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Water Electrolysis (Splitting Water Using Electricity)
Department of Mechanical Engineering,
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Department of Mechanical Engineering, Yuan Ze University
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Materials for Water Electrolysis Cells
•
Hydrogen generation can be accomplished via traditional DC electrolysis of aqueous
solutions at temperatures less than about 100 oC.
•
However, electrolysis of stream can also be accomplished at higher temperatures at
the cathode of electrolytic cells utilizing solid membranes. The solid membranes
typically are electronic insulators and need to be gas-tight (hermetic), but have the
special property of being able to conduct ions via fast diffusion through the solid.
•
Hydrogen production via the conventional electrolysis largely depends upon the
availability of cheap electricity (e.g. , from hydroelectric generators). Consequently,
only about 5 % of the world hydrogen production is via electrolysis.
•
The only complete hydrogen production process that is free of CO2 emissions is water
electrolysis (if the electricity is derived from nuclear or renewable fuels).
•
However, 97% of the hydrogen currently produced is ultimately derived from fossil
energy.
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Low-Temperature Electrolysis of Water
Solutions
• The reversible electrical potential (ΔG/nF = Erev) to split the O-H bond in
water is 1.229 V.
• In addition, heat is needed for the operation of an electrolysis cell.
• If the heat energy is supplied in the form of electrical energy, then the
thermal potential is 0.252 V (at standard conditions), and this voltage must
be added to Erev (i.e., add entropic term TΔS to ΔG).
• The (theoretical) decomposition potential for water at standard conditions
(for ΔH ≈ ΔHo) is then 1.480 V.
• Anode and cathode reactions for electrolysis (see figure 2.1) are:
1
Anode: 2OH  O2  H 2O  2e 
2
Cathode: 2 H 2O  2e   H 2  OH 

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(2.1)
(2.2)
3
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• For alkaline electrolysis, OH- ions must be able to move through the
membrane (under influence of the electric field) from the cathode chamber
into the anode chamber to supply OH- to participate in the reaction
(equation 2.1) at the anode.
• Irreversible processes that occur at the anode and cathode and the electrical
resistance of the cells cause the actual decomposition potential (voltage) to
increase to about 1.85 to 2.05 V.
• This means that the electrolysis efficiency will be between 72 and 80 %. The
total electrical resistance of the cell is dependent upon the conductivity of
the electrolyte, the ionic permeability of the gas-tight diaphragm that
separates the anodic region from the cathodic region, and the current
density (normally in the fairly moderate range of 0.1 to 0.3 A ).
• Higher KOH concentrations (up up 47 %) yield higher conductivity, but this
usually greatly increases the corrosion of various cell components.
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• Common aqueous electrolytes are better conductors at
slightly elevated temperatures (70 to 90 oC), so the
electrolysis cells are operated at these conditions.
• The original discovery of electrolytic water splitting used
acidic (dilute H2SO4) water, but in industrial plants an alkaline
(e.g., 25 wt % KOH) medium is preferred because corrosion is
more easily controlled and cheaper materials can be utilized.
• In order to reduce the actual cell voltage downward toward
the 1.48 value (reduce energy consumption), many different
catalytic materials have been examined for use as anodes or
cathodes (or coatings on underlying electrodes).
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• Low-Temperature PEM-Type Electrolyzers
– Proton exchange membrane or PEM-type water electrolyzers utilize thin
films (e.g., 0.25 mm) of a proton-conducting ion exchange material
instead of a liquid electrolyte.
– When a reverse polarity is applied to a PEM fuel cell, the fuel cell
reactions are reversed and become water electrolysis reactions (see
equation 2.6 to 2.8).
– PEM fuel cells have been the subject of research and development for
decades. In the 1960s NASA used PEM cells for their HOPE, Gemini, and
Biosatellite missions.
– After a lull in the 1980s, a rush of development began in the early 1990s
for transportation applications.
– This was initiated by improvements in bonded electrodes, which enabled
much higher current densities.
– These improvements can be advantageous to PEM cells used as
electrolyzers.
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• The PEM cells typically use sulfonated polymer (e.g., NafionTM)
electrolytes that conduct the protons away from the anode to the cathode
(in electrolysis mode).
• For smaller generators, the solid polymer can be more attractive than a
dangerous, caustic electrolyte.
• A complicating factor is that the solid-state conduction of the protons is
accompanied by multiple water molecules (H2O)nH+.
• Also, the membrane must be kept hydrated to sustain the conduction
mechanism.
• Therefore, water recycling becomes a large consideration since water is
constantly removed from the anode and reappears at the cathode (mixed
with the hydrogen).
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• At temperatures less than 100 oC, gaseous hydrogen is easily
removed from liquid water, but the hydrogen still contains
water vapor that most likely requires dehumidification (e.g.,
pressure swing absorption dryer).
• Electrodes generally have utilized finely divided platinum
black or, more recently, IrO2 or RuO2 (for increased electronic
conductivity) as catalysts.
• Research is currently being conducted into PEM-type
membranes that have better kinetics, yet are chemically
stable at elevated temperatures such that they could operate
in steam.
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• PEM water electrolysis cells have a potential advantage over traditional
low-temperature electrolysis cells (e.g., KOH in water electrolytes with
palladium, titanium, or alternative metal or ceramic electrodes) because
PEM devices have been shown to be reversible.
• They can ”load level” by generating electricity from hydrogen (and oxygen)
operating as a fuel cell when needed (peak) and reverse to operate as an
electrolyer by consuming electricity to produce hydrogen (and oxygen).
• This is convenient if excess electrolysis is available during low periods of
consumption (off-paek).
• PEM electrolysis cells could also be used in hybrid systems utilizing solar
energy.
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• Anode and Tanaka have recently used a Nafion electrolyte in
electrolysis mode to decompose two water molecules to
simultaneously generate one molecule of hydrogen and one of
hydrogen peroxide (used in paper/pulp and chemical
industries).
• They do this by using a high applied voltage (1.77 to 2.00 V) in a
two-electron transfer process:
Cathode: 2e   2 H   H 2
Anode: 2 H 2O  HOOH  2 H   2e 
and a NaOH anolyte collection solution. No oxygen is generated.
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• Low-Temperature Inorganic Membrane Electrolyzers
– Electrolyzers operated at low temperatures do not take full advantage of
thermodynamic efficiency advantages.
– The required cell voltage drops considerably (to Eoo = 0.9 V at 927 oC) because of
the positive entropy value (ΔGo = ΔHo - TΔSo) when operating at high
temperatures.
– However, sealing bipolar plate devices should be easier at low temperatures
since thermal cycling would not result in high stresses due to thermal expansion
mismatches between cell components and sealing material.
– Also, inorganic membranes will be more chemically stable in the 200 to 300 oC
temperature range than most organic proton-conducting membranes.
– A typical pressurized-water nuclear reactor heats water from 285 to 306 oC (at
2150 psia) in its core and might be a heat source (heat-exchanged steam at
temperatures significantly lower than the core temperature) for a lowtemperature electrolysis device.
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• Solid inorganic materials exhibiting fast proton conduction at low
temperatures seem to be more prevalent than fast oxygen ion conductors.
• Some proton-conducting glasses achieve high proton mobility due to
incorporation of water (bonded to POH groups).
• These glasses can be fabricated by sol-gel techniques at low temperatures.
• However, the gels are deliquescent and also are easily fractured into
pieces when heated.
• This limits the practical application of these glasses to very low
temperatures, and therefore limits the flux values of hydrogen that can be
achieved.
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• Fabrication of proton-exchanged ”-alumina compositions is difficult
because waters of hydration are lost during firing, and therefore the
crystal structure is irreversibly destroyed.
• One approach used to solve this problem, for ”-alumina, has been to
fabricate a potassium ion crystal structure by firing to high temperatures.
Then, at room temperature, protons can be electrochemically ion
exchanged into the crystals from a mineral acid.
• Since the potassium ion is larger than the sodium ion, using the potassium
composition lessens lattice strain during the proton exchange process. In
these oxide ceramics, two protonic species can exist. The first type is a
H2O molecule associated with a proton as hydronium ion (H3O+). The
second type is proton bound to an oxygen ion of the crystal lattice (=OH+).
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• Moderate-Temperature Inorganic Membrane
Electrolyzers
– Steam electrolysis is feasible at moderate temperatures
using cells constructed with solid inorganic (ceramic)
membranes.
– These temperatures could range from approximately 500
to 800 oC using ceramic membranes that are either oxygen
ion or proton conductors.
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• Moderate-Temperature Oxygen Ion Conductors
– The electrolysis reactions to produce hydrogen using oxygen ion
conductors are:
Cathode: H 2O  2e   H 2  O 2
(2.3)
1
Anode: O 2  O2  2e 
(2.4)
2
1
Overall : H 2O  H 2  O2
(2.5)
2
– During the electrolysis reaction, oxygen is removed from the reaction site
via the membrane (oxygen ion conductor), leaving hydrogen gas and any
unreacted steam on the cathode side. In order to obtain pure hydrogen
gas, the hydrogen must be separated from the steam by using one of a
number of methods. Methods could include condensation of the steam
(followed by drying) or the use of a hydrogen-conducting membrane (likely
used at elevated temperature and perhaps elevated pressure).
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• Moderate-Temperature Proton Conductors
– Using proton-conducting ceramics as an electrolyte for a steam electrolyzer
involves the same reactions as for a low-temperature proton-conducting
polymer membrane:
1
Anode: H 2O  2 H   O2  2e 
2
Cathode: 2 H   2e   H 2
1
Overall : H 2O  H 2  O2
2
(2.6)
(2.7)
(2.8)
– For fuel cell operations, the proton-conducting cells have a thermodynamic
advantage over oxygen ion-conducting cells (due to product water being
swept from the cathode by excess air required for cell cooling).
– Applications that are driven by maximizing efficiency at the expense of
power density favor proton cells.
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•
•
•
Proton conductors like the cerates (BaCeO3 and SrCeO3) have been studied for number of
years, while doped barium zirconate (BeZrO3) has been advancing strongly in the last
couple of years due to reports of high conductivity and good chemical resistance to CO 2
(not relevant for steam electrolysis).
The aliovalent doping creates oxygen vacancies; an incorporation example is given by
equation 2.9:
Water vapor in the cell can react with the oxygen vacancies to from protons per equation
2.10:
X
'
2BaO Gd2O3 intο BaCeO3 lattice  2BaBa
 2GdCe
 5OOX  VO"
•
2.9
The OHoo species is a proton bound to an oxygen ion in the lattice. However, the proton
can hop from one oxygen ion to another, giving rise to proton conductivity.
H2O  VO"  OOX  2OHOo
(2.10)
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• Moderate-Temperature Bipolar Plates (Interconnects)
– At low to moderate temperatures new possibilities arise for using various metals as
bipolar plate (for series connected cells in a bipolar stack arrangement).
– Most metals have too high (e.g., 15 E-6 oC-1) of thermal expansion to match that of
zirconia (10.5 E-6 oC-1).
– In order to get a lower thermal expansion metal (to match zirconia), SOFC
developers originally tried to use special high-chromium alloys like 95 Cr4- 5 Fe
(Plansee alloy) or 94 Cr - 5 Fe -1 Y2O3.
– However, they ran into the problem of high temperature Cr oxidation.
– The reaction is Cr2O3 + ½ O2  2CrO3 (high vapor pressure gas).
– The presence of alloying element in the interconnect tends to minimize the
tendency for the Cr oxidation to take place (especially after oxide scale formation).
– Alloy elements like Y, Ce, Hf, Zr, and Al are reported to slow scale growth. However,
these elements tend to form scales with low electronic conductivity, whereas
Cr2O3 scales are semiconductors.
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• High-Temperature Inorganic Membrane Electrolyzers
– The most common high-temperature cells being investigated are solidoxide fuel cells (SOFCs) using yttria-or scandia-stabilized zirconia (cubic
phase) electrolytes that are rapid oxygen conductors.
– Over many years, yttrium and scandium have been used to substitute
on the zirconium lattice site to stabilize the cubic structure and
increase oxygen ion diffusion by creating oxygen vacancies to
compensate for their aliovalent (Y 3+ or Sc 3+ on Zr 4+ site) charges.
– Yttria provides excellent structural stabilization and good ionic
conductivity, but at significant additional material cost.
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– Loss of conductivity for Scandia-stabilized zirconia has been reported due to
phase changes upon aging at high temperatures (i.e., 1,000 oC).
– This instability certainly would be less of a problem for cells operated at lower
temperatures (e.g., 800 oC ).
– For long-life operation at high temperatures, it is very important to use suitable
electrodes that do not interact (e.g., interdiffuse) unduly with the electrolyte or
lose their activity (e.g., sintering).
– Fuel cells using zirconia electrolytes have traditionally used Ni-ZrO2 and doped
LaMnO3 electrodes. These combinations have proven to be structurally and
chemically stable at high temperatures for long periods with fuel cells
operating for up to 25,000 h with performance degradation of less than 0.1 %
per 1,000 h.
– Some interdiffusion and formation of nonconductive compounds (e.g., La2Zr2O7)
has been reported. These interactions are more severe at high temperatures
and long times.
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High-Temperature Electrolysis
•
•
•
The electrolysis reaction can be expressed as:
Cathode reaction
H 2O  2e  H 2  O2
1
O2  2e 
2
Anode reaction
O 2 
Overall reaction
1
(3.3)
H 2O  H 2  O
2
2
(3.1)
(3.2)
Energy
The enthalpy of the overall reaction is ∆H = 242 kJ/mole at 298 K and 248 kJ/mole at
1,000 K.
A schematic of an electrolysis cell using an oxygen ion conductor is shown in figure 3.1.
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• The benefit of high-temperature electrolysis (HTE) stems from the fact that
a portion of endothermic heat of reaction can be supplied by thermal
energy instead of electric energy.
• Figure 3.2 shows the energy input required for electrolysis of steam.
– It can be seen that at higher temperatures substantial energy is provided
as thermal energy, resulting in considerable reduction of primary
(electrical) energy.
• The high temperature also allows high current density operation as both
ohmic resistance losses from the electrolyte and electrode materials, and
non-ohmic resistance losses from the electrode reaction processes are
thermally activated.
• Hydrogen production via room temperature electrolysis of liquid water has
the disadvantage of much lower overall thermal-to -hydrogen efficiencies of
24 to 32 % (including power generation), while at higher temperatures
practical efficiency can be as high as 50 to 60 %.
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• Materials and Design
– The high operating temperature that is necessary for an efficient electrolysis
process requires the use of materials that are stable at those temperatures.
– In general, the materials and fabrication technology that are used for hightemperatures solid-oxide fuel cells (SOFCs) are directly applicable to hightemperature electrolysis devices.
– The high-temperature electrolysis cell is commonly referred to as the solidoxide electrolysis cell (SOEC).
1. Series-Connected Tubes
 The cell used traditional SOFC materials
such as 9 mol % yttria-doped zirconia (YSZ)
as the electrolyte, a cermet mixture of 50:
50 wt. % nickel and YSZ as the hydrogen
electrode, and Ca-doped LaMnO3
perovskite (La0.5Ca0.5MnO3) as the air
electrode.
 The electrolyte was about 300 m, and the
electrodes were about 250 m.
 The
typical cell diameter was 14 mm, with
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an Yuan
active
University
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2. Tubular Stack Design






The cell materials consist of YSZ electrolyte, 10 mol % yttria, Sr-doped
LaMnO3 air electrode, and a nickel-zirconia cermet hydrogen electrode.
The individual tubes are electrically connected using a Mg-doped LacrO3
interconnection layer.
The primary difference in the material of construction is the interconnect
material. Unlike the series-connected tube design, a strip of interconnect
is used.
The Mg-doped LaCrO3 perovskite is known for its stability over the range
of oxygen partial pressures the interconnect must face.
While Mg-doped LaCrO3 exhibits a low electrical conductivity, it shows
excellent stability and low loss of oxygen, minimizing ionic short circuit in
the interconnect as well as change in lattice dimensions.
Thus, in the tubular design where a small cross-section of the
interconnect is used to connect the cells, a low-conductivity material is
favored for its stability.
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3. Planar Stack Design

The advantage of the planar design stems from the fact that the
current path of the device has a much larger area and shorter
lengths favoring low electrical resistance.
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• Modes of Operation
– Unlike the SOFC mode where the reaction is exothermic, the electrolysis mode of operation is
endothermic.
– In both modes of operation heat is released from the ohmic loss due to the resistance to
current flow.
– In the SOFC mode, as the stack voltage is decreased, the current increases, causing the stack
to release heat.
– In fact, heat removal is one of the challenging design and operational issues that limits
materials selection, operating point (i.e., current density), and stack footprint.
– In contrast, the endothermic electrolysis reaction and the exotherm of ohmic loss move in
opposite directions.
– At a certain cell operating voltage, the two balance, resulting in no net heat release. This
voltage is referred to as the thermal neutral voltage, Etn, defined as
Etn 
H
nF
– When an SOEC stack is operated at the thermal neutral voltage, the stack operation is
isothermal, whereas it is exothermic above and endothermic below that voltage.
– In general, operating the stack near Etn, which is approximately 1.3 V, has certain benefits, in
particular the reduced need for cooling air for heat removal, or the need to supply the heat
for the reaction.
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• Advanced concepts for high-temperature
electrolysis
1. Natural Gas – Assisted Mode



Higher operating temperature allows for a reduction in the electricity needed for
electrolysis.
However, materials constraints such as oxidation of metal interconnect or other
metallic manifold components and continued sintering of porous electrodes may
result in performance degradation at high temperatures.
Pham et have proposed a method for reducing the voltage necessary for steam
electrolysis, thereby reducing the electric power consumption. The process,
known as natural gas-assisted steam electrolysis (NGASE), uses natural gas as the
anode reactant in place of commonly used air or steam as the sweep gas for
removing the oxygen evolved in the anode compartment. Thus, the oxygen
transported through the electrolyte membrane partially or fully oxidizes the
natural gas, which in effect provides a significant portion of the driving force for
the oxygen transport through the membrane.
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2. Hybrid SOFC-SEOC stacks
 The SOFC mode of operation is exothermic while the SOEC mode can be endothermic,
thermal neutral, or exothermic depending on the operating voltage.
 The hydrogen production efficiency, defined as the ratio of heating value of
generated hydrogen to electric power input, is 100 % at the thermal neutral voltage,
higher in the endothermic mode as the operating voltage moves closer to the opencircuit voltage, and lower when the voltage is higher than thermal neutral. The
efficiency can be as high as 140 % near the open-circuit voltage.
 Operating near the thermal neutral voltage is generally considered favorable from
both the operational and hydrogen production perspectives. As the SOEC can be
operated with minimal requirement for heat supply or removal, it can potentially be
scaled up to large-footprint devices, unlike SOFC, where the heat removal
requirement constrains the overall footprint. Thus, in a reversible fuel cell, one that
operates in SOFC and SOEC modes, the cell area is constrained by the cooling
requirements in the SOFC mode.
 In order to overcome the heat removal constraints, a hybrid stack concept has been
proposed. By integrating both SOFC and SOEC cells in a single stack, the exothermic
SOFC and endothermic SOEC operations can be used to reduce the cooling air
requirement, and thus allow for larger-footprint devices.
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3. Integration of primary energy sources with hightemperature electrolysis process
 The attraction of the high-temperature steam electrolysis process comes
from the fact that a portion of the required energy for the process is
supplied as thermal energy, thereby reducing the electrical need.
 The concept of using electricity to produce hydrogen, which in turn will be
used to produce electricity, makes sense only if the electric power for
electrolysis is inexpensive or from excess capacity, and thus the hydrogen
becomes an energy carrier.
 Additionally, the compression of hydrogen for transport typically consumes
10 % of the energy content. Considering the overall environmental effect,
combining high-temperature electrolysis with a renewable energy source is
a good option－in particular when the electricity generation is intermittent
or the demand is low.
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• Materials Challenges
 High-temperature operation  physical and chemical
changes to the cell materials  performance degradation
(resistance increases with time)
 Interconnect that joins the individual cells to form a stack.
 Corrosion scale near the seal area
 Evaporation and condensation of chromium vapor from
the interconnect onto the air electrode.
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