Hygroscopicity-driven physicochemical and hydrogen evolution behavior of a proton conducting ionic liquid

A protic ionic liquid, N, N-diethylammonium trifluoroacetate ([DEA–H] + [TFA] − ), was synthesized via a solvent-free proton-transfer reaction between diethylamine and trifluoroacetic acid and investigated as a potential electrolyte for hydrogen production and energy storage. Structural formation was confirmed by 1 H NMR and FT-IR spectroscopy. Karl–Fischer analysis indicated a low water content of 0.99 %, while thermogravimetric analysis showed thermal stability up to ∼250 °C. Hygroscopicity studies revealed strong hydrogen-bonding interactions between absorbed water and the [TFA] − anion, with water uptake following pseudo-first-order kinetics. Increasing water content significantly affected the physicochemical properties, resulting in increased density and ionic conductivity and reduced viscosity. The ionic liquid exhibited a wide electrochemical stability window of 3.4 V, which decreased with increasing water content. Hydrogen evolution proceeds through two proton-conduction pathways involving hydronium ions and the [DEA–H] + cation. Water addition enhanced proton mobility and HER activity, with a 50 % water–PCIL mixture showing lower overpotential compared to mixtures containing 10 % and 0.99 % water. These results highlight the potential of water–[DEA–H] + [TFA] − systems as efficient electrolytes for hydrogen evolution and electrochemical energy applications.