Enhancing the Pseudocapacitive Energy Storage of Coordination Polymers by Artificially Constructed Defective Sites Anchoring Redox-Active Species

Abstract

Coordination polymers (CPs) have emerged as potential energy storage materials for supercapacitors due to their tunable chemical composition, structural diversity, and multielectron redox-active sites. However, besides poor cycling stability, the practical application of dense CPs in supercapacitors is generally limited by low specific capacitance and high resistance, which are caused by their low specific surface area and dense frameworks, resulting in insufficient redox reactions of metal sites and poor ion diffusion, respectively. Here, we synthesize a new dense CP {CP-1: [Ce(obb)(HCOO)]_∞} via self-assembly of the Ce cation and 4,4′-oxidibenzoate (obb^2–). The specific capacitance of CP-1 increases by 156.7 times, and its lifetime after charge–discharge for 5000 cycles is elevated from 62 to 86.7%; meanwhile, the resistance of the positive electrode is reduced from 1.05 to 0.73 Ω through this defect engineering strategy (DES), i.e., cyclic voltammetry sweep in a sulfuric acid electrolyte to artificially construct defects for anchoring the redox-active species (ferricyanide anions). To investigate the universality of this strategy, we have applied it to the other two previously reported CPs {CP-2: [Ce₄(obb)₆(H₂O)₉·(H₂O)]_∞ and CP-3: [Ce₂(obb)₃(OH)(H₂O)(DMF)]_∞}, which are also obtained by self-assembly of the Ce cation and obb^2– ligand. By comparing the electrochemical performances of the three pristine CPs and their corresponding defect-engineered CPs obtained through the DES, we have found that (i) this strategy is effective in enhancing the electrochemical performances for all three CP materials and (ii) the effect of this strategy on improving the electrochemical performance of CP-1 with a three-dimensional dense network is better than that of CP-3 with a layered structure, and both are better than that of CP-2 with small pores. This work demonstrates a new effective universal strategy for boosting the electrochemical performances of CPs, thus advancing their application in the energy storage field.