The authors of this paper present a new RFB system that uses polysulfides and high-concentration ferricyanides (up to 1.6 M) as reactants. The RFB cell demonstrates high battery performance, with a capacity retention rate of 96.9% after 1500 cycles (equivalent to a capacity decay rate of 0.002% per cycle or 0.10% per day), and the cost of reactants is as low as $32.47 per kilowatt-hour. Additionally, the neutral aqueous electrolyte is environmentally benign and cost-effective. Moreover, the authors have pioneered the assembly and study of a stack consisting of 10 cells with an effective geometric area of 1700 square centimeters. The assembled battery stack showed a low capacity decay rate of 0.021% per cycle over 642 charge-discharge steps (spanning 60 days). This indicates that this neutral polysulfide/ferricyanide RFB technology has characteristics such as high safety, long duration, low cost, and ease of scaling up, making it an innovative design for storing massive amounts of energy.

The authors first conducted reversibility tests using cyclic voltammetry (CV). As shown in Figures 1B and 1C, a pair of redox peaks appeared in the potential window between -0.1 V and 0.65 V for the 0.1 M K3[Fe(CN)6] solution, and between −1.3 V and 0.3 V for the 0.1 M K2S solution, corresponding to the redox processes of the Fe and S elements, respectively.

The authors then tested the electrochemical reversibility and long-term cycling stability of the cathode and anode redox reactants. In the neutral PFRFB cell they designed, both the charging and discharging curves show only one plateau, as illustrated in Figure 2A, confirming that the polysulfide species dominating on the anolyte side is S22−. After 200 consecutive cycles, the cell's discharge capacity remains at 95.07% of the theoretical capacity, and the discharge capacity decay by the 1500th cycle is very limited, demonstrating an ultra-high capacity retention rate and excellent stability. The narrow separation between the charge and discharge curves indicates minimal ohmic polarization and IR drop from the 200th to the 1500th cycle. The capacity retention rate remains high at 96.9% over 1,500 cycles (equivalent to a capacity decay rate of 0.002% per cycle, lasting for a month), indicating the cell's outstanding cycling stability, as shown in Figure 2B, which is one of the best cycle performances reported for RFBs.

In the initial cycles shown in Figure 2C, the Coulombic Efficiency (CE) and Energy Efficiency (EE) are 98.63% and 80.95%, respectively. The CE maintains unprecedented stability throughout the long-term cycling test, even reaching a higher value of 99.76% by the 1500th cycle, indicating the cell's ultra-high cycling stability. After continuous charge-discharge operations, the EE slowly decreases to 70.00% by the end of the 1500th cycle, which may be related to the increase in the area-specific resistance of the membrane, confirmed by the pale yellow precipitate observed on the anolyte side of the membrane after the cycling test.

The authors further investigated the impact of current density on cell performance and its rate capability, as illustrated in Figure 3A. At all applied current densities, the cell's CE remains almost constant and above 95.0%, indicating high stability and electrochemical reversibility of the redox reactants on both sides. As the current density increases from 20 mA cm−2 to 140 mA cm−2 (in increments of 20 mA cm−2), the Voltage Efficiency (VE) and EE significantly decrease, suggesting that larger electrodes are required for higher power output. Additionally, the authors further studied the relationship between ferricyanide concentration and cell performance, as shown in Figure 3B. The polarization curves indicate that the peak power density at constant current increases with the concentration of the electrolyte on the capacity-limiting side. By using solutions with different concentrations of ferricyanide (0.1 M, 0.5 M, and 0.8 M, close to the solubility limit of K3[Fe(CN)6] at room temperature), the peak power density increases from 153.4 mW cm−2 at 292.4 mA cm−2 to 189.3 mW cm−2 at 403.0 mA cm−2, and further to 213.9 mW cm−2 at 417.6 mA cm−2. Under higher concentration conditions, the voltage and current density follow a typical Ohmic relationship, indicating that voltage loss is mainly attributed to internal resistance. In contrast, at low concentrations, both Ohmic polarization and concentration polarization coexist and lead to voltage loss, which can be proven by the difference in the linear correlation between voltage and current density.

Further constant current charge/discharge curve tests were conducted on neutral PFRFB cells with different initial ferricyanide concentrations, as shown in Figures 3C and 3D. It was found that two cells using 0.5 M K3[Fe(CN)6]+2.0 M KCl and 0.8 M K3[Fe(CN)6]+0.5 M KCl solutions in the cathode both achieved very high CE values (>96.0%), without significant decline during the cycling process, and the capacity retention rate in the initial cycle was as high as 100%, indicating that the electroactive reactants have high stability. However, the faster capacity decay indicates more severe voltage loss due to undesirable side reactions under high concentration conditions.

The authors further demonstrated that the energy density of an RFB is determined by either the catholyte or the anolyte. In this work, based on a neutral solution with KCl as the supporting electrolyte, the authors used a complex mixture of K4[Fe(CN)6] and Na4[Fe(CN)6] to obtain a high-concentration solution of the [Fe(CN)6]4−/[Fe(CN)6]3− redox pair, which can easily achieve a maximum concentration of 1.6 M [Fe(CN)6]4− at room temperature, breaking through the basic solubility limit, as shown in Figure 4A. The specific capacity of the PFRFB cell based on the 1.6 M [Fe(CN)6]4− catholyte in a 1.0 M KCl solution was significantly improved, as illustrated in Figure 4B, with the achieved specific capacity (42.8 Ah L−1) approaching the theoretical value (42.9 Ah L−1), indicating that the mixed [Fe(CN)6]4− has excellent electrochemical activity and reversibility as a catholyte. Furthermore, the CE of the cells containing catholytes with 1.4 M [Fe(CN)6]4− and 1.6 M [Fe(CN)6]4− remained stable and high during the cycling process, as shown in Figures 4C and 4D, demonstrating that the ultra-high concentration [Fe(CN)6]4− mixed catholyte has good stability. Current research indicates that concentrating [Fe(CN)6]4− can significantly enhance cell performance.

In conclusion, the authors assembled cell stacks with varying concentration combinations of K3[Fe(CN)6] and K2S and conducted electrochemical studies, as shown in Figure 5A. Among them, the cell stack composed of 20.0 L of 0.2 M K3[Fe(CN)6] in 1.0 M KCl as the catholyte and 20.0 L of 1.0 M K2S in 1.0 M KCl as the anolyte demonstrated high cycling stability, with a low capacity decay rate of 0.021% per cycle, as illustrated in Figure 5B. Furthermore, after 642 repeated charge-discharge cycles (approximately 50 days) at a current of 34A, the CE value remained almost unchanged, above 97.40%, while the VE and EE values fluctuated slightly, around 82.86% and 80.97%, respectively, which strongly proves the high stability and excellent performance of the cell stack. Additionally, at a current of 51A, the cell stack with a more concentrated catholyte exhibited a low capacity decay rate of 0.056% per cycle and a high average CE value of 97.55% over 204 constant current charge-discharge cycles (spanning 20 days), indicating that the cell stack based on the proposed neutral PFRFB system is reliable for practical applications, as shown in Figure 5C.

Moreover, the techno-economic analysis indicates that the installation cost of this method is comparable to that of Pumped Hydro Storage (PHS) and Compressed Air Energy Storage (CAES), which is very promising for the large-scale adoption of intermittent renewable power. Compared to the state-of-the-art vanadium-based RFB at $124.40 per kWh and other recently proposed RFB systems, as shown in Figure 5D, the material cost of the proposed neutral PFRFB method is significantly lower at $32.47 per kWh. Additionally, the neutral operating conditions also significantly reduce system costs and maintenance expenses. In this study, a Nafion 212 membrane was used as the separator, which is expected to be replaced by a cheaper homemade membrane under mild and neutral conditions, and it is believed that this could further reduce system costs.

Combining the inherent electrochemical stability, reversibility, and low wholesale prices of potassium ferricyanide and potassium sulfide, the authors have demonstrated a neutral PFRFB system. The proposed battery provides exceptional high capacity retention rates and CE, as well as high power density output over an ultra-long cycle of 1,500 cycles (a month of testing time), as shown in Figure 5E. In addition to the high battery performance at the laboratory scale, a cell stack composed of 10 single cells with an effective geometric membrane area of 1700 cm² also demonstrates the high stability and practicality of the proposed system. The high CE and low capacity decay rate achieved over 642 cycles (spanning nearly two months) promote the potential grid-scale energy storage of this method over a decade of operation. Furthermore, the neutral PFRFB system shows very low material costs, which is beneficial for the anticipated low system costs. With the demonstrated high stability, robust battery performance, inherent low material costs, high scalability, and environmental friendliness, the neutral PFRFB provides a simple solution for sustainable and economical energy storage strategies.

In terms of limitations, the authors propose that in the current research, carbon felt without any catalyst coating is used as the electrode. Considering the slow kinetics of the polysulfide redox reaction, the power density of the proposed PFRFB still needs to be improved for practical applications. In future work, by utilizing efficient catalysts tailored for polysulfide species to modify the electrodes, the power density of this RFB system will be significantly enhanced.

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