Research Highlights

The S/Fe redox flow battery (RFB) with abundant sulfur and iron as redox-active materials shows great potential in energy storage, characterized by low cost, high safety, and operational flexibility. However, due to the low solubility limit of [Fe(CN)6]4-, the volumetric capacity of reported S/Fe RFBs has been too low to meet commercial requirements. In this study, the authors present an alkaline S/Fe RFB with high volumetric energy density and improved cycle stability due to the poly-ion effect in the catholyte. In a 0.50M KOH solution, the concentration of [Fe(CN)6]4- reaches 1.52M at room temperature, which is twice the solubility limit of K4[Fe(CN)6] in deionized water (0.76M), corresponding to a theoretical volumetric capacity of up to 40.74 Ah L−1. When the catholyte contains 1.30M [Fe(CN)6]4-, the alkaline S/Fe RFB exhibits high performance, including a long cycle life of 3153 hours (over 4 months), a high Coulombic efficiency (CE) of over 99%, and a slow capacity decay of only 0.0166% per cycle (0.1134% per day). Furthermore, the concentrated catholyte maintains excellent cycling performance at higher temperatures and larger scales, proving the high reliability of this strategy in practical applications for enhancing the performance of cost-effective S/Fe RFB systems.

The alkaline S/Fe RFB consists of two current collectors, a positive electrode carbon felt (13.5 cm²), a negative electrode nickel sulfide foam (13.5 cm²), and a K+ exchange N212 membrane serving as a cation exchange membrane within the electrochemical cell. Additionally, there are two electrolyte tanks with peristaltic pumps outside the battery. A battery stack composed of three individual S/Fe RFBs was assembled, with each cell having a geometric area of 16.0 cm².


The authors first studied the electrochemical reversibility of the redox-active materials. Figure 1a shows the design principle of the proposed alkaline S/Fe RFB, with the catholyte and anolyte composed of [Fe(CN)6]4-/3- and S22-/S2- pairs in KOH supporting electrolyte, respectively. The electrochemical reversibility of the redox-active materials was tested using cyclic voltammetry (CV), and a pair of redox peaks were observed in both the catholyte and anolyte (Figures 1b and c). Clearly, [Fe(CN)6]4-/3- has a higher reduction potential and stronger electron-accepting ability, serving as the catholyte.

Subsequently, the authors investigated the electrochemical performance of the alkaline S/Fe RFB with diluted catholyte. The experiments utilized an alkaline S/Fe RFB composed of diluted [Fe(CN)6]4- (0.1 M K4+0.1 M Na4), which generates a poly-ion effect in the catholyte, while an excess of K2S was used in the anolyte. At 20 mA cm−2, the charge and discharge capacities of the battery showed some decay by the 1200th cycle, indicating good stability (Figure 2a). When the current density was changed from 20 mA cm−2 to 100 mA cm−2, the Coulombic Efficiency (CE) remained at a high value close to 100%, proving the high stability of the chemical substances on both sides (Figure 2b), while the Energy Efficiency (EE) and Voltage Efficiency (VE) values decreased with the increase in current density. Concurrently, with the increase in current density, both the charge and discharge capacities decreased (Figure 2c), which may be due to more severe polarization, reflected in higher charge voltage and lower discharge voltage at high current densities (Figure 2d). It is noteworthy that even at the high current density of 100 mA cm−2, the battery can still exhibit relatively high EE and VE values of about 60%, indicating its practical application value. During the extended cycling process, the catholyte at 100% State of Charge (SOC) and 100% State of Discharge (SOD) with different cycle numbers was monitored using UV-Vis absorption spectroscopy and Raman spectroscopy (Figures 2e-h). The characterization results from 2e to 2h in each figure have the same peak positions and intensities, verifying that the catholyte at the same SOC and SOD remains unchanged during the long-term cycling process, indicating its high chemical and electrochemical stability. The results show that, due to the high stability of the electrolyte, the alkaline S/Fe RFB exhibits a long cycle life close to 1472 hours (2 months) at 20 mA cm−2 (Figure 2i). Moreover, after 2000 cycles, the CE still remains at a high value above 99.0%. The battery's capacity retention rate is 88.75%, equivalent to a decay rate of only 0.0056% per cycle (Figure 2j). These results strongly prove that the alkaline S/Fe RFB has a poly-ion effect in the catholyte and possesses strong stability.

Subsequently, the authors investigated the relationship between the concentration of [Fe(CN)6]4-/3- and battery performance. As the concentration of [Fe(CN)6]3- increased, the peak power density rose from 137.98 mW cm−2 at 264.88 mA cm−2 to 171.99 mW cm−2 at 347.50 mA cm−2 (Figure 3a). From the relationship between voltage and current density, when the concentration of [Fe(CN)6]3- (0.1M) is low, the battery's voltage and current density exhibit both linear and curved parts, indicating the presence of both ohmic polarization and concentration polarization, leading to significant voltage loss and lower peak power density. On the other hand, when the concentration of [Fe(CN)6]3- (0.3M) is high, the voltage is in line with the typical ohmic relationship with current density, indicating that voltage loss is primarily allocated to internal resistance, thus achieving a higher peak power density. Therefore, increasing the concentration of [Fe(CN)6]4-/3- in the catholyte can provide higher performance for the alkaline S/Fe RFB system.

To further increase the concentration of [Fe(CN)6]4-/3- and surpass the reported room temperature solubility limit, equimolar ratios of K4[Fe(CN)6] and Na4[Fe(CN)6] were mixed and dissolved in a KOH solution. It was found that a 1.0M [Fe(CN)6]4- solution could not be obtained from a single source of K4[Fe(CN)6] in a 0.5M KOH solution, with undissolved precipitates at the bottom of the bottle. When 0.5M K4[Fe(CN)6] and 0.5M Na4[Fe(CN)6] were mixed in equimolar fractions, a completely dissolved solution was obtained, confirming the positive effect of multiple ions on increasing solubility. Using this strategy, a maximum of 1.52M [Fe(CN)6]4- was achieved in a 0.5M KOH supporting electrolyte (Figure 3b), which is three times higher than the reported RFB systems based on [Fe(CN)6]4-/3-. At the same time, compared with the diluted catholyte (10.4 Ah L-1), the concentrated catholyte (38.9 Ah L-1) is close to the theoretical value (39.1 Ah L-1), and the battery's charge/discharge capacity is significantly improved (Figure 3c). At a current density of 20 mA cm-2, the alkaline S/Fe RFB containing concentrated [Fe(CN)6]4- (1.30 M) can operate stably for 900 cycles, with a CE value close to 100%. The battery has an ultra-long cycle life of over four months (3152 hours), with a capacity decay rate of only 0.0166% per cycle (i.e., 0.11% per day), which is in the low to medium range (Figure 3d). The capacity decay is mainly due to the penetration of polysulfide ions through the membrane to the cathode, resulting in the loss of the anode redox-active material, and the yellow sulfur precipitate observed on the cathode carbon felt after long-term cycling tests confirms the permeation of polysulfide ions from the anode to the cathode. In addition, the battery shows a stable charge/discharge voltage platform during the cycling process, proving the high stability of the concentrated catholyte (Figure 3e).

Finally, the authors assessed the reliability and practicality of the concentrated catholyte, studying the battery containing 1.3M [Fe(CN)6]4- in the catholyte at 40°C and 50°C (Figures 4a and b). Under both conditions, the battery operated stably at 20mA cm−2, with high Coulombic Efficiency (CE) and capacity retention (Figures 4c and d). Furthermore, a battery pack composed of three single cells containing concentrated [Fe(CN)6]4- (1.30M) was assembled (Figure 4e), with the battery pack's average discharge voltage at 2.67V. After 50 cycles, the charge/discharge curves were similar to the 2nd cycle, with minimal capacity decay (Figure 4f). After 100 cycles, the battery pack still performed well, with high CE (97.87%), Energy Efficiency (EE) (81.56%), Voltage Efficiency (VE) (83.33%), and high capacity retention (93.15%) (Figure 4g). When further expanded to 7 single cells, with each cell's expanded geometric area at 340.0 cm², the battery pack showed a longer cycle life, and with electrolyte replenishment, CE, EE, and VE were restored to high values.

Due to the heteropoly ion effect of K+ and Na+ in the KOH supporting electrolyte, the concentration of [Fe(CN)6]4- in the catholyte is significantly increased to 1.52M, breaking through the solubility limit of K4[Fe(CN)6] or Na4[Fe(CN)6] at room temperature, thereby tripling the charge and discharge capacity of the catholyte. When used in conjunction with the highly soluble polysulfide anolyte, the alkaline S/Fe RFB with a concentration of 1.30M exhibits high stability and long cycle life, with the battery operating stably for over 3153 hours (4 months), a CE close to 100%, and a capacity decay rate of only 0.0166% per cycle. Moreover, the battery shows excellent performance at higher temperatures. Additionally, as the size is expanded, the battery pack demonstrates good performance. These experimental results confirm the positive impact of the ion diversity effect on increasing the solubility of [Fe(CN)6]4- and achieving high-performance RFBs. Given the aforementioned advantages, the proposed alkaline S/Fe RFB system shows great promise in practical applications as an energy storage device.

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