We have synthesized the known [Cu20Cl(OH)(24)(H2O)(12)(P8W48O184)](25 center dot) (1) and report here its bromide and iodide analogues, [Cu20Br(OH)(24)(H2O)(12)(P8W46O184)](25-) (2) and [Cu20I(OH)(24)(H2O)(12)(P8W48O184)](25-) (3). These polyanions were characterized in the solid state by IR spectroscopy and 2 single-crystal X-ray diffraction Magnetic susceptibility and magnetization data over 18-300 K show that the Cu2+ ions in 1-2 are antiferromagnetically coupled, leading to a diamagnetic ground state. The effective exchange coupling constant J(eff) was estimated as similar to -3 K for both 1 and 2. Electron paramagnetic resonance measurements were made on 1 and 2 over 5-295 K at microwave frequencies of 9.5, 34, and 220 GHz. The observed (weak) signals were characteristic of randomly distributed Cu2+ ions only, with g values and hyperfine constants typical of the unpaired electron in a 3d(x2-y2) orbital of Cu2+. No signals attributable to the copper-hydroxo cluster were detected, supporting the conclusions from the magnetization measurements. DFT calculations were performed as well to obtain additional information on the anionic guest inside the cavity created by the copper-hydroxo cage related to electronic structure and energies of encapsulation. The polyanions 2 and 3 were also characterized by cyclic voltammetry (CV) in a pH 5 medium. Their CVs are composed by an initial two-step reduction of the Cu2+ centers to Cu-0 through Cu+, followed at more negative potential by the redox processes of the W centers. A comparison with the CV characteristics of the previously studied compound 1 indicates that the potential locations of the Cu or W waves of the three analogues do not depend significantly upon the identity of the central halide X. This observation is in accordance with conclusions of DFT calculations. The modified electrodes based on 2 and the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetra fluoroborate triggers an efficient reduction of nitrate. To our knowledge, this is the first example of electrocatalytic nitrate reduction by a polyanion entrapped in room-temperature ionic liquid films