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Design and Materials




A vanadium redox battery consists of an assembly of power cells in which two electrolytes are separated by a proton exchange membrane.

Electrode
The electrodes in a VRB cell are carbon based. Several types of carbon electrode used in VRB cell has been report such as carbon felt, carbon paper, carbon cloth, graphite felt, and carbon nanotubes. Carbon-based materials have the advantages of low cost, low resistivity and good stability. Among them, carbon felt and graphite felt are preferred because of their enhanced three-dimensional network structures and higher specific surface areas, as well as good conductivity and chemical and electrochemical stability. The pristine carbon-based electrode exhibits hydrophobicity and limited catalytic activity when interacting with vanadium species. To enhance its catalytic performance and wettability, several approaches have been employed, including thermal treatment, acid treatment, electrochemical modification, and the incorporation of catalysts. Carbon felt is typically produced by pyrolyzing polyacrylonitrile (PAN) or rayon fibers at approximately 1500°C and 1400°C, respectively. Graphite felt, on the other hand, undergoes pyrolysis at a higher temperature of about 2400°C. To thermally activate the felt electrodes, the material is heated to 400°C in an air or oxygen-containing atmosphere. This process significantly increases the surface area of the felt, enhancing it by a factor of 10. The activity towards vanadium species are attribute to the increase in oxygen functional groups such as carbonyl group (C=O) and carboxyl group (C-O) after thermal treatment in air. There is currently no consensus regarding the specific functional groups and reaction mechanisms that dictate the interaction of vanadium species on the surface of the electrode. It has been proposed that the V(II)/V(III) reaction follows an inner-sphere mechanism, while the V(IV)/V(V) reaction tends to proceed through an outer-sphere mechanism.

Electrolyte
Both electrolytes are vanadium-based. The electrolyte in the positive half-cells contains VO2+ and VO2+ ions, while the electrolyte in the negative half-cells consists of V3+ and V2+ ions.

In the early stage of VRFB research, electrolyte was prepared with VOSO4 rather than V2O5 due to the 10 times higher solubility of VOSO4 in sulfuric acid solution than V2O5.

The electrolytes can be prepared by several processes, including electrolytically dissolving vanadium pentoxide (V2O5) in sulfuric acid (H2SO4). The solution is strongly acidic in use.

xxxxxxxxxxxxxxx vanadium electrolytes often consist of H2SO4 solutions, sometimes with a small concentration of H3PO4 in order to increase stability xxxxxxxxxxxxxx This ion is believed to occur in solution as the aquo ion [V(H2O)6]2+. VII is a strong reducing agent and it has been reported to be oxidized by water with the evolution of hydrogen. This would suggest that VII should be unstable in aqueous acidic solutions; however, VII is stablized in the presence of sulfate ions.

Flow Field
Vanadium redox flow batteries: Flow field design and flow rate optimization

The flow field directly affects the flow characteristics of the electrolyte, which in turn affects the liquid phase mass transfer process on the electrode surface, and ultimately affects the battery performance. The flow characteristics of the electrolyte in the flow field are mainly affected by the uniformity of electrolyte distribution and the size of the flow rate. The flow field significantly influences the electrolyte's flow characteristics, impacting the mass transfer process at the electrode surface and thus the battery performance. The goal of flow field are to provide high flow distribution and minimize various losses (pump loss, voltage loss, etc.)

Membrane
The most common membrane material is perfluorinated sulfonic acid (PFSA or Nafion). However, vanadium ions can penetrate a PFSA membrane and destabilize the cell. A 2021 study found that penetration is reduced with hybrid sheets made by growing tungsten trioxide nanoparticles on the surface of single-layered graphene oxide sheets. These hybrid sheets are then embedded into a sandwich structured PFSA membrane reinforced with polytetrafluoroethylene (Teflon). The nanoparticles also promote proton transport, offering high Coulombic efficiency and energy efficiency of more than 98.1 percent and 88.9 percent, respectively.