About the Choice of the Protogenic Group in PEM Separator Materials for Intermediate Temperature, Low Humidity Operation: A Critical Comparison of Sulfonic Acid, Phosphonic Acid and Imidazole Functionalized Model Compounds

• Published in: Fuel cells (Weinheim an der Bergstrasse, Germany) Germany), 2005-08, Vol.5 (3), p.355-365
• Main Authors: Schuster, M., Rager, T., Noda, A., Kreuer, K. D., Maier, J.
• Format: Article
• Language: English
• Subjects: Cyclo Voltammetry; Electrochemical Stability; Imidazole; PEM Fuel Cell; Phosphonic Acid; Proton Conductivity; Proton Diffusion; Sulfonic Acid
• ISSN: 1615-6846; 1615-6854
• DOI: 10.1002/fuce.200400059

Traditionally, sulfonated polymers are used as separator materials in PEM fuel cells. Based on recent experimental results on model compounds this paper critically discusses the potentials and limits of sulfonic acid and alternatively phosphonic acid and heterocycles (imidazole) as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures (T > 100 °C) and low humidification. Apart from transport properties, the stability and reactivity of mono‐functionalized model compounds (1‐heptylsulfonic acid (S‐C7), 1‐heptylphosphonic acid (P‐C7) and 2‐heptyl‐imidazole (I‐C7)) and a few diphosphonic acids are examined under wet and dry conditions. These are characterized with respect to their proton conductivity (ac impedance spectroscopy), proton diffusion coefficient (pulsed‐field gradient NMR), thermo‐oxidative stability (TGA under air), electrochemical stability (cyclic voltammetry) and their hydration behavior (TGA under water vapor). The sulfonic acid functionalized compound shows reasonable properties only when a minimum hydration level is guaranteed, while phosphonic acid functionalized compounds combine satisfactory proton conductivity even in the water‐free state at intermediate temperatures (T