H2Tech - Q4 2021 - 18

SPECIAL FOCUS FUTURE OF HYDROGEN ENERGY
pores. The pore electrolyte consists
of proton-conducting polymer that
has been infused into the porous electrodes.
H2
from external gas channels is
transported in the gas-filled pores and
dissolved in the pore electrolyte. It is
then transported through a thin film
of pore electrolyte to the active catalyst
site (white dashed circle in FIG. 1)
and oxidized to produce hydrogen ions
(protons) at the active site. The electrons
released in the oxidation are conducted
through the anode material to
the outer circuit.
Once the hydrogen ions have migrated
to the cathode, they may react with O2
at the active sites at the cathode (FIG. 2).
O2
is transported through the gas-filled
pores in the cathode and through a thin
film of pore electrolyte before it reaches
the active sites. At the active site, O2
and
protons receive electrons, over the outer
circuit and through conduction in the
cathode electrode material, to produce
water. The reaction at the active site depends
on the local electrode potential
in relation to equilibrium, the local O2
concentration and the local water activity.
The formed water molecules can be
transported out of the cathode as vapor
or as liquid water. Precipitation of liquid
water in the gas-filled pores may occur
depending on the pore structure and on
the local water vapor partial pressure.
The migration of hydrogen ions from
the anode to the cathode also depends
on the water content of the membrane.
Each hydrogen ion drags a few water
molecules over the membrane electrolyte
from the anode to the cathode.
So, there are transport processes in
the gas phases in both electrodes, transport
in the pore electrolyte, transport of
water and protons in the membrane, and
kinetic expressions for the charge transport
relations at the active sites. Processes
may also be added that describe
the deterioration of solid particles (e.g.,
through oxidation). The model equations
describing these processes are coupled
and depend on each other.
The solution of the model equations
reveals the losses in the different reaction
and transport processes. For example,
if water precipitates in the gas-filled
pores at the cathode, the transport of O2
gas through the gas-filled pores is slowed
dramatically. If the model predicts a deterioration
of the particles (e.g., by oxidation
that causes them to detach from the
pore electrolyte or the rest of the electrode
material), then the electrons cannot
be transported to and from the active
sites, causing losses in performance.
Time scales. The contribution of the
different processes to the losses in the
cell are difficult to estimate experimentally.
Here, combining models with experiments
is a great help. The key to understanding
the losses in the cell is that
the different processes occur at different
time scales.
The transport processes in the pore
electrolyte and in the gas-filled pores,
the conduction of ions, the conduction
of electrons and the charge transfer
reactions all occur at different time
scales. Diffusion in the pore electrolyte
is several orders of magnitude slower
than diffusion in the gas-filled pores.
Ionic and electronic conduction are extremely
fast processes. Charge transfer
reactions can be slow (cathode) or relatively
fast (H2
), but are relatively fast
FIG. 2. The transport and reaction processes that occur at the cathode.
compared to the diffusion in the pore
electrolyte. Transient techniques, such
as current interrupt and impedance
spectroscopy, can be modeled and then
compared to and validated using experiments.
The contribution of the different
losses can also be followed over
time for different operating conditions
during the aging of a cell.
The principle of impedance spectroscopy
is quite simple. An average voltage
(V0
) is applied with a small sinusoidal
perturbation over time. As a consequence,
a corresponding sinusoidal current
is obtained as a response to the voltage
perturbations (FIG. 3).
The current response may have a shift
FIG. 3. A perturbation in electrical potential over the cell results in a current response.
18 Q4 2021 | H2-Tech.com
in time (δt) compared to the voltage. A
shift can be caused by processes that delay
the response of the current to the sinusoidal
perturbation in voltage. For example,
at low frequencies, slow processes

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H2Tech - Q4 2021

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