H2Tech - Q4 2022 - 21

FUTURE OF HYDROGEN ENERGY
In the electrolyzer, steam is dissociated
molecules and oxygen ions (water
at 650°C-1,000°C at the cathode to form
the H2
reduction reaction). The oxide ions migrate
from the cathode to the anode and
release electrons to the external circuit to
become oxygen gas via an oxygen evolution
reaction. High temperatures are
required to thermally activate oxide ion
migration and facilitate electrochemical
reactions on both electrodes. As a result,
the overall efficiency is improved.
* Cathode reaction: At the H2
electrode-electrolyte interface,
the steam is split into H2
and
oxygen ions (Eq. 1):
2 H2
O + 4 e- } 2 H2 + 2 O2-
(1)
* Anode reaction: Oxygen ions
are drawn through the ceramic
electrolyte, at the electrolyteoxygen
electrode interface,
and oxygen gas generates (Eq. 2):
2 O2-
} O2
ode while the H2
+ 4 e-
(2)
The oxygen then flows along the an-along
with some steam
mixture-passes along the H2
electrode
on the opposite side of the electrolyte.
Downstream of the electrolyzer, the H2
-
rich product stream is cooled down after
exchanging heat with the inlet process
stream, and then passes through a separator
to separate H2
water stream.
A fraction of the product H2
from the condensed
is recycled
and mixed with inlet steam (5%-10% H2
in steam) to maintain reducing conditions
and avoid oxidation of the nickel in the H2
electrode. As a result, HT-SOECs can be
operated at high current densities that allow
large production capacities using comparatively
small cell areas. Practically, an
electricity-to-H2
efficiency of about 90%
[on a higher heating value (HHV) basis]
appears to be realistically achievable.
High temperature is one of the concerns
for heat resources during startup. To
overcome that, self-heated (by electricity)
standby SOECs can be helpful. While giving
electricity due to resistance in the cell,
Joule heating occurs, keeping the cell in
comparatively hot condition and making it
easier to use standby SOECs during startup.
Conversely, H2
from an external source
may be needed to keep the cathode in reducing
conditions during standby mode.
Preheated air or steam can be used as
a sweep gas to remove oxygen from the
FIG. 2. Typical scheme of SOEC water electrolysis plant.
H2Tech | Q4 2022 21
FIG. 1. Solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) electrochemical
reaction.
stack. The purpose of the sweep gas is to
dilute the oxygen concentration and therefore
decrease corrosion of the oxygenhandling
components. Pure oxygen can
be produced by the stack and would be a
valuable commodity if satisfactory materials
and coating could be developed to construct
the oxygen-handling components.
SOECs use solid ion-conducting ceramics
as the electrolyte, enabling operation
at significantly higher temperatures.
Potential advantages include high electrical
efficiency, low material cost and the
option to operate in reverse mode as a fuel
cell or in co-electrolysis mode producing
syngas [carbon monoxide (CO) and H2
from water steam (H2
]
O) and CO2
.
SPECIAL FOCUS
A key challenge is severe material degradation
due to the high operating temperatures.
Current research is focused on
stabilizing existing component materials,
developing new materials and lowering the
operating temperature to 500°C-700°C
(from 650°C-1,000°C) to enable the commercialization
of this technology. Even
LPS can be used to enhance commercialization.
Furthermore, even if required,
lowering the temperature below 500°C,
integrating SOECs with high-temperature
processes like conventional reforming, etc.,
can reduce combined H2
production costs.
TABLE 1 shows a comparison of SOECs
with other commercial electrolysis technologies.

H2Tech - Q4 2022

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