1. Application: A novel, non-destructive method which provides characterization of the three-phase interface in both catalyst and diffusion layers, between gas, liquid, and solid (i.e. porous carbon matrix and/or supported catalyst/polymer electrolyte). Objective: To measure the voltammogram and impedance spectra of the Oxygen Reduction Reaction (ORR) on Gas Diffusion Electrode (GDE) as a function of GDE structure and additive composition (i.e. Teflon and Nafion content). The reaction is carried out in oxygen saturated 0.1M H 2 SO 4 electrolyte at 293K. Application of Electrochemical Impedance Spectroscopy (EIS) as a Characterization Tool for PEM Fuel Cell Gas Diffusion Electrode Derek Cheng 1,2, Siyu Ye 2, and Elod Gyenge 1 1 University of British Columbia - Department of Chemical and Biological Engineering. 2 Ballard Power Systems Inc. SampleDescriptionCFP Carbon Fibre Paper HCFP Hydrophobic CFP – made by soaking a plain CFP in PTFE dispersion of various concentrations. The wet substrate is then dried and sintered at elevated temperature. NHCFP Nafion-coated HCFP – made by spraying a layer of ionomer dispersion (Nafion) over the HCFP. The sprayed substrate is then dried to remove moisture. PNHCFP Catalyst coated NHCFP – made by brush-coating a Pt- supported catalyst slurry over the NHCFP. Composition of the catalyst slurry is allowed to vary but the Pt loading was kept constant at 0.1 mg/cm 2. The coated substrate is then dried on a hotplate to remove moisture. Catalyst slurry The catalyst slurry is made by first mixing the supported catalyst (40% Pt supported on acetylene black) with Nafion dispersion at pre-defined mass ratios. Drops of IPA (Iso- Propilic Alcohol) are also added to aid dispersion of the catalyst powder in the mixture. The mixture is heated and then sonicated before brush-coating on the substrate. Experimental Setup: MSE Ref. Electrode Working Electrode (2.25 cm 2 active area) Graphite Rod Counter Electrode Electrolyte (0.1M H 2 SO 4 saturated with O 2 ) Electrochemical Impedance Spectroscopy (EIS) Cyclic Voltammetry (CV) Mercury Intrusion Porosimetry (MIP) Result: Scanning Electron Microscopy (SEM) Summary: The Oxygen electro-reduction on the carbon fiber paper (a 2-electron process) was a strong function of Teflon and Nafion content as revealed by both CV and EIS. Hence, oxygen electroreduction could be used as an electrochemical ‘sensor’ for the non-destructive analysis of the gas diffusion layer properties. The Oxygen electro-reduction on the carbon fiber paper (a 2-electron process) was a strong function of Teflon and Nafion content as revealed by both CV and EIS. Hence, oxygen electroreduction could be used as an electrochemical ‘sensor’ for the non-destructive analysis of the gas diffusion layer properties. Good correlation was shown between EIS and other electrochemical and structural measurement techniques, such as CV, MIP, and SEM. Good correlation was shown between EIS and other electrochemical and structural measurement techniques, such as CV, MIP, and SEM. Effect of Teflon and Nafion content was studied and found to have significant effect on the oxygen reduction reaction on carbon fiber paper. Effect of Teflon and Nafion content was studied and found to have significant effect on the oxygen reduction reaction on carbon fiber paper. Catalyst composition, between 25% to 50% Nafion content, did not seem to have significant effect on CV and impedance sweep. Catalyst composition, between 25% to 50% Nafion content, did not seem to have significant effect on CV and impedance sweep. Future Work: Further studies of catalyst layer composition: Pt/C support ratio, ionomer content. Quantitative interpretation of the data and mathematical modeling. Quantitative interpretation of the data and mathematical modeling. Scanning electrochemical impedance spectroscopy. Scanning electrochemical impedance spectroscopy. Acknowledgement: The following individuals are acknowledged for their contribution to this work: Joanna Kolodziej, P.Eng. (SEM support) Joanna Kolodziej, P.Eng. (SEM support) Gina Yin-Chi Chu (MIP support) Gina Yin-Chi Chu (MIP support) Kelly Maclean (Sample preparation) Kelly Maclean (Sample preparation) Principles of EIS: The ability of a circuit element to resist current flow when a potential is applied is called the electrical resistance. For an ideal resistor, the relationship between voltage, current, and resistance is given by Ohm’s Law (R = E/I). Extending this to the general case, when a small sinusoidal potential E(t) = E 0 exp (jt) is applied to an electrochemical cell, the current response I(t) = I 0 exp(jt-j) is also sinusoidal with the same frequency, but shifted in phase. This leads to the definition of the Complex Impedance Z() = E/I = Z 0 (Cos  + j Sin ), where kinetics parameters of an electrochemical system can be estimated by studying the impedance response of the electrochemical system as a function of frequency. R ct (charge transfer resistance ) R s (electrolyte resistance )  Effect of Increasing Teflon content of the HCFP Random carbon fibre matrix in CFP Macro Porosity (filled with epoxy) Teflon web (increasing hydrophobicity and lower porosity)