Researchers develop a new ultra-high-density sulfonic acid polymer electrolyte membrane for fuel cells, which can be used for vehicles and combined heat and power systems.
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A group of researchers in Japan have created a novel sulfonic acid polymer electrolyte membrane with ultra-high density, specifically designed for use in fuel cells.
A fundamental element of eco-friendly PEM fuel cells is the PEM, which produces electric power via a reaction between hydrogen and oxygen gases. FCVs and fuel cell CHP systems are among the operational fuel cells.
The most well-known PEM is a perfluorosulfonic acid polymer membrane, like Nafion, which DuPont created in the 1960s. It has a decent proton conductivity of 0.1 S/cm when moist and heated to 70-90 °C. In such conditions, the sulfonic acid groups can release protons. Proton conduction in these membranes is usually reliant on proton transport among protons, sulfonic acid groups, and water molecules. Ordinarily, a higher density of sulfonic acid groups in the membrane leads to a higher density of released protons, resulting in a higher proton conductivity.
However, it is challenging to create PEMs with a high sulfonic acid group density using traditional synthesis techniques. To increase the sulfonic acid group density in a poly(styrenesulfonic acid)-based PEM, the sulfonation reaction must be performed for extended periods or in severe conditions, often involving highly oxidizing substances such as fuming sulfuric acid and chlorosulfonic acid. Unfortunately, these conditions can result in unwanted side reactions, including backbone chain cleavage in the polymer. Consequently, commercially available PEMs are usually synthesized with a low sulfonic acid group density to avoid undesirable side reactions during polymer synthesis. The ion exchange capacity (IEC), which is an indicator of the acid group density, is generally less than 1.0 meq./g for commercially available PEMs like Nafion or poly(styrenesulfonic acid)-based Selemion by AGC.
In their article, Atsushi Noro and colleagues from Nagoya University's Graduate School of Engineering and Institutes of Innovation for Future Society, have produced a poly(styrenesulfonic acid)-based PEM with an ultra-high density of sulfonic acid groups. The PEM's IEC reached 5.0 mequiv./g, which is five times more than the typical commercially available PEMs, such as Nafion or Selemion. The PEM's proton conductivity at 80°C with 90%RH, a standard operational condition for polymer electrolyte fuel cells, was 0.93 S/cm. This conductivity is six times higher than Nafion's (0.15 S/cm) or Selemion's (0.091 S/cm) conductivity under the same test conditions.
As fuel cell technology advances, future fuel cells will need to operate under more demanding conditions, including higher temperatures and lower humidity levels. The research conducted by Noro and colleagues will aid in the creation and advancement of next-generation, higher-performance PEMs that demonstrate good conductivity of 0.1 S/cm or greater under such challenging conditions. This study will also assist in achieving the goal of a net-zero carbon society by promoting the use of environmentally friendly fuel cell technology.
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