A higher voltage Fe(ii) bipyridine complex for non-aqueous redox flow batteries

Abstract

A new family of tunable iron bipyridine coordination complexes has been synthesized and tested in a non-aqueous flow battery.

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

“…Redox flow batteries were assembled in lab‐scale cells using the cycling conditions optimized for the symmetric Fe(bpy) 3 (BF 4 ) 2 system; 0–80 % targeted state of charge (SOC) based on conversion of 80 % of active species in solution using an applied voltage (all flow cells required different voltage limits); 0.5 M TEABF 4 /PC supporting electrolyte; and Fumasep FAP‐450 anion exchange membrane [11] . Catholyte and anolyte solution reservoirs were prepared at the same concentrations to prevent electroactive species diffusion across the membrane and balanced by volume to account for number of electrons involved in each redox process.…”

“…In a similar fashion, we protected the bpy nitrogens with methyl groups, knowing the BF 3 adducts result in irreversible by‐products, as found in our previous work. The advantage of methylating bpy was three‐fold: (1) stabilize bipyridinium radicals upon reduction, [21] (2) form the salt to increase solubility in polar electrolyte, [22] and (3) prevent by‐product formation with hydrolyzed electrolyte [11] …”

“…Anhydrous propylene carbonate (PC) purchased from Sigma‐Aldrich was dried over activated molecular sieves and stored under inert atmosphere. Iron(II)tris(bipyridine) tetrafluoroborate (Fe(bpy) 3 (BF 4 ) 2 , iron(II)tris(4,4‐di(trifluoromethane)‐2,2’‐bipyridine) tetrafluoroborate (Fe(bpyCF 3 ) 3 (BF 4 ) 2 , iron(II)tris(4,4’‐dicarbomethoxy)‐2,2’‐bipyridine) tetrafluoroborate (Fe(bpyCO 2 Me) 3 (BF 4 ) 2 , and 1‐methyl‐4‐pyridone were prepared according to literature procedures [11,24,25] . Multinuclear NMR measurements were collected on a Bruker AVANCE III 500 and referenced to residual solvent signal.…”

“…The advantage of methylating bpy was three-fold: (1) stabilize bipyridinium radicals upon reduction, [21] (2) form the salt to increase solubility in polar electrolyte, [22] and (3) prevent byproduct formation with hydrolyzed electrolyte. [11] Various 4,4'-disubstituted-2,2'-bipyridine compounds were methylated to the corresponding 1,1'-dimethyl-4,4'-disubstituted-2,2'-bipyridinium tetrafluoroborate salts, summarized in Table 1. The bipyridines were converted to bipyridinium salts using Meerwein's salt under air-free reaction conditions (Figure 2).…”

“…[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.…”