Revealing Ion Adsorption and Charging Mechanisms in Layered Metal-Organic Framework Supercapacitors with Solid-State Nuclear Magnetic Resonance

Abstract

Conductive layered metal-organic frameworks (MOFs) have demonstrated promising electrochemical performances as supercapacitor electrode materials. The well-defined chemical structures of these crystalline porous electrodes facilitate structure-performance studies, however there is a fundamental lack in the molecular-level understanding of charge storage mechanisms in conductive layered MOFs. To address this, we employ solid-state nuclear magnetic resonance (NMR) spectroscopy to study ion adsorption in nickel 2,3,6,7,10,11-hexaiminotriphenylene, Ni3(HITP)2. In this system, we find that separate resonances can be observed for the MOF’s in-pore and ex-pore ions. The chemical shift of in-pore electrolyte is found to be dominated by specific chemical interactions with the MOF functional groups, with this result supported by quantum-mechanics/molecular-mechanics (QM/MM) and density functional theory (DFT) calculations. Quantification of the electrolyte environments by NMR was also found to provide a proxy for electrochemical performance, which could facilitate the rapid screening of synthesised MOF samples. Finally, the charge storage mechanism was explored using a combination of ex-situ NMR and operando electrochemical quartz-crystal microbalance (EQCM) experiments. These measurements revealed that cations are the dominant contributor to charge storage in Ni3(HITP)2, with anions contributing only a minor contribution to the charge storage. Overall, this work establishes the methods for studying MOF-electrolyte interactions via NMR spectroscopy. Understanding how these interactions influence the charging storage mechanism will aid the design of MOF-electrolyte combinations to optimise the performance of supercapacitors, as well as other electrochemical devices including electrocatalysts and sensors.