Benchmarking ORR Activity of Pt-Alloy/C Electrocatalysts Utilizing Improved TF-Rde Methods

Thin-film rotating disk electrode (TF-RDE) measurements have been performed to screen the Oxygen Reduction Reaction (ORR) activity of  electrocatalysts for PEMFC applications. Through progressive refinement of the technique by several researchers, a number of factors that play a critical role -on measured ORR activity -have been recently reported. Some of the factors include catalyst loading (electrode thickness), film uniformity (spin coating), Nafion cap thickness, ink dispersion/formulation, corrections for iR and background currents, etc.  However, for most of these studies, Nafion was typically employed either to improve the catalyst ink dispersion or to glue catalysts on a glassy carbon substrate.

In our previous study, we verified the effect of Nafion on the ORR activity as a function of I/C ratio. The measured activity enhancement was reported ~1.8 times for a Pt/HSC catalyst when Nafion was completely eliminated. Further studies demonstrated that the activity could be further enhanced (~2.7 times compared to a Nafion-based catalyst layer) for extremely thin, uniform films that were Nafion-free.  The thin films of low Pt loading were prepared either by incorporation of a surfactant in the dispersion or by drying of the electrodes under controlled conditions.

While relative standard deviation (RSD; based on ~30 samples for each method) for Nafion based catalyst films ranged ~9–15%, RSD for Nafion-free thin, uniform films was ~6%. This clearly demonstrates high reproducibility of measured ORR activity with the Nafion-free thin, uniform films.  Although the reason of the increase has not been elucidated yet, O₂ transport and proton transport resistances within the catalyst layer are the most probable causes.  Comparison of Tafel plots for Nafion-free and Nafion-based films to a polycrystalline Pt electrode provides some insight into the mechanism.  Definitive anion adsorption features of Nafion ionomer were not observable for the Pt/C catalysts in our studies.  Although further study is necessary to identify the cause of the large increase, the use of thin films without Nafion seems justified to reduce O₂ transport and proton transport resistance within the catalyst layer.

In this work, we have studied the ORR activity for various commercially available Pt based alloy catalysts to provide improved baselines using our new techniques.  Figure 1 is a plot comparing the ORR activity of baseline Pt/C to PtCo/C catalysts films deposited by conventional Nafion incorporation method (thick coffee ring structured film) and the Nafion-free method (thin-uniform film).

The measured specific activity (Figure 1) for Nafion-free, thin-uniform Pt/C films compared to that for a similar Nafion-based film increased by a factor of ~2.7 and is very similar to the enhancements observed for PtCo/C films.  Additionally, the specific activity enhancements due to alloying with Co are similar for Nafion-based and Nafion-free films.