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Proton conductivity of oligomeric poly[(1, 2‑propanediamine)‑alt‑(oxalic acid)] thin films onAl̲2O̲3 substratesAuthor(s)Nagao, Yuki; Naito, Nobuhiro; Iguchi, Fumitada;Sata, Noriko; Yugami, Hiroo

Citation

e‑Journal of Surface Science and Nanotechnology,
7: 530‑532

Issue Date

2009‑04‑18

TypeJournal Article

Text version

publisher

URL

http://hdl.handle.net/10119/10632

RightsCopyright (C) 2009 日本表面科学会. Yuki Nagao,Nobuhiro Naito, Fumitada Iguchi, Noriko Sata, andHiroo Yugami, e‑Journal of Surface Science andNanotechnology, 7, 2009, 530‑532.
http://dx.doi.org/10.1380/ejssnt.2009.530

Description

Japan Advanced Institute of Science and Technology

e-Journal of Surface Science and Nanotechnology

18 April 2009

Conference - ISSS-5 -e-J. Surf. Sci. Nanotech. Vol. 7 (2009) 530-532

Proton Conductivity of Oligomeric Poly[(1,2-Propanediamine)-alt-(Oxalic Acid)
Thin Films on Al2 O3 Substrates∗
Yuki Nagao,† Nobuhiro Naito, Fumitada Iguchi, Noriko Sata, and Hiroo Yugami
Department of Mechanical Systems and Design, Graduate School of Engineering,Tohoku University, 6-6-01 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan(Received 7 November 2008; Accepted 22 January 2009; Published 18 April 2009)
Proton transport properties of three thin films (110, 240, and 360 nm thick films) on R-plane (1102) sapphire
substrates for oligomeric poly[(1, 2-propanediamine)-alt-(oxalic acid)] were investigated. The maximum proton
conductivity of the thin film is 1.7 × 10−3 S cm−1 at the RH of 80%, which is five times higher than that of thebulk sample (3.0 × 10−4 S cm−1 ). The activation energies of the 110, 240, and 360 nm thick films are found to0.74, 0.67, and 0.68 eV at the RH of 80%, respectively. [DOI: 10.1380/ejssnt.2009.530]
Keywords: Electrical transport; Enhancement; Proton conduction

I.

INTRODUCTION

The design of highly proton-conductive solid electrolyte
is essential to many applications in the research field ofsolid state ionics [1, 2]. One of the fundamental methods to make highly proton-conductive material is chemical
modification. The properties of chemical stability and mechanical strength are often unfavorable by using chemical
modification like sulfonation, however the proton conductivity exhibits excellent value [3, 4]. Due to such a tradeoff relation between the proton conductivity, mechanicalstrength, and chemical stability, a new method to makehighly proton-conductive material has been expected fora long time.
The ionic conductivity enhancement in the 2-D system
has been known for more than 20 years [5–16]. These
anomalies are supposed to be attributed to interfacial
effects like structural effects and/or effects of a spacecharge region [17]. Recently we observed the enhancement
phenomenon of the proton conduction only by preparingorganic thin films on SiO2 substrates [18]. The maximum proton conductivity of the thin film exhibit 10 times
higher value than that of the bulk sample. However its
origin for the enhancement phenomenon of the proton
conduction is still unknown, it is supposed that applying this phenomenon is a candidate for a new method tomake highly proton-conductive material, which solves thetrade-off relation between the proton conductivity, mechanical strength, and chemical stability.
The amide groups exhibit the proton exchange property because of the amide/imidic acid tautomerism.
We have investigated a series of the proton-conductive
oligomers containing amide groups [18, 19]. In this paper,
we synthesized proton-conductive polyamide oligomers,
oligomeric poly[(1, 2-propanediamine)-alt-(oxalic acid)],
and investigated the proton transport properties of thethin film on another substrate (sapphire substrate) tostudy the enhancement of the proton conductivities.

∗ This paper was presented at International Symposium on Surface

Science and Nanotechnology (ISSS-5), Waseda University, Japan,
9-13 November, 2008.
† Corresponding author: y_nagao@energy.mech.tohoku.ac.jp

II.

EXPERIMENTAL

The synthesis of oligomeric poly[(1, 2-propanediamine)alt-(oxalic acid)] has been reported as shown in Scheme(Fig. 1) in previous study [18]. The quality of the sample was checked by the X-ray diffraction, FT-IR spectra,
1
H NMR, Matrix Assisted Laser Desorption/Ionization
Time of Flight (MALDI-TOF) mass spectrum [18]. The
synthesized sample is amorphous, and composed of a mixture of the oligomers with the different end groups andlow-molecular-weights including the macrocyclic amides.
Three thin films were prepared on R-plane (1102) sapphire substrates by using a Active ACT-200 Spincoater.
The thickness of the thin film was determined by usinga KLA-Tencor P-10 contact stylus profiler. The obtained
thin films are flat, and their thicknesses are 110, 240, and360 nm, respectively.
Impedance measurements (1 − 106 Hz) of the thin filmsof oligomeric poly[(1, 2-propanediamine)-alt-(oxalic acid)]
were carried out in the relative humidity (RH) of 80%
at 298 K with a Solartron 1260 Impedance/Gain-Phaseanalyzer and a 1296 Dielectric Interface system. The RH
and temperature were controlled with an Espec Corp. SH221 humidity- and temperature-controlled chamber. Theactivation energy was studied in the temperature rangeof 288-308 K. In the case of the impedance measurementsof the thin films, the electrode configuration was selected
to obtain measurements of the current flow in the plane
parallel to the substrate surface. The conductivity of theR-plane (1102) sapphire substrate is negligible because its
resistance is at least two orders of magnitude higher thanthat of the film over the film thickness range of 110-360
nm.

III.

RESULTS AND DISCUSSION

In the previous study, we reported that the oligomericpoly[(1, 2-propanediamine)-alt-(oxalic acid)] exhibitedthe proton conduction [18].
The typical complex
impedance plots of the bulk sample for oligomeric poly[(1,2-propanediamine)-alt-(oxalic acid)] under the RH of 80%
and 298 K are shown in Fig. 2. A resistance is directly
given by the intersection of a depressed semicircle withthe real axis. The obtained electrical conductivity at thec 2009 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)
ISSN 1348-0391 °530

e-Journal of Surface Science and Nanotechnology

Volume 7 (2009)

FIG. 1: Scheme: Synthesis of oligomeric poly[(1, 2-propanediamine)-alt-(oxalic acid)].

FIG. 2: Impedance plots of the bulk sample for oligomericpoly[(1, 2-propanediamine)-alt-(oxalic acid)] under the RH of80% and 298 K.

FIG. 3: Thickness dependence of the proton conductivity withbulk and thin films of oligomeric poly[(1, 2-propanediamine)alt-(oxalic acid)].

RH of 80% are found to be 3.0 × 10−4 S cm−1 .
Figure 3 shows thickness dependence of the proton conductivity (σ) for oligomeric poly[(1, 2-propanediamine)alt-(oxalic acid)] thin films on sapphire substrates. Theproton conductivity of the 110 nm thick film is found to be
1.7 × 10−3 S cm−1 at the RH of 80%. This value is foundto be five times higher than that of the bulk sample ofFIG. 4: Thickness dependence of the activation energy forthe proton conduction with thin films of oligomeric poly[(1,2-propanediamine)-alt-(oxalic acid)] on Sapphire substrates.

oligomeric poly[(1, 2-propanediamine)-alt-(oxalic acid)],which value is one order of magnitude lower than that ofNafion 117 membrane at the RH of 80% [20]. The proton
conductivities of the 240 and 360 nm thick films exhibit
1.7 × 10−3 and 1.1 × 10−3 S cm−1 at the RH of 80%, respectively. We observed the enhancement of the proton
conductivity by the synthesis of the thin films on Sapphire
substrates, however the maximum proton conductivity ofthe thin films on Sapphire substrates is relatively lower
value than that on SiO2 substrates (4.0 × 10−3 S cm−1 ).
Figure 4 shows thickness dependence of the activation energy for the proton conduction with thin filmsof oligomeric poly[(1, 2-propanediamine)-alt-(oxalic acid)]on Sapphire substrates. The activation energies of the110, 240, and 360 thick films are found to 0.74, 0.67, and0.68 eV, respectively. The activation energy for the proton conduction of the bulk sample is ∼0.8 eV at the RHof 80%. The activation energy of the thin films is foundto be lower than that of the bulk sample of oligomericpoly[(1, 2-propanediamine)-alt-(oxalic acid)]. In this system, water molecules play an important role in the proton
conduction from a result of relative humidity dependenceof the proton conductivity [18]. The water molecules canstrongly contribute to the proton conduction at the RHof 80%.
In many kinds of the inorganic ionic conductors, sometheoretical models including the space charge models forunderstanding the enhancement phenomena of the ionic

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Nagao, et al.

conductivities have been discussed [17]. Considering to
the contribution of the space charge model around the interface between the substrate and thin film, it is supposedthat the effective length of the space charge region is much
shorter than that of the thickness of the thin films in thissystem. However we have not yet estimated the quantitative contribution of space charge region at the interface,its degree of contribution is low in the thickness range of110-360 nm. The obvious thickness dependence has notbeen observed yet. Therefore it is difficult to explain thisresult according to the only space charge model. In thissystem, these results could be originated from the structural effects like a molecular orientation by preparing of
the thin film on the substrate rather than the interface
effects like the space charge models. The degree of themolecular orientation by the measurements of polarized
IR spectra is under investigation.
IV.

CONCLUSIONS

Proton transport properties of oligomeric poly[(1, 2propanediamine)-alt-(oxalic acid)] thin films on Sapphire[1] P. Colomban (Ed.), Proton Conductors (Cambridge University Press, Cambridge, 1992).
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532

substrates were investigated. The maximum proton conductivity of the thin film is 1.7 × 10−3 S cm−1 at the RHof 80%, which is five times higher than that of the bulksample (3.0 × 10−4 S cm−1 ). The activation energies of
the 110, 240, and 360 thick films are found to 0.74, 0.67,
and 0.68 eV, respectively. The activation energy of thethin films is found to be lower than that of the bulk sample. These results could be originated from the structuraleffects like a molecular orientation by the synthesis of thethin film on the substrate rather than the interface effects
like the space charge models in this system.
Acknowledgments

The authors thank M. Sasaki for helping the synthesisof the proton-conductive oligomers. This study was supported by Program for Improvement of Research Environment for Young Researchers from Special Coordination
Funds for Promoting Science and Technology (SCF) commissioned by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan.

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