Evolution of an Electrochemically Inactive Metal–Organic Framework to Reticulated Porous Carbon Particles with High Supercapacitance

Abstract

An iron-containing porphyrin-based porous metal–organic framework (PPMOF) has been synthesized and subsequently pyrolyzed to obtain the carbon structure with homogeneous distribution of nitrogen. The pyrolysis is carried out at different temperatures and in the presence of two different environments; in argon to obtain PPMOF-Ar-700, PPMOF-Ar-800, and PPMOF-Ar-900, and in nitrogen to obtain PPMOF-N-800. The motivation here is to study the change in the electrochemical properties that take place on conversion of the metal–organic framework to its corresponding carbons. The materials have been characterized by powder X-ray diffraction, nitrogen adsorption/desorption, scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, among others. It was observed that PPMOF almost did not show any electrochemical response and was unstable, whereas the carbonized frameworks showed good supercapacitive behavior and stability. The surface area increases from 353 to 838 m2·g–1, when the pyrolysis gas is switched over from nitrogen to argon. It also results in the formation of uniform smooth spherical particles in PPMOF-Ar-800, in contrast to agglomerated broken particles in PPMOF-N-800. PPMOF-Ar-800 shows a remarkable specific capacitance value of 500 F·g–1 at a current density of 1 A·g–1 which is retained at a high value of 111 F·g–1 at a very high current density of 50 A·g–1 in galvanostatic charge/discharge studies, whereas for PPMOF-N-800, the specific capacitance is 400 F·g–1 at 1 A·g–1 and its discharge time is also shorter than that for PPMOF-Ar-800. This indicates that the pyrolysis environment plays a crucial role in the formation and stabilization of the carbon structures.