Stable Hydrazyl Radicals as Redox Active Materials

Introduction

Although lithium-ion batteries are nowadays found in everyday life, such as in portable devices and vehicles, there is a high demand for a better technology to overcome the current conceivable maximum power and storage capacity, and also to address some issues regarding the environmental challenges. Stable organic compounds with unpaired electrons (open-shell molecules) are known as free radicals, and usually they possess fascinating and unique properties—the most important being their redox behavior. Therefore, organic radical batteries offer promising improvements on all characteristics of classical batteries, including freedom from rare metals, faster charging time, environmental friendliness, and so on. Stable hydrazyl free radicals are ideal redox species for such new batteries. Therefore, the well-known DPPH free radical (2,2-diphenyl-1-picrylhydrazyl) was tested as a redox mediator in a lithium–graphene battery.

Methods

Our work is primarily focused on the synthesis and characterization of a large number of such stable hydrazyl (di)radicals, tailoring their redox properties based on chemical design. After synthesis, structural characterization was performed by NMR, IR, UV-Vis, MS, (para)magnetic measurements (ESR, SQUID), and cyclic voltammetry, which allow for the evaluation of the redox properties.

Results

Structures of the DPPH free radical derivatives were confirmed by different means. Electrochemistry was performed for both stable and persistent free (di)radicals. As expected, stable radicals showed a full reversible redox process. The oxidation potential usually ranges in the domain range of 0.5–1.5 V, with higher values recorded for poly-nitrated radicals and diradicals. The bond dissociation energy of the -NH- group (hydrazine-hydrazyl) is around 70-90 kcal/mol. Further experiments are underway.

Conclusions

The use of stable hydrazyl radicals in organic batteries as redox active materials can potentially be an important step towards a new technology for the generation and storage of electrical energy.