“Precise” Polymers Promote Fast Proton Transport

Original paper: Self-Assemble Highly Ordered Acid Layers in Precisely Sulfonated Polyethylene Produce Efficient Proton Transport

Global climate change has necessitated the development of ways to harvest electricity from renewable sources, such as the wind and the sun. However, because the wind isn’t always blowing and the sun isn’t always shining, we need to store some of the harvested energy for later use. We can store this energy by converting it into fuels such as methanol and hydrogen, but we need a way to convert it back into electricity when it’s needed. One device that allows us to do this is the fuel cell.

Figure 1: Example proton exchange membrane fuel cell. Hydrogen gas mixed with water vapor enters on the left-hand side of the device and reacts to produce protons and electrons. The protons drift rightward through the center of the device to react with oxygen to produce water. [1]
Fuel cells are electrochemical devices that can convert a stored simple fuel, like hydrogen or methanol, directly into electricity. Because these fuels can be stored and easily converted near homes, vehicles, and businesses, fuel cells can be used to produce energy on demand. One type of fuel cell is the proton exchange membrane (PEM) fuel cell, which uses hydrogen and oxygen gases to make electricity and water. Hydrogen gas reacts at the anode (the negatively charged electrode) to lose its electrons, forming protons (denoted H+ in Figure 1). The electrons flow first through the fuel cell’s load (e.g. a home or business), powering it, and subsequently to the fuel cell’s cathode. The protons must drift across the proton exchange membrane to meet the electrons and oxygen to form water.

One of the greatest challenges of making PEM fuel cells is designing the membrane after which they are named. Engineers would love to have a membrane that transports protons quickly and efficiently. Although there are polymeric materials (e.g. Nafion) that can do this fairly well, there is still a need for faster transport. This week’s paper investigates the role of a polymer’s “precise” structure in facilitating fast proton transport.

Designing polymers, which are large molecules consisting of repeating units called monomers, with a given role relies on interspersing functional groups along the polymer chain. These functional groups are atoms that may contain some important features, such as electrical charge, necessary for the role of polymer. Under most synthetic schemes, the spacing between these functional groups is random. In the case of a typical proton exchange membrane, such as Nafion, negatively charged groups known as sulfonic acids love to interact with each other and with water present in the fuel cell, forming channels that facilitate proton transport, as shown in Figure 2.

Figure 2: Structure of (left panel) disordered and (right panel) ordered polymer materials. Black lines are the polymer backbone, yellow spheres are sulfonic acid groups, blue spheres are water molecules, and green spheres are water molecules with protons attached. Adapted from current work.

However, because the locations of these groups along the polymer chain are random, each chain cannot organize uniformly, forming disordered channels (Figure 2 left panel). In contrast, Edward Trigg and coauthors were able to attach these sulfonic acids at controlled, repeating locations along the polymer chain. Because the sulfonic acid groups are uniformly dispersed along the polymer (every twenty one carbon atoms to be exact), the chains can fold precisely in the same way, forming a uniformly layered structure and ordered channels filled with protons and water and lined with sulfonic acid (Figure 2 right panel).

The authors of this work first compared this material with Nafion, a widely used PEM polymer that is currently one of the best performing materials available. Like Nafion, this material readily absorbs water into the channels formed by the sulfonic acid groups from the air. The water widens the channels and transports protons more quickly as a result. At high levels of humidity, the new polymer performs as well as Nafion. However, as mentioned earlier, Nafion’s structure is amorphous and disordered because of the random placement of its functional groups (see Figure 2 left panel). Why then is a membrane structure so different from Nafion able to transport protons just as well?

The authors used simulation to answer this question. Specifically, they examined the dynamics of water contained in channels. Faster proton transport is facilitated by faster water dynamics. They compared the ordered and disordered versions of their polymer. The water dynamics in the ordered structure were faster than those in the disordered structure, suggesting slower proton transport in the disordered material. They attributed the slower water dynamics to the nonuniform size and the poor connectivity of the water channels in the disordered structure. These findings suggest that PEM membrane materials like Nafion can be further improved by creating ordered, rather than disordered, channels.

In short, Edward Trigg and his colleagues opened a new potential path to better performing fuel cells through the design of a well-ordered polymeric proton exchange membrane. They demonstrated that the ordered polymeric structure within their PEM leads to faster proton transport than in a disordered version of the structure. With further refinement of the synthesis techniques, membranes like these may yield faster proton transport than is currently achievable, leading to exceptional performance in PEM fuel cells. With better performance, PEM fuel cells may be more readily available to quickly convert stored energy for use in domestic and industrial applications when renewable sources are not immediately available.

[1] https://en.wikipedia.org/wiki/Fuel_cell

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