A Perfect Sandwich

Scientists discover why the right amount of moisture in the membrane plays a key role in fuel cell efficiency

What makes a perfect sandwich? Besides good bread and a tasty combination of fillings and condiments, you need the right amount of moisture to convey the flavor in your mouth. If the sandwich is too dry, it may seem less flavorful, and if it is too soggy, the flavor may seem watered down.

The art of sandwich making may be far removed from the science and technology of hydrogen fuel cells, but in both cases, the amount of moisture in the sandwich is important. In a polymer electrolyte membrane (PEM) fuel cell, the electrolyte membrane is sandwiched between an anode (negative electrode) and a cathode (positive electrode), as shown in Figure 1. After the catalyst in the anode splits the hydrogen fuel into protons and electrons, the PEM transports the protons to the cathode, allowing the separated electrons to flow along an external circuit as an electric current. But the PEM needs the right amount of moisture for efficient proton transport — with too much or too little water, power output will drop.

Figure 1. Schematic diagram of a polymer electrolyte membrane fuel cell.

A fundamental understanding of the relationship between membrane nanostructure and the dynamics of water molecules is needed for the development of efficient, reliable, and cost-effective membranes to advance PEM fuel cell technology. The structure and dynamics of the polymer membranes under different levels of hydration cannot be directly observed in experiments, but they can be modeled in molecular dynamics simulations, as shown in a series of three papers published in the Journal of Physical Chemistry B by Ram Devanathan, Arun Venkatnathan, and Michel Dupuis of Pacific Northwest National Laboratory (PNNL).[1-3]

“Experimental studies are inadequate to understand proton dynamics, because it occurs below nanoscale,” said Devanathan. “This is where NERSC’s computing power becomes indispensable. By using advanced computer models, we are getting a grasp of the complex processes at the molecular level in polymer membranes.” The simulations for these three papers were run on Jacquard and Bassi.

The research is part of President Bush’s Hydrogen Fuel Initiative, which aims to develop commercially viable hydrogen fuel cells. Using this clean and efficient technology would help to reduce the world’s reliance on fossil fuels and lessen greenhouse gas emissions.

The PNNL researchers’ three Journal of Physical Chemistry B papers all studied a polymer membrane manufactured by DuPont called Nafion, which has been the subject of numerous experiments and is considered a good starting point for the development of next-generation polymer electrolytes. “Nafion 117 has excellent proton conductivity and good chemical and mechanical stability, but the atomic-level details of its structure at various degrees of hydration are not well characterized or understood,” the authors wrote in their first paper, which was featured on the cover of the journal’s June 28, 2007 issue (Figure 2).

Figure 2. The June 28, 2007 cover of the Journal of Physical Chemistry B showed snapshots of ionized Nafion and hydronium ions at various degrees of hydration: λ = 3.5 (a), λ = 6 (b), λ = 11 (c), and λ = 16 (d) at 350 K. The black area corresponds to the polymer backbone that is not shown. The pendant side chain (green), sulfonate (yellow and red), hydronium ions (red and white), and water molecules (steel gray) show the structural changes associated with changes in hydration. (Click either image for larger view)

Hydration and temperature

In this paper the researchers set out to create simulations that examined the impact of hydration and temperature on the positively charged hydrated protons and water molecules. Understanding these dynamics could lead to polymer membranes that are better engineered for transporting protons while controlling electrode flooding by the water molecules. One of the goals is to develop PEM membranes that need little water.

Using classical molecular dynamics simulations, the research team investigated the impact of four levels of hydration and two different temperatures. The scientists calculated structural properties such as radial distribution functions, coordination numbers, and dynamical properties such as diffusion coefficients of hydronium ions (H₃O⁺) and water molecules.

The results of their calculations showed that protons and water molecules are bound to sulfonate groups in the membrane at low hydration levels. As the hydration level increases, the water molecules become free and form a network along which protons can hop (Figure 2). This leads to a dramatic increase in proton conductivity. Temperature was found to have a significant effect on the absolute value of the diffusion coefficients for both water and hydronium ions. These findings have helped in interpreting experimental results that indicate a major structural change taking place in the membrane with increasing hydration.

Figure 3. Orthographic projection (~42 Å x 30 Å) of hydrated Nafion for the following λ values: (a) 3; (b) 5; (c) 7; (d) 9; (e) 11; and (f) 13.5. Water molecules, hydronium ions, sulfonate groups, and the rest of the membrane are represented in blue, red, yellow, and gray, respectively. (Click image for larger view)

In the second paper, the authors used all-atom molecular dynamics simulations to systematically examine eight different levels of membrane hydration to closely mirror two experimental studies. They also simulated bulk water to develop a novel criterion to identify free water in Nafion. This enabled them to quantify the fraction of free, weakly bound, and bound water molecules in the membrane as a function of hydration.

The researchers found that at low hydration levels, strong binding of hydronium ions to sulfonate groups prevents transport of protons. Multiple sulfonate groups surrounding the hydronium ions in bridging configurations hinder the hydration of the hydronium and the structural diffusion of protons (Figure 3). As the hydration level increases, the water molecules mediate the interaction between hydronium ions and sulfonate groups, moving them farther apart. These results provide atomic-level insights into structural changes observed in Nafion by infrared spectroscopy.

Structure and dynamics

In the third report, Devanathan, Venkatnathan, and Dupuis computed the dynamical properties of water molecules and hydronium ions in Nafion and related them to the structural changes reported previously. They confirmed other researchers’ finding that the behavior of water molecules within nanoscale pores and channels of PEMs, especially at low hydration levels, is remarkably different from that of molecules in bulk water.

At low hydration, fewer than 20% of the water molecules are free (bulklike). With increasing hydration, the diffusion coefficients of hydronium ions and water molecules increase, and the mean residence time of water molecules around sulfonate groups decreases. These results provide a molecular-level explanation for the proton and water dynamics observed in neutron scattering experiments.

Because the structure and dynamics of the membrane under different levels of hydration cannot be directly observed in experiments, there is no universally accepted model of the structure of Nafion. This research makes a significant step toward that goal and toward the development of the next generation of PEMs.

Characteristics of the ideal PEM include high proton conductivity at low hydration levels; thermal, mechanical, and chemical stability; durability under prolonged operation; and low cost. None of the existing membranes meet all these requirements, and developing new membranes requires a molecular-level understanding of membrane chemistry and nanostructure. Molecular dynamics simulations like these, together with experiments, are laying the foundation for future breakthroughs in fuel cells.

This article written by: John Hules and Ucilia Wang, Berkeley Lab.

[1] A. Venkatnathan, R. Devanathan, and M. Dupuis, “Atomistic simulations of hydrated Nafion and temperature effects on hydronium ion mobility,” J. Phys. Chem. B 111, 7234 (2007).
[2] R. Devanathan, A. Venkatnathan, and M. Dupuis, “Atomistic simulation of Nafion membrane: 1. Effect of hydration on membrane nanostructure,” J. Phys. Chem. B 111, 8069 (2007).
[3] R. Devanathan, A. Venkatnathan, and M. Dupuis, “Atomistic simulation of Nafion membrane. 2. Dynamics of water molecules and hydronium ions,” J. Phys. Chem. B 111, 13006 (2007).