In this study, we report the first application of a cobalt(II)/(III) tris(2,2′-bipyridine) based aqueous electrolyte in the fabrication of dye-sensitized solar cells. An improvement in short circuit current was observed by adjusting the concentration of polyethylene glycol in the aqueous electrolyte, which is attributed to better diffusion of the redox mediator within the electrolyte. Replacement of platinum with a composite ITO/platinum counter electrode material results in a further increase in energy conversion efficiency to above 5%. Electrochemical impedance measurements indicate a dramatic decrease in charge transport resistance at the interface between the counter electrode and aqueous electrolyte when ITO/Pt was used. Moreover, significantly improved device stability over previously reported water based DSCs under 1 sun illumination suggests that cobalt(II)/(III) tris(2,2′-bipyridine) is a promising redox mediator for highly efficient aqueous dye-sensitized solar cells.