Holistic design principles for flow batteries: Cation dependent membrane resistance and active species solubility

High‐Energy‐Density Chelated Chromium Flow Battery Electrolyte at Neutral pH

“…This suggests that for the E‐620(K) RFB, both the large VE decrease and the voltage behavior deviations are not due to any change in K + conductivity, but rather due to an electrode change. Furthermore, the E‐620(K) RFB did not show membrane discoloration after cycling and the Fe(CN) 6 solution remained yellow as expected, while the Nafion 212 RFB exhibited the previously reported membrane yellow discoloration and the Fe(CN) 6 solution turned red indicating CrPDTA crossover, confirmed by a static crossover experiment with Nafion 212 (90.3 μA cm −2 /9.36×10 −10 mol s −1 cm −2 ) (Figures S2, S3) [11,20] …”

“…Furthermore, the E-620(K) RFB did not show membrane discoloration after cycling and the Fe(CN) 6 solution remained yellow as expected, while the Nafion 212 RFB exhibited the previously reported membrane yellow discoloration and the Fe(CN) 6 solution turned red indicating CrPDTA crossover, confirmed by a static crossover experiment with Nafion 212 (90.3 μA cm À 2 /9.36 × 10 À 10 mol s À 1 cm À 2 ) (Figures S2, S3). [11,20] To determine the source of the E-620(K) R ct increase, possible causes of this electrode-based resistance increase were investigated. We have shown that the pH of CrPDTA increases during charging and decreases during discharging.…”

“…[9,10] These non-H + cations often have lower membrane conductivity and make high performance and high current density RFB operation more difficult to achieve. [11] Furthermore, few of these electroactive species have been able to demonstrate the high concentrations that FeCr RFBs and VRFBs have demonstrated. [9,10] This limits the energy density of these electrolytes, as active species concentration, as well as cell potential, factor into the energy density of an RFB.…”

“…[16,18] KCrPDTA has been operated at a concentration of 1.0 M at room temperature with no additives, and up to 1.5 M with urea as an additive. [11,18,19] Fumasep E-600(K) series membranes are a promising membrane for use with KCrPDTA, as they have higher K + conductivity than other CEMs, such as Nafion 212, and exhibit no CrPDTA crossover. [20] Extended cycling (� 100 cycles) of a CrPDTA RFB at � 100 mA cm À 2 has not yet been demonstrated with an E-620(K) membrane, as it has been with Nafion 212.…”

Isolation and Characterization of a Highly Reducing Aqueous Chromium(II) Complex

“…CrPDTA exhibited electrochemically pseudo-reversible CV features on a glassy carbon electrode (GCE), with a clear reduction and reoxidation peak present, thus suggesting that water is inert to reduction or reaction with the reduced species . Further, RFB operation of the [CrPDTA] 1–/2– negolyte has exhibited quantitative coulombic efficiency and enabled cell potentials up to 2.13 V (0.9 V greater than the thermodynamic stability window of H 2 O), indicating that the aqueous solvent remains stable in the presence of the reduced species. − This, however, is not the only example of aqueous RFBs that exceed 1.23 V, including the commercialized vanadium redox flow battery that operates at 1.4 V. − …”