Effect of Ionic Liquid Modification on the ORR Performance and Degradation Mechanism of Trimetallic PtNiMo/C Catalysts

Abstract

Ionic liquids (ILs) modification, following the concept of “solid catalyst with ionic liquid layer (SCILL)”, has been demonstrated to be an effective approach to improving both activity and stability of Pt-based catalysts for the oxygen reduction reaction. In this work, the SCILL concept has been applied to a trimetallic PtNiMo/C system, which has been documented recently to be significantly advantageous over the benchmark PtNi-based catalysts for oxygen reduction. To achieve this, two hydrophobic ILs ([BMIM][NTF2] and [MTBD][BETI]) were used to modify PtNiMo/C with four IL-loading amounts between 7 and 38 wt %. We found that the Pt mass activity (@0.9 V) could be improved by up to 50% with [BMIM][NTF2] and even 70% when [MTBD][BETI] is used. Exceeding a specific IL loading amount, however, leads to a mass transport related activity drop. Moreover, it is also disclosed that both ILs can effectively suppress the formation of nonreactive oxygenated species, while at the same time imposing little effect on the electrochemical active surface area. For a deeper understanding of the degradation mechanism of pristine and IL modified PtNiMo/C, we applied identical location transmission electron microscopy and in situ scanning flow cell coupled to inductively coupled plasma mass spectrometry techniques. It is disclosed that the presence of ILs has selectively accelerated the dissolution of Mo and eventually results in a more severe degradation of PtNiMo/C. This shows that future research needs to identify ILs that prevent the Mo dissolution to leverage the potential of the IL modification of PtNiMo catalysts.

<i>Operando</i> X-ray Absorption Spectroscopic Study on the Effect of Ionic Liquid Coverage upon the Oxygen Reduction Reaction Activity of Pd-core Pt-shell Catalysts

“…Comparing the CVs of Pt/ C and Pt/Pd/C with/without IL, the H adsorption peaks for the ILmodified system were slightly suppressed, and the peaks shifted to a lower potential range, which agreed with previous reports. 25,37 These results suggest that the presence of IL suppresses the access of H on the Pt surface by reducing the bonding strength of Pt and H through a ligand effect [37][38][39] and/or the occupation of certain Pt sites by IL molecules. 38 leads to slow ORR kinetics.…”

“…21,22 A hydrophobic ionic liquid (IL) modification of Pt surface has recently emerged as another method to enhance the activity of Pt-based catalysts for ORR. [23][24][25] Many groups have investigated the ability of IL modifications to enhance ORR activity with a variety of IL. [26][27][28] Snyder et al firstly reported the development of a new architecture in which high surface area and nanoporous Ni-Pt alloy nanoparticles encapsulated with a hydrophobic [7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)][bis(pentafluoroethylsulfonyl)imide (BETI)] IL.…”

“…30 By systematically changing the side chain length of the imidazolium cations, ([C n C 1 im][bis(trifluoromethylsulfonyl)imide(NTf 2 )], n = 2, 4, 6, and 10), a clear dependence of Pt catalytic activity on ORR for side chain length was observed, with the highest activity obtained using medium chain length IL (n = 4). 27 While the mechanism of the observed activity enhancement of IL remains elusive, the increased oxygen solubility, 24,31 inhibition of Pt oxidation, 27,30,32 and lower adsorption of nonreactive species 25,27,30 are considered among the factors that enhance ORR activity. A recent study by Zhang et al concluded that the presence of IL improved the electrochemical stability of Pt/C catalysts by suppressing the formation of oxygenated species and/or the initial oxidation of low-coordinating Pt sites at edges, corners, and/or defect Pt sites.…”

Engineering the Electrochemical Interface of Oxygen Reduction Electrocatalysts with Ionic Liquids: A Review

“…Later on, synthetic procedures have been introduced that allow for controlled and smaller loading of ILs. They consist of the dilution of the catalyst and the desired amount of IL in isopropanol, [ 13,29,31,13–13 ] ethanol, [ 12 ] or ethanol/water mixtures, [ 33 ] generally followed by mixing, sonication, and evaporation. Du and coworkers utilized 10 min sonication, followed by 3 h of magnetic stirring, partial solvent evaporation by vacuum desiccation, and centrifugation to isolate the catalyst.…”

“…From top to bottom presented data were originally reported as follows: Pt/C, [ 29 ] Pt/C, [ 35 ] Pt/C, [ 31 ] Pt/C, [ 72 ] np‐NiPt, [ 30 ] np‐NiPt, [ 28 ] np‐NiPt/C, [ 73 ] PtNiMo/C. [ 36 ] Detailed information about the experimental parameters are presented in Table S2, Supporting Information. b) Oxygen reduction current, obtained by IL‐modified PGM‐free catalysts.…”