Exploration of Vitamin B6-Based Redox-Active Pyridinium Salts towards the Application in Aqueous Organic Flow Batteries

We have examined the utility of pyridoxal hydrochloride, a vitamin B6 vitamer and a biobased feedstock, as a starting point towards organic redox flow battery materials. Pyridoxal hydrochloride was synthetically converted to a series of diverse vitamin B6-based redox-active benzoyl pyridinium salts. These compounds were electrochemically characterized through cyclic voltammetry (CV) measurements in neutral and basic aqueous electrolytes (1 M KCl and 0.1 M NaOH). Based on the CV results, which demonstrated reversibility under basic conditions, two of the most promising salts were subjected to laboratory-scale redox flow battery tests involving galvanostatic cycling at 10 mM with 0.1 M NaOH as the supporting electrolyte. These results showed that the battery was charged completely, corresponding to the transfer of two electrons to the electrolyte, but no discharge was observed. Both CV analysis and electrochemical simulations confirmed that the redox wave observed in the experimental voltammograms corresponds to a two-electron process. To explain the irreversibility in the battery tests, we conducted bulk electrolysis with the benzoyl pyridinium salts, affording the corresponding benzylic secondary alcohols. This process involves the transfer of two electrons and two protons to reduce the ketone group to alcohol. Computational studies suggest that the reduction proceeds in three consecutive steps: first electron transfer (ET), then proton-coupled electron transfer (PCET) and finally proton transfer (PT). 1H NMR deuterium exchange studies indicated that the last PT step is not reversible in 0.1 M NaOH, rendering the entire redox process irreversible. The apparent reversibility observed in CV at the basic media likely arises from the slow rate of the PT step at the timescale of the measurement.