Template-Free Ethylene Glycol-Assisted Etching Method to Increase the Porosity of Metal-Organic Framework Derived Carbons for Supercapacitors

Metal-organic frameworks (MOFs) have drawn great interests as both carbon sources and self-templates for creating carbon materials with intrinsic porosity, tuneable chemical and physical properties and high surface area. Hard or soft template methods have been primarily used in the fabrication of hierarchical porous carbons. The template-based approaches, however, require uniform combination of the carbon sources with templates and post-synthetic template removal, making the fabrication procedures costly and complex. Herein, we demonstrate a facile template-free etching strategy for enlarging the micropores in bimetallic zeolitic imidazolate framework (ZIF) particles into mesopores at room temperature. Hierarchical porous bimetallic ZIF particles (etched Zn₃₃Co₆₇-ZIF) exhibiting both micropores and mesopores have been successfully prepared while maintaining the framework integrity using the ethylene glycol-assisted aqueous etching method. After pyrolysis, the etched Zn₃₃Co₆₇-ZIF particles are transformed into cobalt- and nitrogen-doped hierarchical porous carbon (i.e., etched Zn₃₃Co₆₇-C) particles with increased mesoporosity. The etched bimetallic carbon (etched Zn₃₃Co₆₇-C) displays 1.6- and 14.8- times higher specific capacitance at a high scan rate of 100 mV s⁻¹ than unetched bimetallic carbon (Zn₃₃Co₆₇-C) and Zn-only carbon (Zn-C), respectively as electrochemical double layer supercapacitor electrode materials. Additionally, the etched Zn₃₃Co₆₇-C exhibits good electrochemical stability after 5,000 cycles. The presented ethylene glycol-assisted aqueous etching process provides an useful method for enlarging the porosity of MOFs, and their corresponding porous carbon materials and improve their supercapacitor performance.