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Title

Attribution of H+ Transportation of Nafion-Film on
Different Substrates Using Planar Inter-DigitatedElectrodes

Author(s)Bhardwaj, Rahul; Karan, Kunal; Nagao, Yuki

Citation

ECS Transactions, 109(9): 303

Issue Date2022-10Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18790

RightsCopyright (C)2022 ECS. Rahul Bhardwaja, Kunal
Karanb, and Yuki Nagao, ECS Transactions, 109 (9),2022, pp.303. This work is available under theCreative Commons Attribution-NonCommercialNoDerivatives 4.0 International license (CC BY-NCND 4.0) [http://creativecommons.org/licenses/bync-nd/4.0/]. This is the Accepted Manuscript
version of an article accepted for publication in
ECS.

Description

Japan Advanced Institute of Science and Technology

Attribution of H+ Transportation of Nafion-Film on Different Substrates Using
Planar Inter-Digitated Electrodes
Rahul Bhardwaja, Kunal Karanb, and Yuki Nagaoa*aSchool of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi city, Ishikawa, 923-1292, Japanb
Calgary Advanced Energy Storage and Conversion Research–Technologies (CAESRTech) Group, The University of Calgary, 2500 University Dr. NW, Calgary, T2N 1N4,
CanadaIn-plane proton transport property of Nafion thin-film on carbon andquartz was investigated in N2 environment by electrochemical
impedance spectroscopy (EIS) using interdigitated electrodes (IDEs)of platinum on quartz substrate. The Pt IDEs with and without carbon
pad were fabricated with precise geometry and excellent currentvoltage (I-V) response in few hundreds femtoampere (fA). The
morphology of spin-coated Nafion thin-film over IDEs was
confirmed to adapt the geometry of IDE structure. The EIS impedance
response of Nafion thin-film consists of single semi- circularcomponent in either IDEs. Since Pt-C IDE has both ionomer/quartz
and ionomer/carbon interface therefore H+ conductivity at quartz andcarbon was estimated from the quantitative attribution of H+ transport
resistance. The estimated H+ conductivity at quartz from Pt-C IDE
and Pt-Q IDE shows comparable conductivity values except the highrelative humidity (RH) and indicates that H+ transport resistance atcarbon substrate in Pt-C IDE is absent in N2 environment.

Introduction
Proton transport is a key functional property of the ionomer used in energy device (1–3).
Nafion is the most extensively employed ionomer which serves as free standing
membrane and as thin film coating the electrocatalyst in polymer electrolyte membrane-based fuel cell (PEMFC) (4–6). In the cathode catalyst layer of PEMFC, thin film ofionomer plays crucial role and determines the performance of fuel cell. In a fuel cell
catalyst layer, ionomer thin films coating the carbon-supported Pt-based catalysts (Pt/C)
is distributed in 3D spatially heterogeneous manner with variation in thickness andconnectivity (7). Recently, Nafion thin-film has attracted attention and have been
extensively investigated because of different phase segregated structure and protontransport property compared to thick Nafion membrane (8–18). It is now well establishedthat hydration and proton conduction properties of Nafion films are thickness- andsubstrate-dependent. Because of their non-uniform distribution (varying thickness andconnectivity) as well as interfacing with different substrates (carbon and Pt), it is difficult
to clarify the quantitative attribution of the true source of effective proton resistance.
Despite the tremendous significance of ionomer thin film in fuel cell and other energy
devices, the efforts to explore the quantitative attribution of substrate-dependance on H+
conductivity has not sufficiently advanced. Only a handful of studies had been reported
about the influence of substrate and confinement effect on the structure and property of
Nafion’s thin films specifically wherein conductivity of ionomer film of uniform andcontrolled thickness on well-defined planar substrate has been reported. These studies
have focused on understanding the morphology or proton conductivity of the Nafion thinfilms, the quantitative relationship between morphology and proton conductivity of the
Nafion thin films on Pt and/or carbon substrates has not been clarified (16,19–23).
Yamamoto’s group has investigated the substrate dependent H+ conductivity of uniform
and controlled Nafion thin film using Au-IDE with Pt and C pad in N2 environment andshows that Pt-supported Nafion thin-film exhibited higher H+ conductivity than carbon
supported Nafion films (22). Our group, Ono et. al., has also investigated the H+
conductivity at Pt and SiO2 substrate using parallel Au electrodes and concluded that H+
conductivity on Pt deposited surface was one order of magnitude higher than that on SiO2(23). Karan’s group has extended the investigation with more details using planar Pt-IDEs
in N2 and H2 environment and concluded that the protonic resistance of the ionomer thinfilm on top of conductive substrate is not accessible unless measurement was performed
under H2 environment using Pt electrodes (21,24). However, despite the larger area
between IDE was covered with substrate material (ionomer/substrate interface), theattribution of H+ transport at ionomer/quartz part was absent in their work.
In this work, we are using planar interdigitated electrodes of Pt with fixed thickness

of Nafion. Electrochemical Impedance Spectroscopy (EIS) was employed for impedance
measurement of the Nafion thin film on inter-digitated array (IDA) of Pt electrode withand without carbon pad between the electrodes. Film resistance as a function of relative
humidity was determined. The proton transport resistance from EIS was quantitatively
attributed based on the ionomer/substrate interfacial area to estimate the Nafion thin-film
proton conductivity over quartz and carbon substrate. A comparison of the conductivityof the Nafion thin film on carbon and quartz in N2 environment is reported and discussed.

Experimental Section
IDE Fabrication
A schematic fabrication process of the Pt IDEs with carbon pad (step 1-8) andwithout carbon pad (step 1, 6-8) is illustrated in Fig. 1, and referred to as Pt-C IDE and PtQ IDE respectively (Q stands for quartz). The IDE has 108 substrate carbon pads of 100µm width with 120 µm-pitch. Firstly, the quartz substrates (20 × 20 × 0.5 mm) werecleaned by sonication in acetone (EL grade) and isopropyl alcohol (EL grade) for 5 and
10 min each. The quartz substrates were cleaned with piranha solution followed by runningmili-Q (Ultrapure) water to remove deposited organic impurities only if the substrates
were used for high temperature pyrolysis process. The 1,1,1,3,3,3-hexamethyldisilazane(HMDS) solution was spin coated to improve the adhesion before photoresist coating.
The 30% v/v AZP 4210 photoresist (diluted with AZ EBR7030 solvent) was spun coatedat 3000 rpm for 150 s and pre-annealed at 110℃ for 90 s. To prepare the carbon pad patternout of photoresist film, a maskless aligner system (MLA150 HEIDELBERG-instruments)
was used for photolithography and then developed in NMD-3 reagent for 55 s. The quartz
with photoresist pattern was plasma exposed in O2 for 20 s at 15 W to remove residual
resist nanofilm between patterns. The patterned photoresist was carbonized at 1000 ℃ in
2% H2/N2 environment. The quartz substrate with carbonized patterns was again exposed
to plasma for 20 s at 15 W. After that, the electrodes were prepared by the lift-off process.
The LOL-2000 and TSMR-8900 (20cp) photoresist was spin coated at 3000 rpm for 30 s
and dried at 110℃ for 90 s before lithography. The Pt was physically deposited by
sputtering at 100 ℃ for 12 min with Ti (~2-5 nm) as adhesion layer.

Figure 1. A schematic step by step illustration of IDEs fabrication process.
Thin-Film Preparation
The 5wt% Nafion dispersion stock solution (DE521 CS type; Wako Pure Chemical
Industries Ltd.) was diluted to 1.2wt% in isopropyl alcohol and sonicated for 30 min. The
dispersion was equilibrated for 24 h. Before ionomer coating, IDEs were again washedwith acetone and IPA by sonication followed by plasma treatment (Cute-MP; Femto
Science, Korea), and ionomer dispersion was ultrasonicated for 15 min. The Nafion’s thinfilm was prepared from diluted dispersion concentration by spin-coating method (ACT200; Active) at room temperature. After coating, Nafion’s film outside the IDE structure
was removed manually with a swab using Mili-Q water and film was dried at 40℃ undervacuum overnight.
IDE Characterizations and Thin-Film Thickness Measurement
After fabrication, IDE alignment precision was confirmed using an optical
microscope (Company name). The I-V response in the range of 0-1 V using
semiconductor parameter analyzer (E5250A Agilent) and a measurement probe (TN-1910
Vector Semiconductor) was recorded to check the resistance of interdigitated electrodes

with and without carbon pad. The thickness of carbon pad, Pt electrode and ionomer thinfilm coated on IDEs was measured using white light interferometric microscope (BWS506; Nikon Corp.) and AlphaStep D-100 surface profiler (KLA Tencor).
Nafion Thin Film Conductivity Measurement
A small humidity box was newly constructed for thin-film H+ conductivity
measurement in a humidified N2 environment. All measurements were conducted attemperature 25 ℃ (controlled using a computer-controlled chamber SH-221; Espec Corp.)
under various RH generated by separate humidity generator (me-40DP-2PW; Micro
Equipment Inc.). For thin-film impedance measurement, a two-probe method was applied
to obtain in-plane proton conductivity parallel to the thin-film. Thin gold wires (dia 0.5µm,
Y-soft Tanaka Denshi Kogyo K. K. Japan) were used to connect probes with Pt electrodes
contact pad using porous gold paste (SILBEST No. 8560; Tokuriki Chemical Research
Co. Ltd.). Impedance data were collected at alternating amplitude frequency of 50 mV andfrequency range from 10 MHz and 0.1 Hz. The Nafion thin-film resistance was obtained
from collected impedance data using the semi-circle fitting method and line fitting method
in ZView software (version 3.3f). The H+ conductivity of thin film (σ) was calculated fromthe resistance (R) measured on IDEs using the following equation [1].
𝜎=𝑑R∗L∗(N−1)∗w[1]

In the above equation [1] denotes the gap between Pt electrodes (110 µm), L is dry
thickness of Nafion thin film (~55 nm), N is the number of electrodes (109), and w is
width of Pt electrode (8 mm).
Before the measurement was initiated, the film was dried completely at 40 ℃
overnight under vacuum to remove moisture from the film. During the vacuum drying,
the relative humidity inside the vacuum chamber was approximately 3%. The in-plane
proton transport impedance of Nafion film was measured at various relative humidity
starting from 20% up to 95% using humidity generator in N2 atmosphere. Before EIS
measurement, IDEs with Nafion thin film was kept in dry N2 environment for 1h toremove any remaining moisture from the film. During dry N2 flow, humidity reaches to
5% and expected to remove moisture from chamber and film. The humidity was increased
sequentially starting from 20% and impedance was recorded at each RH until stabilized.

Results and Discussions
IDE Fabrication
The optical microscope images of fabricated IDE with their specification arepresented in Figure 2. Neither apparent micro-defect nor cracks were observed on carbon
pad and Pt confirming that the device structure was successfully fabricated. Moreover,precise alignment of the device was achieved. The carbon pads have length (L) and width(w) of 8 mm and 100 µm respectively and precisely aligned between Pt IDE at 5 µm
distance from each Pt electrode of 10 µm width. This clean and precise fabrication of
IDEs is an important feature to achieve the reproducibility of Nafion thin film H+
conductivity. The current-voltage response of IDEs (with and without carbon pad) in
range of 0-1 V was recorded ~100 fA (Data is not shown here). It shows high resistanceof fabricated IDEs with Pt electrodes distanced at 110 µm, and the introduction of
conductive carbon pad distanced at 5 µm from Pt electrodes at both sides doesn’t influence
the I-V response. The I-V response of IDEs also supports the high resistance of quartz
substrate between electrodes. The thickness of the carbon pad and sputtered Pt electrode
was measured using a surface profiler and showed 30 nm and 35 nm respectively. Top
photograph of the Pt-C and Pt-Q IDE is also shown in Fig. 2.

Figure 2. Optical microscope images of different parts from fabricated IDEs withgeometry measurement and photograph of Pt-C IDE and Pt-Q IDE.

Nafion Thin-Film Characterization
The Nafion thin film of ~55 nm thickness was optimized from various concentration
dispersions and film thickness was confirmed by white light microscope and surface
profiler (data not shown here). After spin coating over IDE, film thickness andmorphology of thin film were analyzed using surface profiler (applied stylus force of 0.3
mg). Figure 3 (a & b) shows the illustration of expected and measured Nafion film
morphology over the Pt-C IDE. In the illustrations (Fig. 3), Nafion film is shown over
IDE structure only because Nafion film other than IDE structure was manually removed
using swab and mil-Q. The Nafion thin film was expected to have flat morphology over
IDE however the measured film morphology was surprisingly seeming to adapt the exact
morphology of IDEs. It is the first time to report the morphology of Nafion film over an
IDE for H+ conductivity measurement. In surface profiler data (Fig. 3c), the thickness of
Nafion film at the edge is slightly thicker resulting from film accumulating during swab
removal. The sharp peaks shown in Nafion film surface profile is the result of systemmeasurement error. Film thickness over IDE was determined to be around 60 nm fromprofiler data (confirmed by white light microscope, data not shown here).

Figure 3. (a & b) Illustrations of Nafion’s thin film expected morphology and obtained
film morphology on IDEs. (C) Nafion thin film’s thickness and morphology over IDE
measured by surface profiler with stylus force of 0.03 mg with 0.1 mm/sec speed rate.

Impedance Measurement of Nafion Film using IDEs
The impedance response of Nafion’s thin film consists of an inductive part, a
semicircle at the high-frequency region (corresponds to H+ resistance at substrate) and a
vertical-line like response in the low frequency region (corresponds to infinite diffusion)(Fig. 4).

Figure 4. The Nyquist plots of Nafion thin film prepared on Pt-C IDE and Pt-Q IDE. The
EIS measurement was performed at 25℃ and various RH conditions in N2 environment.
The magnified plots are shown in figure insets at RH 58%, 80% and 95%.
The Nyquist plots show that the Nafion thin-film H+ transport resistance in Pt-Q
IDE is high compared to the Pt-C IDE at the same RH. In Pt-C IDE Nyquist plot, at high
RH (80% & 90%) no semi-circle is observed in impedance plot which indicates high H+
transport in Nafion film at carbon substrate since most area between the Pt electrodes iscovered with carbon (91% area). In the Nyquist plots of Pt-C IDE, it is expected that there
should be two responses corresponding to the ionomer/quartz (~9%) and ionomer/carbon(91%) interfaces. Approximately, 91% of the area between the Pt electrodes is carbon(100 µm) and the rest is quartz (10 µm) compared to Pt-Q IDE where the whole area
between Pt electrodes is quartz (110 µm). So, the obtained H+ transport impedance from
Pt-C IDE could be attributed to quartz and carbon in a ratio of 1:9. Figure 5 shows theillustration for the attribution of obtained impedance in both IDEs. To obtainthe H+ conductivity of Nafion thin-film on quartz and carbon pad in Pt-C IDE, it isimportant to normalize the obtained resistance values according to ionomer/substrateinterfacial area.

Figure 5. The Nyquist plot and illustrations of Pt-C IDE and Pt-Q IDE at 38% RH in
N2 environment and the attribution of H+ transport impedance.
Firstly, the Nafion’s H+ conductivity at quartz from Pt-Q IDE and normalized H+
conductivity at quartz from Pt-C IDE was estimated. In case of Pt-C IDE, to estimate thenormalized H+ conductivity at quartz and carbon, the d value (distance between electrode)
was considered to be 10 µm (i.e., the distance representing the ionomer/quartz interfacialarea) and 100 µm (i.e., the distance representing the ionomer/carbon interfacial area)
respectively. Figure 6(a) illustrates the IDEs structure and equation for H+ conductivity
estimation with normalized d value for both IDEs. Figure 6(b) shows averaged H+
transport resistance obtained by ZView fitting from Pt-Q IDEs and Pt-C IDEs with error
bars. The proton transport resistance from IDE with carbon pad is approximately one
order of magnitude lower than the IDE with quartz interface. It suggests that the Nafion
H+ transport property is affected by the substrates and supports the previous research on
the substrate dependent changes in thin-film H+ transport (21–23,25). To understand theattribution of enhanced H+ transportation in Pt-C IDE, the H+ conductivity at quartz andcarbon from impedance data was estimated using normalized d value and compared with
the H+ conductivity on quartz from Pt-Q IDE. The H+ conductivity at quartz in Pt-C IDE
and Pt-Q IDE is shown in Fig 6(c). Interestingly, the Nafion’s H+ conductivity values atquartz are comparable in each IDEs at RH < 90% however at 90% and 95% RH, the H+
conductivity in Pt-C IDE does not increases expectedly and shows saturation.

Figure 6. (a) Shows the equation and illustration for the normalization of H+ conductivity
at quartz in Pt-Q IDE and Pt-C IDE. (b) The averaged H+ transport resistance of Nafion
thin film in IDEs with error bars (EIS measurement was recorded with two separate IDEs).
(c) The normalized H+ conductivity (averaged) at quartz in Pt-C IDE compared with H+
conductivity at quartz from Pt-Q IDE with error bar (EIS measurement was recorded with
two separate IDEs).
The H+ conductivity value at quartz from Pt-Q IDE (at 95% RH) and Pt-C IDE (at
94% RH) is 15 mS cm-1 and 4.9 mS cm-1 respectively. The reason for lower H+
conductivity at quartz in Pt-C IDE at high RH (>90%) is not clear. Further, the Nafion H+
conductivity at carbon from Pt-C IDE was estimated from H+ transport resistance using
equation [1] with normalized d value (100 µm) according to the carbon/ionomer
interfacial area. Figure 7 shows the H+ conductivity at quartz in Pt-Q IDE and comparedwith the normalized H+ conductivity at carbon in Pt-C IDE. The graph shows highH+ conductivity at carbon substrate at the same RH. The proton conductivity of Nafion’s
thin film at carbon substrate was more than an order of magnitude higher than that ofquartz substrate in low RH (20%). At 95% RH the conductivity at carbon stabilizes andreaches to 49 mS cm-1 compared to H+ conductivity at quartz in Pt-Q IDE which shows
conductivity value of 15 mS cm-1. The H+ conductivity value at quartz at high RH (95%)
is matched with our lab’s previous work (23). Devproshad K. Paul et. al., and our group

Yutaro Ono et. al., previously reported the H+ conductivity of Nafion film of thickness 455 nm at quartz with Au-Q IDE and Nafion film of 75 nm thickness on parallel electrodes
respectively, showed similar H+ conductivity at high RH (95%) (23,24). However, at low
RH (20% and 40%) the H+ conductivity value at quartz substrate shows slightly lower
values in presented result (Fig. 7) than the previous research (24). Since the thin-film
preparation method and choice of electrode material is different, it is hard to explain thereason for slightly lower conductivity at low RH (20% and 40%). On the other hand, H+
conductivity at carbon was also compared with previous work using Au-C IDEs of
comparable geometry (22). Our results show approximately one-order magnitude higher
H+ conductivity on carbon substrate. The H+ conductivity strongly correlated with thewater uptake content however the water uptake content of Nafion thin film over carbon
and quartz substrate do not show very high-water uptake content (21). Thus, it can beconcluded that the water uptake of Nafion film on SiO2 and carbon are not remarkably
different to explain the increase in conductivity.

Figure 7. (a) The normalized H+ conductivity at carbon in Pt-C IDE and at quartz in PtQ IDE with error bar. Normalization distance between Pt electrodes is mentioned infigure text.
Similarly, Gao et. al., estimated H+ conductivity of Nafion thin-film (50 nm) over

carbon substrate using Au-C IDE of comparable geometry in N2 environment and showsan order magnitude lower of H+ conductivity at 95% RH (22). Interestingly at low RH(20%), there is approximately two order magnitude difference in H+ conductivity in our
results than the reported values from Gao et. al. work. Although this comparison of
H+ conductivity at carbon is not completely reasonable as electrode material, Nafion
dispersion solvent, IDEs geometry, quality of carbon pad is different (resistance andstructure) and thin-film pretreatment condition is unclear.
Comparing our results with previous work on H+ conductivity at carbon substratefrom one of coauthors’ work, the significant increase in H+ conductivity in Pt-C IDE
cannot be explained (21). Moreover, the normalized H+ conductivity at quartz from Pt-C
IDE and H+ conductivity at quartz in Pt-Q IDE shows similar values. It indicates that theobtained H+ transport resistance from Pt-C IDE is truly H+ transport resistance at quartz
only, and the normalized resistance is comparable with the resistance from Pt-Q IDE
which is also supported by the work recently published by Karan’s group (21). They
concluded that in N2 environment, the H+ transport resistance over carbon substrate isabsent and can only be obtained in H2 environment using Pt IDE. Therefore, H+
conductivity measurement of Nafion’s thin-film using either Pt-Q IDE or Pt-C IDE resultsin H+ conductivity over quartz substrate. In our results, the reason for lower H+
conductivity at high RH is unknown and will be further investigated with various carbon
pad widths to understand the effect of carbon pad on Nafion H+ conductivity in detail.

Conclusions
Substrate dependent H+ conductivity of Nafion thin film has the potential tounderstand the attribution of elements of the catalyst layer for fuel cell performanceenhancement. Previous studies have shown the H+ conductivity of Nafion on Pt, C andquartz using Au and Pt based IDEs however the attribution of elements is still poorly
understood. In this work, in-plane proton transport of Nafion thin film was investigated
and the attribution of H+ transportation on carbon and quartz using Pt-C IDE
and Pt-Q IDE in N2 environment was proposed. The Pt IDEs were fabricated with highprecision and reproducible geometry. The current-voltage response of IDEs in range of 01 V was found to be in femtoampere (fA). For the first time, the morphology of spin-coated
Nafion thin- film was shown and it is accustomed to the similar IDE morphology. H+
transport of spin coated Nafion thin film (~55 nm) was measured by EIS in N2 environment.
The attribution of H+ transport resistance was quantitatively decided to calculate the H+
conductivity in Pt-C IDE. The normalized H+ conductivity was obtained from resistanceof Pt-C IDE Nyquist plots based on carbon/ionomer and ionomer/quartz interfacial area
between electrodes. The comparison of estimated H+ conductivity at quartz from Pt-C

IDE after normalization (d value = 10 µm), and H+ conductivity from Pt-Q IDE shows
comparable values except at the high RH. The H+ conductivity at carbon substrate cannot
be estimated as the H+ transport resistance at carbon is absent in N2 environment, and thiswork was supported by the previous research from Kunal’s group. The result supports thequantitative attribution of resistance to estimate the H+ conductivity at quartz substrate.
Nafion’s H+ conductivity at carbon can be estimated from the measurements under H2environment and will be estimated in future work.

Acknowledgments
This work was supported by research funding from JSPS KAKENHI Grant
Number JP21H00020, The Murata Science Foundation, and JST CREST Grant Number
JPMJCR21B3.

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