States and Distributions of Water inside a Membrane of a PEMFC Above 100°C at Constant Water Vapour Pressure Studied Using Raman Spectroscopy
Authors: Dzramado Eric Selorm , Wakolo Solomon Wekesa , Inukai Junji 
Published: December 2024

Introduction

High-temperature applications of PEMFCs under low humidity conditions have the potential to decrease the cost of heavy-duty vehicles, like trucks and buses. Water plays a crucial role in PEMFCs as it assists in the transportation of protons from the anode to the cathode for the redox reaction. To better understand the behaviour of water states and their distribution inside the membrane, a study was conducted using Raman Spectroscopy. This study was done to observe the different states of water in the Nafion 211 (25 µm) membrane at a constant water vapour of 36.85 kPa at cell operating temperatures ranging from 80 to 110°C.

Experiment

A single-cell fuel cell setup was constructed for this study, as illustrated in Figure 1. A commercial Nafion 211 (25 µm) was placed between two (2 cm x 2 cm) 22BB gas diffusion layers (GDLs). One of the GDLs had a centred 0.5 mm diameter hole, while the other had a thin film sheet of platinum with the same diameter. Both the anode and cathode were supplied with humidified nitrogen gas in a crossflow manner at a dew point temperature of 74.65°C, equivalent to a water vapour pressure of 36.85 kPa. The experiment was conducted at a range of temperatures (80, 90, 95, 105, and 110°C) and corresponding relative humidities of (80.0, 54.7, 44.4 30.9, and 26.0%) respectively with Raman Spectroscopy (LabRAM HR 800) measurements taken at a depth of 15 µm, or at the centre of the membrane using the laser wavelength of 633 nm[1]. Before each measurement, the membrane stability was ensured for 3 hours. The exposure time for measurement was 90 s.

Results and Discussion

The spectrum was first baseline corrected and smoothened using the LapSpec software. It was then normalized to show the different intensities of the O-H stretching band of water between the Raman shift of 3000 to 3800 cm-1 as shown in Figure 2. Using the Gaussian function, the O-H stretching band of water present in the Nafion 211 membrane was deconvoluted with Igor Pro 8 software.

Five major peaks were identified, including two species of protonated water - Eigen (H3O+) and Zundel (H5O2
+) - at 3080 and 3345 cm-1 respectively. Another peak was found at 3326 cm-1, which indicates H bonding to the sulfonic acid group (SO3H). Additionally, there was a peak at 3471 cm-1, indicating H-bonded water (H2O --- H2O) in bulk water, and a peak at 3663 cm-1, which indicates non-H-bonded water [2], [3]. The relative area, AOH/ACF, of the O-H stretching band of water was determined and plotted against the cell temperatures. Using the gravimetric water uptake obtained from Nafion 211, the number of water molecules per sulfonic acid group (λ) was determined, and each water component λi is expressed by λi = λTotal × fi

Upon analysing the data, it was observed that the number of water molecules per sulfonic acid group (λi) of protonated water, hydrogen bonded to the sulfonic acid group (SO3H), and that of non-hydrogen bonded water remained relatively consistent through the temperature changes. However, the number of water molecules hydrogen bonded to one another, forming H2O---H2O bonds, decreased from λ = 5 at 80°C to λ = 1.6 at 110°C illustrated in Figure 3. In conclusion, we succeeded in determining the states of water existing in Nafion membranes at temperatures higher than 100°C.

References

[1] M. Hara, J. Inukai, K. Miyatake, H. Uchida, and M. Watanabe, “Temperature dependence of the water distribution inside a Nafion membrane in an operating polymer electrolyte fuel cell. A micro-Raman study,” Electrochim. Acta, 58, 449 (2011).

[2] M. A. Barique, E. Tsuchida, A. Ohira, and K. Tashiro, “Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion,” ACS Omega, 3, 349 (2018).

[3] H. Nishiyama, S. Takamuku, K. Oshikawa, S. Lacher, A. Iiyama, and J. Inukai, “Chemical States of Water Molecules Distributed Inside a Proton Exchange Membrane of a Running Fuel Cell Studied by Operando Coherent Anti-Stokes Raman Scattering Spectroscopy,” J. Phys. Chem. C, 124, 9703 (2020).

Figure 1