H2Tech - Q4 2022 - 20

SPECIAL FOCUS FUTURE OF HYDROGEN ENERGY
green H2
generation. Low-temperature
electrolysis is based on either a liquid or a
solid polymer electrolyte. The water molecule
is dissociated by applying an electrical
current in both cases. The operating
temperature is restricted to < 100°C.
Alkaline water electrolysis. Alkaline
water electrolysis is composed of an anode
and cathode separated by a gas-impermeable
membrane. The electrolyte, usually an
aqueous solution, comprises 20 wt%-40
wt% of sodium hydroxide (NaOH) or potassium
hydroxide (KOH) concentration.
While employing electrical energy into
two electrodes, water molecules dissociate
at the cathode to give H2
and negatively
charged hydroxide ions by reduction. At
the anode, hydroxide ions oxidize, and oxygen
and water molecules arise, releasing
electrons. TABLE 1 provides further details.
Polymer electrolyte membrane water
electrolysis. Polymer electrolyte membrane
or proton exchange membrane
(PEM) water electrolysis is a contemporary
development. In this electrolysis,
water is electrochemically dissociated into
H2 at the cathode and oxygen at the anode,
and a solid proton-conducting membrane
separates the electrodes.
During electrolysis, oxidation of the
water molecule leads to the formation of
oxygen gas and positively charged H2
ions
at the anode. The external power circuit
facilitates the flow of electrons while the
H2
ions move to the cathode through the
semipermeable proton exchange membrane.
At the cathode, these electrons recombine
with the two protons to give one
molecule of H2
The H2
by the process of reduction.
gas generated has a very high purity
of 99.99%. TABLE 1 provides further details.
High-temperature electrolysis (HTE).
HTE is not yet commercialized, but systems
have been developed and demonstrated
on a laboratory scale. This technology
holds promise for the future. HTE
is an electrolysis method where steam is
dissociated to H2
and O2
at temperatures
between 650°C and 1,000°C. In electrolysis,
system efficiencies increase with
increasing operating temperatures. Lowpressure
steam (LPS) can be used with
reduced efficiency than the mentioned
TABLE 1. Main characteristics of AEC, PEMEC and SOEC systems
Electrolysis technology
Alkaline electrolysis
Anode reaction
Cathode reaction
Charge carrier
Operating temperature, °C
Operating pressure, bar
Anode material
Cathode material
Electrolyte material
Stack lifetime, hr
Current density, A/cm2
Maturity
Cell voltage, V
Voltage efficiency, %HHV
-1
Capital cost, € kWel
2OH- j 1/2O2
2H2O + 2e-
+ H2O + 2e-
j H2 + 2OH-
OH-
50-80
< 30
Ni, Ni-Co alloys
Ni, Ni-Mo alloys, Cd, Pb,
Cu, Ag, Pt, Pd, etc.
KOH, NaOH, H2SO4
60,000-90,000
Low at 0.2-0.4 because of
large resistance across the thick
diaphragm and liquid electrolyte
Mature
1.8-2.4
62-82
1,000-1,200 (Low due to
inexpensive electrode)
Stack energy, kWh/Nm3
H2
4.2-5.9
*Perovskite-type lanthanum strontium manganese (La0.8Sr0.2MnO3)
# Lab scale development; in general, SOEC available at lower pressure (atm to < 5 bar)
20 Q4 2022 | H2-Tech.com
Commercial
1.8-2.2
67-82
High 1,860-2,320 (Platinum-group metals
used and corrosion resistant coating
are estimated to be 60x more expensive
than the cost of AWE per unit area)
4.2-5.5
6 3.2
Demonstration
0.7-1.5
< 110
> 2,000
H2O j 1/2O2
2H+
PEM electrolysis
+ 2H+
+ 2e-
H+
j H2
20-100
< 200
IrO2, Ir-Sn Oxide, Rh, RhO2
Pt, Pt/activated Carbon, Pt-Pd
temperature. If this heat source is a clean
one-such as geothermal, solar or nuclear-HTE
produces H2
with nearly zero
greenhouse gas (GHG) emissions.
SOECs are fundamentally the reverse
counterpart of solid oxide fuel cells (SOFCs).
Refer to FIG. 1 for the reverse reaction.
DETAILED PROCESS SCHEME
The typical flow scheme of the SOEC
H2 production system is presented in FIG. 2.
The system is designed to produce H2
by
using electricity and water. The system's
main components consist of SOEC stacks
in series and a balance of plant (BOP). The
BOP includes a water pump, heat exchangers,
steam generator, etc. Water is heated
in a series of heat exchangers to recover
the heat from the SOEC outlet gas stream.
Preheated water is introduced to the steam
generator to produce steam and then to
the electric heater to superheat steam. To
minimize electricity demand and improve
SOEC system efficiency, steam is heated in
multiple heat exchangers by the exiting H2
and oxygen streams. However, in case of
availability of external steam, the scheme
shown in FIG. 2 can be further optimized.
+ 2e-
High-temperature electrolysis (SOE)
O2- j 1/2O2
+ 2e-
H2O + 2e- j H2 + O2-
O2-
650-1,000
< 25#
Ni, YSZ/LSM*
Ni/YSZ
PEM membrane: Nafion,
Flemion, etc.
20,000-60,000
Substantially higher at 0.6-2.0
YSZ [ZrO2 stabilized by Y2
O3
(Yttria stabilized Zirconia)], MgO or CaO
8,000-20,000
0.3-4.0 at 750°C-800°C
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