Mechanistic Insights into Proton Transport in Pure and Aqueous Phosphoric Acid

The transport of protons plays a key role in a variety of electrochemical processes and technologies. There are two major mechanisms underlying proton transport: the vehicular mechanism where a protonic defect moves with the aid of a molecular entity; and the ‘Grotthuss mechanism’ where a proton diffuses by being transferred between ‘chains’ of host molecules via elementary reactions within the hydrogen bond networks. The latter is regarded as the most efficient proton conductivity mechanism. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has thus far been reported. Recently, we observed the first direct experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid by employing dielectric spectroscopy, and quasi-elastic neutron scattering (QENS). Complementary to these experiments, ab initio molecular dynamics (AIMD) simulations were performed to gain molecular insight into the underlying mechanisms of proton transport allowing for the breaking and forming of covalent bonds since it calculates the forces from electronic structure methods ‘on the fly’. We calculated the self-intermediate scattering function of the protons, indicating three processes at high Q range consisting of two fast Q-independent processes and one slow Q-dependent process, which are in good agreement with QENS results. Also, the self-part of van Hove function of the protons for the fast characteristic time shows surprisingly short proton jumps of only ~0.5 - 0.7 Å confirmed by analysis of an individual proton trajectory. Furthermore, AIMD demonstrates that proton hopping plays an essential role in the long-range proton diffusion through the breaking and forming of hydrogen bonds. Our analysis confirmed the strong proton-proton correlations in these proton jumps. However, this correlated proton transport unexpectedly leads to a decrease of conductivity in these systems. Based on these results, we propose that the expected Grotthuss-like enhancement mechanism of conductivity cannot be realized in bulk liquids where ionic correlations always reduce conductivity, most likely because of the requirement for the momentum conservation. Our findings will propel the design of electrochemical materials with proton transfers involved in charge transport for achieving high proton conductivity in electrochemical applications including fuel cells.