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Charge Zn2++2e– ⇄ Zn(s) Discharge Charge Zn(OH)42– + 2e– ⇄ Zn + 4OH– Discharge E0 =–0.76Vvs.SHE(1) E0 = –1.25 V vs. SHE (2) The standard potentials of common positive electrode reactions and parasitic reactions in zinc RFBs are given in Table 1. It can be seen that numerous RFB chemistries are possible with acid and alkaline electrolytes, as shown in Table 2. In general, high cell potentials are achieved by these systems compared to other proposed RFBs. Thermodynamically, the main competing reaction is H2 evolution at the Zn electrode, and thus high coulombic efficiencies are only attainable by inhibiting this process. H2 evolution must also be avoided during open-circuit, as it can result in self-discharge via proton reduction. As shown in Figure 1, several RFBs have taken advantage of these properties, exhibiting the highest cell potentials among aqueous systems, which are least 400 mV higher than the all vanadium cell in most cases. Yet, Zn-based RFBs compromise with relative low charging efficiency, difficult uniform zinc electrodeposition and some degree of self-discharge [28]. The controlled electrodeposition of Zn is critical to produce compact, dendrite-free deposits, uniformly coating the negative side of the bipolar electrodes, in order to prevent short- circuiting. Similarly, uniform and efficient redissolution of the metallic deposit is necessary to avoid its accumulation over repeated battery cycling. A timeline of developments in rechargeable zinc batteries is presented in Figure 2. These cells have a long history and, in some cases, have achieved high reliability, e.g., classical Leclanché’s zinc/carbon cells [42, 43]. Nickel/zinc and zinc/air and batteries are also well- known. In the field of RFBs, the zinc-bromine system is the most researched and 6PDF Image | hybrid redox flow batteries with zinc negative electrodes
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