Enhancing\nthe Pseudocapacitive Energy Storage of Coordination\nPolymers by Artificially Constructed Defective Sites Anchoring Redox-Active\nSpecies

Coordination\npolymers (CPs) have emerged as potential energy storage\nmaterials for supercapacitors due to their tunable chemical composition,\nstructural diversity, and multielectron redox-active sites. However,\nbesides poor cycling stability, the practical application of dense\nCPs in supercapacitors is generally limited by low specific capacitance\nand high resistance, which are caused by their low specific surface\narea and dense frameworks, resulting in insufficient redox reactions\nof metal sites and poor ion diffusion, respectively. Here, we synthesize\na new dense CP {CP-1: [Ce(obb)(HCOO)]<sub>∞</sub>} via self-assembly\nof the Ce cation and 4,4′-oxidibenzoate (obb<sup>2–</sup>). The specific capacitance of CP-1 increases by 156.7 times, and\nits lifetime after charge–discharge for 5000 cycles is elevated\nfrom 62 to 86.7%; meanwhile, the resistance of the positive electrode\nis reduced from 1.05 to 0.73 Ω through this defect engineering\nstrategy (DES), i.e., cyclic voltammetry sweep in a sulfuric acid\nelectrolyte to artificially construct defects for anchoring the redox-active\nspecies (ferricyanide anions). To investigate the universality of\nthis strategy, we have applied it to the other two previously reported\nCPs {CP-2: [Ce<sub>4</sub>(obb)<sub>6</sub>(H<sub>2</sub>O)<sub>9</sub>·(H<sub>2</sub>O)]<sub>∞</sub> and CP-3: [Ce<sub>2</sub>(obb)<sub>3</sub>(OH)(H<sub>2</sub>O)(DMF)]<sub>∞</sub>},\nwhich are also obtained by self-assembly of the Ce cation and obb<sup>2–</sup> ligand. By comparing the electrochemical performances\nof the three pristine CPs and their corresponding defect-engineered\nCPs obtained through the DES, we have found that (i) this strategy\nis effective in enhancing the electrochemical performances for all\nthree CP materials and (ii) the effect of this strategy on improving\nthe electrochemical performance of CP-1 with a three-dimensional dense\nnetwork is better than that of CP-3 with a layered structure, and\nboth are better than that of CP-2 with small pores. This work demonstrates\na new effective universal strategy for boosting the electrochemical\nperformances of CPs, thus advancing their application in the energy\nstorage field.