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Redox Flow Batteries: Fundamentals and Applications 107 http://dx.doi.org/10.5772/intechopen.68752 Hydrogen evolution reaction has been observed as a parasitic side reaction at the anode for some flow battery systems. Such behaviour has been used to store electricity and to generate hydrogen simultaneously (2V2þ þ 2Hþ ! H2 þ 2V3þ) as demonstrated in a vanadium-cerium flow battery [6]. Hydrogen generated can be then used to produce electricity in fuel cells. The ionic conductivity and selectivity of membranes often significantly affect the overall cell performance for many redox flow batteries. High area resistance of membrane restricts the practical operation only at low current densities. Crossover of active species through mem- brane leads to performance loss over cycling. Redox chemistry of active species with formation of electrodeposits leads to another type of cell configuration without membranes and with only one electrolyte reservoir [7] (Figure 2c). Some selected membrane-free redox flow batte- ries are listed in Table 1 [8–14]. Reasonable energy efficiencies and cycling stability have been observed. Considering the high cost of most commercial ion exchange membranes, such membrane-free cell configuration could enable simple operation and cost-effective applica- tions. Deposited anodic species should have slow dissolution rate in the presence of oxidized catholyte species as a self-discharge reaction. A direct reaction between the deposited metal and the other electroactive species in the electrolyte should be negligible or inhibited. Self- discharge effects must be minimized compared to a targeted rapid charging/discharging reaction. Acidic-supporting electrolyte is not suitable for anodic metal deposition. Solid-phase reactions in general have poor kinetics, in comparison with those in liquid electrolytes. The voltage efficiencies in most of the membrane-free flow batteries are relatively low (60–80%) restricted by mass transport and charge transfer kinetics. Compared to the flow-by configura- tion, an undivided battery with flow-through electrodes may assure enhanced mass transport. However, the flow rate will be largely limited. A laminar flow battery using two-liquid flowing media, pumped through a slim channel without lateral mixing or with very little mixing, enables membrane-free operation. H2 (flowing across anode with pumped liquid hydrobromic acid) aqueous bromine laminar flow Flow Energy efficiencies batteries Pb/PbO2 65% Zn-NiOOH 86% Cu-PbO2 About 83% at 20.8 mA cm2 Zn-Ce About 75% at 20 mA cm2 Zn-Quinone About 40–70% at 30 mA cm2 H2-Br2 High round-trip efficiency at high current density up to 1 A cm2 Symmetric About 20% at about 2 mA cm2 Ru(acac)3 Table 1. Membrane-free redox flow batteries. Cycling stability Ref. Limited by the dendrite growth of Pb and formation [8] of unwanted phase of β-PbO2 Stable over 1000 cycles with $600% Zn excess [9] Stable over 450 cycles [10] Limited by Zn negative electrode, Zn residual on [11] electrode after discharge Stable for low concentration quinone (50 mM) [12] Not given [13] Low coulombic and voltage efficiencies [14]PDF Image | Redox Flow Batteries Fundamentals and Applications
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