introduction to solid oxide electrolytes
=materials =composites =chemistry =energy =technology =explanation
Batteries, fuel cells, and electrolyzers all involve:
- an
electrolyte which allows passage of some ions with a single charge state
(such as Li+ or Na+)
- material with variable charge state on both sides
of the electrolyte (such as Li metal and Fe+3 in LFP batteries)
-
electrodes which transfer charge to/from material
Batteries typically use liquid
electrolyte and variable-charge-state material that's insoluble in it.
A solid oxide electrolyte (hereafter SOE) is an electrolyte which is
solid and conducts oxide ions, which I'd denote as [O]-2 if they weren't
specially named. Oxide has high charge density, so it has strong
interactions, so the activation energy for it shifting is high, so
conducting it requires high temperatures.
Solid oxide fuel cells (SOFCs)
using SOEs have been manufactured.
doping
The main current SOE
types are doped ceria (eg
GDC) and
doped
zirconia. When an oxide crystal with +4 cations has some replaced with
+3 cations, some of the oxygen positions in the crystal are vacant. At high
temperatures, other oxygen atoms can move into those vacancies.
As
the amount of dopant increases, conductivity increases, reaches a maximum,
and decreases. Why does conductivity start decreasing before the crystal
structure is destabilized? That's because dopants tend to form clusters at
high concentrations, typically with 4 or 6 dopant atoms. Those clusters
stabilize some oxygen vacancies, increasing the activation energy for oxygen
moving into them.
The most important property of dopants besides
their charge state is their ionic radius. Mismatch between dopant size and
the crystal lattice causes strain, which often destabilizes the normal state
of the crystal relative to the intermediate state of an oxygen moving into a
vacancy, reducing activation energy.
Strain is relieved at crystal
surfaces, so dopants have a tendency to move towards the surface, and that
tendency is proportional to the strain they cause. As the crystal surfaces
thus have a higher dopant concentration, dopant clusters first become a
significant problem near the crystal surface.
By using 2 dopants with
different ionic radius, generally using more of the one that causes less
strain, it's possible to somewhat mitigate cluster formation and thus
slightly increase peak conductivity. So, there are
many papers on co-doped ceria.
composites
People noticed
that the conductivity of doped ceria can be greatly increased by mixing it
with molten carbonate. There are
many papers on such ceria-carbonate composite electrolytes. The enhanced
oxide conductivity happens at the ceria-carbonate interface.
Often,
composite materials are more expensive, but here, carbonate composites are
easier to make. Maximizing density of ceramics is difficult, and normally,
minimizing gaps between particles is important for high conductivity. With a
ceria-carbonate composite, people can make low-density doped ceria, then
simply soak it in molten carbonate to infiltrate it.
Ceria-carbonate composites also have lower electrical conductivity
than doped ceria SOEs, which helps improve efficiency. Resistive loss from
electron flow is wasted energy.
My view has been that the enhanced
conductivity of these composites comes mainly from the cations solvating
oxide on the surface. Some facts that agree with my view are:
-
ceria-chloride composites and ceria-sulfate composites also show increased
conductivity
- undoped ceria
also works
Other molten salts would
probably work too, such as chloride-carbonate eutectic or carbonate-cryolite
eutectic. NaAlCl4 would probably work too if it didn't release AlCl3 gas at
the relevant temperatures.
The molten carbonate salt itself also
conducts protons, and it obviously can conduct carbonate ions. The
conductivity of carbonate ions is even higher than the oxide conductivity of
the ceria composites. That being the case, can you add CO2 to the side with
oxygen and conduct carbonate? Yes; that design is called a
molten
carbonate fuel cell. They were made several decades ago, but they
weren't economical because the electrodes corroded too quickly. There was a
decent amount of research into electrode materials, and no solution was
found.
Perhaps you see my concern about these ceria-carbonate
composite electrolytes: why would electrode corrosion be any less of an
issue for them than for molten carbonate fuel cells? If the electrode
corrosion issue was solved, then people could just use molten carbonate
electrolyte and not bother with these composites.
This brings me to a
minor idea I proposed a few years back:
At high temperatures, BaCO3 and
CaCO3 have fairly high solubility in eutectic LiNaK carbonate. If ceria was
infiltrated by such a carbonate solution and then the temperature was
lowered, BaCO3 or CaCO3 would precipitate. Ostwald ripening should cause
crystals to mostly fill some pores in the ceria, with molten carbonate only
remaining there at the ceria surface. This should mostly block the migration
of electrode material (eg nickel ions) through the carbonate salt, slowing
corrosion.
applications
CHP
A commonly proposed
application for SOFCs was combined heat and power generation. Gas turbines
don't scale down to small sizes well, but SOFCs can be fairly small. The
idea was that homes would have a SOFC in the furnace room connected to a
natural gas line, and they'd generate electricity when heat was wanted,
instead of just burning the natural gas. This was a major justification for
SOFC research.
The currently-used systems to compare that to are:
- combined
cycle natural gas turbines + heat pumps for heating
- natural gas
turbines + hot water piped to homes
SOFCs would need to be cheaper per watt than those systems to compete, and they aren't.
combined-cycle power
SOFCs
operate at high temperature, so their waste heat is at a high temperature,
so some of it could be converted to power with gas turbines, eg:
turbine
compressor
-> fuel cell with recirculation
-> burner
-> turbine
expander
-> heat exchanger to a Rankine cycle
The net efficiency of such
systems can be even higher than current combined-cycle gas turbines. It's
been
estimated at slightly over 70%. If SOFCs were cheaper, that might be
practical.
electrolysis
With
large-scale SOE production, high-temperature water electrolysis with SOEs
would be cheaper than ambient-temperature water electrolysis. It's also more
efficient, because waste heat drives more of the electrolysis reaction. But
it's still too expensive. People aren't serious enough about water
electrolysis to pay what it actually costs on a large scale.
SOEs can
also do electrolysis of CO2 to CO, which is somewhat more valuable for
chemical production than hydrogen, but that's also too expensive.
methane reforming
As I
previously wrote,
unlike water electrolysis, electrochemical reforming of methane seems
competitive with current systems and worth pursuing. This involves a SOE
transferring oxygen from steam or CO2 to methane. One side can produce pure
H2 or CO, and the overall H2/CO ratio can be controlled which is valuable
for eg methanol production.
H2 or CO can also be used for steel
production. When people use coke for steelmaking, what actually does the
reduction is generated CO; a blast furnace is just an inefficient way of
making syngas, and coke is needed for its porosity and mechanical strength.
It's much more efficient to use a purpose-built system to convert methane to
syngas and use that to reduce iron ore. If cheap natural gas is available
that's cheaper, so
it's done now,
but China prefers to use coal and make its own steel, and America doesn't
make much new steel anymore. Iron electrolysis doesn't work well because of
Fe+2/Fe+3 shuttling, so full electrification of steel production would
involve water or CO2 electrolysis.