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Title

Interfacial Structure and Proton Conductivity of
Nafion at the Pt‑deposited Surface

Author(s)Ono, Yutaro; Nagao, Yuki

Citation

Langmuir, 32(1): 352‑358

Issue Date

2015‑12‑10

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/14270

RightsYutaro Ono and Yuki Nagao, Langmuir, 2015, 32(1),pp.352‑358. This document is the unedited
author's version of a Submitted Work that was
subsequently accepted for publication in
Langmuir, copyright (c) American Chemical Society
after peer review. To access the final edited andpublished work, see
http://dx.doi.org/10.1021/acs.langmuir.5b02623

Description

Japan Advanced Institute of Science and Technology

Interfacial Structure and Proton Conductivity of
Nafion at the Pt-deposited Surface
Yutaro Ono and Yuki Nagao*
School of Materials Science, Japan Advanced Institute of Science and Technology,

1-1 Asahidai, Nomi, Ishikawa 923-1292 Japan

Abstract
Understanding the Nafion-Pt interface structure is important because fuel cell reactions occur
at the three-phase boundary. Infrared (IR) p-polarized multiple-angle incidence resolution
spectrometry (p-MAIRS) technique was used to investigate the in-plane (IP) and out-of-plane(OP) spectra in the identical substrate. Our previous study revealed that the proton
conductivity of the Nafion thin films decreased at the MgO and SiO 2 surfaces. We proposedthat the origin for the lower proton conductivity can be derived from the highly oriented
structure at the interface. However, the interface structure of the Nafion-Pt interface remains
unclear. In this study, Nafion thin films were prepared by spin-coating on a Pt-deposited MgOsubstrates. The IP spectrum exhibited a well-known spectrum, but the OP spectrum was quite
differed considerably from the IP spectrum. Furthermore, thickness dependence of the degreeof orientation for this OP band was observed at the Nafion-Pt interface. This OP band can be
assigned as the vibration mode of the mixture of the CF 2 and sulfonic acid groups. At the1low-RH region, proton conductivity of the Nafion thin film on the Pt-deposited surface wasone order of magnitude higher than that on the SiO 2 surface. Furthermore, the activationenergy was 0.4–0.5 eV, which is lower than that of the SiO 2 surface. These results, whichsuggest that the Pt surface influenced the proton transport property of Nafion thin film, cancontribute to understand the relationship between the proton transport property and thin film
structure on the Pt-deposited surface at the three-phase boundary for fuel cells.

21. Introduction
Polymer electrolyte fuel cells (PEFCs), which are anticipated for use as a next-generation
power source, have been investigated extensively for use in energy conversion for automotive,
portable device and other applications. The membrane electrode assembly (MEA) comprises
a catalyst layer and the electrolyte membrane. Understanding this interface has become
necessary because reactions occur at the three-phase boundary of a proton conductive
ionomer, catalyst (Pt or Pt alloy), and gas. Therefore, the structure and properties of thematerials at the interface must be elucidated [1]. Some reports have described that theinterfacial confinement of polymer electrolyte structure strongly affects the proton transport
property [2–6].
Nafion, the most well-known and widely studied ionomer developed by DuPont, has highproton conductivity in addition to excellent thermal and chemical stability [7]. The chemical
structure of Nafion consists of a hydrophobic polytetrafluoroethylene (PTFE) backbone withperfluorinated ether side chains terminated by hydrophilic sulfonic acid groups, as shown in
Scheme 1.
Scheme 1. Nafion structure.

CF2 CF2 x CF2 CF y
CF3
O CF2 CF O CF2 CF2 SO3H
3

The structure in commercial bulk Nafion membrane has been investigated thoroughly using
small-angle X-ray scattering technique [8–11]. A phase separation structure in hydrated
Nafion has been reported. Moreover, Nafion is used in a solution state with various solvents
for the preparation of MEA. The ideal ionomer layer in the MEA is regarded as extremely
thin [12,13]. Paul et al. have reported the properties of ultrathin Nafion films [14–17]. The
structure and properties of bulk Nafion membranes have been investigated widely in several
studies described the literature, but the character of its confined thin film in the catalyst layer
is still not well understood.
Several studies of Nafion thin films have been reported on the structure, water uptake,proton transport, and other characteristics. Neutron reflectivity (NR) [18–22], grazing
incidence small-angle X-ray scattering (GI-SAXS) [23–27], and X-ray specular reflectivity(XRR) [28] techniques have revealed the effects of confinement and substrates. The NR
study has revealed a lamellar structure between the SiO 2 substrate and Nafion thin film [19].
Moreover, a single hydrated layer appeared at the metal interface such as Pt and Au surfaces[18, 19]. Surface hydrophilicity and the water–vapor interface contributed to the structure of
the Nafion surface [23, 24]. Results of another GI-SAXS study have demonstrated that thedegree of the phase separation losses with decreasing the thickness [26]. Furthermore,
Kusoglu et al. reported that the annealed films also exhibit the loss of microphase separation[27]. Our previous studies conducted using infrared (IR) p-polarized multiple-angle incidence4resolution spectrometry (p-MAIRS) technique [29, 30], which had been developed by
Hasegawa [31–34], revealed the highly oriented molecular structure of Nafion thin films on
both MgO(100) and Si wafer.
Some studies of proton transport have reported a decrease of the proton conductivity withdecreasing thickness [16, 26, 30, 35], Moreover, proton conductivity has been described asquite lower than that of the commercial bulk Nafion membrane [15, 30]. Recent
investigations have revealed that the hydrophilicity at the surface of Nafion ultrathin films isaltered drastically by annealing, with conductivity changed by around one order of magnitude
according to relative humidity [17].
Systematic analyses have examined the relationship between the structure and the proton
conductivity of Nafion thin films. However, the relation between the interfacial behavior andproton transport property is not understood completely. For example, the different interfaces
suggest the results of different water uptake [26, 27, 36–38]. Kongkanand reported that water
uptake decreases concomitantly with decreasing the film thickness [36]. In contrast, Dishari
et al. reported that the thin film at the SiO 2 interface had higher water uptake than thicker
films [38]. These results suggest that the effects of the interface cannot be neglected. Bass et
al. reported the surface hydrophilicity dependence of the micelle orientation [24]. However,proton conductivity has not been reported for the different interfacial conditions.

5For use in fuel cell operations, the structure of confined Nafion thin films on a Pt surface
has not been investigated sufficiently. As described herein, IR p-MAIRS technique wasconducted to investigate the molecular structure of the Nafion thin films on Pt-deposited
surface. IR p-MAIRS is increasingly regarded as a powerful spectroscopic tool for revealing
molecular orientation in thin films. The p-MAIRS analysis showed that in-plane (IP) andout-of-plane (OP) transition dipoles are distinguishable in an identical infrared transparent
substrate. However, because Pt is not transparent for IR light, we prepared an ultrathin
Pt-deposited surface on the transparent substrate for p-MAIRS. This ultrathin Pt-deposited
surface has enabled the measurement of proton conductivity because the electronic
conductivity of the ultrathin Pt-deposited surface is lower than the proton conductivity of the
Nafion thin film. This result can help to understand the relation between the proton transport
property and the thin film structure on the Pt-deposited surface at the three-phase boundary
for fuel cells.

2. Experimental
2.1 Nafion Thin Film Preparation on Pt Modified Surface
For this study, MgO (Furuuchi Chemical Industries Corp., Japan) and SiO 2 substrates (each15 × 15 × 0.5 mm) were used respectively for IR and proton conductivity measurements. This
study examined the relation between the thin film structure and proton conductivity on the6

Pt-deposited surfaces that had been prepared on each substrate using radio frequency (RF)magnetron sputtering system (KXS-110; Kenix Co. Ltd.). First, Pt-deposited surfaces wereprepared by RF sputtering of a pure platinum (99.99%, Kojundo Chemical Laboratory Co.
Ltd.) metal target. The RF power was applied, 20 W and 15 W respectively to MgO and SiO 2
substrates. Deposition was done in 20 s at room temperature with an Ar atmosphere. To
obtain thin film thickness of Pt layer, deposition parameters were optimized. The thickness of
the Pt-deposited layer was c.a. 15–20 nm (Figure S1), determined using atomic force
microscopy (AFM, VN-8000; Keyence Co.). The Pt-deposited surface was characterizedusing X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD; Shimadzu Corp.). As areference, MgO and SiO 2 substrates were used without Pt deposition. Before using them,
plasma cleaning (Cute-MP; Femto Science, Korea) was conducted.
For thin film preparation by spin-coating (ACT-200; Active Co. Ltd.), 5 % Nafion
dispersion (DE521 CS type; Wako Pure Chemical Industries Ltd.) was used. Thicknesses of
20–400 nm were found using a surface profiler (P-15; KLA-Tencor Corp.). The thickness wascontrolled by adjustment of the Nafion solution concentration using ethanol and water. The
Nafion thin films were dried for at least 12 hr in a desiccator.

2.2 Infrared (IR) p-polarized Multiple-angle Incidence Resolution Spectrometry(p-MAIRS)
7

To investigate IP and OP molecular vibrations in an identical thin film, the IR p-MAIRS
technique was performed. The p-MAIRS measurements were taken using an FT-IR
spectrometer (Nicolet 6700; Thermo Fisher Scientific Inc.) equipped with a mercury–cadmium–telluride (MCT) detector. The optical configuration is shown in Figure 1. To obtain
the p-polarized light, a ZnSe polarizer was used. Single-beam spectra were collected from
38° through 8° in 6° steps between the angles of incidence. The wavenumber resolution was
4 cm-1. The number of scans was 64 for each angle of incidence. Dry air or N 2 gases passed
through the sample compartment and inside of the spectrometer for air purge. The humidityof the sample compartment was less than 5% under room temperature. The aperture was fully
opened (size of 150). A metal plate with small pores was placed in the light path of theincidental beam to prevent saturation. The p-MAIRS analysis from the collected spectra wasconducted automatically using p-MAIRS analyzer software (Thermo Fisher Scientific Inc.).

8

Electric
field

Substrate
Incidentangle
IR light
Thin film
Pt

Figure 1. Configuration of the polarized incident beam path and incident angle.

2.3 Proton Conductivity Measurement of the Nafion Thin Films
The proton conductivity of thin films was measured at relative humidity (RH) range of 40–
95% and temperature of 298–333 K using an impedance / gain-phase analyzer (SI1260;
Solartron Analytical) with a dielectric interface system (SI1296; Solartron Analytical) withhumidity-controlledandtemperature-controlledchamber(SH-221;

Espec

Corp).

Measurements were taken parallel to the substrate. The Au electrodes were fabricated at theedge of thin film. The impedance data were collected at the frequency range of 10 MHz and 1
Hz with amplitude of 50 mV. Proton conductivity (σ) was calculated as follows,

σ=

d
Rlt

(1)

where d is the distance between the Au electrodes, R is the resistance value from impedance, land t are the length of the contact electrodes and thickness of the film respectively.

92.4 DFT Calculations
For the attribution of IR spectra, density functional theory (DFT) calculations were
performed using the DMol3 package in Materials Studio (Accelrys Software Inc.). The
Perdew−Burke−Ernzerhof (PBE) function was chosen. Convergence threshold for the
maximum force and maximum displacement for normal geometry optimization were set
respectively to 0.004 Ha Å−1 and 0.005 Å.

3. Results and Discussion
The p-MAIRS technique has never been applied to the Pt-deposited surface. To confirm thevalidity of the p-MAIRS technique using a Pt-deposited surface on the MgO substrate, avalidity check was carried out according to the literature [33]. Figure S2 shows the
single-beam spectra with different incident angles. The intensity of the single-beam in thecase of thicker Pt-deposited substrate was almost zero. However, the intensity for the thin
Pt-deposited MgO (Figure S2(b)) was only 10% lower than that of the bare MgO substrate(Figure S2(a)). Each spectrum had a similar shape in the case of Pt-deposited and bare MgOsubstrates. The intensity of the single-beam spectra gradually decreased when the angle ofincident decreased from 38° to 8°. Therefore, we inferred that this p-MAIRS technique isapplicable with the Pt-deposited surface. To confirm the reproducibility for Pt deposition,
Figure S2(c) shows the Pt-deposited MgO surface of three samples at 38°. The three samples
10

show similar spectra. The single-beam spectra of the SiO 2 showed different spectra comparedto the MgO substrate, because SiO 2 has low IR transmittance (Figure S2(f)) as a result of thelarge absorption band at less than ca. 2100 cm-1. The intensity from 4000 to 2200 cm-1 forthin Pt-deposited SiO 2 (Figure S2(e)) was a 4–10 % decrease compared with the bare SiO 2
substrate (Figure S2(d)). This value is approximately equal to that of the Pt-deposited MgO.
For Pt, an electroconductive material, the direction used for the impedance measurement isparallel to the Pt-deposited surface. For thick Pt thin film, proton conductivity cannot bemeasured because the resistance of Pt and Nafion thin films should be regarded as a parallel
circuit. Therefore, a thin Pt-deposited surface is necessary to evaluate proton conductivity.
Figure S3 shows the RH dependence of the resistance of Pt-deposited surface on the SiO 2
substrate. The resistance of the Pt-deposited surface was found to be independent of the RH:
it was 1011 ohm. In contrast, the resistance of the 20-nm-thick Nafion thin film deposited on
Pt / SiO 2 substrate was strongly dependent on the RH. The resistance of the Pt-deposited
surface was one order of magnitude higher than that of the Nafion thin films in the whole RH
region. Therefore the proton conductivity can be evaluated in comparison to the Pt electronic
conductivity. Two reasons for the higher resistance of the Pt-deposited surface areconsidered: the thin Pt-deposited surface is oxidized; alternatively, Pt is an island structure
rather than a smooth thin film. To assess these possibilities, XPS was used. Figure S4 shows
XPS spectra of Pt 4f and Si 2p electrons for the Pt / SiO 2 surface. The binding energies of Pt
11

4f 7/2 , 4f 5/2 electron peaks were found respectively to be 71.5 eV and 74.7 eV. These values
show good agreement with those of Pt 4f 7/2 , 4f 5/2 electron peaks that have been reported in
the literature [39]. The oxidation state of Pt 4f peaks was observed in curve-fitted XPS
spectra (Figure S5). The relative peak areas were, respectively, ca. 25% (Pt+2 4f 5/2 ) and 75%(Pt+0 4f 5/2 ), respectively (Table S1). This result suggests that the Pt-deposited surface waspartially oxidized. The Si 2p electron peak can be assigned as SiO 2 . It can be supposed that
the Pt-deposited surface has an island shape on the SiO 2 substrate. The surface morphologyof the bare surface and Pt-deposited surface was obtained using AFM. Figure S6 shows
topographic AFM images of the bare surface and Pt-deposited surface. Each image shows a
10 μm × 10 μm area of the surface. The smooth surface images were obtained respectively
from bare MgO and SiO 2 surfaces in Figure S6(a) and S6(b), respectively. In contrast,
rougher surfaces compared to bare surface were observed in both Pt-deposited MgO andSiO 2 surfaces, as shown in Figure S6(c) and S6(d). These results support that the
Pt-deposited surface has island morphology on both substrates.

Figure 2 shows the incident angle dependence of the transmission IR spectra of the
300-nm-thick Nafion film on the Pt-deposited surface. The absorbance around 1260 cm-1
depended on the incident angle. This result suggests that the thin film has an oriented
structure. For a low incident angle, the peak around 1260 cm-1 was not observed, whichsuggests that the IP transition dipole moment does not have so much. The shoulder peak
12

appeared gradually with the incident angle, meaning that OP transition dipole moments were
excited by polarized light.

Absorbance

0.18

0.12

0.06

38 deg32 deg26 deg20 deg14 deg
8 deg

01450 1400 1350 1300 1250 1200 1150 1100

Wavenumber / cm

-1

Figure 2. Incident angle dependence of the transmission IR spectra of the 300-nm-thick
Nafion film on the Pt-deposited surface.
To discuss the details of the molecular orientation, p-MAIR spectra after the collection of
single-beam spectra are shown in Figure 3. The Nafion membrane consists of a hydrophobic
PTFE backbone with perfluorinated ether side chains terminated by hydrophilic sulfonic acidgroups. The IP spectrum shows the well-known spectrum of Nafion membrane [40, 41]. The
bands at 1150 and 1210 cm-1 are assigned respectively to ν s (CF 2 ) and ν as (CF 2 ). The
shoulder band at 1300 cm-1 is assigned to ν as (CF 3 ). These vibration modes can be observed
13

in the IP spectrum. However, the OP spectrum differed greatly from the IP spectrum. The
characteristic absorption band at 1260 cm-1 was observed only in the OP spectrum. Many
investigations of IR spectra have been conducted using commercial Nafion films with several
hundred or tens of micrometer order thickness. Two groups have reported this peak located at1260 cm-1 experimentally by polarization modulation infrared reflection absorption
spectroscopy (PM-IRRAS) [42, 43]. They assigned different attributions of the band at 1260
cm-1 as ν (CF 2 ) [42] and ν as (CF 3 ) + δ s (COC) [43] vibration mode, respectively. Zeng et al.
assigned ν as (CF 3 ) vibration mode at the 1269 cm-1 in the Raman spectra [44]. Malevich et al.
and Korzeniewski et al. assigned attributions of the band at 1250–1350 cm-1 and 1249–1275
cm-1 as –SO 3 - from a deconvoluted IR spectrum [45, 46]. In our previous report, this IR bandat 1260 cm-1 was attributed to –SO 3 H vibration modes between the two sulfonic acid groups
with hydrogen bonds by the DFT calculation [30]. Similar calculation results have beenreported by another group [47]. Their calculations suggest that the dimer structure of triflic
acid shows absorption near the 1260 cm-1. They assigned this band as SOH in-plane bending
vibration mode. Furthermore, Warren and McQuillan assigned the measured band at 1255–
1289 cm-1 as sulfonic acid groups of perfluoro(2-ethoxyethane) sulfonic acid [48]. For further
consideration the attribution of this band, DFT calculations were conducted as a modified
model. Figure 4 shows a perfluorinated sulfonic acid as a model compound of Nafion for
DFT calculation. This study employed the new model based on the Nafion equivalent weight14

with a longer backbone than that described in our previous report. Vibration analysis was
performed after geometry optimization. In our previous calculation with the short main chainof perfluorinated sulfonic acid, the band at 1260 cm-1 had not been suggested [30]. However,
the CF 2 mode around 1260 cm-1 was suggested in this work using the longer main chain.
Therefore, the band at 1260 cm-1 can be assigned as the vibration modes of the mixture of the
CF 2 and sulfonic acid groups. That abrupt conclusion for this attribution seems inappropriate:
further investigation is needed for this attribution. The peak at 1060 cm-1 [49], whichcorresponds to sulfonic acid group, is helpful for this attribution. However, this IR region
cannot be discussed because of IR absorption by the MgO substrate.

15

Absorbance

0.5
0.4

Out-of-Plane
In-Plane x2

0.3
0.2
0.1
0
1450 1400 1350 1300 1250 1200 1150 1100

Wavenumber / cm

-1

Figure 3. IR p-MAIR spectra of 300-nm-thick Nafion thin film on the Pt-deposited surface.

Figure 4. Nafion-like model for DFT calculation: carbon (gray), fluorine (light blue), oxygen(red), sulfur (yellow), and hydrogen (white).

Some literatures describe the thickness dependence by other measurements in the confined
Nafion thin films. The proton conductivity [16, 26, 29, 35], water uptake [26, 27, 36–38], anddegree of hydrophilicity depend on the thickness [14]. The thin film thickness may affect theinterface structure. To investigate the interface structure of the confined Nafion thin films on
16

the Pt-deposited surface, Figure 5 shows the thickness dependence of the oriented structureon the Pt-deposited surface. The absorbance of the band at 1260 cm-1 depends strongly on
decreasing thickness and decreases along with it.

0.1

(a)
AbsorbanceAbsorbance0.2

Out-of-Plane
In-Plane x2

0.1

01450 1400 1350 1300 1250 1200 1150 1100

Wavenumber / cm

0.08

-1

Out-of-Plane
In-Plane x2

0.04
0.02
0
1450 1400 1350 1300 1250 1200 1150 1100

0.05

Absorbance

0.06

Absorbance

0.06

(b)

Wavenumber / cm

0.06

(c)
0.04

0.08

Out-of-Plane
In-Plane x2

0.02

0.04
0.03

-1

(d)
Out-of-Plane
In-Plane x2

0.02
0.01

0
1450 1400 1350 1300 1250 1200 1150 1100

01450 1400 1350 1300 1250 1200 1150 1100

Wavenumber / cm

Wavenumber / cm

-1

-1

Figure 5. IR p-MAIR spectra of a Nafion thin film on the Pt-deposited surface. The
thickness of Nafion thin films is (a)170 nm, (b)80 nm, (c)50 nm, and (d)35 nm thick.

17

Figure 6 depicts the thickness dependence of relative intensity ratios based on the strongest
bands located at 1260 cm-1 (OP) and 1215 cm-1 (IP). The relative intensity ratio is calculated
as follows,
Intensity ratio =𝐼𝐼𝑂𝑂𝑂𝑂 (1260 𝑐𝑐𝑐𝑐−1 )𝐼𝐼𝐼𝐼𝐼𝐼 (1215 𝑐𝑐𝑐𝑐−1 )(2)where I OP and I IP are derived from peak absorbance of IR p-MAIR spectra. The threshold wasobserved around 50–70 nm for the case of the Pt-deposited surface. The thin film structurechanges drastically at this region of the thickness. A few literatures are available for the
discussion between Pt and sulfonic acid groups [27, 44, 50, 51]. The possibility of acontribution by the difference of the surface morphologies between the Pt-deposited surface
and MgO cannot be excluded based on our experimental results. And no apparent
surface-enhanced IR effect was observed. The reason for the abrupt change in the intensity
ratio remains under investigation. Additional information for the thin film structure can beconsidered at 1400 cm-1. A new peak appeared for 35-nm-thick Nafion thin films on the
Pt-deposited surface. This new peak can be assigned as (S=O) of the SO 3 H [49]. Our modelcompound of Nafion also suggests the S=O and S-OH stretching modes of the -SO 3 H groups
at 1400 cm-1. The dissociation state of the -SO 3 H groups depends on the thickness. Although
the proton of -SO 3 H groups in the thicker thin film is dissociated, the proton of -SO 3 Hgroups at the interface on the Pt-deposited surface is not dissociated in the dry atmosphere.
The dissociation state might derive from the structure.
18

Thickness dependence has not been discussed for the previous case of the MgO substrate.
Figure S7 shows the thickness dependence of the p-MAIR spectra. The absorbance did notdepend as much as in the case of the Pt-deposited surface. As shown in Figure 6, thickness
dependence of the intensity ratio between the Pt-deposited surface and MgO substrates
showed a different trend. The threshold was not observed in the case of the MgO substrate
which suggests that the thin film structure depends on the type of the substrate. Similar
thresholds of the proton conductivity and surface hydrophilicity were reported respectively

2IP
OP

-1

Intensity Ratio-1[ I (1260cm ) / I (1215cm ) ]by Modestino et al. and Paul et al. using the thermal treated SiO 2 surface, [14, 26].

1.6
1.4
1.2
1.0
0.8

Nafion / Pt
Nafion / MgO

0.6
0

50

100

150

200

250

300

350

400

Thickness / nm
Figure 6. Thickness dependence of relative intensity ratios based on the strongest bands
located at 1260 cm-1 (OP) and 1215 cm-1 (IP).

19The thickness dependence of the proton conductivity on the SiO 2 substrate has beenreported [16, 26, 29, 35]. Modestino and co-workers reported that this conductivity change
derives from the change of the interface structure from the GI-SAXS results. However, theproton transport on the Pt-deposited surface has not been sufficiently discussed. The
thickness dependence of proton conductivity of the Nafion thin film for various RH at 298 K
is shown in Figure 7. Proton conductivity is dependent on thickness and RH. These proton
conductivities were found to be 10-3 − 10-2 S cm-1 at 95% RH. However, their conductivities
were lower than that of the commercial bulk Nafion membrane. Furthermore, stronger RH
dependence of the proton conductivity was observed in the thin film case. At the low RH
region, the proton conductivity depends on the surface type. Proton conductivity on the
Pt-deposited surface was one order of magnitude higher than that on the SiO 2 substrate. Thisdifference might derive from the different thin film structure and/or state of proton
dissociation of the sulfonic acid groups, as discussed in the section of the p-MAIR results.

20

-1

(a)

-1

298 K

(b)
-2
-1

Log (σ / S cm )-1

Log (σ / S cm )

-2

298 K

-3

-4

-5
RH
40 %
80 %
60 %
95 %

-6
0

-3

-4

-5
RH
40 %
80 %
60 %
95 %

-6

20 40 60 80 100 120 400

0Thickness / nm

20 40 60 80 100 120 360

Thickness / nm

Figure 7. Thickness dependence of the proton conductivity for the Nafion thin films atvarious RH: (a) on SiO 2 and (b) on Pt-deposited surface.

The activation energy of each RH can provide useful information related to the proton
conduction mechanism. To obtain the activation energy, the temperature dependence of the
proton conductivity is shown in Figure S8. All log(σT) depended lineally on the 1000T-1 inthis temperature region. The activation energy for the 40-nm-thick Nafion thin films on SiO 2
and Pt-deposited surfaces is shown in Figure 8 as a function of the RH. At the high RH region,
all activation energies exhibited similar values (ca. 0.3–0.4 eV). However, at the low RH
region, the activation energies depended strongly on the surface. The activation energy
21depends strongly on RH in the case of the SiO 2 surface. A similar trend was reported by Paulet al. [16]. In the case of the Pt-deposited surface as new data, the activation energy was 0.4–
0.5 eV, which has lower activation energy than in the SiO 2 surface case. The reason for the
relatively higher proton conductivity at the low RH on the Pt-deposited surface is derived
from this lower activation energy.
To understand the origin of the lower activation energy on the Pt-deposited surface, it isessential to consider the Nafion interface on the Pt-deposited surface. Water uptake and the
diffusion coefficient of the water were informative physical parameters for the further
discussion of the proton transport. Eastman et al. reported that the diffusion coefficient of thewater decreased drastically at the interface [28]. These data are based on the PM-IRRAS with
the Au surface and QCM measurements with the SiO 2 surface. The diffusion coefficient
between the metal and SiO 2 surfaces shows no difference. Regarding water uptake, Murthi etal. and Kusoglu et al. reported that the water content (λ) at the interface on the Pt [18] surface
is less than that of the bulk [27]. Modestino et al. reported that the water content at theinterface on the SiO 2 surface is greater than that of the bulk. This greater water uptake
enhances the dilution and isolation of the sulfonic acid groups. [26]. This contributes to thedecrease of the proton conductivity and to the high activation energy. Based on thedescription presented above, the Nafion interface on the Pt-deposited surface might hold thelow water contents. The appropriate amount of water might develop a favorable structure for22the proton conduction. The proton conductivity at further low RH around 0% was not
obtainable because of limited machine specifications.

0.8
Nafion / SiO2 (40-nm-thick)
Nafion / Pt (40-nm-thick)Ea / eV

0.7
0.6
0.5
0.4
0.3

40

50

60

70

80

90

100

RH / %
Figure 8. Activation energy for the 40-nm-thick Nafion thin films on SiO 2 and Pt-deposited
surface as a function of the RH.

4. Conclusion
The interfacial confinement of polymer electrolyte structure strongly affects the protontransport property. Although structure and properties of Nafion bulk membranes have been
investigated widely, the character of their confined thin films on the Pt-deposited surface as amodel of the three-phase boundary remains unclear. Nafion has an orientation structure at the
Pt-deposited surface by p-MAIRS. The degree of orientation on the Pt-deposited surface
23

depends on the thickness. A different dissociation state of sulfonic acid groups was also
observed. At the low-RH region, proton conductivity depends on the Pt-deposited and SiO2
surfaces. Proton conductivity on the Pt-deposited surface was one order of magnitude higher
than that on SiO2 substrate, but its conductivity remained lower than that of the bulk
membrane. This difference might derive from the different thin film structure and/or the
dissociation state of the protons at the sulfonic acid groups. For the Pt-deposited surface, theactivation energy is 0.4–0.5 eV, which has lower activation energy than in the case of the
SiO2 surface. These results elucidate the relationship between the proton transport property
and thin film structure on the Pt-deposited surface at the three-phase boundary for fuel cells.

Acknowledgments
The authors thank all reviewers to improve our manuscript. This work was financially
supported by the Japan Society for the Promotion of Science (JSPS) through the Funding
Program (GR060) for Next Generation World-Leading Researchers (NEXT Program),initiated by the Council for Science and Technology Policy (CSTP). This work was partially
supported by research funds from the Ogasawara Foundation for the Promotion of Scienceand Engineering, The Kyoto Technoscience Center, and the Research Foundation forElectrotechnology of Chubu (REFEC).

24

ASSOCIATED CONTENT
Supporting Information

AFM image of cross-section of Pt-deposited surface, validity and reproducibility checks for
p-MAIRS using Pt-deposited surface, RH dependence of the resistance of Pt-deposited
surface and Nafion thin film, XPS spectra for Pt-deposited surface, AFM images of
Pt-deposited and bare surfaces, thickness dependence of p-MAIR spectra, and Arrhenius
plots for the proton conductivity of the Nafion thin films at various RH. These materials areavailable free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION
Corresponding Author*ynagao@jaist.ac.jp Phone: +81(Japan)-761-51-1541, Fax: +81(Japan)-761-51-1149,
Address: 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan

Notes

The authors declare that they have no competing financial interest related to this study.

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