Cobalt metal–organic framework microcrystalline particles with strong electrocatalytic activity: amine controlled morphology and OER activity

Abstract

Metal organic framework (MOF) based electrocatalysts have received increased attention in recent years because of their tailorable structure, functionality and porosity. Herein, cobalt-terephthalic acid (CoTPA) based microcrystalline MOFs particles are synthesized in the presence of four different organic amines (DABCO (D), butylamine (B), hexylamine (H) and triethylamine (T)) and the electrocatalytic oxygen evolution reaction was explored. Powder X-ray diffraction (PXRD) analysis confirmed the formation of crystalline CoTPA MOFs with a similar structure but with different faceted orientation. Further, morphological studies revealed the formation of CoTPA MOFs microparticles with different morphologies depending on the amine utilized in the reaction. BET analysis confirmed the porosity with a different surface area and pore diameter. Thermogravimetric analysis indicated the water coordination in all four MOFs. X-ray photoelectron spectroscopy (XPS) analysis indicated the coexistence of Co²⁺ and Co³⁺ in the MOFs. All four MOFs exhibited strong OER activity in alkaline medium. CoTPA-D exhibited relatively higher activity and required an overpotential of 273 mV to produce the benchmark current density of 10 mA cm⁻² compared to CoTPA-B (281 mV), CoTPA-H (287 mV) and CoTPA-T (278 mV). The low Tafel slope and charge transfer resistance indicated the faster reaction kinetics whereas the higher electrochemical surface area and double layer capacitance (C_dl) suggested the availability of more active sites in CoTPA-D MOFs. The post catalytic analysis suggested the conversion of CoMOFs into CoOOH during electrocatalysis, which resulted in the enhanced OER reaction. Thus, the present work reports the facile preparation of CoMOFs with varied porosity and functionality that resulted in different electrocatalytic OER activity.