The Impact of Pretreatments and Conditioning on the Structure and Performance of NSTF ORR Alloy Catalysts

Pt-alloy nanoparticles and extended structures are under development for use as the oxygen reduction reaction (ORR) catalyst in polymer electrolyte membrane (PEM) fuel cell cathodes [1,2].  The addition of transition metals such as Co, Fe, or Ni can result in surface structures with enhanced specific activity [3,4], and in some cases increased specific area (Pt₃Ni₇/NSTF, Pt₁Ni₃ foils and NPs), which in combination can lead to overall increase in mass activity.  Although highly active, alloy catalysts can pose serious durability issues due to rapid dealloying of the transition metal during fuel cell operation.  Thus, ongoing development of these alloy catalysts aims not only at meeting critical cost and activity milestones, but also improving durability.

A variety of advanced scanning transmission electron microscopy (STEM) techniques have been employed to elucidate the complex structural-performance relationship of Pt₃Ni₇ nanostructured thin film (NSTF) catalysts after various dealloying and thermal treatments.  These STEM methods include aberration-corrected Z-contrast imaging, energy dispersive X-ray spectroscopy, and electron tomography, which have been utilized to explore the impact of various alloy catalyst pretreatments (dealloying, annealing, etc.) on morphology and composition, and, consequently, performance and durability. 

Electrode conditioning on either rotating disk electrodes or in membrane electrode assemblies (MEA) has a significant impact on alloy catalyst morphology and composition.  An example of this is presented in Figure 1.  While a particular pretreatment results in the catalyst morphology presented in Figure 1a, the conditioned catalyst, Fig. 1b, has a very different morphology and composition.  In this case, the dissolution of Ni during conditioning results in the formation of nanopores in the NSTF catalyst and a new catalyst composition (Pt₇Ni₃).  Since the catalytic metrics of the cathode (mass activity, specific activity, specific area, etc.) are measured after conditioning, it is the post-conditioned catalyst that must be characterized in order to establish accurate structure-performance relationships.  

Electron tomography is a powerful tool for visualizing nanoscale materials in three dimensions, as shown for a Pt₃Ni₇ NSTF catalyst before and after drive cycle testing in Figure 2.  In this work, electron tomography will not only be used to visualize the porous network formed during catalyst conditioning and cycling, but also to quantify changes in surface area, pore density, and pore volume resulting from different catalyst pre-treatments. 

References

1. M. K. Debe, Nature, 486, 43 (2012).
2. S. M. Alia et al., ACS Catalysis, 4, 1114 (2014).
3. V. R. Stamenkovic et al., Nat. Mater., 6, 241 (2007).
4. J. Snyder et al., J. Am. Chem. Soc., 124, 8633 (2012).

 Acknowledgements

 This work was supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy under Award Number DE-EE0005667 and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.