In this study, the practical energy density of the [Fe(CN)6]3-/4- based catholyte is significantly increased from 10.5 Wh L−1 to 92.8 Wh L−1 by combining the counterion effect and the Solid-Mediated Redox Transformation (SMRT) reaction between [Fe(CN)6]3-/4- and Prussian Blue (Fe4[Fe(CN)6]3, PB)/Prussian White (PW). Paired with a concentrated K2S anolyte, the authors demonstrate a PB-Fe/S flow battery based on neutral aqueous SMRT, which has an ultra-long life exceeding 7000 cycles (4500 hours), and the chemical cost of the electrolyte in the battery is extremely low, at only $19.26 per kWh. Notably, in the presence of PB particles in the catholyte, under the influence of the SMRT reaction, the capacity of the PB-Fe/S flow battery after 7000 cycles reaches 181.8% of the initial capacity without PB.

Research Content

The authors first conducted cyclic voltammetry (CV) tests. In a 1.0 M KCl solution, within the potential range of 0V-0.4V vs. Hg/Hg2Cl2, both K3[Fe(CN)6] and PB were tested, and a pair of reversible redox peaks were obtained for each, with similar electrochemical potentials, providing a thermodynamic basis for the Nernstian potential-driven SMRT reaction, as shown in Figure 1b. It was further discovered that the addition of mixed cations in the electrolyte positively affected the concentration of [Fe(CN)6]4-, showing an increase of more than three times. When combined with PB, the maximum concentration of [Fe(CN)6]3-/4- in the catholyte can theoretically reach 10.0 M at room temperature, corresponding to a volumetric capacity and energy density of the catholyte containing [Fe(CN)6]3-/4- of 268.0 Ah L−1 and 260.0 Wh L−1, respectively, as illustrated in Figure 1c.

Further research found that after adding 5.2 mM PB particles to the catholyte at the end of the 7th cycle, the capacity of the symmetric cell increased sharply from 219.7 to 562.1 mAh, with a solid utilization rate as high as 61.0%, as shown in Figure 2a. The symmetric cell with SMRT reactions occurring between [Fe(CN)6]3-/4- and PB/PW demonstrated long cycle stability over 500 cycles (4200 hours), with a cumulative capacity decay of less than 0.03%, as illustrated in Figure 2b. Subsequently, a PB−Fe/S full cell with diluted catholyte (0.1M K3Fe(CN)6) and excess anolyte was assembled and studied, as shown in Figure 2g. Initially, without PB particles, the initial discharge capacity was only 39.0 mAh. After 7 cycles, PB was introduced into the catholyte reservoir, and through the SMRT reaction, the battery capacity more than doubled the initial value, reaching 70.9 mAh. The capacity did not increase immediately after the addition of PB, as it did in the symmetric cell, but gradually increased over 1000 cycles (500 hours) to reach its maximum value, indicating that activating the SMRT reaction in diluted catholyte requires some time. Additionally, the cell exhibited excellent cycle stability over 7000 cycles (4500 hours), with the Coulombic Efficiency (CE) consistently maintained at a high level around 100%.

Subsequently, the authors investigated the stability of the flow battery at high temperatures and found that it could operate stably at temperatures as high as 50°C. Charge/discharge tests of the PB-Fe/S flow battery were conducted at 30°C, 40°C, and 50°C, and the results indicated that the flow battery exhibited good cycle stability within the range of 30 to 50°C, with a Coulombic Efficiency (CE) around 100%, as shown in Figures 3a-c. Additionally, after adding PB to the catholyte after the first cycle, the capacity increased by 109.5 mAh, 112.5 mAh, and 130.6 mAh at 30°C, 40°C, and 50°C, respectively, as illustrated in Figures 3d-f, demonstrating that the utilization rate of solid energy storage materials in the catholyte improves with increasing temperature. However, as the temperature rises, the capacity decay becomes more pronounced, which is due to the accelerated crossover of active materials through the ion exchange membrane at higher temperatures. Furthermore, from the selected charge/discharge voltage curves, the potential values of the battery decreased with the addition of PB and the increase in temperature, as shown in Figures 3d-f, which is related to the negative shift of the equilibrium potential with PB and the negative temperature coefficient of the [Fe(CN)6]3-/4- potential. The good thermal adaptability of the PB−Fe/S RFB cell is beneficial for expanding its application under different climatic conditions and reducing the cost of cooling systems.

Due to the counterion effect, the concentration of [Fe(CN)6]3-/4- in the catholyte increases, shortening the response time of the SMRT reaction and simultaneously enhancing the battery's energy density. By using a mixture of Na+ and K+ counterions, the concentration of [Fe(CN)6]4− (K4[Fe(CN)6](K4) + Na4[Fe(CN)6](Na4)) significantly increases to 1.62 M, corresponding to a volumetric capacity of 43.4 Ah L−1 and an energy density of 42.1 Wh L−1, as shown in Figure 4a, which is markedly higher than the solubility limit of 0.80 M K3[Fe(CN)6](K3) reported in KCl supporting electrolyte, with a theoretical volumetric capacity of 21.4 Ah L−1. Combined with the SMRT reaction, the volumetric capacity and actual energy density of the concentrated [Fe(CN)6]4− catholyte (K4+Na4 with PB) increase to 87.9 Ah L−1 and 85.3 Wh L−1, and further improve to 95.7 Ah L−1 and 92.8 Wh L−1 during cycling.

Furthermore, the shortened response time of the SMRT reaction indicates that the reaction rate of solid energy storage materials in concentrated electrolytes is faster than in dilute electrolytes. Considering that the majority of the electrolyte is located in the battery chamber and tubing, with a porosity of the solid of about 50.0%, the theoretical packing amount of PB particles in the catholyte reservoir reaches 1.0 g mL−1, the theoretical volumetric capacity and energy density of the catholyte can reach unprecedented high values of 155.7 Ah L−1 and 151.0 Wh L−1. Additionally, after the addition of PB particles in the catholyte reservoir, the output power of the PB-Fe/S flow battery increases from 247.2 mW cm−2 to 284.7 mW cm−2, as shown in Figure 4b. The SMRT reaction between [Fe(CN)6]4− and PB in the catholyte regenerates [Fe(CN)6]3−, thereby extending the duration of high SOC, which is beneficial for improving output power.

Moreover, the CE of the PB-Fe/S flow battery containing concentrated [Fe(CN)6]4− remains above 99.6% after 100 cycles (382 hours), demonstrating good cycle stability at a current density of 30 mA cm−2, as shown in Figure 4c. In extended cycle testing, both charge and discharge volumetric capacities gradually increase without any decay, validating the boosting effect of PB particles in the catholyte reservoir. When the current density increases from 30 mA cm−2 to 120 mA cm−2, in increments of 30 mA cm−2, the CE remains stable around 100%, supporting the high cycle stability of the PB-Fe/S flow battery, as shown in Figure S8. Although the energy efficiency (EE) and voltage efficiency (VE) decrease with the increase in current density due to severe polarization of the battery at higher current densities, the battery performance recovers when the current density returns to the initial value, proving the excellent stability of the proposed battery.

To further evaluate the feasibility and scalability of the proposed PB-Fe/S RFB system, a three-cell stack was assembled, as shown in Figure 4d. The catholyte containing 1.30 M [Fe(CN)6]4− (0.65 M K4[Fe(CN)6] + 0.65 M Na4[Fe(CN)6]) was used. The PB-Fe/S cell stack operated stably for 200 cycles (480 hours) at 30 mA cm−2, with the CE consistently above 98.0%, as shown in Figure 4e. During the cycling process, due to the SMRT reaction between [Fe(CN)6]3−/4− and PB/PW, the volumetric capacity of the catholyte containing [Fe(CN)6]3−/4− increased from 34.1 Ah L−1 to 49.0 Ah L−1. Additionally, the PB-Fe/S cell stack exhibited good rate performance, as shown in Figure 4f. At a high current density of 120 mA cm−2, the CE remains at a high value of about 98.9%. Moreover, when the current density returns to 20 mA cm−2, both EE and VE recover to high values, indicating good reversibility and stability of the chemicals on both sides. After the addition of PB, the output peak power density of the cell stack increased from 215.0 mW cm−2 to 242.3 mW cm−2, as shown in Figure S11.

The combination of mixed counterion catholytes and the SMRT reaction between [Fe(CN)6]3−/4− and PB/PW effectively increases the concentration of [Fe(CN)6]3−/4− and accelerates the kinetics of the redox reaction. As a result, the concentration of [Fe(CN)6]4− in the catholyte is elevated to 1.62 M at room temperature, rapidly activating the SMRT reaction with PB, enabling the actual volumetric capacity and energy density of the [Fe(CN)6]3−/4− containing catholyte in the PB−Fe/S flow battery to reach 95.7 Ah L−1 and 92.8 Wh L−1, respectively. Moreover, the chemical cost of the electrolyte in the battery is reduced to as low as $19.26 per kWh, as shown in Figure 4g. The PB-Fe/S flow battery exhibits an ultra-long lifespan, capable of 7000 cycles over 4500 hours, and outstanding capacity, reaching 181.8% of the capacity without PB at 15 mA cm−2, as illustrated in Figure 4h. Additionally, it demonstrates good thermal adaptability up to 50°C. Unlike all-vanadium RFBs in strong acid solutions, the PB-Fe/S flow battery operates in a neutral solution, reducing maintenance costs during system operation. Economically efficient homemade membranes can replace expensive Nafion membranes and perform well under mild conditions, which will further enhance battery performance and reduce costs. In summary, compared to other neutral aqueous flow batteries, the PB-Fe/S system shows exceptional performance and economy. These advantages make it a promising candidate for large-scale energy storage in commercial applications.

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