Faradaic processes on quinone-grafted carbons in protic ionic liquid electrolyte

Abstract

The redox activity of anthraquinone-grafted carbons is demonstrated in a protic ionic liquid, triethylammonium bis(trifluoromethanesulfonyl)imide, [(C2H5)3N+H][TFSI]. The materials were synthesized by spontaneously reducing the anthraquinone diazonium salt using microporous activated carbon, micro-mesoporous activated carbon xerogel and carbon black (CB). The impact of carbon texture/structure on electrical double-layer capacitance (CEDL) and redox reaction characteristics has been investigated. Grafted CB appeared the most advantageous, with relatively high CEDL and well-defined redox peaks. For all the modified materials, protons donated from (C2H5)3N+H dissociation are involved in the reversible reduction of grafted anthraquinone, yielding phenoxide anions. As (C2H5)3N is simultaneously liberated, the local alkalinity increases within the pores, leading the redox reaction to occur at AQ electroactivity potential in basic electrolytes. Hence, the redox peaks progressively diminish as the low vertex potential decreases during voltammetry scans, revealing the detachment of AQ moieties due to their electrostatic repulsion with the negatively charged carbon substrate. Therefore, HTFSI was added to the PIL electrolyte to counterbalance the local alkalinity increment, and provide stable performance of the anthraquinone-modified carbon. On the example of CB-AQ, HTFSI in 0.05 M ratio vs. PIL was sufficient to upshift the redox potential and induce two-proton-two-electron transfer resulting in the formation of 9,10-dihydroxyanthracene.