High energy density redox flow device

An energy storage device, comprising:

• a positive electrode current collector, a negative electrode current collector, and an ion-permeable membrane separating the positive current collector and the negative current collector;
• a positive electrode disposed between the positive electrode current collector and the ion-permeable membrane;
• the positive electrode current collector and the ion-permeable membrane defining a positive electroactive zone accommodating the positive electrode;
• and
• a negative electrode disposed between the negative electrode current collector and the ion-permeable membrane;
• the negative electrode current collector and the ion-permeable membrane defining a negative electroactive zone accommodating the negative electrode;
• wherein at least one of the positive electrode and the negative electrode includes a semi-solid or condensed liquid ion-storing redox composition, the semi-solid or condensed liquid ion-storing redox composition including a conductive additive selected from metal carbides, metal nitrides, carbon black, graphitic carbon powder, carbon fibers, carbon microfibers, vapor-grown carbon fibers (VGCF), fullerenes, carbon nanotubes (CNTs), multiwall carbon nanotubes (MWNTs), single wall carbon nanotubes (SWNTs), graphene sheets, and materials comprising fullerenic fragments that are not predominantly a closed shell or tube of the graphene sheet, and mixtures thereof, and
• wherein the semi-solid or condensed liquid ion-storing redox composition is capable of taking up or releasing ions, remains substantially insoluble during operation of the cell, and has a thickness of about 250 μm to about 800 μm.