Study of Nafion Membrane below Freezing Temperature By Using Near Infrared-Spectroscopy and Electrochemical Impedance Spectroscopy
Authors: Kouno Yuu , Matsushima Hisayoshi , Ohtsuka Toshiaki , Ueda Mikito 
Published: February 2020

Introduction

The conductive polymer is one of the important materials for polymer electrolyte fuel cell (PEFC). Nowadays many researchers have been developed the membrane in order to improve the durability and power performance of PEFC. Kunimatsu et al. analyzed Nafion membrane by FT-IR spectroscopy and reported that the electric conductivity depended strongly on the state of water molecule hydrated around a hydrophilic group[1]. In the membrane, the sulfonic acid groups are aggregated and form a nano-tube structure called “proton conducting channel”[2]. Water molecules confined in the nano-tube are considered to take various hydrogen-bond states (free water, bound water and unfreezing water)[3]. However, the relationship between water state and proton conductivity has not comprehensively investigated.
The in-situ measurement by near infrared spectroscopy (NIR) is superior to detecting the absorbance of the water molecules, since the other signals are not appeared in near infrared region. In this study, we apply NIR for analyzing the hydrated state of water molecules in Nafion below freezing temperature and simultaneously measure the in-plane resistance by the electrochemical impedance spectroscopy (EIS). The mechanism of proton conductivity is discussed by comparing NIR spectroscopy with Arrhenius plots.

Experimental

Nafion membrane (NRE-212) was set between two copper plates (3.5×1.0 cm2) for the in-situ transmission measurement of NIR. To penetrate the infrared incident light, the hole of 5 mm was made a hole at the center of the holder. The membrane was hydrated by exposing to humidified N2gas for 1 hour at 20 ̊C before measurement.
The NIR measurement was conducted by the use of NIR detector (Ocean Photonics Co., NIR256-2.1) and halogen light source which was connected with optical fibers. The range of wavenumber was from 7700 to 5000 cm−1 and the resolution was 4.0 cm−1. A cryostat using liquid nitrogen as a coolant was used to adjust the experimental temperature between −140 ̊C and 20 ̊C.
The impedance spectroscopy was carried out by frequency-response analyzer (Toyo Co., Solartron). The frequency was between 10 MHz and 50 mHz and the amplitude of ±100 mV. A two-terminal AC impedance method was employed and Au wires (φ=0.5 mm) were placed in contact with membrane as a sensor. The distance between two wires was 9 mm.

Results and discussion

Figure 1 (a) shows the NIR spectra during cooling and heating hydrous Nafion membrane. Two peaks attributed to vibration of water molecules were observed. The one arround 7000 cm−1 was assigned to combination tone of symmetric stretch and asymmetric stretch. The other arround 5200 cm−1was done to deformation vibration and asymmetric stretch. Both peaks were shifted to higher wavenumber with decreasing in temperature. This suggests that the individual O-H bonding force was weakened. The hydrated number per one water molecule was increased by lessening the thermal molecular motion at low temperature. This probably change increases the number of “bound water” and “unfreezing water”.
Figure 1 (b) shows the Arrhenius plots of the electrical conductivity obtained from EIS measurement. The conductivity was significantly decreased with decreasing in temperature. As indicated by the broken line in Figure 1 (b), the slope could be divided into three regions, i.e. first, second and third are from 20 ̊C to −40 ̊C, from −40 ̊C to −100 ̊C and from −100 ̊C to −140 ̊C, respectively. Activation energy at the first, second and third are 27 kJ mol−1, 52 kJ mol−1 and 23 kJ mol−1, respectively. By comparing NIR result, it was suggested that conduction mechanism of proton depended on the hydrated state such as “bound water” and “unfreezing water”.

Reference

[1] J. D. Kim, Y. Park, K. Kobayashi, M. Nagai, M. Kunimatsu, Solid State Ionics, 140 (2001) 313-325.
[2] W. Y. Hsu and T. D. Gierke, Journal of Membrane Science, 13 (1983) 307-326.
[3] N. Hara, H. Ohashi, T. Ito, and T. Yamaguchi, Journal of Physical Chemistry, 113 (2009) 4656-4663.

Figure 1