International Journal of Hydrogen Energy 28 (2003) 35 – 41

www.elsevier.com/locate/ijhydene

Existence of optimum space between electrodes on hydrogen

production by water electrolysis
N. Nagaia; ∗ , M. Takeuchia , T. Kimurab , T. Okaa
a Department of Mechanical Engineering, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan
b Advanced Fibro-Science in Graduate School, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

Abstract
The e/ect of bubbles between electrodes on e0ciency of hydrogen production by water electrolysis was experimentally
investigated. The water electrolysis of 10 wt% potassium hydroxide aqueous solution was conducted under atmospheric
pressure using Ni–Cr–Fe alloy as electrodes. In order to examine void fraction between electrodes, the following parameters
were controlled: current density, with or without separator, system temperature, space, height, inclination angle and surface
wettability of electrodes. The e0ciency of water electrolysis was qualitatively evaluated by the voltage drop value at a certain
current density. The experimental results showed that increase of void fraction between electrodes by decreasing the electrode
space brought about decrease of the electrolysis e0ciency; i.e. there is an optimum condition of water electrolysis at a certain
current density. In addition, a physical model of void fraction between electrodes was presented, which was found to represent
a part of the qualitative tendency of experimental results. ? 2002 International Association for Hydrogen Energy. Published
by Elsevier Science Ltd. All rights reserved.

Keywords: Hydrogen energy; Water electrolysis; Bubble; Void fraction

1. Introduction solution as shown in Fig. 1 [3]. On purpose to realize good

e0ciency of water electrolysis, many researches have been
Water electrolysis is a very important technology for a conducted so far, mainly focused on decrease of reversible
large scale of hydrogen production. Hydrogen energy is potential and overvoltage by realizing water electrolysis
expected to be useful as secondary energy in the near under high temperature and pressure or developing
future (for example, [1,2]), applicable to fuel for vehicle and new electrode materials [4]. However, little attention has
rocket, chemical use, Ni–H2 electric cell, thermal engine been paid to ohmic loss in aqueous solution from hydrody-
using hydrogen storage alloys, direct combustion for heat, namic and two-phase Gow point of view. LeRoy et al. [3]
and so on. In addition, hydrogen energy can be used to build pointed out that the increase of volume fraction of hydrogen
up dispersive energy system together with electric power by or oxygen bubbles between electrodes, i.e. increase of void
using water electrolysis and fuel cell. In such an energy sys- fraction, would cause the increase of electric resistance in
tem, water electrolysis will become a key technology, and aqueous solution, resulting in e0ciency decrease of water
high performance of water electrolysis should be achieved. electrolysis. Funk and Thorpe [5] presented an analytical
The voltage needed to realize water electrolysis consists model of void fraction and current density distributions be-
◦
largely of reversible potential (=1:23 V at 1 atm; 25 C), tween electrodes, from view point of two phase Gow. Hine
overvoltage on electrodes, and ohmic loss in aqueous and Sugimoto [6] obtained detailed information on void
fraction, rising velocity and diameter distributions of bub-
bles. Bongenaar-Schlenter et al. [7] measured void fraction
∗ Corresponding author. Tel.: +81-776-27-8537; and current density distributions, and proposed a “bubble
fax: +81-776-27-8748. di/usion model” for ohmic resistance between electrodes.
E-mail address: nagai@mech.fukui-u.ac.jp (N. Nagai). Janssen and Visser [8] also measured void fraction, ohmic

0360-3199/02/$ 22.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 0 2 7 - 7
36 N. Nagai et al. / International Journal of Hydrogen Energy 28 (2003) 35 – 41

there is an optimum condition of hydrogen production in

Nomenclature water electrolysis.
E voltage between electrodes, V
F Faraday constant (=9:65 × 104 , C=mol)
H height of electrode, m 2. Experimental apparatus and method
p system pressure, Pa
R universal gas constant, J=mol K The water electrolysis of 10 wt% KOH aqueous solution
T
◦
system temperature, C or K was conducted under atmospheric pressure using Ni–Cr–Fe
u rising velocity of bubbles, m=s alloy (Inconel 600) as electrodes. In order to vary void frac-
W width of electrode, m tion between electrodes, parameters as follows were con-
x coordinate, m trolled: current density, with or without separator, system
temperature, and space, height, inclination angle, surface
Greek letters wettability of electrodes. As easily postulated from hydro-
void fraction dynamic and two-phase Gow point of view, the increase of
 space between electrodes, m void fraction would occur by following conditions; increas-
 current density, A=m2 ing current density, with separator, higher temperature, nar-
rower space, larger height, horizontal setting of electrodes,
and higher wettability.
Fig. 2 shows outline of the experimental apparatus used.
Inside the liquid container (360 mm width × 200 mm
depth × 300 mm height) made of vinyl chloride, the elec-
trodes were completely immersed and Nxed in parallel with
a certain space. The height of electrodes was chosen to
one of 100; 50; 10 mm, while the width of electrodes was
Nxed to 50 mm. A separator was set at the middle position
between electrodes. The separator sheet tested was either
polyGon Nlter sheet of 0:4 mm thickness or without separa-
tor. DC power supplier enabled DC current up to 60 A and
DC voltage up to 6 V between electrodes: current density

1 2 3

13

12
4
5

Fig. 1. Components of voltage between electrodes [3].

resistance and current density. Recently, Riegel et al. [9]

examined bubble di/usion, convection and transportation
between electrodes in detail. These former works [3,5 –9] 11
9
were successful to generally explain the e/ects of bubbles
10
on water electrolysis e0ciency at a rather low current den- 8
sity or a rather large electrode space. It is easily postulated,
6 7
however, that there is an optimum electrode space under
high current densities; i.e. when the current density is rather 1. DC power supplier 8. Liquid vessel
high and the space is rather small, the void fraction between 2. Standard resistance 9. Aqueous solution
electrodes gets rather large resulting in increasing elec-
3. Voltmeter 10. Water vessel
tric resistance between electrodes, and then decreasing the
4. Separator 11. Water
e0ciency of water electrolysis.
In this report, therefore, the e/ect of bubbles between elec- 5. Electrode 12. H2 collector
trodes on e0ciency of water electrolysis was studied by con- 6. Cartridge heater 13. Stainless steel rod
ducting water electrolysis of potassium hydroxide (KOH) 7. Thermometer
aqueous solution in various experimental conditions. Espe-
cially, the authors’ attention was focused on whether or not Fig. 2. Outline of experimental apparatus.
 N. Nagai et al. / International Journal of Hydrogen Energy 28 (2003) 35 – 41 37

Table 1
Experimental conditions tested

Electrodes Material Ni–Cr–Fe alloy (Inconel 600)

Space  = 1–20 mm
Height H = 10; 50; 100 mm
Width W = 50 mm
Inclination Vertical or horizontal
Wettability Lower wettability with silicone or without treatment
Pressure Atmospheric
◦
Temperature T = 20; 40; 60 C
Current density  = 0:1–0:9 A=cm2
Separator PolyGon Nlter sheet of 0:5 mm thickness or without separator

ranged from 0.1 to 0:9 A=cm2 . Hydrogen gas generated was

collected to H2 collector bottle through water, while oxygen 6
gas was released to open air. The temperature of the KOH
◦ ◦ ◦
aqueous solution was controlled to 20 C, 40 C, or 60 C by
cartridge heaters. The inclination angle of electrodes was

Voltage E, V
either vertical or horizontal. Rotation of the whole liquid
container enabled horizontal setting of electrodes. The
4
surface of electrodes was polished after several experi-
ments to keep same overvoltage on electrodes. The surface
wettability was tested to either lower surface wettability
with silicone oil treatment or without treatment.
The e0ciency of hydrogen production by water electrol-
ysis was qualitatively evaluated and compared by the volt- 2 Current density
age value at a certain current density. Since the amount of Φ , A/cm2
Electrodes : 50x100mm 0.1 0.2
hydrogen gas is proportional to electric current, the volt- 0.3 0.4
Temperature : 20˚C
age value becomes good index to represent electric power Separator : Polyflon filter 0.5 0.6
necessary to produce a certain mass Gux of hydrogen when 0.7
compared among data of the same current density. The volt- 0
age between electrodes was measured by voltmeter, while 0 10 20
the DC current was estimated by measuring voltage drop of Space between electrodes δ, mm
standard resistance (=0:5 mP).
The experimental conditions are summarized in Table 1. Fig. 3. E/ects of current density and space between electrodes on
e0ciency.

3. Results and discussion without surface treatment. While current density was lower
( = 0:1–0:5 A=cm2 ), the voltage decreased as the space
3.1. E8ects of current density and space between became smaller. It is postulated from this tendency that the
electrodes on e9ciency of water electrolysis electric resistance between electrodes basically becomes
smaller as the space gets closer while void fraction is rather
In this section, the e/ects of current density and space small. However, when current density was rather high,
between electrodes on e0ciency of water electrolysis are beyond 0:6 A=cm2 , the voltage increased a little as the
discussed. As stated in the previous chapter, the voltage space got closer in the small-space region ( = 1–2 mm).
qualitatively represents electric power necessary to produce These results can be explained as follows; when the current
certain mass Gux of hydrogen. In other words, lower voltage density is rather high and the space is rather small, the
means higher e0ciency of water electrolysis. void fraction between electrodes gets rather large resulting
The experimental results show that current density in increasing electric resistance between electrodes, and
and space between electrodes have signiNcant e/ects on then decreasing the e0ciency of water electrolysis. It is
the e0ciency of water electrolysis. Fig. 3 illustrates the presumed, therefore, that there is an optimum space as to
relation between voltage, E(V ), and space between the e0ciency of water electrolysis and the optimum space
electrodes,  (mm), at the following condition as an depends on current density and other experimental condi-
example: height of electrodes, H = 100 mm, system tem- tions. In this case (Fig. 3), the optimum space is 1–2 mm
◦
perature T = 20 C, vertical setting, polyGon separator, and when the current density is over 0:5 A=cm2 .
38 N. Nagai et al. / International Journal of Hydrogen Energy 28 (2003) 35 – 41

6 6

4 4

Voltage E, V
Voltage E, V

Current density Current density

2 Φ, A/cm
2 2 Φ, A/cm2
0.1 0.2 0.1 0.2
Electrodes : 50x50mm 0.3 0.4 Electrodes : 50x10mm 0.3 0.4
Temperature : 20˚C 0.5 0.6 Temperature : 20˚C 0.5 0.6
Separator : Polyflon filter 0.7 0.8 Separator : Polyflon filter 0.7 0.8
0.9 0.9
0 0
0 10 20 0 10 20
Space between electrodes δ, mm Space between electrodes δ, mm

Fig. 4. E/ects of height of electrodes on e0ciency (H = 50 mm). Fig. 5. E/ects of height of electrodes on e0ciency (H = 10 mm).

3.2. E8ects of the other parameters on e9ciency of water

electrolysis 6

In this section, the e/ects of the other parameters on the

e0ciency of water electrolysis are discussed one after an-
other: (1) height of electrodes, (2) system temperature, (3)
with or without separator, (4) inclination of electrodes, and 4
Voltage E, V

(5) surface wettability. The experimental results here are

shown as the relation between voltage and space between
electrodes, as in Fig. 3.

3.2.1. Height of electrodes Current density

2 Φ, A/cm2
Fig. 4 and 5 illustrate the experimental result when the
height of electrodes, H , was 50; 10 mm, respectively, while 0.1 0.2
Electrodes : 50x100mm 0.3 0.4
the other parameters were the same with those of Fig. 3. Temperature : 60˚C 0.5 0.6
Comparing Figs. 3–5, it is found out that the higher ef- Separator : Polyflon filter 0.7 0.8
Nciency of water electrolysis arises in the smaller height 0.9
of electrodes when current density is rather large. This re- 0
0 10 20
sult can be explained as follows; the average void fraction
Space between electrodes δ, mm
between electrodes of larger height is bigger than that of
smaller height if the mass Gux of gas is uniform on both Fig. 6. E/ects of system temperature on e0ciency (T = 60 C).
◦

electrodes, because hydrogen and oxygen bubbles densely

pack at the upper part between electrodes.
◦ ◦ ◦
It is also worth noticing that there is no clear optimum for T = 40 C was just between T = 20 C and 60 C. As
space in the result of 10 mm height (Fig. 5). This means the seen from Figs. 3 and 6, the electrolysis e0ciency become
existence of optimum space depends on not only the current higher as the system temperature gets higher, especially in
density but also the height of electrodes. the region of smaller space between electrodes. This result
can be interpreted as follows; the higher system tempera-
3.2.2. System temperature ture causes the increase of bubble volume and the decrease
Fig. 6 shows the experimental result when the system of reversible potential as stated in Section 1. The increase
◦
temperature, T , was 60 C, while the other parameters were of bubble volume is related to both the direct increase of
the same with Fig. 3. The tendency of experimental results void fraction and the decrease of rising velocity of bubbles
 N. Nagai et al. / International Journal of Hydrogen Energy 28 (2003) 35 – 41 39

6 6

4 4
Voltage E, V

Voltage E, V
Current density
2 Φ, A/cm2 2 Current density
0.1 0.2 Φ, A/cm2
0.3 0.4 0.1 0.2
Temperature : 20˚C 0.5 0.6 Electrodes : 50x100mm
Temperature : 20˚C 0.3 0.4
Without separator 0.7 0.8 0.5 0.6
0.9 Without separator
Horizontal setting 0.7
0
0 10 20 0
0 10 20
Space between electrodes δ, mm Space between electrodes δ, mm
Fig. 7. E/ects of with or without separator. Fig. 8. E/ects of inclination of electrodes.

which results also in the increase of void fraction. Combin-

ing these two antithetical e/ects, the e0ciency is considered
to become higher as the temperature gets higher. 6

3.2.3. With or without separator

Fig. 7 shows the experimental result without separator
while the other parameters were the same with Fig. 3. As
shown in Figs. 3 and 7, the electrolysis e0ciency without 4
Voltage E, V

separator is higher than that of with separator. The existence

of separator obstructs rising movement of bubbles to cause
the increase of void fraction and at the same time increases
electric resistance between electrodes, thus resulting in the
decrease of e0ciency. It must be noted that the thickness 2
Current density
and material of separator may have e/ects on e0ciency, Φ, A/cm2
0.1 0.2
which was not examined in this experiment. Electrodes : 50x100mm 0.3 0.4
Temperature : 20˚C 0.5 0.6
3.2.4. Inclination of electrodes Without separator 0.7 0.8
Silicone oil treatment 0.9
Fig. 8 is the experimental result when the electrodes were
0
set horizontal while the other parameters were the same 0 10 20
with Fig. 7. As easily expected, the electrolysis e0ciency Space between electrodes δ, mm
of horizontal setting is lower than that of the vertical one
when the current density is rather high. In this case, the Fig. 9. E/ects of surface wettability.
exhaust of generated bubbles from between electrodes is
restrained at horizontal setting, which causes the decrease
of void fraction. mum electrode space is about 2 mm when the current den-
sity is over 0:7 A=cm2 . The silicone oil treatment makes the
3.2.5. Surface wettability surface of lower wettability, which may cause larger size of
Fig. 9 shows the experimental result with silicone oil bubbles to increase rising velocity of bubbles. On the other
treatment on electrodes while the other parameters were the hand, the silicone oil treatment may changes the overvoltage
same with Fig. 7. As seen from Figs. 7 and 9, the electrol- of electrodes, which was not estimated in this experiment.
ysis e0ciency with silicone oil treatment becomes higher Therefore, the e/ect of surface wettability on electrolysis
than that of without surface treatment. In this case, the opti- e0ciency has not been cleared yet, open to further study.
40 N. Nagai et al. / International Journal of Hydrogen Energy 28 (2003) 35 – 41

3.2.6. Summary of this section

As summary of this section, the e/ects of several param-
eters on e0ciency of water electrolysis discussed above can
be qualitatively explained by void fraction between elec-
trodes in addition to the e/ects of current density and space
between electrodes.

3.3. Modeling of void fraction between electrodes

The physical modeling of void fraction between elec-

trodes is discussed in this section. In most of industrial water
electrolyzers, the size of electrodes is much larger (∼1 m)
than that tested in this report and the system is operated
◦
at higher temperature (∼100 C), larger current density and
beyond atmospheric pressure. Therefore, in order to obtain
useful information from the experimental results, we need a
precise physical modeling of bubble diameter, rising veloc-
ity of bubbles, void fraction and current density, which can
be developed from several models presented so far [5,7,8].
However, two-phase Gow simulation for bubble Gow with
very small diameter of bubbles (∼0:1 mm) or high void
fraction (over 0.3) is said to be almost impossible at the mo-
ment, which means that we cannot obtain a precise expres-
sion among bubble diameter distribution, rising velocity of
bubbles and void fraction. Therefore, a simpliNed physical
model on the void fraction between electrodes was consid- Fig. 10. Coordinates in modeling of void fraction between elec-
ered in vertical setting of electrodes for discussing experi- trodes.
mental results.
Fig. 10 shows the coordinates for modeling average void
fraction between electrodes. As water electrolysis is in (=9:65 × 104 ; C=mol);  the current density, A=m2 , and W
progress, the following reactions occur at both electrodes. the width of electrodes, m.
at cathode: Therefore, considering that void fraction, , is the func-
tion of position, x, and assuming the rising velocity of all
H2 O + e− → OH− + 12 H2 (1) bubbles is constant, u, m=s, the bubble volume balance be-
tween electrodes in the region, x ∼ x + d x, leads to the
at anode:
following equation:
1
2
H2 O → H+ + 14 O2 + e− : (2) 3 RT W
uW ( + d ) = uW + d x; (4)
4 p F
Since the mass Gux of hydrogen and oxygen gas is pro-
portional to current density, the volume Gux of hydrogen and then,
oxygen gas generated from the region, x ∼ x + d x, shown
3 RT 
in Fig. 10 can be represented as follows. d = d x; (5)
4 p Fu
at cathode:
1 RT W where  denotes the space between electrodes.
d x; Solving Eq. (5), the local void fraction, = (x), and the
2 p F
average void fraction of whole region between electrodes,
at anode: av , can be obtained as follows:
1 RT W 3 RT 
d x; = x; (6)
4 p F 4 p Fu
then, total:

1 H 3 RT H
av = dx = : (7)
3 RT W H 0 8 p Fu
d x (m3 =s); (3)
4 p F
Thus, the average void fraction between electrodes can be
where p is the pressure, Pa, R the universal gas constant, expressed in Eq. (7) by the experimental parameters except
J=mol K, T the temperature, K, F the Faraday constant the rising velocity of bubbles, u.
 N. Nagai et al. / International Journal of Hydrogen Energy 28 (2003) 35 – 41 41

The rising velocity of bubbles is considered to have which was found to represent a part of the qualitative ten-
close relationship with bubble diameter, liquid viscos- dency of experimental results.
ity, and the number density of bubbles. Although, these
terms were not estimated in this experiment, the rising References
velocity of bubbles may be related to the existence of
separator, system temperature, current density, surface [1] Winter CJ, Nitsch J, editors. Hydrogen as an energy carrier,
wettability and inclination angle of electrodes. Therefore, technologies, systems, economy. Berlin: Springer, 1988.
it can be said that Eq. (7) explains a part of qualitative [2] Sandstede G, Wurster R. In: White RE, et al., editors. Modern
tendency of the experimental results discussed in the pre- aspects of electrochemistry, vol. 27. New York: Plenum Press,
1995. p. 411.
vious section. Namely, the increase of void fraction would
[3] LeRoy RL, Janjua MBI, Renaud R, Leuenberger U. Analysis
occur by following conditions; increasing current den- of time-variation e/ects in water electrolyzers. J Electrochem
sity, with separator, higher temperature, narrower space, Soc 1979;126:1674–82.
larger height, horizontal setting of electrodes, and higher [4] Abe, et al., In: Proceedings of the Fifth World Hydrogen Energy
wettability. Conference 1984. p. 727–36.
[5] Funk JE, Thorpe JF. Void fraction and current density
distributions in a water electrolysis cell. J Electrochem Soc
1969;116:48–54.
4. Conclusion [6] Hine F, Sugimoto T. Gas void fraction in electrolytic cell. Soda
to Enso 1980;131:347–62.
The optimum condition on hydrogen production by water [7] Bongenaar-Schlenter BE, Janssen LJJ, Van Stralen SJD,
Barendrecht E. J Appl Electrochem 1985;15:537–48.
electrolysis was found to exist from the experimental result
[8] Janssen LJJ, Visser GJ. Distribution of void fraction, ohmic
that the decrease of electrolysis e0ciency occurs by the in- resistance and current in a tall vertical gas-evolving cell. Proc
crease of void fraction between electrodes along with varia- Electrochem Cell Des Optim 1991;123:361–85.
tion of the experimental parameters such as space, height of [9] Riegel H, Mitrovic J, Stephan K. Role of mass transfer on
electrodes, current density, and so on. In addition, a physical hydrogen evolution in aqueous media. J Appl Electrochem
model of void fraction between electrodes was presented, 1998;28:10–7.