Tuning the Electrocatalytic Performance of Ionic Liquid\nModified Pt Catalysts for the Oxygen Reduction Reaction via Cationic\nChain Engineering

Abstract

Modifying Pt catalysts\nusing hydrophobic ionic liquids (ILs) has\nbeen demonstrated to be a facile approach for boosting the performance\nof Pt catalysts for the oxygen reduction reaction (ORR). This work\naims to deepen the understanding and initiate a rational molecular\ntuning of ILs for improved activity and stability. To this end, Pt/C\ncatalysts were modified using a variety of 1-methyl-3-alkylimidazolium\nbis­(trifluoromethanesulfonyl)­imide ([C_<i>n</i>C₁im]­[NTf₂], <i>n</i> = 2–10)\nILs with varying alkyl chain lengths in imidazolium cations, and the\nelectrocatalytic properties (e.g., electrochemically active surface\narea, catalytic activity, and stability) of the resultant catalysts\nwere systematically investigated. We found that ILs with long cationic\nchains (C6, C10) efficiently suppressed the formation of nonreactive\noxygenated species on Pt; however, at the same time they blocked active\nPt sites and led to a lower electrochemically active surface area.\nIt is also disclosed that the catalytic activity strongly correlates\nwith the alkyl chain length of cations, and a distinct dependence\nof intrinsic activity on the alkyl chain length was identified, with\nthe maximum activity obtained on Pt/C-[C₄C₁im]­[NTf₂]. The optimum arises from the counterbalance between more\nefficient suppression of oxygenated species formation on Pt surfaces\nand more severe passivation of Pt surfaces with elongation of the\nalkyl chain length in imidazolium cations. Moreover, the presence\nof an IL can also improve the electrochemical stability of Pt catalysts\nby suppressing the Pt dissolution, as revealed by combined identical-location\ntransmission electron microscopy (TEM) and in situ inductively coupled\nplasma mass spectrometry (ICP-MS) analyses.