Acid‐Base Chemistry Provides a Simple and Cost‐Effective Route to New Redox‐Active Ionic Liquids

Abstract

Redox‐active ionic liquids (RAILs) hold great promise as high density electrochemical energy storage materials, but are hampered by high costs and low bulk conductivities. In this work, we introduce and electrochemically characterise novel redox‐active protic ionic liquids (RAPILs) formed by acid‐base neutralisation from cheap and plentiful starting materials. We also demonstrate a novel RAIL‐in‐IL solvent system for electrochemical characterisation of RAPILs, which enables efficient and cost‐effective determination of redox potentials and screening for electrochemical reversibility. Of the redox‐active ionic liquids tested in this work, only propylammonium 4‐nitrophenylacetate demonstrates completely reversible electrochemistry and preservation of ionic character upon redox cycling under acidic or neutral conditions. Propylammonium 2,5‐dihydroxyphenyl‐carboxylate also demonstrates two reversible redox processes, but is unstable to oxidation at 0.27 V vs Ag (−0.14 V vs Fc), most likely forming an uncharged benzoquinone species. Using the lessons learned from this prototypical set of RAPILs, we propose design criteria to guide future experimental and computational work.

Protic ionic liquids for sustainable uses

“…Recent work highlighting the need to electronically decouple the redox-active centre from any ionizable functional group in PIL systems promises greater investigation in this vein. 246 Increasing the accessible energy density and widening the operating temperature window of the vanadium system by selection of the appropriate PIL may also deliver tangible improvements to the incumbent technology if long-term cyclability is established. 243 Proton battery technology is nascent but PILs appear to be ideal candidates due to their inherently high proton conductivity.…”

“…Recently, three main avenues have been pursued, that of using PILs as solvents to access higher concentrations of active species, 243,244 using PILs as part of the fabrication of novel membranes, 245 or indeed using PILs as redox-active species themselves. 246 In 2020, Zhou et al showed that [HN 122 ][TfO] could be used effectively to produce asymmetric porous membranes for use in vanadium-based RFBs. 245 The PIL was dissolved in a sulfonated poly(ether sulfone) (SPES) solution and via phase inversion and PIL removal, a membrane with a dense SPES layer and a highly porous layer was fabricated.…”

“…Recently, three main avenues have been pursued, that of using PILs as solvents to access higher concentrations of active species, 243,244 using PILs as part of the fabrication of novel membranes, 245 or indeed using PILs as redox-active species themselves. 246…”

“…244 A recent investigation of a range PILs as redox-active species for use in RFBs is particularly encouraging in this area. 246 The authors state that it is straightforward to synthesise redox-active PILs by combining redox-active organic or organometallic acids with liquid amines and present voltammetric measurements for a range of PILs. Conclusions drawn from this study were that PILs may be able to supply protons to electrochemical reactions even in a basic environment, that it is wise to avoid precursor molecules with acidic functional groups attached directly to aromatic rings, and that is it important to select redox-active species with large free energy differences between both their oxidised and deprotonated forms, and their reduced and protonate forms.…”