Electrospinning and MEA Conditioning As a Tool for Designing Optimized Electrode Structures for Fuel Cell Applications

Abstract

Complimentary to materials improvements, the rational design of electrode structures and interfaces have emerged as potential routes for increasing fuel cell efficiency. Traditional MEA fabrication methods are dependent upon the deposition of uniform ink compositions that couple the integration of ionomer and catalyst. Subsequent electrode morphology then depends solely on the material properties and interactions of the catalyst, ionomer and solvents being utilized. In Pt based systems, unfavorable interactions between ionomer and Pt active sites can lead to reductions in activity and additional oxygen transport resistance. [1,2] Additionally, in Platinum Group Metal (PGM)-free electrodes, inhomogeneous coverage of larger particles results in poor catalyst utilization and reduced proton conductivity throughout the layer. Electrospinning can be used to combat such limitations. Electrospinning can produce nanofiber threads (typically ~500 nm in diameter) formed by applying an electrostatic voltage from the deposition needle to the grounded substrate.[3] Electrospun electrode development gives an opportunity for us to dictate electrode properties and interactions that are more defined and desirable, by allowing us to decouple electrode structural aspects from solution or ink-based behavior. Such advantages include: (i) the ability to produce homogeneous ionomer distributions across both electrocatalyst agglomerates and the electrode thickness, (ii) greatly improve the ionomer distribution for larger catalysts structures/agglomerates, such as MOF based PGM-free electrocatalysts and (iii) utilize multiple ionomers, each of which may be specifically tailored based on local or bulk electrode requirements. Here, we will discuss the incorporation of two classes of electrocatalysts; (i) PGM based (e.g. Pt/C and Pt alloy/C)s and (ii) and PGM-free (e.g. FeNx/C based) . The electrospun electrodes were fabricated with different ionomer (Nafion), carrier polymer, (PVDF, PAA) and solvent blends (IPA, DMF, Acetone). The effect of varying synthetic and fabrication parameters (flow rates, humidity, and operating potential) on electrode structure were investigated in order to improve high current density performance. The electrodes were characterized using various microscopic techniques (TEM, SEM, EDS), and in-situ diagnostics such as oxygen reduction reaction (ORR) mass activity, electrochemical surface area (ECA) and H2-Air/O2 performance, oxygen transport resistances (Rnf) and Electrochemical Impedance Spectroscopy (EIS) in the catalyst layers were measured to elucidate the effects of different electrode structures on PEMFC performance. Preliminary results show improvement in mass activity (determined at 0.9V, 150 kPaabs, 80oC and 100% RH) when compared to traditional ultrasonically sprayed catalyst coated membrane (CCM) electrodes. Reductions in non-Fickian transport resistances (RnF) were also observed on nanofiber electrodes, suggesting a re-arrangement in ionomer coverage and distribution on Pt-surface. The results from this study were subsequently applied to thicker (~100 μm) platinum group metal free (PGM-Free) electrodes where optimization of gas and proton transport are more impactful.