Perfluorinated Polymer Electrolyte Membrane Durability
Perfluorinated Polymer Electrolyte Membrane Durability
复制标题
全氟聚合物电解质膜的耐久性
DOI：
--
复制
发表时间：
2006
期刊：
影响因子：
--
通讯作者：
D. Schiraldi
中科院分区：
文献类型：
--
作者：
D. Schiraldi
耐久性是所有燃料电池系统中的关键问题，对于PEM膜聚合物当然如此。目前，全氟化离聚物通常在PEM系统中受到青睐;这种系统的使用寿命可能受到损害的限制，包括膜变薄、重量损失和燃料电池运行期间催化剂材料的再分布。过氧化氢仍然是膜降解的最可能的罪魁祸首，在燃料电池催化条件下很容易作为副产物产生。一旦生成，过氧化氢可以容易地均裂成能够破坏聚合物组成键的过氧化物自由基。市售PEM膜降解的主要机制是从PTFE主链上的残留羧酸末端提取氢原子。这种原子提取引发系统的链式氧化成二氧化碳和氟化氢，其在流出水中被检测到。羧酸末端通过羧基的还原大大减少了降解并增加了耐久性。在这些条件下，聚合物降解似乎也在活性较低的位置通过过氧化物攻击进行，可能比羧酸基团反应慢2-3个数量级;这种较低的反应性可以通过聚合物上相当高浓度的非羧酸酯位置来平衡。在未来的几年里，关键的问题要回答包括在稳态下运行的燃料电池内的实际过氧化物自由基浓度的测定，启动/关闭和扰乱条件下，更好地了解羧酸端基以外的具体攻击位点，以及减少聚合物降解的程度和影响的方法，以延长PEM燃料电池的寿命。
Durability is a critical issue in all fuel cell systems, certainly so for PEM membrane polymers. Perfluorinated ionomers are generally favored in PEM systems at the present time; the useful lifetimes of such systems can be limited by damage including membrane thinning, weight loss, and redistribution of catalyst materials during fuel cell operation. Hydrogen peroxide remains the most likely culprit for membrane degradation, being readily produced as a by‐product under fuel cell catalysis conditions. Once generated, hydrogen peroxide can be readily homolysed into peroxide radicals capable of breaking of polymer constituent bonds. The leading mechanism for degradation of commercially‐available PEM membranes is initiated by abstraction of a hydrogen atom from residual carboxylic acid ends on PTFE backbones. Such atom abstraction initiates a systematic chain oxidation to carbon dioxide and hydrogen fluoride, which is detected in the effluent water. Reduction of the carboxylic acid ends by fluorination substantially reduces degradation and increases durability. Under these conditions, polymer degradation appears to proceed by peroxide attack at less active positions as well, perhaps 2–3 orders of magnitude slower to react than at carboxylic acid groups; this lower reactivity can be balanced by the considerably higher concentration of non‐carboxylate positions on the polymer. In the years to come, critical questions to be answered include determination of actual peroxide radical concentrations within operating fuel cells under steady state, start up/shut down and upset conditions, a better understanding of specific sites of attack beyond carboxylic acid end groups, and the methods for reducing the extent and the impact of polymer degradation in order to prolong PEM fuel cell lives.