Pseudocapacitance improvement of polymolybdates-based metal–organic complexes via modification with hydrogen molybdenum bronze by electrochemical treatment

Abstract The preparation of highly pseudocapacitive electrode materials is important for development of supercapacitors. In this work, a kind of new electrode material, namely, hydrogen molybdenum bronze (HMB) modified polymolybdate-based metal–organic complex {Cu(3-H2bptzh)(H2O)2[γ-Mo8O26]}·(3-H2bptzh)·H2O (1) (1-HMB) was prepared by electrochemical treatment (3-bptzh = 1,4-bis(5-(3-pyridyl)tetrazolyl) -hexane), aiming for promoting the pseudocapability of polymolybdates-based complexes. The carbon cloth-based electrode with 1-HMB (1-HMB/CC-0.15), which was obtained through electrochemical treatment between potential range of − 0.15 and 0.5 V (vs. SCE) achieved a high specific capacitance of 807.2F g−1 at the current density of 1 A g−1. The active sites and the mixed valence brought by the amorphous HMB layer give rise to the improved pseudocapacitance. 1-HMB/CC-0.15 can maintain 89.7% of its initial capacitance after 5000 galvanostatic charge/discharge cycles. In order to probe the effect brought by the structure of complex precursor, two new polymolybdates-based complexes {Cu(3-H2bptzpe)2[γ-Mo8O26]} (2) and {Cu2(3-bptzp)2(H2O)4[γ-Mo8O26]} (3) (3-bptzpe = 1,4-bis(5-(3-pyridyl)tetrazolyl)-pentane, 3-bptzp = 1,4-bis(5-(3-pyridyl)tetrazolyl)-butane) were synthesized and used as electrode material precursors as well. However, no HMB was produced on the surface of complexes 2 and 3 by similar electrochemical treatment. 1-HMB/CC-0.15 is taken as cathode to assemble an asymmetric supercapacitor (ASC) device, which demonstrates an ultrahigh energy density of 23.71 Wh kg−1 at the power density of 0.7 KW kg−1. After 5000 galvanostatic charge/discharge cycles, the aqueous ASC device can retain 86.8% of its capacitance, showing good cyclic stability. The effects brought by the electrochemical treatment and the structure of complexes on the pseudocapacitive behavior are discussed, respectively.