Molecular analysis of hydrogen-bond structures in polymer electrolyte membrane in polymer electrolyte fuel cells below freezing temperatures
Authors: Nishizawa Hiroki , Mabuchi Takuya , Uene Naoya , Tokumasu Takashi 
Published: February 2024

Fuel cells that generate electricity by using hydrogen and oxygen have been attracted to achieve carbon neutralization in the world because fuel cells don’t produce the greenhouse gases like carbon dioxide. The Polymer Electrolyte Fuel Cell (PEFC) has been used for many applications, such as cars and home stationary power supplies. Protons are transported in the polymer electrolyte membrane (PEM) and the transportation efficiency affects power generation efficiency. The internal state of the PEM has a great impact on the proton transport; however, the state is still not unclear below freezing temperatures because contained water might be frozen. Below freezing temperatures, proton-hopping, which is the mediation of proton conduction by water molecules, is important for proton transport. Proton-hopping is the mediation of proton conduction by water molecules. Therefore, we aim to understand the internal state of PEM below freezing temperatures.
We performed reactive force-field molecular dynamics (ReaxFF MD) simulations to incorporate the proton-hopping. Our simulations consisted of two parts. First, bulk water was simulated at different temperatures to find the melting point of water by ReaxFF MD simulation. Radial distribution function (RDF) was used to find out whether the water is frozen or not. RDF of bulk water have a peak. When water is frozen, RDF of water have two distinct peaks and water molecules do not have translational or rotational motion. The size of the simulation box was 30.0 Å×30.0 Å×30.0 Å and the number of water molecules is 865. The temperature of the system was set at 1 K, 100 K, 200 K and 300 K. The time step was set at 0.50 fs and the sampling interval was 0.50 fs and the sampling interval was 100 steps. Second, the water condition in Nafion membrane was simulated. We used 4 Nafion polymer chains, 40 hydronium ions and 120 water molecules (water content: λ=4). The time step was set at 0.25 fs and the sampling interval was 100 steps. System of hydronium ions, water and Nafion changes faster than bulk water system and requires shorter timesteps. The temperature of the system was set at 300 K and 200 K. The annealing process was applied in order to equilibrate the system. After annealing, the production run was 250 ps using NVT ensemble. We performed the same analysis of the first simulation, and we analyzed the condition of water in the Nafion membrane.
The RDF from the first simulation is shown in left figure. This figure shows that the two peaks were clearly visible at 1 K, 100 K and 200 K, although the second peak was barely visible at 300 K. Based on these results, the melting point of water in the ReaxFF potential was expected between 200 K and 300 K. The results of the second simulation are shown on the right figure. The results show that the heights of the two peaks are higher at 200 K than at 300 K. From these results, we could not find out whether the water in Nafion membrane was frozen or not only by these analysis methods.
In this study, the melting point of water in the ReaxFF potential was found, and the state of water in polymer below freezing temperatures was analyzed. It was discussed in a paper by Qiangu Yan et al. According to them, water inside the Nafion membrane can be divided into three states. These are free water, bound water, and antifreeze water. While free water freezes, bound water has a lower melting point than normal water, and antifreeze water is not freeze. The specific heat of water in polymers should be analyzed in the future. Proton diffusion considering proton-hopping should also be analyzed in the future.

Figure 1