Electrochemical and Rheological Investigations of Catalyst-Solvent-Polymer Ink Formulations for Electrospun Fuel Cell Electrode Structures

Abstract

The rational design of electrode structures and interfaces has emerged as a potential route for increasing the efficiency of electrochemical devices. As a fabrication technique, electrospinning allows for the creation of electrode structures possessing purported improvements in proton conductivity, gas transport and durability[1], while economic assessments [2] have shown electrospinning to be cost competitive with other Roll-to-Roll manufacturing techniques.[3] However, at present, very little is known about the particle-solvent-polymer interactions which occur at the ink level. Understanding how the electrospun electrode morphology relies not only on precursor material properties, but also on the interactions between the catalyst, ionomer and solvents being utilized can help improve the ability to dictate and tune relevant interactions within the electrode structure. Unfavorable interactions between ionomer, carrier polymer and Pt active sites can lead to reductions in activity and fuel cell performance. Hence, its imperative to understand the properties that dictate the fundamental chemistry and materials properties of various electrospun ink formulations in order to rationally design electrode structures. In support of this process-focused study, detailed exploration of particle-polymer-solvent interactions in the subject inks and slurries will be undertaken utilizing both electrochemical in-situ diagnostics as well as ex-situ advanced characterization techniques to achieve a fundamental understanding of various ink fabrication processes and colloidal structure of inks and dispersions as a function of solids content, solvent type, and particle size. In particular, we will investigate different electrospun ink formulations for fuel cell electrodes including small-scale studies of various material sets, carrier polymers, slurry compositions and excitation conditions. The resulting ink level interactions will be discussed in conjunction with varying synthetic and fabrication parameters. The resulting electrode structure, ionomer-Pt interactions, and coverage of ionomer on both Pt and carbon, will be related to performance related phenomena at high current density (i.e. oxygen transport resistance and proton conductivity). [4] The electrospinning inks will be characterized via ink/dispersion surface charge stabilization (zeta potential measurements) and rheological studies as function of ink composition and processing variables.[5] Morphological properties of electrospun nanofiber electrodes will also be characterized using various spectroscopic and microscopic techniques (USAXS, TEM, SEM, EDS). Following ink/fiber optimization, electrodes will be conditioned [6] and examined with in-situ electrochemical diagnostics such as electrochemical surface area (ECA), CO-displacement measurements, oxygen transport resistances (Rnf) and electrochemical impedance spectroscopy (EIS) in the catalyst layers will be measured to elucidate the effects of different ink compositions, excitation parameters and electrode structures on PEMFC performance. Preliminary results have demonstrated significant improvements in mass activity, up to a factor of 2, for electrospun electrodes compared to traditional ultrasonically sprayed catalyst coated membrane (CCM) electrodes. Moreover, rheological measurements of Pt/Vu-ionomer (Nafion), Pt/Vu-polymer(PAA) and ionomer-polymer ink slurries have shown that the concentration of carrier polymer (PAA) in the ink slurries plays a significant role in determining rheological properties, which may ultimately play a role in the final electrode structure, ionomer coverage and interaction with the Pt-surface.