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Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and High Temperature Water Electrolysis

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Water electrolysis is a quite old technology started around two centuries back, but promising technology for hydrogen production. This work reviewed the development, crisis and significance, past, present and future of the different water electrolysis techniques. In this work thermodynamics, energy requirement and efficiencies of electrolysis processes are reviewed. Alkaline water electrolysis, polymer electrolysis membrane (PEM) and High temperature electrolysis are reviewed and compared. Low share of water electrolysis for hydrogen production is due to cost ineffective, high maintenance, low durability and stability and low efficiency compare to other available technologies. Current technology and knowledge of water electrolysis are studied and reviewed for where the modifications and development required for hydrogen production. This review paper analyzes the energy requirement, practical cell voltage, efficiency of process, temperature and pressure effects on potential kinetics of hydrogen production and effect of electrode materials on the conventional water electrolysis for Alkaline electrolysis, PEM electrolysis and High Temperature Electrolysis .
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International Journal of Engineering and Advanced Technology (IJEAT)
ISSN: 2249 – 8958, Volume-4 Issue-3, February 2015
80
Published By:
Blue Eyes Intelligence Engineering
& Sciences Publication Pvt. Ltd.
Hydrogen Production by Water Electrolysis: A
Review of Alkaline Water Electrolysis, PEM Water
Electrolysis and High Temperature Water
Electrolysis
Md Mamoon Rashid, Mohammed K. Al Mesfer, Hamid Naseem, Mohd Danish
Abstract:- Water electrolysis is a quite old technology started
around two centuries back, but promising technology for
hydrogen production. This work reviewed the development, crisis
and significance, past, present and future of the different water
electrolysis techniques. In this work thermodynamics, energy
requirement and efficiencies of electrolysis processes are
reviewed. Alkaline water electrolysis, polymer electrolysis
membrane (PEM) and High temperature electrolysis are
reviewed and compared. Low share of water electrolysis for
hydrogen production is due to cost ineffective, high maintenance,
low durability and stability and low efficiency compare to other
available technologies. Current technology and knowledge of
water electrolysis are studied and reviewed for where the
modifications and development required for hydrogen
production. This review paper analyzes the energy requirement,
practical cell voltage, efficiency of process, temperature and
pressure effects on potential kinetics of hydrogen production and
effect of electrode materials on the conventional water
electrolysis for Alkaline electrolysis, PEM electrolysis and High
Temperature Electrolysis .
Index Terms: Hydrogen Production, Water electrolysis,
Electrolyte, Electrode, Electrocatalyst, PEM.
I. INTRODUCTION
The atmosphere is polluted by plenty of greenhouse gases;
SO
x
, NO
x
, CO
2
and CO from hydrogen production by
hydrocarbon source that are fossil fuel sources which can
affect seriously the ecosystem [1–3]. Hence the clean
technology is needed for production of hydrogen that can be
achieved if hydrogen is produced by renewable source like
water electrolysis and no emission of SO
x
, NO
x
, CO
2
and
CO will be possible and to achieve “hydrogen economy” [ 4,
5]. There are many important non-fossil fuel based
processes like Water electrolysis, photocatalysis processes
and thermochemical cycles for hydrogen productions in
practice [6 - 15]. The use of solar energy and wind energy
are sustainable methods for hydrogen production by water
electrolysis with high purity, simple and green process [16].
Manuscript Received on February 2015.
Md Mamoon Rashid, Department of Chemical Engineering, King
Khalid University, Abha, Kingdome of Saudi Arabia.
Mohammed K. Al Mesfer, Department of Chemical Engineering, King
Khalid University, Abha, Kingdome of Saudi Arabia.
Hamid Naseem, Department of Electrical Engineering, King Khalid
University, Abha, Kingdome of Saudi Arabia.
Mohd Danish, Department of Chemical Engineering, King Khalid
University, Abha, Kingdome of Saudi Arabia.
For hydrogen production, water electrolysis has its various
merits like pollution free process if renewable energy
sources use purity of high degree, very simple process and
plenty of resources [17]. Water electrolysis is an around 200
year old technology; around 1800 AD the principle
demonstrated by experiment by J. W. Ritter in Germany. In
the same year William Nicholson and Anthony Carlise
decompose water into hydrogen and oxygen in England. The
application of this technology started to use after tens of
year. The French military in 1890 AD constructed a water
electrolysis unit to generate hydrogen for use in airships by
Charles Renard. Around 1900 AD more than 400 industrial
electrolyzers were operating worldwide. Around 1930 AD
different types of alkaline electrolyzer were developed. In
the 1970s AD, the development of the PEM electrolyzer
offered several advantages over alkaline electrolyzers with
limited use in small hydrogen and oxygen production
capacities due to expensive materials and a limited lifetime
[18]. As hydrogen could be produced at lower cost by steam
reforming, water electrolysis technology advanced only
slowly. The hydrogen production in total around the world
is about 500 bill. Nm³/year, mostly steam reforming. Only 4
% of hydrogen produced by water electrolysis as shown in
figure 1. Due to low efficiency of production processes [19].
Currently, the efficiency hydrogen production by water
electrolysis is too low to be economically competitive [20].
The low gas evolution rate and high energy consumption are
serious problems of water electrolysis. In average 4.5–5.0
kWh/m
3
H
2
energy is needed for conventional industrial
electrolyzer [16]. In water electrolysis for hydrogen
production processes the efficiency is a very important
parameter. Many researchers in their work have done for
analyzing the energy consumption, efficiency of hydrogen
production systems. The authors of [ 21 23] defined the
energy, energy analysis, energy efficiencies, different
driving energy inputs, definition of the efficiency,
thermodynamic analysis, thermodynamic electrochemical
characteristics, thermodynamic losses, system boundary
and heat flows across the process of a hydrogen production
process in different electrolyzer plants. This review paper
analyzes the energy requirement, practical cell voltage,
efficiency of process, temperature and pressure effects on
potential, bubble mechanics and effects , kinetics of
hydrogen production and effect of electrode materials on the
conventional water electrolysis for Alkaline electrolysis,
PEM water electrolysis and High temperature electrolysis .
Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and
Figure1: Annual global hydrogen product share [19].
Abbreviations and Nomenclatures
II.
CONCEPT AND FUNDAMENTALS
When a water molecule passes through electrochemical
process water molecules spilt in hydrogen and oxygen gases,
this process is called water electrolysis. Electricity is used
for the splitting the hydrogen and oxygen into their gaseous
phase. The basic eq
uation of water electrolysis is written as
Eq.1. This technique produces clean energy without
emission of pollution by utilizing electricity.
     
Natural gas
240
48
Hydrogen Production through different sources
PEM
Polymer electrolyte membrane
AWE
Alkaline water electrolysis
SPE
Solid polymer electrolysis
HTEL
High temperature electrolysis
SOEL
Solid oxide electrolysis
HER
Hydrogen evolution reaction
OER
Oxygen evolution reaction
STP
Standard temperature pressure
LHV Lower heating
value,
HHV
Higher heating value,

Change in Gibbs free energy of reaction, J/mol

Enthalpy change of reaction,

Entropy change of reaction, J/mol K
Operating temperature, K
E
theo
Theoretical energy consumption,

Reversible cell voltage, V
Number of moles
Faraday’s constant, C/mole
!"
#
Enthalpy voltage, V
ƞ
Overpotentials, V
$
Anode and cathode constant
%
Anode and cathode constant
&
Current density, A/cm
Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and
High Temperature Water Electrolysis
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Figure1: Annual global hydrogen product share [19].
CONCEPT AND FUNDAMENTALS
When a water molecule passes through electrochemical
process water molecules spilt in hydrogen and oxygen gases,
this process is called water electrolysis. Electricity is used
for the splitting the hydrogen and oxygen into their gaseous
uation of water electrolysis is written as
Eq.1. This technique produces clean energy without
emission of pollution by utilizing electricity.
(1)
For water electrolysis the energy is required as electrical
energy from a DC power source. At room temperature the
splitting of water is very small, approximately 10
moles/liter because pure water is the very poor co
electricity. Therefore, acid
or base is used to improve the
conductivity. I
n an alkaline electrolyzer, KOH, NaOH and
H
2
SO
4
solution mainly is used with water. The solution
splits into ions positive and negative ions and these ions
readily conduc
t electricity in a water solution by flowing
from one electrode to the other.
technology can de divides into three main classifications on
the basis of electrolyte used in the electrolysis cell.
Oil Coal
Electrolysis
150
90
20
30 18
Hydrogen Production through different sources
Total production(in bcm)
Percent Share
Polymer electrolyte membrane
'
"(")
Total resistance, (ohm)
Alkaline water electrolysis
*
++
Membrane resistance,(ohm)
Solid polymer electrolysis
*
)
Electrolyte resistance,(ohm)
High temperature electrolysis
*
,-,
Bubble resistance,(ohm)
Solid oxide electrolysis
*
.
Circuit
Hydrogen evolution reaction
)
Real(actual) cell voltage, V
Oxygen evolution reaction
ƞ
/
Anode ( oxygen) overpotentials, V
Standard temperature pressure
ƞ
0
cathode ( hydrogen) overpotentials, V
value,
kWh per kg
1
23
Energy efficiency
Higher heating value,
kWh per kg
4
5
Hydrogen gas out flow rate, Kg/hr
Change in Gibbs free energy of reaction, J/mol
6
Electrical power supply, kW
Enthalpy change of reaction,
J/mol
42
Heat exchanger energy input , J
Entropy change of reaction, J/mol K
7
Redundant energy required, J
Operating temperature, K
1
)".
Electricity generation efficiency
Theoretical energy consumption,
J/mol
Environmental temperature, K
Reversible cell voltage, V
External heat source temperature, K
Number of moles
1
8()"9
Voltage efficiency
Faraday’s constant, C/mole
1
:7;
Current( Faraday) efficiency
Enthalpy voltage, V
1
))
Total cell efficiency
Overpotentials, V
6
Operating pressure, atm.
Anode and cathode constant
Anode and cathode constant
Current density, A/cm
2
Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and
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For water electrolysis the energy is required as electrical
energy from a DC power source. At room temperature the
splitting of water is very small, approximately 10
-7
moles/liter because pure water is the very poor co
nductor of
or base is used to improve the
n an alkaline electrolyzer, KOH, NaOH and
solution mainly is used with water. The solution
splits into ions positive and negative ions and these ions
t electricity in a water solution by flowing
from one electrode to the other.
Water electrolysis
technology can de divides into three main classifications on
the basis of electrolyte used in the electrolysis cell.
Electrolysis
4
Percent Share
Total resistance, (ohm)
Membrane resistance,(ohm)
Electrolyte resistance,(ohm)
Bubble resistance,(ohm)
Circuit
resistance,(ohm)
Real(actual) cell voltage, V
Anode ( oxygen) overpotentials, V
cathode ( hydrogen) overpotentials, V
Energy efficiency
Hydrogen gas out flow rate, Kg/hr
Electrical power supply, kW
Heat exchanger energy input , J
Redundant energy required, J
Electricity generation efficiency
Environmental temperature, K
External heat source temperature, K
Voltage efficiency
Current( Faraday) efficiency
Total cell efficiency
Operating pressure, atm.
International Journal of Engineering and Advanced Technology (IJEAT)
ISSN: 2249 – 8958, Volume-4 Issue-3, February 2015
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Use of Liquid Electrolyte : Alkaline Water
Electrolysis (AWE)
Electrolysis in acid ionomer environment: Polymer
Electrolyte Membrane Electrolysis (PEM)/Solid
Polymer Electrolysis (SPE)
Use of Solid Oxide Electrolyte: Steam electrolysis
(High temperature electrolysis
-
HTEL or SOEL)
The figure 2 shows the fundamental principle for
electrolysis cell. The general principle for all three
technologies is the same. When a high voltage is applied to
an electrochemical cell in presence of water, hydrogen and
oxygen gas bubbles evolve at cathode (negative electrode)
and anode (positive electrode) respectively.
Figure 2: The fundamental of water electrolysis process.
Three approaches for the hydrogen evolution reaction
(HER) and the oxygen evolution reaction (OER), the typical
temperature range and the ions acting as the charge carrier
through the diaphragm/membrane is given in Table 1.
Table 1: Basic Chemical reactions and Operating temperature range for different types of Water electrolysis [18].
Electrolysis Technology Alkaline Electrolysis Membrane Electrolysis High Temperature
Electrolysis
Anode Reaction
Oxygen Evolution Reaction
(OER)
<=>
?
@A
<=
<
 >
<
=
 <B
?
>
<
= @ A
<=
<
 <>
C
 <B
?
=
<?
@A
<=
<
 <B
?
Cathode Reaction
Hydrogen Evolution
Reaction (HER)
>
<
=
<
B
?
@
>
<
<=
>
?
<
>
C
<
B
?
@
>
<
>
<
=
<
B
?
@
>
<
=
<
?
Charge Carrier
?
C
?
Operating Temperature
Range 40 – 90
O
C 20-100
O
C 700- 1000
O
C
III. ENERGY, VOLTAGE AND EFFICIENCY
A. Energy Consumption
The energy required for decomposing one mole of water
into hydrogen and oxygen corresponds to the enthalpy of
formation of one mole of water. The minimum amount of
the enthalpy of reaction that has to be applied as electrical
energy is the free energy of reaction G
reac
(change in Gibbs
free energy) defined in terms of enthalpy of reaction, H
reac
,
Thermodynamic temperature, T and the Entropy of reaction,
S
reac
, by below equation Eq.2.



 

(2)
Minimum energy required is given by Gibbs free energy
relation deduce from Eq. 2.

 

D 

(3)
At STP the thermodynamic decomposition voltage of water
in theoretical condition is 1.23V and the current efficiency is
100%. Therefore, the theoretical consumption of energy
(E
theo
) for producing 1m
3
of H
2
is 2.94 kWh/m
3
H
2
. However,
for gas evolution the voltages need 1.65–1.7V. Therefore in
industries the voltage of about 1.8–2.6 V use. Hence the
practical energy consumption is nearly 1.5 to 2.2 times more
than the theoretical energy consumption. Hence the actual
efficiency is between 48% and 70% [16].
B. Voltage and Overpotentials Required
For water electrolysis there are need of two voltages for
energy calculation, the water electrolysis voltage (reversible
cell voltage) and enthalpy voltage (thermo-neutral voltage).
The minimum cell voltage required for the decomposition of
Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and
water is the reversible cell voltage V
rev
expressed as Eq.4 where n is the number of moles of
electrons per mole of products, that is, n =2; and F is the
Faraday constant (F
= 96 500 coulombs mole

EF
GHIJ
!K
The minimum cell voltage required for water electrolysis is
related to the enthalpy of reaction and is called the enthalpy
voltage (thermo-neutral cell voltage); V
enth
as Eq.5 [25].
!"#
L4
GHIJ
!K
The cell voltage of an operating electrolysis cell is
significantly higher than the theoretical reversible cell
voltage derived from thermodynamics. Because some extra
voltage is needed to overcome the irreversibilities resulting
from reactant products, tr
ansportation, charge transfer and
resistance of electrolyte and electrodes in the electrolysis
process, increase the actual requirement [26]. The empirical
Tafel equation is used to evaluate the relationship of over
potential η and current density i is exp
ressed by Eq. 6 as
below, Where A and B depends over anode and cathode
material [16].
ƞ $  %MNO&
The resistances due to membrane R
mem
bubble R
bub
and circuit R
cir
are also sums the high energy
consumption and called o
hmic voltage drop, the total ohmic
voltage drop is given by Eq.7 for current density i.
 P '
"(")
  P *
++
 *
)
 *
,-,
*
The real cell voltage V
real
can be regarded as the sum of the
reversible cell voltage Vrev
, the voltage drop i*R
by the area specific ohmic resistance of the cell at a certain
current density i, and the overpotentials (oxygen and
hydrogen overpotentials) at the anode and cathode
electrodes, ƞ
A
and ƞ
C
, respectively, that is expressed
The share variation of ohmic resistance voltage, oxygen
overpotentials and hydrogen overpotentials with respect to
current density is plotted in figure 3 [18].
)
 

 Qƞ
/
Q  Qƞ
0
Q   R *
"(")
Figure 3: Typical cell
voltage versus current density
characteristic of a polymer electrolyte membrane (PEM)
electrolysis cell [18].
1
1.2
1.4
1.6
1.8
2
040 200 400 600
800
Cell Voltage (V)
Current Density mA/cm2
Reverseble Voltage
Ohmic Resistance Voltage
Oxygen Overpotential
Hydrogen Overpotential
Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and
High Temperature Water Electrolysis
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rev
which can be
expressed as Eq.4 where n is the number of moles of
electrons per mole of products, that is, n =2; and F is the
= 96 500 coulombs mole
-1
) [24]
(4)
The minimum cell voltage required for water electrolysis is
related to the enthalpy of reaction and is called the enthalpy
enth
can be expressed
(5)
The cell voltage of an operating electrolysis cell is
significantly higher than the theoretical reversible cell
voltage derived from thermodynamics. Because some extra
voltage is needed to overcome the irreversibilities resulting
ansportation, charge transfer and
resistance of electrolyte and electrodes in the electrolysis
process, increase the actual requirement [26]. The empirical
Tafel equation is used to evaluate the relationship of over
ressed by Eq. 6 as
below, Where A and B depends over anode and cathode
(6)
mem
, electrolyte R
elec,
are also sums the high energy
hmic voltage drop, the total ohmic
voltage drop is given by Eq.7 for current density i.
*
.
(7)
can be regarded as the sum of the
, the voltage drop i*R
total
caused
by the area specific ohmic resistance of the cell at a certain
current density i, and the overpotentials (oxygen and
hydrogen overpotentials) at the anode and cathode
, respectively, that is expressed
as Eq.8.
The share variation of ohmic resistance voltage, oxygen
overpotentials and hydrogen overpotentials with respect to
(8)
voltage versus current density
characteristic of a polymer electrolyte membrane (PEM)
C. Efficiency
The energy of hydrogen out flow per unit time per unit of
electrical power supplied is defined as energy efficiency
1
32
In
water electrolysis process, as expressed in Eq.9. This
efficiency is based on the electrical power supply for
electrolysis.
1
23
L4
GHIJ
K
S5
T48
2U
Whereas the lower heating value (LHV) of hydrogen
kWh per kg corresponding to a
1.254 V
is most often used as the reference. Efficiency
values based on the higher heating value (HHV)
kWh per kg corresponding to the thermo
1.48 V
are also reported in the literature. Some of the most
us
ed relations for efficiencies based on HHV are as below:
According to [23,27 -
29] the efficiency is defined in terms
of out flow rate of hydrogen, electrical power input, heat
supplied to heat exchanger and redundant heat needed. The
expression of efficie
ncy is expressed as Eq.10.
1
23
K
S5
448
2UC2
SV
C2
GHW
According to [11, 28, 30 -
32] the efficiency is defined in
terms of out flow rate of hydrogen, electrical power input,
heat supplied to heat exchanger and redundant heat needed
including with electricity generation efficiency. The
expression of efficiency
is expressed as Eq.11
1
23
K
S5
448
2U 1
HXHJYGZJ
C2
SV
C2
GHW
The electrical efficiency has wide range from 25% to 80%
and this is free from electrolysis cell system [
33]. In the other efficiency relation given by r
27, 28 and 33] included the effect of environmental
temperature and external heat source temperature on the
thermal energies and expression is given as Eq.12. So this
expression has more credibility to describe the water
electrolysis efficie
ncy compare to Eq. 10 to Eq. 12
efficiencies expressions.
1
23
K
S5
448
2U 1
HXHJYGZJ
C2
SV
[?\
\
H
C
further we can calculate the total efficiency of hydrogen
production plant including the power input starting
water treatment , hydrogen production, hydrogen drying
pump power and hydrogen purification can be taken into
account. For comparing many different electrolysis systems,
it is necessary to take similar operating pressure and
boundary conditions. At
room temperature and without any
consideration of additional heat to the process voltage
efficiency measured conveniently based on HHV. The
voltage efficiency is the ratio of enthalpy voltage to the
actual voltage of cell and expressed as Eq.13.
1
8()"9
8
H]Y^
8
GHIX
The equation13 is valid when the process follows or
assumes to follow the faraday’s law of decomposition.
While during process the condition to faraday assumption
cannot be maintain due to the gas diffusion,
behavior within the cell and leakage of gases to
800
1000
Ohmic Resistance Voltage
Hydrogen Overpotential
Hydrogen Production by Water Electrolysis: A Review of Alkaline Water Electrolysis, PEM Water Electrolysis and
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The energy of hydrogen out flow per unit time per unit of
electrical power supplied is defined as energy efficiency
water electrolysis process, as expressed in Eq.9. This
efficiency is based on the electrical power supply for
(9)
Whereas the lower heating value (LHV) of hydrogen
33.3
kWh per kg corresponding to a
n electrolyzer voltage of
is most often used as the reference. Efficiency
values based on the higher heating value (HHV)
of 39.4
kWh per kg corresponding to the thermo
-neutral value of
are also reported in the literature. Some of the most
ed relations for efficiencies based on HHV are as below:
29] the efficiency is defined in terms
of out flow rate of hydrogen, electrical power input, heat
supplied to heat exchanger and redundant heat needed. The
ncy is expressed as Eq.10.
(10)
32] the efficiency is defined in
terms of out flow rate of hydrogen, electrical power input,
heat supplied to heat exchanger and redundant heat needed
including with electricity generation efficiency. The
is expressed as Eq.11
(11)
The electrical efficiency has wide range from 25% to 80%
and this is free from electrolysis cell system [
11, 31, and
33]. In the other efficiency relation given by r
eferences [21,
27, 28 and 33] included the effect of environmental
temperature and external heat source temperature on the
thermal energies and expression is given as Eq.12. So this
expression has more credibility to describe the water
ncy compare to Eq. 10 to Eq. 12
C
2
GHW
[?\
\
H
 
(12)
further we can calculate the total efficiency of hydrogen
production plant including the power input starting
from
water treatment , hydrogen production, hydrogen drying
pump power and hydrogen purification can be taken into
account. For comparing many different electrolysis systems,
it is necessary to take similar operating pressure and
room temperature and without any
consideration of additional heat to the process voltage
efficiency measured conveniently based on HHV. The
voltage efficiency is the ratio of enthalpy voltage to the
actual voltage of cell and expressed as Eq.13.
(13)
The equation13 is valid when the process follows or
assumes to follow the faraday’s law of decomposition.
While during process the condition to faraday assumption
cannot be maintain due to the gas diffusion,
currents
behavior within the cell and leakage of gases to
International Journal of Engineering and Advanced Technology (IJEAT)
ISSN: 2249 – 8958, Volume-4 Issue-3, February 2015
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surroundings. These disturbance reduces the real production
rate of hydrogen from the ideally calculate production rate.
Hence faraday efficiency (Current efficiency) calculates
these lose and expressed mathematically as Eq.14.
1
:7;
K
S5_GHIX
K
S5_`WHIX
K
S5_GHIX
a!K
bc
(14)
Thus the total cell efficiency 1
0))
(losses due to the power
consumption of peripheral devices are not considered) is the
product of the voltage efficiency and the current efficiency,
as Eq.15.
1
))
1
()"9
d 1
:7;
(15)
IV. ALKALINE ELECTROLYSIS
Clean energy is need of this world while the world
generating lots of pollutants enough to change adversely the
ecosystem. The hydrogen production by alkaline water
electrolysis is one of the environmental friendly, zero
emission of carbon dioxide if this process combined with
renewable energy sources [34-36]. Alkaline water
electrolysis is old technology but this is one of the easiest,
simplest and suitable methods for hydrogen production but
alkaline water electrolysis face the crisis of relatively high
energy consumption, installation cost, maintenance cost,
durability and safety [37, 38]. Alkaline electrolysis is a
mature process: suitable electrolyzers are industrially
manufactured. If the electricity is generated by CO
2
-free
processes (renewables, nuclear), alkaline electrolysis is a
sustainable way to produce hydrogen. The recently
developed zero gap system in the Chlor–Alkali electrolyzer
together with new electrode technologies introduces
superior performance [37]. Alkaline electrolyzer
decomposes water at the cathode to hydrogen and HO
-
. The
latter migrates through the electrolyte and a separating
diaphragm/membrane, discharging at the anode liberating
the O
2
. The electrolyte is an aqueous solution containing
either NaOH or KOH with a typical concentration of 20–40
wt. % and operation temperatures are between 343 and 363
K and operating pressure upto 3MPa.
A. Cell Components
i. Diaphragm: for separating the anode and cathode the
materials used for diaphragm are:
a) Asbestos: Now asbestos not used due to safety
regulation of health.
b) Composite material based on ceramic materials or
microporous materials, few examples are ;
Polyethersulfone(PES) a reinforced, microporous
polymer membrane , glass reinforced
polyphenylene sulfide(PPS) compounds, Nickel
Oxide layer on a mesh with Titanium Oxide and
potassium Titanate fine pored, predominantly
ceramic
ii. Electrodes: Electrodes are cathode and anode on which
the hydrogen and oxygen gas separate respectively. A
cathode is the electrode of an electrochemical cell at which
reduction occurs, positive charges usually move towards the
cathode. In a device which consumes power, the cathode is
negative, and in a device which provides power, the cathode
is positive. Usually negative charges move towards the
anode. The anode is positive in a device that consumes
power, and the anode is negative in a device that provides
power. Metal materials as cathodes for HER are divided into
three classes: (a) Metals with high overpotentials: Cd, Ti,
Hg, Pb, Zn, Sn etc. (b) Metals with middle overpotential: Fe,
Co, Ni, Cu, Au, Ag, W etc. (c) Metals with low
overpotentials: Pt, Pd [16]. Metal materials as anode for
OER is generally of Ni and its alloy and also the same
material as cathode can use for anode also.
B. Electrocatalyst
Electrocatalyst is very important material for enhancing the
efficiency of water electrolysis process of hydrogen
production because by diverting the reactions pathway to
lower activation energy. The kinetics of both the HER and
the OER depend strongly on the activity of the
electrocatalyst. In this regard many different combination of
metals and oxides like Raney-Nickel-Aluminum, activated
electrode can be enhanced by adding cobalt or molybdenum
to the alloy , galvanic deposition of Ni–Zn, Ni–Co–Zn, or
Fe–Zn alloys on the electrode support (perforated plates), or
the vacuum plasma spraying (VPS) technique. Several
combinations of transition metals, such as Pt
2
Mo, Hf
2
Fe,
and TiPt, have been used as cathode materials and have
shown significantly higher electrocatalytic activity than
state-of-the-art electrodes. A large number of mixed oxides
have been investigated with the goal of minimizing the
anode overpotential. The oxide ruthenium oxide (RuO
2
),
Spinel type oxide (cobalt(III) oxide (Co
3
O
4
) and Nickel
cobaltite (NiCo
2
O
4
)), perovskite type oxide (LaCoO
3
,
LaNiO
3
and LaCo
x
Ni
1-x
O
3
) and pyrochlore type oxide
(Tl
2
Ru
x
Ir
2−x
O
7
) used as electrode and showed the catalyst
character and decrease the potential requirement
significantly [18].
C. Electrolyte
The electrolytes used in conventional water electrolysis are
KOH and NaOH also H
2
SO
4
use but not as much as KOH
and NaOH used. These are corrosive solution, damage the
electrodes its catalytic activity decreases and increase the
operating cost of process. Therefore additions of some
foreign materials are necessary to neutralize the corrosive
nature of electrolytes. BIMBF
4
molecular electrocatalyst [Ni
(P
2
N
2
)
2
](BF
4
)
2
[39], and many more combinations of ionic
liquid solution with different electrodes are discussed and
optimized in [40,41]. There are many additives are mix in
electrolytes in order to increase the ionic activation. This
way the energy requirement can be decrease for water
electrolysis process. Generally for this purpose Na
2
MoO
4
,
Na
2
WO
4
ethylenediamine based metal chloride complex ([M
(en)
3
]Cl
x
, M=Co, Ni, etc.) [42-44]. In other research [45]
author added ethylene di-amine cobalt (III) chloride
complex ([Co(en)
3
]Cl
3
) or trimethylenediamine cobalt(III)
chloride complex ([Co(tn)
3
]Cl
3
) into KOH solution as
catalyst for HER evolution. In [46] author added Na
2
MoO
4
and [Ni (en)
3
]Cl
2
as ionic activators. In [47] author added
hexadecyltrimethyl ammonium bromide (HTMAB), cationic
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surfactant [48, 49], into H
2
SO
4
electrolyte to enhance water
electrolysis. In a research work [50] NaCl addition in the
different electrolyte studied to see the effect on hydrogen
production. In this work liquids use as electrolyte tap water,
margine, gas liquor, waste water from cooking, puckered
olive, urine, vinegar of pink, municipal waste water and
finally milk water. The addition of NaCl in the produced
more hydrogen and increase the efficiency was noted in this
work. Hence adding of catalyst compound in the electrolyte
solution is easy and efficient method to enhance the process
output. Also using or adding the ionic liquid into or as
electrolyte solution increase the life of electrodes and so
economically sound.
D. Electrolyzer design
In conventional electrolyzer, the perforated electrodes sheet
fixed in the middle of compartment. There is some gap
among cathode, diaphragm and anode in the conventional
design. This design cause a disadvantage and results high
ohmic loses due to gap in cathode and anode. Hence in the
recent development ‘zero-gap’ electrode design to reduce
the gap between the electrodes consequently ohmic loses
reduce. The zero-gap cell has become state-of-the-art in
modern alkaline electrolyzer. Zero gap system is able to
reduce cell voltage in alkaline water electrolysis [18, 37].
Plots of cell voltage versus current density at different
temperatures for an optimized electrolysis cell are presented
in Figure 4 below. Both electrodes consist of highly active
Raney nickel made by the VPS technique. In the same figure
for a conventional alkaline electrolysis cell with electrodes
made of untreated iron (cathode) and nickel-plated iron
(anode) for comparison. The graph shows the considerable
cut in voltage for high temperature and for active Raney
nickel electrodes. For real applications a number of cells are
assembled as one unit called cell stack and this cell stack is
core component of an electrolysis system. In past unipolar
concept designs was commercially in application. Now
bipolar series connection of the cells alkaline electrolyzer
used also called filter-press assembly.
E. Innovations & Development
The water electrolysis has a great age of development, and
even now need a long way to develop. There are many
theoretical aspects and as well as practical also where
development is needed. Using the natural sources like solar,
wind, cyclone, steam etc. energy in place of electricity is
interesting and desirable aspect of this process. These
innovations are made toward to pollution control, reducing
cost, increasing efficiencies. While the new innovation are
quite appealing but also have its own limitations. Among the
several recent developments photovoltaic electrolysis and
steam electrolysis are discussed here. Photovoltaic (PV)
Figure 4: A comparison between two different types of
electrode A and B [18].
electrolysis: Solar energy is in abundant quantity and its
utilization in different need of humanity is well known.
Solar energy now also using in water electrolysis as a
sources of electricity. Firstly author of [51] used this
concept by using titanium oxide electrodes. In this
technology the capturing of photovoltaic energy and
converting into required electricity is main concern. The
electrode in photovoltaic electrolysis is called
photoelectrodes in which energy of sun light absorbs. So
basically here the system consists of two cells one is
photovoltaic cell and other is electrolysis cell. Theoretical
efficiency is investigated upto 18.3% [52], while for large
scale production it is quite low upto 6% [53]. Costly
photoelectrical active material, low operation current
density, low solar energy density, variation of sun radiation
are the key problems for the new technology[2] and has to
study in detail to use this technology in practical level
production.
V. POLYMER/PROTON ELECTROLYTE
MEMBRANE ELECTROLYSIS (PEM)
In the row of development of water electrolysis technology
for efficient hydrogen production General Electric (GE)
developed the first water electrolyzer based on a solid
polymer electrolyte concept [54]. Later Grubb [55, 56] use a
solid sulfonated polystyrene membrane as an electrolyte in
the development of this technology. This new technology
then named as polymer electrolyte membrane or proton
exchange membrane (PEM). When an acidic solid polymer
is used as the electrolyte in place of liquid electrolyte is
called polymer electrolyte membrane (PEM) electrolysis or
proton exchange membrane electrolysis. Only deionized
water without any electrolytic additive is fed to the cell. The
membrane functions both as the gas separator and the
electrolyte. The half-cell reactions (HER and OER) are
expressed according to the equations in Table1. Polymer
electrolyte water electrolysis system (PEM) [57, 58]
produce best alternative for hydrogen production other than
alkaline water analysis (AWE). Polymer electrolyte water
electrolysis (PEM) have more advantages over alkaline
0
0.5
1
1.5
2
2.5
3
3.5
4
0 200 400 600
Cell Voltage ( V)
Current Density (mA/cm2)
A - cathode: untreated iron ; anode: nickel-plated iron
B - cathode : molybdenum - raney nickel ; anode : raney
nickle and cobalt spinel
A-313 K
B 313 K
A 333 K
B 333 K
A 353 K
B 353 K
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water electrolysis, advantages are ecological cleanness,
small size and mass, high purity of hydrogen gas, low gas
crossover, lower power consumption, high proton
conductivity, control over electrical power variations, high
pressure operation, higher safety level ,easy handling and
maintenance [59]. The operation cost of PEM was more
comparatively but the continuous development in research
and investigations on PEM electrolysis for hydrogen
production are focused greatly [60]. In this regard new
catalysts [59, 61]; new PEM electrolytes [62] current
collector methods are going to develop to sustain this green
technology [38]. For PEM electrolyzers, the current
efficiency (Faraday efficiency) is assumed to be over 99%
[63].Requirements of PEM must achieve with high ionic
conductivity, good oxidative stability, mechanical, chemical
and thermal stability, low permeability for gases, good
electric insulator and ionomer must have high stability to
withstand the harsh conditions in a PEM electrolysis cell.
A. PEM electrolysis Components
Membrane Electrode Assembly (MEA)
o Membrane
o Anode and Cathode electrode with
electrocatalyst
Gas diffusion Layer (Current collector)
Bipolar plates
The fundamental design of a PEM electrolysis cell, as
shown in Figure 5 below, the two half-cells are separated by
the membrane. The components of PEM are membrane
electrode assembly (includes membrane, anode and cathode
electrodes), gas diffuser (current collector), gasket, bipolar
plates and interconnector. The core component of PEM cell
is membrane electrode assembly (MEA) in which electrodes
is coated directly. Gas diffuser (current collector) and
gasket are used to enable an electric current to flow between
the bipolar plates and the electrodes. The bipolar plates are
electrically conductive, support to transport liquid water at
the anode and oxygen and hydrogen out of the electrolysis
cell. Materials such as titanium and coated stainless steel
have to be used for constructing the bipolar plate, current
collector, and, if necessary, the support for catalysts.
B. Membrane Electrode Assembly (MEA)
i. Membrane:
The core component of PEM cell is membrane electrode
assembly (MEA). MEA combined membrane, cathode,
anode and coated electrocatalyst. The membrane is
backbone of this assembly and called supported membrane a
dimensionally stable membrane (DSM
TM
). Perfluorosulfonic
acid polymers – PFSA membranes are used such as
Figure 5: Fundamental of PEM electrolysis.
Nafion
®
, Flemion
®
, Fumapem
®
and Aciplex
®
. These PFSA
membranes are known for high oxidative stability, high
strength and high efficiency, dimensional stability with
temperature change, high proton conductivity and good
durability. Lifetimes approaching several 10 000 h and
proton conductivities as high as 0.1 S/cm
have been
reported. The typical membrane thickness varies from
approximately 100µm to 200µm, which is a for low ohmic
drop, high mechanical stability, and high gas permeability.
Nafion 110, 115 and 117 from DuPont are the most
commonly employed membranes in PEM electrolysis cells
to date. At current density 1 A/cm
2
, 80
O
C and atmosphere
pressure there are lots of work has done on membranes. In
[64] author did work on three types of polymer electrolyte
membranes (PEMs): Nafion® 115 membrane (127 µm),
Nafion® 212 membrane (51 µm) and Nafion® 211
membrane (25 µm) and catalysts used for anode and cathode
layers were IrO
2
cathode (20 wt.% Pt, 40 wt.% Pt, 60 wt.%
Pt and Pt black ). Loading for anodes and cathodes 3mg/cm
2
of IrO
2
and 0.5mg/cm
2
of Pt and, respectively. Author also
reviewed lots of works in same scale and tabulate. The
author outcomes compare to the other previous are tabulated
below in table 2 from there we can easily see that the
Nafion® 211 produced the best result. To reduce material
costs there are many alternative membranes are used such as
bi-Phenyl Sulfone Membrane (BPSH),
Hydrocarbon/Phosphonate Membrane Inexpensive starting
materials, PFSA membranes (700 EW & 850 EW), m-
phenylene-bis (5,50-benzimidazole) (PBI) , pyridine groups
(H
3
PO
4
doped) aromatic polyethers containing, Sulfonated
aromatic polymers such as polyether ether ketone (PEEK)
and PSf, in [74] author did work on 30wt.% PES/SPEEK
with MEA are 4 cm
2
, anode Ir black , cathode 20 wt.% Pt/C
at temperature 80
O
C. author found by use of this cost
effective membrane can achieve high current density upto
1600 mg/cm
2
good performance and economically
competitive.
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Table 2: Membrane activity summary of some authors [64].
Membrane Thickness
(µm) Anode Cathode Nobel Metal
Loading Cell Voltage(V) References
Nafion® 115 127 IrO2 60 wt.% Pt 3.5 1.585 [64]
Nafion® 212 51 IrO2 60 wt.% Pt 3.5 1.534 [64]
Nafion® 211 25 IrO2 60 wt.% Pt 3.5 1.523 [64]
Nafion® 115 127 IrO2 20 wt.%
Pt/C 2.12 1.64 [65-67]
Nafion® 115 127 IrO2 30 wt.%
Pt/C 2.65 1.70 [68]
Nafion® 115 127 IrO2 30 wt.%
Pt/C 3.18 1.70 [69]
Nafion® 115 127 IrO2 30 wt.%
Pt/C 3.18 1.72 [70]
Nafion® 115 127 IrO2 Pt black 4.5 1.60 [71]
Nafion® 112 50 Ir black Pt black 2.5 1.70 [72]
Nafion® 212 51 Ir-Sn Oxide, 40 wt.%
Pt/C 1.8 1.580 [73]
ii. Electrode and Electrocatalyst
In MEA, typically membrane is coated directly with noble
metal electrodes (at a loading for cathode upto 2mg/cm
2
and
for anode upto 6 mg/cm
2
) and their oxides as
electrocatalysts (platinum, iridium, iridium dioxide (IrO
2
),
rhodium, rhodium oxide (RhO
2
). Another approach is to
apply the electrocatalysts on the current collector, for
example, by spraying. In a second step, the electrode and
current collector are fixed to the membrane. Hot pressing
method generally used for manufacturing the MEA for PEM
electrolysis. Mixture of catalyst and ionomer used in order
to enlarge the area of three-phase-boundary where half-cell
reactions take place. Hydrogen (cathode) side Supported or
unsupported Platinum black (loading: 1-6 mg·cm-2) and
oxygen (anode) side: Unsupported iridium black, ruthenium
and their oxides and mixtures (loading: 1-2 mg·cm-2)
assemble. The MEA is of high cost intensive due to catalyst
high cost. The cost can be reduce by reducing the catalyst
load on cathode and anode and other way substituting the
high cost catalyst by other low cost material for optimizing
the production and capital cost [54]. Starting from the above
standard configuration, considerable progress has been made
in improving the performance of PEM electrolysis. The
rigorous and valuable study related to electrocatalyst can be
found in two series of work. On the OER the work were
done by Burke and Moynihan and on the HER the work
were done by Furuya and Motoo [75-88]. The OER activity
is enhanced and can be improved by using other catalyst
systems. Mixed oxides of metals from the platinum group,
for example, mixed oxides of iridium and ruthenium.
Ruthenium oxide (RuO
2
) is the most active material for
OER; yet it is highly unstable (the oxidation of RuO
2
to
RuO
4
occurs at potentials more positive than 1.387V)
Iridium oxide (IrO
2
) is the standard material compromising
activity and stability. In the following years, studies on PEM
water electrolysis had been focused on the survey of
electrocatalysts in order to mitigate the drawback of the
OER irreversibility and slowness. Miles and Thomason
experimented over many different metals for the activity of
HER and the OER at 0.1 mol/L H
2
SO
4
at 353K by cyclic
voltammetric techniques. In their observation they found for
the HER the activity in order of Pd > Pt > Rh > Ir > Re > Os
> Ru > Ni and for the OER, the order was Ir > Ru > Pd > Rh
> Pt > Au > Nb. However the oxides of each catalyst
affecting their electrocatalytic activities for the OER greatly
[89]. Among all the RuO
2
showed that oxygen overvoltage
much lower than Ru, Pt or any other material tested and also
gave the high metallic conductivity similar with IrO
2
[90,91]. In many research works it is found that RuO
2
and
Ru go through oxidation and these results the forming of
RuO
4
in acid electrolyte. Consequently Ru is leached out
from catalyst layer of membrane and precipitated and
deposited. Ru ions which may be complexes, negative ionic
state, back-diffuse from the cathode, in cracks, or in the gas
passages [92, 93]. Hence a lot of research works had been
done to finding the alternative to control corrosion nature of
RuO
x
, to use of RuO
x
because of its abundance over IrO
2
[91, 93]. In the direction of work the researchers found that
the mixing of IrO
2
(20%) into RuO
2
improves the stability
and decrease the erosion rate by 4% [94]. Ru(RuO
2
)
compare to Ir(IrO
2
) is more active but use of Ru(RuO
2
) is
limited due its corrosive nature. So for improving the
efficiencies reducing the capital cost corrosive control must
be taken into account for durability of electrodes and
electrolyte system. Lots of work has been done and still
working on numbers of possibilities and alternatives for
OER and HER activity. Binary catalysts are emerging
concepts like mixing of IrO
2
with SnO
2
[95] and among
mixture (TaO2, SbO2, TiO2, Ti4O7 …) also improved
performance of MEAs [96]. Material and mixture of Ta
2
O
5
,
Nb
2
O
5
and Sb
2
O
5
also tried by authors [97-101]. Coating of
Ti over IrO
2
[102], Ti over IrO
2
– Ta
2
O
5
[98] also used. The
Nano sizes (2, 3,7,12 nm) of IrO
2
catalysts activity also
studies over TaC [68-70, 103]. In other works [104, 105] the
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catalyst consisted of pure metal oxide (IrO
2
, SnO
2
and
RuO2) binary mixtures (Ir2RuO
16
, RuIrO
4
) and tertiary
metal oxide (Ir
2
RuSnO
8
) studied. Ternary unsupported
catalyst also taken in great consideration like Ir
x
Ru
y
Ta
z
O
2
,
Ir
x
Ru
y
Mo
z
O
2
and at 353 K, cell voltages of less than 1.6 V
at 1A/cm
2
have recently been observed with anode catalysts
based on Ir
x
Ru
y
Ta
z
O
2
[66,106]. The best result was obtained
with an Ir
0.6
Ru
0.4
O
2
anode and a 20 wt. % platinum–carbon
cathode (1.567 V at 1 A/cm
2
, Nafion 115). At 363 K,
carbon-supported palladium catalysts on the cathode (40 wt.
% palladium–carbon, 2.4 mg/cm
2
; 1.65V at 1A/cm
2
) yielded
higher efficiency comparatively. For HER electrocatalyst
activity many other combinations are tried like MoS
2
with
graphite [107, 108], MoO
3
[109], Cu
1-x
Ni
x
WO
4
[110], PWA
with CNT [111], WO
3
nano rods [112, 113], Co and Ni
glyoximes [114, 115], Pd/CNTs [116,117], Cu/Pt [118]
studied and proving alternatives of good performance. There
are many factors that can be effect on overall OER and HER
electrode qualities, acidic media activity, durability of
coatings, electron conductivity, and electrocatalyst activity
with operation time. Hence for the future and sustainability
of PEM electrolysis process work should be done in
expanding surface area, improving electron activity, change
storage capacitance, transport component of impedance,
high catalyst utilization and high mass transport.
iii. Innovations & Development
There are many positive outcomes in the continuous
development of catalyst, electrode materials, and methods
etc. for PEM electrolysis process. PEM is most desirable
method to produce hydrogen by electrolysis and yet to
optimize the method in great need. In this way further
development are urged like Core-Shell catalyst, Bulk
metallic Glasses and nanostructured films technologies and
reviewed greatly by author [38]. Nanostructured thin films
are emerging and advanced technique for PEM electrolysis
cell design. This technology reduces total cost, increases
durability of cathode and anode, corrosion resistance,
specific activity and mass activity also good OER and HER
activities [119, 120]. Pt
68
Co
29
Mn
3
, Pt
50
Ir
50
and Pt
50
Ir
25
Ru
25
nanostructured thin films are study compare to Pt-black and
author found higher performance [120]. Catalysts for core
shell in order to increase the efficiencies and decrease metal
loading and reducing the cost a core – shell model is very
appealing and promising method. The core – shell is a
bimetallic alloy in which a metallic core substrate supports a
metallic monolayer. In [121] Pt use as metallic mono layer
and Cu use as core substrate. There are very less research
are done for core-shell combinations for PEM, but Pt-Cu is
testified by its very high catalytic reactivity. It has been
reported in the work [122] that if Pt-Cu nanoparticles are
used as core – shell catalyst the OER can be accelerate
uniquely high and the total cost for Pt use in conventional
PEM can be reduce upto 80% because of dramatic reduction
in loading below to 0.2 mg/cm
-2
and there are lots of
indication to improve more. Hence the new structure as
Core- Shell catalyst could change the present scenario of
hydrogen production by PEM. The high surface area is
always helpful to increase the activity of catalysis, precious
metal catalysts can utilize to develop inform of alloys of
multicomponent and nanowire structure to enhanced the
surface are. A multicomponent alloy increases the charge
transfer between them so the electronic band structure [122].
In this direction of development some researchers
demonstrated alternative catalyst called bulk metallic
glasses (BMGs) [123-126]. In [123] author also found that
Pt-BMG is very suitable for high efficiency PEM
electrolysis.
VI. HIGH TEMPERATURE WATER
ELECTROLYSIS
The high temperature electrolysis is favorable due to its
thermodynamics. At high temperature the total electricity
demand decreases significantly compare to rise in thermal
demand. Ionic conductivity of the electrolyte and rates of
electrochemical reactions at the electrode surfaces increases
at high temperature. High temperature can be reused from
waste heat of the process like nuclear origin, Solar,
Geothermal, fossil and from any high thermal process. The
below Figure 6 [15] demonstrate the electrical energy and
thermal energy requirement as temperature of electrolysis
process increases.
Figure 6: the high temperature electrolysis process
energy requirement with variation of temperature.
In [127] author did an experiment in which 19M KOH
electrolytes with distilled water has taken. The electrode
assembly; cathode is Monel wire [Nickel 400 (66.5% Ni,
31.5% Cu, 1.2% Fe, 1.1% Mn)] fixed with various
combinations of anode which included Ni, Li-Ni, and Co-
Ni. Author experimented for electrolysis at atmospheric
pressure, electrolysis at elevated pressure and electrolyte
hygroscopy at high temperature. The experiments are
carried out at 35, 80,200,250,300,350 and 400
O
C. and it is
found that the electricity requirement decreases significantly
with rise of temperature. The best result found at a
temperature of 400
O
C and pressure of 8.7 MPa on a Co-Ni
anode. In [128] work author did work on two different plan
of hydrogen production by water electrolysis. In this work
authors use the external energy sources for achieving high
temperature upto 1000K and high pressure upto 100Mpa. In
the result it is very clear indication that the high temperature
and pressure electrolysis need smaller energy inputs. The
splitting reaction potential of water molecule is reduce with
increasing in temperature. The efficiency of Electrolysis
process is enhanced with decrease in energy consumption
for any current supply due to the raised temperatures [45,
129]. This due increase in surface reaction and ionic
conductivity of electrolyte with temperature rise [130]. At a
high temperature steam electrolyzer (HTSE) it is found high
temperature water electrolysis to need less energy compare
to conventional low temperature electrolysis processes.
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0.9
1
1.1
1.2
1.3
1.4
1.5
250 350 450 550 650 750
Voltage (V)
Temperature (K)
Electrolysis Voltage Enthalpy Voltage
Overall efficiency of the process achieved 59% at 1000
o
C
water electrolysis compare to 33% initial value [131]. In
another study the process efficiency of high temperature and
high pressure electrolyte solutions found that at high
temperature the less amount of voltage required for achieve
the desired current density [127]. In the heated cell the
reversible potential and overpotentials shifted down [132].
At high pressure the gas bubble shrinks so in result there is
drop on ohmic voltage so consequently the power reduce. In
many research it is found the high pressure electrolysis
process need low power in the process. In an experiment of
Appleby et al. [133] author find the when the pressure is
increased from 1 to 10 atm there is fast reduction in power
consumption and upto 30 atm pressure the total potential
drop of about 100mV. The similar work are also done by
LeRoy et al. [134] and Onda et al. [135], B. Laoun[ 25]
experienced the same fact that the power consumption
decreases with increase in pressure irrespective of
temperature. B. Laoun [25] in his work use the following
equations Eq.16 and Eq.17 for electrolysis voltage and
enthalpy voltage, respectively with the relation of
temperature and pressure change and noted the effects for
the pressure range 1 atm to 1000 atm.

_ 6 e 

_ fgh L

_ 6D
L

_ fghi (16)
!"#
_ 6 
!"#
_ fgh L

_ 6D
L

_ fghi (17)
Author did experiment for constant pressure at 1 atm. with
temperature variation from 298K to 1000K and noticed in
the voltage temperature graph the slope of electrolysis
voltage reduce sharply compare to the enthalpy voltage
slope, but the both voltages are drop with increase in
temperature. The figure 7 closely describes the pattern found
by author in his work.
Figure 7: Variation of electrolysis and enthalpy voltage
with temperature at 1 atm. Reproduced data from [25].
Hence the above equations, graphs and tables are advocating
the good effect on efficiency increase as the temperature and
pressure increases. These effects are also studied by [139-
139] in their research works.
VII. COMPARISON BETWEEN TECHNOLOGIES
The below table 3 is a comparison table in this comparison
the alkaline water electrolysis is overcome other electrolysis
process currently mature, reasonable efficiency, relative cost
effective to the other emergent water electrolysis
technologies.
Table 3: A Comparison between technologies [2, 38, 140].
TECHNOLOGY ADVANTAGES DISADVANTAGES
Alkaline Electrolysis
Technology: Oldest and Well established
Cost: Cheapest and effective
Catalyst type: Noble
Durability: Long term
Stacks: MW range
Efficiency: 70%
Commercialized
Current Density: Low
Degree of Purity: Low( crossover of
gases)
Electrolyte; Liquid and Corrosive
Dynamics: Low dynamic operation
Load range: Low for partial load
Pressure: Low operational pressure
PEM electrolysis
Current density: High
Voltage efficiency: High
Load range: Good partial load range
System Design: compact
Degree of Purity: High gas purity
Dynamic: high dynamic operation
Response: rapid system response
Technology: New and partially
established
Cost: High cost of components
Catalyst type: Noble catalyst
Corrosion: acidic environment
Durability: comparatively low
Stack: Below MW range
Membrane: limited and costly
Commercialization is in near term
High Temperature electrolysis
Efficiency = 100%
Thermal neutral efficiency> 100% w/hot
steam
Catalyst: Non noble
Pressure: High pressure operation
Technology: In laboratory phase
Durability: low due to high heat,
Ceramics
System Design: Bulk system design
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VIII. CONCLUSION
Alkaline water electrolysis is easiest and simplest methods
for hydrogen production. Less efficiency is one of the great
disadvantages in order to widespread use of this system.
Effort for development and research needed to over comes
the disadvantages like energy consumption, cost and
maintenances, durability, reliability and safety. The
thermodynamic analysis shows the energy requirements
theoretical and actual, resistances offered by system and also
discussed different efficiencies, these parameters will help
to identify the key problems in way to improvement. The
kinetic analysis indicates the reaction rate in alkaline
solution, ion transfer electrode surface activity and also
effect of different electrolytes and additives on production.
In the direction of improving this application the research
have to consider significantly for reduce electrochemical
reaction resistance, possibilities of low cost electrodes,
electrocatalysts, electrolytes and its additives to increase
ionic mass transfer, corrosive resistive electrolytes and
electrodes for durability of electrolyzer to reduce electrode
surface tension, electrode surface profile modifications and
surface coatings, and more importantly, managing the gas
bubble resistances. Improve the catalytic activity for HER
and OER by using binary, ternary or quaternary alloys with
an advanced design, improving the electrochemical active
surface area, catalyst utilization, and stability against
corrosions, development of highly conductive supportive
catalyst, Understand and improve the triple-phase-boundary
improve the proton transport across the catalytic layer,
understand the water transport across the triple-phase
boundary. For the anode, find catalyst alternatives to replace
scarce iridium or unstable ruthenium will be considered a
great achievement. New catalyst configurations or designed
structures (e.g.: core-shells, BMGs, NTSFs, nanostructures,
tuned alloys) could provide the necessary condition to
decrease the amount of iridium or stabilize the ruthenium
dissolution over time. For the cathode, improve the catalyst
stability (especially when supported on carbon materials),
explore alternative supports other than carbon and
investigate metal-free N-CNTs catalysts. Also important, is
to explore the use of high surface area carbon materials
(carbon blacks, CNTs, graphenes) with adjusted pore size,
functional groups, grafted polymers and electrical
conductivities for the purpose of achieving higher activities
and stability. Use innovative synthesis methods to produce
new support materials, catalysts, and electrode systems.
Development of membrane alternatives to Nafion® with
advanced membrane synthesis methods, resulting in
electrolytes with higher proton transport but providing at the
same time lower gas crossover and higher durability is
required. This could be done by; using membrane
composites or blends, adding inorganic or organic fillers, or
introducing molecular barriers to the electrolyte. This
review has also captured and documented in one place much
of the varying jargon that has been used throughout the early
development to today. Finally, we outlined our idea of the
direction the future research should proceed in order to
develop PEM electrolyzers as a reliable, cost effective
solution to help solve the issues related with renewable
energy. High-temperature alkaline electrolysis cell
performances are tested at various current densities a wide
range of pressure and temperature with various anode and
cathode materials are compared and showed a great deal of
performance enhancement. Although the results are
encouraging, further study is required to completely
understand the reasons for the observed dramatic decrease in
terminal potential, the possible effects of product mixing
and/or electrode corrosion must be determined. The
application of superheated steam to the cell for water
replenishment, and other improvements required for long-
term cell characterization. In summary, this review could
help provide a basis for the development of a new
generation of alkaline electrolysis systems based on very
high temperature and pressure operation. Pressurized
electrolysis required consistently smaller work and total
(work and heat) energy inputs. Further, the percentage of
work composing total energy for pressurized electrolysis is
greater; suggesting the prospects of integrating an
electrolysis system with external thermal sources needs
consider both operating temperature and product
pressurization. Owing to the high efficiency of water
decomposition at elevated temperatures, HT steam
electrolysis could be an option in the future, but only in the
long term. Since HT heat (e.g., from a nuclear or solar
power plant) and base-load operation are required, this
technology would be favorable for centralized and large-
scale hydrogen production plants. At present, research and
development work is focused mainly on the realization of
long-lasting materials to extend both the lifetime and the
performance of electrolysis stacks. Reduction in system
complexity also remains a major challenge.
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Md Mamoon Rashid, is presently Lecturer in the
Chemical Engineering Department, King Khalid
University in the Kingdom of Saudi Arabia. He has
over 5 years of teaching and research experience.
He has completed his B.E in chemical engineering
from RGPV Bhopal, India and M.Tech. Degree
major in chemical engineering specialized in
computer aided process plant design from Indian
Institute of Technology Roorkee (IIT Roorkee),
UK. India. His research interest lies in modeling
and simulation, transport process, renewable and sustainable energy and
computer application in chemical engineering.
Mohammed K. Al Mesfer, received his B.S. and
M.S. in Chemical Engineering in 2000 and 2006,
respectively, from King Saud University, Riyadh,
Saudi Arabia. He worked in the education field for
almost nine years before joining the Missouri
University of Science and Technology, Rolla,
Missouri, USA, where he earned his Ph.D. in
Chemical Engineering in May 2013. Recently, he is
an assistant professor and chairman of chemical
engineering department at King Khalid University,
Abha, Saudi Arabia. His research interests lie in the area of chemical
reaction engineering, chemical and Petrochemical industry, and Catalysis
and Chemical Reactor Design.
Hamid Naseem, is presently Lecturer in the
Electrical Engineering Department, King Khalid
University in the Kingdom of Saudi Arabia. He has
over 4 years of teaching and research experience.
He has completed his B.E. and M.Tech. degree in
Electrical Engineering from Zakir Hussain College
of Engineering, Aligarh Muslim University,
Aligarh. UP India. He is a member of Institute of
Electrical and Electronics Engineers and
International Association of Engineers. His area of research includes Smart
Grid, Virtual Instrumentation and Renewable energy. He has published
several papers and attended many conferences/ workshops.
Mohd Danish, received B.Tech (Chemical
Engineering) and M.Tech (Chemical Engineering)
with specialization in Process Modeling and
Simulation from Aligarh Muslim University,
Aligarh (India) in 2001 and 2005 respectively. He
worked as Assistant Professor and Head of
Chemical Engineering, MIET, Meerut (India) from
2006-2010.Prior to this, he worked as Lecturer of
Chemical Engineering, Jaipur National University,
Jaipur(India).Currently, he is working as Lecturer in Chemical Engineering
Department, King Khalid University Abha, Kingdom of Saudi Arabia. His
research interest includes Modeling and Simulation, Reaction Engineering
and Catalysis, Separation processes and Renewable energy.
... It is relatively cheap because it does not require as many highvalue materials, while also maintaining a relatively long lifetime of around 75,000 h (International Energy Agency, 2019) It is considered a low-temperature type of electrolysis, as it operates between 40 and 90°C. (Rashid et al., 2015) The weaknesses of AE electrolysis include a low partial load range (10-110%) and operating pressure (1-30 bar). It currently makes up the largest share of electrolysers in commercial use. ...
... (International Energy Agency, 2019) PEM electrolysis was first developed by General Electric in the 1960s to overcome the weaknesses of AE electrolysis (Shiva Kumar and Himabindu, 2019) and uses more expensive noble metals such as platinum or iridium as electrode catalysts while utilizing a solid acidic polymer membrane to transmit protons from the anode to the cathode. (Rashid et al., 2015) They also operate at relatively low temperatures between 20 and 100°C. Advantages of PEM electrolysis include a compact design, high efficiency, small footprint (0.048 m 2 /kW compared to 0.095 m 2 / kW for AE electrolysers), easy scalability, high H 2 purity, and perhaps most importantly, the H 2 can already be produced at high pressures of 30-60 bar, reducing the need for energyintensive compression after production. ...
... (International Energy Agency, 2019) The third main kind of electrolysis technology is solid oxide electrolyser (SOE) cells. These are run at very high temperatures of 700-1,000°C and are the least developed and used of the three major technology options, (Rashid et al., 2015) despite being theoretically able to reach stack efficiencies of almost 100%. (Brauns and Turek, 2020) Steam is used, requiring a high energy input, although the investment costs are relatively low as the electrodes are made of ceramic. ...
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... In the third order, the generation of hydrogen by means of the water splitting method employs efficient and diversified catalysts [41][42][43][44][45][46][47][48]; including porphyrin derivatives [49] is in great demand in current research. ...
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... In the 1960s, industrial hydrogen production shifted to fossil fuels, the main source of hydrogen production to this day [28]. In 2015, only about 4% of global hydrogen production is based on electrolysis [29]. Water electrolysis is a process in which water is split into hydrogen and oxygen by applying electricity. ...
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Hydrogen as an energy carrier can play a significant role in reducing environmental emissions if it is produced from renewable energy resources. This research aims to assess hydrogen production from wind energy considering environmental, economic, and technical aspect for the East Azerbaijan province of Iran. The economic assessment is performed by calculation of payback period, levelized cost of hydrogen, and levelized cost of electricity. Since uncertainty in the power output of wind turbines may affect the payback period, all calculations are performed for four different turbine degradation rates. While it is common in the literature to choose the wind turbine based on a single criterion, this study implements Multi-Criteria Decision-Making (MCDM) techniques for this purpose. The results of Step-wise Weight Assessment Ratio Analysis illustrates that economic issue is the most important criterion for this research. The results of Weighted Aggregated Sum Product Assessment shows that Vestas V52 is the most suitable wind turbine for Ahar and Sarab cities, while Eovent EVA120 H-Darrieus is a better choice for other stations. The most suitable location for wind power generation is found to be Ahar, where it is estimated to annually generate 2914.8 kWh of electricity at the price of 0.045 $/kWh, and 47.2 tons of hydrogen at the price of 1.38 $/kg, which result in 583 tons of CO2 emission reduction.
... The development of methods for H 2 production is motivated by a vision to move away from fossil fuels due to their finite availability and the environmental damage caused by their combustion [7][8][9][10]. There are many ways to generate H 2 including water electrolysis [11], chemical reactions [12], steam reforming of hydrocarbons [13], and from biological sources [14]. These methods are not amenable to portable-use systems, as they are either too expensive or initiated under extreme conditions. ...
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... Hydrogen is a clean energy carrier with the highest specific energy density and is the preferred alternative to fossil fuels to satisfy the huge global energy demand because the energy-releasing process is environmentally friendly. Hydrogen gas can be produced using alkaline water electrolysis (AWE) [1][2][3][4], proton exchange membrane (PEM) electrolysis [5][6][7][8], and a solid oxide electrolyzer cell (SOEC) [9,10]. Water electrolysis is commonly used because it requires simple equipment and produces very pure hydrogen. ...
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Water electrolysis is one of the most common methods to produce hydrogen gas with high purity, but its application is limited due to its low energy efficiency. It has been proved that an external magnetic field can reduce energy consumption and increase hydrogen production efficiency in water electrolysis. In this study, electrodes with different magnetism were subjected to a perpendicular magnetic field for use in hydrogen production by water electrolysis. Gas bubbles that evolve from the surface of a horizontal electrode detach faster than the bubbles from a vertical electrode. The locomotion of the bubbles is facilitated if the horizontal electrode faces a magnet, which induces the revolution of bubbles between the electrodes. However, the magnetic field does not increase the current density effectively if the electrodes are more than 5 cm apart. A paramagnetic (platinum) electrode has a more significant effect on bubble locomotion than a diamagnetic (graphite) material and is able to increase the efficiency of electrolysis more effectively when a perpendicular magnetic field is applied. The conductivity of platinum electrodes that face a magnet increases if the distance between the electrodes is less than 4 cm, but the conductivity of graphite electrodes does not increase until the inter-electrode distance is reduced to 2 cm. On the other hand, horizontal graphite electrodes that are subjected to a perpendicular magnetic field will generate a higher gas production rate than a platinum electrode without a magnetic field if the inter-electrode distance is less than 1 cm.
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In this paper, the electrochemical performance and temperature distribution of a polymer electrolyte membrane water electrolyzer (PEMWE) are studied using a numerical model. The effect of three important parameters including operating pressure, operating temperature, and thickness of the membrane on the thermal and electrochemical performance of the electrolyzer are investigated. The results of numerical modeling are verified against experimental data. Higher temperature is observed over the anode because the exothermic process at the anode is dominant in PEMWE. By increasing the operating temperature and decreasing the operating pressure, the temperature distribution is more uniform and the performance of the electrolyzer improves. By increasing temperature from 333 K to 353 K, the mean temperature difference decreases by 4.5%. In addition, by increasing membrane thickness from 127μm to 254μm, the mean temperature difference of the electrolyzer cell increases by 0.18 K, and the voltage of the electrolyzer increases by about 3.63%.
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Sandstone is the most abundant and favourable medium for such storage as carbonate rock might not be suitable due to potential geochemical reactions. It is well established in the literature that interaction of the host rock-fluid and injected gas plays a crucial role in fluid flow, residual trapping, withdrawal, and more generally storing capacity. Such data for the hydrogen system is extremely rare and are generally limited to contact angle measurements, while being not representative of the reality of rock-brine-hydrogen interaction(s). Therefore, we have conducted, for the first time, a series of core flooding experiments using Nuclear Magnetic Resonance (NMR) to monitor hydrogen (H2) and Nitrogen (N2) gas saturations during the drainage and imbibition stages under pressure and temperature that represent shallow reservoirs. To avoid any geochemical reaction during the test, we selected a clean sandstone core plug of 99.8% quartz (Fontainebleau with a gas porosity of 9.7% and a permeability of 190 mD). Results show significantly low initial and residual H2 saturations in comparison with N2, regardless of whether the injection flow rate or capillary number were the same or not. For instance, when the same injection flow rate was used, H2 saturation during primary drainage was 4% and it was <2% after imbibition. On other hand, N2 saturation during the primary drainage was 26% and it was 17% after imbibition. However, when the same capillary number of H2 was utilised for the N2 experiment, the N2 saturation values were ~15% for initial gas saturation and 8% for residual gas saturation. Our results promisingly support the idea of hydrogen underground storage; however, we should emphasise that more sandstone rocks of different clay mineralogy should be investigated before reaching a conclusive outcome.
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The application of proton exchange membrane water electrolyzer (PEMWE) technology has long been limited by the excessive energy consumption and poor catalyst durability because of the harsh corrosive and oxidative conditions that are related to the anodic oxygen evolution reaction (OER) in acidic electrolytes. Herein, we circumvent this challenge by adopting alternative hydrazine oxidation reaction (HzOR) as the anodic half-reaction, integrated with the cathodic hydrogen evolution reaction (HER) for sustainable hydrogen production. To this end, we further developed a PtCo alloy nanosheets electrocatalyst that can efficiently catalyze both the HzOR and HER with ultralow potentials. Specifically, the overall hydrazine splitting driven by the PtCo alloy requires only 0.28 V at 10 mA cm⁻² along with outstanding stability of more than 3000 h. We further proposed a PEM hydrazine electrolyzer (PEMHE) design to promote the practical application. The device can not only produce hydrogen with a high yield rate of 1.87 mmol h⁻¹ cm⁻² at a practical current density of 100 mA cm⁻² with a long durability of 60 h, but also effectively decontaminate hydrazine sewage with the hydrazine removal efficiency up to 100%. Our work provides a new solution to simultaneous mass hydrogen fuel production and hydrazine hazard removal from acidic waste water at minimized energy consumption.
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Electricity-driven proton exchange membrane water electrolyzers are a promising technology to produce dihydrogen gas (H2). However, this technology is limited by the anodic oxygen evolution reaction that controls the overall operation efficiency, since only few electrocatalysts are efficient under the strongly acidic and oxidative conditions. Accordingly, there is a need for electrocatalysts with high activity, high stability, and low cost for the reaction of acidic oxygen evolution. Here, we review electrocatalysis using the acidic oxygen evolution reaction. We discuss two mechanisms of the oxygen evolution reaction: the adsorbate evolution mechanism and the lattice oxygen mechanism. We then summarize strategies to improve the performance of electrocatalysts by active site engineering, electron distribution optimization, interaction modulation, vacancy engineering, and lattice strain regulation. Challenges include the understanding of mechanisms of the oxygen evolution reaction, the operation durability at industrial currents, the flow reactor design of proton exchange membrane water electrolyzers, the alternative reactions, the development of nonprecious metal-based electrocatalysts, and large-scale synthetic approaches of electrocatalysts. Finally, precautions for electrochemical tests are proposed.
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This study deals with the preparation and characterisation of catalyst-coated membranes for an alkaline water electrolysis process. For this purpose, a chloromethylated anion-selective block copolymer of styrene-ethylene-butylene-styrene with 1,4-diazabicyclo[2.2.2]octane functional groups was used both as an alkaline polymer electrolyte membrane and as an ionomer binder. Non-PGM catalysts (platinum group metals), specifically NiCo2O4 and NiFe2O4, were used on the anode and cathode side of the membrane, respectively. Air-brush deposition or computer-controlled ultrasonic dispersion of the catalytic ink were used to deposit the catalyst layers. The influence of the composition of the catalyst layer on its stability and the resulting electrolysis cell performance was investigated under typical membrane alkaline water electrolysis conditions (1–15 wt.% KOH, 45 °C). The optimal catalyst-to-binder ratio in the catalyst layer was identified as 93/7 using a catalyst loading of 2.5 mg cm⁻² on each side of the membrane. The membrane electrode assembly prepared under optimal conditions showed high stability over 140 h at a current density of 250 mA cm⁻². At this current load, the cell exhibited a voltage of 2.025 ± 0.010 V. The increase in cell voltage observed during the stability test did not exceed 1 μV h⁻¹.
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Water based electrolyzers offer a promising approach for generating hydrogen gas for renewable energy storage. 3M's nanostructured thin film (NSTF) catalyst technology platform has been shown to significantly reduce many of the performance, cost and durability barriers standing in the way of H-2/air PEM fuel cells for vehicles. In this paper we describe results from the first evaluations of low loaded NSTF catalysts in H-2/O-2 electrolyzers at Proton OnSite and Giner, Inc. Over two dozen membrane electrode assemblies comprising nine different NSTF catalyst types were tested in 11 short stack durability tests at Proton OnSite and 14 performance tests in 50 cm(2) single cells at Giner Electrochemical Systems. NSTF catalyst alloys of Pt68Co29Mn3, Pt50Ir50 and Pt50Ir25Ru25, with Pt loadings in the range of 0.1 to 0.2 mg/cm(2), were investigated for beginning-of-life performance and durability up to 4000 hours as both electrolyzer cathodes and anodes. Catalyst composition, deposition and process conditions were found to be important for meeting the performance of standard PGM blacks on electrolyzer anodes while using only 10% as much PGM catalyst. Analyses of MEA's after the durability tests by multiple techniques document changes in catalyst alloy composition, loading, crystallite structure and support stability.
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The production of hydrogen, vector of energy, by electrolysis way and by using photovoltaic solar energy can be optimized by suitable choice of electrolytes. Distilled water, usually used, due to membrane presence may be substituted by wastewaters, which enters more in their treatment. Waste water such as those of the Cleansing National Office, and also of the factories such as those referring with ammonia, the margines, and even urines that make it possible to produce much more hydrogen as distilled or salted water, more especially as they do not even require an additive or membranes: conventional electrolysers with two electrodes. This study seeks to optimize the choice among waste water and this, by electrolysis in laboratory or over the sun according to produced hydrogen flow criteria, electrolysis efficiency and electric power consumption. The additive used is NaCl. The most significant results are on the one hand the significant increase in the produced hydrogen flow by the addition of the additive; on the other hand the advantage of gas liquor and urine compared to the others tested electrolytes.
Article
Pressure effects on the water electrolysis performance of the cell using Nafion 117 for electrolyte were investigated. It was found that the internal resistance of the cell decreased with increasing the pressure, while it is expected that the chemical equilibrium shifts to the reactant side. Detail analysis of the internal resistance by a current interruption method suggests that decrease in the internal resistance results from the decreased IR loss, which is assigned to the increased conductivity of Nafion 117 film. On the other hand, the overpotential of cathode and anode slightly increases as the pressure increases. In particular, the concentration overpotential on both electrodes increased under an elevated pressure. Since the internal resistance decreased by elevating water pressure, water electrolysis efficiency was improved to a value of 65% at 20 MPa, 100 mA/cm2. Consequently, this study reveals that pressure shows the prospective effects on the water electrolysis performance of the cell using Nafion 177 electrolyte in an initial short period.