Project title: Revealing the Role of Ionic Liquids in Promoting Fuel Cell Catalysts Reactivity and Durability Authors: Arezoo Avid, Jesus L—pez Ochoa, Ying Huang, Yuanchao Liu, Plamen Atanassov, Iryna V. Zenyuk Samples of rotating disk electrode data recorded by Gamry or Biologic potentiostats can be found in *.DATA or *.mpt format accessible via any text editor. Name of RDE data files is in format ÒRDE_name of material_type of experimentÓ. Imidazolium-derived ILs used in the study are: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim]+[NTf2]-), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim]+[NTf2]-), and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim]+ [NTf2]-). Any data corresponding to the first material will be in this format Ò_EthylÓ Any data corresponding to the second material will be in this format Ò_ButylÓ Any data corresponding to the third material will be in this format Ò_DimethylÓ Type of experiments performed by RDE were cyclic volmtammetry (CV), linear sweep voltammetery polarization curves (ORR), and electrochemical impedance spectroscopy (EIS). Fuel cell data is in *.fcd format recorded by Scribner software. These files are accessible via any text editor. Fuel cell data are provided at the beginning of life before performing accelerated stress tests and also after 30000 cycle aging. Beginning of life data has ÒBOLÓ in their file name and end of life data has ÒEOLÓ. Fuel cell data represent polarization curves, Tafel plots for calculation of mass acitivty, and CO displacement which is transient current response from the introduction of CO. Oxygen mass transport resistance data are provided in the following format, for example: ÒFuelCellData_Butyl_PtC_EOL_1__2__4__100kpa.fcdÓ in which Butyl Pt/C is the catalyst used in this measurement, with inlet dry mole fraction of oxygen in nitrogen maintained at 1,2, and 4 % and outlet pressure controlled at 100 kPa. Polarization curves for C4mim (Butyl) in dry condition at different relative humidities (RH) are also provided in this format: ÒFuelCellData_Butyl_polarization_curve_dry_condition-X__RH.fcdÓ. X% RH is the relative humidity in which the measurement was performed at. The loading of ionic liquid to carbon for all fuel cell data is 1.28 unless mentioned otherwise, for example: ÒFuelCellData_Butyl_PtC_ILtoC_2.56_BOL-polarization_curve.fcdÓ Nitrogen physisorption raw data files are provided in *.xls format readable by any text editor. Water uptake data is in *.xls format. This file has four different sheets including a measurement for baseline Pt/C, and Pt/C modified with Ethyl, Butyl, and Dimethyl. The most important column is the evolution of weight as a function of time. Mass gain in mg is reported in the paper as a function of time. This file also shows that the experiments were performed at room temperature. Zeta potential raw data is in *.txt format measured by Horbia SZ-100. The first section shows the experimental parameters including: Temperature of the holder (C), Dispersion viscosity (mPa*S), conductivity (mS/cm), etc. Also, this section shows the Mean Zeta Potential (mV) values that we reported. The second section shows the distribution of the measured Zeta Potential values which the software automatically takes the average of. The third section shows the mobility, intensity and fitting curves. These sections are labeled and their units specified within each .txt file. The last two sections show the measured frequency. XPS files are .txt files and can be open with Notepad or Microsoft Excel XPS data includes the following: 1. BE_Pt 4f: binding energy (x-axis). 2. CPS_Pt 4f: intensity (raw data) All the following are from fitting (methods described in the manuscript): 3. Pt0_1_Pt 4f: Pt 4f7/2 at 0 status. 4. Pt0 4f 5/2_2_Pt 4f: Pt 4f5/2 at 0 status. 5. Pt2+_3_Pt 4f:Pt 4f7/2 at +2 status. 6. Pt2+ 5/2_4_Pt 4f: Pt 4f5/2 at +2 status. 7. Pt4+_5_Pt 4f: Pt 4f7/2 at +4 status. 8. Pt4+ 5/2_6_Pt 4f: Pt 4f5/2 at +4 status. 9. Background_Pt 4f: background of the spectrum. Substracted during fitting 10. Envelope_Pt 4f: fitting envelop for the whole Pt4f spectrum The data files and their corresponding figure: 1. Baseline-before CO introducing-FigS3a.txt: Fig. S3a 2. Baseline-after CO posisoning-FigS3b.txt: Fig. S3b 3. PtCbutyl-before CO introducing-FigS3c.txt: Fig. S3c 4. PtCbutyl-after CO posioning-FigS3d.txt: Fig. S3d 5. PtCethyl-before CO introducing-FigS3e.txt: Fig. S3e 6. PtCethyl-after CO posioning-FigS3f.txt: Fig. S3f 7. PtCdimethyl-before CO introducing-FigS3g.txt: Fig. S3g 8. PtCdimethyl-after CO posioning-FigS3h.txt: Fig. S3h