Supercapacitors operating in physiological electrolytes are of great relevance for both their environmentally friendly aspect as well as the possibility to be employed for powering implantable microelectronic devices using directly biological fluids as electrolytes. Polyoxometalate (POMs) have been proven to be useful for supercapacitors in acidic media. However, in neutral pH, POMs are usually not stable. One relevant alternative is to stabilize POMs by pairing them with organic moieties to form hybrids. In this study, we combined K6P2Mo18O62 center dot 12H(2)O (P2Mo18) with Ru(bPY)(3)Cl-2 center dot 6H(2)O (Ru(bpy)). The synthesis was carried out with and without the mild reducing agent KI. The hybrids were characterized by CHN analysis, TEM, FT-IR, XRD, TGA and cyclic voltammetry. CHN elemental analysis revealed that one mole [P(2)Mo(18)O62](6-) is paired with 3 mol [Ru(bPY)(3)](2+) to form [Ru(bpY)(3)](3)PMo18O62 center dot nH(2)O. With KI present, [P2Mo18O62](6-) is linked to 3.33 mol to yield [Ru(bPY)(3)](3.33)PMo18O62 center dot mH(2)O. Excess of Ru(bpy) in [Ru(bPY)(3)](3.33)PMo18O62 center dot mH(2)O was further confirmed by TEM, FT-IR, XRD, TGA and cyclic voltammetry. In turn, hybrid composition is found to strongly influence the supercapacitor behavior. The hybrid rich in Ru(bpy) is found to perform better for supercapacitors in physiological electrolytes, 125 F g(-1) and 68 F g(-1) are the capacitance values obtained with [Ru(bPY)(3)](3.33) PMo18O62 center dot mH(2)O and [Ru(bpy)(3)](3)PMo18O62 center dot nH(2)O, respectively. In terms of specific energy densities, 3.5 Wh kg(-1) and 2 Wh kg(-1) were obtained for both hybrid simultaneously. The difference in supercapacitor performance between both hybrids is also noticed in impedance spectroscopy which showed that [Ru(bpy)(3)](3.33)PMo18O62 center dot mH(2)O has lower electron transfer resistance if compared to [Ru(bpy)(3)](3)PMo18O62 center dot nH(2)O. Finally, if compared of parent K6P2Mo18O62 center dot 12H(2)O, the stability of both hybrids is found to be highly improved. (C) 2015 Elsevier B.V. All rights reserved.