The Intricacies of Proton Transport: Structure vs. Vehicular Diffusion

Understanding the mechanisms behind proton transport is crucial, especially in contexts like fuel cell technology. Two primary diffusion mechanisms—structure diffusion and vehicular diffusion—play significant roles in how protons (H⁺) move through different environments. The contributions of these mechanisms vary based on numerous factors, including temperature, pressure, and the types of ions present in the medium.

Temperature impacts structure diffusion directly; as the temperature rises, the effect of structure diffusion diminishes. Conversely, increasing pressure enhances this mechanism's contribution, peaking at around 0.6 GPa. An important aspect to consider is the relationship between acid concentration and structure diffusion. As the concentration of acid increases, structure diffusion tends to decrease significantly. This phenomenon is attributed to alterations in hydrogen-bond patterns, where there are more proton donors than acceptor sites, thereby hindering the proton transfer process.

Moreover, the mobility of defect protons, such as hydroxide ions (OH⁻), is essential in alkaline environments, particularly for alkaline fuel cells. Contrary to what one might assume, the transport mechanism for defect protons differs from that of excess protons. Recent simulations have shown that hydroxide ions are coordinated with an average of 4.5 water molecules, creating a unique planar configuration that influences proton transfer dynamics. This "hyper-coordination" is believed to inhibit direct proton transfer, as it disrupts the typical bond angles necessary for effective movement.

Interestingly, while acidic solutions exhibit suppressed structure diffusion with increasing concentration, the transference number for OH⁻ in concentrated KOH solutions remains unexpectedly high, even at concentrations of about 3M. In pure water, excess and defect protons coexist at similar concentrations, but their diffusion can be considered quasi-independent due to their low ambient concentration.

Controversies still exist regarding the coordination states of hydroxide ions. While traditional views suggest tricoordinated OH⁻ as the predominant species in aqueous environments, newer spectroscopic studies may challenge this notion, particularly in concentrated solutions. These findings underscore the complex nature of proton transport and the need for continued research to unravel the various interactions at play in different chemical environments.