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

Herein, four new cadmium metal-organic frameworks (Cd-MOFs), [Cd(bib)(bdc)](infinity) (1), [Cd(bbib)(bdc)(H2O)](infinity) (2), [Cd(bibp)(bdc)](infinity) (3), and [Cd-2(bbibp)(2)(bdc)(2)(H2O)](proportional to) (4), have been constructed from the reaction of Cd(NO3)(2)center dot 4H(2)O with 1,4-benzenedicarboxylate (H(2)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 Schafli symbol of [4(3)center dot 6(3)](2)center dot[4(6)center dot 6(16)center dot 8(6)]. 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 m(2)g(-1)) and total pore volume (1.37 cm(3)g(-1)) of this series of carbon materials. Consequently, APC-bib demonstrates the highest specific capacitance of 164 Fg(-1) at a current density of 0.5 Ag-1, and also excellent retention of capacitance (approximate to 89.4% after 5000 cycles at 1 Ag-1). Therefore, APC-bib has great potential as the electrode material in a supercapacitor.