Unlocking the Capacity of Iodide for High-Energy-Density Polyiodide-Based Redox Flow Batteries

Introduction Flow battery is one of the most promising technologies for storing electricity generation from intermittent renewables, owing to its design flexibility in decoupling energy and power1-3. Developing high-energy-density flow batteries can reduce the system footprint and storage size and expand their usage to both stationary and transportation applications1-3. Iodide has been identified as one of the most promising redox active species for redox flow battery owing to its high solubility in both aqueous4,5 and nonaqueous media6, fast kinetics4,7 and high reversibility.4,7 However, only 2/3 of the iodide ions contribute to useable capacity and 1/3 of the iodide ions are used as the complexing agent to stabilize the free iodine in most iodide-based battery systems. In this work, we exploit bromide ions (Br–) as the complexing agent to stabilize the free iodine, forming iodine-bromide ions (I2Br–) as a mean to free up iodide ions for charge storage (as shown in Figure 1).8 With this strategy, we here demonstrated a novel zinc/iodine-bromide (I2Br–) battery (ZIBB) with an energy density of 101 Wh L-1 posolyte+negolyte (or 202 Wh L-1 posolyte), which is the highest energy density achieved experimentally for aqueous flow batteries to date. This strategy can be further generalized to nonaqueous iodide-based batteries (e.g. lithium/polyiodide battery), offering new opportunities to improve high-energy iodide-based energy storage technologies.8 Results and Discussion The proposed zinc/iodine-bromide redox flow battery (ZIBB) consists of a positive electrode of graphite felt (GF) operating in a mixed solution of ZnI2 and ZnBr2, Nafion membranes and a GF negative electrode in zinc polyhalide aqueous solution.8 The designed reactions of the ZIBB is shown in equations (1) to (3), with an theoretical cell voltage similar to that of zinc/iodide redox battery (ZIB) c.a. 1.3 V 4. During charge, I2Br– ions will be generated in the posolyte while the Zn2+ ions are reduced to form zinc metal on the negative electrode. Such process is accompanied by the movement of the Zn2+ions from the posolyte to negolyte serving as the charge carrier. During discharge, the reverse reactions occur. +ve: 2I- + Br- I2Br- + 2e- Eo= 0.594 VSHE(1) -ve: Zn2+ + 2e- Zn Eo= -0.76 VSHE(2) Overall: 2I- + Br- + Zn2+ I2Br- + Zn Vcell= 1.354 V (3) Figure 1 compares the galvanostatic voltage profiles of a ZIBB (5.0 M ZnI2 : 2.5 M ZnBr2), and an iodide-only system (5.0 M ZnI2) under the continuous flow mode. A bromide-only (2.5 M ZnBr2) system with two cut off voltages (1.5 V, red curve) and (2.0 V pink curve) are also included for comparison. With the same cut-off voltages, the cell with Br– (ZIBB) achieves higher charge and discharge capacity compared with the iodide-only cell (ZIB). In addition, two control bromide-only cells directly demonstrate that the additional capacity observed in the ZIBB cell (cut at 1.5 V) is not originated from the oxidation of Br– to Br3 –. The origin of the additional capacity and, the influences of the key factors (e.g. electrolyte concentration, applied current density and membrane thickness etc.) on the electrochemical behavior/performance will be discussed in details. Figure 1. Concept illustration of bromide as the complexing agent to stabilize iodine. Galvanostatic voltage profiles of the ZIBB systems with 5 M ZnI2 + 2.5 M ZnBr2 as both posolyte and negolyte at a flow rate of 10 mL min-1. The performance of iodide-only system (5.0 M ZnI2) under the same condition included. In addition, a bromide-only (2.5 M ZnBr2) system with two cut off voltages (1.5 V, red curve) and (2.0 V pink curve) are included for comparison. The charge/discharge current density is 5 mA cm-2.8 Acknowledgments This work is supported by a grant from the Research Grants Council (RGC) of the Hong Kong Administrative Region, China, under Theme-based Research Scheme through project No. T23-407/13-N, a RGC project No. CUHK14200615 and a grant from the Innovation and Technology Commission of Hong Kong Special Administrative Region, China (Project No. ITS/248/14FP). Figure 1