Reactive Grotthuss Transport of H₃O⁺ and OH^- Through Aqueous Solutions: Proton Donors Behind the Scenes

Hydronium (H₃O⁺) and hydroxide (OH^-) are fundamental acidic and basic water ions present in the aqueous solutions ubiquitous in chemistry, biology, and materials science. These water ions can transport at anomalously high mobilities compared with similar-sized ions via structural diffusion, in which a water ion exchanges a proton (thus switching identities) with one of its hydrogen-bonded solvent H₂O molecules. Such reactive structural/Grotthuss diffusion not only plays a fundamental role in the aqueous acid-base chemistry but also provides unique avenues to transport charge carriers (e.g., in proton-exchange membrane and anion-exchange membrane fuel cells). The Grotthuss mechanism is typically understood via the presolvation concept, which is centered around the similarity in the solvation structures between the proton acceptors and their products after the proton transfer. In this work, we will uncover the hidden role of the proton donors in Gortthuss diffusion using deep potential machine-learning force fields trained at the van der Waals inclusive hybrid density functional theory (DFT). To achieve this reliable level of theory, we devised a recently developed SeA framework to enable accurate, efficient, and high-throughput hybrid DFT evaluations for these large-scale condensed-phase systems. By incorporating the role played by the proton donors, we will present a unified quasiparticle theory of this reactive transport process for these fundamental water ions.