Proton transport in aqueous acid solutions

Revealing the mechanisms of proton transport is a fundamental problem critical for many energy storage and conversion applications. In this study we present comprehensive and comparative analysis of proton transport in well-known model systems: pure phosphoric acid and aqueous solutions of sulfuric and phosphoric acids [1, 2]. We employed various spectroscopic techniques covering vast frequency range such as quasielastic neutron scattering, broadband dielectric spectroscopy, light scattering, rheology, and PFG-NMR measurements combined with ab initio molecular dynamic simulations.

Analysis of the results shows that in all systems, proton transport occurs through very short ‘jumps’ about ~0.5Å, and that structural relaxation is much slower than the proton jump rate. There are fundamental differences in proton transport in phosphoric and sulfuric acid systems. One of the key differences is that the proton hopping is coupled to structural relaxation in the aqueous solution of sulfuric acid, while these processes are decoupled in phosphoric acid. Neutron scattering indicates that at least one proton per sulfuric molecule remains slower than other protons in the system, while all protons have the same fast mobility in phosphoric acid systems. The analysis also reveals that proton correlations suppress conductivity, suggesting that the well-known Grotthuss-like enhancement of conductivity is unlikely to occur in bulk liquids.