Fe 2+ /Fe 3+ Redox Kinetics Are Faster Than V 2+ /V 3+ on Glassy Carbon Due to Increased Anion Coverage and Stronger Anion Complexation

Redox flow batteries (RFBs) are a favorable technology for long-duration energy storage due to their scalability and stability. All-Vanadium RFBs are the most studied RFB system and have the advantage of using the same element in both negative and positive electrolytes which results in manageable crossover throughout multiple cycles. All-Iron RFBs make use of this same advantage, with the added benefit of iron electrolytes being much less expensive than vanadium electrolyte. However, all-iron RFBs are still not used readily as all-vanadium RFBs due to the limitations of the battery performance which are not completely understood. The vanadium RFBs (VRFBs) and iron RFBs both depend upon a (II/III) transition metal ion redox reaction of transition; V 2+ /V 3+ (-0.26 V vs. SHE) and Fe 2+ /Fe 3+ (0.77 V vs. SHE). For VRFBs, the V 2+ /V 3+ reaction is generally considered the slower half reaction in the system, and so one hypothesis is Fe 2+ /Fe 3+ might also limit all-Fe RFB performance. At first glance, one would expect the V 2+ /V 3+ and Fe 2+ /Fe 3+ reactions to be very similar, as they both involve one-electron charge transfer of a transition metal ion between 2+ and 3+ state. However, an open question in literature is whether the V 2+ /V 3+ redox kinetics are slower or faster than Fe 2+ /Fe 3+ redox kinetics and whether they have a similar mechanism. To the best of our knowledge, there has not been a direct comparison of their kinetics under identical concentrations and other conditions. Our group has previously studied the kinetics of V 2+ /V 3+ reaction on glassy carbon in two commonly used electrolytes: sulfuric acid (H 2 SO 4 ) and hydrochloric acid (HCl) and demonstrated that the redox kinetics are faster in HCl than H 2 SO 4 . 1 The V 2+ /V 3+ kinetics follow a bridging anion mechanism where the V 2+ /V 3+ reaction occurs through an adsorbed anion (X) bridge, ∗[X-V 3+ ]. The faster V 2+ /V 3+ kinetics in HCl are due to the difference in energy of the adsorbed vanadium intermediate compared to H 2 SO 4 , which causes a decrease in the activation energy barrier in HCl. 2 Here, we employ a similar methodology to study the Fe 2+ /Fe 3+ reaction under the same electrolyte compositions and electrode materials. By measuring the exchange current densities as a function of temperature using two methods: (1) generating Tafel plots and (2) measuring charge transfer resistance from electrochemical impedance, we show that while both V 2+ /V 3+ and Fe 2+ /Fe 3+ kinetics are enhanced in HCl, their reaction mechanisms differ. Fe 2+ /Fe 3+ kinetics are ~14 times faster in HCl as opposed to V 2+ /V 3+ kinetics which are only ~2 times faster. Unlike V 2+ /V 3+ , the Fe 2+ /Fe 3+ activation energies are relatively similar in HCl and H 2 SO 4 , but the apparent frequency factor significantly increases in HCl. We also show that under the same conditions Fe 2+ /Fe 3+ kinetics are faster than V 2+ /V 3+ . We propose that the experimental results are consistent with an anion-coverage mechanism where the faster Fe 2+ /Fe 3+ kinetics are due to the increase of anion coverage in HCl compared to H 2 SO 4 . With the increase in anion coverage, the concentration of active intermediates, ∗[X-Fe 3+ ], also increases resulting in more available sites where charge transfer can occur. We also show that Fe 3+ complexes more strongly with chloride anions than V 2+ /V 3+ at the same concentrations by using Raman spectroscopy. We hypothesize that the reason Fe 2+ /Fe 3+ and V 2+ /V 3+ have different mechanisms and Cl enhancements are due to two factors: (1) difference in the charge of the glassy carbon electrode Fe 2+ /Fe 3+ reaction conditions (0.29 to 1.05 V vs. SHE) and compared to V 2+ /V 3+ (-0.44 to 0.18 V vs. SHE) and (2) stronger complexation of Fe ions by chloride. The consequences of faster Fe 2+ /Fe 3+ kinetics are that future work should be devoted to understanding the Fe deposition and dissolution half-cell reaction for successful all-Fe RFBs, as this may be the slower step in those systems. References: Agarwal, H., Florian, J., Goldsmith, B. R., & Singh, N. (2019). V 2+ /V 3+ Redox Kinetics on Glassy Carbon in Acidic Electrolytes for Vanadium Redox Flow Batteries. ACS Energy Lett 4, 2368−2377. Agarwal, H., Florian, J., Goldsmith, B. R., & Singh, N. (2020). The Effect of Anion Bridging on Heterogenous Charge Transfer for V 2+ /V 3+ . Cell Reports Physical Science 2, 100307.