H2Tech - Q4 2022 - 22

SPECIAL FOCUS FUTURE OF HYDROGEN ENERGY
Thermodynamics of SOECs. From an
overall efficiency point of view, electrolysis
systems should be operated close to
thermoneutral potential. In SOECs, it is
possible to operate with a higher current
density operation; therefore, a higher
H2
production rate is possible. Hightemperature
electrolysis of water occurs
in the vapor phase, so the total energy
demand of electrolysis is reduced by the
heat of vaporization. Vaporization can be
done by using inexpensive thermal energy
rather than electric energy. The Gibbs
free energy of formation (for electrolysis
reaction) decreases with increasing temperature-decreasing
electricity input
can be seen with increasing temperature
in FIG. 3. Electricity input is ~35% lower
than conventional electrolysis in high
temperatures at approximately 800°C.
The electricity input can be further lowered
to ~50% when the temperature is
increased to as high as 900°C.
The HT-SOE process is advantageous
due to its high overall thermal-to-H2
efficiency
when coupled with heat integration.
Once vaporization is achieved, high
temperature is required to superheat
steam to achieve high enough ionic conductivity
in the electrolyte (steam). This
extra heat can be provided by combining a
waste heat source, recovering the sensible
heat of the produced H2
and oxygen, and
self-heating of the cell due to its inherent
electrical resistance.
Temperature-related efficiency gains
are far higher for SOECs when steam enters
the stack at higher temperature using
external heat sources. For splitting steam,
SOECs operated at a thermoneutral potential
(the potential at which the cooling
effect from the endothermic electrolysis
process is balanced by the Joule heating
caused by the resistances in the cell) of
1.29 V will attain an electrolysis current
density of ~1.5 A/cm2
, whereas a PEM
electrolyzer operated at a thermoneutral
potential of 1.47 V attains a current density
of ~0.5 A/cm2
.
Lower cell voltage means lower operational
costs (lower electricity demand per
quantity of produced gas), while higher
current densities are associated with lower
capital costs as fewer electrolyzers are
needed to achieve the required capacity
for gas production when compared with
low-temperature electrolyzers. Therefore,
the economic motivation for the
wider adoption of SOEC technology remains
high (FIG. 4).1
SOEC materials selection. In the
SOEC unit, the electrochemical cell is the
main electrolyzer component in which
the electrochemical reaction occurs. It
is composed of three ceramic layers: a
dense electrolyte and two porous electrodes
(cathode and anode, where H2
and
oxygen are respectively produced) placed
on both sides of the electrolyte. Given
the high operating temperature range, the
electrochemical cell is made of ceramics
(solid oxide membrane electrolyte). For
example, Y2
O3-stabilized ZrO2 [yttriastabilised-zirconia
(YSZ)] acts as a gas
separator and electrolyte. It is used where
oxygen ions start migrating from cathode
to anode when a voltage is applied. As a
result, they show superior ionic conductivity
at elevated temperatures.
The electrodes must be both electron
FIG. 3. Energy demand vs. temperature for water/steam electrolysis.
FIG. 4. Typical performance ranges for AE, PEM and SOE technologies for H2
22 Q4 2022 | H2-Tech.com
O splitting.
FIG. 5. Schematic SOEC configurations:
(A) tubular SOEC and (B) planar SOEC.
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