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Title

OH^- Conductive Properties and Water Uptake ofAnion Exchange Thin Films

Author(s)Wang, Fangfang; Wang, Dongjin; Nagao, Yuki

Citation

Chemsuschem, 14(13): 2694-2697

Issue Date

2021-04-29

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18096

RightsThis is the peer reviewed version of the following
article: Copyright (C) 2021 Wiley-VCH. Fangfang
Wang, Dongjin Wang, Yuki Nagao, Chemsuschem,
14(13), 2021, pp.2694-2697, which has been
published in final form at
https://doi.org/10.1002/cssc.202100711. This
article may be used for non-commercial purposes inaccordance with Wiley Terms and Conditions for
Use of Self-Archived Versions. This article may not
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Description

Japan Advanced Institute of Science and Technology

COMMUNICATION
OH- Conductive Properties and Water Uptake of Anion Exchange
Thin Films
Fangfang Wang, Dongjin Wang, and Yuki Nagao*
F. Wang, D. Wang, Dr. Y. Nagao
School of Materials Science, Japan Advanced Institute of Science and Technology
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
E-mail: ynagao@jaist.ac.jp
Supporting information for this article is given via a link at the end of the document.

forms is higher than that of FAA3 bulk membrane.[16] Kimura et al.
studied the interface between hydrated BAF-QAF (Cl- form) anion
exchange ionomer on SiO2 and Pt substrates using neutronreflectometry.[17]Elucidating the OH- conductive properties of thin films isimportant. However, experiments to assess OH- conductivity
measurement are more challenging than other anion conduction
experiments in thin films because they must be shielded from theinfluence of CO2 from the air in all processes from film preparation
to measurement. No report of the relevant literature has describeda study of OH- conductive properties in the thin film form of anion
conductive polymers. For this study, we established in situ OHconductivity and water uptake measurements in thin film form.
Diverse backbones and cationic groups were developed toobtain high anion conductivity and good alkaline stability
AEMs.[18–25] Recently, fluorene-based polymers have been used
extensively as AEMs.[26–31] Lee et al. reported poly[(9,9-bis(6′(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4benzene)] (PFB+) (Figure 1) as a high-performance AEM. PFB+
bulk membrane displayed high OH- conductivity at 30°C in waterand good chemical stability (1 M NaOH at 80°C).[26] Consequently,
for this study, the OH- form of PFB+ was used as a model ionomer.
The objective of this study is clarification of the OH- conductive
properties and water uptake in thin film forms. In situ quartz crystal
microbalance
(QCM)
and
impedance
spectroscopy
measurements were established under different RH. Allprocesses including transfer of samples and measurements wereconducted in N2 atmosphere. Higher OH- conductivity than Brconductivity in the thin film form of AEM was confirmed. The OHconductivity of 273-nm-thick PFB+ film shows a comparable value
to the reported value of PFB+ bulk membrane. Both reduced OHconductivity and water uptake were observed in the thinner PFB+
film.

Abstract: Several investigations have indicated that proton
conduction and hydration properties of acidic ionomers differ fromthose of membranes. However, relations between the OHconductivity and water uptake in thin film forms of anion exchange
membranes have not been reported yet. For this study, new in situ
measurements were established to elucidate the OH- conductivity andwater uptake without allowing any influence of CO2 from the air.
Poly[(9,9-bis(6′-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt(1,4-benzene)], denoted as PFB+, was synthesized as a modelionomer. The highest OH- conductivity of 273-nm-thick PFB+ film is
5.3 × 10-2 S cm-1 at 25°C under 95% relative humidity (RH), which iscomparable to the reported OH- conductivity of PFB+ membrane.
Reduced OH- conductivity was found in the thinner film at 95% RH.
The decreased OH- conductivity is explainable by the reduced numberof water molecules contained in the thinner film. The OH- conductivity
was reduced only slightly under the same water uptake.

Fuel cells, electrochemical devices that produce electricalenergy from chemical reactions, have potentially higher electricalefficiency than conventional power-generating systems. Becauseof their alkaline operating conditions, anion exchange membrane
fuel cells (AEMFCs) offer advantages of using non-precious metal
catalysts and higher oxygen reduction reaction (ORR) kinetics
than proton exchange membrane fuel cells (PEMFCs). [1]Anion exchange membranes (AEMs) are a key component of
AEMFCs: they allow hydroxyl ions to migrate from the cathode to
the anode. In fact, OH- ion conduction and hydration properties ofbulk AEMs have been investigated widely.[2–8] At the electrodes,
the electrochemical reactions of fuel cells occur on the triplephase interface, which comprises ion conductive ionomer,
catalyst, and fuel/oxidant. The ionomer, which serves as a binderand ion conduction channel in this triple-phase interface, iscrucially important for enhancing the electrochemical
performance of fuel cells. Therefore, understanding the OH- ion
transport properties and water uptake in the thin film form of AEMs
is important.
It is particularly interesting that the proton transport
properties and water uptake in thin film are completely different
from bulk membrane properties. Thin-film confinement can affectwater uptake and proton conductivity by changing ionic-domain
morphology through polymer–substrate interaction.[9–13] High
proton conductivity can be achieved by the organized structure inalkyl sulfonated polyimide (ASPI) thin films with humidity-induced
lyotropic liquid crystalline (LC) property.[14,15] Several studies haveexamined anion (F-, Cl-, Br-, and HCO3-) transport properties ofanion conducting ionomers.[16,17] Shrivastava et al. demonstrated
that anion conductivity of FAA3 thin film in F-, Br-, and HCO3-

Figure 1. Chemical structure of PFB+X- (X=Br and OH).

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lower than OH- conductivity. These results indicate successful
measurement of the OH- conductivity of PFB+ thin film in this study.
We confirmed that OH- conductivity is higher than Br- conductivity
in the thin film form by developing the RH in situ measurement.
Lee et al. reported that the OH- conductivity of PFB+ bulk
membrane is 5.0 × 10-2 S cm-1 at 30°C in water.[26] Our OHconductivity of PFB+ thin film is comparable to the reported OHconductivity of PFB+ bulk membrane. For the transport of F-, Br-,and HCO3- ions, thin films have higher ionic conductivity than thatreported for bulk membranes.[16] This report is the first
demonstrating that, through transport of OH- ions, a thin film canhave comparable OH- conductivity to that of a bulk membrane.

The synthetic route of monomer and polymers is shown in
Scheme S1. Briefly, poly[(9,9-bis(6-bromohexyl)-9H-fluorene)-alt(1,4-benzene)] (PFB) was synthesized using a Pd-catalyzed
Suzuki cross-coupling reaction. The weight-average molecularweight (Mw) of PFB was found to be 32 kDa, as estimated using
gel permeation chromatography (GPC). The Br- form of PFB+ was
obtained after the quaternization reaction of PFB. The chemical
structures of PFB and PFB+ were confirmed from 1H NMR and
FTIR spectra (Figures S2 and S3).
Water uptake of the OH- form of anion conductive polymers
is known to be higher than that of Br- form of bulk membrane. [32,33]
Nevertheless, no report describes thin film forms of AEMs. The
water uptake of PFB+ thin films in both Br- and OH- forms was
confirmed using our RH-controlled in situ QCM. Figure 2 depicts
the RH dependence of water uptake of PFB+ thin films in Br- andOH- forms at 25°C. Actually, λ (number of water molecules perfunctional group) has been used widely to describe the water
uptake. For both Br- and OH- forms of PFB+ thin films, the water
uptake increased concomitantly with increasing RH. The water
uptake increased slowly at lower RH. Sharp enhancement isvisible in the range of higher RH (>80%). The water uptake of
PFB+ thin film in OH- form was higher than that of PFB+ thin filmin Br- form, especially for 80–95% RH. The water content of PFB+
thin film in OH- form at 95%RH was 77 wt%, which is comparable
to that of bulk membrane (71 wt% at 30°C in water).[26]Figure 3. Anion conductivity of 273 nm thick PFB+ films as a function of water
uptake at 25°C.

To illustrate the relations between OH- conductivity and water
uptake, a thinner, 30-nm-thick film in OH- from was newly
prepared. Figure 4 presents the RH dependence of OHconductivity of PFB+ thin films at each thickness at 25°C. The OHconductivity of 30-nm-thick film was lower than that of 273-nmthick film in the RH range of conductivity measurements. The
highest OH- conductivity of 30-nm-thick film was 3.5 × 10-2 S cm-1
at 95%RH.
Figure 2. Water uptake of 273 nm thick PFB+ films as a function of RH at 25°C.

Figure 3 portrays the Br- and OH- conductivity of PFB+ thinfilms as a function of water uptake at 25°C: Br- and OHconductivity increased concomitantly with increasing water
uptake. The Br- and OH- conductivities respectively reached 2.2 ×
10-2 and 5.3 × 10-2 S cm-1 at 25°C under 95%RH. The OHconductivity of PFB+ thin film was much higher than Brconductivity at high hydration conditions. Because of the higher
mobility in dilute aqueous solutions and higher dissociation
degree of OH-, higher OH- conductivity than Br- conductivity wasobserved.[33,34]
The OH- ions are converted easily to CO32- and HCO3- ions
during the conductivity measurement process.[35] Therefore, theRH dependence of HCO3- conductivity at 25°C was also
investigated (Figure S6). Results show that the conductivity of
PFB+ thin film was lower in its HCO3- form than in its OH- form
within the RH range of conductivity measurement. The highest
HCO3- conductivity is 1.7 × 10-2 S cm-1 at 95%RH, which is much

Figure 4. OH- conductivity of PFB+ thin films in different thicknesses as afunction of RH at 25°C.

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Figure 5 presents the water uptake of PFB+ thin films of theOH- form with different thicknesses as a function of RH.
Compared to the 273-nm-thick film, a lower water uptake result
was observed for 30-nm-thick PFB+ thin film. The reduced ionic
conductivity in thinner films is also apparent in the proton
conduction of acidic ionomers. The decreased conductivity ofacidic ionomers in thinner film can be derived from the reduced
water uptake.[36] For further clarification of the relation between
OH- conductivity and water uptake, the OH- conductivities of PFB+
thin films for the respective thicknesses are shown in Figure 6 asa function of water uptake. Similar curves were obtained for both273-nm-thick and 30-nm-thick films. As one might expect, the273-nm-thick PFB+ film included more water molecules than the
30-nm-thick PFB+ film. As the number of water molecules
contained increased, the OH- conductivity of PFB+ thin film also
increased. Given the same water content amount, the OHconductivity was barely reduced with decreasing film thickness.
The results presented above indicate clearly that the lower water
uptake is directly responsible for the reduced OH- conductivity inthinner PFB+ films, as reported also for acidic ionomers.

For this work, PFB+ was chosen as a model ionomer. The OHconductivity and water uptake of PFB+ thin films were investigatedat 25°C under various RH using newly established in situ
measurements. Results confirmed the higher OH- conductivity
than Br- conductivity in the thin film form of PFB+. The 273-nmthick PFB+ film showed a comparable OH- conductivity value tothe reported value of PFB+ bulk membrane. As the PFB+ thin film
thickness decreased from 273 nm to 30 nm, the OH- conductivity
decreased from 5.3 × 10-2 S cm-1 to 3.5 × 10-2 S cm-1. Moreover,a lower water uptake result was observed in the thinner PFB+ film.
Results revealed that the decreased OH- conductivity in thinner
PFB+ film is explainable by the reduced number of watermolecules contained in the thinner film. The OH- conductivity was
reduced only slightly under the same number of water molecules.

Acknowledgments
This work was supported by research funding from JSPS
KAKENHI Grant Number JP18K05257, 21H00020, and
21H01997.
Keywords: OH- conductivity • Water uptake • Thin film
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Entry for the Table of Contents
Insert graphic for Table of Contents here.

Poly[(9,9-bis(6′-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4-benzene)] (PFB+) was used as a model ionomer to investigate
the OH- conductivity and water uptake in thin film form. Reduced OH- conductivity was observed in the thinner film at 95% relative
humidity, which can be attributed to the decreased amount of water content. The OH- conductivity was only slightly reduced under thesame water uptake.
Institute and/or researcher Twitter usernames: Nanoprotonics

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