Studying Ru Dissolution in Pt, Ru, and Pd Core-Shell Nanostructures As a Function of Stress Test Coupled with CO Stripping Analyses

Abstract

Hydrogen gas contaminated with CO, when used as a reactant on the fuel cell anode, has a detrimental impact on its performance. A low CO contamination of 5 ppm can decrease the fuel cell power density to less than half the value obtained with pure H2.1 To combat CO poisoning of the anode, PtRu/C is currently used on the fuel cell anode instead of the nominally more active Pt/C catalyst. The PtRu/C catalyst reduces CO poisoning by oxidizing this adsorbate within the potential range of hydrogen oxidation. The addition of a third metal such as Pd, Sn, or Co to this bimetallic alloy has demonstrated further improvements to the CO tolerance of the catalyst. The wide application of these alloys is, however, restricted due to the instability of non-noble metal, Ru, at high potentials experienced during fuel cell startup, shutdown, and fuel starvation conditions.2 Dissolution of Ru leads to structural modifications to the nanocatalyst and, thus, directly impacts the CO tolerance of the catalyst because of its structure-property relationship. The dissolution of Ru not only decreases the CO tolerance of the anode, but also reduces the oxygen reduction activity of the cathode due to Ru migration, causing an overall decline in fuel cell performance.3 These structural changes result in a complex heterogeneous particle surface with varying amounts of Pt atoms on each catalytic site, as observed at the atomic level using TEM imaging techniques. The use of TEM does, however, require a visual inspection of the sample. Thus, obtaining results based on TEM analysis alone is not only time-consuming but can also be subjective.4 Studying the structural changes of an ensemble of nanoparticles using a series of electrochemical techniques can help to better understand the degradation mechanism(s) of a catalyst. In this work, modifications to the structure of an ensemble of nanocatalysts because of Ru dissolution during stress tests were evaluated by conducting CO stripping analyses after a well-defined period of testing.Nanoparticles of Pd@PtRu, PtRu@Pd, and PdRu@Pt (core@shell) structures along with a PtRuPd alloy were prepared using a water-in-oil microemulsion synthesis method and examined for their structural stability. These nanoparticles were each adsorbed onto separate carbon supports. The successful formation of an alloy or core-shell structure was assessed by electron diffraction and X-ray spectroscopy techniques. The process of alloying metals and creating core-shell structures can alter the electronic structure of a material, which was studied using X-ray photoelectron spectroscopy. Given the applicability of these catalysts to the anode of the fuel cell, hydrogen oxidation activity was investigated using rotating disk electrode studies, while CO tolerance was assessed using CO stripping analysis. Their structural durability was tested by cycling the applied potential between 0.6 to 0.95 V (vs RHE) for 5000 complete cycles while immersing the electrocatalyst in 0.5 M H2SO4. Changes in the position and shape of the CO oxidation peak were analysed after 200, 500, 1000, 3000 and 5000 cycles to gain insights into the structural modifications occurring to each catalyst throughout these stress tests. The results provide insights into the structural changes and the processes of these modifications that take place during these tests. These studies also suggest an optimum structural configuration of Pt, Ru, and Pd metals among the tested alloys and nanostructures to provide a relatively high degree of structural stability while maintaining tolerance to CO poisoning. This type of comparative analysis can also be utilized in guiding the optimization of additional nanocatalysts for other electrocatalytic reactions. References H.-F. Oetjen, V. M. Schmidt, U. Stimming and F. Trila, J. Electrochem. Soc., 143, 3838 (1996).E. Antolini, J. Solid State Electrochem., 15, 455–472 (2011).L. Gancs, B. N. Hult, N. Hakim and S. Mukerjee, Electrochem. Solid-State Lett., 10, B150 (2007).A. Hrnjic et al., Electrochim. Acta, 388, 138513 (2021).