Understanding hydrogen oxidation and oxygen reduction reactions' kinetics in non-stoichiometric protic ionic liquids

• Stoichiometric PILs show poor ORR and HOR due to lack of free protons. • Excess triflic acid enables high HOR and ORR currents in [Bu-Pyrr][TfO] and [Et-Pip][TfO]. • ORR becomes O 2 -limited beyond 300–400 mM acid; HOR stays proton-limited to 500 mM. • PIL cation adsorption on Pt raises overpotentials. • Reliance on acid additives challenges PILs’ benign and stable electrolyte design. Protic ionic liquids (PILs), prepared from a stoichiometric acid-base mixture, can function as electrolytes in proton exchange membrane fuel cells (PEMFCs) under low humidity. However, using platinum (Pt) microelectrode studies, we showed that in stoichiometric PILs, oxygen reduction (ORR) and hydrogen oxidation (HOR) kinetics are sluggish due to the lack of free protons for proton-transfer reactions. This can be addressed by adding excess acid to form non-stoichiometric PILs. In this work, we introduce new PILs: Butyl pyrrolidinium triflate [Bu-Pyrr][TfO] and Ethyl piperidinium triflate [Et-Pip][TfO], featuring thermally stable cations (boiling points: 155°C and 131°C). Adding 200 mM triflic acid enhanced Pt activity, yielding ORR currents of -7.17 and -3.08 mA/cm 2 at 0 V vs. RHE, and HOR currents of 2.31 and 1.43 mA/cm 2 at 0.75 V vs. RHE for [Bu-Pyrr][TfO] and [Et-Pip][TfO], respectively. ORR was limited by O 2 solubility beyond 300 and 400 mM added acid for [Bu-Pyrr][TfO] and [Et-Pip][TfO], respectively, while HOR currents remained linearly dependent on added acid up to 500 mM. This study concludes that stoichiometric PILs are unsuitable for PEMFCs. Although non-stoichiometric PILs support electrochemical activity, cation adsorption on Pt increases overpotentials and reduces limiting currents; [Bu-Pyrr] raised ORR and HOR overpotentials by ∼10% and 6%, respectively.