ABSTRACT
Liquid-redox flow batteries (LRFBs) are rechargeable batteries that store electrical energy in two soluble redox couples contained in two separate external electrolyte tanks. The sizes of the electrolyte tanks can be adjusted to contain different amounts of electrolyte solutions according to application requirements. Figure 21.1 shows a schematic diagram of a typical LRFB [1]. It can be seen that the aqueous liquid electrolytes containing active redox species are pumped from the storage tanks to the flow-through electrodes, where chemical energy is converted to electrical energy (discharge) or vice versa (charge). The electrolytes flowing through the cathode
and anode are usually different and referred to as anolyte and catholyte, respectively. Like proton-exchange membrane fuel cells, there is a membrane between the anodic and cathodic compartments that selectively allows the cross-transport of nonactive species (e.g., H+, Na+, etc.) to maintain the electronic neutrality and electrolyte balance of the whole battery system. Ideally, the membrane should be impermeable to electroactive redox species to avoid the cross-contamination of electrolytes and subsequent battery capacity loss. Different from the traditional batteries that store energy in electrodes, LRFBs are more like regenerative fuel cells in which the chemical energy stored in the incoming fuels is converted into electricity at the electrodes. Therefore, the power and energy
21.1 Introduction ..................................................................................................................................................................... 495 21.2 Types of LRFBs ............................................................................................................................................................... 496
21.2.1 All-Vanadium Redox Flow Batteries (VRBs) ....................................................................................................... 496 21.2.2 Other LRFBs ....................................................................................................................................................... 498
21.2.2.1 Iron/Chromium Flow Batteries (ICBs) ................................................................................................. 498 21.2.2.2 Polysulfide Bromide Flow Batteries (PSBs) ......................................................................................... 499 21.2.2.3 Zinc Bromide Batteries (ZBBs) ............................................................................................................ 499
21.3 LRFB Components .......................................................................................................................................................... 500 21.4 Electrolyte Chemistries and Properties ........................................................................................................................... 500
21.4.1 Solubility of VOSO4 ............................................................................................................................................ 500 21.4.2 State of Dissolved V(IV) and V(V) ..................................................................................................................... 500 21.4.3 Stability of Vanadium Species ............................................................................................................................. 501 21.4.4 Cyclic Voltammetry of Vanadium Species .......................................................................................................... 501
21.5 Electrode Materials .......................................................................................................................................................... 502 21.5.1 Graphite Felt ........................................................................................................................................................ 502 21.5.2 Modification of Graphite Felt .............................................................................................................................. 502
21.5.2.1 Heat Treatment ...................................................................................................................................... 502 21.5.2.2 Chemical Treatment .............................................................................................................................. 503 21.5.2.3 Electrochemical Oxidation ................................................................................................................... 503 21.5.2.4 Metal Doping Treatment ....................................................................................................................... 503
21.6 Catalyst Materials ............................................................................................................................................................ 504 21.7 Membranes ...................................................................................................................................................................... 504
21.7.1 Cation-Exchange Membrane ............................................................................................................................... 504 21.7.1.1 Nafion and Modified Nafion Membrane ............................................................................................... 504 21.7.1.2 Modified Daramic Membrane .............................................................................................................. 507 21.7.1.3 Fluorinated or Partly Fluorinated Membrane ....................................................................................... 507
21.7.2 Anion-Exchange Membrane ................................................................................................................................ 509 21.8 Materials of Flow Plate/Bipolar Plate ............................................................................................................................. 509 21.9 Material Challenges of LRFBs ........................................................................................................................................ 509 References ..................................................................................................................................................................................510
