UC Irvine

In view of growing climate concerns and energy demands, polymer electrolyte fuel cells (PEFCs) are promising zero-emission energy-conversion technologies for stationary and mobile applications due to high thermodynamic efficiencies and power densities. However, their relatively short life and high cost are significant barriers to their widespread commercialization. Cathode catalyst layer (CL) plays a key role in design criteria of PEFC technology as significant performance decays occur there. Performance limitations stem from sluggish oxygen reduction reaction (ORR) kinetics, as well as proton and oxygen transport resistances in the CL. As a result, high loading of precious platinum (Pt) is needed to achieve sufficient current densities. ORR takes place at the interface of Pt and electrolyte; therefore, a deep understanding of the CL interface is vital for proposing novel designs to enhance Pt utilization. Moreover, degradation mechanisms of CL components are not completely understood, and it is crucial to fully comprehend the interactions influencing the durability of the stack.

We have explored the ORR mechanism and reaction pathways using kinetic isotope effect (KIE) approach. The rate-determining step for ORR on dispersed Pt/C electrocatalyst and polycrystalline Pt was identified to be proton-independent. Nevertheless, when Pt is dispersed on a high surface area (HSA) support, morphological confinement becomes critical as it dictates proton and oxygen transport in the CL. We presented an overview of electrostatics and mass transport induced confinement that can be caused by ionic liquids (ILs) integration. We explored three imidazolium-derived ILs, selected for their high proton conductivity and oxygen solubility and incorporate them into HSA carbon black support. Nanoconfinements interpretation and their role on transport properties near Pt surface helped understand the ORR kinetics dependence on the morphology of the carbon-supported electrocatalyst. Further, we established a correlation between the physical properties and electrochemical performance of the IL-modified catalysts to provide guidance for catalyst interface modification and importantly design of highly durable and active Pt-based catalysts for PEFC application.

To understand the evolution of Pt|ionomer and carbon|ionomer interfaces in the CL during life cycle of PEFCs, electrodes with HSA and durable carbon supports were investigated using accelerated stress tests (ASTs). Electrochemical characterizations, as well as X-ray photoelectron spectroscopy (XPS) were utilized to assess degree of components degradation. Catalyst AST along with analytical methodologies brought thorough insight into the role of Pt|ionomer interface in fuel cell polarization loss. In addition, support AST results for HSA carbon showed that the degree of ionomer degradation along with evolution of ionomer|carbon interface is responsible for performance decay.