Stable electron-donating organic compounds are of interest for numerous applications that require reversible electron-transfer reactions. Although many organic compounds are stable one-electron donors, removing a second electron from a small molecule to form its dication usually leads to rapid decomposition. For cost-effective electrochemical energy storage utilizing organic charge-storage species, the creation of high-capacity materials requires stabilizing more charge whilst keeping molecular weights low. Here we report the simple modification of N-ethylphenothiazine, which is only stable as a radical cation (not as a dication), and demonstrate that introducing electron-donating methoxy groups para to nitrogen leads to dramatically improved stability of the doubly oxidized (dication) state. Our results reveal that this derivative is more stable than an analogous compound with substituents that do not allow for further charge delocalization, rendering it a promising scaffold for developing atom-efficient, two-electron donors.

Electron-withdrawing substituents are introduced onto the phenothiazine core to raise its oxidation potential for use as a redox shuttle in high-voltage lithium-ion batteries. A perfluorinated derivative oxidizes at 4.3 V vs. Li+/0, and functions for ca. 500 h of 100% overcharge in LiNi0.8Co0.15Al0.05O2/graphite coin cells at a charging rate of C/10.

Two new small-molecule enzyme mimics of carbonic anhydrase were prepared and characterized. These complexes contain the salen-like ligand bis(hydroxyphenyl)phenanthroline. This ligand is similar to the salen-type ligands previously incorporated into carbonic anhydrase mimics but contains no hydrolyzable imine groups and therefore serves as a promising ligand scaffold for the synthesis of a more robust CO2 hydration catalyst. These homogeneous catalysts were investigated for CO2 hydration in concentrated primary amine solutions through which a dilute CO2 (14%) fluid stream was flowed and showed exceptional activity for increased CO2 absorption rates.

The stability and reactivity of the multiple oxidation states of aromatic compounds are critical to the performance of these species as additives and electrolytes in energy-storage applications. Both for the overcharge mitigation in ion-intercalation batteries and as electroactive species in redox flow batteries, neutral, radical-cation, and radical-anion species may be present during charging and discharging processes. Despite the wide range of compounds evaluated for both applications, the progress identifying stable materials has been slow, limited perhaps by the overall lack of analysis of the failure mechanism when a material is utilized in an energy-storage device. In this study, we examined the reactivity of phenothiazine derivatives, which have found interest as redox shuttles in lithium-ion battery applications. We explored the products of the reactions of neutral compounds in battery electrolytes and the products of radical cation formation using bulk electrolysis and coin cell cycling. Following the failure of each cell, the electrolytes were characterized to identify redox shuttle decomposition products. Based on these results, a set of decomposition mechanisms is proposed and further explored using experimental and theoretical approaches. The results highlight the necessity to fully characterize and understand the chemical degradation mechanisms of the redox species in order to develop new generations of electroactive materials.

A variety of mechanisms lead to the failure of lithium-ion batteries. One is overcharge, a condition in which a battery's voltage rises above its designed end-of-charge potential. Electrolyte additives called redox shuttles limit cell potential by preferentially oxidizing, and cycling between the cathode and anode in their radical cation and neutral forms. Currently, testing requires coin cell assembly and repeated cycling, which can be an expensive and time consuming process. It is commonly accepted that degradation of the radical cation form of a redox shuttle leads to overcharge protection failure. We thus studied the stability of the radical cation forms of a series of redox shuttle additives to determine if there is a correlation between radical cation stability and the number of cycles of overcharge protection. While the reversibility of oxidations in cyclic voltammetry did not correlate to trends in overcharge performance, results from both UV-vis and electron paramagnetic resonance spectroscopy showed a correlation between stability and overcharge protection. Our results reveal trends within a few hours for what otherwise takes months of battery cycling to determine, providing a fast and relatively inexpensive method for predicting redox shuttle performance.

3,7-Bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) was evaluated as a redox shuttle for overcharge protection in lithium-ion batteries. Constant-charging experiments were performed to compare the compound to 1,4-di-tert-butyl-2,5-dimethoxybenzene and N-ethylphenothiazine. BCF3EPT showed significantly longer overcharge protection when compared to either benchmark at the same concentrations in LiFePO4/graphite batteries.

3,7-Disubstituted N-ethylphenothiazine derivatives were synthesized as redox shuttle candidates for lithium-ion batteries. Battery cycling results show that three derivatives prevent overcharge.

We report homogeneous catalysts that are soluble and stable in primary amine-based CO2 capture solvents. The zinc(II) and cobalt(III) complexes, which contain electron-donating multi-dentate anionic ligands, perform catalytic CO2 hydration at unparalleled observed rates under conditions conducive to industrial post-combustion carbon capture processes.

Overcharge, a condition in which cell voltage rises to undesirably high potentials, can be prevented in lithium-ion batteries by incorporating redox shuttles into the battery electrolyte. Although extensive overcharge protection has been demonstrated in batteries with LiFePO4 cathodes, the redox shuttles that work in these batteries are incompatible with higher voltage cathodes. Designing stable additives with higher oxidation potentials is necessary to protect high voltage batteries from overcharge. Toward that goal, we synthesized diarylamines with varied structures, including fused heteroaromatic ring systems and electron-withdrawing substituents. We found that trends in oxidation potentials correlated with those in calculated adiabatic ionization potentials. Some diarylamine derivatives protected batteries from overcharge with varying degrees of success.

Electron-withdrawing substituents are introduced onto the phenothiazine core to raise its oxidation potential for use as a redox shuttle in high-voltage lithium-ion batteries. A perfluorinated derivative oxidizes at 4.3 V vs. Li+/0, and functions for ca. 500 h of 100% overcharge in LiNi0.8Co0.15Al0.05O2/graphite coin cells at a charging rate of C/10.

We report homogeneous catalysts that are soluble and stable in primary amine-based CO2 capture solvents. The zinc(II) and cobalt(III) complexes, which contain electron-donating multi-dentate anionic ligands, perform catalytic CO2 hydration at unparalleled observed rates under conditions conducive to industrial post-combustion carbon capture processes.

3,7-Disubstituted N-ethylphenothiazine derivatives were synthesized as redox shuttle candidates for lithium-ion batteries. Battery cycling results show that three derivatives prevent overcharge.

The stability and reactivity of the multiple oxidation states of aromatic compounds are critical to the performance of these species as additives and electrolytes in energy-storage applications. Both for the overcharge mitigation in ion-intercalation batteries and as electroactive species in redox flow batteries, neutral, radical-cation, and radical-anion species may be present during charging and discharging processes. Despite the wide range of compounds evaluated for both applications, the progress identifying stable materials has been slow, limited perhaps by the overall lack of analysis of the failure mechanism when a material is utilized in an energy-storage device. In this study, we examined the reactivity of phenothiazine derivatives, which have found interest as redox shuttles in lithium-ion battery applications. We explored the products of the reactions of neutral compounds in battery electrolytes and the products of radical cation formation using bulk electrolysis and coin cell cycling. Following the failure of each cell, the electrolytes were characterized to identify redox shuttle decomposition products. Based on these results, a set of decomposition mechanisms is proposed and further explored using experimental and theoretical approaches. The results highlight the necessity to fully characterize and understand the chemical degradation mechanisms of the redox species in order to develop new generations of electroactive materials.

3,7-Bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) was evaluated as a redox shuttle for overcharge protection in lithium-ion batteries. Constant-charging experiments were performed to compare the compound to 1,4-di-tert-butyl-2,5-dimethoxybenzene and N-ethylphenothiazine. BCF3EPT showed significantly longer overcharge protection when compared to either benchmark at the same concentrations in LiFePO4/graphite batteries.

Two new small-molecule enzyme mimics of carbonic anhydrase were prepared and characterized. These complexes contain the salen-like ligand bis(hydroxyphenyl)phenanthroline. This ligand is similar to the salen-type ligands previously incorporated into carbonic anhydrase mimics but contains no hydrolyzable imine groups and therefore serves as a promising ligand scaffold for the synthesis of a more robust CO2 hydration catalyst. These homogeneous catalysts were investigated for CO2 hydration in concentrated primary amine solutions through which a dilute CO2 (14%) fluid stream was flowed and showed exceptional activity for increased CO2 absorption rates.