Researchers, led by a team from the University of Pennsylvania, have used a polymer-folding mechanism to develop a new and versatile kind of solid polymer electrolyte (SPE) that currently offers proton conductivity faster than Nafion by a factor of 2, the benchmark for fuel cell membranes.

As reported in a paper in Nature Materials, the layered sulfonated polyethylene-based structure offers an innovative and versatile design paradigm for functional polymer membranes, opening doors to efficient and selective transport of other ions and small molecules on appropriate selection of functional groups.

The researchers’ new structure self-assembles into hairpin shapes, resulting in acid-lined channels that allow for efficient transport of protons across the electrolyte.

Proton exchange membrane fuel cells depend on membranes such as Nafion to transport protons between electrodes while providing a mechanical and electronic barrier. Nafion has a complex multiscale phase-separated morphology with well-connected hydrophilic domains in which sulfonate-lined water channels percolate through a hydrophobic, semicrystalline polytetrafluoroethylene-like matrix. While the exact structure remains controversial, the water channels are thought to be nominally cylindrical. For decades, researchers have sought to develop new membranes with lower production cost, higher operating temperature, lower operating humidity and other desirable attributes, using percolating hydrated domains as a design rule. Most of these new polymers have amorphous, poorly controlled morphologies like Nafion, and are typically not on par with Nafion’s performance. In striking contrast, controlled hairpin polymer folding is a cutting-edge, versatile strategy featuring a highly ordered morphology that holds promise for the development of new membranes to efficiently transport protons, ions and even small molecules.

… Our innovative approach controls polymer folding to achieve a desirable, well-ordered, highly crystalline morphology with high proton conductivity. The success of this approach provides striking new insight into the design of proton—or other—ion-conducting synthetic membranes. Control of chain folding and morphology is made possible by precise control of the chain microstructure via acyclic diene metathesis synthesis. … Here, sulfonic acid groups, which are highly hygroscopic moieties, replace carboxyl groups, producing hydrated layers with high proton conductivity. Our simulations show that the ordered, layered structure enhances diffusion relative to the tortuous water channels of an amorphous polymer. The layered poly-ethylene-based structure is a new design paradigm for functional polymer membranes, opening doors to efficient and selective transport of protons, ions and small molecules on appropriate chemistry selection.

—Trigg et al.

The study was led by Karen I. Winey, TowerBrook Foundation Faculty Fellow, professor and chair of the Department of Materials Science and Engineering, and Edward B. Trigg, then a doctoral student in her lab. Demi E. Moed, an undergraduate member of the Winey lab, was a co-author.

They collaborated with Kenneth B. Wagener, George B. Butler Professor of Polymer Chemistry at the University of Florida, Gainesville, and Taylor W. Gaines, a graduate student in his group. Mark J. Stevens, of Sandia National Laboratories, also contributed to this study, as well as Manuel Maréchal and Patrice Rannou, of the French National Center for Scientific Research, the French Alternative Energies and Atomic Energy Commission, and the Université Grenoble Alpes.

Nafion, which is widely used in proton-exchange membrane fuel cells, is a sheet of flexible plastic that is permeable to protons and impermeable to electrons. After absorbing water, protons can flow through microscopic channels that span the film.

A thin, SPE like Nafion is especially enticing for fuel cells in aerospace applications, where every kilogram counts. Much of the bulk of portable batteries comes from shielding designed to protect liquid electrolytes from punctures. Systems using liquid electrolytes must separate the electrodes further apart then their solid electrolyte counterparts, as metal build-up on the electrodes can eventually cross the channel and cause a short. Nafion addresses those problems, but there is still much room for improvement.

Nafion is something of a fluke. Its structure has been the subject of debate for decades, and will likely never be fully understood or controlled.

—Karen Winey

Nafion is hard to study because its structure is random and disordered. This fluorinated polymer occasionally branches off into side chains that end with sulfonic acid groups. It’s these sulfonic acids that draw in water and form the channels that allow for proton transport from one side of the film to the other. But because these side chains occur at random positions and are of different lengths, the resulting channels through the disordered polymer are a twisty maze that transports ions.

With an eye toward cutting through this maze, Winey’s group recently collaborated with Stevens to discover a new proton-transporting structure that has ordered layers. These layers feature many parallel acid-lined channels through which protons can quickly flow.

This new structure is the result of a special chemical synthesis route developed by Wagener’s group at the University of Florida. This route evenly places the acid groups along a polymer chain such that the spacing between the functional groups is long enough to crystalize. The most detailed structural analysis to date was on a polymer with exactly 21 carbons atoms between carboxylic acid groups, the polymer that initiated the Penn-Florida collaboration a decade ago.

While Winey’s group and Stevens were working out the structure and noting it’s potential for transporting ions, Wagener’s group was working to incorporate sulfonic acid groups to demonstrate the diversity of chemical groups that could be attached to polyethylenes. Both teams realized that proton conductivity would require the stronger acid.

Precisely placing the sulfonic acid groups along polyethylene proved to be our biggest synthetic challenge. Success finally happened in the hands of Taylor Gaines, who devised a scheme we call ‘heterogeneous to homogeneous deprotection’ of the sulfonic acid group ester. It was this synthetic process which finally led to the formation of the precision sulfonic acid polymers.

—Kenneth Wagener

The details of this process were also recently published in the journal Macromolecular Chemistry and Physics.

With the chains forming a series of hairpin shapes with a sulfonic acid group at each turn, the polymer assembles into orderly layers, forming straight channels instead of the tortuous maze found in Nafion.

The disordered structure of Nafion, left, means the path protons take through the electrolyte is hard to predict or control. The researchers’ new structure, right provides a straighter path. (Nafion illustration adapted from Kreuer. J., Membr. Sci. 2001, 185, 29–39, Fig. 2)

The group’s next step is to orient these layers in the same direction throughout the film.

We’re already faster than Nafion by a factor of two, but we could be even faster if we aligned all of those layers straight across the electrolyte membrane.

—Karen Winey

More than improving fuel cells where Nafion is currently employed, the crystallization-induced layers described in the researchers’ study could be extended to work with functional groups compatible with other kinds of ions.

Better proton conduction is definitely valuable, but I think the versatility of our approach is what is ultimately most important. There’s still no sufficiently good solid electrolyte for lithium or for hydroxide, another common fuel cell ion, and everyone who is trying to design new SPEs is using a very different approach than ours.

—Karen Winey

At the University of Pennsylvania, this study was supported by the National Science Foundation through grants DMR 1506726 and PIRE 1545884, and by the Army Research Office through grant W911NF-13-1-0363.

Resources

• Edward B. Trigg, Taylor W. Gaines, Manuel Maréchal, Demi E. Moed, Patrice Rannou, Kenneth B. Wagener, Mark J. Stevens & Karen I. Winey (2018) “Self-assembled highly ordered acid layers in precisely sulfonated polyethylene produce efficient proton transport” Nature Materials doi: 10.1038/s41563-018-0097-2

• K.D. Kreuer (2001) “On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells,” Journal of Membrane Science Volume 185, Issue 1, Pages 29-39 doi: 10.1016/S0376-7388(00)00632-3