Factors Impacting the Nuclear Magnetic Resonance Spectra of Electrolyte Adsorbed in Layered Metal-Organic Frameworks

Abstract

Electrically conductive layered metal-organic frameworks (MOFs) have a wide range of electrochemical applications including in sensors, batteries, spintronics, magnetic semiconductors, and supercapacitors. In these devices, MOF structure strongly influences performance, often through MOF-electrolyte interactions. However, few studies have directly probed these interactions at the electrochemical interface. Recent work showed that 19F NMR spectroscopy can probe organic electrolyte environments in the layered MOF Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), revealing that the chemical shifts of in-pore anions are influenced by specific chemical interactions with the MOF functionality. Here, we expand this approach to study ion adsorption in a range of layered MOFs and study the factors influencing the in-pore chemical shifts. We find that all MOF-electrolyte systems display positively shifted in-pore electrolyte resonances, with calculations indicating that both aromatic ring currents and metal-center-induced currents significantly contribute to the observed shifts. We also find that paramagnetic MOFs exhibit additional paramagnetic shifts of the in-pore resonance when specific MOF-electrolyte interactions are present, with paramagnetic NMR calculations linking these to specific coordination geometries and revealing ion binding sites. Finally, MOF particle morphology also strongly affects the appearance of the NMR spectra, with rod-like morphologies leading to slower exchange and better peak resolution. Overall, our results reveal the key factors that influence the NMR spectra of electrolyte sorption in layered MOFs and demonstrate the power of NMR spectroscopy to probe electrochemical interfaces and guide materials design.