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and OER, necessary to avoid parasitic loss on charging due to the OER mechanism. To eliminate other possible interpretations of the highly reversible electrochemical process other than the proposed ClO2- /ClO2 couple, we performed cyclic voltammetry of NaClO3 and NaClO in 1M KOH, and did not notice any electrochemical signals in either case (see Figure S3, Supporting Information). Further evidence supporting chlorine dioxide formation in the electrochemical reaction was the observation of its characteristic yellow color in the aqueous electrolyte after oxidation. This was observed in bulk electrolysis half-cells (see Figure S1a, Supplementary Information), as well as in full cells discussed later. The electrochemical reversibility of the ClO2/ClO2- reaction was further investigated in Figure 1b, where CV curves in near-neutral solution at room temperature are shown for sweep rates from 100 to 5 mV/sec. There is a minimal drift of the equilibrium voltage (<2mV) with scan rate. The electrolyte iR corrected peak-to-peak separation slightly increases from 72mV at 5mV/s to 95mV at 100 mV/sec. Figure 1c shows the Randalls-Sevcik analysis of the results in Fig. 1b, showing highly linear cathodic and anodic fits, indicative of a diffusion controlled electrochemical reaction. The diffusivity of the “reduced / discharged” active species (i.e. chlorite) is estimated from the slope of the anodic peak current (i.e. above the X-axis) to be 7.0x10-6 cm2/sec. The diffusivity of the “oxidized / charged” active species (i.e. chlorine dioxide) is estimated from the slope of the cathodic peak current (i.e. below the X-axis) to be 5.7x10-6 cm2/sec. A difference in diffusivity of the charged and the discharged species is expected due to different solvation structures for the chlorite ion and the chlorine dioxide molecule in the electrolyte. As a storage electrode, ClO2 could conceivably be stored in either a gaseous or liquid phase. The current experiments span the boiling point of ClO2, which is 11°C at one atmosphere pressure. We confirmed that the reaction has good electrochemical reversibility both above and below the boiling point, as shown in Figure 1d. Indeed, the separation in potential between oxidation and reduction peaks, as well as the magnitude of the peak currents, are nearly the same between 5°C and 20°C. For practical application in battery systems, storage as a condensed phase is desirable for higher energy density and ease of containment. Note that liquid ClO2 is immiscible with aqueous electrolytes and has a higher density of 1.64 g/cm3. These features allow the storage of ClO2 as a separate liquid phase without resorting to high pressures, and facilitates battery designs that use density-based separation, as illustrated later. There is a less oxygen evolution reaction (OER) signal at 5°C than at 20°C, which may be attributed to the decreased kinetics of OER at a lower temperature. Such a suppressed OER may possibly lead to a higher coulombic efficiency in a battery operation using ClO2-/ClO2 as the positive electrode reaction. We have two hypotheses about the increased cathodic peak at 5°C compared with that at 20°C: (1) higher solubility of ClO2 (i.e. less ClO2 escaped from the electrolyte) in the aqueous electrolyte at a lower temperature; and (2) less oxygen evolution reaction at 5°C, which caused less side reaction between ClO2 and O2. In order to demonstrate use of the ClO2-/ClO2 couple in a full cell, we prepared and tested Zn-ClO2 cells operating with near-neutral electrolyte at temperature of 0.5 ± 0.5°C, where any ClO2 phase produced will be liquid. The Zn-ClO2 couple has the highest equilibrium cell voltage of the anode-cathode combinations considered above, 1.72V (Table 1), which is about 0.5V higher than that of several well- known aqueous rechargeable chemistries such the Fe-Ni “Edison” battery (1.4V),[11] Ni-Cd battery (1.2 V),[11] vanadium redox flow battery (1.26V),[12] and the all-Fe redox battery (1.2V).[13] In alkaline media, if Zn metal and NaClO2 (dissolved in the electrolyte) are used as the starting materials, it is necessary to perform a first charging step to produce ClO2, at the negative electrode, during which hydrogen evolution occurs at the positive electrode, according to the half-cell reactions: Negative electrode half-cell reaction 2H2O + 2e- ↔ H2 + 2OH- (Eq. 1) 4PDF Image | Reversible Chlorite Chlorine Dioxide Anion Redox Storage
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