Controlling the BET Surface Area of Porous Carbon by Using the Cd/C Ratio of a Cd–MOF Precursor and Enhancing the Capacitance by Activation with KOH

Abstract

Herein, four new cadmium metal-organic frameworks (Cd-MOFs), [Cd(bib)(bdc)]_∞ (1), [Cd(bbib)(bdc)(H₂ O)]_∞ (2), [Cd(bibp)(bdc)]_∞ (3), and [Cd₂ (bbibp)₂ (bdc)₂ (H₂ O)]_∞ (4), have been constructed from the reaction of Cd(NO₃ )₂ ⋅4 H₂ O with 1,4-benzenedicarboxylate (H₂ bdc) and structure-related bis(imidazole) ligands (1,4-bis(imidazol-1-yl)benzene (bib), 1,4-bis(benzoimidazol-1-yl)benzene (bbib), 4,4'-bis(imidazol-1-yl)biphenyl (bibp), and 4,4'-bis(benzoimidazol-1-yl)biphenyl (bbibp)) under solvothermal conditions. Cd-MOF 1 shows a 2D (4,4) lattice with parallel interpenetration, whereas 2 displays an interesting 3D interpenetrating dia network, 3 exhibits an unusual 3D interpenetrating dmp network, and 4 presents a 3D self-catenated pillar-layered framework with a Schäfli symbol of [4³ ⋅6³ ]₂ ⋅[4⁶ ⋅6¹⁶ ⋅8⁶ ]. The structural diversity indicates that the backbone of the bis(imidazole) ligand (including the terminal group and spacer) plays a crucial role in the assembly of mixed-ligand frameworks. By using the pore-forming effect of cadmium vapor, for the first time we have utilized these Cd-MOFs as precursors to further prepare porous carbon materials (PCs) in a calcination-thermolysis procedure. These PCs show different porous features that correspond to the topological structures of Cd-MOFs. Significantly, it was found that the specific surface area and capacitance of PCs are tuned by the Cd/C ratio of the MOF. Furthermore, the as-synthesized PCs were processed with KOH to obtain activated porous carbon materials (APCs) with higher specific surface area and porosity, which greatly promoted the energy-storage capacity. After full characterization, we found that APC-bib displays the largest specific surface area (1290 m² g^-1 ) and total pore volume (1.37 cm³ g^-1 ) of this series of carbon materials. Consequently, APC-bib demonstrates the highest specific capacitance of 164 F g^-1 at a current density of 0.5 A g^-1 , and also excellent retention of capacitance (≈89.4 % after 5000 cycles at 1 A g^-1 ). Therefore, APC-bib has great potential as the electrode material in a supercapacitor.