In-depth understanding of electrochemical energy storage efficiency in a series of new 3d–4d mixed metal polyoxometalates: experimental and theoretical investigations

Abstract

Transition-metal complexes, with their reversible redox properties, are the basis for electrochemical energy storage devices, such as rechargeable batteries and supercapacitors. In order to comprehend the variation in the electronic properties and electrochemical activity of concurrent transition elements existing in identical coordination environments, we have pursued extensive multi-method spectroscopic and electrochemical studies of three new isostructural 3d–4d mixed metal polyoxometalate complexes, viz. the vanado-molybdate Na₂(NH₄)₅[(Mo^VI₂O₅)₂(V^IIIO₂){O₃P-C(O)(CH₂-4-C₅NH₄)-PO₃}₂]·10H₂O (1a), the chromo-molybdate (NH₄)₅[H₂(Mo^VI₂O₅)₂(Cr^IIIO₂){O₃P-C(O)(CH₂-4-C₅NH₄)-PO₃}₂]·10H₂O (2a) and the mangano-molybdate Na(NH₄)₆[H(Mo^VI₂O₅)₂(Mn^IIIO₂){O₃P-C(O)(CH₂-4-C₅NH₄)-PO₃}₂]·9H₂O (3a). The structures of the polyanions have been determined using single-crystal X-ray diffraction studies, with all three polyanionic complexes having identical connectivity and arrangement in the solid state. The electronic properties of the complexes have been analyzed thoroughly with electronic paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) methods. EPR measurements for all three complexes are characterized by Δm_s = ±2 signals (forbidden half-field transition) at 77 K in the solid state, confirming the existence of V^III, S = 1 for 1a; Cr^III, S = 3/2 for 2a; Mn^III, S = 2 for 3a. Using our original and novel binder-free approach to prepare supercapacitor electrodes, thorough comparative electrochemical energy storage studies have been performed on the three compounds using cyclic voltammetry, galvanostatic charge–discharge, electrochemical impedance spectroscopy, and cycling stability tests. The oxo-vanado-molybdate complex has shown superior electrochemical performance, with respect to specific capacitances, energy density, power density, and electrochemical stability, followed by the oxo-chromo-molybdate and the oxo-mangano-molybdate complexes. The electrochemical performance trend has been corroborated by DFT calculations at the B3LYP-D3(BJ)/Def2-SVP level of theory in a polar medium, with the same order of promptness of undergoing oxidation of the polyanionic species [M²⁺ to M³⁺; M = V (1′) > Cr (2′) > Mn (3′); ΔG(M³⁺–M²⁺) = +48 (1′), +58 (2′), +81 (3′) kcal mol⁻¹], corroborating the experimental observations. The oxidation of these anionic species from 8-(M^II) to 7-(M^III) has thus been computed to be highly favourable in the gas phase [∼−328 (1′), −311 (2′), and −309 (3′) kcal mol⁻¹]. The studies on the HOMO–LUMO structures of the complexes also corroborate the trend of the electrochemical behavior.