Silver complex-modified tetravanadium-substituted polyoxomolybdate-containing ordered tunnels for supercapacitors and electrocatalytic CO2 reduction

Abstract

Assembly of rigid ligands and metal–organic units into 4 V-substituted cap-type Keggin arsenomolybdate was performed to generate two hybrid derivatives, (Hbipy)₂[As^III₂AsMo^VMo^VI₇V₄O₄₀]·5H₂O (AMV-1) and (bipy)_0.5[{Ag₂(bipy)₄}{(VO)(As^IIIAsMo₈V₄O₄₀)}]·H₂O (AMV-2) (bipy = 4,4′-bipyridyl) via a hydrothermal method. AMV-1 is a 3,6-connected porous supramolecular network modified with bipy ligands with a {4;6²}₂{4²;6⁸;8⁵} topology. In AMV-2, {Ag₂(bipy)₄} and {Ag(bipy)₂} are alternately connected by sharing bipy to yield an Ag-bipy metal–organic layer. Keggin clusters are suspended between adjacent layers, forming a unique polyoxometalate-based organic–inorganic hybrid network with ordered tunnels. Compared to AMV-1, AMV-2 demonstrates superior capacitive performance. An asymmetric supercapacitor using AMV-2-CPE as the negative electrode showed a specific capacitance of 171.8 F g⁻¹ at 2 A g⁻¹, with a high energy density (37.2 Wh kg⁻¹) and a power density (1249.2 W kg⁻¹). After 10 000 charge–discharge cycles, 92.8% of the initial capacitance is retained. This excellent electrochemical performance arises from the synergistic effect between the porous host-guest architecture induced by vanadium substitution in the Keggin cluster and the conductive Ag-bipy framework. Additionally, AMV-2-CPE achieved a CO₂ reduction current density of 25 mA cm⁻² at −0.8 V vs. RHE and operated stably for over 50 hours, with a Faraday efficiency of 90.9% for CO production. DFT calculations indicate that the {Ag₂(bipy)₄} fragments create a localized positive electrostatic potential around the Ag sites, enabling stable CO₂ adsorption. The synergistic catalysis between the Ag-bipy metal–organic layer and the POM significantly lowers the free energy barrier of the rate-determining step.