Proton Conduction and Oxygen Reduction Kinetics in PEM Fuel Cell Cathodes: Effects of Ionomer-to-Carbon Ratio and Relative Humidity

The electrode in a proton exchange membrane (PEM) fuel cell is composed of a carbon-supported Pt catalyst coated with a thin layer of ionomer. At the cathode, where the oxygen reduction reaction occurs, protons arrive at the catalyst sites through the thin ionomer layer. The resistance to this protonic conduction (R H+,each ) through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The R H +, eath in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a H 2 /N 2 cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting R H+,eath , cathode capacitance (C eath ), and high frequency resistance to a simplified transmission-line model with the assumption that the proton resistance and the pseudocapacitance are distributed uniformly throughout the electrode. The proton conductivity in the given types of electrode starts to drop at I/C ratios of approximately <0.6/1 or an ionomer volume fraction of ~ 13% in the electrode. The comparison to H 2 /O 2 fuel cell performance shows that the ohmic loss in the electrode can be quantified by this technique. The cell voltage corrected for ohmic losses is independent of relative humidity (RH) and the electrode's I/C ratio, which indicates that electrode proton resistivity ρ H+cath (ratio of R H+,cath over cathode thickness) is indeed an intrinsic RH-dependent electrode property. The effect of RH on the ORR kinetics was further identified to be rather small for the range of RH studied (≥35% RH).

[1]  P. Pickup,et al.  CHARACTERIZATION OF IONIC CONDUCTIVITY PROFILES WITHIN PROTON EXCHANGE MEMBRANE FUEL CELL GAS DIFFUSION ELECTRODES BY IMPEDANCE SPECTROSCOPY , 1999 .

[2]  H. Gasteiger,et al.  Catalyst Development Needs and Pathways for Automotive PEM Fuel Cells , 2006 .

[3]  J. Jorné,et al.  Study of the Exchange Current Density for the Hydrogen Oxidation and Evolution Reactions , 2007 .

[4]  Shimshon Gottesfeld,et al.  Thin-film catalyst layers for polymer electrolyte fuel cell electrodes , 1992 .

[5]  Hubert A. Gasteiger,et al.  Handbook of fuel cells : fundamentals technology and applications , 2003 .

[6]  J. Jorné,et al.  Determination of Electrode Sheet Resistance in Cathode Catalyst Layer by AC Impedance , 2007 .

[7]  Hubert A. Gasteiger,et al.  PEM Fuel Cell Operation at − 20 ° C . II. Ice Formation Dynamics, Current Distribution, and Voltage Losses within Electrodes , 2008 .

[8]  Hubert A. Gasteiger,et al.  Oxygen Reduction Reaction Kinetics in Subfreezing PEM Fuel Cells , 2007 .

[9]  Mark W. Verbrugge,et al.  A Mathematical Model of the Solid‐Polymer‐Electrolyte Fuel Cell , 1992 .

[10]  Hubert A. Gasteiger,et al.  Dependence of Electrode Proton Resistivity on Electrode Thickness and Ionomer Equivalent Weight in Cathode Catalyst Layer in PEM Fuel Cell , 2008 .

[11]  Bill Diong,et al.  Fuel Cells : Modeling, Control, and Applications , 2009 .

[12]  J. Jorné,et al.  Cathode Catalyst Utilization for the ORR in a PEMFC Analytical Model and Experimental Validation , 2007 .

[13]  Hubert A. Gasteiger,et al.  Handbook of Fuel Cells , 2010 .

[14]  P. Pickup,et al.  Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading , 2003 .

[15]  A. Kornyshev,et al.  Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells , 1999 .

[16]  Michael Eikerling,et al.  Water Management in Cathode Catalyst Layers of PEM Fuel Cells A Structure-Based Model , 2006 .

[17]  Richard C. Alkire,et al.  Advances in electrochemical science and engineering , 1990 .

[18]  Hubert A. Gasteiger,et al.  Effect of Relative Humidity on Oxygen Reduction Kinetics in a PEMFC , 2005 .

[19]  Shimshon Gottesfeld,et al.  High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells , 1992 .

[20]  Hubert A. Gasteiger,et al.  Determination of Catalyst Unique Parameters for the Oxygen Reduction Reaction in a PEMFC , 2006 .

[21]  S. Srinivasan,et al.  Measurements of proton conductivity in the active layer of PEM fuel cell gas diffusion electrodes , 1998 .

[22]  S. Mukerjee,et al.  Oxygen Reduction Kinetics in Low and Medium Temperature Acid Environment: Correlation of Water Activation and Surface Properties in Supported Pt and Pt Alloy Electrocatalysts , 2004 .

[23]  Fernando H. Garzon,et al.  Determination of Ionic and Electronic Resistivities in Carbon/Polyelectrolyte Fuel-Cell Composite Electrodes , 2002 .

[24]  James M. Fenton,et al.  Effect of Elevated Temperature and Reduced Relative Humidity on ORR Kinetics for PEM Fuel Cells , 2005 .

[25]  M. Mathias,et al.  Measurement of Catalyst Layer Electrolyte Resistance in PEFCs Using Electrochemical Impedance Spectroscopy , 2005 .

[26]  H. Angerstein-Kozlowska,et al.  The real condition of electrochemically oxidized platinum surfaces , 1973 .

[27]  H. Gasteiger,et al.  Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs , 2005 .

[28]  Hui Xu,et al.  Analysis of proton exchange membrane fuel cell polarization losses at elevated temperature 120 °C and reduced relative humidity , 2007 .

[29]  John Newman,et al.  A theoretical study of membrane constraint in polymer-electrolyte fuel cells , 2004 .