Expanding the potential of redox carriers for flow battery applications

Abstract

Using theoretical modelling to guide our approach, substituents were selected to improve the negative and positive potentials associated with a representative metal-based redox carrier, a phenyl spaced nickel bispicolinamide complex,...

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.…”

“…While the Ni(bpb) complex displayed low currents due to low solubility, it was found that it could be a good model compound to study because it contained qualitatively reversible positive and negative waves that might be influenced with judicious incorporation of functional groups. Using theoretical modeling to guide their approach, [86] EDGs were selected and installed on the pyridyl moieties, [Ni(bpb-(NMe 2 ) 2 )], while EWGs were incorporated on the phenyl linker, [Ni(bpb-R)] (R: À F, À CF 3 , and À NO 2 ) ( Figure 7C). It was found that regardless of the solvent (DMF or MeCN), for complexes with an EDG or EWG, the oxidations follow intuition with EDG making the process more facile and more difficult with EWG substitutions.…”

“…[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.…”

Impact of Pendent Ammonium Groups on Solubility and Cycling Charge Carrier Performance in Nonaqueous Redox Flow Batteries

“…This agrees with the appreciable amount of the spin density on the amide in 3′ 2+ (−0.17) and 3 1+ (−0.19) as compared to the Ni center in these complexes (1.11 and 1.12, respectively) (Tables S13, S14). Our previous studies of the bispicolinamide Ni complex with a phenyl linker, Ni­(bpb), showed that introducing a redox active linker instead of an alkyl chain or adding electron donating/withdrawing groups may alter the nature of the redox events in the negative waves, making the metal- and ligand-centered reactions energetically indistinguishable . The first reduction of both Ni complexes populates the α-HOMO orbital with primarily Ni AOs (∼45.1% in 3′ 0 and ∼46.4% in 3 1– ) and some amide AOs (∼35.2 and ∼33.7%, respectively), substantiated by the dominant Mulliken spin densities on Ni in both complexes.…”