Electrochemical and NMR Characterization of ZIF-67

Abstract

Metal Organic Frameworks (MOFs) are attractive materials for their tunable porosities and chemistries. Zeolitic Imidazolate Frameworks (ZIF) are a class of MOFs recognized for their relative ease in synthesis and stability. In this work we study ZIF-67 for energy storage applications. We use a host of characterization techniques, including XRD, BET, EDS and 1H and 13C nuclear magnetic resonance (NMR), to better understand the neat ZIF-67. The ZIF-67 particle size is shown to be 150-300 nm, exhibiting good crystallinity with specific surface area of 850 m2/g. NMR reveals hyperfine interactions of the paramagnetic cobalt with 1H and 13C that are in good agreement with calculated values from first principles. Additionally, high resolution NMR spectra of “guest” molecules within the pores of the MOF are demonstrated. Composite electrodes based on ZIF-67 are characterized for their energy storage capacity in KOH electrolytes using cyclic voltammetry and electrochemical impedance spectroscopy. These electrodes are shown to display capacities greater than traditional carbons. Cyclic voltammetry is used to distinguish redox contributions from double layer capacity. Finally, in-situ NMR spectra of two electrode devices are presented to understand charging and discharging mechanisms.