Platinum-decorated carbon nanotubes for
hydrogen oxidation and proton reduction in solid
acid electrochemical cells
†
V. Sara Thoi, Robert E. Usiskin and Sossina M. Haile
*
Pt-decorated carbon nanotubes (Pt-CNTs) were used to enhance proton reduction and hydrogen
evolution in solid acid electrochemical cells based on the proton-conducting electrolyte CsH
2
PO
4
. The
carbon nanotubes served as interconnects to the current collector and as a platform for interaction
between the Pt and CsH
2
PO
4
, ensuring minimal catalyst isolation and a large number density of active
sites. Particle size matching was achieved by using electrospray deposition to form sub-micron to
nanometric CsH
2
PO
4
. A porous composite electrode was fabricated from electrospray deposition of a
solution of Pt-CNTs and CsH
2
PO
4
. Using AC impedance spectroscopy and cyclic voltammetry, the total
electrode overpotential corresponding to proton reduction and hydrogen oxidation of the most active
electrodes containing just 0.014 mg cm
1
of Pt was found to be 0.1 V (or 0.05 V per electrode) at a
current density of 42 mA cm
2
for a measurement temperature of 240
C and a hydrogen-steam
atmosphere. The zero bias electrode impedance was 1.2
U
cm
2
, corresponding to a Pt utilization of
61 S mg
1
, a 3-fold improvement over state-of-the-art electrodes with a 50
decrease in Pt loading.
Introduction
Fuel cells are promising alternatives to combustion engines for
converting chemical energy into electrical energy due to their
high e
ffi
ciencies and benign byproducts.
1
–
4
However, current
fuel cell technologies have focused on polymeric electrolytes
that require critical water management and signi
cantly drive
up the production cost. Emerging as an attractive class of fuel
cells, solid acid fuel cells (SAFCs), which utilize a proton-con-
ducting membrane of a solid-state acid electrolyte, have the
potential to mitigate these issues as well as operate at inter-
mediate temperatures to take advantage of higher kinetics and
enhanced tolerance to fuel impurities.
5
–
7
In addition, a solid-
state electrolyte inherently avoids challenges with catalyst
dissolution and fuel crossover.
Of all the known solid acids, cesium dihydrogen phosphate
(CsH
2
PO
4
) has been the most developed for fuel cells and
hydrogen separation applications due to its high proton
conductivity (2
10
2
Scm
1
at 230
–
250
C), compatibility with
catalysts, and stability.
8
–
11
A fuel cell peak power density of
415 mW cm
2
(at 0.35 V) has been achieved at 250
C using a
CsH
2
PO
4
membrane and composite electrodes of Pt black, Pt on
carbon, and the electrolyte itself, in a 3 : 1 : 3 mass ratio.
12
–
15
Such devices show good activity for hydrogen oxidation and
proton reduction, but at rather high Pt loadings; the perfor-
mance quoted above required a loading of 7.7 mg cm
2
at each
electrode. Accordingly, the mass normalized activity of Pt, or Pt
utilization (de
ned as the area speci
c electrode conductance,
Scm
2
, divided by the Pt loading, mg cm
2
) for such a device is
rather low, 2.2 S mg
1
. Pre-commercial e
ff
orts have increased
the Pt utilization at the anode of CsH
2
PO
4
fuel cells to
21 S mg
1
, using 0.8 mg cm
2
of Pt in composite structures.
15,16
Remarkably, the Pt utilization for a dense thin-
lm Pt electrode
is 19 S mg
1
,
17
comparable to that in the optimized, porous, pre-
commercial electrodes, despite the nominal absence of triple
phase boundaries in the former. This surprising similarity
suggests that a large fraction of the Pt in the composite elec-
trodes is catalytically inactive as a result of electrical isolation,
consistent with the still high Pt loadings required to achieve
attractive fuel cell power outputs. Thus, enhancing electrical
connectivity is suggested as a means of lowering the Pt loading
while maintaining or even increasing the Pt utilization.
Beyond electrical interconnectivity, a second factor shown to
impact electrode overpotential is the size of the electrolyte
particles in the composite electrode. Speci
cally, it has been
shown that decreasing the CsH
2
PO
4
particle size at a
xed Pt
loading lowers the electrode overpotential, a result attributed to
a concomitant increase in the number density of electrolyte
–
catalyst
–
gas triple-phase boundary points.
15
Electrospray depo-
sition is an attractive approach for achieving the goal of small
electrolyte particles. By controlling the drying rate of the elec-
trosprayed droplets, sub-micron to nanometric particles can be
Departments of Materials Science and Chemical Engineering, California Institute of
Technology, 1200 California Blvd, Pasadena, CA 91125, USA. E-mail: smhaile@
caltech.edu
†
Electronic supplementary information (ESI) available. See DOI:
10.1039/c4sc03003f
Cite this:
Chem. Sci.
,2015,
6
,1570
Received 30th September 2014
Accepted 10th December 2014
DOI: 10.1039/c4sc03003f
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generated and ultimately deposited to form a high surface area
structure.
18
We recently demonstrated the use of electrospray
deposition to form Pt
–
CsH
2
PO
4
composite structures with
electrolyte particle sizes in the 50
–
300 nm size range and a 35
times enhancement in Pt utilization at 2.2 S mg
1
over elec-
trodes of identical composition prepared by mechanical
milling.
19
In a subsequent study, we further showed that carbon
nanotubes (CNTs), grown directly onto the carbon
bers in the
carbon paper current collector, could enhance the electrical
interconnectivity of the Pt nanoparticles to the exterior circuit.
The resulting structure displayed a mass normalized activity of
6.6 S mg
1
, a 3-fold increase relative to electrosprayed elec-
trodes without CNTs.
20
The results achieved to date underscore the importance of
architectural control over electrode features in maximizing Pt
utilization in solid state electrochemical devices. Here, we build
on the concepts of carbon nanotubes as Pt interconnects and
electrospray deposition of high surface area structures to create
high activity electrodes for solid acid electrochemical cells.
Speci
cally, we incorporate free-standing Pt-decorated carbon
nanotubes into CsH
2
PO
4
composite electrodes. While Pt-deco-
rated CNTs have been employed in polymer electrolyte
membrane (PEM) fuel cells and indeed have led to enhanced Pt
utilization,
21
–
23
the anhydrous nature of the solid acid electrolyte
employed here eases the need to
rmly anchor the Pt to the CNT
on functionalized sites to prevent nanoparticle detachment.
Using a relatively simple preparation, we achieve a 3-fold
enhancement in Pt utilization and a 50
decrease in Pt loading
relative to state-of-the-art composite electrodes, with only a
small penalty in absolute activity.
Experimental
General
Commercial multi-walled carbon nanotubes (MWCNTs, 15
5 nm diameter, 1
–
5
m
m length) were purchased from Nanolab,
Inc. These CNTs were not subject to any explicit chemical
treatment for functionalization. In-house CsH
2
PO
4
was
synthesized by dissolving stoichiometric quantities of Cs
2
CO
3
and H
3
PO
4
(85% assay) in deionized water, followed by a
methanol-induced precipitation. The resulting precipitate was
dried at 120
C for 12 h. Untreated Toray carbon paper
(TGP-H-120, Fuel Cell Earth, LLC.) was used as the current
collector in electrochemical cells and as the substrate for elec-
trospray deposition. Polyvinylpyrrolidone (Alfa Aesar,
M
w
8000 g mol
1
) and Nanosperse AQ (Nanolab, Inc.) were used as
dispersants for suspending carbon nanotubes in aqueous
solutions in the electrospray step. Scanning electron micros-
copy images were collected at the Department of Geological and
Planetary Sciences at Caltech (ZEISS 1550VP FESEM) and at
Northwestern University's Atomic and Nanoscale Characteriza-
tion Experimental Center (Hitachi SU8030). Thermogravimetric
analysis was conducted on Netzsch STA 449 C Jupiter and
Netzsch STA 449 F3 Jupiter thermal analyzers. X-Ray powder
di
ff
raction was performed using a PANalytical X'Pert
PW3040-PRO (Cu K
a
). Raman spectra were collected with a
Renishaw M1000 Micro Raman Spectrometer System, using a
green laser at 514.5 nm.
Synthesis of Pt-decorated CNTs (Pt-CNTs)
Pt decoration of the CNTs followed a procedure reported by Xue
et al.
that involves essentially direct solid state reaction between
a metal salt and the CNTs
24
and eliminates the need for explicit
functionalization of the support to create metal nanoparticle
anchor sites. 100 mg of MWCNTs was dispersed by sonication
for 2 h in an aqueous solution of H
2
PtCl
6
with the metal salt
concentration
xed to obtain an ultimate target Pt weight
percent (30
–
50 wt%). The water was allowed to evaporate and
the CNTs were dried at 60
C in air for 18 h. The resulting
powder was transferred to an aluminum boat and then heated
in a tube furnace (Lindberg/Blue M Mini-Mite) to 200
Cata
rate of 15
C min
1
under a
ow of argon. Reduction of the Pt
precursor was accomplished by
owing 250 sccm (standard
cubic centimeter per minute, gas-phase velocity of 85 cm min
1
)
of hydrogen for 60 min at 200
C. The system was cooled to
room temperature under a
ow of argon. The actual Pt weight
percent achieved was determined by thermogravimetric anal-
ysis (TGA). The steps for CNT decoration are summarized in
Scheme 1.
Electrospray deposition
Two types of Pt-bearing solutions were utilized for electrode
preparation by electrospray deposition. The
rst solution was a
suspension of Pt-decorated CNTs in a solvent of 50 mol%
methanol in DI water.
19,20
The loading was set at 10 mg of Pt-
CNT per 20 mL of solution, and one drop of a commercial
dispersant (Nanosperse AQ, Nanolab, Inc.) was added (to a
20 mL solution) to prevent agglomeration. The second solution
was one in which both CNTs and CsH
2
PO
4
were incorporated,
termed herea
er a composite solution. This was prepared by
dissolving 20 mg of CsH
2
PO
4
and 10 mg of polyvinylpyrrolidone
in 6 mL of DI water. A
er complete dissolution, 10 mg of Pt-CNT
was added and the solution was diluted with 14 mL of methanol
to yield a
nal methanol concentration of 50 mol% in DI water.
Both solutions were sonicated for two hours, resulting in a
stable suspension suitable for electrospray deposition. As a
note, Pt-CNTs can spark if placed in direct contact with neat
methanol; accordingly, the Pt-CNT powder was always added to
water prior to the introduction of methanol.
The electrospray deposition was performed using a custom-
built apparatus, shown schematically in Fig. S1.
†
19,20
A voltage of
Scheme 1
Synthetic approach for Pt-decorated carbon nanotubes
(Pt-CNTs).
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5.5 kV was applied across a distance of 19 mm between the
substrate and the tip of the stainless steel capillary (Sigma
Aldrich, 0.5 mm inner diameter, 1.6 mm outer diameter, 15 cm
length) through which the solution was supplied. To encourage
rapid solvent evaporation and ensure deposition of dry particles
(that would not be subject to the rapid agglomeration expected
of deposition of damp particles), N
2
heated at 140
C was
owed
through the chamber at 1 L min
1
. The substrate, a 19 mm
diameter piece of carbon paper, was held in place using a
15 mm diameter mask and was similarly heated to 140
C. The
apparatus con
guration positions the substrate above the
capillary such that the spray is directed upwards and any large
droplets are pulled by gravity away from the deposition area.
The deposition mass was determined by comparing the mass of
the carbon paper substrate before and a
er deposition, using a
Cahn C-35 Ultra-Microbalance (with a sensitivity of 1
m
g) to
obtain the starting mass. Prior to the initial measurement, the
bare carbon paper was heat-treated at 140
C for 5 h.
For example, a typical two-hour deposition of 46 wt%
Pt-CNT
–
CsH
2
PO
4
yielded a total mass (inclusive of all three
components) of 200
m
gor
0.11 mg cm
2
. For each Pt loading
value, at least three cells were evaluated. Two variations of this
overall fabrication approach are discussed.
Electrochemical characterization
From the electrosprayed structures, electrodes were formed by
cutting out two 6 mm diameter circles from the electrosprayed
15 mm diameter substrate, con
ning the active electrode area
to be 0.28 cm
2
. Geometrically symmetric cells were fabricated by
pressing two of these electrodes on either side of 500 mg of
CsH
2
PO
4
powder in a 19 mm die at 69 MPa for 20 min. The
thickness of the CsH
2
PO
4
electrolyte was typically 0.6 mm with
density
90% of the theoretical value.
Stainless steel gas di
ff
usion layers (McMaster-Carr, Type 316,
mesh size 100
100) were placed on either side of the cell. The
assembly was pressed together in a custom-made holder and
tightened by four screws. Each electrode was connected to the
impedance analyser by Ag wire leads. The holder was placed
inside a stainless steel tube (40 mm diameter) and the
temperature was ramped up to 140
Cat2
C min
1
in air.
Under a
ow of humid argon (0.4 atm H
2
O, 6 cm min
1
), the
temperature was increased to 240
Cat2
C min
1
. Humid
hydrogen (0.4 atm H
2
O, 6 cm min
1
) was introduced 30 min
prior to the
rst measurement to allow for gas equilibration.
Gases were humidi
ed by
owing through a water bubbler
heated to 80
C. All electrochemical measurements were per-
formed at a single temperature (240
C) and gas atmosphere
condition (pH
2
¼
0.4 atm, balance H
2
O).
AC impedance spectra were collected using a Solartron 1260
Impedance/Gain-Phase Analyzer. The perturbation voltage was
20 mV and the sampling frequency range was 10
6
to 0.05 Hz.
Because the cells were symmetric, the electrode impedance
measured was the summed result of both proton reduction and
hydrogen oxidation. Data were visualized and analyzed using
the commercial so
ware package ZView 2. Polarization curves
were measured using a Metrohm AutoLab PGSTAT-302 at a scan
rate of 1 mV s
1
. Again, because of the symmetric nature of the
cell and the gas environment, the measured voltage drops
included contributions from both forward and reverse
reactions.
Results and discussion
X-ray powder di
ff
raction (XRD) patterns of the as-prepared
Pt-CNTs (a
er reduction in hydrogen) show broad peaks that
can be assigned to C, Pt (111), Pt (200) and Pt (220),
25
as expected
for Pt on CNTs (Fig. 1a). The Pt peaks are broad, characteristic of
nanoparticles, and the grain/particle sizes are estimated, based
on the full-width half-max of the (111) peak (a
er subtraction of
the instrumental broadening) and the Scherrer equation,
26
to be
9.4 nm and 13.7 nm for 30 and 46 wt% Pt-CNTs, respectively.
The very small intensity of the CNT (002) peak at 26
2
Q
in
Fig. 1a is consistent with the high Pt loading.
27
Characteristic D,
G, D
0
, and G
0
bands of carbon nanotubes in the region of
500
–
3000 cm
1
were observed by Raman spectroscopy
(Fig. 1b).
28
The spectra collected before and a
er Pt decoration
are similar, implying that the CNTs are largely unchanged
through the processing steps. The slight increase in the relative
intensity of the D band at
1400 cm
1
upon treatment suggests
a small change in the structure surrounding defect sites, as may
be expected if Pt nanoparticle nucleation and growth is initiated
at such sites. SEM images show that the CNTs are structurally
Fig. 1
(a) X-ray powder di
ff
raction of 46 wt% Pt-CNTs (red) along with a reference trace
25
(black), and (b) Raman spectra of as-received (red) and
46 wt% Pt-decorated (black) commercial CNTs showing that the carbon nanotube structure is largely unchanged after decoration.
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unchanged as a result of the processing (Fig. 2). Here, images of
as-received CNTs and 46 wt% Pt-treated CNTs are compared.
Bright spots, in the size range of 3
–
17 nm diameter are evident
(only) in the Pt-treated CNTs using both secondary and back-
scatter electron detectors. The sizes of these nanoparticles are
consistent with the size estimation by XRD for the Pt particles.
Representative thermogravimetric traces (under
owing air,
200 mL min
1
, heating rate of 2
C min
1
, total sample size of
10 mg), Fig. 3, show that Pt-decorated CNTs are resistant to
oxidation in air to
350
C. Negligible mass changes (<1%) were
observed upon the introduction of a reducing gas (2% H
2
bal.
Ar, Fig. S2
†
) at 800
C, indicating that PtO
x
does not contribute
to the residual mass. From the weight of this mass, it was found
that loadings of 30 and 40 wt% Pt could be reproducibly ach-
ieved. In contrast, several attempts to reach 50 wt% loading
consistently yielded a value of 46 wt% of Pt. The reason for this
behavior was not explored, but may re
ect a saturation of native
defect sites on the nominally unfunctionalized commercial
CNTs onto which Pt precipitation can occur. It is noteworthy
that, at high (but unspeci
ed) Cu loadings, Chen
et al.
observed
a transition in deposit morphology from isolated nanoparticles
to continuous coatings, using a nearly identical approach.
29
No
such transition is evident here. Traditional Pt-deposition
approaches in which a Pt-bearing salt is reduced on the surface
of oxidized (functionalized) CNTs routinely yield Pt loadings as
high as 30 wt%.
30
These loadings can be increased to as much as
60 wt% using more exotic techniques.
31,32
Thus, the loadings
obtained here fall within the ranges possible by other methods,
but with the bene
t of greatly simpli
ed procedures.
The morphological characteristics of electrosprayed
structures from Pt-CNT suspensions and composite
Pt-CNT
–
CsH
2
PO
4
solutions are presented in Fig. 4 and 5,
respectively; for illustrative purposes, the image of a drop-cast
structure, obtained simply by allowing a drop of the Pt-CNT
suspension to dry on carbon paper, is also shown. A dramatic
di
ff
erence is evident between dropcast and electrosprayed
morphologies. In the former, the CNTs are highly agglomerated
and non-uniformly distributed (Fig. 4a), whereas the electro-
sprayed structure of the same Pt-CNT suspension shows a
homogenous distribution of material over the carbon
bers
(Fig. 4b). The composite electrosprayed structure (Fig. 5) addi-
tionally displays a relatively uniform distribution of the
CsH
2
PO
4
electrolyte particles. These particles are formed
directly on the CNTs, a desirable con
guration, and have a size
ranging from as small as 10 nm up to
1
m
m. The appearance of
large particles has been attributed elsewhere to incomplete
drying of the electrosprayed droplets prior to deposition,
19,20
however, this was not con
rmed in the experiments reported
here.
The impedance behavior of two symmetric cells constructed
using two di
ff
erent electrode fabrication strategies (Scheme 2)
from 30 wt% Pt-CNTs are compared in Fig. 6. In the
rst case, a
layered-composite electrode was prepared using sequential
electrospray deposition of 30 wt% Pt-CNTs for 90 min (0.5 g L
1
in 50 mol% methanol in DI water, solution
ow rate of 1 mL h
1
)
and then CsH
2
PO
4
(10 g L
1
with 5 g L
1
polyvinylpyrrolidone in
50 mol% methanol in DI water, solution
ow rate of 0.3 mL h
1
)
for 30 min. In the second case, a 30 wt% Pt-CNTs
–
CsH
2
PO
4
Fig. 2
High resolution SEM images of (a) as received and (b) 46 wt%
Pt-decorated commercial CNTs. Pt nanoparticles with diameters of
<17 nm are evident in the latter. Bundling and agglomeration of CNTs is
evident in both images.
Fig. 3
TGA pro
fi
les of 30 (black) and 46 wt% (red) Pt-CNTs (measured
under
fl
owing air at a heating rate of 2
C min
1
).
Fig. 4
SEM images of (a) drop-cast 30 wt% Pt-CNTs and (b) electro-
sprayed 30 wt% Pt-CNTs (inset: image of electrosprayed Pt-CNTs on a
carbon paper substrate).
Fig. 5
SEM images of electrosprayed 46 wt% Pt-CNT
–
CsH
2
PO
4
composites, showing direct contact between the electrolyte particles
with the carbon nanotubes: (a) low and (b) high magni
fi
cation.
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composite solution was sprayed for 90 min (1 g L
1
CsH
2
PO
4
,
0.5 g L
1
polyvinylpyrrolidone, and 0.5 g L
1
Pt-CNT in 50 mol%
methanol in DI water, solution
ow rate of 1 mL h
1
) to obtain a
direct-composite structure. The combination of component
concentrations,
ow rates, and deposition times were selected
to create electrodes of comparable total mass and a 2 : 1 mass
ratio of electrolyte to Pt-CNTs. Cross-sectional SEM analyses of
the as-deposited electrodes showed that the layered composite
(strategy 1) was about 50
m
m thick, whereas the co-sprayed
composite (strategy 2) was much thicker, about 200
m
m
(Fig. S3
†
). This unanticipated di
ff
erence may result because the
sequential depositions of Pt-CNT and CsH
2
PO
4
in strategy 1
does not require interactions between hydrophobic and
hydrophilic components. In contrast, deposition from a single
solution (strategy 2) may involve repulsive interactions between
components such that the electrode porosity and hence thick-
ness increase. Regardless of these di
ff
erences, the impedance
spectra of both of these representative electrodes show a char-
acteristic o
ff
set along the real axis, readily attributed to the
resistance of the electrolyte, along with a single arc, which
corresponds to the electrode processes. The o
ff
set resistance,
R
1
, of 2.7
U
cm
2
is precisely equal to the expected value implied
by the electrolyte dimensions (thickness of 0.06 cm, area of
0.28 cm
2
) and conductivity of 2.2
10
2
Scm
1
at 240
C.
Accordingly, the impedance response in all cases was described
using a
R
1
–
R
2
(CPE) equivalent circuit (as indicated in the inset
to Fig. 6),
33
with
R
2
representing the electrochemical reaction
resistance and CPE being a constant phase element
34
repre-
senting the capacitive behavior of the electrodes.
In general, the two 30 wt% Pt-CNT-containing cells, which
are representative of all the samples examined, display excep-
tional performance in light of the ultra-low Pt loadings. The
layered-composite electrode has a Pt loading of just 4.2
m
gcm
2
of Pt yet an area-speci
c electrochemical reaction resistance of
3.7
U
cm
2
. These values imply a Pt utilization of 64 S mg
1
,3
times higher than that of state-of-the-art solid acid fuel cell
electrodes.
15,16
The composite electrode of Pt-CNTs and
CsH
2
PO
4
has a resistance value of just 2.3
U
cm
2
, a signi
cant
improvement over Pt-CNTs alone, while maintaining a high Pt
utilization. Overall, it was found that Pt utilization between the
two approaches was similar, but measurably higher loadings
could be achieved with direct-composite fabrication, suggesting
a better deposition e
ffi
ciency by this approach and resulting in
lower electrode impedances. For reference, cells with non-
treated CNTs were also evaluated (Fig. S4
†
). Not surprisingly,
these displayed extremely high electrochemical reaction resis-
tance values (
6500
U
cm
2
,
tted to the same equivalent circuit
as described above), indicating that any residual metals from
the CNT growth contribute negligible activity for hydrogen
electro-oxidation/proton reduction.
Because of the enhanced activity of direct-composite elec-
trodes over layered-composite electrodes, all subsequent
Scheme 2
Two strategies for electrospray deposition of Pt-CNTs.
Fig. 6
Symmetric cell impedance measurements of 30 wt% Pt-CNT based electrodes prepared by (a) strategy 1 (see Scheme 2) in which Pt-CNT
bearing and CDP bearing solutions were electrosprayed sequentially, and (b) strategy 2 in which a single Pt-CNT and CDP bearing solution was
sprayed. In (a), the Pt loading and utilization are 4.2
10
3
mg cm
2
Pt and 64 S mg
1
, respectively, whereas in (b) they are 5.1
10
3
mg cm
2
Pt
and 86 S mg
1
. Measurements are performed at 240
C in a dynamic atmosphere of 0.4 atm H
2
O and balance H
2
supplied at a gas velocity of
6 cm min
1
and spectra are taken after 4 hours (insets: equivalent circuit used for
fi
tting).
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experiments employed direct-composite electrodes. A further
improvement to the catalysis was achieved by increasing the Pt
loading on the carbon nanotubes. The 46 wt% Pt-CNT
–
CsH
2
PO
4
composite electrode (Fig. 5) has an electrode resistance value of
just 1.2
U
cm
2
(Fig. 7a). Given the Pt loading of 0.014 mg cm
2
,
this result implies a Pt utilization of 61 S mg
1
. Thus, the global
activity can be enhanced by increasing the loading while
maintaining a high Pt utilization. The results for all the
measurements are summarized in Table 1. In the case of Pt
loading, ranges are reported due to the di
ffi
culty of measuring
small masses accurately (on the
m
g scale). In addition, there is a
possibility that the soluble CsH
2
PO
4
preferentially deposits on
the carbon substrate due to accumulation of the suspended
Pt-CNTs at the tip of the capillary. Therefore, the Pt utilization
and loading reported here are conservative estimates, repre-
senting the lower limits of activity. Irrespective of this uncer-
tainty, the general trend of extremely high Pt utilization at low
resistance values is evident, as is a tendency towards higher
global activity at
xed utilization with increasing Pt loading.
Two additional features are noteworthy. First, the Pt utilization
in the Pt-CNT electrodes of the present work exceeds the value
obtained from a thin-
lm electrode in which presumably the
entirety of the Pt
lm was electrically connected and thus
electrochemically accessible. The higher Pt utilization obtained
here implies that not only is all of the Pt accessible, it has higher
inherent mass normalized activity than a
lm 7.5 nm in thick-
ness. Perhaps not surprisingly, the implication is that nano-
particle Pt provides triple phase boundary sites not present in
the hydrogen permeable
lm. Second, the Pt particle sizes
obtained here are larger than those typically obtained from
solution reduction methods,
22
yet high activity is achieved.
Given the general observation that catalytic activity improves
with decreasing particles size, a clear path for enhancing
performance emerges. Furthermore, if one presumes that the
observed trend of
xed utilization with increased loading
continues, even without relying on smaller particles, one can
extrapolate to an expected electrode resistance of about
0.05
U
cm
2
at a Pt loading of just 0.25 mg cm
2
. This repre-
sents a factor of 3 reduction in Pt loading from state-of-the-art
pre-commercial electrodes.
15,17
Accordingly, e
ff
orts are under
way to enhance the deposition procedures to achieve higher
loadings without loss of the desirable architectural features.
Beyond initial performance, stability is an essential param-
eter in the evaluation of an electrode. As shown in Fig. 7b, both
the 30 and 46 wt% Pt-CNT samples, represented in Fig. 6c and
7a, respectively, displayed an initial decrease in electrochemical
reaction resistance which persisted for the
rst 4
–
5 h (the full
spectra shown correspond to the minimum impedance condi-
tions), and then reverted towards an increase. Furthermore,
both the initial increase in activity and the subsequent
Fig. 7
Symmetric cell impedance measurements Pt-CNT based electrodes prepared by strategy 2: (a) Nyquist plot for 46 wt% Pt-CNT
–
CsH
2
PO
4
composite after 4 hours (inset: equivalent circuit used for
fi
tting) and (b) temporal evolution of the electrode resistance values of 30 wt% (black)
and 46 wt% (red) Pt-CNT
–
CsH
2
PO
4
composites, showing higher stability in the electrolyte with higher Pt loading.
Table 1
Progress of Pt-CNT
–
CsH
2
PO
4
electrodes and their comparison with previous work
Electrode
R
(
U
cm
2
)
Pt loading (mg cm
2
)
Pt Utilization (S mg
1
)
30 wt% Pt-CNTs
3.3
–
3.8
3.2
–
4.6 (
10
3
)64
–
81
30 wt% Pt-CNT
–
CsH
2
PO
4
composite
2.0
–
3.0
4.9
–
5.7 (
10
3
)68
–
88
40 wt% Pt-CNT
–
CsH
2
PO
4
composite
1.6
–
1.9
6.3
–
11 (
10
3
)45
–
94
46 wt% Pt-CNT
–
CsH
2
PO
4
composite
1.2
–
1.3
10
–
14 (
10
3
)61
–
78
7.5 nm sputtered Pt
17
3.1
17
10
3
19
Pt : Pt/C : CsH
2
PO
4
15,16
0.06
0.8
21
Pt : Pt/C : CsH
2
PO
4
(3 : 1 : 3) mech. mix
13,14
0.06
7.1
2.2
Pt : CsH
2
PO
4
(1 : 2) electrosprayed on carbon
19
1.5
0.3
2.2
Pt : Pt/C : CsH
2
PO
4
(3 : 1 : 3) electrosprayed on
CNT overgrown on carbon
20
0.5
0.3
6.6
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degradation with time are more pronounced for the electrode
with lower Pt loading. The reasons for these changes, which are
representative of all samples examined, are unknown. The
initial improvement in performance may be due to enhanced
interfacial contacts under the pressure of the measurement
apparatus; the later degradation in performance may be due to
loss in triple phase boundaries between the gas, the catalyst,
and the electrolyte, also as a result of compaction under pres-
sure. The high plasticity of superprotonic solid acids is well
documented
13,15,35
and may contribute to morphological evolu-
tion, with higher Pt loadings serving to provide enhanced
mechanical stabilization by increasing the chemical interaction
between the electrolyte and conductive phase. The large pores
in the carbon paper current collectors are areas lacking
mechanical support and may exacerbate morphological evolu-
tion. Coarsening of Pt particles may also be a contributing
factor to long term degradation, and this might be expected to
be less pronounced for the higher loading compositions in
which the initial Pt size is larger. Given the thermal behavior
documented in Fig. 3, oxidation of the CNTs under the condi-
tions of electrochemical measurement is considered unlikely to
contribute to degradation. On the expectation that closing o
ff
the pore structure in the carbon paper electrode would enhance
stability, direct-composite Pt-CNT
–
CsH
2
PO
4
electrodes were
electrosprayed onto a current collector onto which a dense
forest of carbon nanotubes had been directly grown. Prelimi-
nary results showed this approach to be a promising avenue for
enhancing stability and con
rm the detrimental role of the
highly open porosity in the current collector.
An IR-corrected
36
polarization curve of 46 wt% Pt-CNTs
–
CsH
2
PO
4
is shown in Fig. 8, where the ohmic contribution is
determined from the AC impedance behavior. The slope of the
curve near 0 mV (1.2
U
cm
2
per electrode) is essentially identical
to the area-normalized electrode resistance measured in AC
impedance spectroscopy, showing consistency between the two
methods. The DC behavior is remarkably linear, and stands in
stark contrast to what would be expected for Butler
–
Volmer
kinetics.
37
Even accounting for the fact that proton reduction
occurs at one electrode and hydrogen oxidation at the other in
these experiments, conventional electrochemical theory
predicts a non-linear response with decreasing instantaneous
resistance as current increases. Where oxidation and reduction
have been probed separately at the Pt|CsH
2
PO
4
interface, the
polarization curve is also rather linear, but reveals slight
asymmetry with reduction being more facile than oxidation.
11
The observed linearity suggests that perhaps electron or ion
transport to/from the reaction sites (ohmic processes) may be
rate-limiting as opposed to the electrochemical reaction itself,
but no further evidence is available in support of this hypoth-
esis. Furthermore, it is unknown whether the anodic reaction
occurs
via
direct electrochemical reaction of H
2
O (to generate
oxygen and protons) or oxidation of H
2
also supplied to the
anode. Returning to the performance characteristics, the
current density reaches an attractive value of 42 mA cm
2
at
only 0.1 V, where this voltage is the summed electrode over-
potential and implies an average of 0.05 V per electrode. While
further improvements are desirable and may be possible by
increasing absolute Pt loadings as indicated above, these
characteristics demonstrate the potential of Pt-CNT based
electrodes for enhancing solid acid electrochemical cells for a
range of applications from fuel cells and electrolyzers to
hydrogen pumps and even sensors.
Conclusions
We have shown that Pt-decorated carbon nanotubes can be
used to create electrodes with ultralow loadings of Pt that are
capable of catalyzing both proton reduction and hydrogen
oxidation at low overpotentials in solid acid electrochemical
cells. Direct growth of Pt nanoparticles onto carbon nanotubes
ensured minimal catalyst particle isolation, whereas electro-
spray deposition of electrolyte and electrocatalyst-bearing
solutions led to composite electrode structures with uniformly
distributed and size-matched components. Electrode imped-
ance generally decreased with increasing Pt loading, while Pt
utilization was largely independent of loading, remaining at a
high value relative to previous studies. With this fabrication
method, the Pt utilization has been increased by an order of
magnitude over that of other composite electrodes formed of Pt
and CsH
2
PO
4
. This dramatic improvement is likely a result of
electrical access to all Pt particles in the structure. The lowest
interfacial impedance attained here was 1.2
U
cm
2
, achieved
with a direct-composite electrode of 46 wt% Pt-CNT
–
CsH
2
PO
4
in
which the Pt loading was only 0.014 mg cm
1
of Pt. The DC
polarization behavior revealed that a current density of 42 mA
cm
2
can be obtained at the low overpotential of 0.1 V, or 0.05 V
per electrode. If the Pt-decorated CNTs perform in a manner
that maintains a high Pt utilization (
80 S mg
1
) in an electrode
structure thicker than can be readily created by a single-nozzle
electrospray process, one can reasonably anticipate that a target
anode impedance of 0.05
U
cm
2
could be achieved with a Pt
loading of only 0.25 mg cm
2
. Moreover, further increases in
activity may be possible by decreasing Pt nanoparticle sizes
from the 9
–
14 nm employed here to the 3
–
6 nm sizes that are
typical of polymer electrolyte membrane fuel cells. While the
total electrode impedance obtained to date exceeds target
Fig. 8
IR-corrected polarization curve of a 46 wt% Pt-CNT
–
CsH
2
PO
4
composite electrode at 1 mV s
1
, showing high current density at low
overpotentials for both proton reduction and hydrogen oxidation.
1576
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,2015,
6
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–
1577
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values for technological implementation, the high Pt utilization
achieved by electrospray deposition of Pt-CNT
–
CsH
2
PO
4
composites points towards a clear path for moving solid acid
electrochemical cells into a signi
cant role in a sustainable
energy future.
Acknowledgements
This work was supported by the Dow-Bridge Program through
the Resnick Sustainability Institute at Caltech, as well as by a
Resnick Graduate Student Fellowship (R.E.U.). We thank Dr
Tim Davenport, Michael Ignatowich, and Webster Guan for
their assistance in TGA measurements, and Nate Thomas and
Anupama Khan for their assistance in imaging. We also
acknowledge Prof. George R. Rossman for his assistance with
Raman Spectroscopy, Ben Myers from Northwestern Uni-
versity's Atomic and Nanoscale Characterization Experimental
Center (NUANCE) for SEM imaging (Fig. 2). In addition, we
acknowledge Caltech's Kavli Nanoscience Institute (KNI) for
access to additional imaging instrumentation and the Molec-
ular Materials Research Center at Caltech for the use of the
Cahn C-35 Ultra-Microbalance.
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This journal is © The Royal Society of Chemistry 2015
Chem. Sci.
,2015,
6
,1570
–
1577 |
1577
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Chemical Science
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