che_february-2025 - 32

water electrolysis is a highly efficient
and high-purity hydrogen
production method.
TABLE 1. CONSIDERATIONS FOR EFFECTIVE PEM ELECTROLYSIS
Safety
Electrolysis
efficiency
Making an effective PEM
Reducing production costs for
clean hydrogen is a particular
challenge. Described in the following
sections and in Table 1 are four considerations
that can help to reduce
costs in electrolysis processes.
Efficiency. The first consideration is
the improvement of electrolysis efficiency,
particularly the reduction of
cell voltage. Resistance comes from
various phenomena, such as oxidation
and reduction reactions, proton
transport across the PEM interface
and mass transport, which dictates
water's access to a catalyst layer.
The cell voltage is the total of the
voltages derived from these resistances.
As the PEM is involved in
the proton transport, it is a contributor
to total cell voltage, and must be
considered for the reduction of resistance
that is needed to improve
electrolysis efficiency.
Durability. The second design consideration
to focus on is ensuring
durability that allows for operation
over ten years or more. During water
electrolysis, hydroxyl radicals are
generated through the Fenton reaction
of hydrogen peroxide (a side
reaction). Because hydroxyl radicals
have tremendous oxidative reactivity,
PEMs need to have high chemical
stability to give them high durability.
Also, when performing differentialpressure
water electrolysis, the PEM
continuously experiences the pressure
of the cathode, demanding high
mechanical strength.
Safety. The third crucial characteristic
to think about is safety. In highpressure
water electrolysis, hydrogen
permeates the PEM and mixes
with oxygen. The lower explosive
limit (LEL) of hydrogen is 4 vol. %.
Therefore, it is necessary to enhance
the hydrogen-shielding ability and
the mitigation ability of the permeated
hydrogen. Hydrogen shielding
refers to the technically usable
amount of hydrogen per the theoretical
amount of generated hydrogen.
The mitigation ability is the mitigated
amount of permeated hydrogen per
the amount of permeated hydrogen
32
Required PEM
characteristics
PEM resistance,
hydrogen shielding
ability
Durability
Chemical stability,
mechanical strength
without a mitigation strategy, such as
a gas recombination catalyst (GRC).
More details on GRC technologies
are described later in this article.
Compatibility. The fourth characteristic
is process compatibility.
Normally, dry PEMs swell three-dimensionally
when hydrated. In-plane
expansion significantly affects the
yield and quality of PEM manufacturing
and subsequent processes.
Therefore, high dimensional stability
in the in-plane direction is required.
To realize these four characteristics,
trade-offs must be considered. For
example, reducing PEM thickness is
an effective method to reduce membrane
resistance, but it is impossible
to avoid a decrease in safety due to
an increase in the amount of permeated
hydrogen. Therefore, developing
membrane-design strategies that reduce
the need for such trade-offs is a
high priority.
PFSA polymers
A perfluorinated sulfonic-acid (PFSA)
polymer is the most commonly used
polymer for PEMs in water electrolysis
applications, and a representative
PFSA polymer is shown in Figure
2. This type of polymer is obtained
through the copolymerization of tetrafluoroethylene
and perfluoroethylene
with a sulfonic acid group. Due
to their perfluorinated structure, PFSA
materials have high chemical stability.
Furthermore, the hydrophobic
main chain (with a polytetrafluoro
ethylene structure), and the hydrophilic
side chain (with a sulfonic acid
Hydrogen shielding
ability, hydrogen
mitigation ability
Process
compatibility
Dimensional stability
when hydrated
group), form a phase-separated
structure, creating an ion cluster
where sulfonic acid groups are accumulated
[6]. A structure of ion clusters
has been proposed to be linked
with narrow channels by Gierke [7],
which is a probable reason for the
high proton conductivity of perfluorinated
sulfonic acid polymers
(Figure 3) [6-8].
Proton conductivity is known to depend
on the number of sulfonic acid
groups. The ion-exchange capacity
(IEC; typical units: meq/g) is an index
that represents the amount of sulfonic
acid (meq) per unit weight (g) of a polymer.
For a PFSA polymer synthesized
by changing the copolymerization
ratio, the membrane resistance, as
well as IEC, can be tuned. The higher
the IEC is, the lower the distance between
ion clusters is, which is a probable
reason that polymers with high
IEC values also exhibit high proton
conductivity [8, 9]. In addition, the IEC
and molecular structure of the polymer
also affect hydrogen permeability and
mechanical strength. Perfluorinated
sulfonic acid polymer has been used
in various applications, such as chloralkali
electrolysis and fuel cells because
of these characteristics.
PEM design strategy
The following paragraphs describe
several design techniques for PEMs
that can help realize the four required
characteristics for efficient
water electrolysis.
A PEM consists of polymer, reinforcing
materials and additives. One
FIGURE 2. A molecular structure of a representative perfluorinated sulfonic acid (PFSA) polymer is shown
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