Catalyst Layer Structure Properties and Electrode Performance with the Addition of Polyethylene Glycol

Abstract

Reducing the platinum loading in Proton Exchange Membrane Fuel Cells to mitigation costs is of topical interest. Lowering the Pt content beyond critical thresholds has a direct impact on the kinetic conversion efficiency of O2, and result in local oxygen transport resistance in the catalyst layer at high current densities. Improvements in the Pt use efficiency can be achieved by affecting the catalyst layer structure properties with improved Pt dispersion and increased ionomer interface1-3.In this study, we are investigating how the structure properties of the catalyst layer are affected by the addition of polyethylene glycol (PEG) during the ink step. PEG has a high boiling point (>200°C) and is known to have pore forming properties4. It has also been reported to extend the reach of the ionomer and its resulting protonic connectivity. Increasing the porosity in the catalyst layer can increase the triple phase boundaries and reduce interfacial resistances 5.Membrane electrode assemblies (MEAs) have been made with inks containing increasing wt.% PEG. The resulting inks were coated onto membranes to form catalyst layers containing 0.2 mgPt/cm2. The addition of PEG affected the binding of the ionomer and the catalyst, which will likely affect the packing of the agglomerates in the resulting catalyst layer structure. The catalyst layer thickness, surface area and porosity were determined. The coated substrates were assembled into single cells for electrochemical evaluation. The resulting MEAs showed that the inclusion of PEG reduced the kinetic losses at low current density due to improved electronic connectivity between the catalyst and the ionomer. Overall, the performance increased by ≈30mV at 1A/cm2.However, PEG content higher than 3% resulted in a significant decrease in performance in the high current density region. This is due to a higher packing density and therefore higher mass transport resistance with higher PEG content.The inclusion of PEG into the catalyst ink reduced the amount of flammable organic content and improved the MEA performance especially at low current density. Catalyst layer deposition was smoother with reduced cracking. Ultimately, the ambitions for low loading MEAs can be achieved by optimising the structure performance properties in the catalyst layer. PEG is considered a viable candidate for performance enhancement for low loading MEAs, yet a number of practical obstacles were experienced and require some optimization.