Journal of Power Sources 258 (2014) 83e88

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Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour

Short communication

X-ray evaluation of the boundary between polymer electrolyte andplatinum and carbon functionalization to conduct protons in polymer
electrolyte fuel cells
Kazuki Oka, Yuta Ogura, Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japanh i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Boundary between proton conducting polymer and Pt surface was
evaluated by synchrotron X-rays.
 The boundary in PEFC catalyst wasmonitored based on the electronic
state of Pt sites.
 Proton conductivity increased byfunctionalizing
sulfonate/sulfate
groups over carbon black.
 Functionalized sulfonate/sulfates on
C were proton conductive on asimilar level to Naﬁon.
 Functionalized sulfonate/sulfates on
C were energetically inferior to sulfonates of Naﬁon.

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 19 December 2013
Received in revised form
15 January 2014
Accepted 10 February 2014
Available online 18 February 2014

In polymer electrolyte fuel cells (PEFCs), it is important to secure proximate diffusion paths of reactantsand electrons. One approach is to optimize the boundary between polymer electrolyte and Pt nanoparticle surface. Based on synchrotron X-ray absorption ﬁne structure to monitor directly the status of
catalysts in PEFCs, it was found that Pt sites were reduced to Pt0 by alcohols contained in polymer
electrolyte dispersion solution during the preparation of cathode of PEFC. As in membrane electrolyte
assembly, only the Pt sites not covered by polymer electrolyte re-oxidized to Pt2þ/4þ. Thus, the interface
between Pt and polymer electrolyte was evaluated. The other approach is to functionalize carbon surface
with sulfonate/sulfate group to conduct protons. Similar level of proton conductivity was observed incurrentevoltage dependence compared to using polymer electrolyte, but polymer electrolyte was advantageous to lose less voltage for activation. Based on this comparison, optimum catalyst on cathode is
proposed comprising surface sulfonate/sulfate group on carbon mixed with polymer electrolyte. Further
optimization of cathode catalyst is proposed to functionalize carbon with sulfonate group linked to
ﬂuorocarbon branch.
Ó 2014 Elsevier B.V. All rights reserved.

Keywords:
Polymer electrolyte fuel cell
Platinum
XANES
Sulfonate/sulfate group
Surface functionalization
Proton diffusion

1. Introduction* Corresponding author. Fax: þ81 43 290 2783.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
http://dx.doi.org/10.1016/j.jpowsour.2014.02.040
0378-7753/Ó 2014 Elsevier B.V. All rights reserved.

In polymer electrolyte fuel cells (PEFCs), active site exists at the
triple phase boundary in the catalysts [1]. The solid, liquid, and gas
phases play the roles of electron conduction, proton diffusion, and84

K. Oka et al. / Journal of Power Sources 258 (2014) 83e88

reactant gas diffusion, respectively. However, coincidence of thethree factors depends on luck during preparation conditions ofmembrane electrolyte assembly (MEA), and triple phase boundaryis in general very limited as points. Thus, it is important to extend
the two-phase boundary from line to face to maximize the numberof active sites of PEFC at the triple phase boundary.
First approach is to evaluate the boundary between active
platinum nanoparticles and proton-conducting polymer. Epting
et al. observed Pt particle agglomeration and the pore spaces in the
Pt/C catalyst layer using 3-dimensional transmission electron microscope (TEM) [2]. This was a direct evaluation of reactant diffusion path. However, it is still a difﬁcult task to evaluate how muchthe surface of metal nanoparticle is covered with light, soft constituent, e.g. polymer electrolyte. In this study, the boundary formation between Pt nanoparticles and proton conducting polymer
was monitored as the electronic state change of Pt sites by Pt L3edge X-ray absorption ﬁne structure (XAFS). The electronic state isquite sensitive to the surrounding environment, i.e. proton conducting polymer and oxidative/reductive gas. Pt L3-edge XAFS wasrecently applied to probe one monolayer of Pt over Rh(111) crystal
applied by 0e1.6 V [3].
Second approach is to maximize the boundary between electron
conducting carbon (C) and proton conducting medium. The
approach in this study is to functionalize the surface of electron
conducting C to conduct also protons. C functionalization by sulfonate/sulfate group was reported for solid Brønsted acid catalysts[4e7]. The point-like triple phase boundary was expected to extend
to line-like using functionalized C material to conduct both electrons and protons in PEFC electrodes.
2. Experimental
2.1. Electronic state measurements of platinum in replica PteC
catalyst
The replica PteC catalyst was prepared following the procedurereported in ref. 8. In brief, Pt(NH3)4(OH)2 was ion-exchanged with
AleMCM-41 (Al2O3:SiO2 molar ratio ¼ 1:79, 1.11 wt% Al; Aldrich) at
353 K for 48 h. Obtained white powder was heated at 573 K in H2
and then at 973 K in the ﬂow of ethyne and nitrogen at a rate of
200 mL min1 with the ratio of 1:9. Next, AleMCM-41 template
was removed by washing with 15% HF aqueous solution to obtainreplica PteC powder. Similar synthesis route to this replica PteC
was also reported using SBA-15 as a template for PteC [9] and Pt/
RueC [10].
50 mg of above obtained replica PteC powder (6.1 wt% of Pt;
mean Pt particle size 2.0 nm) was suspended in 0.2 mL of Naﬁon
dispersion solution [diluted to 2.8e1.5% by mixing 5% solution(DE521, Wako Pure Chemical) with ethanol] and dried (replica Pte
CeNaﬁon). Separately, replica PteC sample was immersed in
0.2 mL of ethanol and dried. Pt supported on Ketjen C black(EC300J) was provided from Ishifuku Metal Industry (IFPC40-II;
39.5 wt% Pt, mean Pt particle size 4.5 nm). Pt L3-edge XAFS spectra
were measured at 290 K in transmission mode in the Photon Factory at the High Energy Accelerator Research Organization (KEK,
Tsukuba) on the beamlines of 7 C, 9 C, and 12 C [8,11]. The storage
ring energy was 2.5 GeV and the current was 180e450 mA. Si(111)
double crystal monochromator was used. The data were analyzed
using the software package XDAP [12].
2.2. C functionalization with sulfonate/sulfate group
Five types of C powder samples (Ketjen black EC300J, Vulcan
XC72R, Darco G-60, Norit SX-Plus, and Norit SX-II) were used asreceived. 1 g of C powder was mixed with 50 mL of fuming sulfuric

acid (15% of free SO3). The suspension was heated at 313e363 K for
10 h in ﬂask equipped with a reﬂux condenser. Then, the suspension was ﬁltered by a polytetraﬂuoroethene (PTFE)-based membrane ﬁlter (Omnipore JGWP04700, Millipore) with a pore size of
0.2 mm and washed with hot water (353 K) until the pH of ﬁltrateincreased to 6.0. All the procedures were performed under argon
atmosphere. Thus-obtained powder is denoted as HSO3eC (precursor C powder name). The proton contents in HSO3eC were
determined by reverse titration. Namely, slightly excessive sodiumhydroxide solution (10 mmol L1) was added to HSO3eC powder
than to neutralize and well mixed using ultrasonic. The suspension
was ﬁltered using a PTFE-based membrane ﬁlter (Omnipore
JGWP04700). The amount of excessive sodium hydroxide in theﬁltrate was determined by the titration of oxalic acid solution(10 mmol L1).
2.3. PEFC performance tests using cathode catalysts consisting of C
functionalized with sulfonate/sulfate group
As the cathode in MEA, functionalized C of HSO3eC(Norit SX-II)
prepared at 353 K (0.49 mmol-Hþ g1) and/or 0.1 mL of 5% Naﬁon
dispersion solution (DE521) were mixed with Pt/C (TEC10E50E-HT,
Tanaka Precious Metal Industry; 50.8 wt% Pt, mean Pt particle size
4.8 nm). The polymer amount (4.8 mg) contained in the Naﬁon
dispersion solution was essentially equivalent to the amount(5.2 mg) in 0.2 mL Naﬁon dispersion solution diluted to 2.8% forreplica PteC (see Section 2.1). The mixing ratios of HSO3eC, Naﬁon,and Pt/C were listed in Table 2, entries aed. The mixture wasmounted in an area of 5 cm2 on water-repellent C paper (TGP-H060H, Chemix) and dried.
As the anode in MEA, Pt/C powder on C paper (EC-20-10-7,
Electrochem; 20 wt% Pt, mean Pt particle size 4.8 nm) was mixed
with 5% Naﬁon dispersion solution (DE521) and dried. Thusobtained cathode and anode were set on the each side of 50 mm
thick Naﬁon ﬁlm (NR212, Dupont), and pressed by applying apressure of 2 MPa and 393 K for 2 min using a tabletop press (Model
SA-302, Tester Sangyo Co.).
PEFC performance tests were performed equipped with the MEA
comprising HSO3eC(Norit SX-II) and Pt/C on the cathodeside. Control tests with MEA in which Naﬁon was used instead of
HSO3eC(Norit SX-II), Naﬁon was added to HSO3eC(Norit SX-II), or no
addition of proton conductor to Pt/C were also performed. Obtained
MEA was set in a PEFC single cell (Model EFC-05-02, Electrochem). O2
and H2 gases were ﬂowed at a ﬂow rate of 100 mL min1 to thecathode and anode, respectively, both through water bubbler
maintained at 353 K (partial pressure of moisture: 47 kPa).
Impedance measurements were performed for two MEA models
to evaluate the effects of sulfonate/sulfate functionalization. On
cathode, 20 mg of HSO3eC(Norit SX-II) or C powder (Norit SX-II)
Table 1
Energy position (eV) and normalized intensity of whiteline peak near the Pt L3 absorption edge for various PteC and the reference samples.
SampleEnergy (eV)Normalized
intensity(a) Replica PteC under air(b) Replica PteC with 1.5% Naﬁon and dried(c) Replica PteC with 2.8% Naﬁon and dried(d) Replica PteC in ethanol(e) Pt/C(Ketjen black) under air(f) Pt/C(Ketjen black) with 1.5%
Naﬁon and dried(g) Pt/C(Ketjen black) with 2.8%
Naﬁon and dried(h) 5 mm thick Pt metal foil(i) PtO2

11,566.7
11,566.8
11,566.8
11,566.7
11,566.5
11,566.5

1.59
1.28
1.27
1.26
1.44
1.24

11,566.4

1.23

11,565.6
11,567.3

1.28
2.07

K. Oka et al. / Journal of Power Sources 258 (2014) 83e88

85

Table 2
Contents of cathode layer in MEA for PEFC tests.
Entry

Pt/C (mg)

HSO3eC(Norit SX-II)
(mg)a

Naﬁon
(mg)

Proton content(mmol)

a
b
c
d

9.8
9.9
9.9
9.9

0
9.7
0
4.8

0
0
4.8
4.8

0
4.8
4.6
7.0

a

Prepared at 353 K.

was mounted in an area of 5 cm2 on TGP-H-060H. On anode, Pt/C
powder (EC-20-10-7) was used in similar manner to the MEA
above. The cathode, Naﬁon ﬁlm (NR212), and anode were pressedat 2 MPa and 393 K for 2 min using a tabletop press (SA-302). Thus
prepared MEA models (effective catalyst area 5 cm2) were sandwiched by two ﬂat copper plates coated with gold with an area ofgreater than 5 cm2. An alternating voltage (10 mV) with the frequency of 102e106 Hz was applied to the MEA model and theimpedance and phase angle were monitored using a potentio/galvanostat (Model VersaSTAT3-500 with an option of frequency
response analyzer, Princeton Applied Research).
3. Results
3.1. Whiteline peak intensity near the Pt L3-edge absorption for Pt
samples supported on C
Pt L3-edge X-ray absorption near-edge structure (XANES)
spectra for various PteC and the reference samples are depicted inFig. 1. The whiteline peaks at 11,565e11,567 eV higher than the Pt L3
absorption edge (11,562 eV for Pt metal) were more intense forreplica PteC (Fig. 1a and Table 1a) and Pt/C(Ketjen black) (Fig. 1e
and Table 1e) under air (1.44e1.59) than 1.28 for Pt metal foil (h).
The whiteline peak intensity for replica PteCeNaﬁon (b, c) was
1.28e1.27, weaker than 1.44 for replica PteC in air (a). Furthermore,
immersed in pure ethanol, the whiteline peak intensity for replica
PteC (d) decreased to 1.26, that was the smallest intensity among
replica PteC samples (aed). Similar trend was observed of the intensity for Pt/C(Ketjen black): 1.44 (under air) > 1.24 (immersed in
1.5% Naﬁon dispersion solution and dried) > 1.23 (immersed in
2.8% Naﬁon dispersion solution and dried) (eeg).
3.2. Proton contents in functionalized C with sulfonate/sulfate
groupFig. 2A illustrates the proton contents in HSO3eC(Norit SX-II)
samples as a function of the heating temperature in fuming sulfuric acid during the preparation. The proton content in HSO3eC(Norit
SX-II) samples was maximized by heating at 353 K (0.49 mmol g1)
and rather decreased to 0.38 mmol g1 when the heating temperature further increased to 363 K. One of the reasons of the decrease
was that the equilibrium ratio of free SO3 molecules in gas phaseincreased above 353 K [13], resulting not to react with C powder
anymore. Fig. 2B depicts proton contents in HSO3eC prepared fromseveral types of C powder. The proton contents in HSO3eC samples
were positively correlated to speciﬁc surface area (SA) of precursor C
powder, indicating that surface sites to react with SO3 or H2SO4
simply increased as the speciﬁc SA grew. The edge of graphene sheet
is considered as the reacting site with SO3 [4].
3.3. PEFC performance using HSO3eC and/or Naﬁon with Pt
catalysts
Currentevoltage characteristics were tested for PEFC using
C(Norit SX-II) functionalized with sulfonate/sulfate and/or Naﬁon

Fig. 1. Pt L3 absorption edge XANES spectra for replica PteC under air (a), replica PteC
with 1.5% Naﬁon and dried (b), replica PteC with 2.8% Naﬁon and dried (c), replica Pte
C in ethanol (d), Pt/C(Ketjen black) under air (e), Pt/C(Ketjen black) with 1.5% Naﬁon
and dried (f), Pt/C(Ketjen black) with 2.8% Naﬁon and dried (g), 5 mm thick Pt metal foil(h), and PtO2 (i).

(Fig. 3). The cell voltage in entry a in which no proton-conducting
functional group/polymer was added to Pt/C catalyst (TEC10E50EHT; 50.8 wt% of Pt) in cathode (Table 2a) was the lowest and thedata values were unstable. This is because the C was waterrepellent and the protons were conducted to Pt sites only at an
instant when liquid water was in contact with active Pt
nanoparticles.
HSO3eC(Norit SX-II) was mixed with Pt/C in the cathode forentry b (Table 2b). The signal/background ratio in the currente
voltage curve was apparently improved compared to data for entry
a. It was apparent that HSO3eC(Norit SX-II) played a role of proton
conducting path. However, the cell voltage in entry b was by w1.6 V
lower than that in entry c in which Naﬁon was used to prepare thecathode Pt/C catalyst instead of mixing HSO3eC(Norit SX-II)(Table 2c, b).
The currentevoltage characteristics were analyzed as a linear ﬁt
to the range between 15 and 30 mA cm2 and an extrapolated dataof the ﬁt function below the range to current density zero, i.e.
voltage axis intercept. The slope evaluated voltage loss dependent
on currents and thus should be related to electron and proton
conductivity. The voltage axis intercept evaluated overpotential.
The effects of sulfonate/sulfate functionalization were comparedto that of Naﬁon by ﬁxing the proton content (4.6e4.8 mmol) in
cathode catalysts (Table 2, entries b and c). The slope for entry b

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K. Oka et al. / Journal of Power Sources 258 (2014) 83e88

Fig. 2. Proton contents in HSO3eC as a function of preparation temperature in the range of 313e363 K using C black (Norit SX-II) precursor (A) and as a function of speciﬁc SA ofvarious precursor C powders at ﬁxed preparation temperature of 353 K (B).

(2.4 V mA1 cm2) was equivalent or more ﬂat than the one forentry c (2.7 V mA1 cm2), but the voltage axis intercept in entry b(0.69 V) was apparently smaller than the one for entry c (0.85 V).
Therefore, HSO3eC(Norit SX-II) exhibited comparable or slightly
better proton conductivity compared to Naﬁon, but the overpotential was by 0.16 V higher than that using Naﬁon.
In the entry d, both HSO3eC(Norit SX-II) and Naﬁon were mixed
to prepare the cathode (Table 2d). When 9.7 mg of HSO3eC was
added to 9.9 mg of Pt/C with Naﬁon, the amount of Naﬁon suspension solution seemed insufﬁcient to mix well with totally 20 mgof powder. Instead, the amount of HSO3eC for mix was reduced to ahalf in entry d. The slope was most ﬂat (2.1 V mA1 cm2) among
all tests in Fig. 3 and the voltage-axis intercept value was as high as
0.86 V. This was the optimized cathode catalyst in which theboundary between C for the electron conduction and protonconducting polymer was maximized by functionalizing the C surface by sulfonate/sulfate groups. The currentevoltage characteristics for entries c and d were stable and reproducible in repeated
MEA performance tests.
In Bode plot based on the impedance measurements (Fig. 4A),
the impedance for MEA model comprising HSO3eC(Norit SX-II) was
2.4  103 U versus 14.2  103 U for MEA model comprising C(Norit
SX-II) at 102 Hz. This impedance difference was a general trend in
all the frequency range, suggesting the greater proton conductivity
when C surface was functionalized with sulfonate/sulfate. This

trend was also supported in ColeeCole plot (Fig. 4B). The (part of)impedance circle for MEA model comprising HSO3eC(Norit SX-II)
was apparently smaller than that for MEA model comprising
C(Norit SX-II), suggesting smaller impedance (greater proton conductivity) for MEA model comprising HSO3eC(Norit SX-II).
4. Discussion
4.1. Correlation between platinum electronic state and Naﬁoncoverage
The changes of whiteline peak intensity that appeared at
11,565e11,567 eV higher than the Pt L3 absorption edge wereexplained due to the change of oxidation states of Pt sites byforming the boundary between Pt nanoparticles and Naﬁon (Fig. 5).
Under air, the valence of surface Pt sites was the mixture of 4þ, 2þ,and 0 (Fig. 5A) [14]. Similar oxidation/reduction trends of surface
sites were monitored based on peaks near the X-ray absorption
edges of V, Cu, and Sn K and Au and Pb L3 [15e22].
Next, the oxidic surface Pt sites were reduced to Pt0 when the
PteC catalyst was immersed in Naﬁon dispersion solution (Fig. 5B).
The reduction was found by the reaction not with Naﬁon but withalcohol(s). 14e26% of 1-propanol and 46e72% of ethanol were
contained in Naﬁon dispersion solution used (2.8e1.5%). The Pt
sites in replica PteC sample were totally reduced to Pt0 in a blanktest by just immersed in ethanol based on the whiteline intensity
became smaller (1.26) to essentially the same as that for Pt metal(1.28; Table 1d, h). This seems not only the reduction of surface Ptsites but (nearly) total reduction of 2.0e4.5 nm of Pt nanoparticle
bulk based on the peak intensity changes via the following reaction.
mCnH2nþ1  OH þ PtOm / mCn1H2n1  CHO þ mH2O þ Pt0(m ¼ 1, 2; n ¼ 2, 3,.)Fig. 3. Currentevoltage characteristics in MEA performance test. Entry a (black), entryb (orange), entry c (blue), and entry d (red). The dotted lines are the ﬁt in the region
between 15 and 30 mA cm2 and the lines were extrapolated to the V-axis. (Forinterpretation of the references to color in this ﬁgure legend, the reader is referred to
the web version of this article.)

Please note that most of Pt sites of replica PteC immersed inethanol were reduced (Table 1d) whereas they were re-oxidized to
Pt2þ and/or Pt4þ upon exposed to air based on Pt L3 XANES. In clear
contrast, negligible re-oxidation of Pt0 was detected when Naﬁon
dispersion solution was used in Pt L3 XANES (Table 1b, c, f, g)
probably because Naﬁon totally covered Pt nanoparticles for thedried replica PteC and Pt/C (Ketjen black) samples. Althoughsimilar amount of Naﬁon was used on the cathode in the MEA
performance tests (Fig. 3) to that for X-ray study (Fig. 1), but the insitu environment of Pt nanoparticle surface was not exactly identical with the one for dried samples of Table 1b, c, f and g because
the electrodes during the MEA performance tests were surroundedby pseudo-liquid Naﬁon aqueous solution.

K. Oka et al. / Journal of Power Sources 258 (2014) 83e88

87

Fig. 4. Bode plot (A) and ColeeCole plot (B) based on impedance measurements for MEA model using HSO3eC(Norit SX-II) (a; solid line) and C(Norit SX-II) (b; dotted line) as
cathode. Common Pt/C powder (EC-20-10-7) was used as anode.

The reason why the PteC(Ketjen black) was more effectively
reduced to Pt0 was considered compared to replica PteC once
immersed in reductive medium (Table 1). The major reason is that
the Pt nanoparticles were more surrounded by C atoms synthesizedfrom ethyne gas decomposed over Pt for replica PteC and thus theinterface Pt sites with C cannot contact with Naﬁon/alcohol. Theinterface between Pt and C for replica PteC was extensively
investigated by state-sensitive Pt L3-edge XAFS spectroscopy [8].
When the samples immersed in 2.8e1.5% Naﬁon dispersion
solution were dried (Table 1b, c, f, g), a part of Pt surface protectedwith Naﬁon thin ﬁlm remained as Pt0 and the other part of Pt
surface in direct contact with air was re-oxidized to Pt2þ and Pt4þ(Fig. 5C). In summary, Pt L3 whiteline peak intensity for PteC catalysts in contact with Naﬁon was a sensitive indicator of the Naﬁoncoverage over the Pt surface and thus also the formation of theboundary between Naﬁon and Pt surface sites.
4.2. Efﬁciency of proton conduction for HSO3eC and Naﬁon
Surface-functionalized HSO3eC(Norit SX-II) was an effective
proton conductor to reduce the voltage loss at cathode in the PEFC
performance tests (Fig. 3a, b). Furthermore, impedance

measurements for model MEA samples supported improved proton
conductance due to the C surface functionalization by sulfonate/sulfate in comparison to bare C black (Fig. 4a, b).
A proton conduction model in cathode is proposed (Graphicalabstract, right). Using Naﬁon to prepare the cathode catalyst layer(Fig. 3c), protons would diffuse by hopping via sulfonate groups ofNaﬁon. Using HSO3eC(Norit SX-II) in cathode (Fig. 3b), protons
would diffuse also by hopping via sulfonate/sulfate groups offunctionalized C. The proton conductivity via HSO3eC(Norit SX-II)
was comparable or slightly superior to via Naﬁon, but apparently
extra energy was required for the proton conduction via HSO3eC(Norit SX-II) based on the evaluation of overpotential as thevoltage-axis intercept values (Fig. 3b, c). One of the reasons wasthat the sulfonates of Naﬁon are neighboring bonded to strong
electron withdrawing CF2 group and would exhibit in strong acidcharacter in contrast that functionalized sulfonate/sulfates boundto C surface should be less acidic for HSO3eC(Norit SX-II) [23].
Therefore, protons in more acidic medium, i.e. Naﬁon, were moremobile and leading to smaller overpotential compared to in
HSO3eC.
The major proton conduction path should be via the sulfonatesof Naﬁon in both cases. However, the proton conduction path will

Fig. 5. Reduction and oxidation of surface Pt sites in PteC catalyst with Naﬁon. Pt was oxidized under air (A), reduced by alcohol in disperse medium (B), and partially re-oxidized
under air (C).

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K. Oka et al. / Journal of Power Sources 258 (2014) 83e88

be widened by the presence of sulfonate/sulfates of HSO3eC(Norit
SX-II) in addition to Naﬁon as compared to the interface with
Naﬁon only (Graphical abstract, right) as demonstrated in Fig. 5d.
Further optimization with respect to triple phase boundary will be
expected by functionalizing the C black surface with CF2-branchedsulfonates (CF2)neSO3H in which equivalent strong acid sites are
expected to the side branches of Naﬁon.
5. Conclusions
It was possible to evaluate the boundary between proton conducting polymer and surface Pt sites based on the synchrotron Xray monitoring of Pt valence state change dependent on theamounts and concentrations of Naﬁon dispersion solutions used.
Moreover, the support C was functionalized by sulfonate/sulfates to
conduct protons as demonstrated in MEA performance tests and
impedance measurements.
Acknowledgments
This work was supported by Japan Society for the Promotion of
Science (JSPS) KAKENHI Grant Number 24006495 (KO) and fromthe Sumitomo Foundation for Basic Scientiﬁc Research (No. 070110,
YI). The XAFS experiments were performed under the approvals of
the Photon Factory Proposal Review Committee (Nos. 2011G523,
2009G531, 2008G167).
References[1] E. Antolini, J. Appl. Electrochem. 34 (2004) 563e576.

[2] W.K. Epting, J. Gelb, S. Litster, Adv. Funct. Mater. 22 (2012) 555e560.
[3] D. Friebel, D.J. Miller, C.P. O’Grady, T. Anniyev, J. Bargar, U. Bergmann,
H. Ogasawara, K.T. Wikfeldt, L.G.M. Pettersson, A. Nilsson, Phys. Chem. Chem.
Phys. 13 (2011) 262e266.
[4] K. Nakajima, M. Hara, ACS Catal. 2 (2012) 1296e1304.
[5] Q. Shu, J. Gao, Z. Nawaz, Y. Liao, D. Wang, J. Wang, Appl. Energy 87 (2010)2589e2596.
[6] S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi,
M. Hara, J. Am. Chem. Soc. 130 (2008) 12787e12793.
[7] X. Mo, D.E. López, K. Suwannakarn, Y. Liu, E. Lotero, J.G. Goodwin Jr., C. Lu,
J. Catal. 254 (2008) 332e338.
[8] K. Oka, Y. Shibata, T. Itoi, Y. Izumi, J. Phys. Chem. C 114 (2010) 1260e1267.
[9] J.H. Nam, Y.Y. Jang, Y.U. Kwon, J.D. Nam, Electrochem. Commun. 6 (2004)
737e741.
[10] F. Li, K.Y. Chan, H. Yung, C. Yang, S.W. Ting, Phys. Chem. Chem. Phys. 15 (2013)
13570e13577.
[11] Y. Izumi, T. Itoi, S. Peng, K. Oka, Y. Shibata, J. Phys. Chem. C 113 (2009) 6706e
6718.
[12] M. Vaarkamp, H. Linders, D. Koningsberger, XDAP Version 2.2.7, XAFS Services
International, Woudenberg, The Netherlands, 2006. http://www.xsi.nl.
[13] J. Schrage, Fluid Phase Equilib. 68 (1991) 229e245.
[14] Y.C. Chiang, J.R. Ciou, Int. J. Hydrogen Energy 36 (2011) 6826e6831.
[15] Y. Izumi, K. Konishi, D. Obaid, T. Miyajima, H. Yoshitake, Anal. Chem. 79 (2007)
6933e6940.
[16] Y. Izumi, H. Nagamori, F. Kiyotaki, D. Masih, T. Minato, E. Roisin, J.P. Candy,
H. Tanida, T. Uruga, Anal. Chem. 77 (2005) 6969e6975.
[17] M. Morikawa, N. Ahmed, Y. Yoshida, Y. Izumi, Appl. Catal. B 144 (2014)
561e569.
[18] Y. Yoshida, Y. Mitani, T. Itoi, Y. Izumi, J. Catal. 287 (2012) 190e202.
[19] Y. Izumi, D. Obaid, K. Konishi, D. Masih, M. Takagaki, Y. Terada, H. Tanida,
T. Uruga, Inorg. Chim. Acta 361 (2008) 1149e1156.
[20] Y. Izumi, F. Kiyotaki, T. Minato, Y. Seida, Anal. Chem. 74 (2002) 3819e3823.
[21] Y. Izumi, F. Kiyotaki, H. Nagamori, T. Minato, J. Electron Spectrosc. Relat.
Phenom. 119 (2001) 193e199.
[22] H. Oyanagi, M. Ishii, C.H. Lee, N.L. Saini, Y. Kuwabara, A. Saito, Y. Izumi,
H. Hashimoto, J. Synchrotron Radiat. 6 (1999) 155e157.
[23] D.A. Evans, The Evans Group Website, Evans pKa Table, November 8, 2013.
http://evans.harvard.edu/pdf/evans_pKa_table.pdf.