In a research funded by the New Energy and Industrial Technology Development Organisation (NEDO), scientists at Nagoya University in Japan have developed poly(styrenesulfonic acid)-based PEMs with a high density of sulfonic acid groups.
A polymer electrolyte membrane (PEM), which generates electrical energy through the interaction of hydrogen and oxygen gases, is a key component in eco-friendly polymer electrolyte fuel cells. Fuel cell vehicles (FCVs) and fuel cell combined heat and power (CHP) systems are examples of real-world fuel cell uses.
The membrane made of a perfluorosulfonic acid polymer, such as Nafion, which was created by DuPont in the 1960s, is the most well-known PEM. In humid environments, it exhibits a good proton conductivity of 0.1 S/cm at 70–90 °C (158–194 °F). Sulfonic acid groups may release protons under these circumstances.
Proton transport between protons, sulfonic acid groups, and water molecules is typically what drives proton conduction in these membranes.
larger proton conductivities are typically the result of larger sulfonic acid group densities since these groups can release protons at higher densities the higher the density of sulfonic acid groups in the membrane.
However, it is challenging to make PEMs with a high density of sulfonic acid groups using a typical synthetic method. For instance, the sulfonation reaction must be carried out over a lengthy period of time or in harsh conditions in order to increase the density of sulfonic acid groups in a poly(styrenesulfonic acid)-based PEM. Most frequently, it makes use of highly oxidising materials like fuming sulfuric acid and chlorosulfonic acid.
Unfortunately, this has unfavourable side effects, such as breakage of the polymer's backbone chains. Therefore, commercially produced PEMs are often synthesised to have a low density of sulfonic acid groups in order to prevent undesirable side reactions during polymer synthesis.
The PEM had an IEC of 5.0 mequiv/g. The IEC of common commercially available PEMs like Nafion or Selemion is five times lower than this. It had a proton conductivity of 0.93 S/cm at 80 °C and 90% RH, which is a typical working temperature and humidity for polymer electrolyte fuel cells. Under the identical measurement conditions, this conductivity is six times greater than that of nafion (0.15 S/cm) or selemion (0.091 S/cm).
Future fuel cells will need to operate in more demanding settings, like those requiring greater temperatures and lower humidity.
With the help of this work, next-generation, higher-performance PEMs with strong conductivity of at least 0.1 S/cm under such harsh conditions will be synthesised and developed. The research will also help us get closer to becoming a net-zero carbon society.