Quantifying the Influence of C-H···pi Interactions on Non-Aqueous Electrolyte Solubility

For redox active organic molecules (ROMs) used in grid-scale energy storage applications, such as redox flow batteries, solubility is an essential physicochemical property. Specifically, solubility is directly proportional to the volumetric energy density of an energy storage device and thus affects its corresponding spatial footprint. Recently pyridiniums have been introduced as a class of ROMs with high persistence in multiple redox states at low potentials. Unfortunately, solubility of pyridinium salts in non-aqueous media remains low (generally less than 1 M), and relatively few practical molecular design strategies exist for generalized improvement of ROM solubility. Herein, we convey the extent to which discrete, attractive interactions between C-H groups and the p-electrons of an aromatic ring (C-H···pi interactions) can describe the solubility of N-substituted pyridinium salts in a non-aqueous solvent (acetonitrile). We find a direct correlation between the extent of crystalline C-H···pi interactions for each pyridinium salt and its solubility in acetonitrile (R2 = 0.93, solubility range = 0.3 – 2.1 M). The presence of C-H···pi interactions reveals how large disparities in solubility between (e.g.) N-(p-tolyl)-4-phenyl-2,6-dimethylpyridinium (0.32 ± 0.03 M) and N-(p-tolyl)-4-(p-tolyl)-2,6-dimethylpyridinium (1.06 ± 0.03 M) tetrafluoroborate may arise despite differing in structure by only three atoms. The correlation presented in this work highlights a surprising consequence of disrupting strong electrostatic interactions with weak dispersion interactions, showing how minimal structural change can have dramatic effects on ROM solubility

Catalyst-Free, Highly Selective Synthesis of Ammonia from Nitrogen and Water by a Plasma Electrolytic System

There is a growing need for scalable ammonia synthesis at ambient conditions that relies on renewable sources of energy and feedstocks to replace the Haber-Bosch process. Electrically driven approaches are an ideal strategy for the reduction of nitrogen to ammonia but, to date, have suffered from low selectivity associated with the catalyst. Here, we present a hybrid electrolytic system characterized by a gaseous plasma electrode that facilitates the study of ammonia formation in the absence of any material surface. We find record-high faradaic efficiency (up to 100%) for ammonia from nitrogen and water at atmospheric pressure and temperature with this system. Ammonia measurements under varying reaction conditions in combination with scavengers reveal that the unprecedented selectivity is achieved by solvated electrons produced at the plasma-water interface, which react favorably with protons to produce the key hydrogen radical intermediate. Our results demonstrate that limitations in selectivity can be circumvented by using catalyst-free solvated electron chemistry. In the absence of adsorption steps, the importance of controlling proton concentration and transport is also revealed