M15 - Elucidation of Naphthoquinone Square Reduction Pathway and Side Products via Raman Spectroscopy

In fifteen words or less, explain the significance of this contribution (Novel Aspect).: This contribution clarifies naphthoquinone reduction and reduction side products, which have been previously uninvestigated.

Abstract Text:

Menadione is a redox-active molecule that shows promise as a negolyte for use in microemulsion-based redox flow batteries (RFBs). One limitation to achieving higher current density is understanding which menadione species are present in the microemulsion. Menadione is a 1,4-naphthoquinone and, like all quinones, follows a square mechanism for its reduction pathways. Due to the lack of previous studies on the different intermediate species, identifying which specific reduced forms are present in the microemulsions has proven difficult. This prevents the RFB from being fully understood and hinders optimization of efficiency and energy capacity. If the fully reduced form of menadione (menadiol) is achievable through multiple pathways, other forms may be more easily achievable and/or possess higher reversibility.  We can maximize electron transfer and reversibility by selecting chemical conditions to generate these menadione forms. In this work, we synthesize the intermediates along the square reduction mechanism of menadione via the use of oxidizing and reducing agents.  We then determine the identity of each species using Raman spectroscopy (RS), surface-enhanced Raman spectroscopy (SERS) and UV-Vis (UV) spectroscopy, allowing for the assignment of each observed species to its location along the square reduction mechanism. This work clarifies the square reduction mechanism for menadione and, by extension, other quinones. Through elucidating this reaction pathway, we provide a more robust knowledge about what intermediate quinone species and side products may be advantageous for altering RFB results. This improved understanding of the chemical behavior of menadione could lead to longer lifespans and greater long-term energy storage capacity for RFBs that utilize it or related quinone molecules as redox-active species.