Computational Discovery of Hydrogen Bond Design Rules for Electrochemical Ion Separation

Selective ion separation is a major
challenge with far-ranging
impact from water desalination to product separation in catalysis.
Recently introduced ferrocene (Fc)/ferrocenium (Fc+) polymer
electrode materials have been demonstrated experimentally and theoretically
to selectively bind carboxylates over perchlorate through weak C–H···O
hydrogen bond (HB) interactions that favor carboxylates, despite the
comparable size and charge of the two species. However, practical
application of this technology in aqueous environments requires further
selectivity enhancement. Using a first-principles discovery approach,
we investigate the effect of Fc/Fc+ functional groups (FGs)
on the selectivity and reversibility of formate–Fc+ adsorption with respect to perchlorate in aqueous solution. Our
wide design space of 44 FGs enables identification of FGs with higher
selectivity and rationalization of trends through electronic energy
decomposition analysis or geometric hydrogen bonding analysis. Overall,
we observe weaker, longer HBs for perchlorate as compared to formate
with Fc+. We further identify Fc+ functionalizations
that simultaneously increase selectivity for formate in aqueous environments
but permit rapid release from neutral Fc. We introduce the materiaphore,
a 3D abstraction of these design rules, to help guide next-generation
material optimization for selective ion sorption. This approach is
expected to have broad relevance in computational discovery for molecular
recognition, sensing, separations, and catalysis.