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

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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.