J. Electrochem. Sci. Technol., 2019, 10(3), 302-312
Hydrogen Production from Water Electrolysis Driven by High
Membrane Voltage of Reverse Electrodialysis
Ji-Hyung Han, Hanki Kim, Kyo-Sik Hwang, Namjo Jeong, and Chan-Soo Kim*
Jeju Global Research Center, Korea Institute of Energy Research, 63357, 200 Haemajihaean-ro, Gujwaeup, Jeju, South
The voltage produced from the salinity gradient in reverse electrodialysis (RED) increases proportionally with the number
of cell pairs of alternating cation and anion exchange membranes. Large-scale RED systems consisting of hundreds of cell
pairs exhibit high voltage of more than 10 V, which is sufficient to utilize water electrolysis as the electrode reaction even
though there is no specific strategy for minimizing the overpotential of water electrolysis. Moreover, hydrogen gas can be
simultaneously obtained as surplus energy from the electrochemical reduction of water at the cathode if the RED system
is equipped with proper venting and collecting facilities. Therefore, RED-driven water electrolysis system can be a prom-
ising solution not only for sustainable electric power but also for eco-friendly hydrogen production with high purity without
CO emission. The RED system in this study includes a high membrane voltage from more than 50 cells, neutral-pH water
as the electrolyte, and an artificial NaCl solution as the feed water, which are more universal, economical, and eco-friendly
conditions than previous studies on RED with hydrogen production. We measure the amount of hydrogen produced at max-
imum power of the RED system using a batch-type electrode chamber with a gas bag and evaluate the interrelation between
the electric power and hydrogen energy with varied cell pairs. A hydrogen production rate of 1.1 × 10 mol cm h is
obtained, which is larger than previously reported values for RED system with simultaneous hydrogen production.
Keywords : Salinity Gradient Power, Reverse Electrodialysis, High Membrane Voltage, Water Electrolysis, Hydrogen Production
as Surplus Energy
Received : 2 January 2019, Accepted : 30 April 2019
With the progression of climate change and global
warming, the development of alternative energy for
reducing the use of fossil fuels is an inevitable and
crucially important undertaking. Reverse electrodial-
ysis (RED) is receiving growing attention as a sus-
tainable energy technology for generating electric
power from salinity gradients without thermal pollu-
tion or CO2 emission [1-3]. Membrane potential is
established from alternating anion exchange mem-
branes (AEMs) and cation exchange membranes
(CEMs) and diffusion transport of ions is converted
to electric current by redox reaction at the electrodes.
The theoretical energy generated from RED is esti-
mated to be 1.7 MJ when 1 m3 sea water is mixed
with the same amount of river water. Unlike intermit-
tent solar and wind energy, RED can be continuously
available for conversion into electric power for 24 h.
Since Pattle  first demonstrated a membrane
stack for electric power in the early 1950’s and Wein-
stein  emphasized engineering, economic, and
environmental considerations that must be resolved
in the 1970’s, there have been significant achieve-
ments  in development of RED components [7-12]
(e.g., membranes and spacers) and extensive analy-
sis [13-17] on the optimal operating conditions based
on the interrelation between various parameters (e.g.,
flow rate and concentration of feed solutions, feed
channel thickness, and pumping cost). Consequently,
the gross power density has increased considerably
since Pattle obtained power output of 0.2 W/m2 .
The best power output reported to date using the con-
centration difference by mixing 17 mM river water
*E-mail address: firstname.lastname@example.org
This is an open-access article distributed under the terms of the Creative Commons
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which permits unrestricted non-commercial use, distribution, and reproduction in any
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Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312 303
and 500 mM sea water is 2.4 W/m2 , which was
achieved by using ultrathin pore-filling membranes
and specific spacers with a high open area.
By comparison, the choice of redox species for
RED in an electrode system is very limited. For
instance, Fe2+/Fe3+ or ferri-/ferrocyanide [18-21] are
the most commonly used redox species in the afore-
mentioned studies. These redox species have low
overpotential for electron transfer, which leads to low
electrode resistance and maximum electric power.
Because the ferri-/ferrocyanide can be decomposed
into cyanide under sunlight and oxygen, Fe2+/Fe3+ is
regarded as the more stable redox species in pilot
scale systems . Recent papers [22,23] have
reported large-scale RED systems utilizing Fe2+/Fe3+
redox couples with precious metal oxide electrodes
under acidic pH media. However, several problems
have been identified with such couples during long-
term operation; for instance, iron precipitates can
form when the pH is not properly controlled, leading
to channel clogging, membrane fouling, and a
decrease in current. In addition, leaking of the elec-
trode solution into the discharge water is unavoidable
due to the non-ideal permselectivity of ion exchange
Another redox reaction is water electrolysis, which
generates hydrogen and oxygen at the cathode and
anode, respectively. This reaction has been avoided
due to its large overpotential and its products have
thus far been regarded as waste gas. Further, hazard-
ous gaseous mixtures of hydrogen and oxygen and
toxic chlorine gas require proper collection/venting
systems. However, water is a very safe chemical sub-
stance and does not give a negative impact on dis-
charge water or the surrounding environment .
Moreover, hydrogen gas can be simultaneously
obtained as surplus energy from water reduction at
the cathode if the RED system is equipped with
proper venting and collecting facilities. It is well
known that hydrogen has been regarded as a promis-
ing energy carrier with good flexibility and the high-
est energy density on a mass basis .
A few studies on RED with water electrolysis have
been reported to date (Table 1). Scialdone et al.
demonstrated the performance of RED with oxida-
tion/reduction of water and oxidation of chlorine in
water/Na2SO4 or water/NaCl systems . The max-
imum power density of 40 cells was roughly mea-
sured at 0.2 W/m2. Turek et al. simulated 5 cell pairs
of RED in which the electrodes were connected to a
rectifier to simulate an energy consumer, as the cell
voltage did not otherwise exceed the overpotential
for water electrolysis [28,29]. Some reports sug-
gested special means to minimize the overpotential
for water electrolysis, given the potential of the RED
system as a method for renewable hydrogen gas pro-
duction. For instance, the Logan group [30-32] intro-
duced bacteria on the anode to oxidize organic matter
in a microbial RED that can avoid overpotential of
the electrode reaction. Hatzell et al. utilized ammo-
nium bicarbonate (AmB) [33,34] not only to create a
salinity gradient but also to reduce the electrode over-
potential for the hydrogen evolution reaction (HER).
Chen et al. reported that a conversion efficiency
of hydrogen was close to 90% for 20 cells when a
HCl solution was used as the catholyte, which mini-
mized the overpotential for the HER.
However, these previous works were limited to
small-scaled membrane stacks with fewer than 50
cells. In terms of a pilot-scale system, their purpose is
contradictory to the general assumption [5,36,8,24]
that electrode resistance caused by the overpotential
of the electrochemical reaction can be minimized by
using a large number of cell pairs (e.g., 50-500). This
means that minimizing the overpotential for the elec-
trode reaction is not necessary for operating large-
scale REDs. The report by Adriana et al. on a
pilot-scale RED with 500 cell pairs has addressed this
point. When a NaCl solution was used as the electro-
lyte, the maximum power was 1.31 W/m2, which is
only 13% smaller than that produced by the Fe2+/3+
redox system. They explained that the voltage loss by
water electrolysis has a smaller effect on the overall
cell voltage in the presence of a higher number of cell
pairs. Practical cell voltages in industrial water elec-
trolysis are ~1.8-2.6 V , which are much smaller
than the membrane voltage of RED with hundreds of
cell pairs. We recently confirmed the difference in the
maximum power between water electrolysis and
ferri-/ferrocyanide is only 3% in a large-scale RED
system with 1,000 cells when the flow rate of the
electrode solution is set to be sufficiently low to max-
imize the net power . This suggests that even
though there is no specific strategy for minimizing
the overpotential of water electrolysis, water oxida-
tion/reduction occurs effectively at the electrodes in
pilot-scale RED systems and can thus provide stable
long-term operation without negatively affecting
304 Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312
either the performance or the environment.
In this work, we present experimental results on
the electric power and hydrogen production of a RED
system feeding artificial NaCl solution with more
than 50 cells in which the membrane voltage was
large enough to achieve water electrolysis. Neutral-
pH water containing small amounts of NaCl was
used as the catholyte and anolyte, which is advanta-
geous relative to strong acid or alkaline electrolytes
as they are highly corrosive to the device, difficult to
handle, and environmentally unfriendly . The
conditions employed herein are more universal, eco-
nomical, and eco-friendly than previous studies
(Table 1) on RED with hydrogen production. We
confirm that hydrogen evolution occurs effectively at
cathode due to its high stack voltage. To separate and
collect hydrogen from oxygen or chlorine gas during
RED operation, batch-type electrode chambers are
used. The correlation of hydrogen energy with elec-
tric energy at maximum power is evaluated and com-
pared with a conventional RED system circulating
electrolyte to discern the effect of electrode resistance
on the electric power relative to the number of cell
pairs. Broadly, this study is aimed at the long-term
operation of pilot-scale REDs with water electrolysis.
2.1 Reverse electrodialysis (RED) operation
The membrane stack of the RED system consisted
of cation exchange membranes (CEM-Type 1, Fujif-
ilm, Netherlands), anion exchange membranes
(AEM-Type 1, Fujifilm), spacers (thickness 100 μm),
and gaskets (PTFE, thickness 100 μm). Artificial sea
water (0.5 M) and river water (17 mM) were used by
dissolving NaCl salt (Daejung, Republic of Korea) in
the waters. River and sea waters enter their respective
compartments using peristaltic pumps (Masterflex,
Cole-Parmer, US) at a flow rate of 4 mL min-1 per
cell. Distilled water with a pH of ~6 as the electrode
solution was added in batch-type chambers (50 mm ×
50 mm × 50 mm) (Fig. 1b, termed batch-type RED).
Gas bags (Dalian delin gas packing co.,ltd, China)
were connected to the chambers to collect the
produced gas (See a photo of the real RED system in
supporting information, Fig. S1). For comparison, the
performance of a conventional RED (Fig. 1c) was
also evaluated, in which thin (1 mm) electrode cham-
bers were used. The electrode solution was circulated
from the anode to the cathode at a flow rate of 50 mL
min-1 and gas collection was achieved using a con-
tainer connected to the electrode chambers. Plati-
num-coated titanium mesh (thickness 1 mm,
geometric area 19.625 cm2) was used as the anode
and cathode in both types of RED systems. The
effective area of an individual membrane in the
batch-type RED was 19.625 cm2 (3.14 × 2.5 cm × 2.5
cm), which is identical to that of the conventional
2.2 Performance evaluation
A potentiostat (ZIVE SP2, Wonatech, Republic of
Korea) was used to measure polarization (I–V) curves
with varied cell pairs at a scan rate of 40 mV s-1. The
maximum power (Pmax) and the corresponding cur-
rent density were determined from peak values of the
parabolic curves. The measured Pmax was converted
to the maximum power density (Pd,max) based on the
cell pair area.
Where Nm is the number of cell pairs and A is the
effective area of individual membrane.
Table 1 Previous studies on RED system using water electrolysis as electrode reaction
Cell pairs Special means for water electrolysis Hydrogen production rate Ref.
40 Only membrane voltage - 26
5 Rectifier - 28, 29
5 Bacteria on the anode, organic matter 1.6 m H day m -anolyte 31
20 Ammonium bicarbonate (AmB)
as seawater and electrode solution 7.1E-6 mol h cm -electrode 33
20 Ammonium bicarbonate (AmB) as sea water 8.7m H day m -catholyte 34
20 2 M HCI as the catholyte 0.72 mL h cm -electrode 35
Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312 305
Applying a constant load at which maximum
power was achieved in the I–V curves, we evaluated
the average electric energy and the amount of hydro-
gen of the two types of RED systems with different
cell pairs (25, 50, 75, and 100). All measurements
were performed in duplicate with the low variability
(~5%). Ion chromatography (ICS-1600, Thermo
scientific, USA) was used to analyze the ion concen-
trations of the catholyte and anolyte after 1 h of the
constant load tests.
2.3 Hydrogen production rate and hydrogen energy
The produced gas was collected in the gas bag of
the electrode chamber during the constant load test
for 1 hour. And then the gas bag was connected to a
gas chromatography (GC 2014, Shimadzu, Japan) to
evaluate the gas component and the amount of hydro-
gen. Hydrogen production rate (mL h-1 cm-2-elec-
trode) was calculated as the measured hydrogen
volume was divided by the geometric area of elec-
The energy extracted from the produced hydrogen
was calculated based on the volume-specific chemi-
cal energy at standard temperature and pressure
(STP) conditions . Given that the product water
in fuel cells is produced as a liquid (2), we used the
higher heating value (HHV, 285.83 kJ/mol) of hydro-
gen to calculate its chemical energy:
–HHV 285.83 kJ
Fig. 1 (a) Flow of ions and electrons in RED stack, (b) batch-type RED with wide chambers for isolating the catholyte
from the anolyte, and (c) conventional RED with thin chambers circulating the electrolyte.
306 Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312
in which the molar gas volume at STP is V0,mol
= 22.414 Nl/mol and Nm3 is normal cubic meters.
The current efficiency was calculated as the experi-
mental hydrogen production rate was divided by the
theoretical rate (mL h-1 cm-2-electrode).
Theoretical hydrogen production rate
Where I is the measured current, n is the stoichio-
metric number of electrons consumed in the elec-
trode reaction (n = 2 for hydrogen production from
water electrolysis), F is the Faraday constant, A is the
geometric area of electrode.
2.4 Theory: voltage and resistance
The membrane voltage with no current, open cir-
cuit voltage (OCV), produced by one cell consisting
of an AEM and a CEM is given by the Nernst equa-
where z is the ion valence (i.e., 1 for NaCl), F is the
Faraday constant, R is the universal gas constant, and
the factor of 2 accounts for the CEM and AEM. α is
the permselectivity of the ion exchange membrane
and a is the activity of the concentrated (ac) and
diluted (ad) solutions.
The membrane voltage of the stack consisting of
the total number (Nm) of cell pairs is:
The current through an ideal RED is given by :
where Ri is the internal resistance of the stack and Ru
is the external load resistance.
The produced electric power (Pu) from Ru is:
Pu approaches its maximum value when Ru is equal
to Ri. In this case the Estack,Pmax is half the value of
, where Estack,Pmax is the voltage between the
cathode and anode at maximum power.
Ri can be divided into ohmic area resistance (Rohmic)
and non-ohmic resistance (Rnon-ohmic). Rnon-ohmi c con-
siders the concentration change in the boundary lay-
ers (RBL) and in the bulk solution (RΔC). Rohmic is
determined by the resistances of the membranes
(RAEM and RCEM), two feed solutions (Rriver and Rsea),
the electrode system (Relectrode), and the number (Nm)
of cell pairs :
Rohmic =Nm(RAEM +R
The voltage loss ( ) of the electrode
system originates from the thermodynamic electrode
potential, overpotential, and concentration polariza-
tion for the redox reaction. We measured the voltage
loss by using Ag/AgCl reference electrodes (Fig. S2),
which read the membrane voltages of the stack at
maximum power ( ):
2.5 Gross energy efficiency
The maximal available free energy per second
(Xmax) was calculated from the Gibbs free energy of
mixing river water with sea water .
where R is the universal gas constant (8.314 J mol-1
K-1) and T is the temperature (K). CR and CS are the
NaCl concentrations (mol m-3) of river water and sea
water, respectively, and ΦR and ΦS are their
respective flow rates (m3 s-1). CM is the equilibrium
The gross energy efficiency is defined as follows:
3. Results and Discussion
Fig. 1a shows the flow of counter-ions across the
ion exchange membranes by the concentration dif-
ference in the RED system. When distilled water is
used as the electrolyte, the electrode chamber of
batch-type RED (Fig. 1b) does not ideally contain
--------------- 285.83 kJ mol⁄
22.414 Nl mol⁄
×=3.54 kWh N m
-------------- ln a
Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312 307
chloride ions during operation because a CEM was
used as the shielding membrane. In actual experi-
ments, however, a considerable amount of NaCl
salt was found in the catholyte and anolyte during
RED operation. Ion chromatography (IC) showed
that the cathode and anode chambers adjacent to
the sea and river waters contained ~40 mM and
~2.5 mM NaCl, respectively, during 1 h of opera-
tion. There are two possible reasons for this obser-
vation; the first concerns the non-ideal property of
ion exchange membranes; hence, co-ion transport
by imperfect membrane selectivity must be consid-
ered. Another reason for the observation of NaCl is
leakage of the sea and river waters into the elec-
trode chambers through small gaps between the
gaskets and spacers due to pressure imbalance over
the membrane area.
Given the inflow of NaCl into the electrode cham-
ber, when water electrolysis is used as the redox reac-
tion for RED operation, the cathodic and anodic
reactions in neutral pH condition are represented as
4H2O+4e−→2H2↑+4OH−E0 = -0.83 V (13)
O2+2H2O+4e−→4OH −E0 = 0.40 V (14)
2H2O→4e−+4H++O2↑E0 = -1.23 V (15)
4Cl−→4e−+2Cl2↑E0 = -1.36 V (16)
The reduction reactions of water (13) and oxygen
(14) occur at the cathode and NaCl does not
participate in electrochemical reduction. At the
anode, however, water oxidation (15) competes with
the oxidation of chloride anions (16).
In conventional RED with electrolyte circulation
(Fig. 1c), the current and electric power are influ-
enced by the number of protons (17), hydroxide (18),
and hydrogen (19) dispersed in solution, which are
products of water reduction/oxidation. They move in
the opposite electrode chamber by convection and
therefore undergo electron transfer reactions at the
anode and cathode as:
Fig. 2. (a) Polarization curves (I-V curves) of batch-type RED with varied cell pairs, (b) measured power of batch-type
RED, (c) plots of maximum power (P) of batch-type RED compared with that of conventional RED, and (d) P ratio
of batch-type RED to conventional RED with varied cell pairs.
308 Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312
4H++4e−→2H2↑E0 = 0.00 V (17)
4OH −→O2↑+2H2O+4e−E0 = -0.40 V (18)
2H2→4H++4e−E0 = 0.00 V (19)
The polarization curves (Fig. 2a) were measured to
evaluate the effect of the number of cell pairs on the
power output of RED system using water electrolysis
as the electrode reaction. The maximum current den-
sity ( jmax) increased from 26 A/m2 to 95 A/m2 with
the number of cell pairs. On the other hand, an incre-
ment in jmax decreased with increasing cell pairs,
which can be ascribed to the leakage current that
passes through the feed and drain channels. Previous
research showed that the leakage current increases
with the increase of the number of cell pairs and
causes the current efficiency to decrease [42,45]. Fig.
2b shows the measured power as a function of the
current density derived from Figure 2a. The Pmax
increased from 15 mW to 366 mW as the number of
cell pairs increased from 25 to 100. The current den-
sity (Fig. S3) and Estack (Fig. S4a) at Pmax also
increased with increasing cell pairs. This indicates
that increasing the stack voltage activates water
decomposition, leading to increased electric power.
We also monitored Em,OCV and Em(Fig. S4a) at Pmax by
using two Ag/AgCl reference electrodes during the
I–V curve measurements. The difference between Em
and Estack at Pmax indicates the voltage loss
(Relectrode·I) by water electrolysis, which was ~2.2 V
regardless of the number of cell pairs. Compared
with Em at Pmax with 25 cell pairs, the voltage loss of
water electrolysis is relatively large, leading to a volt-
age ratio of 0.80 (Fig. S4b). As the number of cell
pairs increases, however, the contribution of voltage
loss by water electrolysis diminishes, leading to a
decrease in the voltage ratio to a value of 0.35.
Fig. 2c shows plots of maximum power (Pmax) with
respect to the number of cell pairs of batch-type and
conventional RED systems. The Pmax of conventional
RED also increased proportionally with the number
of cell pairs. Fig. 2d shows the Pmax ratio of batch-
type RED to conventional RED derived from Fig. 2c.
Compared with conventional RED with 25 cells, the
electric power of the batch-type RED system was
decreased by half, which we attribute to a decrease in
electrode resistance by electrolyte circulation. The
reaction products (i.e., H+ ions) at the anode in con-
ventional RED move into the cathode chamber and
become electroreduced at the cathode, producing
hydrogen. Because the overpotential for H+ reduction
is much lower than that for water reduction, the cur-
rent and electric power increase. An analogous pro-
cess occurs for the electrooxidation of OH- ions at the
anode. However, the effect of electrolyte circulation
was markedly decreased when more than 50 cell
pairs were employed. The difference in electric
power between the batch-type and conventional RED
systems was only 5%. This means that the Em,Pmax
(Fig. S4a) for more than 50 cells is sufficient to
achieve electrochemical oxidation and reduction of
water molecules in neutral-pH media.
To monitor average electric energy and hydrogen
production rate of batch-type RED, the constant
external load at which maximum power was obtained
in the I–V curves was applied for 1 h (Fig. 3a). Gases
produced at the cathode and anode were collected
separately. The RED with 100 cells had the largest
concentration percentage of hydrogen (71%) (Table
S1). The remaining gases were oxygen and nitrogen.
As mentioned above, leakage of sea and river waters
into the electrode chamber occurred during RED
operation. The water level within the chamber slowly
rose, which caused the air in the cylindrical connec-
tor to diffuse into the gas pack during the constant
load experiments. This phenomenon can explain why
the concentration percentage of hydrogen was less
than 100%. If the leakage problem were solved, the
production of highly pure hydrogen would be
expected in the batch-type RED. From the concentra-
tion percentage, the hydrogen production rates with
different cell pairs were calculated (Fig. 3b). Overall,
the production rate was proportional to the number of
cell pairs and current density. A hydrogen produc-
tion rate of 2.5 mL cm-2 h-1 was obtained for 100
cells, corresponding to 1.1 ×10-4 mol cm-2 h-1, which
is larger than previously reported values for RED-
driven hydrogen production [46,33,6,34]. Tufa et
al.’s work  showing high hydrogen production
rate (2.0 ×10-3 mol cm-2 h-1) is out of scope for our
study. Because hydrogen was produced from alkaline
polymer electrolyte water electrolysis operated by
excess power of RED, which is not simultaneous pro-
duction of hydrogen during RED operation. It should
be noted that this high hydrogen production rate was
achieved from water electrolysis in NaCl solution at
Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312 309
maximum power of RED with 100 cell pairs feeding
an artificial NaCl solution with a salinity ratio of 30.
Compared with previous research (Table 1), our sys-
tem is more universal, economical, and eco-friendly,
which are requisite conditions for the development of
scaled-up RED systems with hundreds of membrane
The current efficiency, which is the ratio between
the experimental hydrogen production rate and theo-
retical one, was calculated for batch-type RED sys-
tems with varied cell pairs (Fig. 3c). As the cell pairs
increased, the current efficiency steadily increased.
Notably, the electric power decreased by half when
N2 purged aqueous solution was used, indicating that
some of the parasitic current originated from the oxy-
gen reduction reaction (ORR). However, the effect of
this parasitic current decreased as the number of cell
pairs increased, with the current efficiency approach-
ing 0.85 for the RED system with 100 cells. This
result demonstrates that water reduction predomi-
nantly occurred at the cathode under large stack volt-
age. The batch-type RED with more than 120 cells is
expected to show near 100% conversion efficiency.
We confirmed that gas crossover caused by leakage
between the cathode and anode chambers was negli-
gible on the basis of the concentration percentage of
hydrogen in the gas pack of the anode chamber being
slightly larger than that of the air.
Fig. 3d shows the hydrogen-to-electric energy
ratios of the batch-type RED. The ratio for 25 cells
was ~0.9, which suggests that a considerable amount
of Em,Pmax was applied at the electrode due to high
electrode resistance, which arises from the relatively
large overpotential of water electroreduction/oxida-
tion. At a result, the voltage (Estack,Pmax) applied at the
external resistance was much smaller than Em,Pmax
(Fig. S4a), indicating that water electrolysis disturbed
production of electric power. At more than 50 cell
Fig. 3 Constant load experiments for 1 h. (a) Changes in the electric power at 100 cells of batch-type RED (red line) and
conventional RED (black line) over time. (b) Hydrogen production rate of batch-type RED with different cell pairs. (c)
Current efficiency of hydrogen production with varied cell pairs of batch-type RED. The current efficiency is defined as the
ratio between the experimental hydrogen volume and the theoretical hydrogen volume. (d) Energy ratio of hydrogen to
electricity with varied cell pairs of batch-type RED.
310 Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312
pairs, however, the ratio of hydrogen energy
remained ~0.4. As the number of cell pairs increased,
the contribution of the electrode resistance to the total
resistance (including AEM, CEM, sea water, and
river water) became much smaller. Moreover, Em,OCV
of more than 50 cell pairs is large enough to achieve
water electrolysis. Therefore, water electrolysis is an
available redox reaction to generate electric power in
RED with a sufficient number of cell pairs. In addi-
tion, this condition decreased the susceptibility of
water electrolysis (or hydrogen production) to the pH
of the electrode solution. When starting the constant
load test, electrode solution was distilled water with a
pH of ~ 6. As the RED system operates, the catholyte
and the anolyte become more basic and acidic,
respectively. The resultant pH values of the catholyte
and the anolyte were 12 and 3, respectively.
Nevertheless, 100 cells of batch-type RED main-
tained a constant current for 1 h (Fig. 3a). This indi-
cates that the electrochemical reduction/oxidation of
water molecules occurred successfully in the pres-
ence of the large RED stack voltage even though the
pH of the electrolyte changes sharply. This abrupt
change in pH when using the batch-type electrode
chamber can be mitigated by switching feed solu-
tions on a regular cycle, which leads to electric cur-
rent in the opposite direction.
We also conducted constant load experiments of
conventional RED (Fig. 3a, black line) with different
cell pairs for 1 h. Fig. 4 shows the hydrogen energy
of batch-type and conventional RED systems with
varied cell pairs. As seen, the hydrogen energy of
batch-type RED was larger than that of conventional
RED for all cell pairs, and the difference in the
hydrogen energy became larger as the number of cell
pairs increased. It is expected that as more cell pairs
are added to a conventional RED, more hydrogen
will be dispersed into solution and circulate between
the anode and cathode; it will then oxidize to produce
protons at the anode, where the overpotential is much
lower than water oxidation. This is why the electric
energy of conventional RED is slightly larger than
batch-type RED and the amount of hydrogen in the
former was smaller than that in the latter.
It should be noted that while hydrogen collected
from batch-type RED is expected to be pure, the gas
pack of conventional RED contains a mixture of
hydrogen and oxygen gas. Thus, hydrogen produced
from conventional RED should be separated from
oxygen and other gases, which means conventional
RED requires installation and maintenance costs for
Fig. 4 Hydrogen energy of batch-type and conventional
RED systems with varied cell pairs.
Fig. 5 (a) Power density and (b) energy efficiency of batch-type
and conventional RED systems with varied cell pairs.
Ji-Hyung Han et al. / J. Electrochem. Sci. Technol., 2019, 10(3), 302-312 311
the gas separation device. Moreover, the hydrogen
and oxygen mixture is potentially explosive when its
minimum activation energy is reached. Therefore, it
could be more economical and safer to produce only
electric power in conventional RED. Fig. 5a shows a
comparison of power density between batch-type and
conventional RED systems, with only electric energy
considered for the latter. As seen, the increase in
power density of batch-type RED was larger than
conventional RED as the number of cell pairs
increased. At 25 cells, where the practical voltage for
water oxidation/reduction is relatively large com-
pared to membrane voltage, the power density of
batch-type RED was lower than conventional RED.
However, RED with more than 50 cells had sufficient
driving force to achieve water electrolysis; all
hydrogen gas produced at the cathode can be
available as energy, leading to an increase in the
power density, unlike conventional RED with
electrolyte circulation where a considerable amount
of hydrogen is consumed at the anode. This trend was
directly reflected in the gross energy efficiency (Fig.
5b). As the number of cell pairs increased, (1) the
energy efficiency of both systems sharply increased,
indicating that a larger stack voltage reduces the
effect of electrode resistance on the electric power,
(2) the difference in energy efficiency between the
batch-type and conventional systems became larger,
which means the contribution of hydrogen energy to
the total energy was larger.
We have demonstrated that even though there is no
specific method to reduce overpotential for water
electrolysis, high membrane voltage of RED with
more than 50 cells can utilize the water oxidation/
reduction in neutral-pH media containing small
amounts of NaCl as electrode reaction. As the num-
ber of cell pairs increased, the effect of voltage loss
by electrode resistance on the produced electric
power became negligible. Therefore, for stable long-
term operation of the pilot-scale RED systems con-
sisting of hundreds of cell pairs, water electrolysis
with a neutral-pH electrolyte (e.g. tap water) can be
an available redox reaction to produce not only elec-
tric power but also hydrogen energy. Moreover, non-
noble metal catalysts (e.g., Ni, Mn, and Ti) [48,35]
can be available as cathodes for water electrolysis
because the high membrane voltage can compensate
for low electrocatalytic ability of these metal cata-
lysts. We recently confirmed that Ti mesh can be
used for a cathode for 1,000 cell pairs of RED with-
out loss of power . If the properties of the ion
exchange membrane and channel design are opti-
mized to increase the current density of the RED sys-
tem, it is expected that pure hydrogen can be
produced at a higher production rate from RED sys-
tems. We expect that pilot-scale RED operation with
simultaneous hydrogen production will be a repre-
sentative example of the successful combination
[37,49-51] of water electrolysis and renewable
Conflicts of Interest
There are no conflicts to declare.
This work was conducted under the framework of
the research and development program of the Korea
Institute of Energy Research (B7-2441).
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