Tuning the Electrocatalytic Performance of Ionic LiquidModified Pt Catalysts for the Oxygen Reduction Reaction via CationicChain Engineering

摘要

Modifying Pt catalysts using hydrophobic ionic liquids (ILs) has been demonstrated to be a facile approach for boosting the performance of Pt catalysts for the oxygen reduction reaction (ORR). This work aims to deepen the understanding and initiate a rational molecular tuning of ILs for improved activity and stability. To this end, Pt/C catalysts were modified using a variety of 1-methyl-3-alkylimidazolium bis(trifluoromethanesulfonyl)imide ([CnC1im][NTf2], n = 2–10) ILs with varying alkyl chain lengths in imidazolium cations, and the electrocatalytic properties (e.g., electrochemically active surface area, catalytic activity, and stability) of the resultant catalysts were systematically investigated. We found that ILs with long cationic chains (C6, C10) efficiently suppressed the formation of nonreactive oxygenated species on Pt; however, at the same time they blocked active Pt sites and led to a lower electrochemically active surface area. It is also disclosed that the catalytic activity strongly correlates with the alkyl chain length of cations, and a distinct dependenceof intrinsic activity on the alkyl chain length was identified, withthe maximum activity obtained on Pt/C-[C4C1im][NTf2]. The optimum arises from the counterbalance between moreefficient suppression of oxygenated species formation on Pt surfacesand more severe passivation of Pt surfaces with elongation of thealkyl chain length in imidazolium cations. Moreover, the presenceof an IL can also improve the electrochemical stability of Pt catalystsby suppressing the Pt dissolution, as revealed by combined identical-locationtransmission electron microscopy (TEM) and in situ inductively coupledplasma mass spectrometry (ICP-MS) analyses.