materials
Article
Novel PEFC Application for Deuterium
Isotope Separation
Hisayoshi Matsushima *, Ryota Ogawa, Shota Shibuya and Mikito Ueda
Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Sapporo, Hokkaido 060-8628, Japan;
[email protected] (R.O.); [email protected] (S.S.); [email protected] (M.U.)
* Correspondence: [email protected]; Tel.: +81-11-706-6352
Academic Editors: Vincenzo Baglio and David Sebastián
Received: 18 February 2017; Accepted: 15 March 2017; Published: 16 March 2017
Abstract: The use of a polymer electrolyte fuel cell (PEFC) with a Nafion membrane for isotopic
separation of deuterium (D) was investigated. Mass analysis at the cathode side indicated that D
diffused through the membrane and participated in an isotope exchange reaction. The exchange of D
with protium (H) in H2 O was facilitated by a Pt catalyst. The anodic data showed that the separation
efficiency was dependent on the D concentration in the source gas, whereby the water produced
during the operation of the PEFC was more enriched in D as the D concentration of the source gas
was increased.
Keywords: hydrogen isotope separation; fuel cell; CEFC; isotope exchange reaction
1. Introduction
The heavy isotopes of hydrogen, deuterium (D) and tritium (T) play essential roles in nuclear energy
production [1,2]. In current heavy-water nuclear fission reactors, D is used as a neutron-moderator.
Similarly, in nuclear fusion reactors, which are expected to represent the next generation of nuclear
power, the reaction of D and T is responsible for the energy-production stage.
Because D and T are not directly obtainable as pure isotopes, methods to separate them from the more
common, lighter isotope, protium, are required. Many researchers have studied various isotope-separation
methods, including water distillation [3], molecular sieving [4], water electrolysis [5–8] and combined
electrolysis catalytic exchange [9,10]. The water electrolysis yields the most effective separation but
consumes enormous amounts of electricity. Such large consumption has led to a search for other
methods that are more energetically efficient. In particular, a new separation technology for tritium is
urgently required at the Fukushima Daiichi Nuclear Power Plant in Japan.
We previously proposed a new hydrogen-separation system: the combined electrolysis fuel
cell (CEFC) process [11]. Here, hydrogen and oxygen were produced by electrolysis and used for
power generation in a fuel cell. By recycling the energy generated from the produced hydrogen,
the electricity consumption of the isotope separation process was reduced More recent work has
reported D separation via the hydrogen isotope effect during the anodic reaction in polymer electrolyte
fuel cells (PEFCs) [12–14] and alkaline membrane fuel cells [15]. We reported that the water produced
by these power sources was enriched in D. This was caused entirely by the kinetic isotope effect
during the hydrogen oxidation reaction (HOR) on a Pt catalyst. However, several other factors must be
investigated to fully realize the potential of CEFC systems. The dependency of the separation efficiency
on the isotope concentration is important from the practical viewpoint. The mass balance of the isotopes
in fuel cells must be strictly controlled when radioactive species are involved. Therefore, this paper
focuses on measuring D separation by a PEFC and investigates the factors influencing separation at
both the cathode and anode, using isotopically mixed gases with several D concentrations.
Materials 2017, 10, 303; doi:10.3390/ma10030303
www.mdpi.com/journal/materials
Materials 2017, 10, 303
Materials 2017, 10, 303
2 of 7
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2.2.Experimental
Experimental
AA JARI
JARI standard
standard cell
cell (FC
(FC Development
Development Corp.,
Corp., Tsukuba,
Tsukuba, Japan)
Japan) was
was employed
employed as
as aa PEFC.
PEFC.
The
membrane
electrode
assembly
(50
×
50
mm)
was
composed
of
Nafion
electrolyte
(NRE-212)
The membrane
assembly (50 × 50 mm) was composed of Nafion electrolyte (NRE-212) and
−2 The PEFC was operated at
and
catalytic
layers
loaded
platinum
catalyst
mg−2·cm
twotwo
catalytic
layers
loaded
withwith
platinum
catalyst
(Pt, (Pt,
0.520.52
mg·cm
). The).PEFC
was operated at 298 K
298
andpower
the power
generation
controlled
under
constant
current
mode
(0.0–1.2
adjustedby
bya
andKthe
generation
was was
controlled
under
constant
current
mode
(0.0–1.2
A)A)adjusted
avariable
variableresistor
resistorunit
unit(PLZ
(PLZ164WA,
164WA,Kikusui
KikusuiElectronics
Electronics Corp.,
Corp., Yokohama,
Yokohama, Japan).
The
Thedeuterium
deuteriumseparation
separationfactor,
factor,α,
α,of
ofthe
thePEFC
PEFCwas
wasmeasured
measuredby
byquadrupole
quadrupolemass
massspectrometry
spectrometry
(QMS)
(QMS) (Qulee-HGM
(Qulee-HGM 202,
202, Ulvac
Ulvac Corp.,
Corp., Chigasaki,
Chigasaki, Japan).
Japan). Figure
Figure 11 shows
shows aa schematic
schematic of
of the
the
−1−1. Mixture gases
experimental
setup.
Humidified
O
gas
was
supplied
into
the
cathode
at
40
mL
·
min
experimental setup. Humidified O
2 2 gas was supplied into the cathode at 40 mL·min . Mixture gases
of
of HH22 and
and DD22 were
were used
used at
at the
the anode.
anode. The
The mixing
mixing ratio
ratio was
was adjusted
adjusted by
by aa mass
mass flow
flow controller
controller
(MC-200SCCM-D,
Alicat
Scientific,
Tucson,
AZ,
USA).
The
composition
of
the
exhaust
gas
from
(MC-200SCCM-D, Alicat Scientific, Tucson, AZ, USA). The composition of the exhaust gas fromeach
each
of
ofLines
LinesI–III
I–IIIwas
wasmonitored
monitoredby
byQMS.
QMS.Ion
Ioncurrents
currentsof
ofmass
massnumbers
numbers(m)
(m)== 2,2, 3,3,44were
wererecorded
recordedatat
Lines
LinesI–II
I–IIand
andthose
thoseof
ofmm==18,
18,19
19at
atLine
LineIII.
III.
Figure1.1.Schematic
Schematicillustration
illustrationof
ofexperimental
experimentalmeasurement
measurementof
ofthe
thedeuterium
deuteriumseparation
separationfactor
factorof
of
Figure
PEFC.1.1.HH22gas;
gas;2.2.D
D22 gas;
gas; 3.
3. O22 gas; 4. Mass flow controller;
controller; 5.
5. Gas mixture
mixture unit;
unit;6.
6. Bubbler;
Bubbler;7.7. Anode;
Anode;
PEFC.
Electrolytemembrane
membraneassembly;
assembly;9.9.Cathode;
Cathode;10.
10.Variable
Variableresistor;
resistor;11.
11.Q-mass.
Q-mass.
8.8.Electrolyte
Resultsand
andDiscussion
Discussion
3.3.Results
andDD22 was
was supplied
supplied to
to the
the anode
anodeof
ofthe
thePEFC.
PEFC.The
Theeffect
effectof
ofthe
thePt
Ptcatalyst
catalyst
mixturegas
gasofofHH22and
AAmixture
onthe
themixture
mixturegas
gaswas
wasinvestigated
investigatedby
bycomparing
comparingthe
thegas
gascomponent
componentcomposition
compositionbefore
beforeand
andafter
after
on
introducing
the
anode.
With
the
PEFC
switched
off,
the
effect
of
the
catalyst
could
be
studied
in
the
introducing the anode. With the PEFC switched off, the effect of the catalyst could be studied in the
absence
of
electrochemical
kinetic
factors
arising
from
power
generation
by
the
cell.
Figure
2
shows
absence of electrochemical kinetic factors arising from power generation by the cell. Figure 2 shows
−2. With
2 . With
the QMS
QMS data
(m(m
= 2–4)
when
the ratio
of D/H
PEFCthe
switched
the
data of
ofeach
eachmass
massnumber
number
= 2–4)
when
the ratio
of was
D/H10was
10−the
PEFC
off,
the
QMS
data
of
Line
I
indicated
no
change
in
the
isotopic
composition
of
the
gas
over
time.
switched off, the QMS data of Line I indicated no change in the isotopic composition of the gas over
However,
a
trace
amount
of
a
species
with
m
=
3
was
detected.
This
can
be
attributed
to
HD,
formed
time. However, a trace amount of a species with m = 3 was detected. This can be attributed to HD,
via the via
fragmentation
of H2 and
D2 and
during
ionizing
processprocess
in the QMS
chamber.
formed
the fragmentation
of H
D2 the
during
the ionizing
in the
QMS chamber.
2
The
mixture
gas
from
the
PEFC
outlet
side
was
monitored
from
Line
II.The
Thearrow
arrowin
inFigure
Figure22
The mixture gas from the PEFC outlet side was monitored from Line II.
indicates the
the time
time when
when the
the gas
gas line
line was
was switched
switched from
from Line
Line II to
to II.
II. The
The ion
ion current
current of
of mm == 2,
2,
indicates
corresponding
to
H
2, remained almost constant, and was independent of passing through the Pt
corresponding to H2 , remained almost constant, and was independent of passing through the
catalytic
layer.
In contrast,
the the
ratioratio
of HD
to D2towas
showing
a substantial
increase
of HD.
Pt
catalytic
layer.
In contrast,
of HD
D2 inverted,
was inverted,
showing
a substantial
increase
From
ionthe
currents
of each
number
it wasit calculated
that more
thanthan
95%95%
of the
2 gas was
of
HD. the
From
ion currents
of mass
each mass
number
was calculated
that more
of D
the
D2 gas
converted
to
HD.
As
reported
previously
[12],
the
generation
of
HD
gas
is
proceeded
by
an
isotope
was converted to HD. As reported previously [12], the generation of HD gas is proceeded by an isotope
exchange
reaction,
as
expressed
in
Equation
(1),
exchange reaction, as expressed in Equation (1),
H2 + D2 → 2HD
H2 + D2 → 2HD
(1)
(1)
Materials 2017, 10, 303
Materials 2017, 10, 303
3 of 7
3 of 7
Figure
of of
mass
numbers
m =m2 =(H22(H
, green
line),
3 (HD,
red line)
and
Figure 2.
2. Transient
Transientbehavior
behaviorofofQ-Mass
Q-Massspectra
spectra
mass
numbers
2, green
line),
3 (HD,
red line)
−1 )−1and D (0.1 mL·min−−1
1 ) was passed
4and
(D24, (D
blue
line)
at
the
anode
side.
A
mixture
of
H
(10.0
mL
·
min
2 H2 (10.0 mL·min ) and 2D2 (0.1 mL·min ) was passed
2, blue line) at the anode side. A mixture of
through
through the
the PEFC
PEFC for
for 22 hh and
and then
then passed
passed directly
directlyto
tothe
theQ-Mass
Q-Massfor
for33hhwithout
withoutpower
powergeneration.
generation.
The exchange
exchange reaction
reaction is
is reported
reported to
to be
be enhanced
enhanced on
on aa Pt
Pt surface
surface [16,17].
[16,17]. The
The high
high kinetic
kinetic rate
rate of
of
The
exchange can
canbebeattributed
attributed
to the
well-developed
catalyst
structure
in PEFCs
[18].
The catalytic
exchange
to the
well-developed
catalyst
structure
in PEFCs
[18]. The
catalytic
activity
activity
is
also
promoted
by
the
use
of
nano-sized
particles
and
the
uniform
distribution
of
these
is also promoted by the use of nano-sized particles and the uniform distribution of these particles
on
particles
on thematerials.
supporting
materials. the
Additionally,
thelayer
gas increases
diffusion the
layer
increases
the degree
of
the
supporting
Additionally,
gas diffusion
degree
of contact
between
contact
between
the the
mixture
gas and the catalyst.
the
mixture
gas and
catalyst.
Assuming that the ion currents of
of each
each mass number were proportional
proportional to the
the numbers
numbers of each
each
Assuming
molecular species,
species, the
the effect
effect of
of the
the membrane
membrane on
on separation
separation was
was evaluated.
evaluated. The
The total
total amount
amount of
of D
D in
in
molecular
Line II
II was
was about
about 0.1%
0.1% less
less than
than that
that of Line I. This loss may have occurred
occurred with
with the penetration
penetration of
of D
D
Line
into
Line
III
or
the
uptake
of
D
by
the
Nafion
membrane.
into Line III or the uptake of D by the Nafion membrane.
The composition
composition of the gas
gas from
from the
the cathode
cathode was
was analyzed
analyzed by
by the
the QMS
QMS connected
connected to Line
Line III
III
The
(Figure
1).
Pure
O
2
gas
was
supplied
to
the
PEFC.
This
gas
was
fully
humidified
by
protium
water
(Figure 1).
O2
was fully humidified
protium water
before being
being let
let in
in to
to the
the PEFC.
PEFC. The
The bubbler
bubbler was
was maintained
maintained at
at 298
298 K.
K. The
The two
two ion
ion currents
currents detected
detected
before
at this
this line
line had
had mass
mass values
values of
of m
m == 18 and
and 19.
19. The species with m = 18 was normal molecular water,
at
H22O, while the other was
was H33O, produced by the fragmentation
fragmentation of
of H
H22O. It appears
appears that
that H
H22O was easily
H
decomposed by
by ionization
ionization in
in the
the QMS
QMS chamber,
chamber,probably
probablybecause
becauseof
ofits
itslarge
largemolecular
molecularsize.
size.
decomposed
Figure 33 shows
shows the
the variation
variation of
of the
the ion
ion currents
currents of
of m
m == 18
18 and
and 19
19 when
when the
the isotopically
isotopically mixed
mixed
Figure
gas
was
supplied
to
the
anode
under
the
same
conditions
as
in
Figure
2.
The
ion
current
of
m
18
gas was supplied to the anode under the same conditions as in Figure 2. The ion current of m == 18
decreased over
over time,
time, while
while that
that of
of m
m == 19
19 increased
increased such
such that
that the
the ratio
ratio of
of the
the latter
latter to
to the
the former
former was
was
decreased
doubled.
Assuming
that
the
frequency
of
the
fragmentation
by
QMS
was
independent
of
the
anode
doubled. Assuming that the frequency of the fragmentation by QMS was independent of the anode
condition, this
this result
result directly
directly suggests
suggests that
that the
the increase
increase of
of the
the D
D content
content resulted
resulted from
from the
the formation
formation
condition,
of HDO.
HDO. The
The H
H22 and
and D
D22 species
the mixture
mixture gas
gas were
were oxidized
oxidized at
at the
the anode,
anode, resulting
resulting in
in their
their
of
species in
in the
+ and D+
+ ions, respectively. The dissociated ions diffused through the conducting
+
conversion
into
H
conversion into H and D ions, respectively. The dissociated ions diffused through the conducting
polymer toward
toward the
the cathode.
cathode. The
The Pt
Pt catalyst
catalyst facilitated
facilitated the
the exchange
exchange of
of the
the deuterons,
deuterons, D
D++,, for
for H
H in
in
polymer
molecules
of
H
2
O.
This
isotope
exchange
reaction,
expressed
by
Equation
(2),
was
responsible
for
the
molecules of H2 O. This isotope exchange reaction, expressed by Equation (2), was responsible for the
increased content
content of
of HDO.
HDO.
increased
D+ + H2O → H+ + HDO
(2)
D+ + H2 O → H+ + HDO
(2)
The D/H ratio at Line III was larger than that at the counter-anode. This difference indicates the
The D/H
ratio the
at Line
III wasrates
larger
that
difference
between
diffusion
of than
H+ and
D+atinthe
thecounter-anode.
membrane. This difference indicates the
+ and D+ in the membrane.
difference
between
the
diffusion
rates
of
H
The PEFC was connected to the variable resistor and the power performance of the PEFC was
The PEFC
was
connected
to the
variable resistor
and used
the power
performance
the PEFC
was
−1, while
measured
under
current
control.
Humidified
O2 gas was
at a flow
rate of 40of
mL·min
−
1
measured
under
control.
O2 gas
used at a flow
rate of
·min
, while
−1. Humidified
dry H2 was
inlet current
at 20 mL·min
Figure 4 shows
the was
current-voltage
curves
of 40
themL
pure
H2 gas
anddry
the
−1 . Figure 4 shows the current-voltage curves of the pure H gas and the
H
was
inlet
at
20
mL
·
min
2
2
mixture
gases. The open circuit voltage of the pure H2 gas was smaller than that of the mixture
gases.
mixture
gases.
The open
circuitwere
voltage
the pure
smallerprobably
than thatcorresponded
of the mixture to
gases.
2 gas was
Since the
cathode
conditions
theofsame,
theHsmall
difference
the
isotope effect on the equilibrium potential of HOR [12,19].
Materials 2017, 10, 303
4 of 7
Since the cathode conditions were the same, the small difference probably corresponded to the isotope
Materials
303
4 of 7
effect
on2017,
the10,
equilibrium
potential of HOR [12,19].
Figure 3. Transient behavior of Q-Mass spectra of mass numbers m = 18 (H2O, blue line) and 19 (HDO,
red line) at the cathode side. Arrow indicates the onset time, when the mixture of H2 (10.0 mL·min−1)
and D2 (0.1 mL·min−1) was passed to the anode side.
When the PEFC was in operation, generating electric power, the current-voltage curves of both
gases showed almost identical behavior. The cell voltage decreased abruptly at about 1.5 A. In such
cells, the cathode potential dominates the cell voltage with an increasing output current, because
oxygen reduction on Pt catalysts is inactive under these conditions and gas diffusion is slow.
The gas composition was monitored in situ. Several inlet H2/D2 gases with a range of D isotopic
concentrations (D/H = 10−2–10−4) were compared. Figure 5 shows the mass analysis data of the gases
from Line II. Before the power generation, the ion current of HD (m = 3) changed in accordance with
the D concentration of the inlet gas. When the PEFC was operated at 1.2 A, the ion current of HD
significantly decreased. The degree of this decrease was lessened with the decreasing D
concentration.
The
contrast
with
behavior
ofofHmass
2,of
which
showed
am
constant
evidence
Figure
3. Transient
Transient
behavior
of Q-Mass
spectra
mass
numbers
(Hion
O,current,
blue
and
2line)
Figure 3.
behavior
of the
Q-Mass
spectra
numbers
m = 18
(H=2O,18blue
and line)
19 is
(HDO,
that the
D
isotope
reacted
preferentially
during
HOR
[12].
The
D
selectivity
was
clearly
dependent
19
(HDO,
red
line)
at
the
cathode
side.
Arrow
indicates
the
onset
time,
when
the
mixture
of
red line) at the cathode side. Arrow indicates the onset time, when the mixture of H2 (10.0 mL·minH−12)
−1 ) and
−1 was passed to the anode side.
on the
isotope
concentration.
(10.0
·min
mL·min
−1)D
2 (0.1
2 (0.1
mL·min
was
passed
to the) anode
side.
and
DmL
When the PEFC was in operation, generating electric power, the current-voltage curves of both
gases showed almost identical behavior. The cell voltage decreased abruptly at about 1.5 A. In such
cells, the cathode potential dominates the cell voltage with an increasing output current, because
oxygen reduction on Pt catalysts is inactive under these conditions and gas diffusion is slow.
The gas composition was monitored in situ. Several inlet H2/D2 gases with a range of D isotopic
concentrations (D/H = 10−2–10−4) were compared. Figure 5 shows the mass analysis data of the gases
from Line II. Before the power generation, the ion current of HD (m = 3) changed in accordance with
the D concentration of the inlet gas. When the PEFC was operated at 1.2 A, the ion current of HD
significantly decreased. The degree of this decrease was lessened with the decreasing D
concentration. The contrast with the behavior of H2, which showed a constant ion current, is evidence
that the D isotope reacted preferentially during HOR [12]. The D selectivity was clearly dependent
on the isotope concentration.
Figure
Figure 4.
4. Cell
Cell current-voltage
current-voltage curves
curves when
when PEFC
PEFC was
was in
in operation
operation with
with pure
pure H
H22 gas
gas (black
(black line)
line) and
and
mixture
gases
of
H
and
D
(red
line).
2
2
mixture gases of H2 and D2 (red line).
When the PEFC was in operation, generating electric power, the current-voltage curves of both
gases showed almost identical behavior. The cell voltage decreased abruptly at about 1.5 A. In such cells,
the cathode potential dominates the cell voltage with an increasing output current, because oxygen
reduction on Pt catalysts is inactive under these conditions and gas diffusion is slow.
The gas composition was monitored in situ. Several inlet H2 /D2 gases with a range of D isotopic
concentrations (D/H = 10−2 –10−4 ) were compared. Figure 5 shows the mass analysis data of the gases
from Line II. Before the power generation, the ion current of HD (m = 3) changed in accordance with
the D concentration of the inlet gas. When the PEFC was operated at 1.2 A, the ion current of HD
significantly decreased. The degree of this decrease was lessened with the decreasing D concentration.
The contrast with the behavior of H2 , which showed a constant ion current, is evidence that the
D isotope
preferentiallycurves
during
HOR
[12].
The
D selectivity
dependent
on the
Figurereacted
4. Cell current-voltage
when
PEFC
was
in operation
with was
pure clearly
H2 gas (black
line) and
isotope
concentration.
mixture
gases of H2 and D2 (red line).
Materials 2017, 10, 303
5 of 7
Materials 2017, 10, 303
5 of 7
Materials 2017, 10, 303
5 of 7
Figure
5. Transient
behavior
spectraofofm m
at several
D concentrations,
= 10−2
Figure
5. Transient
behaviorof
of Q-Mass
Q-Mass spectra
= 3=at3 several
D concentrations,
D/H = D/H
10−2 (blue
−
3
−
4
(blueline),
line),
D/H
10 line)
(green
= 10Arrow
(redindicates
line). Arrow
indicates
thethe
onset
D/H
= 10−3=(green
andline)
D/H and
= 10−4D/H
(red line).
the onset
time, when
PEFCtime,
was
on. The
data ofon.
m =The
2 (black)
and
(orange)
were
when
themeasured
mixture gas
when
theswitched
PEFC was
switched
data of
mm
= =2 4(black)
and
m =measured
4 (orange)
were
when
Figure
5.gas
Transient
spectra of m = 3 at several D concentrations, D/H = 10−2 (blue
−2Q-Mass
−2 wasbehavior
supplied.
with D/H
= 10with
the mixture
D/H
= 10of
was supplied.
line), D/H = 10−3 (green line) and D/H = 10−4 (red line). Arrow indicates the onset time, when the PEFC
−2 were
was
on. The
of m =detectable
2 (black) and
m =when
4 (orange)
measured
mixture
The switched
ion current
of data
D2 was
only
the were
mixture
gaseswhen
withthe
D/H
= 10gas
−2 were
Thewith
ionD/H
current
of D
−2 was
2 was detectable only when the mixture gases with D/H = 10
supplied.
=
10
investigated. Detection was not possible with lower D concentrations because the current was below
investigated. Detection was not possible with lower D concentrations
because the current was below
the detection limit of QMS. The ion current at D/H = 10−2−2is also shown in Figure 5. The HOR
−2The
the detection
limit
of
QMS.
ion
current
at
D/H
=
10
is
also
in
Figure
5.
HOR
The
ion
current
of
D
2The
was
detectable
only
when
the
mixture
gases
with
D/H
= 10
were
selectively consumes D2 in preference to HD. This is a typical isotopeshown
effect,
where
the
variation
investigated.
Detection
was
not
possible
with
lower
D
concentrations
because
the
current
was
below
selectively
preference
to HD. This is a typical isotope effect, where the variation degree
2 inmass
degree consumes
depends onDthe
number.
the detection
limitnumber.
of
QMS. The
ion was
current
at D/Hby=the
10−2following
is also shown
in Figure 5. The HOR
depends
on the
mass
The
deuterium
separation
factor
calculated
equation,
selectively
consumes
D2 in preference
HD. This is
typical
isotopeequation,
effect, where the variation
The deuterium
separation
factor wastocalculated
byathe
following
(3)
degree depends on the mass number. α = ([H]/[D])a/([H]/[D])b
The
deuterium
separation
factor
calculated
byand
thedeuterium,
following equation,
where
[H]
and [D] are
the atomic
fractions
of protium
and the subscripts (a) and (b) (3)
α was
= ([H]/[D])
a /([H]/[D])
b
refer to after and before starting the power
generation.
The species D2 was not considered in (3)
the
α = ([H]/[D])
a/([H]/[D])b
present
study
its lowfractions
concentration,
as shown
Figure 5. The
very
ion current
of (b)
where
[H] and
[D]because
are theofatomic
of protium
andindeuterium,
and
thesmall
subscripts
(a) and
where
[H]not
and [D] are theaffect
atomic fractions
of protium and deuterium, and the subscripts (a) and (b)
= 4after
did
values.
refermto
andappreciably
before starting the
theαpower
generation. The species D2 was not considered in the
refer The
to after
and before starting
the power
generation.
The species
Dshown
2 was not considered in the
separation
calculated
at various
concentrations
are5.
in Figure
Thecurrent
error of
present study
because factors
of its low
concentration,
asDshown
in Figure
The very
small6.ion
present
study
because
of
its
low
concentration,
as
shown
in
Figure
5.
The
very
small
ion current
of
bars
indicate
the
maximum
and
minimum
values
among
several
experiments.
The
α
values
exhibited
m =m
4 did
notnot
appreciably
affect
the
ααvalues.
=
4
did
appreciably
affect
the
values.
concentration dependency. D was separated more effectively at higher D concentrations, as expected.
TheThe
separation
factors
calculated
at various D
D concentrations
concentrationsare
are shown
in Figure
6. The
separation
factors
calculated
6. The
errorerror
However,
it should be
emphasized
thatatαvarious
approached
a certain limitingshown
value in
at Figure
low concentrations
barsbars
indicate −5thethe
maximum and
minimum values
among several
several experiments.
The
α values
exhibited
values
The
α values
(D/Hindicate
< 10 ). Themaximum
PEFC was and
ableminimum
to produce
wateramong
enriched in D.experiments.
Even dilute mixture
gasesexhibited
could be
concentration
dependency.
DD
was
separated
more
effectivelyatathigher
higher
D
concentrations,
as expected.
concentration
dependency.
was
separated
more
effectively
D
concentrations,
as
expected.
dispersed in the gas diffusion layer, resulting in extensive contact between the gas and the catalyst.
However,
it should
be be
emphasized
certainlimiting
limiting
value
low
concentrations
However,
it should
emphasizedthat
thatααapproached
approached aa certain
value
at at
low
concentrations
−
5
−5
(D/H
ThePEFC
PEFCwas
was able
able to produce
D.D.
Even
dilute
mixture
gases
could
be be
(D/H
< 10< 10). ).The
producewater
waterenriched
enrichedinin
Even
dilute
mixture
gases
could
dispersed
in the
diffusion
layer,resulting
resultingin
inextensive
extensive contact
gas
and
thethe
catalyst.
dispersed
in the
gasgas
diffusion
layer,
contactbetween
betweenthe
the
gas
and
catalyst.
Figure 6. Dependency of separation factor, α, on fuel gas concentration of D when PEFC was operated
at 1.2 A.
Figure
6. Dependency
of of
separation
gasconcentration
concentrationofof
when
PEFC
operated
Figure
6. Dependency
separationfactor,
factor,α,
α, on
on fuel gas
DD
when
PEFC
waswas
operated
at 1.2
at A.
1.2 A.
Materials 2017, 10, 303
6 of 7
4. Conclusions
The D isotopic mass flow in a PEFC with a Nafion membrane was investigated by mass analysis
of the mixture gases from both the anode and cathode. Before the power generation was switched on,
the mixture gases of H2 and D2 were almost completely converted to HD at the anode side. A small
amount of D could diffuse through the membrane as D+ ions and then form HDO at the cathode side
by isotopic exchange with protium in H2 O. The mass balance of D indicated the partial accumulation
of D in the membrane.
The power generation of the PEFC was not affected by the introduction of D-containing mixture
gases, while the open circuit potential was shifted to a more anodic potential than the equilibrium one
of value for isotopically pure H2 gas. The D content from the anode side was significantly diluted by
HOR. The value of α depended on the D concentration, decreasing from about 4 at D/H = 10−1 to
about 2 at D/H = 10−5 .
Acknowledgments: The authors gratefully acknowledge financial support from the Takahashi Industrial &
Economic Research Foundation and the Inamori Foundation in Japan.
Author Contributions: Hisayoshi Matsushima conceived the experiments and wrote the paper; Ryota Ogawa
and Shota Shibuya performed the experiments and analyzed the data; Mikito Ueda contributed the discussion of
the data.
Conflicts of Interest: The authors declare no conflict of interest.
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