Reassembly and Immobilization of Heteropolyoxometalate Guided by Flexible N-Rich Ligand with Pseudocapacitive Activity

Heteropolyoxometalates can provide potential pseudocapacitive performance in aqueous solution, showing promise for electrochemical energy storage. Aiming to retain their electrochemical activity, introducing heteropolyoxometalates into an organic-inorganic hybrid has become an attractive research direction. In this paper, a flexible N-rich ligand with a N-containing spacer, N-(pyridin-3-yl)pyrazin-2-amine organic ligand (nppa), was used to combine with different heteropolyoxometalates under hydrothermal conditions. Three kinds of organic-inorganic hybrid materials, H[Cu(nppa)₂PMo₁₂O₄₀] (1), [Zn₂OH(Hnppa)₃TeMo₆O₂₄(H₂O)₂] (2), and [(Hapze)(apze)AlMo₁₄O₄₄]·H₂O (3) (apze = 2-aminopyrazine), were obtained and structurally characterized. The coordination and in situ transformation of the organic ligands were observed. This phenomenon induced the immobilization and reassembly of heteropolyoxometalates. Among them, the cleavage products of the ligands, serving as potential inducers, steered the transformation of alumomolybdates from Anderson type to capped Keggin type. The carbon cloth-based electrode decorated by the title hybrids was fabricated. At a charge-discharge current density of 1 A g^-1, 1/CC reached a specific capacitance of 223.22 F g^-1. The [PMo₁₂O₄₀]^3- anions were fixed in a 3D structure, giving rise to the highest pseudocapacitive activity. The related asymmetric capacitor can achieve a high energy density of 32.91 Wh kg^-1 at a power density of 0.75 kW kg^-1. Additionally, the carbon-paste-based electrode decorated by the hybrid can serve as an electrochemical sensor for detecting NO₂^- and AA with a small detection limit.