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

Abstract

Energy storage is becoming the chief barrier to the utilization of more renewable energy sources on the grid. With independent service operators aiming to acquire gigawatts in the next 10–20 years, there is a large need to develop a suite of new storage technologies. Redox flow batteries (RFB) may be part of the solution if certain key barriers are overcome. This Review focuses on a particular kind of RFB based on nonaqueous media that promises to meet the challenge through higher voltages than the organic and aqueous variants. This class of RFB is divided into three groups: molecular, macromolecular, and redox‐targeted systems. The growing field of theoretical modeling is also reviewed and discussed.

Controlling Ni redox states by dynamic ligand exchange for electroreductive Csp3–Csp2 coupling

“…Specifically, (pyridyl)Ni II complexes often exhibit discrete 1e – redox couples, rather than 2e – redox couples, and require reduction to Ni I before a second reduction forms Ni 0 at a more negative potential (Fig. 1C) ( 33 , 34 ). Even when Ni 0 is formed, the complex undergoes rapid comproportionation with remaining Ni II in solution to form the undesired Ni I intermediate ( 22 , 35 , 36 ).…”

A Mediated Li–S Flow Battery for Grid-Scale Energy Storage

“…Redox flow batteries (RFBs), on the other hand, offer a safe, easily scalable architecture amenable for grid scale energy storage. , Physical separation of anode and cathode limits thermal runaway concerns, while power generation can be scaled independently of the energy storage capacity of the system. Nonaqueous RFBs, in particular, offer the ability to access higher voltages outside the water voltage stability window and are compatible with ultrahigh energy density alkali metal anodes. , Unfortunately, the relatively high cost of nonaqueous solvents, compared to water, necessitates increasingly high concentrations, ideally ≥5 M, of redox-active species to be cost-competitive . One strategy to increase concentration involves pumping semisolid slurries of known battery chemistriesLiCoO 2 , LiNi 0.5 Mn 1.5 O 4 , Li 4 Ti 5 O 12 , S, and electronically conductive carbonsthrough flow cells, increasing effective concentrations >10 M. , While exemplary performance has been seen, removing conductive carbons could decrease pumping costs and self-discharge concerns over long times.…”

Elucidating Instabilities Contributing to Capacity Fade in Bipyridine‐Based Materials for Non‐aqueous Flow Batteries

“…A recent review by Palmer et al . summarizes the variety of metal‐based charge carriers used in nonaqueous flow batteries in the past 5 years [10] . In our previous work, we optimized, modified, and evaluated the performance of symmetric and asymmetric iron bipyridine (bpy) based non‐aqueous flow batteries originally published by Mun et al [11–13] .…”

“…[6][7][8][9] A recent review by Palmer et al summarizes the variety of metal-based charge carriers used in nonaqueous flow batteries in the past 5 years. [10] In our previous work, we optimized, modified, and evaluated the performance of symmetric and asymmetric iron bipyridine (bpy) based nonaqueous flow batteries originally published by Mun et al [11][12][13] By varying the electronics of bpy ligands, we tuned the redox potentials of the catholyte and anolyte with the goal of increasing operating voltage in symmetric iron bpy systems. Additionally, the iron bpy complexes with the highest and lowest redox potentials were evaluated as catholyte and anolyte to form a higher potential, asymmetric battery.…”