A Stable Organo-Aluminum Analyte Enables Multielectron Storage for a Nonaqueous Redox Flow Battery

Abstract

Redox flow batteries (RFBs) operate by storing electrons on soluble molecular anolytes and catholytes, and large increases in the energy density of RFBs could be achieved if multiple electrons could be stored in each molecular analyte. Here, we report an organoaluminum analyte, [(I2P–)2Al]+, in which four electrons can be stored on organic ligands, and for which charging and discharging cycles performed in a symmetric nonaqueous RFB configuration remain stable for over 100 cycles at 70% state of charge and 97% Coulombic efficiency (I2P is a bis­(imino)­pyridine ligand). The stability of the analyte is promoted by the kinetic inertness of the anolyte to trace water in solvents and by the redox inertness of the Al­(III) ion to the applied current. The solubility of the analyte was optimized by exchanging the counteranion for trifluoromethanesulfonate (triflate), and the cell was further optimized using graphite rods as electrodes which, in comparison with glassy carbon and reticulated vitreous carbon, eliminated deposition of analyte on the electrode. Proof-of-principle experiments performed with an asymmetric NRFB configuration further demonstrate that up to four electrons can be stored in the cell with no degradation of the analyte over multiple cycles that show 96% Coulombic efficiency.

Investigation of Iron(III) Tetraphenylporphyrin as a Redox Flow Battery Anolyte: Unexpected Side Reactivity with the Electrolyte

“…Redox flow batteries (RFBs) store energy in the form of oxidized and reduced redox couples in physically separated solutions allowing long-term storage and facile scalability. − The most well-developed RFB chemistry consists of vanadium salts in acidic solutions, which limit the cell potential to the electrochemical window of water. − The use of a nonaqueous RFB permits a greater cell potential and greater synthetic control. This has led to RFBs that use organic − and organometallic − electrolytes with high cell potentials, multiple electron transfers per molecule, , or both. , …”

Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands

“…Octahedral NIL−Al I 2 P complexes are stable in MeCN or in the presence of trace water despite highly reduced oxidation states. 54 The unusual electronic properties of the new molecular Al complexes described above have given rise to new reaction chemistry. NIL−Al(III) complexes support ET reactions, including those where both 1 and 2 electron oxidation and reduction reactions occur.…”

“…For Al­(III), an analogous series of compounds was prepared with electron-withdrawing and -donating ligands, PhF 5 I 2 P and p ‑PhNMe 2 I 2 P, respectively, where delocalization was reduced and unchanged, respectively. Access to 5 charge states separated by one electron in octahedral I 2 P Al complexes also lends to their use as flow battery analytes which can store many electrons to achieve high charge density …”

Too Much of a Good Thing? Assessing Performance Tradeoffs of Two-Electron Compounds for Redox Flow Batteries

“…For example, many redox-active organic molecules that undergo multi-electron transfer in aqueous electrolytes (e.g., phenazines, 13 phenothiazines, 14 and quinones [15][16][17] ) typically exhibit sequential redox reactions occurring at similar potentials due to hydrogen bonding interactions present in these environments. 18 Conversely, similar molecules used in nonaqueous electrolytes (e.g., bipyrimidines, 19 bispyridinylidenes, 20 phenazines, 21 phenothiazines, 9,10 quinones, 22 and viologens 23,24 ), some used in aqueous electrolytes (e.g., viologens [25][26][27][28] ), and metal coordination complexes containing non-innocent ligands 11,[29][30][31] often feature sequential electron transfer events with disparate and easily discernable redox potentials, separated by 200-800 mV. Compared to the concerted mechanism, which presents minimal voltage losses (vide infra), the sequential mechanism imposes significant losses that increase with the potential difference between redox events.…”