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
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
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DOI：
10.1002/fuce.200400059
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发表时间：
2005-08
期刊：
影响因子：
2.8
通讯作者：
M. Schuster;T. Rager;A. Noda;K. Kreuer;J. Maier
中科院分区：
文献类型：
--
作者：
M. Schuster;T. Rager;A. Noda;K. Kreuer;J. Maier
传统上，质子交换膜燃料电池通常采用磺化聚合物作为隔膜材料。基于模型化合物的最新实验结果，本文批判性地讨论了在中温(T>；100℃)和低增湿条件下运行的质子交换膜燃料电池电解液的原生基--磺酸、膦酸和杂环(咪唑)的潜力和局限性。除了输运性质外，还考察了单官能团化模型化合物(1-庚基磺酸(S-C7)、1-庚基膦酸(P-C7)和2-庚基咪唑(I-C7))和少数二膦酸在干湿条件下的稳定性和反应活性。从质子导电性(交流阻抗谱)、质子扩散系数(脉冲梯度核磁共振)、热氧化稳定性(空气中的TGA)、电化学稳定性(循环伏安法)和水化行为(水蒸气中的TGA)等方面对它们进行了表征。只有在保证最低水合水平的情况下，磺酸官能化化合物才能显示出合理的性能，而即使在中温(T<；200°C)的无水状态下，磺酸官能化化合物也具有令人满意的质子导电性、相对较高的热氧化和电化学稳定性以及电化学反应活性(铂表面的氢氧化和氧还原)。水的存在导致适度的吸水率，即使在室温下也能保持合理的导电率，并防止在较高温度下发生缩合反应。咪唑体系表现出最大的电化学稳定窗口，但其中等的质子导电性和热氧化稳定性以及很高的铂上氧还原过电位严重阻碍了预期的应用。
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 < 200 °C), comparatively high thermo‐oxidative and electrochemical stability and electrochemical reactivity (hydrogen oxidation and oxygen reduction at platinum surfaces). The presence of water leads to moderate water uptake allowing for reasonable conductivities even at room temperature and prevents condensation reactions at higher temperature. The imidazole based system shows the largest electrochemical stability window, but its moderate proton conductivity and thermo‐oxidative stability and the very high overpotential for oxygen reduction on platinum turn out to be severe disadvantages for the envisaged application.