High Capcity, High Voltage Hybrid Sodium-Based Flow Batteries with Aqueous Catholyte
In this work, the feasibility of using the aqueous catholyte in HNFBs to utilize multi-electron transfer redox reactions per active ion and multiple active ions  for catholytes has been evaluated and demonstrated. The catholyte chemistry can be the same as or similar to that of traditional RFBs. For example, the reactions in both the anolyte and catholyte in all vanadium RFBs can potentially be used in the catholyte of HNFBs, as shown by reactions (1) to (3) below.
Cathode: VO2+ + H2O ↔ VO2+ + e- Eo = +1.0 V vs. SHE (1)
V3+ + H2O ↔ VO2+ + 2H+ + e- Eo = +0.34 V vs. SHE (2)
V2+ ↔ V3+ + e- Eo = -0.26 V vs. SHE (3)
Anode: Na ↔ Na+ + e− Eo = -2.7 V vs.SHE (4)
Based on these redox reactions of the catholyte and assuming that the catholyte contains 2.5M active V ions, one can obtain a theoretical specific energy of 483.7 Wh/kg, which is 18 times the specific energy provided by conventional all vanadium RFBs (~25 Wh/kg) .
The feasibility of using the aqueous catholyte in HNFBs is evaluated using VOSO4 (V4+) solution. The CV test (Figure 1a) shows the stable electrochemical potential window for the aqueous catholyte solution and the redox potential of V ions. Addition of BiCl3 in V aqueous electrolytes has been found an effective approach to improve the reversibility of V ion redox reactions . The A, B, C redox peaks are attributed to the V4+/V5+, V3+/V4+, and V2+/V3+ redox reaction couples, respectively, while the strong D peaks are generated by the redox reaction of Bi3+/Bi0. Figure 1a also shows that the O2 evolution reaction (OER) and H2 evolution reaction (HER). Based on that and further considering the overpotential in the cell, the practical threshold can be 2.2 – 4.0 V vs. Na+/Na.
The charge/discharge behavior of the HNFB with aqueous catholyte and a flat β”-Al2O3 membrane are examined and shown in Figure 1b. Three plateaus can be observed on the 1st cycle of both discharge and charge curves, corresponding to the redox reactions of V(IV)/V(III), Bi3+(III)/Bi(0), and V(III)/V(II) as labelled in the figure. Therefore, for the first time the concept of multiple electron transfer redox reactions per active ion in aqueous electrolytes (2 redox per V active ion) has been validated. Moreover, the result shows multiple electron transfer obtained by two active species, i.e. V and Bi ions.
It should be noted the low capacity for each redox reaction shown in Figure 1b is mainly attributed to the insufficient mass transportation in the catholyte due to weak stirring and thick, porous graphite felt electrode making the mass transportation inside the felt slow and difficult. The investigations on improving the capacity are undergoing.
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Figure 1. (a) CV curves of aqueous electrolytes with and without BiCl3 in half cells at a scan rate of 10 mV/s in which a graphite felt, Ag/AgCl, and Pt wire were used as the working, reference, and counter electrodes, respectively. (b) The charge/discharge profile of a full cell with the 0.01M VOSO4 -0.05M Na2SO4 -1.5M HCl -0.002M BiCl3aqueous solution as the catholyte.