H2Tech - Q4 2021 - 17

SPECIAL FOCUS: FUTURE OF HYDROGEN ENERGY
Modeling and simulation are key to
developing FCEVs
E. FONTES and H. EKSTRÖM, COMSOL, Stockholm, Sweden
A potential way to help reduce greenhouse
gas emissions and slow climate
change is replacing internal combustion
engine vehicles with electric vehicles.
Electric vehicles also offer the advantage
of reducing pollutant emissions in densely
populated areas, thereby improving
air quality for citizens. Electric vehicles
powered by wind electricity have very
small greenhouse gas emissions during
operation. However, substantial emissions
levels arise during the manufacturing
stage of these vehicles.
Fuel cell electric vehicles (FCEVs) offer
several advantages compared to battery-powered
electric vehicles (BPEVs).
They can achieve a higher energy density
(especially for heavy vehicles) and
higher efficiency-if the comparison is
made assuming that the electricity for
charging the batteries is produced using
H2
-and they do not require capacity to
deliver very high power from the electric
grid when refueled, compared to the recharge
of the battery-powered vehicles.
The main limitations of fuel cells for
electric vehicles are the manufacturing
cost, limited service life, and relatively
low power density.1
The design limitations. The three
limitations mentioned previously all boil
down to the microscopic design of the
active layer in the oxygen (O2
gas diffusion electrode: the cathode in
the fuel cell. Other aspects are important,
as well, but the design of the active
layer is paramount.
The catalyst used in the active layer
is platinum. The platinum loading of the
active layer determines the lower limit
for the manufacturing cost. The manufacturing
cost of almost everything else
in the fuel cell can be reduced, but it is
difficult to reduce the cost of platinum.
It is, therefore, necessary to develop active
layers that require a very low catalyst
loading without reduced performance.
Service life is limited by different degradation
mechanisms, such as proton
reduction, platinum dissolution, carbon
corrosion, the formation of radicals that
attack the membrane electrolyte in the
active layer, adsorption of impurities
on the catalyst sites, and accumulation
of impurities in the pore electrolyte.2
Changes of hydrophobicity in the cathode's
active layer may cause flooding of
the cathode.
The limitation in power density may
be caused by the limited catalytic activity
of the cathode: the O2
electrode. This
activity can be increased by a higher catalyst
loading. However, this also means
a higher cost and a shorter service life
since a higher power density requires a
higher current density.
The active layer. To improve the design
of the active layer for vehicle fuel
cells, engineers and scientists must understand
the fundamental transport
phenomena, electrode kinetics, thermodynamics,
electrolyte chemistry and
catalytic surface activity involved in the
charge transfer reactions in this layer at
the microscopic level.
Let us look closer at the transport and
reaction processes that may occur in the
active layer in a fuel cell electrode. We
can consider a proton exchange membrane
fuel cell (PEMFC), which is the
strongest fuel cell candidate for use in
electric vehicles. The reactions at the anode
and cathode are detailed here.
The electrons released at the anode
are conducted by the electronic conducting
electrode material to the outer
circuit. In the outer circuit, the electrons
are conducted over a load and then to the
cathode. The protons (hydrogen ions)
are transported in the electrolyte and the
separator to the cathode. At the cathode,
the protons react with O2
from the cathode
gas, electrons are received from the
external circuit, and water is formed.
FIG. 1 shows a schematic drawing of
the processes at the anode's active layer.
Note that the active layer contains the
anode material with the catalyst (blue),
pore electrolyte (green) and gas-filled
)-reducing
FIG. 1. The processes that occur in the active layer in a PEMFC anode.
H2Tech | Q4 2021 17
H2Tech - Q4 2021

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