CHIN. PHYS. LETT. Vol. 30, No. 6 (2013) 065204
Hydrogen Generation from the Dissociation of Water Using Microwave Plasmas
Yong Ho Jung** , Soo Ouk Jang, Hyun Jong You
Plasma Technology Research Center, National Fusion Research Institute, Jeonbuk 573-540, Korea
(Received 30 January 2013)
Hydrogen is produced by direct dissociation of water vapor, i.e., splitting water molecules by the electrons in
water plasma at low pressure (<10–50 Torr) using microwave plasma discharge. This condition generates a high
electron temperature, which facilitates the direct dissociation of water molecules. A microwave plasma source
is developed, utilizing the magnetron of a microwave oven and a TE10 rectangular waveguide. The quantity of
the generated hydrogen is measured using a residual gas analyzer. The electron density and temperature are
measured by a Langmuir probe, and the neutral temperature is calculated from the OH line intensity.
PACS: 52.50.Dg, 52.70.−m, 88.30.E−
DOI: 10.1088/0256-307X/30/6/065204
Hydrogen itself does not exist in nature, and can
only be obtained by applying extra energy to natural
resources through methods such as the reformation of
hydrocarbon gas or by the electrolysis of water.[1−5]
Hydrogen is not a natural energy resource, but has
the potential to serve as an energy carrier at the core
of carbon-based systems of energy production. Moreover, when energy resources such as oil, coal, and natural gas are depleted, hydrogen could serve as a primary source of energy. Further, it must be produced
from water if it is to play such a role as a key source
of alternative energy.
Hydrogen and oxygen dissociate if dielectric breakdown takes place in water. The most effective way to
transfer a higher energy density to water molecules is
to produce a plasma discharge in the water. In this
research, hydrogen is produced by plasma, which is
used in various industrial fields, such as semiconductor manufacturing processes, the surface treatment of
materials, and the removal of environmentally hazardous gases.[6] Plasma sources, devices that generate
plasma, have been investigated by many researchers
after Langmuir first coined the term plasma in the
1920s to describe ionized gas.[7] Thermal plasmas such
as dc or ac torches can reach temperatures close to
10000 K, and non-thermal plasmas, such as corona
discharges and dielectric barrier discharges,[6] have attempted to produce hydrogen from natural gas or water in an atmospheric pressure environment.[8−10] The
Commissariat à l’Energie Atomique (CEA) has reported the production of hydrogen by thermal plasma
water splitting (dc torch) with a 1 g H2 /kWh energy yield.[11] In non-thermal plasmas, such as dielectric barrier discharges,[12] pulsed corona discharges,[13]
and micro-hollow cathode discharges,[14] the energy
yields for hydrogen production from water are 0.0016,
0.25, 1.4 g H2 /kWh, respectively.
The hydrogen produced in this study is by direct
dissociation, i.e., the cracking of water vapor at low
pressure (<10–50 Torr) using a microwave plasma discharge without an inert gas[15] such as argon or molecular gases,[16] e.g., nitrogen. This is generated mainly
from the physical reaction of electrons from the water
plasma.[17] According to CEA calculations, between
1500 K and 9000 K, the thermal dissociation of water begins significantly only around 3000 K, to produce the radicals and molecules OH, H2 , O2 , H and
O. From 4500 K, all the chemical bonds are broken
and only radicals H and O are found.[11] The bonding energy between hydrogen and oxygen, which are
the constituent elements of water, is 5 eV.[18] The water discharge plasma produced by 2.45 GHz microwave
power below 10–50 Torr pressure is characterized by a
higher electron temperature than that of the other discharge methods, such as lower frequency rf discharges
and dc discharges. It is difficult for water to be split
(direct dissociation) by atmospheric pressure thermal
plasma since the high electron collision rate at above
10–50 Torr pressure causes a very strong decrease in
the electron temperature due to the high radiation
pumping (or radiative cooling), which is the photon
loss from the excited atoms and molecules produced
by collisions with high energy electrons. Namely, if
there are too many atoms and molecules that collide
with electrons, the electrons will not be able to ionize them or dissociate the molecules, only excite the
atoms and molecules. Therefore, to maintain a high
dissociation rate for water, the plasma should be produced in an environment with a pressure of less than
10–50 Torr, and should not become thermal,[19,20] as
the electrons will not have enough energy to break up
the water.
Microwave plasma discharge is a well-known technology. Its basic principle is to induce a strong microwave electric field, an electromagnetic wave, in
the discharge space and to ionize the target gas.
** Corresponding author. Email: yhjung@nfri.re.kr
© 2013 Chinese Physical Society and IOP Publishing Ltd
065204-1
CHIN. PHYS. LETT. Vol. 30, No. 6 (2013) 065204
Microwave power can provide various discharges,
from glow discharges to arcs at high density (1010 –
1016 cm−3 ), and maintain a neutral temperature from
300 K to 10000 K. Moreover, a typical microwave
plasma has a higher electron temperature (5–15 eV)
than that of the plasma generated by a lower frequency rf discharge or a dc discharge, and has a
broad pressure range (from µTorr to atmospheric
pressure).[6,21] The high electron temperature and low
operating pressure of microwave discharges are advantages that allow both high ionization and dissociation
rates.
by the RGA with the gas that is actually collected in a
gas cylinder. Table 1 shows the microwave plasma parameters, such as discharge pressure, neutral temperature, electron density, electron temperature, plasma
length, and dissociation rate. From Table 1, the dissociation rate is 20–40% at a vapor flow rate of 1 lpm and
a discharge power of 500–900 W. The energy yield for
hydrogen production is 1.8–2.7 g H2 /kWh according to
the microwave discharge tube type (single, double and
hollow tube type).
30000
OH (309 nm):
Line intensity (Counts)
Power 500 W
Coolant out
Microwave
pressure (104 Torr)
Coolant in
60
H2O
H2O vapor
40
20
0
0
Secondary
pumping
for RGA
H2
O2
10
20
30
MASS (amu)
Power 700 W
20000
H
Power 900 W
(656 nm):
Power 500 W
Power 700 W
15000
Power 900 W
10000
5000
0
Valve
40
40
Analysis
50
60
70
80
90
100
110
Distance to the center of discharge tube (mm)
Residual
Gas
Analyzer
(RGA)
Fig. 2. The measured OH (309 nm) and H𝛼 (656 nm) line
intensities at a discharge power of 500–900 W.
Cooling
fan
Magnetron
(2.45 GHz, ~ 2 kW) Power
usually used for
meter
microwave oven
25000
Production
Stub
tuner
Water
vapor
supply
Water
vapor
line
Plunger
Fig. 1. A device to generate hydrogen using microwave
plasmas. Water vapor flows into the discharge space, i.e.,
a quartz tube, in which the strong electric field due to the
tapered waveguide is applied and the water molecules are
dissociated by electrons from the water (H2 O) plasma.
We developed a simple microwave plasma source
using the magnetron of a kitchen microwave oven and
a rectangular TE10 waveguide. The magnetron used
provides up to 2 kW power at a frequency of 2.45 GHz.
The discharge tube is 10 mm in diameter and 450 mm
in length. Figure 1 shows the microwave plasma
source based on the magnetron, waveguide, tuners,
and discharge tube. The pressure in the device could
be varied from 500 mTorr to 20 Torr (by a diaphragm
gauge), and the flow rate of the vapor could be adjusted from 0.1 lpm to 2 lpm. The dissociation rate is
defined as the ratio of the quantity of hydrogen generated to the quantity of water vapor supplied. The
quantity of the produced hydrogen is measured by a
residual gas analyzer (RGA, Transpector2, Inficon),
which consists of an ion source, a mass spectrometer,
and a measurement section. In order to determine the
dissociation rate in the experimental measurements of
the hydrogen quantity, we compare the gas measured
A Langmuir probe can be used to measure the
plasma density, electron temperature, and potential
by analyzing the current collected by the probe for a
specific applied voltage. The plasma density and electron temperature is calculated from Eqs. (1) and (2)
in the presence of collisions,[22]
√︂(︁
)︁−1
1
8𝑘𝑇e )︁(︁ 3 𝑎 𝑎
+𝐾
𝐼es = 𝑒𝐴p 𝑛
,
4
𝜋𝑚e
4𝜆𝑎+𝜆
(1)
−𝑒
ln(|𝐼e (𝑉 )|) =
(𝑉p − 𝑉 ) + ln(|𝐼es |),
(2)
𝑘𝑇e
where 𝐼es is the electron saturation current, 𝑒 the electron charge, 𝐴p the probe area, 𝑛 the plasma density,
𝑘 the Boltzmann constant, 𝑇e the electron temperature, 𝑚e the ion mass, 𝑎 the probe radius, 𝜆 the meanfree path for electron-neutral collisions, 𝐾 is 1/2, 𝐼e
the electron current to the probe, 𝑉p the plasma potential, and 𝑉 the probe bias voltage. In electronegative plasmas, 𝑛+ = 𝑛− + 𝑛e (𝑛, +, − and 𝑒 denote
the density, positive ion, negative ion and electron,
respectively) and the negative ion current can be neglected due to the small ratio of the electron mass
to the ion mass. Therefore, the negative saturation
current equals 𝐼es + 𝐼s− ≈ 𝐼es (𝑠 denotes saturation).
In this experiment, the electron density and temperature measured by a cylindrical Langmuir probe
(𝑎 = 0.5 mm), which is located 37.5 mm from the center of waveguide, are 3–22 × 1011 cm−3 and 3.5–6 eV,
respectively, at discharge pressures of 1–10 Torr, (Table 1). As the pressure increases, the fraction of the
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CHIN. PHYS. LETT. Vol. 30, No. 6 (2013) 065204
negative ions (𝑛− /𝑛e ) decreases (from the ratio of the
negative saturation current to the positive saturation
current). This means that the production probability
of negative ions by the dissociation of water molecules
(by electron attachment) is decreased.
Table 1. The microwave plasma parameters used to generate hydrogen. The plasma length is estimated by the location
where the OH line light intensity goes to zero.
Power (W)
Vapor flow (lpm)
Discharge pressure (Torr)
Rotational temperature of OH (K)
Electron density (1011 cm−3 )
Ratio of negative saturation current
to positive saturation current
Electron temperature (eV)
Plasma length (mm)
Dissociation rate (%)
Energy yield of 1g H2 /kWh
500
1
2.4
1501
3.2
700
1
3.4
1565
9.7
900
1
3.6
1643
22
2.7
5.6
65
24
2.1
4.3
3.9
75
32
2.1
12.2
3.7
83
38
2.1
The rotational temperature is calculated from the
OH line intensity.[23−25] Measurements of the intensity
𝐼nm of the OH bands for the temperature 𝑇 * = 3000 K
are reported by Dieke and Crosswhite as follows:[23−25]
*
𝐼nm = 𝐼nm
exp
(︁ −𝐸 (𝑇 * − 𝑇 ) )︁ 𝑄(𝑇 * )
n
,
𝑇𝑇*
𝑄(𝑇 )
(3)
where 𝐸𝑛 is the statistical weight and 𝑄(𝑇 ) is the
partition function. In experimental spectroscopy, it
is necessary to measure the apparatus function either
by employing a laser line or by employing sharp lines
emitted by a spectral lamp. The apparatus function
can be well fitted with a Gaussian profile as a function
of the wavelength 𝜆:[23−25]
𝐺(𝜆) =
(︁ (𝜆 − 𝜆 )2 )︁
2
0
√ exp −
,
(Δ/2)2
Δ 𝜋
(4)
where Δ represents the full width at 1/𝑒 of the maximum located at the wavelength 𝜆0 . In our experiment, the apparatus function is measured from a
mercury lamp at 𝜆0 = 296 nm[23] using a spectrometer (HR4000, Ocean Optics Inc.) and we obtain
Δ = 0.12 nm from the fitted data of the apparatus
function. For a practical determination of the temperature from an experimental OH intensity (306–
310 nm), we can select two groups of unresolved rotational lines, 𝐺0 and 𝐺1 , the amplitudes of which are
very sensitive to the rotational temperature. The typical evolution of the maximum amplitude of 𝐺0 /𝐺ref
and 𝐺1 /𝐺ref as a function of the rotational temperature is given in the results of the numerical simulations for the apparatus function width. The rotational temperature of the OH radical has a value which
in a thermal equilibrium state is close to the neutral
temperature.[23−25] For low pressure plasmas, which
are not in a thermal equilibrium state, the rotational
temperature of OH can be used to estimate the gas
temperature: the initial rotational population distribution is non-thermalized, while the lifetime of OH is
longer than the time needed for the collisional relaxation to the thermalized state, for example a high rate
of OH quenching by H2 O.[27−29] In this experiment, a
ratio 𝐺0 (306.42 nm)/𝐺ref (309.04 nm) for the discharge
power ranges of 500, 700 and 900 W are 0.405, 0.426
and 0.451, respectively. At Δ = 0.12 nm, the rotational temperature calculated by using the OH line
intensity is 1500–1650 K (Table 1).[23]
Figure 2 shows the measured OH (309 nm) and H𝛼
(656 nm) line intensities. In order to determine the
plasma length by measuring the line intensities, we
compare the decrease in the line intensity versus distance with the decrease in the probe ion current versus
distance. In Table 1, the plasma lengths in the range
65–85 mm for a discharge power range of 500–900 W,
are estimated by the location where the OH line light
intensity goes to zero, which is also the location where
the probe ion current goes to zero.
In conclusion, the technology for producing hydrogen from water through the direct dissociation of water molecules appears to be a microwave plasma discharge at low pressure (<10–50 Torr) in water vapor.
This generates a high electron temperature, which
makes the direct dissociation of water possible. In this
case, higher electron temperature raises the fraction
of high energy electrons above 5 eV (where 5 eV is the
bonding energy between hydrogen and oxygen).[18] It
is difficult for water to be split (direct dissociation) by
an atmospheric pressure thermal plasma since a high
electron collision rate above a pressure of 10–50 Torr
causes a very strong decrease in electron temperature
due to radiative cooling, which is photon loss from the
excited atoms and molecules produced by collisions
with high energy electrons.
References
[1] Yurum Y 1994 Proceedings of the NATO Advanced Study
Institute on Hydrogen Energy System, Utilization of Hydrogen and Future Aspects (Turkey)
[2] Springmann S, Friedrich G, Himmen M, Sommer M and
Eigenberger G 2002 Appl. Catal. A 235 101
[3] Deluga G A, Salge J R, Schmidt L D and Verykios X E
2004 Science 303 993
[4] D O E 2004 The Hydrogen Economy: Opportunities,
Costs, Barriers and RD Needs (Washington DC: National
Academy Press)
[5] Nakajima H, Sakurai M, Ikenoya K, Hwang G J, Onuki K
and Simizu S 1999 The 7th International Conference on
Nuclear Engineering (Japan)
[6] Roth J R 1995 Industrial Plasma Engineering (Bristol and
Philadelphia: Institute of Physics Publishing)
[7] Langmuir I and Mott-Smith H M 1923 Gen. Electr. Rev.
26 731
[8] Mutaf-Yardimci O, Saveliev A V, Fridman A A and
Kennedy L A 1998 Int. J. Hydrogen Energy 23 1109
[9] Deminsky M, Jivotov V, Potapkin B and Rusanov V 2002
Pure Appl. Chem. 74 423
065204-3
CHIN. PHYS. LETT. Vol. 30, No. 6 (2013) 065204
[10] Thanyachotpaiboon K, Chavadej S, Caldwell T A, Lobban
L L and Mallinson R G 1998 AICHE J. 44 2252
[11] Boudesocque N, Vandensteendam C, Lafon C, Girold C and
Baronnet J M 2006 The 16th World Hydrogen Energy Conference (France)
[12] Kabashima H, Einaga H and Futamura S 2001 Chem. Lett.
30 1314
[13] Kirkpatrick M J and Locke B R 2005 Indust. Eng. Chem.
Res. 44 4243
[14] Koo I G, Choi M Y, Kim J H, Cho J H and Lee W M 2008
Jpn. J. Appl. Phys. 47 4705
[15] Sekiguchi H and Mori Y 2003 Thin Solid Films 435 44
[16] Chen X, Suib S L, Hayashi Y and Matsumoto H 2001 J.
Catal. 201 198
[17] Hueso J L, Rico V J, Cotrino J, Mateos J M J and Elipe A
R G 2009 Environ. Sci. Technol. 43 2557
[18] Itikawa Y and Mason N 2005 J. Phys. Chem. Ref. Data 34
1
[19] Lin T L 2001 Proc. 2001 International Workshop on
Plasma Processing for Nuclear Applications (Korea)
[20] Jung Y H and Chung K S 2002 J. Korean Phys. Soc. 40
856
[21] Moisan M and Pelletier F 1992 Microwave Excited Plasmas
(London: Elsevier)
[22] Chen F F 1965 Electric Probes in Plasma Diagnostic Techniques (New York: Academic Press)
[23] Izarra C 2000 J. Phys. D: Appl. Phys. 33 1697
[24] Pellerin S, Cormiery J M, Richardy F, Musiolz K and
Chapelle J 1996 J. Phys. D: Appl. Phys. 29 726
[25] Diecke G H and Crosswhite H M 1962 J. Quant. Spectrosc.
Radiat. Transfer 2 97
[26] Craig J S, Salit M L and Reader J 1996 Appl. Opt. 35 74
[27] Raud J, Laan M and Jogi I 2011 J. Phys. D: Appl. Phys.
44 345201
[28] Bruggemann P, Schram D C, Kong M G and Leys C 2009
Plasma Process Polym. 6 751
[29] Verreycken T, Schram D C, Leys C and Bruggemann P 2010
Plasma Sources Sci. Technol. 19 045004
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