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Chapter 6 Redox Flow Batteries time of writing, over 100 different companies are developing RFBs, each with their own electrolyte formulations. 2.1.1.1. Iron-Chromium Originally invented by NASA in the late 1970s, the iron chromium (Fe-Cr) system was the first RFB electrolyte system developed [8, 9]. It consists of an Fe2+/3+ catholyte coupled with a Cr2+/3+ anolyte in an acidic aqueous electrolyte. On discharging, the following redox reactions occur: Catholyte: Fe3+ +e- Fe2+ E0= 0.77 vs. SHE (1) Anolyte: Cr2+ Cr3+ + e- E0= -0.41 vs. SHE (2) A cell voltage of 1.2 V is typical, with metal ions dissolved at ~1 M concentrations. In general, the Cr2+/3+ redox reactions are sluggish compared to other chemistries, requiring use of a catalyst [3]. Moreover, the dissimilar catholyte and anolyte mean that transport of catholyte to anolyte, and vice versa, leads to permanent loss in battery capacity. Nevertheless, NASA has demonstrated this chemistry on a 1kW/13kWh scale, and even larger scale by others, as described later in this chapter. 2.1.1.2. All-Vanadium Currently, the most widely commercialized RFBs all use vanadium-based electrolytes. The basis for this chemistry was first developed by Skyllas-Kazacos and coworkers in 1984 [10, 11] Here, the anolyte and catholyte both consist of aqueous acidic solutions of vanadium. Using the same electrolyte for anolyte and catholyte is beneficial in that if transport of one to another occurs it does not permanently damage RFB capacity. Upon discharge, the following redox reactions occur in a vanadium RFB: Catholyte: V5+ + e- V4+ (3) Anolyte: V2+ V3+ + e- (4) Note that the V5+ and V4+ are typically oxo-complexes such as VO2+ and VO2+, respectively. Vanadium concentrations are typically on the order of 1-3 M. Coupled with a nominal cell voltage of 1.6 V, an energy density of around 20 Wh/L is observed. The exact composition of the electrolyte is the focus of much research, with various groups experimenting with different acid types, complexing agents, and vanadium purities, leading to improvements in operating temperature and energy density, among others [2, 12, 13]. 2.1.1.3. All-Iron To decrease the cost of RFBs, researchers have developed all-iron systems. There are several variations of this chemistry, though fundamentally all employ the Fe2+/3+ couple in the anolyte. The catholyte may contain the Fe2+/0 couple or the Fe2+/3+ complexed with different ligands from those in the anolyte. Use of the Fe2+/0 couple requires electroplating solid Fe metal on the anode while charging. Sometimes the use of a solid electrode in an RFB is referred to as a “hybrid redox flow battery.” The catholyte chemistry can be fairly sophisticated, with various Fe-complexes (e.g., Fe(CN6)3-/4-) tailored to enhance RFB cell voltage and stability [14-16]. The exact cell voltage depends on the chemistry employed but is typically in the 0.75-1.2 V range. 4PDF Image | REDOX FLOW BATTERIES Chapter 6
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