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Recent Progress in Alkaline Water Electrolysis for
Hydrogen Production and Applications
Kai Zeng and Dongke Zhang *
Centre for Petroleum, Fuels and Energy (M050)
The University of Western Australia
35 Stirling Highway, Crawley, WA 6009
Australia
(A Manuscript offered for Progress in Energy and Combustion Science)
* Author for correspondence
Email: Dongke.Zhang@uwa.edu.au
Phone + 61 8 6488 8668
Postal address:
Dongke Zhang
Centre for Petroleum, Fuels and Energy (M050)
The School of Mechanical Engineering
The University of Western Australia
35 Stirling Highway, Crawley, WA 6009
AUSTRALIA
Kai Zeng
Postal address: Centre for Petroleum, Fuels and Energy (M050)
The School of Mechanical Engineering
The University of Western Australia
35 Stirling Highway, Crawley, WA 6009
AUSTRALIA
Email: kzeng@mech.uwa.edu.au
Phone + 61 8 6488 3782
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Recent Progress in Alkaline Water Electrolysis for
Hydrogen Production and Applications
Kai Zeng and Dongke Zhang *
Centre for Petroleum, Fuels and Energy (M050)
The University of Western Australia
35 Stirling Highway, Crawley, WA 6009
Australia
Abstract
Alkaline water electrolysis is one of the easiest methods for hydrogen production,
offering the advantage of simplicity. The challenges for widespread use of water
electrolysis are to reduce energy consumption, cost and maintenance and to increase
reliability, durability and safety. This literature review examines the current state of
knowledge and technology of hydrogen production by water electrolysis and identifies
areas where R&D effort is needed in order to improve this technology. Following an
overview of the fundamentals of alkaline water electrolysis, an electrical circuit analogy
of resistances in the electrolysis system is introduced. The resistances are classified into
three categories, namely the electrical resistances, the reaction resistances and the
3
transport resistances. This is followed by a thorough analysis of each of the resistances,
by means of thermodynamics and kinetics, to provide a scientific guidance to
minimising the resistance in order to achieve a greater efficiency of alkaline water
electrolysis. The thermodynamic analysis defines various electrolysis efficiencies based
on theoretical energy input and cell voltage, respectively. These efficiencies are then
employed to compare different electrolysis cell designs and to identify the means to
overcome the key resistances for efficiency improvement. The kinetic analysis reveals
that the dependence of reaction resistances on the alkaline concentration, ion transfer,
and reaction sites on the electrode surface, the latter is determined by the electrode
materials. A quantitative relationship between the cell voltage components and current
density is established, which links all the resistances and manifests the importance of
reaction resistances and bubble resistances. The important effect of gas bubbles formed
on the electrode surface and the need to minimise the ion transport resistance are
highlighted. The historical development and continuous improvement in the alkaline
water electrolysis technology are examined and different water electrolysis technologies
are systematically compared using a set of the practical parameters derived from the
thermodynamic and kinetic analyses. In addition to the efficiency improvements, the
needs for reduction in equipment and maintenance costs, and improvement in reliability
and durability are also established. The future research needs are also discussed from
the aspects of electrode materials, electrolyte additives and bubble management, serving
4
as a comprehensive guide for continuous development of the water electrolysis
technology.
[Keywords: Electrochemistry; Electrolyte; Gas bubbles; Hydrogen; Renewable energy;
Water electrolysis;]
5
CONTENTS
ABSTRACT.........................................................................................................................................2
NOMENCLATURE...........................................................................................................................8
1INTRODUCTION...................................................................................................................11
2ELECTROLYSIS FUNDAMENTALS..............................................................................16
2.1Chemistry of Water Electrolysis ....................................................................... 16
2.2Electrical Circuit Analogy of Water Electrolysis Cells .................................. 17
3THERMODYNAMIC CONSIDERATION......................................................................21
3.1Theoretical Cell Voltages ................................................................................... 21
3.2Cell Efficiencies .................................................................................................. 24
4ELECTRODE KINETICS....................................................................................................29
4.1Hydrogen Generation Overpotential ............................................................... 34
4.2Oxygen Generation Overpotential ................................................................... 36
4.3Cell Overpotential .............................................................................................. 37
6
5ELECTRICAL AND TRANSPORT RESISTANCES....................................................39
5.1Electrical Resistances ......................................................................................... 39
5.2Transport Resistances ........................................................................................ 43
5.3Bubble Phenomena ............................................................................................ 44
6PRACTICAL CONSIDERATIONS....................................................................................47
6.1Cell Configurations ............................................................................................ 47
6.2Operating Conditions ........................................................................................ 49
6.3Water Quality Requirements ............................................................................ 51
6.4System Considerations ....................................................................................... 53
7HISTORICAL DEVELOPMENT OF WATER ELECTROLYSIS............................55
8RECENT INNOVATIONS....................................................................................................62
8.1Photovoltaic (PV) Electrolysis ........................................................................... 62
8.2Steam Electrolysis .............................................................................................. 64
8.3Comparison of Technologies ............................................................................. 65
7
9RESEARCH TRENDS...........................................................................................................66
9.1Electrodes ............................................................................................................ 66
9.2Electrocatalysts ................................................................................................... 67
9.3Electrolyte and Additives .................................................................................. 70
9.4Bubble Management .......................................................................................... 72
10SUMMARY...............................................................................................................................74
ACKNOWLEDGEMENTS..........................................................................................................75
REFERENCES................................................................................................................................76
FIGURE CAPTIONS.....................................................................................................................86
LIST OF FIGURES........................................................................................................................88
TABLE CAPTIONS.....................................................................................................................102
LIST OF TABLES........................................................................................................................103
8
Nomenclature
Symbol Definition Typical unit Page
A surface area of electrode
or cross-section area of conductance
cm2
or m2
A frequency factor dimensionless
C concentration
coulomb (electrical charge)
mol·m-3
C
E electrode potential
or energy
V
J or kJ
EA activation energy kJ
F Faraday’s constant 96485 C·mol-1
f volume fraction ratio dimensionless
G Gibbs free energy J
H Enthalpy J·
i or I current A
i0 exchange current density A·m-2
j current density A·m-2
k reaction rate constant mol L-1s-1
K Kohlrausch coefficient dimensionless
9
Ksp solubility constant dimensionless
l or L length m
n number of electrons transferred dimensionless
N the number of one species dimensionless
O oxidation production of R dimensionless
Q electrical charge C
r production rate m3·h-1
R electrical resistance
gas constant
or reduction production of O
Ω
8,314J·K-1 mol-1
or dimensionless
t time, s
T temperature or K
U electrical voltage V
V volume
or unit of the voltage
m3
V
x distance m
α transit coefficient dimensionless
γ surface tension N
η efficiency dimensionless
η overpotential mV
10
gas surface coverage
or contact angle
dimensionless
or ° (degree)
electrical conductivity Ω-1· m-1
ρ resistivity of gas solution mixture Ω·m
molar conductivity of an electrolyte Ω-1·m2 mol-1
∆ difference operator
∑ summation operator
standard
11
1 Introduction
Hydrogen is mainly used in petroleum refining[1, 2], ammonia production [3, 4] and, to
a lesser extent, metal refining such as nickel, tungsten, molybdenum, copper, zinc,
uranium and lead [5, 6], amounts to more than 50 million metric tonnes worldwide in
2006 [7]. The large scale nature of such hydrogen consumptions requires large scale
hydrogen production to match them. As such, the hydrogen production is dominated by
reforming of natural gas [8] and gasification of coal and petroleum coke [9, 10], as well
as gasification and reforming of heavy oil [11, 12]. Although water electrolysis to
produce hydrogen (and oxygen) has been known for around 200 years [13, 14] and has
the advantage of producing extremely pure hydrogen, its applications are often limited
to small scale and unique situations where access to large scale hydrogen production
plants is not possible or economical, such as marine, rockets, spacecrafts, electronic
industry and food industry as well as medical applications. Water electrolysis represents
only 4% of the world hydrogen production [15, 16].
With ever increasing energy costs owing to the dwindling availability of oil reserves,
production and supply [17] and concerns with global warming and climate change
blamed on man-made carbon dioxide (CO2) emissions associated with fossil fuel use
[18], particularly coal use [2], hydrogen has in recent years become very popular for a
number of reasons: (1) it is perceived as a clean fuel, emits almost nothing other than
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water at the point of use; (2) it can be produced using any energy sources, with
renewable energy being most attractive [19]; (3) it works with fuel cells [20-22] and
together, they may serve as one of the solutions to the sustainable energy supply and use
puzzle in the long run, in so-called “hydrogen economy” [23, 24].
Water electrolysis can work beautifully well at small scales and, by using renewable
electricity, it can also be considered more sustainable. In a conceptual distributed energy
production, conversion, storage and use system for remote communities, as illustrated in
Figure 1, water electrolysis may play an important role in this system as it produces
hydrogen using renewable energy as a fuel gas for heating applications and as an energy
storage mechanism. When abundant renewable energy is available, excessive energy
may be stored in the form of hydrogen by water electrolysis. The stored hydrogen can
then be used in fuel cells to generate electricity or used as a fuel gas. A number of
studies have been reported according to the different renewable energy sources.
Isherwood et al [25] presented an analytical optimisation of a remote system for a
hypothetical Alaskan village. In this paper, wind and solar energies are utilised to
reduce the usage of diesel for electricity generation. The electricity generated by the
renewable energy is either merged into the grid or used to produce hydrogen or zinc.
With such a hybrid energy system, 50% of diesel fuel and 30% annual cost savings by
wind turbines were estimated. Energy storage devices such as phosphoric acid fuel cell
and zinc-air fuel cell were found to be helpful to reduce the fuel consumption further.
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Young et al [26] considered the technical and economic feasibility of using renewable
energy with hydrogen as the energy storage mechanism for remote community in the
mountain area of Sengor, Bhutan. The abundant hydro power, at 840 MWh·year-1, can
not only satisfy the need of local lighting and other household uses, but can also be
exported to India. Electrolyser capable of producing hydrogen at the rate of 20Nm3 h-1
is proposed. The practical problems of extending the grid over long distance and
mountainous terrain could then be solved by using such a system. Hanley et al [27]
applied two economic appraisal techniques to evaluate three renewable energy options
for a remote community in North West Scotland. Economic benefits, environmental
implications and tourism are taken into account. The authors believe that the renewable
energy development may well be beneficial for remote rural communities. Although
Hanley et al’s work does not include hydrogen, we believe that if the renewable energy
options referred to in their study are coupled with hydrogen production and use, it will
provide much greater flexibility and reliability of the systems.
Remote areas with abundant solar and/or wind electricity resources can take advantage
of the water electrolysis to produce hydrogen to meet their energy need for households
such as lighting and heating [28], powering telecommunication stations [29] and small-
scale light manufacturing industry applications, electricity peak shaving, and in
integrated systems, both grid-connected and grid-independent [30]. Hydrogen produced
by renewable energy has a great advantage, mobility, which is essential to the energy
14
supply in remote areas away from the main electricity grid. Agbossou et al [31] studied
an integrated renewable energy system for powering remote communication stations.
The system is based on the production of hydrogen by water electrolysis whereby
electricity is generated by a 10kW wind turbine and a 1kW photovoltaic array. When
power is needed the electricity is regenerated from the stored hydrogen via a 5kW
proton exchange membrane (PEM) fuel cell system. The system gives stable electrical
power for communication stations. Degiorgis et al [32] studied the feasibility of a
hydrogen fuelled trial village which was based on hydrogen as the primary fuel. In this
work, the hydrogen is produced by water electrolysis and stored for the use in hydrogen
vehicles and for thermal purposes (heating requirement of three buildings). Water
electrolysers are designed to produce 244,440 Nm3 year-1 of H2, with an energy
efficiency of 61%. The light industry applications of water electrolysis may include
mechanical workshops where hydrogen and oxygen gases produced from water
electrolysis can replace oxygen-acetylene for metal braising, cutting and welding [33,
34].
Small scale water electrolysers can avoid the need for a large fleet of cryogenic, liquid
hydrogen tankers or a massive hydrogen pipeline system. The existing electrical power
grid could be used as the backbone of the hydrogen infrastructure system, contributing
to the load levelling by changing operational current density in accordance with the
change in electricity demand [35]. A small scale pure hydrogen and oxygen can find
15
diverse applications including gases in laboratories and oxygen to life-support system in
hospitals [36].
While possessing these advantages of availability, flexibility and high purity, to achieve
widespread applications, hydrogen production using water electrolysis needs
improvements in energy efficiency, safety, durability, operability and portability and,
above all, reduction in costs of installation and operation. These open up many new
opportunities for research and development leading to technological advancements in
water electrolysis. This literature review aims to identify such new research and
development opportunities. We begin with an overview of the fundamentals of water
electrolysis in the context of electrochemistry, laying a theoretical basis for scientific
analysis of the published electrolysis systems and data. We then analyse various water
electrolysis techniques in a broad range of applications and examine recent trends in
research and innovations to identify the gaps for improvements – the needs for further
research and development.
16
2 Electrolysis Fundamentals
2.1 Chemistry of Water Electrolysis
A basic water electrolysis unit consists of an anode, a cathode, power supply, and an
electrolyte, as illustrated in Figure 2. A direct current (DC) is applied to maintain the
electricity balance and electrons flow from the negative terminal of the DC source to the
cathode at which the electrons are consumed by hydrogen ions (protons) to form
hydrogen. In keeping the electrical charge (and valence) in balance, hydroxide ions
(anions) transfer through the electrolyte solution to anode, at which the hydroxide ions
give away electrons and these electrons return to the positive terminal of the DC source.
In order to enhance the conductivity of the solution, electrolytes which generally consist
of ions with high mobility are applied in the electrolyser [37]. Potassium hydroxide is
most commonly used in water electrolysis, avoiding the huge corrosion loss caused by
acid electrolytes [38]. Nickel is a popular electrode material due to its high activity and
availability as well as low cost [39]. However, the introduction of these conductive
components could also bring about some side effects, which will be discussed in the
following sections. During the process of water electrolysis, hydrogen ions move
towards cathode, and hydroxide ions, move towards the anode. By the use of a
diaphragm, gas receivers can collect hydrogen and oxygen, which form on and depart
from the cathode and the anode, respectively.
17
The half reactions occurring on the cathode and anode, respectively, can be written as
Cathode:
(R1)
Anode:
(R2)
The overall chemical reaction of the water electrolysis can be written as
(R3)
2.2 Electrical Circuit Analogy of Water Electrolysis Cells
For this electrochemical reaction process to proceed, a number of barriers have to be
overcome, requiring a sufficient electrical energy supply. These barriers include
electrical resistance of the circuit, activation energies of the electrochemical reactions
occurring on the surfaces of the electrodes, availability of electrode surfaces due to
partial coverage by gas bubbles formed and the resistances to the ionic transfer within
the electrolyte solution. It is important that these barriers are analysed in the contexts of
thermodynamics and kinetics as well as transport process principles.
Figure 3 shows the resistances (the barriers) presented in a typical water electrolysis
system. The first resistance from the left-hand-side is the external electrical circuit
resistance including the wiring and connections at anode. is originated from the
overpotential of the oxygen evolution reaction on the surface of the anode.
18
is the resistance due to partial coverage of the anode by the oxygen bubbles, hindering
the contact between the anode and the electrolyte. The resistances come from electrolyte
and membrane are noted as and, respectively. Similarly,
roots from the blockage of the cathode by hydrogen bubbles; is the resistance
caused by the overpotential for oxygen evolution reaction and
is the electrical
resistance of the wiring and connections at cathode. Thus, the total resistance can be
expressed in Equation 1 below.
(1)
These resistances in electrolysis systems can be classified into three categories, the first
category includes all the electrical resistances; the second includes the reaction
resistances; and the third includes the transport resistances.
Electrical resistances
The electrical resistances can be calculated using the Ohm’s law: [37], in
which is the current when voltage is applied only at the circuit. Or, it can be
calculated from the physics equation: , in which , and are the
length, specific conductivity and cross-sectional area of the conductor, respectively.
and
belong to this category and are usually considered as one integral part.
19
Transport-related resistances
These are the physical resistances experienced in the electrolysis process such as gas
bubbles covering the electrode surfaces and present in the electrolyte solution,
resistances to the ionic transfer in the electrolyte and due to the membrane used for
separating the H2 and O2 gases. , , and are
considered as transport resistances.
Both electrical resistances and transport resistances cause heat generation according to
the Joule’s law [37] and transport phenomena [40] and thus inefficiency of the
electrolysis system. The lost energy due to these resistances is also known as the ohmic
loss [41].
Electrochemical reaction resistances
The reaction resistances are due to the overpotentials required to overcome the
activation energies of the hydrogen and oxygen formation reactions on the cathode and
anode surfaces, which directly cause the increase in the overall cell potential. These are
the inherent energy barriers of the reactions, determining the kinetics of the
electrochemical reactions [42].
20
The reaction resistances or overpotentials are inherent resistances of the electrochemical
reactions depending on the surface activities of the electrodes employed. and
are reaction resistances.
Clearly, the strategies in any effort to improve the energy efficiency of water
electrolysis and thus the performance of the system must involve the understanding of
these resistances so as to minimise them.
21
3 Thermodynamic Consideration
3.1 Theoretical Cell Voltages
Water is one of the thermodynamically most stable substances in the nature and it is
always an uphill battle to try to pull water molecules apart to make its elements into
hydrogen and oxygen molecules. No pain, no gain. If we want hydrogen (and oxygen)
from water by electrolysis, we have to at least overcome an equilibrium cell voltage, ,
which is also called “electromotive force”. With established reversibility and absence of
cell current between the two different electrode reactions, the open cell potential is
called the equilibrium cell voltage, it is defined as equilibrium potential difference
between the respective anode and cathode [43] and is described by Equation 2 below.
(2)
Equation 3 relates the change in the Gibbs free energy of the electrochemical
reaction to the equilibrium cell voltage as follows.
(3)
where is the number of moles of electrons transferred in the reaction, and is the
Faraday constant. The overall water electrolysis cell reaction, E°(25°C) is 1.23V and the
Gibbs free energy change of the reaction is + 237.2 kJmol-1 [44], which is the minimum
amount of electrical energy required to produce hydrogen. The cell voltage at this point
22
is known as reversible potential. Hence the electrolysis of water to hydrogen and
oxygen is thermodynamically unfavourable at room temperature and can only occur
when sufficient electrical energy is supplied. In contrast, when the electrolysis process
is performed under adiabatic conditions, the total reaction enthalpy must be provided by
electrical current. Under this circumstance, the thermo-neutral voltage is required to
maintain the electrochemical reaction without heat generation or adsorption [45].
Therefore, even when the equilibrium potential is met, the electrode reactions are
inherently slow and then an overpotential η, above the equilibrium cell voltage is
necessary in order to kick start the reaction due to the activation energy barrier, low
reaction rate and the bubble formation [42, 46]. According to the resistances mentioned
above, input of additional energy is also essential to drive the ionic migration process
and overcome the resistance of the membrane as well as the electrical circuit. This extra
energy requirement causes a potential drop,
, (where i is the current through the
cell and RCell is the sum of electrical resistance of the cell, a function of electrolyte
properties, the form of the electrodes and cell design) within the cell. The cell potential
can be written as Equation 4, which is always 1.8 – 2.0 V at the current density of
1000-3000 A m-2 in industry water electrolysis [47]. The total overpotential is the sum
of overpotentials or barriers from the hydrogen and oxygen evolution reactions,
electrolyte concentration difference and bubble formation. If one has a mild condition
under which gas bubble and concentration differences can be neglected, the sum of
23
overpotential can be calculated using Equation 5, where is the current density
(current divided by electrode surface area) at which electrolysis cell operates. Both of
the overpotential and the ohmic loss increase with current density and may be regarded
as causes of inefficiencies in the electrolysis whereby electrical energy is degraded into
heat which must be taken into account in any consideration of energy balance.
(4)
(5)
Figure 4 shows the relationship between the electrolyser cell potential and operating
temperature [17, 48]. The cell potential – temperature plane is divided into three zones
by the so-called equilibrium voltage line and thermo-neutral voltage line. The
equilibrium voltage is the theoretical minimum potential required to dissociate water by
electrolysis, below which the electrolysis of water cannot proceed. The equilibrium
voltage deceases with increasing temperature. The thermo-neutral voltage is the actual
minimum voltage that has to be applied to the electrolysis cell, below which the
electrolysis is endothermic and above which, exothermic. The thermo-neutral voltage
naturally includes the overpotentials of the electrodes, which are only weakly dependent
on temperature. Thus, the thermo-neutral voltage only exhibits a slight increase with
temperature. We denote thermo-neutral voltage as . If water electrolysis takes place
in the shaded area in Figure 4, the reaction will be endothermic.
24
3.2 Cell Efficiencies
Energy efficiency is commonly defined as the percentage share of the energy output in
the total energy input. However, there are a number of ways of expressing the efficiency
of electrolysis, depending on how the electrolysis system is assessed and compared.
Generally, in the electrochemistry sense, the voltage efficiency of an electrolysis cell can
always be calculated using Equation 6 [17, 49] below.
(6)
The physical meaning of this equation is the proportion of effective voltage to split
water in the total voltage applied to the whole electrolysis cell. It is a good
approximation of the efficiency of the electrolysis system.
There are two other efficiencies calculated based on the energy changes of the water
electrolysis reaction, known as the Faradic efficiency and the thermal efficiency. They
use the Gibbs free energy change and enthalpy change of water decomposition reaction
as the energy input, respectively. Both and adopt the theoretical
energy requirement plus energy losses as the energy input. As shown in the Equations 7
and 8 below.
(7)
25
(8)
Both equations can be simplified using cell potential and total cell voltage as shown in
Equations 9 and 10 below
(9)
(10)
where the is cell voltage. and are the equilibrium and thermo-neutral
voltages, respectively.
The physical meaning of Equation 7 is the percentage of the theoretical energy needed
to force apart the water molecules in the real cell voltage and is a measure of the cell
efficiency purely from the cell voltage point of view. On the contrast, Equation 8 means
that an additional cell voltage, above the reversible voltage, is required to maintain the
thermal balance and the percentage of the actual energy input in the real voltage defines
the thermal efficiency. It is then possible that the thermal efficiency of a water
electrolysis cell may exceed 100% as the system may absorb heat from the ambient if it
operates in endothermic mode (in the shaded area of Figure 4).
The Gibbs free energy and the enthalpy of the reaction are also a function of
temperature as illustrated in Figure 4. Equations 9 and 10 give the efficiencies at 25°C.
The values of Faradic efficiency are always less than 1 because there are always losses.
26
While the thermal efficiency can be higher than 1 provided the water electrolysis
operates under a voltage lower than the thermo-neutral voltage. This phenomenon is due
to that heat is absorbed from the environment. When the denominator in Equation 8 is
1.48 V, the electrolysis operates at the efficiency of 100%. No heat will be absorbed
from or released to the environment.
In practice, if the potential drop caused by electrical resistance is 0.25 V and 0.6 V for
the cathode and anode overpotentials at 25°C, respectively, the Faradic efficiency is
, and the thermal efficiency is
. The
electrolysis cell is exothermic at cell potential above 1.48V, and endothermic at cell
potential below this value. The Faradic efficiency investigates the electrolysis reaction
while the thermal efficiency takes the whole thermal balance into account.
Yet another means to compare and evaluate the efficacy of a water electrolysis systems
is to consider the output of hydrogen production against the total electrical energy
applied to the system, in both terms of hydrogen production rate and energy (the high
heating value of hydrogen) carried by the hydrogen produced.
(11)
where the U is the cell voltage, i is the current, t stands for time. V is the hydrogen
production rate at unit volume electrolysis cell. The physical meaning of Equation 11 is
27
the hydrogen production rate per unit electrical energy input. It is a way for direct
comparison of hydrogen production capacity of different electrolysis cells,
or
12
where 283.8 kJ is the high heating value (HHV) of one mole hydrogen and is for the
time needed for one gram hydrogen produced.
An alternative expression of the energy efficiency is to subtract the energy losses from
the total energy input as shown in Equation 13 below.
(13)
where can be expressed in terms of the resistances discussed in Equation 1. Those
resistances cause respective energy losses. By considering these resistances an
analogous electrical unit, each of them can be calculated using the Joule’s Law.
Therefore,
(14)
28
Equation 14 identifies all the components of energy losses, which can then be rated,
allowing the efficiency to be improved by targeting the key causes of energy loss
components.
From the discussion above, we can conclude that there are two broad ways of energy
efficiency improvement: one is to thermodynamically reduce the energy needed to split
water to yield hydrogen, such as by increasing the operating temperature or pressure;
the other is to reduce the energy losses in the electrolysis cell, which can be realised by
minimising the dominant components of the resistances.
In addition to the thermodynamic analysis of water electrolysis, various system
parameters such as electrode materials, electrolyte properties and reaction temperatures
can affect the performance of electrochemical cells. It is necessary to discuss the
kinetics of the electrode reactions.
29
4 Electrode Kinetics
The rate of the electrode reaction, characterised by the current density, firstly depends
on the nature and pre-treatment of the electrode surfaces. Secondly, the rate of reaction
depends on the composition of the electrolytic solution adjacent to the electrodes. These
ions in the solution near the electrodes, under the effect of electrode, form layers,
known as double layer [37], taking cathode for example, the charge layer formed by
hydroxyl ions and potassium ions according to the charge of the electrodes. Finally, the
rate of the reaction depends on the electrode potential, characterised by the reaction
overpotential. The study of electrode kinetics seeks to establish the macroscopic
relationship between the current density and the surface overpotential and the
composition of the electrolytic solution adjacent to the electrode surface [50].
The double layer is illustrated in Figure 5 (a). The accumulated ions form two mobile
layers of solvent molecules and adsorbed species. The one nearer the electrode surface
is relatively ordered, termed the inner Helmholtz layer (IHL). The other one with less
order is called outer Helmholtz layer (OHL) [51]. The electrical charges on the surface
of the electrodes are balanced by ionic counter-charges in the vicinity of the electrodes.
The potential distribution is also plotted against the distance from the electrode surface
in Figure 5 (b). It can be clearly seen that the interfacial potential difference exists
30
between the electrode surface and the solution due to the existence of the double layer
[43].
The phenomenon of the double layer formation is a non-faradic process [43]. It leads to
the capacitive behaviour of the electrode reactions. This capacitor property of electrode
surfaces should be taken into consideration in the kinetics.
According to the Faraday’s law, the number of moles of the electrolysed species (H+ or
O2-), N, is given by
(15)
where Q is the total charge in Coulomb transferred during the reaction, n is the
stoichiometric number of electrons consumed in the electrode reaction (n=2 for both
reactions R1 and R2), F is the Faraday constant. The rate of electrolysis can be
expressed as
(16)
can be noted as Faradic current [42].
Generally the surface area at which the reaction takes place needs to be taken into
account. The rate of the electrolysis reaction can be expressed as
(17)
31
where j is the current density.
The rate constant of a chemical reaction can be in general expressed by the Arrhenius
equation.
(18)
where, stands for the activation energy, kJ·mol-1, A is the frequency factor. R is the
gas constant, and T is reaction temperature. Although the equation is oversimplified, it
reveals the relationship between the activation energy and the rate constant.
For one-step, one-electron reaction, through the relationship between the current and the
reaction rate, the dependence of the current density on the surface potential and the
composition of the electrolytic solution adjacent to the electrode surface is given by the
Butler-Volmer equation [42]:
°
° (19)
where A is the electrode surface area through which the current passes, is the
standard rate constant, α refers to the transfer coefficient its value lies between 0 and 1
for this one electron reaction, is the ratio. t and 0 in the bracket are,
respectively, the specific time at which this current occurs and the distance from the
electrode. For the half reaction R1, stands for the concentration of reaction
32
species at cathode in the oxided state, the hydrogen ions (H+), while is the
concentration of reaction product hydrogen
, which is in the reduced state.
Equation 19 is derived using the transition-state-theory [42]. The theory describes a set
of curvilinear coordinates in the reaction path as shown in Figure 6 (a). The potential
energy is a function of the independent positions of the coordinates in the system. When
a potential increases by ΔE, it will cause the relative energy of the electron to decrease
by F(E-E°) as illustrated in Figure 6 (a). The decrease in turn reduces the Gibbs free
energy of the hydrogen ions in the hydrogen evolution reaction by (1-α)(E-E°) and, on
the contrast, increases the Gibbs free energy of hydrogen by α(E-E°), respectively.
Therefore, provided that there is no mass transfer limitation, the Butler-Volmer equation
can be derived from Equations 17 and 18 using the Gibbs free energy changes in Figure
6.
The Butler-Volmer equation can be simplified as:
(20)
where is known as the exchange current density [52], which is the current of the
reversible water splitting reaction. From the simplified equation above, we can derive
the over-potential at each electrode, respectively. In the absence of the influence of
mass transfer and at the large overpotentials (> 118mV at 25 °), one of the terms in
equation 14 can be neglected. For example, at large negative overpotential,
33
. the relationship between i and can be written in the Tafel
equation [53]:
(21)
where
and
The linear relationship between the overpotential and the logarithm of current density is
characterised by the slope b and exchange current density . The slope is also known
as Tafel slope. Both parameters are commonly used as kinetic data to compare
electrodes in electrochemistry.
From the above analysis, the rate of the electrolysis can be expressed by the current or
current density. Furthermore the current can be reflected by , which is the current
associated with the reversible reaction on the surface of the electrodes. The rate of the
reaction is also directly determined by the overpotential, which depends on a number of
factors. One of the important factors is the activation energy, , which is strongly
influenced by the electrode material, thus a focus of continuing research effort. To
reduce the activation energies of the electrode reactions, or reduce the overpotential, it
is therefore necessary to consider how they are related to the electrode materials and
surface configurations.
34
4.1 Hydrogen Generation Overpotential
The mechanism of the hydrogen evolution reaction is widely accepted [17] to be a step
involving the formation of adsorbed hydrogen
(R4)
which is followed by either chemical desorption
(R5)
or electrochemical desorption
(R6)
where the subscript represents the adsorbed status.
The overpotential of hydrogen is generally measured by the Tafel equation
(22)
In this equation, i0, the exchange current density of the reaction, which can be
analogised as the rate constant of reaction, is a function of the nature of the electrode
(cathode) material [50]. The overpotential of the hydrogen production means extra
energy barrier in the process of hydrogen formation.
35
The overpotential on the cathode is directly related to the formation of hydrogen in the
vicinity of the electrode. The formation of hydrogen is intrinsically determined by the
bond between hydrogen and the electrode surface. Pd has the lowest heat of adsorption
of hydrogen (83.5 kJ·mol-1) as compared to 105 kJ·mol-1 for Ni [54]. Meanwhile, the
hydrogen formation is also influenced by the electrode properties, the type and
concentration of electrolyte and temperature. By comparing the kinetic data including
the exchange current density and Tafel slope, the relationship between these factors can
be revealed. Table 1 compares the kinetic parameters, represented by the current density
and Tafel slope, of the hydrogen evolution reactions on different metal electrode
materials.
For hydrogen evolution reaction, it is necessary to indentify the rate determining step. If
the hydrogen adsorption, R4, is the rate determining step, electrode material with more
edges and cavities in its surface structure which favour easy electron transfer will create
more electrolysis centres for hydrogen adsorption. If the hydrogen desorption, R5 and
R6, are the rate determining step, physical properties such as surface roughness or
perforation will either increase the electron transfer by adding reaction area or
preventing the bubbles from growing, which in turn increase the rate of electrolysis.
Increasing the overpotential could lead to a mechanism change. In other words, the rate
determining step will alter within different potential ranges. When the potential is low,
the electron transfer is not as fast as desorption. The hydrogen adsorption will be the
36
rate determining step. On the contrast, when the potential is high enough to enable the
hydrogen adsorption rate to be greater than the desorption rate, the hydrogen desorption
will be the rate determining step.
4.2 Oxygen Generation Overpotential
The mechanism of oxygen evolution reaction is more complex compared to the
pathways suggested for the hydrogen evolution reaction. There are a number of theories
presented and discussed in the literature and the most generally accepted mechanism
involves the following steps:
(R7)
(R8)
(R9)
One of the charge transfer steps is rate controlling. The dependence of transfer
coefficients α in Equation 19 and Tafel slope variations can be used to identify the rate
determining step. For example, a slow electron transfer step (R7) determines the
reaction at low temperatures, on the contrast, a slow recombination step (R9) controls at
high temperatures on nickel electrode. The different Tafel slopes between the steps can
be used to judge the mechanisms [51, 59].
37
The overpotential of oxygen evolution reaction is generally measured by the Tafel
equation
(23)
The reaction rate decreases with increasing activation energy, so reducing the activation
energy is always favoured for more efficient water electrolysis. Furthermore, the
activation energy increases with increasing current density and can be lowered by using
appropriate electrocatalysts. Table 2 below compares the kinetic parameters, again
represented by the current density and Tafel slope, of oxygen evolution reactions on
different metal electrode materials.
Generally speaking, the overpotential of oxygen evolution is more difficult to reduce
than that of hydrogen evolution, owing to the complex mechanism and irreversibility.
Alloys of Fe and Ni have been found to be able to reduce the overpotential to some
extent [63].
4.3 Cell Overpotential
As shown above, the hydrogen and oxygen overpotentials can be expressed by
Equations 22 and 23. A typical plot in Figure 7 is the Tafel plot as a function of
Equation 21 in water electrolysis. The parameters used to compare the electrode kinetics
38
are the exchange current i0 and the Tafel slope. A higher exchange current density and
lower slope indicate a higher electrode activity.
Since the cell potential contains both anode and cathode reactions, indentifying the
contributions of each of anode and cathode to the cell voltage and factors influencing
them is necessary to understand the overpotentail resistance. The typical effect of
temperature on the overpotential is summarised by Kinoshita [47]. As shown in the
Figure 8 an increase in temperature will result in a decrease in the overpotential at the
same current density.
The overpotential is not only a function of temperature but also a function of current
density [38]. As can be seen from Figure 9, the overpotentials from hydrogen and
oxygen evolutions are the main sources of the reaction resistances. The other obvious
resistance at high current densities is the Ohmic loss in the electrolyte, which includes
resistances from the bubbles, diaphragm and ionic transfer. Understanding these
resistances opens up opportunities to enhance the efficiency of the water electrolysis.
39
5 Electrical and Transport Resistances
5.1 Electrical Resistances
The electrical resistances are the direct reasons of heat generation which leads to the
wastage of electrical energy in the form of heat formation according to the Ohms law.
The electrical resistances in a water electrolysis system have three main components:
(1) the resistances in the system circuits; (2) The mass transport phenomena including
ions transfer in the electrolyte; (3) The gas bubbles covering the electrode surfaces and
the diaphragm.
The resistances of electrodes and connection circuits are determined by the types and
dimensions of the materials, preparation methods, and the conductivities of the
individual components. It can be expressed as follows:
(24)
where is the electrical conductivity and has the unit of Ω-1·m-1, subscript stands for
each component of the circuit, including wires, connectors and the electrode. This part
of the resistance can be reduced by reducing the length of the wire, increasing the cross-
section area and adopting more conductive wire material.
Ionic transfer within the electrolyte depends on the electrolyte concentration and
separation distance between the anodes and cathodes, the diaphragm between the
40
electrodes. Different from the conductance rate in the metallic conductor, the Molar
conductivity is adopted to replace the conductivity and can be expressed as follows:
(25)
where is the electrolyte concentration. The unit of the molar conductivity is
. It is also a function of concentration and the mass transfer rate of the
ions. As strong electrolytes are commonly applied in the water electrolysis, the
empirical relationship between and is given in
(26)
where is the mole conductivity extrapolated to infinite dilution which is known. K is
the Kohlrausch coefficient, a proportionality constant of the linear relationship between
molar conductivity and square root of concentration [52]. In terms of ionic resistance,
improvements can be made by increasing the conductivity of the electrolyte by altering
its concentration or adding appropriate additives.
The presence of bubbles in the electrolyte solution and on the electrode surfaces causes
additional resistances to the ionic transfer and surface electrochemical reactions. One of
the accepted theoretical equations to study the bubble effect in the electrolyte is given as
follows [64]:
(27)
41
where is the specific conductivity of the gas-free electrolyte solution; f is the volume
fraction of gas in the solution [51]. Quantitative illustration of the bubble resistance in
terms of the bubble coverage on the surface and the bubble existence in the electrolytes
needs to be considered. If we take bubble coverage into consideration, the bubble
coverage is denoted as , which represents the percentage of the electrode surface
covered by the bubble. The electrical resistance caused by the bubble formation on the
electrode surface can be calculated as follow [65],
(28)
where, is the specific resistivity of the gas free electrolyte solution. If a diaphragm is
used to separate the hydrogen and oxygen formed for collections, respectively, the
presence of the diaphragm presents another resistance to the ionic transfer. The resistive
effect associated with the diaphragm is expressed by MacMullin [66] for the apparent
conductivity:
(29)
where m is the hydraulic radius and p is the permeability. The effective resistance of a
membrane frequently amounts to between three to five times more than the resistance of
the electrolyte solution of the same thickness as that of the membrane [51].
42
By dividing the overpotential by the current density, all of the resistances can be unified
on the unit of ohm, which makes it possible to compare energy losses caused by
different resistances as illustrated in Figure 10, where, E loss, electrolyte includes energy
losses due to the bubbles in the electrolyte and ionic transfer resistances. Figure 10 also
shows that the energy losses caused by the reaction resistances increases relatively
slowly as the current density increases. The energy loss in the electrical circuit is
relatively small. However, the energy loss due to the ionic transfer resistance in the
electrolyte becomes more significant at higher current density. The dot and dash lines
are the bubble resistance and total resistance. The energy loss due to bubble coverage on
the electrode surfaces, and thus the total energy loss are hypothetical, on the base of
50% electrode surface being covered by bubbles.
Although the relationship between the current density and energy loss in Figure 10 does
not specify all of the resistances mentioned before, it approximately presents the
relationships among the losses. More interestingly, the energy loss due to bubbles
formed on the electrodes should be considered as the major contribution to the total
energy loss. Therefore, minimising the bubble effect holds a key to the electrolyser
efficiency improvement.
43
5.2 Transport Resistances
Convective mass transfer plays an important role in the ionic transfer, heat dissipation
and distribution, and gas bubble behaviour in the electrolyte. The viscosity and flow
field of the electrolyte determine the mass (ionic) transfer, temperature distribution and
bubble sizes, bubble detachment and rising velocity, and in turn influence the current
and potential distributions in the electrolysis cell. As the water electrolysis progresses
the concentration of the electrolyte increases, resulting in an increase in the viscosity.
Water is usually continuously added to the system to maintain a constant electrolyte
concentration and thus the viscosity.
However, better mass transfer does not mean more hydrogen production. It is true that
the mass transport leads to greater reaction rates, but the large number of gas bubbles
formed, resulting from the increased reaction rate, can adversely hinder the contact
between the electrodes and the electrolyte. The recirculation of electrolyte can be
applied to mechanically accelerate the departure of the bubbles and bring them to the
collectors.
The recirculation of the electrolyte is helpful in preventing the development of an
additional overpotential due to the differences in electrolyte concentration in the cell.
The velocity of fluid in the electrolyser can prompt the removal of the gas and vapour
bubbles from the electrodes. On the other hand, the recirculation of the electrolyte can
44
also help distribute the heat evenly within the electrolyte. At start-up, electrolyte
circulation can be utilised to heat up the electrolyte to the operating temperature which
is recommended to be 80-90°C [47, 67].
5.3 Bubble Phenomena
As electrolysis progresses, hydrogen and oxygen gas bubbles are formed on the surfaces
of the anode and cathode, respectively, and are only detached from the surface when
they grow big enough. The coverage of the electrode surfaces by the gas bubbles
directly add to the electrical resistance of the whole system, by reducing the contact
between the electrolyte and the electrode, blocking the electron transfer, and increasing
the ohmic loss of the whole system. Understanding the bubble phenomena is therefore
an important element in the development of any water electrolysis systems.
Mechanically circulating the electrolyte can accelerate the detachment of bubbles,
providing a possible means to reduce the resistance due to gas bubbles. Alternatives are
to consider the use of appropriate additives to the electrolyte solution to reduce the
surface tension of the electrolyte and modifications of the electrode surface properties to
make them less attractive to the gas bubbles.
Understanding the dynamics of the bubble behaviour is important in order to determine
the conditions for the departure of the bubbles from the electrodes. The general
thermodynamic condition for the three phase contact between the gas bubble, electrode
45
and the electrolyte is a finite contact angle at the three phase boundary [68, 69] as
illustrated in Figure 11.
The Young’s equation defines the contact angle in terms of the three interfacial tensions
[70],
(30)
where , and are the surface tensions of the solid/vapour, solid/liquid and
liquid/vapour interfaces, respectively. The Gibbs free energy change accompanying the
replacement of the unit area of the solid/ liquid interface by a solid/vapour interface
(31)
The detachment of the bubbles depends on the replacement of the electrolyte at the
solid/solution interface, which is known as wettablity [71, 72].
Two kinds of electrode surfaces can be defined according to the surface tension,
namely, hydrophobic and hydrophilic. The electrode which favours water is
hydrophilic, and the one does not is hydrophobic. Appropriate surface coating can
therefore be applied to make the electrode surfaces more hydrophilic in order to reduce
the surface coverage by the gas bubbles.
46
Therefore there are some broad approaches to manage the bubble phenomena. One is to
treat the electrode surfaces to make them more hydrophilic so that water is more likely
to take place of bubbles. Another is to use additives in the electrolyte solution to reduce
surface tension so that bubbles are easy to depart from electrodes. In addition,
controlling flow pattern to force bubbles to leave electrodes mechanically is also a
means.
Intensive studies have been given to the bubble behaviour in the electrolysis systems
[71, 73-76]. It is a key issue to be resolved to overcome or reduce bubble resistance.
Further detailed studies are necessary to further reduce the negative effects of the
bubbles.
47
6 Practical Considerations
In order to evaluate different electrolysis systems, it is necessary to relate a number of
practical parameters to the performance of different electrolysers. The important
parameters are categorised and discussed in the following discussion. These practical
parameters for comparison of electrolysers includes
Cell configurations: bipolar and monopolar configuration, the gap between the
electrodes as well as the flow velocity of the electrolytes.
Operating conditions: including cell potential, current density, operating
temperature and pressure, type and concentration of electrolytes as well as the
stability of electrode material.
External requirements: water quality requirement and system issues such as time
space yield the quality of gases produced and safety issues.
6.1 Cell Configurations
There are two alkaline electrolysis cell configurations, namely, the monopolar and the
bipolar as shown in Figure 12. In the monopolar arrangement, Figure 12 (a), alternate
electrodes are directly connected to the opposite terminals of the DC power supply,
respectively, giving a number of individual cells in parallel with one another. The total
voltage applied to the whole electrolysis cell is essentially the same as that applied to
48
the individual pairs of the electrodes in the cell. For the bipolar arrangement, Figure 12
(b), only two end electrodes are connected to the DC power supply. Thus every two
adjacent electrodes form a unit cell, and these unit cells are electrically linked, via the
electrolyse solution as the conducting media, in series with one another. The total cell
voltage is the sum of the individual unit cell voltages.
Due to the difference in the electrode arrangements, the reactions on the electrodes and
the operation potentials are different for these two arrangements. In the monopolar
configuration, the same electrochemical reaction, either the hydrogen evolution reaction
or the oxygen evolution reaction, occurs on both sides of each electrode. However, in
the bipolar configuration, the two different reactions evolving hydrogen and oxygen
respectively, take place simultaneously on the opposite sides of the same electrodes that
are not directly connected to the power source, that is, one side of an electrode acts as a
cathode and the other as an anode (although both sides of the same electrode are at the
same electric potential), except the two end electrodes that are connected to the DC
power source. The cell operation potentials as the total voltage supplied by the DC
power source are quite different for these two basic configurations; the typical value is
normally 2.2 V for the monopolar configuration and 2.2 ×·(n – 1) V for the bipolar
configuration (where n is the number of electrodes) for industrial processes [17].
From the manufacturing point of view, the monopolar configuration is simple and easy
to fabricate and maintain but suffers from high electrical currents at low voltages,
49
causing large ohmic losses. The bipolar configuration reduces the ohmic losses on the
electrical circuit connecters but demands much greater precision in design and
manufacturing to prevent the electrolyte and gas leakage between cells [17].
Cell configurations also include the gaps between the electrodes. The gap between
electrodes is the distance that the ions have to travel in the electrolyte [77]. A smaller
gap has the advantage of less resistance for ionic transportation. However, if the gap is
too small, it would introduce electric sparks, posing an explosion hazard. Therefore, an
optimal gap between electrodes has to be identified.
Another configuration factor, the electrolyte flow, determines the mass transport in the
electrolyte. Circulating the electrolyte forces the species movement in the form of
convection. At high current densities, electrochemical reactions are likely to be limited
by the mass transfer of the electrolyte. High electrolyte flow conditions through rapid
stirring or turbulence promoters eradicate the concentration difference and thus enhance
the ions and mass transfer in the electrolyte.
6.2 Operating Conditions
The overarching parameter is the operating cell voltage, which determines the energy
consumption and electricity efficiency directly. A higher voltage at the same current to
produce equivalent hydrogen means inefficiency.
50
The second important parameter is the operating current density, another parameter
related to the energy efficiency directly. Conventional water electrolysers always run
under the current density ranging from 1000 to 3000 A·m-2. The current density
determines the rate of the hydrogen production. A higher current density means a
greater electrochemical reactions rate. However, the rapid bubble formation resulted
from increased gas production rate will increase the overpotential due to the greater
bubble resistance. Consequently, the operating current density should be maintained
within a certain range with compromises between gas production rates and energy
efficiencies.
The operating temperature is another important parameter. Most of the conventional
alkaline water electrolysers are designed to run at a temperature around 80 to 90. As
discussed in Section 3.1, the equilibrium voltage decreases as temperature increases.
However, the higher the operating temperature, the greater water loss due to evaporation
and the more stringent demands for materials for the structural integrity [47].
Furthermore, the heat management and the material required for the diaphragm bring
more engineering issues at higher operating temperatures.
Depending on the end use of the hydrogen, the pressure at which the electrolyser
operates could be higher than atmospheric pressure. The elevated pressure cells
operating at 3.5 MPa reduce the bubble sizes, minimising ohmic loss due to bubbles.
Generally, the efficiency of pressured cells is not significantly superior to that of
51
ambient pressure cells [78]. Pressurised operating environments increase the
proportions of dissolved gas and require a more endurable diaphragm.
The type and concentration of the electrolyte are also important in the electrolysis due to
the ionic transfer in the electrolyte. Good conductance of an electrolyte helps ionic
transfer in the solution. As indicated in Section 5.2 above, the electrolyte concentration
also plays an important role in determining the electrical resistance of the electrolyte.
25%-30% potassium hydroxide is widely adopted in commercial electrolysers [79].
The stability of the electrode material is essential to the longevity of the electrolysers
which are expected to serve for as long a time as possible to minimise the operating and
maintenance costs from the economical point of view. The electrodes operate in very
corrosive alkali environments, thus need to be resistant to the alkali attacks. Nobel
metals have the alkali resistance and high electrochemical activities desired but are too
expensive for widespread applications in water electrolysis [43]. Transition metals such
as iron and copper possess good electrochemical activities but are less resistant to the
alkali attacks. Nickel is the best electrode material for alkaline water electrolysis with
good alkali resistance and electrochemical activity, while not being too expensive.
6.3 Water Quality Requirements
The purity of water is crucial for endured operations of electrolysers as impurities can
accumulate in the electrolysers, deposit on the electrode surfaces and in the membrane,
52
thus hampering the ions transfer and electrochemical reactions. The impurities in the
electrolyte such as magnesium, calcium ions and chloride ions can also cause side
reactions. Due to the alkaline environment in electrolysis cell, the concentrations of
magnesium and calcium ions should be sufficiently low to avoid the blockage on the
surface of electrode or diaphragm, hindering the mass and electron transfer [80]. The
chloride ions in the alkaline solution are oxidised when the current density exceeds so
called hydroxyl ions limiting current [55], leading to the formation of chlorine at the
anode surface, which is highly corrosive to the metal structures of the electrolysers.
The deposition of salts formed from the impurity metal ions is ruled by their own
solubility product constant () which is the limiting value for deposition to happen
[52]. The constant is the chemical equilibrium between solid and dissolved states of a
compound at saturation. When the product of the concentration value of each of these
impurity metal ions and the power of its stoichiology number in the molecule reaches
this limitation, the deposition will form. Table 3 lists those of possible depositions
with their ions to provide the criteria for choosing alkaline concentrations.
Take a solution with a pH of 14 under 25°C for example, the concentration of hydroxyl
ions is 1 mol/L and then the critical concentration of Mg2+ is 1.8×10-11 mol/L. Mg(OH)2
deposition will form when the concentration of Mg2+ ions exceeds this value.
53
6.4 System Considerations
The hydrogen production capacity of an electrolyser is an important parameter for
judging the performance of an electrolyser. According to the need of energy or
hydrogen, electrolysers of different scales are designed to meet the varying needs of the
end users. It can be expressed in terms of hydrogen production rate in the unit of Nm3 h-
1. Commercially, an electrolyser can vary from the several kW to several hundred MW
in terms of power consumption [78]. Alternatively, embracing the volume of the
electrolyser, the hydrogen production rate in the unit of Nm3 m-3 h-1 symbolises the
mobility and capacity of the electrolyser.
As discussed in Section 3.2, the efficiency is an important parameter to compare the
efficacy of different electrolyser technologies. The efficiency is a critical criterion either
from energy or hydrogen production point of view. considers the efficacy
of an electrolyser from the net energy point of view; an efficiency of around 50% for
low temperature alkaline water electrolysis is considered to be good [81]. On the other
hand, is useful when considering the hydrogen production at per volume
electrolysis at unit time, for example, around 2.3 m3·m-3·h-1·kWh-1 for a momopolar
water electrolyser [78].
From the safety point of view, the wide range of the flammability limits of hydrogen
and oxygen mixture demands careful design of the system configurations and the
54
membrane. Leakage of the electrolyte is also a safety issue. Due to the corrosive nature
of the electrolyte, the leakage is more likely to occur at the connections and the seals of
the electrolyser. Bipolar cell configuration, which employs a more complex design,
poses a higher risk of electrolyte leakage than the monopolar design.
The durability of the cell is also an important criterion for electrolysers. Materials for
cell construction determine the cell lifespan. For alkaline electrolysis, these materials
should be resistant to the high concentrations of alkaline electrolyte. Corrosion always
happens more readily at the joints and connections, therefore the joint seal materials
should also be stable under the operating environments.
55
7 Historical Development of Water Electrolysis
This section reviews the development of the water electrolysis over the past two
hundred years since the discovery of the water electrolysis. From the discovery of the
phenomenon of electrolytic split of water into hydrogen and oxygen to the development
of various versions of industrial technique to meet the hydrogen demands of various
applications, the water electrolysis development has gone through several landmarks.
Some historical events of water electrolysis are listed in the Table 4.
Those events in Table 4 promoted the development of this technology. Generally, we
may classify the developments of water electrolysis into five stages
1. The discovery and recognition of water electrolysis phenomena (1800s-1920s).
2. The technology became industrialised and mature for hydrogen production for
industrial uses such as ammonia production and petroleum refining (1920s-
1970s).
3. Systematic innovations were initiated to improve the net efficiency due to the
fear of energy shortage and environmental considerations. The advancement in
the space exploration drove the development of proton exchange membrane
(PEM) water electrolysis, which is essentially the reverse of the PEM fuel cell
operations, and military needs provoked the development of high pressure
compact alkaline water electrolysis for submarine applications (1970s- present).
56
4. Rapidly evolving conceptual development to integrate water electrolysis with
renewable energy technologies as a means for distributed energy production,
storage and use as well hydrogen gas utilities, especially in remote communities
(present).
5. Emergence of new water electrolysis concepts such as photovoltaic (PV)
electrolysis that integrates the photoelectric effect and water electrolysis into one
coherent operation, and steam electrolysis that employs a solid state electrolyte
to effect the split of water molecules in steam (recent developments). These will
be discussed in some detail in Section 8 on Recent Innovations.
In the first stage, following the discovery of electricity, the phenomenon of water
electrolysis was observed. Out of curiosity, the phenomenon was studied; the gases
produced by electrolysis were finally identified to be hydrogen and oxygen [22], and
gradually recognised for its potential applications. With the development of
electrochemistry, the proportional relationship between the electrical energy
consumption and the amount of gases produced became established through the
Faraday’s law of electrolysis. Finally the concept of water electrolysis was scientifically
defined and acknowledged [52].
The second stage was the “golden” age for water electrolysis technology development.
Most traditional designs were developed in this stage. Driven by the industrial need of
hydrogen and oxygen, the knowledge established in the first stage was applied to the
57
industrialisation of water electrolysis technologies. Stereotype of commercial water
electrolysis developed at this stage included some important technology components
that are still being used today [84].
One of the concepts still being applied in electrolyser is membrane. The function of
membrane is to selectively allow the ions to pass through but not the gases. It realises
the separation of the hydrogen and oxygen in the water electrolysers with some
inhibition on the ionic transfer. It was demonstrated that the benefits of being able to
separate the hydrogen and oxygen gases out weight the ohmic resistance brought about
by the diaphragms. The first commercialised membrane was asbestos which was
popular in the early stage. However, asbestos was found not very resistant to corrosion
due to the strong alkaline environment at elevated temperatures. More recently, due to
its seriously adverse health effect, asbestos was gradually replaced by other materials
[85]. Since the 1970s, the gas separation material has gradually shifted to polymers such
as perfluorosulphonic acid, arylene ether and polytetrafluoroethylene [85, 86].
Another significant development was the configurations of the electrolysis cells. As
introduced before, they are monopolar and bipolar designs presented in the history.
Typical conventional tank cells, with monopolar configuration, have the advantages of
simplicity, reliability and flexibility. In contrast, filter press cells, with bipolar
configuration, have the advantages of low ohmic loss and being more compact. High
pressure electrolysers using bipolar configuration would be hard to achieve with the
58
monopolar cells. The disadvantages of bipolar cells are their structural complexity and
requirement of electrolyte circulation and gas electrolyte separators.
The electrode material selection is based on a dedicated balance amongst the desires for
the corrosion resistance, high conductivity, high catalytic effect and low price [43].
Stainless steel is a cheap electrode material with low overpotential, however, steel could
not resist high concentration alkaline solutions. Other materials such as lead and noble
metals are either not resisted to the alkaline or too expensive to be used as bulk
materials for the electrodes. Nickel has been identified to be a very active material with
better corrosion resistance to the alkaline than other transition metals. It became popular
in the water electrolysers during the electrolyser development. As compared in Table 1,
nickel exhibits reasonable high hydrogen generation activity. Much research effort has
been devoted to the understanding of the influence of physical property and the effects
of the nickel-based alloys [87-89].
These developments stimulated the commercialisation of the electrolysis. The history of
the commercial water electrolysis dates back to 1900, when water electrolyser technique
was still in its infancy. Two decades later, large size plants, rated at 100MW, were
developed in Canada, primarily for the ammonia fertilisers [38].
Electrolyser manufacturers all over the world made a great effort to build their own
energy systems to meet different needs. By late 1980, Aswan installed 144 electrolysers
59
with a nominal rating of 162 MW and a hydrogen generation capacity of 32,400 m3 h-1.
Another highly modularised unit is the Brown Boveri electrolyser which can produce
hydrogen at a rate of about 4 to 300 m3·h-1. A number of electrolyser companies and
their water electrolysis units are listed and compared in Table 5:
There are also some more companies not mentioned in this table. Stuart Cell (Canada) is
the only monopolar tank-type cell manufacturer. Hamilton Sundstrand (USA), Proton
Energy Systems (USA), Shinko Pantec (Japan) and Wellman-CJB (UK) are among the
manufacturers of the latest PEM electrolysers.
The key driver for the development of the water electrolysis technology in the first half
the 20th century was the need of hydrogen for the production of ammonia fertilisers
which was also facilitated by the low cost of hydroelectricity. As the massive
hydrocarbon energy was increasingly applied in the industry, the economical advantage
of water electrolysis gradually faded as coal gasification and natural gas reforming
became able to produce hydrogen in large scales at much lower costs. This resulted in
the cease of the progress of the water electrolysis technology as a means for hydrogen
production. However, the oil crisis in the 1970s provoked a renewed interest in water
electrolysis worldwide and hydrogen was considered as the future energy carrier [17].
After the energy crisis in the 1970’s, hydrogen as an energy carrier was considered a
promising method to solve the energy security and sustainable energy supply problems,
60
in the hydrogen economy ideology. Hydrogen production by water electrolysis received
renewed interest and improving efficiency becomes a major goal. Some novel
breakthroughs have been achieved at the cell system level with the emergence of
pressurised electrolysers and PEM electrolyser [78].
Compact high pressure water electrolysers have been utilised to produce oxygen on
board of nuclear powered submarines as part of the life support system. An important
feature of the design is the elimination of gaskets between cells which necessitates high
precision machining of the cell frames. The deficiency of high pressure electrolyser is
the characteristic pressure of the system up to 3.5 MPa, posing a great demand for
safety [17].
For special energy need in the space area, a thin Nafion membrane was first applied by
General Electric in 1966 [78]. The discovery of proton exchange membrane (PEM),
realised the PEM water electrolysis, also named as solid polymer electrolysis (SPE). In
an operation that reverses the PEM fuel cell, the PEM functions as the electrolyte to
transfer the proton. Intensive studies have been carried out in order to reduce the cost of
the membrane manufacturing. Subsequently, small scale PEM water electrolysers were
used for military and space applications in the early 1970s. However, the short
durability of membrane makes PEM electrolysers too expensive for general applications
[82].
61
PEM water electrolysis systems offer several advantages over traditional alkaline water
electrolysis technologies including greater energy efficiency, higher production rates,
and more compact design [20, 90]. However, there are several disadvantages of the
PEM electrolysis. PEM electrolysers have more special requirements on the
components, including expensive polymer membranes and porous electrodes, and
current collectors [30].
A comparison of typical alkaline and PEM electrolysers are summarised in Table 6
62
8 Recent Innovations
There have been a number of exciting new developments in the field of water
electrolysis. This section discusses the recent innovations and research trends of the
water electrolysis technology.
From the theoretical aspect, the water electrolysis has to reduce several resistances
inherent in the process. Innovations have been made towards reducing the energy
losses. These recent innovations and improvements are novel relative to the existing
concepts of the water electrolysis but inevitably have their own limitations.
8.1 Photovoltaic (PV) Electrolysis
The utilisation of solar energy as the source of electricity for water electrolysis was first
reported by Fujishima [91]. A titanium dioxide electrode was applied to capture the
energy of ultraviolet light. The captured energy, in the form of electricity, was applied
to decompose water into hydrogen and oxygen directly as illustrated in Figure 13. As
renewable energy is receiving increasingly more interest, the PV electrolysis becomes
another innovation of the water electrolysis for hydrogen production.
The PV electrolysis has two circuits in the system, as shown in Figure 13, photovoltaic
cell and electrolysis circuit. The key component in the PV electrolysis is the photo-
electrodes which have to absorb energy from the sunshine and release electricity to split
63
water through electrons. Most of the efforts have been devoted to developing
semiconductor photo-electrically active materials.
The major deficiency of the PV electrolysis is the low energy conversion. Typical
efficiency values of 2%-6% restrict the large scale hydrogen production by the PV
electrolysis [92]. Lichta et al reported a theoretical maximum net efficiency of 18.3%
[93]. However, this figure was derived by assuming that all the electricity produced by
the PV was converted to hydrogen. In other words, the Faraday efficiency was assumed
to be 100%.
Practically, the applications of PV electrolysis on large scales are not optimistic at this
stage of its development as compared to other existing and emergent technologies due
to the low solar energy density, variations of sun radiation energy, low operation current
density and the expensive and unstable electrode materials [94].
There are also research efforts aimed to combine the PV module and electrolyser
together through the current control for the voltage regulation. By attaching a Co and
Mo alloy and the Fe and Ni oxide electrodes for hydrogen and oxygen evolution,
respectively, to a silicone PV solar cell, water electrolysis could be conducted simply by
dipping the device in an electrolyte and irradiating it with ultraviolet light. This
achieved a conversion efficiency of 2.5% from solar energy to hydrogen [95].
64
8.2 Steam Electrolysis
Steam electrolysis or vapour electrolysis is performed using solid oxide electrolysis cell
(SOEC) as shown in Figure 14. This may be viewed in simple terms as the reverse
operation of a solid oxide fuel cell. It is high temperature water electrolysis adopting the
solid oxide as the electrolyte instead of liquid alkaline. It adapts a high temperature
operating environment, around 820K to 1073K, at which water splitting is
thermodynamically favoured [96].
The stream passes through the cathode side of the solid electrolyte where hydrogen ions
are reduced to hydrogen, releasing oxide ions in the process. The oxide ions then
migrate through the electrolyte to the anode where they combine to form oxygen
molecules, releasing an electron current following back to the power source [96, 97].
The reactions on two electrodes are, respectively,
Cathode:
(R10)
Anode:
(R11)
Hydrogen production via steam electrolysis may involve less electrical energy
consumption than conventional low temperature water electrolysis, reflecting the
improved thermodynamic and kinetic operating conditions at elevated temperatures. For
an average current density of 7000 A m−2 and an inlet steam temperature of 1023 K, the
65
predicted electrical energy consumption of the stack is around 3 kWh per normal m3 of
hydrogen, which is significantly smaller than 4.5 kWh of low temperature alkaline
water electrolysis cells commercially available today [98]. However, this result does not
include the heating energy circulation and loss. Furthermore, construction materials,
safety issues and strict temperature control have to be addressed as well.
8.3 Comparison of Technologies
A comparison of different water electrolysis technologies is listed in Table 7. However,
these data have different definitions of efficiencies. For alkaline, PEM and
photoelectrolysis electrolysers, the hydrogen yield efficiency is calculated based on the
high heating value of hydrogen. The efficiency of the solid oxide electrolysis cells is the
net efficiency, considering the thermal energy losses of the system.
The above comparison clearly illustrates that the alkaline water electrolysis is currently
mature with reasonable efficiency relative to the other emergent water electrolysis
technologies. The PEM electrolyser with highest performance in terms of efficiency still
needs to overcome the difficulties mentioned in Section 7. SOEC and PV electrolysis
are faced with more challenges due to its severely corrosive operating environment and
engineering issues such as high operating temperature and scaling up requirements.
Therefore, modifications to the alkaline electrolysis to achieve a better efficiency offer a
more realistic solution for large scale hydrogen production in the near future.
66
9 Research Trends
Despite the emergence of the new concepts of water electrolysis as discussed in the
previous section, the low temperature alkaline water electrolysis still holds the promise
for commercial applications and deserves further development. Intensive studies have
been done in these areas to promote the development of water electrolysis [99]. The
current research trends of alkaline water electrolysis are discussed from several aspects
including electrodes, electrolytes, the ionic transport and bubble formation. These
research efforts are not only significant to the water electrolyser but also fundamentals
of the electrochemistry.
9.1 Electrodes
Metal electrodes are normally adopted in the gas evolving processes. As discussed in
kinetics and the development of electrolyser sections earlier, the most widely used
electrode material is nickel because of its stability and favourable activity. However,
deactivation is a main problem of the electrode material even for nickel. The mechanism
of the deactivation of the nickel electrodes is nickel the hydride phase formation at the
surface of the nickel electrodes due to high hydrogen concentration. The iron coating
prevents nickel hydride phase from forming and hence prevents deactivation of the
electrode [87]. Dissolved vanadium species are also found to activate nickel cathodes
during hydrogen evolution in the alkaline media [53]. Addition of iron to the
67
manganese–molybdenum oxides enhanced the stability of electrodes. The iron addition
also enhanced the oxygen evolution efficiency. The formation of the nickel triple oxides
seems responsible for the enhancement of both oxygen evolution efficiency and stability
[100]. Therefore, electrocatalysts are the key to enhancing and stabilising the electrode
activity.
Apart from material selection, electrode modifications in cell design are also important
in water electrolysis. The electrode surfaces are commonly modified by slits and holes
to facilitate the escape of gas bubbles. The holes must be appropriate to prevent the gas
trapping. Typical diameters for electrode perforation in alkaline water electrolysis are
0.1 and 0.7 mm for hydrogen and oxygen, respectively [43]. Louvered, finned or slotted
electrodes are also used to remove bubbles.
9.2 Electrocatalysts
To some extent, the electrode itself is a catalyst by affecting the activation energy of the
electrochemical reaction. However, doping or coating more stable and active layer is
always used in electrode design. Similar to catalysts, electrocatalysts facilitate charge
transfer or chemical reaction, reducing the activation energy of the reaction. The
obvious effect of an electrocatalyst is to reduce the overpotential of either or both of the
two half reactions. The role of the electrocatalyst is effected by the electronic structure
of the electrodes. In the hydrogen evolution reaction, Ni, Pd, Pt with d8s2, d10s0, and d9s1
68
electronic configurations, exhibiting minimum overvoltage values and Zn, Cd, Hg with
d10s2 electronic configuration showing maximum values. The spillover theory in
electrocatalysis by Bockris and Mchardy enables the understanding of the interaction
between substances [17].
Alloys, with different electronic distributions in the metal, are adopted to improve the
activity of electrodes. For example, alloy of Mo and Pt was found to be a significant
upgrade of the electrolytic efficiency in comparison with its individual components and
conventional cathode materials [101]. More examples are listed in Table 8. The Tafel
slope and exchange current density of the hydrogen evolution reaction in alkaline
solutions at temperature near 70 are used to compare the activities of Ni and Ni based
alloy.
The doping material could be chosen from a wide range of metals. Noble metals are
commonly used as electrocatalysts. Ruthenium dioxide (RuO2), prepared by pyrolysis
and calcinations, clearly shows the electrocatalytic activity for oxygen evolution
reaction.[106]. An anode electrocatalyst with the formula IrxRuyTazO2 has been claimed
to achieve overall voltage of 1.567V at 1A·cm−2 and 80, equating to an energy
consumption of 3.75kWhNm−3 H2 and an efficiency of 94% with the total noble metal
loading less than 2.04 mg cm−2 [90]. Non-noble metals also find their electrocatalytic
activities. The Li doping increases the electrical conductivity of these materials. The key
69
to better performance is that the roughness factor increases with Li percentage up to 3%
of Li, favouring oxygen evolution [107].
The physical properties of electrode materials also influence the electrocatalytic
activity. Larger BET surface area and porosity of the oxide catalyst powder is found by
the small La addition by Singh et al [108]. They observed a reduction in the charge
transfer resistance for the oxygen evolution reaction on the electrode made of oxide
powder.
Nanostructures have also received much attention as it enlarges the material surface area
and enables a unique electronic property. The increased active area of the
nanostructured electrode reduces the operating current density of the electrolyser. A
25% reduction in overpotential and 20% reduction in energy consumption were
achieved by the use of the Ru nano-rod cathode compared to the planar Ru cathode. The
improvement was attributed to the increased active area of the nanostructured electrode
which reduces the operating current density of the electrolyser [44]. Prashant V. Kamat
[109] also proposed different nanostructures for improving the performance of
photoelectrolysis facilitating the charge transfer, which has the potential to be applied as
electrodes for water electrolysis.
The preparation methods of electrodes are an important factor in terms of effecting
electrode surface properties such as roughness. Coatings are another common technique
70
in electrode preparation. For example composite of Ni, Fe and Zn prepared from the
electrodeposition showed good stability for up to 200 h under the current density of
1350 A cm−2. This material showed good activity as well; in 28% KOH under 80, its
overpotential is about 100 mV which is significantly lower than that of mild steel (400
mV) [110]. A catalyst-coated membrane (CCM) with a five-layer structure was
developed [111]. The five-layer CCM exhibits the highest performance and stability,
attributed to the expansion of the triple-phase boundaries for electrochemical reactions
and the improvement of contact and mass transfer resistance.
Tables 9 and 10 list several electrocatalysts which are found helpful to reduce the
overpotential or stabilise the electrodes of the industrial water electrolysis.
To sum up, the physical modifications of electrodes help the removal of the gas from
the electrodes. The electrode material influences the overpotential significantly. The
electronic property and the surface property determine electrocatalytical performance of
the coating or doping. Alloys, nano-structured materials, transition metals and noble
metals could be used to improve the electrode activity.
9.3 Electrolyte and Additives
Most commercial electrolysers have adopted alkali (potassium or sodium hydroxide)
solutions as the electrolyte. Energy consumption during the electrolysis of water was
71
significantly reduced by small quantities of activating compounds by the effect of ionic
activators [121, 122].
Ionic liquids (ILs) are organic compounds. At room temperature, they are liquids solely
consisting of cations and anions, thus possessing reasonably good ionic conductivities
and stability [123]. Imidazolium ILs were used as an electrolyte for hydrogen
production through water electrolysis. The current densities higher than 200 A m-2 and
efficiencies greater than 94.5% are achieved using this ionic liquid in a conventional
electrochemical cell with platinum electrodes at room temperature and atmospheric
pressure. The catalytic activity of the electrode surface was not affected during the
electrolysis mainly due to the chemical stability of the ILs [124]. However, the ILs
normally have high viscosity and low water solubility which is not favoured for mass
transport, resulting low achievable current densities and thus low hydrogen production
rates. Therefore, more suitable ionic liquids with high conductivity and solubility are
needed to facilitate electron transfer and water electrolysis, respectively.
Compared to the research on electrocatalysts, developmental effort on new electrolyte is
relatively low. However, there is still a potential to improve the overall efficiency by
using electrolyte additives to enhance ionic transfer by reducing the electrolyte
resistance. On the other hand, the adoption of electrolyte additives could tune the
affinity between electrolyte and electrodes and help to manage the bubble behaviour.
72
9.4 Bubble Management
The bubble formation and its transportation are major causes of extra ohmic losses. Not
only the dissolution of gas, but also the interface of gas between electrodes and
electrolyte lead to resistances to water electrolysis.
The bubble behaviours are intensively studied [68, 71, 125] in the sense of
electrochemistry, but no mechanisms or models have been applied to alkaline water
electrolysis. Microgravity condition was used to study the bubble behaviours without
the buoyancy effect. Water electrolysis under microgravity resulted in stable froth layer
formation, and the accompanying ohmic resistance increased with the froth layer
thickness. The contributions of electrode surface coverage by bubbles and electrolyte-
phase bubble void fraction to the ohmic drop were also studied [72]. Under territorial
gravity, the bubble sizes are smaller. Therefore, reducing the residence time of bubble
staying in the electrode is the key to minimising the bubble size and thus reducing the
bubble resistance.
According to the theory of surface tension, a hydrophilic electrode prefers water rather
than bubbles. It means the bubble sizes are not easy to grow. The mass transfer and
ionic transfer between electrodes and electrolyte could be enhanced. Similarly,
surfactant additive can be used to reduce the surface tension, which can minimise the
bubble size or accelerate the departure of the bubbles and then achieve the same effect
73
of hydrophilic materials. At the same time, these additives should be inert to the
electrochemical reaction [126] and stable during the process.
Fluid mechanic means, by circulating the electrolyte solution to sweep the bubbles off
the electrode surfaces can also be applied. To sweep the bubbles off the electrode
surface, the velocity of the fluid should be high enough, which in turn will benefit the
mass transfer of the electrolyte and eliminate the concentration difference. Therefore,
mechanically forcing the bubble to depart from the electrode surface is an alternative
way to eliminate the bubble formed on the electrode surface.
74
10 Summary
Alkaline water electrolysis combined with renewable energy can be integrated into the
distributed energy system by producing hydrogen for end use and as an energy storage
media. Compared to the other major methods for hydrogen production, alkaline water
electrolysis is simple but currently less efficient. The challenges for widespread use of
water electrolysis are also the durability and safety. These disadvantages require further
research and development effort.
This paper has examines the fundamentals of the water electrolysis and compared the
performance of various water electrolysis designs as well as introduced several
emergent water electrolysis technologies.
Based on the thermodynamic and kinetic analyses of the alkaline water electrolysis, a
number of resistances hindering the efficiency of the alikaline water electrolysis process
have been identified. These include resistances due to bubbles, reaction activation
energy, ionic transfer and electrical resistances in the circuit. The bubble resistance is
suggested to be reduced by electrode modification and electrolyte additives. Reaction
overpotential can be optimised by electrode material selection and preparation. In
addition, transport-related resistances such as bubble resistance and electrolyte
resistance can be reduced by improving mass transport such as bubble elimination by
electrolyte circulation. By identifying the resistances causing extra energy losses, this
75
study opens the opportunities to minimise the energy input especially at high current
density.
Practical considerations of industrial electrochemical engineering and electrolyser
development have led to the conclusion that the alkaline water electrolysis is still a
better means for hydrogen production. Further R&D efforts to improve the efficiency
are needed to widespread the application of the alkaline water electrolysis. These
include developing electrocatalysts to significantly reduce electrochemical reaction
resistance, electrolyte additives to facilitate the electron transfer and ionic transfer and
to reduce electrode surface tension, electrode surface profile modifications and surface
coatings, and more importantly, managing the gas bubble resistances.
Acknowledgements
This research was supported under Australian Research Council's Linkage Projects
Scheme (project number LP0669575).
76
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Figure Captions
Figure 1 A schematic illustration of a conceptual distributed energy system with water
electrolysis playing an important role in hydrogen production as a fuel gas and energy
storage mechanism
Figure 2 A schematic illustration of a basic water electrolysis system
Figure 3 An electrical circuit analogy of resistances in the water electrolysis system
Figure 4 Cell potential for hydrogen production by water electrolysis as a function of
temperature
Figure 5 A schematic illustrations of electrical double layer and the potential
distribution near an electrode surface
Figure 6 Effect of potential change on Gibbs energy energies: (a) the overall
relationship between energy change and state of reaction and (b) Magnified picture of
shaded area of (a)
Figure 7 Typical Tafel plots for both hydrogen and oxygen evolution
Figure 8 An illustration of the contributions of anode and cathode polarisation to the
cell voltage of an alkaline water electrolysis cell
87
Figure 9 Compositions of the typical cell voltage of an alkaline water electrolysis cell
Figure 10 A qualitative comparison of the energy losses caused by reaction resistances,
ohmic resistance, ionic resistance and bubble resistance
Figure 5 An illustration of the contact angle at the three phase boundary of the gas
bubble, electrode and the electrolyte
Figure 12 Schematics of cell configurations of monopolar (a) and bipolar (b)
electrolysers
Figure 13 A schematic diagram of semiconductor based solid state photovoltaic
electrolysis
Figure 14 A schematic of SOEC planar stack unit for steam electrolysis
88
List of Figures
End Use of Electricity
End Use of Fuel Gas
Intermittent Electricity
Wat er
Electrolysis
Power from
Fuel Cell
Renewable Energy
Hydrogen
Sun
Excess Electricity
Figure 1
89
DC Power
Electron Flow
Cathode
Anode
Diaphragm
Electrolyte
Hydrogen ReceiverOxygen Receiver
H
+
OH
-
O
2
H
2
+
+
‐++
+
‐
‐
‐
Electrolyte
Figure 2
90
R1 Ranode Rmembrane
Rions Rbubble,H2 Rcathode R’1
Rbubble,O2
-
e
+
Figure 3
91
0 200 400 600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Temperature ( oC)
H2 Generation Impossible
Equilibrium Voltage
Endothemic Reaction
Thermoneutral Voltage
Exothomic Reaction
Electrolyser Cell potenial (V)
Figure 4
92
(b)
E
Electrode
E
Solution
E
x
(a)
IHL
-
-
+
+
-
-
+
+
+
-
-
OHL
-
+
+
+
+
-
-
Figure 5
93
Gibbs Engery
Reaction Coordinate
G
c
oG
c
At Eo
At ( Eo
+E)
F
( E-Eo
)G
a
G
a
o
R
O+e
Reaction Coordinat
e
a
F( E-Eo
)
Gibbs Engery
F( E-Eo
)
F( E-Eo
)
b
At Eo
At (E
o
+E)
Figure 6
94
1 10 100 1,000
-0.4
-0.2
1.4
1.6
1.8
Overpotential (V)
Current Density (A m-2)
Oxygen Overpotential
Hydrogen Overpotential
i0
k=b
Figure 7
95
50 100 150 200
0
1
2
Potential (V)
Temperature (oC)
Anode
Cathode
Theoretial
Decompostion
Potential
Figure 8
96
0 1000 2000 3000 4000 5000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ohmic Loss(electrode)
Oxygen Overpotential
Ohmic Loss(electrolyte)
Hydrogen Overpotential
VCELL (V)
Current density (A m-2)
Eo
Figure 9
97
0 1000 2000 3000 4000 5000
0
200
400
600
800
1000
1200
Eloss / mJ s-1
Current density (A cm-2)
E loss, electrolyte
E loss, andoe
E loss, cathode
E loss, circuit
E loss, Bubble
E loss, Total
Figure 10
98
SolidGas
Liquid
γ
lv
γ
sv
γ
sl
θ
Figure 11
99
H
2
(b)
2.2V
O
2
+ +
-
O
2
-
+
-
H
2
O
2
+
-
H
2
O
2
H
2
2.2
(
n-1
)
V
+
+
O
2
2.2V
-
(a)
+
-
+
- -
H
2
H
2
H
2
O
2
O
2
Figure 12
100
+ -
-
Electrolysis cell
p n
O
2
H
2
e
-
e
-
hν
Photovoltaic cell
+
Water
Figure 13
101
Anode
Solid Electrolyte
Inlet H
2
O
Interconnection
Interconnection
Outlet O
2
Outlet H
2
/H
2
O
Cathode
Figure 14
102
Table Captions
Table 1 Kinetics parameters of hydrogen production on different electrode metals
Table 2 Kinetic parameters of oxygen production on different metals
Table 3 Solubility product constants of impurities at 25°C
Table 4 Historical Events of Water Electrolysis
Table 5 Water electrolyser developers and cell operating conditions
Table 6 A comparison of the two types of commercialised electrolysers
Table 7 Comparison of different electrolyser technologies
Table 8 Tafel slopes of Ni alloys
Table 9 Oxygen overpotential of different electrode materials
Table 10 Hydrogen overpotential of different electrode materials
103
List of Tables
Table 2 Kinetics parameters of hydrogen production on different electrode metals
Metal Heat of H2 adsorption
kJ·mol-1
Electrolyte Temperature
i0
A·m -2
Tafel slope
mV
Ni [55] 105 1M NaOH 20 1.1× 10-2 121
Fe [56] 109 2M NaOH 20 9.1 × 10-2 133
Pb [57] N/A 6N NaOH 25 4 × 10 -2 121
Zn [57] N/A 6N NaOH 25 8.5× 10-6 124
Co[58] N/A 0.5M NaOH 25 4.0× 10
-3 118
Pt [17] 101 0.1N NaOH 22 4.0 105
Au [17] N/A 0.1N NaOH 25 4.0× 10 -2 120
104
Table 2 Kinetic parameters of oxygen production on different metals
Metal Electrolyte Temperature
i0
A·m -2
Tafel slope
mV
Pt [60] 30% KOH 80 1.2× 10 -5 46
Ir [61] 1 N NaOH N/A 1.0× 10 -7 40
Rh [61] 1 N NaOH N/A 6.0× 10 -8 42
Ni [62] 50% KOH 90 4.2× 10 –2 95
Co [60] 30% KOH 80 3.3× 10 –2 126
Fe [60] 30% KOH 80 1.7× 10 –1 191
105
Table 3 Solubility product constants of impurities at 25°C
Ions / deposit (at 25°C)
Ca2+ / Ca(OH)2 5.5×10-6
Mg2+ Mg(OH)2 1.8×10-11
Ni2+ / Ni(OH)2 2.0×10-15
Ag+/AgCl 1.6×10-10
Zn2+/Zn(OH)2 1.8×10-14 a
a measured under 20°C
106
Table 4 Historical Events of Water Electrolysis
Year Landmark Event
1800 Nicholson and Carlisle discovered the electrolytical splitting of water [22]
1920s Several large 100 MW size plants were built worldwide [38]
1948 First pressurized electrolyser was built by Zdansky/Lonza [22]
1966 First solid polymer electrolyte system was built by General Electric Company [82]
1970s First solid oxide water electrolysis was developed [83]
107
Table 5 Water electrolyser developers and cell operating conditions [47]
Parameter
De Nora S.A
P
Norsk Hydro
Electrolyzer
Corp.Ltd.
Teledyne
Energy
systems
General
Electric
Cell type B-FP B-FP M-T B-FP B-FP
Anode
Expanded
Ni-plated
mild steel
Activated
Ni-coated
steel
Ni-coated
Steel
Ni screen
PTFE- bonded
noble metal
Cathode
Activated
Ni- Plated
steel
Activated
Ni-coated
steel
Steel Ni screen
PTFE- bonded
noble metal
Pressure (Mpa) Ambient Ambient Ambient 0.2 0.4
Temperature °C 80 80 70 82 80
Electrolyte 29% KOH 25% KOH 28% KOH 35% Nafion
Current density (Am-2) 1500 1750 1340 2000 5000
Cell voltage (V) 1.85 1.75 1.9 1.9 1.7
Current efficiency (%) 98.5 98.5 >99.9 NR NR
Oxygen purity (%) 99.6 99.3-99.7 99.7 > 98.0 > 98.0
Hydrogen purity (%) 99.9 98.9-99.9 99.9 99.99 >99.0
108
Table 6 A comparison of the two types of commercialised electrolysers [78]
Parameter Monopolar Alkaline electrolyser PEM electrolyser / Cell
Cell voltage 1.85 2V
Number of cells N/A 7-51
Current density 0.25 A cm-2 1.075 A cm
-2
Temperature 70 65 (outlet)
Current 10kA 1kA (maximum)
Scale 200kW N/A
Hydrogen production rate 42 m3 h-1 0.42 m3 h-1
Oxygen production rate 21 m3 h-1 0.21 m3 h-1
Hydrogen gas purity H2>99. 5% H2>99.995%
Oxygen gas purity O2>99% O2>99%
Demineralized water conductivity N/A <0.25 S cm -1
109
Table 7 Comparison of different electrolyser technologies
Technology Efficiency Maturity Reference
Alkaline electrolyser 59-70%a Commercial [94]
PEM electrolyser 65-82%a Near term [94]
Solid oxide electrolysis cells 40-60% b Mediate term [94]
Photoelectrolysis 2-12% a Long term [93]
a the efficiency based on the hydrogen yield
b the net efficiency
110
Table 8 Tafel slopes of Ni alloys
Material and Preparation
method
Electrolyte Temperature
Tafel slope
mV
i0
(A cm-2)
250
a
(mV)
Ni (wire) [102] 30% NaOH 70 99 5.5×10-5 362
Ni79Mo20Cd[103] 1M NaOH 70 125 N/A N/A
Ni70Mo29Si5B5
(amorphous) [104]
30% KOH 70 118 1.8×10-6 489
Ni60Mo40 mixture(ball-
milled 20h) [105]
30% KOH 70 50 1.7×10-2 58
a Hydrogen evolution reaction overpoential under 2500 A m-2
111
Table 9 Oxygen overpotential of different electrode materials
Composition Method T Electrolyte C j Ref.
Formula (°C) mol·dm-3 A·m-2 mV
Ni + Spinel type
Co3O4
Thermo-
decomposi
tion
25 KOH 1 1000 235±7 [108]
Ni + La doped Co3O4 Thermo-
decomposi
tion
25 KOH 1 1000 224±8 [108]
MnOx Modified Au Electro-
deposition
25 KOH 0.5 100 300 [112]
Li10% doped Co3O4 Spray
Pyrolysis
RT 1 KOH 1 10 550 [107]
Ni N/A 90 KOH 50wt% 1000 300 [113]
La0.5Sr0.5CoO3 Spray-
stiner
90 KOH 50wt% 1000 250 [113]
Ni0.2Co0.8LaO3 Plasma Jet
Projection
90 KOH 50wt% 1000 270 [113]
Note: 1. Room temperature.
112
Table 10 Hydrogen overpotential of different electrode materials
Composition Method T Electrolyte
C j Ref.
Formula (°C) mol·dm-3 A·m-2 mV
Ni-Fe-Mo-Zn Co-deposition 80 KOH 6 1350 83 [114]
Ni-S-Co Electro-
deposition
80 NaOH 28wt% 1500 70 [115]
Ni50%-Zn Electro-
deposition
N/A NaOH 6.25 1000 168 [116]
MnNi3.6Co0.75M
n0.4Al0.27
Arc melting 70 KOH 30wt% 1000 39 [117]
Ti2Ni Arc melting 70 KOH 30wt% 1000 16 [118]
Ni50% Al Melting 25 NaOH 1 1000 114 [119]
Ni75%Mo25% Co-deposition 80 KOH 6 3000 185 [120]
Ni80%Fe18% Co-deposition 80 KOH 6 3000 270 [120]
Ni73%W25% Co-deposition 80 KOH 6 3000 280 [120]
Ni60%Zn40% Co-deposition 80 KOH 6 3000 225 [120]
Ni90%Cr10% Co-deposition 80 KOH 6 3000 445 [120]
