Quantitative and Structural Analysis of Water Molecules in Anion Exchange Membrane Affecting the Fuel Cell Performance
Authors: Shirase Yuto , Inukai Junji , Ohara Koji , Nagai Tetsuro , Yoshida Koji , Miyatake Kenji 
Published: December 2024

Introduction
Anion-exchange membrane fuel cell (AEMFC) has a potential use of non-precious metal catalysts under an alkaline environment. Furthermore, anion-exchange membranes (AEMs) are also expected to be used for water electrolysis. One of the disadvantages of AEMFC is a lower performance at low relative humidity (RH). One of the reasons for this decrease in performance at low RH is the decrease in conductivity of the AEM and the increase in polarization of each electrode, mainly due to the decrease in back-diffusion of water. In order to solve this problem, it is necessary to elucidate the water behavior inside the AEM.
Experiment
As an AEM, QPAF-4 developed in the University of Yamanashi was used. Water uptake and swelling measurements were combined to investigate the quantitative behavior of water. Neutron reflectometry was conducted to analyze the water content associated with the humidification inside QPAF-4. Synchrotron radiation X-ray scattering was used to analyze the chemical bonding between water molecules inside QPAF-4. Small angle X-ray scattering (SAXS) was carried out to analyze the change in membrane structure associated with humidification inside QPAF-4. Conductivity measurements in the gas phase were carried out at different RHs.
Result & Discussion
From the measurements of the water uptake (The counter ion of QPAF-4 is Cl- ) and swelling (The counter ion of QPAF-4 is OH- ), it was understood that the water swelling abruptly increased at the RH higher than approximately 70%, whereas the water uptake steadily increased with increasing the RH. Therefore, there expected was a change in water states in the QPAF-4 membrane at high RHs.
Fig. 1 shows the neutron reflectometry (The counter ion of QPAF-4 is Cl- ). At low RHs, the topmost sublayer was relatively humidified. Only at 90% RH, all sublayers in the membrane were hydrated. This result suggests that at 90% RH, the state of water in the membrane was different from those at lower RHs.
Fig. 2 shows the radial distribution function of the QPAF-4 at different RHs obtained by synchrotron radiation X-ray scattering (The counter ion of QPAF-4 is Cl- ). A peak around 2.8 Å changed, showing the change in the interactions between OH in water molecules. The intensity of this peak increased with increasing RH. The peak width also changed with increasing RH especially from 70% to 90%RH, showing that at 90% RH, bonds between water with different distances appeared.
Fig. 3 shows the results of SAXS (The counter ion of QPAF-4 is Cl- ). The scattering intensity and d-value of hydrophobic domain increased as the RH increased, but the difference was pronounced at 90% RH. Therefore, the periodicity and the repeating distance of the hydrophobic domain abruptly changed at 90% RH.
Fig. 4 shows the results of OH- conductivity of QPAF-4 (The counter ion of QPAF-4 is OH- ). At low relative humidity, the conductivity did not change. Around 90%RH, the conductivity significantly increased. Therefore, the behaviors of water and conductivity were closely related. The hydrophilic paths and the states of water effective for the OH- might effectively appear at a high RH of 90%.

Figure 1