catalysts
Article
Carbon-Supported Pt-SnO2 Catalysts for Oxygen Reduction
Reaction over a Wide Temperature Range: Rotating Disk
Electrode Study
Ruslan M. Mensharapov 1 , Nataliya A. Ivanova 1 , Dmitry D. Spasov 1,2 , Elena V. Kukueva 1 ,
Adelina A. Zasypkina 1 , Ekaterina A. Seregina 1 , Sergey A. Grigoriev 1,2,3, * and Vladimir N. Fateev 1
1
2
3
*
Citation: Mensharapov, R.M.;
Ivanova, N.A.; Spasov, D.D.;
Kukueva, E.V.; Zasypkina, A.A.;
Seregina, E.A.; Grigoriev, S.A.; Fateev,
V.N. Carbon-Supported Pt-SnO2
National Research Center “Kurchatov Institute”, 1, Akademika Kurchatova sq., 123182 Moscow, Russia;
Mensharapov_rm@nrcki.ru (R.M.M.); ivanovana.1989@outlook.com (N.A.I.);
spasovdd@outlook.com (D.D.S.); elena.kukueva@gmail.com (E.V.K.); Zasypkina_AA@nrcki.ru (A.A.Z.);
Seregina_ea@nrcki.ru (E.A.S.); Fateev_VN@nrcki.ru (V.N.F.)
National Research University “Moscow Power Engineering Institute”, 14, Krasnokazarmennaya St.,
111250 Moscow, Russia
HySA Infrastructure Center of Competence, Faculty of Engineering, North-West University,
Potchefstroom 2531, South Africa
Correspondence: sergey.grigoriev@outlook.com
Abstract: Pt/C and Pt/x-SnO2 /C catalysts (where x is mass content of SnO2 ) were synthesized using
a polyol method. Their kinetic properties towards oxygen reduction reaction were studied by a
rotating disk electrode (RDE) technique in a temperature range from 1 to 50 ◦ C. The SnO2 content
of catalyst samples was 5 and 10 wt.%. A quick evaluation of the catalyst activity, electrochemical
behavior and average number of transferred electrons were performed using the RDE technique.
It has been shown that the use of x-SnO2 (through modification of the carbon support) in a binary
system together with Pt does not reduce the catalyst activity in the temperature range of 1–30 ◦ C.
The temperature rising up to 50 ◦ C resulted in composite catalyst activity reduction at about 30%.
Catalysts for Oxygen Reduction
Reaction over a Wide Temperature
Range: Rotating Disk Electrode Study.
Keywords: PEM fuel cell; cathode catalysts; oxygen reduction reaction; rotating disk electrode;
catalyst activity; hybrid carrier; tin oxide; activation energy
Catalysts 2021, 11, 1469. https://
doi.org/10.3390/catal11121469
Academic Editor: Chao Su
1. Introduction
Increasing interest in renewable energy sources and hydrogen energy makes the
research and development of polymer electrolyte membrane fuel cells (PEMFCs) an imAccepted: 28 November 2021
portant task. A key element of PEMFCs is a membrane electrode assembly (MEA), which
Published: 30 November 2021
includes the anode and cathode catalytic layers. Since the kinetics of the hydrogen oxidation reaction on the anode catalytic layer is rather rapid, the rate-limiting process in the
Publisher’s Note: MDPI stays neutral
PEMFC is the oxygen reduction reaction (ORR) at the cathode (Figure 1), which is a slow
with regard to jurisdictional claims in
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four-electron transfer reaction. The low ORR rate calls for high-effective cathode catalysts
published maps and institutional affilshowing high activity in this process.
iations.
Received: 30 October 2021
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Figure
Simplifiedscheme
schemeofofthe
theORR
ORR
with
direct
indirect
pathways.
Inscheme:
the scheme:
Figure 1.
1. Simplified
with
direct
andand
indirect
pathways.
In the
ki and kk’i i
and
k’i are
the reaction
rate constants
at stages
various
of theinprocess
in theand
forward
and
reverse
are the
reaction
rate constants
at various
ofstages
the process
the forward
reverse
directions,
respectively.
directions,
respectively.
However, the addition of tin dioxide particles may have a negative effect on the activity of the platinum catalysts due to a possible blockage of Pt active sites or formation of
large agglomerates of particles [19,20]. Therefore, a detailed
study of activity of composite
Catalysts 2021, 11, 1469. https://doi.org/10.3390/catal11121469
https://www.mdpi.com/journal/catalysts
tin dioxide catalysts in the ORR is important for evaluation of their applicability in PEM-
Catalysts 2021, 11, 1469
2 of 13
Pt-based carbon-supported catalysts are commonly used at the PEMFC cathode [1,2].
Such electrocatalysts have high activity and sufficient stability under standard operating
conditions of PEMFCs. However, widening of the range of external operating conditions
(temperature and humidity) involves a number of challenges. First, drying of the ionexchange polymer is observed at high temperatures and low humidity, which leads to
a deterioration in proton conductivity both in the catalytic layer ionomer and in the
membrane [3–6]. Second, at negative temperatures, ice crystals are formed inside MEA
components, and destruction of the catalytic layers and membrane occurs during freezethaw cycles [7–13]. These problems can be solved by applying additives with high water
retention ability, such as tin dioxide [14–18].
However, the addition of tin dioxide particles may have a negative effect on the activity
of the platinum catalysts due to a possible blockage of Pt active sites or formation of large
agglomerates of particles [19,20]. Therefore, a detailed study of activity of composite tin
dioxide catalysts in the ORR is important for evaluation of their applicability in PEMFCs
over a wide range of operating conditions.
The technique of a rotating disk electrode (RDE) in a three-electrode electrochemical
cell is commonly used for the determination of electrocatalyst activity towards ORR. This
technique allows for estimating kinetic and diffusion components of the reaction current.
An extension of this method to low and high temperatures may aid in evaluating the
efficiency of the catalysts over a wide temperature range and determining the activation
energy in the ORR.
Most studies [21–24] are dedicated to the evaluation of the activity of tin dioxide
composite catalysts in the alcohol oxidation reaction since Pt-SnO2 hetero-clusters enhance
oxidation of COads intermediates. Fewer studies [25–27] have focused on investigation
of the activity in the ORR of hybrid Pt-SnO2 catalyst with a high tin dioxide loading,
co-catalyst properties of SnO2 have been reported.
As reported in our previous work [20], the composite electrocatalysts with the tin
dioxide content of 5 and 10 wt.% demonstrate a significantly improved stability in the
course of the accelerated stress testing and good efficiency within the PEMFC. This effect is
achieved due to formation of Pt-SnO2 hetero-clusters, providing an improvement in the
electrocatalyst durability. However, the presence of tin dioxide led to a reduction in the
electrochemically active surface area (EASA), and may also affect the ORR kinetics.
This paper is dedicated to further investigation of composite Pt-SnO2 catalysts. The
kinetics of different electrocatalysts in the ORR was investigated using the Koutecky–Levich
approach. The study of the kinetics in the temperature range from 1 to 20 ◦ C allows for
evaluating the catalyst efficiency in conditions of starting the PEMFC at low ambient
temperatures (“cold start”). The upper limit of the measurement temperature of 50 ◦ C is
explained by the increased evolution of acid vapors and accelerated corrosion of the RDE
unit at higher temperatures.
2. Results and Discussion
2.1. Catalyst Characterization
High quality RDE measurements require thin and uniform films over the entire surface
of the electrode [28]. Figure 2 shows micrographs of catalyst films obtained using catalyst
ink of the proposed composition. The film in the left image was produced by the method
of rotational drying at a certain angle, which is detailed below in Section 3.2, and the film
in the right image was produced by stationary drying. Figure 2a shows the film with a
uniform structure over the entire surface of the glassy carbon (GC) electrode. The catalyst
film in Figure 2b is unevenly distributed over the GC electrode; defects and voids are
observed. Thus, according to the micrographs, the proposed method for catalytic ink
drying allows obtaining films with a high degree of uniformity.
Catalysts 2021, 11, 1469
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(a) (a)
(b) (b)
Figure
2. Optical
micrographs
of aof
film
ononthe
surface
of
electrode
fabricated
byrotational
rotaFigure
2.
micrographs
ofcatalytic
catalytic
film
on
surface
of the
electrode
fabricated
by rotaFigure
2. Optical
Optical
micrographs
aacatalytic
film
thethe
surface
of the
the
fabricated
by
tional
(a)
and
stationary
(b)
drying.
tional
(a)
and
stationary
(b)
drying.
(a) and stationary (b) drying.
40 /C (40 wt.% of Pt
40(40
Figure
shows
thesurface
surface
theGC
GCelectrode
electrode
coated
with
Figure
3 shows
thethe
surface
of the
GC
electrode
coated
with
Pt40Pt
/CPt
wt.%
of Pt
Figure
33shows
ofofthe
coated
with
/C
(40
wt.%
of on
Pt on
on
carbon
carrier)
before
and
after
the
RDE
measurements
in
the
HClO
electrolyte.
The
4 electrolyte.
TheThe
dis-discarbon
carrier)
before
andand
after
thethe
RDE
measurements
in the
HClO
4 electrolyte.
carbon
carrier)
before
after
RDE
measurements
in the
HClO
4
distribution
of
the
catalyst
particles
is
very
even,
free
from
formation
of
agglomerates
and
tribution
of the
catalyst
particles
is very
even,
freefree
from
formation
of agglomerates
andand
tribution
of the
catalyst
particles
is very
even,
from
formation
of agglomerates
defects,
despite
formation
of
islands
(Figure
3a,c)
sub-micrometer
defects,
despite
unavoidable
formation
of some
islands
(Figure
3a,c)
atat
the
sub-micromedefects,
despite
unavoidable
formation
of some
some
islands
(Figure
3a,c)
atthe
the
sub-micromeduring
drying
[29].
The
image
layer
surface
after
RDE
measurements
ter ter
level
during
drying
[29].
The
image
ofof
the
layer
surface
after
thethe
RDE
measurements
level
during
drying
[29].
The
image
ofthe
the
layer
surface
after
the
measurements
(Figure
3c)
did
not
show
significant
changes
in the
the
surface
structure
in comparison
comparison
with
(Figure
3c) 3c)
diddid
notnot
show
significant
changes
in the
surface
structure
in comparison
with
(Figure
show
significant
changes
in
surface
structure
in
with
the
initial
layer
structure
(Figure
3a).
thethe
initial
layer
structure
(Figure
3a).3a).
initial
layer
structure
(Figure
(a) (a)
(b) (b)
(c) (c)
(d) (d)
40/C-coated
Figure
3. SEM
images
of Pt
electrodes
before
(a,b)
andand
after
(c,d)(c,d)
RDE
measurements.
Figure
3.
images
of
Pt
electrodes
before
(a,b)
RDE
measurements.
Figure
3. SEM
SEM
images
of
Pt4040/C-coated
/C-coated
electrodes
before
(a,b)
andafter
after
(c,d)
RDE
measurements.
The
image
of the
the
film
before
the
RDE
measurements
taken
athigher
higher
magnification
TheThe
image
of the
filmfilm
before
thethe
RDE
measurements
taken
at aat
magnification
image
of
before
RDE
measurements
taken
aa higher
magnification
(Figure
3b)
shows
thin
µm-sized
patches
of
the
ionomer
layer
(circled
in
red),
which
is
(Figure
3b)3b)
shows
thinthin
μm-sized
patches
of the
ionomer
layer
(circled
in red),
which
is is
(Figure
shows
μm-sized
patches
of the
ionomer
layer
(circled
in red),
which
described
in
[30].
The
ionomer
performs
several
important
functions
such
as
attaching
described
in [30].
TheThe
ionomer
performs
several
important
functions
such
as attaching
described
in [30].
ionomer
performs
several
important
functions
such
as attaching
catalyst
particles
one
another,
obtaining
homogenous
and
well-dispersed
ink,
and
catalyst
particles
to one
another,
obtaining
homogenous
andand
well-dispersed
ink,ink,
andand
uni-unicatalyst
particles
totoone
another,
obtaining
homogenous
well-dispersed
uniform
application
ofink
the[31].
ink
[31].
Such patches
ionomer
patches
disappeared
after
the
form
application
of the
ink
[31].
Such
ionomer
disappeared
after
thethe
series
of
RDE
form
application
of the
Such
ionomer
patches
disappeared
after
series
ofseries
RDE
of
RDE
measurements
(Figure
3d),
which
points
to
the
dissolution
of
the
ionomer
film
measurements
(Figure
3d),3d),
which
points
to the
dissolution
of the
ionomer
filmfilm
in the
elecmeasurements
(Figure
which
points
to the
dissolution
of the
ionomer
in the
elecin
the
electrolyte.
The
dissolution
of
the
ionomer
may
lead
to
an
increase
in
the
catalyst
trolyte.
TheThe
dissolution
of the
ionomer
may
lead
to an
in the
catalyst
layer
activity
trolyte.
dissolution
of the
ionomer
may
lead
to increase
an increase
in the
catalyst
layer
activity
layer
due
totransport
better of
mass
oxygen
to active
sites
and
lower
electrical
duedue
to better
mass
transport
oxygen
to active
sites
and
lower
electrical
resistance
toactivity
better
mass
of transport
oxygen
toofactive
sites
and
lower
electrical
resistance
resistance [31,32]. However, the amount of ionomer in the film and the number of patches
Catalysts 2021, 11, x FOR PEER REVIEW
Catalysts 2021, 11, 1469
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[31,32]. However, the amount of ionomer in the film and the number of patches were rather
small,
sosmall,
the ionomer
dissolution
should not
havenot
a significant
effect oneffect
the quality
were
rather
so the ionomer
dissolution
should
have a significant
on the
ofquality
measurements.
of measurements.
The
Theabsence
absenceofofsignificant
significantchanges
changesininthe
thelayer
layersurface
surfacestructure
structureallows
allowsfor
forobtaining
obtaining
high-quality
high-qualityresults
resultsduring
duringthe
theentire
entireseries
seriesofofRDE
RDEmeasurements
measurementsininthe
thewide
widetemperatemperature
turerange.
range.
2.2.Electrochemical
ElectrochemicalStudies
Studies
2.2.
Figure4 4presents
presentscyclic
cyclic
voltammograms
(CVs)
of different
catalysts.
The EASA
Figure
voltammograms
(CVs)
of different
catalysts.
The EASA
val2 g−1 Pt for Pt40 /C, Pt20 /C, Pt20 SnO 5 /C and Pt20 SnO 10 /C,
values
are
42,
83,
55
and
57
m
2
−1
40
20
20
5
20
10
2
2
ues are 42, 83, 55 and 57 m g Pt for Pt /C, Pt /C, Pt SnO2 /C and Pt SnO2 /C, respecrespectively.
tively.
40
20 /C), 20 wt.%-Pt/5 wt.%-SnO /C
Figure 4.
4. CVs
CVs of
of 40
Figure
40 wt.%-Pt/C
wt.%-Pt/C (Pt
(Pt40/C),
/C), 20
20 wt.%-Pt/C
wt.%-Pt/C (Pt
(Pt20/C),
20 wt.%-Pt/5 wt.%-SnO22/C
5
10 /C) recorded in the 0.1M HClO solution
10/C)2 recorded
(Pt2020
SnO
wt.%-Pt/10
wt.%-SnO
/C20SnO
(Pt202SnO
(Pt
SnO
25/C),
20 20
wt.%-Pt/10
wt.%-SnO
2/C 2(Pt
in the 0.1M HClO4 solution
at 25
2 /C),
4
at(scan
25 ◦ Crate
(scan
°C
20rate
mV 20
s−1mV
). s−1 ).
TheCVs
CVsdemonstrate
demonstrate
well-defined
hydrogen
adsorption/desorption
peaks
inpothe
The
well-defined
hydrogen
adsorption/desorption
peaks
in the
potential
range
of
0.05–0.40
V
vs.
RHE.
Two
peaks
in
the
hydrogen
desorption
region
tential range of 0.05–0.40 V vs. RHE. Two peaks in the hydrogen desorption region at 0.13at
0.130.20
andV0.20
V correspond
to platinum
(110)(100)
and active
(100) active
sites, respectively,
[26,33].
and
correspond
to platinum
(110) and
sites, respectively,
[26,33].
The
The sample
10 wt.%
demonstrates
a shift
thehydrogen
hydrogendesorption
desorption
2 content
sample
with with
SnO2SnO
content
of 10ofwt.%
demonstrates
a shift
of of
the
peaktotomore
morepositive
positivepotentials,
potentials,which
whichmay
maybe
beattributed
attributedtotothe
theeffect
effectofofthe
thepresence
presenceof
of
peak
tin
dioxide
on
the
surface
structure
of
platinum
nanoparticles
and
various
distributions
tin dioxide on the surface structure of platinum nanoparticles and various distributionsof
EASA
catalysts can
can be
beexplained
explainedby
by
ofcrystal
crystalorientations
orientations[34].
[34].The
Thesmaller
smaller
EASA for
for composite
composite catalysts
− species, which impedes hydrogen adsorption and by
the
more
facile
adsorption
of
OH
the more facile adsorption of OH– species, which impedes hydrogen adsorption and by
possibleblockage
blockageofofPt
Ptsites
sitesby
bySnO
SnO2 2nanoparticles.
nanoparticles.AAsmall
smallpeak
peakatat0.73
0.73VVfor
forthe
theCVs
CVsof
of
possible
the
modified
catalysts
may
be
attributed
to
the
oxygen
adsorption
from
the
dissociation
the modified catalysts may be attributed to the oxygen adsorption from the dissociationof
water on the surface of tin dioxide particles [35].
of water on the surface of tin dioxide particles [35].
Measurement of the electrolyte solution resistance is an important factor for obtaining
Measurement of the electrolyte solution resistance is an important factor for obtaincorrect values from processing the data acquired by the RDE technique. An almost two-fold
ing correct values from processing the data acquired by the RDE technique. An almost
decrease in resistance with increasing temperature was observed (Figure 5). The obtained
two-fold decrease in resistance with increasing temperature was observed (Figure 5). The
values of electrolyte solution resistance were used in further calculations and plotting of
obtained values of electrolyte solution resistance were used in further calculations and
polarization curves.
plotting of polarization curves
Figure 6 shows the polarization curves for a Pt20 /C-coated electrode at different electrolyte temperatures and at the electrode rotation speed of 1600 rpm. The current density
on the diffusion-limited plateau of the polarization curves increases with temperature, indicating that the decrease in oxygen solubility with temperature was lower than the increase
in the oxygen diffusion coefficient [36]. A shift of the half-wave potential of the polarization
curves to the high-potential region is also observed with increasing temperature, which
characterizes the positive dependence of the reaction rate on temperature.
Catalysts 2021, 11, 1469
Catalysts 2021, 11, x FOR PEER REVIEW
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5 of 14
Figure 5. Resistance of 0.1 M HClO4 solution at different temperatures in the 1−50 °C range.
Figure 6 shows the polarization curves for a Pt20/C-coated electrode at different electrolyte temperatures and at the electrode rotation speed of 1600 rpm. The current density
on the diffusion-limited plateau of the polarization curves increases with temperature,
indicating that the decrease in oxygen solubility with temperature was lower than the
increase in the oxygen diffusion coefficient [36]. A shift of the half-wave potential of the
polarization curves to the high-potential region is also observed with increasing temper◦range.
ature,
characterizes
the positive
dependence
of the reaction
rate
on
Figurewhich
5. Resistance
Resistance
44solution
atatdifferent
ininthe
1−50
°Ctemperature.
Figure
5.
of 0.1 M HClO
HClO
solution
differenttemperatures
temperatures
the
1−
50
C range.
Figure 6 shows the polarization curves for a Pt20/C-coated electrode at different electrolyte temperatures and at the electrode rotation speed of 1600 rpm. The current density
on the diffusion-limited plateau of the polarization curves increases with temperature,
indicating that the decrease in oxygen solubility with temperature was lower than the
increase in the oxygen diffusion coefficient [36]. A shift of the half-wave potential of the
polarization curves to the high-potential region is also observed with increasing temperature, which characterizes the positive dependence of the reaction rate on temperature.
20/C-coated
Figure 6. RDE
RDE polarization curves
for the
the ORR at a Pt20
/C-coateddisk
diskelectrode
electrode rotating
Figure
curves (20
(20mV
mVss−−11)) for
◦ C.
at
at 1600 rpm in a 0.1 M HClO44 solution
solution at
at different
different temperatures
temperatures in the range from 1 to 50 °C.
Table 11 shows
shows the
the values
valuesofofEASA
EASAand
andactivities
activitiesofofthe
the
catalysts
under
study
Table
catalysts
under
thethe
study
at
◦ C and the values obtained by the other groups. The polyol method is the most
at 20
20
°C and
the values obtained by the other groups. The polyol method is the most comcommonly
method
forsynthesis
the synthesis
of platinum
catalysts.
monly
usedused
method
for the
of platinum
catalysts.
Table 1. Comparison of catalyst parameters: EASA, area-specific (Sa ) and mass-specific (Ma ) activity
◦ C and at potential
Figure
6. RDE
polarization
curves (200.9
mV
of
catalysts
at 20
V.s−1) for the ORR at a Pt20/C-coated disk electrode rotating
at 1600 rpm in a 0.1 M HClO4 solution at different temperatures in the range from 1 to 50 °C.
Catalyst
Synthesis Method
Pt /C [38]
Pt40 /C [39]
Pt20 /C
20
Pt /C [26]
Pt20 /C [26]
Pt20 /C [38]
Pt20 /C [28]
Pt20 SnO2 5 /C
Pt20 SnO2 10 /C
Pt20 SnO2 20 /C [26]
Pt20 SnO2 20 /C [27]
Polyol method
Polyol method
Polyol method
Carbonyl route
Photo-deposition
Polyol method
Polyol method
Polyol method
Photo-deposition
Polyol method
EASA,
2 −1
Sa ,
2
Ma ,
−1
Scan Rate,
−1
m g activities
mA
mA mgunder themV
s
Table 1 shows the values of EASA and
ofcm
the catalysts
study
at
40 /C
Polyol
method
42
0.14
±
0.01
56
±
1
10
Pt
20 °C and the values obtained by the other groups. The polyol method is the most comPolyol method
49 ± 1
0.73 ± 0.03
359 ± 14
20
Pt40 /C [37]
monly
used method for
the synthesis of platinum
catalysts.
40
49 ± 1
43
83
63 ± 2
72 ± 2
55 ± 10
66
55
57
33 ± 1
53
0.48 ± 0.03
0.21
0.25 ± 0.04
0.37 ± 0.01
0.41 ± 0.02
0.33 ± 0.06
0.20
0.37 ± 0.01
0.36 ± 0.01
0.44 ± 0.05
0.3
230 ± 10
90
207 ± 4
75 ± 1
94 ± 5
180 ± 6
160
205 ± 2
205 ± 5
49 ± 4
156
20
10
10
10
10
20
5
10
10
10
10
Catalysts 2021, 11, 1469
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The electrochemical surface area and activity values for the Pt40 /C catalyst proved to
be lower than the values reported in literature. The relatively low values can be explained
by the catalyst preparation technique [20], in which the carbon support was preliminarily
impregnated with a precursor followed by reduction. At the same time, with an increase
in the platinum content, the average size of platinum nanoparticles exceeded 3.5 nm,
which was described in our previous work [40]. According to [41], for particles with a
size less than 3.5 nm, predominant for catalysts with a lower platinum content, the crystal
orientation of the surface (110) dominates, and its activity in ORR is higher than for (100)
Pt sites [42].
The values of EASA and mass activity of the Pt20 /C are comparatively higher than
the values reported in the literature. The difference in the values may be due to different
techniques used for recording polarization curves and catalyst film preparation. However,
a comparative analysis of the obtained values is possible.
Catalysts with hybrid support and SnO2 loading of 20 wt.% have the lower values of
EASA and mass activity, which can be attributed to the increase in the particle size with an
increase in the concentration of tin dioxide [20].
Figure 7 shows the Koutecky–Levich (K-L) plots for catalysts with a platinum content
of 20 wt.% at 50 ◦ C. The plots remain almost parallel up to a potential of 0.9 V, pointing to a weak dependence of the number of transferred electrons on the potential and
applicability of the K-L theory for increased temperatures. The number of transferred
electrons for all samples is close to 4, which suggests high selectivity of samples up to
elevated temperatures.
Table 2 shows the values of kinetic current and catalyst activity at various temperatures.
Table 2. Catalyst activity parameters: kinetic current density (jk ), area-specific (Sa ) and mass-specific
(Ma ) activity of catalysts in the temperature range from 1 to 50 ◦ C at potential of 0.9 V.
T, ◦ C
1
10
Pt40 /C
cm−2
jk , mA
Sa , mA cm−2
Ma , mA mg−1
3.7 ± 0.2
0.12 ± 0.01
47 ± 2
20
(EASA = 42
4.3 ± 0.3
0.13 ± 0.01
51 ± 4
m2
30
50
5.2 ± 0.2
0.16 ± 0.01
62 ± 2
5.7 ± 0.2
0.17 ± 0.01
69 ± 2
6.9 ± 0.1
0.31 ± 0.01
256 ± 2
11.5 ± 0.4
0.52 ± 0.02
428 ± 15
6.3 ± 0.1
0.52 ± 0.01
286 ± 6
7.9 ± 0.3
0.65 ± 0.02
357 ± 13
7.1 ± 0.2
0.44 ± 0.01
248 ± 6
8.0 ± 0.1
0.49 ± 0.01
280 ± 4
g−1 )
4.6 ± 0.1
0.14 ± 0.01
56 ± 1
Pt20 /C (EASA = 83 m2 g−1 )
jk , mA cm−2
Sa , mA cm−2
Ma , mA mg−1
4.2 ± 0.2
0.19 ± 0.01
154 ± 7
4.8 ± 0.2
0.22 ± 0.01
178 ± 8
5.6 ± 0.1
0.25 ± 0.04
207 ± 4
Pt20 SnO2 5 /C (EASA = 55 m2 g−1 )
cm−2
jk , mA
Sa , mA cm−2
Ma , mA mg−1
2.9 ± 0.1
0.24 ± 0.01
132 ± 1
3.6 ± 0.1
0.30 ± 0.05
165 ± 3
4.5 ± 0.1
0.37 ± 0.01
205 ± 2
Pt20 SnO2 10 /C (EASA = 57 m2 g−1 )
jk , mA cm−2
Sa , mA cm−2
Ma , mA mg−1
3.8 ± 0.1
0.24 ± 0.04
134 ± 2
5.0 ± 0.1
0.31 ± 0.01
175 ± 2
5.9 ± 0.2
0.36 ± 0.01
205 ± 5
The Pt40 /C catalyst demonstrates low activity values over the whole temperature
range due to the larger platinum nanoparticle size and the smaller fraction of the platinum
surface involved in the ORR.
The polarization curves for catalysts with platinum content of 20 wt.% have close
values of kinetic current density and activity up to 30 ◦ C. The structure of the sample
with the lowest tin dioxide content Pt20 SnO2 5 /C is mainly represented by individual
highly dispersed Pt and SnO2 particles, which participate as a co-catalyst in ORR. Thus,
Pt20 SnO2 5 /C catalyst has close values of activity in comparison with Pt20 /C up to 50 ◦ C.
The Pt20 SnO2 10 /C sample shows mass activity reduction at about 30% at the temperature
Catalysts 2021, 11, 1469
7 of 13
Catalysts 2021, 11, x FOR PEER REVIEW
of 50 ◦ C,
7 ofas
14
which can be explained by an agglomeration of tin dioxide particles, as well
the formation of Pt-SnO2 hetero-clusters.
(a)
(b)
(c)
20/C (a), Pt2020
5 (b),(b),
20 SnO
◦ C.
Figure7.7.K-L
K-L plots
plots calculated
calculated from the Pt20
25/C
Pt20Pt
SnO
210/C 10
RDE
polarization
curves
at 50 °C.
Inset:
Figure
/C (a), Pt SnO
SnO
/C
(c) RDE
polarization
curves
at 50
2 /C
2 (c)
n value
as a function
of theofelectrode
potential.
Inset:
n value
as a function
the electrode
potential.
Table 2on
shows
the values
of kinetic
current with
and catalyst
activity
at various
temperaBased
the obtained
values,
the sample
tin dioxide
content
of 5 wt.%
has
tures. similar to that of the standard catalyst over the entire temperature range, which
activity
together with high stability of this sample [20], makes it promising for use in PEMFCs
Table 2.
Catalyst
activity
parameters:
kinetic current
density (jwith
k), area-specific
a) and mass-specific
under
a wide
range
of operating
conditions.
The sample
tin dioxide(Scontent
of 10 wt.%
◦
(M
a
)
activity
of
catalysts
in
the
temperature
range
from
1
to
50
°C
at
potential
of
0.9
V.
also demonstrates high activity, however, at 50 C, the activity was comparatively lower.
Nevertheless,
under conditions
of increased
temperature
the use
T, °C
1
10
20 and low humidity,
30
50of this
catalyst in PEMFC is justified taking
into
account
the
high
water
retention
ability
of tin
Pt40/C (EASA = 42 m2 g−1)
dioxide nanoparticles.
jk, mA cm−2
3.7 ± 0.2
4.3 ± 0.3
4.6 ± 0.1
5.2 ± 0.2
5.7 ± 0.2
Sa, mA cm−2
0.12 ± 0.01
0.13 ± 0.01
0.14 ± 0.01
0.16 ± 0.01
0.17 ± 0.01
Ma, mA mg−1
47 ± 2
51 ± 4
56 ± 1
62 ± 2
69 ± 2
Pt20/C (EASA = 83 m2 g−1)
jk, mA cm−2
4.2 ± 0.2
4.8 ± 0.2
5.6 ± 0.1
6.9 ± 0.1
11.5 ± 0.4
Catalysts 2021, 11, 1469
Based on the obtained values, the sample with tin dioxide content of 5 wt. % has
activity similar to that of the standard catalyst over the entire temperature range, which
together with high stability of this sample [20], makes it promising for use in PEMFCs
under a wide range of operating conditions. The sample with tin dioxide content of 10
wt.% also demonstrates high activity, however, at 50°C, the activity was comparatively
8 of 13
lower. Nevertheless, under conditions of increased temperature and low humidity, the
use of this catalyst in PEMFC is justified taking into account the high water retention ability of tin dioxide nanoparticles.
20
10 /C samples under study at the
The
and Pt
Pt2020SnO
SnO2102 /C
TheArrhenius
Arrheniusplots
plotsfor
forthe
thePt
Pt20/C
/C and
samples under study at the popotential
0.9VVare
areshown
shownininFigure
Figure8b,d.
8b,d.Some
Someplots
plotsdemonstrate
demonstratenon-linear
non-linearbehavior.
behavtential ofof0.9
ior.
This
could
be
due
to
activation
of
additional
platinum
catalytic
centers
at
increased
This could be due to activation of additional platinum catalytic centers at increased temtemperatures
larger
kinetic
current
error
than
calculated
one.
peratures andand
larger
kinetic
current
error
than
thethe
calculated
one.
y = −2.4x + 14.1
y = −3.0x + 16.5
Catalysts 2021, 11, x FOR PEER REVIEW
(a)
y = −3.6x + 18.5
(c)
9 of 14
(b)
y = −3.0x + 16.3
(d)
20SnO255/C (c) Pt2020
2020/C (b) Pt20
10 (d).
Figure8.8.Arrhenius
Arrheniusplots
plotsfor
forthe
theORR
ORR
HClO
4 for the samples: 40
Pt40/C (a) Pt
SnO210/C
Figure
inin
0.10.1
MM
HClO
4 for the samples: Pt /C (a) Pt /C (b) Pt SnO2 /C (c) Pt SnO2 /C
The
mass
transfer
is
adjusted,
the
currents
are
normalized
to
the
concentration
of
dissolved
oxygen.
(d). The mass transfer is adjusted, the currents are normalized to the concentration of dissolved oxygen.
Accordingtotothe
the
slope
Arrhenius
plots,
activation
energies
forORR
the ORR
20
According
slope
of of
thethe
Arrhenius
plots,
activation
energies
for the
are 20are
± 1,
−1
40
20
20
5
20
10
−
1
40
20
20
5
20
10
± 1,
1, ±
301± and
1 and
kJ mol
mol for
PtPt/C,/C,
Pt Pt
SnOSnO
2 /C and
/C, respec25
±25
1, ±30
2525
±±22kJ
forPt
Pt /C,
/C,
andSnO
Pt 2 SnO
2 /CPt
2 /C,
−1, respectively. The activation
energies
obtained
in ourinstudy
are −28
± 321
kJ±mol
respectively.
The activation
energies
obtained
our study
areand
−2821and
3 kJ
mol−1 ,
tively, whichwhich
agrees
well well
withwith
the values
for the
catalyst
withwith
Pt content
of 20
wt.%
rerespectively,
agrees
the values
for the
catalyst
Pt content
of 20
wt.%
ported ininpapers
for the
the composite
compositecatalysts
catalystsand
and
reported
papers[43,44].
[43,44]. Close
Close activation
activation energy values for
referenceplatinum
platinumsamples
samplesisisindicative
indicativeofofhigh
highefficiency
efficiencyofofcatalysts
catalystswith
withtin
tindioxide
dioxideinin
reference
10
5 /C
20
20
20
10
20
theORR.
ORR.Lower
Lowervalues
values of
of the
the activation
activation energy for Pt SnO
than
forfor
PtPtSnO
25/C
the
SnO22 /C/C
than
SnO
2 may
may
be attributed
the presence
the Pt-SnO
and higher
activation
be attributed
to thetopresence
of theofPt-SnO
2 hetero-clusters
and higher
activation
energy
2 hetero-clusters
5
energy
of the
SnO2 centers
active centers
of20the
2 active
of the Pt
SnOPt2520
/CSnO
sample.
of the SnO
2 /C sample.
Thus, note should be made of the low dependence of the ORR kinetics on temperature and high efficiency of the electrocatalysts down to temperatures close to 0 ℃. Such
catalysts allow maintaining the PEMFC performance during the “cold start” from low
ambient temperatures to standard operating conditions. Catalysts modified with tin dioxide have activity comparable to that of Pt20/C, despite the partial blockage of Pt sites by
Catalysts 2021, 11, 1469
9 of 13
Thus, note should be made of the low dependence of the ORR kinetics on temperature
and high efficiency of the electrocatalysts down to temperatures close to 0 °C. Such catalysts allow maintaining the PEMFC performance during the “cold start” from low ambient
temperatures to standard operating conditions. Catalysts modified with tin dioxide have
activity comparable to that of Pt20 /C, despite the partial blockage of Pt sites by SnO2
nanoparticles. The high activity of the composite catalysts suggests that tin dioxide participates in the ORR as a co-catalyst, and high water adsorption ability of SnO2 makes these
catalysts efficient at low temperatures due to prevention of formation of ice crystals in MEA,
and at increased temperatures by preventing the PEMFC components from overdrying.
3. Materials and Methods
3.1. Preparation of Catalysts
Pt/x-SnO2 /C and Pt/C catalysts were synthesized using a polyol method in ethylene
glycol described in our previous works [20,24].
3.2. Electrode Preparation
Thick catalyst films on a polished GC disk electrode (0.102 cm2 ; Volta, Russia) were
prepared by dropping catalyst ink with an Eppendorf micropipette. The catalyst ink
included Nafion® used as a binder in the amount of 7 wt.% of the specified catalyst weight.
The catalyst inks were prepared by ultrasound treatment. The mixture consisting of
12 mg of catalyst powder, 0.18 mL of isopropanol and 0.2 mL of 0.5% Nafion® solution in DI
water [45] was treated in ultrasonic homogenizer (the catalyst amount was ca. 60 mg mL−1 )
for 1 h. Five microliters of the prepared catalyst ink were dropped with an Eppendorf
micropipette onto the surface of polished GC electrode. The drying method was obtained
from experiments with the used ink composition. The GC electrode was neatly tilted at
an angle of 45◦ and evenly rotated at 60 rpm until the ink became dry. The electrode was
dried in air at room temperature. The final catalyst loading was about 0.2 mg cm−2 .
3.3. Electrochemical Studies
The cyclic voltammograms (CVs) were measured in 0.1M HClO4 at 25 ◦ C using a
three-electrode glass cell equipped with a polished GC working electrode, a Pt wire counter
electrode placed in a fritted glass tube, and RHE as a reference electrode connected to the
electrochemical cell by a Luggin capillary. We used the RHE because it has a low level of
impurities and does not require potential correction [27].
The electrode was activated in an N2 saturated 0.1 M HClO4 solution at the potential
range of 0.05 to 1.20 V at a 50 mV s−1 sweep rate for about 30 cycles until a stable CV was
obtained. The CVs registered at the potential range of 0.05 to 1.20 V and at a 20 mV s−1
sweep rate were used to characterize the catalyst.
The measurements were performed using a CorrTest CS350 electrochemical workstation (Wuhan, Corrtest Instruments Corp., Ltd., Hubei, Wuhan, China). The catalyst EASA
was calculated using the hydrogen adsorption-desorption peaks at 0.05–0.40 V vs. RHE as
described in [24,46].
After taking the CV measurements, the background current was measured by running
the ORR sweep profile in an N2 -purged electrolyte solution at the electrode rotation
speed of 1600 rpm in the potential range of 0.05–1.05 V and at the 10 mV s−1 sweep rate.
Then the electrolyte was purged with oxygen for 1.5–2.0 h. The ORR polarization curves
were recorded at the electrode rotation speeds of 500, 700, 900, 1200 and 1600 rpm at the
10 mV s−1 sweep rate and in the potential range of 0.05–1.05 V. The electrolyte was purged
by O2 for at least 5 min between the measurements.
To obtain polarization curves at different temperatures (1, 10, 20, 30 and 50 ◦ C), a
thermostatically controlled chamber of the three-electrode cell was connected to a water
thermostat. The upper temperature was limited by increased acid evaporation and accelerated corrosion of the RDE unit, and the lower temperature limit was determined by
freezing of the water used as cooling/heating medium in the thermostat. To accurately
Catalysts 2021, 11, 1469
10 of 13
control the electrolyte temperature, a thermocouple was installed inside the cell. The
polarization curves were recorded after reaching thermal equilibrium. At each temperature
the impedance spectroscopy curves were recorded to take the electrolyte solution resistance
into consideration in the Koutecky–Levich calculations. The electrochemical impedance
spectroscopy curves were plotted with the CorrTest CS350 electrochemical workstation at
frequencies of 105 to 0.1 Hz.
3.4. Catalysts Surface Structure Characterization
The uniformity of the formed films was evaluated using a Levenhuk DTX 90 digital
optical microscope (USA) with 10–300× magnification.
The electrode surface morphology was observed using a Versa 3D DualBeam scanning
electron microscope (SEM) (FEI, Hillsboro, OR, USA) under vacuum. Low vacuum images
were obtained using a low vacuum secondary electron detector (LVSED) and a concentric
backscattered detector (CBS).
3.5. Calculation Methods
The Koutecky–Levich equation was used for calculation of the kinetic and masstransfer components of the measured currents:
1
1
1
1
= + =−
−
j
jk
jd
| jk |
1
2
3
1
1
b ω2
0.62nFDO2 ν− 6 CO
2
(1)
where j (mA cm−2 ) is the measured current density, jk (mA cm−2 ) is the kinetic current
density, jd (mA cm−2 ) is the diffusion-limited current density, n is the average number of
electrons transferred per O2 molecule, F (C mole−1 ) is the Faraday constant, DO2 (cm2 s−1 )
is the diffusion coefficient of O2 in the 0.1M HClO4 solution, ν (cm2 s−1 ) is the kinematic
b (mol cm−3 ) is the oxygen concentration in the electrolyte, and ω (rad s−1 ) is
viscosity, CO
2
the electrode angular rotation rate.
The area-specific (Sa ) and mass-specific (Ma ) catalyst activity were determined by the
following equations:
ik
Sa =
,
(2)
EASA × m Pt
Ma =
ik
,
m Pt
(3)
where mPt (mgPt ) is the mass of Pt on the electrode and ik (mA) is the measured kinetic
current.
To evaluate the average number of transferred electrons n at temperatures different
from room temperature, it is important to take into consideration the temperature dependence of the diffusion coefficient, kinematic viscosity and concentration of dissolved
oxygen. The concentration of dissolved oxygen at each temperature was calculated according to Henry’s law:
COb e[
2
∆sol H
1
R (− T )]
= const
(4)
where ∆sol H is the enthalpy of solution, R is the universal gas constant and ∆solR H is a
constant, equal to 1700 K for oxygen [47]. The values of kinetic viscosity at different
temperatures were taken from [48], and the oxygen diffusion coefficient was calculated by
the Stokes−Einstein equation [49]:
DO2 ν/T = const
(5)
The activation energy of the ORR according to the Arrhenius equation was estimated
by multiplying the gas constant by the slope of the log (ik ) vs. 1/T plot. The kinetic
current was normalized with the respect to the concentration of dissolved oxygen at
each temperature.
Catalysts 2021, 11, 1469
11 of 13
4. Conclusions
The ORR kinetics on composite carbon-supported Pt-SnO2 catalysts were investigated
over a wide temperature range. The CVs show a reduction in the EASA for the composite
catalysts due to possible blockage of Pt active sites by SnO2 nanoparticles. The RDE
technique was used for determination of kinetic and mass components of the ORR current.
K-L plots at 50 ◦ C remain almost parallel up to the potential of 0.9 V, which suggests that
the number of transferred electrons weakly depends on the potential, and that the K-L
theory can be applied for increased temperatures. The number of transferred electrons
for all samples is close to 4, which points to high selectivity of samples at temperatures
up to 50 ◦ C. The catalysts modified with tin dioxide demonstrate high activity despite the
partial blockage of Pt active sites by SnO2 nanoparticles and agglomeration. The relatively
high activity of the composite catalysts is explained by participation of tin dioxide in the
ORR as a co-catalyst. The Pt20 SnO2 5 /C catalyst has close values of activity in comparison
with Pt20 /C up to 50 ◦ C, but the Pt20 SnO2 10 /C sample shows mass activity reduction at
about 30% at the temperature of 50 ◦ C, which can be explained by the agglomeration of
tin dioxide particles, as well as the formation of Pt-SnO2 hetero-clusters. According to the
slope of the Arrhenius plots, the activation energies for the ORR lie in the range from 20 to
30 kJ mol−1 (at temperatures ranging from 1 to 50 ◦ C). The values of activation energies
obtained in our study are in a good agreement with the values of ~28 and 21 ± 3 kJ mol−1
reported in the literature. Thus, the approach used in our study to synthesize modified
catalysts allows for producing hybrid catalysts having not only improved stability, but
also high activity at the temperature up to 50 ◦ C for the Pt20 SnO2 5 /C and up to 30 ◦ C for
the Pt20 SnO2 10 /C.
Author Contributions: Conceptualization, V.N.F., N.A.I., R.M.M., D.D.S.; methodology, R.M.M.,
N.A.I. and D.D.S.; software, R.M.M. and A.A.Z.; validation, N.A.I., V.N.F. and S.A.G.; investigation,
R.M.M., E.V.K., D.D.S., N.A.I. and E.A.S.; resources, V.N.F.; writing—original draft preparation,
R.M.M. and N.A.I.; writing—review and editing, R.M.M., N.A.I., E.A.S. and S.A.G.; visualization,
D.D.S., A.A.Z. and R.M.M.; supervision, S.A.G.; project administration, V.N.F.; funding acquisition,
V.N.F. All authors have read and agreed to the published version of the manuscript.
Funding: The synthesis of catalysts and morphological study of catalytic films were financially
supported by the Russian Foundation for Basic Research (project No. 20-08-00927). The electrochemical studies of catalysts over a wide temperature range were financially supported by the Russian
Foundation for Basic Research (project No. 18-29-23030).
Conflicts of Interest: The authors declare no conflict of interest.
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