Catalyst-Inspired Charge Carriers for High Energy Density Redox Flow Batteries

Abstract

We introduce a theoretical design approach aiming at improving energy density of redox flow batteries (RFBs) via the utilization of redox non-innocent ligands capable of stabilizing a metal center in a wide range of oxidation states. Our findings suggest that this promotes the possibility of multiple redox events as well as high open circuit voltages. Specifically, we have proposed two Fe-coordination complexes (I, Fe(Me2 Pytacn)(C 2 N 3 H 2), and II, Fe(H 2 pmen)(C 2 N 3 H 2)) combining two different types of ligands, i.e., catalyst-inspired scaffolds and triazole ring, which were previously shown to promote high and low oxidation states in transition metals, respectively. These complexes exhibit as many as six theoretical redox events in the full range of charge states +4 → −2, several of which reside within the electrochemical window of acetonitrile. Electronic structure calculations show that the Fe center exhibits oxidation states ranging from the very rare Fe 4+ to Fe 1+. Values of the reduction potentials as well as nature of the redox events of both complexes is found to be similar in their high +4 → +1 charge states. In contrast, while exhibiting qualitatively similar redox behavior in the lower 0 → −2 range, some differences in the electronic ground states, delocalization patterns as well as reduction potential values are also observed. The calculated open circuit voltages can reach values of 5.09 and 6.14 V for complexes I and II, respectively, and hold promise to be experimentally accessible within the electrochemical window of acetonitrile expanded by addition of ionic liquids. The current results obtained for these two complexes are intended to illustrate a more general principle based on the simultaneous utilization of two types of ligands responsible for the stabilization of high and low oxidation states of the metal that can be used to design the next-generation charge carriers capable of supporting multi-electron redox and operating in a broad range of charge states, leading to RFBs with greater energy density.

A Comparative Review of Metal‐Based Charge Carriers in Nonaqueous Flow Batteries

“…This approach demonstrated good correlation between calculated and experimental values ( R 2 =0.97), making it possible to predict redox potentials with a high level of confidence [105] . It has recently been successfully applied to various Ni‐ and Fe‐based charge carriers [84–87] …”

“…While the main emphasis in the development of RFBs has been on experimental efforts to improve their performance, a number of recent attempts have been made to rationally design the structural, electronic, and other properties of RFBs from a theoretical point of view. [81][82][83][84][85] In these and other studies, [86][87][88][89][90][91] where experiment is complemented with computational modeling, density functional theory (DFT) calculations are usually employed to model the redox properties of the charge carriers with the aim of increasing cell voltage and solubility of active species, enhancing stability and reversibility of redox species, and shedding light into reaction kinetics. Since the energy density of an RFB is a function of cell potential and solubility, improving these two parameters represents a major challenge for researchers working in this field.…”

“…[105] It has recently been successfully applied to various Ni-and Fe-based charge carriers. [84][85][86][87] In order to address the challenge of increasing energy density of RFBs by increasing the cell potential, two approaches were previously undertaken based on: a) rational design of redox non-innocent ligands and using specific metal centers, capable of storing as many electrons as possible; and b) tuning the redox waves via installation of electron-withdrawing groups (EWGs) or electron-donating groups (EDGs). Building on the first idea, Popov et al have recently proposed a theoretical approach to use two different types of ligands, which could stabilize metal in both high and low oxidation states and facilitate the possibility of multiple redox events within the electrochemical window (EW) of non-aqueous solvent.…”

A Comparative Review of Electrolytes for Organic‐Material‐Based Energy‐Storage Devices Employing Solid Electrodes and Redox Fluids

“…Metal complexes with organic ligands are promising for improving the energy density by utilizing multielectron reactions . However, insufficient solubility (typically <0.1 m ) of these organometallic compounds in ILs and poor electrochemical reversibility restrict their applications.…”

1,2,4-Triazole Derivatives in Medicine and Pharmacy and Application Prospects

“…The researchers [22] proposed two coordination complexes Fe [22] (Fig. 19), which combine two different types of ligands.…”