Engineering the Ionic Polymer Phase-Fluid Interface of the PEM Fuel Cell Catalyst Layer for Higher Performance

氢-氧质子交换膜燃料电池(PEMFC)是一种将氢气与氧气反应的化学能转化为电能的装置。质子交换膜燃料电池用于运输、推进和分布式电力应用的全面商业化的主要障碍之一是无法在高功率密度和高能源效率下运行。低功率密度使系统成本居高不下，低能效使其运行成本居高不下。目前，高功率密度操作受到限制，因为氧催化层中产生的水湿润并填充了阴极催化层中聚合物粘结剂的气孔。这阻碍了氧气向催化剂反应部位的运输。这项工作将确定是什么控制了这种离子聚合物粘结剂相的表面润湿性，并在此基础上将这种离子聚合物粘结剂的表面修饰为亲水或疏水。在PEMFC应用的情况下，疏水表面是首选的，因为它有助于从催化层的孔中排斥副产物水，并允许氧气更好地进入催化层中的活性位置以产生更多的能量。如果这项工作取得成功，将导致燃料电池汽车的大规模商业化。此外，膜/流体界面性质的发现将有利于其他领域，如水处理、化学分离和大规模储能应用，从而加强我们的科学知识和技术进步，加强我们在世界上的领导地位，以及经济和国家安全。最后，这项工作将产生开发和商业化这些新技术所需的人力资源，并保持科学和技术进步。在这个基础工程科学项目中，我们研究了一种新的工艺，以创建一种最适合两相(输入气态氧气反应物和输出液态水产品)传输的燃料电池催化层。该工艺基于对全氟磺酸(PFSA)聚合物的最新发现，这些发现表明，当改变与聚合物接触的气体的相对湿度(RH)时，PFSA聚合物的表面形态会发生变化。最近，PI的团队开发了一种用于PFSA膜的新的热处理工艺，以创建永久的疏水或亲水表面结构。因此，假设通过控制燃料电池催化层内离子聚合物粘结剂的热处理条件，可以将催化层气孔内的聚合物-气体界面设计为疏水或亲水。对催化层进行热处理，使离子磺酸基团聚集在聚合物-气体界面上(在阴极气孔中的高相对湿度下)，将产生亲水性的聚合物-气体界面。然而，热处理以产生富聚四氟乙烯(PTFE)的聚合物-气界面(在低相对湿度下)将导致疏水的聚合物-气界面。随后冷却阴极催化层将结晶聚合物的聚四氟乙烯相并锁定表面形态。对于具有液体反应物的电极，如PEM电解槽，最好使用亲水性离子聚合物-流体界面。对于含有气态反应物的电极，如氢-空气PEMFC，首选疏水聚合物-流体界面。制备了与PEM燃料电池相关的具有疏水催化层的电极，并采用PI的逐层去除方法，利用扫描电子显微镜、X射线光电子能谱、中子反射仪(NR)和表面增强拉曼散射(SERS)对其内部孔结构的聚合物层形貌进行了表征。这些电极将在PEMFC中进行测试，以确定将导致最高功率密度和能效性能的催化层和电极膜组件制造工艺条件。将研究催化剂油墨中使用的溶剂类型对改性聚合物结构结晶度的影响，并将通过水沸腾和高温湿度循环试验来评估聚合物界面性能的长期耐久性。最后，将开发一个质子交换膜燃料电池阴极催化层的模型，以研究疏水离子聚合物-气体界面对催化层气孔中的两相流体传输、聚合物层中的氧气传输以及燃料电池性能的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

A hydrogen-oxygen proton exchange membrane fuel cell (PEMFC) is a device that converts chemical energy of hydrogen reacting with oxygen into electrical power. One of the primary barriers to full-scale commercialization of PEMFCs for transport propulsion and distributed power applications is the inability to operate at high power density and high energy efficiency. Low power density keeps the system cost high, and low energy efficiency makes its operational cost high. High power density operations are currently limited because water produced in the oxygen catalyst layer wets and fills the gas pores of polymer binder in the catalyst layer of the cathode. This blocks the transport of oxygen to the catalyst reaction sites. This work will determine what controls the surface wettability of this ionic polymer binder phase, and based on this understanding, will modify the surface of this ionic polymer binder to be either hydrophilic or hydrophobic. In the case of the PEMFC application, a hydrophobic surface is preferred because it helps repel the by-product water from the pores of the catalyst layer and allow oxygen gas better access to the active sites in the catalyst layer to generate more power. If this work is successful, it will lead to large-scale commercialization of fuel cell vehicles. Moreover, discoveries in membrane/fluid interfacial property could benefit other areas such as water treatment, chemical separations, and large-scale energy storage applications, thus strengthening our scientific knowledge and technological advancement and leadership in the world and economic and national security. Finally, this work will generate the human resources needed to develop and commercialize these new technologies and sustain the scientific and technological progress.In this fundamental engineering science project, a novel process is researched to create a fuel cell catalyst layer that is optimal for two-phase (incoming gaseous oxygen reactant and outgoing liquid water product) transport. The process is based on recent discoveries with perfluorosulfonic acid (PFSA) polymers, which have shown that the surface morphology of PFSA polymers changes when varying the relative humidity (RH) of the gas in contact with the polymer. Most recently, the PI's group developed a new heat treatment process for PFSA membranes to create a permanent hydrophobic or hydrophilic surface structure. Therefore, it is hypothesized that by controlling the heat treatment conditions of the ionic polymer binder within the fuel cell catalyst layer one can engineer the polymer-gas interface inside the gas pores of the catalyst layer to be either hydrophobic or hydrophilic. Heat treating the catalyst layer such that the ionic sulfonate groups accumulate at the polymer-gas interface (under high RH in the cathode gas pores) will result in a hydrophilic polymer-gas interface. Whereas, heat treating to create a polytetrafluoroethylene (PTFE)-rich polymer-gas interface (under low RH) will lead to a hydrophobic polymer-gas interface. Subsequent cooling of the cathode catalyst layer will then crystallize the polymer's PTFE phase and lock-in the surface morphology. A hydrophilic ionic polymer-fluid interface is preferred for electrodes with liquid reactants, such as the PEM electrolyzer. A hydrophobic polymer-fluid interface is preferred for electrodes with gaseous reactants, such as the hydrogen-air PEMFC. Electrodes with hydrophobic catalyst layer, relevant to PEM fuel cells, will be fabricated, and using the PI's layer-by-layer removal method, the polymer layer morphology of their inner porous structures will be characterized by the SEM, XPS, neutron reflectometry (NR), and surface-enhanced Raman scattering (SERS). These electrodes will be tested in a PEMFC to determine the catalyst layer and electrode-membrane-assembly fabrication process conditions that will lead to the highest power density and energy efficiency performance. The effect of the solvent type used in the catalyst ink on the crystallinity of the modified polymer structure will be investigated, and the long-term durability of the polymer interfacial property will be evaluated using the water boiling and high temperature humidity cycling tests. Finally, a model of the cathode catalyst layer of a PEMFC will be developed to study the effect of the hydrophobic ionic polymer-gas interface on two-phase fluid transport in the catalyst layer gas pores, oxygen transport through the polymer layer, and fuel cell performance.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.