Simultaneously Enhancing the Redox Potential and Stability of Multi-Redox Organic Catholytes by Incorporating Cyclopropenium Substituents

Abstract

High redox potential, two-electron organic catholytes for nonaqueous redox flow batteries were developed by appending diaminocyclopropenium (DAC) substituents to phenazine and phenothiazine cores. The parent heterocycles exhibit two partially reversible oxidations at moderate potentials [both at lower than 0.7 V vs ferrocene/ferrocenium (Fc/Fc + )]. The incorporation of DAC substituents has a dual effect on these systems. The DAC groups increase the redox potential of both couples by ∼300 mV while simultaneously rendering the second oxidation (which occurs at 1.20 V vs Fc/Fc + in the phenothiazine derivative) reversible. The electron-withdrawing nature of the DAC unit is responsible for the increase in redox potential, while the DAC substituents stabilize oxidized forms of the molecules through resonance delocalization of charge and unpaired spin density. These new catholytes were deployed in two-electron redox flow batteries that exhibit voltages of up to 2.0 V and no detectable crossover over 250 cycles.

Development of High Energy Density Diaminocyclopropenium‐Phenothiazine Hybrid Catholytes for Non‐Aqueous Redox Flow Batteries

“…[46] Again, these are comparable to those for other catholyte materials,i ncluding 5,10-dihydro-5,10-dimethylphenazine (5.53 10 À3 cm s À1 ), [7] V(acac) 3 (6.5 10 À4 cm s À1 ), [43] and 2-DAC (first couple:2 .53 10 À3 cm 2 s À1 ; second couple:3.47 10 À3 cm 2 s À1 ). [33] Ah igh concentration flow cell was assembled using amixture of 4-DMPP (0.30 Min0.50 MMeCN/TBAPF 6 )and…”

Development of High Energy Density Diaminocyclopropenium‐Phenothiazine Hybrid Catholytes for Non‐Aqueous Redox Flow Batteries

“…As discussed above,a ppending diaminocyclopropenium substituents onto the phenothiazine core (2-DAC)results in adramatic increase in redox potential as well as stabilization of twoelectron cycling compared to the parent catholyte 1. [33] However, 2-DAC displays poor solubility in MeCN and has an undesirably high molecular weight ( % 1000 gmol À1 ), which limits the maximum achievable concentration in an RFB. [34] Literature precedent suggests that modifying the DACs ubstituents could be effective for enhancing solubility.…”

“…Theo bserved values for 4-DMPP (first couple: 5.77 10 À6 cm 2 s À1 ;s econd couple:4 .51 10 À6 cm 2 s À1 )a re comparable to those for other organic catholyte materials, including ferrocene derivative Fc1N112-TFSI (4.25 10 À7 cm 2 s À1 ) [44] and dialkoxybenzene derivative DBMMB (5.77 10 À6 cm 2 s À1 ), [45] as well as the related pheonthiazine derivatives 2-OEO (0.8 10 À6 cm 2 s À1 ) [28] and 2-DAC (first couple:5 .29 10 À6 cm 2 s À1 ;s econd couple:4 .99 10 À6 cm 2 s À1 ). [33] Theh eterogeneous electron-transfer rates were determined to be 4.84 10 À3 cm s À1 (first couple) and 4.62 10 À3 cm s À1 (second couple) using the Nicholson method (Figure S5). [46] Again, these are comparable to those for other catholyte materials,i ncluding 5,10-dihydro-5,10-dimethylphenazine (5.53 10 À3 cm s À1 ), [7] V(acac) 3 (6.5 10 À4 cm s À1 ), [43] and 2-DAC (first couple:2 .53 10 À3 cm 2 s À1 ; second couple:3.47 10 À3 cm 2 s À1 ).…”

“…[46] Again, these are comparable to those for other catholyte materials,i ncluding 5,10-dihydro-5,10-dimethylphenazine (5.53 10 À3 cm s À1 ), [7] V(acac) 3 (6.5 10 À4 cm s À1 ), [43] and 2-DAC (first couple:2 .53 10 À3 cm 2 s À1 ; second couple:3.47 10 À3 cm 2 s À1 ). [33] Ah igh concentration flow cell was assembled using amixture of 4-DMPP (0.30 Min0.50 MMeCN/TBAPF 6 )and Angewandte 5 (0.60 Mi n0 .50 MM eCN/TBAPF 6 )a sb oth the catholyte and anolyte. [47] Forthis experiment only the first reduction of 5 was accessed due to the modest solubility and stability of the doubly reduced viologen at high concentration.…”

“…[28] Additionally,o ur group recently showed that installing resonance electronwithdrawing diaminocyclopropenium substituents at the 3and 7-positions (2-DAC)r esults in both stabilization of the second redox couple and an % 600 mV increase in redox potential relative to 1-OMe (to 0.6 and 1.2 V). [33] However, 2-DAC remains limited by extremely poor solubility in the electrolyte solution (0.09 Mi nM eCN). Additionally, 2-DAC derivatives bearing different substituents are not readily accessible due to challenges associated with the CÀCb ondforming step that couples the DACtothe phenothiazine core.…”