text: The hybrid flow battery uses one or more electroactive components deposited as a solid layer. The major disadvantage is the loss decoupled energy and power as seen in full flow batteries from using a solid state electrode. The cell contains one battery electrode and one fuel cell electrode. This type is limited in energy by the electrode surface area. Hybrid flow batteries include the zinc–bromine, zinc–cerium, soluble lead–acid, and iron-salt flow batteries. Weng et al. reported a vanadium–metal hydride rechargeable hybrid flow battery with an experimental OCV of 1.93 V and operating voltage of 1.70 V, relatively high values among rechargeable flow batteries with aqueous electrolytes. This hybrid battery consists of a graphite felt positive electrode operating in a mixed solution of and , and a metal hydride negative electrode in KOH aqueous solution. The two electrolytes of different pH are separated by a bipolar membrane. The system demonstrated good reversibility and high efficiencies in coulomb (95%), energy (84%), and voltage (88%). They reported further improvements of this redox couple with achievements of increased current density, inclusion of larger 100 cm electrodes, and the operation of 10 large cells in series. Preliminary data using a fluctuating simulated power input tested the viability toward kWh scale storage. In 2016, a high energy density Mn(VI)/Mn(VII)-Zn hybrid flow battery was proposed.
  A prototype zinc–polyiodide flow battery demonstrated an energy density of 167 Wh/L (watt-hours per liter). Older zinc–bromide cells reach 70 Wh/L. For comparison, lithium iron phosphate batteries store 233 Wh/L. The zinc–polyiodide battery is claimed to be safer than other flow batteries given its absence of acidic electrolytes, nonflammability and operating range of  that does not require extensive cooling circuitry, which would add weight and occupy space. One unresolved issue is zinc buildup on the negative electrode that can permeate the membrane, reducing efficiency. Because of the Zn dendrite formation, Zn-halide batteries cannot operate at high current density (> 20 mA/cm) and thus have limited power density. Adding alcohol to the electrolyte of the ZnI battery can help with the problem. The drawbacks of Zn/I RFB lie at the high cost of Iodide salts (> $20 / Kg); limited area capacity of Zn deposition also losing the decoupled energy and power; and Zn dendrite formation.
  When the battery is fully discharged, both tanks hold the same electrolyte solution: a mixture of positively charged zinc ions () and negatively charged iodide ion, (). When charged, one tank holds another negative ion, polyiodide, (). The battery produces power by pumping liquid from external tanks into the batterys stack area where the liquids are mixed. Inside the stack, zinc ions pass through a selective membrane and change into metallic zinc on the stacks negative side. To further increase the energy density of the zinc–iodide flow battery, bromide ions () are used as the complexing agent to stabilize the free iodine, forming iodine–bromide ions () as a means to free up iodide ions for charge storage.
  Proton flow batteries (PFB) integrate a metal hydride storage electrode into a reversible proton exchange membrane (PEM) fuel cell. During charging, PFB combines hydrogen ions produced from splitting water with electrons and metal particles in one electrode of a fuel cell. The energy is stored in the form of a solid-state metal hydride. Discharge produces electricity and water when the process is reversed and the protons are combined with ambient oxygen. Metals less expensive than lithium can be used and provide greater energy density than lithium cells.