PEMFC催化层中纳米尺度Nafion树脂的质子传导机理研究

Although great progress has been made, the commercialization of PEMFCs is still hindered by the high system cost. To overcome this bottleneck and realize the ultra-low platinum loading, it is necessary to increase the catalyst utilization which requires making full use of the active sites (triple phase boundary - TPB) on Pt surfaces. However, there are limited reports focusing on the proton conduction mechanism in Nafion ionomer of the catalyst layer, as a result, the TPB formation principle is still unclear and the catalyst layer design guidance is not available. For this reason, this project will study the proton conduction mechanism in nanosized Nafion ionomer of the catalyst layer. Since proton conduction is coupled with microstructure of Nafion ionomer, we will first prepare a thickness-controllable Nafion film on model substrates via self-assembly approach to investigate the microstructure of Nafion ionomer and build the microstructure model. Then, based on the proposed microstructure model, the proton conduction mechanism in nanosized Nafion ionomer will be studied by experimental test and theoretical calculation. Furthermore, the influences of PEMFC operation conditions (e.g., temperature, humidity, pressure) and MEA fabrication conditions (such as hot-pressing treatment) on microstructure of Nafion ionomer and proton conduction will be investigated. Finally, we will design model catalyst layer and perform single cell test to verify the proton conduction mechanism. This project will help to improve the TPB formation principle, providing a theoretical basis for the design of novel catalyst layer with high catalyst utilization.

限制质子交换膜燃料电池商业化应用的原因在于成本问题。降低燃料电池成本的关键在于提高催化层中铂催化剂的利用率，充分利用有限的铂表面活性位（三相界面），以实现PEMFC超低铂载量运行。然而，催化层中质子传导的机理研究鲜有报道，成为明确三相界面构建规则缺失的一环，无法为催化层设计提供理论指导。因此，本项目拟开展催化层中纳米尺度Nafion树脂的质子传导机理研究。质子传导与Nafion树脂的微观结构紧密相关，本项目将首先利用自组装技术于模型基底上制备厚度可控的Nafion薄层，探索纳米尺度下Nafion的微观结构并建立相应结构模型；在此基础上结合理论计算和实验测试，揭示质子传导机理，并阐明温度、湿度等电池运行条件及热压处理等电极制备条件对微观结构及质子传导的影响；最后设计模型催化层进行单电池验证，确认质子传导机理的准确性，以完善三相界面的构建规则，为设计高催化剂利用率的催化层提供科学基础。

为了推动质子交换膜燃料电池的商业化应用，必须降低催化层中催化剂铂的载量，从而降低燃料电池成本。因此，需要明确三相界面构建规律，提高催化剂利用率。本项目开展了催化层中纳米尺度Nafion树脂的质子传导机理研究，以期完善三相界面构建规律。首先，针对催化层结构过于复杂以至于无法对Nafion树脂进行微观结构表征、电导率测试等问题，本项目开发了基于分子自组装的Nafion树脂薄膜可控制备方法，厚度精度达到纳米级。在此基础上，研究随厚度变化Nafion树脂微观结构及质子电导率的演变规律，结果表明当Nafion薄膜厚在50 nm以下时，质子电导率仅有~0.01 S/cm，比微米级Nafion膜低一个数量级，结合GI-SAXS技术，证实在限域条件下，Nafion树脂中亲-憎水微相分离显著减弱，进而提出纳米尺度Nafion树脂的结构假想模型。项目进一步研究了聚合物支链长度、分散溶剂、合金催化剂引起的过渡金属阳离子污染对Nafion树脂微观结构及质子传导的影响规律。研究表明短链树脂的磺酸基团间可以形成较强的相互作用，从而产生离子团簇强化微相分离，改善质子传导；介电常数较小的醇类溶剂可以使Nafion聚集体在溶剂中形成棒状胶束，成膜后与基底结合较弱，更易形成连续的水分子通道，使Nafion薄膜的质子电导率得到提高；此外，发现过渡金属阳离子与磺酸基团结合强于质子，金属离子污染后，Nafion树脂的质子电导率大幅下降，并且当树脂中H+在阳离子位点的占有率小于50％时，质子传导的活化能剧增，表明当离聚物污染到达一定的程度，树脂内质子传导机理发生了改变。