Oxygen reduction and transport characteristics at a platinum and alternative proton conducting membrane interface Lei Zhang, Chengsong Ma, Sanjeev Mukerjee * Department of Chemistry, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA Received 13 January 2004; accepted 8 February 2004 Available online 11 March 2004 Abstract Kinetic and mass-transport properties were investigated for the oxygen reduction reaction for Nafion Ò 117 and sulfonated poly(arylene ether sulfone) membranes, both pre- and post-sulfonated analogs (SPES-40 & SPES-PS) under 100% relative humidity and as a function of pressure (1–4 atm total pressure, 323 K) and temperature (303–353 K, 3 atm) using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass-transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)jproton exchange membrane (PEM) interface. The oxygen reduction kinetics were found to be similar for all the membranes at the Pt microelectrode interface. The temperature dependence of O 2 permeation parameters showed identical trends for the membranes studied while the pressure dependence of O 2 permeation parameters displayed some differences. Despite higher ion exchange capacities and hence higher water uptake, the two SPES membranes exhibited relatively lower values of D as compared to Nafion Ò 117. The results are discussed in the context of their different microstructures. Ó 2004 Published by Elsevier B.V. Keywords: Sulfonated poly(arylene ether sulfone); Oxygen diffusion coefficient; Oxygen solubility; Oxygen permeability; Microelectrode 1. Introduction Proton exchange membrane fuel cells (PEMFCs) are candidate power sources for vehicular transportation, residential and consumer electronics [1]. Amongst the key components is the polymer electrolyte membrane (PEM) that provides the ionic pathway and acts as a gas separator. The current state-of-the-art is based on per- fluorinated sulfonic acid chemistry, such as those from Dupont (Nafion Ò ), Asahi chemicals (Aciplex Ò ) and others. These membranes achieve good performance when operating at 80–90 °C and high relative humidity (>80% RH) [2–4]. They have good mechanical strength, chemical stability (up to 60,000 h of operation at 80 °C) and high proton conductivity [1]. However, these membranes remain expensive and have several limiting factors such as low conductivity at low relative humidity [5], high methanol permeability [6,7], and a low T g (glass transition temperature) [8] which restricts its application to below 100 °C. Transitioning to temperatures above 100 °C provides for several attractive options which include higher CO tolerance [9,10], better water and heat management re- lated to interfacing fuel cells with other system compo- nents such as the fuel processor unit. Alternative hydrated membranes to the perfluori- nated sulfonic acid based systems possessing high pro- ton conductivity at lower relative humidity and stability at elevated temperatures are currently the focus of much research and development. Most of them are based on engineering polymers with high thermo-chemical sta- bility [11], typically with a high degree of aromatic character, where the monomer consists of a variety of fused phenyl rings linked together with a number of bridging moieties (referred herein as membranes with aromatic backbones). Sulfonation of these materials involves either using a sulfonated monomer in the polymer synthesis or using a variety of methods for * Corresponding author. Tel: +1-617-373-2382; fax: +1-617-373- 8949. E-mail address: smukerje@lynx.neu.edu (S. Mukerjee). 0022-0728/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jelechem.2004.02.003 Journal of Electroanalytical Chemistry 568 (2004) 273–291 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry