![]() The stronger interaction between K 4Fe(CN) 6 and NaOH reduces the lattice energy of K 4Fe(CN) 6 in solution, thus suggesting a higher solubility value. From the explicit solvation models, the adsorption energy of K 4Fe(CN) 6 in NaOH solution is 252.9 kcal/mol, which is significantly higher than that of K 4Fe(CN) 6 in KOH solution (238.5 kcal/mol, Fig. S1a and b). To verify this hypothesis, ab initio calculations based on density functional theory (DFT) were performed. In other words, it is hypothesized that if K 4Fe(CN) 6 is mixed with NaOH in aqueous media, the diverse ions may change the chemical activity and ionic strength of the solute, leading to solubility increasement of Fe(CN) 6 4− ions. Therefore, we propose that whether the ‘diverse ion effect’ may operate in the opposite way to improve solubility. The Fe(CN) 6 4−/Fe(CN) 6 3− redox pair is commonly used under alkaline environment, while the maximum concentration of K 4Fe(CN) 6 is further reduced when KOH is present due to the ‘common ion effect’, ,, ,, , ,29,31]. Therefore, increasing the maximum concentration of Fe(CN) 6 4− in supporting electrolyte is challenging yet beneficial toward advanced aqueous RFB systems with higher energy density. However, further improvement of these RFBs is obviously hindered by the low solubility of Fe(CN) 6 4−/Fe(CN) 6 3− in water at room temperature (the values of Na 4Fe(CN) 6 and K 4Fe(CN) 6 are 0.56 M and 0.76 M, respectively). The ferrocyanide/ferricyanide (Fe(CN) 6 4−/Fe(CN) 6 3−) redox pair with abundant, stable, and nontoxic natures as well as good electrochemical features has been commonly coupled with various anolytes, such as Zn(OH) 4 2−/Zn, anthraquinone derivatives, and alloxazine 7/8-carboxylic acid (ACA), to build high-performance RFBs, ,, ,, ,, ,, ,, ,, ,, , ]. However, when considering large-scale applications with energy density, capital cost, safety, and durability requirements, these well-known RFB systems are still unqualified, such as the corrosive halide oxidants (i.e., Br 2 and I 2), ,, ], the volatile price of vanadium, and the flammable and poorly conductive nonaqueous organic solvents (i.e., acetonitrile). A broad scope of RFB systems has been extensively studied. Redox flow batteries (RFBs) that can store and utilize intermittent renewable energy (e.g., wind and solar energy) are promising devices to meet the emerging demand for sustainable energy resources, ,, ,, , ].
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