Hexacyanometallate aqueous flow battery

Abstract

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.

Mitigating Jahn–Teller Effects by Fast Electrode Kinetics Inducing Charge Redistribution

“…[ 32 ] The second behavior is that E 1/2 shifts to a higher potential as the scan rate increases, indicating a more dominant shift of E a , which is hindered by Na‐ion desertion ( k l.str ) (e.g., J–T effect). [ 33,34 ] Third, E 1/2 remains unchanged as the scan rate increases, indicating a reversible insertion and desertion of Na ions without any of the aforementioned obstacles. For 1 m SIC, E 1/2 tends to decrease at the reaction of CMn I/II and CMn II/III because Na ions, solvated by a large organic solvent, cause rapid concentration gradient (low k el ).…”

Surpassing water-splitting potential in aqueous redox flow batteries: insights from kinetics and thermodynamics

“…This greater reversibility of redox kinetics suggests that no significant side reactions take place. 45 More recently, Jang et al 174 reported a novel Fe-Cr RFB framework by employing the strong field cyanide ligand (CN À ) to diminish the impact of Jahn-Teller 175 distortion and enhance the operating voltage with trivial hydrogen evolution. The reported novel electrolyte, K 3 [Cr(CN) 6 ], exhibited a lower redox potential of À1.15 V SHE , retaining the stability against the HER and suppressing cross-over effects owing to the bulk size of [Cr(CN) 6 ] 4À , which coupled with a strong field p-acceptor ligand (CN À ) possesses vacant orbitals that interact with the d orbitals of the metal, leading to the stabilization of t 2g orbitals.…”