Aqueous redox flow batteries (RFBs) have attracted significant attention as energy storage systems by virtue of their inexpensive nature and long-lasting features. Although all-vanadium RFBs exhibit long lifetimes, the cost of vanadium resources fluctuates considerably, and is generally expensive. Iron–chromium RFBs take advantage of utilizing a low-cost and large abundance of iron and chromite ore; however, the redox chemistry of CrII/III generally involves strong Jahn–Teller effects. Herein, we introduce a new Cr-based negolyte coordinated with strong-field ligands capable of mitigating strong Jahn–Teller effects, thereby facilitating low redox potential, high stability, and rapid kinetics. Density functional theory (DFT) calculations reveal that the complex of [Cr(CN)6]4− prefers low-spin states, facilitating a stable and fast redox reaction. The prototype full-cell configuration features a high-energy density of 11.4 Wh L− 1 and a stable lifetime of 250 cycles. Consequently, our proposed system opens new avenues for the development of high-performance RFBs.
Article
Hexacyanometallate aqueous flow battery
https://doi.org/10.21203/rs.3.rs-568177/v1
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Sustainable energy storage systems (ESSs) have received a great deal of attention to satisfy emerging demands at grid-scale without compensation for large-scale and long-time operation; additionally, they have low maintenance and cost 1,2. However, the current lithium-ion battery (LIB) technology might not be chosen as an ideal candidate to replace active materials in the positive and negative electrodes in grid energy storage because faded LIBs result from the degradation of active materials (cathode and anode) or the dried electrolyte. Maintenance of LIBs incur relatively high costs and involve complicated processes. In this regard, aqueous redox flow batteries (RFBs) have been widely employed as an alternative for economical ESSs 3. One noticeable advantage of aqueous systems is their very high dielectric constant, allowing soluble redox couples to dissolve with large quantities in an aqueous solution with dissociation. The high solubility also benefits to increase the available energy densities and the output current would be controlled by the size of the electrode stacks in RFBs. Of course, aqueous systems are advantageous in many aspects of: they are inexpensive, environmentally benign, and highly soluble rather than the use of organic liquid systems 4. As shown in Fig. 1a replacing or rebalancing negolytes and posolytes can be readily altered while maintaining infra-tank and pump systems; such properties are well-suited for stationary energy storage 5. All-vanadium redox flow batteries (VRFB) have been widely studied since the same elements are employed in both the negolyte (VII/III) and posolyte (VIV/V) and since such systems feature long calendar and cycle lives with more than 200,000 cycles 6,7. Nevertheless, the price of vanadium source, V2O5, has been highly fluctuated up to 28.8 USD/lb at the end of 2018, resulting in high costs and performance as 1,000 USD/kWh approximately 8.
The concept of iron-chromium RFBs using ferrous/ferric (FeII/III) and chromous/chromic (CrII/III) ions is the first chemistry taking advantage of low cost and widely abundant iron and chromite ore 2,9,10. These FeII/III redox couples are expected to be more stable as a posolyte with a combination of cyanide ligands that have extensively used to a posolyte material in RFB reference 11–14. However, the redox chemistry of CrII/III in chromium-based negolytes generally involves strong Jahn-Teller effects due to the unequal occupation of electrons in eg orbitals of high-spin CrII (d4) as shown in Fig. 1b. This can lead to simultaneous splitting of the electronic states and a symmetry-lowering distortion. This phenomenon limits the electrochemical reactions as a low current density and energy efficiency, resulting in the sluggish kinetics of the CrII/III redox couple (k0 < 10-6) 14–17 and low Coulombic efficiency, which is corroborated by a severe hydrogen evolution reaction (HER) arising from the low reduction potential (-0.41 V vs. SHE) under acidic conditions 18. Interestingly, the d4 electron configuration of CrII can alter its spin states from high-spin to low-spin depending on the increase of the energy gap (∆o) between eg and t2g orbitals. Coordination compounds composed of a metal ion and ligands offer the benefit of pi (π) interactions, which facilitate dramatic effects on the triply degenerate t2g orbitals. Ligands such as carbonyls (CO) and cyanides (CN-) are π acceptors with empty orbitals that can interact with metal d orbitals in a π fashion, leading to the stabilization of t2g orbitals. It has been suggested that the significant electronic stabilization is beneficial to stable CrII/III redox reactions enabled by the low-spin configuration of CrII. As shown in Fig. 1c, the energy split between eg and t2g orbitals can increase from the free metal ion state to coordination with a strong-field ligand, playing a crucial role in shifting a redox potential more negative figures according to spectrochemical series 19. In particular, when CrIII changes its oxidation state to CrII at a high-spin state, electrons are transferred into the eg orbitals, whereas electron transfer can occur into t2g orbitals in the coordination complex covalent with strong-field ligands, manipulating the redox potential of CrII/III via the use of suitable ligands and simultaneously mitigating strong Jahn-Teller effects.
Although various studies have provided some negolytes in a chromium-based system, there is a need for negolytes containing low-redox-potential with fast kinetics and stable cyclability 13,14,20–22. Bae et al. showed that chromium-ethylenediaminetetraacetic acid (Cr-EDTA) can exhibit a low reduction potential (-0.99 V vs. SHE) with reversible cathodic and anodic processes. However, the designed battery have shown slow kinetics operated at very low current density of 2.5 mA cm-2, and a very low energy efficiency of less than 7% 20,21 due to the large overpotential of chromium-based redox couples. Slow kinetics inhibit the power density of batteries and occur in the comparable reaction of hydrogen evolution, although chromium complexes have merits of low redox potential, allowing for an overall high voltage. Marshak et al. introduced 1,3-propylenediaminetetraacetic acid chelated chromium (Cr-PDTA) as a negolyte material using a strategy of chelating ligands. The full-cell couple with the posolyte, [Fe(CN)6]4-/3- showed a high discharging potential of approximately 1.5 V and an improved the energy efficiency of 78.1% during cycling at ±100 mA cm-2, which provides the best performance among the new chromium-based RFBs. However, such cycle life of 70 still needs to be ameliorated. Chen et al. found various chromium complexes for Cr-based RFBs, such as Cr-bipyridine ([Cr(bipy)2(H2O)2]3-), Cr-dipicolicic acid ([Cr(DPA)2]-), Cr-iminodiacetate ([Cr(IDA)2]-), and chromium 3-((2,6- bis (ethoxycarbonyl) pyridin-4-yl)oxy)-N,N,N-trimethylpropan-1-aminium bromide ([Cr(f-DPA)2]+) 14. This work assessed that the charge transfer kinetics, stability, and solubility of a complex would be corroborated by the suitable molecular design. Therefore, the better designs of material chemistry are indubitably required in order to resolve the performance issues associated with the aforementioned systems.
In this work, we introduce a new Fe-Cr RFB system coordinated by strong-field cyanide ligands to mitigate strong Jahn-Teller effects for high-energy and stable performances. Chromium complexes coordinated with strong-field cyanides can allow low redox potentials compared to complexes coordinated with weak-field ligands due to the electron transfer to t2g orbitals rather than eg orbitals. Since this system is introduced for the first time, we have carefully evaluated diverse assessment methods for the unique features with low redox potential, high stability, and fast kinetics below the potential of hydrogen evolution reactions as shown in Fig. 2a. A thermodynamic analysis of the redox reaction energetics using density functional theory (DFT) calculations suggests that the [Cr(CN)6]4- complex is preferentially associated with low-spin states relative to complexes coordinated with weak-field ligands, facilitating a stable and fast redox reaction. In order to enable further development of redox flow system, we matched the negolyte with a K3[Fe(CN)6] posolyte as a full cell and performed various investigations to confirm the proper redox at different SOCs. Benefiting from the aforementioned uniqueness, this hexacyanometalate-based RFB is capable of reversible redox reactions with relatively high potential as an aqueous system taking advantage of the low potential of [Cr(CN)6]4-/3- and improved cycling performance at high current densities. Furthermore, we expand on the considerable degradation mechanisms that play a crucial role in governing the kinetics and stability of the reaction and provide important insights into the design of a stable RFB.
Electrochemical properties of hexacyanochromate with respect to low redox potential, high stability, and fast kinetics
Facile synthetic K3[Cr(CN)6] was employed as a source of [Cr(CN)6]4-/3- in our experiment; this enabled us to take advantage of the low chemical cost of chromium(III) acetate, which is beneficial compared to the onerous price of commercially available K3[Cr(CN)6] (Sigma Aldrich, 99.99%). X-ray diffraction (XRD) profiles in Supplementary Fig. 1 reveal that as-synthesized K3[Cr(CN)6] corresponds to the XRD patterns of both commercialized K3[Cr(CN)6] and PDF 00-027-1350. Cyclic voltammograms (CVs) in Fig. 2a,b show reversible redox reactions of [Cr(CN)6]4-/3- anions, which possess the reduction and oxidation peaks at -1.19 and -1.11 V vs. SHE, respectively. Whereas CV of CrII/III in CrCl3 redox reaction denotes serious irreversibility with a considerably small oxidation peak near -0.11 V and a negligible reduction peak, even at the same scan rate of 20 mV s-1. The CrCl3 reaction also shows poor activity with very low current density. The reliable electrochemical properties of [Cr(CN)6]4-/3- are unique because of its low redox potential of approximately -1.15 V, which is lower than the potential of HERs in an aqueous system. The redox potential could arise from complexation effects, leading to the stabilization of the energy level of t2g orbitals in the complex 19. Simultaneously, controlling supporting electrolytes facilitate redox reactions with high stability and fast kinetics. To validate the plausible properties of [Cr(CN)6]4-/3-, various aspects influencing such formidable properties are needed to discuss in more detail in the next section.
Hexacyanometallates are known to be more stable at acidic pH than basic pH due to the ligand exchange of CN- to OH-, resulting in the destruction of the hexacyanometallates 23. This ligand exchange is the dominant loss mechanism during electrochemical reactions in RFBs, thereby the condition mitigating this ligand exchange is suggested to extend their lifetimes. However, reductive electrolysis to hydrogen could occur at higher potential in acid, which in turn limits the use of negolytes with low reaction potentials. Supporting electrolytes can open up new opportunities to suppress undesirable ligand exchanges during the reaction. For example, high concentrations of supporting electrolytes limit trace solubility, potentially extending the cathodic electrochemical stability window of water beyond the thermodynamic level of -0.8 V vs. SHE 24. In addition, concentrated supporting electrolytes decrease the chemical activity of water, resulting in less ligand exchange to OH-. As a cyanide-based supporting electrolyte, sodium cyanide was selected for the excellent electrochemical stability, especially against very reductive potentials, combined with a high solubility in water. Long-term CV data in Fig. 2c,d compares the stability of [Cr(CN)6]4-/3- redox reactions in the cyanide-based supporting electrolyte (NaCN) to the NaCl supporting electrolyte. The initial cycles using both NaCl and NaCN supporting electrolytes in Fig. 2c,d clearly show the reversible reduction and oxidation reactions near -1.2 and -1.1 V, respectively. Whereas the current density when using the NaCl electrolyte rapidly diminished and was rarely detected after 70 cycles, suggesting that the NaCl supporting electrolyte did not alleviate the ligand exchange and the redox species might result in forming Cr(OH)x via side reactions; such material might be deposited on the glassy carbon electrode 25. On the other hand, the current density and redox potentials using NaCN supporting electrolyte were maintained even after 500 cycles as shown in Fig. 2d, which was expected since the ligand exchange was limited by the number of cyanide ligands. Unlike the chloride based solutions, the excessive cyanide ions played a significant role in suppressing decomposition and maintaining a high stability of [Cr(CN)6]4-/3-.
The fast kinetics of electrochemical reactions can be understood by analyzing associations between the CV scan rate and the corresponding current density as shown in Fig. 2e,f. The CV data, which were scanned from 5-500 mV s-1, show small potential shifts from 81-134 mV (Fig. 2e and Supplementary Table 1) and exhibit high current densities during redox processes, even at the high scan rate of 500 mV s-1, indicative of a stable redox system with high charge and discharge conditions. The corresponding peak currents exhibit a linear relationship with respect to the square root of the scan rates as shown in Fig. 2f; this means that the [Cr(CN)6]4-/3- redox couple has a quasi-reversible electrochemical property. The small ∆Ep is comparable with the value in the diffusion-controlled kinetic regime such that the slope in Fig. 2f can thus be used to determine the diffusion coefficient (D0) via the Randles-Sevcik equation. Since the slopes of the cathodic and anodic processes are ±0.0002, the diffusion coefficient D0 is calculated as 7.16 ×10-6 cm2 s-1. The charge transfer rate (k0) can also be calculated using the Nicholson method that provides the information about the relationship between ΔEp and k0 (Table S2) 26. According to this method, the k0 is calculated as 6.03 × 10-3 cm s-1, facilitating that [Cr(CN)6]4-/3- redox couples show outstanding performance without any catalysts compared to other Cr-based materials 18,20–22. Table S3 compares the electrochemical properties of the reduction potential, diffusion coefficient, and charge transfer rate in the hexacyanochromate complex with those obtained by previous studies. In particular, k0 of [Cr(CN)6]4-/3- redox couple is approximately 6,000 times higher than CrII/III redox couple in CrCl3 without catalysts and even six times higher than that with Bi catalyst. Additionally, it is higher than that of CrII/III-PDTA redox couple (1.7×10-4 cm s-1), which had previously been considered to have the best performance among Cr-based systems. Our electrochemical analyses of the [Cr(CN)6]4−/3− complex indicate that it exhibits desirable behavior that enables fast kinetics with high stability at low redox potentials below the hydrogen evolution reaction, which can be a promising candidate as a negolyte for sustainable RFB systems.
Simulated electronic configurations, Jahn-Teller distortions, and redox reaction energies of CrII-based compounds. a, Energy levels of electronic orbitals. Upward arrows represent electrons with up-spin and downward arrows represent electrons with down-spin. Black solid and dashed lines denote occupied molecular orbitals (MOs) for up-spin and down-spin, respectively. Blue solid and dashed lines indicate unoccupied MOs for up-spin and down-spin, respectively. b, Molecular structures with bond lengths of ligands and distortion index D of octahedral complex. c, Calculated energy changes during CrII/IIIredox reactions in these compounds.
Theoretical consideration by DFT simulation of chromium-based compounds
We performed molecular simulations to understand the complex chemistry behind the stable redox reaction of the [Cr(CN)6]4-/3- coordination compound at low potential. By calculating the electronic structures and redox reaction energies of various CrII-ligand complexes using density functional theory (DFT), we revealed the complex interplay between ligands and electronic structure in governing redox potential. Since frontier molecular orbitals (MOs), including the highest occupied and lowest unoccupied MOs, are important for the redox behaviors of coordination compounds, we compared electronic configurations of the frontier MOs of representative CrII-based compounds using DFT calculation as presented in Fig. 3a. We considered triplet (t2g4) and quintet (t2g3eg1) states for each CrII-based compound in order to find the energetic stable spin states (Table S4). [CrCl6]4-, [Cr(H2O)6]2+, and [Cr(NH3)6]2+ are stable in their quintet states (high-spin state), whereas [Cr(CN)6]4- is stable in its triplet states (low-spin state); these finding are consistent with conventional understanding that strong-field ligands such as CN- result in low-spin states due to their large d orbital splitting.
Since the CrII-based compounds with high-spin state have single electron at eg states, thereby leading to strong Jahn-Teller distortions, bonds with two ligands along z direction are more elongated than the bonds with other fours, as shown in Fig. 3b. In contrast, [Cr(CN)6]4- has weak Jahn-Teller distortions due to unevenly occupied t2g orbitals in the low-spin state, allowing for negligible elongation of bonds along the z direction. To compare distortion degree, we calculated the distortion index using the following equation:
Full redox flow battery using hexacyanometallates
After carefully validating the desirable properties, we built a full RFB cell composed of [Cr(CN)6]4-/3- and [Fe(CN)6]4-/3- redox couples as shown in Fig. 1a and Supplementary Fig.2. The preliminary half-cell data in Fig. 2a uggest that the standard thermodynamic potential of the two negolyte and posolyte is 1.64 V. However, it is notorious that a half-cell CV test does not always correlate to full-cell data measured in a constant current mode because the potential-derived characterization is imprecise due to the overlapped faradaic response for the forward process, especially in the potential regime below HER. In this regard, we have verified the galvanostatic profile for the full-cell configuration, showing a highly reversible capacity of 10,200 mAh L-1 at a discharging current density of 50 mA cm-2 with an average potential of 1.43V (Fig. 4a. In particular, the redox mediators of [Cr(CN)6]4-/3- are coordination compounds that are well known to exhibit vivid colors depending on the d orbital energy gap of transition metals. During the discharge process, the color of the negolyte in Fig. 4a changes from dark brown to light red, as also shown in Supplementary Movie S1. The UV-vis spectra depending on the state of charge (SOC), shown in Fig. 4b, also reveals corresponding species such that the peak at 326 nm indicates the charged state of [Cr(CN)6]4- 27–29. As the color changes to light red, the correlated peaks at 307 and 377 nm increase together with increased concentrations of [Cr(CN)6]3-, implying that comprehensive redox change occurred in the full-cell configuration with excellent reductive stability. Another post-mortem XRD analysis also manifests successful conversion to [Cr(CN)6]4- as shown in Fig. 4c. We used dried powder obtained by evaporating solvent of the negolyte at the different SOCs of 0, 50, and 100 %. Asterisks (*) in Fig. 4c indicate the information of sodium cyanide (NaCN) reference, which was exploited to calibrate peak positions across different samples. The enlarged XRD patterns at 32-35 degrees in Fig. 4c clearly elucidate that the dried samples show good agreement with the conventional K3[Cr(CN)6] and K4[Cr(CN)6] phases upon the SOCs, implying a successful phase transition from [Cr(CN)6]4- to [Cr(CN)6]3-.
Cell polarization curves and power densities in Fig. 4d, E enable to the fast kinetics of [Cr(CN)6]4-/3- to be deduced for the full-cell configuration. This advance allows accurate demonstration of the electrochemical behavior of [Cr(CN)6]4-/3- as well as the implementation of hexacyanometallates for a genuine energy storage application with competitive performance. Using our characterization of the current-voltage behavior at various SOC values (0 %, 10 %, 25 %, 75 %, and 100 %), we achieved a remarkable performance of [Cr(CN)6]4-/3- even without using any catalysts, such as bismuth, titanium, or a porous current collector, especially at high current densities above 700 mA cm-2 30–33. Furthermore, the maximum peak power density reaches 410 mW cm-2 as shown in Fig. 4e, denoting that the material is a promising candidate for large-scale energy storage field. This charge transfer kinetics in the full-cell configuration does not seem to limit power output, which is in good agreement with the rapid kinetics presented in Fig. 2e,f. Finally, we tested the long-term cycling performance of the full-cell composed of [Cr(CN)6]4-/3- and [Fe(CN)6]4-/3- complexes at a current density of 100 mA cm-2 (Fig. 4f and Supplementary Fig. 3). No obvious decay occurred with respect to Coulombic efficiency, energy efficiency, and normalized capacity for 250 cycles. Coulombic efficiency is another vital factor for evaluating the electrochemical performance of hexacyanometallates because it determines the reversibility of the redox processes and the inactivity of HER. The Coulombic efficiencies of the hexacyanometallates present above 99% to 250 cycles, inferring the truly reversible reaction of redox mediators. The energy efficiency was also maintained to 76-77% attributed by overpotential between charge and discharge at the high current density. The capacity decay was calculated to be less than 0.02% per cycle, denoting that there are no significant side reactions such as fatal decomposition of active materials or HER. Based on these tests, we have confirmed that the fast kinetics with high stability continuously occur from [Cr(CN)6]4-/3- and [Fe(CN)6]4-/3- complexes, enabling the stable operation of even the full-cell system. Our results show that the full cell exhibits a prolonged cycle life with promising electrochemical properties, which are compared to previous results in Table S5.
For a complete assessment of the study materials for practical applications, we have to consider the significant degradation mechanisms of RFBs, such as crossover, self-decomposition of active materials, and electrolyte side reaction 5. Crossovers denote the ion-transport of active electrolytes across ion-conducting membranes leading to undesirable migration of ions in posolytes, negolytes, and/or supporting electrolytes. In this work, we employed a cation-exchanged Nafion membrane that transports mobile cations and excludes the negatively charged anions either by size or Donnan exclusion effects (Fig. 1a). Since [Cr(CN)6]4-/3- and [Fe(CN)6]4-/3- hexacyanometallates are surrounded by strong-field ligands of cyanides, the outer region of the octahedral redox species is strongly negative, resulting in complete exclusion dominated by electrostatic interactions within the membrane. Bakajin et al. measured ion exclusion coefficients using the Donnan model, suggesting that K3Fe(CN)6 showed near-complete exclusion due to its high cation-anion valence (1/3), which indicates high electrostatic repulsion 34. The self-decomposition of active materials and electrolyte side reactions should also be considered for stable RFBs since the chemical decomposition of redox species causes deterioration of the intrinsic property of active materials. [Fe(CN)6]4-/3- is widely used as a reference posolyte due to its highly stable performance without compromising the self-decomposition of active materials. In the case of [Cr(CN)6]4-/3-, we observed that the charged state of [Cr(CN)6]4- tends to partially exchange cyanide with hydroxyl (OH-) ligands via photoaquation mechanism, which is well-known in metal complexes 27–29 (Supplementary Fig. 4a). It is also known that the photo-excitation of the hexacyanometallates might lead either to its dissociation into CN- and [Cr(CN)5]3- fragments or the formation of [Cr(CN)5(H2O)]3- in a general aqueous medium29,35. However, cyanide complexes tend to show different features depending on the solvent interaction, thereby influencing the electronic structure. A feasible mechanism of ligand stability in this system is that the cyanide-based supporting electrolyte can offer an opportunity to control the condition of the aqueous solvent. As shown in Supplementary Fig. 4b, although as-synthesized K3Cr(CN)6 is coordinated with aqua ligands observed in UV-vis spectra, the subsequent complex after first cycle verifies ligand exchange to cyanides forming K3Cr(CN)6 28,29. Our results provide clear examples both of the energy dependence of ligand exchanges and of the role of supporting electrolytes in the reactivity of metal complexes. We therefore conclude that by harnessing this advantage, the suggested RFB system can operate the prolonged cycles without crossover problems and chemical decomposition, including ligand exchange.
Although hexacyanometalate-based RFBs have shown promising electrochemical performances against the aforementioned issues, some unexpected degradation, such as leakage of electrolytes and precipitation as salts, may occur in the system, thereby facilitating the replacement or rebalancing the ratio of the active material solutions while maintaining the infra-tank and pump systems. To evaluate conceptual and technological potential, we prepared an intentionally degraded negolyte and replaced it with a new K3[Cr(CN)6] solution as shown in Supplementary Fig. 5. Following the replacement, the specific capacity reaches the original performance after a couple of cycles for aging processes that are required to match the counterpart capacity with the posolyte. It is also recommended to adjust the posolyte and negolyte volume with maintaining a similar concentration at both sides in order to avoid water migration by osmotic pressure to design the commercially applicable RFBs. However, the solubilities of posolytes and negolytes do not generally correspond with each other, minimizing the overall energy density. Herein, we employed 0.4 M K3[Cr(CN)6] as a negolyte because of the limited solubility of K4[Fe(CN)6] used for the posolyte. Considering the solubility of K3[Cr(CN)6] per se, up to 1.1 M can be dissolved in water with 2 M NaCN supporting electrolytes. The solubility in water with 2 M NaCN supporting electrolyte was measured by fitting the UV-vis spectra as shown in Supplementary Fig. 6. The peak intensity of the fully saturated solution is 1.28014 at a wavelength of 432 nm. Compared to values at other concentrations of 0.05 to 0.4 M, we can deduce the unknown concentration to be approximately 1.1 M, allowing energy density beyond 30 Wh L-1 in our RFB system. Otherwise, according to previous studies, the solubility of ferrocyanide could increase to 1.5 M by cation exchange to ammonium ions, resulting in the high-energy density 36.
The outstanding properties of hexacyanometallates are not only limited to RFBs, possible exploring their application in a variety of ambitious projects. The main advantage of the [Cr(CN)6]4-/3- redox system is the very low redox potential of the negolyte due to the coordination with strong-field ligands retaining high stability even against HER. A comparison against other well-known aqueous RFB systems (Fig. 5) shows that this high overall potential is contributed by the potential of the negolyte, [Cr(CN)6]4−/3−, suggesting a higher overall potential matching with high redox posolytes. We note that the low redox potential of [Cr(CN)6]4-/3- will benefit from knowledge obtained by other electrochemical technologies, such as CO2 electroreduction processes and biomass conversion 37. Therefore, we believe that this invention will contribute significantly to stationary energy storage in other areas that prefer to combine with continuous-flow electroreduction systems.
In the quest for sustainable RFBs, we have discovered the new negolyte, K3[Cr(CN)6], which exhibits a low redox potential of -1.15 V vs. SHE, stable cycling performance, and fast charge transfer kinetics. The coordination with strong-field ligands observed herein is expected to play important roles in governing the low-spin state of CrII, thereby allowing low redox potential during electrochemical cycling. On the basis of a wide variety of RFB chemistries beyond hexacyanometallates, our results, coupled with high stability and fast kinetics, will help to guide the development of RFBs with high-energy densities and prolonged lifetimes. Our thermodynamic DFT calculation also confirms that the cyanide-coordinated molecules can promote the fast kinetics attributed by virtue of their facile redox reaction via weak Jahn-Teller effects rather than the strong Jahn-Teller effects characterizing other compounds. The prototype full-cell configuration highlights a high-energy density of 11.4 Wh L− 1 and a stable lifetime to 250 cycles when using a 0.4 M concentration of both active electrolytes. As K3[Cr(CN)6] itself, it can dissolve up to approximately 1.1 M concentration, expecting to achieve a higher energy density than 20 Wh L− 1 beyond the challenges in vanadium redox flow battery. Consequently, we believe that our new proposed [Cr(CN)6]4−/3− systems represents a novel and scientifically intriguing material that can push forward the development of grid energy storage.
Synthesis of Potassium Hexacyanochromate (K3[Cr(CN)6])
K3[Cr(CN)6] was synthesized following a modified method based on previous literature 38,39. Cr(CH3COO)3 (JUNSEI, Cr 21-25 %) was dissolved in 20 mL distilled (DI) water and this solution was transferred into a KCN (Alfa Aesar, 96.0 %) solution (8.93g of KCN in 30mL DI water). 0.24g of activated charcoal was added during the reaction After several minutes, activated charcoal was filtered and the filtered solution was partially dried in order to evaporate the remnant solvent. Finally, the solution was chilled to precipitate K3[Cr(CN)6]. This precipitated K3[Cr(CN)6] was filtered and washed by ethanol five times, then dried in a vacuum oven at 40 oC. The synthesized K3[Cr(CN)6] was characterized by XRD and compare with commercial K3[Cr(CN)6] (Sigma Aldrich, 99.99%).
Half-cell characterization
Cyclic voltammetry (CV) measurements were conducted using an EC-Lab potentiostat (BioLogic). A three-electrode system was employed with an Ag/AgCl reference electrode filled with a 3 M KCl solution, Pt wire as a counter electrode, and a 3 mm diameter-glassy carbon working electrode. A half-cell test was performed using the 5 mM and 0.1 M solutions containing synthesized K3[Cr(CN)6] with 1 M supporting electrolytes. To compare the redox reaction of chromium, we conducted experiments using the CrCl3 redox system as a reference. 5 mM CrCl3 (Sigma Aldrich, ≥ 98.0 %) was dissolved in DI water with the supporting electrolyte of 1 M HCl.
Flow cell (Full-cell) characterization
30 mL of K3[Cr(CN)6] solution (0.1, 0.4, and 1 M) was used as an anolyte with 2 M NaCN (Alfa Aesar, 95 %) as the supporting electrolyte. As a catholyte, 0.15 M K3[Fe(CN)6] (Sigma Aldrich, 99 %) and 0.3 M K4[Cr(CN)6] . 3H2O (Sigma Aldrich, 98.5 % - 102.0 %) were dissolved in 2 M NaCl (Alfa Aesar, 99.0 %) supporting electrolyte, for which the total volume was 40 mL. The flow battery cell (TS CHEM) was assembled with 5 cm2 (2 cm × 2.5 cm) carbon-felt electrode (XF-30a, TOYOBO), graphite plate, copper current collector, and Nafion membrane. Nafion (NRE-212, Sigma Aldrich, NafionTM perfluorinated membrane, thickness 0.002 inch) was soaked in a 0.1 M HCl solution for 30 min then rinsed in DI water. The carbon felt electrode was heated to 500 oC for 5 hrs under an air atmosphere. The anolytes and catholytes were circulated in a tube (Masterflex Norprene tubing, I.D 3.18 mm, O.D 6.35 mm) by pump drives (JenieWell, JWSE600) at an average flow rate of approximately 40 mL/min, where all carbon felt electrodes was compressed by ~70% in a stack cell.
A galvanostatic charge-discharge test was performed using an EC-Lab potentiostat at room temperature. Before taking measurements, we circulated both the anolyte and catholyte for 2 hrs without applying a bias. A constant current was applied during the charging and discharging processes (100 mA cm-2) for a voltage cutoff range from 1 to 1.9 V. Power tests were conducted by potentiostat (IviumStat) at each state of charge (SOC). Current scanning was employed during the discharging state in order to obtain peak power densities, which the cell was charged at a constant current density of 30 mA cm-2.
The state of the anolyte during full-cell operation was investigated using UV-vis spectroscopy. The cell was charged at 30 mA cm-2 and discharged at 50 mA cm-2 from 1.0 to 1.9 V. We extracted 0.2 mL of the solution at each SOC (0 %, 25 %, 50 %, 75 %, and 100 %,) and diluted them 1/100 times (0.004 M).
Computational details
All first-principles calculations were conducted according to spin‐unrestricted density functional theory (DFT) using the Gaussian 09 package 40. Geometry optimizations and molecular orbital energy level calculations were carried out following Becke–Lee–Yang–Parr (B3LYP) hybrid functional 41 and the 6-31G basis set. The solvation effect of water was modeled using the integral equation formalism polarizable continuum model (IEFPCM). Various spin states from singlet to septet were considered and the state with the lowest DFT energy was selected for each molecule.
Solubility test
Various concentrations of K3[Cr(CN)6] solution were prepared (0.05 M, 0.1 M, 0.2 M, 0.3 M, 0.4 M, and unknown (saturated solution)). The unknown solution was made by adding K3[Cr(CN)6] into the 2M NaCN solution until the solid could no longer be dissolved, after which the residue was filtered. The concentration of the unknown solution was evaluated uinsg UV-visible spectroscopy. Absorbance is proportional to concentration. A pre-calibrated absorbance-concentration plot was obtained through the peak intensity at 429~432 nm for other solutions, and the solubility was calculated using this plot.
Acknowledgments
This work was supported by the 2020 Research Fund (1.200115.01) of UNIST and Individual Basic Science & Engineering Research Program (NRF-2019R1C1C1009324) through the National Research Foundation of Korea funded by the Ministry of Science and ICT.
Author Contributions
J.J. and H.-W.L. conceived the idea, designed the experiments, and analyzed the data. J.J. and R.K. conducted all experiments with the assistance of C.L., S.K., J.R., S.J., J.C., and W.C. D.-H.S. completed DFT simulations. J.J., R.K., D.-H.S., and H.-W.L. interpreted the results. J.J., D.-H.S., and H.-W.L. co-wrote the paper. All authors discussed the results and commented on the manuscript.
Competing Interests
The authors declare no competing interest.
Data Availability
The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
Additional Information
Correspondence and requests for materials should be addressed to D.H.S. and H.W.L.
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- SupplementaryMaterialsNatureEnergy.docx
Supplementary information
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Supplementary video