One of the main challenges for certain electricity production technologies (e.g. renewables) is the capability to supply energy according to the demand. It follows that storing energy is a possible solution to the inadequacy between production and demand. Among various technologies, electrochemical energy storage is a particularly good solution. Redox flow batteries, as electrochemical energy storage technology are scalable, durable, robust and quite safe, but as compared to other technologies, they have a quite poor volumetric capacity (10-40 Wh/L) determined by the solubility limit of the redox-active species dissolved in the electrolytes. In this thesis, an approach to overcome the low storage capacity in aqueous redox flow batteries is presented. The approach consists of adding solid-phase materials (herein called solid boosters) into the liquid electrolytes contained in the battery reservoirs. These boosters can receive charge from the redox-active species reacting in the flow cell, if the respective Fermi levels are aligned to those of the liquid electrolyte materials. This concept is denoted as “redox-mediated” charge transfer, where solid boosters become the primary storage media, bypassing the solubility limit issue that restraints the storage capacity. This work opens a series of future studies to enable bulk-phase charge storage in the reservoirs of redox flow batteries and it looks promising for industrial demonstrators.