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106 Redox - Principles and Advanced Applications Figure 2. Configurations of (a) a conventional redox flow battery with two divided compartments containing dissolved active species, (b) a hybrid redox flow battery with gas supply at one electrode, (c) a redox flow battery with membrane- less structure and (d) a redox flow battery with solid particle suspension as flowing media. Hybrid redox flow batteries such as zinc-bromine and zinc-cerium systems use metal strip- ping/plating reactions (Zn2þ/Zn, 0.76 V vs. [standard hydrogen electrode] SHE) on one of the electrodes inside the cell and the other side with normal soluble flowing electrolyte. Similarly, redox flow lithium batteries in non-aqueous electrolytes have been explored to make use of the low redox potential of Liþ/Li couple (3.04 V vs. SHE). During charging, zinc or lithium is deposited from the electrolyte and during discharging, Zn2þ or Liþ dissolves into the solution again. A shortcoming of such hybrid redox flow battery is that the energy storage capability is limited by the free space inside the cell accommodating the metal deposits. A second-type hybrid redox flow batteries use gas such as Cl2, O2 and H2 as the reaction medium or with gas evolution reaction at the cathode or anode (Figure 2b). For instance, oxygen reduction reaction (O2 þ 4Hþ þ 4e ⇄ 2H2O) with high positive potential can be used as the cathode. The cell capacity is then only determined by the capacity of anolyte. Oxygen reduction reaction in non-aqueous electrolytes with the presence of lithium ions can proceed through: O2 þ 2Liþ þ 2e ⇄ Li2O2. Moreover, oxygen reduction and oxidation during discharging and charging can be catalysed chemically with redox mediators [5]. Interestingly, the use of electrocatalysts for the oxygen reduction and oxidation as in a conventional system can be avoided. Note that the formation and deposition of Li2O2 proceed at porous matrix, which can be held statically in a gas diffusion tank over charging/discharging. Such a concept may maintain the character of decoupled energy and power for flow batteries. For aqueous electrolytes, oxygen and hydrogen gas evolution reactions by electrolysis of water take place during charging at very positive and negative electrode potentials, respectively.PDF Image | Redox Flow Batteries Fundamentals and Applications
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