Unraveling the Proton Transfer Mechanism in Water

The study of proton conductors, particularly in the context of fuel-cell applications, reveals fascinating insights into the mechanisms governing proton transfer in water. At the heart of this process are two major configurations: the Zundel and Eigen complexes. While simulations suggest that both configurations occur comparably, experimental data indicates a slight stabilization of the Eigen complex by about 2.4 kJ/mol. Despite this minor difference, the fundamental characteristics of the proton transfer mechanism remain intact.

One significant aspect of proton transfer lies in the potential energy surface associated with contracted hydrogen bonds. In both Zundel and Eigen complexes, the time-averaged potential surfaces exhibit near symmetry, allowing for the proton to hover close to the bond center. The proton's location occasionally shifts due to the surrounding hydrogen-bond pattern, emphasizing the dynamic nature of this transfer process. This phenomenon is often described as "structured diffusion," where the protonic defect moves in tandem with the breaking and forming of hydrogen bonds.

As the proton traverses through these hydrogen-bonded networks, it does so almost adiabatically, influenced primarily by the solvent’s characteristics. The low energy barriers associated with this transfer mechanism allow for a classical description of the proton's motion, with quantum tunneling playing a minimal role. Interestingly, this defect can become delocalized across multiple hydrogen bonds due to quantum fluctuations, further showcasing the complexity of proton dynamics in water.

Traditional models, such as the Grotthuss mechanism, suggest a concerted transfer of protons within elongated hydrogen-bonded chains, which accounts for the high conductivity of protons in aqueous environments. However, recent findings challenge this notion, indicating that the energy required to create a corresponding dipolar moment in a high dielectric constant environment is too significant for a fast, low-energy process. Instead, the propagation of a protonic defect in low-dimensional water structures appears to be a hybrid of both concerted and step-wise mechanisms.

Moreover, the rapid diffusivity of water contributes to the overall proton conductivity, as the movement of protonated water molecules—like hydronium ions—plays a crucial role. The diffusion coefficient of water at room temperature is noteworthy, allowing for a substantial contribution (up to 22%) from protonated water molecules to the total conductivity. Nonetheless, the strong hydrogen bonding present in the first hydration shell may hinder the diffusion of these protons, adding another layer of complexity to the understanding of proton transfer in liquid water.