Low PGM PtCo alloy catalysts for low loading Polymer Electrolyte Membrane Fuel Cells

Munro, Tiaan

Low PGM PtCo alloy catalysts for low loading Polymer Electrolyte Membrane Fuel Cells

Thesis / Dissertation

2023

Abstract

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are a promising means of generating clean energy in especially transportation applications. The electrochemical conversion of energy is driven within membrane electrode assemblies (MEAs) over costly Pt catalyst which limits PEMFC cost viability. PtCo alloy catalysts are attractive alternatives to Pt, with similar or higher catalytic activity at a reduced cost. The catalyst is combined with a perfluorosulfonic acid (PFSA) ionomer which serves as the proton conductor and binder to form the catalyst layer (CL). The ionomer contains both hydrophilic and hydrophobic groups, and therefore plays a key role in water management in the fuel cell. In every MEA design, ionomer content needs to be optimised to maximise fuel cell performance, ensuring adequate reactant-catalyst interaction, without flooding the MEA. This optimal ionomer content depends on the properties of the ionomer as well as catalyst characteristics (such as platinum loading, carbon to metal ratio and particle size), as well as fuel cell operating conditions. To establish an ideal low PGM MEA design, this thesis investigated two commercial PtCo catalysts (PtCoU30 and PtCoT50). Physical and electrochemical characterisation of the PtCo catalysts were investigated and compared to an in-house Pt benchmark (PtH40). The morphology, composition, size distribution, physical surface area, electrochemical surface area, performance and durability of these catalysts were established. Electrochemical characterization of the two catalysts showed that the higher Pt loading catalyst (PtCoT50) had lower mass activity compared to the lower Pt loading catalyst (PtCoU30). The PtCoT50 catalyst, however, exhibited similar specific activity. The electrochemical surface area of PtCoT50 was smaller than that of PtCoU30, which was consistent with the larger particle sizes observed for PtCoT50. Ex-situ accelerated stress tests showed that the higher metal loading catalyst was more susceptible to PtCo metal degradation but had greater carbon support durability. The ionomer contents for PtCoU30 and PtCoT50 MEAs were optimised at 30 wt% and 24 wt% respectively. This suggested that the higher metal content catalyst (PtCoT50) required less ionomer than the PtCoU30, due to the thinner catalyst layer thickness. Both optimised PtCo MEAs outperformed the PtH40 benchmark. This trend was consistent with the activities seen in ex-situ RDE. The impact of incorporating ionomer in different ink preparation stages/phases on performance and durability was determined. A 1-phase catalyst ink was prepared by adding the ionomer to catalyst prior to heat treatment, and solvent addition. A 2-phase ink was made by adding additional ionomer to the 1-phase ink, without being heat treated. It was shown that the performance for the PtCo catalysts was higher when all the ionomer was added in the heat treatment phase. However, the 2-phase MEAs showed slightly improved carbon corrosion and particle stability. It was suggested that the minor improvement in stability for 2-phase MEAs did not override the 1-phase MEA performance benefit . The recommended design for a low PGM PtCo MEA based on these findings is a 30 wt% PtCo/C MEA with 30 wt% ionomer content added during the heat treatment phase.

Keywords

Engineering

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