaIn10, 90% Gallium and 10% Indium by

weight, with a melting point of about 16 C) was dropped into

the solution, and a slide of Al foil (0.03 g) was immersed in the

solution and contacted with the liquid metal. A soap film

flowmeter was used to measure the real-time hydrogen yield

and a camera (Sony Cyber-shot Dsc-W690, Japan) was

configured to record the dynamic hydrogen generation pro-

cess. Comparative experiments were conducted using a glass

petri dish as substrate. All those experiments were carried out

at room temperature (about 20 C).

Results and discussions

Hydrogen evolution mode

Fig. 1 shows the different hydrogen evolution modes under

four different conditions, including the liquid metal activated

Alewater reaction on stainless steel substrate (Fig. 1(a))and on

glass substrate (Fig. 1(b)), and Al foil without liquid metal in

NaOH solution (Fig. 1(c)) and NaCl solution (Fig. 1(d)) for

comparison. When a slice of Al foil was put in contact with the

liquid metal droplet in aqueous solution, it was tightly

attracted by the liquid metal and then embrittled into small

grains, caused by the LME effect, and finally swallowed into

the liquid metal droplet. The hydrogen evolution process of Al

fed liquid metal can be divided into two main sub-processes,

namely the embrittlement and swallow process at the


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beginning and the subsequent escape process. Here, “escape”

means that the embrittled tiny Al grains escape out from the

Al-liquid metal amalgam to its surface. The differenthydrogen evolution modes induced by the two different sub-

strates mainly occurred in the later process, namely the

escape process, and our attention will be focused on that

process.

On the glass substrate, there were two hydrogen evolution

forms. Some hydrogen was generated on the almost homo-

genously scattered sites on the bottom of the liquid metal, and

some hydrogen was generated on the side surface of the liquid

metal through the aggregated Al grains, in Fig. 1(b). From the

hydrogen evolution yield viewpoint, the second form was

dominant. The generated tiny bubbles beneath the liquid

metal would merge to form a bigger bubble. And finally the

bigger bubble was released from the edge of the liquid metalbottom. At the same time, aqueous solution compensated into

the bottom space, thus the reaction happened there was kept

running. The aggregated Al grains on the side surface reacted

violently with the surrounding solution and intensive tiny

bubbles were generated there.

When the substrate was replaced with stainless steel,

different hydrogen evolution mode was observed, in Fig. 1(a).

Intensive tiny hydrogen bubbles were ejected from the whole

edge of the liquid metal bottom, and big bubbles covered on

the whole upper surface of the liquid metal. The big bubbles

desorbed from the liquid metal surface due to buoyancy force

and then were quickly compensated by newly generated

bubbles. In addition, vertical oscillation of the liquid metal

was observed, which did not occur on the glass substrate. The

oscillation was not very regular and the frequency was about

2e

5 Hz. This oscillation, in some degree, promoted thedesorption of the hydrogen bubbles and the compensation of

solution beneath the liquid metal, thus accelerated the

hydrogen evolution process. In addition, the intensive

hydrogen ejection and liquid metal oscillation enhanced the

solution flow and take the reaction byproduct Al(OH)3 away.

This is beneficial to the efficient and continuous hydrogen

evolution process.

As controls, Fig. 1(c), (d) show the hydrogen evolution

modes of Al foil without liquid metal in NaOH and NaCl so-

lutions, respectively. In NaOH solution, tiny hydrogen bubbles

were generated on the whole surface of the Al foil after its

surface oxide layer was destroyed by NaOH. While in NaCl

solution, no hydrogen bubble was generated due to the pro-tection of the surface passivation layer.

Hydrogen evolution rate

Comprehensive experiments on the hydrogen yields in about

18 min under the aforementioned four conditions were per-

formed with typical outputs presented in Fig. 2. For compari-

son purpose, the mass of Al foil used for all of the 4 conditions

are kept the same, namely 0.03 g. In addition, the concentra-

tion of the solution is a constant of 0.5 mol/L and the volume

of liquid metal is a constant of 1 mL. The shadowed area on

the left part of the picture presents the embrittlement and

swallow process, while the rest presents the escape process. It

Fig. 1 e Hydrogen evolution modes under 4 different conditions. a) Al fed liquid metal (0.03 g Al, 1 mL GaIn10 ) in 0.5 mol/L

NaOH solution, on stainless steel substrate; b) Al fed liquid metal in 0.5 mol/L NaOH solution, on glass substrate; c) Al foil

without liquid metal in 0.5 mol/L NaOH solution, on stainless steel substrate; d) Al foil without liquid metal in 0.5 mol/L NaCl

solution, on stainless steel substrate.

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can be seen that, the most intensive hydrogen evolution re-action happened in the embrittlement stage, and the

hydrogen generation performance on both substrates in this

processhad almost no difference. While in the escape process,

the stainless steel substrate showed significant superiority,

due to its long lasting intensive reaction, as shown in Fig. 1(a).

About 20 min later, the hydrogen evolution reaction on the

stainless substrate was almost completed, while the reaction

on the glass lasted about 1 h. It is clearthat, on a stainless steel

substrate, the Al fed liquid metal displays the best hydrogen

production performance.

Enhancement mechanism discussion

To clarify the reason why the hydrogen generation rate was

enhanced on the stainless steel substrate, it is necessary at

first to reveal the basic hydrogen evolution mechanism on the

glass substrate. According to our former experimental results

[22,23], after the embrittlement and swallow process, the

embrittled tiny Al grains can be roughly considered uniformly

dispersed in the whole Al-liquid metal amalgam, as shown in

Fig. 3(a). As the density of Al is about two times lower than

that of the liquid metal, some Al grains, which have larger

volume and thus larger buoyancy, float up to the amalgam

surface due to buoyancy force, while some very tiny grains

will not float up due to the viscous drag. Since the Al grain is

mechanically instable on the quasi-atomically smooth arcsurface of the liquid metal [24], it slips down to the side sur-

face.The escaped Al grain is still tightlyabsorbed by theliquid

metal and will react with the surrounding solution, and the

surface tension of the amalgam there will become larger,

which is caused by the electrochemical reaction induced

charge variation of the surface electric double layer (EDL).

More details can refer to our previous work [25]. The Al grain

site where has the largest surface tension pulls surrounding

surface liquid to flow there, which is called Marangoni flow,

and can be easily observed in the experiment. To compensate

the pulled away surface liquid, the inner liquid will flow out,

and a flow loop is thus formed, as shown in Fig. 3(b). The Al

grains inside the amalgam are carried out by the dual driving

force from the flowing liquid metal and the buoyancy, and

then aggregate at the side surface of the amalgam where the

surface tension is the largest, as shown in the experimental

result in Fig. 1(b) and in Fig. 3(b), respectively. The aggregated

Al grains released hydrogen violently both through direct

hydrolysis reaction with the surrounding solution and the Al/

liquid metal bipolar electrochemical reaction. At the same

time, some of the larger grains are embrittled into smallergrains and swallowed by the liquid metal once again, and then

participate in the loop flow, until all of the Al grains are hy-

drolyzed. It is worthy to note that, the situation shown in

Fig. 3(a) in fact hardly exists, for that in the embrittlement

process, Marangoni flow occurs, and the situation in Fig.3(b) is

already present.

It can be concluded from the aforementioned analysis that

the buoyancy force and the Marangoni flow are the main

reason that incessantly provides the Al grains for hydrogen

evolution reaction, accelerate the Al grains supply rate will

lead to the enhancement of the hydrogen generation rate.

Bearing this in mind and based on the experimental obser-

vation, it is presumed that, on the stainless steel substrate,tremendous Al grains were attracted and quickly aggregated

at the bottom of the amalgam, thus violent hydrogen evolu-

tion reaction happened there, as shown in Fig. 3(c). This

assumption can be easily verified by a simple and intuitive

demonstrative experiment, in Fig. 3(d). When a stainless steel

bar was inserted into the amalgam, rapidly, tremendous tiny

Al grains aggregated near the bar, and significant hydrogen

evolution reaction happened both on the bar and on the Al

grains surface. The stainless steel induced attraction effect

can be explained by the bipolar electrochemical reaction

happened there. Al acted as anode: Al þ 3OH/AlðOHÞ3 þ 3e,

and the stainless steel bar acted as cathode:

3H2O þ 3e/3=2H2 þ 3OH. The mutual attraction betweentwo opposite electrodes promotes the Al grains to move to the

bar.

The same analysis can be applied to the situation where

the reaction happens on a stainless steel substrate. Different

from the situation on the glass substrate, where the Al grains

aggregate on certain side of the amalgam, the stainless steel

substrate attracts all of the escaped Al grains to the bottom of

the amalgam. In this situation, the escaped Al grains are not

passively draggedby the surface flow, but actively and quickly

move towards the substrate. It will not retard the surface flow

as that of the situation on the glass substrate, but in turn

accelerate the surface flow. Since the surface liquid flows

faster, the compensation from the inner liquid acceleratesaccordingly, and a more efficient Al grain supply loop flow is

thus formed, and the enhancement effect is obtained.

Effect of surface roughness

Another factor that may cause varied hydrogen generation

performances on both substrates is the surface roughness. To

rule out this potential influence factor that may induce the

different hydrogen revolution modes aforementioned, addi-

tional experiments were conducted. The glass petri dish was

scratched by a 200 mesh sand paper (M200, which means that

the diameter of the sand is about 74 mm) to get a rougher

surface, and similar hydrogen production experiments as

Fig. 2 e Hydrogen yield curves over time under 4 different

conditions.



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stated before were carried out. The result showed that, on the

rough glass substrate, the hydrogen evolution mode was the

same as that of the former smooth one, and the hydrogen

generated on the side surface of the liquid metal was still

dominant, almost no hydrogen generation enhancement ef-

fect was achieved.

Although the surface roughness of the substrate cannot

induce the change of hydrogen evolution mode, it does can

affect the hydrogen revolution process beneath the liquid

metal, and this effect is not significant for the reaction on

glass substrate. However, on stainless steel, where the

hydrogen evolution reaction mainly occurs on the bottom of

the liquid metal, this effect cannot be neglected. Fig. 4 shows

the hydrogen yield curves on stainless substrate with

different surface roughness. The substrates were scratched by

M200 and M80 sand papers, respectively. In addition, a sub-

strate without surface treatment was set as control group.

Here, only the corresponding hydrogen yields in the escape

process were measured and presented. It can be seen that, a

rougher surface can promote the hydrogen evolution process,

which may be because that such rough surface facilitated the

compensation of solution beneath the liquid metal and pro-

vided tremendous nucleation sites for the hydrogen evolution

reaction.

Different conductive substrates

Further, copper and graphite plates, had also been investi-

gated as substrates. When a drop of liquid metal was placed

on the copper substrate, it dissolved some copper and its

surface tension was deduced, thus caused the liquid metal to

spread out and adhere on the copper substrate [26]. Different

from the hydrogen evolution mode on stainless steel sub-

strate, hydrogen bubbles were mainly generated on the whole

copper surface. However, the corrosion of copper by liquid

metal will pollute this catalyst, especially in alkaline solution

Fig. 3 e Schematic diagrams for illustrating the different hydrogen evolution mechanisms on glass/stainless steel

substrates. a) Section view of the Al-liquid metal amalgam after the embrittlement and swallow process; b) Marangoni flow

inside and on the surface of the amalgam on glass substrate; c) On the stainless steel substrate; d) Stainless steel bar

induced Al grains aggregation on the liquid metal surface near the bar and corresponding intensive hydrogen evolution

reaction happened both on the Al grains surface and the bar surface.

Fig. 4 e Hydrogen yield curves on stainless substrate with

different surface roughness. The substrates were scratched

by M200 and M80 sand paper respectively, and a substrate

without surface treatment was set as control.



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and high temperature situation, thus it is unsuitable to select

copper as the substrate for future practices.

As for graphite substrate, similar hydrogen evolution mode

as that on stainless steel was observed, while inferior

hydrogen generation performancewas observed. To conclude,

some prerequisites must be satisfied when selecting the

conductive substrate material. The substrate should be

conductive. Further, it should have high chemical stabilityand do not react with the aqueous solution, and cannot be

corroded by liquid metal. In addition, the substrate material

should be readily available, easy to fabricate, and clearly low

cost. Taking all those requirements into consideration,

stainless steel is an excellent candidate.

It is worthy to note that, the liquid metal GaIn10 is essen-

tially used for destroying the oxide film layer on the Al surface

and embrittling Al into tiny grains, thus activating the

Alewater reaction and increasing the reaction area. The metal

substrate used here is only for promoting the escape of the Al

grains from the liquid metal-Al amalgam, and thus acceler-

ating the hydrogen evolution process. Both the liquid metal

and the metal substrate can be seen as the cathode of thebipolar electrochemical reaction, and are not consumed in the

whole reaction processes, which can be verified by the

composition analysis of the after-reaction aqueous solution

examined by Inductively Coupled Plasma Emission Spec-

trometer (ICP, Varian 710-ES,USA). 2.5 mg/mL Ga and 0.2 mg/mL

Fe was measured, with total volume about 50 mL, which

means that 1.8 mmol Ga and 0.18 mmol Fe were tested, far less

than the amount of the reactant Al 1.11 mmol. The tested Ga

and Fe are thought to be due to dissolution by the solution,

similar to the situation in Tang 's work [27], thus it is believed

that both the liquid metal and the substratedid not participate

in the hydrogen generation reaction.

Last but not least, at the end of the reaction happened onstainless steel substrate, brownish-black chemical substance

was generated on the liquid metal surface, which might be

gallium oxide. This oxide layer can be easily removed by

transferring the liquid metal into NaOH solution on a glass/

plastic substrate. To avoid such oxidation reaction, one can

transfer the liquid metal out from the container with stainless

steel substrate to one with glass substrate before the end of

the hydrogen evolution reaction, thus ensuring that the liquid

metal is not polluted and can be used repeatedly, well

resembling the same functions that “catalyst” can achieve. On

graphite substrate, no such oxidation reaction will occur, thus

a compromise must be made when choosing stainless steel or

graphite as the substrate.

Conclusion

Liquid metal can easily destroy the oxide film layer of Al and

embrittle it into tiny grains at room temperature, thus activate

the Alewater reaction and increase the reaction area. In such

a liquid phase hydrogen production method, the reaction

process is much different from that of the conventional solid

Al rich alloy hydrogen production method. To better utilize

the strategy, conductive materials, typically stainless steel,

was found to be able to evidently enhance the hydrogen

evolution reaction of the Al fed liquid phase Gae

In alloy inside

aqueous solution. According to our experimental in-

vestigations, such an enhancement effect is thought to be

achieved by the attraction of the substrate and the Al grains,

which accelerates the Marangoni flow of the amalgam and

thus accelerates the Al grains supply rate. Stainless steel is a

good choice to be the conductive substrate, due to its high

chemical stability and low cost. Rough substrate surface

cannot change the hydrogen evolution mode, but areconductive to the hydrogen production rate, especially on the

stainless substrate, where the hydrogen evolution reaction

mainly occurs on the bottom of the liquid metal. Overall, the

present metal enhanced liquid phase hydrogen generation

method could be possibly used for straightforward real-time

and on-demand hydrogen production and utilization, espe-

cially in situations where water or salt water is readily avail-

able, thus only bulk Al and a small amount of GaeIn alloy are

needed.

Acknowledgment

This work is partially supported by the Dean's Research

Funding of the ChineseAcademy of Sciences. Great thanks are

given to Mr. Si-cong Tan, Mr. Yu-jie Ding and Ms. Lu Tian in

the lab for their help in the experiments.

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