Understanding Proton Conductivity in Polymers: The Role of Acids

Proton conductivity is a crucial property in various applications, particularly in fuel cells and electrochemical devices. Recent studies have revealed that the structure of the surrounding chemical groups, such as the sulfonic acid in Nafion and phosphoric acid in polybenzimidazole (PBI) complexes, significantly impacts the mobility and dissociation of protons. This is vital for enhancing the efficiency of these materials in conducting protons.

Nafion, a well-known proton-conducting polymer, shows how changes in chemical groups surrounding sulfonic acid can hinder proton transport. For instance, substituting a perfluoro methylene with an aromatic carbon alters the dissociation dynamics of protons from the acidic group. The dielectric constant of the surrounding environment, particularly the water of hydration, also plays a key role in this process. As the hydration level varies, the dielectric properties and proton charge carrier concentration fluctuate, impacting overall conductivity.

In contrast, complexes of PBI and phosphoric acid are emerging as highly efficient proton conductors. Phosphoric acid not only provides high mobility for protons but also maintains a high concentration of intrinsic protonic defects. This unique combination allows for effective proton conduction even at varying levels of polymer content. Moreover, interactions between PBI and phosphoric acid lead to the formation of stable hydrogen-bonded complexes, enhancing the overall conductivity of the system.

Research indicates that as the concentration of phosphoric acid increases within these complexes, the conductivity approaches that of pure phosphoric acid. This trend demonstrates that the polymer itself does not significantly participate in the conduction process. Instead, the mobility of protonic charge carriers remains considerably higher than that of phosphate species, allowing for effective transport even in high acid concentrations.

Investigations into the macroscopic transport properties of these polymer-acid systems reveal that the proton mobility and self-diffusion coefficients decrease with increasing polymer content. This phenomenon is akin to the behavior observed in hydrated acidic polymers, where the percolation within the liquid-like portion of the acid domain diminishes, leading to lower conductivity. It highlights the intricate balance between polymer structure, acid concentration, and the resultant transport properties.

Understanding these complex interactions is essential for advancing the development of new materials with improved proton conductivity. As researchers continue to explore the chemical and physical properties of these systems, the potential for enhanced applications in energy conversion and storage technologies remains promising.