H2Tech - Q4 2022 - 23

FUTURE OF HYDROGEN ENERGY
and oxide ion-conducting, and maximizing
the active surface area is essential for
efficient operation. For the H2
electrode,
a cermet (combination of ceramic and
metal) of nickel and YSZ is often used
with ~30% porosity, whereas a lanthanum
strontium manganite (LSM)-YSZ mix is
utilized for the oxygen electrode. Materials
of different layers are shown in FIG. 5B.
Commonly used SOEC materials are
earth-abundant materials, such as yttria
and zirconia, which have attracted the
use of SOECs. Solid oxide cells providing
1 TW of power in fuel cell mode would
require just 1 mos worth of global ZrO2
production and 21 mos worth of Y2
O3
.
In contrast, the same power provided by
a PEM fuel cell system would require 53
mos worth of global Pt production.2
SOEC configuration. Single cells are the
smallest units of SOEC and can be in either
tubular configuration or planar configuration,
as shown in FIG. 5. In the tubular
SOEC, steam is fed through the inside
of the tube and reduced to H2
gas and
oxygen ions. The oxygen gas is extracted
from the outer layer of the tubular SOEC.
Compared with planar SOECs, the
tubular SOEC exhibits higher mechanical
strength and facilitates sealing. Tubular
SOECs have the specific advantage
of high-pressure operation over a planar
configuration, although the interconnector
design is a challenge (FIG. 5). Despite
the larger sealing length between the anode
and cathode compartment, the planar
cells have better manufacturability
and higher electrochemical performance.
The planar SOEC system performed
better than its tubular counterpart due
to its uniform distribution of gas species
on planar SOECs, as well as easier mass
production of planar cells-the planar
SOEC system configuration is advantageous
and should be further investigated.
TABLE 2 shows some features of planar
and tubular SOEC design.
Cell-to-system configuration. To increase
the production rate, the active area
of the electrolyzer should be increased.
By increasing the single cell dimension,
it is challenging to increase the active
area; so, many single cells are connected
to build a stack. Other than cells, a stack
consists of metallic interconnects, glass
sealings, flow channels, etc. Several stacks
build a module, and several modules
build an SOE system of the desired area
to achieve the desired production rate.
FIG. 6 illustrates the typical SOEC system
configuration from cell level to plant
level.
SOEC CHALLENGES AND
ADVANCEMENTS
Challenges. The main challenge of
SOEC technology that requires further
improvement remains the lifetime of electrodes
(particularly H2
electrodes), which
are limited by degradation and the longduration
performance of the cells.
One of the main causes of cell degradation
is the effect of impurities. In H2
electrodes,
silica-containing impurities can
block the electrocatalytically active sites
by nonconducting phases, causing degradation
and increased polarization resistance.
However, cell degradation reduces if
the quality of stack inlet gas is maintained.
For H2
electrodes during long-term
operation at high over-potentials (~300
mV), the percolating nickel (Ni) network
closest to the electrolyte is destroyed. Ni
migrates from the electrolyte electrode
interface to the support layer, resulting in
irreversible loss of electrochemical performance.
For future improvements of
SOECs, this must be addressed.
High temperature is also a leading
challenge associated with thermal expansion
mismatch among different layers
and diffusion between layers of material
in the cell. Reducing stack operating temperature
minimizes interconnect corroAdvancements.
Developments continue
in the field of SOECs. Some advancements
towards the enduring future of SOECs are
discussed here.
Cell level improvement: Currentvoltage
curves recorded in steam electrolysis
reveal that the initial performance of
SOECs has increased by more than a factor
of 2.5 over the past 15 yr (FIG. 7) due
to a drop in area-specific resistance from
0.71 ohm.cm2
to 0.27 ohm.cm2
electrode.
at 750°C.
This has been achieved through modifications
like improved cell layers' processing,
especially the H2
Cell degradation rates tend to decrease
over time by a factor of 100 over a 10-yr period.
From 2005-2015, cell tests conducted
at a current density of 1 A/cm2
(all cells
were supported by a Ni-YSZ electrode and
had an active area of 16 cm2
) found a decrease
in long-term degradation rate from
SPECIAL FOCUS
sion and the reaction between different
stack components.
Challenges also remain in pressurized
operations. Low-pressure SOE operation
has the advantage of using easily available
low-pressure steam at a comparatively
lower temperature than that of high-pressure
steam. Conversely, pressurized operation
can provide several benefits (e.g.,
pressurization can increase cell power
density and reduce the size of auxiliary
components). The development of manufacturing
techniques and assembling
large area cells can reduce the overall cost
of a commercial plant.
FIG. 6. SOEC system configuration.
TABLE 2. Planar and tubular SOEC design features
Features
Power density
Volumetric power density
High-temperature sealing
Fabrication cost
Thermal cycling stability
Planar design
High
High
Diffi cult
Low
Low
Tubular design
Low
Low
Easy
High
High
H2Tech | Q4 2022 23
H2Tech - Q4 2022

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